SPICE Simulation Challenge

[Carl-NC]

The B-H curve becomes a B-H line for small H, yet even the weak ferrous responses produce 90-180? shift.
When you observe the inductive frequency response curves of steels, you'll find a resistive magnitude stronger at lower frequencies, followed by decline, and finally with a constant resistive response at high frequencies. It may convince you the remanence is dominant in the softest of ferrites, but that would explain viscosity far better than ferrous behaviour.
In impedance plane you may notice the steel responses working as if the non-ferrous response is merely translated to the ferrous side (by means of μr). Even the lift-off curves look as if they are merely translated from the non-ferrous quadrant.
The main difference is that resistive response at high frequenvies of non-ferrous has a flat top, while ferrous has a flat bottom. The big difference is at low frequencies, and I may already see implications for PI.

My understanding is that the B-H curve tends toward a tilted ellipse at small signal, sort of like a Lissajous pattern. Which still results in a 0-90 deg phase range. Many steel targets -- especially flat steel, like bottle caps -- have a dominating eddy response that pushes the overall response to the non-ferrous side. For a ferrous test target I like to use a nail-on-end, which has practically no eddy component.

[Davor]

The B-H curve becomes a B-H line for small H, yet even the weak ferrous responses produce 90-180? shift.
When you observe the inductive frequency response curves of steels, you'll find a resistive magnitude stronger at lower frequencies, followed by decline, and finally with a constant resistive response at high frequencies. It may convince you the remanence is dominant in the softest of ferrites, but that would explain viscosity far better than ferrous behaviour.
In impedance plane you may notice the steel responses working as if the non-ferrous response is merely translated to the ferrous side (by means of μr). Even the lift-off curves look as if they are merely translated from the non-ferrous quadrant.
The main difference is that resistive response at high frequenvies of non-ferrous has a flat top, while ferrous has a flat bottom. The big difference is at low frequencies, and I may already see implications for PI.

[Carl-NC]

Personally I do not think the difference between ferrous and non-ferrous targets can be attributed to an inductance change, nor to a shift in resonance. It's the way the magnetic field is distorted by these different targets that is the root cause. This field distortion causes an imbalance in the coil coupling.

Correct.

If you have a VLF detector with a balanced coil system, the RX loop has a residual signal when no target is present. If a non-ferrous target gets near the coil, there is a redistribution of the magnetic field that causes an imbalance. For a concentric coil, this results in an increase in the amplitude of the RX signal, and all target types cause a left shift in the phase. If a ferrous target gets near the coil, there is some absorption of the magnetic field that also causes an imbalance, but in this case the RX signal experiences a decrease in amplitude. The phase shift is used by the GEB channel to detect the presence of a metal target, whereas the amplitude changes are used by the DISC channel to indicate ferrous or non-ferrous.

If an IB loop has a perfect null, there is no residual signal and either magnetic or eddy responses will cause an increase in RX amplitude. It's only when the coil is mis-nulled to one side or the other that there is a residual signal and a target causes a decrease in amplitude. Whether it is ferrous or non-ferrous that does that depends on which side of null the coil starts out at. This is true for either DD or concentric.

Both magnetic targets and eddy targets cause a distortion in the TX field. In VLF, the magnetic response is due to the gap in the B-H curve which produces a phase response that is limited to 0-90 degrees (all my phases are WRT the TX coil current). Ferrite is 0 deg because it (ideally) has no B-H gap. Eddy responses incur an automatic 90 deg phase shift in developing the surface EMF, so those target responses are limited to 90-180 deg, a fortuitous coincidence.

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.

[Davor]

By in-phase i meant resistive - in PI there is little point speaking in terms of phase.

[Davor]

But the whole point is that PI is inherently an in-phase detector. The whole difference in balanced vs. mono coil is the possibility to look at the time where excitation field still collapses, and quadrature response still creeps through.
However... there is a difference in in-phase frequency response of ferrous materials against the non-ferrous. Up to ~ 30kHz the Ferrous in-phase response is a low pass, while non-ferrous is a high pass. Because EF knocks the response tail to zero, a Fe response, with a fat low pass tail, may sink below non-Fe responses for the early samples. And there you have it.

I believe my simple model from post #31 may prove useful. The only thing I'm missing are the different material data. You may find some data scattered on Internet, but fun part is that you may tune this model to accommodate. See this paper: Effect of nanograin?boundary networks generation on corrosion of carburized martensitic stainless steel but especially the Figure 3, and compare those to my model. I'd say I nailed it

[Qiaozhi]

My attempt with a mono coil. Thought I could see a difference in the beginning based on what I saw with the IB figure8 coil. Didn't see a difference with the mono coil so repeated test with IB figure8. Can see a difference. Any suggestions why I don't see a difference with the mono coil? Maybe something to try?

The same reasons I presented in post #30. The effects of redistribution and absorption are simply not visible when using a mono coil, and can only be seen with a balanced coil arrangement.

[green]

So my conclusion (at the moment) is that real effective ferrous / non-ferrous discrimination cannot be achieved with a mono coil.
Now, go and prove me wrong ... not with an LTspice simulation, but with some real hardware.

P.S. I'm talking about a standard PI setup here, and not other variations that use truncated sine or triangular waves, etc.

My attempt with a mono coil. Thought I could see a difference in the beginning based on what I saw with the IB figure8 coil. Didn't see a difference with the mono coil so repeated test with IB figure8. Can see a difference. Any suggestions why I don't see a difference with the mono coil? Maybe something to try?

[Davor]

I think I was on the right track with my previous Fe model. It was based on a negative inductance, something that naturally happens in vicinity of transformers, and it placed the response in a proper quadrant. However, response was completely wrong. Instead of a response circling from left to right with frequency, it did the opposite. But at least the quadrant was correct.

