What can a PLC do? Why do we use them?

The meaning of PLC…

PLC” means “Programmable Logic Controller”, that’s clear. The word “Programmable” differentiates it from the conventional hard-wired relay logic. It can be easily programmed or changed as per the application’s requirement. The PLC also surpassed the hazard of changing the wiring.
The PLC as a unit consists of a processor to execute the control action on the field data provided by input and output modules. In a programming device, the PLC control logic is first developed and then transferred to the PLC.
So, what can a PLC actually do?
  1. It can perform relay-switching tasks.
  2. It can conduct counting, calculation and comparison of analog process values.
  3. It offers flexibility to modify the control logic, whenever required, in the shortest time.
  4. It responds to the changes in process parameters within fractions of seconds.
  5. It improves the overall control system reliability.
  6. It is cost effective for controlling complex systems.
  7. It trouble-shoots more simply and more quickly
  8. It can be worked with the help of the HMI (Human-Machine Interface) computer
There are many other things this little ‘mean’ thing can do, but one thing I’m sure – that PLCs are irreplaceable in many industry applications and control projects.
Here is an example of wired ABB’s AC500 programmable logic controllers.
Wired ABB's PLCs
Figure 1 – Wired ABB’s PLCs (photo credit: us.profinet.com)

Basic block diagram

Figure 1 shows the basic block diagram of a common PLC system.
Block diagram of a PLC
Figure 2 – Block diagram of a PLC

As shown in the above figure, the heart of the “PLC” in the center, i.e., the Processor or CPU (Central Processing Unit).
  • The CPU regulates the PLC program, data storage, and data exchange with I//O modules.
  • Input and output modules are the media for data exchange between field devices and CPU. It tells CPU the exact status of field devices and also acts as a tool to control them.
  • A programming device is a computer loaded with programming software, which allows a user to create, transfer and make changes in the PLC software.
  • Memory provides the storage media for the PLC program as well as for different data.

Size of the PLC system

Usually they are classified on the basis of their size:
  • A small system is one with less than 500 analog and digital I/Os.
  • A medium system has I/Os ranging from 500 to 5,000.
  • A system with over 5,000 I/Os are considered large.

Components of the PLC system

CPU or processor: The main processor (Central Processing Unit or CPU) is a microprocessor-based system that executes the control program after reading the status of field inputs and then sends commands to field outputs.
I/O section: I/O modules act as “Real Data Interface” between field and CPU. The PLC knows the real status of field devices, and controls the field devices by means of the relevant I/O cards.
Programming device: A CPU card can be connected with a programming device through a communication link via a programming port on the CPU.
Operating station: An operating station is commonly used to provide an “Operating Window” to the process. It is usually a separate device (generally a PC), loaded with HMI (Human Machine Software).

PLC Configurations

There are two basic configurations that commercial manufacturers offer:
1. Fixed Configuration
Fixed PLC configuration
Fixed PLC configuration

2. Modular Configuration
Modular type PLC 'SLC 500'
Modular type PLC ‘SLC 500’ (photo credit: ab.rockwellautomation.com)

PLC Applications (VIDEOs) //

Real world applications


An Application for Industrial Process Control


PLC Bottling Application

PLC application color mixing


PLC Suited To Bottling Line Application

5 guides to study PLCs //

  1. PLC – Programmable Logic Controller – Hugh Jack
  2. PLC Programming – OMRON
  3. PLC – Theory and Implementation – L.A. Bryan; E.A. Bryan
  4. Industrial Training – SCADA System and PLC – Mr. Sonu Kumar Yadav

References //
  • Industrial Automation Pocket Book – IDC Technologies
  • Overview of Programmable Overview of Programmable Logic Controllers – Dr. Fernando Rios-Gutierrez

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