Abstract: Automation of heat treatment production lines in steel plants is of great significance for improving product quality. This paper introduces the hierarchical structure design concept of automation systems and, in combination with practical experience, divides the system into hierarchical layers and control functions. Based on the communication mechanism of field control bus, the communication process between the host computer and the slave computer is explained. Keywords: PROFIBUS; fieldbus; hierarchical structure; configuration software 0 Introduction The MINI-MPM continuous rolling mill for steel pipes produces high-precision products with good surface quality, and many steel plants have adopted it. To ensure that the final product also has high quality, all subsequent processes must have corresponding advanced technology and equipment. To optimize the control of advanced equipment in the heat treatment line and reliably guarantee the advanced process, the entire processing line needs to be automated, and advanced technical measures and reliable technical equipment must be adopted to ensure system redundancy and stability. This automated production line utilizes digital communication and network technologies and adopts a hierarchical design approach, consisting of two parts: basic automation and production process automation. Basic automation is responsible for controlling the process equipment in each production area; production process automation is responsible for the entire production line, including production planning, production process monitoring and material tracking, as well as communication and interlocking between the basic automation areas. 1 System Design 1.1 Production Line Heat Treatment Process After the steel pipes are processed by the MINI-MPM unit, the heat treatment process flowchart is as follows: The process flow to be implemented is divided into: (1) Feeding area process flow; (2) Quenching-tempering area process flow; (3) Normalizing-tempering area process flow; (4) Sizing-straightening area process flow; (5) Cooling area process flow; (6) Bending degree inspection-dust collection area process flow; (7) Non-destructive testing area process flow; (8) Grinding and waste cutting area process flow; (9) Vision inspection and weighing collection area process flow; 1.2 Layered Design Concept The entire automated heat treatment production line adopts a layered structure, divided into basic automation part and process automation part. The basic automation part is controlled by PLC according to the divided areas. According to the heat treatment process, the design scope can be concentrated in the following four areas: (1) Cooling bed area 1#, 2#; (2) Cooling bed area 3#, 4#; (3) Cooling bed area 5#, 6#; (4) Grinding and cutting waste area and visual inspection and weighing collection area; Each area is equipped with a PLC control system to complete the point-to-point intelligent control on site. The PLC adopts CPU318-2DP in the S7-300 series. Since the entire processing process occupies a large area and has many control loops, in order to avoid laying a large number of control lines, Siemens ET200 modules are used to place some I/O in places that are far from the central control room and where the control points are relatively concentrated. Remote I/O is formed by using the PROFIBUS-DP communication protocol. The production process automation part connects the control systems of each area of ​​the entire production line through Ethernet, as well as the communication between each control system; completes data acquisition and statistical processing, material tracking, setting and calculating process control parameters; provides operation guidance through human-machine interface, records production data, alarms and displays operating status. The system's host computer uses an industrial computer, and the configuration software is Siemens WinCC 5.0. It communicates with the slave computer through a CP5611 communication card. 1.3 System Block Diagram 2 PROFIBUS Fieldbus Technology PROFIBUS is a fieldbus that has become increasingly popular internationally in recent years. It has a fast data transmission rate (up to 12M baud) and is widely used in many fields. The PROFIBUS network protocol is based on the seven-layer reference model of the OSI standard issued by ISO, only simplifying layers three to six, and it has strong standard adaptability. In addition, its three modules (FMS, DP, and PA) can adapt to different application objects and communication rate requirements, and have good openness. The PROFIBUS fieldbus master station communicates with the slave station in a master-slave mode. The bus control right between each master station is determined by the token protocol, and the number of nodes can reach 127. Based on the maturity and openness of PROFIBUS fieldbus technology, as well as the economic benefits after practical application, it is adopted as the underlying control bus in this automation control system. 3 System Control Function Division 3.1 Basic Automation Part 1) Control all electrical equipment within the scope of the design. 2) All electrical equipment control and interlocking programs are completed by PLC. 3) Interlocking between PLCs is achieved through both I/O wiring and network modes. 4) Safety interlocking is implemented by area and level. 5) It has three operating modes: manual, semi-automatic, and fully automatic. 3.2 Process Automation Part 1 Basic 2 Process Automation Part 1) Interlocking between basic automation systems: Interlocking signals between various devices are designed according to process requirements and equipment conditions, and are achieved through two methods: I/O signals and PLC networking. 2) Work area control zoning: Unified planning and design, separating the production supply and control of some equipment, and entrusting the control of these devices to the individual equipment manufacturers closely connected to their processes. 