<%BANNER%>

Design of an Ultra-Low-Power Control System for a Self-Powered Wireless Sensor

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PAGE 1

DESIGN OF AN ULTRA LOW POWER CONTROL SYSTEM FOR A SELFPOWERED WIRELESS SENSOR By DAVID EDWARD JOHNSON A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2006

PAGE 2

Copyright 2006 by David Edward Johnson

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ACKNOWLEDGMENTS I would like to thank my advisors, Toshikazu Nishida, Khai Ngo, Jenshan Lin, and Juan Nino, who provided encouragement and support in completing my thesis. My advisors also had an abundance of patience with me as I balanced a full time career with working on a masters degree, and for this I am deeply thankful. Specifically I would like to thank Toshikazu Nishida for giving me a job with IMG as an undergraduate, which provided me with the opportunity to work on remarkable research projects. The position with IMG definitely instilled a desire to continue my education. Many thanks go to my fellow students that worked on sections of the self-powered wireless sensor design. Specifically, thanks go to Jerry Jun and Ed Koush for their work and help with the RF transmitter characterization as well as their work with the hydrogen sensor. Thanks go to Shengwen Xu for his work on the energy reclamation circuit. Thanks go to Anurag Kasyap for his work on the piezoelectric beams. I would like to thank all of the members of IMG who allowed me to work freely in the lab and provided much needed assistance with the use of equipment. Thanks go to the staff at the University of Florida, including Linda Kahila, Shannon Chillingworth, and Janet Holman. My employer, Cisco Systems, deserves special thanks for allowing me to continue my education while working full-time. I would like to explicitly thank my manager Phil Van Atta for his proctoring as well as his understanding and encouragement in completing my degree. I would also like to thank Frank Juliano for his assistance in covering my work at Cisco while I took trips to school in order to work on my thesis. iii

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Special thanks go to a college roommate, Gregory Schaefer, who helped me in deciding to pursue an undergraduate degree in engineering, and ultimately a masters degree in engineering. I would like to thank Dr. Roland Federico and his family for allowing me the use of their condominium when I visited Gainesville to work on my thesis. I would like to thank my family and friends for their love and support. My deepest thanks go to Barbara Fleener, whose love and support have kept me motivated for the past few years. Finally, my thanks go to God for instilling the desire to learn within my soul. iv

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TABLE OF CONTENTS page ACKNOWLEDGMENTS .................................................................................................iii LIST OF TABLES ............................................................................................................vii LIST OF FIGURES .........................................................................................................viii ABSTRACT .........................................................................................................................x INTRODUCTION ...............................................................................................................1 Research Goals .............................................................................................................3 Thesis Organization ......................................................................................................4 THEORETICAL DEVELOPMENT ...................................................................................6 Background Information ...............................................................................................6 Previous Work ..............................................................................................................7 Design Challenges ........................................................................................................9 DESIGN OF A CONTROL SYSTEM FOR A SELF POWERED WIRELESS SENSOR .....................................................................................................................11 Selection of a Microcontroller ....................................................................................11 Design of Programming Code ....................................................................................14 Level Monitoring .................................................................................................15 Data Transmitting ................................................................................................16 Programming Techniques for Minimizing Power Consumption ........................18 RF Transmitter ............................................................................................................21 Design of RF Transmitter ....................................................................................21 Power Consumption for a RF Transmitter ..........................................................22 Free Space method ..............................................................................................22 Plane Earth method ..............................................................................................24 Techniques for minimizing power consumption .................................................25 Construction of a Prototype ........................................................................................26 EXPERIMENTAL METHODS AND SETUP ..................................................................31 Energy Sources ...........................................................................................................31 v

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Energy from Light ...............................................................................................31 Energy from Vibrations .......................................................................................32 Data Sensor .................................................................................................................33 Testing Methodology for the Control System ............................................................36 Methods for Measuring Power Consumption ......................................................36 Methods for Characterizing RF Transmission ....................................................37 Testing of a Self-Powered Wireless Sensor ................................................................39 EXPERIMENTAL RESULTS ...........................................................................................42 RF Transmitter Characterization ................................................................................42 ADC Testing ...............................................................................................................44 Testing of the Control System ....................................................................................45 RF Transmission ..................................................................................................45 Power Consumption ............................................................................................51 Functional Verification ........................................................................................64 System Integration ......................................................................................................65 CONCLUSIONS ................................................................................................................67 Conclusions .................................................................................................................67 Future Work ................................................................................................................68 MICROCONTROLLER PROGRAM CODE ...................................................................70 Code for Level Monitoring .........................................................................................70 Code for Data Transmitting ........................................................................................72 LIST OF REFERENCES ...................................................................................................75 BIOGRAPHICAL SKETCH .............................................................................................79 vi

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LIST OF TABLES Table page 3-1 Power characteristics of microcontrollers. ...............................................................12 3-2 Added features of microcontrollers. .........................................................................13 5-1 Maximum transmission distances for different antenna locations. ..........................51 vii

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LIST OF FIGURES Figure page 2-1 Simple state machine of a control system for a wireless sensor. ...........................10 3-1 System level block diagram of self powered wireless sensor. ...............................11 3-2 Microcontroller flow chart for level monitoring. ...................................................15 3-3 Microcontroller flow chart for data transmitting. ..................................................17 3-4 Frequency versus current consumption for MSP430 operating at 3V. ..................19 3-5 Supply voltage versus current consumption for MSP430 operating at 32 kHz. ....20 3-6 Schematic of TX-99 RF transmitter. ......................................................................22 3-7 Transmission distance versus power loss for given frequencies. ..........................23 3-8 Power loss vs. transmit distance for various transmitter and receiver heights. .....25 3-9 Schematic for control system prototype. ...............................................................27 3-10 Top side PCB layout for control system prototype. ...............................................28 3-11 Bottom side PCB layout for control system prototype. .........................................29 3-12 Control system prototype with RF transmitter. ......................................................30 3-13 Top side of control system prototype. ....................................................................30 4-1 Bimorph PZT composite beams. ............................................................................33 4-2 Direct charging circuit for energy reclamation from vibrations. ...........................33 4-3 Schematic of biasing circuit for hydrogen sensor. .................................................35 4-4 Picture of biasing circuit with hydrogen sensor. ....................................................36 4-5 Whip antenna. ........................................................................................................38 4-6 Floor layout of the atrium area in NEB. ................................................................38 viii

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4-7 Test setup. ..............................................................................................................39 4-8 Gas chamber for testing hydrogen sensor. .............................................................40 5-1 Transmit power from Ming TX-99 RF transmitter. ...............................................44 5-2 Received spectrum at 1 meter from transmitter. ....................................................46 5-3 Received spectrum at 8 meters from transmitter. ..................................................47 5-4 Received power at varying distance from transmitter. ..........................................48 5-5 Received signal at 8 meters. ...................................................................................49 5-6 Received signal at 10 meters. .................................................................................50 5-7 Received signal at 14.5 meters. ..............................................................................51 5-8 Scope capture of voltage across series resistor during initialization. ....................53 5-9 Scope capture of voltage across series resistor during idle state. ..........................54 5-10 Transmit power for a single data bit. .....................................................................55 5-11 Zoomed in view of transmit power for a single data bit. .......................................56 5-12 Scope capture of transmission power for data pattern '1011101011'. ....................58 5-13 Scope capture of transmission power for data pattern '1101100101'. ....................59 5-14 Scope capture of transmission power for data pattern '1010000110'. ....................60 5-15 Different data patterns for two "1" valued bits. .....................................................61 5-16 Maximum average transmission power vs. number of "1" valued bits. ................62 5-17 Duty cycle vs. average power for level monitoring. ..............................................64 ix

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Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science DESIGN OF AN ULTRA-LOW POWER CONTROL SYSTEM FOR A SELF-POWERED WIRELESS SENSOR By David Edward Johnson May 2006 Chair: Toshikazu Nishida Major Department: Electrical and Computer Engineering Wireless sensors are becoming more commonplace in applications where harsh environments or remote locations make it difficult to run wires. Making these sensors self-powered is essential as battery replacement is extremely difficult. The drawback of being self-powered is that the system must rely on the environment for power. With the small size of a wireless sensor in mind, the amount of energy available is minimal. Hence there is a need for a control system that manages when power is distributed to different portions of the design. This thesis focuses on the design of a control system that manages the components of a self-powered wireless sensor, with optimization for ultra-low power. The control system was designed with several flexibilities, so it can used to interface a variety of sensors. The control system consists of a Texas Instruments MSP430 microcontroller and a MING TX-99 RF transmitter. The MSP430 microcontroller has an analog to digital converter which is used to encode data from the sensor. One of the x

PAGE 11

output ports on the microcontroller is used to interface the TX-99 RF transmitter. One of the keys to minimizing power consumption is for the microcontroller to stay in a low power mode. The microcontroller supplies the data and power to the RF transmitter; so power can be minimized further by applying power to the transmitter only when data is being transmitted. The power consumption of the control system is partitioned into three main portions: sensor sampling, data transmission, and idle time. The microcontroller code was written so that the average power used to sample data from a sensor was the same as the power to remain idle, which is 2.5W. The average power used to transmit a single bit is 261W. The code running on the microcontroller was written to be able to vary the idle time between sampling and transmissions. By allowing the idle time to be varied the total average power can be varied according to how often the end user wants to transmit data. Two modes of operation were designed, (1) threshold based and (2) data based, so that the design would have the flexibility of providing either level detection or data logging. The flexibility in the design while maintaining minimal power requirements allows this control system to work for a range of wireless sensor applications. The control system was tested with a ZnO nano-rod hydrogen sensor while being powered from both solar and piezoelectric energy. xi

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CHAPTER 1 INTRODUCTION Applications for wireless sensors are growing in many fields from environmental and health monitoring to disaster relief. Further applications for wireless sensors include space exploration, military uses, chemical processing and other fields where the environmental conditions make it extremely difficult to achieve the desired task without having a remote wireless sensor. Harsh environmental conditions that make it difficult to access the sensor combined with limited battery life create a need for wireless sensors to be self-powered. Self-powered systems harvest ambient energy from the environment that would normally be unused. An abundant number of sources for environmental energy exist, including light, air flow, fluidic flow, heat, acoustics, vibrations, and chemical reactions to name a few. One major challenge in designing a self-powered wireless sensor is to minimize the amount of power required by the system. Minimizing the power requirements is necessary due to limitations on the amount of energy that can be scavenged from any particular environment, as well as the size of the energy collector required to harvest any given amount of energy. For instance photovoltaic solar cells only use about 15% of the energy available from sunlight [1], so to get more electrical energy multiple solar cells must be combined into an array, hence increasing the size of the energy collector. Minimizing the size of the system is important so that the system can be conveniently placed in various remote locations. 1

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2 The need for low power requirements in a wireless sensor becomes apparent when examining the lifetime of the device. Batteries start to lose their power density over time and must be replaced in order to continue operating the system. Several wireless sensor systems exist that make battery replacement extremely difficult. For instance a wireless biomedical sensor that is implanted under the skin cannot be easily accessed to replace a battery [2]. One option is to use a rechargeable battery, however even rechargeable batteries lose their power density over time, albeit at a much lower rate then standard batteries [3]. Another option for power storage is to use a super-capacitor, which are similar to standard capacitors except with much higher capacitance on the order of a hundred millifarads to several farads [4]. Ultimately the selection of an energy storage device is dependant on the expected lifetime for the final application. This thesis minimizes the average power so that the lifetime of the system can be extended based on the lifetime of the storage device, thus making selection of a storage device much easier. Most previous work has focused on application specific wireless sensors, and while ultimately the final system tested in this thesis is for a very specific hydrogen sensor the design is extremely flexible so that it can be easily modified to suit any application. While this thesis focuses on the application of a wireless sensor, there are other simple control systems with similar low power constraints that should be looked at for ideas on decreasing the required power. Apart from wireless sensors, simple control systems that gather and utilize data from a sensor are prevalent in everything from digital tire pressure gauges [5] to a new pair of Adidas sneakers that have a built-in microcontroller which uses data from sensors to control the support and cushioning in the shoe [6]. Complete self-powered wireless sensor systems already exist, in particular the MICA Mote [7], and

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3 Smart Dust [8] both of which were developed at UC Berkeley. However these systems incorporate a sensor, control system, and transmitter and therefore are limited to specific applications. Furthermore Smart Dust consists of a custom designed ASIC that integrates the control system with the energy harvesting circuitry. Commercially available wireless sensor systems are on the market. For example, MicroStrain has two wireless sensor systems which are low power and are small in size: EmbedSense TM which is a passive device that receives power wirelessly and only transmits when queried [9], and StrainLink TM which operates off a battery and still draws power on the order of milliwatts [10]. The design discussed in this thesis will differentiate itself from other designs in that the design will be modular, flexible in its applications, consist of off the shelf components, be able to transmit without querying the system, and maintain ultra-low power requirements on the order of microwatts. Research Goals Rather then focusing on addressing the problems related with increasing the amount of available power this thesis focuses on design techniques for minimizing the amount of power required by the wireless sensor system. The main drain on power is the control system which consists of a microcontroller interfaced to a RF transmitter. The microcontroller also provides the interface to the sensor for data collection. The utmost importance is to find a microcontroller that operates at an extremely low power level while also providing the necessary interfaces to sample data from the sensor as well as send data to the RF transmitter. The goal of this thesis is to design a control system for a self-powered wireless sensor that operates with an average power requirement on the order of a few microwatts. Several techniques for limiting the power consumption of the microcontroller will be

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4 examined including operating the microcontroller at a slower processor speed and using the low power modes available during idle states. Techniques for decreasing the power consumption of the RF transmitter will be examined however the RF transmitter will not be redesigned for this thesis. All parts used in the construction of the control system will be off the shelf components so that reproducibility and ease of reuse is simple. An additional goal of this thesis is for the control system to be designed with a high level of flexibility, so that the control system does not become application specific. The microcontroller code will be written with variables that are easily modified to change the timing between sampling and transmitting data. The RF transmitter will be a module so that a different transmitter could easily replace the one used without effecting the operation of the system. The microcontrollers sensor interface will be a simple analog to digital converter so that many different sensors can be used. The microcontroller code will be written with a very basic modular structure so that modifying the code to encode or store the data can be done very easily with minor modifications to the code. Thesis Organization This thesis is organized into six chapters. This chapter provides background information about wireless sensors as well as providing an introduction to the research goals of this thesis. Chapter 2 reviews the previous work and theoretical information involved with designing a control system for a self-powered wireless sensor. Chapter 3 discusses the actual design process for the control system as well as an introduction to the energy harvesting circuits used. Chapter 4 explains the experimental methods used for testing the components of the control system as well as integrating the system into a self-powered wireless sensor. Chapter 5 discusses the results of testing the control system and the results of the system level testing of the complete self-powered wireless sensor.

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5 Chapter 6 outlines conclusions made from the testing, and provides insight into possible future work to further decrease the power requirements of a self-powered wireless sensor.

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CHAPTER 2 THEORETICAL DEVELOPMENT Background information about self-powered wireless sensors, specifically their control systems is presented in this chapter. Aspects of other peoples work in the area of wireless sensors are discussed in this chapter, including the growing amount of research being done on wireless sensors networks. Finally this chapter will review the challenges associated with designing an ultra-low power control system for a self-powered wireless sensor. Background Information The simplest form of a wireless sensor is a RF transmitter connected to a data sensor. However a RF transmitter without any control circuitry will continuously transmit. Continuous transmission requires a continuous power source. In order to minimize the average power required a simple control circuit needs to be implemented. The control circuit is used to control the time the transmitter requires power as well as providing an interface between the data sensor and the RF transmitter. The control system also provides the ability to encode the data being transmitted as well as make decisions about transmissions based on the data from the sensor. A control system cannot be designed without having some knowledge about what is being controlled. In the case of a self-powered wireless sensor, the control system controls the sampling rate of data from the sensor, the frequency of transmission, the data rate to the transmitter, and if any decision making or data encoding needs to be done prior to transmission. The precision level of the microcontrollers interface to the sensor must 6

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7 be known in order to determine if any external amplification of the sensors signal is needed. The type of sensor also matters, since passive sensors require a biasing circuit to create a voltage that represents the data value from the sensor. For specific applications a custom microcontroller could be designed to incorporate any extra circuitry needed to interface the specific sensor. This thesis focuses on the design of a control system which is generic in that it is very flexible and has been designed with a high level of modularity so that the control system can be used in a variety of different wireless sensor applications. Additionally the control system will be designed using commercially available components, so that the design process is simpler and quicker then attempting to design a custom controller. Using off the shelf components also provides a better opportunity for success over a custom controller since the components have already been tested by the manufacturer. Ultimately the techniques used in this thesis to lower the average power of the control system could be applied to custom designed controllers as well. Previous Work Over the past decade research in the area of wireless sensors has been growing. The majority of research has been toward creating wireless sensor networks with their own operating systems and communication protocols. In fact, several companies have developed wireless sensor nodes that are specifically designed for use in creating a wireless sensor network. Startup companies like Coronis Systems [11], Dust Networks [12], Crossbow Technology Inc. [13], and Cirronet, Inc. [14] all have developed wireless sensor networks. Even larger companies such as Intel are researching wireless sensor networks as a potential new revenue stream [15]. With the increasing number of different sensor networks there has been more and more research into the standardization

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8 of communication protocols for wireless sensor networks. The University of California, Los Angeles has developed a sensor network-specific media access control protocol called S-MAC [16]. The goal of S-MAC is to reduce the amount of energy wasted due to constant listening, collisions, overhearing and overhead. A more industry wide standard is IEEE 802.15.4 [17], which defines both the physical and media access control protocols for wireless sensor networks. Several companies have combined to create the ZigBee Alliance [18], which promotes the IEEE 802.15.4 standard. S-MAC and IEEE 802.15.4 are just two of the protocols being developed; other protocols include the use of Bluetooth [19], PicoRadio [20], and various simple RF communications. One of the more interesting ideas is the use of TCP/IP protocol to allow wireless sensor networks to connect to the Internet [21]. Connecting wireless sensor networks to the Internet would allow for remote monitoring from anywhere with Internet access. One of the main challenges to wireless sensors is maintaining an ultra-low average power consumption. Back in 1995 a group from the University of Michigan designed a low-power wireless sensor system that operating using 700W of power, however the system used a 6V battery and barely made it halfway through a year before needing to replace the battery [22]. Since then the average power of most wireless sensor systems has been targeted at under 100W, with most people aiming to reach average power on the order of a few microwatts or less. A variety of techniques for lowering the average power consumption, other then changing the data protocols, have been and are currently being developed. One such technique is designing ultra-low power transmitters, such as the CMOS RFIC being developed by Mohammed Ismail in Sweden [23]. A more common technique for lowering the average power is to build a custom microcontroller.

