Tip of the Day 168: Of Pinched Piezo Cables Signal Drift

At a recent customer visit we were testing the tool on the bench. We were pushing on pins while watching values on the Cycle Values screen and noticed one changing when we were not touching it. The value was continuously going down.

The cause? A pinched piezo wire.

Moral of the story: Be very careful of piezo cables. Do not pinch, crush or kink them

                                                   (or fold, staple or mutilate).

Any damage to a piezo cable can cause drift in the signal. It also can cause other weird pressure data behavior like plateaus on the graph or other behaviors not natural to plastic.

Some Technical Discussion

If you recall from tip #165 any dirt or water that can provide a path between the signal wire and the shield can cause drift. This is due to the natural behavior of the piezo amplifier circuit injecting electrons across the dirt.

In the case of a pinched wire the electrons can get in from outside. Any hole punctured in the shield provides a path. A very small hole or pinch may leave a tiny hole with a large resistance. Furthermore the insulation resistance of a crushed cable will decrease because the insulating material is squashed thinner.

But the resistance with the hole is not as large as a sealed cable. Thus any pinched point, especially while it remains pinched, can create another path from ground into the inside of the cable. Hence the drift.

Note: Our specification for drift is less than 0.2% in 1 minute. Usually the Lynx amplifiers and cables are much better than that.