1. PLC features:

Modern industrial production is complex and diverse, and their requirements for control are also different. As soon as the programmable controller appeared, it was welcomed by the majority of engineering and technical personnel. Its main features are as follows:

(1) Strong anti-interference ability and high reliability

Although the microcomputer has very strong functions, it has poor anti-interference ability. The electromagnetic interference, power fluctuation, mechanical vibration, temperature and humidity changes in the industrial field can make the general purpose microcomputer unable to work normally. PLC draws on the long-term industrial control experience of manufacturers in electronic circuit, mechanical structure and software structure. The main modules are large-scale and super large-scale integrated circuits. The I / 0 system is designed with perfect channel protection and signal conditioning circuit; In the structure, the heat-resistant, moisture-proof, dust-proof, seismic and so on are accurately considered; In the hardware, the anti-interference measures such as isolation, shielding, filtering and grounding are adopted; In software, digital filter and other anti-interference and fault diagnosis measures are adopted; All these make PLC have high anti-interference ability. The average time between failures of PLC is usually more than tens of thousands of hours, which is unmatched by ordinary microcomputer.

Although the relay contactor control system has good anti-interference ability, it uses a large number of mechanical contacts, which makes the equipment wiring complex, and the contacts are vulnerable to arc damage when opening and closing, with short service life and poor system reliability. PLC adopts microelectronic technology, a large number of switching actions are completed by contactless electronic memory devices, most of the relays and complicated wiring are replaced by software programs, so the service life is long and the reliability is greatly improved.

(2) The structure of the control system is simple and universal

There are many kinds of PLC and peripheral modules, which can be flexibly combined into control systems of various sizes and requirements. In the control system composed of PLC, it only needs to connect the corresponding input and output signal lines on the terminals of PLC, without physical devices such as relays and a large number of complicated hard wiring lines. When the function of the control system needs to be changed, the programmer can modify the program online or offline. The same PLC device is used for different control objects, but the input and output components and application software are different. The input and output of PLC can be directly connected with strong current such as AC 220 V, DC 24 V, and has strong load capacity.

(3) Programming is convenient and easy to use

PLC is a user-oriented equipment, PLC designers fully consider the skills and habits of field engineering and technical personnel. The programming of PLC adopts ladder diagram or simple instruction form for industrial control. Ladder diagram is similar to relay schematic diagram. This programming language is visual and easy to master. It does not need special computer knowledge and language. As long as the personnel with certain electrician and process knowledge can learn it in a short time.

(4) Function complete alarm

The input and output system of PLC has perfect function and reliable performance, and can adapt to the input and output of various forms and properties of switching and analog quantities. Because of the use of microprocessor, it can easily realize many functions such as timing, counting, latch, comparison, jump and forced I / O. it not only has the functions of logic operation, arithmetic operation, number system conversion and sequence control, but also has the functions of analog operation, display, monitoring, printing and report generation. In addition, it can also form a distributed or decentralized control system with other microcomputer systems and control equipment, and realize the functions of group data transmission, matrix operation, closed-loop control, sorting and table lookup, function operation and quick interruption. Therefore, PLC has a strong adaptability, which can well meet the needs of various types of control.

(5) The period of design, construction and debugging is short

To complete a control project with relay contactor control, we must first draw the electrical schematic diagram according to the process requirements, and then draw the layout and wiring diagram of relay panel (cabinet) for installation and debugging. It is very inconvenient to modify later. PLC control, because of its software control, hardware circuit is very simple, and modular building block structure, and has been commercialized, so only according to the performance, capacity (input and output points, memory size) selection and assembly, and a large number of specific programming can also be carried out before the arrival of PLC, thus shortening the design cycle, so that the design and construction can be carried out at the same time. Because the hard wiring is replaced by software programming to realize the control function, the heavy installation and wiring work is greatly reduced, and the construction period is shortened. Because PLC completes the control task through the program, adopts the industrial programming language which is convenient for users, and has the function of forcing and simulation, so the design, modification and debugging of the program are very convenient, which can greatly shorten the design and operation cycle.

