SPICE Simulation Challenge

[green]

An exponential curve implies varying tc along the curve, however small or large.

TC is the same if I did the chart correctly.

[dbanner]

An exponential curve implies varying tc along the curve, however small or large.

[green]

Thinking US nickel should have a TC close to 10us anywhere in the decay it's tested.

[waltr]

Target geometry usually has several axes of symmetry causing responses which are a sum of exponentials. Additionally ferrous targets, like magnetic soils, may have magnetic particles wth a size distribution causing an additional sum of exponential decays. Discrimination would then involve obtaining a spectrum of time constants. The difficulty being that fitting a sum of exponentials is a very well known ill-posed problem. The manufacturer whose name must not be said has a patent for best fitting two time constants as an approximate discrimination technique.

Yes, I have played a little with summing exponentials and curve fitting to explain data I have measured (see the Plot I posted).
The TC of each target in the plot fitted to portions 24-64usec & 44-84usec of the curves are:
None: 25/35
Brass: 20/20 Note this is a large disk that produces a very strong signal
nail: 28/38
US nickle: 17/25
US cent: 29/32
US Quarter: 33/39
gold ring: 16/19

I originally expected the TC to be the same regardless of where on the curve the fit was done but quickly realized that the curve is a sum of exponentials with at least the coil itself having a decay TC. Also, when the target signal is strong then its sum becomes dominate (Brass disk) whereas if the signal is weak then the coil, etc exp sums dominate.

[waltr]

Could you do the averaging of multiple readouts in linear domain, and then present it in a log scale? I think I already see what I expected.

Is this question to me?
If so then the above plots are sums (average without the divide) of 8 sets of ADC samples (after 8 TC pulses).
Then I put the data into a code function that emulates an Op-amp integrator which is a type of filter.
The data was sampled a 2usec intervals in linear then put in Excel. The Excel plot has a log y-axis to see the difference amoung targets.
I could in Excel average more sets of data which should help smooth the curves (remove noise).
Which targets would you like to see?

[Davor]

Could you do the averaging of multiple readouts in linear domain, and then present it in a log scale? I think I already see what I expected.

[waltr]

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

That is my plan. Here is a plot, log/lin, of some targets sampled at 2usec intervals.
I am calculating the TC over two intervals, 24-64usec & 44-84usec, with an exp curve fit.
Note the curve of the Nail verse the Cent, at first glace they look the same but there is a difference at the very beginning, the Nail has a slight lag then the curves diverge later giving a different 2nd TC.
I am exploring all possibilities that I have read about & can think of.

[Teleno]

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.

Target geometry usually has several axes of symmetry causing responses which are a sum of exponentials. Additionally ferrous targets, like magnetic soils, may have magnetic particles wth a size distribution causing an additional sum of exponential decays. Discrimination would then involve obtaining a spectrum of time constants. The difficulty being that fitting a sum of exponentials is a very well known ill-posed problem. The manufacturer whose name must not be said has a patent for best fitting two time constants as an approximate discrimination technique.

[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.

[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]

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.

[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.

[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.

[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]

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.