What are PLC's?

If you're into Automation you might have come across PLC commonly known as Programmable Logic Controllers. So what are they and how are they different than a Normal PC. Let's find out..

In old days, Relay logic was commonly used to operate a process plants. Control logic for manufacturing was mainly composed of relays, Cam timers, Drum sequencers and dedicated closed-loop controllers. Due to the hard wiring of relays it was a painstaking task for engineers to iterate the Logic and document it again in a proper form. A seriously tedious task. 

To change this hardwire iteration problem engineers at Bedford Associates in 1969 developed a Programmable Logic Controller so that the iteration could be carried out in software. Resulting in saving time and clumsy effort of rewiring. 

PLC is a Controller designed to provide programming capabilities for controlling continuous processes while withstanding harsh industrial environments.

A PLC can be Programmed by its Application from a PC as per the design requirements. It is easy to program as it has many languages namely: 

• Ladder logic: A Relay logic diagram easy to program due to the graphical arrangement of Input and Outputs,
• Sequential Function Charts
• Function Block Diagram
• Structured Text
• Instruction List

Types of PLC

• Compact 

  

  Siemens S7-1200 compact PLC

The compact PLC's are the ones where we need to fit a controller on a machine which requires less space. It has Power supply, I/O's, communication module, Etc. inside it. 

• Modular

  

  Allen Bradley Logix Modular
  

The moduar are mostly the ones which has external I/O modules and communication modules, etc. This is because it could satisfy more functionality and that is why it is more found in control rooms. 

Components of PLC 

• Input Module
• CPU ( processor with memory)
• Output Module
• Power Supply
• Bus / communication system

Advantages of PLC are

• Small in size, reduced space
• Shorter project time 
• Energy saving
• Ease of maintenance / trouble shooting 
• Economical 
• Greater life & reliability in operations 
• Tremendous flexibility in control techniques
• Easier storage, archiving and documentation 
• Online / offline modification
• In - house simulation & testing of project
• Speed in operation
• Ability to communicate with computer system in plant
• Security

Disadvantages of PLC are

•  Higher initial cost
•  Difficult to find errors and require skillful  work force  
•  When  a  problem  occurs,  hold-up time is indefinite
•  Initially  it  was  manufactured   to   replace   relays, counters, timers but now it is  to  be interfaced with smart transmitters 
•  There's too much work required in connecting wires 
• There's  difficulty  with  changes or replacements.

Some Manufacturers of PLC are: Siemens, Allen Bradley, Omron, Yokogawa, Honeywell, Delta, ABB, Mitsubhishi Electric, Etc.