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Introduction to PLC Basic Knowledge

In the field of automation control, PLC is an important control device. At present, more than 200 manufacturers in the world produce more than 300 varieties of PLC products, which are used in automobiles (23%), food processing (16.4%), chemical / pharmaceutical (14.6%), metals / mines (11.5%), pulp / paper ( 11.3%) and other industries. In order to make it easier for beginners to understand PLC, this article gives a brief introduction to the basic knowledge of PLC development, basic structure, configuration, application, etc., in the hope that it will be helpful to all netizens.
First, the development of PLC
In the industrial production process, a large number of digital switches are sequentially controlled. It performs sequential actions in accordance with logical conditions, and performs chain protection control in accordance with logical relationships, as well as a large number of discrete data collections. Traditionally, these functions have been achieved through pneumatic or electrical control systems. In 1968, the United States GM (General Motors) company proposed to replace the electrical control device. In the second year, the American Digital Corporation developed a control device based on integrated circuits and electronic technology. It was the first time that programmatic methods were used for electrical control. It is the first generation of programmable controller, called Programmable Controller (PC).
After the personal computer (PC for short) was developed, for convenience and to reflect the functional characteristics of programmable controllers, the programmable controller was named Programmable Logic Controller (PLC). At present, PLC is often referred to as PC. There are many definitions of PLC. The International Electrotechnical Commission (IEC) defines PLC as: a programmable controller is an electronic system of digital arithmetic operations, designed for applications in industrial environments. It uses programmable memory to store internal instructions that perform logical operations, sequence control, timing, counting, and arithmetic operations, and control various types through digital and analog inputs and outputs Machinery or production process. The programmable controller and related equipment should be designed according to the principle that it is easy to form a whole with the industrial control system and easy to expand its functions. From the 1980s to the mid-1990s, PLC was the fastest-growing period, with an annual growth rate of 30 to 40%. During this period, PLCs have greatly improved their analog processing capabilities, digital computing capabilities, human-machine interface capabilities, and network capabilities. PLC has gradually entered the field of process control, replacing DCS, which is dominant in the field of process control, in some applications. system
PLC has the characteristics of strong versatility, easy to use, wide adaptability, high reliability, strong anti-interference ability, simple programming and so on. The position of PLC in industrial automation control, especially sequence control, cannot be replaced in the foreseeable future.
Composition of PLC
In terms of structure, PLC is divided into fixed type and combined type (module type). Fixed PLC includes CPU board, I / O board, display panel, memory block, power supply, etc. These elements are combined into a non-removable whole. Modular PLC includes CPU module, I / O module, memory, power module, backplane or rack. These modules can be configured according to certain rules.
Third, the composition of the CPU
The CPU is the core of the PLC and functions as the nerve center. Each PLC has at least one CPU. It receives and stores user programs and data according to the functions given by the PLC system program, and collects the data sent by the field input device in a scanning manner. The status or data is stored in the specified registers. At the same time, the working status of the power supply and the internal circuits of the PLC and the syntax errors during programming are diagnosed. After entering the operation, the instructions are read one by one from the user program memory, and after analysis, the corresponding control signals are generated according to the tasks specified by the instructions to command the relevant control circuits.
The CPU is mainly composed of an arithmetic unit, a controller, a register, and a data, control, and status bus that implements the connection between them. The CPU unit also includes a peripheral chip, a bus interface, and related circuits. Memory is mainly used to store programs and data, and is an indispensable component of PLC.
From the user's point of view, it is not necessary to analyze the internal circuit of the CPU in detail, but there should still be sufficient understanding of the working mechanism of each part. The controller of the CPU controls the work of the CPU, which reads instructions, interprets instructions, and executes instructions. But the working rhythm is controlled by the shock signal. The arithmetic unit is used for digital or logical operation, and works under the command of the controller. Registers participate in the operation and store the intermediate results of the operation. It also works under the command of the controller. CPU speed and memory capacity are important parameters of the PLC. They determine the working speed of the PLC, the number of IOs, and the software capacity, which limits the control scale.
Fourth, I / O module
The interface between the PLC and the electrical circuit is done through the input / output section (I / O). The I / O module integrates the PLC's I / O circuit. Its input register reflects the state of the input signal, and its output point reflects the state of the output latch. The input module converts the electrical signal into a digital signal and enters the PLC system. The output module is the opposite. I / O is divided into digital input (DI), digital output (DO), analog input (AI), analog output (AO) and other modules.
The switching value refers to a signal with only two states of on and off (or 1 and 0), and the analog value refers to a continuously changing amount. The common I / O classification is as follows:
Switching value: according to voltage level, there are 220VAC, 110VAC, 24VDC, according to the isolation method, relay isolation and transistor isolation.
Analog: divided by signal type, there are current type (4-20mA, 0-20mA), voltage type (0-10V, 0-5V, -10-10V), etc., according to accuracy, there are 12bit, 14bit, 16bit, etc. .
In addition to the general IO mentioned above, there are special IO modules, such as thermal resistance, thermocouple, pulse and other modules.
The module specifications and quantity are determined according to the number of I / O points. The number of I / O modules can be more or less, but the maximum number is limited by the ability of the basic configuration that the CPU can manage, that is, the maximum number of backplanes or rack slots.
Five, power module
The PLC power supply is used to provide working power for the integrated circuits of the PLC modules. At the same time, some also provide 24V working power for the input circuit. Power input types are: AC power (220VAC or 110VAC), DC power (commonly 24VAC).
Six, base plate or rack
Most modular PLCs use a backplane or rack, and its role is: to electrically achieve the connection between the modules, so that the CPU can access all the modules on the backplane, mechanically, to achieve the connection between the modules, so that each module constitutes a overall.
Seven, other equipment of the PLC system
1. Programming equipment: The programmer is an indispensable device for PLC development and application, monitoring operation, inspection and maintenance. It is used for programming, making some settings for the system, and monitoring the working conditions of the PLC and the system controlled by the PLC, but it does not directly participate. On-site control operation. Small programmers PLCs generally have handheld programmers. At present, computers (running programming software) are generally used as programmers.
2. Human-machine interface: The simplest human-machine interface is indicator lights and buttons. At present, LCD-based (or touch-screen) integrated operator terminals are more and more widely used. Computers (running configuration software) serve as human-machine interfaces. Very popular. 3, input and output devices: used to permanently store user data, such as EPROM, EEPROM writer, bar code reader, input analog potentiometer, printer, etc.
Communication networking of PLC
Relying on advanced industrial network technology can quickly and efficiently collect, transmit and manage production and management data. Therefore, the importance of the network in the automation system integration project is becoming more and more obvious. Some people even put forward the view that "the network is the controller".
PLC has the function of communication and networking. It enables the exchange of information between PLC and PLC, PLC and higher-level computers and other intelligent devices, forming a unified whole, and achieving decentralized and centralized control. Most PLCs have RS-232 interfaces, and some have built-in interfaces that support their respective communication protocols.
PLC communication has not yet achieved interoperability. IEC has stipulated a variety of fieldbus standards, which are used by PLC manufacturers. For an automation project (especially a medium to large-scale control system), choosing a network is very important. First, the network must be open to facilitate the integration of different equipment and future system scale expansion; second, to choose the network form for the transmission performance requirements of different network levels, it is necessary to understand the network standard protocols, Under the premise of the mechanism; once again, comprehensive consideration of specific issues such as system cost, equipment compatibility, and field environment applicability to determine the network standards used at different levels.

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