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Briefly describe the structure and working principle of PLC
PLC is usually composed of CPU, memory, input / output (I / O) channel, timer (T), counter (C) and other units. Sometimes external devices such as printers and CRT monitors are connected, and the control program is input by the programmer. The structure of PLC is similar to that of computer, and the functions of each component are:
â‘ Central processing unit CPU. It is the core component of PLC, which is composed of processor, intermediate variable memory, etc., similar to the microcomputer CPU. Its function is to receive and store the user program and data input from the programmer according to the function given by the system program in the PLC; receive the status and data of the field input device in the way of scanning, and store it in the human data register; diagnostic Power Supply, PLC internal circuit Grammatical errors during working status and programming. When the PLC is running, read the user programs from the memory one by one, and generate the corresponding signals according to the tasks specified by the program to turn on and off the relevant circuits; perform data access, transmission, combination, and comparison in time and channel And conversion, complete the tasks of logic control or operation specified in the user program; update the status of the relevant flags or the relevant content in the data register according to the operation results, to achieve output control, tabulation and data communication.
â‘¡Program memory. The system program memory is used to store system engineering programs (monitoring programs), function subprograms and management programs. The user program memory is used to store the control algorithm coefficient (analog quantity) and the program compiled by the user. PLC user memory usually represents the memory capacity in units of words (16 bits / word). The memory type and capacity given in the PLC product data refer to user memory. Since the user memory is powered by a lithium battery with a spare cycle, even if the power grid fails, the programs in the user memory will not be lost.
⑨ The input and output channel I / OI / O is the connection part between the CPU and the on-site vacuum device or other external equipment. PLC provides I / O for various purposes, various operating levels and driving capabilities for users to choose. I / O has level shifting, electrical isolation, string-pie-line exchange, data transmission, error check, A / D or D / A conversion, and other functional controls.
â‘£Programmer. It is used for the preparation, debugging and monitoring of user programs, and can call and display some internal status and system parameters of PLC. It uses the communication interface to communicate with the CPU to complete a human-machine dialogue. The programmer has various function keys and indicator lights for programming, as well as a programming / monitoring switch. The programmed keyboard uses ladder language key symbols or command language mnemonic key symbols, and can also be edited through on-screen dialogue using the function key symbols specified by the software. There are two types of programmers: simple and smart. The former can only be programmed online, while the latter can be programmed online or independently.
⑤External equipment. Generally PLC is equipped with cassette recorder, printer, EPROM writer, screen display device, etc.
⑥ The power supply is a general Industrial Power Supply. Due to the different functions and instructions of various PLCs, their working principles are not exactly the same. The PLC enters and stores the user program in the user memory of the CPU through a programmer or other input device. When the PLC is put into operation, the CPU continuously scans cyclically according to the order specified by the system program, completes the status and data collection of each input point, and stores it in the input register of the input channel. The prescribed content performs logical operations on the corresponding input and output switching values ​​and intermediate variables, stores the operation results in the output latch or intermediate variable memory, drives the actuator, and completes the control of the vacuum device and process. After the execution of an instruction is completed, the next instruction is immediately fetched and executed. At this time, if the transfer instruction is set in the program, the program will jump to the instruction at the specified address and execute it in the new order. The scan period of a general program is tens of milliseconds.