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82-1536
STRUCTURAL FLEXIBILITY OF THE SHUTTLE REMOTE MANIPULATOR SYSTEM MECHANICAL ARM
by
P.K. Nguyen, R. Ravindran, R. Carr, D.M. Gossain
Spar Aerospace Limited
Toronto, Ontario
K.H. Doetsch
National Research Council of Canada
Ottawa

Abstract
The dynamics of the 15m long remote manipulator system which is the major element of the payload deployment and retrieval system of the orbiter of the NASA Space Transportation System are, by virtue of the large range of payload masses to be manoeuvred throughout the operating envelope, significantly affected by the flexibility of the structural components.
The paper addresses the finite-element analyses and computer simulations which were carried out by the Canadian design team to ascertain both that arm and orbiter induced manoeuvring loads were maintained below design limits, and that adequate level of stability and controllability were achieved throughout the dynamic operating envelope of the system in the presence of structural flexibility influences.

Introduction
The Shuttle Remote Manipulator System (SRMS) is a part of the Space Transportation System (STS) designed mainly for the deployment and retrieval of payloads from and to the Orbiter Cargo Bay. Under a memorandum of understanding signed with NASA in 1974, the National Research Council of Canada has undertaken the design, development, manufacture and qualification of the SRMS. Spar Aerospace Limited was then awarded the contract to provide the SRMS in conjunction with other manufacturers. The SRMS was tested successfully in space in November, 1981 during the second flight of the Orbiter Columbia (STS-2) and continued its evaluation of STS-3 in March, 1982.

The SRMS (Figure 1) comprises a 15m long, six-degree-of-freedom, anthropomorphic manipulator arm. The arm is attached at one end to the Manipulator Positioning Mechanism of the Orbiter and the other end carries an End Effector which is designed to capture payloads. The six joints are distributed along the arm as follows: two joints (Shoulder Yaw, Shoulder Pitch) near the arm attachment point, three joints (Wrist Pitch, Wrist Yaw, Wrist Roll) near the arm tip and one joint at the 'elbow'. The shoulder and elbow joints are designed to mainly provide the three dimensional translation of the End Effector, whereas the wrist joints provide the three dimensional rotation of the End Effector. The Shoulder Pitch joint and Wrist Pitch joint are connected by two 0.343m diameter graphite-epoxy tubular booms.

The Orbiter is designed to accommodate two manipulator arms which may be mounted on the port and starboard longerons. The port arm is normally installed. If a particular mission requires the inclusion of two arms, they can only be operated in a serial mode. Each manipulator arm consists of: 
1. A mechanical arm assembly, 
2. A standard End Effector, 
3. Arm mounted CCTV cameras and viewing light, 
4. Thermal protection for Arm and End Effector. 

Along with the computer support system, the manipulator arm, functionally, can be broken down into four major subsystems: 
1. Mechanical arm subsystem, 
2. Displays and controls subsystem, 
3. Electrical subsystem, 
4. Software subsystem. 

Brief descriptions of these subsystems and their functions have been given in [1] and [2]; further details can be found in [3]. 

From the structural design viewpoint, the mechanical arm has been designed to meet stringent geometry, weight and stiffness requirements. For example, the constraint of arm stowage requires 0.343m maximum arm boom diameter and 0.381m dynamic envelope for the arm cross-section. Design goals of 450Kg SRMS control weight and 1662 N/m stiffness at the tip of the arm, when fully extended, were established. The stiffness and weight distribution of the arm structure along its length has been optimized to keep the design cost effective (e.g. using the commonality of parts) and, at the same time, to satisfy the stiffness and weight requirements. As a result, the wrist joint structures are within a smaller diametrical envelope, thus more flexible, than those at the shoulder and at the elbow. 

Copyright (c) American Institute of Aeronautics and Astronautics, Inc., 1982. All rights reserved. 

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