Basic knowledge of PLC

PLC is a kind of digital operation electronic equipment specially designed for industrial environment application. It uses programmable memory to store instructions for performing logical operations, sequential operations, timing, counting and arithmetic operations, and can control various types of machines or production processes through digital or analog input and output. PLC and its related peripheral equipment should be designed according to the principle that it is easy to form a whole with industrial control system and expand its functions.

Basic composition of PLC

abstract

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.

The essence of PLC is a computer dedicated to industrial control, and its hardware structure is basically the same as that of a microcomputer, which basically includes:

Power?Supply?

PLC power supply is used to provide working power for the integrated circuit of PLC module, which plays a very important role in the whole system. Without a good and reliable power supply system, it can not work normally, so PLC manufacturers also attach great importance to the design and manufacture of power supply. The AC voltage generally fluctuates within the range of+10%(+ 15%), so PLC can be directly connected to the AC power grid without taking other measures. At the same time, some also provide 24V working power for the input circuit. The power input types are: AC power (220VAC or 1 10VAC) and DC power (usually 24VDC).

cpu

Central processing unit (CPU) is the control center and core of PLC, and plays the role of nerve center. Each PLC has at least one CPU. It receives and stores user programs and data input from programmers according to the functions given by PLC system programs. Check the status of power supply, memory, I/O and alarm timer, and diagnose syntax errors in user programs. When PLC is put into operation, it firstly receives the status and data of various input devices on site through scanning and stores them in the I/O image area, then reads the user programs from the user program memory one by one, interprets the commands, and sends the results of logical or arithmetic operations according to the instructions to the I/O image area or data register. After all user programs are executed, the output status of the I/O image area or the data in the output register are finally transmitted to the corresponding output devices, and so on until the operation stops.

In order to further improve the reliability of PLC, in recent years, large-scale PLC also adopts dual CPU to form redundant system, or adopts three CPU voting system. In this way, even if a CPU fails, the whole system can still run normally.

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.

Storage; stock

The memory that stores system software is called system program memory.

The memory for storing application software is called user program memory.

Input/output interface circuit (input/output 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.

1. The field input interface circuit is composed of optocoupler circuit and microcomputer input interface circuit, which is used as the input channel of PLC and field control interface.

2. The field output interface circuit is composed of output data register, gating circuit and interrupt request circuit, which plays the role of PLC and outputs corresponding control signals to the field execution components through the field output interface circuit.

Commonly used I/O classifications are as follows:

On-off value: divided by voltage level, there are 220VAC, 1 10VAC, 24VDC, divided by isolation mode, including relay isolation and transistor isolation.

Analog quantity: according to the signal type, there are current type (4-20mA, 0-20mA) and voltage type (0- 10V, 0-5V,-10- 10V). , and according to the accuracy, there are 12bit, 65438+. In addition to the above general IO, there are special IO modules, such as thermal resistance, thermocouple, pulse and so on.

The specification and quantity of modules are determined according to the number of I/O points. I/O modules can be more or less, but the maximum number is limited by the basic configuration capacity that the CPU can manage, that is, by the maximum number of backplane or rack slots.

Floor or frame

Modular PLC mostly adopts backboard or rack, whose function is: electrically, to realize the connection between modules, so that CPU can access all modules on the backboard, mechanically, to realize the connection between modules and make them form a whole.

Functional micro-assembly

Such as counting, positioning and other functional modules.

communication module

Such as Ethernet, RS485, Profibus-DP communication module, etc.

Programming equipment

Programmer is an indispensable equipment for the development, application, monitoring and maintenance of PLC. It is used to program, set up the system and monitor the working state of PLC and its controlled system, but it does not directly participate in on-site control and operation. Small programmer PLC generally has a handheld programmer. At present, a computer (running programming software) generally acts as a programmer.

Man-machine interface

The simplest man-machine interface is indicator light and button. The integrated operator terminal with LCD screen (or touch screen) is more and more widely used, and the computer (running configuration software) is very popular as man-machine interface.

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Characteristics of PLC

High reliability and strong anti-interference ability

PLC replaced a large number of intermediate relays and time relays with software, leaving only a small amount of hardware related to input and output. The wiring can be simplified to110 ~1100 of the relay control system, and the faults caused by poor contact are greatly reduced.

High reliability is the key performance of electrical control equipment. Because PLC adopts modern large-scale integrated circuit technology and strict production technology, its internal circuit adopts advanced anti-interference technology and has high reliability. For example, the average time between failures of F series PLC produced by Mitsubishi Company is as high as 300,000 hours. The average trouble-free working time of PLC with redundant CPU is longer. As for the external circuit of PLC, compared with the relay contactor system of the same scale, the electrical wiring and switch contacts are reduced to hundreds or even thousands, and the faults are greatly reduced. In addition, PLC has the function of hardware fault self-checking, which can send out alarm information in time when there is a fault. In the application software, users can also write the fault self-diagnosis program of peripheral equipment, so that the circuits and equipment in the system except PLC can also obtain fault self-diagnosis protection. It is not surprising that the whole system has high reliability.

