A programmable logic controller (PLC) refers to an industrial computer control system that monitors input devices’ state. It then makes decisions on a custom program to control the state of output devices.

Nearly any production line, machine function, or process can use a PLC. But, probably its most significant benefit is its ability to change and replicate an operation or a process while collecting and communicating vital information.

A PLC is also modular, which means you can mix and match various input and output devices to best suit your needs.

Other interesting terms…

Read More about a “Programmable Logic Controller”

How Did Programmable Logic Controllers Come to Be?

The first PLCs were designed and developed by Modicon, now Schneider Electric, a global corporation that caters to the electric industry. These PLCs acted as relay replacers for GM and Landis. They eliminated the need for rewiring and adding additional hardware for each new logic configuration. As such, they drastically increased control functionality while reducing the cabinet space that housed the logic.

The first PLC, dubbed “Model 084,” was invented by Dick Morley in 1969. But the first PLC that reached commercial success was “Model 184,” which Michael Greenberg designed.

What Are the Elements of a Programmable Logic Controller?

A PLC system typically has three components—inputs, a central processing unit (CPU), and outputs.

The CPU has an internal program that tells the PLC how to perform functions, such as:

  • Execute control instructions contained in a user’s program that is stored in “nonvolatile” memory, which means it will not be lost if the system loses power
  • Communicate with other devices that include but are not limited to input and output devices, programming devices, networks, and other PLCs
  • Do housekeeping activities that include communications and internal diagnostics, among others.

How Does a Programmable Logic Controller Work?

Four basic steps make up all PLCs’ operation—input scan, program scan, output scan, and housekeeping. These steps occur in a repeating loop.

  • Input scan: Detects all input devices connected to the PLC.
  • Program scan: Executes the user-created program logic.
  • Output scan: Energizes or de-energizes all output devices connected to the PLC.
  • Housekeeping: Includes communications with programming terminals, internal diagnostics, and others.

Watch this video to see a PLC in action:

What Input and Output Devices Do Programmable Logic Controllers Use?

The various input devices you can use with PLCs are:

  • Switches and pushbuttons
  • Sensing devices
    • Limit switches
    • Photoelectric sensors
    • Proximity sensors
  • Condition sensors
  • Encoders
    • Pressure switches
    • Level switches
    • Temperature switches
    • Vacuum switches
    • Float switches

The output devices, meanwhile, include:

  • Valves
  • Motor starters
  • Solenoids
  • Actuators
  • Horns and alarms
  • Stack lights
  • Control relays
  • Counters/Totalizers
  • Pumps
  • Printers
  • Fans

What You Need to Consider to Choose the Right Programmable Logic Controller

Apart from cost, it would help if you answered the following questions to decide which PLC will best suit your needs:

  • Will the PLC be powered by alternating current (AC) or direct current (DC)?
  • Does it have enough memory to run my user program?
  • Is it fast enough to meet my application’s requirements?
  • What type of software does it use for programming?
  • Will it be able to manage the number of inputs and outputs my program requires?
  • If needed, can it handle analog inputs and outputs, apart from discrete ones?
  • How am I going to communicate with it?
  • Does it have network connectivity?
  • Will it be located in one place or spread out over a large area?

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PLCs are critical to sectors that use industrial control systems (ICSs). If you work in such an industry, knowing the answer to “What is a programmable logic controller?” is crucial for you.