## 3 Development of a Software for LNG Cargo Welding Automation Using Mobile Robots
### Fundamental Functions of the Developed Software
Once the MOI was set, it was necessary to determine the actual seam for laser welding, as represented by (f) in Figure 4. In conventional laser welding using robots, the seam has a continuous trajectory; however, in this development, it is expressed as a sequential arrangement of membrane edges. This is because actual welding involves overlapping welding between the membrane edges. Thus, among the two overlapped membrane edges, that above is the welding target. Generally, the lower membrane among the overlapped membranes in the vertical direction and the left membrane among the overlapped membranes in the horizontal direction were considered, although the opposite was also possible. While it is possible to arbitrarily determine the order by considering only the robot's workspace without considering the quality of welding, in practice, the order is typically determined based on field expertise considering welding quality. It is essential to note that laser welding of the internal membranes of the target LNG cargo window was previously performed by humans, and various techniques for high-quality welding have been developed. This development software does not consider separate methods for improving the welding quality, aiming to capture the actions of human operators as much as possible to maintain the quality of welding. Figure 4: Designed software structure for LNG cargo laser welding process automation. Figure 5: Topology description file to express the mutual positions between membranes. Through the preceding process, the target and sequence for robot welding were determined. However, the trajectory of the welding target seam was not considered in the previous steps, and it was addressed in the (h) robot command generation step shown in Figure 4. For the specific generation of robot trajectories, the development software defines the (e) Membrane Description File (MDF). Figure 6 illustrates the concept of MDF. MDF has the membrane type as the first field and the membrane shape as the second field. The membrane shape provides information about the CAD file for the 3D simulation. The most crucial part of MDF is the edge information section. Every membrane has top, bottom, left, and right edges; for each edge, the actual robot-assisted laser welding method is included in the edge information. Edge information can contain two types of data: the trajectory information of the edge for robot simulation and the robot command set for the actual welding operation. To achieve high-quality welding, the motion of the robot arm, synchronized laser intensity, and angles are crucial, as determined through the preliminary experiments. The welding method and procedure for all edges in all directions were predetermined based on the membrane type. The development program outputs the preset robot system operation in sequential order according to the determined seam-welding sequence in Figure 4f. In the simulation, the simulated robot follows the sequentially arranged robot trajectory, allowing confirmation of the robot's movement and posture, but does not account for the welding quality.