First, the development history of PLC
In the industrial production process, a large number of switching quantities are sequentially controlled, which are sequentially operated according to logical conditions, and control of interlocking protection actions according to logical relationships, and a large amount of discrete data acquisition. Traditionally, these functions have been achieved through pneumatic or electrical control systems. In 1968, the United States GM (General Motors) proposed to replace the requirements of electrical control devices. In the second year, American Digital Corporation developed a control device based on integrated circuits and electronic technology. For the first time, it was applied to electrical control by means of programmed means. It is the first generation of programmable controller, called Programmable Controller (PC).
After the development of the personal computer (referred to as PC), in order to facilitate the function of the programmable controller, the programmable controller is named Programmable Logic Controller (PLC). Now, PLC is often referred to as PC.
There are many types of PLC definitions. The International Electrotechnical Commission (IEC) defines PLC as: A programmable controller is an electronic system for digital computing operations designed for use in industrial environments. It uses a programmable memory for storing instructions that perform operations such as logic operations, sequence control, timing, counting, and arithmetic operations, and controls various types through digital, analog inputs and outputs. Mechanical or production process. The programmable controller and its related equipment should be designed in such a way that it is easy to integrate with the industrial control system and easily expand its functions.
From the 1980s to the mid-1990s, it was the fastest period of PLC development, and the annual growth rate has remained at 30-40%. During this period, PLC has greatly improved the ability to handle analog, digital computing, human-machine interface 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, convenient use, wide adaptability, high reliability, strong anti-interference ability and simple programming. The position of PLC in industrial automation control, especially sequential control, cannot be replaced in the foreseeable future.
Second, the construction of PLC
From the structural point of view, PLC is divided into fixed type and combined type (modular type). Fixed PLCs include CPU boards, I/O boards, display panels, memory blocks, power supplies, etc. These elements are combined into a non-detachable unit. Modular PLCs include CPU modules, I/O modules, memory, power modules, backplanes, or racks. These modules can be configured in accordance with certain rules.
Third, the composition of the CPU
The CPU is the core of the PLC and functions as the nerve center. Each set of PLC has at least one CPU. It receives and stores the user program and data according to the functions given by the PLC system program, and collects the data sent by the field input device by scanning. Status or data, and stored in the specified registers, at the same time, diagnose the operating state of the power supply and PLC internal circuits and syntax errors during programming. 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 circuit.
The CPU is mainly composed of an arithmetic unit, a controller, a register, and a data, control and status bus for realizing the connection therebetween. The CPU unit further 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 it should have sufficient understanding of the working mechanism of each part. The CPU controller controls the CPU to read, interpret, and execute instructions. But the working rhythm is controlled by the oscillating signal. The operator is used for digital or logical operations and operates under the direction of the controller. The register participates in the operation and stores the intermediate result of the operation, which also works under the direction 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, thus limiting the scale of control.
Fourth, I / O module
The interface between the PLC and the electrical circuit is done through the input and output section (I/O). The I/O module integrates the I/O circuitry of the PLC, its input register reflects the state of the input signal, and the output point reflects the state of the output latch. The input module converts the electrical signal into a digital signal into the PLC system, and the output module is reversed. I/O is divided into digital input (DI), digital output (DO), analog input (AI), analog output (AO) and other modules.
The amount of switching refers to the signal of only the two states of on and off (or 1 and 0), and the analog quantity refers to the amount of continuous change. Commonly used I/O classifications are as follows:
Switching quantity: According to the voltage level, there are 220VAC, 110VAC, 24VDC, divided by isolation, with relay isolation and transistor isolation.
Analog quantity: according to the signal type, there are current type (4-20mA, 0-20mA), voltage type (0-10V, 0-5V, -10-10V), etc., according to the accuracy, there are 12bit, 14bit, 16bit, etc. .
In addition to the above general IO, there are special IO modules, such as thermal resistance, thermocouple, pulse and other modules.
I/O modules can be determined by I/O points according to the number and number of I/O points, but the maximum number of I/O modules can be limited by the maximum number of baseboards or rack slots.
Five, power module
The PLC power supply is used to supply working power to the integrated circuits of each module of the PLC. 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 (usually 24VAC).
Sixth, the bottom plate or the rack
Most modular PLCs use a backplane or a rack. Their functions are: electrical, to 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, monitoring the working status of the system controlled by PLC and PLC, but it is not directly involved. Field control operation. Small programmer PLCs generally have a handheld programmer, which is currently generally used by a computer (running programming software) as a programmer.
2. Man-machine interface: The simplest human-machine interface is the indicator light and button. At present, the LCD operator (or touch screen)-type integrated operator terminal is more and more widely used, and the computer (running configuration software) acts as a human-machine interface. Very popular.
3. Input and output devices: used to permanently store user data, such as EPROM, EEPROM writer, barcode reader, input analog potentiometer, printer, etc.
Eight, PLC communication network
Relying on advanced industrial network technology, production and management data can be collected and transmitted quickly and efficiently. Therefore, the importance of the network in the automation system integration project is becoming more and more significant. Some people even put forward the idea that "the network is the controller".
PLC has the function of communication networking, which enables information exchange between PLC and PLC, PLC and host computer and other intelligent devices to form a unified whole and realize distributed centralized control. Most PLCs have an RS-232 interface, and some have built-in interfaces that support their respective communication protocols.
PLC communication has not yet achieved interoperability. The IEC has specified a variety of fieldbus standards, which are used by all PLC manufacturers.
For an automation project (especially for medium and large scale control systems), it is very important to choose a network. First, the network must be open to facilitate the integration of different devices and the expansion of future system scales. Second, for the transmission performance requirements of different network layers, choose the form of the network, which must be a deeper understanding of the protocol of the network standard, Under the premise of the mechanism; again, comprehensive consideration of system cost, equipment compatibility, site environment applicability and other specific issues to determine the network standards used at different levels.
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