Lightweight, Low-Backlash Robot Wrist With Epicyclic Drive
- Monday, 10 September 2007
Several design features contribute synergistically to versatility and efficiency.
A unique lightweight wrist with three degrees of rotational freedom has been developed as a prototype of wrists for future anthropomorphic robots that would perform a variety of tasks on Earth and in outer space. The three degrees of freedom (two rolling, one bending) intersect at the center of the wrist. Included in the wrist is a power transmission with an epicyclic ring-gear configuration that enables efficient packaging and provides a wide internal passage through the center of rotation for routing of wires and drive cables. The power transmission combines lowbacklash planetary gearing and a tripleinput differential with a triple-loadpath, cable-driven output stage that generates minimal radial bearing loads and no thrust (that is, axial) bearing loads.
The forearm portion of the wrist contains three high-performance motors — one for each input to the differential. The motors are mounted side by side, with their shafts parallel to the axis of the forearm and with room between them to route wires through the center of rotation. The motions in the three degrees of freedom are coupled; in other words, all three motors contribute to the output motion of each axis. As a result, the load capacity of the wrist for each degree of freedom can be as much as three times that of a wrist of traditional design in which each degree of freedom is actuated by a single dedicated motor. From a slightly different perspective, if one takes advantage of the possibility of combining the outputs of all three motors to actuate a given degree of freedom, then one can use motors with a maximum power, size, and weight of only one-third of those that would otherwise be needed.
Each motor drives one of the input pinions of the ring-gear epicyclic transmission. Each input pinion is supported on both ends to reduce (in comparison with support on one end) shaft-flexing and bearing loads. Each input pinion drives two ring gears. For compactness, the ring gears are not supported by bearings; instead, each pair of ring gears is supported by its drive pinion and by two freely spinning (idler) pinions. The pinions bear all the radial loads on the ring gears. Pairs of ring gears are separated by low-friction washers to reduce wear.
Cables are wrapped around the ring gears and around mating pulleys. The ring gears in each pair are wrapped with a single cable and are loaded against a drive pinion in opposite directions to suppress backlash. Cables are terminated inside pulleys with preloading devices to compensate for thermal expansion and wear. Each cable is routed immediately around an adjoining pulley to change its direction without imposing thrust or radial load on any of the pulleys or gears, even under preload. Cables are wrapped immediately from one pulley to the next, minimizing the total length of cable and reducing susceptibility to stretch and thermal expansion. To minimize adverse effects of flexing and wear of cables, the diameters of the pulleys are made more than 30 times the diameter of the cables.
This work was done by Joseph Matteo of Matteo Automation and Robotics Co. for Johnson Space Center.
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
Matteo Automation and Robotics Co.
721 Summit Lake Court
Knoxville, TN 37922
Tel. No. (423) 777-0577
Fax No. (423) 777-0578
Refer to MSC-23005, volume and number of this NASA Tech Briefs issue, and the page number.