DEEPER PI DETECTION DEPTH

[Davor]

So it seem John left White's. Hopefully for something even better.

[Aziz]

Like all patents, it lists the "inventor" (John Earle) and the "assignee" (White's Electronics).

Great!
I hope, we will see it's successful implementation and (cheap) commercialisation.
Another big and greedy metal detector company needs a small knock-out (competition).

Aziz

[Carl-NC]

I was wondering about the fact, that the mentioned hybrid patent is registered under his own name. No White's mentioned there.

Maybe Carl can say more about this. Carl?

Aziz

Like all patents, it lists the "inventor" (John Earle) and the "assignee" (White's Electronics).

[Tinkerer]

Tinkerer,

the TEM transmitter is the best, versatile, cheapest and simplest TX transmitter. It is even better than the half-sine current type. Trust me.
(And it's prior art. No one can patent it. )

You can energize it with high voltage as well (100 V) in conjuction with higher PPS rate. But you shouldn't let saturate the coil current (t-on << TC of TX) as it gets energy inefficient.


Aziz

I agree, TEM is the best. The Truncated half sine gives some advantage for targets of very short TC, but gives less signal response for the same power consumption.

There is a modality using an IGBT instead of a Mosfet, that gives quite different results. I have not fully investigated that.

Tinkerer

[Aziz]

Here is a concrete proposal for TX parameters, that can be switched for a large coil of 1 meter diameter and a smaller coil of 0.5m diameter. It is the best I can come up with.

If you find something better, I would like to try it.
...

Tinkerer,

the TEM transmitter is the best, versatile, cheapest and simplest TX transmitter. It is even better than the half-sine current type. Trust me.
(And it's prior art. No one can patent it. )

You can energize it with high voltage as well (100 V) in conjuction with higher PPS rate. But you shouldn't let saturate the coil current (t-on << TC of TX) as it gets energy inefficient.


Aziz

[Aziz]

Left for coffee or left for good? The former would be much better for Whites.

I was wondering about the fact, that the mentioned hybrid patent is registered under his own name. No White's mentioned there.

Maybe Carl can say more about this. Carl?

Aziz

[Davor]

Left for coffee or left for good? The former would be much better for Whites.

[Carl-NC]

So you ARE impersonating Whites here. Good to know - I'm still a rookie here. Please pay John my respects, I like what he does.

Yup, I work for White's... no secret. It was agreed when I joined White's that I could continue running Geotech with no interference. Ergo the neutral nature of the forums.

John just left my office. He's a clever engineer, and an interesting person.

[Davor]

I'm glad you like it, so do I! We call this "truncated half-sine." Yes, it really works.

So you ARE impersonating Whites here. Good to know - I'm still a rookie here. Please pay John my respects, I like what he does.

[Davor]

Hi Tinkerer,

Yeah, spice gets confused when same inductances appear twice in two K statements, but it is usually due to some mistake on our own side. I'll check your circuit tomorrow, now I'll hit the hay.

[Tinkerer]

Hi Tinkerer:

I would specify your mutual inductances just two at a time, because specifying more at once causes every combination of two to be specified, and some don't really apply.

You don't really need coupling from target to target, for example, although it's so small probably won't hurt.

Also, you don't want to specify .0001 between your two RX coils -- I think your two Rx coils representing a center-tapped coil probably need to be coupled with a coefficient close to 1 (or -1), in which case you should end up with an inductance approx equal to 1000u combined (is that what you want?).

Specifying the separate coupling between the Tx and Rx coils -- oooh, don't make my brain hurt. The phasing may require a negative coupling for one of them.

I'm not sure about the bucking coil -- seems you need to couple that to the RX coils also, although I'm not sure what physical configuration you are modeling -- concentric coils??? Maybe you should just assume the bucking coil works perfectly and omit all the couplings between the TX coils and the RX coils?

But I think specifying only pair at a time is the way to go.

Regards,

-SB

Thanks SB,

This is for a concentric coaxial IB coil. The coil works great, but I would like to be able to simulate it, because it is easier to simulate many variations than building many coils.

