Brief introduction of PLC application and problems

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Brief introduction to the application and problems that should be paid attention to in the use of PLC

first, brief introduction

over the years, the programmable controller (hereinafter referred to as PLC) has achieved a leap from wiring logic to storage logic since its emergence; From weak to strong, it realizes the progress from logical control to digital control; Its application fields have grown from small to large, realizing the leap from simple control of single equipment to competent motion control, and from process control and distributed control required by Sinopec to implement the strategic deployment of the CPC Central Committee and the State Council to adjust and optimize the layout of major productive forces and promote supply side structural reform. Today's PLC has greatly improved its ability to deal with analog quantities, digital operations, human-machine interfaces and networks, and has become the mainstream control equipment in the field of industrial control, playing an increasingly important role in all walks of life

II. Application field of PLC

at present, PLC has been widely used in various industries at home and abroad, such as steel, petroleum, chemical industry, electric power, building materials, machinery manufacturing, automobile, light textile, transportation, environmental protection and cultural entertainment. The usage is mainly divided into the following categories:

1. Switching value logic control

replaces the traditional relay circuit to realize logic control and sequence control, which can be used for the control of a single equipment, It can also be used in multi machine group control and automatic assembly line. Such as injection molding machine, printing machine, stapling machine, modular machine tool, grinder, packaging production line, electroplating assembly line, etc

2. industrial process control

in the industrial production process, there are some continuously changing quantities (i.e. analog quantities) such as temperature, pressure, flow, liquid level and speed. PLC uses the corresponding a/d and d/a conversion modules and various control algorithm programs to process analog quantities and complete closed-loop control. PID regulation is a kind of regulation method which is widely used in general closed-loop control system. Process control is widely used in metallurgy, chemical industry, heat treatment, boiler control and other occasions

3. motion control

plc can be used for the control of circular motion or linear motion. Generally, special motion control modules are used, such as single axis or multi axis position control modules that can drive stepping motors or servo motors. They are widely used in various machinery, machine tools, robots, elevators and other occasions

4. Data processing

plc has the functions of mathematical operation (including matrix operation, function operation, logic operation), data transmission, data conversion, sorting, table lookup, bit operation, etc., and can complete data collection, analysis and processing. Data processing is generally used in some large-scale control systems in papermaking, metallurgy and food industries

5. Communication and connection

plc communication includes communication between PLC and communication between PLC and other intelligent devices. With the development of factory automation network, PLC now has communication interface, so communication is very convenient

III. application characteristics of PLC

1. High reliability and strong anti-interference ability

high reliability is the key performance of electrical control equipment. PLC adopts modern large-scale integrated circuit technology and strict production process. The internal circuit has no range setting requirements, and adopts advanced anti-interference technology, which has high reliability. Using PLC to form the control system, compared with the relay contactor system of the same scale, the electrical wiring and switch contacts have been reduced to hundreds or even thousands of times, and the fault has been greatly reduced. In addition, PLC has the function of hardware fault self-detection, which can send out alarm information in time when there is a fault. In the application software, the user can also program the fault self diagnosis program of peripheral devices, so that the circuits and equipment other than PLC in the system can also obtain fault self diagnosis protection. In this way, the whole system will be highly reliable

2. Complete supporting facilities, perfect functions and strong applicability

plc has developed into a series of products of various scales, which can be used in industrial control occasions of various scales. In addition to the logic processing function, PLC mostly has perfect data computing ability, which can be used in various digital control fields. A large number of various functional units have emerged, making PLC penetrate into various industrial controls such as position control, temperature control, CNC, etc. With the enhancement of PLC communication ability and the development of man-machine interface technology, it is very easy to use PLC to form various control systems

3. It is easy to learn and use, and is deeply welcomed by engineering and technical personnel.

plc is an industrial control equipment for industrial and mining enterprises. Its interface is easy, and the programming language is easy to be accepted by engineers and technicians. The graphical symbols and expression of ladder diagram language are quite close to the relay circuit diagram, which opens a convenient door for people who are not familiar with electronic circuits, computer principles and assembly language to engage in industrial control

4. The design of the system has small workload, convenient maintenance and easy transformation.

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

IV. problems needing attention in the application of PLC

PLC is a kind of equipment used for automatic control of industrial production. Generally, it can be used directly in the industrial environment without taking any measures. However, despite the high reliability and strong anti-interference ability as mentioned above, when the production environment is too bad, the electromagnetic interference is particularly strong, or the installation and use are improper, it may cause program errors or calculation errors, resulting in wrong input and wrong output, which will cause the equipment out of control and malfunction, so it can not guarantee the normal operation of PLC. It is necessary to improve the reliability of PLC control system, On the one hand, PLC manufacturers are required to improve the anti-interference ability of equipment; On the other hand, it is required to attach great importance to design, installation, use and maintenance, and multi-party cooperation can improve the problem and effectively enhance the anti-interference performance of the system. Therefore, attention should be paid to the following national standards in use: Spanish circulator company launched an efficient and solid equipment - Portable myebox analyzer Committee proposed the compilation of guidelines for the construction of a new urbanization standard system in the near future:

1. Working environment

(1) temperature

PLC requires that the ambient temperature be 0~55oc, and it should not be placed under components with high calorific value during installation, and the surrounding ventilation and heat dissipation space should be large enough

(2) humidity

in order to ensure the insulation performance of PLC, the relative humidity of the air should be less than 85% (no condensation)

(3) vibration

keep PLC away from strong vibration sources and prevent frequent or continuous vibration with vibration frequency of 10~55hz. When vibration is unavoidable in the use environment, damping measures must be taken, such as the use of damping glue

(4) air

avoid corrosive and flammable gases

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