The Nanhai City Second Water Plant is designed to supply 1 million m³ of water per day, providing water for production and daily life in most towns north of the Beijiang River. The first phase, with a daily supply of 250,000 m³, was completed and put into operation in April 1997. The second phase, with a daily supply of 250,000 m³/d, is nearing completion. The automation project mainly involves upgrading the first phase's automation system and constructing the second phase's automation system, while also reserving interfaces for future expansion. Production Process The production process of the water purification plant is shown in Figure 1, mainly consisting of the following processes: [align=center] Figure 1: Water Purification Plant Production Flow[/align] ■ Water intake: Surface water from rivers is pumped into the water purification plant using multiple large centrifugal pumps. ■ Chemical dosing: Coagulants and chlorine are added according to specific process requirements to achieve coagulation and disinfection. ■ Flocculation: The surface water reacts with the coagulant, and the precipitated sludge is discharged. ■ Horizontal sedimentation: Water reacting with the coagulant flows slowly through a horizontal sedimentation tank to allow suspended particles to settle, and the precipitated sludge is discharged. ■ Filtering and settling water involves passing it through granular media (quartz sand) to remove suspended impurities and clarify the water, with periodic backwashing of the quartz sand. ■ Water delivery utilizes multiple large centrifugal pumps to deliver tap water to the municipal water supply network at a specific pressure and flow rate. Control Scheme Due to the following characteristics of the tap water production process: ■ Each production process segment is relatively independent, with numerous individual devices; ■ The collected data is large in volume and diverse in type, but there are few upstream and downstream related production parameters; ■ Tap water production is continuous, irreplaceable, and uninterrupted; ■ The process segments are far apart, the equipment is dispersed, and the network is relatively complex. Based on these characteristics, this system uses OMRON's small and medium-sized PLCs for decentralized control of each process segment or equipment. A network is formed through OMRON Protocol and Controller Link. Each process segment control room and the central control room are equipped with a host computer to build a human-machine interface for production management and subsequent processing of production data. The overall plant control network is shown in Figure 2. [align=center]Figure 2: Plant-wide Control Network Diagram[/align] The hardware configuration of the monitoring system is as follows: The host computer is a high-reliability microcomputer, expanded with Controller Link support card 3G8F7 CLK211-E, and equipped with 8 sets of medium-sized PLCs OMRON C200HG, 1 set of OMRON CS1H, 8 sets of small PLCs OMRON CQM1, and 8 sets of CPM2A. All medium-sized PLCs and the host computer form an Omron Controller Link network through the Controller Link cable communication unit CLK21 and the communication unit 3G8F7-CLK21-E extended on the operator station. The small PLCs are connected to the medium-sized PLCs with relevant functions through the OMRON Protocol. OMRON's Controller Link network is OMRON's main FA (Factory Automation) level network, a network that uses token bus communication. Each node in the network can act as a master station to send and receive data. By setting data links, nodes can automatically exchange data within preset areas. In this network, the node controlling communication is called the token issuing unit. It controls the tokens, checks the network, and performs related tasks. This bus topology offers maximum flexibility, ease of expansion and maintenance, and meets the system's scalability requirements. The use of distributed control technology ensures the Controller Link network will not collapse due to a single site failure, improving system stability. Shielded twisted-pair cable is used as the communication medium for the Controller Link network in this system. The entire network is divided into two segments by a bridge, primarily to meet communication distance requirements and accommodate future expansion. Due to the relatively large distances between nodes, the transmission rate is set at 500kbps, which meets the system's real-time requirements. This control scheme uses small to medium-sized PLCs for distributed control of major production equipment, while simultaneously connecting them tightly via a network for centralized management. This reduces the risk of failure, improves reliability, and is an economical and feasible solution. In the water intake and delivery process section, the main equipment consists of multiple large centrifugal water pumps and 10kV high-voltage DC motors. Therefore, each high-voltage distribution cabinet uses a Sepam2000 (a small PLC manufactured by Schneider Electric, specifically designed for distribution cabinet control) for data acquisition and control. The PLCs are connected to a network via RS485 interfaces. The