PLC control cabinet refers to the complete set of control, electrical cabinet that can realize the control of motor and switch.
PLC control cabinet has functions such as overload, short circuit and phase loss protection. The utility model has the advantages of compact structure, stable operation and complete functions, and can be combined according to the actual control gauge size, and can realize automatic control of single cabinets, and can also realize multi-cabinet distributed control (DSC) control system through industrial Ethernet or industrial field bus network. It can adapt to industrial automation control occasions of various sizes and is widely used in electric power, metallurgy, chemical, papermaking, environmental sewage treatment and other industries.
PLC control cabinet can complete equipment automation and process automation control, achieve perfect network function, stable performance, scalability, strong anti-interference and so on. It is the core and soul of modern industry. Users can design PLC control cabinets, frequency conversion cabinets, etc. according to their own needs, and can be equipped with human-machine interface touch screens for easy operation.
Typical applications include constant pressure water supply, air compressors, fan pumps, central air conditioners, port machinery, machine tools, boilers, paper machinery, food machinery, and more.
Let's spread the basic knowledge to everyone~
Components of the PLC control cabinetOpen
A total air switch, which is the power control of the entire cabinet, is something that every cabinet must have.
2.PLC
This should be chosen according to the needs of the project. If the project is small, you can use an integrated PLC directly; if the project is relatively large, you may need modules and cards, and you may need redundancy (that is, two sets of alternate use).
3.24VDC power supply
A 24VDC switching power supply, most of the PLCs are self-contained 24VDC power supply, you need to determine whether you want this switching power supply according to the actual situation.
4. Relay
The general PLC can directly send the command to the control loop, but it may also be relayed by the relay first. If the output of your PLC is 24VDC, but the picture drawn in your control loop needs 220VAC for the node, then you must add a relay to the PLC output, that is, the relay will act when the command is issued. Let the node of the control loop be connected to the normally open or normally closed point of the relay. This is also the choice of whether to use the relay depending on the situation.
5. Terminal block
This is definitely an essential part of every cabinet, and it can be configured according to the number of signals. If it is just a simple PLC control cabinet, basically these things are needed. If you need other equipment in your control cabinet, you will see an increase. For example, if you are likely to supply power to some on-site instruments or small control boxes, you may have to increase the number of open spaces. Or if you want the PLC to connect to the host computer, you may need to increase the switch. Subject to availability.
Power supply: DC DC 24V, two-phase AC 220v, (-10%, +15%), 50Hz
Protection level: IP41 or IP20
Environmental conditions: The ambient temperature is between 0 °C and 55 °C to prevent direct sunlight; the relative humidity of the air should be less than 85% (no condensation). Keep away from strong vibration sources and prevent frequent or continuous vibrations with a vibration frequency of 10-55HZ. Avoid corrosive and flammable gases.
basic structureFirst, the power supply
The power of the programmable logic controller plays an important role in the overall system. If a good, reliable power system does not work properly, manufacturers of programmable logic controllers also place great emphasis on the design and manufacture of power supplies. Generally, the AC voltage fluctuation is in the range of +10% (+15%), and the PLC can be directly connected to the AC grid without taking other measures.
Second, the central processing unit (CPU)
The central processing unit (CPU) is the control center of the programmable logic controller. It receives and stores user programs and data typed from the programmer in accordance with the functions assigned by the programmable logic controller system program; checks the status of power, memory, I/O, and watchdog timers, and can diagnose syntax errors in the user program.
When the programmable logic controller is put into operation, it firstly receives the status and data of each input device in the field by scanning, and stores them in the I/O image area respectively, and then reads the user program one by one from the user program memory. After the command is interpreted, the result of the logic or arithmetic operation is executed in the I/O image area or data register as specified by the instruction. After all the user programs have been executed, the output status of the I/O image area or the data in the output register is finally transferred to the corresponding output device, and the operation is cycled until the operation is stopped.
In order to further improve the reliability of the programmable logic controller, in recent years, large-scale programmable logic controllers have also adopted a dual CPU to form a redundant system, or a three-CPU voting system. This way, even if a CPU fails, the entire system will still operate normally.
Third, the memory
The memory that holds the system software is called the system program memory.
The memory that holds the application software is called the user program memory.
Fourth, the input and output interface circuit
1. The field input interface circuit consists of an optical coupling circuit and an input interface circuit of the microcomputer, which functions as an input channel of the interface between the programmable logic controller and the field control.
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 corresponding control signals to the execution components of the field through the field output interface circuit.
Five, function module
Such as counting, positioning and other functional modules.
Sixth, communication module
Working principle: When the programmable logic controller is put into operation, its working process is generally divided into three stages, namely, input sampling, user program execution and output refresh. Completing the above three phases is called a scan cycle. During the entire run, the CPU of the programmable logic controller repeatedly performs the above three phases at a certain scan speed.
1. Input sampling stage
During the input sampling phase, the programmable logic controller sequentially reads all input states and data in a scan mode and stores them in the corresponding cells in the I/O map area. After the input sampling is finished, it is transferred to the user program execution and output refresh phase. In these two phases, the status and data of the corresponding unit in the I/O map area does not change even if the input status and data change. Therefore, if the input is a pulse signal, the width of the pulse signal must be greater than one scan period to ensure that the input can be read in any case.
2, the user program execution stage
During the user program execution phase, the programmable logic controller always scans the user program (ladder) in order from top to bottom. When scanning each ladder diagram, the control lines formed by the contacts on the left side of the ladder diagram are always scanned first, and the control lines composed of the contacts are logically operated in the order of first left, then right, first up and then down. And then refreshing 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 refreshing the state of the corresponding bit of the output coil in the I/O map area; or determining whether to execute the ladder diagram The specified special function instructions.
That is, during the execution of the user program, only the state and data of the input point in the I/O map area will not change, while other output points and soft devices are in the I/O map area or the system RAM storage area. The status and data are subject to change, and the ladder diagrams listed above, the program execution results will work on the ladder diagrams that use these coils or data; instead, the ladder diagrams below, The state or data of the refreshed logic coil can only be applied to the program that is listed above in the next scan cycle.
I/O points can be accessed directly if immediate I/O instructions are used during program execution. Even if I/O instructions are used, the value of the input process image register will not be updated. The program will take values ​​directly from the I/O module. The output process image registers will be updated immediately, which is somewhat different from the immediate input.
3. Output refresh phase
When the scan user program ends, the programmable logic controller enters the output refresh phase. During this time, the CPU refreshes all the output latch circuits according to the corresponding state and data in the I/O map area, and drives the corresponding peripherals through the output circuit. At this time, it is the real output of the programmable logic controller.
Programmable Logic Controller Features
1. The system is flexible in structure and easy to expand. It is controlled by switch quantity control. It can also carry out PID loop control of continuous process. It can also form complex control system with host computer, such as DDC and DCS, to realize the comprehensive automation of production process. .
2. Easy to use, simple to program, using a simple programming language such as ladder diagram, logic diagram or statement list, without computer knowledge, so the system development cycle is short, and on-site debugging is easy. In addition, the program can be modified online, changing the control scheme without disassembling the hardware.
3. It can adapt to all kinds of harsh operating environments, with strong anti-interference ability and high reliability, which is much higher than other various models.
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