Abstract: This article describes in detail the load distribution, speed chain, system network configuration, and communication aspects of the electrical drive control system for a paper machine using an AB PowerFlex 4 frequency converter. This fully digital control system based on a three-level control system of S7-300 PLC is suitable for the high-speed and high-reliability control performance requirements of high-speed paper machines. Keywords: Paper machine, speed chain, load distribution Introduction The paper drive control system designed in this paper uses a control network composed of an AB frequency converter and a Siemens PLC to complete the electrical control system design for the paper machine; its electrical drive control system is an AC variable frequency speed regulation control system based on a three-level control system of S7-300 PLC. 1. Requirements of the Paper Machine for the Electrical Drive Control System The overall system structure diagram of the paper machine is shown in Figure 1. Figure 1 Overall System Structure Diagram The basic requirements of the electrical drive control system for the normal operation of the paper machine are as follows: 1.1 The paper machine drive system must have a certain degree of speed stability and fast dynamic response. The steady-state accuracy is ±0.02–0.01%, and the dynamic accuracy is 0.1%–0.05%. 1.2 The working speed should have a wide and uniform adjustment range to adapt to the needs of producing different varieties and quantities. The adjustment range is between I=1:10. 1.3 The speed ratio between each transmission section should be stable and adjustable. To ensure the paper machine produces good paper sheets and improves its normal operating time, the speed of each section of the paper machine must be stable and adjustable. The speed adjustment range for each section is ±8–10%. 1.4 Creep speed. To facilitate inspection and cleaning of the polyester wire and press blanket, as well as to check the operation of each section, each section should have an adjustable creep speed of 15–30 m/min. However, such low-speed operation time should not be too long to reduce ineffective operation and mechanical wear; 1.5 For transmission components with rigid or flexible connections, in addition to maintaining the speed chain relationship, there must also be a function for dynamic load adjustment to avoid some components being overloaded and overcurrented, while others are overloaded due to light load, caused by dynamic load transfer; 1.6 Each component has micro-lifting and micro-lowering functions, and necessary display functions, such as linear speed, current, running, and fault signals. Related components have single-action and linkage functions; 1.7 The paper machine drive control system should have good interface capabilities and be able to connect to the upper-level industrial control computer and factory management computer with subsystems such as QCS control and steam control; 2. Paper Machine Control System Structure Our selection principle is: optimized design, universal program, and aesthetically pleasing interface, so that the entire control system has good stability, high reliability, and strong robustness. The paper machine control system structure diagram is shown in Figure 2. This control system adopts AC variable frequency drive control and a three-level control mode. The first stage is the drive stage, using AB series frequency converters, which form a closed-loop control system with a closed-loop control encoder feedback board. The second stage is the PLC control system, using a Siemens S7-300 PLC. The S7-300 and the frequency converter form a Modbus bus control network with a communication rate of up to 19.2 Kbit/s, and complete the mechatronics functions of automatic winding and auxiliary parts. The third stage is the upper control system, using a DELL industrial computer for monitoring the status of the paper machine drive system and realizing the automatic control of the entire paper machine. It can also be networked with the QCS system, DCS system, and plant-level management system via industrial Ethernet, enabling optimized control of the paper machine control system. Figure 2 Control System Structure Diagram 3. Design of Paper Machine Electrical Drive Control System 3.1 System Hardware Selection Based on the principles of system control accuracy, communication speed, response time, high cost-effectiveness, and high reliability, the hardware selection uses a SIEMENS S7 314 PLC and a CP340 communication processor as the main control unit to control the entire system. The host computer is a DELL industrial computer, configured as "PIV2.0G/21", used for monitoring the status of the paper machine's drive system. The frequency converter is an Allen-Bradley PowerFlex series high-performance vector frequency converter, which is compact, space-saving, and provides powerful motor speed control functions. Its maximum starting torque can reach 150% of the motor's rated torque; variable PWM allows the frequency converter to output larger current at low frequencies; digital PID function improves application flexibility; timer, counter, basic logic, and step sequence logic functions reduce hardware design costs and simplify the control scheme. In short, the ingenious design of the PowerFlex series high-performance vector frequency converter can ideally meet the high transmission performance requirements of this machine. The electrical control schematic diagram of the paper machine is shown in Figure 3. Figure 3 Electrical Control Schematic Diagram of the Paper Machine 3.2 System Software Design and Function Implementation The program uses a modular structure design, with various functions implemented through subroutines that are called in a timely manner; the program uses a cyclic scanning method to process the transmission points on the speed chain, improving program execution efficiency; the