# Wall-Climbing Mobile Robot for Inspecting DEMO Vacuum Vessel
###### Abstract  
The vacuum vessel (VV) inside and outside inspection of the Demonstration Fusion Power Plant (DEMO) is very difficult due to various constraints, such as non-magnet effect material requirements, constrained space, and neutrons on its surfaces. We propose a design method for wall-climbing mobile robots (WMR) based on the vortex principle and investigate key technologies to meet VV inspection requirements. We developed a kinematic model based on the robot's motion control requirements and a trajectory tracking control algorithm according to the tractrix principle, enabling the robot to follow the path for autonomous inspection. The impeller is designed based on the vortex principle. The aerodynamic characteristics and structural strength of the impeller were also analysed and optimised. A sliding-mode robust pressure control system was designed for the robot's negative pressure adsorption, and its effectiveness was verified by simulation. Finally, an initial test prototype verified the structural design and vortex adsorption performance. We also addressed the potential applications of the WMR in DEMO and other fusion reactors.  
demonstration fusion power plant; wall-climbing mobile robot; trajectory tracking; sliding mode control 202  
wall because if the wall surface is rough or has a groove, the air leakage reduces the adsorption force, and the robot will fall [10; 11]. To address the effect of air leakage on the robot's adsorption force, ref. [12] designed a wall-crawling robot that can achieve rough wall-crawling by generating and maintaining a negative pressure adsorption force through the rotational inertia effect of air. Ref. [13] describes a multi-chambered WMR for concrete wall inspection. The adsorption force comprises seven controlled vacuum chambers and a large gas storage chamber, reducing the effects arising from air leakage. Ref. [14] investigated the optimisation of the magnetic adsorption unit's (MAU) structure based on virtual prototyping techniques to improve the working performance of the robot. For the WMR control system, refs. [15; 16] established a numerical simulation model of a WMR based on a computational fluid dynamics (CFD) package and studied its liquid-sealing control technology for negative pressure adsorption. Ref. [17] proposed a design method for negative absorption systems providing stable pressure at variable flow rates using CFD software. Ref. [18] designed a sliding mode control method with a sliding mode backstepping controller (SMBC) and a terminal sliding mode controller to increase the dynamic system regulation's robustness. Ref. [19] presented a small negative pressure WMR based on an ARM microprocessor system and analysed its dynamic characteristics. For the trajectory control algorithm, ref. [20] studied the intelligent path planning algorithm and obstacle avoidance of a mobile robot based on a swarm intelligence algorithm, proving the algorithm's correctness through software simulation. Ref. [21] proposed a nonlinear model predictive control (NMPC)-based trajectory tracking algorithm for mobile robots. Ref. [22] proposed a trajectory-tracking algorithm based on Improved Dual-Heuristic Dynamic Programming, and its effectiveness was verified by simulation. Integrating existing technologies and developing a WMR that meets the needs of VV inspection tasks for the confined spaces within DEMO are key points of this study.  
We introduced a WMR based on the vortex technique, which runs on all material surfaces and achieves tasks by switching the carried tool, such as the visualization tool used to inspect the weld defect and VV inner wall surface. The proposed robot has size and flexibility advantages and does not require modification or additional construction on existing VV. The design, motion control, dynamic analysis, and capability analysis are presented in this paper. Finally, a prototype was built, and the robot's functionality was approved.