2025

Pernu, Tapio; Saarilahti, Jaakko; Sillanpää, Teuvo

This paper presents a comprehensive study on a low-cost, real-time frequency matching circuit for a high sensitivity gas flow sensor based on cMUTs (Capacitive Micromachined Ultrasonic Transducer). The gas flow sensor is designed with a focus on the mechanical resonance of the moving membrane. It measures gas velocity using the differential transit time method. Custom cMUTs with a resonance frequency of 2.44 MHz were designed and fabricated. The fabrication process involves a 7-step lithography process with thin-film surface micro-machining technology. The frequency matching circuit, implemented around a microcontroller, uses conductance measurement and bias voltage sweeping to match resonance frequencies of two cMUTs to the excitation frequency. The flow sensor was tested over a full-scale range with air flow of ± 50 standard cubic centimeters per minute (sccm), showing a maximum flow error within 0.8 % of the full-scale (FS), demonstrating the state-of-the-art sensitivity of cMUT-based gas flow sensors in the literature. The real-time frequency matching circuit significantly reduces zero-flow drift over a temperature range of 0–50 °C, from 1.3 % FS to 0.3 % FS. The sensor offers high accuracy and low-cost advantages for ultrasound-based gas flow measurement applications.