Surface-acoustic-wave (SAW) piezoelectric motors of a proposed type would be capable of operating in multiple degrees of freedom (DOFs) simultaneously and would be amenable to integration into diverse structures and mechanisms. These motors would be compact and structurally simple and would not contain bearings or lead screws. One example of a particularly useful motor of this type would be a two-dimensional-translation stage. Another such example would be a self-actuated spherical joint that could be made to undergo controlled, simultaneous rotations about two orthogonal axes: Such a motor could serve as a mechanism for aiming an "eyeball" camera or as a compact transducer in, and an integral part of, a joint in a robot arm.
The multiple-DOF SAW piezoelectric motors as now proposed would be successors to the ones reported in "Multiple-DOF Surface-Acoustic-Wave Piezoelectric Motors" (NPO-20735), NASA Tech Briefs, Vol. 24, No. 12 (December 2000), page 5b. The basic principle of operation of a multiple-DOF SAW piezoelectric motor is a straightforward extension of that of single-DOF SAW piezoelectric motors, which have been reported in several previous NASA Tech Briefs articles: For example, in the case of a linear SAW piezoelectric motor, piezoelectric transducers at opposite ends of a stator excite surface acoustic waves that travel along the surface of the stator. An object (denoted the slider) is pressed against the stator with sufficient pressure (in practice ≈300 MPa) that it remains in frictional contact with the stator at all times. The slider rides the crests of the waves and is thereby made to move along the surface of the stator. The direction of motion (forward or backward) is controlled by selecting the relative phase of waves generated by the two piezoelectric transducers. The speed increases with the amplitude of the waves and thus with the magnitude of the voltage applied to the transducers.
The extension of this actuation principle to multiple degrees of freedom can be illustrated via the example of a two-dimensional-translation stage as now proposed (see Figure 1). In this case, the slider would be clamped between an upper and a lower stator and there would be two pairs of SAW piezoelectric transducers (instead of a single pair) on each stator. Small bosses on the upper and lower surfaces of the slider would make contact with the upper and lower stators, respectively. The pairs of transducers would be oriented orthogonally so that they could generate orthogonally propagating waves, making it possible to move the slider along either of two orthogonal axes. Like other SAW piezoelectric motors, a motor as now proposed would hold its position when not energized because the static friction generated by the clamping force would act as a braking force.
The ability of a surface acoustic wave to travel on a curved surface would make it possible to design a spherical 2-DOF SAW actuator like that depicted in Figure 2. In this case, two pairs of SAW piezoelectric transducers would be oriented orthogonally on a concave spherical stator instead of a flat stator, and the slider would be a sphere of nearly equal radius pressed against the stator.
In general, the minimum actuation step size would be approximately inversely proportional to the SAW excitation frequency. At contemplated maximum excitation frequencies of the order of tens of megahertz, minimum step sizes of nanometers could be achieved. Another advantage of using high excitation frequencies is that it would make it possible to achieve high force densities, thereby enabling the design of relatively small, lightweight actuators.
This work was done by Yoseph Bar-Cohen, Xiaoqi Bao, Anthony Hull, and John Wright of Caltech for NASA's Jet Propulsion Laboratory.
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Refer to NPO-20859.