Multi Frequency Acoustic Emission Micromachined Transducers for Structural Health Monitoring

Abstract

One of the well-established passive Non-Destructive Evaluation (NDE) methods is Acoustic Emission (AE) in which elastic waves produced by sudden energy release of active flaws are detected. Low power consumption and lightweight characteristics along with high sensitivity are important features of AE transducers. Low power and lightweight AE transducers can be developed using Micro-Electro- Mechanical-Systems (MEMS). Unlike the capacitive MEMS transducers, piezoelectric MEMS transducers do not require bias voltage and have lower polarization voltage. Typically, MEMS transducers suffer from low sensitivity. In this paper, piezo-MEMS AE transducers operating in plate flexural mode driven by piezoelectric effect are presented. Piezo-MEMS AE transducers consist of two different frequency transducers tuned to 40 kHz and 200 kHz. Using Piezoelectric Multi-User MEMS Process (PiezoMUMPs) offered by MEMSCAP Co., transducers are fabricated by 5-mask level Silicon-On-Insulator (SOI) patterning and etching process. This process is followed by deposition of 0.5 micron Aluminum Nitride (AlN) which is sandwiched between top (metal layer) and bottom (doped silicon layer) electrodes and functioning as the sensing layer of the transducers. The micro-mechanical layers coupled with the piezoelectric layer are modeled in COMSOL Multiphysics software to understand their performance numerically. To tackle the problem of low sensitivity, two different frequency transducers are connected in series in order to increase the bandwidth and sensitivity. The electromechanical characterization of the transducers is presented. The independent performance of each transducer is compared with the case that they are connected in series to detect the simulated AE events. Taking into account size, power consumption and weight, the designed piezo-MEMS AE device has significant advantages as compared to conventional bulky AE transducers.