Relative Position And Attitude Coupled Control For On-orbit Servicing Spacecraft Rendezvous And Docking To An Out-of-control Target

With the development of autonomous on-orbit servicing technology, it will bean important field in the future to make small spacecraft perform on-orbit rescue andrepair for the out-of-control target spacecraft. In such a mission, the servicingspacecraft needs autonomous rendezvous and dock to the out-of-control target, andafter that, on-orbit servicing operations could be carried out. The coupled controlproblem of relative position and attitude for the on-orbit servicing spacecraftautonomous rendezvous and docking to an out-of-control target is one of the keytechnologies and urgent issues in the autonomous on-orbit servicing for anout-of-control target mission. Focusing on the coupled control problem that theservicing spacecraft, which is small satellite here, performs close-range operationson the out-of-control target, this dissertation intensively investigates the relativeposition and attitude coupled control algorithm of the servicing spacecraft withrespect to an out-of-control target. The main contents of the dissertation are asfollows:The motion of the out-of-control target in space is analyzed. Then, a controlstrategy of autonomous rendezvous and docking between the on-orbit servicingspacecraft and the out-of-control target is presented for the most complex motion(which is also the most common motion) of the out-of-control target. The motion ofthe out-of-control target is divided into two situations according to its characteristicsof moment of inertia, i.e. the motion without nutation and the motion with nutation.Further, the motion of the two situations are also divided into three classesrespectively, according to the geometric relationship between the direction of theout-of-control target’s angular velocity and the direction of the docking port. Amongthese situations, the most complex motion of the out-of-control target is the motionwith nutation, whose angular velocity vector is neither parallel nor vertical to thedocking port’s direction. The proposed control strategy for the most complx motionsituation of the out-of-control target can effectively avoid the local collision duringthe approach of the servicing spacecraft to the out-of-control target.Choosing the relative position and the vector part of relative attitude quaternionas the system state and considering the relative position and attitude coupled (alsocalled the control input coupled) which is produced by the propulsion installation,the relative position and attitude coupled dynamic model of the servicing spacecraftwith respect to the out-of-control target is established in two order form of thesystem state. The control objective of the relative position and attitude coupledcontrol system is analyzed, and the relative position tracking coupled (which is also called the control command coupled) is also analyzed according to the relativeposition tracking command.Considering the coupled control problem of the on-orbit servicing spacecraftautonomous rendezvous and docking to the out-of-control target under unknown butbounded disturbances, a relative position and attitude coupled control algorithm ispresented based on the feedforward-feedback control principle. The robustness ofthe feedforward-feedback-based coupled control algorithm to the boundeddisturbances is proven by Lyapunov stability theory, and the effectiveness androbustness are verified via numerical simulations. This feedforward-feedback-basedcoupled control algorithm has simple mechanism, easy implementation and highreliability, but the shortage is that the robustness to the system uncertainties is notproved in theory.In order to enhance the robustness of the coupled control system, threesliding-mode-based relative position and attitude coupled control algorithms arepresented with consideration of the unknown but bounded disturbances, systemuncertainties and measurement noise. The convergence of sliding mode is analyzed,and the effectiveness and robustness of the three sliding-mode-based coupledcontrol algorithms are verified via numerical simulations. Compare to thefeedforward-feedback-based coupled control algorithm, the threesliding-mode-based coupled control algorithms show better control accuracy androbustness. Among the three sliding-mode-based coupled control algorithms, thetransient response of the relative distance and position corresponding to thetraditional-sliding-mode-based coupled control algorithm has one oscillating withsmall amplitude, while the double-sliding-mode-based coupled control algorithmshows good control accuracy and transient response.In order to improve the performance of the coupled control system, acompound-control-based relative position and attitude coupled control algorithm ispresented with consideration of the unknown but bounded disturbances, systemuncertainties and measurement noise. The characteristic of the integral sliding modeis analyzed under control saturation and measurement noise. And the effectivenessand robustness of the compound-control-based coupled control algorithm is verifiedvia numerical simulations. This compound-control-based coupled control algorithmshows high control accuracy, strong robustness and very steady transient response.In order to add optimality as well as robustness to the coupled control system,two optimal-sliding-mode-based relative position and attitude coupled controlalgorithms are presented with consideration of the unknown but boundeddisturbances, system uncertainties and measurement noise. The twooptimal-sliding-mode-based coupled control algorithms can make a balance betweenthe control accuracy and energy assumption. The effectiveness and robustness of the two optimal-sliding-mode-based coupled control algorithms are verified vianumerical simulations. And the energy assumption is compared among all kinds ofdesigned relative position and attitude coupled control algorithms in thisdissertation.