Abstract: This paper introduces a communication network design based on ProfiBus-DP and Ethernet, along with its system security protection design, suitable for the DCS control requirements of the alkali recovery evaporation section. Their implementation provides a good foundation for implementing conventional PID control and complex intelligent control in the alkali recovery evaporation section. Keywords: Evaporation section; ProfiBus; Ethernet (Industrial Ethernet); Communication 1 Introduction The paper industry is one of the important pillar industries of the national economy. Currently, alkaline pulping is a widely used pulping method in the paper industry both domestically and internationally, and alkali recovery is an important component of modern alkaline pulping. Alkali recovery can provide alkali for cooking in alkaline pulping plants, thereby achieving the goals of energy saving, consumption reduction, and environmental protection. As more and more automated equipment such as sensors and PLCs are applied to the control of the paper industry, the implementation of a communication network with high-speed data transmission has become an indispensable part of the electrical drive and control system. 2 Control Strategy for Alkali Recovery Black Liquor Evaporation Process Alkali recovery includes the evaporation section, combustion section, and causticizing section. The evaporation section evaporates the black liquor sent from the pulping workshop into high-concentration black liquor required by the combustion section through a multi-effect evaporator. The quality of evaporation directly affects the combustion effect of the concentrated black liquor in the combustion section. Therefore, the evaporation section is an important section of the alkali recovery system [1,2]. The main control objectives of the alkali recovery evaporation section are to stabilize the concentration of concentrated black liquor and reduce steam consumption. The following parameters must be controlled: the concentration and flow rate of the dilute black liquor entering the effect; the pressure and flow rate of the fresh steam entering the effect; the concentration of the concentrated black liquor exiting the effect; the vacuum degree of the secondary steam in the last effect; the level of the black liquor exiting the effect and the cooling water; and the temperature of the clear water in the condenser. In view of the characteristics of the evaporation section having many process parameters and complex control loop structure, in order to achieve reliable and flexible control requirements, DCS, i.e., distributed control system, is suitable for its control [3]. 3. System Hardware Configuration When selecting hardware equipment, the number of AI, AO, DI, and DO signals to be monitored should first be determined based on the process requirements of the alkali recovery evaporation section and the technical data provided by the plant. This will allow for the reasonable configuration of the system's I/O module units. Simultaneously, the main control unit, power supply module, operator station, engineer station, monitoring station, communication network, printer, and UPS power supply required by the DCS system should be configured. Based on the control requirements of the evaporation section and the characteristics of the controlled signals, the hardware of our designed DCS system mainly includes: a main control unit (including CPU, 100M Ethernet card, and DP master station card), power supply module, analog input module, analog output module, digital input module, digital output module, ProfiBus-DP repeater, terminal modules, etc. 4. Communication Network Structure The DCS system collects and processes field data through hardware (including controllers and I/O modules). To use this data for upper-level computer display and control adjustment, network configuration is required for this data acquisition hardware. In current industrial control network communication, the commonly used are ProfiBus fieldbus and Ethernet (industrial Ethernet). 4.1 ProfiBus Fieldbus Technology ProfiBus fieldbus technology is a standardized specification jointly developed by 13 companies and 5 research institutes, including Siemens in Germany. ProfiBus includes three compatible variants of the same type, namely ProfiBus-FMS, PA, and DP, to meet the requirements of different fields [4]. ProfiBus-FMS (Fieldbus Technology Information Specification) is designed to solve workshop monitoring level communication tasks and provide a large number of communication services; ProfiBus-PA (Process Automation) uses the extended ProfiBus-DP protocol for data transmission and describes the PA rules for field device behavior, which is suitable for process automation; ProfiBus-DP (Distributed Peripheral Devices) is used for high-speed data transmission at the field level. 4.2 Ethernet (Industrial Ethernet) With the rapid development of information technologies such as Intranet/Internet, enterprises are required to achieve comprehensive and seamless information integration from the field control layer to the management layer. Since they have the same communication protocol, Ethernet and TCP/IP can be easily integrated into IT (Information Technology) networks and can run different transmission protocols on the same bus, thereby establishing a public network platform or infrastructure for enterprises. 