The rewinder consists of a rewinding roll (also known as a paper unwinding roll), a guide roll, a slitting mechanism, a pressure roll, and a take-up bottom roll (including a front bottom roll and a rear bottom roll), as shown in Figure 1. [IMG=Rewinder]/uploadpic/THESIS/2007/12/2007121813515839757D.jpg[/IMG] The rewinder requires the control system to: gradually reduce the pressure of the pressure roll on the paper roll; provide a reverse torque to generate constant tension on the paper surface to ensure that the paper is not broken and loose during the rewinding process, and that the finished paper is tight inside and loose outside; distribute the load to the front and rear bottom roll motors; maintain a stable linear speed over a long period and have a wide speed range and fast dynamic response. In response to this situation, this system is designed with a speed control system that uses PROFIBUS-DP fieldbus as the communication method, PLC as the control core, Siemens vector control frequency converter as the actuator, and OP panel as the human-machine interface. 1 Fieldbus and its network structure Fieldbus port [2] is a system used in production sites and microcomputer-based measurement and control equipment to realize bidirectional serial multi-node digital communication. It is also known as an open, digital, multi-point communication underlying control network. PROFIBUS fieldbus is the national industrial fieldbus protocol standard formulated by Germany in the early 1990s. The protocol structure is based on the ISO7498 international standard and takes the Open Systems Interconnection (OSI) model as a reference. The ISO/OSI protocol model is divided into 7 layers. The PROFIBUS protocol structure adopts the first physical layer, the second data link layer, and the seventh application layer of OSI. The physical layer defines the physical characteristics. It is connected to the data link layer above and the medium below. When transmitting, the physical layer encodes and modulates the information from the data link layer and drives the medium with physical signals. When receiving, the physical layer is used to demodulate and decode the signals of the medium. The data link layer defines the bus storage protocol, executes the bus communication rules, and handles error detection, error recovery, arbitration, and scheduling. The application layer defines the application functions, completes the translation of information instructions, and understands the structure and meaning of data. The user layer is the data application software. PROFIBUS bus is divided into three compatible communication methods: PROFIBUS-PA, PROFIBUS-FMS, and PROFIBUS-DP. Among them, PROFIBUS-DP (split communication system) adopts the first and second layers, so the communication speed is fast, the data transmission volume is large, and it has good scalability, making it the communication method commonly used by users. PROFIBUS-DP is an electrical network that communicates through the RS 485 bus standard interface and can have up to 126 stations. PROFIBUS-DP and PROFIBUS-PA can be connected through a DP/PA coupler or a link. PROFIBUS-DP adopts a master-slave system, where the master station periodically polls the slave station and exchanges data with it. 2 Structure of the rewinding machine control system based on fieldbus Due to the intelligence and digitization of fieldbus [3] equipment, compared with analog signals, it fundamentally improves the accuracy of measurement and control and reduces transmission errors. At the same time, due to the simplification of the system structure, the number of connections between devices is reduced, the internal functions of field instruments are strengthened, the round-trip transmission of signals is reduced, and the reliability of the system is improved. Furthermore, the field control equipment possesses self-diagnostic and simple fault handling capabilities, and transmits relevant equipment operation information and diagnostic maintenance information to the human-machine interface via digital communication. Users can monitor the operating status of all equipment, query the causes of faults, and set and modify process parameters (such as tension) according to different requirements of the rewinding machine process. Therefore, applying fieldbus technology and HMI technology to rewinding machines has become the development trend and mainstream of automatic control for rewinding machines. Thus, this system uses an S7-300 PLC as the control center, a Siemens 6SE70 vector frequency converter with powerful and rich control functions as the actuator, and an OP270 operation panel as the operation unit to form a fully digital fieldbus AC drive control system for the rewinding machine. The system design scheme is shown in Figure 2. [IMG=System Design Scheme]/uploadpic/THESIS/2007/12/20071218135205471248.jpg[/IMG] In the system, the PLC is a type 1 master station, the OP panel is a type 2 master station, the frequency converter and remote I/O are slave stations. The frequency converter performs speed closed-loop control, torque control, current control, starting link, overcurrent and overvoltage protection control, process adjustment control, fault detection and protection, motor temperature monitoring, etc. on its own drive points. The PLC, as the master station, is the control center of the entire system. It mainly realizes the functions of starting, stopping, crawling, running, micro-lifting, micro-lowering, speed chain and load distribution, tension control, etc. of the frequency converter. The OP panel realizes the stopping, crawling, running, micro-lifting, micro-lowering, jogging, tension setting operation and adjustment, and user management of the frequency converter through the PLC. 3 Network Hardware Configuration The topology of the PROFIBUS-DP network [3] has three types: pure master-slave, master-master, and multi-master-slave hybrid system. The master station includes: PLC, PC, PG programmer, OP operation panel, etc.; the slave station mainly includes: frequency converter, remote I/O, PLC, etc. As shown in Figure 2, this rewinding machine system is a multi-master-one-slave hybrid system. Therefore, the hardware configuration of the entire network includes three parts: master station configuration, slave station configuration, and OP panel configuration. The master station uses an S7-300 PLC, and the CPU is a 315-2DP. This CPU has two DP interfaces, one of which can be set as an MPI port for communication with the host computer and PG programmer, and the other DP port is used for PROFIBUS-DP network communication. The OP270 panel itself has a DP port that can be connected to the bus. The frequency converter is connected to the network through the PROFIBUS communication card CBP2 to realize communication with the PLC and OP panel. 