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Programmable Logic Controller (PLC) is a digital operation electronic system designed specifically for applications in industrial environments. It uses a programmable memory to store instructions for performing logic operations, sequential control, timing, counting, and arithmetic operations inside it. It controls various types of mechanical equipment or production processes through digital or analog input and output.
brief introduction
Programmable Logic Controller (PLC) is a digital arithmetic controller with a microprocessor for automatic control, which can load control instructions into memory at any time for storage and execution. The programmable controller is composed of functional units such as CPU, instruction and data memory, input/output interface, power supply, digital to analog conversion, etc. In the early days, programmable logic controllers only had the function of logic control, so they were named programmable logic controllers. Later, with continuous development, these computer modules with simple functions at the beginning had various functions, including logic control, timing control, analog control, and multi machine communication. The name was also changed to Programmable Controller, However, due to the conflict between the abbreviation PC and the abbreviation Personal Computer, and due to customary reasons, people still often use the term programmable logic controller, and still use the abbreviation PLC.
Programmable logic controllers used in industry are already equivalent to or close to the mainframe of a compact computer, and their advantages in scalability and reliability make them widely used in various industrial control fields. Regardless of whether it is in a computer direct control system, a centralized distributed control system DCS, or a field bus control system FCS, there is always a significant use of various types of PLC controllers. There are many manufacturers of PLC, such as Siemens, Schneider, Mitsubishi, Delta, and almost all manufacturers involved in the field of industrial automation will provide their PLC products.
Functional characteristics
(1) High reliability. Most PLCs use single chip microcomputers, resulting in a high degree of integration, coupled with corresponding protection circuits and self-diagnosis functions, improving the reliability of the system.
(2) Programming is easy. PLC programming mostly uses relay control ladder diagrams and command statements, with a much smaller number than microcomputer instructions. Except for medium and high-end PLCs, general small PLCs only have about 16. Because the ladder diagram is vivid and simple, it is easy to master and use, and even can be programmed without requiring computer expertise.
(3) Flexible configuration. Due to the modular structure of the PLC, users can flexibly change the function and scale of the control system by simply combining them. Therefore, it can be applied to any control system.
(4) The input/output functional modules are complete. One of the greatest advantages of PLC is that for different field signals (such as DC or AC, switching, digital or analog, voltage or current, etc.), there are corresponding templates that can be directly connected to devices in industrial fields (such as buttons, switches, sensing current transmitters, motor starters, or control valves, etc.), and connected to the CPU motherboard through a bus.
(5) Easy installation. Compared to computer systems, the installation of PLC requires neither a dedicated computer room nor strict shielding measures. When in use, it is only necessary to correctly connect the detection device to the I/O interface terminals of the actuator and PLC to ensure normal operation.
(6) Fast operation speed. Because PLC control is performed by program control, its reliability and operating speed are unmatched by relay logic control.
In recent years, the use of microprocessors, especially with the widespread adoption of single-chip microcomputers, has greatly enhanced the ability of PLC, and has made the difference between PLC and microcomputer control systems smaller and smaller, especially for high-end PLCs.
There are a wide variety of PLC products. The types of PLCs vary, corresponding to their structural form, performance, capacity, instruction system, programming method, price, etc., and the applicable occasions also have different emphasis. Therefore, reasonable selection of PLC is of great significance for improving the technical and economic indicators of PLC control systems.
PLC model
The selection of PLC should mainly be comprehensively considered in terms of PLC model, capacity, I/O module, power supply module, special function module, communication networking capability, etc. The basic principle for selecting PLC models is to strive for the best performance price ratio on the premise of meeting functional requirements, ensuring reliability, and convenient maintenance. During selection, factors such as reasonable structural type, selection of installation method, corresponding functional requirements, response speed requirements, system reliability requirements, and model uniformity should be mainly considered.
structural style 
PLC mainly has two types of structure: integral type and modular type.
The average price of each I/O point of an integrated PLC is cheaper than that of a modular PLC, and its volume is relatively small. It is generally used in small control systems where the system process is relatively fixed; The modular PLC has flexible and convenient function expansion, wide choice in terms of I/O points, the ratio of input points to output points, the type of I/O modules, and convenient maintenance. It is generally used in more complex control systems.
Installation method
The installation methods of PLC systems are divided into centralized, remote I/O, and distributed, networked multiple PLCs.
Centralized type does not need to set up remote I/O hardware to drive, resulting in fast system response and low cost; Remote I/O is suitable for large systems, with a wide range of devices distributed. Remote I/O can be installed scattered near on-site devices with short connections, but additional drivers and remote I/O power supplies are required; The distributed networking of multiple PLCs is suitable for situations where multiple devices are independently controlled and interconnected. Small PLCs can be selected, but additional communication modules are required.
Functional requirements
Generally, small (low level) PLCs have functions such as logic calculation, timing, and counting, which can be satisfied for devices that only require on-off control.
For systems that primarily rely on switching control with a small amount of analog control, an enhanced low-level PLC with A/D and D/A conversion units, plus/minus arithmetic operations, and data transfer functions can be selected. For complex control, it is required to achieve PID calculation, closed-loop control, communication networking, and other functions. Depending on the size and complexity of the control, medium or high-end PLCs can be selected. However, medium and high-end PLCs are more expensive and are generally used in large-scale process control and distributed control systems.
response speed
PLC is a general-purpose controller designed for industrial automation, and the response speed of different grades of PLC can generally meet the needs of its application range. If you want to use a PLC across a range, or if some functions or signals have special speed requirements, you should carefully consider the response speed of the PLC. You can choose a PLC with high-speed I/O processing functions, or choose a PLC with fast response modules and interrupt input modules.
The reliability of PLC for general systems can be satisfied. For systems with high reliability requirements, it should be considered whether to adopt redundant systems or hot standby systems.

Copyright: NIIC (suzhou) Su ICP Bei No. 2020061042-1

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