1. Introduction The 4400/550 high-grade corrugated base paper making machine newly put into operation in Linqing, Shandong Province, has 24 drive points, four load distribution groups, and three tension control points, with a total power of 2216.5 kW. The system has many information acquisition points, a large amount of communication data, and requires a short communication response time. Using Siemens S7200 series PLCs and Modbus communication cannot meet the system requirements. Therefore, a large Siemens S7-400 CPU with a 414-DP was selected, employing a PROFIBUS-DP network with high communication speed (12 Mbps) and stronger information processing capabilities. Five operation panels were set up on-site for real-time monitoring of the paper machine's operation. It can also exchange data at high speed with workshop management computers, DCS control systems, QCS control systems, and industrial Ethernet. It is a domestically advanced paper machine automation control system. 2. PROFIBUS Fieldbus Technology PROFIBUS, short for Process Fieldbus, is a fieldbus technology used for factory automation workshop-level monitoring and field device data communication and control. It enables distributed digital control and field communication from the field device level to the workshop level, providing a feasible solution for achieving comprehensive factory automation and intelligent field devices. The PROFIBUS network protocol is based on the ISO-issued OSI standard seven-layer reference model, simplifying layers three through six, thus exhibiting strong standard adaptability. Furthermore, its three modules (DP, FMS, and PA) can adapt to different application objects and communication rate requirements, offering good openness. 2.1 Paper Machine Control System Structure This control system adopts AC variable frequency drive control, with a three-level control mode, as shown in Figure 1. The first level of the drive system is the frequency converter control level. The frequency converter uses an ABB ACS600 series direct torque control (DTC) frequency converter, equipped with a closed-loop control encoder feedback board, forming a closed-loop control system. The frequency converter is also equipped with a PROFIBUS-DP communication board NPBA-12, which forms a PROFIBUS-DP fieldbus control network with the host PLC. [align=center] Figure 1 Topology diagram of the paper machine control system[/align] The second level of the drive system is the PLC control system. The PLC uses a Siemens large S7-400 with a 414-2DP CPU. The operator console uses a Siemens OP27 color operator screen. The S7-400, the frequency converter, and the OP27 operator screen form a PROFIBUS-DP fieldbus control network to complete the operation and control of the entire paper machine. The third level of the drive system is the upper-level optimization control system, which uses a Dell industrial control computer configured as a PIV1.7G/21" for monitoring the status of the entire paper machine drive system. The upper-level computer uses Siemens WinCC configuration software and can be networked with QCS upper-level computers, DCS upper-level computers, workshop management level, and plant management level via industrial Ethernet to achieve optimized and automatic control of the paper machine drive control system. 2.2 PROFIBUS DP Network Hardware Configuration: Each frequency converter is connected to the PROFIBUS-DP network via an NPBA-12 adapter card, all using fiber optic connections. Therefore, each frequency converter requires one NPBA-12. The S7-400 PLC is connected to the DP network via an RS485 interface. The PROFIBUS-DP network can be constructed using shielded twisted-pair cables. Because the system has 24 main drive points with long distances, a color operation screen OP27 is installed in different locations for easy operation and monitoring, totaling 5 units. They are connected to the S7-400 via the PROFIBUS bus. The PLC is connected. Two additional S7-200 PLCs, each equipped with an EM227 PROFIBUS-DP interface, are connected to the DP network, acting as slave stations to the master S7-400. One is used for automatic roll changing on the paper machine, and the other performs integrated electrical auxiliary control. The host computer and the S7-400 PLC communicate via PROFIBUS-DP, forming a host-server optimized control system. Furthermore, the open PROFIBUS-DP network can connect to DCS control systems, QCS control systems, and industrial Ethernet, exchanging information and data to form a modern automatic control system. 2.3 PROFIBUS-DP Network Software Configuration The host computer software is configured using Siemens WinCC configuration software. WinCC and the S7-400 form a PROFIBUS-DP network, allowing free communication without additional configuration. The S7-400 PLC uses STEP7 v5.1 programming software. STEP7 v5.1 supports multiple Windows operating systems and can run on Windows. Programming is convenient and quick under operating systems such as NT, Win98, and Win2000. The S7-200 PLC uses MicroWin programming software, and can be programmed using ladder diagram, statement list, and function chart languages ​​according to the different habits of programmers. The OP27 is a large color operation screen, equipped with a dedicated ProTool programming tool. 3. Profibus-DP communication function and transmission technology The structure orientation of the Profibus protocol is based on the ISO 7498 international standard and takes the Open Systems Interconnection (OSI) as a reference model. Profibus-DP only uses the first layer, the second layer, and the user interface. The third to seventh layers are not described [2], as shown in Figure 2. [align=center] Figure 2 Profibus-DP network seven-layer structure[/align] The first layer – PHY – specifies the line medium, type of indoor connection, and electrical characteristics. PROFIBUS-DP achieves current connection via RS485 with differential voltage output. Twisted-pair connection is used in linear topology. The second layer – MAC – describes the bus access method connected to the transmission medium. PROFIBUS-DP uses a hybrid access method. The second layer – FLC – specifies the interface to lower layers. The