A Capacitance Ultrasonic Transducer With Micromachined Backplate for Fast Flow Measurements in Hot Pulsating Gases

A novel high-temperature resistant capacitance ultrasonic transducer is presented. It is designed for an ultrasonic transit-time gas flowmeter and meets two main requirements not fulfilled by common piezoelectric transducers: First, a special construction based on an oxidized and patterned silicon backplate combined with a metallic membrane enables transducer operation at elevated gas temperatures of up to 600 degC. Second, the geometry and material parameters were chosen to obtain a broadband device that allows high signal slew rates and pulse repetition rates. As proven by measurements in an automotive combustion engine test bed environment, this new transducer suits for internal combustion engine exhaust flow measurements in between the catalytic converter and the end of the exhaust pipe. Preliminary results for the exhaust mass flow (up to 160 kg/h) of a typical automotive engine measured with these novel transducers are given and compared with the mass flow calculated from fuel consumption and air/fuel ratio (lambda)