Microfabricated Shear Stress Sensors, Part 2: Testing and Calibration

Abstract

Three designs of surface-micromachined shear stress sensors have been tested and calibrated in a continuum e ow channel. The e rst design, for moderate shear stress conditions, is composed of passive sensors with optically determined sensitivities of 9 and 5.5 Pa/ πm of e oating-element dee ection for two variants. The second-generation design features e oating elementsintegratedwithon-chipelectronics.Thedee ection isthusmeasured witha voltage output that displays signie cant nonlinearities due to the limitations of drive electronics. Complete calibration of the third design was performed, as these sensors were integrated with complex active element control circuitry. These devices demonstrated a device sensitivity of 1.02 V/Pa § 5% over a sensor range of 0.5‐ 3.7 V. HE design and fabrication of the sensors investigated are described in Ref. 1and use e oatingelements that displace against springsuspensionsystemswhen exposedtoshearforces.Thisdirect measurement of shear stress has the potential for accurate measurements because the spring restoring force is calculated from the dee ection of the e oating element and must counteract the shear force inasteady-statecondition.Becausetheshearforceistheshearstress operating over the area of the e oating element, there exists a direct correlation between element dee ection and shear stress under any e ow conditions. One dife culty in employing e oating-element shear stress sensors stems from the millimeter-range thickness of boundary layers for practical wall shear stress conditions. 2 The e uid e ow’ s ine uence on the e oating element is very sensitive to misalignment of the surface of the element to the surrounding surface. 3 Floating elements that protrude into the e uid e ow alter the e ow over the sensor relative to the surrounding surface. Similarly, a recessed element would encourage boundary-layer separations at the upstream and downstream edges, also resulting in an altered e ow over the face of the element. Both of these conditions can lead to spurious or inaccurate measurements. Because of these accuracy limitations, which are dependent on sensor fabrication, microfabrication techniques described in Ref. 1 are used to manufacture the sensors in this work, an example of which is shown in Fig. 1. This paper describes the effort to test and to calibrate these sensors for both mechanical and electrical characteristics.