SPICE Simulation Challenge

[green]

Tried charting coil decay with ferrous and non ferrous target. Used a small mono coil and fairly large targets to get a good signal difference. Non ferrous target charted same as simulation.

[dbanner]

Wait a minute, doesn't both ferrous and non ferrous cause increasing amplitude(IB coils), provided the coil is at zero residual voltage?
If in the simulation the ferrous target is supposed to show a decrease(as with real appropriately misbalanced coil) but shows an increase, then would the opposite be true as well. That is to say non ferrous should give a decrease on the flip side of the zero residual voltage point. Simulation would be wrong as it would show an increase as well?
That's curious. There is something funny about the cancelling field in a concentric coil which is really an anti-phase field. Is it not? A visualization of the tx and cancelling fields would be interesting. Both fields would be stronger with a ferrous metal present, both weaker when non ferrous metal present. But it is the cancelling field which determines the amount of coupling(TX, RX). Not so?
At residual zero, the tx field and bucking coil "anti-field" renders the Rx coil "blind".
I think spice cannot contemplate this scenario in terms of (not zero degrees phase difference) but " neutral phase" at zero residual voltage.
I am not equipped with sufficient mathematical and physics prowess to dig deeper. Just some daydreaming thoughts.
I really should go back to school, if only to do justice to my imaginative skill.
Spice does contemplate the polarity of magnetic lines of force, doesn't it?
Well then, has anyone considered the coupling which exists between the TX and bucking coil in the presence of ferrous/ non ferrous? A scenario which does not exist in the DD type arrangement.

[Qiaozhi]

Don't know how to simulate a ferrous target to see if coil decay time increases. Does it make sense that coil should decay faster with a non ferrous target?

That's the whole crux of the problem. There's a dichotomy here, in as much as the phase-shift of the ferrous target is incompatible with the amplitude change. If your model displays the correct amplitude decrease, the phase-shift is wrong ... and vice versa. Unfortunately, it looks like we would need to model the magnetic field structure around the coil in order to get the correct results. How it would react with a PI is the question. If we fudge the model to work for a VLF, then you just cannot rely on the results if tested with a PI.

[Davor]

That would depend on a ferrous material.

The whole point is that there was no precedent in literature giving a generic ferrous target model so far. Mine seem to work, with a few touches, but I have to perfect the non-linear B-H behaviour. If I make it, we'll have the first model of a ferrous target.

As for the Chan model, It is very simple to feed with parameters. There is a LTspice how-to appearing on different places on Internet, but I don't think I saw the original source. Enjoy...

[green]

Yes, but (again) that's just a fudge, and it doesn't allow the same underlying mechanism to be modeled in a PI simulation.
Unless someone else has a brainwave, I think that's all folks ... at least for the present.

[ATTACH]46024[/ATTACH]

Notice the coil in the image.

Not a brainwave but a question. Most of the thread has been trying to get the ferrous or non ferrous target simulation to act the same as a VLF. How should they act with a PI? If I look at Rx with a IB coil, signal during coil decay is same polarity as signal after coil decay with a ferrous target. Opposite polarity with a non ferrous target. Thinking mono coil decay should be faster with a non ferrous target. Used the simulation posted earlier to look at Tx. Increased K3 coupling from .01 to .2 so I could see decrease in decay time like I was thinking. Don't know how to simulate a ferrous target to see if coil decay time increases. Does it make sense that coil should decay faster with a non ferrous target?

[Qiaozhi]

Well, that sounds like the end of the road. Close but no cigar.
Someone will have to write code to upgrade spice software.
Or you could just subtract double the increase at the Rx output.

Yes, but (again) that's just a fudge, and it doesn't allow the same underlying mechanism to be modeled in a PI simulation.
Unless someone else has a brainwave, I think that's all folks ... at least for the present.



Notice the coil in the image.

[waltr]

Yes, very interesting thread even if the finial conclusion is negative.
Thanks.

[KingJL]

Qiaozhi,
Thank you for this thread. I think it has been an extremely informative discussion and, for the most part, was not diverted from the original theme of the discussion.

[dbanner]

I problem is that the amplitude decrease is caused by a distortion in the magnetic field pattern whereby the applied field is absorbed by the ferrous target, and this mechanism is not modeled by the LTspice simulation..

Well, that sounds like the end of the road. Close but no cigar.
Someone will have to write code to upgrade spice software.
Or you could just subtract double the increase at the Rx output.

[Qiaozhi]

I may have reached an impasse with this endeavour.

As a sanity check I monitored the RX pre-amp output of Raptor versus the TX signal and compared the signal change for various targets. For the concentric coil used, all targets caused a phase-shift left. Non-ferrous (eddy) targets produced an increase in amplitude, and ferrous (magnetic) targets caused a decrease in amplitude. When Carl mentioned that the magnetic response is limited to 0 to , and eddy responses incur an automatic shift to give a range of 90 to , this is referring to:
which I can clearly measure on the scope by sampling at zero-crossing and quadrature points of the RX waveform wrt the no-target response.

