1. Overview of Submerged Arc Welding Control for Rolls Submerged arc welding is an important welding method. Its advantage is that the arc light is buried during welding, minimizing its impact on people and the environment. Submerged arc welding involves three basic steps: evenly applying sufficient granular flux to the weld seam on the workpiece; connecting the contact tip and the workpiece to the two poles of the welding power source to generate a welding arc; and automatically feeding the welding wire and moving the arc and workpiece to perform welding. Automatic control technology for submerged arc welding includes two aspects: sequential control of the automatic welding process to achieve control of the three basic steps; and automatic control of the arc welding equipment and welding process, including control of the arc welding machine's output characteristics, automatic tracking of the welding process, and adaptive control of the welding process. 2. Overview of Fieldbus Fieldbus is an industrial data bus that provides a fully digital, distributed, bidirectional, and multi-point communication link between intelligent field devices and automation systems. Fieldbus technology and standards are completely open; from bus standards and product inspection to information dissemination, everything is public and accessible to all product manufacturers and users. Intelligentization, digitalization, informatization, networking, miniaturization, and decentralization represent the mainstream direction of today's automation technology development, and fieldbus is a crucial technology that drives control systems to adapt to this development direction. FCS (Fieldbus Control System) is a new generation control system following DCS (Distributed Control System) and is currently one of the hot topics in industrial automation development. Its outstanding feature is that it overcomes the defects of DCS communication, which is achieved through a closed dedicated network system. It transforms the centralized and decentralized DCS control structure into a new fully distributed structure, completely delegating control functions to the field, relying on the intelligent field devices themselves to achieve basic control functions. Fieldbus control systems utilize fieldbus technology to transform traditional distributed control systems, achieving digital information transmission and significant breakthroughs in communication quality and connection methods. The FCS for roll welding equipment abolishes the three-layer master-slave structure of "operator station-control station-field device" used in DCS, distributing the functions of input/output units and control stations to intelligent field units. A failure in any one unit only affects that unit and does not jeopardize the entire system. This thoroughly decentralized control system makes the system more reliable. The FCS (Fieldbus Control System) for roll welding equipment can adopt various network topologies, greatly facilitating the construction of automated systems and saving costs compared to DCS (Distributed Control System). Based on the advantages of FCS, this design uses fieldbus technology to form a fieldbus control system to realize the automated control of the welding equipment. 3. Hardware Design of Roll Welding Control System 3.1 Fieldbus Selection PROFIBUS fieldbus is a fieldbus standard developed and formulated by Siemens and is a popular fieldbus internationally. The PROFIBUS network protocol is based on the seven-layer reference model of the OSI standard issued by ISO, simplifying layers three to six, and has strong standard adaptability. PROFIBUS fieldbus is based on a token protocol combined with master-slave polling access control method. The master station communicates with the slave station in a master-slave manner, and the bus control right between masters is determined by the token protocol. Due to the maturity and openness of PROFIBUS fieldbus technology, as well as its cost advantages in practical applications, PROFIBUS fieldbus is adopted in the roll welding equipment control system. 3.2 Composition of Fieldbus Control System PROFIBUS-DP is more suitable for networking single production machines, and PROFIBUS-DP fieldbus technology is used for networking in this design. Both the master and slave stations of the roll welding equipment are PLCs. PROFIBUS-DP is used for communication between unit-level control devices and distributed I/O in the automation system, and is particularly suitable for communication between PLCs and field devices. PROFIBUS-DP scans 1000 I/O points in less than 1 ms, supports communication rates from 9.6 kbps to 12 Mbps, and has repeater expansion capabilities. The FCS network of the roll welding equipment adopts a bus network topology, using RS-485 standard shielded twisted-pair cable as the network medium. PROFIBUS-DP is a protocol widely used in manufacturing automation, a high-speed, low-cost communication solution that meets the speed and real-time requirements of long roll welding processes. The FCS configuration of the roll welding equipment is quite flexible, with various networking forms. The fieldbus control system of the roll welding equipment is shown in Figure 1. The implementation of the PROFIBUS network is mainly completed by a central processing unit or communication module with a PROFIBUS communication interface, including DP master stations and DP slave stations. This system is a hybrid DP system with multiple masters and slaves, and