Abstract: This paper introduces a paper machine drive control system based on PROFIBUS DP. The system is a three-level control system consisting of a supervisor computer, PLC, and multidrive inverters. It implements the speed chain structure, load distribution, pressure closed-loop control, tension control, and related auxiliary control requirements, ensuring the stable and reliable operation of the paper machine production line. Key Words: PROFIBUS DP, PLC, speed chain, load distribution I. Project Introduction Shandong Dezhou Xingtai Paper Industry Co., Ltd. is a newly built high-grade paperboard production enterprise with leading domestic level by Shandong Zhaodongfang Paper Industry Group. Its high-grade paperboard project is a national key technological transformation "high-efficiency and high-quality" project approved by the State Economic and Trade Commission. Located in the Longmen Economic Development Zone East District, Pingyuan County, Shandong Province, the company covers an area of ​​over 500 mu (approximately 33 hectares) with a total investment of 500 million yuan. It mainly produces 100-300g/m² environmentally friendly high-grade industrial paperboard. The designed production capacity is 200,000 tons/year. The project adopts advanced production technology and equipment from Japan, Europe, and the United States, and is equipped with a domestically advanced 4400/450 paperboard machine production line, making it one of the largest production lines in China. The project's process uses American waste (European waste, domestic waste) and wood pulp as the main raw materials. See Figure (I) below. Figure (I) The 4200/450 paper machine's drive control system uses a Siemens S7-400 PLC as the main controller, five S7-200 PLCs as auxiliary controllers, and five OP270 operation panels as the system operation control panel. Since the drive system has 40 transmission points, plus the master-slave control PLC and the operation panel, the total number of nodes in the system is 50. A high-speed communication network is necessary for real-time data exchange and control. Therefore, the Siemens PROFIBUS DP high-speed fieldbus communication network is used to construct the entire drive system, ensuring rapid and timely system response and reliable operation. II. Control System Composition The structure diagram of the paper machine control system is shown in Figure (II): Figure (II) The system uses a three-level control method. The first level of the drive system is the frequency converter control level. The frequency converter uses the ABB ACS800 series multidrive controller, equipped with a closed-loop control encoder feedback board, forming a closed-loop control system. The frequency converter is also equipped with a DP communication board, forming a PROFIBUS-DP fieldbus control network with the PLC for real-time high-speed communication. The second level of the drive system is the PLC control system. The PLC uses a Siemens large S7-400 CPU 414-2DP, and the operation control uses a Siemens OP270 operation panel. The S7-400, frequency converter, OP270 operator panel, and S7-200 auxiliary PLC form a PROFIBUS-DP fieldbus control network to complete the entire paper machine operation control. The third level of the drive system is the upper-level optimization control system, using a DELL industrial control computer for monitoring the status of the entire paper machine drive system. The upper-level computer uses Siemens configuration software WINCC 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. III. Functions Performed by the Control System In the paper machine drive control system, the following main controls need to be implemented according to process requirements: 1. Speed ​​Chain Control: The speed chain structure adopts a binary tree data structure algorithm. First, mathematical abstraction is performed on each transmission point to determine the number of each transmission point in the speed chain. This number should be consistent with the address set in the drive unit (in this system, the inverter). In other words, any transmission point is positioned in the speed chain by three data points ("parent-child-elder brother" or "parent-child-younger brother"), and the corresponding values ​​are filled into the position register to form the entire speed chain structure. Algorithm design: Figure (III) shows the speed chain control structure. We take the first section of the paper machine as the main node in the speed chain, that is, its given speed determines the working speed of the entire paper machine, and adjusting its given speed adjusts the speed of the entire paper machine. In the PLC, when we detect the speed adjustment signal, we change the speed unit value. The speed at point 1 is the running speed set value of the first frequency converter, which is sent to the first frequency converter for execution and to the second frequency converter for calculation. The speed value of the first section multiplied by the transformation ratio b1/a of the second section is the given value of the