# Hybrid-Compliant System for Soft Capture of Uncooperative Space Debris
###### Abstract
The most sophisticated study accomplished in this field is MEDUSA. MEDUSA has flexible arms actuated by electrical inputs that can grasp nearly any object. When a simple electrical signal triggers the nitinol wires, the arms of MEDUSA begin to adapt their shape and grasp space debris [29]. Many detailed experiments showed the great robustness of the mechanical contact. However, despite promising on-ground facility results, these capturing methods have not been tested in space yet, making their performance fuzzy for on-site space applications. In addition, more flexible mechanisms take advantage of the gecko-inspired dry adhesive to stick to the debris surface. However, they are, so far, either not suitable for small autonomous integration applications [30] or fitting a specific debris shape [31; 32]. Moreover, a critical factor for a capturing system is its ability to absorb the first impact with the target; a strong and rigid impact can lead to mechanical failure and, thus, to mission failure or debris generation. To deal with this, researchers have integrated either passive [33] or active [34] compliance into their systems. However, to ensure adequate contact time with the debris for the adhesive to stick to the debris surface during the impact, passive compliance, although essential to dissipate the impact energy, is not enough; controlled active compliance of the interaction is required [35]. To the authors' knowledge, no such hybrid system, i.e., a system with both active and passive compliance, has yet been proposed. Therefore, this paper proposes the following:  
* A concept for an active debris removal capturing phase (Section 2);
* A hybrid-compliant system for the soft capture of space debris (Section 3);
* An impedance control design for the proposed hybrid-compliant system (Section 4). The proposed flexible, versatile hybrid-compliant system of class _ET2_[9], custom-built to fit a CubeSat, is displayed in Figure 1. This new system targets a vast range of small debris, enabling a profitable one-to-many solution. In contrast to previous concepts,the mechanism's compliance is hybrid. It incorporates active (with linear actuators and impedance controller) and passive (thanks to revolute joints) compliance to dissipate the impact energy, allow adequate contact time, and successfully help capture a broader range of space debris. Impedance control (IC) is an example of active interaction control, incorporating lumped parameters [36]. For a mechanism in contact with debris, IC can regulate the relationship between the mechanism's tip position and the impact force [37; 38]. An essential part of IC is the proper tuning of its gains. By adjusting them regarding the mass of the debris to be captured, the capturing system can target a wider range of debris. In this paper, a simulation study was conducted. It presented the correlation between the debris mass, the ADR system's hybrid compliance, and the contact time, providing the required data for appropriate IC gains tuning. In addition, the necessity of hybrid compliance and the IC was validated. This paper is organized as follows. Section 2 introduces the space debris capture problem with a brief on space environment statistics, focusing on LEOs to determine which shape is the most common and must be targeted first. Additionally, the section presents the proposed concept of operations (ConOps) for an ADR Capturing Phase. Section 3 presents the proposed hybrid-compliant system, and its integration into the proposed capturing phase is described. Section 4 introduces the impedance controller, a critical component of the proposed hybrid-compliant system. Finally, Section 5 presents the simulation study and discussion of the results, and Section 6 presents the conclusions and direction of future work.