Basics of Programmable logic controller - Black keyhole

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Basics of Programmable logic controller

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What is PLC and why it has been used in industries?

Today in our blog, we will be going to take look about PLC. before move on to further discuss what is PLC and why it has been using industries? PLC stands for "Programmable logic controllers". PLC is a computer specially designed operate under harsh industrial environments. Such as extreme temperature, wet, dry, and or dusty conditions. PLC are used to automate industrial processes such as manufacturing, processing plant, wastewater treatment plant. PLC can work on a rough industrial environment. PLC can thought of as a ruggedized digital computer that manages the electromechanical processes of an industrial environment. PLC can be programmed according to the operational requirement of the process. In the manufacturing industry according to the needs it can be reprogrammed. 

It has been used in various industries such as steel industry, glass industry,cement industry, paper mill, coal mine, automobile industry, chemical industry, texttile industry,robotic system, food processing system.

Manufacturers in PLC

    1. Siemens

       Probably the biggest name out there in the automation and PLC world is Siemens.

   2. Allen-Bradley

   3. ABB

   There are more  manufacturers in  PLC but the above companies are standing top on the list

Basic components of PLC

  1.Input and Output Modules

  2. Power Supply

  3. Control Processing Unit(CPU)

  4. Memory System

  5. Communication Protocols

  6. Programming

You can see the components in the below figure

   (image from here )

Now let us take look at the basic components one by one

Input and Output modules

   Input modules in PLC are two types. It can be either digital or analog. \In PLC to take input and to return Output there are an I/O modules available.

Input and output switch in plc

for example when the user press the button the light will turn on. Here the light works as an Output and the switch works as an Input device. Here the Input modules are providing interface for the different Input devices such as push-button switch, selector switch, limited switch, sensors and etc. As the same way the Output modules give an interface for the output devices such as Motor, fan, relay, light, lamp, electrical heater, solenoid valve, buzzer, etc...

Power Supply

   There is no doubt if we need to work on any device that requires a power supply. here we need an electrical power supply to operates PLC. The power supply provides power to all the components of PLC such as Input/ output modules, memory system, and processor. The power supply can be AC or DC to operate PLC. Most of the PLC works on 220V Ac or 24V DC.

Central Processing Unit(CPU)  

   The central processing unit is the heart of the PLC system. It stores and run the PLC software programs which were programmed by the PLC programmer. It helps to perform arithmetic, logic, controlling, Input and output operations given by Instructions. It has three subjects such as processor, memory, power supply.

Memory System

 A memory is used to store and retrieve data and information. It consists of different types of memory such as RAM, ROM, EEPROM, and Flash memory. Overall all memory is classified into four different types of data it stores. such as 

     Input or Output Image memory

     data memory

     User memory

     Executive memory

Read about Machine learning in control system

Communication Protocols

     communication protocols are useful  for exchanging the data between connected device through a network


  when using PLC, it's important to know the design and implement concepts of your particular use case. In PLC programming there is two main classifications of PLC programming languages, which are further divided into many sub- classification types.

    1. Textual Language

          Instruction List

           Structured List

     2. Graphical Form

               Ladder Diagrams(LD)

                Function Block Diaagram (FBD)

                Sequential Function Chart(SFC)

How Does PLC work

       The most important working principle of PLC is -the PLC is operated by continuously scanning programs. Scanning happens every millisecond per hour. So it is called the scan cycle. For this scan cycle PLC requires little amount in the range of milliseconds or ms.

This scan cycle consists of three main basic steps. such as read the input, execute the program by CPU, update the output.

    Read/sense the input

                 The PLC reads the input on/off signals from the input devices such as switches, pushbuttons, proximity sensors, limit switches, pressure switches, etc and it gets stored in the input memory.

Execute the logic by the processor

      the input data stored in the input memory will be transferred to the processor. the processor executes the program based on the given input. after the execution, the result will be stored in the device memory.

Update / write the output:

   when the programs execute the last instruction, the status(on/off) will be sent to the output device memory. The output include solenoids, valve,motos, actuators, and pumps. 

 Main types of PLC 

   There are two types of PLC that are used for commercial or industrial purpose.

       1. Compact PLC

           It is also called as integrated or Fixed PLC. The compact PLC has a fixed number of inputs and outputs modules along with the power supply and CPU.

Compact PLC – Martinetc

(picture from here)

      2. Modular PLC

             In Modular PLC there are various input and output.  if the the user wish can add more inputs and outputs to the PLC System. 

Siemens Modular PLC, Expandable Compact PLC, Programmable Controller,  Danfoss Programmable Logic Controllers, Danfoss PLC, Yaskawa Programmable  Logic Controllers in Loni Industrial Area, Ghaziabad , Dreamz Automation  Systems Private Limited | ID ...

(picture from here)


     You can check out my previous post about Introduction of PLC.

I hope You have get the basic idea of PLC. please share this blog who wants to get started with PLC




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