# Hybrid-Compliant System for Soft Capture of Uncooperative Space Debris
## 3 Hybrid-Compliant System for a Soft Capture of Space Debris
### Passive Compliance Unit (PCU)  
The PCU, as shown in Figure 6, has two main functions: to ensure a softer impact with the debris, as well as to adhere to the debris surface, both preventing it from moving away. This unit comprises three items: six articulated legs, six spherical joints, and six adhesive pads. The choice of having six legs lies in finding the right balance between the geometry of the system, its weight, and reliability, as fewer legs would question the system's redundancy. This part is the first of the whole system to encounter the target's surface.  
* **Articulated Legs** Each articulated leg, as shown in Figure 7, is composed of three aluminium parts linked together by revolute joints: the lower leg, the upper leg, and the link between the leg and the plate. The latter separates the PCU and the ACU. The passive compliance and flexibility feature is then made possible thanks to torsional springs located in the joints of the legs. The choice of adding torsional springs is for two reasons; to have a softer impact and better safety concerns regarding the system's integrity by avoiding high compression and bending constraints.  
The torsional springs' stiffness will determine the PCU's maximum displacement in the axis of capture. This parameter plays an important role in the design of the overall hybrid compliance of the soft capture. Depending on the debris parameters (such as its mass), a too-low stiffness of the PCU could result in a longer displacement of the legs, and as a result, the system could break under the generated constraints. This point will be discussed later in Section 5.
* **Spherical Joints** To link the legs to the adhesive pads, spherical joints are integrated. They give the pads two more mechanical degrees of freedom (in our case, two free rotations and the rotation in the axis of capture blocked), thus the better possibility to adapt to the debris surface. Indeed, in the case of a slight misalignment between the chaser satellite and the debris surface, the adhesion might not occur. In that regard, ensuring the parallelism of the pads with the debris surface is of utmost importance for efficient adhesion [43].
* **Gecko Adhesive Pads** At the tip of each leg, a gecko-inspired dry adhesive [44] component is integrated as a thin layer under the pads. This dry, yet sticky, material must be activated by applying a shear force [43]. As a result, the microscopic \"hairs\" bend, creating a wider contact area between the pad and the target's surface, then making adhesion possible to many different material surfaces. These pads would also include a contact sensor [31] so that the control algorithms know exactly when to activate the adhesives. The shear force is created thanks to the active shrinkage of the legs towards the capture axis. This bio-inspired dry adhesive fits well for our case for two main reasons. Firstly, adhering within the required contact time to the debris surface is one way to avoid the action-reaction effect while creating sufficient time for securing the debris-chaser link. Moreover, selecting a dry adhesive that requires shear force to activate fulfils some of the requirements for this concept: besides being able to be used in a space environment [30], no additional normal force is required to adhere. Indeed, applying more contact force when one tries to avoid pushing the debris away sounds paradoxical. To that extent, getting a dry adhesive activated by shear force is the most suitable solution for catching space debris more reliably.