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Six key differences between PLCs and computers

On the surface, programmable logic controllers (PLCs) and computers are similar, but PLCs can perform discrete and continuous control functions that a computer can't. See video.

Frank Lamb, Automation Primer
02/06/2017

On the surface, programmable logic controllers (PLCs) and computers are similar, but PLCs can perform discrete and continuous control functions that a computer can't. Courtesy: Automation PrimerWhat makes a programmable logic controller (PLC) different from a typical computer? After all, computers are used to control things; they can even run a software PLC.

So what exactly is a PLC? A PLC is a digital computer used to control electromechanical processes, usually in an industrial environment. It performs both discrete and continuous control functions and differs from a typical computer in several important ways:

  1. It has physical input/outputs (I/Os); electrical inputs and outputs bring real world information into the system and control real world devices based on that information. A PLC's, inputs can be thought of as "senses" like vision and touch; outputs can be thought of as arms and legs.
  2. It is deterministic and processes information and reacts to it within defined time limits. PLCs operate on a timescale of milliseconds or even microseconds.
  3. It is often modular; it can have I/O modules, communication modules or other special purpose modules added to it for expansion. PLCs may also take the form of a computer or a small single module.
  4. It is programmed using several defined languages. Some languages allow the program to be changed while the machine or system being controlled is still running.
  5. Software and hardware are platform specific; components and programming software usually can't be used between different manufacturers. There are exceptions however.
  6. It is rugged and designed for use in industrial environments.

Also, unlike computers, PLCs are made to run 24/7 and are designed to resist harsh physical and electrical environments.

Where are PLCs used? PLCs are used in many different kinds of applications and industries. In a 2012 Control Engineering magazine poll, 87% of machine control applications used a PLC as the control platform. This includes assembly, packaging and other manufacturing operations. 58% of process control applications used PLCs, in such industries as chemical processing and the oil and gas industry. Power plants and wastewater treatment also fall into this category. 40% of motion control and robotics, 26% of batch control and 18% of diagnostic or testing applications used PLCs.

Many applications are a combination of these, which just goes to show that PLCs are used anywhere and everywhere.

Frank Lamb is the founder of Automation Consulting Services Inc. This article originally appeared on the Automation Primer blog. Automation Primer is a CFE Media content partner. Edited by Chris Vavra, production editor, Control Engineering, CFE Media, cvavra(at)cfemedia.com.

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