OMRON C200HG medium-sized PLC in the control room communicates with them using the OMRON Protocol to read and write data and perform unified scheduling. This saves a lot of data acquisition cables, and when a PLC fails, it can be easily disconnected without affecting the normal production of other equipment. For the control of the sludge removal vehicle in the sedimentation tank, since the vehicle moves back and forth on the nearly 100-meter-long sedimentation tank, the small PLC used for control communicates with the control room via a C200HG radio through an RS232 interface (1:N). The radio model is MDS-SCADA-24810, a direct digital modulation and demodulation radio with an operating frequency range of 2.4 GHz to 2.4835 GHz. It supports standard asynchronous communication protocols, ensuring stable and reliable operation. The protocol used is the OMRON Protocol, and the software is developed using OMRON-CX-Protocol. The second-phase filter uses multiple small PLCs (OMRON CQM1H) for distributed control, effectively solving the problem of all filter shutdowns due to control equipment failures. Program Structure The control of all equipment in this system is performed by PLCs, and the PLC programs are developed using OMRON-CX-Programmer software. The control programs for each process section and individual equipment are relatively independent, with some identical processes using a subroutine model. Therefore, the program structure is relatively simple, facilitating debugging and maintenance. Human-Machine Interface (HMI) The system's HMI is developed using the iFix 3.0 configuration software platform and consists of several screens: a main screen (water treatment process of the water plant), process diagrams for each system, alarm windows, etc. To enhance readability and visual appeal, the main screens are presented in 3D (drawn using 3ds, Flash, etc.) format, displaying all major operating parameters of the equipment in relevant locations. Equipment control is accessed by clicking on the equipment; shift + left mouse button opens the equipment's help file, including equipment profiles and operating procedures. Communication between iFix and the OMRON PLC is handled by OMRON's FinsGateway and Intellution's drivers OMF or OMR, which is crucial for the normal operation of the entire system. ■ Main Screen: Displays the entire water treatment process of the water plant (3D diagram format), from water intake, chemical dosing, chlorination, flocculation and sedimentation, filtration to water supply. Key control parameters for water treatment and important equipment are displayed in relevant locations, and users can click to access individual substations. ■ System Process Diagrams: These mainly include water intake process diagrams, alum dosing process diagrams, chlorination process diagrams, flocculation tanks, sludge removal trucks, filter tanks, water delivery process diagrams, and high and low voltage power distribution diagrams. Except for the power distribution diagrams, all are presented in 3D format, making the images intuitive and eye-catching, and conveying more information than 2D diagrams. ■ Alarm Window: All alarms are displayed simultaneously, and the horn will sound continuously until confirmed. Alarms can also be displayed in categories as needed. ■ Equipment Control Parameter Setting: During parameter setting, the system checks whether the input parameters are correct (incorrect parameters cannot be entered) and whether the parameters have been correctly downloaded to the PLC. If an error occurs, it will report to the operator. ■ Production Reports: Two types of reports are provided: production status (equipment operating parameters) and production statistics. The old system lacked production status reports, and production statistics reports could not be generated correctly. To address this, we completely modified the PLC program, and to save storage space and facilitate querying, we stored daily production data in a historical database, allowing for immediate report generation when needed. ■ Historical Curves: Historical data for all major operating parameters of the entire plant can be queried. To facilitate equipment operation analysis, historical and current operating data can be displayed simultaneously on the same screen for comparison. ■ To prevent equipment control errors, all equipment has four control modes: central control (controlled by the central control room's host computer), local control (controlled by the workshop's host computer), automatic, and local (equipment not controlled by the PLC). These modes can be switched as needed. ■ All workstations throughout the plant can view the overall plant operation status. Conclusion This project is a distributed control system composed of industrial computers and small to medium-sized PLCs. The computer enables control of all production equipment and the setting, adjustment, and monitoring of process parameters, meeting the automatic control requirements of a large-scale waterworks. The entire solution is economical, practical, easy to program, operate, and maintain, and has been successfully applied at the Guangdong Nanhai Second Water Plant.