DEEPER PI DETECTION DEPTH

[Davor]

Aziz, I like what you do, and it is in many ways fundamental. But if you really wish to drive attention, and also keep people on the same track, you could provide some hints as well

Thank you

[Aziz]

Hi all,

anyone thought of what the AC response says to you?
I'm talking about the bode plot in the following post (right side, no modifications):

http://www.geotech1.com/forums/showpost.php?p=143373&postcount=221

Again, don't look at the phase response yet. Don't look at the discrimination ability. Forget the discrimination at all.

We want to focus to the fundamentals only.
Go ahead guys.

Cheers,
Aziz

[Davor]

Yeah, it is a bit frustrating not to see it as an animated powerpoint presentation .

Aziz put a nice LTSpice model with a quoted post, where all the components had regular values. By first glance you'll see nothing much going on, but there is. You may easily play with values and see how the whole shebang gets affected. Everything is in AC (mag/phase) domain.

Now, with receiver signal having a 0 DC, it is obvious that whatever happens in time domain right after a PI pulse, must go through zero, and a whole signal integrated from pulse to pulse is zero. If target's tau is not affected by Tx coil's resistance (you may observe it as a transformer), you'll get a Rx signal that passes through zero with the same delay from PI pulse, regardless of depth/size, hence discrimination.

Attached is a LTSpice simulation of a White's oscillator working as a PI exciter, and a step voltage excitation I was meditating upon. There are ".step param" options to play with, but these should work as they are. Please note how clean the signal gets with stepped exciter.

It is just a thought ...

[simonbaker]

Exactly!
I'm new to this MD stuff, but I have some experience with spice. It dawned to me unfortunately completely independent from this discussion, as it could spare me a lot of time and effort. I played with a Tx of 3900's circuit and to do so I modeled a "coin", yet with values scaled up - works the same. Took me some time to come up with it
Anyway, Rx loop is a LP filter. When its time constant - or better say impedance match is achieved, you'll not lose much in amplitude, but the phase curves become steeper. The peak voltage is much lower (lower high frequency content), but the zero crossings become sensible. I mean REALLY sensible.
To cut the crap short - it is a no-go for high impedance amplifiers.

High series resistance gets coupled with target resistance, and when it is high it completely obliterates phase information. They both act as HP so High Tx series resistance is a bad thing.

With all these filters in a system, a short pulse is a bad thing because it is actually made of two events: rise and fall. Both of them are propagated in time domain and screwing up detection. And it comes with high voltage spikes.

Stepped voltage at Tx produces single events which are more sensible and have neat zero crossing. Because there are no overlapping events it will be a better choice for discrimination.

So, with all due respect to all, good detection/discrimination may begin when phase response is taken care of - both in PI and IB. Tau of various targets is the same for both of them.

I can't quite understand what you are saying.

Can you explain using some formulas/numbers or show your Spice simulations that demonstrate what you are trying to say?

Regards,

-SB

[Davor]

Hi SB,

no I don't say do the FFT on the response signal. I'm just saying look at the AC response in the posting ( http://www.geotech1.com/forums/showp...&postcount=221 ).
I'm just saying to do more AC response spice simulations. It answers some fundamental (but interesting) questions.

So, what's the simple graph is telling us?

Put the following command line somewhere and change the transmitter inductance (don't remove the L=300µ command):

.step param L list 150µ 300µ 600µ

And look what happens.

Report all here, what you have found.

Cheers,
Aziz

Exactly!
I'm new to this MD stuff, but I have some experience with spice. It dawned to me unfortunately completely independent from this discussion, as it could spare me a lot of time and effort. I played with a Tx of 3900's circuit and to do so I modeled a "coin", yet with values scaled up - works the same. Took me some time to come up with it
Anyway, Rx loop is a LP filter. When its time constant - or better say impedance match is achieved, you'll not lose much in amplitude, but the phase curves become steeper. The peak voltage is much lower (lower high frequency content), but the zero crossings become sensible. I mean REALLY sensible.
To cut the crap short - it is a no-go for high impedance amplifiers.

