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The Single Joint Drive Mode enables the operator to control the arm on a joint-by-joint basis with full GPC support. The operator selects the desired joint using a selector switch and supplies a fixed drive signal via a toggle switch on the D&C panel. In response, the software algorithm supplies the rate demands to drive the selected joint and maintains the remaining joints in position hold. The Direct Drive is a contingency mode, which enables the operator to provide a direct drive command to the Motor Drive Amplifier (MDA) via hardwires, thus bypassing the MCIU, the GPC, the data busses and the servo control loop. During operation in Direct Drive, brakes are automatically applied to all uncommanded joints, and a constant positive or negative voltage is applied to the driven motor. The display information may be available to the operator in this mode. The Backup Drive Mode is an additional contingency mode used when no prime channel drive modes are available. This mode again allows joint-by-joint movement, but without status information available from the D&C panel. Backup Drive is designed to fulfill the failsafe requirement of the SRMS by using only electro-mechanical drive train prime channels of the selected joints, driven through a separate backup drive amplifier which bypasses the rest of the system. System requirements The major performance requirements of the SRMS are the following: • Deploy or retrieve up to 5 payloads during a single mission within a miximum [[maximum]] range of 15m from the attach point to the manipulator arm to the orbiter. •Deploy, or return without release, maximum envelope (18.3m length, 4.5m diameter) pay-loads with masses of up to 30,000 kg. • Capture and retrieve free-flying maximum envelope payloads with masses of up to 15,000 kg whose relative rate with respect to the shoulder is less than or equal to 0.03m/sec. •Arm end point rates are limited to the values shown below. [[3 Columned Table]] |Load | Rate Limits m/Sec | Rate Limits Deg/Sec | |---|---|---| |Unloaded|0.6|4.76| |Loaded | | | |(15,000 Kg.)|0.06|0.476| |(30,000Kg.)|0.03|0.238| • Ability to stop from maximum speed within 0.6m under all loading conditions. •Ability to position end effector relative to shoulder attach point to within +0.05m and +1 degree. • Release a 30,000 kg maximum envelope payload to within 5 attitude error with tipoff rates less than 0.015 deg/sec. •In the manual augumented mode maintain end effector rate accuracies of +-0.01m/sec and +-0.09 deg/sec. •The minimum end effector force/moment capability is a combined 53.3N. shear force and 217Nm bending moment and a 3 torque about the end effector roll axis. •To be fail-safe •To remain within an overall system weight budget of 450kg. Control System Philosophy Due to the limited memory and processing time allocated to SRMS in the onboard computer (GPC) the control structure of the SRMS was selected to minimize the requirement for the above resources. A hierarchical control organization was chosen. Such an organization leads to the the following: •Permits the design of members of the lower levels of the hierarchy in isolation, from bottom up, based on requirements placed on the next level, thus simplifying the design process. •Members of the higher levels control several members of the level below them in parallel and coordinate them. •Members of the higher levels may observe all levels below them but effect control only through the hierarchical command structure. •Localization of control functions eliminates delays and thus improves the stability and performance of the total system. The control hierarchy of the SRMS consists of the operator as the task controller, the arm control algorithms and the joint servos. The primary function of the joint servos is to control the joint rates and angles to satisfy the commands. The commands can either be provided by the operator from the D&C panel as in the case of 'joint-by-joint' control or by the control algorithms. The function of the control algorithms is to receive commands from the operator in terms of the required end point (POR) variables and to satisfy these by computing the required commands for the joint sevos on the basis of estimated (from measured joint angles and rates) end point variables. The operator observes the true state of the arms either by direct vision or from CCTV views and the estimated end point variables displayed and provides commands to satisfy the requirements based on the task to be performed. Display and Control Algorithms The SRMS Display and Control Algorithms have been implemented in the SRMS software. The primary function of these algorithms are the following: 459
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