Programmable logic controller is a kind of digital operation electronic system specially designed for application in industrial environment. It uses a kind of programmable memory to store the instructions for executing logic operation, sequence control, timing, counting, arithmetic operation and other operations, and controls various types of mechanical equipment or production process through digital or analog input and output. [1]

Computer and automation of disciplines such as reliable point work and programmability

catalogue

1 Introduction

2 Development history

▪ origin

▪ development

3 Basic structure

▪ Power Supply

▪ Central processing unit

▪ storage

▪ input unit

▪ output unit

4 Working principle

▪ Input sampling

▪ User program execution

▪ Output Refresh

▪ Summary

5 Functional Features

6 System integration

7 Selection Rules

▪ Point estimation

▪ Memory capacity

▪ Control function selection

▪ Controller type

▪ Input/output type

▪ Conversion principle

8 Select model

▪ PLC model

▪ structural style

▪ Installation method

▪ Functional requirements

▪ response speed

▪ reliability

▪ The model shall be as uniform as possible

9 Application Overview

▪ Open loop control

▪ Analog closed loop

▪ Digital quantity control

▪ Data acquisition monitoring

Introduction editing broadcast

PLC

PLC

Programmable Logic Controller (PLC), a digital operation controller with microprocessor for automatic control, can load control instructions into memory at any time for storage and execution. The programmable controller is composed of CPU, instruction and data memory, input/output interface, power supply, digital analog conversion and other functional units. In the early days, the programmable logic controller only had the function of logic control, so it was named as the programmable logic controller. Later, with the continuous development, these computer modules with simple functions have already had various functions, including logic control, timing control, analog control, multi computer communication, and the name has also been changed to programmable controller, However, due to the conflict between its abbreviation of PC and personal computer, and the habit, people still often use the name of programmable logic controller, and still use the abbreviation of PLC. [2]

The programmable logic controller used in industry is equivalent to or close to the host of a compact computer, and its advantages in scalability and reliability make it widely used in various industrial control fields. No matter in the computer direct control system, the centralized and decentralized control system DCS, or the field bus control system FCS, there is always a large number of PLC controllers. There are many manufacturers of PLC, such as Siemens, Schneider, Mitsubishi, Delta, etc. Almost all manufacturers involved in industrial automation will have their own PLC products. [2]

Development history editor broadcast

origin

The development of the production technology requirements of the American automobile industry promoted the emergence of PLC. In the 1960s, General Motors found that the relay and contactor control systems were difficult to modify, bulky, noisy, inconvenient to maintain, and poor in reliability when adjusting the factory production line, so it proposed the famous "General Ten" bidding index. [3]

In 1969, Digital Equipment Company of the United States developed the first programmable controller (PDP-14), which was tested on the production line of General Motors, with remarkable results; In 1971, Japan developed the first programmable controller (DCS-8); In 1973, Germany developed the first programmable controller; In 1974, China began to develop programmable controllers; in 1977, China promoted PLC in industrial applications. [3]

The initial goal was to replace mechanical switching devices (relay modules). However, since 1968, the function of PLC has gradually replaced the relay control board, and modern PLC has more functions. Its application extends from single process control to the control and monitoring of the whole manufacturing system. [4]

development

Microprocessors appeared in the early 1970s. It was soon introduced into the PLC, so that the PLC added the functions of calculation, data transmission and processing, and completed the industrial control device with real computer characteristics. At this time, the PLC is the product of the combination of microcomputer technology and conventional relay control concept. After the development of personal computers, in order to facilitate and reflect the functional characteristics of programmable controllers, programmable logic controllers are named Programmable Logic Controllers (PLC). [5]

In the middle and late 1970s, the programmable logic controller entered the stage of practical development, and computer technology has been fully introduced into the programmable controller, making its function leap. Higher operation speed, super small size, more reliable industrial anti-interference design, analog calculation, PID function and high cost performance have established its position in modern industry. [5]

In the early 1980s, PLC has been widely used in advanced industrial countries. The number of countries producing programmable controllers in the world is increasing, and the output is increasing. This indicates that the programmable controller has entered a mature stage. [5]

