1.Overview and Structure of PLC
PLC is an electronic system device designed specifically for digital operation in industrial field applications. It uses programmable memory to store and execute logical operations, sequential control, timing/counting, and arithmetic operations, and controls various types of machinery or production processes through digital or analog input/output interfaces.
The basic structure of PLC includes a Central Processing Unit (CPU), memory, input/output interfaces (I/O interfaces), communication interfaces, and a power supply. The CPU is the core of PLC, responsible for running user programs, monitoring I/O interface states, making logical judgments, and processing data; memory is used to store system and user programs and data; I/O interfaces serve as the bridge connecting PLC to input/output devices, responsible for receiving and sending signals; communication interfaces are used for information exchange between PLC and other devices; the power supply provides energy for the entire system.
2. Working Principle and Scanning Process of PLC
The working principle of PLC is based on a “sequential scanning and continuous cycling” method. When PLC is running, the CPU scans the user program stored in the user memory according to the control requirements prepared by the user, performing periodic cyclic scanning according to the instruction sequence number (or address number). If there are no jump instructions, it starts executing the user program sequentially from the first instruction until the program ends. Then it returns to the first instruction and begins the next round of scanning.
The scanning process of PLC mainly consists of three stages: input sampling, program execution, and output refreshing. In the input sampling stage, PLC sequentially reads the on/off states or input data from all input terminals temporarily stored in the input latch and writes them into the corresponding input status registers. In the program execution stage, PLC performs logical operations and processing according to the user program. In the output refreshing stage, PLC outputs the results of the program execution to the control devices.
3. Programming Languages and Functions of PLC
The programming languages of PLC mainly include Ladder Diagram (LAD), Function Block Diagram (FBD), and Structured Control Language (SCL). Among them, Ladder Diagram is a graphical language that is intuitive and easy to master, making it the most widely used. It uses terms and symbols similar to relay contacts, coils, and series/parallel connections to represent the logical relationships between PLC inputs and outputs based on control requirements.
PLC has powerful functions, including control functions, data acquisition and storage processing functions, input/output interface conditioning functions, communication networking functions, and programming/debugging functions. The control function is the most basic function of PLC, including logical control, timing control, counting control, and sequential control. The data acquisition and storage processing function is used to collect, store, and process input signals and output the processing results. The input/output interface conditioning function is used to control and adjust analog quantities. The communication networking function enables PLC to exchange information and control networking with other devices. The programming/debugging function allows users to conveniently program, debug, and monitor PLC operations.
4. Applications and Selection of PLC
PLC is widely used in almost all industrial fields requiring automation control. In manufacturing, PLC can be used for controlling automated assembly lines and automated packaging lines; in transportation, PLC can control traffic lights and baggage handling systems at stations and airports; in petrochemical production processes, PLC can be used for process control and real-time monitoring and adjustment of safety parameters.
When selecting a PLC, multiple factors need to be considered, including model selection, capacity selection, I/O module selection, and the selection of auxiliary devices such as power supply modules and programmers. Model selection should be based on structural form, functional requirements, response speed, and reliability; capacity selection should be based on the number of I/O points and user storage capacity; I/O module selection includes the choice of digital and analog modules as well as the needs for special function modules; power supply modules and programmers are also important considerations when selecting PLC.
5. Summary and Outlook
Through this article, it is believed that you have gained a deeper understanding of the key knowledge of PLC. As the core device of modern industrial automation, the importance of PLC is self-evident. With the continuous development of technology, the application range of PLC will further expand. In the future, PLC will focus more on intelligent, networked, and integrated development trends, providing more efficient, reliable, and intelligent solutions for the field of industrial automation.
As practitioners or enthusiasts in the electrical field, we should continuously learn and master the latest technologies and application methods of PLC, enhancing our professional quality and practical skills. Only in this way can we keep pace with the times and contribute to the development of the industrial automation field.