Nut Detection Challenge Pt. 1

Nut Detection Challenge Pt. 1

, by Jim Ryan, 1 min reading time

Sensing a nut with an inductive prox is a challenge when the nut is fastened or clamped behind a metal sheet.  How can I sense the nut and not sense the sheet metal, since the prox sees the metal sheet first and then doesn't care about the nut? 

And to make matters worse, the thicker the sheet, and the farther away the nut is - the greater the sensing challenge becomes! Take this example:  1/2" nut through a 9/16" hole in 1/4" steel plate.


Under-substrate detection is a non-intrusive sensing method, where the prox is positioned facing the hole in the metal sheet and "looks" though the substrate  to detect the nut on the other side. In this case we need to used a 18mm programmable inductive, and establish 2 sensing widows.

Looks simple enough, but we have some parts and placement requirements necessary to make this work:

  1. Nut ID must be smaller than the hole. In this case 1/16" gives enough interactivity with the sensor to make a positive detection.
  2. The sensor needs to touch, or nearly touch the 1/4" steel plate (substrate) with high repeat accuracy. This is the Z axis, which can be addressed with fixturing and various types of mechanical mounting devices and techniques.  This sensor is a one-piece solid stainless face/body that can take a massive beating and still operate - we have excellent results in the field in parts riding on the face, but if a small gap ca be acheived, with repeatability, this is even better.
  3. Consistent part-to-sensor placement in the X-Y means basically centering the sensor face to the substrate hole
  4. Repeatable parts - sometimes that's an issue.


If the application cannot assure these requirements, then you might still be in luck with another sensing method using our 5mm probe.

That's what we'll look at in Part 2.

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