Multiple-DOF Surface-Acoustic-Wave Piezoelectric Motors

NASA's Jet Propulsion Laboratory, Pasadena, California

Surface-acoustic-wave (SAW) piezoelectric motors capable of operating in multiple degrees of freedom (DOFs) simultaneously have been proposed. A particularly useful motor of this type could provide both linear and rotary motions. Another particularly useful motor of this type would feature a spherical joint that could be made to undergo controlled, simultaneous rotations about two orthogonal axes; such a motor could serve as a compact actuator in a joint of a robot arm and could be constructed as an integral part of the arm.

Figure 1. Surface Acoustic Waves are launched by suitably phased interdigital piezoelectric transducers at opposite ends of a stator. The slider ball rides the crests of the waves. Unlike in ocean surfing, the direction of motion of the ball is opposite the direction of propagation of the waves.

Single-DOF SAW piezoelectric motors have been reported previously [for example, in "High-Torque Ultrasonic Motors" (NPO-19835), NASA Tech Briefs, Vol. 20, No. 10 (October 1996), page 12b]. Though the practical implementation can be fraught with difficulties, the basic principle of operation in the single-DOF case is straightforward: Piezoelectric transducer(s) excite waves that travel along the surface of a stator. A round object (e.g., a rod or a ball) in frictional contact with the stator rides the crests of the waves and is thereby made to move along the surface of the stator (see Figure 1). The resulting rotary or linear motion is coupled to an output shaft. The direction of motion (left or right in the example of Figure 1) is controlled by activating individual piezoelectric transducers. The speed increases with the amplitudes of the waves and thus with the magnitude of the voltage applied to the transducers.

Figure 2. A Hemispherical Motor with two degrees of freedom would operate according to the basic principle illustrated in Figure 1, and would take advantage of the fact that waves propagate along curved surfaces as well as along flat ones.

Figure 2 depicts a simple example of a hemispherical motor that would tilt a shaft in two degrees of freedom. The shaft would be anchored in a spherical bearing nominally located at the center of the spherical coordinate system. A slider ball on the inner end of the shaft would make contact with the hemispherical surface, along which surface acoustic waves would be launched by two orthogonally located pairs of piezoelectric transducers (one pair for each degree of freedom). The bearing would be mounted in a plate connected to the hemispherical surface by use of screws, which would be adjusted to obtain the desired force of contact between the slider ball and the hemispherical surface.

This work was done by Yoseph Bar-Cohen of Caltech for NASA's Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to