Overview A water treatment plant's water production system is a complex nonlinear system with multiple variables (such as flow rate, temperature, and pressure), multiple tasks (such as water delivery, chemical dosing/chlorination control, screen and sludge removal control, filter control, effluent pump start/stop control, pressure control, and data management), and multiple devices (such as motors, valves, and pumps), exhibiting time-varying, coupled, and stochastic characteristics. Figure 1 illustrates the water purification process flow. Most existing water treatment plant systems in China use outdated conventional instrumentation and manual operation. In recent years, computer control systems using DCS technology have replaced conventional instrumentation and manual operation, achieving some progress in realizing decentralized automatic control and centralized monitoring of water production. However, DCS-based systems have some significant drawbacks. Firstly, traditional DCS systems are self-contained distributed systems, making it difficult to achieve inter-device interaction and sharing, as well as between the system and the outside world, turning automation systems into "information silos." Secondly, the one-to-one physical connections and analog signal transmission between field sensors and data acquisition units in traditional DCS systems result in extensive wiring, causing significant problems for on-site construction. Furthermore, the signal transmission's anti-interference capability is poor. With the rapid development of computer, communication, and control technologies, especially the establishment of the Open Systems Interconnection (OSI) protocol and the widespread application of the TCP/IP protocol, advanced control models are constantly emerging in the industrial control field, gradually forming enterprise information systems based on network-integrated automation systems. Fieldbus (FIELDBUS) is a new technology that has developed in response to this trend. Based on fieldbus technology, we designed a fieldbus water production system for a water plant. It uses the PROFIBUS fieldbus as its core, intelligent PLCs as the control body, and an IPC-610 industrial computer and a PC as an integrated platform for system programming, configuration, maintenance, monitoring, and management. This system effectively solves the problems existing in traditional DCS systems, realizing the integration of water production site network communication with computer control systems and industrial television monitoring systems. Fieldbus Technology Fieldbus is an advanced control technology developed internationally in the mid-1980s. Currently, the most popular fieldbuses internationally include PROFIBUS, Foundation Fieldbus (FF), LonWorks bus, and CAN bus. Many automation technology manufacturers worldwide have launched products supporting a mainstream fieldbus standard. PROFIBUS is a standard specification jointly developed by Siemens and 80 other companies. In recent years, PROFIBUS has become the leading fieldbus application, widely used in manufacturing automation, process automation, building automation, and other fields. PROFIBUS is a fieldbus technology with multiple protocols coexisting; data from different protocols is transparently transmitted within the system, making PROFIBUS a highly effective fieldbus technology for connecting the past, the field, and the future. Therefore, PROFIBUS is the natural choice in the design of the water plant network control system. System Network Structure Design The core of fieldbus technology is network technology. The computer network of the control system is a real-time network, and information processing must meet the requirements of real-time performance, integrity, consistency, and reliability. Based on the characteristics of the water production system, the system adopts an integrated structure of Enterprise LAN and fieldbus, realizing the integration from field measurement and control to the water plant's internal information management system, as well as integration with external information interaction. The entire system comprises management workstations, production scheduling workstations, monitoring engineer stations, monitoring operator stations, and data management servers, forming the system's production and scheduling, planning and management, and decision support information network layer. This layer transmits management data, which is relatively large in volume but not requiring high real-time performance, so Ethernet is used. To improve system reliability and prevent data loss and loss of control due to communication cable failures, industrial Ethernet is used for interconnection, giving the system redundancy. The network effectively integrates all functional information related to water plant decision-making, planning, management, operation, and scheduling, making all functional departments and workshops of the water plant a unified whole. Simultaneously, the network also allows access to source water information, enabling large-scale data sharing. The main functions of each computer in the system network are as follows: The production scheduling workstation automates production scheduling in each workshop of the water plant and automatically generates water production arrangement business orders; the management workstation manages real-time and historical data, generates and prints various statistical information reports necessary for water plant management; the data management server uses a large database, SQL Server 2000, to manage data, providing standard SQL queries and connecting via ODBC, DDE, or other databases; the monitoring engineer workstation sets and modifies system process parameters, alarm limits, configures equipment, and displays and queries historical data on system operation, production, and alarms; the monitoring operator workstation performs data acquisition and processing, data storage and exchange, and display. The field control network layer transmits real-time data, which is small in volume but highly real-time. Therefore, PROFIBUS is used for the field control network. The main control PLCs can interconnect via PROFIBUS-FMS and also via Ethernet. The main control PLCs and the sub-control PLCs are connected via PROFIBUS-DP fieldbus. Each sub-control PLC can operate independently. PROFIBUS-DP enables distributed PLCs, frequency converters, soft starters, various intelligent sensors, and peripheral I/O devices to be networked with the main control PLC or computer. PROFIBUS-FMS is used for communication tasks