Abstract: This paper introduces a drive control system for a five-layer corrugated cardboard production line based on the Profibus DP bus. A three-level control system consisting of a host computer, PLC, and multiple drive drivers is constructed, realizing control requirements such as speed chain structure, pressure closed-loop control, tension control, and related auxiliary interlocking control, ensuring the stable and reliable operation of the production line. Keywords: Profibus DP; PLC; Speed ​​chain; Load distribution I. Introduction In recent years, China's economy has maintained a high-speed growth trend. The rapid development of domestic demand and exports of Tuopai Group's alcoholic beverages, food, and beverages has spurred rapid growth in related packaging demand. Corrugated cardboard boxes account for a considerable proportion of its packaging business. Therefore, Tuopai Group's key technological transformation project in 2007 proposed to upgrade its original production line to improve product quality and efficiency, achieving integrated management and control, and enabling the production of packaging boxes to be controlled based on the output of alcoholic beverages, food, and beverages. The corrugated cardboard production line is a production line consisting of roll paper that has been pressed into corrugations, glued, bonded and shaped, slitting and pressing, and cut into standard cardboard, and finally stacked and output. It is also called the corrugated board production line. The production line structure is shown in Figure 1[1]. [align=center] Fig.1. The five corrugated cardboard production lines[/align] The original production line used centralized drive, and each level of transmission was completed by mechanical gearbox and clutch. The control accuracy was poor, the operation was difficult for workers, and product adjustment required stopping the machine to reset the ratio, resulting in low efficiency[2]. It was decided to use a distributed control system based on programmable logic controller (PLC) and frequency converter. The PLC is used as the field acquisition and control station, and Profibus DP is used for data communication. At the same time, Ethernet can be used to form a higher management level to achieve integrated management and control[3]. II. Control System Composition The structure diagram of the cardboard production line control system is shown in Figure 2 below: [align=center] Figure 2. The structure of control system of cardboard production lines[/align] The system uses a three-level control method. The first level of the drive system is the frequency converter control level. The frequency converter adopts the Mitsubishi FR-A540 series multi-drive controller, 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 module (FR-A5NP) to complete the speed synchronization function. At the same time, relevant speed fine-tuning parameters can be set through the human-machine interface, and then the main station CPU communicates and sets with the frequency converter system through the network module, 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 adopts the Siemens large S7-400 CPU 414-2DP, which performs input and output logic processing, calculates synchronous data, and transmits relevant information to each station through the PROFIBUS-DP network. The PROFIBUS-DP master station is located in the central control room. The PLC is responsible for collecting various information through PROFIBUS-DP and displaying the entire workshop operation status on the simulation screen in real time. Once an abnormality occurs, an alarm will be triggered to remind the operator to make corresponding adjustments at the first time. The S7-400, frequency converter, OP270 operation screen and S7-200 auxiliary PLC form a PROFIBUS-DP fieldbus control network to complete the operation control of the entire production line [4][5]. The third level of the transmission system is the upper-level optimization control system, which uses DELL industrial control computer for monitoring the status of the entire paper machine transmission system. The upper computer uses Siemens configuration software WINCC, which can be connected to the QCS upper computer, DCS upper computer, workshop management level, plant management level and other network control through industrial Ethernet to realize the optimization control and automatic control of the paper machine transmission control system [6]. III. Functions of the Control System In the paperboard production line transmission control system, the following main controls need to be realized according to the process requirements. 1. Speed ​​Chain Control: The speed chain structure adopts a binary tree data structure algorithm. First, each transmission point is mathematically abstracted to determine the number of each transmission point in the speed chain. This number should be consistent with the address set in the transmission unit (in this system, the inverter). That is, any transmission point is determined by three data ("parent-child-sibling" or "parent-child-sibling") to determine its position in the speed chain, and the corresponding value is filled into the position register, thus forming the entire speed chain structure. [align=center] Fig. 3. The structure of control system of speed chain[/align] As shown in Fig. 3, we take the gluing part of the cardboard production line as the main node in the speed chain. That is, its given speed determines the working speed of the entire cardboard production line. Adjusting its given speed adjusts the speed of the entire cardboard production line. 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 then 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 gives the setpoint for the second frequency converter. If the speed of the second section does not meet the operating requirements, it indicates that the transformation ratio of the second section is unsuitable. This can be addressed 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 meet production requirements. This is equivalent to having a high-precision gearbox inside the PLC, allowing for stepless speed adjustment. If the transformation ratio is suitable during normal production, and for some reason, paper tightening or loosening is required, pressing the paper tightening or loosening button in that section will cause the PLC to add a positive or negative offset to the speed chain, thus achieving the paper tightening or 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, which can form a speed chain structure with arbitrary branches. This speed chain design not only meets the transmission control requirements of the cardboard production line but also provides the possibility for subsequent computer-optimized control. The PLC has a highly precise transmission ratio, designed with an accuracy of 0.001%, which can be further improved through parameter settings. This precise transmission ratio allows the host computer to accurately memorize the transmission process