3.1 Master Station Configuration The master station is configured in a network configuration mode. In STEP 7, create a new project, insert an S7-300 station, and select the PLC CPU model 315-2DP during hardware configuration. When configuring the PLC hardware, create a bus when placing the CPU in the rack and set the master station address to 2. In network parameter configuration, set the transmission rate to "1.5 Mb/s", the pattern to "DP", and set it as the master station (DP Master). 3.2 Slave Station Configuration: In the PROFIBUS-DP options, select IM153 and drag it onto the DP network, setting the addresses to 3 and 4, then assigning I/O addresses. Then, in the PROFIBUS-DP options, select "Vector Control DCUVC", drag it to the DP network, and define the slave station address as 5. Besides setting the network address and communication rate, the inverter's communication area also needs to be configured. Double-click the inverter slave station icon to enter the communication interface area. Select PZD as "Enterlength", and according to the data transmission requirements of the rewinder, set the MasterDrive PPO type to PPO2, the PKW communication area start address to 256, and the PZD communication area start address to 264. Then configure another slave station until all three inverter slave stations are configured. 3.3 OP Panel Configuration in the Network The OP panel can also be used as a slave station to access the DP network. However, the OP panel itself requires high real-time performance, and data modifications need to be sent promptly. If the slave station mode is used, the system data transmission method is master station query mode, which cannot meet the real-time requirements. Therefore, the master station mode is used to access the network, forming a hybrid system. In STEP7, the OP panel only needs to be dragged and dropped onto the ROBIBUS network, and the address is set to 1 by default. The entire network system is then configured, as shown in Figure 3. [IMG=Network System Configuration]/uploadpic/THESIS/2007/12/2007121813521286703X.jpg[/IMG] 4. Communication Methods of PROFIBUS-DP Networks PROFIBUS-DP network communication includes two aspects: communication between the PLC and the inverter and remote I/O, and communication between the PLC and the OP panel. Communication between master stations uses a token-based method, and communication between master and slave stations uses a master-slave sequential polling method. Slave stations cannot access master stations, and slave stations cannot access each other. Communication between the OP panel and the inverter and remote I/O is completed through the PLC. In the system, master station communication mainly involves data transmission between the PLC and the OP operation panel. The PLC belongs to the first type of master station, and the OP operation panel belongs to the second type of master station. Due to the use of the token ring method, the PLC is automatically responsible for generating tokens and managing the distribution of tokens during system initialization. The station with the token has the right to control the bus and send or read data from a certain station. The PLC first arranges the master stations on the bus into an ascending sequence according to their station numbers, and then sets the first station in the sequence as the next station after the last station, thus forming a logical loop. Communication between the PLC and the frequency converter is achieved through the CBP2 communication module board inserted into the frequency converter. The ET200M communicates with the master PLC via the IM153 interface on PROFIBUS-DP, and the PLC master periodically polls the slave stations. The PLC operates the ET2020M in the same way as the PLC's integrated I/O ports. The PLC reads and writes to the frequency converter slave stations by calling the S7-300 PLC's internal system functions SFC14 and SFC15. During configuration, 6 PZD words are used simultaneously in PPO2, used to send control words and motor set frequencies, receive frequency converter status words and frequency return values, and monitor parameters such as frequency converter voltage, current, motor torque, and motor power; 4 PKW words are used to modify motor or frequency converter parameters via the PROFIBUS-DP bus. The data transmission frame structure is shown in Figure 4. [IMG=Frame Structure Diagram of Data Transmission]/uploadpic/THESIS/2007/12/2007121813521841027M.jpg[/IMG] 5 Conclusion This fieldbus-based rewinding machine control system is an advanced control system. Because it adopts a fully digital AC drive control system composed of fieldbus and PLC technology, it effectively avoids interference, improves system control accuracy, and shortens the equipment installation and commissioning cycle. The use of an OP panel for digital operation simplifies operation, allows dynamic monitoring of the controlled equipment's operation and status, modification and setting of process parameters, and access to alarm information, making maintenance more convenient, greatly reducing the labor intensity of workers, and improving production efficiency and reliability.