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9 While custom microcontrollers can operate on less power than most commercial microcontrollers, the design process for generating and testing a custom IC is long and tedious. Furthermore, a corporation that focuses on producing a generic microcontroller that is ultra-low power has an advantage over universities and small groups that try to develop a custom device due to the man power and engineering time available to the corporation. One of the ultimate goals of low power wireless sensors is to operate off low enough power that the energy required can be harvested through ambient energy sources, thus making the system self-powered. Shad Roundy has written a book [24] that focuses on the area of energy scavenging for wireless sensors. Additionally, Shad Roundy has created a 1.9GHz RF transmit beacon [25], that while it does not sense anything and only acts as a transmitter the design operates of environmentally scavenged energy. A group at the University of California, Berkeley has developed a wireless temperature sensor that operates off the vibrations in a staircase [26]. More and more work on the area of self-powered wireless sensors seems to be available everyday as progress in the area continues at a strong and fast pace. This thesis differentiates itself from other work in that the design is modular and focuses purely on techniques for lowering the power requirements of the control system, while using commercially available components. Design Challenges Several challenges exist when designing an ultra-low power control system for a self-powered wireless sensor. Minimizing the power consumption is the main challenge in designing a control system for a self-powered wireless sensor. Several aspects of the design must be analyzed to determine methods for decreasing the power required. The

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10 control system can be broken down into a state machine consisting of sampling data, transmitting data, and remaining idle. Figure 2-1 shows a simple state machine that represents the cycle of a control system for a wireless sensor. The duty cycle of each state determines the total amount of power required for each particular state across a single cycle. Each state must be analyzed to determine methods for reducing the power consumption of the system while in the particular state. While most of the challenges have to do with minimizing the power consumption of the control system, other challenges include designing the interface to the sensor and the transmitter as well as determining the format of the data to transmit. Figure 2-1. Simple state machine of a control system for a wireless sensor.

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CHAPTER 3 DESIGN OF A CONTROL SYSTEM FOR A SELF POWERED WIRELESS SENSOR A self-powered wireless sensor consists of four major components: energy reclamation, data sensor, microcontroller, and a RF transmitter. Figure 3-1 shows a block diagram of the system. Each component has been designed independently however information from the design of each component was taken into account by the other components. This chapter focuses on the design of the control system, which consists of the microcontroller and RF transmitter. Figure 3-1. System level block diagram of self powered wireless sensor. Selection of a Microcontroller When considering a microcontroller it is important to compare several different features. The most important aspect to consider for the application of a self-powered wireless sensor is for the microcontroller to use the least amount of power. The majority of power used is during the microcontrollers active state. However the standby current 11

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12 is just as important, since the microcontroller will still be drawing power during its sleep mode. The standby current is particularly important since the power source for a self powered wireless sensor may not be constant. So that power is not wasted on the microcontroller operation, it is desired to have a very short wakeup time. Additionally, to conserve power a low port leakage is desired. A comparison of power characteristics for several microcontrollers is shown in Table 3-1. Table 3-1. Power characteristics of microcontrollers. Manufacturer and Model Active Current Standby Current Wakeup Time Brown-Out Reset Port Leakage TI MSP430F1122 14uA@32kHz, 2.5uA@4kHz and 2.2V 4 low power modes from 0.7uA to 0.1uA 6us 50nA 50nA Microchip PIC16F73 20uA@32kHz 1uA 1ms 85uA 1uA Motorola MC9S08G 812uA@1MHz 3 Stop modes: from 4.3uA to 25nA 2.4ms 70uA 25nA Atmel Atmega169 27uA@32kHz 5 Sleep modes, lowest is 0.2uA 10us 19uA 1uA EM Micro EM6617 9uA@32kHz 0.6uA standby 0.1uA sleep NA NA NA XEMICS XE88LC01A 10uA@32kHz 1uA in hibernating mode 0.1uA in sleep mode NA NA NA In order to completely compare the microcontrollers, a comparison of all of the features of the microcontroller is required. For a self-powered wireless sensor the microcontroller must contain an ADC to interface the sensor, a serial output to interface the RF transmitter, and enough memory to hold the runtime code as well as store data from the sensor. A comparison of the features for several microcontrollers is shown in Table 3-2.

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13 Table 3-2. Added features of microcontrollers. MSP430 PIC16F73 MC9S08G Atmega 169 EM6617 XE 88LC01A Oscillator 1MHz internal, input for 32kHz external 32kHz external 32kHz external 1MHz internal, input for 32kHz external 32kHz crystal 100kHz -4MHz RC oscillator, 32kHz external ROM NA NA NA NA 6kB 22kB, 8kB RAM 512B-2kB depending on package 192 bytes 1K-4K depending on package 1KB SRAM (2) 64x4 bit 8 bytes low-power RAM, 512 bytes E 2 PROM NA NA NA 512 bytes 64x8 bit NA ADC 8-channel 12-bit 5-channel 8-bit 8-channel 10-bit 8-channel 10-bit 2-channel 8-bit 16 +10 bit Zooming ADC UART 2 USART USART NA Serial NA Serial UA I/O Ports 48 I/O lines 22 I/O lines 34-56 I/O lines depending on package 53 general purpose I/O lines (1) 4-bit input, (2) 4 bit bi-dir, (1) serial write buffer 24 I/O lines FLASH 16kB -60kB depending on package 4Kx14 bit 16K-60K depending on package 16KB NA NA In terms of features, all of the microcontrollers have adequate features for the design of a self-powered wireless sensor. From the power characteristics listed in the Table 3-1, the MSP430, EM6617, and XE88LC01A look like clear favorites (all are around 10 A active current down to 0.1 A sleep mode current). However neither the EM6617 not XE88LC01A have given data for port leakage or wakeup time. The final decision was to go with the TI MSP430 since it has an abundance of available resources and since TI gave samples and offered support.

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14 Design of Programming Code The code for the microcontroller is designed with flexibility in mind. This is done so that the design can be tailored to any specific application with minimal code redesign required. Along those same lines two different versions of code were written. Each version of code presents a slightly different mode of operation. The two modes of operation are listed below. 1. Level Monitoring Constantly monitors sensor and sends a single emergency RF pulse when the sensors data goes above a given threshold value. 2. Data Transmitting Constantly monitors sensor and sends data every given number of seconds. The delay is overridden if the sensors data goes above a given threshold value. The program code was written in the C programming language. The C code for both modes of operation is given in Appendix A. Regardless of the mode of operation, the structure of the code is very similar. The microcontroller code is broken into two main sections: initialization and interrupt routines. This allows the microcontroller to constantly be in a sleep mode with the CPU turned off, functioning in an interrupt driven architecture. The initialization section is common for both modes of code. The sampling delay is defined in the initialization section as CCR0. CCR0 is defined in terms of a number of 32 kHz clock cycles. For example, if CCR0 is set to 500, then the delay equals 500 divided by 32 kHz or about 15 ms. The interrupt routines consist of a timer interrupt and an ADC interrupt. The timer interrupt is used to initiate ADC samples, as defined by the amount of time CCR0 is set to in the initialization section. The ADC interrupt occurs once an ADC sample has been

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15 taken. The ADC interrupt is used to determine what if anything to send out over the RF transmitter. Level Monitoring For the case of level monitoring the microcontroller conserves power by only transmitting a signal when the sensors data goes above the threshold value. The microcontroller runs a very basic state machine consisting of the following states: initialization, collect data, analyze data, transmit data, and sleep. A timer is used to delay sampling the sensor data. A basic flow chart of the microcontrollers state machine is shown in Figure 3-2. Figure 3-2. Microcontroller flow chart for level monitoring. The state machine is designed to run using a single timer to cycle through sampling data and sleep mode. However if the data is above a threshold value then the microcontroller immediately sends a single RF transmission and resets the timer. One foreseen problem is that when the sensors data is above the threshold value the control system will constantly be transmitting until the sensors data drops below the threshold value. The solution to this problem is to use a static variable that is set in the code once a

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16 transmission occurs. This variable gets cleared when the sensors data falls back below the threshold level. The case of level monitoring has two settings that can be modified: threshold value and transmit pulse length. The ADC sample is examined in the line if (ADC10MEM < 0x1FF) where 0x1FF is the threshold value. Since the ADC sample is 10 bits the maximum value is 0x3FF and the minimum is 0x000, so 0x1FF is halfway between the two. The threshold value is defined as a function of the supply voltage, so for a 2 V supply voltage each bit is equal to 2 V / (2 10 ) = 1.95 mV. For the case of a sensor that needs to be monitored for a falling voltage level, the code can be modified so that the comparative statement is if (ADC10MEM > 0x1FF). The data pulse is defined as just a single voltage pulse on the data line to the RF transmitter. The length of time the data line is held high is defined by the variable pulse_length, which is defined in terms of a number of 32 kHz clock cycles. The shorter the length of the pulse the less average power the system takes, as the pulse is used to turn on the RF transmitter. Data Transmitting For the case of data transmitting, the microcontroller cycles between sampling data and transmitting the data. The microcontroller runs a very basic state machine consisting of the following states: initialization, collect data, analyze data, store date, transmit data, and sleep. The state machine is designed to run using timers to add delays between collecting data and transmitting the data. However if the data is above a threshold value then the microcontroller immediately sends a single RF transmission and resets the timers. The code has the same foreseen problem as the level monitoring code, that when the sensors data is above the threshold value, the control system will constantly be transmitting until the sensors data drops below the threshold value. The same static

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17 variable solution from the level monitoring code is used for the data transmitting code. A basic flow chart of the microcontrollers state machine is shown in Figure 3-3. Figure 3-3. Microcontroller flow chart for data transmitting. The case of data transmitting has three settings that can be modified: transmission delay, transmitted data, and threshold value. The threshold value is defined similarly to the threshold value for the level monitoring code. The transmission delay is defined by the variable transmit_dly, which is defined in terms of a number of 32 kHz clock cycles. The transmitted data is currently defined simply as the sampled ADC value, where each bit represents 1.95 mV for a 2 V supply voltage. This is the simplest method for saving processing power, however once a sensor is chosen a coding scheme can be defined that would decrease the number of bits needed to transmit, and hence decrease the amount of power even further.

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18 Programming Techniques for Minimizing Power Consumption One of the keys to minimizing power consumption is to put the microcontroller in a sleep mode for as long as possible. The MSP430 has 5 modes of operation: Active Mode and Low Power Modes 0-4 (LPM0-LPM4). As the level increases from LPM0 to LPM4, the amount of the microcontroller that is active decreases. The CPU portion of the microcontroller is turned off in all low power modes. The peripherals like the ADC and USART can be enabled and disabled individually. By individually enabling and disabling the peripherals as they are needed power is not wasted on inactive portions of the microcontroller. Another method for decreasing the amount of power consumed by the microcontroller is to decrease the clock frequency. The MSP430 has an internal 1MHz clock, however the microcontroller can be operated using an external 32 kHz crystal oscillator. Both the ADC and USART can be operated off the 32 kHz oscillator, which means the 1 MHz built in clock can remain off at all times except during initialization. Using an external clock allows the MSP430 to run in LPM3 which is the second lowest low power mode. The datasheet for the MSP430 [27] gives the following equation that correlates how the current consumption changes as the clock frequency changes. I(AM) = I(AM) [1 MHz] f(System) [MHz] (3.1) Where I(AM) is the current for active mode operation, I(AM) [1 MHz] is the current for active mode operation at 1 MHz, and f(System) is the operating frequency. The equation shows a linear relationship between the current consumption and the operating frequency. According to the datasheet the current consumed at 1 MHz is 500 A, thus the equation can be simplified to

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19 I(AM) = (500 A / MHz) f(System) (3.2) Figure 3-4 shows a graphical representation of the current consumption versus frequency, for a supply voltage of 3 V. Frequency vs Current Consumption05010015020025030035040045050002004006008001000Frequency (kHz)Current (uA) Figure 3-4. Frequency versus current consumption for MSP430 operating at 3V. The next major way to decrease the power consumption for the microcontroller is to decrease the supply voltage. The microcontroller can operate with a supply voltage between 1.8 V and 3.6 V. The MSP430 datasheet provides the following equation that correlates the supply voltage to the current consumption based on a 1 MHz operating frequency. I(AM) = I(AM) [3 V] + 210 A/V (V CC 3 V) (3.3)

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20 Where I(AM) [3 V] is the current consumption when operating at 3 V and V CC is the supply voltage. This equation provides a linear relationship between supply voltage and current consumption and is shown graphically in Figure 3-5. Supply Voltage vs Current Consumption2002503003504004505005506006501.822.22.42.62.833.23.4Supply Voltage (V)Current Consumption (uA) Figure 3-5. Supply voltage versus current consumption for MSP430 operating at 32 kHz. The original design for the programming code involved using the UART to serially transmit data to the RF transmitter. Even with the ability to enable and disable the UART around a sleep cycle, the default value being output on the UART was a . With the UART outputting a the RF transmitter would be transmitting a which would be a waste of power. In order to minimize the power, the code was re-written to use a single output port of the microcontroller and simply serially shift the data bit by bit to RF transmitter. One additional benefit of using an output port instead of a UART is that the data pattern does not have to follow the parameters defined for a UART. Having open

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21 options for the data pattern means that the data can be any length and can be as many or as few bits as desired. RF Transmitter The used of a RF transmitter is required to wirelessly transmit the data from the sensor. An RF transmitter has several design considerations such as transmission frequency, transmit distance, data rate, and power usage. The power usage is the most important of these design criteria in the design of a self-powered wireless sensor. However the transmit distance has a minimal length to be useful. Thus the design needs to be optimized for lowest possible power for a useful transmission distance. Design of RF Transmitter RF transmitters typically consist of four parts: frequency determining device, amplifier, feedback circuit and output. A relatively simple design is the key to keeping the RF transmitters power usage low. Obviously the fewer active components in a design the less power that is required. The simplest form of a frequency determining device is a LC oscillator. The LC oscillator uses an inductor and a capacitor to provide initial oscillations. The resulting oscillation of the inductor and capacitor is then input into an amplifier via a feedback circuit. The amplifier then provides the amplified oscillating signal to the output. The following simple formula is used to determine the oscillation frequency based on the values of the inductor and capacitor. (3.4) A Colpitts Oscillator is an example of a circuit design that employs a LC oscillator and is a prime candidate for a very simple RF transmitter. The Colpitts Oscillator consists of a LC oscillator along with a single transistor acting as a unity amplifier and

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22 feedback circuit. Ming Microsystems Inc. has fabricated a RF Transmitter (TX-99) based on the Colpitts Oscillator design. Figure 3-6 shows the schematic for the TX-99. The datasheet [28] provides full operational details for the TX-99. Figure 3-6. Schematic of TX-99 RF transmitter. Power Consumption for a RF Transmitter The power required for data transmission depends on the transmission distance, transmission frequency, and the data rate. There are two methods for estimating the power required for RF transmission: Free Space and Plane Earth. The Free Space method assumes unobstructed transmission in all directions. The Plane Earth method takes into account that the transmission is done on a plane and hence depends on the height of the transmitter and receiver above the plane. Free Space method The Free Space method yields the following equation [29] relating the power loss to the transmission frequency and the distance between transmitter and receiver, assuming a dielectric constant of 1. Power Loss = 20*log(4**D*f/c) (3.5)

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23 Where D is the distance between the transmitter and receiver, f is the frequency, and c is the speed of light. Power loss is defined as the difference in the power of the transmitted signal between the transmitter and the receiver. A receiver requires the transmitted signal to have some minimum amount of power, in order to interpret the signal. Hence the minimum power required for transmitting is primarily dependant on the minimum amount of power required in the transmitted signal at the receiver. The frequency of the transmission and the distance between the transmitter and receiver add to the total power required for transmitting. Figure 3-7 shows a plot of transmission distance versus power loss for given frequencies. Power Loss vs Transmit Distance 0204060801001201101001000Distance (m)Power Loss (dBm) 49MHz 300MHz 434MHz 916MHz 2400MHz Figure 3-7. Transmission distance versus power loss for given frequencies. As the transmission frequency decreases, the power loss for a given distance decreases. Similarly for a given frequency, as the distance between transmitter and receiver increases the power loss increases. Thus, to minimize the amount of power lost

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24 in transmission, and hence minimize the amount of power required for transmitting, a low frequency is desired. Plane Earth method The Plane Earth method takes into account the height above the Earth of the transmitter and receiver. One benefit of reviewing the Plane Earth method is that the equation that describes power loss is valid for an entire frequency band. Removing the dependence on frequency provides an equation that only depends on the geometry of the distance between the transmitter and receiver. According to the Plane Earth method the following equation is valid for frequencies in the UHF band (300 MHz to 3 GHz). Power Loss = 20*log[D 2 /(H T *H R )] (3.6) Where D is the distance between the transmitter and receiver, H T is the height of the transmitter, and H R is the height of the receiver. The above equation makes it obvious that as the distance between the transmitter and receiver increases the power loss increases, however as the height above the ground of the transmitter and/or receiver increases the power loss decreases. Thus, one method for minimizing power would be to increase the height of the transmitter and receiver. A graphical representation of the Plane Earth equation for power loss is shown in Figure 3-8.

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25 Power Loss vs Transmit Distance 020406080100120140101001000Distance (m)Power Loss (dBm) Ht = 0.5m, Hr = 0.5m Ht = 1m, Hr = 0.5m Ht = 1m, Hr = 1m Ht = 5m, Hr = 1m Ht = 5m, Hr = 5m Figure 3-8. Power loss vs. transmit distance for various transmitter and receiver heights. Techniques for minimizing power consumption From the Plane Earth and Free Space methods, the obvious ways to minimize power consumption are to use a low frequency and to increase the height of the transmitter and receiver. In terms of the operation of the transmitter, one method for lowering the required power is to use On/Off Keying (OOK) which synchronizes the power supplied to the transmitter with the data being supplied. Compared to other methods such as Frequency Shift Keying (FSK), OOK transmitters are simpler, require at least 50% less transmitter current, and require less bandwidth [30]. The control system designed for this thesis uses the OOK method by connecting the output port of the microcontroller to both the data and supply voltage inputs of the transmitter. Thus the transmitter is only powered when it is transmitting a .

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26 Another method for reducing the power is to decrease the number of bits transmitted per transmission. However the number of bits transmitted depends on the end application for a wireless transmitter and what information is desired to be transmitted. Similarly, the number of transmissions per second directly effects the amount of power required. The following equation correlates the number of transmissions per second to the average power consumed by the system every second. P avg = [P uC (1 (t tx N tx )) + P tx t tx N tx ] (3.7) Where P uC is the power consumed by the microcontroller, t tx is the time for a single transmission, N tx is the number of transmissions per second, and P tx is the power required for a single transmission. Construction of a Prototype A prototype for the control system described in this thesis is constructed using a double-sided copper clad printed circuit board (PCB). Protel software [31] was used to design the electrical schematic and mechanical layout for the PCB as shown in Figure 3-9, Figure 3-10, and Figure 3-11. An LPFK [32] computer-controller surface milling machine was used to etch the design into the circuit board. The layout is designed on both sides of the PCB; one side contains the electrical components and the other side has connectors to interface the various components of the prototype. BNC connectors are used to interface the power source, data sensor, and RF transmitter to the microcontroller. A 14 pin header is used for programming the microcontroller via the microcontrollers JTAG pins. The electrical components consist of the microcontroller, a crystal oscillator, a 50 k resistor used to pull the reset pin of the microcontroller to V cc and filter capacitors as recommended by the MSP430 datasheet. A photograph of the prototype

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27 with the RF transmitter is shown in Figure 3-12. The photograph shows the bottom side of the board, which contains the interface connectors. The microcontroller and other electrical components are on the top side of the board and are shown in Figure 3-13. Figure 3-9. Schematic for control system prototype.

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28 Figure 3-10. Top side PCB layout for control system prototype.

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29 Figure 3-11. Bottom side PCB layout for control system prototype.

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30 Figure 3-12. Control system prototype with RF transmitter. Figure 3-13. Top side of control system prototype.

