Fault Tolerant Attitude Control And Control Allocation For Spacecraft With Actuator Failure

Failures and uncertainties do occur inevitably in spacecraft actuators and other components during their long-time working in the severe space environment,which deteriorate the stability,reliability and security of the spacecraft attitude control system.And finally the on-orbit missions may fail completely for just this reason.More specifically,almost 44% failures of the attitude control system occurred in control actuators.Therefore,it is of great theoretical and practical significance to investigate the autonomous fault tolerant attitude control technique under actuators’ failures,which is able to improve the reliability and the percentage of successfully accomplished missions.On the other hand,redundant actuators are generally equipped to improve the capacity of autonomous operation and the reliability of the spacecraft system,which will lead to the problem of how to distribute control signals to the individual actuator.Then,it is important to design an optimal control allocation approach considering actuator constraints and attitude control performance indexes,such as the minimum energy consumption,or minimum control allocation errors,etc.In view of these issues,this dissertation focuses on the establishment of the attitude fault tolerant control and control allocation schemes for spacecraft in the present of actuators’ failures,model uncertainties and external disturbances,to address the significant problems such as real-time fault diagnosis,finite time attitude fault tolerant control,closed-loop optimal control allocation and its stability analysis.The main contents and achievements of this dissertation are presented in the following aspects.Observer based fault diagnosis approaches are firstly investigated to reconstruct the actuator failures or uncertainties of spacecraft with redundant actuators.To estimate the control effectiveness of the actuators,an iterative learning observer based fault diagnosis mechanism is developed for the spacecraft attitude control system under model uncertainties and disturbances.The iterative learning method just needs the past sampling time estimation of the actuators’ effectiveness and the sign of attitude state,rather than the actual value.Furthermore,an extended state observer and fast nonsingular terminal sliding mode control based finite time fault diagnosis method is proposed to estimate and compensate for the specified synthetic uncertainties derived from actuators’ failures,model deviations and disturbances.The detailed derivations of the finite time observer are provided,along with a thorough analysis for the associated ultimately bounded stability and estimation error convergence property in the sense of finite time control.On the basis of the proposed fault diagnosis strategies,two finite time fault tolerant control method are studied for spacecraft attitude stabilization respectively in the presence of actuators’ failures,model uncertainties and external disturbances.With the reconstructed actuators’ effectiveness derived from the iterative learning observer,an adaptive sliding mode based fault tolerant control law is designed to ensure that the attitude control system reaches the real sliding mode surface in finite time.Meanwhile,the singularity and chattering problems have been avoided/restrained via the saturation function and the gain adjusting law respectively.Moreover,another fast nonsingular terminal sliding mode technique based robust fault tolerant attitude control approach is developed by using the synthetic compensated information from the finite time extended state observer.The finite time stability proof of the attitude control system is constructive and accomplished by the development of a novel Lyapunov function candidate.With application of these two control methods,the actuators’ failures and uncertainties are able to be tolerated,and the spacecraft attitude is governed to be high-precise stability rapidly.Then,the optimal control allocation issue is investigated to improve the attitude control performance for the over-actuated spacecraft.A novel null-space based optimal control allocation method is employed to map the specified control command to the redundant actuators.This optimal control solution is obtained by penalizing the control allocation errors at a lower power/energy cost using quadratic programming algorithm.Considering further the actuator misalignments and output magnitude deviations or failures,a modified robust least-squares based control allocator is developed to deal with the problem of distributing the previously designed control signals over the available actuators.The focus of this control allocation is to find the optimal control vector by minimizing the worst-case residual error using programming algorithms.And the control performance especially the robustness of the spacecraft control attitude system is evaluated through a numerical example.At the end of the dissertation,we attempt to address the very challenging problem,that is,design a closed-loop control allocation scheme and analyze the stability of the whole closed-loop attitude control system.An original closed-loop constrained optimal control allocation scheme is firstly designed to distribute the moments synthesized by the proposed controllers over the actuators in the presence of amplitude and rate constraints.The optimal control solution is to be found by penalizing the control allocation errors and rates using quadratic programming,which is also robust to noises or disturbances,and fine performances are achieved.A significant feature of this work is that the asymptotic stability with the closed-loop control allocation is guaranteed theoretically.Furthermore,an novel on-line closed-loop constrained optimal fault tolerant control allocator is proposed in the presence of actuators’ failures and constraints.The stability of the closed-loop fault tolerant control allocation process is analyzed using the theory of discrete time feedback control system.With application of the fault tolerant control allocation,the control effects can be updated in real time by utilizing the proposed previously fault diagnosis mechanism,rather than redesign the control law.This attitude control methodology decreases the complexity in the control reconfiguration process,and improves the reliability and safety of the spacecraft system.Finally,numerical simulations are implemented on the closedloop spacecraft attitude control system consisting of proposed fault diagnosis mechanisms,fault tolerant controllers and control allocation algorithms.Simulation results clearly verify the effectiveness and superiority of the proposed attitude control system schemes,which also demonstrates that the proposed approaches can achieved a lower energy consumption,greater percentage of successfully accomplished missions and higher-precise stability of the spacecraft attitude control.