(6) Small size, easy maintenance and operation

PLC is small, light and easy to install. The input and output system of PLC can intuitively reflect the change state of the field signal, and also intuitively reflect the operation state of the control system through various ways, such as internal working state, communication state, I / o point state, abnormal state and power state, etc. all of these have eye-catching instructions, which are very helpful for the operation and maintenance personnel to monitor the system.

2. PLC classification:

Nowadays, there are many kinds of programmable controllers, and their functions are not the same. According to different principles, there are different classifications.

1. Classification by I / O points and memory capacity

Generally speaking, if there are more I / O points to be processed, the control relationship will be more complex, the program memory capacity required by users will be larger, the PLC instructions and other functions will be more required, and the process of instruction execution will be faster. According to the number of input and output points of PLC, PLC can be divided into the following three categories:

1. Small PLC

The function of Small PLC is mainly controlled by switching value. The total number of input and output points of Small PLC is generally less than 256 points, and the capacity of user program memory is less than 4K words. Now the high-performance Small PLC also has a certain communication ability and a small amount of analog processing capacity. This kind of PLC is characterized by low price and small size, which is suitable for controlling single equipment and developing mechatronics products.

Typical minicomputers include Siemens S7-200 series, Rockwell (AB) SLC500 series, Omron CPM2A series and Mitsubish FX series.

2. Medium PLC

The total number of input and output points of medium PLC is between 256 and 1024, and the capacity of user program memory is more than 8K words. Medium sized PLC not only has the control function of switching value and analog value, but also has stronger digital calculation ability, and its communication function and analog value processing ability are more powerful. The instruction of medium-sized computer is more abundant than that of small-sized computer. Medium-sized computer is suitable for complex logic control system and continuous production process control.

Typical medium-sized machines include Siemens S7-300 series, Rockwell (AB) ControlLogix series and OMRON C200H Series modular PLC.

3. Large PLC

The total number of input and output points of large PLC is more than 1024, and the capacity of user program memory is 8-16m. The performance of large-scale PLC is equivalent to that of industrial control computer. It has the functions of calculation, control and adjustment. It also has powerful network structure and communication networking ability. It can be connected to HMI as system monitoring or operation interface, can express the dynamic process of the process, record various curves, PID adjustment parameter selection diagram, and can be equipped with a variety of intelligent modules to form a multi-functional system. This system can also be interconnected with other types of controllers and connected with the host computer to form a centralized and decentralized production process and product quality control system. Mainframe is suitable for equipment automation control, process automation control and process monitoring system.

Typical large PLC products include Siemens S7-400, Omron CVML and CSL series, Rockwell (AB) ControlLogix and PLC5 / 05 series.

With the rapid development of PLC technology, some small and medium-sized PLCs also have the functions of large-scale PLCs. For example, ControlLogix of Rockwell (AB) company not only has the characteristics of medium-sized computers, but also has the functions and performance of large-scale computers, which is also the development trend of PLC.

2. According to the structural form

(1) Integral (box type)

The feature of the integral structure is that the basic components of PLC, such as CPU board, input board, output board, power board, etc., are compactly installed in a standard cabinet to form a whole and a basic unit (host) or expansion unit of PLC. There is no expansion port on the basic unit, which is connected with the expansion unit through expansion cable. To form different configuration of PLC. The PLC with integral structure has the advantages of small volume, low cost and convenient installation. Micro PLC and small PLC are generally integrated structure.

(2) Rack modular

The PLC with modular structure is composed of some module units. These standard modules, such as CPU module, input module, output module, power module and various function modules, can be inserted on the frame or base plate. The function of each module is independent, the overall dimension is unified, and the inserted modules can be flexibly configured according to the needs. At present, this kind of structure is widely used in medium and large PLC.

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