University of Florida | Journal of Undergraduate Research | Volume 13 Issue 2 | Spring 2012 1 Vibration Energy Harvesting Jennifer Griffith Delgado Erick Macias and Dr. Karl Gugel College of Engineering, University of Florida Th e project's goal was to harvest electrical energy from a vibrating source. We experimented with acoustic and cantilever piezoelectrics. We developed different techniques to strain each type of piezoelectric, thus generating an AC voltage. After trying several methods, we converted th e AC signal into a DC signal with enough voltage and current to turn on an LED. Finally, we attempted to tune the piezoelectrics to find their maximum output at a particular frequency. INTRODUCTION Because the every year, there is a need for more electricity and power to be generated. E nvironmental a nd economic problems arise requiring a new means of energy harvesting. The resources in Earth are finite an d are not replaced as fast they are being consumed. Electric companies have a limited amount of power they can generate, causing the prices of kW hou rs to increase, along with the cost of living. Currently energy is harvested from several sources including windmills solar power, and thermal energy Energy is harvested by transferring the kinetic, solar, and thermal energy into electrical; therefore, f ollowing the law of conservation of energy, where some of the energy dissipates into heat. Following this same principle, we wanted to transfer mechanical energy from the vibration into electrical energy. Vibrating surfaces can be found in our environment in power mills, motors, and the movement of a person walking amongst others. There are different types of vibration energy harvesters includ ing electrostatic, electromagnetic, and piezoelectric. Piezoelectric harvesters ar re quire exter nal voltage sources, making them the main topic of our research since we wanted to harvest energy without the need of electrical energy. Theory The straining of the piezoelectrics is what allows for the mechanical energy to be transferred to ele ctrical energy. Acoustic piezoelectric s are circular devices of different diameters; as the diameters increase the resonant frequency decreases. A sinusoidal voltage applied across the negative and positive nodes allows for the piezoelectric to create a s ound at different frequencies based on the Reversing this process, vibrating the center of the piezoelectric will create a voltage difference across the output of the acoustic piezoelectric. On the other hand, cantilevers are rectangu lar piezoelectrics, which similarly are strained when they are stretched However, instead of straining the center of acoustic piezoelectric, they are strained when they are bent or stretched as it can be seen in Figure 1 Figure 1. 1.3 kHz Acoustic Piezoelectric Hypothesis We hypothesize that using vibration energy harvesting can create a power supply that can power sensors or microcontrollers. Microcontrollers and sensors nowadays require a very small amount of electricity in the mA range For th e most part they are on only for some short time to sample some data or record an event and then go back to low power mode. While they are in low power modes they can be consuming electricity in the uA range. Thus these power harvesters would remov e the need for expensive batteries or the use of external power sources. METHODS Acoustic Piezoelectrics The initial experiments revolved around straining acoustic piezoelectrics by clamping their edges and moving their center back and forth. A piezoelectric with a resona nt frequency close to 1.3 kHz [4 ] was used with a series of different techniques to see which clamping method was most effective. First, three c clamps were used to affix the acoustic piezoelectric to blocks of wood. With the piezoelectric se cured, the apparatus was placed on a 60 Hz shaker table and the output signal was measured. Figure 2 shows the clamping apparatus.
