Micromachined Vibratory Rate Gyroscopes

A vibratory rate gyroscope has been developed and implemented using a micromachined mechanical element and integrated circuitry on the same silicon substrate. The mechanical element is a proof mass suspended by springs which is driven into lateral oscillation with electrostatic forces generated on air-gap capacitors formed from interdigitaged comb fingers. A rotation rate perpendicular to the plane of the substrate will result in Coriolis acceleration applied to the proof mass which is detected as an oscillation orthogonal to the previously mentioned driven oscillation. These deflections are sensed using a bridge circuit comprised of air-gap capacitors. Signal processing circuitry reduces the deflections to an estimate of the input rotation rate. In total, the gyroscope is a system comprising a micromachined sense element, an electrostatic actuators to generate and control oscillations, deflection sense circuitry, and integrated signal processing to produce an estimate of rotation rate.
This vibratory rate gyroscope demonstrates several innovations. Firstly, the gyroscope is fabricated using micromachined polysilicon integrated with CMOS circuitry on a single chip resulting in a complete sensor including signal processing electronics measuring less than nine square millimeters in area. Secondly, the gyroscopic sensing element has a unique configuration which allows for sensing rotation rates about an axis normal to the surface of the substrate. Finally, the rotation rate sensor makes use of electrostatic forces to not only improve sensitivity but also reduce the primary source of error common to vibratory rate gyroscopes, quadrature error.
Several vibratory rate gyroscopes have been designed, implemented, and evaluated. The sensors have been demonstrated to perform as expected. The proof mass oscillates at a resonant frequency of approximately 12-15 KHz. Quadrature error, which is on the order of 1000°/sec can be directly controlled and cancelled using electrostatic springs. Electrostatic "negative" springs have also been employed to match the resonant frequencies of the sense and drive modes. The sense mode resonant frequency has been lowered from 30 KHz to achieve near mode matching and an increase sensitivity of nearly two orders of magnitude. And finally, the gyroscope does in fact measure Z-axis rotation rates. The gyroscopes tested have a measured noise floor of approximately 1 degree/(secHz)
William A. Clark
Publication date: 
December 31, 1997
Publication type: 
Ph.D. Dissertation
Clark, W. A. (1997). Micromachined Vibratory Rate Gyroscopes. United States: University of California, Berkeley.

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