Stabilization For Tethered Space Robot During Target Capture And Post-capture Phase

Tethered space robot(TSR) is a novel kind of space robot, which consists of a gripper, a space tether and a space platform. Compared w ith traditional space robot, TSR features advantages of high operation flexibilit y and long operation workspace, which determines its future use in on-orbit services including on-orbit maintenance, on-orbit refuelling, auxiliary orbit transfer and space debris removal. The capture mission of a TSR is t ypicall y separated into the deployment and approaching, capture and post-capture, and retrieval phase. However, target capture is the precondition for TSR accomplishing on-orbit services. Therefore, the resear ch on target capture and post-capture control is quite important and essential.This paper focuses on the investigation of target capture and post-capture control for TSR and the main innovations are as follows: the impact dynamics of a TSR during target capture phase is simulated and discussed. Based on the dynamic characteristics of the target capture, the stabilization controller of TSR during target capture phase is designed based on the impedance control, and the adaptive and robust control methods to stabilize the post-capture combination are investigated. The main research contents and detailed achievements of this paper are as follows:(1) The dynamic model of TSR is derived using the Lagrange equation based on the bead model of space tether, where three-axis attitude of gripper, the in-plane and out-of-plane motions of space tether are taken into account.(2) The dynamics characteristics of TSR during contact impact with target are investigated. The structure of the TSR’s gripper and the detailed capture component on the target are designed. Then the methods of detecting the collisions between the gripper and target are considered and Hertz model is introduced to describe the impact forces during the capture. Based on the previous work, the dynamics contact characteristics of TSR and target are simulated and anal yzed. The simulation results show that more beads of the space tether leads to a higher simulation accuracy; compared with straight-tether capture, the free-tether capture exerts a greater in fluence on the postion and attitude of TSR during target capture; if the relative linear or angular velocit y between the gripper and target is too high, the target may escape from the gripper and the capture mission may fail.(3) The stabilization control of TSR during target capture is studied. Given the structure of the TSR’s gripper, a position-based impedance controller is proposed. The neural network is used to estimate the uncertainty of the TSR’s dynamic model, and an adaptive and robust controller for TSR is designed. The numerical simulations suggest tha t the proposed control method can achieve stabilization of TSR during target capture; besides, the uncertaint y of the TSR can effectivel y be estimated via adaptive law, which leads to a smaller over shoot, less convergence time and higher control accuracy.(4) After a target is captured by a TSR, the new combination tumbles due to the collisions and the original spinning of target, and should be stabilized. To achieve stabilization of the combination, a robust adaptive backstepping controller is designed. An adaptive law is designed to estimate the disturbance of attitude disturbing torque and inertia uncertaint y, which is compensated in the controller design. Moreover, an auxiliary system is introduce d to reduce the saturations of the thruster and enhance the control performances. Simulation results suggest that the proposed robust adaptive controller can compensate the influences of the parameter uncertaint y, and the control process of combination is smooth with a small overshoot. Besides, the utilization of the auxiliary s ystem can effectivel y reduce the saturations of the thruster and enhance the control performance.(5) As for the stabilization of the post-capture combination with unknown system parameters, including the position of the attachment point, mass and inertial matrix, an adaptive control method is designed based on dynamic inversion theory. An adaptive law is designed to realize online estimatation of the unknown s ystem parameters and an auxiliary s ystem is used to deal with the saturations of the thruster. Simulation results show that the proposed adaptive control can achieve the stabilization of the post-capture combination, lower the saturation of the thrusters and improve the performan ce of the controller.