This paper presents a new methodology for on-line inertia parameters estimation for a rigid space debris captured by a tethered system, based on a new dynamics model of the system where the base satellite (chaser) and the space debris (target) are modeled as rigid bodies and the attachment points of the tether are offset from the centers of mass of the two bodies. Parameters estimation of unknown debris is critical for subsequent tasks in the space debris remediation mission, in particular, for debris retrieval and de-orbiting. In the proposed algorithm, the chaser and target are modeled as rigid bodies, the latter with unknown inertia parameters. Then, the parameters identification problem is formulated and solved in three phases. First, a coarse estimate of the target mass is obtained during the post-capture phase, while the length of tether is much longer than the offsets of base and target satellite, and the rigid body model is degenerated to a mass point model. Then,with a proper tension control scheme and the coarse estimate used as an initial guess, the debris is retrieved smoothly and a precise mass estimate is achieved during the first halfof the retrieval. Finally, when the tether is retrieved relatively short and the rigid body model is used, moments of inertia and the offsets of the space debris will be estimated with a proper tension control scheme for rigid body model.

In order to eliminate the limitation of the Space Tether-Net System in the field of maneuver and control, we propose the Maneuvering-Net Space Robot System (MNSRS) in this paper, which can capture and remove the space debris dexterously. We focus on the approaching phase towards the space debris, which is a challenging problem for the MNSRS, especially the coupled dynamics modelling and configuration control problems. Firstly the system and mission overview of the MNSRS is described in detail. After that, a coupled dynamics modelling, which divides the MNSRS into finite mass points connected with massless springs, is established to describe dynamic characteristics of the MNSRS in approaching phase. Then the configuration variation of the MNSRS in approaching phase is analyzed. Finally the configuration control of the MNSRS in approaching phase is investigated.

Tethered space robots use tethers to replace rigid arms and have more flexibility than a traditionalspace robot, which gives it wide application prospect in future on-orbit servicing missions. Before carrying out elaborate manipulations, tethered operation robots need to approach the target. In order to save fuel in the approaching phase, various coordinated control methods that employ tethers and thrusters together are investigated in the literature. However, the increasing mass of the tether and the distributed force acting on the tether will affect the position and attitude of the robot, which is neglected in previous studies and can degrade the performance of the control system. Here, in order to involve these factors, coupled dynamics and coordinated control theories are combined and applied. Firstly, a coupling dynamics model for the tethered space robot system is built based on the Hamilton principle and the linear assumption. Then, based on the dynamics model, we design an optimal coordinated controller which can minimize the fuel consumption by using the hp-adaptive pseudospectral method and the classical PD controller. Finally, the advantages of the proposed method and the performance of the designed controller are validated by the numerical simulation.

This paper proposes a visual perception system for a tethered space robot’s (TSR) automatic rendezvous from 100 to 0.15 m. The core problem, tracking the entire contour of noncooperative moving targets in real time, is emphasized in this work. Given numerous challenges in a dynamic scene, a novel feature tracking algorithm is developed, i.e., the monocular real-time robust feature tracking algorithm (MRRFT). To generate a robust target model, improved speeded-up robust features (SURF) are used to extract features from a marked target box. The tracker then uses the pyramid Kanade-Lucas-Tomasi (P-KLT) matching algorithm and eliminates mismatched points by a statistical method. The greedy snake algorithm is applied to obtain the exact location of the target box and to update it automatically. A discrete feature filter and an adaptive feature updating strategy are also designed to enhance robustness. A three-dimensional (3D) simulation and a semiphysical system are developed to evaluate the method. Numerous experiments demonstrate that the tracker can stably track satellite models with simple structures with improved accuracy and time savings than good features to track (GFTT)+P-KLT or scale invariant feature transform (SIFT)+P-KLT.

The tethered space robot (TSR) is a new concept of space robot which consists of a robot platform, space tether and operation robot. This paper presents a multi-objective optimal trajectory planning and a coordinated tracking control scheme for TSR based on velocity impulse in the approaching phase. Both total velocity impulse and flight time are included in this optimization. The non-dominated sorting genetic algorithm is employed to obtain the optimal trajectory Pareto solution using the TSR dynamic model and optimal trajectory planning model. The coordinated tracking control scheme utilizes optimal velocity impulse. Furthermore, the PID controller is designed in order to compensate for the distance measurement errors. The PID control force is optimized and distributed to thrusters and the space tether using a simulated annealing algorithm. The attitude interferential torque of the space tether is compensated a using time-delay algorithm through reaction wheels. The simulation results show that the multi-objective optimal trajectory planning method can reveal the relationships among flight time, fuel consumption, planar view angle and velocity impulse number. This method can provide a series of optimal trajectory according to a number of special tasks. The coordinated control scheme can significantly save thruster fuel for tracking the optimal trajectory, restrain the attitude interferential torque produced by space tether and maintain the relative attitude stability of the operation robot.