From the 1980s to the mid-1990s, the PLC was the fastest growing period, with an annual growth rate of 30-40%. During this period, the ability of PLC to process analog quantities, digital operation, human-computer interface and network has been greatly improved, and PLC has gradually entered the field of process control, replacing the DCS system that is dominant in the field of process control in some applications. [5]

At the end of the 20th century, the development of PLC is more suitable for the needs of modern industry. This period witnessed the development of mainframe and subminiature computers, the birth of various special functional units, and the production of various human-computer interface units and communication units, making it easier to match industrial control equipment with programmable logic controllers. [5]

Basic structure editing broadcast

Figure 1 Basic components of PLC

Figure 1 Basic components of PLC

In essence, the programmable logic controller is a computer dedicated to industrial control. Its hardware structure is basically the same as that of a microcomputer. Its basic composition is shown in Figure 1. The basic composition is described in detail as follows: [6]

Power Supply

The power supply is used to convert the AC power into the DC power required by the PLC, and most PLCs use the switch type regulated power supply for power supply. [6]

Central processing unit

Central processing unit (CPU) is the control center of PLC and the core component of PLC. Its performance determines the performance of PLC. [6]

The central processing unit is composed of controller, arithmetic unit and register. These circuits are all concentrated on a chip, and are connected with the input/output interface circuit of the memory through the address bus and control bus. The function of the central processing unit is to process and run user programs, perform logical and mathematical operations, and control the entire system to coordinate them. [6]

storage

Memory is a semiconductor circuit with memory function, which is used to store system programs, user programs, logic variables and other information. The system program is a program that controls the PLC to realize various functions. It is compiled by the PLC manufacturer and solidified into the read-only memory (ROM), which cannot be accessed by the user. [6]

input unit

The input unit is the input interface between the PLC and the controlled equipment. It is the bridge for signals to enter the PLC. Its role is to receive signals from the master command components and detection components. Input types include DC input, AC input and AC/DC input. [6]

output unit

The output unit is also a connecting part between PLC and the controlled equipment. Its function is to transmit the output signal of PLC to the controlled equipment, that is, convert the weak current signal sent by the central processor into a level signal, and drive the executive components of the controlled equipment. The output types include relay output, transistor output and crystal gate output. [6]

In addition to the above parts, PLC also has a variety of external devices according to different models, which are used to help programming, monitoring and network communication. Commonly used external equipment includes programmer, printer, cassette tape recorder, computer, etc. [6]

Working principle edit broadcast

When the PLC is put into operation, its working process is generally divided into three stages, namely input sampling, user program execution and output refresh. The completion of the above three stages is called a scanning cycle. During the whole operation period, the CPU of the PLC repeats the above three stages at a certain scanning speed. [5]

Input sampling

PLC

PLC (2 sheets)

In the input sampling stage, the PLC reads in all input states and data in turn by scanning, and stores them in the corresponding units in the I/O image area. After the input sampling is completed, the user program execution and output refresh phase are started. In these two stages, even if the input state and data change, the state and data of the corresponding unit in the I/O image area will not change. Therefore, if the input is a pulse signal, the width of the pulse signal must be greater than a scanning period to ensure that the input can be read in any case. [5]

User program execution

In the execution phase of the user program, the PLC always scans the user program (ladder diagram) from top to bottom. When scanning each ladder diagram, the control circuit composed of each contact on the left side of the ladder diagram is always scanned first, and the control circuit composed of contacts is logically calculated in the order of left to right, top to bottom, and then the corresponding bit status of the logic coil in the system RAM storage area is refreshed according to the result of the logic operation; Or refresh the status of the corresponding bit of the output coil in the I/O image area; Or determine whether to execute the special function command specified in the ladder. [5]

That is, during the execution of the user program, only the state and data of the input point in the I/O image area will not change, while the state and data of other output points and software devices in the I/O image area or the system RAM memory area may change, and the program execution results of the ladder diagrams listed above will play a role in the ladder diagrams listed below that use these coils or data; On the contrary, the status or data of the logic coil refreshed in the ladder diagram below can only be used for the program above it in the next scan cycle. [5]