Now I have a small correction, and it seem OK. It should be properly tested, of course. It is based on a inductance difference, which effectively gives an equivalent of μr >> 1
Also it is rotating as it should, and it ends at high frequencies as mostly resistive, yet still on ferrous side.

If only I could produce a Cartesian plot in AC simulation... Here are the magnitude/phase, real, and imaginary outputs. I'd say it is close enough, but please knock yourself perfecting it. It should be possible to curve-fit some materials with responses measured on professional equipment.

Here goes:

[Qiaozhi]

OK - here's my latest thoughts on this subject:

If you have a VLF detector with a balanced coil system, the RX loop has a residual signal when no target is present. If a non-ferrous target gets near the coil, there is a redistribution of the magnetic field that causes an imbalance. For a concentric coil, this results in an increase in the amplitude of the RX signal, and all target types cause a left shift in the phase. If a ferrous target gets near the coil, there is some absorption of the magnetic field that also causes an imbalance, but in this case the RX signal experiences a decrease in amplitude. The phase shift is used by the GEB channel to detect the presence of a metal target, whereas the amplitude changes are used by the DISC channel to indicate ferrous or non-ferrous.

However, if you take a PI with a mono coil, the same increase / decrease in amplitude is not detectable. The reason is (of course) because we no longer have a balanced coil system. What we are looking at is the latter part of the eddy current decay curve once the huge flyback signal has decayed away. Redistribution and absorption are still taking place, but there is no balanced coil system there to experience an imbalance. Essentially it's not possible to have true ferrous / non-ferrous discrimination with a PI and a mono coil. It is only possible to have discrimination based on conductivity, which is not the same thing. This is why you see demonstrations of PI detectors on youtube rejecting iron nails, but never rejecting a range of ferrous targets of different sizes and shapes. White's TDI is a good example of this process, where the ground balance feature is highjacked to eliminate small iron nails, and provide 2 different tones for high and low conductivity items.

Others have suggested that it may be possible to provide discrimination with a mono coil by measuring the change in inductance of the loop when a target is present. However, based on a BFO's sensitivity and ability to discriminate, I doubt very much this method would be very effective.

Although this is not strictly speaking a "mono" coil, Deemon presented a very interesting balanced mono arrangement. I tested this arrangement in LTspice and discovered that if you place all four loops within the coil shell, and wind them to have the same inductance, it still works as a balanced system. This removes the need to use external coils which would make it difficult to swap coils. I also built this arrangement and found it surprisingly easy to balance. At the end of day, however, this is still a balanced coil system masquerading as a mono.

So my conclusion (at the moment) is that real effective ferrous / non-ferrous discrimination cannot be achieved with a mono coil.
Now, go and prove me wrong ... not with an LTspice simulation, but with some real hardware.

P.S. I'm talking about a standard PI setup here, and not other variations that use truncated sine or triangular waves, etc.

[Qiaozhi]

@Qiaozhi, you may want to prune the LTspice files prior to zipping. Raw and FFT files are most certainly not needed. There is a setting that will help you maintain your disk less cluttered:

Useful information.

[Qiaozhi]

Theoretically it should be possible to simulate both ferrous and non-ferrous targets correctly by adding a core model. However, the core model in LTspice uses Hc, Bs, Br, A, Lm, N and Lg to replace the inductance value. So you still end up with a calculated value of inductance, but with some hysteresis. Unfortunately the addition of hysteresis does not appear to provide the expected target response seen by the detector within the simulation, and consequently no discrimination between ferrous and non-ferrous.

My latest attempt ( in post #18 ) "apparently" works for a VLF detector, but the results are not correct when simulated for a PI.

Personally I do not think the difference between ferrous and non-ferrous targets can be attributed to an inductance change, nor to a shift in resonance. It's the way the magnetic field is distorted by these different targets that is the root cause. This field distortion causes an imbalance in the coil coupling.

If you go to the Coilcraft site, there are some instructions on how to create models in LTspice for their power inductors. This involves the use of parameterized GLaplace components. and an example is given for a Coilcraft LPS4018-332 inductor. Interesting ... but maybe not an approach that would be useful in this instance.

Another possible approach could be using B elements with an arithmetic function to emulate either redistribution or absorption.

[Davor]

Don't get used to seeing me around
BTW, balance is not that narrowband phenomenon. You may expect 1:2 frequency range with decent balance... provided you are not a perfectionist.

@Qiaozhi, you may want to prune the LTspice files prior to zipping. Raw and FFT files are most certainly not needed. There is a setting that will help you maintain your disk less cluttered:

[dbanner]

If you say they cancel out then I believe you. I was thinking they move in opposite directions, tx frequency go up, Rx resonance frequency go down and vice versa. Or they shift, but at different magnitudes. The relative difference causes the coils to become unbalanced. I suppose there's many ways to make sense of it but only one way it actually works in reality.
I go out for few days.
Nice you dropped by, Davor.

[Davor]

As mentioned above, shifting frequency is there, but it nearly equally affects both Rx and Tx coils, so the effect largely cancels out.

I have a glimpse of an idea that involves a negative capacitance. There are various odd effects related with stray field of a transformer, including negative reactances, so why not. Negative inductance shifts response to a ferrous 2nd quadrant, so it is worth a try.

[dbanner]

Can someone please put a 6.5 decimal frequency meter on a tx coil and see what change if any happens when a target is placed in its field?
I don't have such test gear. Various targets can be plotted, then presented as a model to mimic a target. Ferrous/ non Ferrous should present up or down shift.