Abstract: An automatic control system for water plant filters was designed, based on ControlNet and incorporating a Rockwell Automation NetLinx three-layer network, to meet the production process and requirements of filters in water purification workshops. The system uses Rockwell's ControLogix 5550 as the programmable controller, and leverages the ControlNet and Ethernet platforms, employing RSView32 monitoring and configuration software to achieve automatic operation of the water plant filters in various modes and switching between different modes. Keywords: ControlNet, ControLogix, Rockwell Automation, Automation Abstract: Based on the techniques and requirements of filters in water plants, an automatic control system for water plant filters is designed based on a three-layer network of ControlNet and Rockwell Automation NetLinx. It uses ControLogix 5550 as the controller, ControlNet and EtherNet as the communication network, and RSView as the supervisory configuration software to achieve automatic operation under each mode and automatic switching among different filter modes throughout the plant. Keywords: ControlNet, ControLogix, Rockwell Automation 1. Introduction Water plant production processes are complex and have strict quality requirements. Most existing water plant production lines in China have outdated equipment, low levels of process control, and low automation, with most operations requiring operator intervention. A water plant in Tianjin expanded its existing equipment and implemented a SCADA system during the expansion project, achieving automatic control of the entire water treatment production process. Rockwell Automation’s AB programmable controller (PLC) and its monitoring configuration software RSView32 have been widely used in the domestic water treatment industry [1]. In the expansion project of a water plant in Tianjin, Rockwell’s new generation controller ControlLogix5550 was also used as the main controller. The human-machine interface was programmed using RSView32, and on this basis, a fieldbus control network based on Rockwell’s ControlNet was built to realize the automatic control of the water plant’s filter. 2. Water plant monitoring system composition The water plant filter control system belongs to the water plant monitoring system. The entire water plant network monitoring system includes a central control station, three system monitoring points and eight PLC control stations. The central control station is located in the central control room, which is equipped with two industrial control computers as operator stations and one industrial control computer as engineer station; three industrial control computers are installed on site as system monitoring points; PLC1 control station - PLC6 control station are located in the purification workshop; PLC7 control station is located in the chlorination room; PLC8 control station is located in the ammonia addition room. The entire water plant has a large amount of input/output information and high requirements for system functions. The industrial control computers at each monitoring station in the central control room and at the field monitoring points not only monitor individual sites, but also monitor the entire system. The information processing mechanism centered on PLCs and computers will manage comprehensive information, monitor, schedule, and control the production process. Based on this, the system uses the Rockwell AB brand's ControlLogix series products as its core and adopts a hybrid architecture that includes all three layers of NetLinx networks (i.e., information network, control network, and equipment network). Therefore, the entire system is also set up according to the above three layers. 2.1 Information Layer The information layer adopts an EtherNet/IP network, mainly used to connect one engineer station, two operator stations, and three field monitoring points. The industrial control computers at the two operator stations are connected to the control layer through PCIC network cards, and the other industrial control computers use EtherNet information transmission to indirectly access the ControlNet network through the operator stations to monitor the entire system. This method not only realizes high-speed exchange of large amounts of information between the computer and the controller and monitors the production process, but can also be further used to build an enterprise information network [2-3]. 2.2 Control Layer The control layer adopts a high-speed, deterministic ControlNet field network to realize communication between controllers and control of remote extended I/O [4-5]. The field control station (i.e., each PLC control station) and the two operator station industrial control computers in the central control room are connected by coaxial cable using the ControlNet network protocol to realize high-speed transmission of control signals. Each PLC in the PLC control station automatically monitors the operating status of each process section equipment, the parameter changes of instruments and other field information, and performs automatic control of the equipment. The eight PLC control stations of the system are set up according to process requirements. Among them, PLC5 control station monitors No.1-No.5 and No.12-No.16 filter tanks; PLC6 control station monitors No.6-No.11 and No.17-No.22 filter tanks; PLC3 control station is responsible for the control of the filter tank common flushing equipment and the filter tank flushing queue. 2.3 Equipment Layer The equipment layer adopts a high-efficiency, low-cost DeviceNet network to realize the monitoring of the bottom field equipment [5]. The equipment layer connects the sensors and actuators distributed throughout the water plant, and directly connects the frequency converters, instruments and other components with bus cables. This realizes the digital transmission of data between digital and analog input/output modules and programmable controllers (PLCs), and disperses the I/O signal channels to the vicinity of the actual field equipment, reducing installation and wiring costs and saving costs [6]. 