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- Repair, replacement, and inspection of modular equipment on spacecraft
- Connection and disconnection of umbilicals.
- Attachment and release of equipment restraints.
- Installation, removal or transfer of material samples, protective covers, optical filters, instruments, film cassettes and other items.
The proficiency with which the SRMS could be used to perform the above tasks would be greatly enhanced by some of the following modifications.
Force Sensing
The present SRMS has no provision for force sensing although the maximum force/moment generated can be adjusted by setting the motor current limits in the joints at the required values. Knowledge of tip forces and moments being generated by the SRMS would be necessary if force feedback were to be provided to the operator to assist him in performing tasks which involved constrained motion of the manipulator or payload, or if automatic force/movement limiting through the servo system were desired. A force/moment sensor capable of measuring the three component forces and the three component moments can be mounted between the wrist roll joint and the end effector.
Force Feedback
The forces and moments sensed at the interface between the wrist roll joint and the end effector interface could be fed back to the operator either visually or by tactile means. For visual feedback, the information could be presented through a composite analogue display representing the six control force and moment components. For continuous tactile feedback, the translational and rotational hand controllers would need to incorporate their own force feedback systems with position controlled servos. In either case, the force and moment vectors must be resolved into the component form of the coordinate system being controlled by the operator.
Force Accommodation
The forces and moments sensed by the force/moment sensor could also be resolved into joint torques. These could then be fed back to the joint servos to accomodate the forces and moments on the basis of a force accomodation matrix specified for the task being performed. Force accomodation would be useful in the automatic mode of operation. In the manual mode it could be used as an alternative to tactile force feedback
Increased Number of Joints
To position and orient an object in three dimensional physical space, six degrees of freedom are necessary but not always sufficient. The SRMS with six joints, suffers from insufficient number of joints in some configurations where one or more degrees of freedom are lost. There are three such singular configurations as described below.
When the wrist jaw joint is at +90 [degree], the wrist gimbal lock singularity occurs, resulting in an inability to produce an instantaneous angular velocity component perpendicular to both the wrist pitch and yaw axis.
The shoulder yaw singularity occurs when the wrist yaw joint lies on the shoulder yaw axis, resulting in an inability to impart to the wrist an instantaneous translational velocity component perpendicular to the arm plane (plane containing the upper and lower arm booms).
The elbow singularity occurs when the wrist pitch, elbow pitch and shoulder pitch joints are colinear, implying that the arm cannot reach any farther.
When singularities are encountered, the SRMS slows down to a standstill before proceeding. Similarily, when one or more joints encounter joint stops ,the SRMS cannot be maneuvered along the required trajectory. These problems can be alleviated by providing additional joints in the SRMS.
The addition of an upper arm roll joint would help eliminate the shoulder yaw singularity. Provision of a lower arm roll joint would eliminate the wrist singularity. The elbow singularity could only be eliminated by providing a translational joint, since it is a manifestation of the physical fact that when the arm is fully extended, it can reach no further unless its length is increased. The upper arm and lower arm roll joints would also help to reduce loss of the desired trajectory due to joint stop run-ins.
The introduction of two additional joints would complicate the control algorithms considerably. The Jacobian matrix is rectangular and the resolved rate control algorithm requires the use of the concept of a pseudo-inverse of the Jacobian matrix. By introducing a suitable weighting matrix, an eight joint SRMS could be made to behave in the same manner as the present SRMS everywhere except in the neighborhood of singularities and joint limits.
With eight joints, there would be multiple configurations for the arm corresponding to a given end effector position and orientation. The choice or the appropriate configuration could be made by providing additional constraints such as, for example, specifying the elbow position.
Collision Avoidance
Because of limited computer resources, no provision was made in the SRMS software to allow the prediction of collisions between the arm, payload, and the orbiter or its contents. The operator is responsible for ensuring that no collisions occur, responsible for ensuring that no collisions occur, and in this task he is aided by the use of selectable CCTV camera views. The addition of collision warning software would assist the operator in avoiding collisions both in the manual augmented mode of control, and in the automatic control modes where the same software could be used to generate trajectories which would protect against collisions. If, for example, the trajectory of the end effector would lead to configurations in which collisions could occur, the trajectory would be abandoned or, if the extra joints (upper arm roll and lower arm roll) were available, different configurations which avoid collisions could be automatically selected by the software.

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