Complete hardware, perfect function and strong applicability.

Nowadays, PLC has formed a series of products of large, medium and small sizes, and has realized standardization, serialization and modularization. It is equipped with a full range of hardware equipment for users to choose from, and users can flexibly and conveniently configure the system to form systems with different functions and scales. The installation and wiring of PLC are also very convenient. Usually, terminals are used to connect external wiring. PLC has strong load capacity, can directly drive universal solenoid valves and AC contactors, and can be used in industrial control occasions of various scales. In addition to the logic processing function, most modern PLCs also have perfect data operation ability, which can be used in various digital control fields. In recent years, a large number of functional units of PLC have emerged, which makes PLC penetrate into various industrial controls such as position control, temperature control and numerical control. Coupled with the enhancement of PLC communication ability and the development of man-machine interface technology, it is very easy to form various control systems with PLC.

Easy to learn and easy to use, it is welcomed by engineers and technicians.

As a general industrial control computer, PLC is the industrial control equipment of industrial and mining enterprises. Its interface is simple, and the programming language is easily accepted by engineers and technicians. The graphic symbols and expressions of ladder diagram language are quite close to the relay circuit diagram, and the functions of relay circuit can be realized conveniently with only a few PLC switching logic control instructions. It opens the door for people who are not familiar with electronic circuits, computer principles and assembly languages to use computers to engage in industrial control.

Easy to reform

The design, installation and debugging of the system have small workload, convenient maintenance and easy transformation.

The ladder diagram program of PLC generally adopts sequential control design method. This programming method is very regular and easy to master. For complex control systems, the design time of ladder diagram is much less than that of relay system circuit diagram.

PLC uses storage logic instead of wiring logic, which greatly reduces the external wiring of control equipment, shortens the design and construction cycle of control system, and makes maintenance easier. More importantly, it is possible for the same equipment to change the production process by changing the program. This is very suitable for multi-variety and small batch production.

Small size, light weight and low energy consumption.

Take the subminiature PLC as an example, the bottom size of the newly produced product is less than 100mm, which is only equivalent to the size of several relays, so the size of the switch cabinet can be reduced to the original1/2 ~1/kloc-0. It weighs less than 150g and consumes only a few watts. Because of its small size, it is easy to be installed in machinery, and it is an ideal control device to realize electromechanical integration.

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Trends and tidal currents

First, the development trend of contemporary PLC technology

Rapid development, product upgrading; Develop various intelligent modules to continuously enhance process functions; The combination of PLC and PC; The communication networking function has been continuously enhanced; Develop new programming languages to enhance fault tolerance.

Second, the development trend of contemporary PLC technology

As a user, General Motors of the United States proposed that the new generation controller should meet ten conditions, as follows:

1. Programming is convenient, and the program can be modified on site;

2. Easy maintenance, preferably plug-in;

3. The reliability is higher than that of relay control cabinet;

4. The volume is smaller than the relay control cabinet;

5. Data can be sent directly to the management computer;

6. It can compete with relay control in cost;

7. The input can be AC115v;

8. The output is above AC 1 15V/2A, and the solenoid valve can be directly driven;

9. When expanding, the original system only needs to be slightly changed;

10. The storage capacity of the user program can be extended to at least 4K bytes.

1969 American digital equipment company successfully developed the world's first programmable controller PDP- 14, which was used on the assembly line of general motors for the first time and was successful. Then MODICON Company of the United States also developed 084 control. Since then, this new technology has been rapidly popularized and applied all over the world. 197 1 year, Japan introduced this technology from the United States, and soon developed the first programmable controller DSC- 18. 1973 western European countries have also developed the first programmable controller. China began to develop from 1974, and was popularized and applied in industry from 1977. In 1970s, with the development of microelectronics technology, especially after PLC adopted communication microprocessor, this controller was no longer limited to the original logic operation, and its function was further enhanced. In the 1980s, with the rapid development of microelectronic technology such as large-scale and ultra-large-scale integrated circuits, microcomputer-based PLC composed of 16-bit and a few 32-bit microprocessors enhanced the functions of PLC, making it work faster, smaller in size, more reliable, lower in cost, and more flexible and convenient in programming and fault detection.

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PLC training and certificate

Programmable controller programming language

There are many programming languages of programmable controller PLC, including ladder diagram language, Boolean mnemonic language, function diagram language, function module diagram language and structured statement description language.

