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Figure 1 SRMS Mechanical Arm in Stowed Position

From the dynamics and control viewpoint, the SRMS control system has been designed using the independent joint control philosophy so as to keep the design simple [4].  Furthermore, the arm structural flexibility is ignored in the control system design in the sense that the arm elastic deformations are not actively controlled.  From theories of dynamism and control of flexible structures, it has been known that:

1.  Structural flexibility is capable of degrading a control system in the sense that it can make a stable rigid body system unstable.

2.  Structural loads in response to a disturbance are higher in the flexible body than in a similar rigid body.

3.  The dynamics of a flexible body are more complicated, thus more difficult to analyze, than those for a rigid body because of the larger number of degrees of freedom and the coupling between rigid-body and elastic degrees of freedom.

In view of the above considerations, careful attention was placed in the design on the SRMS controllability, stability, as well as the safe load levels required to maintain the SRMS structural integrity.  Finite-element analyses and dynamics simulations have been carried out during the design, development and verification phase of the SRMS, to predict and verify the arm performance capacity.  This paper is aimed at presenting part of the math modelling, analysis and simulation results mentioned above.

A brief description of the mechanical arm assembly is given in the next section to provide an appreciation for the SRMS arm structure.

Description of the Structure

The mechanical arm assembly structure is provided by the joint housing, electronics housings and arm booms.

Joint Housings

The joints, transition pieces and electronics housings are constructed of aluminum alloy, titanium alloy and steel.  Typically, each joint housing contains a motor module, a gear train and an encoder.

Each motor module (Figure 2) has a reversible, brushless DC motor, primary and back-up commutation scanner assembly systems, a rotary inductosyn tachometer and a joint brake.  The gear

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Figure 2 Motor Module