End Point Control Unit

NASA Langely Research Center, Hampton, Virginia

Future capture and berthing operations require precision telerobotic maneuvering of large payloads by astronauts using the Remote Manipulator System (RMS) of the Space Shuttle. Since the RMS was not designed with this in mind, augmentation of the system may be needed. A requirement is that the augmentation should enable precision payload positioning and active monitoring and control of loads transmitted to the payload with little or no impact on the existing RMS.

The augmentation being developed is an independent device called the End-Point Control Unit (EPCU). The EPCU interfaces between the payload and the end-effector of the RMS and actively controls loads transmitted to the payload using active compliance and/or inertial load relief. Astronaut inputs and power are provided via a Power/Data Grapple Fixture (PGDF) which returns appropriate signals so that the astronaut can monitor safe operation of the unit.

The first laboratory-prototype EPCU design was tested in the Manipulator-Coupled Spacecraft Control Research Testbed at the Marshall Space Flight Center in May, 1994 to develop basic understanding of this novel control technology and requirements for design implementation. The testbed mimics the Space Shuttle RMS arm attached to a large 1363.64 kg payload, e.g. the partially assembled International Space Station. The propotype tested is pictured in the photograph and consists of a drive mechanism, a mechanical linear motion constraints system, motion and load sensors, and controller hardware and software. The drive mechanism is a 5-phase stepper motor with a 2.54 cm pitch diameter gear driving a rack, thus, providing linear motion control. Combined, the drive mechanism and the linear motion constraints allow motion of +/- 2.54 cm of travel in one translational direction only. The motion sensor is an encoder mounted directly to the motor shaft. The load sensor measures force transmitted to the payload along the axis of motion. The controller hardware is a typical laboratory computer VME-bus based system with a real-time operating system.

From the testing and simulation studies accomplished to date the following conclusions were obtained:

1. No controller experienced deflection saturation during runs at 1.87 times the design speed.
2. The EPCU with the stiffness controller has successfully reduced the loads transmitted to the payload by 31%.
3. The EPCU with the velocity input controller reduced the root mean square tracking error by 8% at input speeds up to 187% of the design speed of 2.44 cm/s.

Telerobotics Program page

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Maintained by: Dave Lavery
Last updated: May 10, 1996