Research On Rendezvous Trajectory Optimization And Close-Range Safe Approach Control Of Spacecraft For On-orbit Servicing

On-orbit servicing(OOS)provides new technologies and concepts for spacecraft on-orbit operation and mission execution.OOS can improve on-orbit life,operational stability and mission execution ability of spacecraft effectively in complex space environment.Currently,OOS has great practical application value and has become one of the increasingly important space technologies.For the implementation of on-orbit servicing,it is prerequisite to complete the long-distance rendezvous and proximity approach to space target subject to rapidity,low-cost and high precision.This paper studies optimal design of trajectory for long-distance rendezvous with different thrusts and close-range safe approach control considering input saturation.The main contents are as follows.Firstly,the multi-objective optimization design of three-dimensional trajectory for impulse thrust rendezvous is studied.Based on Lambert’s theorem,the processes of time-open two-impulse and three-impulse rendezvous are analyzed and studied,and a general solution method for orbit maneuvering strategy is given.At the same time,the fuel and time synthesis optimization of three-dimensional impulse thrust rendezvous is analyzed in detail.In order to deal with multi-objective optimization design of impulse thrust trajectory,an improved NSGA-II algorithm that has better searching ability is proposed,and the problem of fuel and time synthesis optimization of three-dimensional impulse rendezvous strategy considering multi-constraints is handled by using the multi-objective optimization algorithm.The simulation results show that the improved NSGA-II algorithm has better solving ability and can effectively deal with the fuel and time synthesis optimization problem of impulse rendezvous in different situations.Secondly,based on shape method,the trajectory optimization design of trajectory subject to continuous thrust is studied.Aiming at the problem of three-dimensional continuous thrust multiple-revolution rendezvous,taking the initial orbital plane as the reference plane,the motion trajectory subject to continuous thrust is designed based on trigonometric function and polynomial function,and the unknown coefficients are solved according to the initial and terminal boundary conditions.The results of the designed trajectory approximation function are taken as the sub-optimal solution of the optimal problem,and the initial value of the costate variables in the Hamilton equation can be estimated.And the continuous thrust trajectory optimal design is further completed by using the the optimal control theory.The simulation results show that the new shape-based method is suitable for initial design of continuous thrust trajectory optimization design with large inclination and multiple-revolutions,and the method has small thrust acceleration and fuel consumption.Combining with the optimal control theory,the optimal design of the continuous thrust trajectory with the lowest fuel consumption can be achieved effectively.Thirdly,the problem of close-range relative position control considering external disturbances,input saturation and safe constraint is studied.Based on the relative position control model of spacecraft close-range approach and spherical collision avoidance function,using backstepping method,second-order tracking differentiator and auxiliary system,two anti-saturation dynamic surface control strategies based on tracking differentiator for relative position safe approach are designed for known and unknown upper bound of external disturbance,respectively.Considering the finite-time stability and the performance of the auxiliary system,using backstepping method,the command filter,the compensation signal and the new auxiliary system,two finite-time anti-saturation dynamic surface control strategies for relative position safe approach are designed for known and unknown upper bound of external disturbance,respectively.Strict theoretical proofs are given for the designed controllers,and simulation results show that the chaser can complete the close-range safe approach subject to input saturation with high precision by using the designed controllers.Finally,the problem of close-range attitude and orbit coupled control considering external disturbances,input saturation,safe constraint and finite-time convergence is studied.Based on the attitude and orbit coupling model of spacecraft close-range approach,a new cone-like safe constraint collision avoidance function is proposed.Using the collision avoidance function,fast terminal sliding mode and auxiliary system,two safe approach finite-time anti-saturation attitude and orbit coupled control strategies based on fast terminal sliding mode are designed for known and unknown upper bound of external disturbance,respectively.In order to relax the restriction on the initial position,a continuously differentiable collision avoidance function is further designed,and an improved fast nonsingular terminal sliding mode with faster convergence speed is proposed.Combining with the auxiliary system,two safe approach finite-time anti-saturation attitude and orbit coupled control strategies based on the improved fast terminal sliding mode are designed for known and unknown upper bound of external disturbance,respectively.Strict theoretical proofs are given for the designed controllers,and simulation results show that the chaser can complete the close-range safe approach subject to input saturation with high accuracy,which can not only meet the relative position requirement,but also meet the attitude requirement.