Research On Close-range Hovering And Proximity Trajectory Planning Method Relative To A Tumbling Target

With the development of space technology,space operations such as on-orbit assembly,on-orbit refueling,on-orbit repair,and debris removal have become a new trend of on-orbit servicing missions.Hovering and proximity are important prerequisites for the realization of the above tasks.This dissertation focuses on the close-range hovering and proximity problems in on-orbit servicing missions.The strategies to achieve close-range safe hovering,safe fly-around maneuver relative to a tumbling target under no-fly zone constraints,and ultra-close approaching a tumbling target under the uncertainty of target’s attitude parameters are studied.The main work is as follows:Firstly,the control methods of two hovering patterns,namely fixed-point hovering,and teardrop hovering,are studied.With these two hovering patterns,the chaser can realize close observation of the tumbling target.For fixed-point hovering,the critical safe hovering surface which separates the regions with and without passive safety is obtained to evaluate the passive safety feature in an arbitrary hovering direction.The location set with the passive safety feature is determined at the same time.Then an adaptive backstepping controller with input saturation is proposed to realize the proximity and hovering operation.An auxiliary system is introduced into the basic backstepping controller to compensate for the effect of input saturation.The asymptotic stability of the traditional controller is improved by designing adjustable scaling factors,which helps to reduce fuel consumption.For the quasi-hovering of teardrop configuration,the necessary condition of the teardrop is derived based on the relative orbital elements(ROEs).The relations of the ROEs,geometry size,and impulse consumption are analyzed.Then an optimal closed-loop control method based on convex optimization and receding horizon optimization is proposed to achieve the teardrop maintenance.The infeasibility of flight time and control command during optimization is considered for the first time.Then,based on the feature of infeasibility,the algorithm to estimate the minimum remaining flight time is embedded in the closed-loop control method to deal with this problem.Therefore,the control algorithm is feasible,optimal,and robust.Secondly,a safe trajectory planning method via successive convexification and multi-resolution technique(MRT)is studied to deal with the problem of the chaser flying around a tumbling target in a close range.The traditional uniform discretization method does not consider the local characteristics of trajectory or control.In the proposed optimization strategy,the MRT is embedded into the successive convexification process to deal with this problem.Convex optimization works as an inner-layer algorithm for trajectory optimization,and the MRT works as an outer-layer algorithm for mesh refinement.This combination method remedies the weakness of the traditional uniform-grid discretization method,and can adaptively adjust the local grid density according to the designed resolution level.So it performs better in computing efficiency than the traditional uniform discretization.Moreover,a mesh refinement strategy for collision avoidance is proposed to make the grid points near the no-fly zone denser,thereby improving the safety of the spacecraft.The affine approximations of the spherical and ellipsoidal no-fly zones are derived by the analytical formula of the tangent plane.Therefore,the trajectory optimization problem with no-fly zone constraints can be solved with a convex programming solver.Finally,the ultra-close trajectory planning method considering the uncertainty of the target’s attitude is proposed to plan the safe trajectory for a chaser approaching the tumbling target.For the pose description in the target’s LVLH frame,the multiple sampling points of the attitude uncertainty are obtained based on the sampling idea of unscented transformation.Multi-no-fly zone constraints are constructed based on the sampling points and attitude motion model.The results obtained by the proposed method have better security than the single no-fly zone method.For the position description in the target’s body frame,the robust trajectory optimization method is proposed based on based on polynomial chaos expansion(PCE).The uncertainty of chaser’s initial state and target’s attitude is described by PCE method.Then the stochastic optimal control problem is converted into a deterministic optimal control problem,which is solved by convex optimization.The proposed method has better robustness and security than the deterministic optimization method.The convex expressions of the dynamic model,no-fly-zone constraints,and robust no-fly-zone constraints are derived,and then a convex programming solver is employed to solve the trajectory optimization problem to improve computing efficiency.In conclusion,the hovering control,fly-around,and ultra-close proximity trajectory planning problems relative to a tumbling target are studied in this dissertation.The safe hovering control strategy and safe proximity trajectory based on convex optimization method are proposed.The effectiveness of the proposed method is verified by numerical simulation.The work in this dissertation is conducive to promoting the development of on-orbit servicing technology,and can provide theoretical and technical support for China’s autonomous proximity operation experiment.