Research on the design of sewage treatment system based on PLC control

introduction

At present, with the rapid development of science and technology in my country, the clarion call of the poverty alleviation war has been sounded, and various small and medium-sized enterprises have sprung up like mushrooms covering the land, driving people out of poverty and becoming rich, but the amount of various industrial wastewater is also skyrocketing. According to relevant data, the investment in the construction of industrial sewage treatment projects in my country has exceeded hundreds of billions of yuan, of which about 30 billion yuan has been invested in industrial sewage treatment equipment. However, due to the backwardness of the sewage treatment automatic control system, if a reasonable plan is not quickly formulated and the high cost of sewage treatment is not solved as soon as possible, it will inevitably hinder the economic development of various regions. So how can we quickly and efficiently treat sewage, reduce environmental pollution, and truly turn waste into treasure?

In today's environment of rapid development of automation, informationization and intelligence, PLC technology is rushing to the forefront of science and technology with overwhelming force. Based on this background, all major secondary and technical colleges and universities have opened courses related to PLC programming and application of mechanical and electrical majors, especially the introduction of sequential function control instructions, which makes complex problems simple and intuitive. Therefore, using PLC sequential control function instructions to design sewage treatment systems is undoubtedly the most environmentally friendly, scientific and efficient choice. Based on this, this paper combines the teaching of PLC programming and application courses to guide students to design a sewage treatment system based on PLC control.

1 System composition design

1.1 System control requirements and overall structure

Figure 1 is a simulated sewage treatment mixing device. The operator turns on the power switch, the sewage inlet valve is energized, the relevant indicator lights up, and the sewage flows into the sewage treatment tank: when the sewage in the tank reaches the liquid level sensor SL1, SL1 is connected, the sewage inlet valve is closed and the sewage inlet indicator light goes out: the No. 1 decontaminant inlet valve is opened and the No. 1 decontaminant indicator light turns on, 30S later, the No. 1 decontaminant inlet valve is closed, the turbidity sensor SL3 is connected, and the sewage pollution situation is judged. If it is heavy sewage, the light and heavy sewage switch is turned on, the No. 2 decontaminant inlet valve is energized and opened, and the No. 2 decontaminant flows into the sewage treatment tank, and the No. 2 decontaminant indicator light turns on, and the No. 2 decontaminant inlet valve is closed after 20S. The stirring motor starts to work and the stirring motor indicator light is on. After 3 minutes, the stirring motor stops and the light goes out. The clean water discharge valve opens to release the decontaminated water. The clean water discharge indicator light is on. When the water is discharged to the liquid level sensor SL2, SL2 is connected, the clean water discharge system is closed and the clean water discharge indicator light is off, the sewage discharge system is opened and the sewage discharge indicator light is on, the sewage discharge system is closed after a delay of 2 minutes, and enters the next cycle.

1.2PLC selection and I/O address allocation

According to the design requirements, control requirements and school laboratory conditions, this paper selects Mitsubishi Fx2N-48MRPLC, which has 24 inputs and 24 (relay) outputs, but this example actually only needs 6 inputs and 17 outputs, and there is a margin for both inputs and outputs, which fully meets the requirements. The I/O address allocation of the sewage treatment system control device is shown in Table 1.

Figure 1 Simulated sewage treatment mixing device

2 Programming

The sewage treatment in the decontamination tank can be divided into 7 control processes according to the task requirements. There is a strict timing relationship between these control processes. Based on this, the state transition diagram SFC language can be used to describe the state programming of the sequential structure, and the SFC program can be converted into a step ladder program. Figure 2 is a step control state flow chart of the sewage treatment device. Based on this state flow chart, the SFC program ladder diagram is written, as shown in Figure 3.

2.1 Instructions for programming using SFC

S0 in the program is the initial state, and S20~S25 are the intermediate states of SFC. In this program, if you want to add or delete certain functions, you only need to add or delete a function block, and the relevant state relays will be automatically assigned.

The short horizontal line in the cross graphic indicates the transfer condition, and each condition must be marked. The long vertical line indicates the transfer direction, which is automatically generated by the system. If you want to jump to the transfer arrow, you need to edit it.

2.2 Some functional instructions

M8002: The PLC starts a scan cycle the moment it is powered on, activating the initial state of the program.

ZRST: Reset combination instruction, all the programs under control are cleared when the trigger signal comes.

3 Conclusion

After programming with Mitsubishi GXdeveloperv8.86 software, this paper simulated the operation with the help of GXSimluaor6cn simulation software and found that the expected functions can be fully realized. The hardware configuration design of Mitsubishi FX2N-48 programmable controller in this design is completely feasible in production, and because PLC has the advantages of high reliability, strong flexibility, low requirements for the working environment and good anti-interference performance, it can completely rescue operators from harsh on-site environments. In addition, in practical applications, if PLC is used to control sewage treatment equipment, not only is the capital investment less and the control accuracy high, but it is also easy to modify the programmable controller program at any time to change the working time and working status of the sewage treatment equipment to meet the needs of different levels of sewage treatment, which has a high promotion value.