In-plane MEMS acoustic emission sensors development and experimental characterization

Abstract

Damage initiation and growth in materials releases elastic waves, which can be detected by surface mounted acoustic emission (AE) transducers. In this paper, new MEMS comb-drive AE transducers, responsive to in-plane motion, manufactured using electroplating technique for highly elevated microstructure geometries are presented. The transduction principle is capacitance change achieved by area/gap change in two separate designs. Mechanism of spring orientation, dimensions and mass have been selected in such a way that they satisfy three design criteria as the frequency range of 100-200 kHz, 2-30 pF capacitance and the functionality under atmospheric pressure. The challenge of coupling the microstructure vibration in out-of-plane and in-plane directions is addressed with differential mode approach and frequency domain responses. The squeeze film damping is reduced with 8 μm gap between moving electrodes so that the transducers are operational under atmospheric pressure. The directional independence of the transducers to two orthogonal directions is demonstrated using laser source as the excitation signal. The Nd: Yag Q switch laser has 3 mm beam diameter and is focused on the top and the edge of the transducer package. The results show a distinct output signal for in-plane and out-of-plane motions due to the directional sensitivity of the MEMS transducers. © The Society for Experimental Mechanics, Inc. 2014.