Air Gap in toroid for metal permeability detection

[Jeswin]

Hi

I am trying to extract the permeability of certain metal objects (coins) and my theory was that if the objects were passed through the air gap of a toroid inductor then the reluctance of the air gap would change (since reluctance is dependant on relative permeablity).

So there are two reluctances in series in the magnetic circuit : the reluctance of the windings and the air gap. And when the metal cuts through the air gap then the TOTAL reluctance of the toroid will change.

But I wasnt seeing any change in inductance at all (assuming the voltage across the coil will increase or decrease) when I passed magnetic metals through the air gap.


The gap is 3mm wide and the toroid - made of steel - has 27mm external diameter and 17mm internal diameter.
Could somebody verify my theory. I might be heading in the wrong direction.
Thanks

 

[MrAl]

Hi,

Most coins are not magnetic right?
Many metal detectors use a different principle of operation, i cant remember offhand what it is now, i think it is based on eddy currents and at least two coils for non magnetic object detection. Should be found with a little web searching though. Some techniques will not work on non magnetic materials however so you'll want to avoid those.
You also did not mention what you are driving your coil with, the frequency in particular.

 

[Jeswin]

Hi,

The British coins 1p and 2p are magnetic. The frequency I was running at was ranging from 10Khz to 200Khz. But ofcourse I did try lower frequencies to see if the results improved.

Yes you are right metal detectors use coils to measure conductivity through eddy current response. But I am not sure how I can measure permeabillity of the 'metal' through this technique.

 

[MrAl]

Hi again,


Oh ok, well then that should make it simpler (the magnetically active coins).

It could be that the permeability of the core you are using relatively too high. In other words, if the u of the core is 5000 a small change may be hard to detect, but if the core was more like 500 it might be easy to detect. Try using a lower permeability core and see if that helps. If you can you might even try an air core, and if you can have the coin pass through the center of the coil.

Another idea is to have one coil on top, the other on bottom, the top coil is the drive coil and the bottom coil is the sense coil. The coupling between coils should increase when the coin is passed between them.

Another idea is to have the coin pass between a good magnet and a linear hall effect device. The HED would detect some nominal magnetic field without a coin, but once a coin is introduced it would cause a higher level field to be detected by the HED, which would respond by putting out a larger or smaller voltage level. Two HED devices might be able to detect different diameters as different outputs in the two.
You can try passing the coin side ways with flat faces toward the HED and magnet, and you can try passing the coin up right where the diameter creates a stronger link between magnet and HED device. Different diameters should produce a different field level.