PLC generally refers to Programmable Logic Controller, full name in English: Programmable Logic Controller, PLC for short.PLC is a kind of intelligent controller, along with the development of science and technology, to adapt to the needs of many varieties, small batch production and the development of a new industrial control device.

Programmable Logic Controller (PLC) is a digital operation electronic system designed specifically for application in industrial environments.It uses a programmable memory, in its internal storage of the implementation of logical operations, sequence control, timing, counting and arithmetic operations and other operations of the instruction, through digital or analog input and output to control various types of mechanical equipment or production process.

Working principle of PLC

When programmable logic 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 scan cycle.During the entire run, the PLC's CPU repeats the above three phases at a scanning rate.

1. Input sampling

During the input sampling phase, the programmable logic controller reads all the input states and data in a scan sequence and stores them into the corresponding cells in the I/O image area.After the end of input sampling, the user program execution and output refresh phase.In both phases, even if the input state and data change, the state and data of the corresponding cell in the I/O image area do not change.Therefore, if the input is an impulse signal, the width of the impulse signal must be greater than one scan cycle to ensure that the input can be read in under any circumstances.

2. User program execution

During the execution phase of the user program, the PLC always scans the user program (ladder diagram) in top down order.When scanning each ladder diagram, and always scan ladder diagram on the left side of the control circuit, composed of various contact and press the left right after the first, after the first order of composed of contact control circuit logic operations, and then according to the result of logic operation, refresh the logic coil in the system RAM memory area corresponds to a state;Or refresh the state of the corresponding bit of the output coil in the I/O mapping area;Or to determine whether to execute the specific functional instructions specified in the ladder diagram.

In other words, during the execution of the user program, only the state and data of the input point in the I/O mapping area will not change, while the state and data of other output points and software devices in the I/O mapping area or the system RAM storage area may change.The result of the program execution will be applied to any ladder chart below that uses these coils or data;In contrast, in the ladder diagram below, the state or data of the logic coil being refreshed is only available to the program above it until the next scan cycle.

I/O points can be accessed directly during program execution if the immediate I/O instruction is used.Even with the I/O instruction, the input process image register value is not updated, the program directly from the I/O module value, the output process image register will be updated immediately, which is a little different from the immediate input.

3. Output refresh

When the user scan program is complete, the PLC enters the output refresh phase.During this period, the CPU flushes all the output latches according to the corresponding state and data in the I/O image area, and then drives the corresponding peripherals through the output circuit.This is the real output of the programmable logic controller.

According to the description of the above process, the characteristics of PLC working process can be summarized as follows:

1. PLC adopts the working mode of centralized sampling and centralized output, which reduces the influence of external interference.

2, PLC work process is the process of cycle scanning, cycle scanning time depends on the speed of execution of instructions, the length of the user program and other factors.

3. The influence of output on input has lag phenomenon.PLC uses centralized sampling, centralized output working mode, when the sampling stage is over, the change of the input state will wait until the next sampling cycle can be received, so the length of the lag time is mainly determined by the length of the cycle.In addition, input filtering time, output circuit lag time and other factors that affect the lag time.

4. The contents of the output image register depend on the results of the scan execution of the user program.

5. The content of the output latch is determined by the data in the output image register during the last output refresh.

6. The current actual output state of PLC is determined by the content of the output latch.

Now the programmable logic controller used in industry has been equivalent to or close to the host of a compact computer, its advantages in expansibility and reliability make it widely used in various industrial control fields at present.Whether it is in the computer direct control system or centralized decentralized control system DCS, or the field bus control system FCS, there are always various types of PLC controllers used in large numbers.PLC manufacturers are many, such as Siemens, Schneider, Mitsubishi, Delta and so on, almost involved in the field of industrial automation manufacturers will have their PLC products to provide.

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