CONCENTRIC SEARCH COIL DESIGN

[DOOLEY]

CHEERS AZIZ ,

was thinking of trying a PI on seperate TX and RX , between 300uH to 400uH each approx , wound together same diameter, with the TX between + and mosfet to - ,
and RX grounded to - and other end going to op-amp , then that way i can use "one" voltage rather than multiple voltages. simples (sorry, meercat quote)

as coil inductance seams to be directly linked to speed of decay , i didnt want to end up with a realy slow coil if somehow they interacted somehow resulting in a combined inductance.

[Aziz]

Hi Dooley,

in the quote, you will find the blue answers.

cheers aziz , as you said before , mono coil apears to take some beating.

can i be a pain and ask another question ??
sure

coulple of PI designs out there that use sepperate TX and RX coils ,

going to try and word this so it makes sense.

if i wind a TX coil to be 400uh

then wind an exact copy for the RX and it's 400uh

if i have them close together , will the inductance of either coil alter ??
No, if no induced current is flowing in the other coil. If a current is flowing, we have then a mutual inductance. RX coil usually senses voltage rather than current. The current is normaly ultra tiny (op-amp input current).

thinking of taking a former of say 12 inches , then winding both TX and RX at the same time , with same number of turns to get between 300uh to 400uh,

will the coils interact to alter the inductance of either ??

or will the inductance of each coil remain the same ??

The coils might be sligthly different due to variation of winding compactness. But generally, they should have quite same inductivity. Again, if an induced current (inductive, capacitive) is flowing in the other coil (RX), the mutual inductance will have an effect to the TX coil.

only asking as dont want to end up with a realy slow coil arangement.

Dooley, you should focus to the easy to build coils. The DD coil arrangement is very simple and you can not make much mistakes.

Aziz

[DOOLEY]

cheers aziz , as you said before , mono coil apears to take some beating.

can i be a pain and ask another question ??

coulple of PI designs out there that use sepperate TX and RX coils ,

going to try and word this so it makes sense.

if i wind a TX coil to be 400uh

then wind an exact copy for the RX and it's 400uh

if i have them close together , will the inductance of either coil alter ??

thinking of taking a former of say 12 inches , then winding both TX and RX at the same time , with same number of turns to get between 300uh to 400uh,

will the coils interact to alter the inductance of either ??

or will the inductance of each coil remain the same ??

only asking as dont want to end up with a realy slow coil arangement.

[Aziz]

Coil Comparisons

Hi all,

below is the comparison of the interesting coils:
Every coil 10 " outer diamater (round)
- Standard mono coil (used in PI's only)
- Standard Double-D IB coil
- Standard concentric co-planar IB coil (R=0.5, half the outer radius)
- Optimized concentric co-planar IB coil (R=0.8, bigger inner radius)
- Coaxial IB coil (80 mm distance between RX and TX/Bucking)

Don't look at the response voltages. This is irrelevant. Look how it compares to other coil types. The x-axis is the detection depth in inch (air). Pity, you can not see how the depth performance goes further. The coaxial coil is becoming better in the distance but not better than the mono coil. Note, the deeper going coils are having less near-distance sensitivity.

As mentioned earlier, not really worth to make all the effort.

Cheers,

Aziz

[Aziz]

Look at the magnetic field lines below. It is a 0.5*R concentric coplanar coil, showing the magnetic field lines without the target response.
If you add all magnetic field vectors (vector addition), you get a zero magnetic field vector (0,0,0) (=balanced coil).

[Aziz]

The way I understand it, we do not receive the eddy currents with the RX coil. In an IB configuration, we measure the distortion caused by the eddy current magnetic field, within the Induction balanced field of the IB assembly.
The usual co-planar concentric IB coils are also quite sensitive to targets outside of the coil assembly.

Hi Tinkerer,

this is only valid for the following condition:
If we place a ferrite (or any other magnetic material having no conductivity and hence no eddy current would exist), the balance is disturbed only due to attraction of magnetic field lines to the ferrite. This inbalance of the flux area of the RX coil is then detected. There are other materials, which repel the magnetic fields, but the effect is much much smaller. The ferromagnetic attraction is much stronger.

For the receive coil however, it is totally irrelevant, what happens outside of it's flux area. It only detects, what is happening inside of it.

