PLC Working Principle and Application Insight Report

Detail

PLC has a wide range of application scenarios and plays an important role in various industrial fields. From aquarium control systems to fitness equipment control, from restaurant automated kitchens to garbage disposal systems, to artificial rain systems, etc. PLCs are present. In terms of intelligent building control, PLCs can be applied to energy management, security monitoring, lighting control and other aspects to achieve energy-saving, safe and comfortable building environments. For example, using PLC to control building energy management systems can achieve energy consumption monitoring, ambient temperature control, and improve building energy efficiency. In industrial production, PLCs can be used in welding and processing, analog control, motion control and other fields to promote the development of industrial intelligence.

With the continuous advancement of technology, PLC technology is also developing, leading the profound transformation of future industrial production. On the one hand, PLC technology is more integrated into industrial, automotive, communication and other products, with increasing miniaturization and integration, and more refined process requirements. On the other hand, with the development of AI technology, intelligent PLC technology continues to emerge, such as the intelligent I/O module of PLC used in industrial control and the intelligent temperature control module in automobiles, which will have more application scenarios in the future. In addition, diversified PLC technology is also developing, including PLC-related programming languages ​​and tools, customized PLC products will increase, and PLC modular technology will also increase. The development of these technologies will make industrial control systems more flexible, efficient and reliable, and promote industrial production to develop in a more intelligent, autonomous and green direction.

The work of PLC is mainly divided into three steps: information collection, program operation, and instruction output. First, in the input sampling stage, the PLC reads all input states and data in sequence in a scanning manner and stores them in the corresponding units in the I/O image area. For example, when a sensor signal is transmitted to the PLC, at this stage, the PLC will read the sensor state and store it in a specific input image register. Next, in the user program execution stage, the PLC always scans the user program in order from top to bottom. In this stage, the PLC will take data from the input image register and the internal component register, perform logical operations and arithmetic operations, and then store the results in related memories such as the output image register. For example, when the program executes a logical judgment instruction, the PLC will judge according to the input state and the data in the internal register, and store the result. Finally, in the output refresh stage, 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.

PLC adopts cyclic scanning working mode, which has significant characteristics and advantages. This working mode has centralized input and centralized output, high reliability and strong anti-interference ability. For example, in a complex industrial environment, there may be various electromagnetic interference and noise, but the cyclic scanning working mode of PLC can ensure the stable operation of the system without being affected by external interference. However, this working mode also has certain disadvantages, namely, delayed response and slow speed. For example, for some application scenarios that require fast response, special measures may need to be taken, such as using high-speed counting modules, interrupt processing, etc. to improve the response speed of the system.

In the input sampling stage, the PLC first scans all input terminals and stores each input state in the corresponding input image register. At this time, the input image register is refreshed. Generally speaking, the width of the input signal should be greater than one scan cycle, otherwise it is likely to cause signal loss. For example, if the input signal of a button lasts less than the scan cycle of the PLC, then this signal may not be correctly collected. This is because after the input sampling is completed, the program execution and output refresh phase begins. The input image register is isolated from the outside world. No matter how the input changes, its content remains unchanged until the input sampling phase of the next scan cycle, when the new content of the input terminal is rewritten.

In the user program execution phase, the PLC scans and executes instructions one by one from top to bottom and from left to right in the order of the ladder diagram program. When the instruction involves input and output status, the PLC "reads" the corresponding input terminal status collected from the input image register and "reads" the current status of the corresponding component ("soft relay") from the component image register. Then the corresponding operation is performed, and the latest operation result is immediately stored in the corresponding component image register. For example, when the program executes an addition instruction, the PLC will read the corresponding value from the input image register, perform the addition operation, and store the result in the component image register. For the component image register, the state of each component ("soft relay") will be refreshed as the program executes.

In the output refresh phase, the states of all output relays in the component image register are transferred to the output latch together in the output refresh phase, and the output is concentrated in a certain way, and finally drives the external load through the output terminal. For example, when the state of an output relay in the output image register is "1", in the output refresh phase, this state will be transferred to the output latch, and then drive the corresponding external load, such as motor, indicator light, etc. through the output circuit. Before the next output refresh phase begins, the state of the output latch will not change, and thus the state of the corresponding output terminal will not change.