Degree-per-hour mode-matched micromachined silicon vibratory gyroscopes
Zaman, Mohammad Faisal
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The objective of this research dissertation is to design and implement two novel micromachined silicon vibratory gyroscopes, which attempt to incorporate all the necessary attributes of sub-deg/hr noise performance requirements in a single framework: large resonant mass, high drive-mode oscillation amplitudes, large device capacitance (coupled with optimized electronics), and high-Q resonant mode-matched operation. Mode-matching leverages the high-Q (mechanical gain) of the operating modes of the gyroscope and offers significant improvements in mechanical and electronic noise floor, sensitivity, and bias stability. The first micromachined silicon vibratory gyroscope presented in this work is the resonating star gyroscope (RSG): a novel Class-II shell-type structure which utilizes degenerate flexural modes. After an iterative cycle of design optimization, an RSG prototype was implemented using a multiple-shell approach on (111) SOI substrate. Experimental data indicates sub-5 deg/hr Allan deviation bias instability operating under a mode-matched operating Q of 30,000 at 23ºC (in vacuum). The second micromachined silicon vibratory gyroscope presented in this work is the mode-matched tuning fork gyroscope (M2-TFG): a novel Class-I tuning fork structure which utilizes in-plane non-degenerate resonant flexural modes. Operated under vacuum, the M2-TFG represents the first reported high-Q perfectly mode-matched operation in Class-I vibratory microgyroscope. Experimental results of device implemented on (100) SOI substrate demonstrates sub-deg/hr Allan deviation bias instability operating under a mode-matched operating Q of 50,000 at 23ºC. In an effort to increase capacitive aspect ratio, a new fabrication technology was developed that involved the selective deposition of doped-polysilicon inside the capacitive sensing gaps (SPD Process). By preserving the structural composition integrity of the flexural springs, it is possible to accurately predict the operating-mode frequencies while maintaining high-Q operation. Preliminary characterization of vacuum-packaged prototypes was performed. Initial results demonstrated high-Q mode-matched operation, excellent thermal stability, and sub-deg/hr Allan variance bias instability.