Research On Drive And Detection Techniques For Linear Tunable MEMS Gyroscope

A micro-machined gyroscope is a sensor that measures the rate of a system. Due to the advantages of small volume, low power consumption and batch production, micro-machined gyroscopes have captured the attention of the academic and industrial circles.In recent years, micro-machined gyroscopes are developing towards high precision. Mode-matching and a great number of other new techniques have been adopted to improve the precision of gyroscope system effectively. This Ph. D. dissertation studies overall techniques, which include the drive, detection and electrostatic tuning technologies, based on the electrostatically driven/capacitive detected slot-structure micro-machined gyroscope with tunable spring constant. New studies have been carried out to solve the disadvantages in the existing gyroscope techniques, which are expected to contribute useful material to the existing body of knowledge. The main content and contributions are given below:1) Research on the resonant characteristic and its identification techniquea) The displacement sensing circuit and velocity sensing circuit are analyzed from the standpoint of electrical coupling suppression, and a method used for electrical coupling suppression in velocity sensing circuit is proposed in this thesis. The suppression is realized by insulating the frequency between the actual mechanical response and electrical coupling signal.b) In this thesis, a digital identification technique based on the coordinate rotation digital computer (CORDIC) algorithm is proposed for the first time. The low efficiency and precision by artificial frequency sweep and the expensive cost and complex operation of a vector network analyzer can be avoided by adopting this identification technique which has found the balance between the precision and flexibility.2) Research on the drive technique for micro-machined gyroscopesc) The widely used PLL-AGC drive technique is optimally designed from the standpoint of transient response improvement. Experiment results show a settling time of only42ms and the amplitude variance of the detected signal of drive mode is52ppm in an hour while the phase standard deviation is2.80×10-4π rad (1σ). A bias instability value of8deg/h is achieved for a gyroscope working at atmosphere with34.7Hz mode-split using this drive technique after optimization. d) The concept of constant-frequency oscillation control for vibratory micro-machined gyroscopes is presented for the first time in this thesis from the standpoint of stability improvement. The constant-frequency control algorithm alters the drive mode dynamics to make the resonant frequency at a specified frequency and maintain specified amplitude and phase of oscillation. As the frequency is constant, the stability of displacement signal and velocity signal can be controlled simultaneously. Experiment results show that the amplitude variance of the detected signal of drive mode is22ppm in an hour while the phase standard deviation is6.97×10-67π rad (1σ). A bias instability value of6deg/h is achieved for a gyroscope working at atmosphere with34.7Hz mode-split using this new oscillation control loop which is better than the existing one.3) Research on the detection technique for micro-machined gyroscopese) Analytical closed-form solutions for force balanced micro-machined gyroscopes have been achieved for the first time in this thesis with the help of constant-frequency oscillation control which reduces the number of system variables. With the analytical closed-form solutions, the design of transient response and bandwidth becomes easily by control parameters tuning for force balanced gyroscope with matched modes. Experiment results show that the overshoot and the stable time are about5%and56ms for step response, respectively. Furthermore, the nonlinearity of the mode-matched gyroscope with force balance loop controlled sense mode is measured to be0.036%with the measurement range of±500°/s and the bandwidth of the force balance loop is extended to24.5Hz from3.3Hz in the open loop, which accords with the design goal. In order to compare with mode-split gyroscope, the bandwidth is re-designed to10Hz, then, a bias instability value of1.2deg/h is achieved which stays ahead of micro-machined gyroscopes working at atmosphere.f) A force-balance based calibration technique for micro-machined gyroscopes without turntable is proposed to avoid the limitation of real turntable. The scale factor, measurement range, linearity and bandwidth can be measured without a turntable by adopting this calibration technique.4) Research on the electrostatic tuning technique for micro-machined gyroscopesg) The automatic and real-time mode-matching techniques are presented using variable-area capacitor with tuning capability. An automatic algorithm for initial mode-matching using the phase information of quadrature-error shows250ms to accomplish the matching process, which is far faster than the existing techniques. Then, a real-time mode-matching method is proposed by introducing a dither signal with external frequency as virtual quadrature error in force-balanced control system. This real-time mode-matching method will not affect the detection of angular rate input.h) The amplifying effect brought by spring constant modulation of the linear tunable structure is discovered for the first time. The spring constant will be modulated if the tuning voltage has AC signal, then, the resonant effect and gyroscope effect can be amplified when the frequency and phase of the AC signal satisfy special conditions. The amplifying effect is verified by experiment results.