SPICE Simulation Challenge

[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.

[waltr]

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

[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.

[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?

[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...

[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.

[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.

[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.

[Davor]

If only it was in a log amplitude scale...

Simulation would be wrong as it would show an increase as well?

Simulation is wrong as long as you don't account for major contributions. In this particular case the coil null is not misaligned as much as it has an orthogonal contribution that can't possibly be cancelled by geometry alone. People spend days trying to achieve a deep null with a simple DD coil, and inevitably fail. Why? Because there is a capacitive coupling between Rx and Tx coil, and even more of it in a concentric between Rx and a bucking coil.
mikebg suggested using a piece of aluminium foil in proximity of a coil and find a spot where it cancels the orthogonal component. It makes sense, foil response is almost entirely orthogonal. I tried it and it worked. But does it make sense? I'd say it doesn't, as long as the output of the Rx preamp is not driven to distortion which would mask weak target responses.

[Qiaozhi]

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.

Essentially you are correct.

A non-ferrous target has eddy currents generated in its surface, and these create a magnetic field that opposes the applied field. In effect, it distorts that overall field and upsets the coil balance. A ferrous target, on the other hand, concentrates the applied field in its interior. This also distorts the overall coil balance. Whether the ferrous or non-ferrous target produces the decrease in amplitude depends on which side the loops are mis-nulled. For the DD coil, you have to negate the coupling coefficient between TX and RX in order to flip over to the other side, whereas the concentric coil can be swept through the balance point by adjusting inductance of the nulling coil and/or its associated coupling coefficient. So, in the case of a VLF detector simulation, it is obviously possible to adjust the coil balance in such a way that it accounts for the overall field distortion for the various targets. Maybe that's the best solution we have for this scenario, and it certainly accounts for the observed effects.

In the case of a PI detector with a mono coil, there is no balance to upset. Hence it appears to be impossible to see the difference between ferrous and non-ferrous targets, or to include these effects in the simulation.

However, before we give up on this, please read the attached application note.

[Davor]

I like this kind of cheating
Only I'm afraid it will perform as described as long as the coil is in the air. But a clever idea anyway.

[Teleno]

However, before we give up on this, please read the attached application note.

It must be synchronicity, I came acroos this same application note two days ago.

The coil is slightly underdamped to provide one cycle of decaying oscillation which is then integrated. What this actually does is to provide a signal that's proportional to the coil's inductance. Ferrous targets slow down tbe decay, non-fe accelerate it. The effect is very tiny and only measurable for shallow targets. It's equivalent to measuring changes in the rise time of the coil's current during the Tx period. Not a great solution because magnetic grounds cause the same increase in L as ferrous tergets and the effect requires strong coupling, it's not sensitive.

[Qiaozhi]

It must be synchronicity, I came acroos this same application note two days ago.

The coil is slightly underdamped to provide one cycle of decaying oscillation which is then integrated. What this actually does is to provide a signal that's proportional to the coil's inductance. Ferrous targets slow down tbe decay, non-fe accelerate it. The effect is very tiny and only measurable for shallow targets. It's equivalent to measuring changes in the rise time of the coil's current during the Tx period. Not a great solution because magnetic grounds cause the same increase in L as ferrous tergets and the effect requires strong coupling, it's not sensitive.

When I read the App Note, I did wonder if that was the case. At least that saves me wasting time investigating yet another red herring.

[waltr]

I also have read that app note this past year but never was able to explore its possibility.
Suspected that it may be sensitive to offset error and may need a strong Fe signal.

I am working on a PI detector that uses fast ADC in a PIC32 and trying to do every thing in software.
So I should be able to try that method.

[Qiaozhi]

I also have read that app note this past year but never was able to explore its possibility.
Suspected that it may be sensitive to offset error and may need a strong Fe signal.

I am working on a PI detector that uses fast ADC in a PIC32 and trying to do every thing in software.
So I should be able to try that method.

You may also want to consider Carl's reply in post #36, where he said "In PI, the same mechanisms cause off-time decay responses. Eddy responses are pretty close to exponential. Magnetic responses are not, they are still controlled by the magnetic lag dictated by the B-H curve. Ferrite again has no gap and therefore no magnetic lag; when the TX field collapses, the ferrite response collapses along with it. Targets with a B-H gap are not instantaneous, they have a time lag in their collapse. Then there are targets with magnetic remanence, which is yet another mechanism."

Sampling along the curve may reveal a difference between ferrous and non-ferrous. That's your mission, if you choose to accept it.