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What are Programmable Logic Controllers Used For?


What are Programmable Logic Controllers Used For?

In Isaac Asimov’s Foundation trilogy, fast-talking salesman Hober Mallow demonstrates an atomic-powered hand tool that cuts, drills holes of any size, planes and welds solid steel. It does anything you want in Asimov’s future utopia. In today’s real industrial world, programmable logic controllers (PLCs) are almost as dazzling. They have one huge advantage over Asimov’s dream machine, though: they exist.

What are Programmable Logic Controllers?

According to the experts at Machine Design, a PLC is “an industrial solid-state computer that monitors inputs and outputs, and makes logic-based decisions for automated processes or machines.”

Some definitions, like that one, need to be defined themselves. A PLC is a computer that can be adapted to control just about any industrial machine, from rugged assembly line robots to delicate sensors. PLCs are designed to be useful across many environments and on many machines. They are very durable, deliberately equipped to handle cold, heat, dust, and high humidity.

Before PLCs

Because today’s PLCs are solid-state circuitry, they have no mechanical parts (switches and relays) and are not apt to fail under difficult conditions. The old mechanical relays that today’s PLCs replaced were prone to breakdown, meaning expensive delays. The older electromechanical relays could arc, causing contacts to weld shut and possibly start fires.

Electromechanical relays have been used, with varying degrees of reliability, in elevators, assembly lines, coalmines, heavy industry, and even in residential applications. These switches could be scaled for both size and voltage, so small relays could be built into automobiles, hair dryers, kitchen appliances and the like. Large relays could be used to control heavy equipment.

Uses of PLCs Today

Because of their straightforward design and basic parts structure, PLCs can be adapted to fit almost any machine control application. The parts are usually the same:

  1. Power supply
  2. Input module
  3. Output module
  4. Central processing unit (CPU)
  5. Programming device

Because of their relative simplicity and easy programming, PLCs can be attached to control just about any machine or system used in commercial and industrial applications.

A partial listing of typical PLC programming functions can be found at SanFoundry:

  • Implement an automatic car-washing process
  • Operate four outputs simultaneously with a time delay
  • Drive motors simultaneously with interlocking
  • Drive motor in forward and reverse direction
  • Control lights in a sequence
  • Control traffic lights and pedestrian Lights
  • Control level of a single tank, series tanks or parallel tanks
  • Heat liquid in a tank using a PLC-controlled heater
  • Control mixing in a tank
  • Maintain the pressure head in a bottle filling system
  • Count parts from a conveyor
  • Count and pack parts from a conveyor
  • Control the sequence of interconnected conveyors
  • Operate Drilling and milling machines
  • Clean and rinse bottles in beverage industries
  • Continuous Stirred Tank Reactor
  • Heat and bend glass tubes

Employees who may once have never had to worry about programming computers may find themselves needing to work with PLCs. These practical controllers affect all types of job descriptions:

  • Engineers
  • Maintenance crew chiefs and crews
  • Operations managers
  • Management personnel
  • IT Technicians
  • Instrumentation technicians
  • Automation technicians
  • Multi-craft personnel


Because they are modular, PLCs can be configured in input/output channels from as few as four to as many as hundreds, all working through one simple program. PLCs are built to handle simple coding, not complex, nuanced tasks. Reading inputs from keyboards, switches, sensors or pushbuttons, the PLCs then send outputs to devices like these:

  • Motor starters
  • Relays
  • Solenoids
  • LED displays
  • Indicator lights

Because their programming is so fundamental, workers troubleshooting or working with PLCs need to know how to diagnose programming errors in automated systems:

  • Assess electromagnetic interference (EMI)
  • “Tell” your machine what is wrong with it
  • Use ladder logic programming language
  • Generate reports to aid in troubleshooting
  • Interpret fault messages
  • Inspect for physical issues like corrosive or conductive contaminants in the local environment

Due to their omnipresence in commercial and industrial applications, PLCs are vital parts of commerce. Any downtime created by having to diagnose and correct a faulty PLC is an enormous expense, one that most companies do not take lightly.

Employees must have the skills to correctly diagnose, reprogram and place online any problematic PLC in as fast a turnaround time as possible. The ability to fix operating errors makes an employee extremely valuable to the organization.

Enrolling in NTT’s seminar on PLCs will provide you with several days of hands-on and classroom learning, so you can understand the advantages and disadvantages of using PLCs, recognize various hardware components of the PLC, and understand how ladder logic software and communication software interact to allow data transfer between the PLC and a programming computer. Contact NTT today to learn more about the learning opportunities afforded by this informative seminar.

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