An Adjustable-Stiffness MEMS Force Sensor: Design, Characterization, and Control

This paper presents a novel one-degree-of-freedom microelectromechanical systems (MEMS) force sensor. The high-bandwidth device contains on-chip sensing and actuation mechanisms, enabling open- and closed-loop modalities. An active compliance mechanism is incorporated to render the device more conducive to characterization of soft samples. When operated in closed loop, the adjustable stiffness enables the sensor to attain a larger dynamic range and minimize the nonlinearities originating from flexures. Analytical models are employed to design and calibrate the sensor. In open loop, the sensing resolution of 23.3 nN within a bandwidth of 2.35 kHz and a full-scale range of ± 42.6 µ N are experimentally obtained. The resolution is enhanced to 9.3 nN by employing an active compliance mechanism. When operated in closed loop, a resolution of 12.9 nN is achieved within a dynamic range of 71.2 dB and a sensing bandwidth of 3.6 kHz is demonstrated. The sensor performance is tested by obtaining the stiffness of an atomic force microscope probe and measuring the force produced by a self-actuated piezoelectric microcantilever.