J ENNIFER G RIFFITH D ELGADO E RICK M ACIAS AND DR KARL GUGEL University of Florida | Journal of Undergraduate Research | Volume 13 Issue 2 | Spring 2012 2 Figure 2. Wood clamps setup For the next clamping method, an acoustic piezoelectric was glued to a 2 inch long piece of PVC pipe of the same diameter as the piezoelectric. This left only the center exposed. Thus, the pipe acted as a funnel, directing computer generated sound waves of varying frequencies to the center and straining the piezoelectric properly. The sound w aves were funneled to the piezoelectric at different frequencies until a peak output signal was obtained. See Figure 3 for a picture of this setup. Figure 3 Speaker and Acoustic Piezoelectric The output of the acoustic piezoelectric was also test ed with varying amounts of weight placed on the area that must be strained to obtain an AC voltage. The results showed the e ffects of adding weight to the center of the piezoelectric on its resonant frequency. Once efficient clamping and straining methods were found, an attempt was made to rectify the signal obtained from the acoustic piezoelectric. A diode bridge rectifier was used in this experiment. See Figure 4. Figure 4. Rectifier circuit for acoustic piezoelectric Cantilever Piezoelectrics A fter concluding experiments with the acoustic piezoelectrics, cantilevers were used. The cantilever piezoelectrics needed to be bent back and forth at a certain angle to be strained prop erly. At first, the LDT0 028K [6 ], a piezoelectric polymer film tab, w as tested. This tab had a weight affixed to the end to allow it to bend in response to vibration. It was held down on the shaker table and the output signal was measured. Once more, attempts were made to rectify the signal. This time, a rectifier chip that did not need to be powered by an external power so urce, the LTC3588 1 [1 ], was used in place of the rectifier circuit from the acoustic experiments. The LTC3588 1 contained a full wave rectifier and was de signed specifically to assist applications similar to those in this project [ 1 ]. See Figure 5 for the circuit required to operate this chip with a cantilever piezoelectric. Figure 5 Linear Technology Rectifier Circuit The same rectifier chip was us ed in experimentation with the V21BL cantilever piezoelectric, which was tested at the end of experiments with the LDT0 028K tabs. See Figure 6 for a picture of the V21BL cantilever
VIBRATION ENERGY HAR VESTING University of Florida | Journal of Undergraduate Research | V olume 13 Issue 2 | Spring 2012 3 piezoelectric. This particular piezoelectric was chosen based on the circ uit described in the LTC3588 1 datasheet as an energy harvesting example. This cantilever needed to be strained in the same way as the tabs described previously. The V21BL, however, consisted of not one but two cantilever piezoelectric elements, clamped to gether at the bases and attached to a beam [2 ]. There were two options for connecting the pair of cantilevers in a circuit: in series for increased voltage or in parallel for increased current. The latter method was chosen because the primary barrier to re ctifying the sign al was having enough current Once the circuit suggested in the application note for the LCT3588 1 was completed, the output from the rectifier chip was taken on the oscilloscope. Figure 6 V21BL Cantilever Piezoelectric Once one pair of cantilevers was successfully tested, anoth er V21BL was connected in parallel with the first to give four cantilever piezoelectrics in total. The output was connected to the LTC3588 1 rectifier, and the output from the chip was once more observed on the oscilloscope. RESULTS The first acoustic exp eriment clamping the piezoelectric to a wooden apparatus and placing this on the shaker table did not yield a clean signal. The test in which the 1.3 kHz acoustic piezoelectric was affixed to the PVC pipe, however, yielded clean sine waves such as that sho wn in Figure 7 on the output. Table 1 and Figure 8 show the peak voltages obtained for varying frequencies of the sound waves used to strain the piezoelectric. The AC signal was unable to be rectified using the rectifier circuit available at the time the a coustic experiments were performed. Figure 7. Output Voltage of Acoustic Piezoelectric
