Acoustic emissions are stress waves released when damage occurs in structures, with typical frequency content between 100 kHz and 1 MHz. Typically, acoustic emission sensors use a piezoelectric sensing element and are limited to sensing in the out-of-plane direction. This research focuses on the design, development, and testing of a multi-axis sensor system for acoustic emission sensing. Multiple capacitive resonant sensors were fabricated on one MEMS chip using the PoIyMUMPs foundry process. The potential advantages of a multi-axis sensor system include improvements in characterization of structural damage by distinguishing between different wave modes and by determining the direction of the source.;One out-of-plane sensor and one in-plane sensor were tested extensively. The out-of-plane sensor features a spring-suspended plate that is an open grill, designed to increase sensitivity over earlier designs. The in-plane sensor is a finger-type design and has low damping for motion in the in-plane direction and relatively high damping for motion in the out-of-plane direction. The in-plane sensor depends on this difference in damping to isolate the unwanted response to out-of-plane motion from the desired response to in-plane motion. Finite element simulations of the mechanical and electromechanical behavior of the sensors were performed and showed good agreement with the parameters measured in laboratory characterization tests.;Experiments were performed to compare the out-of-plane sensor system, consisting of the out-of-plane MEMS sensor and its amplifier, to a commercial piezoelectric sensor. Initial experiments revealed that the out-of-plane sensor system was significantly less sensitive than the commercial sensor. A new low-noise amplifier increased the sensitivity of the out-of-plane sensor system. Additional comparison tests between the out-of-plane sensor system and the commercial sensor were performed.;The in-plane MEMS sensor features a sensitive and a nonsensitive orientation separated by 90 degrees. In experiments, the in-plane sensor showed a stronger response to excitation in the sensitive orientation compared to the nonsensitive orientation. Also, the in-plane sensor showed a long ringing response in both orientations, consistent with a lightly damped device. In another test, the in-plane sensor did not show the expected behavior described above, and several hypotheses explaining the anomalous behavior were investigated.
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