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. IntroductionAcoustic emission (AE) method relies on propagating stress waves due to newly formed damage surfaces. Conventional piezoelectric sensors have bulky geometry and are sensitive to the wave motion in thickness mode and tangential mode due to lateral deformation of the sensor geometry. Understanding the propagating wave direction through differentiating orthogonal wave motions can improve the source localization and characterization. The comb drive sensors have interdigitized fingers formed by the stationary part known as stator and the moving part known as rotor [1]. Comb drive sensors created using Micro-Electro-Mechanical Systems (MEMS) can be designed to be sensitive to tangential wave motion with respect to the structure that the sensor is attached. Harris et al.[2] designed comb drive sensor using the surface micromachining method for detecting the in-plane motion. However, the sensor has coupled response for in-plane and out-of-plane motions. The unwanted out-of-plane motion dominated the sensor response. The surface micromachining method has the limited thickness as 2 μm, which limits the sufficient separation of the resonant frequency modes of in-plane and out-of-plane motions. The lateral instability of the comb drive is a known problem [3].The metalMUMPs incorporates LIGA-like, thick metal electroplating and bulk/surface micromachining processes to develop thick metal layer. The process is available by Memscap Inc since 2003. The process has been implemented by several researchers in the literature. For instance, Tsai et al.[4] designed out-of-plane rotational platform using in-plane electrostatic comb drive actuators. Almeida [5] dev...

Progress of MEMS acoustic emission sensor: a review

“…The sensor’s independence in two orthogonal directions was proven using a laser light source as an excitation signal. The above analysis highlights the effectiveness of using the thick film process with a high aspect ratio to achieve low frequency sensing (Saboonchi and Ozevin, 2014). This finding provides an essential foundation for designing low frequency acoustic emission sensors.…”

The design, characterization, and comparison of MEMS comb-drive acoustic emission transducers with the principles of area-change and gap-change

“…In this study, comb drive sensors are designed with the two principles of gap change and area change as their transduction principle, shown in Figure 1 (Saboonchi & Ozevin, 2014). Figure 1a shows the geometry of the area-change comb drive sensor (IParea).…”

Elastic Wave Measurement Using a MEMS AE Sensor

“…Capacitive-and resonant-type MEMS AE sensors have been actively researched in the academic field [6][7][8][9][10]. Capacitive MEMS sensors have moving electrodes parallel to the fixed ones manufactured by surface micromachining processes.…”