Fixed-Time Coordinated Control For Spacecraft Formation Flying

Spacecraft formation flying(SFF)has the advantages of improving the overall performance,the reliability and the adaptability of the system,and shortening development cycle.It has become an important research direction in space technology and has been widely used in deep space exploration and earth observation missions.Distributed coordinated control algorithm is an important prerequisite and guarantee for SFF.Finite-time control has the advantages of fast convergence,high control precision and strong robustness,but the a ctual settling time of the closed loop system is related to the initial value s of the states.When the initial values of the states are far from the equilibrium point and tend to infinity or unknown,the settling time of the closed loop system will tend to infinity or cannot be calculated.This greatly limits its application in engineering.Fixed-time control makes up for this.The fixed-time control algorithm ensures that the closed loop system is stable in a given upper bound of time,regardless of the change of the initial values of states,known or unknown.The upper bound of the settling time is determined only by the designed parameters.Some SFF missions(Tech Sat-21)need to complete attitude coordination or formation configuration reconstruction within a specified time,which makes it very necessary to study the fixed-time coordinated control of SFF.Based on sliding mode control(SMC),adaptive control,backstepping control,disturbance observer,RBF neural network,auxiliary system,saturation compensator and other technologies,this dissertation studies the fixed-time cooedinated controllers design for SFF.The main contents of this dissertation are as follows:Firstly,the fixed-time control theory based on SMC is studied.The most basic work of SMC is to design a sliding mode surface with certain characteristics so that the system states have corresponding dynamic characteristics.Each chapter designs different forms of sliding mode surfaces,each of them can ensure that the system states converge to 0 in a bounded time and has different characteristics,such as saturation,time-varying,collision avoidance,etc.These sliding surfaces are help ful to the development of fixed-time control theory.Secondly,the display relation between the upper bound of the settling time of the system states and the maximum amplitude of the control input is studied.The dynamic part of the controlled system and the inconsistent terms due to input saturation are compensated by a dynamically adjusted parameter,and a co nservative lower bound of this parameter is obtained,so that the boundary of settling time of the states is directly related to the maximum amplitude of the control input.Thirdly,the fixed-time attitude coordinated control problem of SFF is studied.Under the condition of communication topology for undirected graph,and cons ider constraints such as the external disturbance,model uncertainty,input saturation,actuator failure,a fixed-time attitude coordinated controller,a fixed-tme attitude coordinated controller considering input amplitude constraint,an anti-saturation attitude coordinated controller are proposed respectively based on SMC,adaptive control,and fixed-time observers.Fourthly,the problem of fixed-time position coordinated control for SFF is studied.Aiming at the safety control problem of SFF at close range,the artificial potential function is skillfully combined with the designed sliding mode surface,and a collision avoidance coordinated controller with fixed-time convergence property is designed,which can ensure no collision among spacecrafts or between spacecrafts and obstacles as long as the sliding mode surface is bounded.Aiming at the problem of state constraint in position coordination of SFF,a fixed-time anti-saturation coordinated controller is designed based on prescribed performance control and combined with SMC,backstepping control,observer,saturation compensator and auxiliary system.Simulation results show the effectiveness and superiority of the proposed controllers.Finally,the fixed-time coordinated control problem of SFF with attitude orbit coupling is studied.Based on the RBF neural network and the designed sliding mode surface,a controller that only needs the system states of neighbors is designed to complete the cooperative task.Aiming at the state constraint problem,based on the prescribed performance control,SMC,RBF neural network,backstepping control,saturation compensator,auxiliary system and other technologies,an anti-saturation attitude orbit coupled coordinated controller is designed.It not only has anti-saturation characteristics but also ensures that the states of the controlled system reache the specified control accuracy in a bounded time.Simulation results show the effectiveness and superiority of the proposed controllers.