8 April 2008 Design and testing of a MEMS acoustic emission sensor system
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We present four new findings pertaining to MEMS sensors for acoustic emission detection. Our sensors are resonant-type capacitive transducers, operating with a frequency between 100 kHz and 500 kHz, fabricated in the PolyMUMPS process. The sensitivity of a resonant transducer is related to the sharpness of its resonance, measured by the quality factor Q, and operating in a coarse vacuum will increase Q. We describe a practical laboratory method for sealing and evacuating our MEMS sensor, and present measurements showing Q in the evacuated packages to be 2.4 to 3.6 times greater than under atmospheric pressure. We also describe our theoretical analysis of noise sources in the electromechanical behavior of a resonant, capacitive-type transducer sensitive to out-of-plane motion, with particular interest in noise resulting from mechanical excitation of the moving plate by air molecule impact. We report on a new transducer design to sense out-of-plane motion featuring a moving plate constructed as an open grill rather than as a plate perforated by etch holes. Characterization measurements show the open grill design to have a higher Q than a comparable perforated plate transducer. Finally, we report on another novel transducer designed to sense in-plane motion. The sensor is a comb finger capacitive transducer, and theoretical predications predict the in-plane sensor to have a much higher Q than the out-of-plane sensors. We show experimental measurements confirming these design characteristics, and we show results from pencil lead break experiments.
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David W. Greve, Irving J. Oppenheim, Amelia P. Wright, Wei Wu, "Design and testing of a MEMS acoustic emission sensor system", Proc. SPIE 6932, Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2008, 69321P (8 April 2008); doi: 10.1117/12.775777; https://doi.org/10.1117/12.775777

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