In the industrial production process, there are a large number of sequential control of the switching quantity, and it is required to perform sequential actions according to logical conditions, and perform control of interlocking protection actions according to logical relations, and a large amount of discrete data acquisition and the like. Traditionally, these functions are implemented by pneumatic or electrical control systems. In 1968, the United States GM (General Motors Corporation) publicly invited tenders to propose requirements for the development of control devices that could replace relays. In the second year, Digital Equipment Corporation (DEC) developed control devices based on integrated circuits and electronic technologies. The means used in electrical control, this is the first generation of programmable controllers, called Programmable Controller (PC).

After the personal computer (PC) was developed, the programmable controller was named Programmable Logic Controlle (PLC) for the sake of convenience and to reflect the features of the programmable controller. Now, the PLC is still often referred to as PC.

There are many kinds of PLC definitions. The definition of PLC by the International Electrotechnical Commission (IEC) is: The programmable controller is an electronic system for digital operation and is designed for application in industrial environments. It uses a programmable memory to store instructions for performing logic operations, sequence control, timing, counting, and arithmetic operations in its internal memory, and to control various kinds of signals through digital and analog inputs and outputs. Type of machinery or production process. Programmable logic controllers and related equipment should be designed in such a way that they can be easily integrated with industrial control systems and can easily be extended.

From the 1980s to the mid-1990s, PLC was the fastest growing period, and its annual growth rate remained at 30-40%. During this period, PLC has been greatly improved in the handling of analog capabilities, digital computing capabilities, human-machine interface capabilities, and network capabilities. PLC has gradually entered the field of process control, and has replaced the dominant position in the field of process control in some applications. DCS system.

Nowadays, PLC has the characteristics of strong versatility, easy use, wide adaptation, high reliability, strong anti-interference ability and simple programming. In the foreseeable future, PLC's dominant position in industrial automation control, especially sequential control, cannot be replaced by other control technologies.

Second, the composition of PLC

From the structural point of view, PLC is divided into fixed and modular (module) two. Fixed PLCs include CPU boards, I/O boards, display panels, memory blocks, power supplies, etc. These elements are combined into a non-removable unit (above left). Modular PLCs include CPU modules, I/O modules, memory, power modules, backplanes, or racks. These modules can be combined and configured according to certain rules (see above right figure).

Third, the composition of PLC's various elements and functions:

1, CPU composition and function

The CPU is the core of the PLC and plays the role of a nerve center. It is mainly composed of an operator, a controller, a register, and data, control, and status buses that implement the connection between them. The CPU unit also includes a peripheral chip, a bus interface, and related circuits. Each set of PLC has at least one CPU. It receives and stores user programs and data according to the functions assigned by the system program of the PLC, and collects the status or data sent by the on-site input device by scanning and stores them in the specified registers. At the same time, diagnosis of power supply and PLC internal circuit working status and programming syntax errors. After entering the operation, the instructions are read one by one from the memory of the user program. After analysis, the corresponding control signals are generated according to the tasks specified in the instruction, and the relevant control circuit is commanded.

For the user, it is not necessary to analyze the internal circuits of the CPU in detail, but there should be sufficient understanding of the working mechanism of each part. The controller of the CPU controls the operation of the CPU, and it reads instructions, interprets instructions, and executes instructions, but the operating rhythm is controlled by the oscillating signal. The calculator is used to perform digital or logical operations and work under the direction of the controller. The register participates in the operation and stores the intermediate result of the operation. It also works under the direction of the controller.
CPU speed and memory capacity are important parameters of PLC. They determine the working speed, IO quantity, and software capacity of PLC, thus limiting the control scale.

2, I/O module

The interface between the PLC and the electrical circuit is accomplished through an input/output section (I/O). The I/O module integrates the PLC's I/O circuit. Its input register reflects the status of the input signal, and the output point reflects the output latch status. The input module converts the electrical signal into a digital signal into the PLC system, and the output module is reversed.
I/O types include digital input (DI), digital output (DO), analog input (AI), and analog output (AO).
Switching quantity refers to the signal of only two states of opening and closing (or 1 and 0). Analog quantity refers to the amount of continuous change. Commonly used I/Os are classified as follows:
Switching quantity: According to the voltage level, there are 220VAC, 110VAC, 24VDC, divided by isolation mode, there is relay isolation and transistor isolation.
Analog quantity: According to the signal type, there are current type (4-20mA, 0-20mA), voltage type (0-10V, 0-5V,-10-10V), etc. According to the accuracy, there are 12bit, 14bit, 16bit, etc. .
In addition to the general-purpose IO described above, there are special IO modules such as thermal resistance, thermocouple, and pulse modules.
According to the number of I/O points to determine the module specifications and quantity, I/O modules can be more or less, but the maximum number of CPUs can be managed by the basic configuration of the ability, that is subject to the maximum number of backplane or rack slots.

3, memory

Memory is mainly used to store programs and data and is an indispensable component of the PLC. The memory size of the PLC in different models is not the same. Except for the existing memory area of ​​the host unit, most models can be expanded according to the specific needs of the user.

4, power module

The PLC power supply is used to provide the operating power for the integrated circuits of the PLC modules. At the same time, some also provide 24V operating power for the input circuit. Power input types are: AC power (220VAC or 110VAC), DC power (24VAC is commonly used).

5, floor or rack

Most modular PLCs use backplanes or racks. Their role is: Electrically, to achieve the connection between the various modules, so that the CPU can access all modules on the backplane, mechanically, to achieve the connection between the various modules, so that each module constitutes a overall.

6, other equipment of PLC system

i) Programming Device: The programmer is an indispensable device for PLC development, application, monitoring, and maintenance. It is used to write programs, make some settings for the system, and monitor the operating status of the PLC and PLC controlled system, but it is not directly Participate in on-site control operations. Some PLCs are also equipped with handheld programmers, which are currently used by computers (running programming software) as programmers.
Ii) Human-Machine Interface: The simplest man-machine interface is the indicator light and button. Currently, the integrated operator terminal of the LCD (or touch screen) type is more and more widely used. The computer (run configuration software) acts as the human-machine interface. It is also very popular.
Iii) Input and output devices: Used to permanently store user data, such as EPROM, EEPROM writer, bar code reader, analog input potentiometers, printers, etc.

7, PLC communication networking

As shown in the figure, relying on advanced industrial network technology can quickly and efficiently collect and transmit production and management data. Therefore, the importance of the network in the integration of automation systems is becoming more and more significant. Some people even put forward the idea that "the network is the controller."
PLC has the function of communication networking. It can exchange information between PLC and PLC, between PLC and host computer and other intelligent devices, forming a unified whole and achieving decentralized centralized control. Most PLCs have an RS-232 interface, and some have built-in interfaces that support their respective communication protocols.
The communication of PLC has not realized the interoperability yet, IEC has stipulated many kinds of on-the-spot bus standards, all PLC manufacturers have adopted. For an automation project (especially for medium and large-scale control systems), choosing a network is very important. First of all, the network must be open to facilitate the integration of different devices and the scale of future systems; secondly, for the transmission performance requirements of different network layers, the form of the network must be selected, which must be a deeper understanding of the network standard protocols and Under the premise of the mechanism, once again, considering the specific issues such as system cost, equipment compatibility, and site environmental applicability, determine the network standards used at different levels.

Guangzhou Lingyu Automation Equipment Co., Ltd. Contact: Tu Yan

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