4.1.2 Detailed Control System Analysis The preliminary analytical study performed in support of this contract included an assessment of probable pointing accuracy, control torque requirements, horsepower requirements and control system bandpass frequency. These results were obtained using simplified system models so that the most significant factors could be studied. These simplifications may result in not identifying key dynamics problems associated with spacecraft to antenna structural dynamics and non-linearities in the mechanical elements of the control system. 4.1.2.1 Background A more in-depth study which considers non-linearities and structural dynamics would modify and complicate the control system design. The first step in a refined study would be the development of an accurate set of structural modes of vibration for all frequencies up through that of the control system bandpass frequency. This is required to understand system stability and performance in a realistic manner. These bending modes couple into antenna motions through friction effects in the bearings and gear trains, and by physically perturbing the antenna through motion cross-coupling into its two-axis gimbal system. The preliminary study documented here addressed these effects; however, a much more detailed investigation is required. The torque drive-gear chain system has within it the factors of flexibility, friction, backlash, and hysteresis. These were briefly considered in the preliminary study. They were neglected in the preliminary analysis, but they must be considered in more detail. Disturbance forces which must be considered in further analysis efforts are due to angular momentum cross-coupling, gravity gradient, magnetic field interaction, and solar pressure. Preliminary study indicated that these might be initially neglected because of the dominating influence of friction forces and structural mode oscillations, which were considered. It is possible that detailed study efforts will show that many of these aforementioned factors are critical effects on control design and performance. It may also be found that it is not possible to calculate or estimate some of these factors with sufficient accuracy to provide the necessary control performance accuracy. In that case a type of "adaptive" control design may be required where a special estimator logic (Kalman Filter) can update the knowledge of these factors and adjust the control system gains appropriately.
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