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Charge control of parallel-plate, electrostatic actuators and the tip-in instability

By Joseph I. Seeger, Student Member, Bernhard E. Boser and Senior Member


Abstract—Controlling the charge, rather than the voltage, on a parallel-plate, electrostatic actuator theoretically permits stable operation for all deflections. Practically, we show that, using charge control, the maximum stable deflection is limited by 1) charge pull-in, in which the actuator snaps due to the presence of parasitic capacitance and 2) tip-in, in which the rotation mode becomes unstable. This work presents a circuit that controls the amount of charge on a parallel-plate, electrostatic actuator. This circuit reduces the sensitivity to parasitic capacitance, so that tip-in is the limiting instability. A small-signal model of the actuator is developed and used to determine the circuit bandwidth and gain requirements for stable deflections. Four different parallel-plate actuators have been designed and tested to verify the charge control technique as well as to verify charge pull-in, tip-in, and the bandwidth requirements. One design travels 83% of the gap before tip-in. Another design can only travel 20 % of the gap before tip-in, regardless of whether voltage control or charge control is used. [972] Index Terms—Capacitance, charge transfer, dynamics, electrostatic actuators, extended travel, pull-in, rotation

Year: 2003
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