Characterisation of micromachined capacitive sensors for ultrasonic applications

A single transducer self-reciprocal calibration method has been successfully developed for air-coupled ultrasonic sensors and compared with traditional three-transducer calibration. This proposed new calibration standard is effective for ultrasonic transducers at frequencies of up to 2.5MHz in air. The self-reciprocal method has been applied to the absolute calibration of a set of capacitive air-coupled transducers with various configurations of backplate structures. The frequency range of 100kHz to 2 MHz allows comparison with theory to understand the physical operating principles of the micromachined air-coupled capacitance transducer.; A physical model integrating almost all of the physical phenomena active in an air-coupled capacitive transducer has been developed and verified. This has included a proper numerical approach in both the time and space domains to solve the complex partial differential equation that governs the performance of such transducers.; An improved computer controlled automatic non-contact analysis system was designed and built using an optimal combination of signal prefiltering and pulse compression. A single transducer self-reciprocal calibration method using a large time bandwidth chirp signal was successfully implemented.; The most sensitive and most broadband transducer was found to have a small circular pit size in a rectangular array with a slightly smaller size pit located between neighboring normal pits. The best transducer design also has small static capacitance C0 and should be operated under the highest possible bias voltage. This work has enhanced the design of broadband air-coupled ultrasonic transducers with optimum characteristics for the rapidly growing field of fluid- (or gas-) coupled ultrasound.