Vibration Suppression And Fuzzy Attitude Control Of Flexible Spacecraft

The space technology has received considerable development since the first spacecraft was launched.To reduce the cost,modern spacecraft widely employs flexible structures(such as solar arrays,large antennas,etc.).However,the existence of flexible structures will induce elastic vibration for flexible spacecraft.Moreover,disturbances in space(such as solar radiation torque,aerodynamic torque,gravity-gradient torque,etc.),uncertain physical parameter,actuator saturation and faults will further degrade the precision of attitude control.Clearly,flexible spacecraft is a complex nonlinear system and its attitude control problem is a difficult and challenging work.This thesis is dedicated to investigating the problem of vibration suppression and fuzzy attitude control of flexible spacecraft and proposing some novel design methods.The main contributions are summarized as follows.To solve the vibration control problem of the flexible spacecraft implementing singleaxle maneuver,a novel finite frequency H_?control strategy is proposed,which can attenuate the elastic vibration of flexible appendages very well.Different from conventional H_?control strategy established on entire frequency band,the proposed one notes that the low-order vibration modes of flexible appendages always play a key role in elastic vibration,so it only imposes H_?index on the frequency range where the low-order vibration modes are located and achieves better vibration control results.The reason for it is that the main control energy is applied to chosen frequency band and the constraints of other frequency band are liberalized at the same time.When it comes to the mathematic model of flexible spacecraft,in this work TakagiSugeno(T-S)fuzzy method is employed to construct the dynamic model of flexible spacecraft.T-S fuzzy model is capable of uniformly approximating the nonlinear model of plant with any accuracy by using a set of local linear models.Based on the proposed T-S fuzzy model of flexible spacecraft,novel fuzzy attitude control strategies can be studied.It is worth mentioning that most of the existing attitude control methods for flexible spacecraft are based on nonlinear model.Since the analysis and design tools of nonlinear systems are scarce,it is difficult to design nonlinear attitude control schemes.By contrast,the analysis and deign tools for T-S fuzzy systems are resourceful,therefore,T-S fuzzy method can make it easy and open a new window for the solution to the problems which are hard to solve for nonlinear model.To solve the attitude control problem of the flexible spacecraft with actuator saturation and input delay,a novel control algorithm based on T-S fuzzy model and dissipative theorem is proposed.First,a novel Lyapunov-Krasovskii functional making full use of the information of time delay is constructed to deal with input delay,and asymptotic stability of the closed-loop system is guaranteed.Then,auxiliary-matrix method is used to compensate the effects of actuator saturation.Prescribed dissipative performance enables the closed-loop system to reject the elastic vibration of flexible appendages and external disturbances.Furthermore,the comparison of various types of performances is also implemented.To solve the attitude control problem of the flexible spacecraft with actuator faults and input sampling,a novel reliable sampled-data control algorithm based T-S fuzzy model is proposed.In this study,sampled-data control system is transformed to a continuous-time system with special input delay by input-delay method.Such that the desired controller can be designed based on this continuous-time system directly.Moreover,to cope with the partial loss of actuator effectiveness,the linear-matrix-inequality-based method is used,which converts the actuator faults into one kind of parametric uncertainty of closed-loop system.Finally,H_?performance ensures that the closed-loop system can effectively attenuate the elastic vibration of flexible appendages and external disturbances.To solve the finite-time attitude control problem of the flexible spacecraft with stochastic actuator faults and input sampling,a novel finite-time reliable sampled-data control algorithm based on T-S fuzzy system is proposed.Here,input-delay method is also employed to handle the sampled-data control problem.Then,a novel time-dependent Lyapunov-Krasovskii functional is established,which is able to describe the sawtooth time delay induced by sample-and-hold operation,to reduce the conservatism of the synthesized controller greatly.Moreover,A novel and general input model related to Markovian variables is developed to depict the stochastic actuator faults,which is capable of covering various fault modes.Finally,with the proposed finite-time notion,the designed controller can implement finite-time attitude control of flexible spacecraft.