JLAK’s Law of Troubleshooting

Simply stated, “When a complex system is being troubleshot, an expert in a subsystem always suspects the subsystem he knows least about”.

I arrived at this ‘law’ after repeated observation of this law at play when CNC machine tools were being tested, tuned and qualified. This being a control system with multiple control loops, the effect of one fault would manifest itself in all the variables – jerky motion of the axes, unsteady speed reference to the drive controller, unsteady tacho voltage, unsteady encoder feedback, etc.

This is how the scenario would look:

All the experts have been called because, during the production testing the movement of the axis is found to be uneven and the normal fault tree and troubleshooting chart have not helped.

CNC expert: Have you checked if there is stick-slip? (Being a digital electronics guy he knows a little about the drive electrics. So, he does not question that.)

Drives expert: Are you sure there is no backlash? (Notice that the two suspected causes sound pretty impressive and have a ‘certain something’ (as Asterix would say) about their sound.

The Mechanical expert: Have you tuned the drive?

It would go on and on. Some ‘poor generalist’, who is a down to earth guy, is pottering away at finding the root cause of the problem. Shielding that has come off, a dry solder or whatever would be found and corrected and the system would start behaving itself.

Then all the experts would leave shaking their heads and clucking their tongues muttering “these production guys never do anything right”!

Even in the trivial case of a simple motor controller troubleshooting, the test engineer soldiers on trying to fix the problem all on his own, until he sees a specialist in motors. He immediately starts suspecting all kinds of esoteric problems with the motor. One of the first things the motor man himself would ask is perhaps “is the incoming phase sequence OK?” or “Have you checked the tacho coupling?” – even if the drive is a single phase one or the motor has an integral shaft mounted tacho.-----Contributed by JLAK