SPICE Simulation Challenge

[Qiaozhi]

Golden sabre, in fact all tesoro detectors of similar circuits. Or any coil so balanced( slightly off to one side).
My humble tgsl sometimes beep with big iron target, but not with every sweep, seems intermittent.
With tgsl, false beep on big iron is sure indication of a correctly misbalanced coil.

Just tested a Laser (Tesoro) Hawkeye, and it doesn't beep with an enormous rusty bolt close to the coil, just a bit of clicking. But a Tesoro Bandido does beep with the same bolt.

The coil for the Hawkeye has a 4-pin plug, and the Bandido uses the 5-pin type. These two coil types have vastly different inductances, and may be balanced differently as well. It's possible that the coil is mis-nulled on the other side of the balance point, in which case all ferrous targets will cause an increase in amplitude, and consequently cannot flip the RX signal. Whereas all non-ferrous targets will cause a decrease, and most are of a size that cannot flip the RX signal and cause it to be rejected.

In fact, I've just tested the Hawkeye with a huge biscuit tin lid close to the coil, and it gives a double beep. This is a sure sign that the RX signal has been flipped over.

[dbanner]

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.

[dbanner]

Golden sabre, in fact all tesoro detectors of similar circuits. Or any coil so balanced( slightly off to one side).
My humble tgsl sometimes beep with big iron target, but not with every sweep, seems intermittent.
With tgsl, false beep on big iron is sure indication of a correctly misbalanced coil.

[Qiaozhi]

Now let us say that Rx loop is misbalanced such that a ferrous target causes reduction in amplitude, then if the ferrous target were sufficiently large, would it cause an initial reduction in amplitude then as the Rx loop moves through the perfect null, then an increase in amplitude? A sufficiently large ferrous target would affect the coupling to put Rx loop back to perfect null then beyond, so that it is now misbalanced to the other side resulting in increasing amplitude?

That's exactly what does happen, and is why a large ferrous target will cause a beep when it's close to the coil.

If this is the case, then is this why some detectors cannot discriminate big iron?

As an example, a Garrett GTAx1000 will beep and indicate a target at the top end of the VDI scale. However, it is possible to notch out the top segment on the display and consequently ignore these "false" targets. Unfortunately this causes some reduction in sensitivity, so it's not recommended.
Which particular detectors are you referring to?

[dbanner]

Maybe limiting the decreasing amplitude to zero would help discrimination of big iron targets?

[dbanner]

Ok. What I understand from reading the posts is that a target whether ferrous or non ferrous, placed in the field causes an imbalance in the coupling, either by way of absorption or redistribution of magnetic field. Fine.
An IB Rx loop with perfect null will show rise in amplitude to both ferrous/non ferrous.
Rx loop which is misbalanced will show a reduction in amplitude dependent on which side misbalanced and whether the target is ferrous or non ferrous.
Now let us say that Rx loop is misbalanced such that a ferrous target causes reduction in amplitude, then if the ferrous target were sufficiently large, would it cause an initial reduction in amplitude then as the Rx loop moves through the perfect null, then an increase in amplitude? A sufficiently large ferrous target would affect the coupling to put Rx loop back to perfect null then beyond, so that it is now misbalanced to the other side resulting in increasing amplitude? If this is the case, then is this why some detectors cannot discriminate big iron?

[Teleno]

As ever you like it. Good luck obtaining correct phase response.

As for steel frequency response, seek a paper "FREQUENCY-DEPENDENCE OF RELATIVE PERMEABILITY IN STEEL" by N. Bowler. That paper alone is enough to sink a lot of PI nonsense related to Fe.

Care to explain in plain language the practical implications in metal detection/discrimination for those of us unable to understand the paper?

Thanks.

[Qiaozhi]

As for steel frequency response, seek a paper "FREQUENCY-DEPENDENCE OF RELATIVE PERMEABILITY IN STEEL" by N. Bowler. That paper alone is enough to sink a lot of PI nonsense related to Fe.

Found it.

[Davor]

BTW, a resistor and capacitor in parallel to a current source = leaking integrator. A coil parallel to a current source = differentiator, and not a target inductance.

[Davor]

As ever you like it. Good luck obtaining correct phase response.

As for steel frequency response, seek a paper "FREQUENCY-DEPENDENCE OF RELATIVE PERMEABILITY IN STEEL" by N. Bowler. That paper alone is enough to sink a lot of PI nonsense related to Fe.

[Qiaozhi]

George, I checked your model, and the ferrous part is wrong. I attached a part of your schematic that needs a correction, and I don't expect it to work properly on the first run, but it will respond in a correct quadrant. Also you should try it in AC simulation prior to attaching it to a narrowband RTX.