in the field control unit network. PROFIBUS network equipment mainly consists of a communication processor (CP) and network media. The communication processor is used for bus networking and point-to-point communication, and the RS485 interface enables the system to interconnect with bus systems from other manufacturers, allowing major field devices in the system, such as frequency converters and soft starters, to be plug-and-play. In addition, this network structure should also include an industrial television monitoring system, which consists of hardware such as industrial cameras, lenses, pan-tilt units, video matrix switchers, industrial monitors, outdoor decoders, and video recorders, along with multimedia software, to monitor and manage important workstations in the water production process. System Function Description The water treatment network control system based on PROFIBUS mainly has the following functions: 1. Bar Screen and Sludge Removal Control: There are four bar screens in the bar screen room, controlled by a SIEMENS S7-300 PLC with 32 input points and 16 output points. Control modes include automatic and manual. In automatic mode, the system automatically controls the bar screen's operation based on the timer value set in the PLC and the water level difference before and after the bar screen. The working status of the sand suction machine is also detected and controlled by the PLC. The sedimentation tank PLC sets the sludge suction machine's operating cycle time and mode according to the water turbidity. When the turbidity is high, double-pass sludge suction is used, with a longer operating time; when the turbidity is low, single-pass sludge suction is used, with a shorter operating time. 2. Chemical Dosing and Chlorination Control: The chemical dosing/chlorination control system has a large number of input and output points, and is controlled by a SIEMENS S7-400 PLC with 192 input points and 192 output points. The chemical dosing room has eight coagulant metering pumps and eight flocculant aid metering pumps. Each metering pump is driven by an AC variable frequency drive (VFD). For both metering and dosing methods, six pumps operate and two are on standby. The six metering pumps correspond to six reaction tanks, with one-to-one dosing of coagulant and flocculant aid. Dosing control is a critical aspect of water treatment in the plant; it is a long-delay, strongly coupled, nonlinear system, presenting certain technical challenges. We designed a self-learning fuzzy control algorithm. Based on parameters such as raw water temperature, turbidity, and pH value collected by the PLC, the system adjusts the parameters via the self-learning fuzzy controller. The PLC then controls the AC VFD to change the frequency and stroke of the metering pumps, thereby controlling the dosing amount and achieving good control results. The chlorination room has seven vacuum V-tank chlorinators. Four are used for pre-chlorination at the inlet pipe, and the remaining three are used for disinfection chlorination (post-chlorination). Disinfection chlorination uses a closed-loop control method combining flow rate and residual chlorine (two operating, one on standby), adding chlorine to the outlet pipes of two filter tanks for disinfection. In addition, the main control PLC for dosing/chlorination also needs to handle tasks such as dry powder dissolution in the dosing room, chlorine cylinder switching in the chlorination room, leak alarms, and safety control. 3. Filter Control: Filter control is the most important control link in the water plant's automatic control system. It can be divided into filtration process control and backwashing process control. Due to the large number of filters and the relatively small number of I/O points per filter, 25 SIEMENS S7-300 PLCs with 16 input/output points each are selected for control. During the filtration process, the PLC of each filter automatically adjusts the opening degree of each purified water valve based on the water level, blockage value, and running time of the corresponding filter, maintaining a constant water level in each filter. When the blockage value or filter running time exceeds the set value, the filter PLC sends a backwashing request to the backwashing PLC. Once the request is granted, the filter enters the backwashing process. During the backwashing process, the backwashing PLC needs to control the backwash water flow rate and backwash air pressure, and cooperate with the corresponding filter PLC to control the backwashing time, rinsing time, and the opening and closing of relevant valves during the backwashing process. 4. Pump Group Control: The purified water is pumped into the water pipeline. The pump station houses eight pumps of varying capacities. The system uses a SIEMENS S7-300 PLC with 48 input points and 16 output points for control. The system automatically starts and stops the pumps according to pipeline pressure and production scheduling requirements. Because pipeline pressure can suddenly increase or decrease during valve opening or closing, the system is designed to memorize valve opening or closing commands to avoid malfunctions. Within a certain time, the system automatically stops opening and closing the valves, and resumes doing so automatically once the pipeline pressure stabilizes. The system also monitors and alarms for various faults related to valves, machinery, and circuits. 5. Other Functions: In addition to the above control functions, the system also includes management functions such as production planning and scheduling, information management and decision support, and video monitoring. Conclusion and Outlook The fieldbus-based water production network control system can operate at the production site. Compared with traditional methods, its biggest advantages are low cost, powerful functions, flexible and practical structure, high reliability, simple installation and commissioning, and low development and maintenance costs. The system can be expanded into different modes according to different site conditions and user requirements. The fieldbus control system achieves effective and seamless integration of field communication networks, automatic control systems, and computer networks, essentially fulfilling functions such as field communication networks, field device interconnection, communication line power supply, unified configuration, and open interconnection networks. The fieldbus control system will provide a strong foundation for the comprehensive automation and remote control of industrial production.