1.Working Principle and Characteristics of PLC

1.1 Working Principle of PLC

PLC is the abbreviation of Programmable Logic Controller, and its Chinese name is programmable logic controller. It is composed of six main parts: power supply, central processing unit, read-only memory, interface module, function module, and communication module.

The working principle of PLC is that operators store in the read-only memory in advance the program written in C language and other high-level languages. When the PLC is powered on, the central processing unit calls the program in the read-only memory, so that the functional modules complete the functions set by the program, and through the interface circuit converts the digital signal of the PLC program into analog signal current for controlling motors, etc., thereby realizing precise control of motors or other components.

1.2 Characteristics of PLC

The main characteristics of PLC chips are economical and practical, easy to install, and easy to operate. PLC chips can realize many functions. If functional transformation of some electromechanical equipment is required, PLC chips can be installed in the control circuit of the electromechanical equipment. Although they are not as powerful as computers, in cases without heavy computational load, this transformation method is the most economical and practical. Meanwhile, because PLC chips are small in size and all are standard IC packages, in the transformation of existing equipment control circuits, it is only necessary to install the corresponding PLC chip socket on the circuit board; during use, directly insert the PLC chip into the socket. If the chip is damaged or needs an upgrade, directly remove the chip. With deeper development of PLC chips, various PLC programming languages and software are increasing, which is very convenient for operators to program. These software can realize simulation operation while implementing program design, which not only facilitates programming but also improves the efficiency of program writing.

2 Fields of PLC in Industrial Control Systems

In industrial control systems, the application fields of PLC are mainly to realize automation control, especially in high-precision control such as switching (discrete) control, motion control, centralized control, and remote control.

Below, taking the application of PLC in a beer fermentation production line as an example, the role of the PLC control system is explained. The technological process of the beer fermentation production line is roughly: crushing malt and other grains → gelatinization of malt → filtration → boiling → whirlpool sedimentation → cooling → fermentation → maturation → filtration and filling. Among them, fermentation is the key step in beer production. During fermentation, new substances are produced and a large amount of heat is released. There are two methods of beer fermentation: top fermentation and bottom fermentation. The two fermentation methods are basically consistent; the difference lies in the control of fermentation temperature. The top-fermentation temperature is 15°C–25°C, and the bottom-fermentation temperature is 5°C–10°C. The level and variation of fermentation temperature determine the taste and quality of beer. Therefore, it is necessary to control the temperature during fermentation with high precision, which is the core part of the automation control system in beer production.

2.1 Switching (Discrete) Control

Temperature control in the beer fermentation process is a type of switching control. This is also the most common method in PLC control systems. In the control system, first, the temperature sensor in the fermenter collects the temperature variable of the controlled object. The collected analog signal is converted through the interface circuit (i.e., I/O input/output interface) in the PLC chip into a digital signal that the PLC chip can recognize. These signal data are compared with the values in the preset program. The central processing unit in the PLC chip performs comparative calculation, and further triggers the preset decision program, then transmits this decision action as a digital signal to the I/O input/output interface, which is converted into an analog signal to control the electric regulating valve. In this process, remote control can also be realized through the communication module of the PLC chip. Its working principle diagram is shown in Figure 1.

Table 1 System I/O Allocation Table

2.2 Motion Control

In beer fermentation production, the main method of temperature control is the heat transfer between the cooling medium and the fermenter wall. In this process, it mainly relies on the electric regulating valve to complete, and the switching of the electric regulating valve and the degree of valve opening must be consistent with the trigger initial parameters set in the PLC control system for controlling temperature changes. In the PLC control system, each I/O interface is directly connected with temperature, liquid level, pressure, and flow sensors, comparing each data with the initial parameters in the memory and making corresponding actions. The specific I/O allocation table is shown in Table 1.

2.3 Remote Control

In the beer fermentation process, the PLC control system is connected to the computer in the central control room through its bus, and remote communication functions are carried out through the EM241 intelligent communication module in the PLC. In this way, the master control equipment in the central control room can connect with the PLC control system of the fermenter through LAN or Ethernet, and display the real-time temperature transmitted from the temperature sensor and obtained by the PLC on the control screen, while also displaying the operating status of each electric regulating valve in the I/O interface transmitted by the PLC system. At the same time, the host in the central control room controls the memory of the PLC control system and each I/O interface through OPC (OLE FOR Process Contrrol) embedded-process-based control software. The host stores the designed temperature control program into the memory of the PLC, sets the initial parameters and corresponding actions for temperature control, or directly sends control commands to the PLC through the host.

Conclusion

In the “Made in China 2025” development plan proposed by China, to realize the goal of becoming a manufacturing power, the application of PLC technology in industrial control systems will become more extensive in this process. At the same time, innovation development should be strongly advocated in applications, and problems in PLC applications should be further solved, greatly expanding the fields of PLC application and improving the modernization level of industrial production.

References

Yu, X. (2018). Application of PLC control systems in industrial production. China New Technology and New Products