SIMULATIONS

[Tinkerer]

Here we have the traditional PI simulation.
3 non magnetic targets, TC's 10us, intense blue, 100us, red, 500us, light blue.
With a TX of 50us, we see how the 10us target TX ON eddy currents have reached their peak and are diminishing, while the eddy currents in the longer TC targets are still raising.
The light green trace is the TX coil current. The raise is near linear, because the TX time is very much shorter than the coil TC. At switch OFF, the current drops fast, then raises again, to drop to the 0 level, due to the critical damping resistor.
What causes the transient during the decay curve? It is the stray capacitance, set at 200p, to represent the inter-wire and wire to shield capacitance.

The TX coil also serves as RX coil with the target coupling at 0.0001

The target response traces are separate from the TX trace, but this we can not see on a real circuit. What we see on a real circuit is the sum of the TX wave form and the target wave form and all the other stuff, like environmental noise, ground etc. that the RX coil sees.

On the simulation we can sum the traces by adding <I(L1)+I(L3)> etc. to give us a trace that is more like the trace we see on the scope from the real circuit.

For complex target responses, like magnetic and conductive ground etc. I sum R and X responses. Summing several targets with different TC's and different R and X, seems to be able to reproduce just about any target response wave shape.

This is as far as I got with the simulation. Please, help me here. The better the simulations, the better we can all understand the real circuits.

An interesting add, would be a noise source.

[Tinkerer]

Plated 2p is 93% mild steel core and 7% copper plating. Weight of coin 7.12gms. Copper contribution is about 0.5gm spread over the whole surface. Skin effect will be there but invisible to PI as we know it. Bet that all the response we see is the mild steel core.

Skin effect will be there but invisible to PI as we know it. Bet that all the response we see is the mild steel core.[/QUOTE]
This is one of the problems of traditional PI. We can not see what happens during the first few us after switch OFF. This information information remains hidden in the Flyback and Pre-amp saturation.

Is there a way to see what happens during the first few us? Yes, by using an IB configuration we can largely eliminate the Flyback in the RX coil.
We can also make simulations that show us that time period, but, unless we have a real circuit to compare the simulation with, we do not really know if the simulation results are right.

I have designed a circuit that allows us to see the total cycle time. The simulations are much the same as the wave forms of the real circuit. There are some distortions of the waveform, due to the filtering, but they can easily be recognized as such.

For this purpose I make sure that the pre-amp does not saturate. This means relatively low gain. I make up for that by increasing the TX power, such that the target response can be seen, when using a real detector coil.

Of course, now I have the problem that nobody recognizes the wave form they see, because they have never seen it before.
To get out of this dilemma, I thought to show 2 different simulations side by side, Traditional PI and IB-PI. I am still stuck with that. How do I make a traditional PI simulation to come out right, if I have no means to see a real corresponding circuit?

Attached is the screenshot of the simulation as well as the asc file in zip format.

There are 2 circuits and 3 different targets. Changing the inductance in the targets, changes the TC of the target. Ex, 10pH=TC of 10us.

The problem is with the K coupling between the various coils. For the individual sim's, the K has to be changed, I have not managed yet to run them at the same time.

Maybe somebody can help?

Tinkerer

[Davor]

Guess you'll be a bit surprised again
At VLF these behave funny, provided you have a two tone machine, and in PI they may behave even more funny at fast samples.

I often laugh when I come across such coins with my detector - the sound of junk - you can't miss it.

[Ferric Toes]

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

Plated 2p is 93% mild steel core and 7% copper plating. Weight of coin 7.12gms. Copper contribution is about 0.5gm spread over the whole surface. Skin effect will be there but invisible to PI as we know it. Bet that all the response we see is the mild steel core.

[Ferric Toes]

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

Yes you can. "Soft ferrites" as used in TV line scan or switchmode psu exhibit high permiability/susceptibility but little or no viscosity; much like a sample of California black sand that I have. Ferrites as used in MW and LW radio antennas exhibit viscosity. It is all to do with the grain size. Fine grain has viscosity, coarse grain hasn't.

Soon I wil be able to do log/lin, log/log, and linear plots to see what is going on. Just have to get a small batch of my Magnetic Viscosity Meters finished. The latest linear curves I have done were using the MVM sensor and front end
amplifier.

Just bought a beer with a steel cap and looked at the linear scope plot. Bit surprised at what I saw. Report later.

Eric.

[mikebg]

All depends on sample delay or frequency

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

"Skin effect" is a term of frequency domain. When you use it, you should point at what point of frequency spectrum it dominates. The same is valid for time domain: At what sample delay magnetization of the iron core prevails?
In frequency domain all is clear. When we have combination of conductivity and permeability, there is a resonance frequency (point Q) at which X=0. If you have coins 1, 2 or 5 Eurocents, you can measure the resonance frequency.

[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

[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]

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

[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

[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.

[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

[Davor]

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

[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.

[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.