# Stochastic Models and Control of Anchoring Mechanisms for Grasping in Microgravity
## 2 Anchoring Mechanism  
The asteroid robot can safely land on and locomote on the asteroid with the anchoring mechanism. The configuration of the anchoring mechanisms and the robot are shown in Figure 1a-c. The anchoring mechanism is mounted on the end of the leg through a spherical hinge. The spherical hinge ensures the anchoring mechanism has appropriate passive compliance. When the robot locomotes in an unknown environment, the spherical joint can quickly adapt to the terrain to grasp the surface of the asteroid. As the only power source, the servo motor is installed on the shell of the anchoring mechanism. The driving torque is transmitted to the drive disc through the rack and pinion. Inelastic cables (not shown) passing through the holes of the anchor mechanism base connect the edge of the drive disc to the base of the microspine arrays. The base of the microspine array ensures that all the microspines are dragged the same distance on the surface of the asteroid. The steel needle fixed at the bottom of the microspine conforms to the millimeter-scale asperity. The special configuration of the microspine ensures it has appropriate flexibility in the tensile direction. The flexibility gives all microspines the opportunity to engage with asperities. The independent microspine can still travel on the asteroid surface after its adjacent microspines engaged to asperities.  
Driving the servo motor to rotate drives the rack to move up in a straight line along the positioning rod, and the drive disc fixed at the end of the rack performs the same movement. The rising of the drive disc pulls the cables, thereby dragging all the microspine arrays to retract towards the center of the anchor mechanism. The reverse rotation of the servo motor drives the rack and the drive plate fixed on the rack to move downward. As the cable tension decreases, it gradually approaches the return force of the spring compressed in the previous process. When the tension reaches the critical value, the base of the microspine array is pushed away from the center of the anchoring mechanism under the action of the return force of the recovery spring. When the deformation is zero, that is, when the microspine array returns to its initial state, the microspine is separated from the asteroid surface.  
In order to intuitively show the characteristics of the microspine array, an experiment on the anchoring mechanism was carried out, as shown in Figure 1d,e. Before the anchoring mechanism grasps the trunk, an external force acts on the microspine array of the anchoring mechanism. For the microspine array, appropriate positive pressure helps it grasp the  
Figure 1: 3D model and prototype of the anchoring mechanism. (**a,b**) are front and bottom views of the anchoring mechanism; (**c**) 3D model of asteroid exploration robot; (**d**) The anchoring mechanism grabs the trunk (**e**) The microspine array grabs the trunk.  
rough surface. Because the radius of the curvature of the trunk is small, only two or three microspine arrays directly above and below can grasp the trunk. Steel needles of other microspine arrays cannot contact the trunk because of interference. Take the microspine array in the red rectangle as an example. When the microspine array is pulled the same distance, some of the microspines are engaged to the trunk earlier, such as the four on the left. The fifth and sixth microspine on the left are engaged to the trunk later. As can be seen from Figure 1e, the second and third microspines on the right are not engaged to the trunk. This is precisely because of the independence of the microspine, that is, the travel of the microspine is not interfered with by other microspines, and all microspines have the opportunity to engage with asperities.