As a typical application of industrial automation, PLC was originally produced as a substitute for relay lines and sequential control. Later, PLC manufacturers gradually added functions such as analog calculation, PID function and more reliable industrial anti-jamming technology. With the increasing demand for networking, various communication interfaces have been added. With the development of fieldbus technology and Ethernet technology, the application range of PLC has become more and more extensive.
What is a PLC?
The Programmable Controller is a member of the computer family that is designed and manufactured for industrial control applications. The early programmable controller was called the Programmable Logic Controller (PLC), which was mainly used to replace the relay for logic control.
The International Electrotechnical Commission (IEC) issued the third draft of the draft programmable controller standard in 1987. The programmable controller is defined in the draft as follows:
“A programmable controller is an electronic system for digital computing operations designed for industrial applications. It uses programmable memory for performing logic operations, sequence control, timing, counting, and Commands for arithmetic operations, etc., and control various types of machinery or production processes through digital and analog inputs and outputs. Programmable controllers and their associated peripherals should be easily integrated with industrial systems. , the principle design that is easy to expand its function."
With the development of technology, the function of such a device has greatly exceeded the scope of logic control. Therefore, today such a device is called a programmable controller, referred to as a PC. However, in order to avoid confusion with the abbreviation of Personal Computer, the PLC is referred to as PLC.
The development status of PLC
The world's first PLC
In 1968, General Motors (GE) of the United States, in order to adapt to the continuous updating of automobile models, the ever-changing production process, to achieve small-volume, multi-variety production, hope to have a new industrial controller, which can reduce the re Design and replace electrical control systems and wiring to reduce costs and cycle times.
The design requirement at that time was to take advantage of both the relay and the computer: the relay control system is bulky, low in reliability, complicated in wiring, difficult to change, difficult to find and troubleshoot, and has poor adaptability to changes in production processes, but is easy to understand and price. Cheap; computer is powerful, flexible (programmable), versatile, but difficult to program; using a process-oriented, problem-oriented "natural language" programming, so that people who are not familiar with the computer can quickly grasp the use.
In 1969, the United States Digital Equipment Corporation (DEC) developed the first PLC, which was successfully tested on the GM Automated Assembly Line.
This new type of industrial control device has been widely used in other industrial fields in the United States for its simple and easy to understand, easy to operate, high reliability, versatile and flexible, small size and long service life. By 1971, it has been Successfully used in food, beverage, metallurgy, paper and other industries.
Early 70s
Only the sequential control functions of logic operation, timing, counting, etc., are only used to replace the traditional relay control, usually called Programmable Logic Controller.
Japan introduced this new technology from the United States in 1971 and quickly developed the first PLC in Japan. In 1973, Western European countries also developed their first PLC. China began to develop in 1974. Industrial applications began in 1977.
Mid 70s
The microprocessor technology is applied to the PLC, so that the PLC not only has the logic control function, but also adds functions such as arithmetic operation, data transmission and data processing.
After the 80s
With the rapid development of microelectronics technologies such as large-scale and ultra-large-scale integrated circuits, 16-bit and 32-bit microprocessors have been applied to PLCs, enabling PLCs to develop rapidly. PLC not only enhances control functions, but also improves reliability, reduces power consumption, reduces volume, reduces cost, makes programming and fault detection more flexible and convenient, and has functions such as communication and networking, data processing and image display.
PLC has developed rapidly in recent years
PLC integrates three electric (electric control, electric instrument, telex) as one, high performance and price ratio, high reliability, has become the core equipment of automation engineering. PLC has become a general-purpose industrial control device with computer functions, and its usage is the highest.
PLC has become one of the three technical pillars (PLC, robot, CAD/CAM) of modern industrial automation.
The three major schools of China's PLC market
After China's reform and opening up, the products of American AB, GE, MODICON, TI, Japan OMRON, Mitsubishi, Fuji, Germany and other manufacturers have continuously entered China, and occupy an important position in the industrial control system of all walks of life.
Europe: Siemens (SIEMENS), AEG and TI in France
United States: AB (Allen-Bradly) (its products account for approximately 50% of the US PLC market), GE (General Electric), Modicon (MODICON), Texas Instruments (T1), Gould, Westinghouse the company
Japan: Mitsubishi Electric, OMRON, FUJI (Japan's main development of small and medium-sized PLCs, Japan's products account for about 70% of the world's small PLC market. In China, OMRON products rank first in sales. .)
At present, there are PLC products such as South Korea and Taiwan in the domestic market; now there are serialized domestic PLCs on the market, which are relatively inexpensive and cost-effective.
In 1974, China began to imitate the second generation of PLC products in the United States, but it was not promoted due to component quality and technical problems. It was not until 1977 that China developed the first practical PLC and began mass production and application to industrial process control.
