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Characterization of a MEMS Optical Floating-element Shear Stress Sensor
Microelectromechanical systems (MEMS), a new field of technological advances, consists of integrated devices
or systems that combine electrical and mechanical components. Such systems are commonly fabricated using
silicon, the most dominant material in MEMS devices . These systems can sense, control and actuate on the
micro scale. MEMS applications include accelerometers, pressure, chemical and flow sensors, and optical
scanners. We have explored an optical floating-element fluid shear stress sensor that permits the
direct measurement of wall shear stress based on geometric Moire interferometry. The characterization of this
device includes sampling the displacement of the sensor with variable mean shear stress, and the calculation of
the actual mechanical displacement to check for accuracy of linear output from the sample data.
The Flow of fluid over a surface is a phenomenon that occurs anywhere, for instance in our blood vessels, on
the body of the car, and in a pipe. The motion of a fluid with respect to a surface creates a boundary layer. Flow
in this layer may be laminar or turbulent which is determined by the Reynolds number. Fox and McDonald
defined Reynolds number as "the ratio of inertia to viscous forces" , however there is not unique value at
which transition from laminar to turbulent flow occurs in a boundary layer. Laminar flows refer to flows that go
slowly and smoothly, where particles of fluid move parallel to a surface wall. On the other hand, turbulent flows
refer to flows with fast velocities that are highly unsteady due to more interaction between the surface and the
fluid. The detection and control of turbulent flows has been of importance not only in the fluid mechanics but also
in the chemical and environmental community because of negative effects on viscosity, drag, and shear stress.
The development of an optical floating-element shear stress sensor in this work was motivated by a need for a
direct measurement of shear stress in a water recovery system. Such systems are important for the treatment
of wastewater to produce potable water. Measurements of time resolved shear stress give information on
flow separation, transition from laminar to turbulent flow, and re-attachment. In this project, we explored and
tested the functionality of a MEMS optical floating-element flow shear stress sensor (Fig.1). Schmidt defined
a floating-element shear stress sensor as "an element that is free to displace laterally against the restoring force
set up by springs, flush mounted into the wall" . The floating element sensor is an example of direct
force measurement. Geometric Moire interferometry was used as the optical detection principle to measure
the displacement of the floating element.
Figure: Microscopic top view of the floating-element.
A geometric Moire interference pattern is formed when light impinges on two overlapping gratings with
different spacing of pitch. A grating is defined as a repetitive series of dark and light bars. When superimposing
two sets of gratings of slightly different pitch creates a new image with black as well as illuminated sections of
a specific period known as the Moird effect. When the top grating attached to a floating element is displaced due
to shear stress, it will create a linear amplified fringe shift that can be used to calculate the displacement of
the floating element. Therefore, the sensor provides a direct indication of the impinging flow. The optical
detection scheme is based on the detection of the Moird fringe pattern motion using a CCD imaging array.
Further, this paper will explain in more detail wall shear stress, the Moird effect, and the sensor design
specification in the background section followed by the settings of the actual experiment that includes sampling
the displacement of the sensor with variable mean shear stress and the calculation of the actual
mechanical displacement for a given shear stress. At last, the results of the experiment will be discussed.
Wall Shear Stress
The motion of a fluid with respect to a surface creates a boundary layer (Fig.2) due to the no-
slip boundary condition. The no-slip boundary condition describes the absence of fluid motion on a
fixed surface. When the fluid comes in contact with the surface, it creates a force . The
force perpendicular to the surface is pressure while the force parallel to the surface is known as
shear stress. Measurement of time resolved shear stress gives information on the condition of the
flow laminarr or turbulent), flow separation, and re-attachment. In other words, being able to
measure skin-friction will help to measure, analyze, and control flow as it interacts with the
surface. Further, if we want to measure shear stress in a fluid pipe, it is necessary to have a small
device for high spatial and temporal resolution . Therefore silicon-micromachined flow sensors
have been developed to measure flow, shear stress and accurately capture the turbulent
..................................... , .... U Freestreamrn --
i nlet - -
............................... - . ...............................----
............................... F lo w .y/.e .
Shear stress Wall-Mounted
Figure 2: Boundary Layer, courtesy of V. Chandrasekaran.
The Moire effect is one of the optical detection techniques used as a measurement tool for
the determination of relative displacement . According to Post and Ifju, "The Moire fringes are
broad dark and light bands formed by the superposition of two amplitude gratings each comprised
with opaque bars and clear spaces" . If we have a set of fringes (Fig. 3), and we overlap them (Fig.
4), we create a pattern whose displacement is proportional to the physical displacement. Such a
Moire pattern originates from the geometric distribution of light passing through the gratings.
