Let's made a PC-base metal detector with usb interface !!!

[Aziz]

I'll be interested in how you do with the coaxial coils, even though they do not seem novel to me. I saw a photo once on the web (I can't locate it now) of an MD with coaxials coils and the guy hunting near a fence under some power lines with a claim it reduced EMI. There have been some discussions on this site also of the idea, and I started experimenting with some EMI cancelling dual coil designs, first starting with co-planar because much easier to stabilize, but with intent to stack them coaxial also. The theory makes sense so let's see how practical they are to build and use (weight, vibration, etc.).

Note: need for more preamp gain suggests loss of S/N advantage...???

A long time ago I did a thumbnail analysis that suggested that such coils are worth it if lots of EMI around, but otherwise not. So useful in my neighborhood, yes!

One benefit of the top-hat design is that if there is more trash than treasure around, we can get part-time jobs filling the hat with gum wrappers and tin-foil and make an important contribution to our communities as trash pickers...

Cheers!

-SB

Hi SB,

co-axial coils aren't novel. We have discussed them here too:
http://www.geotech1.com/forums/showthread.php?t=17929

See here too: (CONCENTRIC SEARCH COIL DESIGN - Thread)
http://www.geotech1.com/forums/showt...448#post130448

But this EMI cancelling IB coil is novel and hasn't been invented yet.
(I want to see it if there is a prior art. That would prevent my patent troll friends from stealing it.)


Co-axial coils are very easy to build and balance. You only have to find the number of turns for TX-Bucking coil and the distance of the coil bundle RX-/TX-Bucking coil to the main TX coil. There are many possibilities to induction balance (one more: distance of RX- and TX-Bucking for instance).

Coil vibration isn't an issue to me ("MadLabs Inc." new feature(TM)(R)(C) ) and I can operate even with very large IB coils (very deep seeking IB coils). Weight doesn't matter for large coils (I'll pull/push the frame coil/trolley).

But EMI noise is a big issue with large coils.

Cheers,
Aziz

[simonbaker]

Hi SB,

co-axial coils aren't novel. We have discussed them here too:
http://www.geotech1.com/forums/showthread.php?t=17929

See here too: (CONCENTRIC SEARCH COIL DESIGN - Thread)
http://www.geotech1.com/forums/showt...448#post130448

But this EMI cancelling IB coil is novel and hasn't been invented yet.
(I want to see it if there is a prior art. That would prevent my patent troll friends from stealing it.)


Co-axial coils are very easy to build and balance. You only have to find the number of turns for TX-Bucking coil and the distance of the coil bundle RX-/TX-Bucking coil to the main TX coil. There are many possibilities to induction balance (one more: distance of RX- and TX-Bucking for instance).

Coil vibration isn't an issue to me ("MadLabs Inc." new feature(TM)(R)(C) ) and I can operate even with very large IB coils (very deep seeking IB coils). Weight doesn't matter for large coils (I'll pull/push the frame coil/trolley).

But EMI noise is a big issue with large coils.

Cheers,
Aziz

Ok, I always thought the main point with coaxial coils was for EMI suppression, especially when coils separated by large distance... at least that's how I viewed them, where both RX coils have similar EMI reception but different target reception, so you can cancel (or solve for) the EMI and leave some fraction of the target (hopefully large). Perhaps your use of bucking coils is new, I don't know. But go for it! Of course really small wavelengths won't cancel and you'd need special math to take it out, but for VLF and PI hopefully we don't see that really high stuff anyway.

Hat's off to you!

-SB

[Aziz]

Aziz's Quiz Show

(Don't blame me for this game. But I can't resist. )

What is better (more sensitive, more depth)?

Two magic cylinder coils:
I'll relate to mean coil radius units (1 R = 1 coil radius unit) and will put the distance of RX+/RX- on both configurations at 2 R (one diameter unit). Both coil configurations have the same total outer diameter (2 R) and equivalent coil inductivities.

Coil 1:
TX: 1 R
RX+: 0.8 R
RX-: 0.8 R
TX-Bucking: 0.8 R

Bigger TX, outsize
Smaller RX+/RX-/TX-Bucking, inside

Coil 2:
TX: 0.8 R
RX+: 1 R
RX-: 1 R
TX-Bucking: 1 R

Smaller TX, inside
Bigger RX+/RX-/TX-Bucking, outside


There is a minor performance difference of approx. 15%. Which coil configuration does it better?

Aziz

[Midas]

(Don't blame me for this game. But I can't resist. )

What is better (more sensitive, more depth)?

