Tri-axial optical MEMS accelerometer enables monolithic fabrication

Accelerometers are one of the most widely used sensors. They are essential in many applications such as inertial measurement units (IMUs), airbags safety systems, activity monitoring in biomedical applications, and vibration analysis of industrial machinery. Some of these applications may need high immunity to Electromagnetic Interference (EMI). Optical accelerometers can provide such an advantage and usually shows better sensitivity. However, most of the previous versions involved optical fibers, which hindered their monolithic microfabrication. The few optical accelerometers suited for mass production presented previously in literature suffered from some other drawbacks, besides measuring in one direction (1D) only. In this work, we present a novel optical accelerometer that enables measurements in 3D, besides facilitating monolithic fabrication and simple assembly. The operation principle is based on power modulation technique that does not need complicated processing and achieves real-time measurements. The device consists of a light-emitting diode (LED), a quadrant photodetector and a proof mass suspended between them by springs allowing it to move along the 3-axes. When the proof mass moves due to the applied acceleration, more light will pass to some panels of the quadrant detector while others will receive less light, according to the motion direction. The sensor design, implementation scheme, mechanical simulation and optical modeling are reported. COMSOL finite element analysis (FEA) simulation shows a mechanical sensitivity exceeding 3.7 mu m/G. The modeling for both mechanical, optical and electrical transductions shows a total sensitivity up to 100 mu A/G. The mechanical part of the device is fabricated using the SOIMUMPs process.