second frequency converter. If the speed of the second section does not meet the operating requirements, it means that the transformation ratio of the second section is not suitable. This can be achieved by operating the acceleration and deceleration buttons of the second section. After the PLC detects the button signal, it adjusts b1 to adjust the transformation ratio to adapt to the production requirements. This is equivalent to having a high-precision gearbox inside the PLC, allowing for stepless speed adjustment. If the gear ratio is appropriate during normal production, and for some reason, paper tightening or loosening is required, pressing the corresponding paper tightening/loosening button will cause the PLC to add a positive or negative offset to the speed chain, thus achieving the paper tightening/loosening function. Point 2 in the diagram contains the speed values ​​for speed adjustment and paper tightening/loosening operation commands, which are sent to the second frequency converter for execution and simultaneously to the next level of calculation. This process continues, forming the speed chain control system. The branch design of the speed chain uses a parent-child algorithm, allowing for the construction of speed chain structures with arbitrary branches. This speed chain design not only meets the paper machine drive control requirements but also provides the possibility for subsequent computer-optimized control. The PLC has a very precise transmission ratio, designed with an accuracy of 0.001%, which can be further improved through parameter settings. With precise transmission ratios, the host computer can accurately memorize the paper machine's transmission process parameters. When changing product types or speeds, the host computer can accurately transmit the paper machine's operating parameters to the PLC, which then executes the adjustment to the current operating state. 2. Load Distribution Control: In paper machine transmission control, situations often arise where several motors simultaneously drive the same load. For example, in a two-roll press section, the upper and lower drive rolls each have their own drive motors, operating synchronously through pressurization. Therefore, synchronous motor speed alone is insufficient for such transmissions. The actual system requires the same load rate for each drive point; otherwise, one drive point may experience overcurrent due to overload, while another may experience overvoltage due to being driven, potentially causing alarms or even shutdowns, impacting production. Therefore, load distribution control is necessary between these two drive points. Load Distribution Principle: In multi-motor transmission processes, the load rate of each drive point must be the same, i.e., δ=Pi/Pie (Pi is the load power borne by motor i, and Pie is the rated power of the motor). Furthermore, the load distribution adjustment process must not affect the speed of other sections. Therefore, we adopt a design method combining the main chain and sub-chains of the speed chain. This system has a total of 11 load distribution transmission points, including 4 groups in the mesh section, 3 groups in the pressing section, 3 groups in the drying cylinder section, and 1 group in the sizing section. The PLC collects the operating torque of the load distribution points, calculates the total load torque of each group, and calculates the expected torque value for load balancing based on the total load torque, as shown in the following formula: Where: T is the expected torque value for load balancing; Pie is the rated power of each load distribution point; Ti is the actual torque of each point. The main control PLC obtains the actual torque of each point via communication through the DP bus, calculates the expected torque for load balancing using the above formula as a setpoint, and adjusts the output of the drive unit to make the actual torque values ​​of each point tend to be consistent. In the actual design, the load distribution transmission point is placed on a branch of the speed chain so that adjusting the load at that point will not affect other transmission points; at the same time, output limits must be set when adjusting the output of each load distribution point to avoid excessive speed difference causing equipment damage. 