What can go wrong with programmable logic controllers?

Posted on 11 Feb 2015 by Jonny Williamson

Jonathan Wilkins, of industrial automation components suppler European Automation, discusses the top five problems associated with programmable logic controllers (PLCs).

Jonathan Wilkins, European Automation
Jonathan Wilkins, European Automation.

Programmable Logic Controllers have become a stalwart of the automation industry and can be found in countless automated manufacturing processes across the world.

An iconic and integral component, PLCs have a host of responsibilities, functionalities and capabilities wherever there is a need to control devices, such as pneumatic machines, robots, hydraulic machines and packaging lines.

By performing sequences of instructions such as timing, counting, storing memory, relaying logic and arithmetic computation, the PLC’s duties are crucial and imperative for the complex processes needed.

Such heavy duty, essential machine-kit ought to be problem and troubleshooting free. So, aside from the traditional and universal signs of something going wrong, such as programme bugs and wiring errors, what can possibly go wrong?

1) I nput/Output modules

Everyone generally assumes that when something goes wrong with a PLC, it is due to internal processor problems. This would be wrong.

There is a propensity amongst those unfamiliar with the PLC to fear the mysterious ‘black box’ when troubleshooting. This irrationality can be soothed though as PLCs are in fact easier to troubleshoot than old-school hard-wired control systems, with more open and easy diagnosis due to the black box.

A big percentage of problems are the result of (I/O) modules or field equipment. It’s not difficult to diagnose whether the problem is emanating from the I/O system or in the processor, both types of problems have unique signatures allowing an easy examination, and therefore conclusion.

2) In or out?

If the problem is traced back to a specific I/O module, this means that the cause is usually an external one, like the aforementioned wiring errors. If it is an internal problem, this could result in erratic behaviour, large groups of failures, or even total failure of the PLC system.

One of the first things to do now is assess the integrity of the PLC, i.e. the integrity of the ground has to be electronically checked.

The power and ground wiring need to be further examined to ascertain whether it’s loose, corroded or has damaged connections. The power supply should also be checked using a digital meter, with both AC/DC voltages reading zero.

3) Interference

Another problem can be the effects of electromagnetic interference (EMI) or radio frequency interference (RFI). These can be related to lightning strikes, welding in the area or handheld radio transmitters. Handheld radios, often used by maintenance staff, emit powerful radio frequency radiation, disrupting and interfering with any unprotected electronic equipment.

Improvements in shielding, grounding and power conditioning can help combat any EMI or RFI problems.

PLCs perform sequences of instructions such as timing, counting, storing memory, relaying logic and arithmetic computation
PLCs perform sequences of instructions such as timing, counting, storing memory, relaying logic and arithmetic computation.

4) Corrupted memory

Frequency interference, power and grounding are all problems that can disrupt and corrupt the PLC’s memory, so it is crucial to verify the program is still correct and comparable with a backup copy on tape, disk or in the cloud.

As with all data backups, ensure they are up-to-date and kept away from any sources of EMI and RFI, along with high temperatures and humidity.

5) Confusion

When PLC troubleshooting, a major aim is to find out why the internal status of the PLC (what the PLC thinks is happening) is in conflict and not in agreement with the external situation (what is actually happening).

Our job is to determine the status of the relationship between physical I/O modules and I/O instructions in the PLC program. Different manufacturers have different solutions and schemes, usually a terminal, handheld unit or computer. Any of which can be used to check the internal status of the input/output in question.

PLCs have both advantageous and disadvantageous. Yes, there is and always will be a lot of work required in connecting wires, a difficulty with changes and replacements, and potentially long delays when something does go wrong.

And, when something does go wrong, the best advice is to choose a reliable partner, such as European Automation, that can source and deliver the correct part quickly and efficiently.

But there’s something nostalgically pleasing and reassuring about the familiarity of the PLC.

In today’s high technology world, sometimes it’s advisable to choose equipment or brands that are easily managed, cost effective, flexible, reliable and have troubleshooting aids.