Cooperative approach control of tethered robots for space debris capture

Conventional tether-net systems deployed from a chaser spacecraft are essentially uncontrolled after ejection, which leads to limited pointing accuracy and a very short capture window. This significantly reduces the probability of a successful capture. To overcome these limitations, this study investigates a tethered space net robot (TSNR) system consisting of a flexible tether-net and four small maneuvering spacecraft that are referred to as TSNRs, each connected to a corner of the tether-net. This study aims to develop a cooperative approach control method that allows a tethered robot system to autonomously track a target debris object without requiring knowledge of its absolute position or velocity. The proposed framework combines vision-based relative state estimation and cooperative control. The position of the debris relative to the spacecraft is estimated using stereo vision constructed from multiple monocular cameras mounted on the spacecraft. The desired formation for the spacecraft surrounding the debris is determined based on the relative positions of the spacecraft and the net center. The optimal rotational orientation of the formation is computed such that the TSNR system configuration approaches an ideal square arrangement. To achieve both debris tracking and inter-spacecraft distance regulation, a control strategy based on the artificial potential-field method is introduced. Potential functions are defined for attraction toward the goal formation and for maintaining appropriate distances between TSNRs, including upper and lower bounds that prevent tether overstretching or collisions between TSNRs. Numerical simulations are conducted to evaluate the effectiveness of the proposed approach. The results demonstrate that the TSNRs can cooperatively deploy a net, maintain a stable formation, and pursue approaching debris while avoiding collisions between them.