Early in the program, the decision was made to provide for control and monitoring of the remote manipulator in the Orbiter avionics system. This device, an l8.3-meter (60 foot) arm with shoulder, elbow, and wrist joints, is designed to insert, remove, and otherwise physically manipulate payloads as heavy as 27 216 kilograms (60000 pounds) in and about the payload bay. Because of weight constraints, the arm/joint design which evolved is a very slow moving system with rigid arm segments and joints which can be easily back-driven under load. The integrated control dynamics of the combined Orbiter/loaded arm system, particularly the interaction between the vehicle control system and that of the manipulator, are extremely complex and presented a significant design problem. At issue was the degree of integration of the two control systems. The approaches considered ranged from a single, integrated concept to separate independent systems, with or without crosstalk. After much deliberation, the decision was made to make the control systems and associated software for the Orbiter and manipulator independent and to rely on preflight simulations and operational procedures to avoid possible adverse interactions.
Another issue was the question of collision avoidance. It was possible for the arm, with or without an attached payload, to be commanded to a position which could impact and potentially damage the Orbiter; therefore, a requirement to provide collision detection and prevention measures in the software was originally imposed. Protection against all the possible combinations of arm position and payload geometry proved to be very difficult to mechanize without an unacceptable software penalty, and the requirement eventually evolved to one which called for software reasonableness tests to ascertain that no command inconsistent with the crew inputs was being transmitted.
NASA Office of Logic Design
Last Revised: February 03, 2010
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