A design overview of an eccentric-motion electrostatic microactuator (the wobble motor)

Abstract

A description is given of the design, fabrication and analysis of an electrostatically-driven micro-actuator in which a planetary-motion rotor rolls inside a cylindrically shaped stator cavity. The design has four primary advantages: (1) the motor geometry and the rolling motion enable very small gaps to be achieved, which are accurate and stable, and across which electrostatic forces act, leading to high forces on the rotor; (2) relative motion is achieved by rolling rather than sliding, thus obviating the concern over internal friction; (3) higher output torques can be traded for lower rotor speeds, due to immediate planetary reduction; and (4) the power output should be higher than for systems constructed using two-dimensional silicon fabrication approaches, since woble motor lengths are not limited by such fabrication methods. The stator segment recruitment logic can range from simple, open-loop stepping to full servo-controlled commutation using rotor position sensors.

Two-dimensional analytical and finite-element simulations that estimate motor torque generated by electrostatic fields have been used to determine the influece of: (1) rotor and stator radii; (2) stator segment angular width and position with respect to the contact point; and (3) dielectric properties and dimensions (e.g., insulator thickness on rotor) of motor materials. Dynamic modelling is being used in the comparison of predicted and observed motor behavior, and for the study of the effects of stator segment recruitment logic.

A number of eccentric-motion micromotors constructed via different fabrication techniques, have been operated. Electro-discharge machining (EDM) is the fabrication method of choice for the prototypes presently used for experimental studies. Typical rotor diameters for the EDM motor are about 500 μm, with lengths of 500 μm. Motor operation has been achieved with commutation rates in excess of 120 000 r.p.m.