We usually measure the eddy current response of the targets. These eddy currents cause a secondary magnetic field. The sec. magnetic field distorts the balance in the flux area of the receive coil, which we measure only the secondary magnetic field induction.

Aziz

[ApBerg]

No! Small RX coil is not efficient.
All secondary caused magnetic fields from the target should go through (inside) the RX coil. Every magnetic field outside the RX coil isn't visible to the detector.

The "half radius rule" for the concentric coil isn't really a rule.
Perhaps nobody analysed (or calculated) the optimum radius for the inner coils.

But there is an optimum inner radius for a given concentric coplanar coil.

Best to test it with big targets at depth.

Aziz

Perhaps nobody analysed (or calculated) the optimum radius for the inner coils.

But there is an optimum inner radius for a given concentric coplanar coil.

Hello Aziz

This is right !


Some 15 years ago I have made many inner coils, the outside coils where the transmit and first receive coil 100% coupling e.g. transmit 100 turns, first receive coil 18 turns both clockwise and 22 cm wire 0.25mm
Then made some tests with the second receive coil(s), placed counter clockwise in the centre from the tr. and first rec. coil.
To large sec. receive coils give bad pinpoint property's there was 2 times detection from small coins...in one sweep.

.....I find your coil discussions very nice ! Thanks !

Best regards

Ap

[Tinkerer]

No! Small RX coil is not efficient.
All secondary caused magnetic fields from the target should go through (inside) the RX coil. Every magnetic field outside the RX coil isn't visible to the detector.

The "half radius rule" for the concentric coil isn't really a rule.
Perhaps nobody analysed (or calculated) the optimum radius for the inner coils.

But there is an optimum inner radius for a given concentric coplanar coil.

Best to test it with big targets at depth.

Aziz

Hi Aziz,

right now I have a 80cm coil, with a 48cm Bucking coil and a 11cm, 1000uH RX coil. The assembly is not properly balanced, because I just wound some pieces of wire that I had lying around.
However, moving the Bucking coil and the RX coil around inside the TX coil, I got some kind of balance, stretching the TC coil into an oval shape with the Bucking coil and the RX coil offset to one end of the oval.

This leaves a large part of the TX coil free of other coils. This area also is quite sensitive, but does not have FE discrimination. I wonder what influence the number of turns in the TX has in that TX only part of the coil in respect of the RX sensitivity.

The way I understand it, we do not receive the eddy currents with the RX coil. In an IB configuration, we measure the distortion caused by the eddy current magnetic field, within the Induction balanced field of the IB assembly.
The usual co-planar concentric IB coils are also quite sensitive to targets outside of the coil assembly.

Tinkerer

[Aziz]

This may be the difference between theory and practice. In the real world, Tesoro concentric coils (with their relatively small RX) give very good depth. I have not found any advantage with concentric coils where the RX is closer to the TX. The usual rule for concentric coils is to make the RX half the radius of the TX, but Tesoro have made their RX coils smaller than this, and it appears to work just fine.

No! Small RX coil is not efficient.
All secondary caused magnetic fields from the target should go through (inside) the RX coil. Every magnetic field outside the RX coil isn't visible to the detector.

The "half radius rule" for the concentric coil isn't really a rule.
Perhaps nobody analysed (or calculated) the optimum radius for the inner coils.

But there is an optimum inner radius for a given concentric coplanar coil.

Best to test it with big targets at depth.

Aziz

[Qiaozhi]

Small RX coil is not efficient (fact). RX coil needs some flux coverage area to go deep enough.

This may be the difference between theory and practice. In the real world, Tesoro concentric coils (with their relatively small RX) give very good depth. I have not found any advantage with concentric coils where the RX is closer to the TX. The usual rule for concentric coils is to make the RX half the radius of the TX, but Tesoro have made their RX coils smaller than this, and it appears to work just fine.

[ApBerg]

coils...

In ' clean ' ground the open centre coil is good to use with sensitivity that is almost just as good as a 2D coil, but with better disk and pinpoint options...
In fresh water sears (not salt beach) I prefer the open centre for good disc and pinpoint, look at the open centre coil in the pdf at post no:2 The cable is in the rod so less resistant in the water...http://www.geotech1.com/forums/showthread.php?p=77266#post77266
The open centre coils I found easier to make, this because the nulling is not so hard to make as a 2 D coil, a 2D coil is more mechanic work...
Best regards.
Ap
Off course this is a personal opinion !! I have respect for others ! And are very..very curios in what is new....

