SIMULATIONS

[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

[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

[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.

[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]

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.

[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.

[PiTec]

I have found two typos in my previous post:

1. Horizontal and vertical TCs swapped, i.e. it should read ‘I measured 9-10µs TC vertical over the RX1 coil center, and 12-13µs horizontal …’. So this is of course in accordance with your measurements, Eric.

2. K14 = 0.001001 / K14 = 0.001004 should read K14 = 0.01001 / K14 = 0.01004

My reason for making 3 simulations, with the target
1) over the center of the coil, the magnetic field lines cut through the target vertical, at 0 degrees, south to north
2) over the rim of the coil, the field lines cut through the target at 45 to 90 degrees, depending how close to the rim.
3) Off to the side of the coil, the field lines cut through the target at 180 degrees, the X response of a ferrite should be 180 degrees different from 1).

Therefore, we do not need to rotate the target, changing the position relative to the coil, while maintaining the same orientation should be good enough. ).

1) and 2) should be very similar (horizontal at the center same as vertical over the rim and vice versa), but 3) may have other reactive component amplitudes.

I have about 20 different crown corks here, and they are all quite similar regarding their two main TCs (9-10µs when parallel to the TX field, 12-13µs when perpendicular).They all show a similar and strong reactive response when they are parallel to the TX field, but also a less stronger response when perpendicular to the TX field. The amplitude ratio is approx. 3:1 for equal distances to the TX coil. For comparison, I measured a 5 Eurocent coin (also copper plated steel). Here the ratio is approx. 8:1, so they they are more difficult to identify as iron when buried horizontally.

Here is the LTspice file:
iron_target.rar

Thomas

[Ferric Toes]

I have come to the conclusion that simulating ferrous objects is going to be a nightmare. I have just made some test targets all the same diameter as a 2p coin but different thicknesses. They are made from steel shim stock and of thickness 2, 4, 6, 8 and 12thou. As you might guess the shim stock is rather old. All display like the crown cap with no apparent viscosity lag. Perpendicular to the TX field the decay time appears to be proportional to thickness i.e. the 4thou is twice the time of the 2thou. The other orientation starts shorter and also doubles. It appears that when the objects are parallel to the TX field, there is a certain permiability effect which has the effect of magnifying the smaller X section eddy currents. Bring up a strong ferrite magnet and the decay can be neutralised, indicating saturation. The 0.5mm wire on a 1N4148 diode gives a strong response that disappears when a strong field is applied. Not so the 2p coin but maybe my magnet is not powerful enough.

Eric.

[Tinkerer]

Hi,

basically, the response for iron targets is similar to that of non-iron ones, except that depending on shape, orientation and alloy some reactive response has to be added in case of an IB setup.

The problems with crown corks have been discussed already – and I think this actually makes a useful simulation with LTspice impossible. As proposed, you may limit the simulation to 3 target positions (plus 90° rotation this would make 6), but to really simulate the influence of target position and rotation you would need a suitable software, like the 3D coil simulation software from Aziz, but with a life target response plugin With LTspice, you would have to use experimental values for target TCs and coupling coefficients for each target position and rotation angle.

A very basic simulation that shows the principle could be like this:

• IB setup so the reactive response can be seen as well (double D with two identical RX coils RX1 and RX2 and the TX coil around both). The response of a conventional PI is of course also visible after the flyback pulse.
• Target TC can be varied to simulate in which angle the TX field strikes the target. For simplicity only one single TC, although real iron targets may have several TCs plus other effects that may influence the response, like skin effect or magnetic viscosity.
• The coil coupling coefficient RX1 to target determines the amplitude of the resistive response.
• The coil coupling coefficient TX to RX1 determines the amplitude of the reactive response.

Here is a setup similar to the one I used in the Triangular Wave thread. Ideal TX coil with an additional constant current period so that the decay curve after the driving pulse is visible as well. The TX on/off ratio is limited to 2:1 (50µs to 25µs) to keep the amplitude differences small. The coupling coefficients K24 and K14 will be changed in the different simulations. With K24 = K25 there is no reactive RX signal, and with K14 = K15 there is no resistive RX signal. Only the target with TC=10µs is being used.

