A Portable Proximity Sensing System Based on Capacitive Micromachined Ultrasonic Transducer

This paper reports a resonant proximity sensing system based on a capacitive micromachined ultrasonic transducer (CMUT), which has advantages of low cost, high sensitivity, and fast response. An equivalent circuit model of the CMUT was established and used for the design of the Colpitts oscillating circuit, which tracked the parallel resonance frequency of the sensor. Simulations of the oscillator demonstrated its capability of responding to the loaded mass of water molecules from the fingertip. A signal acquisition system based on the oscillator and the STM32 microcontroller has been built and its frequency stability was examined. The feasibility of the CMUT sensor as a proximity sensor has been experimentally evaluated. A maximum frequency shift of 55 kHz was achieved as the fingertip moved from 20 mm to 2 mm from the surface of the sensor. Short response and recovery times of 0.65 s and 0.85 s were achieved by the sensing system.