As a general-purpose industrial control computer, in the past 30 years, the programmable controller has realized the leap from the wiring logic to the storage logic in the industrial control field; its function from weak to strong, realizes the progress of logic control to digital control; its application From small to large, the field has achieved a simple leap from single device control to competent motion control, process control, and distributed control. Today's programmable controllers are becoming the mainstream control device in the field of industrial control, playing an increasingly important role in the world.
1. Definition of programmable controller
Programmable controller (PLC) refers to a new type of industrial control device based on computer technology. The PLC was defined as follows in the draft PLC standard promulgated by the International Electrical Committee in 1987:
"PLC is an electronic device designed for digital computing operations in industrial environments. It uses a program-programmable memory for performing logic operations, sequential operations, timing, counting, and arithmetic operations in its internal memory. Operational instructions and control of various types of machinery or production processes through digital or analog inputs and outputs. PLCs and their associated peripherals should be easily integrated with industrial control systems to easily extend their functionality. Designed according to the principle."
2. Features of PLC
2.1 High reliability, high anti-interference ability and high performance are the key performance of electrical control equipment. Due to the adoption of modern large-scale integrated circuit technology, PLC adopts strict production process and the internal circuit adopts advanced anti-interference technology, which has high reliability. For example, the F series PLC produced by Mitsubishi has an average time between failures of up to 300,000 hours. Some PLCs that use redundant CPUs have a longer mean time between failures. From the PLC's off-board circuit, PLC is used to form the control system. Compared with the same-scale relay contactor system, the electrical wiring and switch contacts have been reduced to hundreds or even thousands, and the fault is greatly reduced. In addition, the PLC has a hardware fault self-detection function that can issue an alarm message in time when a fault occurs. In the application software, the application can also be programmed into the fault self-diagnosis program of the peripheral device, so that the circuit and the device other than the PLC in the system also obtain the fault self-diagnosis protection. In this way, it is not surprising that the entire system is extremely versatile.
2.2 complete set, complete functions, strong applicability
Since the development of PLC to today, it has formed a series of products of various sizes, large, medium and small. Can be used in industrial control applications of all sizes. In addition to the logic processing functions, modern PLCs mostly have perfect data computing capabilities and can be used in various digital control fields. In recent years, a large number of functional units of PLC have emerged, which has enabled PLC to penetrate into various industrial controls such as position control, temperature control, and CNC. Coupled with the enhancement of PLC communication capabilities and the development of human-machine interface technology, it is very easy to use PLC to form various control systems.
2.3 Easy to learn and use, very popular with engineering and technical personnel
As a general industrial control computer, PLC is an industrial control equipment for industrial and mining enterprises. Its interface is easy and the programming language is easy for engineers to accept. The graphical symbol of the ladder language is quite close to the expression and the relay circuit diagram. The function of the relay circuit can be conveniently realized by only a small amount of switching logic control instructions of the PLC. It is a convenient door for people who are not familiar with electronic circuits, who do not understand computer principles and assembly language to use computers for industrial control.
2.4 system design, construction workload is small, easy to maintain, easy to transform
PLC replaces the wiring logic with storage logic, which greatly reduces the wiring outside the control device, shortens the design and construction cycle of the control system, and makes maintenance easier. More importantly, it is possible to change the production process by changing the program on the same device. This is very suitable for multi-variety, small batch production occasions.
2.5 Small size, light weight, low energy consumption In the case of ultra-small PLCs, the newly produced varieties have a bottom size of less than 100mm, a weight of less than 150g, and a power consumption of only a few watts. Due to its small size, it is easy to fit inside the machine and is an ideal control device for mechatronics.
3. Application fields of PLC
At present, PLC has been widely used in various industries such as steel, petroleum, chemical, electric power, building materials, machinery manufacturing, automobile, textile, transportation, environmental protection and cultural entertainment. The usage can be roughly classified into the following categories.
3.1 Logic control of switch quantity This is the most basic and wide application field of PLC. It replaces the traditional relay circuit and realizes logic control and sequence control. It can be used for control of single equipment, multi-machine group control and automation pipeline. . Such as injection molding machines, printing machines, bookbinding machinery, combination machine tools, grinding machines, packaging production lines, electroplating lines and so on.
3.2 Analog Control In the industrial production process, there are many continuous changes, such as temperature, pressure, flow, liquid level and speed. In order for the programmable controller to process the analog quantity, it is necessary to implement A/D conversion and D/A conversion between analog (Analog) and digital (Digital). PLC manufacturers produce matching A/D and D/A conversion modules to make programmable controllers for analog control.
3.3 Motion Control
The PLC can be used for the control of circular motion or linear motion. From the perspective of the control mechanism configuration, the early use of the switching I/O module to connect the position sensor and the actuator is now generally using a dedicated motion control module. For example, single or multi-axis position control modules that can drive stepper motors or servo motors. The products of all major PLC manufacturers in the world have almost all motion control functions, which are widely used in various machines, machine tools, robots, elevators and other occasions.