Two magic cylinder coils:
I'll relate to mean coil radius units (1 R = 1 coil radius unit) and will put the distance of RX+/RX- on both configurations at 2 R (one diameter unit). Both coil configurations have the same total outer diameter (2 R) and equivalent coil inductivities.

Coil 1:
TX: 1 R
RX+: 0.8 R
RX-: 0.8 R
TX-Bucking: 0.8 R

Bigger TX, outsize
Smaller RX+/RX-/TX-Bucking, inside

Coil 2:
TX: 0.8 R
RX+: 1 R
RX-: 1 R
TX-Bucking: 1 R

Smaller TX, inside
Bigger RX+/RX-/TX-Bucking, outside


There is a minor performance difference of approx. 15%. Which coil configuration does it better?

Aziz

*hand up*
Sir, you've left the answers on the black board! (I bet that's going to get me beaten up after class )

Coil number 2, but I have to admit it's a surprising result considering pretty much every concentric design I've seen has it the other way round. I can't see whats different about the THC that changes that aspect. Is it an all round performance increase or does it perhaps perform worse on small close targets?

Midas

[Aziz]

*hand up*
Sir, you've left the answers on the black board! (I bet that's going to get me beaten up after class )

Coil number 2, but I have to admit it's a surprising result considering pretty much every concentric design I've seen has it the other way round. I can't see whats different about the THC that changes that aspect. Is it an all round performance increase or does it perhaps perform worse on small close targets?

Midas

Hi Midas,

yep, coil number 2 performs slightly better even the TX coil is smaller. Both perform equivalent in the near detection region. But the coil number 2 produces approx. 15% more signal response at more depth. But signal doesn't mean depth!

This is due to slightly reduced turns count of the TX-Bucking coil and the increased RX flux area. BTW, the TX-Bucking coil doesn't counter the main TX magnetic field in contrast to the other concentric co-planar IB coils. It is supporting it.

I'm still wondering, why this great invention has been missed till now?
Anyway, it's free available to everyone now.

Cheers,
Aziz

[Midas]

Hi Midas,

yep, coil number 2 performs slightly better even the TX coil is smaller. Both perform equivalent in the near detection region. But the coil number 2 produces approx. 15% more signal response at more depth. But signal doesn't mean depth!

This is due to slightly reduced turns count of the TX-Bucking coil and the increased RX flux area. BTW, the TX-Bucking coil doesn't counter the main TX magnetic field in contrast to the other concentric co-planar IB coils. It is supporting it.

I'm still wondering, why this great invention has been missed till now?
Anyway, it's free available to everyone now.

Cheers,
Aziz

Hmmm but a reduced turn count bucking coil and increased RX flux area would also be true of a concentric with a bigger than TX, RX, what happens when you simulate such a coil, does that also show the same benefit?

I realize the bucking coil has the same polarity as the main TX in the THC but I still don't really see why that makes bigger than TX, RX more viable.

Midas

[Mechanic]

Hi Aziz,

That is a very interesting observation you have made. I can see fairly much how this works with larger targets at depth, but what about the little tiddlers? Coil 1 would be more suited to the smaller targets would it not?

Also I have been toying with some free oscilloscope/ spectrum analyzer software. "Visual analyzer" seems to be the best I have found so far, but when I run the time domain scope I just don't see what I should be seeing. My soundcard is 24bit 196khz supposedly. Has anybody found any decent laptop soundcard scope software that agrees with what you see on a normal scope?

Cheers Mick

[WM6]

Has anybody found any decent laptop soundcard scope software that agrees with what you see on a normal scope?

Cheers Mick

Hi Mick

If you need scope only, without spectrum analyzer, then try this:

http://www.zeitnitz.de/Christian/scope_en

As you can see here, it is working pretty well:

http://hobidetektori.serbianforum.in...770/u2pin6.jpg

[simonbaker]

Hi Midas,

yep, coil number 2 performs slightly better even the TX coil is smaller. Both perform equivalent in the near detection region. But the coil number 2 produces approx. 15% more signal response at more depth. But signal doesn't mean depth!

This is due to slightly reduced turns count of the TX-Bucking coil and the increased RX flux area. BTW, the TX-Bucking coil doesn't counter the main TX magnetic field in contrast to the other concentric co-planar IB coils. It is supporting it.

I'm still wondering, why this great invention has been missed till now?
Anyway, it's free available to everyone now.

Cheers,
Aziz

Hi Aziz:

How do you define "performance"? Can you show the calculations that give the 15% difference?

-SB

[Aziz]

Hi Aziz:

How do you define "performance"? Can you show the calculations that give the 15% difference?