[Aziz]

True, you want extremely tight magnetic coupling, but you also want zero capacitive coupling, which means having a little spacing. Most manufactured coils wind the RX right on top of the TX-.

That's true. It's more critical, if the TX coil is carrying a high voltage. A good shielding should minimize the capacitive coupling. A minimal gap is always present and necessary however.

As examples, the old Fisher concentrics (from the CZ's) had an RX that was 0.6*TX+, yet Tesoro concentrics have an RX that is 0.3*TX+. The Tesoros go every bit as deep, maybe deeper. I realize this isn't a fair comparison as they are completely different detectors, but the fact that Tesoro uses such small RX coils suggests they found an advantage in doing so.

Small RX coil is not efficient (fact). RX coil needs some flux coverage area to go deep enough.

This kind of concentric is remarkably easy to build, at least in a production environment. However, give us more hints on the more efficient method!

No, I don't think it is easy to build. I am speaking for the amateurs here.

The coaxial coils are more efficient than the concentric coplanar, 4B and DD coils. Particulary the ones, I have mentioned in the other thread (having lesser thickness now). And these coils are far more easier to build. Particularly, if somebody needs bigger IB coil.


This might be interesting to everybody:
The mono coil is equivalent sensitive (depth performance), if you put the same mono coil with another same as RX coil in the OO IB configuration. This however occupies significantly more surface area of the coil. But easy stuff.

If I try to optimize the DD coil (yes, it's also possible), there isn't much room for optimization (performance gain is not much significant as the coils TX and RX are already big enough). It's still possible to make the overlap area bigger and hence the TX and RX coil slightly bigger for the same coil coverage area. This can be achieved with a bucking coil in the overlap region of the DD coil. (same applies to the OO coil)

Funny stuff. But we are talking about very small depth gain. All the effort isn't really worth. I still prefer simple and easy to build coils.

Aziz

[Carl-NC]

Figure 11 configuration is not optimal.

One can get more depth sensitivity, if:
a) coil coupling between receive coil (RX) and bucking coil (TX-) is maximized (as much as possible = as close as possible to each other)

True, you want extremely tight magnetic coupling, but you also want zero capacitive coupling, which means having a little spacing. Most manufactured coils wind the RX right on top of the TX-.

b) provided that a) is achieved, you can increase the radius of TX- and RX (same radius).
(Radius 0.7 .. 0.8 times the outer radius of TX+ coil gives a good range for experimenting, but is dependent on the condition a) )

Maybe, maybe not... a larger RX means you will also pick up a stronger ground signal, which might result in no net SNR improvement.

As examples, the old Fisher concentrics (from the CZ's) had an RX that was 0.6*TX+, yet Tesoro concentrics have an RX that is 0.3*TX+. The Tesoros go every bit as deep, maybe deeper. I realize this isn't a fair comparison as they are completely different detectors, but the fact that Tesoro uses such small RX coils suggests they found an advantage in doing so.

It's too diffucult to build these coil. There are much simpler and more efficient IB coils available (not invented by others yet but I have found them ).

This kind of concentric is remarkably easy to build, at least in a production environment. However, give us more hints on the more efficient method!

- Carl

[DOOLEY]

cheers aziz for putting in the effort into an old design.

i'll think i'll poss move away from the idea then , thank you for you input.

[Aziz]

Hi all,

as you can see (below), one can make more efficient and deep seeking concentric coplanar coils. But it's very important, that the coil coupling between bucking and RX coil should be maximized (coils as close as possible). In this case, the number of turns of the bucking coil decreases and the number of turns for the TX coil increases (for a given inductivity). Additionally, you can make the RX/bucking coil diameter slightly bigger, which effectively has more flux coverage area. In the example below, I could increase the inner diameter up to 0.8 times of the outer diameter. If you make it bigger, the depth performance decreases then. So you should experiment in the range of 0.7 .. 0.8 times the outer diameter.

Anyway, this coil is not for beginners. It's quite difficult to build (to me too). I personally wouldn't invest the effort as there are much simpler coils there.

Cheers,

Aziz