## 3 Development of a Software for LNG Cargo Welding Automation Using Mobile Robots
### How to Ensure Seam-Tracking Precision  
As previously mentioned, a commercially available seam-tracking device with a single degree of freedom was used as the baseline. While it ensures reliability in laser welding, it has limitations in terms of the operational range. Depending on the manufacturing specifications, it can roughly achieve a linear movement of 100 mm along one axis. Therefore, as long as the positional error of the robot end effector concerning the welding line falls within the operational range, welding can proceed without issues. However, in this study, the extensive workspace of the robot led to significant positional errors even with small rotational discrepancies at the end effector. To address this issue, seam-tracking errors were periodically monitored, and corrective measures were taken in the pose of the robot when there was a substantial positional error between the target welding line and the robot's final end effector. Specifically, a dual-control routine structure was employed, as depicted in Figure 7, consisting of an external control routine with a low control cycle that corrects the position of the robot's end effector. Simultaneously, a high-speed control routine is designed to govern the end-effector device of the welding head. The low-frequency external control routine, shown in Figure 7 as (a), (b), (d), generates a corrected end effector position command, \(\bar{p}\), addressing the end effector position error without considering the welding head. Meanwhile, the high-speed control routine, composed of (d) and (e), generates real-time correction drive commands \(\Delta p_{w}\) for the seam-tracker. \(w_{s}\) represents the sensing values recognized by the commercial seam tracker. Figure 6: Membrane description file format for expressing information about membrane objects. When employing robotic laser welding in LNG cargo holds, the considerable size of the holds poses a significant mobility challenge. Simultaneously, specific issues within the laser welding process must be addressed, particularly those concerning LNG tanks equipped with membranes. The membrane served as a unit module comprising the inner walls of the LNG cargo. Although the membrane dimensions may vary, the most commonly utilized size is approximately 3000 mm in width and 1000 mm in height. Given the explosive risks associated with LNG cargo holds, the laser welding quality is of paramount importance. Achieving welding of one side of the membrane in a single pass is essential for safety. This necessitates a robot capable of laser welding a 3000 mm long profile in one uninterrupted operation, requiring a robot arm with a working range exceeding 3000 mm. The membrane incorporates special corrugations designed to accommodate the volume expansion resulting from the elevated temperature of LNG gas. These corrugations pose substantial challenges for robot-based laser welding owing to their pronounced curvatures. In conventional straight-line laser welding, the robot maintains a constant speed and adjusts the wire feed rate, laser intensity, and robot travel speed accordingly. However, determining process variables, such as wire feed speed, quantity, laser intensity, and robot travel speed for corrugations, is intricate and often involves extensive experimentation. Owing to the high curvature of the corrugations, the robot faces challenges in maintaining a constant speed along the path, and there is a possibility of collision owing to the size of the laser-welding head. Adapting to changes in the robot's path profile requires complex and crucial adjustments to parameters, such as the wire feed rate and laser intensity. Figure 8 illustrates the configuration of the robot end effector device welding the corrugations of the membrane, consisting of (a) wire feeder, (b) laser, and (c) vision sensor.