A micro-electro-mechanical-system-based thermal shear-stress sensor with self-frequency compensation

Abstract

Abstract. By applying the micro-electro-mechanical-system (MEMS) fabrication technology, we developed a micro-thermal sensor to measure surface shear stress. The heat transfer from a polysilicon heater depends on the normal velocity gradient and thus provides the surface shear stress. However, the sensitivity of the shear-stress measurements in air is less than desirable due to the low heat capacity of air. A unique feature of this micro-sensor is that the heating element, a film 1 µm thick, is separated from the substrate by a vacuum cavity 2 µm thick. The vacuum cavity prevents the conduction of heat to the substrate and therefore improves the sensitivity by an order of magnitude. Owing to the low thermal inertia of the miniature sensing element, this shear-stress micro-sensor can provide instantaneous measurements of small-scale turbulence. Furthermore, MEMS technology allows us make multiple sensors on a single chip so that we can perform distributed measurements. In this study, we use multiple polysilicon sensor elements to improve the dynamic performance of the sensor itself. It is demonstrated that the frequency-response range of a constant-current sensor can be extended from the order of 100 Hz to 100 kHz.

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“…The relatively low sensitivity at large values of the wall shear stress could be improved by using a differential amplifier in the associated circuit. It is likely that the overall sensitivity of this sensor, when accounting for the circuit in which it is used, will be similar to the results obtained by Huang et al (1999), since the present values for A and B are in the same range as the values they obtained.…”

“…Several approaches for developing practical wall shear stress sensors based on these two approaches can be found in the literature (e.g., Huang et al 1996Huang et al , 1999Kimura et al 1999;Liu et al 1999;Gad-el-Hak 1989Löfdahl & Gad-el-Hak 1999;Padmanabhan et al 1995;Shajii et al 1992;Schmidt et al 1988;Breuer 2000). Of particular relevance for the present study, Schmidt et al (1988) fabricated a microscale floating element sensor with a sensing area of 500 × 500 µm 2 using MEMS techniques, and Huang et al (1996) developed a microscale thermal sensor to measure surface shear stresses, but noted that the resulting sensitivity of their sensors was less than desirable.…”

“…For a more detailed evaluation of k, detailed heat transfer simulations such as those of Huang et al (1999) would be required.…”

“…6.7, and have been largely adapted from Huang et al (1999), although some changes were made to the geometry and type of electrodes used. One key modification is made in Step 3 in Fig.…”

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“…Micro-electromechanical-based sensor (MEMS) devices allow short response time and high sensitivity. Cheap and fast production of these devices is the result of progress in micro-technology in the last decade [8][9][10].…”