Structure Rotation and Pull-in Voltage Control of MEMS Lateral Comb Resonators Under Fault Conditions

Abstract

Lateral comb resonators (LCRs) theoretically possess a symmetric structure; therefore, the electrostatic force is effective only along the axis of displacement and is neutralized elsewhere. This force is not a function of the distance between the stationary and the moving (shuttle) plates in the direction of the movement. However, manufacturing imperfections and fault conditions make the device operate asymmetrically and cause rotation of the shuttle structure in a lateral gap-closing direction, putting the device in an unstable region of operation (pull-in voltage limit). In this region, the rotated structure experiences an extra force which pulls the device into the region of instability beyond the pull-in voltage limits. This paper illustrates the application of variable structure controllers to correct such behavior and to compensate for the uncertainties that exist in the microsystem parameters. The controller is equipped with a self-tuning parameter estimation block and is designed to control the LCRs under both normal and fault conditions. The controller utilizes through-wafer optical monitoring to provide the position feedback signal. The controller is implemented in a real-time control board and is experimentally verified to demonstrate the effectiveness of the control method under different operating conditions. Index Terms-Fault, lateral comb resonator (LCR), pull-in voltage, structure rotation, variable structure control.