First, the development of PLC
In the process of industrial production, a large number of switches are controlled in sequence, acting in sequence according to logical conditions, controlling interlocking protection actions according to logical relations, and collecting a large number of discrete data. Traditionally, these functions are realized by pneumatic or electric control systems. 1968 American GM (general motors) company proposed to replace the electronic control device. In the second year, Digital Company of America developed a control device based on integrated circuit and electronic technology, which was applied to electrical control by programming for the first time. This is the first generation of programmable controller, called programmable controller (PC).
After the development of personal computer (PC for short), it was named as programmable logic controller (PLC) in order to facilitate and embody the functional characteristics of programmable controller. Now, PLC is usually referred to as PC for short.
There are many definitions of PLC. The definition of PLC by the International Electrotechnical Commission (IEC) is that a programmable controller is an electronic system with digital operation, specially designed for industrial applications. It uses programmable memory to store instructions for performing logical operations, sequence control, timing, counting and arithmetic operations, and controls various types of machines or production processes through digital and analog input and output. Programmable controller and its related equipment should be designed according to the principle that it is easy to form a whole with industrial control system and expand its functions.
From the 1980s to the mid-1990s, PLC developed the fastest, with an annual growth rate of 30~40%. During this period, the analog processing ability, digital computing ability, man-machine interface ability and network ability of PLC have been greatly improved, and PLC has gradually entered the field of process control, replacing DCS system, which is dominant in the field of process control, in some applications.
PLC has the characteristics of strong universality, convenient use, wide adaptability, high reliability, strong anti-interference ability and simple programming. The position of PLC in industrial automation control, especially in sequence control, is irreplaceable in the foreseeable future.
Second, the composition of PLC
Structurally, PLC can be divided into fixed and modular. Fixed PLC includes CPU board, I/O board, display panel, memory block, power supply, etc. These elements are combined into an inseparable whole. Modular PLC includes CPU module, I/O module, memory, power module, backplane or rack, which can be combined and configured according to certain rules.
Third, the composition of CPU
CPU is the core of PLC and plays the role of nerve center. Each PLC has at least one CPU, which receives and stores user programs and data according to the functions given by PLC system programs, and collects the status or data sent by field input devices through scanning and stores them in designated registers. At the same time, diagnose the working state of the power supply and the internal circuit of PLC and the grammatical errors during programming. After running, the instructions are read from the user program memory one by one, and after analysis, corresponding control signals are generated according to the tasks specified in the instructions to command the relevant control circuits.
CPU is mainly composed of arithmetic unit, controller, register and data, control and status bus to realize the connection between them. The CPU unit also includes peripheral chips, bus interfaces and related circuits. Memory is mainly used to store programs and data, and it is an indispensable unit of PLC.
In the eyes of users, it is not necessary to analyze the internal circuit of CPU in detail, but it is necessary to fully understand the working mechanism of each part. The controller of CPU controls the work of CPU, which reads, interprets and executes instructions. But the working rhythm is controlled by the oscillation signal. The arithmetic unit is used for digital or logical operations and works under the command of the controller. The register participates in the operation and stores the intermediate result of the operation, and it also works under the command of the controller.
CPU speed and memory capacity are important parameters of PLC, which determine the working speed, IO number and software capacity of PLC, thus limiting the control scale.
Fourth, I/O module.
The interface between PLC and circuit is completed by input and output (I/O). I/O module integrates the I/O circuit of PLC, its input register reflects the state of input signal, and its output point reflects the state of output latch. The input module converts electrical signals into digital signals and enters the PLC system, while the output module does the opposite. I/O is divided into switch input (DI), switch output (DO), analog input (AI), analog output (AO) and other modules.