4.3 Communication Network Structure Selection Based on the process requirements of the evaporation section and the need for flexible and reliable control, and considering the company's future needs for information-based production management, our design combines ProfiBus fieldbus and Ethernet in the evaporation section's communication network. Redundant configuration is used for the main hardware. The communication network structure diagram is shown in Figure 1. The enterprise management-level network topology can be easily adjusted according to the company's future management needs. [align=center] Figure 1 Evaporation Section DCS Communication Network Structure Diagram[/align] ProfiBus-DP is an open system protocol with a transmission rate of 9.6Kbps to 12Mbps. The maximum transmission distance is 100m at 12Mbps and 400m at 1.5Mbps, which can be extended to 10km using repeaters. Twisted-pair cable can be used as the transmission medium. Input/output modules are connected to the ProfiBus-DP line to achieve information transmission with the main control unit of the control cabinet. The main control unit has a built-in CPU, 100M Ethernet card, and DP master station card, etc., to complete data exchange between ProfiBus and Ethernet. The host computer (master/redundant server) transmits information to the main control unit of the control cabinet via 100M Ethernet and 100M redundant Ethernet using the TCP/IP protocol. This means the host computer sends operation commands to the control station and simultaneously sends information collected from the field back to the host computer. Ethernet, with a transmission speed of 100Mbps, offers advantages such as high transmission speed, low power consumption, ease of installation, and good compatibility. Thus, the ProfiBus-DP network enables data transfer between the main control unit and the field I/O units; Ethernet is used for information transfer between the host computer and the main control unit. Therefore, real-time monitoring and control of the industrial field are achieved through the host computer (master/slave server and monitor) and the main control unit of the control cabinet, and the communication network of the evaporation section combines the advantages of high-speed data transmission of ProfiBus-DP and the topology convenience of Ethernet to the enterprise management level. 5. Software Configuration The system's software configuration and settings are mainly implemented using Helishi's ConMaker (controller) software and FacView (human-machine interface) software. ConMaker is a development platform for developing control schemes. Based on the process control requirements of the evaporation section, we developed the MACS_PRG main program and related subroutines. The subroutines mainly include: data conversion, I/O module definition, conventional PID control algorithm, intelligent PID control algorithm, and main steam flow control. Using FacView, the necessary field equipment monitoring pages can be displayed on the monitor. The entire evaporation section includes the following pages: main menu, evaporation section I, evaporation section II, hardware configuration diagram, system status diagram, hardware alarms, process alarms, alarm disabling, analog quantity trends, comparison trends, operation logs, and report outputs. Subsequently, we used FacView to establish a connection with ConMaker through label variables, thus creating a real-time monitoring system based on SmartPro-FacView to monitor the field equipment. 6 System Safety Protection Design To ensure the safe and reliable operation of the evaporation section, in addition to software protection such as permission settings, we also implemented system hardware mechanical protection design. Therefore, even when the system malfunctions, the evaporation section remains in a safe control state. The system safety protection design mainly includes: (1) Communication cable shielding: The communication cable shielding design is adopted to solve the problem of poor ProfiBus-DP communication. In addition, in actual construction, the ProfiBus-DP cable is kept away from the strong power area as much as possible to avoid electromagnetic interference; (2) Effective isolation design: All IO units adopt reliable opto-isolation technology, so that there is no electrical connection between the CPUs of each unit and between the unit and the host computer, thereby improving the anti-interference ability, reliability and security of the system. Isolation measures are also provided between different channels in the same unit to eliminate the damage to the system caused by the ground potential difference on site; (3) Comprehensive redundancy design: Redundancy configuration is adopted for the main hardware parts, such as the controller and Ethernet network. When the continuously executed diagnostics detects the failure of important hardware parts, it automatically switches to the backup part and transmits the alarm information to the operator station. The redundant configuration equipment mainly includes: controller (CPU) and its supporting cards, communication network, power supply unit and I/O cards, etc. 7 Conclusion Through on-site debugging, the distributed control system of the alkali recovery evaporation section based on the ProfiBus-DP and Ethernet communication network has been put into operation in several paper mills in Shandong and Henan since 2002. The operation results show that the communication network based on ProfiBus-DP and Ethernet is stable, simple in structure and easy to maintain, providing a good foundation for the implementation of conventional PID control and advanced intelligent control in the alkali recovery evaporation section. References [1] Wang Mengxiao, Sun Yu, Tang Wei. Measurement and control system and engineering of pulp and paper process [M]. Beijing: Chemical Industry Press, 2003 [2] Zhang Ke, Yu Zhengqian. Technical guide for alkali recovery of wheat straw pulp [M]. Beijing: China Light Industry Press, 1999 [3] Yang Ning, Zhao Yugang. Distributed control system and fieldbus [M]. Beijing: Beijing University of Aeronautics and Astronautics Press, 2003 [4] Yang Xianhui. Fieldbus technology and its application [M]. Beijing: Tsinghua University Press, 1999