parameters of the paperboard production line. When changing product types or speeds, the host computer can accurately transmit the operating parameters of the paperboard production line 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 the gluing section, two rollers are glued together, and the upper and lower drive rollers each have their own drive motors, operating synchronously under pressure. Therefore, synchronous motor speed alone is insufficient for such transmissions. The actual system requires equal load rates for all motors at each transmission point; otherwise, one transmission point may experience overcurrent due to overload, while another may experience overvoltage due to being driven, potentially causing alarms or even shutdowns in the transmission unit, impacting production. Therefore, load distribution control is necessary between these two transmission points. Load Distribution Principle: In multi-motor drive processes, the load rate of each motor at each transmission 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 speed of other components must not be affected during load distribution adjustment. Therefore, we adopt a design method combining the main chain and sub-chain 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 practical design, placing the load distribution transmission point on a branch of the speed chain ensures that adjusting the load at that point will not affect other transmission points. Simultaneously, output limits must be set when adjusting the output of each load distribution point to prevent excessive speed differences from damaging the equipment. 3. Pneumatic Pressure Closed-Loop Control: In the original production line, corrugated pressurization and heat-sealing section main arm pressurization mostly used open-loop control with manual valve operation, resulting in poor pressure regulation accuracy and difficulty in maintaining stable pressure. This system employs a pressure closed-loop control system, which not only improves pressure regulation accuracy but also stabilizes the pressure, thus ensuring normal production. To achieve high control quality, 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 touchscreen. Generally, the calibration range can be divided into three segments, as the linearity of the pressure sensor is poor at the two ends and better in the middle. After calibrating the pressure value, a PID control algorithm is used to implement pressure closed-loop control. Appropriate tuning of the PID parameters achieves stable pressure output control. In actual design, the actual pressure value is collected, filtered, and processed by the S7-200 auxiliary PLC of the pressurizing unit, and then a closed-loop control program is designed. The corresponding data is sent to the main control PLC via the PROFIBUS-DP bus, and then displayed and parameter set via the analog screen of the touch screen. In addition, automatic differential pressure roller lifting, manual/automatic switching, pre-pressurization/pressurization control, and other control links must be considered. 4. Tension Control This system is mainly used in corrugated cardboard production lines. It can automatically detect and control the tension of the raw paper in corrugated cardboard production, thereby eliminating paper breakage caused by changes in the tension of the raw paper during production, greatly improving production efficiency and raw material utilization, and reducing production costs. According to the process requirements of the production line, a tension sensor is added before the cross-cutting machine. The tension sensor sends the tension signal to the PLC, and the tension is set on the operation screen of the control panel. The PLC adjusts according to the tension set value and the feedback value of the tension sensor to maintain a constant and accurate tension. The actual tension is displayed on the operation screen of the control panel. Tension sensors detect the tension signal of the paperboard and send it to the PLC. Based on the tension setting on the control panel, the PLC 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-engaged, 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 signal, providing timely alarms and effectively preventing the impact of tension sensor failures on production. Tension control can be enabled/disabled via the control panel. 5. Automatic Roll Changing Control: To achieve automatic roll changing, the mechanical, hydraulic, pneumatic, and electrical systems of the paperboard production line 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 shaftless paper support to detect the proper functioning of each part during shaftless paper changing. The design employs three control modes: manual, semi-automatic, and fully automatic. In "Manual" mode, operation is entirely manual. In "Semi-Automatic" mode, the entire roll changing process is divided into several consecutive stages. After each stage is completed, the system checks whether each part is in place and provides a prompt signal. If everything is 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 to be malfunctioning, the system immediately stops operation and provides an audible and visual alarm for operator intervention. The proximity switch's detection signal is sent to the S7-200 auxiliary PLC in the support unit and then to the main control PLC via the PROFIBUS-DP bus for processing. The detection signal status and prompt information are displayed on the operation screen, allowing the operator to monitor the automatic roll changing operation status in a timely manner. The entire automatic roll changing program runs in the auxiliary PLC. IV. Summary Since its commissioning in August 2007, the system has undergone a period of mechanical break-in. After operators became proficient, the system's speed regulation accuracy, dynamic response, adjustment effect, and system stability all met design requirements and satisfied production needs . V. Innovations of this Paper The application of PROFIBUS-DP in the carton production line drive system leverages its reliability, flexibility, and ease of use in industrial settings. In particular, the high-speed communication performance and flexible configuration of the PROFIBUS-DP bus facilitate the construction of a three-level control system consisting of a host computer, PLC, and multiple drive drivers. This system achieves control requirements such as speed chain structure, pressure closed-loop control, tension control, and related auxiliary interlocking control, ensuring stable and reliable operation of the production line. Field operation experiments have shown excellent results. References [1] Hong Liang. Corrugated box technology [J]. Packaging Engineering, 2007, 2: 282-285 [2] Li Fei. Rational utilization and technical transformation of corrugated production line [J]. Global Corrugated Industry, 2007, 5: 75-77 [3] Sun Ping. Programmable controller principle and application. 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