A dynamically tuned vibratory micromechanical gyroscope - accelerometer

A comb driving vibratory micro-gyroscope, which utilizes the dynamically tunable resonant modes for a higher rate-sensitivity without an accelerational error, has been developed and analyzed. The Surface micromachining technology is used to fabricate the gyroscope having a vibrating part of 400x600(mu m) with 6 mask process, and the ploy-silicon structural layer(thickness:6.5[(mu m)]) is deposited by LPCVD at 625 degrees(C). The gyroscope and the interface electronics are housed in a hermetically sealed vacumm package for low vibrational damping condition. This gyroscope is designed to be driven in parallel to the substrate by electrostatic forces and subject to coriolis forces along vertically, with a folded beam structure. In this scheme, the resonant frequency of the driving mode is located below than that of the sensing mode, so it is possible to adjust the sensing mode with a negative stiffness effect by applying inter-plate voltage to tune the vibrating modes for a higher rate-sensitivity. Unfortunately this micromechanical vibratory gyroscope is also sensitive to vertical acceleration force, especially in the case of a low stiffness of the vibrating structure for detecting a very small coriolis force. In this study, we distinguished the rate output and the accelerational error by phase sensitive synchronous demodulator and devised a feedback loop to maintain resonant frequency of the vertical sensing mode by varying the inter-plate tuning voltage according to the accelerational output. Therefore, this gyroscope has a high rate-sensitivity without an acceleration error, and also can be used for a resonant accelerometer. This gyroscope was tested on the rotational rate table at the seperation of 50[(Hz)] resonant frequencies by dynamically tuning feedback loop. Also self-sustained oscillating loop is used to apply de 2[V] + ac 30[(mV)pk] driving voltage to the drive electrodes. The characteristics of the gyroscope are 0.1[deg/sec] resolution, 50[(Hz)] bandwidth, and 1.3[(mV)/deg/sec] sensitivity.