New Concentric Co-planar Coil with Strong MF.

[B^C]

Small coils present a greater overall magnetic flux density within their area of influence than a large coil. But I think the practical result is that small coils give better separation of targets, and therefore appear more sensitive. Of course, when you increase the coil size sufficiently, the flux density at the target may be reduced enough to compromise detection of the smaller items.

Gday Qiaozhi,

That's pretty much exactly what i said, not sure about the better seperation of targets though, small coils are definitely more sensitive in the field--each to there own?

It would be interesting to do some testing to check some things out regarding the field strength of say MONO coils of varying sizes.
One would think that with each increase in diameter the field strength would vary, with all things being equal of course. Inductance, Voltage, Amps, etc.

How does that work actually?
It would seem to get the same concentrated field strength as an 8" coil with X Volts & Amps that you would have to increase Voltage & Amps sufficiently to get the same concentrated field strength with every increase in coil diameter?
This is more than likely the reason that larger coils fall in a heap on smaller targets?.

I was just looking at the net & found Garretts opinion on coil size, it's pretty much the same as what we are talking about.
http://www.garrett.com/hobby/techsup...searchcoil.pdf

[tiktak]

Hi,

Hi,
So cant be made bigger coil in such manner that it have equal density with small one?
Like for example changig it a little bit?
Last coils are 45!
I am shore that they dont miss small ithem just this way!
Even oposite!
Realy good discution here

[Aziz]

Thanks to all contributing this thread and giving me some answers to my questions.

Lets make a thought experiment:
We assume, we have a MONOCOIL in PI mode. TX coil and RX coils have same characteristics. Our front-end has a limit of detection level. This level is defined by amplifier stage and SNR-limit (signal-to-noise ratio).
We make a simplification and focusing only on the target. We have a small target with its shape, orientation, flux area, resistance, inductance, capacitance, etc. If we apply some magnetic flux change to the target, it is reacting immediately according to Faraday's law.
Let assume, that a minimum magnetic field strength on the target flux area is necessary, to get the front-end beeping. Below this minimum level, we can not detect it, regardles of its distance to the coil.

According to the last shown graph above, we have a coil radius 10 cm (20 cm diameter) and our target is placed at 10 cm above the center position of the coil. And our front-end needs at least 150 A/m magnetic field strength at target position to detect the signal caused by the target.

Lets see the graph above again. Can we detect our target?
Yes! Right part of the graph shows, how deep this level can be keeped until it is going below 150 A/m. So, according to this primitive sight, we can detect our target up to ca. 12 cm. We are not taking the effect of changing the target distance to the RX coil into account yet. We are making still dump simplifications.

Which coils (radius) are convenient, to detect our target on 10 cm above the coil?
See left part of the graph above. Radius from 9 cm to ca. 28 cm are ok. But if we take the 15 cm radius, we can detect the target on 10 cm distance safer (max. possible magnetic field strength).

Now to discover the fact, why bigger coils are insensitive to small targets, we put all till now known facts together:
1. Magnetic field strength and density is dependend on coil radius and target distance (R,z). See graphs above.

2. If the coil radius gets bigger, a max. magnetic field strength can be seen on a specified position z. Then it will decrease with bigger coils even if they have the same number of windings.

3. If we make the coils bigger, the inductance increases and we have to decrease the number of the windings N therefore to match to front-end specifications. This will flatten the curves on the graphs above and decrease the level of magnetic field strength and therefore our detection level.

4. On the RX side, with bigger coils the front-end sees more flux area for detecting (good for us), but the RX coil has less windings N now (bad). Remember, the detection of signal is proportional to number of windings and the flux area of the RX coil.

The facts are interacting all together. Fact 3 and 4 has a linear dependence and a negative result for us. Fact 1 and 2 has a not linear dependence and is showing mostly the effect, why much bigger coils are insensitive to a small target. In addition of fact 3 and 4 this behaviour is increased.

