Actuator Fault Diagnosis And Tolerant Control For Satellite Attitude Control Systems During Collision Avoidance

With the increasing number of space rubbish, satellites are being collided more easily.Now the research of satellites with collision avoidance is becoming the focus in space area.Since the environment for on-orbit satellites is severe, the actuators of satellites are inevitableto be faulty because of the changing temperature and high radiation. Fault detection andtolerant control for satellites during collision avoidance are greatly concerned.According to the nonlinearities and strongly coupling for satellite attitude control systems duringcollision avoidance, a fault diagnosis approach based on Self-organizing fuzzy neural network(SOFNN) is proposed. The network can update its weights on line and adjust the nodes to optimizethe network, thus providing a new method for fault diagnosis. In this dissertation, two Self-organizingfuzzy neural networks are investigated for actuator fault diagnosis. One SOFNN is designed forhealthy system training off line, which can estimate the uncertainties and nonlinearities, with theoutput as the threshold reference for fault detection. The other is to estimate the actuator fault.Simulation is carried out to show the correctness of the method for fault detection and estimation.Since the diagnosis approach of sliding observer is effective for systems withnonlinearities and disturbances, a fault diagnosis method based on sliding observer foractuators is studied in the presence of large disturbances, model uncertainties and nonlinearitiesfor satellite attitude control systems during collision avoidance. The observer is designed basedon the T-S fuzzy model for satellite attitude control systems. Then if the system encounters withfault, the concept of equivalent output inject, similar to sliding control, is used to estimate fault.Simulation demonstrates that the method can realize fault detection even if the system isequipped with uncertainties and nonlinearities. Besides, the method is simple and can berealized for engineering application.In order to avoid the false alarm caused by the model uncertainties and large disturbances,a fault diagnosis approach based on reduced nonlinear unknown input observer (RO-NUIO) isproposed. The method can relax the condition compared with nonlinear unknown observer,which can not only realizing fault detection but also realizing fault isolation. Simulation resultsprove the effectiveness of the proposed method and show that the method is suitable for thesystem considered in the paper.Considering the thrust may affect the attitude and flexible mode during orbit maneuvering, a backstepping adaptive variable structure method is developed for fault tolerant control. Forthe vibration suppression, the strain-rate feedback control is employed by using piezoelectricmaterials as additional sensors and actuators. Simulation results show the validation of the faulttolerant control methods and the effectiveness of the vibration suppression control which cansuppress the vibration of the flexible modes.Due to the physical constraint of the actuators, fault tolerant control method based on inputsaturation is presented. Since in practical application, the output of the actuators is impossibleto provide infinite torque because of the physical design and output energy constraint. Avariable fault tolerant control is designed with actuator fault and input saturation considered isgiven. The simulation result illustrates that the proposed fault tolerant controller satisfies theinput saturation condition and guarantees the effectiveness of the system to be work in faultycondition.If the system is equipped with redundant actuators, an on-line control allocation method isinvestigated for fault tolerant control. The fault tolerant controller can be carried out withoutreconfiguring the baseline controller, thus reducing the complexity of the designing. Simulationresults shows that control allocation method can allocate the virtual control command on line andoptimize the configuration of the resources, realizing fault tolerant control to improve the reliability ofthe attitude control system.At last, a semi-physical simulation system for fault detection and tolerant control is developedbased on the existing satellite attitude control simulation equipments. Fault detection and tolerantcontrol is studied combined with theory method and engineering practice to illustrate the effectivenessand reasonableness of the methods. Besides, numerical software for satellite attitude control systemsduring collision avoidance maneuver is designed to demonstrate the mechanisms of collisionavoidance, fault detection and tolerant control.