# Tracked Wall-Climbing Robot for Calibration of Large Vertical Metal Tanks
## 1 Introduction
Figure 1: Working principle of the optical reference line method (ORLM): (**a**) Top view of the tank (**b**) side view of the tank (**A**–H horizontal stations, **a**–f vertical stations, **1**—horizontal weld seams, **2**—pulley, **3**—optical reference line, **4**—magnetic trolley, **5**—tank surface, **6**—optical device, **7**—operator #1, **8**—operator #2, **9**—reference circumference). When the trolley is moving vertically to reach the measuring stations, its location can only be confirmed visually, which results in deviations at large elevation angles. The conventional ORLM has exhibited great potential for the applications of wall-climbing robots (WCR), which could solve the above problems. A number of types of WCRs capable of working in the vertical plane with different adhesion and locomotion mechanisms have been studied. Several major adhesion mechanisms are identified as six types. The magnetic adhesion employs permanent magnetization or electrical magnetization for climbing [3; 4; 5; 6; 7]. The suction/propulsion adhesion generates a pressure difference for climbing on the smooth surface [8; 9]. The mechanical grasping adhesion utilizes grasping mechanisms like a bio-inspired spine or tiny hook for climbing [10; 11]. The chemical adhesion makes use of adhesive tapes on the robot feet to climb on the vertical surface [12]. The thermoplastic adhesion employs a kind of special material which provides temperature-dependent adhesion forces for climbing [13], and the electrostatic force is also studied for surface adhesion [14]. With respect to the locomotion mechanisms, six categories are classified, which are arms and legs [15; 16], wheels [4; 5; 6], tracks [3; 7], wires and rails [17], sliding frame [18] and sandwich configuration [19]. Tables 2 and 3 show the comparisons of the adhesion and locomotion mechanisms, respectively. Typically, the design of the adhesion and locomotion mechanisms, which determine the payload, moving velocity, and energy supply, vary according to the working conditions and working content. In a number of applications where the WCRs are designed for heavy work, such as rust-removal [3; 20], wall cleaning [8; 9; 17], and automatic welding [4; 21], the robots are optimized for high loads and high energy consumption to ensure the working progress. The locomotion and adhesion mechanisms of these robots could be large-sized and heavy, and a number of types of locomotion and adhesion mechanisms could even be combined to meet specific requirements. On the other hand, different design concepts when designing WCRs for inspection and measuring, such as portability, low energy consumption, and inspection capacity, could be of more practical benefit to researchers. The WCRs used for capacity calibration or inspection of large vertical metal tanks are types of WCR designed specifically for inspecting and measuring. As the walls of the tanks are made of steel, permanent magnets, with the advantage of significant adhesion capacity without power, are widely \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Type** & **Payload Capacity** & **Limitation** & **References** \\ \hline Magnetic & High & Ferromagnetic surfaces only & [3; 4; 5; 6; 7] \\ Suction and Propulsion & High & Not suitable for rough surfaces & [8; 9] \\ Mechanical Grasping & High & Not suitable for smooth and flat & [10; 11] \\ Chemical Adhesion & Low & Vulnerable to dust & [12] \\ Thermoplastic Adhesion & Low & Slow state-changing process, & [13] \\ Electrostatic Adhesion & Low & Low robustness & [14] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of adhesion mechanisms.