Ladder diagram language and Boolean mnemonic language are basic programming languages, which usually consist of a series of instructions. These instructions can complete the simplest control functions, for example, replacing relays, counters and timers to complete sequential control and logical control. They can also perform other basic operations by extending or enhancing the instruction set.

Function diagram language and statement description language are high-level programming languages, which can perform more effective operations as needed, such as analog control, data manipulation, report printing and other functions that cannot be completed by basic programming languages.

Function module diagram language adopts the form of function module diagram, and completes the required control function through soft connection. It has been widely used not only in programmable controllers, but also in the programming and configuration of distributed control systems. Because of its convenient connection, simple operation and easy to master, it is loved by engineering designers and application personnel.

PLC learning situation

At present, the supply of PLC application talents mainly depends on universities (267 related majors), higher vocational colleges (more than 600) and technical schools (more than 2,000). Its related majors are generally called automation, mechanical manufacturing and its automation, electrical automation and mechatronics.

Schools offering related majors include Tsinghua, Zhejiang University and other domestic first-class universities, as well as various vocational training institutions, covering a wider range of majors. Many schools have taken the application of PLC as a basic course in professional colleges.

The PLC course under the CETTIC project in Ministry of Human Resources and Social Security divides the training into general knowledge, practical knowledge and practical skills, focusing on case analysis and behavior orientation.

Programmable controller PLC programmer training certificate

CETTIC requires students to pass the three-part knowledge examination before they can obtain the vocational training certificate of programmable controller (PLC) programmers at corresponding levels (primary, intermediate and advanced). This certificate is also the only nationally recognized training certificate in the field of PLC, with high gold content.

Programmable controller PLC programmer teacher training certificate

CETTIC certificates are divided into post certificates and teacher certificates, and trainers can also choose to apply for teacher training certificates according to their own abilities. However, because the certificate requires that the applicant must have a bachelor's degree or above and have certain work experience before accepting the registration, it is rare to hold this teacher qualification certificate.

At present, only a few strong training institutions have been authorized by CETTIC to develop corresponding courses and organize teacher training. After passing the exam, students will get CETTIC professional teacher training certificate (described in detail on the website of the National Office for Shortage of Talents).

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Application field of PLC

General introduction

At present, PLC has been widely used in steel, petroleum, chemical industry, electric power, building materials, machinery manufacturing, automobile, textile, transportation, environmental protection, culture and entertainment industries at home and abroad, and its usage can be roughly summarized into the following categories.

Logical control of switching value

This is the most basic and extensive application field of PLC. It replaces the traditional relay circuit to realize logic control and sequence control. It can be used for single machine control, multi-machine group control and automatic production line. Such as injection molding machine, printing machine, stapler, modular machine tool, grinder, packaging production line, electroplating production line, etc.

analog control

In the process of industrial production, there are many continuously changing quantities, such as temperature, pressure, flow, liquid level and speed, which are all analog quantities. In order to make the programmable controller deal with analog quantity, it is necessary to realize A/D conversion and D/A conversion between analog quantity and digital quantity. PLC manufacturers all produce matching A/D and D/A conversion modules, so that programmable controllers can be used for analog control.

electric machine control

PLC can be used to control circular motion or linear motion. From the configuration of control mechanism, I/O module was used to directly connect the position sensor and actuator in the early days, but now it is generally used with special motion control module. Such as a single-axis or multi-axis position control module that can drive a stepping motor or a servo motor. Almost all the products of major PLC manufacturers in the world have motion control functions, which are widely used in various machinery, machine tools, robots, elevators and other occasions.

process control

Process control refers to the closed-loop control of temperature, pressure, flow and other analog quantities. As an industrial control computer, PLC can compile various control algorithms and complete closed-loop control. PID regulation is a widely used regulation method in general closed-loop control system. Large and medium-sized PLC has PID module, and many small PLCs also have this function module at present. General PID processing is to run a special PID subroutine. Process control is widely used in metallurgy, chemical industry, heat treatment, boiler control and other occasions.

data processing

Modern PLC has the functions of mathematical operation (including matrix operation, function operation and logic operation), data transmission, data conversion, sorting, table lookup, bit operation and so on, which can complete data collection, analysis and processing. These data can be compared with the reference values stored in the memory to complete some control operations, and can also be transmitted to other intelligent devices by using communication functions, or printed and made into tables. Data processing is generally used in large-scale control systems, such as unmanned flexible manufacturing systems; It can also be used in process control systems, such as some large-scale control systems in papermaking, metallurgy, food and other industries.

Communication and network

PLC communication includes communication between PLCs and communication between PLCs and other intelligent devices. With the development of computer control and the rapid development of factory automation network, all PLC manufacturers attach great importance to the communication function of PLC and have launched their own network systems. The newly produced PLC has communication interface, which makes communication very convenient.