# Compliant Detachment of Wall-Climbing Robot Unaffected by Adhesion State
## 3 Robot System
### Mechanical Structure of Wall-Climbing Robot  
To emulate the motion performance of a gecko, we designed a gecko-mimetic wall-climbing robot with dry adhesion, called IBSS-8 (dimensions: 440 mm \(\times\) 260 mm \(\times\) 45 mm, mass: 1.98 kg with a 2600 mAh 12.1 V battery, center of mass: 18 mm to the robot chassis, battery life: 25 min on full charge), shown in Figure 3, based on the size ratio of a gecko.  
A single--limb mechanism, the core unit of robot motion, determines the reliability of adhesion and detachment flexibility. To ensure that the robot can realize flexible three-dimensional single--limb movement without redundant actuation, the single--limb design is based on the gecko limb structure. The hip and the knee active joint are used (the wrist active degree of freedom is not used). This design realizes gecko-like limb movement with flexible switching between attachment and detachment states.  
The adhesive unit (see Figure 3) used in the foot is made of polyvinyl siloxane (PVS), which is similar to gecko bristles and can, thus, stably adhere to a smooth surface. The upper surface of the foot is made of polyvinyl chloride (PVC). Foot drive types are important factors affecting the reliability of the detachment, which include active detaching foot, such as stickybot [12], and the passive detaching foot, such as CLASH [14] and Anyclimb [15]. The active detaching foot can make the detachment more reliable, but also need an additional driver to control the toes, which increases the mass of the robot, the passive detaching  
Figure 3: Bionic structure design of IBSS-8 for the body, limb, and adhesion unit. The workspace limitation and 0° position of each joint can be checked. A gecko’s adhesion unit is the bristles of the sole, and that of the robot is PVS [5]. (1) Upper surface of adhesive (PVC). (2) Gecko-like adhesive material (PVS). (3) Passive ball bearing and reset spring.  
foot is chosen in robot design. The shape of the sole also greatly affects the adhesion. The common types of the sole include the toe imitating strip foot [14] and the sole-imitating circular foot [15]. The tangential adhesion strength of the strip foot is anisotropic, while the circular foot is isotropic. The circular foot is more suitable for a multi-contact wall-climbing robot because the detachment normal impact may cause adhesion failure. To achieve the smooth detachment of the robot and provide more adhesive force, we expanded the striped foot into a rectangular foot. Combined with a series of parameters, such as the tangent adhesive strength per unit area (2.2 N/cm2) of the adhesive material, the ratio of the actual adhesion area to the foot area (20%), and the mass of the robot, the size of the adhesive foot was determined to be 90 mm \(\times\) 50 mm. During adhesion contact, the whole foot and the adhesive surface are at a 15\({}^{\circ}\) angle, and the PVC surface is forced to bend and deform, thus providing the required pre-pressure for PVS adhesion. The ball bearing on the foot provides a passive degree of freedom for foot posture self-adaptation during the stance phase. The upper part of the foot is connected to a single limb of the robot via a spring to realize the resetting of the soles. PVS provides a reliable adhesion force for robot climbing movement, but the large required detachment force affects the stability of robot climbing.