THREE COINS IN THE P.I. FOUNTAIN

[Aziz]

Yes, nice plots. Thanks for sharing.

Don't forget, that the first coin is shielding the magnetic field lines so the successive stacked coins won't see much magnetic field strength. See also the Skineffect.

Cheers,
Aziz

[Ferric Toes]

Yes, nice plots. Thanks for sharing.

Don't forget, that the first coin is shielding the magnetic field lines so the successive stacked coins won't see much magnetic field strength. See also the Skineffect.

Cheers,
Aziz

As the first coin shows just a single exponential (straight line on a log lin plot) any individual skin effect must take place prior to 20uS, otherwise there would be an upward deviation at early times. When second coin is in place the first coin acts as a coupled LR damping network such that the magnetic field is that radiated by the first coin's eddy current. This results in a slower switch off and less initial emf in the second coin. This situation is repeated by the slowing of switch off by the second coin in energising the third. Seems as though the coins act as low pass filter sections for the magnetic field step. It's just like skin effect in that each coin is a skin for the next one. At least, that is how it appears to me.

Eric.

[Qiaozhi]

Nice plots . I expect the results will be rather different for other coins, particularly coins that are made of a more conductive metal e.g. silver or copper. Each coin in turn is attenuating the high frequency components of the TX waveform, giving less excitation to the next coin. Lower frequencies still get through to give the enhancement at late times. Star = 1 coin, Circle = 2 coin, Triangle = 3 coin.

Here is a further plot for my coins where you see the initial amplitude reduced for the next coin in line, but the enhancement at late times. I am hoping that some brave person will try a Spice simulation - if it is possible.

Eric.

It looks to me like the plots show an interplay between permeability (absorption) and eddy current effects (redistribution). With the single coin, the cross-sectional area is relatively large when compared to its mass. As more coins are added, the overall target mass increases relative to the cross-sectional area, resulting in an amplitude decrease below 40us and an increase at later times. I would expect this effect to become increasingly prominent as the spacing between the coins in reduced.

[mikebg]

A blast from the past
http://www.youtube.com/watch?v=8YxRNdgY5vg
http://en.wikipedia.org/wiki/Three_C...ain_%28song%29

[Ferric Toes]

A blast from the past
http://www.youtube.com/watch?v=8YxRNdgY5vg
http://en.wikipedia.org/wiki/Three_C...ain_%28song%29

How about a frequency domain rendition ?

Eric.

[mikebg]

How about a frequency domain rendition ?

Eric.

The frequency domain shows that participants of this thread use a term "skin effect" which is not attributable to time domain. There is no skin effect in TD. The suitable term for TD is "relative time" normalized to fundamental timeconstant of target.

The frequency domain shows that your decay curves depend on width of TX pulses and their repetition frequency.
If you increase the TX pulse width, you can obtain the same decay curve for three coins as for a single coin.

The frequency domain shows that each coin has series of several timeconstants inverse proportional to squares of natural odd numbers: TC₂ = TC₁/9 ; TC₃= TC₁/25 ; TC₄ = TC₁/49 etc.

Note that there is no emoticons to this post. All is serious.

[Carl-NC]

Eric, this is classic skin effect. The thickness of the metal determines the depth of penetration of the eddies, and the deeper they penetrate, the more "drag" there is in the decay. You can really see this with aluminum foil; I have a bunch of pieces cut to 1" square, with stacks of 1x, 2x, 4x, 8x, 16x, 24x, and 32x, each stack tightly bound in plastic. The tau (TD) and the phase (FD) get higher for thicker stacks. It's easy to use these stacks to emulate just about any coin response.

A very very thin gap between each layer (relative to the layer thickness, anyway) makes little difference as there are no vertical currents, assuming the target is centered in the coil field. You can test this by using a thin poly food wrap as an insulator. As the gap is widened, the tau increase drops because the magnetic field weakens across the gap. I've never thought much about it, but the boundary conditions across a gap may also give rise to effects which alter the exponential. Probably not.

The decrease in amplitude with thicker metal is interesting... did you increase the pulse width as you stacked the coins? I would expect thicker metal to suffer more from residual reverse turn-off current.

Mike, of course there is skin effect in the TD! Try the experiment, and you can see it for yourself. No, increasing the pulse width will not produce the same decay curves, and pulse frequency is largely irrelevant. The shift in tau is real, and due to skin effect.

- Carl

[green]

I didn't get a difference in amplitude at 10 usec when stacking the coins the other day. I tried again today. I measured 1 coin and 1 coin stacked on a wood spacer = to 2 coins to see if position mattered. Plotted the same. I tested 1,2,and three nickels again with different charge times, 54usec and 145usec. Very little change in TC.