second layer – FMA1/2 completes the setting of specific bus parameters for the second layer – MAC and the first layer settings. PROFIBUS-DP combines master-slave communication and token communication. The system is divided into master and slave stations. Communication between master and slave stations relies on the master's query and the slave's response. Communication between master stations relies on obtaining bus control (token). Specifically, for the PROFIBUS-DP network of the Linqing 4400/550 corrugated paper machine, the command transmission diagram is shown in Figure 3: [align=center] Figure 3 Command Transmission Diagram[/align] The application of fieldbus systems largely depends on the selected transmission technology. RS-485 is the most commonly used PROFIBUS-DP technology, commonly referred to as H2. It uses shielded twisted-pair copper cable, sharing a single conductor. Each RS-485 segment can have up to 32 stations, and with repeaters, up to 127 stations. Transmission speeds range from 9.6kbps to 12Mbps. Once operational, all devices use the same transmission speed. The maximum cable length depends on the transmission speed, as shown in the attached table. 4. Three-Level System Functions 4.1 The upper computer optimization control system can record the operating status of the paper machine's drive system under various conditions, i.e., the machine speed and the speed ratio of each section, based on the different types of paper being produced. When changing the type of paper being produced or the machine speed, the computer can automatically control the PLC based on the recorded drive system operating parameters. This allows the drive system to automatically adapt to the current transmission requirements when production needs change, eliminating the need for paper machine adjustments. This improves the degree of automation and control. It can accept commands from plant-level, workshop-level management, DCS, and QCS host computers to control the working status of the drive system. It completes optimized and automatic control of the paper machine. The system has an alarm function; it automatically alarms when a fault occurs in the drive system and can automatically diagnose the cause of the fault, prompting operators on how to handle it. It can display historical curves of section speed, current, and torque. It can generate real-time and timed reports and can print them out. The computer is also equipped with a simulated paper machine circuit diagram, using animation technology to display the paper machine's operating status, giving you an immersive experience while working. It can be networked with DCS, QCS, and other systems for control. 4.2 S7-400PLC and OP27 Functions The S7-400PLC is the drive control center, whose main functions are to acquire field control signals via PROFIBUS-DP bus, speed chain control and calculation, load distribution control, and constant tension control. The PLC can complete the speed chain control processing according to process requirements and control the operating speed of each section's frequency converter via fieldbus. The system has five load distribution requirements. The PLC can dynamically adjust the load distribution coefficient based on the current and torque data collected on site to complete the load distribution function. The method for calculating the average load torque is shown in the following formula [1]. The PLC adjusts the load based on the comparison between the average expected torque m and the actual torque mli. Tension control is used in the semi-dry pressing upper and lower rollers and the winding section. Constant tension control is automatically performed through the tension feedback signal. The tension sensor is an ABB compression type (pillow type) tension sensor. The S7-400 PLC and the OP27 operation panel are connected through the PROFIBUS-DP bus. The OP27 obtains the data information collected by the PLC through the DP bus and transmits the instructions and data information to the PLC through the DP bus. The S7-400 PLC dynamically adjusts the transmission performance of each part and the constant tension control of each part, and performs real-time monitoring. An auxiliary S7-200 PLC, acting as a slave station to the S7-400 PLC, works in conjunction with the master station to complete the automatic roll changing function. This distributed DCS control system simplifies the system structure and ensures system reliability. Another PLC is installed on the operating console of the pressing section, forming a DP network with the master PLC via EM277 to complete integrated electrical auxiliary control. It controls various interlocking and protection functions. 4.3 Inverter Control Function This system uses ABB's ACS600 and ACS800 series direct torque control (DTC) inverters, equipped with photoelectric encoder speed feedback, forming a speed closed-loop control system. Each inverter is equipped with an NPBA-12, which can connect to the PROFIBUS-DP network, communicate with the PLC and operating panel, receive command information from the field operating panel, and dynamically adjust the motor operating status. 5 Conclusion Since the completion of the debugging, the paper machine has been in normal production for two months. The system is running normally and the PROFIBUS-DP network is stable and fault-free. The master station S7-400 PLC communicates normally with the slave station and communicates well with the upper optimization control system. At the same time, it communicates normally with the DCS control system and QCS control system. The paper machine can be controlled through the field operation panel. Global parameters can be modified on any operation panel, avoiding the need for operators to go back and forth between different operation panels. The system has a fast response speed and high precision. It has made good use of the superior performance of the PROFIBUS-DP network and has been affirmed by users. References [1] Meng Yanjing. Principle and Design of Variable Frequency Drive for Paper Machine. Xi'an: Shaanxi People's Publishing House, 2002. [2] Yang Xianhui. Fieldbus Technology and Its Application. Beijing: Tsinghua University Press, 1999.