What You Should Know About Programmable Logic Controllers

PLCs are the LOGIC in your plant

Was there life before programmable logic controllers?  It doesn’t seem like it even though they are relative newcomers to industrial electronics.  Read on to find out more about PLCs whether you already use them or are looking to purchase.

These miniature industrial computers are everywhere in shops and plants of all sizes.  I think of them this way:  if your plant is the Starship Enterprise, that makes you Captain James T. Kirk, and the PLC is Mr. Spock… reliable, unemotional and always logical.

You might have a few PLCs in your shop already, or maybe you are considering selecting one, or more likely, several.  If so, here are a few things you should know about PLCs.

Advantages of the PLC

Two HUGE advantages and main reasons you are likely to use PLCs:

  • Ability to change and duplicate operations or processes while also collecting and communicating important information
  • Because PLCs are modular, you can mix and match the types of input and output devices that best suit your particular application

The Basics – What Exactly IS a PLC and Where Did They Come From?

  • Developed in the 1960s for General Motors, first called a Modular Digital Controller (MODICON)
  • PLCs are electronic, not mechanical, which eliminated the bulky and troublesome traditional relay-based machine control systems
  • PLCs are miniature industrial computer control systems that continuously monitor the state of input devices and makes “decisions” based upon the customized program you have installed to control the state of the output devices
  • The first commercially successful PLC was the 184 launched in 1973

How Does A PLC work?

This could be a very long explanation but here is the nutshell version:

  1. Input Scan – detects the state of all input devices connected to that PLC (such as pushbuttons, sensing devices, photoelectric sensors, proximity sensors, encoders, and switches (pressure, level, temperature, vacuum, and float)
  2. Program Scan – operates the users customized program logic (the most common is Ladder Logic also referred to as LL)
  3. Output Scan – Energizes (or de-energizes) all output devices that are connected to that PLC (such as valves, motor starters, solenoids, actuators, horns, stack lights, control relays, counter/totalizer, pumps, printers, fans)
  4. Housekeeping – one of the best parts about PLCs is they communicate with programming terminals and internal diagnostics, and collect important data.

How to Select the Right PLC for Your Application

There are numerous manufacturers of PLCs these days including ABB, Allen Bradley, GE Fanuc, Mitsubishi, Siemens, Yaskawa and many more.  Here are a few things to consider on your quest:

  • Is your system in just one place or spread out over a larger area?
  • Does the PLC have enough memory to run your user program?
  • Will the system be powered by AC or DC voltage?
  • What type of software will be needed to handle your application?
  • Will the PLC be able to handle the number of inputs and outputs?
  • Is network connectivity needed and can it be added to your PLC?
  • How will your staff communicate with the PLC?
  • What kind of training is available from the OEM or sales rep?
  • What kind of warranty covers the PLC and for how long?

What About PLC Repairs?

Another great advantage about PLCs is that they are relatively simple and hardy pieces of equipment.  Typically, if PLCs break down, they are suffering from power fluctuations or surges, overheating,  blown I/O cards, bad I/O channels, bad or loose cables, or (and this is so preventable) bad back up batteries.  Remember to back up your program often to prevent loss.  Do this BEFORE you change those back-up batteries.

PLCs are usually repairable.  If it is no longer under warranty by the OEM, choose a reputable industrial electronic repair shop with experience repairing PLCs.  Look for good customer service, no-fee evaluations, competitive pricing, and a good warranty that covers both parts and labor.