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Top 5 problems with programmable logic controllers

Posted: 24 Mar 2015 ?? ?Print Version ?Bookmark and Share

Keywords:programmable logic controller? PLC? manufacturing? I/O modules? electromagnetic interference?

The programmable logic controller, or using the less tongue-tie-inducing acronym--PLC, has become a stalwart of the automation industry and can be found in countless automated manufacturing processes across the world.

PLCs find a home wherever there is a need to control devices, such as pneumatic machines, robots, traffic lights, hydraulic machines and packaging lines. Such heavy duty, essential machine-kit ought to be problem and troubleshooting free, no?

Well despite being a powerhouse of control systems, and widely becoming the device du jour for a wide variety of control tasks, the truth is the PLC is an iconic and integral component in industry with a whole host of responsibilities, functionalities and capabilities. 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. So what possibly can go wrong?

The black box
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.

Putting aside the traditional and universal signs of something going wrong, such as programme bugs and wiring errors, here we will explore the top five things that can go wrong with our beloved industry giant, the PLC.

1) I/O modules
Everyone generally assumes that when something goes wrong with a PLC, it is due to internal processor problems. Wrong!

A big percentage of problems are the result of I/O modules or field equipment. No need to panic either as it is 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 even easier examination, and therefore conclusion.

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

However, one of the first checks to do now is the 'integrity' of the PLC. This doesn't mean questioning whether the PLC is a good, honest citizen with morals and a clean tax record, rather the integrity of the ground has to be electronically checked.

The power and ground wiring also needs to be examined further, to ascertain whether its loose, corroded or has damaged connections. The power supply should be checked; using a digital meter ('re' for unit of measurement) and both AC/DC voltages should be zero. .

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