The structure of the PLC and the role of each part
There are many types of PLCs, functions and command systems are different, but the structure and working principle are similar. Usually, it consists of several main parts: host, input/output interface, power expander interface and external device interface. The hardware system structure of the PLC is shown below:
1, the host
The host portion includes a central processing unit (CPU), system program memory, and user programs and data storage. The CPU is the core of the PLC. It is used to run the user program, monitor the status of the input/output interface, make logical judgments and perform data processing, that is, read the input variables, complete various operations specified by the user instructions, and send the results to the output. And respond to requests from external devices (such as computers, printers, etc.) and make various internal judgments.
There are two types of PLC internal memory, one is system program memory, which mainly stores system management and monitoring programs and programs for compiling and processing user programs. The system programs are fixed by the manufacturer and cannot be changed by users; the other is user programs and Data storage, mainly for user-programmed applications and various temporary data and intermediate results.
2, input / output (I / O) interface
The I/O interface is the part of the PLC that is connected to the input/output device. The input interface accepts control signals from input devices such as buttons, sensors, contacts, travel switches, and the like. The output interface is to drive the output device (such as contactor, solenoid valve, indicator light, etc.) through the power amplifier circuit after processing the host. I/O interfaces generally use optocoupler circuits to reduce electromagnetic interference and improve reliability. The number of I/O points, that is, the number of input/output terminals, is a major technical indicator of PLC. Usually, there are dozens of points in a small machine, hundreds of points in a medium machine, and more than a thousand points in a mainframe.
3, the power supply
In the figure, the power supply refers to a DC switching power supply configured for internal electronic circuits such as CPU, memory, I/O interface, etc., and usually supplies DC power to the input device.
4, programming
Programming is the use of external devices by the PLC to input, check, modify, debug, or monitor the operation of the PLC. Connect the PLC to the computer via a dedicated PC/PPI cable and use dedicated software for computer programming and monitoring.
5, input / output expansion unit
The I/O expansion interface is used to connect an expansion unit that expands the number of external input/output terminals to a base unit (ie, a host).
6, external device interface
This interface can connect external devices such as printers, barcode scanners, and inverters to the host to complete the corresponding operations.
PLC characteristics
(1) High reliability
1. All I/O interface circuits are optically isolated to electrically isolate the external circuits of the industrial field from the internal circuits of the PLC.
2. The RC filter is used at each input end, and the filter time constant is generally 10~20ms.
3. Each module adopts shielding measures to prevent radiation interference.
4. Use a switching power supply with excellent performance.
5. Strict screening of the devices used.
6. Good self-diagnosis function. Once the power supply or other soft and hardware abnormalities occur, the CPU immediately adopts effective measures to prevent the fault from expanding.
7. Large PLCs can also use a redundant system composed of dual CPUs or a three-CPU voting system to further improve reliability.
(2) Rich I/O interface module
PLC targets different industrial site signals such as:
AC or DC;
Switching quantity or analog quantity;
Voltage or current;
Pulse or potential;
Strong or weak electricity.
There are corresponding I/O modules and industrial field devices or devices, such as:
Button
Limit switch
Proximity switch
Sensor and transmitter
Electromagnetic coil
Control valve
direct connection. In addition, in order to improve the operational performance, it also has a variety of interface modules for human-machine dialogue; in order to form an industrial local network, it also has a variety of communication network interface modules, and so on.
(iii) Adopting a modular structure
In order to meet various industrial control needs, in addition to the small PLC of the unit type, most PLCs adopt a modular structure.
The various components of the PLC, including the CPU, power supply, and I/O, are modularized. The modules are connected by racks and cables. The scale and function of the system can be combined according to the needs of users.
(4) Programming is easy to learn
PLC programming mostly uses a ladder diagram similar to the relay control circuit. It does not require computer expertise for the user, so it is easy to be understood and mastered by general engineering and technical personnel.
(5) Simple installation and convenient maintenance
The PLC does not require a dedicated machine room and can be operated directly in a variety of industrial environments. When you use it, you only need to connect the various devices on the site with the corresponding I/O terminals of the PLC. Various modules have operation and fault indication devices to facilitate users to understand the operation and find faults.
Due to the modular structure, once a module fails, the user can quickly restore the system by replacing the module.
PLC application field
PLC is not only used for switching quantity control, but also for analog quantity and digital quantity control. It can collect and store data, and can also monitor the control system. It can also be connected and communicated to realize large-scale and cross-regional control and management. PLC has increasingly become an important player in the family of industrial control devices.
1, for switching control
The ability of the PLC to control the amount of switching is very strong. The number of points of entry and exit controlled is less than ten, tens of points, and many can reach several hundred, several thousand, or even tens of thousands. Because it can be networked, the number of points is almost unlimited, no matter how many points can be controlled.
The logic problems that are controlled can be varied: combined, timed; immediate, delayed; countless, countable; fixed order, random work; and so on.
The hardware structure of the PLC is variable, the software program is programmable, and it is very flexible for control. If necessary, you can write multiple sets, or multiple sets of programs, called as needed. It is very suitable for the needs of multiple working conditions and multi-state transformation in industrial fields.