I don't understand how that could work, as you've removed the target inductance and replaced the mutual coupling capacitance with a resistor and capacitor in parallel.

This method, that uses a combination of controlled sources, is an overly complex solution which attempts to remove the necessity to define coupling coefficients between the various inductors. The simple reason for this is because LTspice does not support the use of its Chan core model together with mutual inductance. The actual target model is the part in the middle, and the two outer sections define the mutual inductances between the TX and RX loops and the target. Although you can now use the core model with this setup, the results are not correct. I suspect this is because the target inductance is calculated implicitly from the core parameters and the geometry, whereas the mutual inductances are calculated from an explicit target inductance value. Hence I've decided to revert back to the use of coupling coefficients, and somehow defining a model for the ferrous target to give a more realistic magnetic response.

As far as I can see, the simple LR target model works just fine for non-ferrous targets that produce an eddy response.

Also, when I tried to use the overly complex model in a PI simulation, there were some difficulties connecting both ends of the model to the a single TX loop. Therefore it would be much easier if it was possible to use the original setup. The problem is not so much getting the correct phase, but to also produce a decrease in amplitude so that it can simulate correctly in a Tesoro-like detector circuit.

[Davor]

George, I checked your model, and the ferrous part is wrong. I attached a part of your schematic that needs a correction, and I don't expect it to work properly on the first run, but it will respond in a correct quadrant. Also you should try it in AC simulation prior to attaching it to a narrowband RTX.

[Qiaozhi]

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.

Perhaps I should have been more specific when I stated that non-ferrous targets cause an increase, and ferrous targets cause a decrease in amplitude. I was talking within the context of Tesoro-like detectors, such as TGSL and Raptor, where the coil is mis-nulled to one side, and the direction of amplitude shift was with reference to commercial Tesoro coils.

So yes, what you said above is absolutely true. This is also the point I've been trying to get across to coil builders who insist on attempting to create the deepest null possible, and then wonder why they cannot get the coil to ground balance, and discrimination doesn't work.

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.

I'll do some more hacking around with the LTspice core model, and try to create a realistic ferrous target. However, I'm not hopeful based on my previous simulation tests.

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.

OK ... so maybe not all is lost. That's something to look at.

Then there are targets with magnetic remanence, which is yet another mechanism.

I'm not sure how you would detect that, unless it's possible to measure the coercive force required to kill off the residual magnetism. Will have to think about it some more.

[ivconic]

"...Bottle caps are all tin or zinc plated. When those eventually rust, there's no non-ferrous response. But the geometry plays a distinct role..."

Bottle caps are tough to beat trash in the real world searching.
If we speak on common bottle caps produced in Europe.
In most of the cases; rusty or not: those will produce spurious and pretty suspicious response, tending to non-ferrous loookalike.
More rust on it; more "crispy" will be the ends of response. Rise doubts, yet reminds on some other desirable targets.
I've been fighting with this so often on my sites. Deus is having nice little feature "Silencer", adjustable from -1 to 4.
To get more better performances on all other targets; it is desirable to set it on -1. Especially if you search for smallest coins (like i do).
But once you step on "bottle cap field" it is a horror. So than i usually do set it up to 4. It reduces the error in significant percentage.
And now we came to the second part of this; the geometry. It reduces the error... much better if geometry is not perfect circle.
If it is remained circle still; "silencer" is struggling.
And now i would add a third factor too; the orientation and position in soil... of such bottle cap.
If it is leveled and parallel to soil surface; it will produce crispy non ferrous response. If it is positioned in some angle; non ferrous response will move to more accurate ferrous response; as the angle is larger.

Silencer at 4 and bottle cap parallel to soil surface = crispy and "chirpy" non ferrous response.
Silencer at 4 and bottle cap by 90 degree to soil surface = perfectly accurate ferrous response.

And this is only a part of a story. True completely only in case of modern produced bottle caps plated with zinc or other similar material.
Much older bottle caps, produced 40-50 years ago; add to this story more details and aspects.
I could write quite a study only on bottle caps as main subject.
Don't know about other modern md models; but on Deus, Gold Max Power and Gmaxx II there is mentioned "Silencer" option.
Interesting to play with it and it's setups. By my humble opinion it is the one of most important features at those machines.

[Davor]

Bottle caps are all tin or zinc plated. When those eventually rust, there's no non-ferrous response. But the geometry plays a distinct role, and nail-on-end, or pliers-in-hand work ferrous every single time.
For that reason we may only combine different target simulants and observe their contributions to the outcome... a bottle cap may work as both ferrous and a non-ferrous target, and if your detector discriminates, and is fast enough, you'll hear them both as da-deee-da while passing over it, and you'll spare yourself a futile digging.