A Closed-loop MEMS force sensor with adjustable stiffness

This paper presents the design, implementation, and control of a novel microelectromechanical system (MEMS) force sensor. The device features bidirectional electrostatic actuators and on-chip piezoresistive displacement sensors. For the fabrication, a standard silicon-on-insulator process is used. Due to the bidirectional geometry, the probe is able to perform force measurement in both push and pull directions. An electrostatic structure is also incorporated for in-situ tuning of the sensor's mechanical stiffness. This stiffness-adjusting mechanism offers an additional control parameter, enabling the sensor to be adaptable on various input force conditions. Exploiting the stiffness-adjusting mechanism, a variation in the resonant frequency from 3 kHz to about 4.4kHz is experimentally demonstrated which translates to a stiffness variation within a factor of two. By implementing a resonant controller together with an integral action, the adverse effect of flexural nonlinearities on the precision of the force measurement is mitigated. The performance of the closed-loop system is also evaluated by exerting a disturbance signal to the closed-loop system simulating an external input force. Finally, the resolution and the measurement range of the sensor in the open-loop and the closed-loop are compared and discussed.