Abstract: This paper introduces a distributed control system for boilers based on Advantech Adam5511 soft PLC. This system has been applied to the control of a university heating boiler system. The system realizes automatic control and combustion optimization control of the heating boiler, which can significantly improve boiler thermal efficiency and reduce pollution. The system is also connected to the Internet through the enterprise intranet, realizing remote monitoring. Keywords: Soft PLC, Distributed Control System, Boiler Control, Internet. I. Overview Boilers are currently a major source of air pollution in cities, especially in northern cities, where the phenomenon of numerous chimneys still exists. One solution to improve this situation is to dismantle small boiler rooms with outdated equipment, low efficiency, and high pollution, and merge them into large boiler rooms for centralized heating, using advanced technologies such as computer control and frequency converters, thereby effectively reducing pollution, improving efficiency, saving energy, and improving heating quality. Many cities are currently implementing this solution. A university originally had four small boiler rooms, which were renovated and merged into one large boiler room. Four new boilers were built, including one 15-ton steam boiler and three 20-ton hot water boilers, responsible for heating the entire campus's teaching area, dormitory area, and staff quarters, as well as supplying steam to the canteen and bathhouse. Based on our years of experience designing boiler control systems, we designed the thermal control section for the new boiler system. The boiler's forced draft, induced draft, grate, and heating circulation pumps all adopt variable frequency speed control. The boiler system uses a self-designed distributed control system, realizing modern control and management of the heating boiler. This article introduces the design and implementation of this distributed control system. II. System Overall Structure The boiler system operates under high temperature and high pressure conditions, which poses certain risks and requires high reliability from the control system. Therefore, we adopted a distributed control system scheme in the system structure. The system mainly consists of three layers: the field control layer, the workshop monitoring layer, and the enterprise management layer. Advantech Adam5511 soft PLCs were selected as the field control units, with each 5511 handling the control of one boiler. The monitoring layer uses a Pentium III industrial control computer as the host computer, displaying real-time data and operation screens. The system includes a database server and a web server, allowing administrators to browse real-time and historical boiler data via the internet and optimize system operation accordingly, forming the system's management layer. Communication between the field control stations and operator stations uses an RS485 bus and MODBUS protocol; the operator stations, engineer stations, and server are connected via Ethernet. The entire system boasts high reliability and advanced control and management functions, while its cost is more than half that of imported DCS, making it the preferred solution for similar systems. The overall system structure is shown in Figure 1. [align=center] Figure 1 Overall System Structure[/align] III. System Functional Design 1) Workshop Monitoring and Management Layers The monitoring layer consists of two (or more) operator stations, one engineer station, and one server. The operator station uses an Advantech Pentium III industrial control microcomputer, primarily for displaying data and performing control operations on the boiler system. The engineer station uses a high-end Pentium IV microcomputer for setting system parameters and maintaining the system. The main function of the operator station is to provide boiler system operators with an intuitive and convenient human-machine interface. The system can have two or more operator stations, each with the same functions and serving as backups for the others. The operator station has the following display screens: * **Flowchart Screen:** Displays the field data and process parameters collected by the field control station at the corresponding positions on the flow chart, visually showing the boiler operating status and various real-time data through animation. Operators can understand the overall operation of the boiler system through this screen. * **Process Parameter Screen:** Displays each process parameter and its corresponding name and unit in real-time in the form of a data table. It can also display the calculation and cumulative results for boiler coal consumption, steam (heat) production, water consumption, etc. * **Adjustment Screen:** Displays the operating status and relevant parameters of each control loop in the form of an adjustment bar graph. It can display the manual/automatic status of the loop. Operators can easily adjust the control parameters of each control loop online using the keyboard or mouse. The alarm screen records when and where alarms occur for relevant personnel to check, and also enables safety interlock control. The historical trend screen records long-term historical trend data of the system's main process parameters, displayed as curves, providing a basis for analyzing system operation and efficiency, and troubleshooting. In addition to all the functions of the operator station, the engineer station also has parameter setting and modification functions, system maintenance functions, etc. It can set the scaling transformation coefficients of each analog measurement point, the linearization parameters of RTDs and thermocouples, orifice plate flow calculation parameters, coal feed calculation parameters, boiler and heating thermal efficiency parameters, configuration parameters of each control loop, and PID parameters, etc. The engineer station is responsible for system printing, and can print real-time alarms, historical alarm records, boiler operation logs, and historical data tables. The system transmits boiler system data and process parameters to the campus network or enterprise intranet via a web server, allowing relevant leaders to view the boiler system's operating status on the intranet and enabling remote system diagnosis and maintenance. The management layer, located in the factory manager's office, provides higher-level management functions. It allows access to system operation data via the internet, monitoring system status, and performing calculations, statistics, and optimizations. Engineering technicians and company leaders can view system data and even perform system maintenance regardless of their location. 