THREE COINS IN THE P.I. FOUNTAIN

[Kev]

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.

Hi Eric,
Should you not get a stronger response?
Are not the three insulated coins acting similar to a laminated transformer core, preserving energy loss i.e. our signal?

Cheers
Kev.

[Carl-NC]

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?

Yes, if the gaps between the 3 coins is negligible.

I think the difference between your plots and green's are due to the proportional TX. For this experiment, I think you want to hold the pulse width constant, and large enough that the thickest target has died out before turn-off. Your plots suggest 100us is about right.

[Carl-NC]

Are not the three insulated coins acting similar to a laminated transformer core, preserving energy loss i.e. our signal?

Transformer laminations are aligned with the magnetic flux lines, so that eddy currents attempt to transverse the laminations, which they can't. In this experiment, the insulated gaps are normal to the flux lines, and eddy currents are unaffected. The results are the same with or without the insulated gap, as there should be no vertical current flow.

[Ferric Toes]

Yes, if the gaps between the 3 coins is negligible.

I think the difference between your plots and green's are due to the proportional TX. For this experiment, I think you want to hold the pulse width constant, and large enough that the thickest target has died out before turn-off. Your plots suggest 100us is about right.

For a nickel, and looking at the linear plot in post 27, you can see that the start value of 1000 is the star at the top of the plot. The single exponential fit did not reach star because there is a faster exponential superimposed due to the skin effect of the single coin. This is with a TX pulse of 40uS (TX is always 4 x delay). At 20uS delay and 80uS TX, and from then on, the single exponential fit is good, indicating that the coin is fully energised for all TX pulses >80uS. It is important that the coil current flat tops at the same value for each TX pulse width i.e. is not inductance limited. Even for the shortest TX pulse (40uS), you have 35uS of constant current. A more conductive coin such as a quarter, would need a much longer TX pulse and reach a single exponential decay considerably later. All this can also be checked on a scope as the delay/TX/sample control is rotated. The decay waveform fills out and then becomes invariant at the point where the TX width is fully energising the target.

I did some tests today with a UK 5 pence coin which is smaller than a nickel but still a cu/ni alloy. Two coins side by side give 2x the signal of 1 coin at any delay. This indicates that the TC of two coins is the same as for one, and just the amplitude is affected. Conversely, two or three coins stacked gives similar results to those above i.e. the TC increases as coins are added.

It's now fairly clear to me what is happening. The stacked coins are acting as mutually coupled inductances where the total inductance is similar to that of a single coin. However, the small inductance of each coin is associated with a parallel resistance (resistivity of the metal) which is relatively high for cu/ni, and even if the coins are insulated, these resistive losses are effectively in parallel. The loss is less, the current is shared, and the TC gets longer.

Basically, 100 individual coins scattered on the surface, even if quite close to one another, will need different detector settings to 100 coins packed into a jar, even if not electrically touching. Of course we all knew that already, but it's nice to know why.

Eric.

[WM6]

The stacked coins are acting as mutually coupled inductances where the total inductance is similar to that of a single coin.

As I understand, each single eddy current space (on coin surface) act as separate inductance. What single coin has, can be only resultant of many (depend on frequency) "eddy current" inductances and not fixed coin inductance per se (cause resultant of coin inductance changes according to coin orientation). Reason that stacked coin has resultant similar ti single coin, is up to shielding effect that upper coin has to other two bottom coin.

[Ferric Toes]

As I understand, each single eddy current space (on coin surface) act as separate inductance. What single coin has, can be only resultant of many (depend on frequency) "eddy current" inductances and not fixed coin inductance per se (cause resultant of coin inductance changes according to coin orientation). Reason that stacked coin has resultant similar ti single coin, is up to shielding effect that upper coin has to other two bottom coin.

At the frequency components we are dealing with in PI, a coin does have at least two, and possibly three eddy current modes. If the coin is thick and/or is made of a very conductive metal such as copper or silver then then at early delays and the coin normal to the magnetic field, you could see an early time skin effect which causes an upward lift to the signal prior to settling down at the main fundamental exponential at 1 tau and onwards. This is inductance no. 1. Inductance no.2 is the fundamental mode which is operative for the rest of the decay and gives a straight line negative slope on a log lin plot. Inductance no. 3 is if there is a component of the field at an angle to the plane of the coin, or if the coin is on edge. This has a considerably faster decay and smaller amplitude than the fundamental decay, but nevertheless it is there and will sum in at the early part of the decay. With most modern coins and especially cupro-nickel, these additional early decays are long gone within the first 10-15uS, so we are just left with the fundamental single inductance decay for each coin.