3.4 Process Control Process control refers to closed-loop control of analog quantities such as temperature, pressure, and flow. As an industrial control computer, PLC can program a variety of control algorithm programs to complete closed-loop control. PID regulation is a much used adjustment method in general closed-loop control systems. Large and medium-sized PLCs have PID modules, and many small PLCs currently have this function module. PID processing is generally a dedicated PID subroutine. Process control has a wide range of applications in metallurgy, chemical, heat treatment, boiler control and other occasions.
3.5 Data Processing Modern PLC has functions such as mathematical operations (including matrix operations, function operations, logic operations), data transfer, data conversion, sorting, table lookup, bit manipulation, etc., which can complete data collection, analysis and processing. These data can be compared with reference values ​​stored in the memory to perform certain control operations, or can be transferred to other smart devices using communication functions, or printed and tabulated. Data processing is typically used in large control systems, such as unmanned flexible manufacturing systems; it can also be used in process control systems such as large control systems in the paper, metallurgical, and food industries.
3.6 Communication and networking
PLC communication includes communication between PLCs and communication between PLCs and other intelligent devices. With the development of computer control, the factory automation network has developed rapidly. Each PLC manufacturer attaches great importance to the communication function of PLC and has launched their own network systems. The newly produced PLCs all have communication interfaces, and communication is very convenient.
4. The status of PLC at home and abroad
The world's first recognized PLC was developed by American Digital Equipment Corporation (DEC) in 1969. Limited to the current component conditions and computer development level, the early PLC was mainly composed of discrete components and small and medium-sized integrated circuits, which can complete simple logic control and timing and counting functions. Microprocessors appeared in the early 1970s. People quickly introduced it into the programmable controller, which enabled the PLC to add functions such as calculation, data transmission and processing, and completed the industrial control device with real computer characteristics. In order to facilitate the use of engineers and technicians familiar with relays and contactor systems, the programmable controller adopts a ladder diagram similar to the relay circuit diagram as the main programming language, and the computer storage components participating in the calculation and processing are named by relays. The PLC at this time is a combination of the microcomputer technology and the conventional control concept of the relay.
In the mid-to-late 1970s, programmable controllers entered the stage of practical development, and computer technology has been fully introduced into programmable controllers, making their functions leap. Higher computing speed, ultra-small size, more industrial anti-jamming design, analog computing, PID function and high cost performance have established its position in the modern industry. In the early 1980s, programmable controllers were widely used in advanced industrial countries. The development of programmable controllers during this period is characterized by large-scale, high-speed, high-performance, and product serialization. Another feature of this phase is the increasing number of countries producing programmable controllers in the world, with increasing production. This marks the beginning of the mature stage of the programmable controller.
At the end of the 20th century, the development of programmable controllers was more adapted to the needs of modern industry. In terms of control scale, mainframes and ultra-small machines were developed during this period; in terms of control capabilities, various special functional units were born for various control such as pressure, temperature, speed, displacement, etc. Occasionally; in terms of the supporting capabilities of the products, various human-machine interface units and communication units have been produced, making it easier to apply industrial control equipment using programmable controllers. At present, the application of programmable controllers in the fields of machinery manufacturing, petrochemical, metallurgical steel, automobile, light industry, etc. has been greatly developed.
The introduction, application, development and production of programmable controllers in China began with the reform and opening up. The programmable controller was heavily used in the introduction of equipment. Next, the application of PLC has been continuously expanded in the production equipment and products of various enterprises. At present, China has been able to produce small and medium-sized programmable controllers. The CF series produced by Shanghai Dongwu Electric Co., Ltd., the DKK and D series produced by Hangzhou Machine Tool Electric Apparatus Factory, the S series produced by Dalian Combined Machine Tool Research Institute, and the YZ series produced by Suzhou Electronic Computer Factory have already achieved a certain scale. And it has been applied in industrial products. In addition, Wuxi Huaguang Company, Shanghai Xiangdao Company and other Sino-foreign joint ventures are also relatively famous PLC manufacturers in China. It can be expected that with the deepening of China's modernization process, PLC will have a broader application horizon in China.
5. Future prospects of PLC
In the 21st century, PLC will have greater development. From a technical point of view, the new achievements of computer technology will be more applied to the design and manufacture of programmable controllers, and there will be varieties with faster computing speed, larger storage capacity and more intelligent; from the perspective of product scale It will further develop in the direction of ultra-small and super-large; from the perspective of product compatibility, the product variety will be more abundant and the specifications are more complete. The perfect human-machine interface and complete communication equipment will better adapt to various industrial control. The demand of the occasion; from the market point of view, the situation of each country producing a variety of products will be broken with the intensification of international competition, there will be a few brands monopolizing the international market, there will be an internationally common programming language; from the network In terms of development, the large-scale control system composed of programmable controllers and other industrial control computer networks is the development direction of programmable controller technology. A large number of programmable controller applications exist in the current distributed control system DCS (Distributed Control System). With the development of computer networks, programmable controllers, as an important part of automated control networks and international common networks, will play an increasingly important role in many fields outside of industry and industry.
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