3) Definition and switching of control modes: According to the overall requirements of process design, three control modes are set: automatic, semi-automatic, and manual. 4) Emergency stop protection in work areas: Emergency stop is an essential device on automated production lines, generally activated in case of malfunctions and other emergencies. The area of ​​emergency stop can be defined according to the actual situation of the production line, and it does not mean that the entire line must stop once the emergency stop button is pressed. 5) Process Parameter Issuance: Since multiple process routes can be implemented on a single production line, different process routes require different operating, interlocking, and protection parameters for equipment. These parameters cannot be known in advance by individual equipment and can only be uniformly set by the process-level system when adjusting the production process. 6) Production Plan Issuance: Production plans are issued to each individual piece of equipment, such as batch number, furnace number, diameter, length, wall thickness, and quantity. 7) Equipment Setting Parameter Upload: After various production plan data and process data are downloaded to each individual piece of equipment, each individual piece of equipment performs statistical analysis, calculation, and processing of this data to determine the equipment's setting parameters and uploads these parameters to the secondary system. 8) Equipment Operating Parameter Upload: During production, key equipment parameters are monitored, such as temperature, pressure, speed, and flaw detection. Statistical analysis of this data can determine product quality and understand the equipment's operating status. 9) Equipment Operating Status Monitoring: System functions are designed and programmed on the host computer to display and monitor equipment operating parameters. 10) System Alarm Coordination and Handling: Alarm types are planned, such as emergency shutdown, equipment accidents, and equipment operating parameters exceeding warning lines. Set alarm levels, such as: Level 1 alarm, full line shutdown; Level 2 alarm, area shutdown; Level 3 alarm, warning given, operation can continue after confirmation; Level 4 alarm, only warning given. Limit alarm areas, different alarm information applies to different operation and control areas. 11) Material Tracking Data Transmission: Material tracking data transmission is a dynamic data interaction process across areas, transmitting the data of the current cycle to each piece of equipment on the production line and uploading the tracking data from each piece of equipment to the secondary system. 4 System Implementation Based on the proposed system functional requirements, the system software implementation is divided into upper-level computer and lower-level computer parts. 1) Lower-level Computer Programming Software: This system uses the SIMATIC S7-300's matching programming tool STEP 7 to complete hardware configuration, parameter setting, PLC program compilation, testing, debugging, and document processing. The user program consists of organization blocks (OB), function blocks (FB, FC), and data blocks (DB). Among them, OB is the interface between the system operation program and the application program under various conditions, used to control the program's operation. FB and FC are user subroutines. DB is a user-defined data storage area, which serves as the data interface between the host computer monitoring software and the STEP7 program in this system. 2) The host computer monitoring software, WinCC, is an integrated Human-Machine Interface (HMI) and Supervisory Control and Data Acquisition (SCADA) system. It combines Siemens' advanced technology in process automation with Microsoft's powerful software capabilities. One of its characteristics is its openness; system integrators can use WinCC as the foundation for system expansion and develop their own application software through open interfaces. WinCC provides various PLC driver software, making the connection between the PLC and the host computer very easy. Users can directly use the variable tables configured in STEP7 when connecting to WinCC, significantly reducing project time. The steps for communication between S7-300 and WinCC are as follows: First, start WinCC, create a new WinCC project, and then in Tag Management, select to add a PLC driver, setting parameters such as node name and node address (the node address must be the same as the one set in the PLC). Second, set the tags under the configured S7-300. Each tag has three settings: tag name, data type, and tag address. The most important element is the tag address, which defines a one-to-one correspondence between the tag and a specific address in the S7-300, such as an input bit, output bit, or intermediate bit. Setting the tag address can be done directly using the variable table configured in STEP7. For example, setting the tag address to Q0.0 indicates that the output address in the S7-300 is Q0.0. This method labels each piece of data that needs to be communicated between the S7-300 and WinCC, effectively completing the connection between the S7-300 and WinCC. Then, in the graphical editor, a production process monitoring screen is created using basic components or graphic library objects, and the variable tags are connected to each object, which is equivalent to connecting each object in the screen to the field equipment. 5. Conclusion This system adopts a hierarchical structure and the highly real-time PROFIBUS field control bus, enabling the comprehensive automation of the entire system to be fully realized. Using Siemens' matching configuration software WinCC, a human-machine interface is generated, allowing operators to clearly manage and optimize the production process. References: [1] Huang Jingwen. Monitoring system for underground conveyor belt in coal mine based on PROFIBUS fieldbus[J]. Electronic Technology Application, 2003, 18(3): 28-30 [2] Li Yaobi. 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