# Tracked Wall-Climbing Robot for Calibration of Large Vertical Metal Tanks
## 1 Introduction  
With the increasing demand for energy and the rapid worldwide growth of the petroleum industry, large vertical metal tanks and various other types of storage tanks, which serve as the basic vessels for storage and turnover of crude oil, have been constructed in large quantities [1]. The tanks undergo mandatory capacity calibrations at set intervals or after maintenance according to the requirement of Chinese national standard JG 168-2005 (Verification Regulation of Vertical Metal Tank Capacity Sector/Industry) and American Petroleum Institute (API) standard. The accuracy of the capacity calibrations is related not only to the trade settlement, cost accounting, and energy consumption of the domestic companies, but also to the trade fairness between countries and the credibility of national measurement. For example, the throughput of oil, natural gas, and their products at ports above designated size in China was 1.066 billion tons in 2018 [2] and, if there was a 0.1% error in capacity calibration, the measurement error would be a million tons. The radial deviation measurement is one of the primary steps of the capacity calibration of the large vertical metal oil tanks, which can be performed through several methods, as shown in Table 1.  
The liquid fill method and the strapping method, which are very time-consuming and inconvenient, are used only on small tanks or as a reference. The electro optical distance ranging method (EODR) is wildly used because of its convenience, but when the tanks are built closer, the EODR instrument must be placed near the tanks, which leads to the large measuring elevation angle, as well as the large ranging error. The optical reference line method (ORLM) is considered as one of the main methods in Chinese national standard JJG 168-2005 and API standard chapter 2.2B, because of its accuracy and wide applicability. In the ORLM, the tank is divided into horizontal and vertical stations, as is shown in Figure 1. The reference circumference is determined by the strapping method. Each ring has two vertical stations which are 1/4 or 3/4 below the horizontal weld seam to obtain its averaged deviation from the reference. In each horizontal station, the magnetic trolley moves vertically and stops at every vertical station where the reference deviation is recorded by the optical device. Finally, the reference circumference and the deviations are used for the calculation of the volume. There remain a number of drawbacks in the conventional ORLM:  
1. This method requires 2-3 persons to spend more than 2 h to perform the task, and one of them has to install the pulleys at the top of the tanks (more than 20 m in the large tanks) which poses a significant risk to the operator. 2. At least one operator is required to pull the rope to lift the trolley and maintain its balance manually. As there could be as many as 200 measuring stations, it could lead to operator fatigue. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Method** & **Advantage** & **Disadvantage** & **References** \\ \hline Liquid Fill Method & Direct and very accurate & Time-consuming excessively & JG 168-2005 \\ \hline Strapping Method & Accurate & Scaffolds is needed, lots of & API 2.2A \\ \hline Optical Reference Line & Accurate, easy to operate & & \\ Method (ORLM) & relatively, low requirements & & A small amount of \\  & for equipment & & aerial work \\ \hline Electro Optical Distance & Automated, fewer operators & Not suitable for rusty & \\ Ranging Method (EODR) & are needed & surface, increased ranging & API 2.2D \\  & are needed & error with an increase of the & \\  & & measuring elevation angle. & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Advantages and disadvantages of different methods.