Introduction to PLC, excerpted from "Control Engineer"

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Introduction to PLC, excerpted from "Control Engineer" [Copy link]

In production process automation devices, PLC and DCS are the two most widely used control systems. Before the 1980s, the I/O cards of these control systems were concentrated in control rooms far away from the site, and the connection lines with the field devices (including AI/AO analog input and output devices and DI/DO switch input and output devices, etc.) were all one-to-one direct wiring. We can still see a large number of cables entering and leaving the control room and large-sized bridges for laying cables in many sites. In the late 1980s, PLC and DCS control systems successively launched remote I/O cards installed far away from the control room. It was installed on site and could be connected to the field devices nearby. These remote I/O cards and the controllers installed in the control room of the PLC and DCS systems exchanged information through a single cable. Before that, in the early 1980s, some relatively small-scale manufacturers took advantage of their advantages in data acquisition, conversion and communication to launch remote control terminals RTU, and used RTU to form a computer SCADA (Supervisory Control And Data Acquisition) system. Sometimes we call it a four-remote (telemetry, telesignaling, remote control, and remote adjustment) system. For example, the IMP remote control terminal developed by the British Schelumberger Company in the early 1980s and the SCADA system composed of it are an example of the early successful application of RTU.
So what is RTU? RTU is the abbreviation of Remote Terminal Unit, which is the basic component unit of the SCADA system. An RTU can have several, dozens or hundreds of I/O points and can be placed on site near the measurement point. RTU should have at least the following two functions: data acquisition and processing, data transmission (network communication). Of course, many RTUs also have PID control functions or logic control functions, flow accumulation functions, etc.
As a product that embodies the idea of "decentralized measurement and control, centralized management", the remote measurement and control terminal RTU was introduced to China in the 1980s and has been widely used. Its advantages in improving signal transmission reliability, reducing the burden on the host, reducing the amount of signal cables, and saving installation costs have also been recognized by users.
In the automation instrumentation market, we can see some products named RTU, such as the IMP remote measurement and control terminal of Schlumberger Company in the UK, the 893-IDCB remote measurement and control terminal of the Microcomputer Measurement and Control Research Institute of the Nanjing Engineering Corps Academy of the People's Liberation Army, the SuperE remote measurement and control terminal of Beijing Ankong Technology Development Co., Ltd., the eNET wireless RTU of Beijing Huaxun Communication Electronic Technology Co., Ltd., the MOSCAD remote terminal equipment RTU of MOTOROLA Company in the United States, the remote measurement and control terminals such as OPTOMUX and SNAP of OPTO 22 Company in the United States, the EP105 integrated RTU of ELPRO Company in Australia, the MOX RTU of SEAT Company in Australia, etc., but some are integrated by system automation companies themselves, such as the Kunming Condel Company introduced below using GSM modules with the MOX of SEAT Company or the UNITRONICS Company in Israel Micro OPLC; for example, the radio in the EP105 integrated RTU of Australia's EPLO company is equipped with RS-232 and RS-485 interfaces, which can communicate with products produced by other companies, such as AB's SLC5/03/04, SIEMENS's S7-300/400 series and ACTION-CONTROLS' products to form a remote measurement and control terminal RTU.

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The emergence of PLC itself is used to replace the relay control cabinet. It has a simple programming language "ladder diagram", which can be learned and used by anyone with a little electrical knowledge. There is no need to design interfaces and other complex hardware circuits, the development cycle is extremely short, and the system upgrade is simple, just modify the software. In addition, various special function modules can be expanded, and the functions are powerful, but users do not need to care about its internal structure at all, so it is very suitable for general equipment developers. In the field of equipment control, except for price, PLC has great advantages in other aspects!

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1. PLC works stably and reliably; All major PLC manufacturers should strictly conduct electrical performance tests on their products in accordance with IEC61131-2. Including performance, function, EMC, safety, reliability, and environmental tests. And their own limits and white box tests. Does the single-chip microcomputer system you make have these guarantees? Do you dare to use it in industrial sites without these professional tests? Although PLCs are all made of single-chip microcomputers, think about which manufacturer that can make PLCs is not a big shot. 2. PLC is a general controller; Engineers can write control processes based on the ladder diagrams they are familiar with. There is no need to know the instructions inside the single-chip microcomputer. Just use the ready-made instructions of the PLC. And the PLC instructions themselves take into account the actual functions of the user. For example, one instruction can send PWM waves, and people who are slightly trained can use it. 3. The hardware interface of the PLC is very simple, and the functions are implemented by the user; The most basic hardware control ports are: IO, AD, DA, RS485. Users can select the type and quantity of ports they need and select appropriate commands to easily achieve: ModBus/ProfiBus communication, temperature measurement, motion control, PID, etc. Users do not need to understand the hardware, as long as they know the PLC commands, they can complete the project in a short time.