PLC stands for Programmable Logic Controller. It is a digital electronic system designed for industrial applications. It uses a programmable memory to store programs, execute user-oriented instructions for logical operations, sequential control, timing, counting, and arithmetic operations, and control various types of machinery or production processes through digital or analog inputs/outputs. It is a core component of industrial control.
Basic structure of a PLC controller
A 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:
1. Power supply
The power supply plays a crucial role in the entire PLC system. 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.
2. Central Processing Unit (CPU)
The Central Processing Unit (CPU) is the control center of the PLC. It receives and stores the user program 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 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 register. After all the user programs have been executed, it finally transmits the output status of each output in the I/O image area or the data in the output register to the corresponding output device. This cycle continues until the PLC stops operating.
To further improve the reliability of 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.
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/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 PLC and the field control.
(2) The field output interface circuit is integrated with the output data register, the gating circuit and the interrupt request circuit. Its function is to output the corresponding control signals to the field execution components through the field output interface circuit.
(3) Functional modules: such as counting, positioning and other functional modules.
(4) Communication modules: such as Ethernet, RS485, Profibus-DP communication modules, etc.
PLC controllers are widely used in industrial control. Before the advent of programmable logic controllers, hundreds or even thousands of relays and counters were typically needed to form an automated system with the same function. Now, simple programmable logic controller modules have largely replaced these large devices. The system program of a programmable logic controller is generally initialized before leaving the factory, and users can edit the corresponding user program according to their own needs to meet different automated production requirements.
