The design of electrostatic actuators optimized for both high force generation as well as long and stable displacement is difficult. Several groups have proposed and/or implemented devices based on "zipping" to achieve improvements in both areas simultaneously. In these designs the implementation of a zipping actuator requires the fabrication of a curved surface on which one electrode touches another. The fabrication of controlled curved surfaces often presents difficulties, especially those

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... pendicular to the wafer surface. In this paper we propose and model a class of devices, two-phase actuators, which can obtain similar effects without need for fabrication of curved structures. INIRODUCTION Practical use of electrostatic actuation often encounters two difficulties: (1) the electrostatic force scales as 1/x, giving very short displacement if large forces are needed, and (2) electrostatic spring-mass systems exhibit pull-in instabilities and so have stable controllability only over a fraction of their travel. Much of the design of electrostatic actuators is devoted to working around or utilizing these two effects. Several groups have proposed and/or implemented devices based on "zipping" to overcome both of these effects [1,2 ,3 ,4 ]. In these designs the implementation of a '.!zipping" actuator with stable motion requires the fabrication of a curved surface on which one electrode touches another. We propose a class of devices, two phase actuators, which can obtain similar effects without the need for fabrication of curved structures. This design concept emerged in our work on MEMCAD and the CoSolve-EM package fGr 3D simulation of coupled electromechanics[S] and contact electromechanics[6 ]. In this paper we use those tools to simulate a voltage controlled torsion mirror (VCM) as a representative example of this class of devices.