Research On Flexible Spacecraft Attitude Control With Prescribed Performance

In recent years,space projects such as deep space exploration,space debris removal,and on-orbit service,have attracted great attention and been gradually implemented.Faced with such complex and diverse missions,the spacecraft attitude control system plays an important role for mission accomplishment.The design of the attitude control system mainly focuses on the following issues.The first one is the robustness.It is required that the attitude control system should be capable of coping with the disturbances and uncertainties arising from the severe space environment for reliability concern.Another one is the vibration suppression of flexible appendages.The structural design of the spacecraft with large size and light weight results in the increase of the flexibility,which will influence the stability of the attitude control system.Moreover,in order to complete the diverse and difficult missions better,the requirement of the control performance is dramatically increased as well.Therefore,the attitude control design is still a challenging research subject.Taking the above problems into consideration,this dissertation studies the high performance and strong robustness attitude control and effective vibration suppression schemes of flexible spacecraft.Its structure and methods have the following striking characteristics:(1)To make full use of the advantages of the centralized control which regards the flexible vibrations as disturbance and the active control,the attitude control strategies of flexible spacecraft based on disturbance compensation and active vibration suppression are researched according to the two thoughts,respectively.(2)The attitude controllers and the active vibration controllers are designed by using the prescribed performance control method,such that the attitude error can obtain the prescribed dynamic and steady-state performance(such as overshoot,convergence rate,and steady-state accuracy)and the modal coordinate of the flexible vibration can be always kept within the prescribed constraint bounds to achieve expected vibration attenuation effect.The works accomplished in this dissertation are as follows:First of all,the required basic knowledge such as definitions and lemmas,as well as the kinematic equations,the dynamic equations,and the vibtation equations are presented.Several disturbance observers are disigned and the observer-based PD control schemes were studeid preliminarily.The features of each disturbance observer are further compared to provide references for the follow-up researches.When dealing with the attitude centralized control problems of flexible spacecraft in the presence of external disturbances,a neural network adaptive and a low complexity model-free prescribed performance control methods are proposed based on the idea of disturbance compensation,respectively.The neural network is used to approximate the sum of disturbances and the influence of the flexible appendages vibrations while the upper bound of the approximation error is estimated adaptively,after which the complete controller is given by combining with the adaptive law.The simulation results indicate that this method can effectively compensate the lumped disturbance and make the attitude control system obtain rapid dynamic process with expected accuracy.When considering the large computation cost and the existence of specific problems of the neural network approximation,a simple form controller is developed by using the prescribed performance control and the idea of backstepping method.The controller does not need any dynamics information and can even handle the uncertainties without depending on the approximation,obsevers,or adaptive methods,which greatly reduces the complexity.The effectiveness of the method is verified by simulation.When dealing with the attitude centralized control problems of flexible spacecraft in the presence of external disturbances,the observer-based prescribed performance control methods are further investigated.In order to improve the convergence properties,a new performance function is proposed.Compared with the commonly used exponential form,the modified one has an explicit prespecified terminal time which determines the maximum convergence time of the attitude control system.A modal observer and a disturbance observer are designed to deal with the flexible vibration and disturbances,respectively.Furthermore,when considering actuator saturation,an improved control strategy is developed with an auxiliary system utilized to compensate the saturation.The stability of the closed-loop system is analyzed by Lyapunov theory.Simulation results show the effectiveness and performance of the proposed methods.When dealing with the attitude active control problems of flexible spacecraft in the presence of external disturbances,the active vibration control and adaptive attitude control methods with prescribed performance are studied by using the piezoelectric sensors and actuators.First,for the flexible space structures subject to external disturbances and parameter uncertainties,an active vibration suppression method with state observer is proposed.To meet the case that the velocity sensors are not available on the flexible structures,a neural network based state observer is designed for the estimation of the modal velocity.The neural network approximation is introduced into the observer design which can effectively dispose the disturbances and uncertainties in order to obtain more accurate observation results.Further,the controller and the adaptive law are derived by using the backstepping technique and independent modal space control method with the stability of the whole closed-loop system proved strictly.Simulation results illustrate the vibration control performance of the proposed method.Then for the spacecraft with flexible appendages,a prescribed performance composite attitude and vibration control strategy is proposed.To reduce the implementation complexity of the active vibration control in practical application,the piezoelectric sensors are not adopted while the modal observer is introduced to estimate the modal information.Based on the observed modal information,an adaptive attitude controller which has the capabilities of rejecting disturbances and possessing predetermined transient and steady-state control performance is designed by combining with an active vibration approach to deal with the vibrations induced by the attitude motions.The stability of the entire closed-loop system is analyzed by Lyapunov theory.Simulation results in different cases show the effectiveness of the proposed composite control strategy.