-SB

Hi SB,

performance is, what the receive coil detects: signal strength (secondary magnetic field induction caused by the target only). If you compare two configurations, you can set the received voltages in relation.

But the performance improvement isn't significant enough. It is slowly increasing. The 15% is only at extreme depth distances significant. You won't be able to measure it as you will end up at the noise floor level before you can probably detect the faint difference.

Even a 50% signal strength improvement doesn't mean a 50% more detection depth improvement. The detection depth is a logarithmic relationship of the signal strength.

In practice, one would say, both configurations do equal.

Aziz

[Aziz]

Hi Aziz,

That is a very interesting observation you have made. I can see fairly much how this works with larger targets at depth, but what about the little tiddlers? Coil 1 would be more suited to the smaller targets would it not?

Also I have been toying with some free oscilloscope/ spectrum analyzer software. "Visual analyzer" seems to be the best I have found so far, but when I run the time domain scope I just don't see what I should be seeing. My soundcard is 24bit 196khz supposedly. Has anybody found any decent laptop soundcard scope software that agrees with what you see on a normal scope?

Cheers Mick

Hi Mick,

I personally prefer the coil 1 option (bigger TX, smaller RX bundle). The reason for this is simple: on highly mineralized ground, one could use a larger TX and limit the ground noise with smaller RX. The RX size would be as much as the ground conditions allow. But the TX coil isn't limitted and can be much larger if you want.

To detect smaller targets, you need more magnetic field strength at the target position(hence smaller TX coils).

Sound card issue:
1. Note, that the sound card has a low-pass (LP) filter on its inputs.
2. The LP frequency might be set too low (for instance 20 kHz) and you can't build the signal with the missing high frequency components. The scope has a much much more bandwidth.
3. You can't measure DC voltages - only AC voltages (>30 Hz or so).
4. Not every sound card software can drive the high sampling rate or high resolution. It requires an extended WDM (Windows Driver Model) interfacing.

I don't use other sound card software. I'm using my own software.

Cheers,
Aziz

[Aziz]

Hmmm but a reduced turn count bucking coil and increased RX flux area would also be true of a concentric with a bigger than TX, RX, what happens when you simulate such a coil, does that also show the same benefit?

I realize the bucking coil has the same polarity as the main TX in the THC but I still don't really see why that makes bigger than TX, RX more viable.

Midas

Hi Midas,

I don't understand your described coil configuration. You have to be more precise. (A software guy needs an exact specification.)
Use the coil radius units (e.g. 1R, 0.8R, 1.2R), coil names (TX, RX+, RX-, TX-Bucking, and so on) and their vertical positions (z=0, z=10 inch, and so on).

Once I have fully understood, I can make an analysis and a comparison.

Cheers,
Aziz

[Midas]

Hi Midas,

I don't understand your described coil configuration. You have to be more precise. (A software guy needs an exact specification.)
Use the coil radius units (e.g. 1R, 0.8R, 1.2R), coil names (TX, RX+, RX-, TX-Bucking, and so on) and their vertical positions (z=0, z=10 inch, and so on).

Once I have fully understood, I can make an analysis and a comparison.

Cheers,
Aziz

The comparison I'm after is between two concentric designs:
Standard smaller RX coil design, TX=1R, RX/BU=0.8R.
Vs Larger RX coil, TX=0.8R, RX/BU=1R.

Cheers
Midas

[Aziz]

The comparison I'm after is between two concentric designs:
Standard smaller RX coil design, TX=1R, RX/BU=0.8R.
Vs Larger RX coil, TX=0.8R, RX/BU=1R.

Cheers
Midas

Ok Midas, I have understood you now.
I don't know, whether your proposed coil design is possible. But I'll try some configurations and will let you know.

Cheers,
Aziz

[Aziz]

Concentric Co-Planar Coil Comparison

Hi Midas and et.al.,

your suggested concentric co-planar IB-coil is in principle possible. But that is the only good news.

Standard concentric co-planar IB-coil:
TX: 1R
RX/TX-Bucking: 0.8R (4 mm z-distance between them)

Modified concentric co-planar IB-coil:
TX: 0.8R
RX/TX-Bucking: 1R (4 mm z-distance between them)

The modified concentric co-planar IB-coil produces a strong signal at the very near detection distance region (good for pin-pointing). The "benefit" is diminishing quickly and is overtaken by the standard coil variant. At far detection distance regions, the standard coil variant produces twice more target response signal.
So the modified coil variant isn't performing good.

Aziz