Surface Touch and State Detection with Ultrasonic Guided Waves Based on PMUTs

Surface touch and state detection are critical for many industrial applications. Conventional methods, such as capacitive and optical sensing, are limited by their restricted coverage and susceptibility to environmental interference. In contrast, ultrasonic guided waves offer wide-area sensing with high sensitivity. Here, we present a compact and broadband piezoelectric micromachined ultrasonic transducer (PMUT) system designed for surface touch and state detection using guided waves. PMUT arrays were encapsulated in PDMS to ensure robust acoustic coupling to the test plate; the packaged array exhibits a resonant frequency of 145 kHz with a broad bandwidth. We demonstrated that the on-chip array geometry and placement have a significant influence on guided-wave transmission and reception. Accordingly, a 10×10 PMUT array positioned adjacent to the plate was selected to maximize guided wave amplitude. In experiments, the S0 guided wave mode amplitude changes enable accurate localization of touch events, while combined amplitude and phase changes in the A0 mode allow precise discrimination of surface contaminants. These results demonstrate that PDMS-packaged PMUT arrays can realize compact, broadband guided wave measurement systems suitable for high-resolution surface sensing, offering a promising route toward integrated, robust industrial sensing solutions.