The RX coils are center tapped, but I just realize that the phase dots are wrong on the circuit drawing. The coils are of the same phase or just one coil, center tapped. The RX coils are wound on the BU coil. Coupling near 1 between the 3 coils.
The BU coil is counter phase of the TX coil, but the coupling is about 0.25.

The RX signal is received by the RX, BU and TX coils. Due to the coupling and precise balancing, the R signal and the X signal are of opposite phase at the output of the RX coil.

Easy FE discrimination.

Tinkerer

[simonbaker]

For illustrative purposes, let's make a gross simplification:

Response "conductivity" is entirely due to the target metal type
Response "inductivity" is entirely due to the target thickness
Response "strength" is entirely due to target surface area

Ergo, a silver dollar and a silver half-dime have identical conductivity, but the silver dollar has a much higher inductivity (and therefore higher tau) and a much higher signal strength.

A silver half-dollar and an English penny have about the same thickness and same surface area, therefore will have the same inductivity and signal strength, but the half-dollar has a little higher conductivity and therefore a higher tau.

Again, this is a gross simplification and things are more complicated than this. But folks need to get away from thinking that conductivity is the major factor in tau variations. It is not. Target thickness is a far bigger factor.

How does thickness affect tau? Consider a small thin ring of metal; the incident TX magnetic field produces a primary eddy current in the ring, which creates a counter-magnetic field. OK, now consider a second identical thin ring that is right behind the first. The TX magnetic field also produces a (smaller) primary eddy current in the second ring, but the counter-magnetic field of the first ring also induces a counter-eddy current in the second ring. Ergo, the first ring creates a "drag" effect in the second ring. Likewise, the second ring induces a (smaller) drag on the first ring. Consider yet a third ring; the process continues, with the third ring having even more drag effect (and a smaller overall eddy current) than the second ring.

You can think of thick targets in the same way. The surface eddies are the strongest, but as you go deeper into the metal the eddies are reduced due to ever-increasing drag of the distributed counter-magnetic effect. The point where the eddies are reduced to 37% of the surface value is called the "skin depth."

At some depth, the eddies will be reduced to near-zero, and at the back side of the target there will be no more magnetic field. In this case, the target looks like a mirror and the resulting counter-magnetic field is about 180-degrees out-of-phase with the TX field. If you reach the back side of the target before the eddies die out, some of the TX magnetic field will appear to have "shown through" the target, as if it is (optically) translucent. In this case, you will have less than 180 degrees of phase shift, depending on the relative depth.

Did I mention all of this is dependent on frequency/slew rate? And it all works, whether you use a sine wave, a square wave, or a pulse. It doesn't matter. However, if you wait to look at the target right after you click off the flashlight then the target is harder to see. But when the flashlight is on, and it is a xenon bulb shining in your eyes, the target is also hard to see.

- Carl

Thanks, that is interesting.

For comparison, I looked at some formulas for a single loop coil -- they are confusing but it seems that most of the formulas suggest that inductance of a single loop scales linearly with diameter. Since the circumference is also proportional to diameter, and resistance should be proportional to circumference, resistance should also scale linearly with diameter.

This implies that L/R is constant for all different sized loops of a given conductivity -- which also means if you're looking for rings, maybe tau will actually help discriminate rings of different metals.

However, the thickness of the wire is also a factor (maybe a log function???), so there is still room for confusion.

This perhaps corroborates Carl's statement that differences in inductance are due to thickness, since simple eddy rings of the same thickness would seem to have the same tau regardless of diameter (or target size), but stacking such rings would change the relationship.

BTW: does conductivity affect skin depth?

-SB

[simonbaker]

Could somebody help me getting the K Spice directive right?

The circuit is for the IB coil configuration. K1 L1 L3 0.25 is about right. But I can not get all the other couplings to work at the same time. K2, K3, K4,

Tinkerer

Hi Tinkerer:

I would specify your mutual inductances just two at a time, because specifying more at once causes every combination of two to be specified, and some don't really apply.

You don't really need coupling from target to target, for example, although it's so small probably won't hurt.

