northwest DOD coils vs DD coil? (The Real Thread)
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australian electronic gold prospecting forum.com  |  Detector Technology and Electronics and new detectors  |  Detector Coils  |  Topic: DOD coils vs DD coil? (The Real Thread) 0 Members and 1 Guest are viewing this topic. « previous next »
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Author Topic: DOD coils vs DD coil? (The Real Thread)  (Read 3478 times)
Aziz
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« on: Saturday December 17 2011 17:54:31 AEDT PM »

Hi all,

this is the real thread "DOD coils vs DD coil". I'll put the relevant posts from the other thread together:

  
DOD coils vs DD coil?
What are the pro's and cons of each coil type? Anyone made one and used it on GB Pi?
doug smile

  
  
Hi all,
I presume, that the DOD coil is in anti-interference configuration (D=RX+, O=TX, D=RX-). Doug, could you please specifiy the configuration of the DOD?
 smile
Aziz

Yes that's the one! happy face
doug smile



To be continued...
Aziz
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« Reply #1 on: Saturday December 17 2011 18:54:19 AEDT PM »

I just want to add some comments:

The specification for the coil comparison isn't finished yet. This will take some time during the discussions until I know it exactly. Otherwise, it would cause an immense work to cover all the possibilities.

I think, the very interesting questions are as follows:
Will the DOD coil perform better than the DD coil?
In what case and operating conditions?

As both coil types differ so much in coil geometrics, the coil size differs and makes the comparison difficult. If each coil windings gets smaller to fit into a given coil housing, then the performance will have an huge impact on the detection depth of course!

But the RX+, RX- coils require as much distance difference to a target as possible. Otherwise, you can forget the depth peformance.

So let's talk about and refine the specifications before I can make a numerical EM analysis.
 smile
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« Reply #2 on: Saturday December 17 2011 20:00:17 AEDT PM »

G'day Aziz,

My thoughts are(thoughts only!) that it will be good on uniform ground, but I suspect when you are on variable ground there will be a false response every time one half of the coil crosses the variation. I also suspect that it will be more sensitive to hotrocks going by Mikebgs' own admission on geotech a few months back.

Cheers Mick
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Aziz
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« Reply #3 on: Saturday December 17 2011 20:15:35 AEDT PM »

Hi Mick,

well, the coil form (geometrics) isn't specified yet. We can not say anything at this stage, whether it will perform good or not.

I used to be stupid and having no background information when there is no exact specification.

So I ask:
Is the RX+ coil inductively balanced to the TX coil (coil coupling coefficient 0, same applies to RX- coil to TX coil)? If so, the coil size gets bigger (wider).

Or is the RX+/RX- half of the TX coil sharing the same center position? You know, this isn't good for depth detection performance. Both RX coil halves aren't spaced much enough. But this coil type is sharing the same coil housing and coil size is equivalent.

There is a difference in TX coil sizes:
In the DOD configuration, the TX coil is bigger (round). In the DD it's smaller (D-shape).

So many questions yet.

Aziz
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« Reply #4 on: Saturday December 17 2011 22:04:24 AEDT PM »

G'day Aziz,

I suspect that Doug saw this post here :   

Cheers Mick

Ps Doug, just about every time I try and post something I get an error FURIOUS good luck
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« Reply #5 on: Saturday December 17 2011 22:08:37 AEDT PM »

  
G'day Aziz,

I suspect that Doug saw this post here :   

Cheers Mick

Ps Doug, just about every time I try and post something I get an error FURIOUS good luck

You are triggering the bad behavior plugin.I suspect its your ISP server configuration. I don't want to turn it off because its trapping spambots that slip through Incapsula.
doug smile
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« Reply #6 on: Saturday December 17 2011 22:14:21 AEDT PM »

I will go sit in the naughty corner then!
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« Reply #7 on: Saturday December 17 2011 22:21:58 AEDT PM »

  
I will go sit in the naughty corner then!

Other than turning it off there is nothing i can do to fix the problem. sad1 At the time it was blocking you a German spambot made 6 attempts to  register and then post on the forum!!! Maybe i have to turn it off when you are online?
doug smile
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« Reply #8 on: Saturday December 17 2011 22:32:44 AEDT PM »

  
G'day Aziz,

I suspect that Doug saw this post here :   

Cheers Mick

Ps Doug, just about every time I try and post something I get an error FURIOUS good luck

Thanks, this clarifies a lot.

Doug, can you confirm this DOD coil type?

I think, it would be better to make a rectangular type for comparison. It would make the work easier to me. And we can focus to the same size coil housings.