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CHAPTER 4 EXPERIMENTAL METHODS AND SETUP Testing of a control system for a self-powered wireless sensor consists of testing and verification of the control system as well as testing the control system integrated with the rest of the self-powered wireless sensor. Before integrating the control system with the rest of the self-powered wireless sensor, the energy sources and data sensor must be designed and tested. This chapter gives an overview of the experimental methods and setup required for testing the control system, as well as describing the energy sources and the data sensors used in testing. Energy Sources A self-powered wireless sensor must get its power from environmentally scavenged energy. Environmental energy sources include light, wind, heat, vibrations, and the flow of water. Light and vibrations are the two sources that are focused on in this thesis. The reasons for choosing those two sources are the availability of energy conversion devices, the ability to replicate and control the sources, and the various locations where these energy sources are available. Energy from Light One of the most readily available environmental sources of energy is the sun. Solar energy is a common energy source used in a wide range of applications from small handheld calculators to traffic signals. Solar cells are used to convert energy from light to electrical energy. The size and efficiency of the solar cell determines the amount of electrical energy available for any given amount of light. Due to the typical constraints 31

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32 on the size of a wireless sensor, a solar cell used for a wireless sensor needs to be small and highly efficient. IXYS Semiconductor makes a 6mm x 6mm monocrystalline high efficiency solar cell with their part number XOD17-04B [33]. The XOD17-04B solar cell has an open circuit voltage of 630 mV and a short circuit current of 12 mA. In order to attain a higher open circuit voltage multiple solar cells are connected in series. A circuit designed by Shengwen Xu [34] is used to transfer the energy from the solar cell into a storage capacitor that is used to provide the power to the control system. Details of the circuit are shown in the conference paper by Shengwen Xu [34]. Energy from Vibrations Vibrations are a common source of environmentally reclaimed energy. The use of piezoelectric materials to convert vibrations to electrical energy is widespread [35]. Selecting a piezoelectric material is dependant on several criteria, specifically vibration frequency, efficiency, size and scalability, and magnitude of vibration. Since this thesis focuses on a control system for an ultra-low power wireless sensor, the selection of a piezoelectric is based primarily on size and efficiency. Piezo Systems, Inc. makes a Double Quick Mounted Y-Pole Bender, D220-A4-203YB [36]. This bimorph beam consists of two parallel connected 1.25 x 0.25 x 0.02 pieces of 5A4E piezoceramic [37] on an Aluminum beam. Four of these bimorph beams are mounted on a shim which is connected to an impedance head, as shown in Figure 4-1. A simple direct charging circuit, shown in Figure 4-2, is used to store the energy reclaimed from the beams. The circuit consists of four Fairchild Semiconductor BAT54 Schottky diodes, a 330F electrolytic capacitor and a Panasonic VL1220 2V battery. The four bimorph beams

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33 were connected in parallel to the input of the circuit, in order to maximize the input current. Figure 4-1. Bimorph PZT composite beams. Figure 4-2. Direct charging circuit for energy reclamation from vibrations. Data Sensor The microcontroller used in the control system requires an analog voltage as the input for the sensor. This means that any type of data sensor used with the control system must either vary voltage in accordance with what the sensor is sensing, or have some

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34 circuitry that converts the output of the sensor into an analog voltage. To control the voltage of the sensor during initial testing a simple reference voltage supply is used as the data sensor input. Once the control systems functionality is verified a solar cell is used as the data sensor input. A solar cell is used as an example of a sensor that provides a varying voltage as the medium it is sensing changes (in this case as the amount of light shined on the sensor varies). The final sensor that is tested is a hydrogen sensor. The hydrogen sensor is made of a layer of ZnO nanowires with palladium deposited on top. The sensor acts like a variable resistor, where the resistance changes with respect to the concentration of hydrogen. A characterization of the sensor is required in order to determine how to interface the sensor with the microcontroller. Details about the sensor including a characterization of the sensor are written in several papers [38]. Since the sensor acts like a variable resistor a biasing current is required in order to get a voltage to input to the microcontroller. The design of a biasing circuit was done by Jerry Jun. A schematic of the biasing circuit is shown in Figure 4-3. A photograph of the biasing circuit with an encased hydrogen sensor is shown in Figure 4-4. The biasing circuit requires two hydrogen sensors, one encased and the other open to the environment. The encased sensor provides a fixed resistance to compare to the data sensors resistance, which is needed since the resistance value can change with variances in temperature as well as hydrogen concentrations. Using two sensors provides a differential voltage, which is amplified before being input to the microcontroller.

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35 Figure 4-3. Schematic of biasing circuit for hydrogen sensor.

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36 Figure 4-4. Picture of biasing circuit with hydrogen sensor. Testing Methodology for the Control System The control system consists of the microcontroller and the RF transmitter, as described in the previous chapter. The focus of the testing of the control system is to determine the power required and to verify the functionality of the system. To verify the functionality of the system, a regulated voltage supply is used as the source of power. By using a regulated voltage supply, the supply voltage is constant and a small series resistor is used to measure the input current and thus determine the input power. Methods for Measuring Power Consumption Measurements are taken to assess the power required to sense data, transmit data, and remain idle. All power measurements are done using a Tektronix TDS5104B digital phosphor oscilloscope; this provides a visual display of the power over a period of time. Using an oscilloscope also provides a method for determining peak power, average power, and total energy. A Tektronix P6248 differential probe connects to the

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37 oscilloscope to measure the voltage drop across the series resistor, between the output of the power supply and the input of the microcontroller. A differential probe is used to isolate the grounds between the oscilloscope and the control system. A small valued resistor is optimal to minimize the effects of the voltage drop across the resistor on the control system. However, too small a value and the voltage drop will be in the noise region of the oscilloscope. From experimentation with various resistor values, the value of 383 Ohms was determined to be optimal for measurements. The transmitted data can consist of different patterns depending on whether a pulse is transmitted or if a data pattern is transmitted. Several measurements are needed to determine the power required to transmit data. These measurements include the power to transmit: a single pulse, a pattern of all 1s, a pattern of all 0s, and a pattern of mixed 1s and 0s. Methods for Characterizing RF Transmission A characterization of the RF transmission is required to determine the maximum transmission distance, the received power, and the frequency spectrum of the received signal. An Agilent E4448A PSA series spectrum analyzer is used to measure the frequency spectrum and the received power. A wave whip antenna made from 22 gauge copper wire is soldered to a SMA connector which attaches to the spectrum analyzer. A picture of the antenna is shown in Figure 4-5. Similar antennas are attached to the transmitter and receiver to determine the additional transmission distance due to the use of antennas. A Tektronix TDS210 two channel digital real-time oscilloscope is used to view the data at the receiver to verify that it matches the data being transmitted. The maximum transmission distance is determined by when the signal at the receiver no longer matches the signal at the transmitter.

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38 Figure 4-5. Whip antenna. The test setup requires a fixed power supply for the control system as well as a fixed power supply for the receiver. A power source for the spectrum analyzer and oscilloscope is also required. Due to the need for multiple power sources, and the measurements being done over a distance, there are limitations on where the measurements can be taken. The atrium area of the New Engineering Building (NEB) at the University of Florida is where the measurements are taken. Figure 4-6 shows a floor layout of the atrium where the testing takes place. Figure 4-6. Floor layout of the atrium area in NEB. The transmitter is held stationary at one end of the hallway, while the receiver and spectrum analyzer are moved away from the transmitter down the hallway, through the atrium and down into the extended hallway. The height above the floor of the transmitter and receiver factors into the amount of power received. Both the receiver and transmitter

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39 are mounted approximately 0.5 meters above the floor. The transmitter is mounted onto the front end of a chair. The receiver is mounted onto a cart that also holds the spectrum analyzer and oscilloscope. Figure 4-7 shows the setup for the transmitter and the receiver. Figure 4-7. Test setup. Testing of a Self-Powered Wireless Sensor After testing of the control system is complete, the control system is integrated with a sensor and reclaimed energy source to create a complete self-powered wireless sensor. Testing of the self-powered wireless sensor is more for verification purposes then detailed measurements. This testing provides proof that the control system works as it was intended as a portion of a self-powered wireless sensor. Two versions of the self-powered wireless sensor are tested, one being powered from solar energy and the other powered from vibrations. The solar powered system uses a series of IXYS solar cells [33] and an energy harvesting circuit [34]. A flashlight is shined directly on the solar cells to provide a fixed light source. The vibration powered

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40 system uses the bimorph PZT composite beams described earlier in this chapter. A Ling Dynamic Systems V408 shaker is used to vibrate the beams. A hydrogen sensor with biasing circuit is used as the data input to the ADC of the control system. The biasing circuit receives its power from the same energy reclamation circuit as the control system. The hydrogen sensor is placed inside a custom built gas chamber as diagramed in Figure 4-8. Figure 4-8. Gas chamber for testing hydrogen sensor. The gas chamber consists of a glass tube with vacuum valves on both sides and a furnace surrounding the middle of the tube. The furnace was not operated for any of the experiments. Entering the gas chamber from the valve on the right is a hollow glass tube with wires running to a HP4156B Semiconductor Parameter Analyzer. The hydrogen sensor is placed inside this glass tube, and the wires from the parameter analyzer are connected to the hydrogen sensor in order to plot the IV characteristics of the sensor. Alternatively these same wires are used to connect the hydrogen sensor to the biasing circuit, which is placed outside of the gas chamber. A valve on the left side of the gas

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41 chamber is connected to a gas cylinder containing 99.99% compressed nitrogen, a gas cylinder containing 500ppm of compressed hydrogen, and a turbo pump. The turbo pump is used to create a vacuum inside the gas chamber. A series of step by step procedures for setting up and operating the gas chamber to perform tests with the hydrogen sensor is described in the following paragraph. Turn off the nitrogen and hydrogen gas lines and close both Valve 1 and 2. Open Valve 1 and wait for the pressure inside the chamber to drop below 0.01 torr (approximately 0 atm). Inside the gas chamber is now an approximate vacuum. Close Valve 1 and turn on the nitrogen gas line at maximum flow rate until the pressure inside the chamber reaches 760 torr (1 atm), then turn off the nitrogen gas line. The nitrogen is used to clear the gas chamber so that the hydrogen sensor starts from 0ppm of hydrogen. Turn on hydrogen gas line at maximum flow rate and open Valve 2. Opening Valve 2 connects the turbo pump, which creates a stream of 500ppm hydrogen gas flowing through the chamber and across the hydrogen sensor. The steps are repeated as many times as necessary to collect enough data and verify the different features of the self-powered wireless sensor.

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CHAPTER 5 EXPERIMENTAL RESULTS Characterization of the RF transmitter is presented in this chapter. Results of testing the ADC input of the microcontroller are given. The results of testing the control system including RF transmission tests, power consumption analysis, and functional verification are presented. This chapter also covers system integration test results for both the solar powered system and the vibration powered system. RF Transmitter Characterization The Ming TX-99 RF transmitter described in Chapter 3 was characterized using a Keithley 2400 Source Meter as a variable voltage supply for the V dd supply input and a Tektronix CFG253 3MHz Function Generator for the data input. The function generator was set to output a 1 kHz square wave. The amplitude of the square wave was varied to determine the minimum data input level to successfully transmit the waveform to a Ming RE-99 RF receiver placed 1 foot away. The receiver was connected to a Tektronix TDS5104B digital phosphor oscilloscope to verify that the received signal matched the data signal from the function generator. The V dd supply input was held constant at 9V, and the minimum data level was found to be 510mV. The next step was determining the minimum V dd supply voltage to properly function with a 1V peak to peak data signal. The function generator was set to output a 1 kHz square wave that was 1V peak to peak with a 0.5V DC offset. The minimum V dd supply voltage for this condition was 0.6V. To prove that the RF transmitter could operate using the on-off key technique, the signal 42

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43 from the function generator was connected to both the data input and the V dd supply input. The minimum peak to peak voltage of the function generator was found to be 530mV in order to get the correct signal at the receiver. Since the transmitter will be operated at 2V from the microcontroller output, further testing was done to determine the transmission power. A SMA connector was soldered to the antenna output of the TX-99 transmitter. An Agilent E3631A DC power supply was connected to the V dd and data inputs on the TX-99. A FLX402#1 1 foot SMA cable was used to attach the SMA connector on the TX-99 to a DC blocker on the input of a Hewlett Packard 8563E spectrum analyzer. The spectrum analyzer measured a transmit power of -4.5dBm from the TX-99. Figure 5-1 shows the screen capture from the spectrum analyzer.

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44 Figure 5-1. Transmit power from Ming TX-99 RF transmitter. ADC Testing Various devices were used to verify the operation of the ADC of the microcontroller. The microcontroller was programmed to simply take the input from the ADC, encode the analog input to a 10 bit value, and then serially shifted the 10 bits out onto an output pin. The output pin was connected to a Tektronix TDS5104B digital phosphor oscilloscope to verify that the output data matched the data input to the ADC. Initially the ADC was connected to a Keithley 2400 Source Meter which was set to output a fixed voltage so that the encoded data would be easy to decode when viewed on the oscilloscope. The data on the scope was a 10 bit serial stream that represented an encoded value based on a percentage of the microcontrollers supply voltage. The

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45 microcontrollers supply voltage was connected to a Tektronix PS281 DC Power Supply which was set to output 2V. The ADC input was varied from 0V to 2V and the bit stream on the scope was visually verified to coincide with the value of the input on the ADC. Testing of the Control System Testing the control system consists of testing the microcontroller connected to the RF transmitter to determine the RF transmission characteristics, power consumption, and to verify the functionality of the system. The microcontroller was programmed with the data transmitting code. Due to the need for a sampled voltage on the ADC input a fixed voltage supply was required in order to control the value of the transmitted data. The power consumption was analyzed over the entire code routine, from initialization through an entire cycle of reading the ADC and transmitting data. Delays in the code were modified to make it easier to separate each step of the code. The functionality of the system was verified using both the level monitoring code and the data transmitting code. RF Transmission The RF transmitter was connected to the microcontroller and experiments were done in order to determine the maximum transmission distance, the received power, and the frequency spectrum of the received signal. For all experiments the microcontrollers supply voltage was connected to a Keithley 2400 Source Meter set to 2V. The microcontrollers ADC was connected to another Keithley 2400 Source Meter so that the transmitted signal could be controlled. An Agilent E4448A PSA series spectrum analyzer was used to measure the frequency spectrum and the received power. Figure 5-2 and Figure 5-3 show the received power and spectrum at 300 MHz for distances of 1 meter and 8 meters with a wave whip antenna connected to the transmitters antenna connector. At distances above 8 meters, the received power was barely above the noise

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46 floor of the spectrum analyzer. Figure 5-4 plots the received power measured from the spectrum analyzer for various distances. The bounce back in the slope of the plot at 10 meters is attributed to a wave guide caused by the shape of the room where testing took place. Figure 5-2. Received spectrum at 1 meter from transmitter.

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47 Figure 5-3. Received spectrum at 8 meters from transmitter.

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48 -75-70-65-60-55-50-45-40-3505101520Distance (m)Received Power (dBm) Figure 5-4. Received power at varying distance from transmitter. The Ming RE-99 receiver was connected to a Tektronix TDS210 two channel digital real-time oscilloscope to verify that the data received matched the data transmitted. Above 8 meters the received signal started to deteriorate. To illustrate the deterioration Figure 5-5 shows the received signal at 8 meters and Figure 5-6 shows the received signal at 10 meters. Tests were continued until the received signal no longer looked like the transmitted signal. Figure 5-7 shows the received signal at 14.5 meters. Antennas were used on the transmitter and receiver to attain an even greater transmission distance. Table 5-1 lists the maximum transmission distances for the different antenna locations.

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49 Figure 5-5. Received signal at 8 meters.

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50 Figure 5-6. Received signal at 10 meters.

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51 Figure 5-7. Received signal at 14.5 meters. Table 5-1. Maximum transmission distances for different antenna locations. Antenna Locations Maximum Distance Receiver Only 14.5 m Transmitter Only 16.8 m Transmitter & Receiver 19.4 m Power Consumption The power consumption of the control system is broken down into the power consumed during different activities of the system. The power was measured for initialization, sensing data, transmitting data and remaining idle. Both modes of operation require the same initialization code, therefore the initialization power is the

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52 same. Figure 5-8 shows a scope capture of the voltage measured across the 383 Ohm resistor during initialization. Since the resistor is constant and the supply voltage is a constant 2V, initialization power has the same shape as the voltage across the resistor. The value of the initialization power at any point along the scope capture is calculated using the equation P init = 2V V meas / 383 Ohms, (5-1) where V meas is the voltage measured in the scope capture. From the scope capture the duration of the initialization phase was found to be 12.5 ms. The peak power during initialization was 7.3 mW. The average power during initialization was found by separating the scope capture into linear segments and calculating the area under each segment then summing the areas together and multiplying the resulting sum by the total time duration for initialization. The average power during initialization was calculated to be 3.07 mW. The energy required for initialization is simply the total initialization time multiplied by the average power during initialization. The initialization energy was calculated to be 38.4 J.

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53 Figure 5-8. Scope capture of voltage across series resistor during initialization. The code was written using sleep modes, so that the ADC sampling does not require any additional power compared to remaining idle. Therefore the power during the idle state and power during the ADC sample state are the same. Figure 5-9 shows a scope capture of the voltage measured across the 383 Ohm resistor during an idle state. Since the voltage is in the noise range of the scope and is less than 50 mV, the average power was found by using a Keithley 2400 Source Meter which has a feature that displays the output supply current and supply power. The displayed average output power was 2.5 W.

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54 Figure 5-9. Scope capture of voltage across series resistor during idle state. The amount of power required to transmit data was measured for various transmission cases. Since the RF transmitter is operating in an on-off key mode, the amount of power required to transmit data is directly related to the data being transmitted. With on-off keying there is no additional power, above the power required to remain idle, to transmit a valued bit. Additional power is only required when transmitting a valued bit. For the case of level detection, the power required is simply the power required to send a single valued bit of data. Figure 5-10 shows a scope capture of the power required to transmit a single valued bit of data, as well as the single bit of data being transmitted. Figure 5-11 shows a zoomed in view of the same scope capture as

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55 Figure 5-10, focusing on the rise time of the transmit power. While the rise time is equal to the same amount of time the transmitted bit is sent, the remaining 5.5 ms of fall time is due to the slow discharge of the LC circuit in the RF transmitter. The average power to transmit a single valued bit was calculated similarly to the initialization power. The calculated value for average power to transmit a single valued bit was 303 W. Since the total time for transmit power is 6 ms, the average energy required to transmit a single valued bit of data is 1.8 J. In Figure 5-11, an apparent overlaying frequency exists on the scope capture of the power. This switching frequency is due to the oscillation of the LC circuit in the RF transmitter which is used to generate the transmitted signals frequency. Figure 5-10. Transmit power for a single data bit.

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56 Figure 5-11. Zoomed in view of transmit power for a single data bit. Measurements were taken during transmission of mixed data patterns. Scope captures of the power along with the data pattern are shown in Figure 5-12, Figure 5-13, and Figure 5-14. The slope of each rising and falling segment of the transmit power is the same as the rising slope and falling slope of the transmit power for a single valued bit. The rising slope represents the power during a valued bit. The falling slope represents the power during a valued bit. The length of time for each segment of rising and falling slope is based on the number of consecutive valued bits or valued bits. The scope captures clearly show that the power is dependant on the bit order, with every 0 valued bit transmitted after a valued bit decreasing the total power.

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57 Therefore, the maximum power would occur when all the valued bits were transmitted in a row. This can clearly be seen in Figure 5-15, which shows 3 different bit patterns all with the two valued bits for six transmitted bits. Note that the area under the waveform after the sixth bit will always be the same for any given number of valued bits. The area under the waveform is greatest when all the valued bits are transmitted one after the other. Thus the maximum energy used occurs when all valued bits are transmitted one after the other. Using the waveform that represents the maximum energy will allow for a simplification in determining the maximum average power, for any given number of valued bits per transmission. The peak of the triangular pulse is simply the peak value for transmitting a single valued bit times the total number of valued bits transmitted. Since the waveform has a triangular shape, the area under the waveform which represents the maximum energy required is equal to the peak power times the total time. The maximum average power required to transmit any given data pattern can be calculated simply by multiplying the average power to transmit a single valued bit by the number of valued bits transmitted. The maximum average power during transmission is calculated using the equation P tx = N 1 P 1 (5-2) where N 1 is the number of valued bits, and P 1 is the average power to send a single valued bit. Figure 5-16 lists the maximum average power during transmission for various number of valued bits transmitted.

