| After more than two decades of development, small satellites have matured and become a very useful tool for space missions. Current attitude control systems have provided the necessary control for these satellites, but they cannot do so efficiently for agile satellites. Agile satellites require an attitude control system that provides rapid multi-target acquisition, pointing, and tracking capabilities. It is crucial to develop advanced and practical control algorithms for agile satellites in that the effectiveness of ACS greatly affects agility performance. In this thesis, an alternative and more efficient means of providing the required agile capability is proposed based on single gimbal control moment gyros. Compared to reaction wheel,SGCMGs have high control torque capability, which is suitable for attitude control of agile satellites. However, as the inherent problem of redundant SGCMGs, singularity becomes one of major constraints on the maneuver ability of agile satellite. When agile satellite are equipped by SGCMGs, momentum saturation, singularity avoidance, and gimbals rate saturation need to be further considered in developing an attitude control system for large-angle, rapid multi-target acquisition and pointing maneuvers.In this research, a geometric study of singularity characteristics of SGCMGs was carried out in order to clarify singularity problems, to construct an effective steering law, and to evaluate this law's performance. Firstly, particular emphasis will be placed on characterizing and visualizing the physical as well as mathematical nature of the singularities, and singular momentum surfaces. Through improved index calculation method, it is not difficult to acquire the accurate configuration indexes and comparison of singular surface between elliptic singularity and hyperbolic singularity. Based on what is mentioned above, the singularity surface could provide information on how much momentum is available from the array in the row, pitch, and yaw axes.Obviously, it is necessary to develop efficient steering law to overcome the singularity problem. Next, the concept and derivation of steering law will be simply introduced. Great emphasis is placed on analysis of the errors caused by steering laws and how to avoid singular states effectively. As rapid maneuvers are often subjected to physical limits of actuators, sensors, spacecraft structural rigidity, and other mission constraints, in this paper a novel steering logic with mechanical gimbal angel constraint is presented, which can decrease the risk of encountering singularities. In fact, there are always differences between current gimbal rate and desired gimbal rate because of the influence of gimbal servo system. To make response of servo system more faster and accurate, voltage control law compensation law is designed, which can maintain proper bandwidth at extremely low levels of gimbal rate command signals and eliminate the interference of servo friction. Based on these results, the performance of the proposed steering law associated with realistic servo system is demonstrated by simulations.In addition, to achieve rapid, large-angle retargeting in the presence of the actuator saturation and slew rate limit, a nonlinear quaternion control algorithm is introduced and analyzed. Based on this algorithm, an improved controller is then developed. The proposed algorithm is cascade-saturation control logic with self-adjusting saturation limit, which is necessary to avoid integrator anti-windup. Simulation results have demonstrated the rapid retargeting capability of the proposed nonlinear algorithm.Finally, an attitude control model which chooses SGCMG as the actuator is designed in Matlab/Simulink. This model can be used in trying out existing control algorithms and optimizing parameters. This model is also be used to clarify the issue of CMG torque amplification and verify rapid retargeting maneuvers of agile satellite.
|
PDF Full Text Request