Development of a spherically focused capacitive-film air-coupled ultrasonic transducer and a leak location array sensor for long-endurance spacecraft
This research work is focused on the development of a spherically focused (no-mirror) capacitive-film air-coupled ultrasonic transducer and a leak location array sensor for long-endurance spacecraft. For the development of a spherically focused capacitive-film air-coupled ultrasonic transducer, two transducers have been designed, fabricated, and their performance characterized, using a spherically deformed backplate and film. One has a 10-mm diameter and 25.4-mm geometric focal length, and another has a 50-mm diameter and 50.8-mm geometric focal length. Both spherically focused transducers have frequency spectra centered at 805 kHz with -6-dB points at 400 kHz and 1200 kHz. By performing rigorous feasibility tests; a flexible copper/polyimide circuit board material is employed as a backplate in place of the conventional silicon substrate. Utilizing its deformability and ease of microfabrication, we have demonstrated that spherically focused air-coupled ultrasonic transducers can be made to function without the need of an external focusing device, such as a zone plate or an acoustic mirror. We have also invented a simple and easily applied method to conform the metalized polymer film onto a spherically curved backplate, while suppressing wrinkling on the film. Good agreement has been shown between measurement and theory, suggesting that our transducers behave as ideal spherically focused piston transducers;For the development of a leak location array sensor for long-endurance spacecraft, we have developed and experimentally demonstrated a sensitive and reliable means to locate an air leak in a plate-like structure. The goals of this work are accomplished by developing a sophisticated leak location algorithm and a two-dimensional PZT array sensor. The proposed leak location algorithm is highly effective in finding the direction of the leaks, using a minimal number of sensors, and needing less computation time while still achieving high accuracy. In addition, it accounts for the multi-mode dispersive characteristics in a plate-like structure, and utilizes structure-borne noise generated by turbulence at an air leak. This leak location algorithm is implemented by a prototype of a 64-element array sensor.