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58 Figure 5-12. Scope capture of transmission power for data pattern '1011101011'.

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59 Figure 5-13. Scope capture of transmission power for data pattern '1101100101'.

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60 Figure 5-14. Scope capture of transmission power for data pattern '1010000110'.

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61 Figure 5-15. Different data patterns for two "1" valued bits.

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62 Maximum Average Transmission Power vs. Number of "1" Valued Bits00.511.522.533.512345678910Number of "1" Valued BitsAverage Power (mW) Figure 5-16. Maximum average transmission power vs. number of "1" valued bits. The average power across an entire cycle of sampling and transmitting data is dependant on the delay times set in the code as well as the number of valued bits being transmitted. The following equation is used to determine the average power for an entire sample and transmit cycle. P cycle = (P ADC t ADC + P idle t idle + P tx t tx ) / t total (5-3) Where P ADC is the average power during ADC sampling, t ADC is the amount of time to perform an ADC sample, P idle is the average power idling, t idle is the amount of time in an idle state, P tx is the average power during transmission, t tx is the amount of time to perform a transmission, and t total is the total amount of time to perform an entire cycle.

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63 Since the average power during ADC sampling is the same as the average power for remaining idle, Equation 5-3 can be simplified to the following equation. P cycle = (P no_tx t no_tx + P tx t tx ) / t total (5-4) Where P no_tx is the average power during ADC sampling and idling or in other words when not transmitting, t no_tx is the amount of time during the cycle when not transmitting data. The total time for a cycle is equal to the time when transmitting plus the time when not transmitting (i.e. t total = t tx + t no_tx ). The time parameters can be re-written in terms of duty cycle. The duty cycle for transmitting data compared to the rest of the cycle time is defined as D tx = t tx / t total. (5-5) Substituting the duty cycle into Equation 5-4 yields the following equation, P cycle = [P no_tx (1 D tx )] + (P tx D tx ) (5-6) which is completely a function of the average powers and transmission duty cycle. Ultimately, this equation can be rewritten using Equation 5-2 so that the only dependencies are the number of valued bits and the duty cycle. The resulting equation for average power during a complete cycle is P cycle = [2.5W (1 D tx )] + (303W N 1 D tx ) (5-7) In the case of level monitoring only one valued bit is transmitted. The transmission duty cycle versus the average power for the case of level monitoring versus is shown in Figure 5-17. The power to remain idle dominates the average power up to a duty cycle of 0.1%. At 1% duty cycle the average power is twice the idling power, and the average power continues exponentially from there before topping off at 303 W for a case when the system is constantly transmitting.

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64 Average Power vs Transmission Duty Cycle0501001502002503003500.010.1110100Transmssion Duty Cycle (%)Average Power (W) Figure 5-17. Duty cycle vs. average power for level monitoring. Functional Verification By performing the transmission tests the functionality of the control system was tested. Both the level monitoring and data transmitting versions of code were verified to work properly. A Keithley 2400 Source Meter was used to control the input to the ADC. The RE-99 receiver was connected to a Tektronix TDS210 two channel digital real-time oscilloscope, to monitor the received signal. For the case of level monitoring, the source meter was set below the trigger level and no data was seen on the oscilloscope. Once the source meter was set above the trigger level a single bit was visually verified on oscilloscope. The source meter was set back below the trigger level and then the experiment was repeated and a single bit was again seen on the oscilloscope.

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65 For the case of data transmitting, the source meter was set to 0 V and no data was seen on the oscilloscope. The source meter was then set to 2 V and a single 5 ms long pulse was seen on the oscilloscope. The source meter was varied from 0 V to 2 V, and varying data patterns were seen on the oscilloscope. System Integration The complete self-powered wireless sensor tested consisted of the control system, the hydrogen sensor, and an energy harvesting circuit. The control system was operated in level monitoring and data transmitting modes. The hydrogen sensor required a biasing circuit as described in Chapter 4. Two different energy harvesting circuits were used, one for solar energy and the other for energy from vibrations. In both cases enough power was generated to operate the control system as well as to provide the power to bias the hydrogen sensor. The level monitoring code was edited so that 100 ppm hydrogen would trigger the transmission of an emergency pulse. With no hydrogen present in the gas chamber the level monitoring mode never transmitted any emergency pulses. Once hydrogen was introduced into the gas chamber within a few minutes an emergency pulse was transmitted. The data monitoring code was run with no hydrogen in the gas chamber and an all 0s valued transmission was received by the RE-99 receiver and displayed on a Tektronics TDS224 Four Channel Real Time Oscilloscope. Once hydrogen was introduced into the gas chamber the received signal started to vary, with the encoded value increasing over time. After a longer period of time the hydrogen sensor reached a steady state value as became apparent when the received data remained constant. Once

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66 steady state was reached the hydrogen was flushed from the gas chamber and the received data started to vary again until finally settling back at a value of all 0s.

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CHAPTER 6 CONCLUSIONS The control system designed in this thesis has conclusively been proven to work. This chapter summarized the work presented in this thesis. Conclusions drawn from the work as well as potential future work are also discussed in this chapter. Conclusions An ultra-low power control system for a self-powered wireless sensor has been designed, tested and its functionality has been verified. The control system has been optimized for minimal power consumption, based on the results of testing the sections of the design. The average power has been measured for all portions of the design, and has been minimized in accordance with the test results. Equations have been generated to calculate the average power required based on the number of transmitted bits and the duty cycle of transmission. The RF transmission has been characterized based on the series of measurements described in this thesis. The average power was ultimately defined as a simple equation with the two variables being the duty cycle of transmission, and the number of 1s valued bits being transmitted. However, regardless of the value of the data being transmitted the frequency that data is transmitted is the main factor in determining the average power. For a transmission duty cycle of less than 0.1% the transmit power barely factors into the total and the average power is dominated by the idle power which is 2.5 W. The transmitter was not optimized other then operating it using on-off keying. The maximum transmission distance with antennas was measured to be 19.4m. Due to 67

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68 constraints on the area available for the RF transmission, the maximum transmission distance may vary from the measurements. However this thesis was not focused on the transmission characteristics, but rather the design of an operational system. The control system was designed to be very flexible so that future designs could easily modify the microcontroller code to adapt to a specific application. The microcontroller code was designed to minimize the power required by holding the microcontroller in a sleep mode once the initialization sequence is complete. Two separate sets of code were written to minimize the processing time for a system that requires transmission only when a sensors data is above a given value versus a system that requires data transmission at a given interval. However simple adjustments in the code could easily be written to make a single system that does both fixed interval transmission as well as emergency pulse transmissions if the sensors data reaches a given value. Future Work The control system was designed for ultra low power however the transmitted power can be minimized by encoding the data in order to minimize the number of bits transmitted. Further development can be done to minimize the power required by the RF transmitter. The RF transmitter should be designed for the exact application and deployment of the system. Using an antenna chamber a more accurate characterization of the RF signal could be attained. More work can also be done on the energy harvesting circuits so that a variety of energy sources can be used simultaneously, thus minimizing the reliance on a single source of energy. The energy harvesting circuits can also be made more efficient so that less total energy is required into the complete system. Similarly more efficient solar cells

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69 and PZT beams would enable operation in systems with lower levels of available energy. Less required energy would translate to smaller sized solar cells or PZT beams, and hence a smaller total package size of the system which would allow deployment of the system in more applications. This system could be used as a single node in a multi-node array of wireless sensors. However some form of protocol would have to be defined for the transmitted data in order to operate several of these sensors within a given area. Additionally, a multi-node array would require a more advanced receiver than the RE-99 used for this design.

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APPENDIX MICROCONTROLLER PROGRAM CODE The program code for the microcontroller is included here. Two versions of code are given, one for level monitoring and the other for data transmitting. Code for Level Monitoring //********************************************************************* // // Description: Level Monitoring // // MSP430F1232 // ---------------------// /|\ | XIN|-----\/ // | | | Crystal // |--|RST XOUT|-----/\ // | | // | P1.0|--> RF Transmitter // sensor-->|ADC | // ---------------------// // David Johnson // University of Florida // August 2005 //********************************************************************* #include void main(void) { WDTCTL = WDTPW + WDTHOLD; // Stop WDT P3DIR |= 0x10; // Set P1.0 to output direction P3OUT = 0x00; // Set the output to 0x0 ADC10CTL0 = ADC10SHT_2 + ADC10ON + ADC10IE; CCTL0 = CCIE; // CCR0 interrupt enabled CCR0 = 500; // Delay for Sampling 70

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71 TACTL = TASSEL_1 + MC_1; // ACLK, upmode _EINT(); // Enable interrupts. ADC10AE |= 0x01; // P2.0 ADC option select LPM3; } // ADC10 interrupt service routine #pragma vector=ADC10_VECTOR __interrupt void ADC10_ISR (void) { static int output_already; // Variable to determine if Transmitted to Output if (ADC10MEM < 0x1FF) { P3OUT = 0x00; // Set output LOW output_already = 0; // Reset variable } else if (output_already == 0) { output_already = 1; // Set variable so transmission only occurs once P3OUT = 0x10; // Set output HIGH int pulse_length; for (pulse_length = 100; pulse_length>0; pulse_length--); // Wait with output HIGH P3OUT = 0x00; // Set the output LOW } else // Occurs if threshold has already been reached { output_already = 1; // Holds variable } } // Timer A0 interrupt service routine #pragma vector=TIMERA0_VECTOR __interrupt void Timer_A (void) { ADC10CTL0 |= ENC + ADC10SC; // Sampling and conversion start }

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72 Code for Data Transmitting //********************************************************************* // // Description: Data Transmitting // // MSP430F1232 // ---------------------// /|\ | XIN|-----\/ // | | | Crystal // |--|RST XOUT|-----/\ // | | // | P1.0|--> RF Transmitter // sensor-->|ADC | // ---------------------// // David Johnson // University of Florida // August 2005 //********************************************************************* #include void main(void) { WDTCTL = WDTPW + WDTHOLD; // Stop WDT P3DIR |= 0x10; // Set P1.0 to output direction P3OUT = 0x00; // Set the output to 0x0 ADC10CTL0 = ADC10SHT_2 + ADC10ON + ADC10IE; CCTL0 = CCIE; // CCR0 interrupt enabled CCR0 = 5000; // Delay for Sampling TACTL = TASSEL_1 + MC_1; // ACLK, upmode _EINT(); // Enable interrupts. ADC10AE |= 0x01; // P2.0 ADC option select LPM3; } // ADC10 interrupt service routine #pragma vector=ADC10_VECTOR __interrupt void ADC10_ISR (void) { static int output_already; // Variable to determine if Transmitted to Output if (ADC10MEM < 0x1FF) { P1OUT = 0x00; // Set output LOW

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73 // Delay data from being transmitted int transmit_dly; for (transmit_dly = 2000; transmit_dly>0; transmit_dly--); int d_out = 0; // Loop variable for number of data bits int temp = ADC10MEM; while (d_out<10) { P3OUT = (temp & 0x1) << 4; int i; for (i = 100; i>0; i--); // Wait with output set to bit value of sensor data temp = temp >> 1; d_out++; } P3OUT = 0x00; // Set output LOW output_already = 0; // Reset variable } else if (output_already == 0) { P1OUT = 0x00; // Set output LOW output_already = 1; // Set variable so transmission only occurs once // Encode and transmit ADC Sample int data_bit = 0; // Loop variable for number of data bits int temp2 = ADC10MEM; while (data_bit<10) { P1OUT = (temp2 & 0x1); int j; for (j = 100; j>0; j--); // Wait with output set to bit value of sensor data temp2 = temp2 >> 1; data_bit++; } P1OUT = 0x00; // Set output LOW } else // Occurs if Emergency Transmit sent already { // Delay data from being transmitted int delay2; for (delay2 = 2000; delay2>0; delay2--); int d_out2 = 0; // Loop variable for number of data bits int temp2 = ADC10MEM;

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74 while (d_out2<10) { P3OUT = (temp2 & 0x1) << 4; int k; for (k = 100; k>0; k--); // Wait with output set to bit value of sensor data temp2 = temp2 >> 1; d_out2++; } P3OUT = 0x00; // Set output LOW output_already = 1; // Reset variable } } // Timer A0 interrupt service routine #pragma vector=TIMERA0_VECTOR __interrupt void Timer_A (void) { ADC10CTL0 |= ENC + ADC10SC; // Sampling and conversion start }

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LIST OF REFERENCES 1. A. Siemens, Facts about Solar Energy, http://www.siemenssolar.com/facts.html Pacific Paradise, Australia, 2003, last accessed March 2006. 2. A. E. Messer, Prototype Under-Skin Glucose Sensor, Medical News Today, http://www.medicalnewstoday.com/medicalnews.php?newsid=31294 Sept. 28, 2005, last accessed March 2006. 3. S. Roundy, P.K. Wright, and J.M. Rabaey, Energy Scavenging for Wireless Sensor Networks with Special Focus on Vibrations, Ch. 1, 22-23, Kluwer Academic Publishers, Norwell, Massachusetts, 2004. 4. Cap-XX, GW1 Series Double Layer Supercapacitor, Product Data Sheet, Cap-XX, Myrtle Beach, SC, 2006. 5. Freescale Semiconductor, High Accuracy Digital Tire Pressure Gauge, AN1953 Product Application Note, Freescale Semiconductor, Austin, TX, 2006. 6. C. Murray, Embedded Developers Do Some Sole Searching, Embedded.com, June 14, 2004. 7. Crossbow Technology, Inc., Product Information on MICA Mote, http://www.xbow.com/Products/Product_pdf_files/Wireless_pdf/MICA.pdf Crossbow Technology, Inc., San Jose, CA, 2006, last accessed March 2006. 8. K. Pister, Smart Dust: Autonomous Sensing and Communication in a Cubic Millimeter, http://robotics.eecs.berkeley.edu/~pister/SmartDust/ Berkeley, CA, 2001, last accessed March 2006. 9. MicroStrain, EmbedSense Wireless Sensor, http://www.microstrain.com/embed-sense.aspx MicroStrain, Williston, VT, 2005, last accessed March 2006. 10. Microcstrain, StrainLink, http://www.microstrain.com/strain-link.aspx, MicroStrain, Williston, VT, 2005, last accessed March 2006. 11. Coronis Systems, Homepage of Coronis Systems, http://www.coronis-systems.com/ Coronis Systems, Cambridge, MA, 2006, last accessed March 2006. 75

PAGE 87

76 12. Dust Networks, Homepage of Dust Networks, http://www.dustnetworks.com/flash-index.shtml Dust Networks, Hayward, CA, 2006, last accessed March 2006. 13. Crossbow Technology, Inc., Wireless Sensor Networks, http://www.xbow.com/Products/Wireless_Sensor_Networks.htm Crossbow Technology, Inc., San Jose, CA, 2005, last accessed March 2006. 14. Cirronet Inc., Homepage of Cirronet Inc., http://www.cirronet.com/ Cirronet Inc., Duluth, GA, 2005, last accessed March 2006. 15. Intel Corporation, Sensor Nets / RFID, http://www.intel.com/research/exploratory/wireless_sensors.htm Intel Corporation, Santa Clara, CA, 2006, last accessed March 2006. 16. W. Ye, J. Heidenmann, and D. Estrin, An Energy-Efficient MAC Protocol for Wireless Sensor Networks, Proceedings of 21 st International Annual Joint Conference of the IEEE Computer and Communications Societies (Infocom 2002), volume 3, pages 3-12, June 2002. 17. IEEE, IEEE 802.15.4 Standard, http://standards.ieee.org/getieee802/download/802.15.4-2003.pdf IEEE, Piscataway, NJ, 2006, last accessed March 2006. 18. ZigBee TM Alliance, Homepage of ZigBee TM Alliance, http://www.zigbee.org/en/index.asp Global Inventors, Inc., San Ramon, CA, 2006, last accessed March 2006. 19. A. Rodzevski, J. Forsberg, and I. Kruzela, Wireless Sensor Network with Bluetooth, Proceedings of Smart Object Conference, Grenoble, France, May 2003. 20. J. M. Rabaey, M. J. A., J. L. da Silva Jr., D. Patel, and S. Roundy, PicoRadio Supports Ad Hoc Ultra-Low Power Wireless Networking, Computer Magazine, Vol. 33, No.7, pages 42-48, July 2000. 21. A. Dunkels, T. Voigt, and J. Alonso, Connecting Wireless Sensor Networks with the Internet, ERCIM News, No. 57, pages 61-62, April 2004. 22. A. Mason, N. Yazdi, K. Najafi, K. D. Wise, A Low-Power Wireless Microinstrumentation System for Environmental Monitoring, The 8 th International Conference on Solid-State Sensors and Actuators, and Eurosensors IX, volume 19-A3, pages 107-110, June 1995.

PAGE 88

77 23. M. Ismail, A Novel Ultra Low Power CMOS Radio for Wireless Sensor Nodes (WiSeNode), http://wireless.kth.se/menu/research/research_pdf/Wireless@KTH_SensorNodes_final_Sept2005.pdf Wireless@KTH, Kista, Sweden, 2005, last accessed March 2006. 24. S. Roundy, P.K. Wright, and J.M. Rabaey, Energy Scavenging for Wireless Sensor Networks with Special Focus on Vibrations, Kluwer Academic Publishers, Norwell, Massachusetts, 2004. 25. S. Roundy, B. P. Otis, Y.-H. Chee, J. M. Rabaey, P. Wright, A 1.9GHz RF Transmit Beacon using Environmentally Scavenged Energy, ISPLED 2003, August 25-27, 2003, Seoul, Korea. 26. E. S. Leland, E. M. Lai, P. K. Wright, A Self-Powered Wireless Sensor for Indoor Environmental Monitoring, 2004 Wireless Networking Symposium, University of Texas, Austin, October 20-22, 2004. 27. Texas Instruments, MSP430x11x2, MSP439x12x2 Mixed Signal Microcontoller, Product Datasheet, Texas Instruments, Dallas, TX, 2004. 28. Ming Microsystem, Inc., TX-99 V3.0 RF Transmitter Board, Product Datasheet, Reynolds Electronics, Canon City, CO, 2005. 29. Radio Innovation, How Far Will My Radio Transmit? http://www.radioinnovation.com/Howto/how_far.htm Radio Innovation, 2005, last accessed March 2006. 30. D. L. Ash, A Comparison between OOK/ASK and FSK Modulation Techniques for Radio Links, http://www.rfm.com/products/apnotes/ookvsfsk.pdf RF Monolithics, Inc., Dallas, Texas, 2006, last accessed March 2006. 31. Protel International Pty. Ltd., Protel v. 4.0.16 Users Guide, Altium Limited, San Diego, CA, 1998. 32. LPKF Laser & Electronics AG, LPKF ProtoMat 91s/HS Manual, Part #106 049, LPKF Laser & Electronics AG, Wilsonville, OR, May 1998. 33. IXYS Corporation, IXOLAR TM High Efficiency Solar Cells, XOD17 Product Datasheet, IXYS Corporation, Santa Clara, CA, 2005. 34. S. Xu, K. Ngo, T. Nishida, G. B. Chung, A. Sharma, Converter and Controller for Micro-Power Energy Harvesting, 20 th Annual IEEE Applied Power Electronics Conference and Exposition, Vol. 1, pp. 226-230, March 2005. 35. S. Meninger, J. O. Mur-Miranda, R. Amirtharajah, A. P. Chandrakadan, and J. H. Lang, Vibration-to-Electric Energy Conversion, IEEE Trans. on VLSI systems, vol. 9, pp. 64-76, February 2001.

