Design, Analysis, And Experimental Results For The Wobble Motor: An Eccentric-Motion Electrostatic Microactuator

Abstract

A description is given of the design, fabrication and analysis of an electrostatically-driven microactuator 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 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 wobble 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 which estimate motor torque generated by electrostatic fields have been used to determine the influence of: (1) rotor and stator radii, (2) stator segment angular width and position with respect to the contact point, (3) stator segment voltage(s), and (4) dielectric properties and dimensions (e.g., insulator thickness on rotor) of motor materials. Dynamic models of motor behavior are also under development. A number of eccentric-motion micro motors, 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 microns, with lengths of 5,000 microns. Motor operation has been achieved with commutation rates in excess of 120,000 RPM.