Research Of Slot-structure MEMS Gyroscope With Tunable Spring Constant

Gyroscope is a kind of sensor to measure angle or angular rate. It is an important part of inertial measurement unit (IMU). Micromechanical gyroscopes have the advantages of small volume, low cost, low power consumption and mass production compare with optical gyroscopes and conventional rotor gyroscopes. Its applications include consumer electronics, automotive industry and navigation.The slot-structure micromechanical gyroscope utilizes the variable-area capacitor to sense the movement. The damping coefficient is low because of the in-plane movement. The silicon-on-glass structure makes the proof mass large enough to get a low Brown noise. Conventional slot-structure micromechanical gyroscope utilizes the variable-area capacitor to drive and sense. The resonant frequencies of the gyroscope cannot be adjusted to make the drive and sense modes matched. The performance of the gyroscope cannot be promoted effectively and the discreteness of the devices due to fabrication error cannot be suppressed. Based on the conventional slot-structure micromechanical gyroscopes, the gyro structure and circuit are improved to make the gyroscope mode matched and thus promote the performance.1) A novel slot-structure variable-area capacitor is proposed based on conventional slot-structure gyroscopes. The fixed electrodes are changed to triangular while the movable electrodes are still rectangular. There will be an electrostatic spring constant when a voltage difference exists between the fixed and movable electrodes. The total spring constant of corresponding mode will changed and thus the resonant frequencies of drive and sense modes can be adjusted to the same. Horizontal flip of the triangular electrodes will introduce a spring constant with contrary sign. The tuning electrodes are distributed in the sense mode according to our design. The objective is to get15Hz resonant frequency shift under15V tuning voltage. The test result of the prototype is consistent with the theory. A15V tuning voltage brings13.5Hz frequency shift.2) The bandwidth of the gyroscope under open loop is analyzed. The bandwidth is about0.54|Δf|when the resonant frequency difference between drive and sense modes is large, fo/2Q when mode matched. The bandwidth under small frequency difference situation is not explained in detail. Generally speaking, the bandwidth and the sensitivity is mutual restraint, larger bandwidth will decrease the sensitivity. The former conclusionsare verified and it is found that there is a break point in the relationship between bandwidth and resonant frequency difference. The location of the break point is related to the working frequency and the quality factor of the sense mode. It is found that the location is about fJQ and the corresponding bandwidth is about1.52fo/Q. The break point is a maxima and the bandwidth and sensitivity can be optimized. The bandwidth of the gyroscope is tested under different frequency split and the existence of the break point is verified.3) Using the tuning electrodes, the frequency tuning and sense force balance can be realized at the same time. A dc voltage and anti-phase ac signals are applied to the differential tuning voltage. The coupling of the gyroscope can be depressed. The sense close loop is analyzed under mode matched condition and the simulation tells that the bandwidth of the system can be expanded while the noise level is about the same. The gyroscope is tested under proper parameters. The close loop bandwidth increases to12.5Hz compare with3.5Hz under open loop. The close loop noise equivalent angular rate is8.467o/hr/√Hz which is about the same with open loop9.218ohr/√Hz. The nonlinerity of the output in±500o/hr is0.0175%(175ppm). The coefficient ratio of one degree term and quadratic term in the quadratic fit equation is0.395ppm.The standard deviation of the gyroscope output is15.65o/hr in1hr and2.7o/hr utilizing Allan variance analysis. The performance stay ahead of the gyroscopes working in atmosphere.