Functions of PLC control cabinet
The PLC integrated control cabinet has protection functions such as overload, short circuit, and phase loss protection. It has compact structure, stable operation and complete functions. It can be combined according to the actual control scale to achieve automatic control of a single cabinet or multiple cabinets to form a distributed control system (DSC) through industrial Ethernet or industrial fieldbus network. PLC control cabinet can adapt to various large and small industrial automation control occasions. It is widely used in electric power, metallurgy, chemical industry, papermaking, environmental protection sewage treatment and other industries.
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Components of PLC control cabinet
1. Air switch: a general air switch, which is the power control of the entire cabinet. I believe it is a must-have item in every cabinet.
2. PLC: This should be selected according to project needs. For example, if the project is small, it can be directly integrated with an integrated PLC. But if the project is relatively large, it may require modules or card types, and it may also require redundancy (that is, two sets are used alternately).
3. 24VDC power supply: A 24VDC switching power supply. Most PLCs come with their own 24VDC power supply. You can decide whether to use this switching power supply based on whether you really need it.
4. Relay: Generally, the PLC can directly send instructions to the control loop, but it may also be relayed by the relay first. For example, if the output port of your PLC is powered by 24VDC, but the diagram drawn in your control loop requires the node supplied by the PLC to be powered by 220VAC, then you must add a relay to the PLC output port, that is, when the command is issued When the relay operates, the node of the control loop is then connected to the normally open or normally closed point of the relay. It also depends on the situation whether to use a relay.
5. Terminal block: This is definitely an essential thing for every cabinet and can be configured according to the number of signals. If it is just a simple PLC control cabinet, it basically needs these things. If you need other things in the control cabinet, it depends on the situation. For example, you may need to supply power to some on-site instruments or small control boxes, and you may need to increase the number of circuit breakers. Or if you want to connect the PLC to the host computer, you may need to add a switch or something. Subject to availability.
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PLC control cabinet can complete equipment automation and process automation control, realize perfect network functions, have stable performance, scalability, strong anti-interference and other characteristics, and is the core and soul of modern industry. PLC control cabinets, frequency conversion cabinets, etc. can be customized according to user needs to meet user requirements, and can be paired with a human-machine interface touch screen to achieve easy operation. The equipment can also transmit data with the DCS bus host computer through modbus, profibus and other communication protocols; control and monitoring can be achieved with industrial computers, Ethernet, etc.
PLC control cabinet usage conditions
Power supply: DC 24V, two-phase AC 220v, (-10%, +15%), 50HZ;
Protection level: IP41 or IP20;
Environmental conditions: The ambient temperature is between 0℃ and 55℃. Avoid direct sunlight; the relative humidity of the air should be less than 85% (no condensation). Stay 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 of PLC control cabinet
The programmable logic controller is essentially a computer dedicated to industrial control. Its hardware structure is basically the same as that of a microcomputer. Its basic composition is:
1. Power supply
The power supply of programmable logic controller plays a very important role in the entire system. Without a good and reliable power supply system, it cannot work properly. Therefore, manufacturers of programmable logic controllers also attach great importance to the design and manufacturing of power supplies. Generally, the AC voltage fluctuation is within the range of +10% (+15%), and the PLC can be directly connected to the AC power 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 the user program and data entered from the programmer according to the functions assigned by the programmable logic controller system program; checks the status of power supply, memory, I/O and alert timers, 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 on site in a scanning manner, 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 logical or arithmetic operation is performed according to the instructions and sent to the I/O image area or data register. After all user programs are executed, each output status of the I/O image area or the data in the output register is finally transferred to the corresponding output device, and this cycle continues until the operation stops.
In order to further improve the reliability of programmable logic controllers, in recent years, dual CPUs have been used to form a redundant system for large programmable logic controllers, or a three-CPU voting system has been adopted. In this way, even if a certain CPU fails, the entire system can still run normally.
3. Memory
The memory that stores system software is called system program memory.
The memory that stores application software is called user program memory.
4. Input and output interface circuit
1. The on-site input interface circuit consists of an optical coupling circuit and an input interface circuit of a microcomputer. It functions as an input channel for the interface between the programmable logic controller and on-site control.
2. The on-site output interface circuit is integrated by the output data register, strobe circuit and interrupt request circuit, and the programmable logic controller outputs corresponding control signals to the on-site execution components through the on-site output interface circuit.
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5. Function 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 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 phase, the programmable logic controller sequentially reads in all input states and data in a scanning manner and stores them in the corresponding units in the I/O image area. After the input sampling is completed, it enters the user program execution and output refresh stages. In these two phases, 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 under any circumstances.
2. User program execution stage
During 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 first left, then right, first up, then down. , and then according to the result of the logic operation, refresh the status of the corresponding bit of the logic coil in the system RAM storage area; or refresh the status of the corresponding bit of the output coil in the I/O image 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 of the input points in the I/O image area will not change, while the status and data of other output points and software devices in the I/O image area or system RAM storage area will not change. The status and data may change, and the program execution results of the ladder diagram listed above will affect any ladder diagram listed below that uses these coils or data; on the contrary, the ladder diagram listed below will have its program execution results affected. The status or data of the refreshed logic coil can only take effect on the program above it until the next scan cycle.
If you use the immediate I/O instruction during program execution, you can directly access the I/O point. Even if I/O instructions are used, the value of the input process image register will not be updated. The program directly obtains the value from the I/O module, and the output process image register will be updated immediately. This is somewhat different from immediate input.
3. Output refresh stage
When the scanning of the 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 status and data in the I/O image area, and then drives the corresponding peripherals through the output circuits. At this time, it is the real output of the programmable logic controller.
Functional features: Programmable logic controller has the following distinctive features.
1. The system structure is flexible and easy to expand, with switching control as its specialty; it can also perform PID loop control of continuous processes; and it can form complex control systems with host machines, such as DDC and DCS, to achieve comprehensive automation of the production process. .
2. Easy to use, simple programming, using concise ladder diagram, logic diagram or statement list and other programming languages without computer knowledge, so the system development cycle is short and on-site debugging is easy. In addition, the program can be modified online and the control scheme can be changed without dismantling the hardware.
3. It can adapt to various harsh operating environments, has strong anti-interference ability and strong reliability, which is much higher than other models.
