Section 2 SUMMARY 2.1 TASK 1 - PRELIMINARY DESIGN 2.1.1 Control Analysis Qualitative estimates of requirements and design options for antenna mechanical steering indicates that pointing accuracy of better than 1 arc-min can be achieved. The mechanical system, if integrated with the electronics microwave beam phase front control, could improve overall system efficiency with minimal impact on system weight and cost. Figure 2-1 summarizes the design environment for mechanical steering. The antenna gravity gradient torques are a major externally induced disturbance. Other factors, such as torque caused by solar pressure, or electromagnetic forces, are small. The most significant torque is the friction torque at the rotary joint. This torque varies as a function of system power level and power transfer technique. Base motions of the SSPS are caused by normal limit cycle operations and by solar array bending dynamics. Figure 2-2 is a composite of system accuracy and torque requirements as a function of mechanical control system frequency. An azimuth accuracy of 40 arc-sec can be achieved with a control system frequency of 1 rad/sec. This control frequency would require 1, 020, 000 N*m (750, 000 ft-lb) peak control torque (measured on load side of the gear train). This control system frequency is well above the first structural frequency of the SSPS and antenna. Peak horsepower requirements at 1 rad/sec is 0.18 and 1.75 hp in azimuth (East-West rotation) and elevation (North-South rotation), respectively. A review of top level methods for implementing mechanical steering favors a motorgearing mechanical system as opposed to a reaction jet system. Because control system frequencies are well above the first structural bending frequencies, no instabilities are foreseen. A mechanical system could be configured against wear by providing sufficient redundancy. The reaction jet approach, in which jets are mounted to the antenna, would be advantageous because the antenna structure could be more readily isolated from spacecraft dynamics than a mechanical system using gear trains. The shortcomings of the jet system, however, include:
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