Also, you don't want to specify .0001 between your two RX coils -- I think your two Rx coils representing a center-tapped coil probably need to be coupled with a coefficient close to 1 (or -1), in which case you should end up with an inductance approx equal to 1000u combined (is that what you want?).

Specifying the separate coupling between the Tx and Rx coils -- oooh, don't make my brain hurt. The phasing may require a negative coupling for one of them.

I'm not sure about the bucking coil -- seems you need to couple that to the RX coils also, although I'm not sure what physical configuration you are modeling -- concentric coils??? Maybe you should just assume the bucking coil works perfectly and omit all the couplings between the TX coils and the RX coils?

But I think specifying only pair at a time is the way to go.

Regards,

-SB

[Carl-NC]

Gee, they made it with complementary transformer gated mosfets and a complementary power supply, EXCELLENT WORK!!!
To make it a power saving PI they use complementary pulses too - brilliant!

OK, they in essence use Current maximum/voltage zero to discriminate ground in IB style, and target detection in PI style, and all of it fully symmetricaly due to the complementary pulses. The current pulse is shortened to gain a voltage spike - something that my exciter lacks. (I'll make it better though...)

They also completely given up monocoil. So basically this design uses IB only for GB, and in a most logical way: at zero crossing.

Complementary pulses are a bonus in case of demining because of the potential triggering of magnetic fuses with pulses of single polarity.

Shortly, I'm impressed. A large smile is flying around my head

(it would be difficult to beat this, but I have some ideas)

I'm glad you like it, so do I! We call this "truncated half-sine." Yes, it really works.

[Carl-NC]

In other words, to me it looks like there is a disconnect between the statements that a) targets have 3 to 4 orders of magnitude tau range and b) that tau roughly corresponds to metal conductivity and can be used for discrimination.

For illustrative purposes, let's make a gross simplification:

Response "conductivity" is entirely due to the target metal type
Response "inductivity" is entirely due to the target thickness
Response "strength" is entirely due to target surface area

Ergo, a silver dollar and a silver half-dime have identical conductivity, but the silver dollar has a much higher inductivity (and therefore higher tau) and a much higher signal strength.

A silver half-dollar and an English penny have about the same thickness and same surface area, therefore will have the same inductivity and signal strength, but the half-dollar has a little higher conductivity and therefore a higher tau.

Again, this is a gross simplification and things are more complicated than this. But folks need to get away from thinking that conductivity is the major factor in tau variations. It is not. Target thickness is a far bigger factor.

How does thickness affect tau? Consider a small thin ring of metal; the incident TX magnetic field produces a primary eddy current in the ring, which creates a counter-magnetic field. OK, now consider a second identical thin ring that is right behind the first. The TX magnetic field also produces a (smaller) primary eddy current in the second ring, but the counter-magnetic field of the first ring also induces a counter-eddy current in the second ring. Ergo, the first ring creates a "drag" effect in the second ring. Likewise, the second ring induces a (smaller) drag on the first ring. Consider yet a third ring; the process continues, with the third ring having even more drag effect (and a smaller overall eddy current) than the second ring.

You can think of thick targets in the same way. The surface eddies are the strongest, but as you go deeper into the metal the eddies are reduced due to ever-increasing drag of the distributed counter-magnetic effect. The point where the eddies are reduced to 37% of the surface value is called the "skin depth."

At some depth, the eddies will be reduced to near-zero, and at the back side of the target there will be no more magnetic field. In this case, the target looks like a mirror and the resulting counter-magnetic field is about 180-degrees out-of-phase with the TX field. If you reach the back side of the target before the eddies die out, some of the TX magnetic field will appear to have "shown through" the target, as if it is (optically) translucent. In this case, you will have less than 180 degrees of phase shift, depending on the relative depth.

Did I mention all of this is dependent on frequency/slew rate? And it all works, whether you use a sine wave, a square wave, or a pulse. It doesn't matter. However, if you wait to look at the target right after you click off the flashlight then the target is harder to see. But when the flashlight is on, and it is a xenon bulb shining in your eyes, the target is also hard to see.

- Carl