| Spacecraft on-orbit servicing and space debris removal are current research hotspots in the field of aerospace,and a common denominator of these missions is that the targets to be serviced or removed are mostly uncooperative targets in space.Depending on the degree of cooperation,the targets can be classified into partially uncooperative targets with known geometric models and fully uncooperative targets with unknown geometric models.Depending on the motion characteristics,uncooperative targets may be in quasistationary,slow-tumbling and fast-spinning states.This thesis addresses the problem of spacecraft close-range rendezvous and docking with slow-tumbling uncooperative targets with known geometric models,and explores the use of image information to achieve integrated control of spacecraft attitude and orbit,taking into account physical constraints,such as field-of-view constraint,path constraint and input saturation to enable the chasing spacecraft to approach the target along a safe trajectory and ensure the target’s visibility.The main research contents of the thesis are as follows:An integrated navigation-guidance-control framework based on image information is proposed for spacecraft rendezvous missions with uncooperative targets in space.Considering the requirement for target’s visibility,the chasing spacecraft needs to keep the target within the field of view of the camera while approaching and tracking the uncooperative target.To this end,a navigation function is introduced to design an offline path planner for generating a set of desired image feature trajectories.Based on the current image features and the generated desired image features,a novel formulation of the system dynamics model is presented for spacecraft 6-degree-of-freedom relative pose tracking.A robust image-based controller is developed to track the desired image feature trajectories using the theory of passivity.In addition,a hyperbolic tangent function is introduced to solve the chattering phenomenon caused by the sign function.The proposed control scheme is evaluated in terms of the target’s visibility,control accuracy and fuel consumption through comparative studies with conventional image-based and pose-based control schemes.Considering that the pixel velocities of image features are not easily obtained in practice,a linear observer is further designed to estimate the image errors and their first-order derivatives online,thus solving the spacecraft proximity and tracking control problem without image velocity feedback.During relative measurements of uncooperative targets such as failed satellites or space debris,the fixed-focal-length camera will limit the observation distance and imaging effect of the target.Therefore,a new image-based attitude/orbit integrated control scheme is proposed,which is capable of autonomously adjusting the camera focal length,thus extending the range of relative measurement and visual servoing,as well as keeping the target within the field of view of the camera.As the camera focal length is time-varying,it is necessary to remap the image features to another image space independent of the camera’s internal parameters,then redefine the image errors and build the image dynamics in the this space.An active zoom strategy is developed to keep the target in the image plane with appropriate size in the process of visual servoing by adjusting the focal length.Based on the established image dynamics model,a finite-time controller is designed,which is robust to the unknown movements of the target and external disturbances.Two visual servoing scenarios with fixed-focal-length and zooming cameras are simulated,respectively.A comparative study with the conventional sliding mode controller is conducted to evaluate the convergence rate and control accuracy of the finite-time controller.In the process of spacecraft rendezvous and docking with uncooperative targets,it is necessary to ensure the path safety,target’s visibility and input saturation.To address such problems,a spacecraft proximity and tracking control strategy with multiple constraints is proposed.A coupled orbit/attitude dynamics model is established with the consideration of kinematic coupling caused by the rotation of the docking port around the target’s center of mass.A docking corridor is designed for the path constraint and the corresponding repulsive potential function is constructed based on its geometrical properties;a restricted region is designated in the image plane for the field of view constraint and a repulsive potential function based on the 2D image information is constructed accordingly.A new sliding mode manifold is defined based on the proposed potential functions and an adaptive sliding mode controller is presented.Considering the physical limits of the actuator,a linear anti-saturation compensator is introduced to balance the impact of input saturation on the stability of the system.Numerical simulations are carried out,and the effectiveness of the spacecraft proximity and tracking control strategy under multiple constraints in terms of path safety and target’s visibility is evaluated by comparing it with the control strategy under unconstrained conditions. |
