Passive Wireless Direct Shear Stress Measurement

This paper presents a passive-wireless interrogation strategy for shear stress measurement.  The wireless system is realized via a hybrid packaging approach that combines a floating-element-type MEMS capacitive shear stress sensor die flush-mounted in a printed circuit board (PCB) substrate.  The sensor die is wire-bonded to a 5-turn inductor on the PCB to form an LC tank circuit with a nominal resonant frequency of 258 MHz and a Q of ~6.  A change in input shear leads to a change in the sensor capacitance and a corresponding shift in the resonant frequency.  A single-turn loop antenna, fabricated on a second PCB substrate and placed in proximity to the sensor is used for wireless interrogation.  The sensor displayed a linear sensitivity of 219 kHz/Pa (849 ppm/Pa) up to the testing limit of 1.6 Pa with a resolution of 45 mPa (limited by instrumentation).  The sensor showed superb stability, with drift variations of only ±0.1% over 3 hrs.  A wireless range of 8 mm was also achieved at 3.4 mW RF source power.