## 2 Materials and Methods  
In this study, the schematic PAW system is shown in Figure 1. The numbers 1-6 in Figure 1 indicate the procedure sequence of the welding system. It mainly includes plasma power (NW-350 AH-III), servo motors, a computer for image processing, a Programmable Logic Controller (PLC) (Omron NX1), and a CMOS camera (BASLER acA1300-60 gmNIR) with an infrared filter. The moving component of this system consists of two servo motors to control the X and Y axes for the movement of base material and plasma torch, respectively. PLC was controlled by the plasma power source and servo motor due to sequence control on the servo motor and power source. The communication speed of a CMOS camera becomes significantly faster when using Gigabit Ethernet (GigE) without taking up a large amount of PC memory. The shutter timing has also been controlled by PLC. The welding current is synchronized with the shuttering speed of the camera to ensure proper image capture. The movement controller of the welding system communicates using Ethernet for efficient data exchange. Additionally, the welding conditions are transmitted to the PLC via RS-232C, which effectively minimizes electrical noise interference. The measurement unit detects the welding current during the welding process. Upon initiating the welding process, the PLC sends two analog signals to the plasma power source. These signals are utilized to control the plasma welding current and regulate the gas flow rate shown in Figure 1, Procedure 1. The PLC also controlled the servo motors for the XY axis. As the welding starts, the PLC controls the plate axis from the start position to the end position, which was settled before welding, as shown in Figure 1, Procedure 2. Meanwhile, the PLC gives trigger signals to capture the weld pool images shown in Figure 1, Procedure 3. After images are taken, the CMOS camera transmits the captured signals to the PC using the GigE communication shown in Figure 1, Procedure 4. The original images were transferred to the computer to be pre-processed by using the dedicated software HALCON.20.11. The welding line position data from imaging processing are transferred to PLC by using RS232C. The heat source of the plasma arc is obtained through a thermal pinch effect by the nozzle electrode. The pilot arc is initiated by applying a high-frequency voltage between the tungsten electrode and the nozzle electrode. This high voltage creates a plasma arc between the tungsten electrode and the base material, enabling the welding process to start. Then, the plasma gas and the shielding gas are used to form a narrow and fused penetration. The state of PAW when the plasma current flowed through the base material is shown in Figure 2.  
During the welding process, the center region of the weld pool is irradiated by a high-energy plasma arc. When the temperature reaches a peak, the metal will be fully melted. Due to the different reflective properties of metal surfaces at varying temperatures, the high-temperature molten metal surface appears in a higher brightness region in the  
Figure 1: Robotic welding control system. Figure 2: Principle of PAW. grayscale image. On the other hand, the edge region of the weld pool experiences a gradual decrease in temperature due to the region being further from the heat source. The metal in this region is not fully melted in the heat-affected zone; thus, the brightness gradually dims. The metal at the edge region is not fully melted. It takes the rougher surface with lower light reflectivity. In the plasma welding process, the plasma arc is typically symmetrical relative to the welding line. The symmetry of the heat source results in the heat being distributed on both sides of the welding line. The weld pool edges must be heated uniformly to form a symmetrical weld pool boundary. The materials being welded are usually homogeneous metal plates. The symmetry of the heat source ensures heat distribution on both sides of the welding line to create symmetrical molten pool edges.