[Ferric Toes]

I didn't get a difference in amplitude at 10 usec when stacking the coins the other day. I tried again today. I measured 1 coin and 1 coin stacked on a wood spacer = to 2 coins to see if position mattered. Plotted the same. I tested 1,2,and three nickels again with different charge times, 54usec and 145usec. Very little change in TC.

I'll give it a try with nickels starting at 10uS and see what results I get.

Eric.

[Dave J.]

Carl got it right from the very start, working with frequency domain discriminators teaches a person these things.

Anyone who might wish to disagree with Carl and myself on this, don't bother arguing with us, I recommend first collect your own data and argue with that.

--Dave J.

[Davor]

I think these experiments speak volumes about target identification. Similar experiment cleared the ferrous materials orientation dependence. With targets buried in ground you can't get clear idea why they appear like this or like that, because in a process of digging you disturb the original orientation and there is no way of learning much.

Shovel is the ultimate discriminator...

[Ferric Toes]

Here are the plots for the 3 Nickels test. As each coin is added it is insulated from the previous one by double sided sellotape. This prevents relative movement of the coins while the measurements are taken. To answer Carl's question about the TX pulse, all my tests are taken with proportional pulsing. This means that the TX width increases proportionally to the delay time and sample width. i.e. 40uS TX for 10uS delay and sample pulse width - 80uS TX for 20uS delay and 20uS sample width - and so on such that at 100uS delay we have 400uS TX and 100uS sample. Constant current TX circuit keeps current the same for all pulse widths. With the Nickels I still see the drop in initial amplitude as each coin is added. This shows up more on the linear plot than it does on the log linear as the top end is obviously more compressed on the log. Actual data is in the data table.

The question I am wondering about is, if I had a piece of identical solid metal equal to the thickness of three coins and the same diameter, would I get the same decay curve as that of three insulated coins?

Eric.

[green]

(Carl got it right from the very start, working with frequency domain discriminators teaches a person these things.

Anyone who might wish to disagree with Carl and myself on this, don't bother arguing with us, I recommend first collect your own data and argue with that.)

I posted my plots since they were different from Erics. Why I don't know. Maybe neither test relates to measuring the coins at one and two times the coil radius. I got very little difference in amplitude at 10usec when adding the coins. At 40 usec adding nickels increased the amplitude. Adding quarters decreased the amplitude. We have a sign at work (One test is worth a thousand expert opinions). Only true if the test is valid and relates to what you want know. Measuring the temperature of a cup of coffee with a large metal dial thermometer gives the right answer but not what the temperature was before measuring it. My test is simple. Anyone with a DSO could do the test. If it doesn't relate to PI metal detecting I would be better off not doing them. I've worked in measurement for many years and for me it's fun. I'm not trying to disagree with anyone. When it comes to metal detectors I don't know enough to agree or disagree. Just wanting to know what testing methods might be valid.

[Ferric Toes]

(Carl got it right from the very start, working with frequency domain discriminators teaches a person these things.

Anyone who might wish to disagree with Carl and myself on this, don't bother arguing with us, I recommend first collect your own data and argue with that.)

I posted my plots since they were different from Erics. Why I don't know. Maybe neither test relates to measuring the coins at one and two times the coil radius. I got very little difference in amplitude at 10usec when adding the coins. At 40 usec adding nickels increased the amplitude. Adding quarters decreased the amplitude. We have a sign at work (One test is worth a thousand expert opinions). Only true if the test is valid and relates to what you want know. Measuring the temperature of a cup of coffee with a large metal dial thermometer gives the right answer but not what the temperature was before measuring it. My test is simple. Anyone with a DSO could do the test. If it doesn't relate to PI metal detecting I would be better off not doing them. I've worked in measurement for many years and for me it's fun. I'm not trying to disagree with anyone. When it comes to metal detectors I don't know enough to agree or disagree. Just wanting to know what testing methods might be valid.

We are all learning, whether we have been in the game for a few years or many. There are many variables which can alter results, but experimenting can be extremely valuable and as you say, Green, it is fun. By experimenting, I have come across things which work, that no amount of time doing calculations would have done. When it works, then see how the theory fits.

I have just done another plot for fun. Using our plentiful 10p coins, which are a similar material to a nickel, I set my instrument to 80uS delay where 1 coin does not quite give a reading. I then stacked up one at a time 15 coins and noted the reading increase. This is still my 30mm diameter solenoid coil, so the coins are getting further away as they stack up. However, the amplitude increase at 80uS shows the lengthening decay time. X axis is number of coins.

Eric.

[green]

Eric
Your plots with nickels look closer to being the same at the start. I averaged my readings from 10usecto 12usec to see the difference. (54usec pulse) 1 coin .0378v,
2 coins .0381v, 3 coins .0403v. (145 usec pulse) 1 coin .0625v, 2 coins .0705v, 3 coins .0691v. Mine were lower at the start. Thanks for all the testing you have been doing with targets and ground response.