PAGE 89

78 36. Piezo Systems, Inc., Standard Double Quick-Mount Bending Actuators, Product Information Page, http://www.piezo.com/prodbm8dqm.html Piezo Systems, Inc., Cambridge, MA, 2006, last accessed March 2006. 37. Piezo Systems, Inc., PSI-SA4E Single Layer Disks, Product Information Page, http://www.piezo.com/prodsheet3disk5A.html Piezo Systems, Inc., Cambridge, MA, 2006, last accessed March 2006. 38. L. C. Tien, P. W. Sadik, D. P. Notron, L. F. Voss, S. J. Pearton, H. T. Wang, B. S. Kang, F. Ren, J. Jun, and J. Lin, Hydrogen-Selective Sensing at Room Temperature with ZnO Nanorods, Appl. Phys. Lett. 87, 222106(2005).

PAGE 90

BIOGRAPHICAL SKETCH David E. Johnson was born on July 14, 1977, in Toms River, New Jersey. He moved to Jacksonville, Florida, where he graduated from Samuel Wolfson High School in 1995. He earned a B.S. in electrical engineering and a B.S. in computer engineering from the University of Florida, in December 2000. During his undergraduate time at the University of Florida he maintained research positions working for two different research groups: Interdisciplinary Center for Aeronomy and Other Atmospheric Sciences (ICAAS) and Interdisciplinary Microsystems Group (IMG). Upon finishing his bachelors degrees, he took a position working for Cisco Systems as a Hardware Engineer. While working for Cisco Systems he continued part time at the University of Florida in pursuit of a M.S. in electrical engineering. He intends to graduate from the University of Florida in May 2006 with a M.S. in electrical engineering. Upon graduation he plans to continue working for Cisco Systems. 79


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DESIGN OF AN ULTRA LOW POWER CONTROL SYSTEM FOR A SELF-
POWERED WIRELESS SENSOR
















By

DAVID EDWARD JOHNSON


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2006

































Copyright 2006

by

David Edward Johnson















ACKNOWLEDGMENTS

I would like to thank my advisors, Toshikazu Nishida, Khai Ngo, Jenshan Lin, and

Juan Nino, who provided encouragement and support in completing my thesis. My

advisors also had an abundance of patience with me as I balanced a full time career with

working on a master's degree, and for this I am deeply thankful. Specifically I would

like to thank Toshikazu Nishida for giving me a job with IMG as an undergraduate,

which provided me with the opportunity to work on remarkable research projects. The

position with IMG definitely instilled a desire to continue my education. Many thanks go

to my fellow students that worked on sections of the self-powered wireless sensor design.

Specifically, thanks go to Jerry Jun and Ed Koush for their work and help with the RF

transmitter characterization as well as their work with the hydrogen sensor. Thanks go to

Shengwen Xu for his work on the energy reclamation circuit. Thanks go to Anurag

Kasyap for his work on the piezoelectric beams. I would like to thank all of the members

of IMG who allowed me to work freely in the lab and provided much needed assistance

with the use of equipment. Thanks go to the staff at the University of Florida, including

Linda Kahila, Shannon Chillingworth, and Janet Holman.

My employer, Cisco Systems, deserves special thanks for allowing me to continue

my education while working full-time. I would like to explicitly thank my manager Phil

Van Atta for his proctoring as well as his understanding and encouragement in

completing my degree. I would also like to thank Frank Juliano for his assistance in

covering my work at Cisco while I took trips to school in order to work on my thesis.









Special thanks go to a college roommate, Gregory Schaefer, who helped me in

deciding to pursue an undergraduate degree in engineering, and ultimately a master's

degree in engineering. I would like to thank Dr. Roland Federico and his family for

allowing me the use of their condominium when I visited Gainesville to work on my

thesis. I would like to thank my family and friends for their love and support. My

deepest thanks go to Barbara Fleener, whose love and support have kept me motivated

for the past few years. Finally, my thanks go to God for instilling the desire to learn

within my soul.
















TABLE OF CONTENTS

page

A C K N O W L E D G M E N T S ................................................................................................. iii

LIST OF TABLES ........ ............ ............... .................... ............ vii

LIST OF FIGURES ............ ............................................. viii

A B ST R A C T ................. .......................................................................................... x

IN TR O D U C TIO N ....................................................................

R research G oals ................................................................. 3
T h esis O rg an izatio n .................................................................................. 4

THEORETICAL DEVELOPMENT .................................................. 6

B background Inform ation............................................ 6
P rev iou s W ork ................................................................... .............................. . 7
D design C challenges ..................... .................................. .... ....... .................

DESIGN OF A CONTROL SYSTEM FOR A SELF POWERED WIRELESS
S E N S O R ............................................................................................ 1 1

Selection of a M icrocontroller ...................................................... .. .............. .. 11
D design of Program m ing Code ......................................................... ............... 14
L evel M monitoring ............................................................. .... ......................15
D ata Transm hitting .................................... ...............................16
Programming Techniques for Minimizing Power Consumption ........................18
R F T ran sm hitter ................................................................................ 2 1
Design of RF Transmitter.................................................21
Power Consumption for a RF Transmitter .................................................... 22
Free Space m ethod ................................................................. 22
Plane Earth m ethod .............................................................. .... ... 24
Techniques for minimizing power consumption ...............................................25
Construction of a Prototype .......................... .............26

EXPERIMENTAL METHODS AND SETUP ................................................... 31

E energy Sources ............. ...................................................................... 3 1



v









Energy from Light .................. ........................... .... ...................3 1
Energy from V ibrations ......................................................... .............. 32
D ata Sensor ................................................................................................................33
Testing M ethodology for the Control System .................................... .................36
Methods for Measuring Power Consumption..........................................36
Methods for Characterizing RF Transmission .......................................... 37
Testing of a Self-Powered W wireless Sensor..................................... ......... ......... 39

EXPERIM EN TAL RESULTS................................................ ............................... 42

RF Transm itter Characterization ........................................ .......................... 42
A D C Testing ............................................................................... ......... ....... ...............44
Testing of the Control System ............................................................................45
R F T ransm mission .......... ............................................................ ...... .... .... 45
Power Consumption ............................... .. .......... .............. ...5 51
Functional V erification............................................... ............................. 64
System Integration ........... .... .......... .. .... ..... ................. .. ...... 65

CONCLUSIONS.................. ........ .. .... .... ....................67

C o n c lu sio n s ........................................................................................................... 6 7
Future Work ........................ .........................68

MICROCONTROLLER PROGRAM CODE ............................... ..................... 70

C ode for L evel M monitoring ................. ......... .........................................................
Code for D ata Transm hitting ......... .................................................... ............... 72

L IST O F R E F E R E N C E S ............................ ............................................... .................. 75

B IO G R A PH IC A L SK E TCH ..................................................................... ..................79
















LIST OF TABLES

Table page

3-1 Power characteristics of microcontrollers. .............................................................12

3-2 Added features of microcontrollers..................................................13

5-1 Maximum transmission distances for different antenna locations......................... 51
















LIST OF FIGURES


Figure p

2-1 Simple state machine of a control system for a wireless sensor.......................... 10

3-1 System level block diagram of self powered wireless sensor .............................11

3-2 Microcontroller flow chart for level monitoring ..............................................15

3-3 Microcontroller flow chart for data transmitting. .............................................17

3-4 Frequency versus current consumption for MSP430 operating at 3V .................19

3-5 Supply voltage versus current consumption for MSP430 operating at 32 kHz.....20

3-6 Schematic of TX-99 RF transmitter............................................. ...............22

3-7 Transmission distance versus power loss for given frequencies .........................23

3-8 Power loss vs. transmit distance for various transmitter and receiver heights. .....25

3-9 Schematic for control system prototype. .................................... .................27

3-10 Top side PCB layout for control system prototype....................................28

3-11 Bottom side PCB layout for control system prototype.......................................29

3-12 Control system prototype with RF transmitter................... ..................................30

3-13 Top side of control system prototype.................................. ....................... 30

4-1 Bim orph PZT com posite beam s....................................... .......................... 33

4-2 Direct charging circuit for energy reclamation from vibrations ..........................33

4-3 Schematic of biasing circuit for hydrogen sensor............... .................... 35

4-4 Picture of biasing circuit with hydrogen sensor................................ ...............36

4-5 W hip antenna. .......................................................................38

4-6 Floor layout of the atrium area in NEB. ........................................ .............38









4-7 Test setup. .......................................... ............................ 39

4-8 Gas chamber for testing hydrogen sensor................................... .................40

5-1 Transmit power from Ming TX-99 RF transmitter............... ............................44

5-2 Received spectrum at 1 meter from transmitter.................... .......................46

5-3 Received spectrum at 8 meters from transmitter. .............................................47

5-4 Received power at varying distance from transmitter. ........................................48

5-5 R received signal at 8 m eters......................................................... ............... 49

5-6 R received signal at 10 m eters........................................... ........................... 50

5-7 R received signal at 14.5 m eters........................................ ........................... 51

5-8 Scope capture of voltage across series resistor during initialization ...................53

5-9 Scope capture of voltage across series resistor during idle state. ..........................54

5-10 Transmit power for a single data bit....................................................................55

5-11 Zoomed in view of transmit power for a single data bit................... ............56

5-12 Scope capture of transmission power for data pattern '1011110011'...................58

5-13 Scope capture of transmission power for data pattern '1101100101'...................59

5-14 Scope capture of transmission power for data pattern '1010000110'...................60

5-15 Different data patterns for two "1" valued bits. .............................................. 61

5-16 Maximum average transmission power vs. number of "1" valued bits. ...............62

5-17 Duty cycle vs. average power for level monitoring............................................64















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

DESIGN OF AN ULTRA-LOW POWER CONTROL SYSTEM FOR A SELF-
POWERED WIRELESS SENSOR

By

David Edward Johnson

May 2006

Chair: Toshikazu Nishida
Major Department: Electrical and Computer Engineering

Wireless sensors are becoming more commonplace in applications where harsh

environments or remote locations make it difficult to run wires. Making these sensors

self-powered is essential as battery replacement is extremely difficult. The drawback of

being self-powered is that the system must rely on the environment for power. With the

small size of a wireless sensor in mind, the amount of energy available is minimal.

Hence there is a need for a control system that manages when power is distributed to

different portions of the design.

This thesis focuses on the design of a control system that manages the

components of a self-powered wireless sensor, with optimization for ultra-low power.

The control system was designed with several flexibilities, so it can used to interface a

variety of sensors. The control system consists of a Texas Instruments MSP430

microcontroller and a MING TX-99 RF transmitter. The MSP430 microcontroller has an

analog to digital converter which is used to encode data from the sensor. One of the









output ports on the microcontroller is used to interface the TX-99 RF transmitter. One of

the keys to minimizing power consumption is for the microcontroller to stay in a low

power mode. The microcontroller supplies the data and power to the RF transmitter; so

power can be minimized further by applying power to the transmitter only when data is

being transmitted.

The power consumption of the control system is partitioned into three main

portions: sensor sampling, data transmission, and idle time. The microcontroller code

was written so that the average power used to sample data from a sensor was the same as

the power to remain idle, which is 2.5[tW. The average power used to transmit a single

bit is 261 [W. The code running on the microcontroller was written to be able to vary the

idle time between sampling and transmissions. By allowing the idle time to be varied the

total average power can be varied according to how often the end user wants to transmit

data. Two modes of operation were designed, (1) threshold based and (2) data based, so

that the design would have the flexibility of providing either level detection or data

logging. The flexibility in the design while maintaining minimal power requirements

allows this control system to work for a range of wireless sensor applications. The

control system was tested with a ZnO nano-rod hydrogen sensor while being powered

from both solar and piezoelectric energy.














CHAPTER 1
INTRODUCTION

Applications for wireless sensors are growing in many fields from environmental

and health monitoring to disaster relief. Further applications for wireless sensors include

space exploration, military uses, chemical processing and other fields where the

environmental conditions make it extremely difficult to achieve the desired task without

having a remote wireless sensor. Harsh environmental conditions that make it difficult to

access the sensor combined with limited battery life create a need for wireless sensors to

be self-powered. Self-powered systems harvest ambient energy from the environment

that would normally be unused. An abundant number of sources for environmental

energy exist, including light, air flow, fluidic flow, heat, acoustics, vibrations, and

chemical reactions to name a few.

One major challenge in designing a self-powered wireless sensor is to minimize the

amount of power required by the system. Minimizing the power requirements is

necessary due to limitations on the amount of energy that can be scavenged from any

particular environment, as well as the size of the energy collector required to harvest any

given amount of energy. For instance photovoltaic solar cells only use about 15% of the

energy available from sunlight [1], so to get more electrical energy multiple solar cells

must be combined into an array, hence increasing the size of the energy collector.

Minimizing the size of the system is important so that the system can be conveniently

placed in various remote locations.









The need for low power requirements in a wireless sensor becomes apparent when

examining the lifetime of the device. Batteries start to lose their power density over time

and must be replaced in order to continue operating the system. Several wireless sensor

systems exist that make battery replacement extremely difficult. For instance a wireless

biomedical sensor that is implanted under the skin cannot be easily accessed to replace a

battery [2]. One option is to use a rechargeable battery, however even rechargeable

batteries lose their power density over time, albeit at a much lower rate then standard

batteries [3]. Another option for power storage is to use a super-capacitor, which are

similar to standard capacitors except with much higher capacitance on the order of a

hundred millifarads to several farads [4]. Ultimately the selection of an energy storage

device is dependant on the expected lifetime for the final application. This thesis

minimizes the average power so that the lifetime of the system can be extended based on

the lifetime of the storage device, thus making selection of a storage device much easier.

Most previous work has focused on application specific wireless sensors, and while

ultimately the final system tested in this thesis is for a very specific hydrogen sensor the

design is extremely flexible so that it can be easily modified to suit any application.

While this thesis focuses on the application of a wireless sensor, there are other simple

control systems with similar low power constraints that should be looked at for ideas on

decreasing the required power. Apart from wireless sensors, simple control systems that

gather and utilize data from a sensor are prevalent in everything from digital tire pressure

gauges [5] to a new pair of Adidas sneakers that have a built-in microcontroller which

uses data from sensors to control the support and cushioning in the shoe [6]. Complete

self-powered wireless sensor systems already exist, in particular the MICA Mote [7], and









Smart Dust [8] both of which were developed at UC Berkeley. However these systems

incorporate a sensor, control system, and transmitter and therefore are limited to specific

applications. Furthermore Smart Dust consists of a custom designed ASIC that integrates

the control system with the energy harvesting circuitry. Commercially available wireless

sensor systems are on the market. For example, MicroStrain has two wireless sensor

systems which are low power and are small in size: EmbedSenseTM which is a passive

device that receives power wirelessly and only transmits when queried [9], and

StrainLinkTM which operates off a battery and still draws power on the order of milliwatts

[10]. The design discussed in this thesis will differentiate itself from other designs in that

the design will be modular, flexible in its applications, consist of off the shelf

components, be able to transmit without querying the system, and maintain ultra-low

power requirements on the order of microwatts.

Research Goals

Rather then focusing on addressing the problems related with increasing the

amount of available power this thesis focuses on design techniques for minimizing the

amount of power required by the wireless sensor system. The main drain on power is the

control system which consists of a microcontroller interfaced to a RF transmitter. The

microcontroller also provides the interface to the sensor for data collection. The utmost

importance is to find a microcontroller that operates at an extremely low power level

while also providing the necessary interfaces to sample data from the sensor as well as

send data to the RF transmitter.

The goal of this thesis is to design a control system for a self-powered wireless

sensor that operates with an average power requirement on the order of a few microwatts.

Several techniques for limiting the power consumption of the microcontroller will be









examined including operating the microcontroller at a slower processor speed and using

the low power modes available during idle states. Techniques for decreasing the power

consumption of the RF transmitter will be examined however the RF transmitter will not

be redesigned for this thesis. All parts used in the construction of the control system will

be off the shelf components so that reproducibility and ease of reuse is simple.

An additional goal of this thesis is for the control system to be designed with a high

level of flexibility, so that the control system does not become application specific. The

microcontroller code will be written with variables that are easily modified to change the

timing between sampling and transmitting data. The RF transmitter will be a module so

that a different transmitter could easily replace the one used without effecting the

operation of the system. The microcontroller's sensor interface will be a simple analog to

digital converter so that many different sensors can be used. The microcontroller code

will be written with a very basic modular structure so that modifying the code to encode

or store the data can be done very easily with minor modifications to the code.

Thesis Organization

This thesis is organized into six chapters. This chapter provides background

information about wireless sensors as well as providing an introduction to the research

goals of this thesis. Chapter 2 reviews the previous work and theoretical information

involved with designing a control system for a self-powered wireless sensor. Chapter 3

discusses the actual design process for the control system as well as an introduction to the

energy harvesting circuits used. Chapter 4 explains the experimental methods used for

testing the components of the control system as well as integrating the system into a self-

powered wireless sensor. Chapter 5 discusses the results of testing the control system

and the results of the system level testing of the complete self-powered wireless sensor.






5


Chapter 6 outlines conclusions made from the testing, and provides insight into possible

future work to further decrease the power requirements of a self-powered wireless sensor.















CHAPTER 2
THEORETICAL DEVELOPMENT

Background information about self-powered wireless sensors, specifically their

control systems is presented in this chapter. Aspects of other people's work in the area of

wireless sensors are discussed in this chapter, including the growing amount of research

being done on wireless sensors networks. Finally this chapter will review the challenges

associated with designing an ultra-low power control system for a self-powered wireless

sensor.

Background Information

The simplest form of a wireless sensor is a RF transmitter connected to a data

sensor. However a RF transmitter without any control circuitry will continuously

transmit. Continuous transmission requires a continuous power source. In order to

minimize the average power required a simple control circuit needs to be implemented.

The control circuit is used to control the time the transmitter requires power as well as

providing an interface between the data sensor and the RF transmitter. The control

system also provides the ability to encode the data being transmitted as well as make

decisions about transmissions based on the data from the sensor.

A control system cannot be designed without having some knowledge about what

is being controlled. In the case of a self-powered wireless sensor, the control system

controls the sampling rate of data from the sensor, the frequency of transmission, the data

rate to the transmitter, and if any decision making or data encoding needs to be done prior

to transmission. The precision level of the microcontroller's interface to the sensor must









be known in order to determine if any external amplification of the sensor's signal is

needed. The type of sensor also matters, since passive sensors require a biasing circuit to

create a voltage that represents the data value from the sensor. For specific applications a

custom microcontroller could be designed to incorporate any extra circuitry needed to

interface the specific sensor.

This thesis focuses on the design of a control system which is generic in that it is

very flexible and has been designed with a high level of modularity so that the control

system can be used in a variety of different wireless sensor applications. Additionally the

control system will be designed using commercially available components, so that the

design process is simpler and quicker then attempting to design a custom controller.

Using off the shelf components also provides a better opportunity for success over a

custom controller since the components have already been tested by the manufacturer.

Ultimately the techniques used in this thesis to lower the average power of the control

system could be applied to custom designed controllers as well.

Previous Work

Over the past decade research in the area of wireless sensors has been growing.