First one with no target response (K25 and K24 = 0.01), but with a reactive signal as if you moved a ferrite core up and down over the RX1 coil (K14 varied). Soil with magnetic susceptibility and mechanical coil instability would show the same response:

[ATTACH]23067[/ATTACH]

Now for comparison a target with a resistive response, like a gold ring (K14 = K15, K24 changed to 0.012):

[ATTACH]23068[/ATTACH]

And finally the crown cork. I measured 9-10µs TC horizontal over the RX1 coil center, and 12-13µs vertical (real hardware, scope readout). As expected, the reactive response is much larger when it is vertical. In the simulations below I modified the coupling coefficients so that the simulation results are very close to the scope readouts.

Left side horizontal crown cork with K14 = 0.001001 and K24 unchanged at 0.012. Right side vertical crown cork with K14 = 0.001004 and K24 unchanged at 0.012. I did not change the target TC (both simulations 10µs) as the real values do not differ so much.

[ATTACH]23066[/ATTACH]

Both curves for the crown cork have basically the same shape as the curve for the gold ring with the same TC. They are just more or less shifted due to the reactive component during the TX on/off periods. To make the curves easier to compare, the amplitudes of the underlying decay curves are identical in each of the above plots. If you rotate a real iron target at the same distance from the coils, the vertical response may be larger, especially if it is a long target like a nail.

Thomas

Thomas, thank you for the simulations and explanations. I will need some time to analyse all that.

For now I have just one comment:

My reason for making 3 simulations, with the target
1) over the center of the coil, the magnetic field lines cut through the target vertical, at 0 degrees, south to north
2) over the rim of the coil, the field lines cut through the target at 45 to 90 degrees, depending how close to the rim.
3) Off to the side of the coil, the field lines cut through the target at 180 degrees, the X response of a ferrite should be 180 degrees different from 1).

Therefore, we do not need to rotate the target, changing the position relative to the coil, while maintaining the same orientation should be good enough.

Somewhere I posted a picture showing the field lines of a TANDEM COIL. This coil arrangement is designed to enhance the response of flat targets at different angles.

Tinkerer

[Tinkerer]

The crown caps are not what I expected. I tested two, which appear to be from different manufacturers, and neither show any magnetic lag (X). They are, however, strongly attracted to a magnet. The total decay is 100uS when perpendicular the the TX field and 75uS when parallel to the field. In other words it is behaving like a non-ferrous target. No wonder they are a problem.

The UK steel cored coins are just as much of a problem here, as 2p and 1p denominations have been made this way since 1992. With them, there is a clear magnetic signature (long decay when // to field), so discrimination should be easy by observing the TC change when scanned, or large X response from a balanced coil.

I can only assume that the crown caps that I have are a steel alloy which has constituents that prevent a magnetic lag.

Eric.

Thank you for the feedback and the TC data.

If we look at the response of steel as a combination of a ferrite and of a low conductivity conductor, we can simulate a similar response. Ferrites come in different grades, according to the frequency at which they are going to be used.

A ferrite core for a high frequency transformer can not have a lag, because then the impedance would be great.

On the other hand, a ferrite sleeve on a 50Hz power line is designed to present low impedance at that frequency.

We could term this to be frequency susceptibility.

Steel can be designed to be frequency susceptible. Adding silicon to the alloy makes for better 50Hz transformer cores.

Adding 14% of Manganese, makes the steel non magnetic and extremely hard, to be used for rock drills. Since such rock drill bits are not attracted to even a very powerful electromagnet, they present a problem for the rock crushers in the mining business.

Some crown caps are plated with zinc or tin or cadmium or whatnot metals. This can make things a bit more complicated still. It is a fact that copper plating on a steel rod changes the HF frequency conductance considerably.

All that may mean that my choice of a crown top as a universal test target turned out to be not universal.

But, crown tops are a big pain when detecting. So it still may be worth defining why they behave as they do.

In one of my PI designs, I tried to differentiate the crown tops by enhancing the X response. This seemed to work OK and might work in the field, if this feature could be switched on only to verify a doubtful target response.

By the way, I use the X response denomination as the response that shows as 180 degrees from the R response, while there is current running in the coil. This is very easy to see during the TX ON time.
When we switch OFF, we still have current flowing in the coil, until the decay curve reaches 0. However, at some time on the decay slope, the X response becomes less than the R response. I call the point where the response crosses over the coil decay curve, the pivot.

