The Dynamic Performance Investigation And Optimization Of Closed-Loop Capacitive Micro-Accelerometer

Micoelectromechanical system (MEMS) accelerometer has become a key research of the scientific research institutes, universities and companies around the world due to its superior performance and application value. In some applications, overshoot of the detection system will affect the performance of the accelerometer. For instance, large overshoot of the detection system will result in false operation of the airbag in the automobile. Therefore, dynamic performance of the microaccelerometer should be studied to prevent the false operation. Meanwhile, digital detection circuit can reduce the effect device mismatch error and improve the system precision. Compared with traditional digital to analog converter (DAC), sigma-delta DAC can achieve higher accuracy while consuming less hardware resource. At the same time, the sigma-delta modulation technique could improve the noise shaping ability of microaccelerometer. Therefore, it is important to investigate the sigma-delta microaccelerometer for obtaining higher accuracy.In this paper, a proportional-integral (PI) controller is proposed to improve the dynamic performance of a typical closed-loop capacitive microaccelerometer. Furthermore, in order to realize the PI controller with digital circuit, a 3 order sigma-delta modulator is designed to realize the analog to digital convertion. The primary work of this paper is listed as follows:(1)The principle and model of the closed-loop capacitive microaccelerometer are analyzed and set up in this paper. Simulation is carried out to analyze the effect of proportional and integral constant on the system dynamic performance. Experimental result compares well with the simulation result.(2)The PCB circuit of closed-loop capacitive microaccelerometer is built to verify the MATLAB simulation results. Effect of parasitic capacitance and resistor on the circuit zero-offset is discussed. Parallel connection of adjustable capacitor is proposed as a method to reduce the zero offset. Experimental result shows that the zero offset is reduced from 62.5mV to 0.5mV.(3)A sigma-delta modulator is designed to achieve the convertion between the digital and analog signal. MATLAB and Modelsim joint simulation is applied to verify the designed sigma-delta modulator. Digital circuit simulation result is consistent with the MATLAB simulation.(4)Design a fifth-order sigma-delta microaccelerometer for improving the performance of noise shaping. The simulation results indicate that the SNR(signal to noise ratio) of fifth-order sigma-delta microaccelerometer is 10.8dB higher than second-order microaccelerometer.