What are the applications of PLC?


PLC’s are used in wide range of industries like petrochemical,biomedical,distillery,beverages,food industries and any other industry that seek less human help to produce the product.With the help of PLC some of the process we can do is listed below
1.Continuous bottle filling system
2.Batch mixing system
3.stage air conditioning system
4.Traffic control

What is a PLC ?

Control engineering has evolved over time. In the past, humans were the main method to control a system. More recently, electricity has been used for control and electrical control is based on relays. These relays allow you to turn the power on and off without a mechanical switch. It is common to use relays to make simple decisions of logical control. The development of low-cost computers has brought the most recent revolution, the Programmable Logic Controller (PLC). The advent of the PLC began in the 1970s and has become the most common option for manufacturing controls.

PLCs have been gaining popularity in the factory and will probably remain predominant for some time to come. Most of this is due to the advantages they offer.

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Advantages of PLC

• Cost effective to control complex systems.
• Flexible and can be reapplied to control other systems quickly and easily.
• Computer skills allow more sophisticated control.
• Troubleshooting accessories make programming easier and reduce downtime.
• Reliable components make them work for years before failure.
• Less manpower for design
• Downsizing and standardization
• Improved maintainability

Internal Architecture of PLC

1.The CPU

The internal structure of the CPU depends on the microprocessor concerned. In general, CPUs have the following:

An arithmetic and logic unit (ALU) that is responsible for data manipulation and carrying out arithmetic operations of addition and subtraction and logic operations of AND, OR, NOT, and EXCLUSIVE-OR.
Memory, termed registers, located within the microprocessor and used to store information involved in program execution.
A control unit that is used to control the timing of operations.

2.The Buses

The buses are the paths used for communication within the PLC. The information is transmitted in binary form, that is, as a group of bits, with a bit being a binary digit of 1 or 0, indicating on/off states.


To operate the PLC system there is a need for it to access the data to be processed and instructions, that is, the program, which informs it how the data is to be processed. Both are stored in the PLC memory for access during processing.

4.Input/Output Unit

The input/output unit provides the interface between the system and the outside world, allowing for connections to be made through input/output channels to input devices such as sensors and output devices such as motors and solenoids.

Types of PLCs

  • Integrated or Compact PLC
  • Modular PLC
  • Small PLC
  • Medium PLC
  • Large PLc

PLC Manufactures

Allen bradley
Mitsubishi Electric
Schneider Electric

Applications of PLC

A programmable logic controller (PLC) is an electronic device used in many industries to monitor and control construction systems and production processes. Unlike PCs and smartphones, which are designed to perform any number of roles, a PLC is designed to perform a single set of tasks, except in the case of limitations in real time and with superior reliability and performance.

To meet the demands of rigorous industrial environments, PLCs are designed to be extremely robust, often capable of withstanding extreme temperatures, humidity, vibration and electrical noise. The logical controllers are commonly responsible for monitoring and controlling a large number of sensors and actuators, and therefore are different from other computer systems in their extensive input / output (I / O) arrangements.
PLC 's are used in several industries like petrochemical ,biomedical,cement manufacturing,oil and gas sector etc

How to choose the right plc based on its application

PLC selection criteria consists of:

  1. SCAN time

Scan time of PLC is duration of:
Read Input
Excute program
Update the output
Therefore, the larger the program is, the longer the Scan Time will be.

  1. Number of I/O s

3. Processing capacity
4. Digital or analog signals
5. System requirements
6. Application requirements
7. Electrical Requirements
8. Speed of Operation
9. Communication

Types of programming langauges used in plc

The majority of PLC systems today adhere to the IEC 61131/3 control systems programming standard that defines 5 languages: Ladder Diagram (LD), Structured Text (ST), Function Block Diagram (FBD), Instruction List (IL) and Sequential Flow Chart (SFC).

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1.Batch mixing system with PLC

where two liquids are mixed in required proportion to form a batch .Rate of the flow is already fixed. We only control the time of the flow. Level of the liquids in the tank are sensed by the level sensor switches


We try a simple blending of water and acid in a container where we only have three level
sensors(L1,L2, and L3) and two liquids flowing in through two solenoid valves, solenoid a(water
control) and solenoid b(acid control)and draining out through solenoid c(blend outflow).The
batch is to be controlled by timer. After required level of blend is sensed (by L1)the mixer runs
for 3 mins. by the motor. They are mixed in ratio of 3:2. The process initiates with the drain
valve open, water and acid valves closed, mixer motor is off, and the tank is empty

Ladder Diagram

When start button is pressed water is filled upto L2 and it ends as L2 is closed. First of all as start
is pressed output O:0/15 turns ON and remains ON until tank is emptied. Rung 2 closes normally
open drain valve, before timer T:4 activates. Rung 3 energises solenoid a until L2 doesn’t signal,
once it signals solenoid a gets de-energised.Then motor is turned ON and mix it for 3 mins.
Similarly acid is filled upto L3 by solenoid b.as level gets detected by L3 solenoid b de-energises
.And then mixer gets started and it runs for 3 minutes. After time delay of 3 mins solenoid c
opens and the blend gets drained out .Once the blend gets out completely ,the process cycle
The ladder diagram was successfully checked in the PLC simulator and all the prescribed
conditions were observed completely

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