High series resistance gets coupled with target resistance, and when it is high it completely obliterates phase information. They both act as HP so High Tx series resistance is a bad thing.

With all these filters in a system, a short pulse is a bad thing because it is actually made of two events: rise and fall. Both of them are propagated in time domain and screwing up detection. And it comes with high voltage spikes.

Stepped voltage at Tx produces single events which are more sensible and have neat zero crossing. Because there are no overlapping events it will be a better choice for discrimination.

So, with all due respect to all, good detection/discrimination may begin when phase response is taken care of - both in PI and IB. Tau of various targets is the same for both of them.

[Tinkerer]

Can you throw in a 2 x 2" as well?

What depth are you detecting at?

Regards,

-SB

t

I have a 3"x3" aluminum foil and a 3"x3" gold leaf that I commonly use.

The 1/2"x 1/2" is only detected at the center of the coil at 0 level with the 1 meter coil, but I think it will work as the minimum target for that coil and my present preamp breadboard, taking the output of the integrator. Also my TX is at 50% power, just making a quick and dirty trial to decide on a setup to do the series of tests.

Using about 6000 PPS.

Tinkerer

[simonbaker]

I have cut a batch of alu foil targets.
1/2"x1/2" is the smallest that I can detect with the 1meter coil, inside the house.

Then I have 1"x1/2", 2"x1/2", 1"x1", 1"x1"x2, 1"x1"x4, 1"x1"x8 layers.

I am working on it, but time is sparse at the moment so please be patient.

In the meantime it would be nice if somebody would give it a try too, so we could compare results.

Tinkerer

Can you throw in a 2 x 2" as well?

What depth are you detecting at?

Regards,

-SB

[Tinkerer]

Thats a fair point. Even face on a laminated target thick enough for the skin depth to come into play would have reduced losses over an identical solid target. You could call it a 'litz target'. Anway of course this is entirely academic and certainly doesn't invalidates their use in the manner that Carl suggests.

I have cut a batch of alu foil targets.
1/2"x1/2" is the smallest that I can detect with the 1meter coil, inside the house.

Then I have 1"x1/2", 2"x1/2", 1"x1", 1"x1"x2, 1"x1"x4, 1"x1"x8 layers.

I am working on it, but time is sparse at the moment so please be patient.

In the meantime it would be nice if somebody would give it a try too, so we could compare results.

Tinkerer

[Midas]

Midas,

excellent observation.

Now, how does this influence the skin effect?

Tinkerer

Thats a fair point. Even face on a laminated target thick enough for the skin depth to come into play would have reduced losses over an identical solid target. You could call it a 'litz target'. Anway of course this is entirely academic and certainly doesn't invalidates their use in the manner that Carl suggests.

[Midas]

Interesting about the tau from 1.6 to 37us. How does the signal amplitude increase from 1x to 32x? This test target gives indeed valuable information. I will have to make one.

For gold nuggets:
If anyone wants to further the cause, I accept donations of gold nuggets in any size and variation. Any quantity.

Tinkerer

I'm also willing to 'take one for the team' however I'm not as generous as you Tinkerer, I have a 5 nugget limit per person per day, and please don't send me nuggets any bigger than 250kg. I don't have any mechanical lifting equipment and the courier got really annoyed the last time that happened.

[Tinkerer]

As I mentioned before, I use varying stacks of 1" square aluminum foil to create target taus of 1.6us (1x) to 37us (32x). The oxide between layers has no effect, as there is no vertical current flow. This is a nice way to get changes in tau while keeping other variables (metal conductivity, surface area) constant. Try it!

Another fabricated target I use is a solder blob. I slowly add solder until I get the exact weight I want, then reflow the whole blob and let surface tension create a nice consistent hemispherical piece. I have accurate 1-gram and 1-dwt pieces.

If you want to exactly emulate the response of gold nuggets, then use... gold nuggets. Of which I also have a variety.