There are many examples of switching control with PLC, metallurgy, machinery, light industry, chemical industry, textiles, etc., which is needed in almost all industrial industries. At present, the first target of the PLC is also unmatched by other controllers, that is, it can be conveniently and reliably used for switching control.
2, for analog control
Analog quantities, such as current, voltage, temperature, pressure, etc., are continuously variable in size. Industrial production, especially in continuous production processes, often requires control of these physical quantities.
As an industrial control electronic device, if the PLC cannot control these quantities, it is a big disadvantage. To this end, various PLC manufacturers have carried out a lot of development in this regard. At present, not only large and medium-sized machines can perform analog control, but also small machines, and such control can be performed.
The PLC performs analog quantity control, and is configured with A/D and D/A units that convert analog and digital quantities. It is also an I/O unit, but a special I/O unit.
The A/D unit converts the analog quantity of the external circuit into a digital quantity and then sends it to the PLC. The D/A unit converts the digital quantity of the PLC into an analog quantity and sends it to the external circuit.
As a special I/O unit, it still has I/O circuit anti-interference, internal and external circuit isolation, and input and output relays (or internal relays, which is also an area of ​​PLC working memory. Read and write) exchange information, etc. Features.
The advantage of using analog control with a PLC is that the amount of switching can be controlled while performing analog control. This advantage is not available in other controllers, or the implementation of control is not as convenient as PLC.
3, for motion control
The actual physical quantity, in addition to the amount of switching, analog, and motion control. Such as the displacement of machine parts, often expressed in digital quantities.
PLC is also based on computer technology and is increasingly perfect. Therefore, it can also be used for digital control.
The PLC can receive counting pulses with frequencies up to a few k to tens of kHz. This pulse can be received in a variety of ways and can be received in multiple ways. Some PLCs also have a pulse output function, and the pulse frequency can reach several tens of k. With these two functions, plus PLC has data processing and computing capabilities, if equipped with corresponding sensors (such as rotary encoders) or pulse servo devices (such as ring distributor, power amplifier, stepper motor), you can rely on The principle of NC implements various controls.
High- and mid-range PLCs are also developed with NC units or motion units for point control. The motion unit also implements curve interpolation to control curve motion. Therefore, if the PLC is equipped with such a unit, it is entirely possible to use the NC method to perform digital control.
4, for data collection
With the development of PLC technology, its data storage area is getting bigger and bigger. For example, the data storage area (DM area) of Deweisen PLC can reach 9999 words. Such a large data storage area can store a large amount of data.
Data acquisition can use a counter to accumulate the number of pulses collected and periodically transfer them to the DM area.
Data acquisition can also be used in the A/D unit. After the analog quantity is converted into a digital quantity, it is transferred to the DM area periodically.
The PLC can also communicate with the computer, and the data of the DM area is read by the computer, and the data is processed by the computer. At this time, the PLC becomes the data terminal of the computer.
Power users have used PLC to record the user's power consumption in real time, so as to realize different charging time and different pricing methods, and encourage users to use more electricity when using electricity to achieve reasonable power consumption and energy saving.
5, for signal monitoring
There are many PLC self-test signals, and there are many internal devices. Most users do not fully play their roles.
In fact, it can be used to monitor the PLC's own work or to monitor the control object.
For a complex control system, especially an automatic control system, monitoring and further self-diagnosis is necessary. It can reduce system failures, find faults, and improve cumulative mean time-to-failure, reduce fault repair time, and improve system reliability.
6, for networking, communication
PLC networking and communication capabilities are strong, and new networked structures are constantly being introduced.
The PLC can be connected to the personal computer for communication, and the computer can be used to participate in programming and control of the PLC, so that the PLC is more convenient to use.
In order to give full play to the role of the computer, a computer can be implemented to control and manage multiple PLCs, up to 32 units. It is also possible to communicate with one or more computers and exchange information to realize the monitoring of the PLC control system.
PLC and PLC can also communicate. One-to-one PLC communication. Can communicate with several PLCs. It can be as many as tens or hundreds.
PLC and smart meters, intelligent actuators (such as inverters), can also communicate online, exchange data, and operate each other. Can be connected to a remote control system with a system range of up to 10 km or more.
Can form a local network, not only PLC, but also high-end computers, various smart devices can also enter the network. Available bus networks, or ring networks. The network can also be set. The network and the network can also be bridged. Networking can organize thousands of PLCs, computers, and smart devices in a single network.
Nodes between networks can communicate or exchange information directly or indirectly.
Networking and communication are adapting to the needs of today's computer integrated manufacturing system (CIMS) and intelligent factory development. It enables industrial control from point to line to line (Aero), enabling equipment-level control, line control, and plant management control to be integrated into one, creating higher levels. Benefits. This infinitely beautiful future has become more and more clearly presented to our generation.
With the development of smart factories and the Internet of Things, the field of equipment and equipment, PLC and PC and other intelligent products, data interaction, big data applications, etc., will greatly promote the close integration of traditional automation and network technology, with China's manufacturing industry. The realization of the sublimation from large to strong, PLC application technology will appear more development.
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