These joints would be relatively compact, lightweight, simple, and inexpensive.

The term "actuated ball-and-socket" (ABS) characterizes a proposed class of ball-and-socket joints that would incorporate ultrasonic motors and other piezoelectric actuators to generate multidimensional actuation. In some applications, ABS joints could supplant traditional joint-and-actuator assemblies that include passive rotary joints actuated by electromagnetic motors via gears. In comparison with such assemblies, ABS joints offer potential advantages of compactness, relative mechanical simplicity, higher torque-to-weight ratios, no backlash, self-braking with power turned off, and lower cost. ABS joints are expected to be particularly attractive for use as robot joints and as general low-power orienting actuators for diverse applications that could include robot hands, tools, and mechanisms for aiming scientific instruments.

Figure 1. Ultrasonically Vibrating Prods would be inserted in the ground,in a manner similar to that of a manaual search for mines. The robotic vehicle could also be equipped with an arm for digging up and removing mines.

Figure 2 depicts an ABS joint. A spherical rotor would fit in a housing that would contain bearings. The housing would also contain three drive units (A, B, and C) — each for rotation about one of three axes. These axes, which span three-dimensional space, intersect at the center of the rotor. (The axes are not necessarily perpendicular.) Power and control cables would be connected to each drive unit.

Figure 2. Small Elliptical Orbital Motion of a point on the surface of the stator under the influence of a traveling wave causes (through friction) the rotor to move in a circumferential direction opposite that of the propagation of the wave

A piezoceramic ring, denoted the thruster, would be located between the thruster block and a shelf in the housing. When not energized, the thruster would not exert any effect. When electrically energized, the thruster would push the thruster block radially outward (against the spring washer), thereby pulling the stator out of contact with the rotor. Thus, energizing the thruster would prevent the drive unit from either driving or braking the rotor.

To avoid working against the holding torque of the other drive units, only one drive unit at a time could be used to produce rotation. For example, in order to obtain rotation about axis A, it would be necessary to (1) excite the stator of drive unit A with appropriately phased waveforms while (2) energizing the thrusters of drive units B and C to prevent these units from braking the rotation about axis A. To move the rotor from an initial orientation to a desired final orientation, it would be necessary to generate a trajectory consisting of a sequence of small rotations about the A, B, and C axes. It would be necessary to develop an electronic data-processing and control system to compute and implement the coordinated, sequential actuation of the three thrusters and the three ultrasonic-motor stators to generate the trajectory. It has been estimated that, in a typical application, the increments of rotation would be characterized by frequencies of the order of a kilohertz.

This work was done by Issa Nesnas of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Machinery/Automation category.

NPO-20984

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