1.The development of PLC

In the process of industrial production, a large number of switching quantities are controlled sequentially, which ACTS sequentially according to logical conditions, and controls interlocking protection actions according to logical relations, and collects a large number of discrete quantities of data. Traditionally, these functions have been achieved through pneumatic or electrical control systems. In 1968, GM (general motors) of the United States put forward the requirement of replacing electric control devices. In the second year, digital corporation of the United States developed control devices based on integrated circuits and electronic technology, and for the first time applied programmatic means to electric control, which is the first generation of Programmable Controller (PC).

With the development of personal computers (PCS), Programmable Logic controllers (PLC) have been designated for convenience and to reflect the functional characteristics of Programmable controllers (PLC). Today, PLC is often referred to simply as PC.

There are many definitions of PLC. The international electrotechnical commission (IEC) defines PLC as: programmable controller is a digital operation of the electronic system, designed for use in industrial environments. It USES programmable memory, used in its internal storage to perform logic operations, sequence control, timing, counting and arithmetic operations and other operations of the instruction, and through the digital, analog input and output, control of various types of machinery or production process. The programmable controller and its related equipment should be designed according to the principle that it is easy to form a whole with the industrial control system and easy to expand its functions.

The 1980s to the mid-1990s, is the fastest development of PLC period, the annual growth rate has been maintained for 30~40%. In this period, PLC in processing analog capacity, digital operation capacity, man-machine interface ability and network ability have been greatly improved, PLC gradually entered the field of process control, in some applications to replace the DCS system in the dominant position in the field of process control.

PLC has the characteristics of strong universality, convenient use, wide adaptability, high reliability, strong anti-interference ability, simple programming and so on. PLC in industrial automation control, especially in the position of sequence control, in the foreseeable future, is irreplaceable.

2.Second, the structure of PLC

From the structure, PLC is divided into fixed and modular (modular) two. Fixed PLC includes CPU board, I/O board, display panel, memory block, power supply, etc. Modular PLC includes CPU module, I/O module, memory, power module, baseplate or rack. These modules can be combined and configured according to certain rules.

3. Composition of CPU

The CPU is the core of PLC, the nerve center of role, each PLC has at least one CPU, which is according to the function of the PLC system program to receive and store the user program and data, collected by scanning way by the state or data from input devices, and deposited in the register of regulations, at the same time, the diagnosis of the power and the working state of the circuit inside the PLC and programming in the process of grammatical errors, etc. After entering the operation, read the instructions one by one from the user program memory, and then generate the corresponding control signal according to the task specified in the instruction after analysis, so as to command the relevant control circuit.

CPU is mainly composed of arithmetic unit, controller, register and data, control and state bus to realize the connection between them. CPU unit also includes peripheral chip, bus interface and relevant circuit. Memory is mainly used to store programs and data, PLC is an indispensable unit.

From the user's point of view, it is not necessary to analyze the internal circuit of the CPU in detail, but it is necessary to have an adequate understanding of the working mechanism of each part. The controller of the CPU controls the work of the CPU, by which instructions are read, interpreted and executed. But the rhythm of the work is controlled by the oscillating signal. The arithmetic unit is used for numerical or logical operations and works under the command of the controller. The register participates in the operation and stores the intermediate results of the operation. It also works under the command of the controller.

CPU speed and memory capacity are important parameters of PLC, they determine the working speed of PLC, IO quantity and software capacity, and so on, and therefore limit the control scale.

4. I/O module

PLC and electrical circuit interface, is through the input and output part (I/O) completed. The I/O module integrates the I/O circuit of PLC. Its input register reflects the state of input signal and output point reflects the state of output latch. The input module converts electrical signals into digital signals and enters the PLC system, while the output module is opposite. I/O is divided into switch quantity input (DI), switch quantity output (DO), analog quantity input (AI), analog quantity output (AO) and other modules.

A toggle quantity is a signal with only on and off (or 1 and 0) states, and an analog quantity is a continuously varying quantity. Common I/O classifications are as follows:

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

Analog quantity: divided by signal type, including current type (4-20ma, 0-20ma), voltage type (0-10v, 0-5v, -10-10v), etc., divided by precision, including 12bit,14bit,16bit, etc.

In addition to the above general IO, there are special IO modules such as thermal resistance, thermocouple, pulse, etc.

The number of I/O modules can be more or less, but the maximum number of I/O modules is limited by the CPU's ability to manage the basic configuration, that is, the maximum number of baseplate or rack slots.

5. power module

PLC power supply is used to provide working power for the integrated circuit of each PLC module. At the same time, some also provide 24V power supply for the input circuit. Power input types are: ac power supply (220VAC or 110VAC), dc power supply (commonly used for 24VAC).

6.Base plate or frame

Most modular PLCS use a baseplate or a rack, whose function is to: electrically, to achieve the connection between the modules, so that the CPU can access all the modules on the baseplate, mechanically, to achieve the connection between the modules, so that the modules constitute a whole.

7.Other equipment of PLC system

1, programming equipment: the programmer is the PLC development and application, monitoring operation, inspection and maintenance of indispensable devices, used for programming, the system to make some Settings, monitoring PLC and PLC control system work, but it does not directly participate in the field control operation. Small PLC programmer generally handheld programmer, currently generally by the computer (running programming software) as a programmer.

2. Man-machine interface: the simplest man-machine interface is indicator light and button. At present, the all-in-one operator terminal with LCD screen (or touch screen) is more and more widely used.

Input/output device: it is used to permanently store user data, such as EPROM, EEPROM writer, bar code reader, potentiometer for input analog quantity, printer, etc.

8.PLC communication network

Relying on advanced industrial network technology, production and management data can be collected and transmitted rapidly and efficiently. Therefore, the importance of network in automation system integration engineering is becoming more and more significant, and some people even put forward the view that "network is controller".

PLC has the function of communication network, it makes PLC and PLC, PLC and the upper computer and other intelligent equipment between the exchange of information, forming a unified whole, decentralized centralized control. Most PLCS have rs-232 interfaces, and some have interfaces built in to support their own communication protocols.

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

For an automation engineering (especially in large scale control system), 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 future systems. Secondly, according to the transmission performance requirements of different network levels, the form of the network should be selected, which must be carried out on the premise of a deeper understanding of the protocol and mechanism of the network standard. Thirdly, considering the system cost, equipment compatibility, site environment applicability and other specific issues, determine the network standards used at different levels.