PLC Definition

Programmable Logic Controller referred to as PC or PLC is an electronic system operated by digital operations, specially designed for application in an industrial environment. It uses a memory that can be programmed to execute instructions for storing logical operations and sequence control, timing, counting and arithmetic operations, and through digital or analog input (I) and output (O) interfaces, control various Type of mechanical equipment or production process. Programmable controller is developed on the basis of electrical control technology and computer technology, and gradually developed into a new type of industrial control device with microprocessor as the core, integrating automation technology, computer technology and communication technology. PLC controller has been widely used in the automatic control of various production machinery and production processes. It has become one of the most important, most popular, and most applicable industrial control devices. It is recognized as the three pillars of modern industrial automation (PLC, robots, CAD/CAM).

Types of PLC

There are many types of PLC controllers, and their specifications and performance are also different. The classification of PLCs is usually roughly classified according to the difference in its structure, the difference in functions and the number of I/O points.

1. Classified by structure

According to the structure of PLC, PLC can be divided into integral type and modular type.

(1) Integral PLC controller

Integral PLC is a combination of power supply, CPU, I/O interface and other components in a box, which has the characteristics of compact structure, small size and low price. Small PLCs generally adopt this integral structure. Integral PLC is composed of basic unit (also called host) and expansion unit with different I/O points. The basic unit has CPU, I/O interface, expansion port connected with I/O expansion unit, and interface connected with programmer or EPROM writer, etc. There are only I/O and power supply in the expansion unit, but no CPU. The basic unit and the expansion unit are generally connected by a flat cable. Integral PLCs can generally be equipped with special function units, such as analog units, position control units, etc., to expand their functions.

(2) Modular PLC controller

Modular PLC is to make each component of PLC into several separate modules, such as CPU module, I/O module, power module (some included in CPU module) and various functional modules. Modular PLC is composed of a frame or base plate and various modules. The module is mounted on the socket of the frame or base plate. The characteristic of this kind of modular PLC is flexible configuration, different scale systems can be selected according to needs, and it is easy to assemble, easy to expand and maintain. Large and medium-sized PLCs generally adopt a modular structure.

There are also some PLCs that combine the characteristics of integrated and modular to form a so-called stacked PLC. Stacked PLC, its CPU, power supply, I/O interface, etc. are also independent modules, but they are connected by cables, and each module can be stacked layer by layer. In this way, not only the system can be flexibly configured, but also can be made compact.

2. Classified by function

According to the different functions of PLC, PLC can be divided into three types: low-grade, middle-grade and high-grade.

(1) The low-end PLC has basic functions such as logic operation, timing, counting, shifting, self-diagnosis, monitoring, etc. It can also have a small amount of analog input/output, arithmetic operation, data transmission and comparison, communication and other functions. Mainly used in single-machine control systems for logic control, sequence control or a small amount of analog control.

(2) In addition to the functions of low-end PLC, mid-range PLC also has strong analog input/output, arithmetic operations, data transmission and comparison, number system conversion, remote I/O, subroutines, communication networking and other functions. Some can also add interrupt control, PID control and other functions, suitable for complex control systems.

(3) In addition to the functions of a mid-range PLC, the high-end PLC also adds signed arithmetic operations, matrix operations, bit logic operations, square root operations and other special function functions, tabulation and table transmission functions. The high-end PLC machine has a stronger communication networking function, which can be used for large-scale process control or form a distributed network control system to realize factory automation.

3. Classified by I/O points

According to the number of I/O points of the PLC, the PLC can be divided into three categories: small, medium and large.

(1)Small PLC-I/O points <256 points; single CPU, 8-bit or 16-bit processor, user memory capacity below 4K words.

(2)Medium PLC——I/O points 256～2048 points; dual CPU, user memory capacity 2～8K.

(3)Large PLC——I/O points> 2048 points; multiple CPUs, 16-bit and 32-bit processors, user memory capacity 8～16K.

PLC Applications

At present, programmable logic controllers have been widely used in various industrial sectors. With the continuous improvement of its performance-price ratio, the scope of application is still expanding, mainly in the following aspects:

1. Logic Control

Programmable logic controller has the ability of logic operations such as "and", "or", "not", etc. It can realize logic operations, using series and parallel contacts and circuits instead of relays for combinational logic control, timing control and sequence logic control. Digital logic control can be used in a single device or in an automatic production line. Its application field is the most popular, including microelectronics and home appliance industries.

2. Motion Control

The programmable controller uses a dedicated motion control module, or flexibly uses instructions to organically combine motion control and sequence control functions. With the widespread use of frequency converters and motor starters, programmable controllers can be combined with frequency converters to provide more powerful motion control functions and are widely used in various machinery, such as metal cutting machine tools, assembly machinery, robots, elevators Wait for the occasion.

3. Process Control

The programmable controller can receive continuously changing analog quantities such as temperature, pressure, flow, etc., and realize the A/D conversion and D/A between the analog quantity (Analog) and the digital quantity (Digital) through the analog quantity I/0 module Convert and implement closed-loop PID (proportional-integral-derivative) control to the controlled analog quantity. Modern large and medium-sized programmable controllers generally have PID closed-loop control functions, which have been widely used in industrial production, heating furnaces, boilers and other equipment, as well as light industry, chemical industry, machinery, metallurgy, electric power, building materials and other industries.

4. Data Processing

The programmable controller has functions such as mathematical operation, data transmission, conversion, sorting and table look-up, bit operation, etc., which can complete data collection, analysis and processing. These data can be intermediate reference values for calculations, or they can be transmitted to other smart devices through communication functions, or they can be saved and printed. Data processing is generally used in large-scale control systems, such as unmanned flexible manufacturing systems, and can also be used in process control systems, such as some large-scale control systems in the papermaking, metallurgy, and food industries.

5. Build network control

The communication of the programmable logic controller includes the communication between the host and the remote I/0, the communication between multiple programmable controllers, the programmable controller and other intelligent control devices (such as computers, inverters) Communication. The programmable controller and other intelligent control equipment can form a distributed control system of "centralized management and decentralized control".

Of course, not all programmable logic controllers have the above-mentioned functions. Users should choose a suitable one according to the needs of the system, so that it can not only complete the control task, but also save money.

The specific applications can refer to Coolmay Solutions. Learn more please click:

https://www.coolmayplc.com/solution

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