The majority of research has been toward creating wireless sensor networks with their

own operating systems and communication protocols. In fact, several companies have

developed wireless sensor nodes that are specifically designed for use in creating a

wireless sensor network. Startup companies like Coronis Systems [11], Dust Networks

[12], Crossbow Technology Inc. [13], and Cirronet, Inc. [14] all have developed

wireless sensor networks. Even larger companies such as Intel are researching wireless

sensor networks as a potential new revenue stream [15]. With the increasing number of

different sensor networks there has been more and more research into the standardization









of communication protocols for wireless sensor networks. The University of California,

Los Angeles has developed a sensor network-specific media access control protocol

called S-MAC [16]. The goal of S-MAC is to reduce the amount of energy wasted due

to constant listening, collisions, overhearing and overhead. A more industry wide

standard is IEEE 802.15.4 [17], which defines both the physical and media access control

protocols for wireless sensor networks. Several companies have combined to create the

ZigBee Alliance [18], which promotes the IEEE 802.15.4 standard. S-MAC and IEEE

802.15.4 are just two of the protocols being developed; other protocols include the use of

Bluetooth [19], PicoRadio [20], and various simple RF communications. One of the

more interesting ideas is the use of TCP/IP protocol to allow wireless sensor networks to

connect to the Internet [21]. Connecting wireless sensor networks to the Internet would

allow for remote monitoring from anywhere with Internet access.

One of the main challenges to wireless sensors is maintaining an ultra-low average

power consumption. Back in 1995 a group from the University of Michigan designed a

low-power wireless sensor system that operating using 700[tW of power, however the

system used a 6V battery and barely made it halfway through a year before needing to

replace the battery [22]. Since then the average power of most wireless sensor systems

has been targeted at under 100[tW, with most people aiming to reach average power on

the order of a few microwatts or less. A variety of techniques for lowering the average

power consumption, other then changing the data protocols, have been and are currently

being developed. One such technique is designing ultra-low power transmitters, such as

the CMOS RFIC being developed by Mohammed Ismail in Sweden [23]. A more

common technique for lowering the average power is to build a custom microcontroller.









While custom microcontrollers can operate on less power than most commercial

microcontrollers, the design process for generating and testing a custom IC is long and

tedious. Furthermore, a corporation that focuses on producing a generic microcontroller

that is ultra-low power has an advantage over universities and small groups that try to

develop a custom device due to the man power and engineering time available to the

corporation.

One of the ultimate goals of low power wireless sensors is to operate off low

enough power that the energy required can be harvested through ambient energy sources,

thus making the system "self-powered". Shad Roundy has written a book [24] that

focuses on the area of energy scavenging for wireless sensors. Additionally, Shad

Roundy has created a 1.9GHz RF transmit beacon [25], that while it does not sense

anything and only acts as a transmitter the design operates of environmentally scavenged

energy. A group at the University of California, Berkeley has developed a wireless

temperature sensor that operates off the vibrations in a staircase [26]. More and more

work on the area of self-powered wireless sensors seems to be available everyday as

progress in the area continues at a strong and fast pace. This thesis differentiates itself

from other work in that the design is modular and focuses purely on techniques for

lowering the power requirements of the control system, while using commercially

available components.

Design Challenges

Several challenges exist when designing an ultra-low power control system for a

self-powered wireless sensor. Minimizing the power consumption is the main challenge

in designing a control system for a self-powered wireless sensor. Several aspects of the

design must be analyzed to determine methods for decreasing the power required. The









control system can be broken down into a state machine consisting of sampling data,

transmitting data, and remaining idle. Figure 2-1 shows a simple state machine that

represents the cycle of a control system for a wireless sensor. The duty cycle of each

state determines the total amount of power required for each particular state across a

single cycle. Each state must be analyzed to determine methods for reducing the power

consumption of the system while in the particular state. While most of the challenges

have to do with minimizing the power consumption of the control system, other

challenges include designing the interface to the sensor and the transmitter as well as

determining the format of the data to transmit.




Acquire
SLEEP
Data








Transmit S
SLEEP
Data




Figure 2-1. Simple state machine of a control system for a wireless sensor.














CHAPTER 3
DESIGN OF A CONTROL SYSTEM FOR A SELF POWERED WIRELESS SENSOR

A self-powered wireless sensor consists of four major components: energy

reclamation, data sensor, microcontroller, and a RF transmitter. Figure 3-1 shows a block

diagram of the system. Each component has been designed independently however

information from the design of each component was taken into account by the other

components. This chapter focuses on the design of the control system, which consists of

the microcontroller and RF transmitter.



Solar Cell
Energy Energy Available
Reclamation Storage Power
Piezoelectric Circuit

Energy Harvesting Circuitry




Data Microcontroller RF
Sensor Transmitter


Figure 3-1. System level block diagram of self powered wireless sensor.

Selection of a Microcontroller

When considering a microcontroller it is important to compare several different

features. The most important aspect to consider for the application of a self-powered

wireless sensor is for the microcontroller to use the least amount of power. The majority

of power used is during the microcontroller's active state. However the standby current









is just as important, since the microcontroller will still be drawing power during its sleep

mode. The standby current is particularly important since the power source for a self

powered wireless sensor may not be constant. So that power is not wasted on the

microcontroller operation, it is desired to have a very short wakeup time. Additionally, to

conserve power a low port leakage is desired. A comparison of power characteristics for

several microcontrollers is shown in Table 3-1.

Table 3-1. Power characteristics of microcontrollers.
Manufacturer Active Current Standby Wakeup Brown-Out Port
and Model Current Time Reset Leakage
TI 14uA@32kHz, 4 low power 6us 50nA 50nA
MSP430F1122 2.5uA@4kHz modes from
and 2.2V 0.7uA to
0O.luA
Microchip 20uA@32kHz luA Ims 85uA luA
PIC16F73
Motorola 812uA@lMHz 3 Stop modes: 2.4ms 70uA 25nA
MC9S08G from 4.3uA to
25nA
Atmel 27uA@32kHz 5 Sleep modes, 10us 19uA luA
Atmegal69 lowest is
0.2uA
EM Micro 9uA@32kHz 0.6uA standby NA NA NA
EM6617 0O.luA sleep
XEMICS 10uA@32kHz luA in NA NA NA
XE88LC01A hibernating
mode
O.luA in sleep
mode

In order to completely compare the microcontrollers, a comparison of all of the

features of the microcontroller is required. For a self-powered wireless sensor the

microcontroller must contain an ADC to interface the sensor, a serial output to interface

the RF transmitter, and enough memory to hold the runtime code as well as store data

from the sensor. A comparison of the features for several microcontrollers is shown in

Table 3-2.









Table 3-2. Added features of microcontrollers.
MSP430 PIC16F73 MC9S08G Atmega EM6617 XE
169 88LC01A
Oscillator 1MHz 32kHz 32kHz 1MHz 32kHz 100kHz -
internal, external external internal, crystal 4MHz RC
input for input for oscillator,
32kHz 32kHz 32kHz
external external external
ROM NA NA NA NA 6kB 22kB, 8kB
RAM 512B-2kB 192 bytes 1K-4K 1KB (2) 64x4 8 bytes
depending depending SRAM bit low-power
on on RAM, 512
package package bytes
E2PROM NA NA NA 512 bytes 64x8 bit NA
ADC 8-channel 5-channel 8-channel 8-channel 2- 16 +10 bit
12-bit 8-bit 10-bit 10-bit channel Zooming
8-bit ADC
UART 2 USART USART NA Serial NA Serial UA
I/O Ports 48 I/O 22 I/O 34-56 I/O 53 general (1) 4-bit 24 /O
lines lines lines purpose input, lines
depending I/O lines (2) 4 bit
on bi-dir,
package (1) serial
write
buffer
FLASH 16kB 4Kx14 bit 16K-60K 16KB NA NA
60kB depending
depending on
on package
package

In terms of features, all of the microcontrollers have adequate features for the

design of a self-powered wireless sensor. From the power characteristics listed in the

Table 3-1, the MSP430, EM6617, and XE88LC01A look like clear favorites (all are

around 10 ptA active current down to 0.1 ptA sleep mode current). However neither the

EM6617 not XE88LC01A have given data for port leakage or wakeup time. The final

decision was to go with the TI MSP430 since it has an abundance of available resources

and since TI gave samples and offered support.









Design of Programming Code

The code for the microcontroller is designed with flexibility in mind. This is done

so that the design can be tailored to any specific application with minimal code redesign

required. Along those same lines two different versions of code were written. Each

version of code presents a slightly different mode of operation. The two modes of

operation are listed below.

1. Level Monitoring. Constantly monitors sensor and sends a single emergency RF
pulse when the sensor's data goes above a given threshold value.

2. Data Transmitting. Constantly monitors sensor and sends data every given number
of seconds. The delay is overridden if the sensor's data goes above a given
threshold value.

The program code was written in the C programming language. The C code for

both modes of operation is given in Appendix A. Regardless of the mode of operation,

the structure of the code is very similar. The microcontroller code is broken into two

main sections: initialization and interrupt routines. This allows the microcontroller to

constantly be in a sleep mode with the CPU turned off, functioning in an interrupt driven

architecture.

The initialization section is common for both modes of code. The sampling delay

is defined in the initialization section as CCRO. CCRO is defined in terms of a number of

32 kHz clock cycles. For example, if CCRO is set to 500, then the delay equals 500

divided by 32 kHz or about 15 ms.

The interrupt routines consist of a timer interrupt and an ADC interrupt. The timer

interrupt is used to initiate ADC samples, as defined by the amount of time CCRO is set

to in the initialization section. The ADC interrupt occurs once an ADC sample has been









taken. The ADC interrupt is used to determine what if anything to send out over the RF

transmitter.

Level Monitoring

For the case of level monitoring the microcontroller conserves power by only

transmitting a signal when the sensor's data goes above the threshold value. The

microcontroller runs a very basic state machine consisting of the following states:

initialization, collect data, analyze data, transmit data, and sleep. A timer is used to delay

sampling the sensor data. A basic flow chart of the microcontroller's state machine is

shown in Figure 3-2.

Initialization

[ SLEEP

NO
Timer Set?

YES
Collect Data NO


Analyze Data T rDetected? Y Transmit Puls




Figure 3-2. Microcontroller flow chart for level monitoring.

The state machine is designed to run using a single timer to cycle through sampling

data and sleep mode. However if the data is above a threshold value then the

microcontroller immediately sends a single RF transmission and resets the timer. One

foreseen problem is that when the sensor's data is above the threshold value the control

system will constantly be transmitting until the sensor's data drops below the threshold

value. The solution to this problem is to use a static variable that is set in the code once a









transmission occurs. This variable gets cleared when the sensor's data falls back below

the threshold level.

The case of level monitoring has two settings that can be modified: threshold value

and transmit pulse length. The ADC sample is examined in the line "if (ADC10MEM <

OxlFF)" where OxlFF is the threshold value. Since the ADC sample is 10 bits the

maximum value is Ox3FF and the minimum is 0x000, so OxlFF is halfway between the

two. The threshold value is defined as a function of the supply voltage, so for a 2 V

supply voltage each bit is equal to 2 V / (210) = 1.95 mV. For the case of a sensor that

needs to be monitored for a falling voltage level, the code can be modified so that the

comparative statement is "if (ADC10MEM > OxIFF)." The data pulse is defined as just

a single voltage pulse on the data line to the RF transmitter. The length of time the data

line is held high is defined by the variable pulse length, which is defined in terms of a

number of 32 kHz clock cycles. The shorter the length of the pulse the less average

power the system takes, as the pulse is used to turn on the RF transmitter.

Data Transmitting

For the case of data transmitting, the microcontroller cycles between sampling data

and transmitting the data. The microcontroller runs a very basic state machine consisting

of the following states: initialization, collect data, analyze data, store date, transmit data,

and sleep. The state machine is designed to run using timers to add delays between

collecting data and transmitting the data. However if the data is above a threshold value

then the microcontroller immediately sends a single RF transmission and resets the

timers. The code has the same foreseen problem as the level monitoring code, that when

the sensor's data is above the threshold value, the control system will constantly be

transmitting until the sensor's data drops below the threshold value. The same static









variable solution from the level monitoring code is used for the data transmitting code. A

basic flow chart of the microcontroller's state machine is shown in Figure 3-3.


Figure 3-3. Microcontroller flow chart for data transmitting.

The case of data transmitting has three settings that can be modified: transmission

delay, transmitted data, and threshold value. The threshold value is defined similarly to

the threshold value for the level monitoring code. The transmission delay is defined by

the variable transmit dly, which is defined in terms of a number of 32 kHz clock cycles.

The transmitted data is currently defined simply as the sampled ADC value, where each

bit represents 1.95 mV for a 2 V supply voltage. This is the simplest method for saving

processing power, however once a sensor is chosen a coding scheme can be defined that

would decrease the number of bits needed to transmit, and hence decrease the amount of

power even further.









Programming Techniques for Minimizing Power Consumption

One of the keys to minimizing power consumption is to put the microcontroller in a

sleep mode for as long as possible. The MSP430 has 5 modes of operation: Active Mode

and Low Power Modes 0-4 (LPMO-LPM4). As the level increases from LPMO to LPM4,

the amount of the microcontroller that is active decreases. The CPU portion of the

microcontroller is turned off in all low power modes. The peripherals like the ADC and

USART can be enabled and disabled individually. By individually enabling and

disabling the peripherals as they are needed power is not wasted on inactive portions of

the microcontroller.

Another method for decreasing the amount of power consumed by the

microcontroller is to decrease the clock frequency. The MSP430 has an internal 1MHz

clock, however the microcontroller can be operated using an external 32 kHz crystal

oscillator. Both the ADC and USART can be operated off the 32 kHz oscillator, which

means the 1 MHz built in clock can remain off at all times except during initialization.

Using an external clock allows the MSP430 to run in LPM3 which is the second lowest

low power mode. The datasheet for the MSP430 [27] gives the following equation that

correlates how the current consumption changes as the clock frequency changes.

I(AM)= I(AM)[1 MHz] x f(System) [MHz] (3.1)

Where I(AM) is the current for active mode operation, I(AM)[1 MHz] is the current

for active mode operation at 1 MHz, and f(System) is the operating frequency. The

equation shows a linear relationship between the current consumption and the operating

frequency. According to the datasheet the current consumed at 1 MHz is 500 ptA, thus

the equation can be simplified to










I(AM) = (500 [A / MHz) f(System) (3.2)

Figure 3-4 shows a graphical representation of the current consumption versus

frequency, for a supply voltage of 3 V.


Frequency vs Current Consumption

500
450
400
350
< 300
2 250
200
o
150
100
50 -
0
0 200 400 600 800 1000
Frequency (kHz)


Figure 3-4. Frequency versus current consumption for MSP430 operating at 3V.

The next major way to decrease the power consumption for the microcontroller is

to decrease the supply voltage. The microcontroller can operate with a supply voltage

between 1.8 V and 3.6 V. The MSP430 datasheet provides the following equation that

correlates the supply voltage to the current consumption based on a 1 MHz operating

frequency.


I(AM) = I(AM)[3 v] + 210 [tA/V (Vcc 3 V)


(3.3)










Where I(AM)[3 V] is the current consumption when operating at 3 V and Vcc is the

supply voltage. This equation provides a linear relationship between supply voltage and

current consumption and is shown graphically in Figure 3-5.


Supply Voltage vs Current Consumption

650

600

: 550

C 500

1 450

I 400
o
S350

300

250

200
1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4
Supply Voltage (V)


Figure 3-5. Supply voltage versus current consumption for MSP430 operating at 32 kHz.

The original design for the programming code involved using the UART to serially

transmit data to the RF transmitter. Even with the ability to enable and disable the UART

around a sleep cycle, the default value being output on the UART was a "1". With the

UART outputting a "1" the RF transmitter would be transmitting a "1" which would be a

waste of power. In order to minimize the power, the code was re-written to use a single

output port of the microcontroller and simply serially shift the data bit by bit to RF

transmitter. One additional benefit of using an output port instead of a UART is that the

data pattern does not have to follow the parameters defined for a UART. Having open









options for the data pattern means that the data can be any length and can be as many or

as few bits as desired.

RF Transmitter

The used of a RF transmitter is required to wirelessly transmit the data from the

sensor. An RF transmitter has several design considerations such as transmission

frequency, transmit distance, data rate, and power usage. The power usage is the most

important of these design criteria in the design of a self-powered wireless sensor.

However the transmit distance has a minimal length to be useful. Thus the design needs

to be optimized for lowest possible power for a useful transmission distance.

Design of RF Transmitter

RF transmitters typically consist of four parts: frequency determining device,

amplifier, feedback circuit and output. A relatively simple design is the key to keeping

the RF transmitter's power usage low. Obviously the fewer active components in a

design the less power that is required. The simplest form of a frequency determining

device is a LC oscillator. The LC oscillator uses an inductor and a capacitor to provide

initial oscillations. The resulting oscillation of the inductor and capacitor is then input

into an amplifier via a feedback circuit. The amplifier then provides the amplified

oscillating signal to the output. The following simple formula is used to determine the

oscillation frequency based on the values of the inductor and capacitor.

f 1
fr 1
S2 L (3.4)

A Colpitts Oscillator is an example of a circuit design that employs a LC oscillator

and is a prime candidate for a very simple RF transmitter. The Colpitts Oscillator

consists of a LC oscillator along with a single transistor acting as a unity amplifier and









feedback circuit. Ming Microsystems Inc. has fabricated a RF Transmitter (TX-99) based

on the Colpitts Oscillator design. Figure 3-6 shows the schematic for the TX-99. The

datasheet [28] provides full operational details for the TX-99.






1 l2
i
--------IT"-----LiA









Figure 3-6. Schematic of TX-99 RF transmitter.

Power Consumption for a RF Transmitter

The power required for data transmission depends on the transmission distance,

transmission frequency, and the data rate. There are two methods for estimating the

power required for RF transmission: Free Space and Plane Earth. The Free Space

method assumes unobstructed transmission in all directions. The Plane Earth method

takes into account that the transmission is done on a plane and hence depends on the

height of the transmitter and receiver above the plane.

Free Space method

The Free Space method yields the following equation [29] relating the power loss

to the transmission frequency and the distance between transmitter and receiver,

assuming a dielectric constant of 1.

Power Loss = 20*log(4*7T*D*f/c) (3.5)










Where D is the distance between the transmitter and receiver, f is the frequency,

and c is the speed of light. Power loss is defined as the difference in the power of the

transmitted signal between the transmitter and the receiver. A receiver requires the

transmitted signal to have some minimum amount of power, in order to interpret the

signal. Hence the minimum power required for transmitting is primarily dependant on

the minimum amount of power required in the transmitted signal at the receiver. The

frequency of the transmission and the distance between the transmitter and receiver add

to the total power required for transmitting. Figure 3-7 shows a plot of transmission

distance versus power loss for given frequencies.


Power Loss vs Transmit Distance

120


100 -

E 80 ---- -49MHz
.- -- --300MHz
S60 ---- 434MHz
-J
--- 916MHz
40 .-- -2400MHz

20

0 --------------------------
1 10 100 1000
Distance (m)


Figure 3-7. Transmission distance versus power loss for given frequencies.

As the transmission frequency decreases, the power loss for a given distance

decreases. Similarly for a given frequency, as the distance between transmitter and

receiver increases the power loss increases. Thus, to minimize the amount of power lost









in transmission, and hence minimize the amount of power required for transmitting, a low

frequency is desired.

Plane Earth method

The Plane Earth method takes into account the height above the Earth of the

transmitter and receiver. One benefit of reviewing the Plane Earth method is that the

equation that describes power loss is valid for an entire frequency band. Removing the

dependence on frequency provides an equation that only depends on the geometry of the

distance between the transmitter and receiver. According to the Plane Earth method the

following equation is valid for frequencies in the UHF band (300 MHz to 3 GHz).

