1 Introduction
Currently, the control systems in my country's papermaking industry mainly adopt distributed control systems (DCS). The controllers and field devices are connected by a large number of I/O cables, which not only increases costs but also reduces system reliability.
The control system transmits 4-20mA signals to monitor field equipment. As a result, due to the limited amount of information obtained by the controller, the online fault diagnosis, alarm, and recording functions of field equipment are relatively weak. On the other hand, it is also difficult to complete the dynamic monitoring, remote parameter setting, and modification functions of field equipment. This results in weak information integration capability and poor maintainability of the papermaking control system, which affects the factory's production efficiency and brings many inconveniences to production management.
With the development of computer network technology, serial fieldbus communication technology has penetrated into various fields of automatic control. This technology can connect programmable logic controllers (PLCs), AC/DC drivers, monitoring computers, remote I/O, and intelligent sensors to achieve distributed computer control. This improves the accuracy of detection and control, enhances the dynamic response speed of the system, and increases system security. Therefore, establishing a fieldbus-based paper machine control system is an effective way to solve this problem.
PROFIBUS is a fully digital, serial, bidirectional, multi-branch communication network used for local area networks (LANs) of instrumentation and control equipment in factories/workshops. PROFIBUS-DP is a factory automation control subset of the PROFIBUS process fieldbus protocol. Therefore, its application in paper machine control systems significantly reduces wiring and cable investment, avoids signal interference, makes the system more reliable, easier to operate, and more intuitive to monitor. Based on these reasons, Shandong Zhongmao Shengyuan Pulp Co., Ltd. adopted PROFIBUS-DP in its paperboard machine project, achieving excellent results in realizing communication and distributed control of the unit.
2. Paperboard manufacturing process analysis
Figure 1 shows a schematic diagram of a papermaking machine, which is a multi-unit, interconnected machine. The wet end includes the pulp delivery system, the wire section, and the press section; the dry end includes the dryer, the cutter, and the feeder. Pulp with suitable papermaking properties enters the pulp delivery system of the papermaking machine. After being distributed and homogenized by the pulp distributor and headbox, it flows evenly and stably onto the moving forming wire. The pulp is gradually filtered and dewatered in the wire section, forming a continuous wet paper web. When the wet paper web reaches a certain dryness, it can be peeled off the wire and sent to the press section for further dewatering. The press section consists of several sets of roller presses. The wet paper web is supported by press blankets and dewatered by mechanical squeezing between the press rollers. To maintain good dewatering performance of the press blankets, the press rollers are equipped with blanket washing devices. After passing through the press section, the dryness of the wet paper web generally reaches about 40%. Then, the wet paper web is further dewatered in an air-cushioned drying chamber. After drying, the cardboard enters the paper cutter via traction rollers, then passes through the longitudinal cutter and feed rollers into the cross-cutting section. The cross-cutting blade cuts the paper web and sends it out. The cut paper is then conveyed to the paper feeder via conveyor rollers, a high-speed conveyor belt, a low-speed conveyor belt, and a pressure belt. Finally, it is packaged and weighed, completing the entire process.
Figure 1 Schematic diagram of a paperboard machine
2.1 Requirements for steady speed
A paper machine involves multiple sections from pulp to paper, making it a multi-unit speed coordination system. The speeds of each section must be strictly coordinated. According to the process flow, the following relationships generally apply: if the speed of any one section is unstable, production cannot be maintained, and the paper web will either break or become loose. If the overall paper machine speed is unstable, the basis weight (weight of paper per square meter) cannot be guaranteed to remain constant. Therefore, it is essential that all sections of the paper machine maintain a stable speed. However, in actual operation, many interfering factors disrupt speed stability, such as fluctuations in mains voltage, frequency changes, load fluctuations, and temperature variations. The requirement for electrical drive automation control is to overcome the effects of these interferences and ensure stable machine speed.
2.2 Requirements for smooth start-up
Some parts of the paper machine require a smooth start. For example, starting the wire section too quickly will damage the copper wire; the drying section has a large transmission inertia, and starting too abruptly will break the mechanical coupling. Therefore, the entire system must be able to start smoothly, and each part must be able to start and stop independently.
2.3 Paper machine speed chain
Since each section conveys paper during the production process, according to the requirements of the papermaking process, the linear speed ratio between each section must be coordinated (the ratio of the linear speed between two adjacent sections should remain constant). Maintaining this ratio with high precision and reliability is crucial for ensuring product quality and normal production operation. Any disruption to this ratio will reduce product quality. Furthermore, this speed ratio coordination of the paper machine should be maintained when changing speed or restarting after a stop, without requiring readjustment. Secondly, this ratio coordination should have a fine-tuning function to adjust the speed difference between adjacent sections, preventing paper slack or tension during transport. The speed fine-tuning should be sensitive and reliable, without significant lag during adjustment. The ratio coordination relationship is as follows:
n1 = k1(n0 + δn0)
n2=k2(n1+δn1)
n3 = k3(n2 + δn2)
n4 = k4(n3 + δn3)
In this system, the PROFIBUS-DP process fieldbus is used in conjunction with the PLC program to control the speed chain, which avoids signal drift in the speed chain setpoint of the operational amplifier and improves stability.