If the immediate I/O instruction is used during program execution, I/O points can be directly accessed. That is, if the I/O instruction is used, the value of the input process image register will not be updated. The program directly takes values from the I/O module, and the output process image register will be updated immediately, which is somewhat different from the immediate input. [5]

Output Refresh

When the scanning user program is finished, the PLC will enter the output refresh phase. During this period, the CPU refreshes all output latch circuits according to the corresponding state and data in the I/O image area, and then drives the corresponding peripherals through the output circuit. This is the real output of the PLC. [5]

Summary

According to the above process description, the characteristics of PLC working process can be summarized as follows: [7]

① PLC adopts the working mode of centralized sampling and centralized output, which reduces the influence of external interference. [5]

② The working process of PLC is a cycle scanning process, and the cycle scanning time depends on the speed of instruction execution, the length of user program and other factors. [5]

③ The effect of output on input has hysteresis. PLC adopts the working mode of centralized sampling and centralized output. When the sampling phase is over, the change of input state will not be received until the next sampling cycle. Therefore, the length of this lag time mainly depends on the length of the cycle. In addition, the factors that affect the lag time include the input filter time and the lag time of the output circuit. [5]

④ The contents of the output image register depend on the results of the user program scan execution. [5]

⑤ The contents of the output latch are determined by the data in the output image register during the last output refresh. [5]

⑥ The current actual output state of the PLC is determined by the contents of the output latch. [5]

Functional features, editing and broadcasting

(1) High reliability. Because PLC mostly adopts single chip microcomputer, the integration degree is high, and the corresponding protection circuit and self diagnosis function improve the reliability of the system. [8]

(2) Easy programming. PLC programming mostly uses relay control ladder diagram and command statements, which are much less than microcomputer commands. Except for medium and high grade PLCs, there are only about 16 small PLCs in general. Because the ladder diagram is visual and simple, it is easy to master and use, and it can be programmed without computer expertise. [8]

(3) Flexible configuration. Because PLC adopts building block structure, users can flexibly change the function and scale of the control system by simply combining, so it can be applied to any control system. [8]

(4) Input/output function modules are complete. One of the greatest advantages of PLC is that for different field signals (such as DC or AC, switching value, digital or analog quantity, voltage or current, etc.), there are corresponding templates that can be directly connected to devices in the industrial field (such as buttons, switches, sensing current transmitters, motor starters or control valves, etc.), and connected to the CPU motherboard through the bus. [8]

(5) Easy to install. Compared with the computer system, the installation of PLC neither requires a dedicated machine room nor strict shielding measures. When in use, it is only necessary to correctly connect the detection device with the I/O interface terminal of the actuator and PLC, and then it can work normally. [8]

(6) Fast running speed. Because the PLC control is executed by the program control, no matter its reliability or running speed, the relay logic control is incomparable. [8]

In recent years, the use of microprocessors, especially with the large number of single chip computers, has greatly enhanced the ability of PLC, and made the difference between PLC and microcomputer control system smaller and smaller, especially for high-end PLC. [8]

System integration editing and broadcasting

In the manufacturing industry, there are a large number of open loop sequential control based on switching value, which performs sequential action number and sequential action according to logical conditions; In addition, the interlocking protection action is controlled according to the logic relationship independent of sequence and time sequence; As well as a large number of switching value, pulse quantity, timing, counter, analog quantity out of limit alarm and other state quantity based discrete quantity data acquisition monitoring. Because of these requirements of control and monitoring, PLC has developed into a product mainly replacing relay circuit and sequential control. PLC manufacturers have gradually added various communication interfaces to the original CPU template, and the field bus technology and Ethernet technology have also developed synchronously, making the application scope of PLC more and more extensive. PLC has the advantages of stability, reliability, low price, complete functions, flexible and convenient application, and convenient operation and maintenance, which is the fundamental reason why it can occupy the market for a long time.