Variable-structure fault-tolerant attitude control for flexible satellite with input saturation

The variable-structure fault-tolerant attitude control system is investigated for an orbiting three-axis stabilized flexible satellite with redundant thrusters. In this system the thruster failures, the control-input saturation and external disturbances are explicitly considered simultaneously. In the proposed controller, a single parameter is dynamically adjusted, independent of the fault detection and mechanism isolation; only the remaining active thrusters are required to be able to produce a sufficient combined force to maintain the satellite in operation for performing the given tasks. By explicitly considering the saturation magnitude of the available control input to the thruster, we develop a direct relation between this magnitude and those of the desired trajectories and disturbances in continuous control. Lyapunov stability analysis shows that the closed-loop system is stable; and the effect of the external disturbances and faults can be reduced by appropriately choosing the design parameters. Numerical examples show that the proposed control algorithms are robust to external disturbances, and are able to handle thruster failures under a limited value of saturation.