Targets frequency response

[Qiaozhi]

Broken or open rings are notorious for their weak signals.
One interesting test to add to the series, would be a full coin but cut to the center.
Tinkerer

Thanks Tinkerer ... I'll try that.

And maybe a full coin with a cut that does not go all the way to the center ... perhaps only 5mm.

[Tinkerer]

This time I used 4x British 2p coins dated 1971. These coins are solid bronze and were modified as follows:

1. Original coin.
2. 3mm hole in center.
3. 17mm hole in center.
4. 17mm hole in center, but with a 3mm gap in the rim.

Again I used the Fisher for some subjective testing. The result was that #1 and #2 were detected at 24cm, #3 was detected at 25cm, and #4 was detected at 18cm. The results for coins 1, 2 and 3 were not so dramatic, so I then performed a more objective test.

In this case the 4 coins were each placed at a distance of 5cm from the center of a metal detector coil, and the DC voltage at the output of the synchronous demodulator in the GEB channel was measured. Coins #1 and #2 each gave a reading of 23.3mV, #3 gave 24mV, and #4 was 6.7mV.

Perhaps removing more material from the center of coin #3 will improve the response (must try this later). However, by far the greatest difference was coin #4. It appears that breaking the ring causes the target response to fall quite distinctly.

Broken or open rings are notorious for their weak signals.
One interesting test to add to the series, would be a full coin but cut to the center.
Tinkerer

[Qiaozhi]

Test continued

This time I used 4x British 2p coins dated 1971. These coins are solid bronze and were modified as follows:

1. Original coin.
2. 3mm hole in center.
3. 17mm hole in center.
4. 17mm hole in center, but with a 3mm gap in the rim.

Again I used the Fisher for some subjective testing. The result was that #1 and #2 were detected at 24cm, #3 was detected at 25cm, and #4 was detected at 18cm. The results for coins 1, 2 and 3 were not so dramatic, so I then performed a more objective test.

In this case the 4 coins were each placed at a distance of 5cm from the center of a metal detector coil, and the DC voltage at the output of the synchronous demodulator in the GEB channel was measured. Coins #1 and #2 each gave a reading of 23.3mV, #3 gave 24mV, and #4 was 6.7mV.

Perhaps removing more material from the center of coin #3 will improve the response (must try this later). However, by far the greatest difference was coin #4. It appears that breaking the ring causes the target response to fall quite distinctly.

[Carl-NC]

Here is what I originally used to test coin vs ring response. I like the idea of making variable-sized holes to see where the best response is. Another idea is to machine a series of aluminum rings that will snuggly nest inside one another.

[Qiaozhi]

Results of initial tests

Today I searched through my coin collection and found 6 identical British 2p coins from 1994. The first thing that surprised me was how hard they were to drill and/or file. Perhaps this should have been my first clue. In fact, they were so hard to drill that I only used 4 of the coins, as follows:

1. Original (undrilled) coin
2. 5mm hole in center
3. 12mm hole in center
4. 20mm hole in center

Now for the unexpected result ... there was no measurable difference in detection distance between these 4 coins.

I used a Fisher 1266-XB with a 10.5" spider coil on minimum sensitivity, and all 4 coins were detected at 24mm. After this test I had a suspicion about the composition of these coins, so I tried a Garrett GTAx-1000 and found it was unable to clearly ID coin #1.

Therefore I did some research and discovered the following. The 2p coin was initially minted from bronze in 1971, but after 1992 it was minted in copper-plated steel. No wonder it was so hard to drill out the holes!
The diameter of each coin is 25.9mm with a thickness of 1.85mm.

Conclusion - removing the center of these coins has no effect on detection distance.

I will next try to search out some pre-1992 2p coins to repeat the test, or acquire some aluminium discs (for easier drilling).

[Qiaozhi]

The problem is that both the original diagrams and the new ones are correct, depending on your viewpoint.

When the TX electromagnetic field intersects a metallic object there are lots and lots of small eddy currents generated in the target. Because these circulating currents intersect there is inevitably some cancellation that occurs. If you plot the current density in the target the result is a series of concentric rings. This gives the illusion that the current flows in a series of discrete rings (like an onion) around the target. This is not the case. Also, the current density does not decrease, from the surface to the interior, in a linear manner. In fact, this is shown in the graphs that WM6 presented in his last post.

I still think the original diagram is basically correct.

I'll let you know the results of my experiment later this week. Unfortunately, there's too many other things getting in the way at the moment.

[WM6]

Some additional view:

The equation for calculation how eddy current density changes in a semi-infinite conductor is:



Where:

d = Standard Depth of Penetration (mm)
p = 3.14
f = Test Frequency (Hz)
m = Magnetic Permeability (H/mm)
s = Electrical Conductivity (% IACS)
Form of field of penetration:



A little different observation than pictured theory I think.

Good lecture about how to eddy current calculations and experiment place together one can find here:

http://www.cwscholz.net/projects/proj3/

[WM6]

Hi Carl,

I think that such a simplified (adapted to the wishes rather than reality) approach together with the idealized and simplified mikes equation is dropped as soon as we make a small intervention in the structure of visualized object such as this:



Maybe equation can be usable if we observe object inside inductor (like in inductive heater in last Aziz post here) but such demo have only in very general simplification something to do with coin detecting in earth.

In case of coin in soil there are not only two main variables (as in given equation) that are enough to make the virtual proof. What is enough for mathematical idealised needs is not enough for reality. What is enough for doctorate in physics is not enough for working detector.

So I do not see how the pictured adjustment was more convincing, regarding state of the art, than the previous original, except that it is more convenient for some from reality isolated calculations.

[Carl-NC]

See if you like this pic any 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.

[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!

[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.

[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.

[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.