SIMULATIONS

[mikebg]

CROWN CORK

Attached image shows why cutoff frequency or timeconstant depends on orientation. Eddy diameter D changes significant. Parameters depend on square of eddy diameter.

[Ferric Toes]

Attached image shows why cutoff frequency or timeconstant depends on orientation. Eddy diameter D changes significant. Parameters depend on square of eddy diameter.

For a non-ferrous bottle cap this is true. D2 will have a faster decay than D1. For a ferrous bottle cap the vertical orientation gives a slower decay than D1 as the magnetic response dominates the D2 eddies.

Eric.

[Davor]

I'd say that magnetisation effects would be a better explanation for this. In VLF you'll mostly see Fe targets in 1st quadrant, and coloured metal are always in the second (or more accurately the 4th). There are two competing effects there, one that rotates a phase counter-clockwise (ferrous) and the other clock-wise (conductivity). There may be a certain frequency where this phase shifts are in equilibrium, and your Fe target will respond as a lidless jar of brine. In fact that may be a wide range of frequencies.

While conductivity-caused eddy currents taus are fairly constant over various material geometry, magnetisation is not. Therefore, steel can covers will always produce funny sounds.

[Davor]

a bit later but in the same line of thinking

[Tinkerer]

a bit later but in the same line of thinking

Maybe we should look at the different magnetic properties of different steels.

The steel used to make the cores of 50/60Hz transformers, contains a certain amount of silicon, to make it "softer" magnetically speaking. I understand this as a steel alloy that has "lower magnetic viscosity".

A "high magnetic viscosity" steel, will have a long decay trail. For magnetically unimportant objects, like a steel bottle top, maybe we might find a variety of different steels in different parts of the world.

So, how can we measure the "magnetic viscosity" of a bottle top?
How long does it take for the magnetization of the bottle top to decay?
Does the collapse of the bottle top's magnetic field induce a current in it's matrix?

If indeed the collapse of the magnetic field induces a current, does this current oppose the eddy currents or aid them?

Magnetic permeability concentrates or "sucks up" the surrounding magnetic field lines. This distorts the magnetic field of the coil and therefore produces a change in the RX.

What is the correlation between the permeability and the viscosity?

Forgive me for harping on bottle tops. I think they serve as fairly universal examples of the kind of steel targets we want to discriminate.

Tinkerer

[ODM]

In a bit of conversation with a friend who designs magnetics for a company, he advised to simulate the magnetizing inductance separately from the coupling inductance. The coil's own magnetizing inductance would thus be an uncoupled inductor, and parallel to it would be the coupling inductor(s) - large enough not to affect the magnetizing inductance for practical accuracy.

http://www.edacafe.com/books/PSpice/san63267_ch02.pdf
So, not series but parallel inductance. I'm a bit rusty

[Ferric Toes]

I didn't have a steel bottle cap handy so I used the next best thing which is a modern UK 2p copper plated steel coin. I compared this with an older 2p which is a copper alloy. Both are the same diameter, thickness and weight. The plots nicely show the edge on eddy decay of the non-ferrous coin which, as you would expect, is fast compared to the horizontal plane.

The ferrous coin shows a faster decay horizontally because steel is less conductive, but the initial amplitude is higher (preamp saturation period is longer). Vertical orientation shows a very much greater signal both in saturation and decay time.

All measurements taken with the coin in a uniform field inside a small solenoid coil.

Off to the beer store now with a magnet to find a bottle with a steel cap.

Eric.

[Qiaozhi]

The ferrous coin shows a faster decay horizontally because steel is less conductive, but the initial amplitude is higher (preamp saturation period is longer). Vertical orientation shows a very much greater signal both in saturation and decay time.

Have you any idea why the ferrous coin should give a greater signal when vertical, than when it was horizontal? Although the percentage of steel would probably dominate the thin copper layer when horizontal, and the copper may dominate in the vertical, I don't understand why the horizontal position does not provide the larger signal due to the overall greater amount of material presented to the coil.

[Davor]

You may imagine the vertically positioned one as a copper solenoid with an iron core.

[ppodg]

Have you any idea why the ferrous coin should give a greater signal when vertical, than when it was horizontal? Although the percentage of steel would probably dominate the thin copper layer when horizontal, and the copper may dominate in the vertical, I don't understand why the horizontal position does not provide the larger signal due to the overall greater amount of material presented to the coil.

vertical iron coin = large magnetic viscosity component + very weak eddy currents

[Ferric Toes]

I don't think there is any contribution from the copper plating, it is just too thin. A magnetic material magnetizes best when its long dimension is parallel to the flux. Put a thin 25mm nail in the solenoid coil and it will give a large signal when oriented parallel but virtually no signal when across the flux. There is obviously some induction but the decay is too fast to see, unless you run at <5uS delay. Metallic iron decay is different than non conductive ironstone. I will see if I have some log/log plots which show the difference more clearly. I get the same result as a 2p coin by using a plain steel washer of the same size. Somewhere I have some very thin steel shim stock which should show little R response in horizontal but large X when vertical.

Eric.

[Tinkerer]

I don't think there is any contribution from the copper plating, it is just too thin. A magnetic material magnetizes best when its long dimension is parallel to the flux. Put a thin 25mm nail in the solenoid coil and it will give a large signal when oriented parallel but virtually no signal when across the flux. There is obviously some induction but the decay is too fast to see, unless you run at <5uS delay. Metallic iron decay is different than non conductive ironstone. I will see if I have some log/log plots which show the difference more clearly. I get the same result as a 2p coin by using a plain steel washer of the same size. Somewhere I have some very thin steel shim stock which should show little R response in horizontal but large X when vertical.

Eric.

Could we compare the non conductive ironstone with a ferrite?
Could we find a ferrite that has the right hardness to be similar enough?

Tinkerer

[Aziz]

Skin effect dominates on copper plated coin with iron core. And the magnetisation of the iron core is there too.
Do you all see the difficulty in implementing a working discrimination, when the orientation or the ground mineralization screws your theoretical models?
Aziz

[Qiaozhi]

I don't think there is any contribution from the copper plating, it is just too thin. A magnetic material magnetizes best when its long dimension is parallel to the flux. Put a thin 25mm nail in the solenoid coil and it will give a large signal when oriented parallel but virtually no signal when across the flux. There is obviously some induction but the decay is too fast to see, unless you run at <5uS delay. Metallic iron decay is different than non conductive ironstone. I will see if I have some log/log plots which show the difference more clearly. I get the same result as a 2p coin by using a plain steel washer of the same size. Somewhere I have some very thin steel shim stock which should show little R response in horizontal but large X when vertical.

Eric.

OK, that makes sense, as I've just realised that you're putting the target inside a solenoid coil. This is not the same situation as you would get using a typical detector coil with a metal target below it. In the latter case the vertical flux lines would tend to cancel due to superposition, resulting in a stronger signal for the horizontal target.

[Aziz]

Who has real-time "FFT-eyes" on the time-domain decay curves of Eric's latest coin response scope pics? *LOL*
It tells you a lot of about the holly grail of the metal detecting.
Aziz