Ultrasound Fill-Level and Touch Sensors Through Acoustic Coupling for CMUTs

Abstract

Capacitive micromachined ultrasonic transducers (CMUTs) are the next-gen ultrasound transducers for diverse applications. CMUTs provide high bandwidth and sensitivity and have excellent integrability with CMOS processes. Additionally, our CMUT design enables mobile and battery driven devices with a small form factor and requiring voltages as low as < 30 V. Here, we present a novel coupling approach and demonstrate two sensor applications: fill level sensing and touch sensing. With a package containing a stack of silicone and epoxy, we achieve excellent coupling into different materials such as mold plastics and aluminum via direct or intermediate acoustic impedance matching. This enables an universal solution for coupling into almost any kind of material. Fill level sensing is demonstrated through a ca. 4 mm-thick aluminum containment wall with levels from 2.3 mm to 600 mm. Furthermore, a touch sensor is realized via detecting a reduced echo amplitude due to the presence of a finger. Measurements through a commercial decor panel and a ca. 4 mm-thick aluminum plate show a maximum signal difference of 6.5 % and 19.2 % in the echo, respectively. Additionally, we investigate several interference and coupling phenomena in the layered system for the touch scenario. By that, we enable a wide field of applications, where CMUTs are coupled to hard materials such as plastics and metals, as found in touch sensors in e.g. consumer electronics, as well as liquid (flow / fill level) monitoring in chemistry, pharmaceutical and food industry. Furthermore, structural health monitoring for smart industry and non-destructive testing are feasible with our approach as well.