Large Displacement Electrostatic Microactuators With Polysilicon Flexure Suspensions


Two electrostatically-driven surface-micromachined microactuators designed for large displacement motion are presented along with a literature review of microactuators that includes over 300 references. The first polycrystalline silicon device, the semaphore, achieves large amplitudes by resonant excitation of a plate mass attached to the end of a cantilever beam.  The semaphore is driven by either linear or curved electrostatic comb drives. The second device, the tangential drive, uses the direct electrostatic force between two charged bars to attain large static displacements in one of the bars while the other is fixed to the substrate. The moving or free bar is suspended by a parallelogram flexure suspension which acts to guide the bar such that it moves nearly parallel to the other bar and across the electric field between the bars. Theoretical models for both the semaphore and tangential drives are derived.

The semaphore is modeled using a lumped mass method which matches the experimental resonant frequencies to within 5% for the first three modes. The former has four distinct perceptible resonant modes while the latter has three. Measured peak-to-peak resonant amplitudes of up to 30 um are measured in air and up to 400 um in vacuum and measured resonant frequencies range from 1650 Hz up to 137.9 kHz.

Two simulations of the tangential drive are derived. A modified parallel plate capacitance method is used to determine the electrostatic forces and is combined with nonlinear small deflection beam theory to simulate the static response. The second model uses nonlinear large deflection beam theory. Surface fits are formed for the horizontal and levitation electrostatic forces on the free bar which are calculated by finite element analysis. An iterative finite difference method is used to solve the differential equations of the beams and include the electrostatic forces. Measured deflections of up to 37 um at an applied potential of 188.3 volts are made for the tangential drive with 500 um long suspension beams. The simulations correspond well to experimental response and predict the tangential drive behavior without need for prior testing of the device. The second simulation is used to perform a sensitivity study with respect to the T-drive design geometry and to fabrication-induced variations from the nominal geometry.

Reid Alyn Brennen
Homayoon Kazerooni
Publication date: 
November 30, 1993
Publication type: 
Ph.D. Dissertation
Brennen, R. A. (1993). Large Displacement Electrostatic Microactuators with Polysilicon Flexure Suspensions. United States: University of California, Berkeley.

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