# Hybrid-Compliant System for Soft Capture of Uncooperative Space Debris
###### Abstract  
Active debris removal (ADR) is positioned by space agencies as an in-orbit task of great importance for stabilizing the exponential growth of space debris. Most of the already developed capturing systems are designed for large specific cooperative satellites, which leads to expensive one-to-one solutions. This paper proposed a versatile hybrid-compliant mechanism to target a vast range of small uncooperative space debris in low Earth orbit (LEO), enabling a profitable one-to-many solution. The system is custom-built to fit into a CubeSat. It incorporates active (with linear actuators and impedance controller) and passive (with revolute joints) compliance to dissipate the impact energy, ensure sufficient contact time, and successfully help capture a broader range of space debris. A simulation study was conducted to evaluate and validate the necessity of integrating hybrid compliance into the ADR system. This study found the relationships among the debris mass, the system's stiffness, and the contact time and provided the required data for tuning the impedance controller (IC) gains. This study also demonstrated the importance of hybrid compliance to guarantee the safe and reliable capture of a broader range of space debris. space debris; active debris removal; impedance controller; in-orbit servicing; uncooperative satellites; gecko-inspired dry adhesive +
Footnote †: journal: Applied Sciences  
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Footnote 1: email: _Applied_  
Footnote †: journal: Applied Sciences  
as the space environment and the crucial nature of the mission are demanding. These requirements give the capturing phase the most critical role in the mission. Capturing mechanisms can interact differently with the debris; an Energy-Transfer Classification (ET-Class) was proposed in [9]. For instance, the _Impact Energy Dissipation (ET2)_ class implies a capture with a decrease of energy at the first impact. In this class, the rigid and flexible capturing methods stand out as the most promising for their reliability. The rigid capturing operation was one of the first capturing methods tried for the realization of mechanical contact in space. However, this method requires, in some cases, extremely expensive motion control since any misalignment during the contact can push the debris far away [10; 11], especially if the object is tumbling at high velocity [12]. Additionally, any rigid capturing mechanism must be lightweight and compatible with different space debris volumes [13]. As a result, the rigid capturing method is more applicable for cooperative targets that have proper docking ports [14]. In the literature, many rigid robotic structures are from single-arm to multiple-arms [15; 16]. Multiple arms are controlled by more complex control algorithms, such as sliding mode control or adaptive control [17]. For single-arm rigid capturing methods, the classical PID control approach is enough to achieve position and velocity control of the end-effector [18]. Nowadays, reinforcement learning (RL)- [19; 20], model predictive control (MPC)- [21], and \(\mathcal{H}_{\infty}\)-based [22] methods are also researched. Yet, the most crucial problem regarding rigid capturing methods remains the same, which is the difficulty of achieving robust mechanical interaction using rigid structures in space, since rigidness lacks appropriate impact energy dissipation in a frictionless environment. Therefore, both academic and industrial research are inclined to focus on flexible capturing methods rather than rigid capturing methods [23]. Regarding flexible capturing methods, shape memory alloys (SMA) and pneumatic capturing mechanisms are nowadays part of the most popular flexible mechanisms [24; 25; 26]. They can be categorized in the _ET2_ category, as capturing mechanisms of this class decrease the impact energy of the debris at the very first contact, according to the ET-Class. For example, capturing mechanisms using SMA material can fully comply with the debris geometry. Moreover, the actuation of SMA does not demand high energy consumption [27; 28].