variable structure control

In this paper, a continuous stable tracking control algorithm is proposed for spacecraft in the presence of unknown actuator failure, control input saturation and external disturbances. The design method is based on variable structure control and has the following properties: 1) fast and accurate response in the presence of bounded disturbances; 2) robust to the partial loss of actuator effectiveness; 3) explicit consideration of control input saturation. In contrast to traditional fault-tolerant control methods, the proposed controller does not require knowledge of the actuator faults and is implemented without explicit fault detection and isolation processes. In the proposed controller, a single parameter is adjusted dynamically in such a way that it is possible to prove the ultimate boundedness of both attitude and angular velocity errors. The stability proof is based on a Lyapunov direct method and the properties of the singularity free quaternion representation of spacecraft error dynamics. Results of numerical simulations state that the proposed controller is successful in achieving high attitude performance in the presence of external disturbances, actuator multiplicative faults, and control input saturation.