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 engineering needs. For example, if the project is small, it can be directly an integrated PLC, but if the project is relatively large, it may require modules and cards, and it may also require redundancy (that is, the two sets are used interchangeably).
3. 24VDC power supply: A 24VDC switching power supply. Most PLCs have their own 24VDC power supply. Whether or not this switching power supply is required depends on whether it is really needed.
4. Relay: In general, PLC can directly send instructions to the control loop, but it may be transferred by the relay first.
For example, if the output of your PLC is powered by 24VDC, but the picture drawn in your control loop requires that the node supplied by the PLC is 220VAC, then you must add a relay to the PLC output, that is, the instruction When the relay is activated, the node of the control loop is then connected to the normally open or normally closed point of the relay. Whether to use the relay is also selected according to the situation.
5. Terminals: This must be essential for every cabinet and can be configured according to the number of signals. If it is just a simple PLC control cabinet, these things are basically needed. If there are other things in your control cabinet, it depends on the situation. For example, you may have to supply power to some on-site instruments or a small control box, and you may have to increase the number of open circuits. Or if you want to connect the PLC to the host computer, you may need to add a switch or something. It depends.
PLC control cabinet can complete equipment automation and process automation control, achieve perfect network function, stable performance, scalability, strong anti-interference and other characteristics, is the core and soul of modern industry.
PLC control cabinets and inverter cabinets can be designed according to user needs to meet user requirements, and can be matched with human-machine interface touch screen to achieve the purpose of easy operation. The device can also communicate with the DCS bus host computer modbus, profibus and other communication protocol data transmission; industrial control computer, Ethernet and other control and monitoring.
Typical applications: water treatment, constant pressure water supply, air compressor, fan and water pump, central air conditioner, port machinery, machine tool, boiler, paper machinery, food machinery, etc.
Overview of PLC control cabinetThe PLC integrated control cabinet has protection functions such as overload, short circuit and phase loss protection. It has compact structure, stable work and complete functions. It can be combined according to the size of the actual control plan. It can not only realize single cabinet automatic control, but also realize multiple cabinets to form a distributed (DSC) control system through industrial Ethernet or industrial field bus network. PLC control cabinet can adapt to various industrial automation control occasions. Widely used in electric power, metallurgy, chemical industry, paper making, environmental protection sewage treatment and other industries.
PLC control cabinet use conditionsPower supply: 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 to prevent frequent or continuous vibration with a vibration frequency of 10-55 Hz. Avoid corrosive and flammable gases.
basic structureThe programmable logic controller is essentially a computer dedicated to industrial control. Its hardware structure is basically the same as that of a microcomputer, and its basic structure is:
1. Power supply
The power supply of the programmable logic controller plays a very important role in the entire system. Without a good and reliable power supply system, it will not work properly. Therefore, manufacturers of programmable logic controllers also attach great importance to the design and manufacture of power supplies.
Generally, the AC voltage fluctuation is in the range of + 10% (+ 15%). You can connect the PLC directly to the AC grid without taking other measures.
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 entered from the programmer according to the functions given by the programmable logic controller system program; checks the status of 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, first it receives the status and data of each input device in the field by scanning, and stores them in the I / O image area, and then reads the user programs one by one from the user program memory. After the command is interpreted, the result of performing logical or arithmetic operations according to the instructions 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 image area or the data in the output register is finally transferred to the corresponding output device, and the cycle runs until it stops.
In order to further improve the reliability of programmable logic controllers, in recent years, dual-CPUs have been used to form redundant systems for large-scale programmable logic controllers, or voting systems using three CPUs. In this way, even if a CPU fails, the entire system can still operate normally.
3. Memory
The memory for storing system software is called system program memory.
The memory for storing application software is called user program memory.
4. Input and output interface circuit
1. The field input interface circuit consists of an optical coupling circuit and a microcomputer input interface circuit, which functions as the input channel of the interface interface between the programmable logic controller and the field control.
2. The on-site output interface circuit is integrated by the output data register, the strobe circuit and the interrupt request circuit, and functions as a programmable logic controller to output the corresponding control signals to the on-site execution components through the on-site output interface circuit.
5. Functional modules
Functional modules such as counting and positioning.
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 stages is called a scan cycle. During the entire operation, the CPU of the programmable logic controller repeatedly executes the above three stages at a certain scanning speed.
1. Input sampling stage
In the input sampling stage, the programmable logic controller sequentially reads all input states and data in a scanning manner and stores them in the corresponding unit in the I / O image area. After the input sampling is completed, it shifts to the user program execution and output refresh phase. In these two stages, even if the input status and data change, the status and data of the corresponding unit in the I / O image area will not 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. User program execution stage
In the user program execution phase, the programmable logic controller always scans the user program (ladder diagram) in order from top to bottom. When scanning each ladder diagram, the control circuit composed of contacts on the left side of the ladder diagram is always scanned first, and logical operations are performed on the control circuit composed of contacts in the order of left first, then right, and then top and bottom. , 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 image area; or determine whether to execute the ladder diagram The specified special function instructions.
That is, during the execution of the user program, only the status and data of the input point in the I / O image area will not change, while other output points and soft devices are in the I / O image area or the system RAM storage area. Both the state and the data may change, and the ladder diagram listed above, the program execution results will affect the ladder diagram that uses these coils or data below; on the contrary, the ladder diagram arranged below, its The status or data of the logic coil being refreshed can only affect the programs arranged on it until the next scan cycle.
If you use immediate I / O instructions during program execution, you can directly access the I / O points. Even if the I / O instruction is used, the value of the input process image register will not be updated, the program directly takes the value from the I / O module, and the output process image register will be updated immediately, which is somewhat different from the immediate input.
3. Output refresh stage
When the scan user program ends, the programmable logic controller enters the output refresh stage. During this period, the CPU refreshes all output latch circuits according to the corresponding state and data in the I / O image area, and then drives the corresponding peripherals through the output circuit. At this time, it is the real output of the programmable logic controller.
The programmable logic controller has the following distinctive features:1. The system configuration is flexible and easy to expand, with the switch quantity control as its specialty; it can also perform PID loop control of continuous processes; and can form a complex control system with the host computer, such as DDC and DCS, to achieve comprehensive automation of the production process .
2. Easy to use, simple programming, using concise 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 to change the control scheme without disassembling the hardware.
3. It can adapt to various harsh operating environments, with strong anti-interference ability and strong reliability, which is much higher than other various models.
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