Functions of PLC control cabinet
The PLC integrated control cabinet features overload, short circuit, and phase loss protection functions. It is compact, stable in operation, and fully functional. It can be combined according to the actual control scale to achieve single-cabinet automatic control or multiple cabinets forming a distributed control (DSC) system via industrial Ethernet or industrial fieldbus networks. The PLC control cabinet is adaptable to various sizes of industrial automation control applications and is widely used in industries such as power, metallurgy, chemical, papermaking, and environmental wastewater treatment.
Components of a PLC control cabinet
1. Circuit breaker: A main circuit breaker, which controls the power supply for the entire cabinet. It's something every cabinet should have.
2. PLC: This depends on the project requirements. For example, if the project is small, an integrated PLC can be used directly. However, if the project is large, modular or card-based PLCs may be needed, and redundancy (i.e., two sets used alternately) may also be required.
3. 24VDC power supply: A 24VDC switching power supply. Most PLCs come with a built-in 24VDC power supply. Whether or not you need this switching power supply depends on whether you actually need it.
4. Relays: Generally, a PLC can directly send commands to the control circuit, but it may also need to be relayed first. For example, if your PLC output port is 24VDC, but your control circuit diagram requires the PLC to supply 220VAC, then you must add a relay to the PLC output port. That is, the relay activates when the command is issued, but the control circuit node is then connected to the relay's normally open or normally closed contact. Whether to use a relay depends on the specific situation.
5. Terminal blocks: These are essential for every cabinet and can be configured according to the number of signals. A simple PLC control cabinet basically only needs these. If your control cabinet requires other components, you'll need to add more. For example, if you need to power certain field instruments or small control boxes, you might need to increase the number of circuit breakers. Or if you need to connect the PLC to a host computer, you might need to add switches or similar equipment. It depends on the specific situation.
PLC control cabinets enable automated equipment and process control, providing seamless network functionality. They are characterized by stable performance, scalability, and strong anti-interference capabilities, making them the core and soul of modern industry. Customized PLC control cabinets and frequency converter cabinets can be designed to meet specific user needs and can be integrated with human-machine interface touchscreens for easy operation. The equipment can also transmit data via DCS bus, Modbus, PROFIBUS, and other communication protocols; and provide control and monitoring via industrial PCs, Ethernet, and other protocols.
PLC control cabinet usage conditions
Power supply: DC 24V, two-phase AC 220V (-10%, +15%), 50Hz
Protection rating: IP41 or IP20
Environmental conditions: Ambient temperature between 0℃ and 55℃, avoid direct sunlight; relative humidity should be less than 85% (no condensation). Keep away from strong vibration sources, and prevent frequent or continuous vibration with a frequency of 10-55 Hz. Avoid corrosive and flammable gases.
Basic structure of a PLC control cabinet
A programmable logic controller (PLC) is essentially a computer specifically designed for industrial control. Its hardware structure is basically the same as that of a microcomputer, and its basic components are:
I. Power Supply
The power supply plays a crucial role in the entire system of a programmable logic controller (PLC). Without a good, reliable power supply, the PLC cannot function properly. Therefore, PLC manufacturers place great emphasis on the design and manufacture of power supplies. Generally, AC voltage fluctuations within +10% (+15%) allow the PLC to be directly connected to the AC mains without additional measures.
II. Central Processing Unit (CPU)
The Central Processing Unit (CPU) is the control center of the Programmable Logic Controller (PLC). It receives and stores user programs and data entered from the programmer according to the functions assigned by the PLC system program; it checks the status of the power supply, memory, I/O, and watchdog timers; and it can diagnose syntax errors in the user program. When the PLC is put into operation, it first receives the status and data of each input device in the field in a scanning manner and stores them in the I/O image area. Then, it reads the user program line by line from the user program memory, interprets the commands, and executes logical or arithmetic operations according to the instructions, sending the results to the I/O image area or data registers. After all user programs have been executed, it finally transmits the output status of each device in the I/O image area or the data in the output registers to the corresponding output devices. This cycle continues until the PLC stops operating.
To further improve the reliability of programmable logic controllers (PLCs), in recent years, large PLCs have adopted redundant systems with dual CPUs or voting systems with three CPUs. This ensures that even if one CPU fails, the entire system can still operate normally.
III. Memory
The memory that stores system software is called system program memory.
The memory that stores application software is called user program memory.
IV. Input/Output Interface Circuit
1. The field input interface circuit consists of an optocoupler circuit and a microcomputer input interface circuit. Its function is to serve as the input channel for the interface between the programmable logic controller and the field control.
2. The field output interface circuit is integrated with an output data register, a gating circuit, and an interrupt request circuit. Its function is to allow the programmable logic controller to output corresponding control signals to the field execution components through the field output interface circuit.
V. Functional Modules
Functional modules such as counting and positioning.
VI. Communication Module
Working principle: When a programmable logic controller (PLC) is put into operation, its operation generally consists of three stages: input sampling, user program execution, and output refresh. The completion of these three stages is called one scan cycle. Throughout the entire operation, the PLC's CPU repeatedly executes these three stages at a certain scan speed.
1. Input sampling stage
During the input sampling phase, the programmable logic controller (PLC) sequentially reads all input states and data in a scanning manner and stores them in the corresponding cells of the I/O image area. After input sampling is completed, the process transitions to the user program execution and output refresh phases. During these two phases, even if the input states and data change, the states and data of the corresponding cells 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 cycle to ensure that the input can be read under any circumstances.
2. User program execution phase
During the user program execution phase, the programmable logic controller (PLC) always scans the user program (ladder diagram) sequentially from top to bottom. When scanning each ladder diagram, it always first scans the control circuit composed of each contact on the left side of the ladder diagram, and performs logical operations on the control circuit composed of contacts in the order of left to right and top to bottom. Then, based on the result of the logical operation, it refreshes the state of the corresponding bit in the system RAM storage area of the logic coil; or refreshes the state of the corresponding bit in the I/O image area of the output coil; or determines whether to execute the special function instruction specified by the ladder diagram.
That is, during the execution of the user program, only the state and data of the input points in the I/O image area will not change, while the state and data of other output points and soft devices in the I/O image area or system RAM storage area may change. Moreover, the program execution result of the ladder diagram above will affect the ladder diagram below that uses these coils or data; conversely, the state or data of the logic coils of the ladder diagram below can only affect the program above it in the next scan cycle.
During program execution, if immediate I/O instructions are used, I/O points can be accessed directly. Even when using I/O instructions, the value of the input process image register will not be updated; the program directly retrieves the value from the I/O module. The output process image register will be updated immediately, which is somewhat different from immediate input.
3. Output refresh phase
After the user program scan is complete, the programmable logic controller (PLC) enters the output refresh phase. During this period, the CPU refreshes all output latch circuits according to the corresponding states and data in the I/O image area, and then drives the corresponding peripherals through the output circuits. This is when the PLC actually outputs.
Features: Programmable logic controllers have the following distinctive features.
1. The system is flexible in configuration and easy to expand, with switch control as its specialty; it can also perform PID loop control of continuous processes; and it can form complex control systems with host institutions, such as DDC and DCS, to achieve comprehensive automation of the production process.
2. Easy to use and simple to program, employing concise ladder diagrams, logic diagrams, or statement lists as programming languages, requiring no computer knowledge. Therefore, system development cycles are short, and on-site debugging is easy. Furthermore, the program can be modified online, changing the control scheme without disassembling the hardware.
3. It can adapt to various harsh operating environments, has strong anti-interference ability, and high reliability, far exceeding other models.
