A motor involves a simple design that can be embedded into a plate structure by incorporating ultrasonic horn actuators into the plate. The piezoelectric material that is integrated into the horns is pre-stressed with flexures. Piezoelectric actuators are attractive for their ability to generate precision high strokes, torques, and forces while operating under relatively harsh conditions (temperatures at single-digit K to as high as 1,273 K).

Electromagnetic motors (EM) typically have high rotational speed and low torque. In order to produce a useful torque, these motors are geared down to reduce the speed and increase the torque. This gearing adds mass and reduces the efficiency of the EM. Piezoelectric motors can be designed with high torques and lower speeds directly without the need for gears.

Designs were developed for producing rotary motion based on the Barth concept of an ultrasonic horn driving a rotor. This idea was extended to a linear motor design by having the horns drive a slider. The unique feature of these motors is that they can be designed in a monolithic planar structure. The design is a unidirectional motor, which is driven by eight horn actuators, that rotates in the clockwise direction. There are two sets of flexures. The flexures around the piezo-electric material are pre-stress flexures and they pre-load the piezoelectric disks to maintain their being operated under compression when electric field is applied. The other set of flexures is a mounting flexure that attaches to the horn at the nodal point and can be designed to generate a normal force between the horn tip and the rotor so that to first order it operates independently and compensates for the wear between the horn and the rotor.

This motor could be stacked to increase the torque on the rotor, or flipped and stacked to produce bidirectional rotation. The novel features of this motor are:

  • A monolithic planar piezoelectric motor driven by high-power ultrasonic horns that can be manufactured from a single piece of metal using EDM (electric discharge machining), precision machining, or rapid prototyping.
  • A plate structure that can rotate a rotor in a plane.
  • A flexure system with low stiffness that accommodates mechanical wear at the rotor horn interface and maintains a constant normal force.
  • The ability to embed many horns in a plate to increase the torque.
  • A rotary actuator that can be designed to rotate clockwise or counterclockwise, depending on the direction of extension of the horn with respect to the center axis of the rotor.
  • A linear actuation mechanism that operates bidirectionally in the plate.
  • A mechanism that is operated with soft flexure springs that maintains constant normal and hence friction forces in a motor.
  • A planar rotary piezoelectric motor that is driven by ultrasonic horns that can be stacked to produce higher torques.
  • Actuator plates that can be flipped and stacked to produce bidirectional drive.

This work was done by Stewart Sherrit, Xiaoqi Bao, Mircea Badescu, and Yoseph Bar-Cohen of Caltech; Daniel Geiyer of Rochester Institute of Technology; and Patrick N. Ostlund and Phillip Allen of Cal Poly Pomona for NASA’s Jet Propulsion Laboratory. NPO-47813

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Planar Rotary Piezoelectric Motor Using Ultrasonic Horns

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This article first appeared in the July, 2011 issue of NASA Tech Briefs Magazine.

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