3. Design of Process Automation System
3.1 Hardware Configuration
Based on the process requirements of the paperboard machine, this control system uses a PROFIBUS-DP configuration in a single master-slave mode, as shown in Figure 2. The master station is a Siemens S7-300 PLC (CPU313C-2DP), with address 2, enabling bus communication control and management, and facilitating periodic data access. The frequency converters (MM440) in the wire section, press section, dryer section, and cutter are slave stations, with addresses 3, 4, 5, 6, 7, 8, 9, and 10 respectively. The field touchscreen is connected to the PLC via the MPI port, with its address set to 1. The host computer is connected to the master PLC via a CP5611, using the default address 0. The remote I/O (ET200M) address of the paper feeder is 11. The master PLC achieves high-speed data communication with the frequency converters and the field touchscreen, completing functions such as speed chain, load distribution, and tension control throughout the paper machine's transmission process. The on-site touchscreen displays the real-time operating status of each distribution point, monitors the operation and fault status of each frequency converter, and enables full control of each drive point. The PLC receives optimized control commands from the host computer and the touchscreen in real time, automatically adjusting the speed of each section to meet production demands. Simultaneously, the PLC sends the operating parameters of each section to the host computer for timely understanding of production status. The entire system adopts PROFIBUS-DP fieldbus control technology, with all system control functions implemented through fieldbus communication. Only a single communication cable is used for transmission, eliminating traditional wiring connections. This significantly improves system reliability and saves on control cables. Furthermore, it achieves full digitalization from operation to control, completely eliminating the impact of on-site interference on the control system's operation.
3.2 Software Design
The PLC is programmed using the S7 series programming software STEP7V5.3 . This software is used to configure the system's network, such as setting communication ports, station addresses, and data transmission rates. Then, the master station S7-300 is configured with hardware. Through this configuration, the CPU313C-2DP can assign addresses to the I/O of each inverter and ET200M. From a programming perspective, the CPU313C-2DP controls the slave stations as if they were its own I/O.
The STEP 7 v5.3 software adopts a modular programming structure. The entire control program consists of OB organization blocks, FC function blocks, and DB data blocks. Control words are the basic means for the fieldbus system to control the drive units. Control words are sent to the drive units by the fieldbus controller (PLC), and the drive units perform corresponding actions according to the bit encoding instructions of the control words. Status words are words containing status information, sent by the drive units to the fieldbus controller (PLC). Organization blocks (OBs) are the interface between the system operating program and the user application under various conditions, used to control the program's operation. Different OBs have different functions. In this design, the organization blocks are OB1, OB20, OB35, OB82, OB86, OB87, OB100, OB121, and OB122.
ob1 is used for the main program loop, and it is used to design the structure of the main loop program. In the user program delay interrupt ob20, system function blocks sfc32 ("srt_dint" to start the delay interrupt), sfc33 ("can_dint" to cancel the delay interrupt), and sfc34 ("qry_dint" to query the delay interrupt status) are called. ob35 is the program loop interrupt organization block; ob82 is the diagnostic interrupt routine, receiving diagnostics from modules with diagnostic capabilities; ob86 is the rack error interrupt, and ob87 is the communication error interrupt; ob100 belongs to the startup organization block and is used for warm-up; ob121 is the program error organization block, and ob122 is the access error organization block, belonging to the fault handling organization block. ob1 is the main program, mainly responsible for system initialization, initial parameter setting, and subroutine calls. fc is a user-defined subroutine block, including function blocks for wire section control, press section control, drying section control, paper cutter control, paper feeder control, fault handling, data acquisition and processing, etc. Data blocks (db) are used to store large amounts of data or variables required for the user program to run. They are also an important means of exchanging, transferring, and sharing data between program blocks. In this system, communication between the host computer and the slave computer is mainly achieved by reading and changing the db blocks of the slave computer. The system is designed with a total of 8 db blocks, representing the actual speed data block, the set speed data block, the current data block, the clock background data block, the alarm data block, the actual temperature data block, the set temperature data block, and the cardboard size data block. By reading the db blocks of the slave computer, the host computer displays the corresponding information such as speed, cardboard size, and alarms. By changing the corresponding db block data of the slave computer through the touch screen, the production can achieve the expected results.
Figure 2 Schematic diagram of the paperboard machine system
The host computer uses Visual C++ for screen display design, obtains real-time data from the PLC via DLLs, and performs animation design, data management, report printing, fault logging, and analysis. The field touchscreen is configured and its screens are created using Siemens' HMI configuration software, ProTool v6.0 . The touchscreen screen is based on the equipment diagram and is segmented and detailed. From the touchscreen, one can easily observe the overall system diagram, individual component diagrams, and even the status of each distributed sensor. Utilizing the touchscreen's input/output, bar graphs, line graphs, characters, help information, passwords, and screen switching functions, one can observe and set the inverter's frequency and speed, as well as the current actual frequency and speed, and the paper machine's operating status.
4. Conclusion
Engineering practice has proven that this control system, employing PROFIBUS-DP network technology for distributed control, significantly reduces the workload and cost of field signal connections, improves signal transmission accuracy and flexibility, lowers system costs, and facilitates installation, commissioning, and equipment maintenance. PROFIBUS-DP networks are fast, reliable, open, and have strong anti-interference capabilities, making them suitable for various industrial control systems and the preferred network for communication between PCs, PLCs, and other intelligent field devices.
