It refers to a new type of industrial control equipment based on computer technology. In the draft PLC standard issued by IEC 1987, PLC is defined as follows:
"PLC is a kind of digital operation electronic equipment, specially designed for industrial environment. 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 peripherals should be designed according to the principle of easy integration with industrial control system and easy function expansion. "
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Characteristics of PLC
2. 1 has high reliability and strong anti-interference ability.
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.
2.2 complete sets, perfect functions and strong applicability.
Today, PLC has developed into a series of large, medium and small-scale products. It 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.
2.3 easy to learn and easy to use, welcomed by engineering and technical personnel.
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.
2.4 The design and construction workload of the system is small, and it is easy to maintain and transform.
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.
2.5 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, the weight is less than 150g, and the power consumption is 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.
3. Application fields of 3.PLC
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.
3. 1 switch logic control
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.
3.2 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.
3.3 Motion 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.
3.4 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.
3.5 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.