# Hybrid-Compliant System for Soft Capture of Uncooperative Space Debris
## 3 Hybrid-Compliant System for a Soft Capture of Space Debris
### Soft Capture Process  
Both the ACU and the PCU will work together towards a successful soft capture of the space debris, as described in Section 2.2. As a reminder, the main goal of the soft capture sub-phase is to absorb the impact and welcome, as softly as possible, the debris while retaining it from moving away. During the capture process, the actions of the soft capture can be depicted in four main steps, as displayed in Figure 9: the Initialization, the first contact, the hybrid compliance operation (active and passive compliance occurs simultaneously), and the adhesive activation. Video S1 attached to this paper provides a visual understanding of the described steps.  
* **Initialisation** At this moment of the process, the ADR system is already deployed, and only a relative distance \(d_{t}\) separates the servicer satellite from the debris. The servicer satellite approaches the debris with a translation motion, as depicted in the first image in Figure 9a.
* **First contact** The PCU is the part which arrives in contact with the flat surface of the debris first, with its gecko adhesive pads parallel to the debris surface, as shown in Figure 9b.
* **Hybrid Compliance Operation** As the contact is made, the flexible legs articulate instantly, as seen in Figure 9c, providing the first damping of the impact's vibrations and not being too close to an elastic collision between the two entities (where both momentum and kinetic energy are conserved). The fixed stiffness of the PCU lets the legs articulate while keeping in contact with the debris surface.  
At this time of the process, the PCU is not the only one acting; the ACU is also activated at the impact. As soon as there is contact between the SCU and the debris, the force exerted in the axis of capture on the ADR system's tip is fed into the controller of the ACU. As a result, the electromechanical linear actuators are put into action accordingly, reducing their length (as shown in Figure 9c) and thus providing an additional set of virtual springs and dampers based on the contact's force. The contact time \(t_{c}\) between the tip of the capturing system and the debris can then be controlled thanks to the ACU.  
Figure 9: Soft capture process of the proposed ADR concept: (**a**) initialisation; (**b**) first contact; (**c**) hybrid compliance operation; (**d**) adhesive activation.  
* **Adhesive Activation** The action of passive and active compliance is performed within that time frame of \(t_{c}\) seconds, giving the required theoretical time for the adhesive activation to occur, which is essential to the mission's success. The last goal of the soft capture sub-phase (retaining the debris from moving away due to the action-reaction effect) is made possible by creating adhesion on the debris surface. Within the contact time \(t_{c}\), the adhesive pad's contact sensors must send a positive signal to the ADR system's process controller and activate the pads' shrinkage, as shown in Figure 9d. Working in opposite pairs, the pads are pulled towards the longitudinal axis of the capture, towards the centre of the ADR tip's plane; shear force is necessary to adhere. That shear force is maintained to keep the adhesives activated, retaining the debris from moving away. Once the bond is created between the servicer and the debris, the soft capture sub-phase is performed, and the capture phase can proceed.