Targets frequency response

[WM6]

To WM6:
Eddy currents are driven by EMV (electromotive voltages) induced in loops.
Experiments and theory show that relations between cutoff frequencies of eddy loops are 1:9:25:49 etc. (timeconstants are inversely proportional).
The attached figure illustrates what I mean with words:
"Nature does not allow in one path to flow currents in opposite directions".

Hi mikebg.
Your sketch is only simplified illustration and, I think, wrong pointed. At his periphery two eddy currents try each other steal electrons and there is virtually no opposing currents due the absence of available electrons at this point. So sketch have to be like this:



What determines the initiation of a particular eddy current? These are the highest points of instability (e.g. due to impurities) of electrons in the material. Similar to avalanches, which may have triggered even by people voices.

Avalanche of electrons will then choose the shortest path to release their energy. Electrons necessary for the long journey are collected by adjacent eddy currents.

Explanation many eddy current in coin and only one in ring is also simplified. Example what about half broken coin like this: (?)



I do not think that one eddy current is impossible on coin in some circumstances (maybe if we put coin inside inductor as seen on Aziz videos) but not as rule.

[Qiaozhi]

Hi WM6,

I don't think your sketch is correct either. The eddy currents will all be in the same direction, but portions of these circulating currents cancel due to the principle of superposition, and the result is a stronger current density at the surface of the target, which diminishes towards the center. Mike-BG's sketch shows the current density of a cross-section of the target (minus the arrows, of course) not the actual eddy currents.

Another reason, a drilled-out coin (or ring) gives an enhanced response to an ordinary coin, is because the current density around the inner surface is much stronger. In other words, for a thin coin the current density is strong around the periphery and diminishes towards the center, whereas a drilled-out coin is strong at both the outer and inner peripheries.

[Qiaozhi]

To add more fuel to this subject:

An interesting experiment would be to drill a small hole in the center of a coin to see if this enhances its response. Then progressively enlarge the hole (testing at each step) until only a small loop remains. Which test will give the highest response?

[Tinkerer]

Eddy current magnetic field

Maybe it would be better explained by looking at the magnetic field that is generated by the eddy currents.
It is this magnetic field that interacts with the search coil.

In the case of a ring, the target's field looks somewhat similar to the field of the search coil.

Magnetically permeable targets have a much stronger field because they concentrate the field lines. A nail is a perfect dipole.

Google for visualizations of magnetic fields of different shaped objects.

Tinkerer

[WM6]

Another reason, a drilled-out coin (or ring) gives an enhanced response to an ordinary coin, is because the current density around the inner surface is much stronger. In other words, for a thin coin the current density is strong around the periphery and diminishes towards the center, whereas a drilled-out coin is strong at both the outer and inner peripheries.

Hi Qiaozhi

according my experience this shape of wire or ring (with unclosed circle) give us even better response than real ring (so where is here periphery circle current?):

[Qiaozhi]

Hi Qiaozhi

according my experience this shape of wire or ring (with unclosed circle) give us even better response than real ring (so where is here periphery circle current?):

This is another experiment to try.

Personally I would expect the ring to give a better response. Unless someone else gets there first, I will do these experiments later this week.

[WM6]

I regularly detect spring washers at unbelievable depths. But yes, rings deeper than coins.

[Carl-NC]

When I drew that picture of the solid coin with the multiple eddy paths, I knew even then that it wasn't technically correct, but I decided to use it because it was an easy way to get across that ring objects create stronger eddy responses than solid objects. Yes, you would use superposition such that directly opposing currents cancel -- there's nothing wrong with drawing currents this way -- and the result would be overall like Mike drew.

In reality, eddy currents in a solid coin don't quite behave even like the multiple small circles, they just flow around as Mike drew. So why does a solid coin have a weaker overall eddy response? Imagine the solid coin made up of concentric conductive rings of wire, each ring insulated from its neighbor (as with magnet wire, ferinstance). The incident magnetic field induces a current in each ring of wire, but each ring of wire also creates a counter-magnetic field. The counter-magnetic field of each wire ring then interacts with neighboring wire rings to reduce their eddy currents. In essence, neighboring wire rings create an eddy "drag" on each other, from the outer ring all the way to the center.

If all you have is the outer ring, then there is no induced eddy drag because the counter-magnetic field sees only air. But does one ring of wire give the strongest eddy response? Or two rings? Probably there is an optimal ring thickness; any thinner and the reduction in primary eddy response dominates; any thicker and additional primary eddy response is overwhelmed by the eddy "drag".

If you follow any of what I am saying...

[Qiaozhi]

When I drew that picture of the solid coin with the multiple eddy paths, I knew even then that it wasn't technically correct, but I decided to use it because it was an easy way to get across that ring objects create stronger eddy responses than solid objects. Yes, you would use superposition such that directly opposing currents cancel -- there's nothing wrong with drawing currents this way -- and the result would be overall like Mike drew.

In reality, eddy currents in a solid coin don't quite behave even like the multiple small circles, they just flow around as Mike drew. So why does a solid coin have a weaker overall eddy response? Imagine the solid coin made up of concentric conductive rings of wire, each ring insulated from its neighbor (as with magnet wire, ferinstance). The incident magnetic field induces a current in each ring of wire, but each ring of wire also creates a counter-magnetic field. The counter-magnetic field of each wire ring then interacts with neighboring wire rings to reduce their eddy currents. In essence, neighboring wire rings create an eddy "drag" on each other, from the outer ring all the way to the center.

Yes, it's difficult to explain. In essence we are all agreeing with each other, although I think Mike's diagram is more representative of the current density in a cross-section of the target. At least it would look more like that if the rings were different colors. I suppose this is similar to using either a distributed or lumped model, and the final result is an approximation.

If all you have is the outer ring, then there is no induced eddy drag because the counter-magnetic field sees only air. But does one ring of wire give the strongest eddy response? Or two rings? Probably there is an optimal ring thickness; any thinner and the reduction in primary eddy response dominates; any thicker and additional primary eddy response is overwhelmed by the eddy "drag".

That's why I plan to to do the experiment of drilling out the coin. My expectation is the same as your's.

[WM6]

That's why I plan to to do the experiment of drilling out the coin.

You need to use a gold coin, otherwise results will not be valid.

[ivconic]

That is simple relation between areas covered by minute alternative fields generated in detected item. My draw is simplified but hopefully enough good to explain the idea.

[Esteban]

When the coin is not in "typical position", the coin seems is an only width wire. In discriminative mode detector tend to show the item as trash.

[ivconic]

That is simple relation between areas covered by minute alternative fields generated in detected item. My draw is simplified but hopefully enough good to explain the idea.

Or...even more crazy explanation:

Ring can act as loop, at certain moment "resonant" with some of TX signal harmonics (tough to beleive, one turn loop is more likely to be resonant with frequencies in MHZ range).
Spring washer (unclosed circle) - even better!

[mikebg]

Step down response of a coin


Differential equation for analysis.

This differential equation is solved by the method of Fourier by splitting the variables r and t.

[Carl-NC]

See if you like this pic any better...