Cheers,
Aziz
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« Reply #9 on: Saturday December 17 2011 22:40:03 AEDT PM »

  
  
G'day Aziz,

I suspect that Doug saw this post here :   

Cheers Mick

Ps Doug, just about every time I try and post something I get an error FURIOUS good luck

Thanks, this clarifies a lot.

Doug, can you confirm this DOD coil type?

I think, it would be better to make a rectangular type for comparison. It would make the work easier to me. And we can focus to the same size coil housings.

Cheers,
Aziz

Yes can confirm that is the DOD coil happy face A rectangular DOD is fine if it makes calculations easier! happy face
doug smile
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« Reply #10 on: Saturday December 17 2011 23:36:31 AEDT PM »

  
Yes can confirm that is the DOD coil happy face A rectangular DOD is fine if it makes calculations easier! happy face
doug smile

That's fine. Makes my life alot easier.  smile

As the physics law predict (without making any calculations yet), the RX+ and RX- coil parts aren't much inductively coupled together (something in the 0.1xxx .. 0.0xxx range). If I'm going to make the comparison for a given RX coil inductivity, the turns count of the RX+/RX- coil in the DOD configuration will be half of the normal DD RX coil (DD RX coil not splitted into two halves in this case).

According to physics law:

L = L1 + L2 + 2*k*sqrt(L1*L2),  where
L1 = inductivity of RX+ coil,
L2 = inductivity of RX- coil,
k = coil coupling coefficient between L1 and L2 (almost 0 here),
L = total RX coil inductivity

At the end, we have nearly L=L1+L2 and if L1 and L2 are identical, the total inductivity L is splitted into two equal halves.

The comparison of the DD RX vs. DOD RX+/RX- coil would be very unfair of course (RX+ and RX- would have less windings turns count compared to the DD RX coil). The DOD RX coil configuration needs compensation for this disadvantage (higher gain required). Well, the disadvantage brings more ground coverage and two hot spots (high sensitivity regions), in which is a benefit of course. If you miss the target in the first hot spot, you may get the signal in the second one.

Unfortunately, the center of the coil does not detect anything (RX+ and RX- coils induce the same amount of the response signal with different polarity, which adds to 0)! This is a real disadvantage.

The anti-interference configuration of the RX+/RX- coils will reduce the EMI by 20 .. 30 dB at least (that's a factor of 10 .. to 31.6) and the gain compensation can be applied easily. We assume, that the amplifier does not add much noise to the signal path (noise-less gain).

Ok so far?

Aziz

PS: added more facts
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« Reply #11 on: Sunday December 18 2011 22:27:11 AEDT PM »

Hi all,

I want to make the comparisons of the following coils:
- a rectangular mono coil
- a rectangular DD coil
- a rectangular DOD coil

so all the coils should cover the same quadratic surface area (sharing the same coil housing). No compensation will be applied to the coil configurations. In the logarithmic view of the comparison, you only have to add some constant value to apply a magnification to the response signal (which is equivalent to add more gain to the RX circuit).

Unfortunately, it causes a lot of work and it takes some time. But you should know, that this is #1 in my priority list (Oz hotrocks have to wait  sad1).

Stay tuned..

Cheers,
 smile
Aziz
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« Reply #12 on: Monday December 19 2011 21:32:05 AEDT PM »

Hi all,

all three coil models are designed. The simpler ones have been already calculated. I'm currently working on the DOD configuration, which requires much much more calculations. Unfortunately, the hotspot region (max. sensitivity) is non-linear and I have to find the max. response for a given depth. So I can't simply sweep the target in the vertical position only. That's the reason, why this coil causes so much work.

The sketch is showing the problem. Please be patient.

To be continued..
Aziz


* DOD-1.jpg (91.83 KB, 800x600 - viewed 295 times.)
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« Reply #13 on: Monday December 19 2011 21:37:37 AEDT PM »

  
Hi all,

all three coil models are designed. The simpler ones have been already calculated. I'm currently working on the DOD configuration, which requires much much more calculations. Unfortunately, the hotspot region (max. sensitivity) is non-linear and I have to find the max. response for a given depth. So I can't simply sweep the target in the vertical position only. That's the reason, why this coil causes so much work.

The sketch is showing the problem. Please be patient.

To be continued..
Aziz

Why is the hot spot region non linear?
doug smile
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Aziz
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« Reply #14 on: Monday December 19 2011 21:50:58 AEDT PM »

  
  
Hi all,

all three coil models are designed. The simpler ones have been already calculated. I'm currently working on the DOD configuration, which requires much much more calculations. Unfortunately, the hotspot region (max. sensitivity) is non-linear and I have to find the max. response for a given depth. So I can't simply sweep the target in the vertical position only. That's the reason, why this coil causes so much work.

