Development of a Closed-loop MEMS Capacitive Force Sensor

Abstract

This thesis describes a closed-loop microelectromechanical system (MEMS) based on lumped-parameter modeling. Analytical models are derived for electrostatic comb drive actuator (CDA) under force-controlled actuation, electrothermal actuator (ETA) under displacement-controlled actuation, capacitive position sensor, including parallel plate capacitive sensor (PPCS) and torsional plate capacitive sensor (TPCS), mechanical equation of motion of a suspended shuttle, viscous air damping, folded exure. These models are implemented and simulated in finite element analysis softwares (ANSYS and FEMM). System level simulation, implementing PID difierential feedback loop, is simulated in a numerical simulation program (MATLAB). The MEMS die is fabricated by following the standard PolyMUMPs process by MEMSCAP. A series of MEMS packaging process and storage are done in the lab. All peripheral circuitries are self-made. A commercial capacitive readout IC (MS3110) is first used for open-loop capacitive sensing, which achieves the resolution of 0.05fF, equivalent to 1nm in displacement. Due to the disadvantage of MS3110 in closed-loop, AC bridge capacitance measurement method is then implemented for closed-loop integration. The resolution of AC bridge sensor reaches 0.02fF, equivalent to 0.4nm in displacement. An additional function of AC bridge sensing is accomplished which is simultaneously sensing and actuation of CDA. In the feedback loop, the traditional analog PID controller is designed to transfer the voltage signal of capacitance measurement to the voltage-force transducer which converts feedback voltages to differential feedback force. Since the differential feedback force is limited by clamped voltage, a force-balanced mode is observed under 5V actuation of CDA.