Indeed, my last simulations shows this behaviour. But only, if I take much bigger coils. So there must be some more hidden reasons for this yet.

Aziz

[B^C]

Now to discover the fact, why bigger coils are insensitive to small targets, we put all till now known facts together:
1. Magnetic field strength and density is dependend on coil radius and target distance (R,z). See graphs above.

2. If the coil radius gets bigger, a max. magnetic field strength can be seen on a specified position z. Then it will decrease with bigger coils even if they have the same number of windings.

3. If we make the coils bigger, the inductance increases and we have to decrease the number of the windings N therefore to match to front-end specifications. This will flatten the curves on the graphs above and decrease the level of magnetic field strength and therefore our detection level.


The facts are interacting all together. Fact 3 and 4 has a linear dependence and a negative result for us. Fact 1 and 2 has a not linear dependence and is showing mostly the effect, why much bigger coils are insensitive to a small target. In addition of fact 3 and 4 this behaviour is increased.

Indeed, my last simulations shows this behaviour. But only, if I take much bigger coils. So there must be some more hidden reasons for this yet.

Aziz

Hi Aziz,

Ground conditons are a big factor!

I have attached a picture --sorry it's a bit out of focus--of some very small nuggets that I can detect with a 250mm coil in some ground but with a 350mm or larger coils are near impossible to detect in the same ground conditions.
As ground conditons change the 350mm coil can detect them but not quite as good as the 275mm coil.
Then also to make it more difficult for small targets, i have had to use larger coils in some ground conditions to cancel out ground noises, only god knows how many small targets i have missed due to this fact.

This may be due to the fact that larger coils lack the punch of the smaller coils & therefor the eddy currents are to weak to detect.
This brings the other question into mind, if we make the larger coils with the same intensity flux as the smaller coils will then the ground noise issues be the same--one would think yes?

Bigger targets are no issue but they produce more eddy currents for detection.
Smaller targets are harder to detect because they produce LESS eddy currents, if the smaller targets are induced with a stronger magnetic field from smaller coils they produce stronger eddy currents for detection.

In some cases larger coils with weaker magnetic fields will of course not produce sufficient eddy currents in small targets, larger targets will produce more eddy currents.

An object with twice the surface area of another, will have a detection signal near twice that of a smaller object but it will not necessarily be detected twice as far away.
The larger target will produce the same detection signal at a distance farther away from the searchcoil than the smaller target.

Three things determine whether the eddy currents produced are sufficient for detection. field strength, target size and surface area.
Field strength for me is the big issue.

Setting the detectors for mineralised soils etc also won't help the situation with large coils & small targets, it seems the more we do to cancel out nature the more we do to cancel out what where after.

[JC1]

Yes

Hi,
No metter Aziz.
I was good Idea.
Maybe the probleme is in compensation coil?
Please try to simulate this coil.I wanned to try it from long time but dont have data.
Thanks

Yes this is a good coil with the two internal receive coils wound opposite

to cancel external noise, and for null of course.

Outside is xmit.

[B^C]

Hi Aziz,

Thought this may be helpful to figure into your equations for real ground condition simulations for your coils. Simulating for air may lead you to some wrong information.

Let me know how you go.

[bbsailor]

Coil Diameter, Ground and Power

Aziz and all,

The optimum diameter of a coil is dependent on the size of the target that you are seeking. See the link below where Eric Fosted discusses a way to optimize the coil size for a particular target size.
http://www.findmall.com/read.php?34,...505#msg-137505

Mineralized ground will affect larger coils more than smaller coils or DD coils.

Coil power can affect detection range. To double the detecting distance in free space or neutral soil you need 2^6 (2 to the sixth power) or 64 times more power to drive the coil. This is 2^3 for the coil-to-target distance and another 2^3 for the return trip back to the coil to be sensed.

Putting in too much coil power on mineralized ground affects detecting the desired targets as the ground becomes one large target, obscuring the desired target's detection.

