Development of a micromachined electrostatically suspended gyroscope

Abstract

In this thesis, a new approach based on an electrostatically suspended gyroscope (ESG) was explored in order to improve the performance of micromachined gyroscopes. Typically, a conventional micromachined gyroscope consists of a vibrating mass suspended on elastic beams that are anchored to a substrate. It measures the rotation rate of a body of interest by detecting rotation-induced Coriolis acceleration of a vibrating structure. Such a gyro is sensitive to fabrication imperfections and prone to cross-coupling signals between drive and sense modes, which degrade its performance. The micromachined ESG, on the other hand, employs a proof mass with no elastic beams connecting it to a substrate. The proof mass is levitated and spun electrostatically. In the presence of rotation, the spinning mass will rotate in the direction perpendicular to the spin and input axes. The displacement of the mass is capacitively sensed by a closed-loop electrostatic suspension system based on a sigma delta modulator (ΣΔM). The system, in turn, produces feedback forces to counteract themovement of the mass, moving it back to its nominal position. These feedback forces are equal to the precession torque and provide a measure of the rotation rate. Electrostatic levitation isolates the proof mass from unwanted inputs (for instance, mechanical friction, wear and stress), and thus the long-term stability of the gyroscope is expected to be improved. Furthermore, the micromachined ESG has a potential to achieve higher device sensitivity than that of a conventional vibrating-type micromachined gyroscope.
This thesis deals with three aspects of the development of the micromachined ESG: device design and analysis, design and simulation of an electrostatic suspension system and device fabrication. Analytical calculations and ANSYS simulations were carried out to predict the behaviour of the micromachined ESG. The micromachined ESG with an electrostatic suspension control system based on a sigma-delta modulator (ΣΔM) was modelled in Matlab/Simulink and OrCAD/PSPICE to evaluate the operation and performance of the closed-loop gyroscope. A front-end capacitive readout circuit was also developed. Initial tests were carried out and the measurement results showed a reasonable good agreement to both theoretical calculation and OrCAD/PSPICE simulation. The fabrication of the prototype micromachined ESG was developed using a triple-stack glass-silicon-glass anodic bonding in combination with a high-aspect-ratio DRIE process. Fabrication results and processing issues were discussed. However, it was found that the rotor of the fabricated gyroscopes was stuck to the substrate. Therefore, a fabricated prototype, which had not yet covered by a top substrate, was used to investigate an alternative approach to provide electrostatic levitation using sidewall electrodes. The analysis of this approach was investigated using 2D electrostatic finite element simulations in ANSYS. Initial tests were also carried out.

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