Figure 3: Gratings of 19mm (a) and 20mm (b) pitch .
Figure 4: Moire Pattern of 19mm and 20mm pitches .
The displacement of the fringes is determined as follows :
where G is the fringe pitch, gl is the pitch of movable grating, and G/g1 is the
The Moire effect may be considered as a spatial amplifier as can be seen by the amplification factor
from the above formula.
The optical floating-element sensor consists of a floating micro-machined silicon plate held by
four tethers patterned with a grating on the bottom surface and a lower Pyrex plate also patterned with
a set of gratings on the top surface (Fig. 5). When the device is illuminated from the bottom of the
Pyrex, the movement of the upper plate in contrast with the bottom one will produce a shift in the
two gratings. Since light is transmitted through the superposed top and bottom gratings, a Moire
fringe pattern is created. When the floating element is displaced due to the shear stress, it will create
a linear amplified fringe shift that can be used to calculate the displacement of the floating element.
The MEMS optical flow-element shear stress sensor was previously developed in the
Interdisciplinary Microsystems group .
BOX (0.4 ?m) SOI (A) SOl (C)
Si(10 ?m) Al(025 ?m) Pyrex
Figure 5: A Schematic of the fabrication sequence: (A) Etch 2 pm recess on SOX
wafers and deposit and pattern device gratings (0.25 pm - Al). (B) Deposit 0.25
mm of aluminum on the Pyrex wafer and pattern handle gratings. (C) Anodically
bond Pyrex and SOI wafers. (D) Aligned DRIE up to the recess to release the
floating element .
Sensor Design Specification
In order to measure shear stress, for instance in a fluid pipe, it is necessary to have a small device
for spatial and temporal resolution. Therefore micromachined silicon flow sensors have been
developed to measure flow, shear stress and accurately capture the turbulent flow fluctuations
. Silicon micromachining is a process that creates microscopic mechanical parts out of the
silicon substrate, for instance the tethers of this sensor. According to Naughton and
Sheplak, "micromachined wall shear stress sensors can be grouped into two distinct
measurement classes: direct techniques such as floating-element type devices, or indirect
techniques such as thermal and optical sensors" . The sensor specified below uses direct
techniques to measure the force produce by the shear stress on the floating element. The MEMS
optical floating-element flow shear stress sensor consists of a 1280pm x 400pm silicon floating
element of 10pm thickness, suspended 2.0pm above the surface of a Pyrex wafer by four tethers (Fig.
6). The Moire fringes are realized by pattern aluminum lines with varying pitch on both the bottom of
the floating element and on the Pyrex wafer .
Transmitted Floauing clement
Figure 6: Top-view and cross sectional schematic of the first-generation optical shear stress sensor
For this experiment, a sensor with a set of gratings of 9.9pm and 10pm was used. The sensor die
was flush-mounted in a Lucite plug with back-side imaging optics and a Thomson-CSF TH78CE13
linescan CCD camera (Fig. 7). The CCD camera contains an array of 1 x 1024 pixels, each having a
width of 10pm. The package was mounted in a 2-D flow cell that provides a variable mean shear
stress. The Moire fringe was captured using the CCD camera. First, we started by grabbing a live image
of the sensor with no flow (zero shear stress) using a frame grabber. Then, flow was turned on
and several images were taken at different pressure levels. Once all the images were collected, it
was necessary to calculate the displacement of the floating element by taking the no-flow image
and comparing it with the ones of active flow. Using Matlab, a least squares curve fit was applied to
the recorded intensity pattern obtained for a given shear stress input. The following formula was
applied for the least-squares curve fit :
y(x) = AO + Al*sin(A2*x+A3)
where AO is the offset, Al is the amplitude, A2 is the frequency, and A3 is the phase.
The least squares method assumes that the best-fit curve is the curve that has the minimal sum of
least square errors from a given set of data. The intensity frequency for such a curve needed
was determined using the least square procedure to fit a sinusoidal intensity pattern to the
measured pattern (zero shear stress) and then determine the amplitude, DC offset, frequency, and
phase of the sinusoid. Once the frequency was determined, then it was used as a constant for
the remaining sample data.
Floating Sensor Package
KLamninar flow crl
Figure 7: Schematic diagram of optical testbed for sensor characterization.
The same procedure is used for a each shear stress and the corresponding intensity pattern to
obtain new phase of the sinusoid. By looking at the phase shift, that is the phase between the no-
flow pattern and the rest of the data, the number of pixels shifted by the Moire fringe is calculated.
Based on the Moire optical amplification, the corresponding mechanical displacement of the
floating element was determined using the following formula
Sensor Displacement= [AMoir6 shift *gJ]/ G
where DMoird shift= (02-01)/ frequency and 1/G= (1/g9)-(1/g2).