The above variables account for the fact that many new PI detectors allow the users to control various circuit parameters to optimize detection on a wide variety of ground conditions as well as choose coil types and sizes optimized for particular target sizes and ground conditions.

Once you analyze each coil parameter (diameter, type, inductance, field strength, etc.) consider analyzing the normal range of that parameter along with how this parameter range intersects with other variables such as coil power and ground conditions. This is where the theory and practice will be most valuable.

bbsailor

[gwzd]

Hi Aziz,

Thought this may be helpful to figure into your equations for real ground condition simulations for your coils. Simulating for air may lead you to some wrong information.

Let me know how you go.

You could play a bit with the formulas in your picture using this software:

http://www.geoscanners.com/gprsim.htm

[B^C]

Aziz,

Some more information that may be helpful.

gwzd,

Thanks for the link.

[Qiaozhi]

Aziz and all,

The optimum diameter of a coil is dependent on the size of the target that you are seeking. See the link below where Eric Fosted discusses a way to optimize the coil size for a particular target size.
http://www.findmall.com/read.php?34,...505#msg-137505

Mineralized ground will affect larger coils more than smaller coils or DD coils.

Coil power can affect detection range. To double the detecting distance in free space or neutral soil you need 2^6 (2 to the sixth power) or 64 times more power to drive the coil. This is 2^3 for the coil-to-target distance and another 2^3 for the return trip back to the coil to be sensed.

Putting in too much coil power on mineralized ground affects detecting the desired targets as the ground becomes one large target, obscuring the desired target's detection.

The above variables account for the fact that many new PI detectors allow the users to control various circuit parameters to optimize detection on a wide variety of ground conditions as well as choose coil types and sizes optimized for particular target sizes and ground conditions.

Once you analyze each coil parameter (diameter, type, inductance, field strength, etc.) consider analyzing the normal range of that parameter along with how this parameter range intersects with other variables such as coil power and ground conditions. This is where the theory and practice will be most valuable.

bbsailor

This is very interesting.
I see that Eric posted a graph of coil radius/range curves in a later post, but that image has now disappeared. Do you have a copy you could post here?

[Aziz]

Thanks B^C and bbsailor,

Indeed, your supplied informations could help me to go on further.

Particularly, the induced eddy current on the target is not calculated correctly yet in my model at the moment. The induced voltage seams to be correct for simplified model (target seen as a coil). I had proofen this some days ago by making some physical measurements.
Unfortunately my volt-meter is measuring only up to 1 kHz AC signal (3 dB cut-off frequency). Signals also must be strong enough, to measure this correct.

So just implementing some code on the PC, to measure very small induced voltages and currents by using the lock-in amplifier. I haven't an oscilloscope or high accurate measurement instruments and have to do this by myself as a poor-man's solution.

Anyway, I do not give up and will implement more details on the physical side of effects.

So just very interested on the lock-in technique and is almost prepared in my code to implement it.

Regards,
Aziz

[bbsailor]

Qiaozhi

I don't have the graph. You might want to Eric Foster's PI forum and ask him to repost it. He gives more textual information at the link below but unfortunately the referenced graph is missing.

http://www.findmall.com/read.php?34,...593#msg-131593

Summary. You can increase the diameter of a coil until the coil radius equals the detecting distance from a particular target to that coil size.

bbsailor

[Stefan]

Eric's original graph attached

Regards,

Stefan

Qiaozhi

I don't have the graph. You might want to Eric Foster's PI forum and ask him to repost it. He gives more textual information at the link below but unfortunately the referenced graph is missing.

http://www.findmall.com/read.php?34,...593#msg-131593

Summary. You can increase the diameter of a coil until the coil radius equals the detecting distance from a particular target to that coil size.

bbsailor

[Qiaozhi]

Eric's original graph attached

Regards,

Stefan

Thanks Stefan!

[Unregistered]

http://www.deeptech-bg.com/pdfroudc/coplanar_150.pdf