Charge control of parallel-plate, electrostatic actuators and the tip-in instability
Seeger, J.I.
Boser, B.E.
Dept. of Electr. Eng. & Comput. Sci., Univ. of California, Berkeley, CA, USA;
This paper appears in: Microelectromechanical Systems, Journal of
Publication Date: Oct. 2003
Volume: 12,
Issue: 5
On page(s): 656- 671
ISSN: 1057-7157
INSPEC Accession Number: 7779797
Digital Object Identifier: 10.1109/JMEMS.2003.818455
Current Version Published: 2003-10-27
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.
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