SIMULATIONS

[Qiaozhi]

Not quite sure what you mean here. The coil I am using is a vertical shielded solenoid of such a length that when I put a 10ml sample holder inside on the platform, it is in a uniform vertical field. This is the situation for soil and rock measurements and OK for a vertical coin, where in the case of the plated 2p, I get the magnetic response. With the coin horizontal as in the picture, the flux/coin situation should equate to the object being under the centre of a standard detector coil.

Eric.

This is very very interesting, so I decided to do a simple experiment.

Using two British 2p coins, one from 2002 (copper plated steel) and one from 1971 (bronze), with a PI detector and a mono coil, I passed each coin slowly across the face of the coil. As expected, the bronze coin gave a stronger signal in the horizontal position than the vertical. However (as you have correctly noted) the plated coin gave a stronger signal in the vertical position. I have to admit that I've never noticed this before.

Next I took a VLF detector with a concentric coil in All-Metal mode, and performed the same test. The bronze coin behaved exactly as expected, but the plated coin (as afar as I could determine) gave the same response for both horizontal and vertical positions. In fact, the orientation of the coin appeared to be irrelevant. Curious.

[Qiaozhi]

Addition:
I then decided to make the same tests using the VLF detector in DISC mode with only iron being rejected. The bronze coin was accepted in both orientations, but was again detected more strongly in the horizontal position, whereas the plated coin was accepted in the horizontal position but rejected in the vertical position (with some chatter). Being primarily a VLF detector user, I guess that's why I expected the plated coin to react in the same way as the bronze coin.

[WM6]

Using two British 2p coins, one from 2002 (copper plated steel) and one from 1971 (bronze), with a PI detector and a mono coil, I passed each coin slowly across the face of the coil. As expected, the bronze coin gave a stronger signal in the horizontal position than the vertical. However (as you have correctly noted) the plated coin gave a stronger signal in the vertical position. I have to admit that I've never noticed this before.

Next I took a VLF detector with a concentric coil in All-Metal mode, and performed the same test. The bronze coin behaved exactly as expected, but the plated coin (as afar as I could determine) gave the same response for both horizontal and vertical positions. In fact, the orientation of the coin appeared to irrelevant. Curious.

Thanks for interesting experiment Qiaozhi.
What is explanation in case of plated Ferro coin in VLF field? Changing EM field that cause impact both component (electric and magnetic) of signal?

[Qiaozhi]

Thanks for interesting experiment Qiaozhi.
What is explanation in case of plated Ferro coin in VLF field? Changing EM field that cause impact both component (electric and magnetic) of signal?

I'm not sure, but perhaps Eric knows the answer.

Just to recap (I ran the experiment again to double-check) the bronze coin was accepted in both the horizontal and vertical positions. However, the vertical position gives a double beep as it crosses the coil but is clearly not being rejected. The horizontal orientation is much stronger. The plated coin is also accepted in the horizontal position, but is definitely being rejected in the vertical position with some chatter. Perhaps the 7% copper plating has a more pronounced effect for VLF, but the steel core dominates in vertical mode.

[Davor]

The coil coupling coefficient TX to RX1 determines the amplitude of the reactive response.

This is downright brilliant
You actually found the easiest way to simulate ferrous behaviour that works equally well for IB and PI. The only unfortunate quirk is that coupling in Spice can't be convinced to play nice over a large frequency span to mimic real life ferrous materials, but knowing this makes it less of a problem. You can always pick a value in a middle of a frequency range of interest.

Thinking about this approach makes it kind of logical. A ferrite antenna kinda squeezes space around it to appear as a much larger loop than it physically is. Reactive component of the ferrous materials does basically the same thing. It actually increases inductance of the nearby coil, and in a process it rotates the vectors ferrous-wise. You may also observe this as a target that possesses some interesting properties negative-inductance-wise which is more of an anomaly than a physical reality. Your approach sorts that out, once and for good.

I'm impressed

[Aziz]

Hi all,

that's a good exercise to see, that a reliable discrimination isn't possible. *LOL*


Just focus to separate X and R from signal S. The relation of R to X gives a good indication about the discrimination but isn't reliable however.

Forget it to simulate the complex magnetic component (frequency, material, shape, orientation dependent).

Cheers,
Aziz