Interesting about the tau from 1.6 to 37us. How does the signal amplitude increase from 1x to 32x? This test target gives indeed valuable information. I will have to make one.

For gold nuggets:
If anyone wants to further the cause, I accept donations of gold nuggets in any size and variation. Any quantity.

Tinkerer

[simonbaker]

About stacking tight:

This is how transformer iron cores are made. The transformer core is made up with many thin sheets of silicon steel, with a thin dielectric coating between. The coating does not actually isolate the layers, it just makes a bad contact. The reason is to contain the eddy currents within the individual sheets. Sometimes we see such a transformer core welded on one edge. This does not seem to cause a big difference with the eddy currents.

If we compare a tightly stacked pack of aluminium foil, we have a similar occurrence. Aluminium has a thin dielectric oxide coating. Although the aluminium oxide is a very good dielectric, because it is so thin, it gets easily scratched or abraded, for example with the stress of being tightly stacked. But, any contact with air generates a new oxide layer.

So what kind of target do we have with a tight stack of aluminum? We have some continuity, but we also have individually insulated sheets.

Do the eddy currents in this stack add up?
Do the eddy currents partially cancel each other?
Anyway, this is not a usual target, we better use test targets that are more similar to the targets we are searching for.

The targets that we are searching for, have a very great variety. If we can define a few categories and then classify the targets into these categories, we already have taken a great step forward in the endeavor to ID the targets.

So let's look for a answer to a first question: What percentage of the target response is generated by the target surface area.

Tinkerer

Good points. If you can get some data it will be great.

Regards,

-SB

[Carl-NC]

As I mentioned before, I use varying stacks of 1" square aluminum foil to create target taus of 1.6us (1x) to 37us (32x). The oxide between layers has no effect, as there is no vertical current flow. This is a nice way to get changes in tau while keeping other variables (metal conductivity, surface area) constant. Try it!

Another fabricated target I use is a solder blob. I slowly add solder until I get the exact weight I want, then reflow the whole blob and let surface tension create a nice consistent hemispherical piece. I have accurate 1-gram and 1-dwt pieces.

If you want to exactly emulate the response of gold nuggets, then use... gold nuggets. Of which I also have a variety.

[Tinkerer]

I think its all about orientation Tinkerer. Since the magnetic field generated by the coil goes primarily vertically through a target it generates primarily horizontally flowing eddy currents. So a laminated target presented 'face' to the coil is probably much the same as an equivalent solid block. Edge on however I would expect it to be very different.

Midas,

excellent observation.

Now, how does this influence the skin effect?

Tinkerer

[Midas]

About stacking tight:

This is how transformer iron cores are made. The transformer core is made up with many thin sheets of silicon steel, with a thin dielectric coating between. The coating does not actually isolate the layers, it just makes a bad contact. The reason is to contain the eddy currents within the individual sheets. Sometimes we see such a transformer core welded on one edge. This does not seem to cause a big difference with the eddy currents.

If we compare a tightly stacked pack of aluminium foil, we have a similar occurrence. Aluminium has a thin dielectric oxide coating. Although the aluminium oxide is a very good dielectric, because it is so thin, it gets easily scratched or abraded, for example with the stress of being tightly stacked. But, any contact with air generates a new oxide layer.

So what kind of target do we have with a tight stack of aluminum? We have some continuity, but we also have individually insulated sheets.

Do the eddy currents in this stack add up?
Do the eddy currents partially cancel each other?
Anyway, this is not a usual target, we better use test targets that are more similar to the targets we are searching for.

The targets that we are searching for, have a very great variety. If we can define a few categories and then classify the targets into these categories, we already have taken a great step forward in the endeavor to ID the targets.

So let's look for a answer to a first question: What percentage of the target response is generated by the target surface area.

Tinkerer

I think its all about orientation Tinkerer. Since the magnetic field generated by the coil goes primarily vertically through a target it generates primarily horizontally flowing eddy currents. So a laminated target presented 'face' to the coil is probably much the same as an equivalent solid block. Edge on however I would expect it to be very different.