The sketch is showing the problem. Please be patient.

To be continued..
Aziz

Why is the hot spot region non linear?
doug smile

That's simple. On the center position of the coil configuration, both RX coils (RX+/RX-) induce the same target response and hence subtracting it to 0.

The more you go left (or right) there will be a distance difference to the RX+/RX- coils, which will be seen a signal response difference in the RX coil finally.

The magnetic field strength diminishes, the more you go left (or right) or the more you put the target heigher (or deeper).

So I have to find out the maximum response for a given depth to be able to compare against the other coil configurations.

Aziz
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« Reply #15 on: Tuesday December 20 2011 09:45:03 AEDT AM »

Hi friends,

I'm tired. 
give me beer

Ok, I have compared same sized 10" x 10" rectangular coils with same inductivity (RX=TX=300µH):
- mono coil (as a reference)
- DD coil (Double-D)
- DOD coil (D-O-D in anti-interference RX coil configuration (RX+/RX-) )

The TX coil of the DOD configuration is the same of the DD TX (fair comparison). To have some reasonable figures, I have taken a high frequency induction. So don't look at the absolute induced target response voltage. Look at the relative comparison of the coils. I also have put three additional DOD configurations with gain cranked up with +20 dB, +30 dB and +40 dB. You know, the interference is cancelled in the anti-interference coil configuration and we can crank up the gain in the RX stage (compensation).

Below is the comparison graph. You can see, that we can outperform a DD coil with +20 dB additional RX gain (which should easily be achievable with EMI rejection). Without gain compensation, the DOD coil configuration performs very bad (yellow).

Now you can put your comments and questions..

Cheers,
 smile
Aziz


* CoilComparison-DDvsDOD.gif (20.65 KB, 966x556 - viewed 288 times.)
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« Reply #16 on: Tuesday December 20 2011 10:11:09 AEDT AM »

  
Hi friends,

I'm tired. 
give me beer

Ok, I have compared same sized 10" x 10" rectangular coils with same inductivity (RX=TX=300µH):
- mono coil (as a reference)
- DD coil (Double-D)
- DOD coil (D-O-D in anti-interference RX coil configuration (RX+/RX-) )

The TX coil of the DOD configuration is the same of the DD TX (fair comparison). To have some reasonable figures, I have taken a high frequency induction. So don't look at the absolute induced target response voltage. Look at the relative comparison of the coils. I also have put three additional DOD configurations with gain cranked up with +20 dB, +30 dB and +40 dB. You know, the interference is cancelled in the anti-interference coil configuration and we can crank up the gain in the RX stage (compensation).

Below is the comparison graph. You can see, that we can outperform a DD coil with +20 dB additional RX gain (which should easily be achievable with EMI rejection). Without gain compensation, the DOD coil configuration performs very bad (yellow).

Now you can put your comments and questions..

Cheers,
 smile
Aziz


thanks for your work Aziz. happy face
doug smile
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« Reply #17 on: Tuesday December 20 2011 16:10:25 AEDT PM »

Hi Aziz,

What happens if the DOD RX coils are in phase but induction ballanced and the combined inductance of the RX coils is around 450 uh like a standard DD is or even both at 300 uh giving a total of 600 uh.

Regards, Ian.
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« Reply #18 on: Tuesday December 20 2011 17:52:51 AEDT PM »

  
Hi Aziz,

What happens if the DOD RX coils are in phase but induction ballanced and the combined inductance of the RX coils is around 450 uh like a standard DD is or even both at 300 uh giving a total of 600 uh.

Regards, Ian.

Hi Ian,

in this case, there will only be one hotspot (at the coils center axis) and the calculation is getting easy (already running). Due to more overlapping coil area, the total RX coil flux area gets slightly bigger (more efficient than a DD-IB coil).

If you double the total RX inductance, the response signal is amplified by 1.41 (sqrt(2)). You also amplify the induced EMI noise by the same amount however. Whereas in the anti-interference coil configuration (anti-phase RX coils), the EMI noise is ideally zero (or very small).

The EMI noise is direct proportional to N*A, where N=RX coil turns count and A=surface flux area.

I'll put this configuration into the comparison too and let's see, what happens.
Cheeers,
Aziz
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« Reply #19 on: Tuesday December 20 2011 18:10:18 AEDT PM »

Well,

the DOD inphase is performing almost equivalent to a DD coil but is slightly poorer than a DD coil.
No real improvement.
 sad1
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australian electronic gold prospecting forum.com  |  Detector Technology and Electronics and new detectors  |  Detector Coils  |  Topic: DOD coils vs DD coil? (The Real Thread) « previous next »
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