Application of automatic control systems in paper manufacturing- -

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Application of automatic control systems in paper manufacturing – With the continuous advancement of papermaking processes and equipment, there are increasing demands for automatic control in this industry, which has also led manufacturers to pay more attention to this aspect. How to select and configure automatic control systems that offer the best performance-to-price ratio in a practical manner has become a critical issue for manufacturers. In papermaking production, automatic control used to focus solely on electrical drive systems; today, its scope has expanded to include pulping control systems, pulp distribution control systems, sizing flow control systems, multi-stage aeration control systems for the drying section of paper machines (heat pump control), as well as moisture content detection systems. These are what are commonly referred to as DCS process control systems and QCS quality inspection systems, and the trend is toward centralized control of the entire factory. I. Electrical Drive Systems: In recent years, with the widespread use of foreign control devices in electrical drive systems, improvements have been seen in aspects such as the stability of speed control, dynamic response, and reliability of these systems. In particular, the adoption of AC variable-frequency control devices has brought significant benefits to manufacturers; it has resolved many of the problems associated with domestic analog control devices, which had long troubled manufacturers, thereby enhancing production efficiency. Currently, transmission systems have evolved from their initial analog configuration to fully digital network communication; the latter offers higher resolution and more precise control. Moreover, as part of the plant’s centralized control system, it facilitates connections with other related components via data networks. For this reason, the market share of controllers with weak communication capabilities or limited openness in their communication protocols is gradually declining, while high-quality controllers from well-known brands are being used more and more often. Our company has been engaged in automation project development for the papermaking industry for many years. Since the successful deployment of the first fully digital electrical drive system for a 1760/300 size paper machine at Guangning People’s Paper Mill in 1994, we have utilized advanced technology from Siemens to develop various control systems, thus creating a relatively complete range of products. On production lines for thicker paper types such as cardboard, where the paper speed is 150 m/min or less, we usually use Siemens’ MDV series of frequency converters and Siemens’ S7-200 series of PLCs, with the USS data communication protocol employed. The biggest advantage of this configuration is its simple structure and cost-effectiveness. On paper machines with a speed of 150 m/min or more, as well as on production lines for tissue paper and specialty papers, we typically use Siemens’ 6SE70 series of industrial frequency converters, along with Siemens’ S7-200 or S7-300 series of PLCs, employing the USS data communication protocol. This configuration is characterized by a simple structure and high control precision. In larger paper machines or when users have high requirements regarding the networking capabilities and configuration level of electrical drive systems, we use Siemens’ 6SE70 series of frequency converters, which represent the current state-of-the-art, along with Siemens’ 57-400 series PLCs. Communication is carried out via the PROFIBUS-DP field data bus network. The main advantages of this configuration are its high communication speed, reaching 12 MB/s, as well as its large capacity – the field data bus can support up to 127 stations. II. Plant-wide integrated control system: With the rapid development of theories related to industrial automation process control and computer technology, higher demands have been placed on such systems. The three main challenges are the increasing volume of data exchange between different levels of the system, the integration and improvement of fieldbus technologies within the system, and the need for safe and reliable software. PCS is a new generation of process control system developed under such circumstances; it is a process control system designed for the entire production process, based on Siemens’ TELEPER M series distributed control systems as well as the S5 and S7 series of PLCs. The PCS7 control system introduced by Siemens consists of SIMATIC PCS7 software and SIEMENS S7 series hardware to form a complete and unified factory automation system. It is a next-generation process control system based on the concept of fully integrated automation. 1. Control structure and communication network: The integrated control system of an entire plant is generally divided into the process control level, the automation level, and the field level. Process control level: The process control level utilizes high-performance PCs, along with standard software and networks, to enable process diagnosis, process monitoring, control, and overall plant information management. With standard display and operation windows, images and charts of various production stages can be shown. All operational parameters and device settings can be called to be modified and adjusted; all data can be copied and archived at any time, and this system is completely open. Automation level: The automation level uses CPUs equipped with either a single processor or multiple processors. Communication between the automation level and the process control level is