MEMS floating element sensor array for wall shear stress measurement under a turbulent boundary layer

A MEMS floating element shear stress sensor array has been designed as a 1 cm x 1 cm chip. The array consists of 256 individual floating elements which are separated into 16 groups with a pitch of approximately 2 mm. Bumps were included on the surface of the device in order to increase interaction with the flow. Although this does increase hydrodynamic forces, it appears to primarily increase pressure gradient sensitivity rather than shear stress sensitivity.;The device was fabricated on a glass substrate using four layers of surface micromachining including copper and nickel electroplating. The chips were packaged either in a ceramic package or on a printed circuit board. A capacitance to voltage readout and a capacitance to digital converter IC were used to measure the differential capacitance change resulting from flow forces.;The sensor chips with half the elements acting in parallel were flushed mounted into the flow channels of a laminar flowcell at Tufts University and an indraft windtunnel at the NASA Ames Research Center. Experimental characterization of a chip in a ceramic package determined that the sensitivity to shear stress was 77 aF/Pa. The achieved resolution was limited by white noise with a level of 0.24 Pa/rtHz at low frequencies (below 1.5 Hz), and linearity was demonstrated to larger than 13 Pa. A second sensor packaged in a printed circuit board was tested under a turbulent boundary layer in the indraft windtunnel. The sensor sensitivity was 90 aF/Pa, and resolution was 1 Pa/rtHz at low frequencies (below 1.5 Hz). Orientation dependence of the sensor output was verified, demonstrating the ability to measure direction as well as magnitude of the shear stress.;In addition to the demonstration of a MEMS shear stress sensor array, a major finding of this work is that pressure gradient sensitivity for these devices can be high. An experimental methodology was conceived and demonstrated that allows independent determination of pressure gradient and shear stress sensitivity. It is particularly important to accurately measure pressure gradient effects when calibrating these devices in a laminar flow cell. If this effect is neglected the calibrated shear stress sensitivity may exhibit large errors. For the device described here, neglecting pressure gradient effects would lead the researcher to erroneously conclude that sensitivity is 173 aF/Pa, rather than the correct value of 77 aF/Pa.