[Abstract] This paper introduces a control system using a Siemens S7-300 PLC as the master controller to backwash six high-speed laminar flow filters . The paper details the hardware and software structure of the electrical control system and provides a new control method for laminar flow filters in industrial water treatment systems. [ Keywords ] PLC; circulating water treatment; laminar flow filter; backwashing High-speed laminar flow filters are one of the main devices in modern industrial water treatment, highly suitable for purifying turbid water (containing iron oxide scale and oil) from steel rolling and continuous casting equipment in steel enterprises, as well as for side-stream filtration in circulating water systems in the coal washing industry. To this end, we jointly developed a backwash control system for six laminar flow filters in the circulating water treatment system of the hot-rolled thin plate plant with Tangshan Iron & Steel Group Corporation – Hot Rolled Thin Plate Plant. 1. System Composition The laminar flow filter is the main component for filtration throughout the entire process of circulating water in the hot-rolled thin plate plant or coal washing plant. Various filter media, such as white coal, yellow sand, and activated carbon, are filled inside the filter to form physical and chemical filter layers for coarse and fine filtration, thereby achieving the goal of purifying the effluent. Because the circulating water in hot-rolled sheet or coal washing plants contains a large amount of iron oxide scale, oil, coal dust and other impurities, the laminar flow filters in their circulating water treatment systems are very prone to clogging. Therefore, after a period of use, each laminar flow filter needs to be backwashed (water and air mixed) to remove impurities from the water, so as to ensure the clean and smooth flow of circulating water and the normal operation of the filters, and to improve the service life of the filters. 1.1 Equipment Composition and Functions of Laminar Flow Filters Our developed laminar flow filter backwashing control system controls six laminar flow filters. Each filter is equipped with six electric valves, which control the opening and closing of pipelines for raw water inlet, purified water outlet, backwash drainage, backwash water inlet, backwash air inlet, and backwash air outlet to meet process requirements. To maximize equipment utilization, the six filters share two blowers and two water pumps as the air and water sources for backwashing. To ensure the normal operation of the entire backwashing system, the two blowers and two water pumps serve as backups, with one blower and one water pump operating and one on standby respectively. Under normal conditions, the backwashing system starts one blower and one water pump as the system's air and water sources. If one blower or water pump fails, the other automatically starts operating to ensure reliable backwashing system operation. Each blower and water pump is equipped with one electric outlet valve to control its pipeline outlet. 1.2 Working Principle and Process Flow of the Laminar Flow Backwash Electrical System The blower motor has a capacity of 380V/132kW. Due to its large power, a soft starter is used to reduce the impact of its starting current on the power grid. The control signals of the soft starter are: Input – Manual/Automatic, Emergency Stop, Start/Stop; Output – Bypass, Fault. The blower outlet valve motor has a power of 380V/1.1kW, with one opening and one closing limit switch (passive contact). The backwash water pump motor has a capacity of 380V/75kW. Considering its relatively small power, a direct start method is used. Its control signals are: Input – Manual/Automatic, Emergency Stop, Start/Stop; Output – Fault. The water pump outlet valve motor has a power of 380V/1.1kW, with one opening and one closing limit switch (passive contact). Figure 1 shows a schematic diagram of a single laminar flow filter. Its control functions are as follows: 1) Raw water inlet valve V1: 1 valve motor (380V/1.1 Kw), 1 opening/closing limit switch. 2) Clean water outlet valve V2: 1 valve motor (380V/1.1 Kw), 1 opening/closing limit switch. 3) Backwash drain valve V3: 1 valve motor (380V/1.1 Kw), 1 opening/closing limit switch. 4) Backwash inlet valve V4: 1 valve motor (380V/1.1 Kw), 1 opening/closing limit switch. 5) Backwash air inlet valve V5: 1 valve motor (380V/1.1 Kw), 1 opening/closing limit switch. 6) Backwash air outlet valve V6: 1 valve motor (380V/1.1 Kw), 1 opening/closing limit switch. [align=center]Figure 1 Schematic diagram of laminar flow filter equipment[/align] The upper computer is controlled and managed by configuration software (WINCC), and the lower control is operated by SIEMENS-PLC, S7-300-314. 