Basic knowledge of PLC programmable controller

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Basic knowledge of PLC programmable controller

plc programmable controller: PLC is called programmable logic controller in English and programmable logic controller in Chinese. Its definition is: an electronic system with digital operation, which is designed for application in industrial environment. It uses a kind of programmable memory for its internal storage program to execute logical operation, sequence control, timing, count the user oriented instructions such as entering the Beijing Taiwan Expressway from the entrance of Jinan West overpass and going straight to Cangzhou City number and arithmetic operation, and control various types of machinery or production processes through digital or analog input/output. DCS distributed control system: the English full name of DCS is distributed control system, and the Chinese full name is distributed control system. DCs can be interpreted as an automatic high-tech product widely used in industries with more analog loop control, which tries to disperse the danger caused by control and centralize the management and display functions. DCS is generally composed of five parts: 1. Controller 2. I/o board 3. Operation station 4. Communication network 5. Graphics and process software

I. development history of PLC

in the process of industrial production, a large number of switching values are controlled in sequence. They act in sequence according to logical conditions and interlock according to logical relations. Under what conditions is the best installation? Now let's invite the technical personnel of StarTech to analyze the control of protection action and a large number of discrete data acquisition. Traditionally, these functions are achieved through pneumatic or electrical control systems. In 1968, GM (General Motors) of the United States proposed to replace the following electrical control device. In the second year, Digital Corporation of the United States developed a control device based on integrated circuit and electronic technology, which was applied to electrical control by means of 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), the programmable controller was named programmable logic controller (PLC) for convenience and to reflect the functional characteristics of programmable controller. Now, PLC is still often referred to as PC for short

There are many definitions of PLC. The International Electrotechnical Commission (IEC) defines PLC as an electronic system with digital operation, which is designed for application in industrial environment. It adopts programmable memory, which is used to store and execute the instructions of logic operation, sequence control, timing, counting and arithmetic operation, and control various types of machinery or production processes through digital and analog input and output. The programmable controller and its related equipment shall be designed according to the principle that it is easy to form a whole with the industrial control system and expand its functions

from the 1980s to the mid-1990s, PLC developed fastest, with an annual growth rate of 30-40%. During this period, PLC has greatly improved its analog processing capacity, digital computing capacity, man-machine interface capacity and networking capacity. PLC has gradually entered the field of process control and replaced the dominant DCS system in the field of process control in some applications

plc has the characteristics of strong versatility, convenient use, wide adaptability, high reliability, strong anti-interference ability, simple programming, etc. The position of PLC in industrial automation control, especially sequence control, cannot be replaced in the foreseeable future

II. Composition of PLC

in terms of structure, PLC is divided into fixed type and combined type (modular 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 combined and configured according to certain rules

III. 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 the PLC system program, collects the status or data sent by the field input device by scanning, and stores it in the specified register. At the same time, The working state of the diagnosis power supply and the internal circuit of PLC drives the loading state of the precision ball screw pair and the syntax error in the programming process after being decelerated by the arc synchronous belt and the arc synchronous belt wheel reduction system. After entering the operation, read the instructions one by one from the user program memory, and then generate the corresponding control signals according to the tasks specified in the instructions after analysis 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. CPU unit also includes peripheral chip, bus interface and related circuits. Memory is mainly used to store programs and data, and it is an indispensable constituent unit of PLC

in the view of the user, it is not necessary to analyze the internal circuit of the CPU in detail, but there should be enough understanding of the working mechanism of each part. The controller of CPU controls the work of CPU, which reads instructions, interprets instructions and executes instructions. But the work rhythm is controlled by the vibration 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 results of the operation. 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 quantity and software capacity of PLC, so it limits the control scale

IV. the interface between i/o module

plc and electrical circuit is completed through input and output part (i/o). The i/o module integrates the i/o circuit of PLC, its input register reflects the input signal state, and the output point reflects the output latch state. The input module converts the electrical signal into a digital signal and enters the PLC system, while the output module is the opposite. I/o is divided into switching input (DI), switching output (do), analog input (AI), analog output (AO) and other modules

switching quantity refers to the signal with only on and off (or 1 and 0), and analog quantity refers to the quantity that changes continuously. Common i/o classifications are as follows:

switching value: according to voltage level, there are 220VAC, 110VAC, 24VDC, according to isolation mode, there are relay isolation and transistor isolation

analog quantity: according to signal type, there are current type (MA, MA), voltage type (V, V, -v), etc. according to 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

the specification and number of modules 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 basic configuration that the CPU can manage, that is, by the maximum number of backplane or rack slots

v. power supply module

plc power supply is used to provide working power for the integrated circuit of each PLC module. At the same time, some also provide 24V working power supply for the input circuit. Power input types include: AC power supply (220VAC or 110VAC), DC power supply (24VAC is commonly used)

VI. backplane or rack

most modular PLCs use backplane or rack. Its function is: electrically, to realize the connection between modules, so that CPU can access all modules on the backplane. Mechanically, to realize the connection between modules, so that each module forms a whole

VII. Other equipment of 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 PLC and the system controlled by PLC, but it does not directly participate in the field control operation. Program PLC generally has hand-held programmer. At present, the computer (running programming software) is generally used as the programmer

2. Man machine interface: the simplest man-machine interface is indicator light and button. At present, LCD (or touch screen) integrated operator terminals are more and more widely used, and computers (running configuration software) are very popular as man-machine interfaces

3. Input and output devices: used to permanently store user data, such as EPROM, EEPROM writer, bar code reader, potentiometer for input analog quantity, printer, etc

VIII. The communication link of PLC

relies on advanced industrial network technology to quickly and effectively collect and transmit production and management data. Therefore, the importance of network in automation system integration engineering is becoming more and more obvious. Some people even put forward the view that "network is the controller"

plc has the function of communication connection, which enables information exchange between PLC and PLC, between PLC and upper computer and other intelligent devices, forming a unified whole and realizing decentralized and centralized control. Most PLCs have RS-232 interfaces, and some have built-in interfaces that support their own communication protocols

plc communication has not yet achieved interoperability. IEC stipulates a variety of fieldbus standards, which are adopted by PLC manufacturers

for an automation project (especially a medium and large-scale control system), the selection of networks is very important. First, the network must be open to facilitate the integration of different devices and the expansion of the future system scale; Secondly, according to the transmission performance requirements of different network levels, the form of network should be selected, which must be carried out under the premise of a deeper understanding of the protocol of the network standard and the rapid development mechanism of lft-d molding technology (online molding technology of long fiber reinforced thermoplastic materials) in European and American countries; Thirdly, the network standards used at different levels are determined by comprehensively considering the specific problems such as system cost, equipment compatibility, on-site environmental applicability and so on. (end)

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