Abstract: This paper introduces an automatic control system design method for decontaminate sewage containing iron oxide impurities using a Siemens S7-224 PLC. The hardware configuration and software design flowchart of the system are detailed, and key programming issues are also discussed. Keywords: PLC; sewage treatment system; sequential function chart; fault diagnosis; hardware; programming skills; competition adjusting calculation method 1. Introduction : In metallurgical enterprises, a large amount of industrial water is used for cooling, consuming a significant amount of water resources daily. Because the used cooling water contains a large amount of iron oxide impurities, it is not suitable for repeated recycling. To protect the environment and conserve water, it is necessary to purify the sewage containing iron oxide impurities. 2. System Introduction: The following is a schematic diagram of the composition of the sewage purification system: [align=center] Figure 1. Schematic diagram of the composition of the sewage purification system[/align] 2.1 Process Flow Introduction: (1) Filtration Process: Open the inlet valve and outlet valve. When the sewage flows through the magnetic filter, if the coil of the magnetic filter is always energized, the iron oxide impurities in the sewage will be adsorbed on the magnet of the magnetic filter, so that the water flowing out of the tank is purified water. (2) Backwashing Process: After filtration for a period of time, the iron oxide impurities attached to the magnet must be cleaned. At this time, just cut off the power supply of the magnetic filter coil, close the inlet valve and outlet valve, open the drain valve and compressed air valve, and let the compressed air forcefully push the water in the tank into the magnetic filter to rinse the magnet, remove the attached iron oxide impurities, and let the rinsed sewage flow into the sewage pool for secondary treatment. 2.2 Control Tasks and Requirements: (1) The filtration process of the two units can be carried out separately or simultaneously. The backwashing process is only allowed for one unit. When one unit is backwashing, the other must wait. When two units request backwashing at the same time, unit 1 takes priority. (2) To ensure the normal operation of the water filtration process, differential pressure detection instruments are installed on the pipelines of each unit. As soon as a "high differential pressure" signal appears, the water filtration process should be stopped immediately and the backwashing process should be started automatically. (3) In order to enhance the reliability of the system, the connection signals of the magnetic filter and each solenoid valve coil of each unit are fed back to the input terminal of the PLC. Once an input signal is abnormal, the system should be stopped immediately to avoid failure. (4) Actuator output fault detection and alarm. 3. Control System Design: 3.1 Hardware Design: (1) Determine the CPU model and expansion module model of the PLC: The following table is the input and output address allocation table of Unit 1 of the water purifier: [align=center]Input/output address allocation table of Unit 1[/align] Because Unit 1 and Unit 2 have the same working principle, the total number of input points of the water purification system is 18 points and the total number of output points is 16 points. For this purpose, the CPU224 programmable controller of Siemens S7-200 series and the digital expansion module EM223 of S7-200 are selected, which can provide 22 input points and 18 output points. (2) Selection of differential pressure detection instrument: The function of differential pressure detection instrument is to detect the differential pressure between the inlet and outlet of the magnetic filter. If the differential pressure is too high, it indicates that the magnetic filter has a blockage fault and needs to enter the backwashing process. The differential pressure detection instrument should have the functions of setting differential pressure, displaying differential pressure, and outputting differential pressure signal. In summary, the 3000IMR series Photohelic pressure gauge/switch from Dwyer Corporation of the United States was selected. (3) The selection of other input and output components is relatively easy, and due to space limitations, it is omitted. 3.2 Software design: The following sequence function diagram is designed according to the process requirements and control tasks of the water purifier unit: [align=center] Figure 2, Sequence function diagram of the control system of the water purifier unit[/align] Based on the above sequence function diagram, the ladder diagram can be written quickly. The ladder diagram is not listed in detail here. Only a few tips for ladder diagram design are listed: (1) The addresses and functions of other programming components of the PLC are clearly marked in the sequence function diagram, so they are not listed. (2) In order to avoid the impact caused by the switching of system processes, the opening and closing of valves and magnetic filters adopt delayed sequential actions. (3) Scheduling algorithm for backwashing process of Unit 1 and Unit 2: If Unit 1 and Unit 2 enter the backwashing process at the same time, Unit 1 will be given priority because Unit 2 has a delay of 0.1 seconds; if they do not enter the backwashing process at the same time, since the backwashing processes of Unit 1 and Unit 2 have an interlock function, the unit that enters the backwashing process first will have to wait for the other unit. (4) Fault diagnosis subroutine: (4.1) Function of fault diagnosis subroutine: Compared with PLC, external output devices such as solenoid valves and magnetic filters are prone to failure. If solenoid valves and magnetic filters fail and are not dealt with in time, it will easily cause the system to work abnormally, or even damage the system. The solution is: if the main external output devices fail, the system must be stopped and an alarm must be set to remind the staff to repair them. (4.2) Design of fault diagnosis subroutine: This control system has a total of 8 fault diagnosis subroutines, and their fault diagnosis algorithms are similar. The specific algorithm is: if a coil is energized, the corresponding normally open auxiliary contact should be closed; if it is not closed, the device is judged to be damaged. If a coil is de-energized, the corresponding normally closed contact should be closed; if it is not closed, the device is judged to be damaged. The following uses the fault diagnosis subroutine 1 as an example to talk about the implementation of the fault diagnosis subroutine. The ladder diagram of the fault diagnosis subroutine is shown on the right. It is explained in the logic language of sequential function as follows: In step M1.1, i.e. {water filtration process}, if the magnetic filter or the outlet valve or the inlet valve is not open, or the drain valve or the compressed air valve is open, an alarm will be triggered and the machine will enter the shutdown state. (5) Differential pressure protection of the magnetic filter: if the differential pressure between the inlet and outlet of the magnetic filter is greater than the set differential pressure, the water filtration process will end unconditionally and the backwashing process will be entered sequentially. 4. The innovation of the author of this paper: The automatic control of the sewage purification system is realized by using PLC, and the hardware design and software design method of the sewage purification system are described in detail. The software design gives the sequential function diagram of the control system and adopts the structured programming method. The hardware design uses a differential pressure detection instrument to ensure the performance indicators of the water filtration process and to prevent the filter from clogging. Since the PLC is used as the controller, the system structure is simpler and more reliable than the traditional control system structure, and the system rarely fails. Since the control algorithm of the control system is implemented by software, it is easy to upgrade the system and easy to network. In order to solve the problem of competition between Unit 1 and Unit 2 in the backwashing process, a delay and interlocking algorithm is adopted. In order to ensure the reliable operation of the system, an external output device self-diagnosis program is designed, which can determine whether the external actuator is working properly and automatically enter the fault handling step. This gives the system a certain degree of self-diagnosis intelligence and ensures the stable and reliable operation of the system. References: [1] Guo Zhaoxin, Li Chao, Zhang Xuedong. Application of PLC in integrated reactor sewage treatment system [J]. Microcomputer Information, 2006.7: 1-3 [2] Liao Changchu. PLC Programming and Application, Beijing: Machinery Industry Press, 2005 [3] Siemens S7-200 Programmable Controller System Manual