Power Loss = 20*log[D2/(HT*HR)] (3.6)

Where D is the distance between the transmitter and receiver, HT is the height of

the transmitter, and HR is the height of the receiver. The above equation makes it obvious

that as the distance between the transmitter and receiver increases the power loss

increases, however as the height above the ground of the transmitter and/or receiver

increases the power loss decreases. Thus, one method for minimizing power would be to

increase the height of the transmitter and receiver. A graphical representation of the

Plane Earth equation for power loss is shown in Figure 3-8.











Power Loss vs Transmit Distance

140

120 .

100 -- -
E -- Ht= 0.5m, Hr= 0.5m
S 80 -- -Ht= 1m, Hr= 0.5m
SHt= m, Hr=lm
60 .-- Ht=5m, Hr= lm
-- -Ht=5m, Hr=5m
o. 40

20 --

0
10 100 1000
Distance (m)


Figure 3-8. Power loss vs. transmit distance for various transmitter and receiver heights.

Techniques for minimizing power consumption

From the Plane Earth and Free Space methods, the obvious ways to minimize

power consumption are to use a low frequency and to increase the height of the

transmitter and receiver. In terms of the operation of the transmitter, one method for

lowering the required power is to use On/Off Keying (OOK) which synchronizes the

power supplied to the transmitter with the data being supplied. Compared to other

methods such as Frequency Shift Keying (FSK), OOK transmitters are simpler, require at

least 50% less transmitter current, and require less bandwidth [30]. The control system

designed for this thesis uses the OOK method by connecting the output port of the

microcontroller to both the data and supply voltage inputs of the transmitter. Thus the

transmitter is only powered when it is transmitting a "1."









Another method for reducing the power is to decrease the number of bits

transmitted per transmission. However the number of bits transmitted depends on the end

application for a wireless transmitter and what information is desired to be transmitted.

Similarly, the number of transmissions per second directly effects the amount of power

required. The following equation correlates the number of transmissions per second to

the average power consumed by the system every second.

Pavg = [Puc (1 (ttx Ntx)) + Ptx ttx Ntx] (3.7)

Where Puc is the power consumed by the microcontroller, ttx is the time for a single

transmission, Ntx is the number of transmissions per second, and Ptx is the power required

for a single transmission.

Construction of a Prototype

A prototype for the control system described in this thesis is constructed using a

double-sided copper clad printed circuit board (PCB). Protel software [31] was used to

design the electrical schematic and mechanical layout for the PCB as shown in Figure 3-

9, Figure 3-10, and Figure 3-11. An LPFK [32] computer-controller surface milling

machine was used to etch the design into the circuit board. The layout is designed on

both sides of the PCB; one side contains the electrical components and the other side has

connectors to interface the various components of the prototype. BNC connectors are

used to interface the power source, data sensor, and RF transmitter to the microcontroller.

A 14 pin header is used for programming the microcontroller via the microcontroller's

JTAG pins. The electrical components consist of the microcontroller, a crystal oscillator,

a 50 kM resistor used to pull the reset pin of the microcontroller to Vo, and filter

capacitors as recommended by the MSP430 datasheet. A photograph of the prototype









with the RF transmitter is shown in Figure 3-12. The photograph shows the bottom side

of the board, which contains the interface connectors. The microcontroller and other

electrical components are on the top side of the board and are shown in Figure 3-13.


IvCC


Figure 3-9. Schematic for control system prototype.

















































Figure 3-10. Top side PCB layout for control system prototype.

















































Figure 3-11. Bottom side PCB layout for control system prototype.




























Figure 3-12. Control system prototype with RF transmitter.


Figure 3-13. Top side of control system prototype.














CHAPTER 4
EXPERIMENTAL METHODS AND SETUP

Testing of a control system for a self-powered wireless sensor consists of testing

and verification of the control system as well as testing the control system integrated with

the rest of the self-powered wireless sensor. Before integrating the control system with

the rest of the self-powered wireless sensor, the energy sources and data sensor must be

designed and tested. This chapter gives an overview of the experimental methods and

setup required for testing the control system, as well as describing the energy sources and

the data sensors used in testing.

Energy Sources

A self-powered wireless sensor must get its power from environmentally scavenged

energy. Environmental energy sources include light, wind, heat, vibrations, and the flow

of water. Light and vibrations are the two sources that are focused on in this thesis. The

reasons for choosing those two sources are the availability of energy conversion devices,

the ability to replicate and control the sources, and the various locations where these

energy sources are available.

Energy from Light

One of the most readily available environmental sources of energy is the sun. Solar

energy is a common energy source used in a wide range of applications from small

handheld calculators to traffic signals. Solar cells are used to convert energy from light

to electrical energy. The size and efficiency of the solar cell determines the amount of

electrical energy available for any given amount of light. Due to the typical constraints









on the size of a wireless sensor, a solar cell used for a wireless sensor needs to be small

and highly efficient. IXYS Semiconductor makes a 6mm x 6mm monocrystalline high

efficiency solar cell with their part number XOD17-04B [33]. The XOD17-04B solar

cell has an open circuit voltage of 630 mV and a short circuit current of 12 mA. In order

to attain a higher open circuit voltage multiple solar cells are connected in series. A

circuit designed by Shengwen Xu [34] is used to transfer the energy from the solar cell

into a storage capacitor that is used to provide the power to the control system. Details of

the circuit are shown in the conference paper by Shengwen Xu [34].

Energy from Vibrations

Vibrations are a common source of environmentally reclaimed energy. The use of

piezoelectric materials to convert vibrations to electrical energy is widespread [35].

Selecting a piezoelectric material is dependant on several criteria, specifically vibration

frequency, efficiency, size and scalability, and magnitude of vibration. Since this thesis

focuses on a control system for an ultra-low power wireless sensor, the selection of a

piezoelectric is based primarily on size and efficiency. Piezo Systems, Inc. makes a

Double Quick Mounted Y-Pole Bender, D220-A4-203YB [36]. This bimorph beam

consists of two parallel connected 1.25" x 0.25" x 0.02" pieces of 5A4E piezoceramic

[37] on an Aluminum beam. Four of these bimorph beams are mounted on a shim which

is connected to an impedance head, as shown in Figure 4-1. A simple direct charging

circuit, shown in Figure 4-2, is used to store the energy reclaimed from the beams. The

circuit consists of four Fairchild Semiconductor BAT54 Schottky diodes, a 330[tF

electrolytic capacitor and a Panasonic VL1220 2V battery. The four bimorph beams









were connected in parallel to the input of the circuit, in order to maximize the input

current.


Input
Inp ut


a ttery


Figure 4-2. Direct charging circuit for energy reclamation from vibrations.

Data Sensor

The microcontroller used in the control system requires an analog voltage as the

input for the sensor. This means that any type of data sensor used with the control system

must either vary voltage in accordance with what the sensor is sensing, or have some









circuitry that converts the output of the sensor into an analog voltage. To control the

voltage of the sensor during initial testing a simple reference voltage supply is used as the

data sensor input. Once the control system's functionality is verified a solar cell is used

as the data sensor input. A solar cell is used as an example of a sensor that provides a

varying voltage as the medium it is sensing changes (in this case as the amount of light

shined on the sensor varies). The final sensor that is tested is a hydrogen sensor.

The hydrogen sensor is made of a layer of ZnO nanowires with palladium

deposited on top. The sensor acts like a variable resistor, where the resistance changes

with respect to the concentration of hydrogen. A characterization of the sensor is

required in order to determine how to interface the sensor with the microcontroller.

Details about the sensor including a characterization of the sensor are written in several

papers [38]. Since the sensor acts like a variable resistor a biasing current is required in

order to get a voltage to input to the microcontroller. The design of a biasing circuit was

done by Jerry Jun. A schematic of the biasing circuit is shown in Figure 4-3. A

photograph of the biasing circuit with an encased hydrogen sensor is shown in Figure 4-

4. The biasing circuit requires two hydrogen sensors, one encased and the other open to

the environment. The encased sensor provides a fixed resistance to compare to the data

sensor's resistance, which is needed since the resistance value can change with variances

in temperature as well as hydrogen concentrations. Using two sensors provides a

differential voltage, which is amplified before being input to the microcontroller.


































Figure 4-3. Schematic of biasing circuit for hydrogen sensor.

























-i






Figure 4-4. Picture of biasing circuit with hydrogen sensor.

Testing Methodology for the Control System

The control system consists of the microcontroller and the RF transmitter, as

described in the previous chapter. The focus of the testing of the control system is to

determine the power required and to verify the functionality of the system. To verify the

functionality of the system, a regulated voltage supply is used as the source of power. By

using a regulated voltage supply, the supply voltage is constant and a small series resistor

is used to measure the input current and thus determine the input power.

Methods for Measuring Power Consumption

Measurements are taken to assess the power required to sense data, transmit data,

and remain idle. All power measurements are done using a Tektronix TDS5104B digital

phosphor oscilloscope; this provides a visual display of the power over a period of time.

Using an oscilloscope also provides a method for determining peak power, average

power, and total energy. A Tektronix P6248 differential probe connects to the









oscilloscope to measure the voltage drop across the series resistor, between the output of

the power supply and the input of the microcontroller. A differential probe is used to

isolate the grounds between the oscilloscope and the control system.

A small valued resistor is optimal to minimize the effects of the voltage drop across

the resistor on the control system. However, too small a value and the voltage drop will

be in the noise region of the oscilloscope. From experimentation with various resistor

values, the value of 383 Ohms was determined to be optimal for measurements.

The transmitted data can consist of different patterns depending on whether a pulse

is transmitted or if a data pattern is transmitted. Several measurements are needed to

determine the power required to transmit data. These measurements include the power to

transmit: a single pulse, a pattern of all l's, a pattern of all 0's, and a pattern of mixed l's

and 0's.

Methods for Characterizing RF Transmission

A characterization of the RF transmission is required to determine the maximum

transmission distance, the received power, and the frequency spectrum of the received

signal. An Agilent E4448A PSA series spectrum analyzer is used to measure the

frequency spectrum and the received power. A /4 wave whip antenna made from 22

gauge copper wire is soldered to a SMA connector which attaches to the spectrum

analyzer. A picture of the antenna is shown in Figure 4-5. Similar antennas are attached

to the transmitter and receiver to determine the additional transmission distance due to

the use of antennas. A Tektronix TDS210 two channel digital real-time oscilloscope is

used to view the data at the receiver to verify that it matches the data being transmitted.

The maximum transmission distance is determined by when the signal at the receiver no

longer matches the signal at the transmitter.














Figure 4-5. Whip antenna.

The test setup requires a fixed power supply for the control system as well as a

fixed power supply for the receiver. A power source for the spectrum analyzer and

oscilloscope is also required. Due to the need for multiple power sources, and the

measurements being done over a distance, there are limitations on where the

measurements can be taken. The atrium area of the New Engineering Building (NEB) at

the University of Florida is where the measurements are taken. Figure 4-6 shows a floor

layout of the atrium where the testing takes place.








C Hallway Atrium Hallway
Cu






O m 3.5 m 10 m 2 m


Figure 4-6. Floor layout of the atrium area in NEB.

The transmitter is held stationary at one end of the hallway, while the receiver and

spectrum analyzer are moved away from the transmitter down the hallway, through the

atrium and down into the extended hallway. The height above the floor of the transmitter

and receiver factors into the amount of power received. Both the receiver and transmitter









are mounted approximately 0.5 meters above the floor. The transmitter is mounted onto

the front end of a chair. The receiver is mounted onto a cart that also holds the spectrum

analyzer and oscilloscope. Figure 4-7 shows the setup for the transmitter and the

receiver.





Receive
Transmitter


oa
LEl





Figure 4-7. Test setup.

Testing of a Self-Powered Wireless Sensor

After testing of the control system is complete, the control system is integrated with

a sensor and reclaimed energy source to create a complete self-powered wireless sensor.

Testing of the self-powered wireless sensor is more for verification purposes then

detailed measurements. This testing provides proof that the control system works as it

was intended as a portion of a self-powered wireless sensor.

Two versions of the self-powered wireless sensor are tested, one being powered

from solar energy and the other powered from vibrations. The solar powered system uses

a series of IXYS solar cells [33] and an energy harvesting circuit [34]. A flashlight is

shined directly on the solar cells to provide a fixed light source. The vibration powered









system uses the bimorph PZT composite beams described earlier in this chapter. A Ling

Dynamic Systems V408 shaker is used to vibrate the beams.

A hydrogen sensor with biasing circuit is used as the data input to the ADC of the

control system. The biasing circuit receives its power from the same energy reclamation

circuit as the control system. The hydrogen sensor is placed inside a custom built gas

chamber as diagramed in Figure 4-8.

Sensor N2








A._----- = 1 Valvel2

Fumace

S Turbo
pump


Figure 4-8. Gas chamber for testing hydrogen sensor.

The gas chamber consists of a glass tube with vacuum valves on both sides and a

furnace surrounding the middle of the tube. The furnace was not operated for any of the

experiments. Entering the gas chamber from the valve on the right is a hollow glass tube

with wires running to a HP4156B Semiconductor Parameter Analyzer. The hydrogen

sensor is placed inside this glass tube, and the wires from the parameter analyzer are

connected to the hydrogen sensor in order to plot the IV characteristics of the sensor.

Alternatively these same wires are used to connect the hydrogen sensor to the biasing

circuit, which is placed outside of the gas chamber. A valve on the left side of the gas









chamber is connected to a gas cylinder containing 99.99% compressed nitrogen, a gas

cylinder containing 500ppm of compressed hydrogen, and a turbo pump. The turbo

pump is used to create a vacuum inside the gas chamber. A series of step by step

procedures for setting up and operating the gas chamber to perform tests with the

hydrogen sensor is described in the following paragraph.

Turn off the nitrogen and hydrogen gas lines and close both Valve 1 and 2. Open

Valve 1 and wait for the pressure inside the chamber to drop below 0.01 torr

(approximately 0 atm). Inside the gas chamber is now an approximate vacuum. Close

Valve 1 and turn on the nitrogen gas line at maximum flow rate until the pressure inside

the chamber reaches 760 torr (1 atm), then turn off the nitrogen gas line. The nitrogen is

used to clear the gas chamber so that the hydrogen sensor starts from Oppm of hydrogen.

Turn on hydrogen gas line at maximum flow rate and open Valve 2. Opening Valve 2

connects the turbo pump, which creates a stream of 500ppm hydrogen gas flowing

through the chamber and across the hydrogen sensor. The steps are repeated as many

times as necessary to collect enough data and verify the different features of the self-

powered wireless sensor.














CHAPTER 5
EXPERIMENTAL RESULTS

Characterization of the RF transmitter is presented in this chapter. Results of

testing the ADC input of the microcontroller are given. The results of testing the control

system including RF transmission tests, power consumption analysis, and functional

verification are presented. This chapter also covers system integration test results for

both the solar powered system and the vibration powered system.


RF Transmitter Characterization

The Ming TX-99 RF transmitter described in Chapter 3 was characterized using a

Keithley 2400 Source Meter as a variable voltage supply for the Vdd supply input and a

Tektronix CFG253 3MHz Function Generator for the data input. The function generator

was set to output a 1 kHz square wave. The amplitude of the square wave was varied to

determine the minimum data input level to successfully transmit the waveform to a Ming

RE-99 RF receiver placed 1 foot away. The receiver was connected to a Tektronix

TDS5104B digital phosphor oscilloscope to verify that the received signal matched the

data signal from the function generator. The Vdd supply input was held constant at 9V,

and the minimum data level was found to be 510mV. The next step was determining the

minimum Vdd supply voltage to properly function with a IV peak to peak data signal.

The function generator was set to output a 1 kHz square wave that was IV peak to peak

with a 0.5V DC offset. The minimum Vdd supply voltage for this condition was 0.6V.

To prove that the RF transmitter could operate using the on-off key technique, the signal









from the function generator was connected to both the data input and the Vdd supply

input. The minimum peak to peak voltage of the function generator was found to be

530mV in order to get the correct signal at the receiver.

Since the transmitter will be operated at 2V from the microcontroller output, further

testing was done to determine the transmission power. A SMA connector was soldered

to the antenna output of the TX-99 transmitter. An Agilent E3631A DC power supply

was connected to the Vdd and data inputs on the TX-99. A FLX402#1 1 foot SMA cable

was used to attach the SMA connector on the TX-99 to a DC blocker on the input of a

Hewlett Packard 8563E spectrum analyzer. The spectrum analyzer measured a transmit

power of-4.5dBm from the TX-99. Figure 5-1 shows the screen capture from the

spectrum analyzer.




































Figure 5-1. Transmit power from Ming TX-99 RF transmitter.

ADC Testing

Various devices were used to verify the operation of the ADC of the

microcontroller. The microcontroller was programmed to simply take the input from the

ADC, encode the analog input to a 10 bit value, and then serially shifted the 10 bits out

onto an output pin. The output pin was connected to a Tektronix TDS5104B digital

phosphor oscilloscope to verify that the output data matched the data input to the ADC.

Initially the ADC was connected to a Keithley 2400 Source Meter which was set to

output a fixed voltage so that the encoded data would be easy to decode when viewed on

the oscilloscope. The data on the scope was a 10 bit serial stream that represented an

encoded value based on a percentage of the microcontroller's supply voltage. The









microcontroller's supply voltage was connected to a Tektronix PS281 DC Power Supply

which was set to output 2V. The ADC input was varied from OV to 2V and the bit stream

on the scope was visually verified to coincide with the value of the input on the ADC.

Testing of the Control System

Testing the control system consists of testing the microcontroller connected to the

RF transmitter to determine the RF transmission characteristics, power consumption, and

to verify the functionality of the system. The microcontroller was programmed with the

data transmitting code. Due to the need for a sampled voltage on the ADC input a fixed

voltage supply was required in order to control the value of the transmitted data. The

power consumption was analyzed over the entire code routine, from initialization through

an entire cycle of reading the ADC and transmitting data. Delays in the code were

modified to make it easier to separate each step of the code. The functionality of the

system was verified using both the level monitoring code and the data transmitting code.

RF Transmission

The RF transmitter was connected to the microcontroller and experiments were

done in order to determine the maximum transmission distance, the received power, and

the frequency spectrum of the received signal. For all experiments the microcontroller's

supply voltage was connected to a Keithley 2400 Source Meter set to 2V. The

microcontroller's ADC was connected to another Keithley 2400 Source Meter so that the

transmitted signal could be controlled. An Agilent E4448A PSA series spectrum

analyzer was used to measure the frequency spectrum and the received power. Figure 5-

2 and Figure 5-3 show the received power and spectrum at 300 MHz for distances of 1

meter and 8 meters with a 14 wave whip antenna connected to the transmitter's antenna

connector. At distances above 8 meters, the received power was barely above the noise









floor of the spectrum analyzer. Figure 5-4 plots the received power measured from the

spectrum analyzer for various distances. The bounce back in the slope of the plot at 10

meters is attributed to a wave guide caused by the shape of the room where testing took

place.


Figure 5-2. Received spectrum at 1 meter from transmitter.







































Figure 5-3. Received spectrum at 8 meters from transmitter.











Distance (m)
0 5 10 15 20
-35
E -40
S-45
S-50
10 -55 -
a-
S-60
Z -65
| -70
-75

Figure 5-4. Received power at varying distance from transmitter.

The Ming RE-99 receiver was connected to a Tektronix TDS210 two channel

digital real-time oscilloscope to verify that the data received matched the data

transmitted. Above 8 meters the received signal started to deteriorate. To illustrate the

deterioration Figure 5-5 shows the received signal at 8 meters and Figure 5-6 shows the

received signal at 10 meters. Tests were continued until the received signal no longer

looked like the transmitted signal. Figure 5-7 shows the received signal at 14.5 meters.

