Vijay Chandrasekharan

A Microscale Differential Capacitive Direct Wall-Shear-Stress Sensor

This paper presents the development of a floating-element-based capacitively sensed direct wall-shear-stress sensor intended for measurements in a turbulent boundary layer. The design principle is presented, followed by details of the fabrication, packaging, and characterization process. The sensor is designed with an asymmetric comb finger structure and metalized electrodes. The fabrication process uses deep reactive ion etching on a silicon-on-insulator wafer, resulting in a simple two-mask fabrication process. The sensor is dynamically characterized with acoustically generated Stokes layer excitation. At a bias voltage of 10 V, the sensor exhibits a linear dynamic sensitivity of 7.66 mV/Pa up to the testing limit of 1.9 Pa, a flat frequency response with resonance at 6.2 kHz, and a pressure rejection of 64 dB. The sensor has a noise floor of 14.9 μPa/√(Hz) at 1 kHz and a dynamic range >;102 dB. The sensor outperforms previous sensors by nearly two orders of magnitude in noise floor and an order of magnitude in dynamic range.

A metal-on-silicon differential capacitive shear stress sensor

The paper presents a direct, capacitive shear stress sensor with performance sufficient for time-resolved turbulence measurements. The device employs a bulk-micromachined, metal-plated, differential capacitive floating-element design. A simple, two-mask fabrication process is used with DRIE on an SOI wafer to form a tethered floating element structure with comb fingers for transduction. Experimental results indicate a linear sensitivity of 7.66 mV/Pa up to the testing limit of 1.9 Pa at a bias voltage of 10V , and a bandwidth of 6.2 kHz . The sensor possesses a dynamic range ≫ 102 dB and a noise floor of 14.9 μPa/Hz at 1 kHz , outperforming previously reported sensors by nearly two orders of magnitude in MDS.

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

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.

Characterization of a MEMS-Based Floating Element Shear Stress Sensor

A microelectromechanical systems (MEMS)-based capacitive floating element shear stress sensor, developed for time-resolved turbulence measurement, is dynamically characterized via Stokes layer excitation. The floating element structure incorporates interdigitated comb fingers forming differential capacitors, which provide electrical output proportional to the floating element deflection. Preliminary sensor characterization reveals a bandwidth of 6.1 kHz and a sensitivity of 23 mV Pa at 4.2 kHz up to the testing limit of

A miniaturized optical package for wall shear stress measurements in harsh environments

We report the development of a time-resolved direct wall shear stress senor using an optical moiré transduction technique for harsh environments. The floating-element sensor is a lateral-position sensor that is micromachined to enable sufficient bandwidth and to minimize spatial aliasing. The optical transduction approach offers several advantages over electrical-based floating element techniques including immunity from electromagnetic interference and the ability to operate in a conductive fluid medium. Packaging for optical sensors presents significant challenges. The bulky nature and size of conventional free-space optics often limit their use to an optical test bench, making them unsuitable for harsh environments. The optical package developed in this research utilizes an array of optical fibers mapped over the moiré fringe. The fiber bundle approach results in a robust package that reduces the overall size of the optics, mitigates vibration between the sensor and optoelectronics and enables in situ measurement. The optical package for sampling the amplified moiré fringe is evaluated using bench top test setups. An optical test bench is constructed to simulate the movement of the moiré fringe on the floating element. High-resolution images of the optical fringe and optical fibers are combined in simulation to model the lateral displacement of the fringe. The performance of several fringe estimation algorithms are studied and evaluated. Based on the optical study, the optical package and post-processing algorithms are implemented on an actual device. Initial device characterization using this approach results in a device sensitivity of 12.4 nm/Pa.

Effect of Dynamic Pressure on Direct Shear Stress Sensor Design

A microelectromechanical systems (MEMS)-based capacitive floating element shear stress sensor, developed for time-resolved turbulence measurement, is studied for its response to unsteady pressure forcing. The floating element structure incorporates interdigitated comb fingers forming differential capacitors, which provide electrical output proportional to the floating element deflection. An equivalent circuit model is provided under simultaneous pressure and shear loading. A design strategy to mitigate pressure sensitivity while enhancing shear stress sensitivity is provided. The significance of the design methodology is experimentally illustrated via comparison of combined shear and pressure response of the sensor with its pressure response. Results indicate a significant effect of pressure on combined sensor response despite 64 dB of pressure rejection.

Microfabricated silicon-on-Pyrex passive wireless wall shear stress sensor

This paper presents the design, fabrication, and characterization of a passive wireless sensor for the measurement of wall shear stress. A micromachined variable-capacitor shear stress transducer is realized using a silicon-on-Pyrex microfabrication process. The design features a diamond-shaped 2.25 mm2 silicon floating-element to accommodate more comb fingers for improved capacitive transduction in a smaller die area. The variable-capacitor device is connected to a fixed inductor on a printed circuit board to enable passive wireless sensing. The nominal resonant frequency of the device is 168 MHz with a quality factor of 8.6. Calibrations of static shear stress in a flow cell show a linear response to over 4 Pa, with a frequency-shift sensitivity of 474 kHz/Pa (1.1% full scale). Theoretically a minimum detectable shear stress of 4.1 mPa can be resolved giving a 61.7 dB dynamic range.