achieved via SIMATIC NET INDUSTRIA ETHERNET, an industrial Ethernet standard. This network system can connect to 1,024 sites at a rate of 10 MB/S. Communication at the field level is achieved through the SIMATIC NET PROFIBUS-DP communication network, with a speed of up to 12 MB/S; the system can support 127 stations. At the field level, there are mainly three types of standard system products connected to the field bus. On-site operation panels OP27, OP25, TP27, etc. Operation panels such as OP27, OP25, TP27, etc., are suitable for decentralized, device-related control and monitoring. Different transmissions and transmission groups can have standard graphical displays and the same operating procedures. Control devices such as AC frequency converters. As drive and control units for motors, the 6SE70 and 6RA70 offer identical performance, a clear hardware structure, as well as straightforward methods for operation and fault diagnosis. Remote intelligent interface SIMMIC S7 ET200. The ET200M I/O series offers digital and analog I/O units with various voltages and protection levels, which can be installed either centrally or dispersed at the field site; other sensors and actuators can be connected directly to this open bus structure. PROFIBUS field bus is used for data exchange in field control and monitoring units. PROFIBUS field bus is an open field bus based on the international standard EN50170; it uses a token scheduling principle for data communication. It mainly includes PROFIBUS—DP (H2), a high-speed bus with a maximum baud rate of 12 M, and PROFIBUS—PA (H1), a low-speed intrinsically safe bus used for process control. The perfect combination of DP and PA gives the PROFIBUS field bus an advantage over other field buses in terms of structure and performance. PROFIBUS is suitable for communication between automation systems and field signal units, as well as for directly connecting transmitters, actuators, drives, and other field instruments and equipment equipped with interfaces in order to collect and monitor field signals; it replaces the traditional use of numerous transmission cables with a pair of twisted pairs. It results in significant savings on cable costs, as well as reduced time and expenses associated with construction, commissioning, and maintenance after the system goes into operation. It is worth noting that in the electrical drive systems of paper machines, foreign manufacturers seem to use the PROFIBUS-DP field data bus network for communication without exception. Undoubtedly, this will be the most widely used network in the future, as it is a field real-time control network with high reliability and security. However, due to factors such as cost, it is less commonly used in medium and small-sized paper machines. 2. Operating systems and software: The SIMATIC fully integrated automation system can currently provide a unified technical environment for all automation applications, namely: unified data management, unified communication protocols, and unified configuration and programming software. SIMATIC PCS uses WinCC as the monitoring software for the upper-level computer; the operating system is Windows NT4.0, while the communication networks consist of industrial Ethernet and PROFIBUS fieldbuses. Industrial Ethernet is used for data communication between system stations, while PROFIBUS is used for communication between I/O stations. SIMATIC PCS uses programming software and field device libraries that comply with the IEC—1131—3 international standard, offering continuous control, sequential control, and advanced programming languages. The field device library provides a large number of common field device information and function blocks, which can **simplify configuration tasks and shorten the project timeline. SIMATIC PCS7 features standard interfaces such as ODBC and OLE, and makes use of open networks like Ethernet and PROFIBUS fieldbuses, thereby offering a high degree of openness. It is easy to connect the host computer management system with control systems from other manufacturers. On the server, the operating system is Windows NT 4.0, and the control software uses the SIMATIC WINCC control system. SIMATIC WINCC is an important component of SIMATIC’s fully integrated automation system. For us, it is possible to link the symbol tables used during configuration to the variables in WINCC, and it is also possible to display the control information related to the controllers directly within the WINCC system. Features of the SIMATIC WINCC control system: In a short period of time, SIMATIC WINCC has developed into one of the most successful HMI/SCADA systems in the world. That is the Windows Control Center; it was the first process monitoring system to use 32-bit technology. Its preemptive multitasking feature is suitable for rapid response to process events. The openness and scalability of WINCC are its most prominent features. The openness is mainly reflected in terms of software: a. It integrates standard database interfaces, ODBC/SQL. When the manufacturer establishes an information and data management system across the entire plant, the data from the production site can be connected to this system through a database interface. b. It provides standard OPC and industrial ActiveX interfaces. Through interfaces, other applications can be integrated into the process interface and exchange relevant data with WINCC. The WINCC control system includes an OPC interface for connecting to other controller drivers that comply with the OPC standard; therefore, WINCC can theoretically support all controllers. c. It provides standard API programming interfaces, allowing software engineers to use other programming languages to access WINCC via these API