The sensor displacement gives a direct measurement of the wall shear stress. The displacement, A,
of floating element as a function of wall shear stress is given as follows :
A= Tw (LeWe/ 4Et)(Lt/Wt)3[1 + 2(LtWt/LeWe)]
where Tw = shear stress, Lt= tether length, Wt= tether width, t= tether thickness, and E=elastic
modulus of tether (Fig. 8).
P- X t
^ Floating Flet-netitr
Figure 8: Schematic plan view and cross-section of a typical floating-element sensor .
The recorded Moire intensity pattern for a shear stress of 0 Pa and 1.6 Pa is shown in grayscale in Fig.
9 and the relative intensities are shown vs. pixel number in fig. 10. The Moire pattern was found to
have a spatial period of 943pm, before the 5x optical amplification, compared to the physical
grating period of 9.9pm. The Moire amplification for the sensor was found to be 95.3 compared to
the theoretical value of designed 100.
Figure 9: Moire fringe pattern for shear stress of 0 Pa (a) and 1.6 Pa (b) as seen by
1024 pixel linescan camera. Successive frames from the camera are stacked vertically
within each rectangle.
I-I - x
---- Fit - 0 Pa
- Fit - 1.6 Pa
50 100 150 200 250 300 350 400 450
Figure 10: Measured relative pixel intensity overlaid with least squares curve
fit for 0 Pa and 1.6 Pa of shear stress.
Furthermore, the number of pixels shifted by the Moire pattern was determined for a range of
applied shear stress. The results are shown in Fig. 11 and 12. The mechanical sensitivity found from
the slope of this curve is 0.56pm/Pa, while the optical sensitivity of the Moire fringe after the 5x
optical amplification is 266.84pm/Pa.
0 0.4 0.8 1.2 1.6
Shear Stress II'a|
Figure 11: Mechanical displacement of the floating element of the shear-stress sensor.
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Sthar Stress (Pa)
Figure 12: Number of pixels shifted by the Moir6 fringe in pixels for a given shear stress.
Using MEMS technology for development and packaging, an optical flow shear stress sensor utilizing a
geometric Moird based transduction technique was statically characterized with applied shear stress from 0.1Pa
to 1.6Pa. In general, a linear trend was seen between the mechanical and optical displacement and the applied
shear stress, showing the potential utility of this sensor and transduction technique. Future work includes
rigorous dynamic and static characterization, noise floor studies, and sensitivity analysis to non-shear stress inputs.
rigorous dynamic and static characterization, noise floor studies, and sensitivity analysis to non-shear stress inputs.
Approximately The author would like to thank Professor Toshikazu Nishida, Professor Mark Sheplak, Professor
Lou Cattafesta, Steve Horowitz, Venkat Chandrasekaran, and Julio Castro for their assistance and cooperation in
1. Gardner, J., Varadan, V., Awadelkarim, 0., Microsensors MEMS and Smart Devices, John Wiley & Sons, New
2. Fox, R. McDonald, A., Introduction to Fluid Mechanics, John Wiley & Sons, New York, 1992.
3. Schmidt, M., Howe, R., Senturia, S., Haritonidis, J., Design and Calibration of a Microfabricated Floating-
Element Shear-Stress Sensor, IEEE Transactions on Electron Devices, Vol. 35, No. 6, pp. 750,1988.
4. Padmanabhan, A., Goldgerg, H., Breuer, K., Schmidt, M., A Wafer-Bonded floating element shear stress
microsensor with optical position sensing by photodiodes, Journal of Microelectromechanical Systems, Vol. 5, No.
4, pp. 307,1996.
5. Naughton W., J., Sheplak, M., Modern developments in shear-stress measurement, Progress in Aerospace
Sciences, pp.516-520, 2002.
6. Kuhnert, R., Bernd, M., Pattern sequential analysis of second-order Moire Interferograms, Physical
Research, Akademic- Verlag Berlin, pp.101-102, 1989.
7. Post, D., Han, B., Ifju, P., High Sensitivity Moire, Springer, New York, pp.103-105, 1994.
8. The MEMS optical floating-element shear stress sensor was designed by Mark Sheplak and Toshikazu Nishida,
with Steve Horowitz and Hamed Kourouma under NASA Langley Research Center support and monitored by Dr.
K. Tedjojuwono. It was fabricated at the Stanford Nanofabrication facility by Venkat Chandrasekaran and
post-procedure at ARL by Steve Horowitz, Kourouma, H., Thesis on Design and Analysis of an optical
detection scheme for micromachined floating-element shear stress sensors, University of Florida, 2002.
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