Piezoelectric ultrasonic micromotors

This thesis describes development of a new actuator technology for integrated machines of the future: piezoelectric ultrasonic micromotors. Ultrasonic motors o er the advantages of low speed, high torque operation without the need for gears. They can be made compact and lightweight and provide a holding torque in the absence of applied power due to the traveling wave frictional coupling mechanism between the rotor and the stator. Whereas ultrasonic motors would typically be made from a bulk ferroelectric ceramic such as lead zirconate titanate, or PZT, this thesis describes the implementation of a new idea { that of using PZT in a sol-gel form deposited directly onto silicon to create high-torque motors compatible with silicon integration technologies. Due to large dielectric constants and increased breakdown strengths of thinlm PZT, ultrasonic micromotors o er a factor of 1000 improvement in energy density over electrostatic micromotors. In a joint project with the Penn State Materials Research Laboratory and MIT Lincoln Laboratory, 2 mm and 5 mm diameter stator structures were fabricated on 1 m thick silicon nitride membranes. Small glass lenses placed down on top spun at 100-300rpm with 4V excitation at 90 kHz. While generation of appropriate traveling bending waves in the stator is fairly well understood, less is known about how the frictional coupling and surface properties at the rotor-stator interface a ect mechanical power output performance. This thesis proposes models for production of torque in rotary ultrasonic motors. Models of line contact, Hertzian contact and linear spring contact for Coulomb and viscous friction have been derived and simulations are presented which predict speed-torque curves, e ciencies and overall output power for various conditions of operating voltage and normal force. To validate these models, a set of 8 mm diameter 3mm tall motors, in a designed experiment, has been fabricated. These devices have demonstrated maximum stall torques of 10 Nm, maximum no-load speeds of 1710 rpm and peak power outputs of 27mW. The resulting peak power density is 108 W kg , more than double that of human muscle. This thesis further describes a laser-etching process which has been developed to fabricate more practical piezoelectric ultrasonic micromotors. This laser-based process produces thinlm PZT-on-silicon stators without the need for mask alignment, wet-etching or fragile membranes. The large power densities and stall torques of these piezoelectric ultrasonic motors o er tremendous promise for integrated machines: complete intelligent, electro-mechanical autonomous systems mass-produced in a single fabrication process.