Detect And Isolate Actuator Faults For Flexible Satellite Attitude Control Systems During Collision Avoidance

With the increasing number of space rubbish, collision avoidance is attracting more and more attentions. Fault detection and isolation can improve the reliability of the flexible satellite during collision avoidance. This thesis focuses on designing orbit and attitude control methods, modeling actuator faults, detecting and isolating faults for satellite attitude control systems during collision avoidance. The key contributions are summarized as follows:The orbit control approach based on Q-law is proposed, which can not only eliminate vibration of orbit elements but also reduce the trajectory errors. Meanwhile, thrusters and momentum wheels are combined as the attitude control actuators and the control strategy can be designed to keep the satellite stable even if the satellite is affected by the disturbance caused by the orbit control force.Various fault classification methods are given, and the models of satellite actuator faults are analyzed. Moreover, the characters of fault diagnosis system in flexible satellite attitude control systems during collision avoidance are surveyed, which illustrate the difficulties of the fault diagnosis systems.A fault detection and isolation approach based on nonlinear unknown input observer is proposed. The method can avoid the false alarm caused by the model uncertainties and large disturbances. However, the existing condition of the proposed observer is very rigorous. Then, an improved reduced order– nonlinear unknown input observer (RO-NUIO) is developed. The improved observer can relax the existing condition of NUIO and it is more suitable for the satellite during collision avoidance.The semi-physical simulation systems based on single axis air-bearing platform is studied to demonstrate the effectiveness and feasibility of the control methods, fault detection and isolation methods in this paper.