PLC control cabinet overload, short circuit, lack of phase protection and other functions. Its compact structure, stable work, complete functions, can be based on the actual control of the size of a combination of control, both single cabinet automatic control can also be achieved through the multi-cabinet industrial Ethernet or industrial field bus network distributed control system (DSC) Can adapt to various sizes of industrial automation and control applications, is widely used in electric power, metallurgy, chemical industry, papermaking, environmental protection, sewage treatment and other industries.

Typical applications are constant pressure water supply, air compressor, fan pump, central air conditioning, port machinery, machine tools, boilers, paper machinery, food machinery and so on.
 

Let's take a look at the basics about PLC control cabinet.

 

PLC control cabinet components

1.Empty

A total air switch, which is the power control of the entire cabinet, each cabinet must have something.

2.PLC

This should be based on the project needs to choose. If the project is small, you can directly use an integrated PLC; if the project is relatively large, you may need the module, card-type, but may also require redundancy (that is, two sets of alternating use).

3.24VDC power supply

A 24VDC switching power supply, most of the PLC comes with 24VDC power supply, need to be based on the actual situation whether to the switching power supply.

4.Relay

General PLC can send commands directly to the control loop, but may also be the first relay relay. If your PLC output port is charged 24VDC, but your control circuit diagram drawn need PLC for the node is 220VAC, then you have to add a PLC output port relay, that is, when the command is issued relay action, Let the node of the control loop be connected to the normally open or normally closed point of the relay. This is also based on the situation choose whether to use the relay.

5.Terminals

This is definitely something that is essential for every cupboard and can be configured according to the number of signals. If only a simple PLC control cabinet, the basic is the need for these gadgets, if your control cabinet also need to have other equipment, see the situation increases. For example, you may want to power some of the field instruments or small control box, you may have to increase the number of open space. Or you have to PLC connected to the host computer, you may need to increase the switch or something. Depending on the situation.

 

PLC control cabinet conditions of use

Power Supply: DC 24V DC, two-phase AC 220v, (-10%, + 15%), 50Hz

Degree of protection: IP41 or IP20 　

Environmental conditions: Ambient temperature at 0 ℃ -55 ℃, to prevent direct sunlight; air relative humidity should be less than 85% (no condensation). Keep away from strong vibration source and prevent frequent or continuous vibration with vibration frequency of 10-55Hz. Avoid corrosive and flammable gases.

 

basic structure

1.the power

The power of a programmable logic controller plays a very important role in the overall system. If not a good, reliable power system is not working properly, so the manufacturer of programmable logic controller power supply design and manufacturing also attach great importance. General AC voltage fluctuations in the range of +10% (+15%), the PLC can be connected directly to the AC grid without taking any other measures.
 

2.Central Processing Unit (CPU)

The central processing unit (CPU) is the control hub of the programmable logic controller. It receives and stores user programs and data typed from the programmer according to functions assigned to the programmable logic controller system program; checks the status of the power supply, memory, I / O, and watchdog timer, and can diagnose syntax errors in the user program.

When the programmable logic controller is put into operation, it first receives the status and data of each input device in the field by scan mode and stores them into the I / O image area respectively, then reads the user programs one by one from the user program memory, After the command is interpreted, the result of executing a logical or arithmetic operation according to the instruction is sent to the I / O image area or data register. After all the user programs are executed, the output status of the I / O map area or the data in the output register is finally sent to the corresponding output device, and the program runs in a loop until it stops.

In order to further improve the reliability of programmable logic controller, large programmable logic controller in recent years also uses dual CPU to form a redundant system, or the use of three CPU voting system. This allows the entire system to function even if one of the CPUs fails.
 

3.Memory

The storage of system software is called system program memory.

The memory that holds the application software is called user program memory.
 

4.Input and output interface circuit

1.Field input interface circuit by the optical coupling circuit and microcomputer input interface circuit, the role of the programmable logic controller and field control interface interface input channel.

2.The field output interface circuit is integrated by the output data register, the strobe circuit and the interrupt request circuit, and the programmable logic controller outputs the corresponding control signal to the field execution unit through the field output interface circuit.
 

5.functional module

Such as counting, positioning and other functional modules.
 

6.Communication module

Working principle: When the programmable logic controller is put into operation, its working process is generally divided into three phases, namely input sampling, user program execution and output refresh three stages. The completion of the above three stages is called a scan cycle. During the entire run, the programmable logic controller's CPU repeatedly performs the above three stages at a certain scan rate.

i.Enter the sampling phase

During the input sampling phase, the programmable logic controller reads all input states and data sequentially in scan mode and stores them in the corresponding cells in the I / O image area. Input sampling is completed, the user program execution and output refresh phase. In both phases, the status and data of the corresponding cells in the I / O map area will not change even if the input status and data changes. Therefore, if the input is a pulse signal, the width of the pulse signal must be greater than one scan cycle to ensure that the input can be read in any case.

ii.User program execution phase

In the user program execution phase, the programmable logic controller always scans the user program (ladder diagram) in the top-down sequence. When scanning each ladder diagram, it always scans the control circuit formed by the contacts on the left side of the ladder diagram first, and then performs the logic operation on the control lines formed by the contacts in the order of left back, right back, first back to back next , And then refresh the state of the corresponding bit of the logic coil in the system RAM storage area according to the result of the logic operation; or refresh the state of the corresponding bit of the output coil in the I / O mapping area; or determine whether to execute the ladder diagram Specified special function instructions.

That is, during the execution of the user program, only the status and data whose input point is in the I / O image area will not change while the other output points and the soft device are in the I / O image area or the system RAM memory area Status and data are subject to change, and the ladder in the top row, the results of the program execution will be in the row below all the use of these coils or data ladder function; In contrast, the lower ladder, which The state or data of the logical coil to be refreshed can not affect the program that is placed on it until the next scan cycle.

I / O points can be accessed directly using the immediate I / O instructions during program execution. Even if the I / O instruction is used, the value of the input process video register will not be updated. The program directly takes values from the I / O module. The output process video register will be updated immediately, which is different from the immediate input.

iii.Output refresh phase

When the scanning user program ends, the programmable logic controller enters the output refresh phase. During this period, the CPU refreshes all the output latch circuits according to the corresponding status and data in the I / O image area and drives the corresponding peripherals through the output circuit. At this point, it is the real output of the programmable logic controller.

 

Programmable logic controller features:

i.The system is flexible and easy to expand, with its advantages of digital control. It also can carry out PID loop control of continuous process. It can form complicated control system such as DDC and DCS with the host computer to realize the comprehensive automation of the production process.

ii.Easy to use, simple programming, using simple ladder diagram, logic diagram or statement table and other programming languages, without the computer knowledge, so the system development cycle is short, on-site debugging easy. In addition, the program can be modified online to change the control scheme without disassembling the hardware.

iii.Can adapt to a variety of harsh operating environment, anti-interference ability, reliability, much higher than the other models.