PLC control cabinet essence knowledge, one second collection
Components of the PLC control cabinet
1. Open: A total air switch, this is the power control of the entire cabinet. I believe that every cabinet must have something.
2, PLC: This should be selected according to the needs of the project. For example, if the project is small, it can be directly an integrated PLC. However, if the project is relatively large, it may require modules and card-type, and may also require redundancy (that is, two sets of alternate use).
3, 24VDC power supply: a 24VDC switching power supply, most PLCs are equipped with 24VDC power supply, depending on whether you really need to determine whether you want this switching power supply.
4. Relay: The general PLC can directly send the command to the control loop, but it may also be relayed by the relay first.
For example, 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 command is issued. The relay acts, but then the node of the control loop is connected to the normally open or normally closed point of the relay. It is also possible to choose whether or not to use the relay depending on the situation.
5. Terminals: This is definitely something that is essential for every cabinet. It can be configured according to the number of signals. If you just need a simple PLC control cabinet, you need these things. If you need other things in your control cabinet, you can see the situation 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.
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.
PLC control cabinets, frequency conversion cabinets, etc. can be designed according to user requirements to meet user requirements, and can be matched with human-machine interface touch screen for easy operation. The device can be connected with the data transmission of the dcs bus host computer modbus, profibus and other communication protocols; the control and monitoring realized by the industrial computer and Ethernet.
PLC application field
Typical applications: water treatment, constant pressure water supply, air compressors, fan pumps, central air conditioners, port machinery, machine tools, boilers, paper machinery, food machinery, etc.
PLC control cabinet overview
PLC integrated control cabinet has protection functions such as overload, short circuit and phase loss protection. It has a compact structure, stable operation and complete functions. It can be combined according to the actual control size and size, which can realize the automatic control of single cabinet, and can also realize the multi-cabinet (DSC) control system through industrial Ethernet or industrial field bus network. PLC control cabinet can adapt to industrial automation control occasions of various sizes. Widely used in electric power, metallurgy, chemical, paper, environmental sewage treatment and other industries.
PLC control cabinet use conditions
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 structure
The programmable logic controller is essentially a computer dedicated to industrial control. Its hardware structure is basically the same as that of a microcomputer. The basic structure is as follows:
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1, 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.
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2, 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.
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3, 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.
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4, 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. The function is 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 a corresponding control signal to the execution component of the field through the field output interface circuit.
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5, function module
Such as counting, positioning and other functional modules.
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6, 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, the 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 controllers have the following distinctive features:
1. The system is flexible in structure and easy to expand, with the switch quantity control as its special feature; it can also carry out the PID loop control of the continuous process; and can form a complex control system with the host computer, such as DDC and DCS, to realize the comprehensive automation of the production process. .
2, easy to use, simple programming, using a simple programming language such as ladder diagram, logic diagram or statement table, without the need of computer knowledge, so the system development cycle is short, on-site debugging is easy. In addition, the program can be modified online, changing the control scheme without disassembling the hardware.
3, can adapt to a variety of harsh operating environment, strong anti-interference ability, strong reliability, far higher than other models.
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