Detection and Mitigation of Electrostatic Pull-in Instability in MEMS Parallel Plate Actuators

Abstract

Electrostatic MEMS actuators are used in a wide variety of applications including micro- machined gyroscopes, high speed mechanical switches, variable capacitors, and vibration isolation devices. MEMS parallel plate actuators (PPAs) are simple to realize, yet complex nonlinear variable capacitors. If a DC voltage is applied in an attempt to move the proof mass greater than 1/3 of the electrode rest gap distance, the device becomes unstable and the electrodes snap into contact. Most research into this pull-in phenomenon is devoted to extending the operational range of motion past the 1/3 instability point. This usually involves the addition of complex external electronics. Many electronics applications, however, only require that the actuator remain out of the pull-in region, and do not require an extended stable range of motion. If detection of the pull-in event is all that is required, then simpler solutions can be realized, minimizing the requirements on the driving signal. Once pull-in is reached, the velocity of the movable plate increases rapidly until the plates make contact. The decreasing distance causes a proportional increase in capacitance. To maintain a constant voltage across the plates, an inrush of current must flow into the actuator in response to the increased capacitance. This work presents a method for detecting the inrush current using a transimpedance amplifier circuit to convert the current to a measurable voltage. Once pull- in is detected, the PPA is electrically shutdown to prevent damage to the actuator or the voltage source, thus mitigating pull-in. A simulation of the expected results was performed using a Simulink model for the actuator structure predicting the expected range of inrush current. This result was then verified using a silicon micro-machined PPA connected to the detection/mitigation circuit on a Printed Circuit Board (PCB). The experimental results follow closely with the simulation allowing precise control in mitigating the pull-in event.