interfaces in order to exchange data. Basic functions of WINCC: Graphic system: Through configuration, a variety of dynamic parameter control interfaces can be created. Trend archiving: Analysis of events and accidents that occur during the production process, as well as relevant trends, can all be recorded in WINCC. Information alarm: WINCC captures process information and local events, stores them in a database, and retrieves them as needed, or transfers them directly to the relevant process screens. Production reports: WINCC provides a reporting system that enables the easy generation of black-and-white and color production reports ； III. Application of PCS Systems in the Papermaking Industry by Our Company In recent years, our company has developed integrated plant control systems based on Siemens’ PCS7 system. Taking into account actual conditions, these systems employ a control structure with multiple CPUs, which effectively addresses the issues of speed and security. This approach enables the integrated plant control system to function at a local level, allowing for both centralized management and enhanced local control capabilities; as a result, excellent performance has been achieved during operation. Taking the dry paper machine at Shandong Zaiping Paper Mill as an example: 1. System control structure: This system adopts a PCS7 control structure, which features a three-layer network architecture from top to bottom: the process control level, the automation level, and the field control level. The field control level features 9 field operation stations and 7 display terminals, which respectively display and control the process parameters of local areas. In accordance with the requirements of the overall control system, the system is divided into nine components: 1) Main drive control system 2) Temperature control system 3) Pressure control system 4) Tension control system 5) Humidity control 6) Paper break detection 7) Glue application control 8) Auxiliary drive control system 9) Air circuit control system. 2. Three functional levels of the system: Field control level: 1) This level consists of 9 SIEMENS S7-type PLCs, 7 display terminals, as well as various frequency converters, intelligent process controllers, tension sensors, humidity sensors, pressure sensors, photoelectric sensors, and transmitters. 2) Powerful and easy to use. Devices such as PLCs and intelligent process controllers adopt a distributed layout based on the arrangement of equipment on site, enabling real-time response to events in the production process. This reduces the number of cables required to connect the field area to the control room, allowing for localized and modular control of the production process. It not only improves system performance and reduces risk levels but also facilitates system expansion and maintenance. 3) The field operation and display terminal (PWS) displays production data in real time via dynamic graphics (X-Y graphs), and field operators can control the on-site equipment through a touch screen. 4) Intelligent process controllers, among others, carry out automatic control of parameters such as pressure and temperature. 5) PLCs, intelligent process controllers, and transmitters exchange various real-time information by means of a field control network, connecting to a central controller to operate the PLCs. Automation control level (central control level): 1) This level is composed of SIEMENS large-scale PLCs S7-414. 2) Utilize the PROFIBUS communication protocol to establish an advanced L2-DP fieldbus network. 3) As the central control unit of the entire machine, it controls the operating status of the main drive point. 4) Achieve coordinated control among the subsystems composed of the various on-site PLCs in accordance with the process requirements. 5) Receive real-time information transmitted from the on-site PLC and intelligent process control stations. 6) Various production data are transmitted through the production management network, which connects the production management-level computers. 7) Execute instructions from the production management computer. Judge and handle overall machine operation events. Process control level: 1) This level consists of one industrial control computer. 2) The system uses SIMATIC PCS7 software, with Windows NT 4.0 as the operating system, and WinCC is employed as the monitoring software for the upper-level computer. 3) Receive production data from the central PLC via the process control network. 4) Real-time display of process parameters at the control points of the process flow. 5) Monitor the overall operation of the device, detect abnormal events, and issue alerts, warnings, and handling commands. 6) Access and modify control parameters. 7) Remote switching of operating status. 8) Fault diagnosis and report printing. 9) Save production reports and alarm information. 10) The process control computer can be connected to the plant-level management network, transmitting various production data to it and receiving various production tasks. 3. Control methods for the main control points of the paper machine: a. Main drive: 1) The main drive of the entire machine refers to B201#–B211#. 2) The motor for the B205# forming roller has not been considered yet. 3) A Siemens 6SE70 full-digit vector frequency converter is used as the drive unit, while a rotary optical encoder serves as the speed feedback unit, thereby forming a closed-loop control system. 4) The three PWS units on the field control panel, as well as the computer interface, can all display parameters such as linear speed and load, and the speed can be set and fine-tuned according to process requirements. 