2 Description of the control system 2.1 System control mode According to the user and site control requirements, there are three system operation modes: 1) Automatic mode, monitored on the configuration screen; 2) Semi-automatic mode, operated individually on the configuration screen; 3) Manual mode, controlled by the manual operation box located on site, which has the highest control level. (The manual operation box is located on site in a place that is convenient for observation and operation) Each of the 6 laminar flow filters can be operated manually. When one filter is in manual mode, the automatic circulation mode of that filter is cut off, and the other laminar flow filters in automatic mode continue to operate in automatic mode. Because the power of each valve motor is relatively small, its circuit control form is: air switch (with thermal trip and short circuit protection), contactor, valve motor (where the thermal trip of the air switch is used as overload protection for the valve motor). 2.2 System Hardware Composition This control system uses one Siemens PLC to control six laminar flow filters. Simultaneously, analog input/output modules are used to control temperature and pressure. The main controller uses an S7-300 series 314 CPU with external input/output modules, including 24 AI points, 8 AO points, 64 DI points, and 40 DO points. The PLC programming software is Siemens Step7; the monitoring software is generated using WINCC visual monitoring software. The host computer and PLC are connected via a Siemens programming communication cable. The hardware configuration of the Siemens PLC used is shown in Table 1. 2.3 Control System Software Design Based on the backwashing process requirements of the laminar flow filters, the design is as follows: six laminar flow filters are backwashed in rotation, with a backwashing cycle of 12 hours. The backwashing time for a single filter is approximately 24 minutes. The control flow for a single filter is shown in Table 2. 3 Control Description of the Configuration Software This control system uses WINCC configuration software to create a host computer management screen, simulating the on-site production process. WINCC communicates with the S7-300-PLC via the CP5611 communication card, linearly mapping the numerical range of the PLC's process variables to the variable definition range of WINCC. WINCC requests data from the PLC by sending a request signal; a periodic data reading method is used here, that is, WINCC registers the read request on the PLC, and the PLC processes the communication within the corresponding cycle. The control screen is shown in Figures 2 and 3: [align=center] Figure 2 Laminar Flow Filter Automatic Control Diagram (Laminar Flow 1#～6#) Figure 3 Laminar Flow Filter Automatic Control Diagram (Fan/Pump)[/align] The main screen includes 3 sub-screens, which can overlap each other and can be switched by the operator using the mouse or keyboard. When a screen button is clicked, the button changes color as an indication that the configuration system has responded. The status of field limit switches, fault signals, etc., is indicated to the operator by flashing the corresponding screen component colors. Through these screens, the working status of field equipment can be monitored in real time, and the field equipment can be easily controlled. Summary: This control system has been in operation for six months and is performing well, fully meeting the process requirements of the circulating water treatment system in the hot-rolled thin plate production line of the plant. Compared with other types of laminar flow filtration equipment in the plant, this control system not only saves water and electricity during backwashing but also significantly shortens the backwashing time of the laminar flow filters, bringing considerable economic benefits to the plant. Innovation of this paper: Although this control system controls six laminar flow filters, the process flow of each filter is the same. Therefore, its hardware and software can be modularized, and the number of laminar flow filters controlled can be arbitrarily combined. Furthermore, by adjusting the opening and closing times of various valves, fans, and pumps, different backwashing process requirements can be met, thus giving the control system a certain degree of versatility. Among current equipment in related industries, our designed control system has certain typicality and advancement, and has value for promotion. References: [1] Siemens. S7-300 Programmable Controller Template Description. 2005 [2] Siemens. S7-300 Programmable Controller Hardware and Installation Manual. 2005 [3] Siemens. NETPRO Network Configuration Manual. 2004 [4] Song Hongwei, Zhen Ran. Design and Implementation of Central Control System for Gas Station. Microcomputer Information. 2006.6.1.106～107