DCS / PLC | Instrumentation blog

PLC - Programmable logic controller Basics | Instrumentation blog

Chemical, Pharmaceutical industries, Petrochemical industries, and every small to big scale company has automation now a days.

PLC means Programmable logic controller. It is registered trademark of Rockwell automation and Allen Bradley Hardware company

PLC is used in small batch process plants. Its reason because it has small memory and high speed.
DCS is used in large continuous process plant. Its reason it has Good memory and can handle larger numbers of Instruments simultaneously.

This blog contain questions

  1. Why DCS / PLC [Programmable logic controller]
  2. Component of DCS / PLC. 
  3. Flowchart of DCS / PLC.
  4. Programming in DCS / PLC

Interview Questions

  1. DCS [Distributed Control system] is available now a days then why PLC is still used in Industries?
  2. What is component of DCS / PLC?
  3. What is working flowchart of DCS / PLC?
  4. Symbols of Flow chart.

1. Why PLC [Programmable logic controller] in small batch process plant?

  1. PLC is Economic and user friendly operation.
  2. PLC is not need hard wired relay logic.
  3. Fast  operation in small scale batch operation
2. Component of PLC.

Basic Components of DCS / PLC
  1. IO Module, Cards
  2. CPU
  3. Programming Device for operation
  4. Memory
  5. Power supply

1. IO Module, Cards

IO module means Input Output module.

Input modules convert Analog / digital input & out put reversibly to 120 VAC, 240 VDC, 4 - 20 mA.
  • Inputs are Pressure ,Temperature, Flow, pH, Level Transmitters
  • Outputs are Motor, Solenoid, Heaters, Lamp or any others Which controls Valves, Motors, Lighting accessories. 
2. CPU -Central Processing Unit.

This is brain of Automation. It contains Input signals which control output signal based on Controlling programs.

3. Programming Device for operation [ HMI - Human Machine Interface]

This is a device which used for plant or machine operation. This is Computer or Panel screen. Sometimes Laptops or Tablets also used.

4. Memory

Memory divided in two categories. We will see example of Android mobile.

1. Data memory
  • In android phone You get 64GB, 128GB, etc this is Data memory
2. Program memory
  • In 64GB Phone you get 50GB memory. Your 14GB is android Program Memory.
PLC is connected to SCADA for data storage and analysis.

5. Power supply

For Operation of PLC panel or Computer and IO Card it requires various rating power.

3. Flowchart of DCS / PLC

PLC - Programmable logic controller Basics | Instrumentation blog
PLC - Programmable logic controller Basics | Instrumentation blog

  • Input devices like PT FT LT send Electricity [4mA -20mA] to IO Module which convert AI[Analog input ] to Digital DI [Digital Input].
  • IO Module sending digital data to CPU which interfered by Operational person.
  • CPU sending data to according to Parameter adjusted set point by Operational person.
  • Again IO Module convert DO [Digital output] To AO [Analog output] to Field Input devices

4. Programming in DCS / PLC

In PLC basic functioning is in Flowcharts or ladder logic or Statement logic or mnemonics

In any batch process algebraic flowchart, Which is set as per process sequence. There is a symbolic presentation 

1. Data symbol is used for stored data
Programming in PLC
2. Process symbol

Programming in PLC
3. Decision symbol

Programming in PLC
4. Document stored in Flowchart

Programming in PLC

5. Start/ End of Flowchart
Programming in PLC
 6. Direct Data

Programming in PLC

7. Manual Input
Programming in PLC
8. Predefined Process

Programming in PLC
9. Process

Programming in PLC

10. Sequential Access Storage
Programming in PLC

 11. Stored data in sequence
Programming in PLC

Algebra using logic operation Which use Gates
  • AND
  • OR
  • NOT
  • NAND
  • NOR

What is Rung in Ladder logic?

In ladder logic single step or single basic statement or one control logic known as Rung.

Common symbol in Ladder Logic

PLC - Programmable logic controller Basics  Instrumentation blog
PLC - Programmable logic controller Basics  Instrumentation blog

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Thanks for reading -
Naitik Patel
Industrial Guide

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