Turbulent shear force microsensor

Abstract

A microbridge is used for the accurate measuring of time varying shear forces in the presence of fluctuating pressure. A microdimensioned plate is suspended by arms to form a microbridge. The microdimensions enable the smallest turbulence scales of interest to be sensed uniformally throughout the entire surface of the plate. The cavity beneath the microbridge is so small that a viscous drag is created in the air within the cavity and dampens normal movement of the plate. The microdimensions in conjunction with the damping effect of the cavity enable the sensor to be substantially insensitive to pressure and thus sense lateral forces independent of normal forces. The microbridge sensor is fabricated by surface micromachining. A sacrificial layer is deposited over a substrate. A structural layer is deposited and patterned to form the plate and support arms over the sacrificial layer. The cavity is formed by a selective etchant removing the sacrificial layer and leaving the rest of the microbridge structure suspended above the substrate. In a differential capacitance readout scheme, a conducting layer in the plate of the microbridge is capacitively coupled with conductors in the substrate. A sensed change in capacitive coupling generates an indication of plate deflection and thereby shear stress independent of vertical movement. Optic readout schemes may also be employed and are readily incorporated in the fabrication process. A mounting member presses the microbridge sensor into a holding plate which fits in a matching slot flush with the target wall.