Basics on PLC

PLC (Programmable Logic Controller) is defined as digital computer used for automation of electromechanical process like relay operations. Below you can see the relay operations.

Now, imagine that someone wants to control a 220Volts 1 K-Watt load with a command that comes from a 5 Volts battery. A load-Relay should be used for this application. The Coil of the relay is driven with the 5 Volts. The contacts from this relay (NO) will be connected in series with the power supply of the load. Thus, the load will only operate when the relay is actuated. Our friend bellow will turn on an electric oven bare-handed!!!

Programmable Logic Controller means we can control the process in industries by programming through PLC. For ex : In power plant industry, if we need to control the flow of steam to produce electricity or control some other parameters in industries just create the logic of the controlling process then program in PLC. we can easily control the whole process without any disturbances. Later we will discuss about the programming in PLC.

Why we need PLC?

Generally most industries having repeated process or controlling the parameters in certain conditions. For these purpose we can't accurately finish the work by manually. Now a days automation is heart of the industries. Main aim of the automation is to reduce the manual work, increase accuracy, increase productivity, etc. These task can't be achieved by manually. In olden days relay logic system is used for automation.

Relay logic system is like a switch. Normal relay is used for automation. I will explain the process how they are used the relay logic system in automation in industries.

Relay logic system is hard wired connections not programmable. I shown below image is one type automation process. There are two switched stop and start and one output CR1 that output is given to the first relay which is normally opened. Same output is given to second relay which is normally closed. Now do the process. Stop switch is normally closed. If i press start switch CR1 output get energized then both relays first goes to on position and second goes to off position. So G output is energized.

If i pressed stop button same process is done but first relay goes to off position and second relay goes to on position so R output is energized. Below diagram is simple automated logic. These connections are hard wired in Relay Logic System it is not programmable. So if any problem occur we need to rewire it. So it is more complex and time consuming.

We have lot of disadvantages in Relay Logic System. Using this same principle PLC is developed with lot of advantages. Same switch concept is applied in PLC using solid state devices. Above example can applied in PLC also. In PLC using digital computer program the logic through corresponding software we can easily program and execute it. Later we will discuss about programming software, etc.

Disadvantages in Relay Logic System :
 1. All connections are hard wired so complex logic connections create confusion.
 2. High power consumption. 
 3. Large Size.
 4. We need one electrician for troubleshooting purpose.
 5. High Maintenance.

Advantages in PLC :
  1. All the logic is programmable so complex logic also easy to access.
  2. Low power Consumption.
  3. Miniaturization.
  4. Less maintenance.
  5. In single PLC we can made lot of input and output connections.
  6. very fast.
  7. using single PLC we can control whole process.


I hope now you have basic knowledge in PLC. In next post i gave very interesting concept in PLC. Please read that. If you like give your comments.