single-manufacturer equipment. The FCS consists of two master stations and two slave stations. The first type of master station primarily coordinates and controls the slave stations, while the second type monitors them. The slave stations primarily control the welding motion. Slave stations are peripheral devices without bus control rights; they only acknowledge received information or send messages when requested by the master station. Siemens S7-300 and S7-200 series PLCs are used to form the basic field control units in the FCS. The communication protocol between the CPU315-2DP, CPU224XP, and TP170A is the PROFIBUS fieldbus protocol. The CPU315-2DP PLC acts as a first-type master station, the TP170 touchscreen as a second-type master station, and the CPU224XP as a slave station. The EM277 can be used as a dedicated PROFIBUS fieldbus module connected to the CPU224XP. Communication between master stations is token-based, while communication between master and slave stations is master-slave. The two slave station systems shown in the diagram have identical components, enabling the separate processing of two short rolls and the joint processing of one long roll. 4. Software Design of Roll Welding Control System In a master-slave system, the master station and slave stations communicate using a master-slave program. The master-slave system allows the master station to send and access information to designated slave devices. These slave stations are passive nodes; the master station can send information to or retrieve information from slave stations. The S7-300 PLC program mainly performs organizational, monitoring, and coordination tasks. The S7-200 PLC includes a main program, interrupt programs, and various functional subroutines, primarily handling specific motion control and communication operations. 4.1 Software Design of a Type-A Master Station CPU315-2DP The S7-300 PLC program includes system configuration, data calculation and processing programs, and communication programs between the host and slave computers. Configuring the hardware is a crucial step in establishing a fieldbus control system network, connecting all hardware in the network into an organic network. This process is not simply a connection of lines, but rather the establishment of a network based on the PROFIBUS fieldbus communication protocol. The application software MICROWIN STEP-7 V5.0 enables hardware configuration and control software editing. The CPU315-2DP control program primarily monitors the slave station's operating status and coordinates the control of the two CPU224XP units during long roll welding. Coordinating the control of the two CPU224XP slave stations during long roll welding is a challenge. First, the CPU315-2DP sends clock signals to both CPU224XP slave stations, which can be achieved using a fieldbus with the PROFIBUS fieldbus communication protocol. Then, the real-time clocks of the two CPU224XP units are set. This allows for precise timing during long roll welding, ensuring the welding process proceeds smoothly and systematically. 4.2 TP170A Master Station Software Design: The TP170A configuration software is SIMATIC ProTool. Parameter selection and settings are performed, the welding operation interface is designed, and finally, the program blocks are stored in the CPU315-2DP memory. The operation interface allows for welding mode selection and parameter setting. The operation interface function selection page is shown in Figure 2. 4.3 Slave Station CPU224XP Software Design PLC system software generally adopts modular programming, with the entire program consisting of several relatively independent functional modules. The powerful mathematical function instructions of the PLC make complex mathematical calculations easy to implement. The S7-200 PLC control program is structurally divided into a main program and functional subroutines. The main program mainly completes operations such as parameter initialization; functional subroutines include welding machine combination position control subroutines, head movement subroutines, wire feeding control subroutines, wire pole oscillation control subroutines, current control subroutines, dual-machine long roll processing subroutines, and manual operation console signal processing subroutines. In actual control, the actions controlled by these programs are required to be coordinated and unified, not solely relying on the main program or a single subroutine for control. When processing long rolls, the two slave station CPU224XPs first receive the clock information sent by the CPU315-2DP, and then set the real-time clock. Next, they compare and determine whether the clocks of the two slave stations are consistent. If they are inconsistent, an error message is sent to the CPU315-2DP, and the CPU315-2DP resends the clock information. By utilizing the clocks of two slave machines to process long rolls according to a specific timing sequence, the welding process can be ensured to proceed normally and orderly. The subroutine control flowchart for dual-machine welding of long rolls is shown in Figure 3. 5. Conclusion The design scheme of the control system for the special equipment for submerged arc welding of rolls adopts a PLC-based PROFIBUS fieldbus control system. The application of fieldbus technology in the submerged arc welding control system can make traditional submerged arc welding processes more dynamic, making welding monitoring and control operations easier, and thus improving labor productivity.