Sampling before the pivot enhances the X response. Sampling after the pivot enhances the R response.

Tinkerer

[Ferric Toes]

The crown caps are not what I expected. I tested two, which appear to be from different manufacturers, and neither show any magnetic lag (X). They are, however, strongly attracted to a magnet. The total decay is 100uS when perpendicular the the TX field and 75uS when parallel to the field. In other words it is behaving like a non-ferrous target. No wonder they are a problem.

The UK steel cored coins are just as much of a problem here, as 2p and 1p denominations have been made this way since 1992. With them, there is a clear magnetic signature (long decay when // to field), so discrimination should be easy by observing the TC change when scanned, or large X response from a balanced coil.

I can only assume that the crown caps that I have are a steel alloy which has constituents that prevent a magnetic lag.

Eric.

[PiTec]

Hi,

basically, the response for iron targets is similar to that of non-iron ones, except that depending on shape, orientation and alloy some reactive response has to be added in case of an IB setup.

The problems with crown corks have been discussed already – and I think this actually makes a useful simulation with LTspice impossible. As proposed, you may limit the simulation to 3 target positions (plus 90° rotation this would make 6), but to really simulate the influence of target position and rotation you would need a suitable software, like the 3D coil simulation software from Aziz, but with a life target response plugin With LTspice, you would have to use experimental values for target TCs and coupling coefficients for each target position and rotation angle.

A very basic simulation that shows the principle could be like this:

• IB setup so the reactive response can be seen as well (double D with two identical RX coils RX1 and RX2 and the TX coil around both). The response of a conventional PI is of course also visible after the flyback pulse.
• Target TC can be varied to simulate in which angle the TX field strikes the target. For simplicity only one single TC, although real iron targets may have several TCs plus other effects that may influence the response, like skin effect or magnetic viscosity.
• The coil coupling coefficient RX1 to target determines the amplitude of the resistive response.
• The coil coupling coefficient TX to RX1 determines the amplitude of the reactive response.

Here is a setup similar to the one I used in the Triangular Wave thread. Ideal TX coil with an additional constant current period so that the decay curve after the driving pulse is visible as well. The TX on/off ratio is limited to 2:1 (50µs to 25µs) to keep the amplitude differences small. The coupling coefficients K24 and K14 will be changed in the different simulations. With K24 = K25 there is no reactive RX signal, and with K14 = K15 there is no resistive RX signal. Only the target with TC=10µs is being used.

First one with no target response (K25 and K24 = 0.01), but with a reactive signal as if you moved a ferrite core up and down over the RX1 coil (K14 varied). Soil with magnetic susceptibility and mechanical coil instability would show the same response:



Now for comparison a target with a resistive response, like a gold ring (K14 = K15, K24 changed to 0.012):



And finally the crown cork. I measured 9-10µs TC horizontal over the RX1 coil center, and 12-13µs vertical (real hardware, scope readout). As expected, the reactive response is much larger when it is vertical. In the simulations below I modified the coupling coefficients so that the simulation results are very close to the scope readouts.

Left side horizontal crown cork with K14 = 0.001001 and K24 unchanged at 0.012. Right side vertical crown cork with K14 = 0.001004 and K24 unchanged at 0.012. I did not change the target TC (both simulations 10µs) as the real values do not differ so much.



Both curves for the crown cork have basically the same shape as the curve for the gold ring with the same TC. They are just more or less shifted due to the reactive component during the TX on/off periods. To make the curves easier to compare, the amplitudes of the underlying decay curves are identical in each of the above plots. If you rotate a real iron target at the same distance from the coils, the vertical response may be larger, especially if it is a long target like a nail.

Thomas

[ODM]

I'm curious about this measurement setup and its coil setup. Is there a separate receiving coil or flux sensor, and what measurements does it yield? There is no closed magnetic loop like in most magnetic characterizing setups.

Some earlier threads are mentioning the MVM, too. I presume it's no longer an active product. What type of setup is/was it?

[Ferric Toes]

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.

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.

[Aziz]

Oh well, I have completely eliminated the magnetic viscosity effects.
But the skin effect is even present at the decaying eddy currents.

Eric, I'm looking forward to your interesting report.

Cheers,
Aziz