What is PLC?

Figure 1: Programmable logic controller (PLC).(Go to Okmarts and pick the PLC you need.)

1. Definition

Programmable logic controller (PLC) is a digital operation electronic system specially designed for application in industrial environment. It adopts a kind of programmable memory which stores the instructions to perform logic operation, sequence control, timing, counting and arithmetic operation. And it controls various types of mechanical equipment or production process through digital or analog input and output.

2. Structure

Programmable logic controller is a computer dedicated to industrial control. Its hardware structure is basically the same as that of microcomputer. The basic composition is described as follows:

2.1 Power Supply

The power supply is used to convert AC into DC required by PLC. At present, most PLCs use switching regulated power supply.

2.2 Central Processing Unit

Central processing unit (CPU) is not only the control center of PLC, but also the core component of PLC. Its performance determines the performance of PLC.

The CPU is composed of controller, arithmetic unit and register. These circuits are concentrated on one chip and connected with the input / output interface circuit of memory through address bus and control bus. The function of CPU is to process and run user programs, carry out logical and mathematical operations, and control the whole system to make it coordinated.

Figure 2: The structure of a PLC.

2.3 Memory

Memory is a semiconductor circuit with memory function for storing system programs, user programs, logical variables and other information.

2.4 Input Unit

The input unit is the input interface between the PLC and the controlled equipment, and it is the bridge for the signal to enter the PLC. Its function is receiving the signal from the master element and the detection element.

2.5 Output Unit

The output unit is also the connecting part between PLC and controlled equipment. Its function is transmitting the output signal of PLC to the controlled equipment. The level of the controlled device is converted into a weak current driving signal from the central processor.

In addition to the above parts, PLC also has a variety of external devices according to different models. It can help programming, monitoring and network communication. Commonly used external equipment include programmer, printer, cassette tape recorder, computer, etc.

3. Working Process

When the PLC controller is put into operation, its working process is generally divided into three stages: input sampling, user program execution and output refresh. Completing the above three stages is called a scanning cycle.

Figure 3: The working process of a PLC.

3.1 Input Sampling Phase

During input sampling, the PLC controller reads in all input states and data in sequence by scanning, and stores them in the corresponding unit in the I / O image area. After the input sampling is completed, it goes to the user program execution and output refresh stage.

3.2 User Program Execution Phase

When executing the user program, the PLC controller always scans the user program (ladder diagram) in order from top to bottom.

3.3 Output Refresh Phase

After scanning the user program, the PLC controller enters the output refresh stage. During this period, the CPU refreshes all output latch circuits according to the corresponding state and data in the I / O image area, and then drives the corresponding peripherals through the output circuit.

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