PLC programmable logic controller: PLC English full name Programmable Logic Controller, Chinese full name programmable logic controller, definition is: a number arithmetic operation electronic system, designed for application in industrial environments. It uses a class of programmable memory for its internal stored programs, performs user-oriented instructions such as logic operations, sequence control, timing, counting and arithmetic operations, and controls various types of mechanical or production processes through digital or analog inputs/outputs. DCS distributed system: DCS English full name DISTRIBUTED CONTROL SYSTEM, Chinese full name distributed control system. DCS can be interpreted as an automated high-tech product widely used in industries with more analog loop control, which disperses the risk caused by control as much as possible and concentrates management and display functions. DCS generally consists of five parts: 1: controller 2: I/O board 3: operation station 4: communication network 5: graphics and traversal software.

1, the development process of PLC In the industrial production process
, a large number of switching sequence control, it according to logical conditions for sequential action, and according to the logical relationship of chain protection action control, and a large number of discrete data acquisition. Traditionally, these functions have been implemented by pneumatic or electrical control systems. In 1968, the American GM (General Motors) company proposed to replace the relay electrical control device requirements, the next year, the American digital company developed a control device based on integrated circuits and electronic technology, the first programmatic means applied to electrical control, which is the first generation of programmable controllers, called Programmable Controller (PC).

After the development of personal computers (referred to as PCs), in order to facilitate and reflect the functional characteristics of programmable controllers, programmable controllers are named Programmable Logic Controller (PLC), and PLCs are still often referred to as PCs.

There are many definitions of PLC. The International Electrotechnical Commission (IEC) defines a PLC as a digital crunching electronic system designed for use in industrial environments. It uses programmable memory to store instructions for performing logic operations, sequence control, timing, counting and arithmetic operations in its interior, and controls various types of mechanical or production processes through digital and analog inputs and outputs. The programmable controller and its related equipment should be designed according to the principle of easy to form a whole with the industrial control system and easy to expand its functions.

From the 80s to the mid-90s of last century, PLC was the fastest developing period, and the annual growth rate has been maintained at 30~40%. During this period, PLC in the processing of analog capabilities, digital computing capabilities, human-machine interface capabilities and network capabilities have been greatly improved, PLC gradually entered the field of process control, in some applications to replace the dominant DCS system in the field of process control.

PLC has the characteristics of strong versatility, easy to use, wide adaptability, high reliability, strong anti-interference ability, and simple programming. The position of PLC in industrial automation control, especially sequential control, cannot be replaced in the foreseeable future.

2, the composition of PLC

From the structural point of view, PLC is divided into two types: fixed type and combined type (modular type). Fixed PLC includes CPU board, I/O board, display panel, memory block, power supply, etc., these elements are combined into a non-removable whole. Modular PLC includes CPU module, I/O module, memory, power module, backplane or rack, which can be configured in combination according to certain rules.

3, the composition of the CPU

CPU is the core of PLC, play the role of nerve center, each set of PLC at least one CPU, it according to the PLC system program to give the function to receive and store user programs and data, by scanning the way to collect the status or data sent by the field input device, and stored in the specified register, at the same time, diagnose the power supply and PLC internal circuit working status and programming process syntax errors, etc. After entering the operation, read the instructions one by one from the user program memory, and after analysis, the corresponding control signals are generated according to the tasks specified in the instructions to command the relevant control circuits.

CPU is mainly composed of arithmetic, controller, register and data, control and status bus to realize the connection between them, CPU unit also includes peripheral chips, bus interfaces and related circuits. Memory is mainly used to store programs and data, and is an indispensable component unit of PLC.

From the user's point of view, it is not necessary to analyze the internal circuitry of the CPU in detail, but there should be enough understanding of the working mechanism of each part. The controller of the CPU controls the operation of the CPU, which reads instructions, interprets instructions, and executes instructions. But the operating rhythm is controlled by the oscillation signal. Combinators are used to perform numerical or logical operations and work under the command of the controller. The registers participate in the operation and store the intermediate results of the operation, which also works under the command of the controller.

CPU speed and memory capacity are important parameters of PLC, they determine the PLC's working speed, IO number and software capacity, etc., so they limit the control scale.

4, I/O module

The interface between the PLC and the electrical circuit is done through the input-output part (I/O). The I/O module integrates the I/O circuit of the PLC, and its input register reflects the input signal status 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 is divided into switching input (DI), switching output (DO), analog input (AI), analog output (AO) and other modules.

Switching refers to signals with only two states (or 1 and 0), and analog refers to the amount of continuous change. Commonly used I/O classifications are as follows:

Switching: according to the voltage level, there are 220VAC, 110VAC, 24VDC, according to the isolation mode, there is relay isolation and transistor isolation.

Analog: 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 above general IO, there are also special IO modules, such as thermal resistance, thermocouple, pulse and other modules.

Determine the specifications and number of modules according to the number of I/O points, and the number of I/O modules can be as many or few, but their maximum number is limited by the ability of the basic configuration that the CPU can manage, that is, by the maximum number of backplanes or rack slots.

5, Power supply module

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

6, the bottom plate or rack Most modular PLC uses the base plate or rack
, its function is: electrically, to realize the connection between the modules, so that the CPU can access all the modules on the baseboard, mechanically, to realize the connection between the modules, so that the modules constitute a whole.

7. Other equipment of PLC system

1. Programming equipment: The programmer is an indispensable device for the development and application, monitoring and operation, inspection and maintenance of PLC. It is used for programming, setting the system, and monitoring the working status of the system controlled by PLC and PLC, but it does not directly participate in the site control operation. Small programmer PLC generally has a handheld programmer, currently generally by the computer (running programming software) as a programmer.

2, man-machine interface: the simplest man-machine interface is the indicator light and button, at present, the LCD screen (or touch screen) type of the integrated operator terminal application is more and more widely used, by the computer (running configuration software) to act as the man-machine interface is very popular.

3. Input and output equipment: used for permanently storing user data, such as EPROM, EEPROM writer, barcode reader, input analog volume potentiometer, printer, etc.

8. PLC communication networking

Rely on advanced industrial network technology to collect, transmit production and management data quickly and efficiently. Therefore, the importance of networks in automation system integration engineering is becoming more and more significant, and some people have even put forward the view that "the network is the controller".

PLC has the function of communication networking, which enables PLC and PLC, PLC and host computer and other intelligent devices to exchange information, form a unified whole, to achieve decentralized centralized control. Most PLCs have RS-232 interfaces, and some have built-in interfaces that support their respective communication protocols.

PLC communication, has not yet achieved interoperability, IEC stipulates a variety of fieldbus standards, PLC manufacturers have adopted.

For an automation project, especially for medium and large-scale control systems, the choice of network is very important. First, the network must be open to facilitate the integration of different devices and the expansion of the scale of the system in the future; Secondly, according to the transmission performance requirements of different network levels, select the form of the network, which must be carried out under the premise of a deeper understanding of the protocol and mechanism of the network standard; Again, consider specific issues such as system cost, equipment compatibility, and applicability of the field environment to determine the network standards used at different levels.