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Design, manufacturing, calibration, and basic characterization of a microelectromechanical systems (MEMS) wall hot wire sensor on a flexible polyimide substrate are presented. A configuration exhibiting bond pads on the top side of the foil, as well as an improved setup featuring a through-foil metallization and bottom side bond pads were established. Both sensor designs make use of a highly sensitive nickel thin-film resistor spanning a reactive ion etched cavity in a polyimide substrate. The polyimide base material enables the sensor to be adapted to curved aerodynamic surfaces, e.g., airfoils and turbine blades. A mismatch of curvature of aerodynamic surface and silicon sensor surface, as observed with previously presented MEMS hot-wire anemometers is avoided. The combination of polyimide's low thermal conductivity and a cavity featuring FEM-optimized dimensions accounts for a very low-power consumption (<25 mW). Fluctuations in wall shear stress up to 85 kHz can be resolved in constant-temperature mode. An average sensitivity of 0.166 V/(N/m2) is achieved in a wall shear stress range from 0 to 0.72 N/m2. The specifically designed through-foil metallization process allows for electrical contacts to be positioned on the backside of the substrate, thus effectively minimizing aerodynamic disturbances.
Date of Publication: Aug. 2007