Antennas were used on the transmitter and receiver to attain an even greater transmission

distance. Table 5-1 lists the maximum transmission distances for the different antenna

locations.




























IiajArl r %NkI I IA


S. .


* 1


Figure 5-5. Received signal at 8 meters.


CM? Ott


i
*. ;


r**
























A.1 ..A U..1. .A 1 .1. L. 1. ). 1.


CH1



..i .l CH1


CH1 844mV


Figure 5-6. Received signal at 10 meters.


2 10.0V


M lrns


,H1 INVr~ C


!:


!,: -i '
M I ;


IN
1q.
JL I A


I I Ii~r





















CHi
2.56V

CH2 Of f



CH1


CH1 ,O' CH2 10,V M lis CH1 844mV



Figure 5-7. Received signal at 14.5 meters.

Table 5-1. Maximum transmission distances for different antenna locations.
Antenna Locations Maximum Distance


Receiver Only 14.5 m

Transmitter Only 16.8 m

Transmitter & Receiver 19.4 m



Power Consumption

The power consumption of the control system is broken down into the power

consumed during different activities of the system. The power was measured for

initialization, sensing data, transmitting data and remaining idle. Both modes of

operation require the same initialization code, therefore the initialization power is the


....................... .........
....... .. .''
... .............







. ......-


l









same. Figure 5-8 shows a scope capture of the voltage measured across the 383 Ohm

resistor during initialization. Since the resistor is constant and the supply voltage is a

constant 2V, initialization power has the same shape as the voltage across the resistor.

The value of the initialization power at any point along the scope capture is calculated

using the equation

Pinit = 2V Vmeas / 383 Ohms, (5-1)

where Vmeas is the voltage measured in the scope capture. From the scope capture

the duration of the initialization phase was found to be 12.5 ms. The peak power during

initialization was 7.3 mW. The average power during initialization was found by

separating the scope capture into linear segments and calculating the area under each

segment then summing the areas together and multiplying the resulting sum by the total

time duration for initialization. The average power during initialization was calculated to

be 3.07 mW. The energy required for initialization is simply the total initialization time

multiplied by the average power during initialization. The initialization energy was

calculated to be 38.4 IJ.







































Figure 5-8. Scope capture of voltage across series resistor during initialization.

The code was written using sleep modes, so that the ADC sampling does not

require any additional power compared to remaining idle. Therefore the power during

the idle state and power during the ADC sample state are the same. Figure 5-9 shows a

scope capture of the voltage measured across the 383 Ohm resistor during an idle state.

Since the voltage is in the noise range of the scope and is less than 50 mV, the average

power was found by using a Keithley 2400 Source Meter which has a feature that

displays the output supply current and supply power. The displayed average output

power was 2.5 [tW.







































Figure 5-9. Scope capture of voltage across series resistor during idle state.

The amount of power required to transmit data was measured for various

transmission cases. Since the RF transmitter is operating in an on-off key mode, the

amount of power required to transmit data is directly related to the data being transmitted.

With on-off keying there is no additional power, above the power required to remain idle,

to transmit a "0" valued bit. Additional power is only required when transmitting a "1"

valued bit. For the case of level detection, the power required is simply the power

required to send a single "1" valued bit of data. Figure 5-10 shows a scope capture of the

power required to transmit a single "1" valued bit of data, as well as the single bit of data

being transmitted. Figure 5-11 shows a zoomed in view of the same scope capture as









Figure 5-10, focusing on the rise time of the transmit power. While the rise time is equal

to the same amount of time the transmitted bit is sent, the remaining 5.5 ms of fall time is

due to the slow discharge of the LC circuit in the RF transmitter. The average power to

transmit a single "1" valued bit was calculated similarly to the initialization power. The

calculated value for average power to transmit a single "1" valued bit was 303 [W.

Since the total time for transmit power is 6 ms, the average energy required to transmit a

single "1" valued bit of data is 1.8 [PJ. In Figure 5-11, an apparent overlaying frequency

exists on the scope capture of the power. This switching frequency is due to the

oscillation of the LC circuit in the RF transmitter which is used to generate the

transmitted signal's frequency.


Figure 5-10. Transmit power for a single data bit.






































Figure 5-11. Zoomed in view of transmit power for a single data bit.

Measurements were taken during transmission of mixed data patterns. Scope

captures of the power along with the data pattern are shown in Figure 5-12, Figure 5-13,

and Figure 5-14. The slope of each rising and falling segment of the transmit power is

the same as the rising slope and falling slope of the transmit power for a single "1"

valued bit. The rising slope represents the power during a "1" valued bit. The falling

slope represents the power during a "0" valued bit. The length of time for each segment

of rising and falling slope is based on the number of consecutive "1" valued bits or "0"

valued bits. The scope captures clearly show that the power is dependant on the bit order,

with every "0" valued bit transmitted after a "1" valued bit decreasing the total power.









Therefore, the maximum power would occur when all the "1" valued bits were

transmitted in a row. This can clearly be seen in Figure 5-15, which shows 3 different bit

patterns all with the two "1" valued bits for six transmitted bits. Note that the area under

the waveform after the sixth bit will always be the same for any given number of"1"

valued bits. The area under the waveform is greatest when all the "1" valued bits are

transmitted one after the other. Thus the maximum energy used occurs when all "1"

valued bits are transmitted one after the other. Using the waveform that represents the

maximum energy will allow for a simplification in determining the maximum average

power, for any given number of "1" valued bits per transmission. The peak of the

triangular pulse is simply the peak value for transmitting a single "1" valued bit times the

total number of"1" valued bits transmitted. Since the waveform has a triangular shape,

the area under the waveform which represents the maximum energy required is equal to

12 the peak power times the total time. The maximum average power required to transmit

any given data pattern can be calculated simply by multiplying the average power to

transmit a single "1" valued bit by the number of"1" valued bits transmitted. The

maximum average power during transmission is calculated using the equation

Ptx = N P1, (5-2)

where N1 is the number of "1" valued bits, and Pi is the average power to send a single

"1" valued bit. Figure 5-16 lists the maximum average power during transmission for

various number of "" valued bits transmitted.







































Figure 5-12. Scope capture of transmission power for data pattern '1011101011'.






































Figure 5-13. Scope capture of transmission power for data pattern '1101100101'.






































Figure 5-14. Scope capture of transmission power for data pattern '1010000110'.















1 0 0 0 0 1


1 0 0 1 0 0


S11 0 0 0


Figure 5-15. Different data patterns for two "1" valued bits.










Maximum Average Transmission Power
vs. Number of "1" Valued Bits

3.5


E2.5
I 2
1.5
I- 1
0.5 .
0
1 2 3 4 5 6 7 8 9 10

Number of "1" Valued Bits


Figure 5-16. Maximum average transmission power vs. number of "1" valued bits.

The average power across an entire cycle of sampling and transmitting data is

dependant on the delay times set in the code as well as the number of"1" valued bits

being transmitted. The following equation is used to determine the average power for an

entire sample and transmit cycle.

Pcycle = (PADC tADC + Pidle tidle + Ptx ttx) / total (5-3)

Where PADC is the average power during ADC sampling, tADC is the amount of time to

perform an ADC sample, Pidle is the average power idling, tidle is the amount of time in an

idle state, Ptx is the average power during transmission, ttx is the amount of time to

perform a transmission, and total is the total amount of time to perform an entire cycle.









Since the average power during ADC sampling is the same as the average power for

remaining idle, Equation 5-3 can be simplified to the following equation.

Pcycle = (Pno tx tno tx + Ptx ttx) / total (5-4)

Where Pno tx is the average power during ADC sampling and idling or in other words

when not transmitting, tno tx is the amount of time during the cycle when not transmitting

data. The total time for a cycle is equal to the time when transmitting plus the time when

not transmitting (i.e. total = ttx + tno tx). The time parameters can be re-written in terms of

duty cycle. The duty cycle for transmitting data compared to the rest of the cycle time is

defined as

Dtx = ttx / total. (5-5)

Substituting the duty cycle into Equation 5-4 yields the following equation,

Pcycle = [Pno tx (1 Dtx)] + (Ptx Dtx) (5-6)

which is completely a function of the average powers and transmission duty cycle.

Ultimately, this equation can be rewritten using Equation 5-2 so that the only

dependencies are the number of "1" valued bits and the duty cycle. The resulting

equation for average power during a complete cycle is

Pcycle = [2.5[tW (1 Dtx)] + (303[tW N1 Dtx) (5-7)

In the case of level monitoring only one "1" valued bit is transmitted. The transmission

duty cycle versus the average power for the case of level monitoring versus is shown in

Figure 5-17. The power to remain idle dominates the average power up to a duty cycle of

0.1%. At 1% duty cycle the average power is twice the idling power, and the average

power continues exponentially from there before topping off at 303 [tW for a case when

the system is constantly transmitting.











Average Power vs Transmission Duty Cycle

350

300 -

S250

200
0

a 150 -

100 -

50

0
0 ----- i i~ ^-------
0.01 0.1 1 10 100
Transmssion Duty Cycle (%)


Figure 5-17. Duty cycle vs. average power for level monitoring.

Functional Verification

By performing the transmission tests the functionality of the control system was

tested. Both the level monitoring and data transmitting versions of code were verified to

work properly. A Keithley 2400 Source Meter was used to control the input to the ADC.

The RE-99 receiver was connected to a Tektronix TDS210 two channel digital real-time

oscilloscope, to monitor the received signal.

For the case of level monitoring, the source meter was set below the trigger level

and no data was seen on the oscilloscope. Once the source meter was set above the

trigger level a single bit was visually verified on oscilloscope. The source meter was set

back below the trigger level and then the experiment was repeated and a single bit was

again seen on the oscilloscope.









For the case of data transmitting, the source meter was set to 0 V and no data was

seen on the oscilloscope. The source meter was then set to 2 V and a single 5 ms long

pulse was seen on the oscilloscope. The source meter was varied from 0 V to 2 V, and

varying data patterns were seen on the oscilloscope.

System Integration

The complete self-powered wireless sensor tested consisted of the control system,

the hydrogen sensor, and an energy harvesting circuit. The control system was operated

in level monitoring and data transmitting modes. The hydrogen sensor required a biasing

circuit as described in Chapter 4. Two different energy harvesting circuits were used, one

for solar energy and the other for energy from vibrations. In both cases enough power

was generated to operate the control system as well as to provide the power to bias the

hydrogen sensor.

The level monitoring code was edited so that 100 ppm hydrogen would trigger the

transmission of an emergency pulse. With no hydrogen present in the gas chamber the

level monitoring mode never transmitted any emergency pulses. Once hydrogen was

introduced into the gas chamber within a few minutes an emergency pulse was

transmitted.

The data monitoring code was run with no hydrogen in the gas chamber and an all

0's valued transmission was received by the RE-99 receiver and displayed on a

Tektronics TDS224 Four Channel Real Time Oscilloscope. Once hydrogen was

introduced into the gas chamber the received signal started to vary, with the encoded

value increasing over time. After a longer period of time the hydrogen sensor reached a

steady state value as became apparent when the received data remained constant. Once






66


steady state was reached the hydrogen was flushed from the gas chamber and the

received data started to vary again until finally settling back at a value of all O's.














CHAPTER 6
CONCLUSIONS

The control system designed in this thesis has conclusively been proven to work.

This chapter summarized the work presented in this thesis. Conclusions drawn from the

work as well as potential future work are also discussed in this chapter.

Conclusions

An ultra-low power control system for a self-powered wireless sensor has been

designed, tested and its functionality has been verified. The control system has been

optimized for minimal power consumption, based on the results of testing the sections of

the design. The average power has been measured for all portions of the design, and has

been minimized in accordance with the test results. Equations have been generated to

calculate the average power required based on the number of transmitted bits and the duty

cycle of transmission. The RF transmission has been characterized based on the series of

measurements described in this thesis.

The average power was ultimately defined as a simple equation with the two

variables being the duty cycle of transmission, and the number of l's valued bits being

transmitted. However, regardless of the value of the data being transmitted the frequency

that data is transmitted is the main factor in determining the average power. For a

transmission duty cycle of less than 0.1% the transmit power barely factors into the total

and the average power is dominated by the idle power which is 2.5 [LW.

The transmitter was not optimized other then operating it using on-off keying. The

maximum transmission distance with antennas was measured to be 19.4m. Due to









constraints on the area available for the RF transmission, the maximum transmission

distance may vary from the measurements. However this thesis was not focused on the

transmission characteristics, but rather the design of an operational system.

The control system was designed to be very flexible so that future designs could

easily modify the microcontroller code to adapt to a specific application. The

microcontroller code was designed to minimize the power required by holding the

microcontroller in a sleep mode once the initialization sequence is complete. Two

separate sets of code were written to minimize the processing time for a system that

requires transmission only when a sensor's data is above a given value versus a system

that requires data transmission at a given interval. However simple adjustments in the

code could easily be written to make a single system that does both fixed interval

transmission as well as emergency pulse transmissions if the sensor's data reaches a

given value.

Future Work

The control system was designed for ultra low power however the transmitted

power can be minimized by encoding the data in order to minimize the number of bits

transmitted. Further development can be done to minimize the power required by the RF

transmitter. The RF transmitter should be designed for the exact application and

deployment of the system. Using an antenna chamber a more accurate characterization of

the RF signal could be attained.

More work can also be done on the energy harvesting circuits so that a variety of

energy sources can be used simultaneously, thus minimizing the reliance on a single

source of energy. The energy harvesting circuits can also be made more efficient so that

less total energy is required into the complete system. Similarly more efficient solar cells









and PZT beams would enable operation in systems with lower levels of available energy.

Less required energy would translate to smaller sized solar cells or PZT beams, and

hence a smaller total package size of the system which would allow deployment of the

system in more applications.

This system could be used as a single node in a multi-node array of wireless

sensors. However some form of protocol would have to be defined for the transmitted

data in order to operate several of these sensors within a given area. Additionally, a

multi-node array would require a more advanced receiver than the RE-99 used for this

design.














APPENDIX
MICROCONTROLLER PROGRAM CODE



The program code for the microcontroller is included here. Two versions of code

are given, one for level monitoring and the other for data transmitting.



Code for Level Monitoring



// Description: Level Monitoring
//
// MSP430F1232

S /1\ | XINI-----V
// Crystal
// --IRST XOUTI-----A
// I I
// P1.01--> RF Transmitter
// sensor-->|ADC
II
/-
I/ David Johnson
// University of Florida
// August 2005


#include

void main(void)
{
WDTCTL = WDTPW + WDTHOLD; // Stop WDT
P3DIR = 0x10; // Set P1.0 to output direction
P30UT = x00; // Set the output to Ox0
ADC10CTLO = ADC10SHT 2 + ADC100N + ADC10IE;
CCTLO = CCIE; // CCRO interrupt enabled
CCRO = 500; // Delay for Sampling









TACTL = TASSEL + MC_

EINT();
ADC10AE 1= Ox01;
LPM3;


// ACLK, upmode


// Enable interrupts.
// P2.0 ADC option select


}

// ADC10 interrupt service routine
#pragma vector=ADC 10VECTOR
interrupt void ADC10_ISR (void)


static int outputalready;
if (ADC10MEM < OxlFF)
{
P30UT = 0x00;
outputalready = 0;


else if (outputalread
{
outputalready = 1;
P3OUT = 0x10;
int pulse length;
for (pulse length =
P30UT = 0x00;


else


// Variable to determine if Transmitted to Output


// Set output LOW
// Reset variable


y == 0)

// Set variable so transmission only occurs once
// Set output HIGH

100; pulse length>0; pulse length--); //Wait with output HIGH
// Set the output LOW


// Occurs if threshold has already been reached


outputalready = 1;


// Timer AO interrupt service routine
#pragma vector=TIMERAO_VECTOR
interrupt void Timer A (void)
{
ADC10CTLO 1= ENC + ADC10SC;


// Holds variable


// Sampling and conversion start









Code for Data Transmitting



// Description: Data Transmitting

// MSP430F1232

I/ /1\ XIN-----V
// Crystal
// --IRST XOUTI-----A


P1.01--> RF Transmitter


sensor-->IADC


// David Johnson
// University of Florida
// August 2005


#include

void main(void)
{
WDTCTL = WDTPW + WDTHOLD; // Stop WDT
P3DIR = 0x10; // Set P1.0 to output direction
P30UT = 0x00; // Set the output to Ox0
ADC10CTLO = ADC10SHT 2 + ADC100N + ADC10IE;
CCTLO = CCIE; // CCRO interrupt enabled
CCRO = 5000; // Delay for Sampling
TACTL = TASSEL_1 + MC_1; // ACLK, upmode


EINT();
ADC10AE I= 0x01;
LPM3;


// Enable interrupts.
// P2.0 ADC option select


}

// ADC 10 interrupt service routine
#pragma vector=ADC 10_VECTOR
interrupt void ADC 10ISR (void)


static int outputalready;
if (ADC10MEM < OxlFF)
{
Pl OUT = Ox00;


// Variable to determine if Transmitted to Output


// Set output LOW









// Delay data from being transmitted
int transmitdly;
for (transmitdly = 2000; transmitdly>0; transmitdly--);
int dout = 0; // Loop variable for number of data bits
int temp = ADC10MEM;

while (d_out<10)
OUT = (temp & ) << 4;
P30UT= (temp & Ox1) < 4;


int i;
for (i = 100; i>0; i--);
temp = temp >> 1;
d_out++;
}

P30UT = 0x00;
outputalready = 0;
}

else if (outputalready == 0'
{
P OUT = 0x00;
outputalready = 1;
// Encode and transmit AD
int data bit = 0;
int temp2 = ADC10MEM;


// Wait with output set to bit value of sensor data




// Set output LOW
// Reset variable


// Set output LOW
// Set variable so transmission only occurs once
C Sample
// Loop variable for number of data bits


while (data bit<10)
OUT = (temp2 & );
Pl OUT = (temp2 & Ox1);


intj;
for (j =100;j>0; j--);
temp2 = temp2 >> 1;
data bit++;
}

Pl OUT = 0x00;


else


// Wait with output set to bit value of sensor data




// Set output LOW


// Occurs if Emergency Transmit sent already


// Delay data from being transmitted
int delay2;
for (delay2 = 2000; delay2>0; delay2--);
int dout2 = 0; // Loop variable for number of data bits
int temp2 = ADC10MEM;










while (d_out2<10)
{
P3OUT = (temp2 & Oxl) << 4;


int k;
for (k= 100; k>0; k--);
temp2 = temp2 >> 1;
d_out2++;
}

P30UT = 0x00;
outputalready = 1;


}

// Timer AO interrupt service routine
#pragma vector=TIMERAO_VECTOR
interrupt void Timer A (void)
{
ADC10CTLO I= ENC + ADC10SC;


// Wait with output set to bit value of sensor data


// Set output LOW
// Reset variable


// Sampling and conversion start
















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BIOGRAPHICAL SKETCH

David E. Johnson was born on July 14, 1977, in Toms River, New Jersey. He

moved to Jacksonville, Florida, where he graduated from Samuel Wolfson High School

in 1995. He earned a B.S. in electrical engineering and a B.S. in computer engineering

from the University of Florida, in December 2000. During his undergraduate time at the

University of Florida he maintained research positions working for two different research

groups: Interdisciplinary Center for Aeronomy and Other Atmospheric Sciences

(ICAAS) and Interdisciplinary Microsystems Group (IMG). Upon finishing his

bachelor's degrees, he took a position working for Cisco Systems as a Hardware

Engineer. While working for Cisco Systems he continued part time at the University of

Florida in pursuit of a M.S. in electrical engineering. He intends to graduate from the

University of Florida in May 2006 with a M.S. in electrical engineering. Upon

graduation he plans to continue working for Cisco Systems.