Design, modelling and simulation of a novel micro-electro-mechanical gyroscope with optical readouts

Abstract

Micro Electro-Machnical Systems (MEMS) applications are fastest development technology present. MEMS processes leverage mainstream IC technologies to achieve on chip sensor interface and signal processing circuitry, multi-vendor accessibility, short design cycles, more on-chip functions and low cost. MEMS fabrications are based on thin-film surface microstructures, bulk micromaching, and LIGA processes. This thesis centered on developing optical micromaching inertial sensors based on MEMS fabrication technology which incorporates bulk Si into microstructures. Micromachined inertial sensors, consisting of the accelerometers and gyroscopes, are one of the most important types of silicon-based sensors. Microaccelerometers alone have the second largest sales volume after pressure sensors, and it is believed that gyroscopes will soon be mass produced at the similar volumes occupied by traditional gyroscopes. A traditional gyroscope is a device for measuring or maintaining orientation, based on the principle of conservation of angular momentum. The essence of the gyroscope machine is a spinning wheel on an axle. The device, once spinning, tends to resist changes to its orientation due to the angular momentum of the wheel. In physics this phenomenon is also known as gyroscopic inertia or rigidity in space. The applications are limited by the huge volume. MEMS Gyroscopes, which are using the MEMS fabrication technology to minimize the size of gyroscope systems, are of great importance in commercial, medical, automotive and military fields. They can be used in cars for ASS systems, for anti-roll devices and for navigation in tall buildings areas where the GPS system might fail. They can also be used for the navigation of robots in tunnels or pipings, for leading capsules containing medicines or diagnostic equipment in the human body, or as 3-D computer mice. The MEMS gyroscope chips are limited by high precision measurement because of the unprecision electrical readout system. The market is in need for highly accurate, high-G-sustainable inertial measuring units (IMU's). The approach optical sensors have been around for a while now and because of the performance, the mall volume, the simplicity has been popular. However the production cost of optical applications is not satisfaction with consumer. Therefore, the MEMS fabrication technology makes the possibility for the low cost and micro optical devices like light sources, the waveguide, the high thin fiber optical, the micro photodetector, and vary demodulation measurement methods. Optic sensors may be defined as a means through which a measurand interacts with light guided in an optical fiber (an intrinsic sensor) or guided to (and returned from) an interaction region (an extrinsic sensor) by an optical fiber to produce an optical signal related to the parameter of interest. During its over 30 years of history, fiber optic sensor technology has been successfully applied by laboratories and industries worldwide in the detection of a large number of mechanical, thermal, electromagnetic, radiation, chemical, motion, flow and turbulence of fluids, and biomedical parameters. The fiber optic sensors provided advantages over conventional electronic sensors, of survivability in harsh environments, immunity to Electro Magnetic Interference (EMI), light weight, small size, compatibility with optical fiber communication systems, high sensitivity for many measurands, and good potential of multiplexing. In general, the transducers used in these fiber optic sensor systems are either an intensity-modulator or a phase-modulator. The optical interferometers, such as Mach-Zehnder, Michelson, Sagnac and Fabry-Perot interferometers, have become widely accepted as a phase modulator in optical sensors for the ultimate sensitivity to a range of weak signals. According to the light source being used, the interferometric sensors can be simply classified as either a coherence interferometric sensor if a the interferometer is interrogated by a coherent light source, such as a laser or a monochromatic light, or a lowcoherence interferometric sensor when a broadband source a light emitting diode (LED) or a superluminescent diode (SLD), is used. This thesis proposed a novel micro electro-mechanical gyroscope system with optical interferometer readout system and fabricated by MEMS technology, which is an original contribution in design and research on micro opto-electro-mechanical gyroscope systems (MOEMS) to provide the better performances than the current MEMS gyroscope. Fiber optical interferometric sensors have been proved more sensitive, precision than other electrical counterparts at the measurement micro distance. The MOMES gyroscope system design is based on the existing successful MEMS vibratory gyroscope and micro fiber optical interferometer distances sensor, which avoid large size, heavy weight and complex fabrication processes comparing with fiber optical gyroscope using Sagnac effect. The research starts from the fiber optical gyroscope based on Sagnac effect and existing MEMS gyroscopes, then moving to the novel design about MOEMS gyroscope system to discuss the operation principles and the structures. In this thesis, the operation principles, mathematics models and performances simulation of the MOEMS gyroscope are introduced, and the suitable MEMS fabrication processes will be discussed and presented. The first prototype model will be sent and fabricated by the manufacture for the further real time performance testing. There are a lot of inventions, further research and optimize around this novel MOEMS gyroscope chip. In future studying, the research will be putted on integration three axis

Gyroscopes in one micro structure by optical sensor multiplexing principles, and the new optical devices like more powerful light source, photosensitive materials etc., and new demodulation processes, which can improve the performance and the interface to co-operate with other inertial sensors and navigation system.