3. Filter monitoring system The filter is located in the purification workshop. This system has 22 rapid gravity filter tanks to remove the small amount of residual solids brought from the purification stage in the water. Each filter tank includes inlet valve, clear water valve, water flushing valve, drain valve, air flushing valve, exhaust valve, level gauge, differential pressure gauge and flow meter. The upper-level monitoring screen of the filter tank is shown in Figure 1. [align=center] Figure 1 Upper-level monitoring screen of the filter tank[/align] Each filter tank has three operating modes: non-service mode, filtration mode and flushing mode. It can automatically switch between the three modes through upper-level operation or operating requirements. The filter tank switching control flowchart is shown in Figure 2. [align=center]Figure 2 Filter Switching Control Flowchart[/align] 3.1 Non-Service Mode The filter does not participate in filtration. In this mode, all valves are closed. If a fault occurs or a command to enter non-service mode is received from the supervisory authority while the filter is in filtration or flushing mode, all valves will be automatically closed, entering non-service mode, and the closure of each valve will be confirmed. 2.2 Filtration Mode The filtration mode can be entered directly from either non-service mode or flushing mode. In non-service mode, upon receiving a command to enter filtration mode, the filter first goes through the "slow start stage" and the "intermediate sedimentation stage," opening the inlet valve and the clear water valve, and finally enters filtration mode. In flushing mode, after going through the flushing stages, it also enters filtration mode. In addition, in flushing mode, if there is a command to return to filtration mode, the filter can enter filtration mode before the "downward drainage" stage begins or after the "slow start" stage. In filtration mode, the PLC uses PID to adjust the opening of the filter's clear water valve so that the filter's effluent flow rate reaches the set value of the filter's effluent flow rate specified by the inlet channel. The PLC monitors the water level of each filter and displays the measured instantaneous value on the SCADA. 3.3 Flushing mode The flushing of the filter is automatically controlled by the PLC, and all 22 filters share a set of flushing equipment, namely two blowers and three flushing water pumps. Before the filter is to be flushed, it first enters the flushing queue through a flushing request, and finally enters the flushing mode after receiving the flushing signal from the queue (this part of the detailed control is executed by the filter queue flushing system). The entire flushing process of the filter is divided into the following stages, and the filter flushing process control flow is shown in Figure 3. [align=center] Figure 3 Flushing process control flow [/align] (1) Downward drainage In the downward drainage stage from the filtration mode to the flushing mode, the filter running time timer is reset to zero. The filter inlet valve enters the closing stage according to the closing time and delayed closing time. The water level of the filter is monitored, and the PID regulation control of the filter clear water valve in the filtration mode is still used to control the opening of the filter clear water valve, so that the remaining water continues to be filtered. When the water level drops to the drainage level, close the filter clean water valve and open the filter drain valve and air flush valve. Confirm that the sewage tank has sufficient capacity. (2) Air flush When the sewage tank has sufficient capacity, enter the air flush stage and start the blower. After confirming that the blower is running, start timing the filter air flush time. When the filter air flush time is up, enter the air-water combined flush stage. (3) Air-water combined flush Open the water flush valve and start the low-speed water flush process (i.e., start a flushing water pump). Start timing the air-water combined flush time and monitor the filter water level. When the filter water level reaches the "low-speed flush" level or the air-water combined flush time exceeds the maximum set value, turn off the blower. Close the filter air flush valve and open the filter exhaust valve. (4) Exhaust After confirming that the filter air flush valve is closed and the filter exhaust valve is open, start timing the exhaust time and high-speed water flush delay time. When the high-speed water flush delay time reaches the set value, enter the high-speed water flush stage. When the exhaust time reaches the set value, close the filter exhaust valve. (5) High-speed water flushing: After the high-speed water flushing delay time expires, start the high-speed water flushing process (i.e., start two flushing water pumps), begin accumulating the high-speed flushing water flow, and shut down the two flushing water pumps that have been started when the set value is reached. Close the filter water flushing valve and the filter drain valve. (6) Filter refilling and intermediate sedimentation confirmation: After the filter water flushing valve and the filter drain valve are closed, open the filter inlet valve according to the refilling time and intermediate sedimentation time. (7) Slow start confirmation: After the filter inlet valve is open, restart the flow control of the clear water valve. The flow control value gradually increases according to the set slope. When the filter flow control value reaches the PID control value determined by the inlet channel, continue to adjust the outflow flow using the PID control value determined by the inlet channel. The filter enters the filtration mode, and the filtration running time starts to reset. 4. Conclusion The innovation of this paper is: the automatic control system for water plant filters based on the ControlNet network is used for the expansion and renovation of water plants, realizing the informatization of centralized data monitoring and management of the filter section. Furthermore, the system boasts high reliability and stability, allowing the water plant to reduce the number of operators required, necessitating minimal staffing to ensure its normal operation. The application of Rockwell Automation's ControLogix series PLCs at this water plant has not only promoted the rapid and healthy economic development of the region and improved the quality of life for urban residents, but also holds significant importance for the development of the region's water treatment industry. References [1] Heng Junshan, Zhen Chenggang. Research on software-based dual-CPU redundancy control [J]. 2005.7. 21 (7-1). Pp. 59-61. [2] W.Bolton.Programmable Logic Controllers [M]., Third Edition. Prentice Hall.2004.3. Pp.15-25. [3] Wang Weibing. PLC system communication extension and network interconnection technology [M]. Beijing: Machinery Industry Press. 2005.3. Pp.1-20. [4] Jon Stenerson. Programming PLCs Using Rockwell Automation Controllers [M]. Prentice Hall.2003.3. Pp.21-25. [5] Jia Aimin, Wang Yu. Programming of Rockwell AB programmable controllers. Beijing: Machinery Industry Press 1999.3. Pp.11-20 [6] Wang Yong. 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