The bottom line is that we have a problem. The simple LR model is ok for simulating non-ferrous (eddy) targets as you can see the phase-shift left and the increase in amplitude, but the ferrous (magnetic) targets (although they also phase-shift left) need to show a decrease in amplitude. Even if the LTspice simulator supported the Chan core model with mutual coupling between inductors (as Pspice does with the Jiles-Atherton model) the results would still be incorrect. To produce a decrease in amplitude when compared to the no-target response, the signal from the target would need to be partially in anti-phase with the no-target RX signal in order to cause the reduction, which clearly violates what we see on the scope. The problem is that the amplitude decrease is caused by a distortion in the magnetic field pattern whereby the applied field is absorbed by the ferrous target, and this mechanism is not modeled by the LTspice simulation. Teleno managed to fudge around the problem to some extent by assuming that the target affects the loop inductance, and hence by messing with the coil inductances you could give the impression that it was working. However, the purpose of this investigation was to create some target models that could be used in both VLF and PI simulations to investigate the possibility of ferrous detection with a PI using a mono coil. Unfortunately, simply fudging the results for a VLF does not achieve this goal.

I also tried using a collection of controlled sources to allow the Chan model to be included in a mutually coupled coil simulation, but with limited success, as the absorption phenomenon is not being modeled that way either. So, I'm stuck...

It looks like (in this instance) that real circuitry and a scope are the only alternative to poking around with LTspice.

[Davor]

Thank you for the explanation. If I uderstood correctly the Fe decay is not exponential (log linear) but more extended in time and can still be significantly present at the end of the Rx period or even above the level of the main sample?

You may observe it as 2 kinds of contributions superimposed. But perhaps the correct approach would be to devise a simplified mathematical expression that would be simple enough to tackle with the known mechanisms. By casually browsing through patents, the manufacturer-who-must-not-be-named sees hyperbolas everywhere. How well it approximates Fe is debatable, but if a piece is small enough, just about any function will do for piecewise approximation. So why not a hyperbola?!

[KingJL]

... Alternating polarity is a way to drop it completely.

Perfectly on target!!!

[Teleno]

As for a steel response paper, it should be noted that there are 2 plateaus, the upper one at lower frequencies, and a lower at a higher frequencies. The PI technology samples are band limited at the lower side by fmin~1/(2t2), where t2 is the end of main sample, and for the short samples you are limited to higher frequency range. However, EF sample is frequency limited much lower. Fe has strong response at low frequency, and therefore the EF will be raised above the proper late sample without Fe. For that reason you have EF sinking the main sample below a no-target response (for some steel targets, but not all), and for some Fe targets appearing to go negative against the non-Fe. Also the schemes for discriminating Fe depend on the slope between plateaus to decide presence of Fe. But when you go through a few different kinds of steel, you'll notice that the slope is in fact mostly in a frequency response range of a PI, but not necessarily so. Hence Fe discrimination, as attempted so far, is a hit or miss.
What can be done? Re-think the role of EF in PI detectors used for Fe-infested terrains. Alternating polarity is a way to drop it completely.

Thank you for the explanation. If I uderstood correctly the Fe decay is not exponential (log linear) but more extended in time and can still be significantly present at the end of the Rx period or even above the level of the main sample?

[Davor]

The whole idea is to fit, within reason, the frequency response of a target, or a target family. Because amplitude may walk from nanovolts to tens of milivolts, only the frequency response is meaningful.

As for a steel response paper, it should be noted that there are 2 plateaus, the upper one at lower frequencies, and a lower at a higher frequencies. The PI technology samples are band limited at the lower side by fmin~1/(2t2), where t2 is the end of main sample, and for the short samples you are limited to higher frequency range. However, EF sample is frequency limited much lower. Fe has strong response at low frequency, and therefore the EF will be raised above the proper late sample without Fe. For that reason you have EF sinking the main sample below a no-target response (for some steel targets, but not all), and for some Fe targets appearing to go negative against the non-Fe. Also the schemes for discriminating Fe depend on the slope between plateaus to decide presence of Fe. But when you go through a few different kinds of steel, you'll notice that the slope is in fact mostly in a frequency response range of a PI, but not necessarily so. Hence Fe discrimination, as attempted so far, is a hit or miss.
What can be done? Re-think the role of EF in PI detectors used for Fe-infested terrains. Alternating polarity is a way to drop it completely.

[Qiaozhi]

How does one know the proper coefficients to apply in simulation?
I imagine that degree to which the Rx is decoupled when a target is presented would vary with the type and size of material and also the distance . I don't think conductivity of metals is linear across the non ferrous spectrum. Rather complex problem.
I'll have to go back to school.

The conductivity of non-ferrous metals is not linear across the spectrum. The targets we are attempting to simulate are just representative (ideal) samples.