Micro Shear-Stress Sensor for Separation Detection During Flight of Unmanned Aerial Vehicles Using a Strain Gauge

A micro shear-stress sensor (MiSS) for real-time detection of flow separation in unmanned aerial vehicles was presented. The direct method was selected for the MiSS, and movement of its floating element was measured using a simple and highly reliable piezoresistive scheme. To realize the MiSS, a polysilicon strain gauge of the piezoresistor type with a very low temperature coefficient of resistivity (TCR) and high gauge factor was first developed. These two characteristics were adjusted by varying the boron concentration during fabrication. The strain gauge was also designed to evaluate the effect of size (width) on these characteristics. The TCR was almost zero and the gauge factor was 32 at a boron concentration of 1.5×10¹⁹ cm^-3. The characteristics were unaffected by its width (in the tens of micrometer range). The MiSS was designed and fabricated using the developed polysilicon strain gauge, and its performance was evaluated using a NACA0012 airfoil section in a wind tunnel. At a 0 ^° angle of attack (AOA), the resistance of MiSS increased with the flow. At a 20 ^° AOA, the resistance did not change owing to detachment of the flow caused by separation. In the real-time separation detection test, the resistance of MiSS decreased as soon as separation occurred. Hence, the separation detection ability of the developed MiSS using a polysilicon strain gauge was verified.