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A stiffness-tunable MEMS accelerometer

This paper reports a novel MEMS accelerometer whose effective stiffness can be adjusted flexibly and even tuned to almost zero by using the electrostatic softening effect of two parallel plate capacitors: the comb-finger capacitor and the triangular capacitor. When applied to a biased voltage, the comb-finger capacitor provides a relatively large and nonlinear negative stiffness in the gap-varying direction, while the triangular capacitor produces a small yet linear negative stiffness in the area-varying direction. The proposed accelerometer is fabricated by using a standard silicon-on-glass manufacturing process and controlled by a customized digital circuit. An analytical model based on the pull-in dynamics as a result of the stiffness tuning is developed and simulated to investigate the dynamic behavior of the nonlinear accelerometer system. The proportion integral differential controller is utilized to improve the performance of the accelerometer while maintaining the stability of the closed-loop system operating with extremely low effective stiffness. The experimental results indicate that a two-fold increase of the sensitivity, a 30% reduction of Allan deviation bias instability, and a 20% reduction of Allan velocity random walk are achieved for the open-loop accelerometer system. A 75% decrease of the standard deviation bias instability and nearly two orders of magnitude improvement of the noise floor at 1 Hz for the closed-loop accelerometer system are realized when the effective stiffness is reduced to almost zero.