Large-angle attitude control of spacecraft with structure spillover

In this paper, a nonlinear control design procedure is proposed for general attitude reorientation of a spacecraft with flexible structures. The method of input-output feedback linearization and Lyapunov stability analysis are the main tools incorporated in this procedure to design an asymptotically stable feedback control law. The spacecraft attitude is considered to be confined to the surface of a four-dimensional sphere centered at the origin of the four-dimensional Euler-parameter space. Instead of forming an attitude transformation relation, the direct subtraction of attitude vector from present to final orientation is utilized. Through feedback linearization, dynamics of the attitude errors is derived as a second-order linear ordinary differential matrix equation and constant coefficients of the equation become the gain matrices of the attitude control law. Stabilization of the flexible structures in the form of adaptive damping is also derived Lyapunov stability analysis and becomes a part of the attitude feedback control law. Overall stability of the attitude control can also be achieved by tuning the control gains obtained in the analysis. The uniqueness of the proposed design approach is that it requires only three independent control torques on the hub. High-frequency modes are added to the system as unmodeled dynamics. Simulation results indicate that the uncontrolled high frequency modes will be excited and cannot be eliminated as the controller receives no direct information on the motions. However, the spacecraft will reach the desired attitude with modeled low-frequency modes stabilized and the spillover of unmodeled high-frequency modes remain bounded.