5) The speed of each section is specified using a cascaded approach, thereby enabling synchronous control while ensuring an appropriate speed difference between the sections. 6) The two feeding motors, B201# and B202#, are interlocked with the fiber separation drive motor D101#; the feeding motors cannot start unless motor D101# reaches its rated speed. 7) The two feeding motors are equipped with manual/automatic switch buttons; in manual mode, they start and stop independently, while in linked mode they operate alternately. 8) The speeds of the two feeding motors are controlled by the signals output by the metering detector. b. Surface temperature of the hot roller (TO1, T02, T22): 1) Infrared temperature sensors are used to measure the surface temperature. The thyristor stepless temperature control device is controlled by an intelligent temperature controller, thereby adjusting the electrical power of the heating element. 2) Display: On-site and computer interface display. 3) Settings: On-site and computer interface settings. 4) Over-limit audio-visual alarm. c. Oven air temperature control (T03-06, T10–T17): 1) Temperature sensors are used together with smart instruments to control the electric control valves V01–V03, thereby regulating the oil supply amount and thus the temperature. 2) Display: On-site and computer interface display. 3) Settings: On-site and computer interface settings. 4) Over-limit audio-visual alarm. d. Pressure control of the heat roller (P02, P04, P07): 1) Pressure sensors are installed at both ends of the heat roller to detect the line pressure. 2) Adjust the pneumatic control valve using the control instrument to maintain a constant air pressure in the air bag. 3) Display: On-site and computer interface display. 4) Settings: On-site and computer interface settings. 5) Over-limit audio-visual alarm. e. Vacuum pressure in the molding section (Y01–Y04): 1) A vacuum pressure sensor is installed at the lower part of the molding section. 2) Intelligent meters are used to adjust the opening degree of the electric valve, thereby controlling the exhaust volume and maintaining a constant negative pressure at the lower level. 3) Display: On-site and computer interface display. 4) Settings: On-site and computer interface settings. 5) Over-limit alarm. f. Tension control: 1) The tension control of the paper sheet in the entire machine is set before winding. 2) Adjust the speed of the relevant main drive using the signal from the tension transmitter to maintain a constant paper tension. 3) Display: On-site and computer interface display. 4) Over-limit audio-visual alarm. g. Humidity control: 1) The humidity control in the molding section is achieved by using humidity control instruments to regulate four solenoid valves in segments. 2) Humidity displayed at the site and on the computer interface. h. Glue spraying control: 1) The glue spraying flow rate is controlled by using a flow controller to regulate the electric valve. 2) Manual control of glue spraying air pressure. 3) The flow electric alarm and the glue spraying air valve open simultaneously. 4) The air valve closes only after the electric valve has been closed for ten seconds. 5) On-site and computer interface display and setting. 4. Settings for the computer monitoring screen: a. The monitoring screen is programmed and created under W.NT in accordance with the requirements of the DCS system screen. b. The screen highlights the entire process flow through thumbnail views. The graphic symbols for the components of the electrical system are set according to **standards**. d. Online dynamic display of the motor operation at the main drive points and key auxiliary drive points; red indicates stopped status, green indicates operating (rotating) status. Clicking to display the window allows for status switching. e. By clicking on each main drive point with the mouse, the corresponding control parameter display window can be activated, showing values such as speed and load, as well as the set values and actual values; these values can also be modified. f. Parameter bar chart and numerical display for the main drive point. g. Important solenoid valve status display: red for closed, green for open. h. Electric valve indication of opening degree; click to display the percentage of opening. i. All temperature and pressure measurement points are displayed digitally online. The control point displays the set value and actual value, and allows the set value to be modified. j. Online digital display of tension control point. k. Online dynamic display of paper break and material break detection points. The red light flashes when paper or material runs out. l. Online digital display of glue flow rate. m. Flash alarm for low level in the rubber compound tank. n. Parameter bar charts and digital displays for temperature and pressure measurement points. o. Status displayed in a matrix table for motors and solenoid valves. P. The ambient temperature, humidity, and paper machine speed are displayed in the column box at the lower left corner of the screen. q. Set a search key to dynamically lock the image, in order to analyze the instantaneous values of various parameters. The pulp and paper manufacturing process is a complex industrial process characterized by high uncertainty, non-linearity, interdependence, large time lags, and incomplete state information. At present, the level of automation in China’s papermaking industry is generally low. Manufacturers should start by working on basic aspects of automation such as the optimized control of production processes. When configuring systems, they must take into full account the requirements of management information systems, paying attention to standardization and openness in order to support the development of the enterprises.