Conference Paper

Experimental Verification of a MEMS Based Skin Friction Sensor for Quantitative Wall Shear Stress Measurement

Authors:
  • Illinois Tech
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Abstract

This paper presents the preliminary wind tunnel characterization of a microelectromechanical systems (MEMS)-based capacitive floating element shear stress sensor. The floating element structure incorporates interdigitated comb fingers forming differential capacitors, which provide electrical output proportional to the floating element deflection. A compact sensor package combined with a synchronous modulation/demodulation system facilitates mounting in a flat plate model located in an open-loop low-speed wind tunnel. Particle image velocimetry is used to measure the boundary layer velocity profiles for laminar, transitional and turbulent flows. The mean wall shear stress estimated from profile curve fits is in agreement with MEMS sensor output.

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... The experimentally verified capacitance of the sensor electrodes is O(10 pF). Historically, a single-ended configuration has been used in capacitive wall shear stress sensors [6,10,11]. For a dual-axis sensor in this configuration, the fixed electrode pairs (E 1+ , E 1− and E 2+ , E 2− in Fig. 1) are excited by a pair of differential bias voltage signals at two different carrier frequencies, and the common floating element electrode (E 3 in Fig. 1) provides a single output signal which is then buffered for transmission back to the SCU. ...
... As a capacitive sensor with the intention of measuring both mean and dynamic inputs, a standard dc biasing scheme is unsuitable [10]. Because an ideal capacitor has an infinite impedance at dc, a sinusoidal biasing scheme is utilized instead. ...
... MEMS skin friction sensors are considered promising sensors in hypersonic wind tunnel experiments owing to their miniature size, high sensitivity, and stability. In recent years, several researchers have developed MEMS sensors to measure skin friction, including the capacitance-type and comb differential capacitance-type [1][2][3][4][5], piezoresistive-type [6][7][8], and piezoelectric-type [9]. For example, Mills et al. [5] reported a fully differential capacitive wall shear stress sensor for low-speed wind tunnels with the high sensitivity of 196 mV/Pa and a minimum detection limit of 12 mPa at 1000 Hz in a range from 0-10 Pa; Von, P. et al. [8] reported a wall shear stress sensor using four piezoresistors in the cantilever, and the resolution was 0.01 Pa in the range of 2 Pa; Kim, T. et al. [9] reported a piezoelectric floating element shear stress sensor for the wind tunnel flow measurement with the high sensitivity of 56.5 pc/Pa and with the frequency range of the sensor up to 800 Hz. ...
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Micro-electromechanical system (MEMS) skin friction sensors are considered to be promising sensors in hypersonic wind tunnel experiments owing to their miniature size, high sensitivity, and stability. Aiming at the problem of short test duration (a few milliseconds) and heavy load in a shock wind tunnel, the fast readout circuit and the sensor head structures of a MEMS skin friction sensor are presented and optimized in this work. The sensor was fabricated using various micro-mechanical processes and micro-assembly technology based on visual alignment. Meanwhile, the sensor head structure was integrated with the fast readout circuit and tested by using a centrifugal force equivalent method. The calibration results show that this sensor provides good linearity, sensitivity, and stability. The measurement ranges are 0–2000 Pa with good performance. The resolution is better than 10 Pa at 3000 Hz detection frequency of the readout circuit for the sensor in ranges from 0 to 1000 Pa. In addition, the repeatability and linearity of static calibration for sensors are better than 1%.
... The upper surface of test-head is its sensing surface, and will be installed evenly with the measurement wall. [7], adopted four supporting cantilevers and comb capacitance sensing and had a measuring range of 0.1-5.0 Pa, however its floating element and comb capacitances were exposed in the flow field, thus it could be only used in low wind tunnels with pure gases. ...
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MEMS-based skin friction sensors are used to measure and validate skin friction and its distribution, and their advantages of small volume, high reliability, and low cost make them very important for vehicle design. Aiming at addressing the accuracy problem of skin friction measurements induced by existing errors of sensor fabrication and assembly, a novel fabrication technology based on visual alignment is presented. Sensor optimization, precise fabrication of key parts, micro-assembly based on visual alignment, prototype fabrication, static calibration and validation in a hypersonic wind tunnel are implemented. The fabrication and assembly precision of the sensor prototypes achieve the desired effect. The results indicate that the sensor prototypes have the characteristics of fast response, good stability and zero-return; the measurement ranges are 0–100 Pa, the resolution is 0.1 Pa, the repeatability accuracy and linearity are better than 1%, the repeatability accuracy in laminar flow conditions is better than 2% and it is almost 3% in turbulent flow conditions. The deviations between the measured skin friction coefficients and numerical solutions are almost 10% under turbulent flow conditions; whereas the deviations between the measured skin friction coefficients and the analytical values are large (even more than 100%) under laminar flow conditions. The error resources of direct skin friction measurement and their influence rules are systematically analyzed.
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A microfabricated floating-element shear-stress sensor for measurements in turbulent boundary-layers is reported. Using surface micromachining of polyimide, a 500- mu m multiplied by 500- mu m probe has been fabricated incorporating a differential-capacitor readout circuit. A model for the sensor response is described and is used for the design of an element to measure shear stresses of 1 Pa in air. The sensor is packaged for calibration in laminar flow, and electrical results obtained match the expected response.
A MEMS shear stress sensor for turbulence measurements Design and characterization of microfabricated piezoresistive floating element-based shear stress sensors
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A MEMS-based shear stress sensor for high temperature applications
  • N Tiliakos
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