J ENNIFER G RIFFITH D ELGADO E RICK M ACIAS AND DR KARL GUGEL University of Florida | Journal of Undergraduate Research | Volume 13 Issue 2 | Spring 2012 4 Figure 8. Frequency vs. Voltage from Acoustic Piezos Table 1. Voltage Output from 1.3kHz: Acoustic Piezoelectric w hen Starined with Sound Waves of Varying Frequencies f (Hz) Vpeak(V, Computer 1) Vpeak(V, Computer 2) 0 0 0 200 0.48 0.44 400 0.52 0.48 600 0.72 0.52 800 3.38 4.84 1000 6.6 3.72 1200 5.6 2.68 1400 5.04 9.29 1600 9.2 9.72 1800 0.32 0.4 2000 0.2 0.24 The LDT0 028K tabs yielded inconclusive results. The output signal was inconsistent, and an accur ate measurement of its value was unable to be made. The LTC3588 1 rectifier chip was unable to rectify the output from the cantilever tabs. Figure 9 shows an oscilloscope measurement from the V21BL experiments using t wo of the cantilever piezoelectric units (giving four total piezoelectrics in parallel). The LTC3588 1 was able to rectify the signal from these cantilever piezoelectrics, giving a stable output voltage around 3.8 V and a stable output current between 3 an d 7 uA. A single V21BL unit had a peak rectified voltage of approximately 5 V and a peak current of 88 uA. Using two V21BLs gave a peak rectified voltage of 4 V and a peak rectified current of 170 uA. Figure 9. Output Voltage of V21BL Varying the output capacitor on the LTC3588 1 rectifier chip during the V21BL experiments changed how fast the rectified signal would come out on the chip. With larger capacitors, the time required for the stable rectified voltage appeared was longer. Using two V2 1BL units changed the amount of time the LED could stay lit, shown in Figure 10. With the second unit, the LED was lit for a longer duration of the blinking cycle. The hardware set up of the shaker table, two V21BLs, LTC3588, and LED can be seen in Figure 11. After stopping and restarting the LED's blinking several times, the circuit stopped rectifying the cantilever output. Replacing the LTC3588 1 chip with a spare fixed the problem, but after several more tests, the spare also no longer rectified the sign al, concluding this set of experiments. Figure 10. LED being powered by LTC3588
VIBRATION ENERGY HAR VESTING University of Florida | Journal of Undergraduate Research | V olume 13 Issue 2 | Spring 2012 5 Figure 11 Shaker Table and V21 BL Circuit Set up DISCUSSIONS The piezoelectric technology at this moment is not far along enough to generate current in the mA range. Thus the goal of being able to power a microcontroller or sensors that require milliamps is not possible. Power supplies for smaller power can be created using cantilever piezoelectrics in parallel, which is expensive, and would require for all the cantilevers to be placed in such a manner where all the piezoelectrics are vibrating at the same freque experiments were inconclusive since the output of the piezoelectrics could not be rectified, unlike the cantilever piezoelectric In the future experiments connecting multiple cantilevers in para llel should be r u n to see by how much the power harvested can be improved. Since the output of the acoustic piezoelectric was a sine wave, experiments on putting acoustic and cantilevers piezoelectrics in parallel could be done to see the effects on the po wer harvested. Moreover more tests could be run on different rectifier circuits connected to the output of the cantilevers and see their durability against the vibration on the piezoelectrics at different frequencies. ACKNOWLEDGEMENTS We thank our faculty mentor, Dr. Gugel, for the guidance and direction he provided on this complex project. We also thank Dr. Nishida for teaching us how piezoelectrics work and how to obtain an output signal to work wi th. We a l s o thank Dr. Fox who was very helpful during the acoustic piezoelectric experiments, giving us the idea to try funneling the sound waves onto the area we needed to strain. L ast ly, we thank the University Scholar Program for giving us the oppor tunity to perform research for a year i n an area outside our major, allowing us to become better engineers. REFERENCES  Linear Technology Corporation. LTC3588 1 Datasheet [Online]. Available: http://cds.linear.com/docs/Datasheet/35881fa.pdf  MIDE. V21BL Datasheet [Online]. Available: http://www.mide.com/pdfs/Volture_Datasheet_001.pdf  G. Simmers and H. Sodano, Increasing El ectrical Power Generation for a Piezoelectric Power Harvester [Online]. Available: http://www.writing.eng.vt.edu/urs/simmers1.pdf  Pui Audio. 1 .3 kHz Buzzer Element Datasheet [Online]. Availa ble: http://www.puiaudio.com/pdf/AB4113B.pdf  Linear Technology Corporation. J. Drew, Design Note 483 [Online] Available: http://cds.l inear.com/docs/Design%20Note/DN483.pdf  Measurement Specialties. LDC with Crimps Vibration Sensor/Switch [Online]. Available: http://www.meas pec.com/product/t_product.aspx?id=2484