What is PLC and What are its applications?

What is PLC?

According to National Electrical Manufacturers Association (NEMA), a Programmable Logic Controller (PLC) is an electronic device that uses a programmable memory to perform various functions such as counting, timing, logic, etc. by monitoring the states of input continuously to control various machines & process.

PLC Components

A PLC has the following components

  • Power Supply
  • CPU
  • Input-Output Module
  • Communication
  • Redundancy

(1) Power supply: Just like any other electronic and electrical equipment, PLC also needs the power to run the primary components of PLC. Most of the PLC controllers either run with 24 VDC or 240 VAC. Power supply systems also monitor and regulate the voltage they are getting from the power source.

(2) Central Processing Unit (CPU): Central Processing Unit (CPU) is considered as the heart of the PLC system. The CPU will perform various logic functions and data manipulation and this will be done with the input/output section. This component will help the PLC to communicate with computers, field devices, and can also communicate with other PLCs.

Different Operating Modes

The different operating modes of PLC CPU are

  • Run
  • Program
  • Remote

Run Mode: The program execution and sending relevant commands to the output devices is performed in this mode. But we cannot change the mode of the processor by using the programmer/operator interface in this model.

Program Mode: In this mode, the processor will be prevented from scanning or program execution and controller output will not be enabled. In this mode it is possible to modify and insert programs.

Remote Mode: In this mode, we can change the processor mode with the help of the programmer/operator interface. We can also run all the modes in remote mode like Remote Run, Remote program, or Remote test mode.

(3) Input/Output (I/O) modules: These modules allow the processor to communicate with various field devices connected in PLC. Analog Input Module, Analog Output Module, Digital Input Module, Digital Output Module, and so on are examples of these modules.

These are used to connect devices like flow transmitters, pressure transmitters, control valves, analyzers, and substation feeders for motor control between the plc and the field.

(4) Communication: Modules for a wide range of industry-standard communication network connections are offered. These enable digital data flow between PLCs and other facility systems.

Modbus communication cards and serial communication are the most widely used modules.

Rather than using separate modules, some PLCs have communications functionality built into the processor.

(5) Communication Protocols : Ethernet, Ethernet/IP, and DeviceNet are the most common “open” communication protocols in PLC.

“Open” systems typically include “plug and play” features in which the system software recognizes and communicates with any compatible device that is connected to it.

Modbus, Profibus, and ControlNet are three other extensively used open protocols.

(6) Redundancy: Many PLCs may be set up to run in a redundant mode, with one processor backing up the other.

A redundancy module, which offers status confirmation and control assertion across the processors, is frequently required for this setup. There’s also the possibility of signal wiring to redundant racks.

How Does a PLC Operate?

There are four basic steps in the operation of all PLCs; They are

  • Input Scan
  • Program Scan
  • Output Scan
  • Housekeeping

1.) Input Scan

It detects the sates of all the input connected to the PLC.

2.) Program Scan

Executes the user created program logic

3.) Output Scan

Turns on or off all output devices connected to the PLC.

4.) Housekeeping

Housekeeping is considered a necessary chore, which are necessary for the normal activity of a particular computer, but are not necessarily part of the algorithm.

Top 10 PLC Manufacturers

Top 10 PLC manufacturers are

  • Siemens
  • Rockwell Automation / Allen Bradley.
  • Mitsubishi Electric
  • Schneider Electric.
  • ABB.
  • Honeywell Process.
  • Omron.
  • Hitachi Industrial Equipment Systems.
  • IDEC
  • B&R Industrial Automation

Major Area of Applications of Programmable Logic Controllers

PLCs are used in many industries such as the steel industry, automobile industry, chemical industry, and power sector.

PLC has also been used to monitor the safety control system as well as to operate lifts and escalators.

Industry of Glass

PLC was first implemented in the glass industry in the 1980s. After that, technology develops, and almost all units is now controlled by PLCs. Production of glass is a complex, as well as a sophisticated process. So the use of PLCs eases the procedure and the company can provide high-quality material due to the fact that PLC can control material ratio efficiently, cold forging of equipment, etc.

Paper Industry

PLCs are used in various process in the paper industry such as controlling and monitor the machines that produce paper products at very high speed. PLC also monitors the production of pages of books, newspapers etc.

Manufacturing of cement

Cement is made by combining various raw ingredients in a kiln. The quality of these raw ingredients, as well as their amounts, have a considerable impact on the end product’s quality. The accuracy of data on such process variables is critical to ensuring the usage of the appropriate quality and quantity of raw materials.

In the industry’s production and management operations, a distributed control system comprised of a PLC in its user mode and a configuration software is used. The PLC, in particular, regulates the ball mill, coal kiln, and shaft kiln.

Other Applications of PLCs are

  • It is useful in civil applications such as the operation of washing machines, elevators, and the regulation of traffic signals.
  • Water tank quenching systems are used in aerospace.
  • Batch processes in the chemical, cement, food, and paper industries are sequential in nature and necessitate time or event-based decisions.
  • It controls the purging, pilot light off, flame safety checks, main burner light off, and valve switching for fuel changeover in the burner management system.
  • It is used in a multistage screen washing system and an Offset web press print register control system in the printing business.
  • It is employed in the travel sector for escalator operation and a safety control system that’s monitored.