3. Hydraulic Pressure Closed-Loop Control: In most domestic paper machines, the press section pressurization and the main arm pressurization of the winding section mostly adopt open-loop control with manual valve operation, resulting in poor pressure regulation accuracy and difficulty in maintaining stable pressure. This system adopts a pressure closed-loop control system, which not only improves pressure regulation accuracy but also stabilizes the pressure, thus ensuring normal production of the paper machine. To achieve high control quality in the design, the pressure value must first be calibrated. Due to the nonlinearity of the pressure sensor itself, piecewise linearization can be used to calibrate the pressure value, ensuring consistency between the displayed value on the field pressure gauge and the value displayed on the control panel. Generally, the calibration range can be divided into three segments, as the linearity of the pressure sensor is poor at both ends and better in the middle segment. After calibrating the pressure value, pressure closed-loop control is implemented through a PID control algorithm. Appropriate tuning of the PID parameters achieves stable pressure output control. In the actual design, the actual pressure value is acquired by the S7-200 auxiliary PLC of the press section, filtered, and used for closed-loop control programming. The corresponding data is then sent to the main control PLC via the DP bus, and displayed and parameter-set on the control panel. In addition, control mechanisms such as differential pressure automatic roll lifting, manual/automatic switching, and pre-pressing/pressurization control must be considered. 4. Tension Control: Based on the paper machine's process requirements, tension sensors are added before the sizing machine and calender. The tension sensors send tension signals to the PLC, where tension is set on the control panel. The PLC adjusts based on the set tension value and the feedback value from the tension sensor to maintain constant and accurate tension. The actual tension is displayed on the control panel. The tension sensor detects the paper tension signal and sends it to the S7-400 PLC. Based on the tension setting on the control panel, it calculates and adjusts the output torque of the subsequent drive roller to maintain constant paper tension, achieving closed-loop tension control. A paper break detector is added before the tension sensor. If a paper break occurs, the system automatically exits tension control mode and switches to speed control mode. Once the paper is re-raised, the paper break signal disappears, and the system automatically switches back to tension control mode. The PLC uses a PID control algorithm with speed limiting to prevent runaway during paper breaks. The PLC analyzes the tension sensor signals, providing timely alarms and effectively preventing the impact of tension sensor malfunctions on production. Tension control can be enabled/disabled via the operation panel. 5. Automatic Roll Changing Control: To achieve automatic roll changing, the mechanical, hydraulic, pneumatic, and electrical systems of the paper machine must achieve high control accuracy and reliability; otherwise, equipment and personnel damage may occur. Therefore, 23 proximity switches are installed in relevant parts of the paper machine to detect the proper functioning of each part during roll changing. The design employs three control modes: manual, semi-automatic, and fully automatic. In manual mode, operation is purely manual. In semi-automatic mode, the entire roll changing process is divided into several consecutive stages. After each stage, the system checks whether each part is in position and provides a prompt signal. If normal, the next stage can proceed; otherwise, the system alarms and stops operation for operator intervention. In fully automatic mode, the system operates completely automatically under normal circumstances. If any part is detected as not in position, the system immediately stops operation and provides an audible and visual alarm for operator intervention. In the design, the detection signal from the proximity switch is sent to the S7-200 auxiliary PLC in the take-up unit, and then to the main control PLC via the DP bus for processing. The detection signal status and prompts are displayed on the operation screen, allowing operators to promptly monitor the automatic roll changing operation. The entire automatic roll changing program runs in the auxiliary PLC. IV. Project Operation Since its commissioning in August 2006, after a period of mechanical break-in and operator proficiency, the system's speed regulation accuracy, dynamic response, load distribution adjustment effect, and system stability have all met design requirements and production needs, ultimately gaining user approval. V. Application Experience Through practical application of hundreds of large, medium, and small Siemens PLCs in paper machine drive systems, their reliability, flexibility, and ease of use in industrial settings have been verified as highly competitive. In particular, the PROFIBUS DP bus, with its high-speed communication performance and flexible configuration, has become one of the international standard buses, and its application in the automated control systems of medium and large paper machines is becoming increasingly widespread. References [1] SIEMENS STEP7 V5.1 Programming Manual. Siemens Corporation, 1998 [2] S7-300 PL in Depth. Automation and Drives Group, Siemens (China) Co., Ltd. Beijing University of Aeronautics and Astronautics Press, 2004 [3] ACS800 Firmware Manual System Software 7.X. Beijing ABB Electric Drive Systems Co., Ltd., 2002 [4] Paper Machine Frequency Conversion Drive Principle and Design. Meng Yanjing. Shaanxi People's Publishing House