MEMS sensor for low frequency vibration detection.

Abstract

Machine or structural health monitoring requires the detection of very low vibration frequencies. Such frequencies are typically in the order of tens of Hertz. Current sensors that detect these frequencies, such as accelerometers, require external power and computation. Additionally, resonant MEMS sensors are not currently possible due to the fact that micro-scale mechanical components tend to have very high resonant frequencies. Also, some designs in the literature which can detect such frequencies have a very low selectivity. The objective of this thesis is to propose a design concept for a sensor that can detect very low frequencies with a high selectivity, without off board computation, and having the potential to be self-powered. The main sensor components are designed for a specific case and simulated predominantly in ANSYS, a finite element analysis software that supports electromechanical systems. The sensor consists mainly of a bistable mechanical component and a frequency detector. The bistable structure responds harmonically to the external vibration. The response takes the form of a pulse train having high frequency content, while the frequency of the pulses is equal to that of the external vibration frequency. This response is obtained with the help of a damping mechanism. Fluidic damping is unattractive because the presence of fluid in the sensor package degrades the detector’s performance. The mechanism used consists of a compact electromechanical circuit which is composed of a Zener diode and a comb-drive. The bistable component transfers its response to the frequency detector via contact. The detector consists of two oscillators having very close natural frequencies which are in turn close to the frequency content of the pulse created by the bistable structure. This detector presents a unique response when the frequency of pulses is equal to the difference in natural frequencies of the oscillators. The simulation results showed that first the Zener diode-based damping mechanism is effecti