15 Basics of PLC Automatic Control You Should Know

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As we all know, industrial production and the development of science and technology are inseparable from the automatic control of PLC.

PLC can be broadly understood as:

Centralized relay extension control cabinet.

In the actual production application, PLC greatly saves the cost of industrial control and strengthens the centralized management and automatic control of equipment.

If you want to learn PLC well, first of all, the foundation of PLC needs to be solid.

1.From the composition of PLC, in addition to CPU, memory and communication interface, what other interfaces are directly related to the industrial site? And explain its main functions.

1) Input interface:

Receive the signal of the controlled equipment, and drive the internal circuit to turn on or off through the optocoupler and input circuit.

2) Output interface:

The execution result of the program is output through the optocoupler and output components (relay, thyristor and transistor) of the output interface to control the on or off of the external load.

2. Which parts does the basic unit of PLC consist of? What role does each play?


The core component of PLC, which commands PLC to carry out various work.

Such as receiving user programs and data, diagnosis, executing programs, etc;

2) Memory:

Storage system and user programs and data;

3) I / O interface:

The connecting part between PLC and the controlled object in the industrial production site is used to receive the signal of the controlled equipment and output the execution result of the program;

4) Communication interface:

Exchange information with monitor, printer and other equipment through communication interface;

5) Power supply.

3.What are the types of PLC switch output interfaces? What are the characteristics of each?

Thyristor output type:

Generally, it can only carry AC load, with fast response speed and high action frequency;

Transistor output type:

Generally, it can only carry DC load, with fast response speed and high action frequency;

Relay output type:

Generally, it can carry AC and DC loads, but its response time is long and its action frequency is low.

4. According to the structure type, what types can PLC be divided into? What are the characteristics of each?

1) Integral:

CPU, power supply and I / O components are all concentrated in one chassis, which has compact structure and low price. Generally, small PLC adopts this structure;

2) Modular:

Each part of the PLC is divided into several separate modules, and different modules can be selected according to the needs to form a system. It has the characteristics of flexible configuration, convenient expansion and maintenance. Generally, this structure is adopted for medium and large PLC.

Modular PLC is composed of frame or base plate and various modules, which are installed on the socket of frame or base plate.

3) Stack type:

Combined with the characteristics of integral and modular, the CPU, power supply and I / O interface of stacked PLC are also independent modules, but they are connected by cables, which makes the system not only flexible configuration but also small volume.

5. What is the scanning cycle of PLC? What is its main influence?

The scanning process of PLC includes five stages: internal processing, communication service, input processing, program execution and output processing.

The time required for scanning in these five stages is called scanning cycle.

The scanning cycle is related to the running speed of CPU, PLC hardware configuration and the length of user program.

6. How does the PLC execute the user program? What are the stages of user program execution?

PLC executes the user program by means of cyclic scanning.

The execution process of the user program includes input sampling stage, program execution stage and output refresh stage.

Schematic diagram of button switch structure

1. Button cap

2. Return spring

3. Moving contact

4. Normally closed static contact

5. Normally open static contact

7. What are the advantages of PLC control system compared with relay control system?

1) Control method:

PLC adopts program mode to realize control, which is easy to change or increase control requirements, and the contact of PLC is unlimited;

2) Working mode:

PLC adopts serial working mode to improve the anti-interference ability of the system;

3) Control speed:

The contact of PLC is actually a trigger, and the execution time of instructions is on the microsecond level;

4) Timing and counting:

PLC uses semiconductor integrated circuit as timer, clock pulse is provided by crystal oscillator, with high delay precision and wide range.

PLC has counting function that relay system does not have;

5) Reliability and Maintainability:

PLC adopts microelectronic technology with high reliability, and its self inspection function can be found out in time.

8. Why does PLC produce output response lag? How to improve I / O response speed?

Because PLC adopts the circular scanning mode of centralized sampling and centralized output, the state of the input can only be read in at the input sampling stage of each scanning cycle, and the execution result of the program can only be sent out at the output refresh stage;

Secondly, the input and output delay of PLC and the length of user program can cause the output response lag.

To improve I / O response sampling, output refresh, or direct input sampling, output refresh, interrupt input and output and intelligent I / O interface.

9. What are the internal soft relays of Siemens series PLC?

Input relay, output relay, auxiliary relay, status register, timer, counter and data register.

10. How to select PLC?

1) Model selection:

The structural form, installation mode, functional requirements, response speed, reliability requirements, model unification and other aspects shall be considered;

2) Capacity selection:

I / O points and user storage capacity should be considered;

3) I / O module selection:

Including the selection of switching value and analog I / O module, as well as the selection of special function module;

4) Selection of power module, programmer and other equipment.

11. Briefly describes the characteristics of PLC centralized sampling and centralized output working mode, and what are the advantages and disadvantages of this working mode?

Centralized sampling:

In a scanning cycle, the sampling of the input state is only carried out in the input sampling stage. When entering the program execution stage, the input end will be blocked.

Centralized output:

In a scanning cycle, only in the output refresh stage, the output related state in the output image register is transferred to the output latch to refresh the output interface, and the output state is always saved in the output image register in other stages.

This working mode can improve the anti-interference ability of the system and enhance the reliability of the system, but it will cause the lag of PLC input / output response.

12. What kind of working mode does PLC adopt? What are the characteristics?

PLC adopts the working mode of centralized sampling, centralized output and cyclic scanning.


Centralized sampling means that in a scanning cycle, PLC samples the input state only in the input sampling stage.

When entering the program execution stage, the input end will be blocked.

Centralized output means that in a scanning cycle, PLC transfers the output related state in the output image register to the output latch only in the output refresh stage, refreshes the output interface, and saves the output state in the output image register in other stages.

Cyclic scanning refers to that PLC needs to perform multiple operations in a scanning cycle.

It adopts the method of time-sharing scanning to execute one by one in order and run repeatedly again and again.

13. What are the main parts of the electromagnetic contactor? Briefly describe the working principle of electromagnetic contactor.

Electromagnetic contactor is generally composed of electromagnetic mechanism, contact, arc extinguishing device, release spring mechanism, support and base.

The contactor works according to the electromagnetic principle:

When the electromagnetic coil is energized, the coil current generates a magnetic field, which makes the static iron core generate electromagnetic suction to attract the armature, and drives the contact action to open the normally closed contact and close the normally open contact.

The two are linked.

When the coil is powered off, the electromagnetic force disappears, and the armature drops under the action of the release spring to restore the contact, that is, the normally open contact is open and the normally closed contact is closed.

14. Brief definition of programmable logic controller (PLC).

Programmable logic controller (PLC) is an electronic device designed for digital operation in industrial environment.

It adopts the memory that can be programmed, which is used to store the instructions for executing logical operation, sequential operation, timing, counting and arithmetic operation, and can control various types of machinery or production processes through digital or analog input and output.

PLC and its related peripheral equipment shall be designed according to the principle that it is easy to form a whole with the industrial control system and expand its functions.

15. Brief answer the difference of working principle between PLC system and relay contactor system.

Different components;

The number of contacts is different;

The methods of control are different.

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