Robotic manipulators are expected to perform various functions in space operations, such as assembly, inspection, repair, retrieval, and exploration. As such, they would be a vital resource on the future U.S. space station. But these robots must be light, have high-load capacity, and be able to manipulate objects smoothly and precisely. Unfortunately, these conflicting requirements rule out conventional robotic systems because either they are too heavy or they don’t operate smoothly enough.

To overcome this problem, a robotic manipulator must have:

• A mechanical drive with low friction and backlash.
• A sophisticated control system that compensates for lessthan- ideal mechanical performance and provides precise control.

Down to earth

A mechanical drive and control system that meets these requirements would also be suitable for servomechanism applications back on Earth. It could perform industrial tasks such as mechanical assembly, grinding and deburring, polishing, scribing, and riveting. Many of these tasks are now done with hand-held power tools because robots using power tools don’t provide satisfactory control of contact forces In an effort to develop a suitable robotic manipulator for space, a Cleveland engineering firm, called NASTEC, designed and built a robotic joint for the Structural Dynamics Branch of the NASA Lewis Research Center, also in Cleveland. Called a pitch-yaw joint, this flexible device is part of a 2-degree-of-freedom (DOF) robotic assembly that manipulates objects by providing rotation (pitch and yaw) around two mutually perpendicular axes.

Using a combination roller-andgear assembly to transmit torque, the pitch-yaw joint, Figure 1, combines the smooth, backlash-free characteristic of a roller traction drive with the high-torque capacity and lower bearing loads of a gear drive. At the bottom of this figure, a pair of bidirectional motors and gearboxes with a 90:1 speed-reduction ratio drives the two input stages of the joint. At the top, a 33- in.-long link is mounted to the output stage of the pitch-yaw joint, creating a 2-DOF manipulator arm capable of moving a 50-lb payload within a spherical workspace.

How it works

The pitch-yaw joint consists of a series of bevel-shaped rollers and bevel gears as shown in Figure 2. Each rollergear pair is mounted as a parallel set.

Each input stage asssembly, consisting of a bevel roller and a bevel gear (bottom), drives additional roller-gear assemblies including an intermediate and transverse stage. Finally, both transversing roller-gears mesh with the output roller-gear assembly (top). When the motors turn in the same direction at equal speed, they produce a pure yaw output motion. Turning in opposite directions at equal speeds produces a pure pitch motion. Any other combination of input motions produces both pitch and yaw motions. The ratio of input speed to output speed in either pure pitch or pure yaw is 3.43:1.

The pitch-yaw joint, which is approximately 6 in. square and 9 in. long, delivers a maximum torque of 1,650 lb-in. to the manipulator arm.

Roller functions

The bevel-shaped rollers, which act as traction-drive components, do two things that are essential to smooth operation and proper control:

First, they remove gear backlash (clearance between mating teeth) from the system. Contacting roller pairs are compressively loaded against each other by springs to obtain smooth, backlashfree motion. In each input stage, a spline locks the bevel roller with its corresponding input bevel gear, letting the rollers move axially in response to spring force.

At startup, when the gear teeth are not fully engaged (because of clearances between teeth), the spring-loaded rollers transmit the torque. Though each roller moves at the same theoretical speed as its corresponding gear, rollers experience a small loss of motion, known as creep, when transmitting torque. This allows the gears to “catch up” and begin transmitting torque soon after the initial motion — usually within a fraction of a revolution.

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