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Solving the "Pull-in" Instability Problem of Electrostatic Microactuators using Nonlinear Control Techniques

The operation of electrostatically actuated microelectromechanical systems (MEMS) devices is significantly limited by a saddle node bifurcation phenomenon to one-third of its full capacitive gap. Under constant voltage actuation conditions, travel beyond this allowable range results in "pull-in" instability. This is due to positive feedback in the electrostatic actuation and is independent of mechanical design parameters such as stiffness and mass. This paper presents an effective control strategy that stabilizes electrostatic microactuators and allows the effective utilization of the entire capacitive gap. We show that with normalized deflection as output, the driven system is feedback linearizable with relative degree 3 (equal to the system order) in the region of interest. A nonlinear tracking controller capable of extending the travel range over the entire capacitive gap while ensuring the desired dynamic performance is discussed. Simulation results show that the proposed nonlinear control scheme not only has good tracking ability, but also has extremely good parameter variation robustness and remains stable under output measurement noise conditions inherent in output feedback control.