2) Field Control Layer: The field control layer uses Advantech's Adam5511 soft PLC. This is a modular industrial control unit with a built-in DOS operating system, supports C language programming, and supports the Modbus communication protocol. Each boiler is handled by one Adam5511 for data acquisition and control, while another Adam5511 handles data acquisition and control for the system's common components. Each Adam5511 is configured with 16 analog inputs, 4 analog outputs, and 16 digital inputs/outputs. It can collect 16 points of boiler operation data, forming four closed-loop control circuits. These control circuits control the water level, steam pressure, furnace negative pressure, and blower of steam boilers, or the outlet water temperature, furnace negative pressure, and blower of hot water boilers. IV. System Software The system's operator station software utilizes the Chinese industrial control configuration software MCGS. MCGS is a fully Chinese industrial automation control configuration software that can run stably on Windows 95/98/NT operating systems. It integrates powerful functions such as animation display, process control, data acquisition, equipment control and output, network data transmission, dual-machine hot standby, engineering reports, and data and curves, resulting in a visually appealing and stable system. The system's field control station software is designed using Turbo C 3.0. The software employs a modular design approach, with a real-time database at its core. Various data acquisition, processing, calculation, and control functions are designed as function blocks. The system's real-time data and the parameters of each function block are stored in the real-time database, and the function blocks exchange data through the real-time database. New systems can be constructed by configuring on the host computer and then downloading the configuration parameters to the 5511. Data exchange between the control station and operator station uses the Modbus protocol. V. Boiler System Control Loop A boiler is a complex controlled object with highly nonlinear control loops and coupling between them. Therefore, the system employs an intelligent modified PID algorithm, combined with advanced control methods such as feedforward, to control each loop of the boiler. The control loops for small steam boilers mainly include steam pressure, drum water level, furnace negative pressure, and blast control loops; hot water boilers include outlet water temperature, furnace negative pressure, and blast control loops. The boiler's steam pressure (or outlet water temperature), furnace negative pressure, and blast control loops constitute the boiler's combustion control system. The control scheme uses steam pressure or outlet water temperature as the primary control variable, adjusting the grate speed to quickly reach the setpoint. Simultaneously, it coordinates with the air-coal ratio to control the blast volume for economical combustion. The furnace negative pressure loop maintains a slightly negative pressure within the furnace. The hot water boiler's outlet water temperature setpoint automatically corrects for changes in outdoor temperature, keeping the indoor temperature constant and achieving economical heating. The temperature setpoint curve can be adjusted according to different heating periods. The boiler water level control loop maintains a constant boiler water level. However, since the boiler water level is significantly affected by steam load, false water levels are easily generated. Therefore, a three-impulse control scheme with feedforward control of steam flow and feedwater flow is introduced into the feedwater control loop to eliminate the influence of false water levels. In addition to the conventional control loop, an automatic optimization algorithm was designed for the boiler combustion control system. During boiler operation, the optimization program automatically adjusts the air-coal ratio based on the calculated boiler thermal efficiency and combustion conditions to achieve optimal combustion, thereby achieving the goals of coal saving and pollution reduction. VI. Conclusion The distributed microcomputer control system for boilers designed in this paper has fully met the design requirements after practical use. The use of this system has brought the control and management of urban community heating boilers to a new level, significantly improving the operating efficiency and heating effect of the heating system, and demonstrating significant effects in energy saving and reducing urban pollution. Preliminary statistics based on the operating results show that after using frequency converters and the distributed microcomputer control system designed in this paper, heating boilers can save 30-40% of electricity and 3-5% of coal, recovering the investment cost within one year. It is the preferred solution for computer control systems in urban centralized heating systems. References: 1. Zhao Yongsheng et al., DMU386 New Boiler Microcomputer Control System, 1997, 4, pp30-32. 2. Zhao Yongsheng et al., Distributed Monitoring and Management System for Industrial Thermal Power Plants, Xinlangchao, 1997, 4, p13-15.