Now with stacked cupro-nickel coins that are insulated from each other, each coin can be regarded as a single inductance in parallel with a resistance and that has a single TC. Stacking one on top of the other means that they will have a mutual inductance and be tightly coupled magnetically. Have a look at series and parallel coupled inductances and you will see that for parallel the total inductance is not far off that of a single inductance. Once you have more than two it become tricky to calculated, at least for me, and then I don't know the coupling factor. What is observable in the tests I did is that the TC increases with each coin, so either the inductance has stayed the same, or gone up, and/or the current path resistance has gone down. Because of the mutual coupling, you can regard the current paths as being in parallel, which will push the TC up. My view is that both effects are taking place. Later, I will try the same tests with coins that are more conductive, such as quarters or copper pennies, and I am sure the results will be quite different. Don't anyone ask me to do plated steel coins.

Eric

[Tepco]

Coin rule?

Very interesting, and probably correct explanation, in situation when TX pulse is long enough, longer than object TC. I played with this time ago, and noticed that actual coin separation is not that critical, very little change from thin tape insulating layer to 1mm plastic, comparable to coin thickness, TC “superposition” takes place. However I find something like “coin rule”, that is, maximal rise in amplitude occurs when spacing between coins is about coin diameter, (18mm for 18mm coin etc) now amplitude superposition takes place, very little impact on TC. Normal coil is fine for this test, tried with few different coin types with about same result. With short TX pulse, slightly shorter than TC, situation is different. Now, stacking coins will increase amplitude but TC will change very little. This on picture below is taken with fast setup and 10uS TX, with empty, one and 3 coins, clearly visible early time response (this remain with longer pulse too) but almost no change in TC, change appears when TX width approach TC. Actually, with very short pulses TC is not constant at all, it varies with pulse width. (sorry for picture quality, scope is rubbish, camera too, scale is 5uS\div.).

[Davor]

Don't anyone ask me to do plated steel coins.

I could send you some Croatian coins if you wish. On a CW machine with two channel discrimination these sound as a machine gun, as their ID is strongly related to orientation. If they are laying flat they sound more like their metal plating, but if they are perpendicular they sound as iron. As moving your coil shifts the relative orientation, these have sharp transitions from Fe to Cu sound, hence the machine gun sound. People tend to lose several at once and you just can't misinterpret such groups: junk coins.

[Tepco]

I don't think Croatian coins are plated, just alloy composition. Similar problem here, in neighboring country, even recent coins in circulation are made of at least 3 alloy types, maybe more, for same coin. Some attracted to magnet, some don't, different TC, on VLF machine VDI is everywhere. No Star Trek made VDI can go thru this. In just one run last summer, I collected 67 of them in just 30m in one resort, total value ~2-3$, and give up. Better to have smelter installed in MD, so far I could have my statue cast in natural size made of this alloy. Not only PI discrimination, but also VDI or multifrequency VLF wont help with this. On the beach, try to follow “trail” of first 2-3m from waterline, coins are usually grouped in trails, pay attention if just one response is different (from 15 usual VDI readings you have to remember), and, wow, you will get equally worthless foreign coin. Good old bottlecaps, I learned to ignore them with ordinary Surf, but this is a plague and no cure for it.



It is much better to use concentric, not DD coil for this, to avoid coil “phasing”, less susceptible to coin orientation.

[Ferric Toes]

I could send you some Croatian coins if you wish. On a CW machine with two channel discrimination these sound as a machine gun, as their ID is strongly related to orientation. If they are laying flat they sound more like their metal plating, but if they are perpendicular they sound as iron. As moving your coil shifts the relative orientation, these have sharp transitions from Fe to Cu sound, hence the machine gun sound. People tend to lose several at once and you just can't misinterpret such groups: junk coins.

Look for a PM from me.

Eric.

[Davor]

Granted

[Carl-NC]

At the frequency components we are dealing with in PI, a coin does have at least two, and possibly three eddy current modes.

I suspect #1 is from edge effects due to the small diameter of the coin relative to the coil. If you use a target much larger than the coil then this may vanish. #3 is due to the vertical eddies in a non-normal target, which then suffer the same edge effects as #1 but due to the thickness, not the diameter.

[Tec]

Great Plots and data, thanks..

Are we still wondering what is happening here ?

Yes the three coins insulated should be the same as three coins not insulated ?
Yes, the three coins will decay slower than a single coin..

Insulated or not the are connected magnetically.

Move them a couple of mm apart and the decay curve time will decrease ?


Kick three fly wheels glued together, or kick one fly wheel that weighs as much as all three..

It will spin for the same amount of time ?

The big fly-wheel will spin for longer.
The small fly-wheel will spin for less time.

[mickstv]

Here's some screen shots taken using 200us TX pulse and frontend gain of 100x, the targets are 50c AU coins. CRO set to 10us per div.

No Target.....





50c AU coin.




Two 50c AU coins no spacer.




Two 50c AU coins using a piece of A4 paper as a spacer.




Two 50c Au coins with a 5mm spacing.

[Tec]

So is this what is happening ?

If this isn't happening, then I am lost.
Amplitude and slop(e).png

I notice your scope, does it have connection to PC ?.. I bought a DIGTECH 40MHz USB scope from Jaycar and am very happy with it.