program design is highly versatile and has necessary protection functions and a certain degree of intelligence. The main program flowchart is shown in Figure 4. Figure 4 Main program flowchart 3.2.1 Speed ​​chain design (1) Speed ​​chain structure design. The speed chain structure adopts a binary tree data structure algorithm. First, the transmission points are mathematically abstracted to determine the number of each transmission point in the speed chain. This number should be consistent with the address set by the frequency converter. That is, any transmission point is determined by 3 data ("parent-child-sibling" or "parent-child-sibling") to determine its position in the speed chain and fill the corresponding value in the position register. Thus, a speed chain structure that meets the normal working needs of the machine can be formed. (2) Algorithm design. The speed chain design adopts the control method of adjusting the transformation ratio to realize the speed chain function. The press is taken as the main node in the speed chain. The speed of this point is the working speed of the paper machine. Adjusting its speed is adjusting the speed of the whole machine. The speed of other sub-points is obtained by multiplying the speed of this point by the corresponding transformation ratio. The speed adjustment signal of other sub-points is detected by the PLC. By operating the increase and decrease buttons of this part to change its speed ratio, the speed of the corresponding sub-point is changed. 3.2.2 Load Distribution Design This paper machine's transmission structure has flexible connection points in the dryer and press sections. These points require not only speed synchronization but also balanced load rates. Otherwise, one transmission point may experience overload and overcurrent, while the other may experience overpressure due to being driven, affecting normal papermaking and potentially tearing the felt, damaging the frequency converter, and other mechanical equipment. Therefore, automatic load distribution control is needed between the transmission points of these two transmission sections. Load Distribution Working Principle: Assume P1e and P2e are the rated power of the two motors, and Pe is the rated total load power, Pe = P1e + P2e. P is the actual total load power, and P1 and P2 are the actual load power of the motors, then P = P1 + P2. The system requires P1 = P * P1e/Pe and P2 = P * P2e/Pe, with a difference of ≤3%. Since motor power is a control parameter, the actual control uses the motor stator torque instead of motor power for calculation. The PLC samples the torque of each motor in each section, calculates the total load torque of each group, and calculates the expected torque value when the load is balanced based on the total load torque. The method for calculating the average load torque is shown in the following formula. Where: M[sub]L1[/sub] and M[sub]L2[/sub] are the actual output torques of the pressing and drying cylinder motors; P[sub]e1[/sub] and P[sub]e2[/sub] are the rated power of the pressing and drying cylinder motors; M is the expected average load torque. The PLC obtains the motor torque through the Modbus bus, and uses the above principle and then applies a PID algorithm to adjust the output of the frequency converter to make the torque percentages of the two motors consistent. This achieves the goal of automatic load distribution. A maximum limit value is set. If the load deviation exceeds the set value, the machine must be stopped to prevent mechanical and electrical damage. The premise of load distribution control is a reasonable speed chain structure, so that the transmission point group of load distribution is on the sub-chain structure. When the load of this part is adjusted, it does not affect other transmission points. Therefore, the speed chain structure adopts a combination of main chain and sub-chain. 3.3 System Network Configuration and Communication This system uses STEP7 software for network configuration. A project is created using STEP7, first selecting the PLC type, then adding the MPI bus, operator panel, industrial computer, and assigning network addresses to the frequency converters. In this system, the host computer and PLC belong to the first type of master station (DPM1), primarily responsible for bus communication control and management. The operator panel belongs to the second type of master station, primarily responsible for data reading and writing at each station, system configuration, and fault diagnosis. The operator panel is designed using SIEMENS ProTool software, while the host computer is designed using SIEMENS WinCC software, enabling real-time monitoring of the entire system. Communication between the host computer and PLC is via a common MPI cable. The Modbus network uses RS485 transmission technology and dedicated shielded twisted-pair cable. Real-time communication between the PLC and operator panel is achieved through data image processing. A cyclic polling method is used between the master and slave stations to complete read and write operations on the frequency converters. 3.4 Integrated Mechanical, Electrical, and Hydraulic Design of Auxiliary Controls The auxiliary electrical systems, including the integrated mechanical, electrical, and hydraulic systems, interlocking and protection mechanisms, automatic roll changing control for the paper machine, and the thin oil station lubrication system, work in coordination to ensure normal system operation and equipment safety. 5. Conclusion This paper machine has undergone nearly a year of actual operation verification at a paper mill in Shandong. The system's speed stability, dynamic response, load distribution effect, paper quality, system stability, and reliability have all been affirmed by the user. This paper machine drive control system based on AB frequency converters and S7-300 PLC is feasible and reasonable.