what is plc

PLC is the abbreviation of "Programmable Controller". Originally, the programmable controller (Progammadble Controller) was abbreviated as PC, but in order to avoid confusion with the abbreviation PC (Personal Computer), the programmable controller was called PLC (Progammadble Logic Controller). It can be seen that PLC is a member of the computer family and is a microcomputer mainly used in the field of industrial automatic control.

IEC (International Electrotechnical Commission) defined programmable controller in 1987 as: (recommended study: PHP video tutorial)

The programmable controller is an electronic system for digital operation, specially designed for application in industrial environments; it uses a type of programmable memory for It stores programs internally to execute user-oriented instructions such as logical operations, sequence control, timing, counting and arithmetic operations; and controls various types of machinery or production processes through digital or analog input/output.

PLC has a very wide range of applications and is widely used in automation control fields in various industries such as steel, petrochemicals, machinery manufacturing, automobile assembly, and power systems at home and abroad.

There are hundreds of PLC manufacturers in the world, and the more famous ones include Siemens of Germany, Rockwell Automation of the United States, Schneider of France, Mitsubishi and Omron of Japan. A large number of PLC products of various types have been produced. The FX series of Mitsubishi Corporation of Japan, the S7-200 CPU22X series of Siemens of Germany, and a small number of Schneider products are widely used in our country.

The basic components of PLC mainly include central processing unit (CPU), memory (RAM, ROM), input and output unit (I/O), power supply and programmer.

The equivalent circuit of the PLC control system can be divided into three parts, namely the input part, the output part and the internal control part. The input part is to collect the input signal, and the output part is the execution part of the system. These two parts are the same as the traditional relay control circuit. The internal control circuit is a logical control realized through programming method, and software programming is used to replace the function of the traditional relay circuit.

The input part of PLC consists of external input circuit, PLC input terminal block and input "relay". The so-called "input relay" here is inside the machine, and only the input terminals are visible from the outside. It can also be understood that each input terminal is equivalent to a "relay", represented by the symbol "I". Because there are many input terminals, digital symbols must be added to distinguish them. The numbers start counting from "0.0", such as the first one. The input terminal (also the first "input relay") is represented by "I0.0". And so on from now on.

The coil of this relay is controlled by external input components (such as control buttons, travel switches, etc.). The coil of the input relay cannot be controlled in the program. This coil then controls the action of its contacts. These contacts are used in the program to control the output by changing the state according to the program requirements. There are only two types of input relay contacts. One is a normally open contact, and the other is a normally closed contact. There are many. The textbook says to use them casually during programming. (See the "input part" circuit diagram on the left side of the picture above)

The output part is composed of an "output relay", an output terminal, and an external load circuit inside the PLC and isolated from the internal control circuit. Like the input, each output terminal is also equivalent to an "output relay". Its symbol is represented by "Q". The numerical symbols also start counting from "0.0". The first "output relay" is represented by "Q0.0" express.

Different from the input, the internal program controls the coil of this "output relay", and its contacts control the external load circuit. In the same way, this "output relay" also has two types of contacts, namely normally open and normally closed contacts. The number is also large and can be used casually in the program.

The intermediate internal control circuit is formed by the user program and uses a so-called "soft relay" to replace the control logic of the actual relay. Its function is to detect, judge, calculate and process the status of input signals and output signals according to the logical relationship specified by the user program, and then derive the corresponding output to achieve the control goals required by the user.

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