Stacked Spiral Coil Windings

[Geotech]

A very interesting thread.

Jason: No, adding ferrite won't help, it'll just make the coil heavy. The only exception is in a pinpointer where a ferrite rod is used to concentrate the field. This works well in a PI or energy theft pinpointer which use a mono coil. When you use an IB coil the RX must be air-cored so, ferinstance, the White's TRX has the TX coil wound on a ferrite rod but the RX coil is on a plastic bobbin. If this concept were used in a regular detector, you would have a big long ferrite for the TX coil with the RX underneath, so the coil is 8" thick and weighs 20 lbs.

Chet: Have you ever tried using FEM software?

[Chet]

Hi Carl

I looked at FEM software but it looked like a little learning curve to get up to speed. And I wasn’t convinced that it would do that much for me. I am so use to Excel that for some things I just do tedious measurements and plot the results.

Have a good day,
Chet
 

[jasong]

18 hours ago, Geotech said:

Jason: No, adding ferrite won't help, it'll just make the coil heavy. The only exception is in a pinpointer where a ferrite rod is used to concentrate the field. This works well in a PI or energy theft pinpointer which use a mono coil. When you use an IB coil the RX must be air-cored so, ferinstance, the White's TRX has the TX coil wound on a ferrite rod but the RX coil is on a plastic bobbin. If this concept were used in a regular detector, you would have a big long ferrite for the TX coil with the RX underneath, so the coil is 8" thick and weighs 20 lbs.

I'm envisioning the opposite of a ferrite core, more like an exterior casing to concentrate flux such that the amount of magnetic field energy is maximized towards the ground and minimized towards open air. Since half of the field of a coil radiates upwards and thus is not particularly useful for detecting, it seems to make sense to put it where it's more useful - if possible?

Here is a cutaway diagram. Would this not have any benefit, at least for near surface flux density concentration? At the field strengths of PI machines I'd guess something like fairly thin 24 gauge steel would be more than enough, saturation is probably ok since the goal would just be some improvement, not capturing all the upwards flux. 

Is there a way to use that other half of the "wasted" (using that term loosely) magnetic field energy more usefully downwards if this doesn't work? Sort of like the idea behind a Halbach Array, except whatever the EM version would be?

Weight alone doesn't have to be a deal breaker. For instance if a hypothetical 12" coil with flux shaping components weighed the same as a 20" standard coil, can it be made to obtain similar depths? If so, then it'd be worth it due to maneuverability with all other things being equal. 

[Geotech]

Any time you place ferrite near a pancake coil it causes the flux lines to bend more. This is why mineralized ground reduces depth, it increases the flux curvature and reduces flux density at depth. Placing a ferrite shell on top of the coil certainly does what you envision (reduces the top-side field); at the same time it increases the bottom-side flux density very close to the coil but this advantage is quickly lost and at depth the flux density is reduced. Here is an FEM plot:

Here is the same coil with the ferrite set to air:

It's possible the ferrite-shelled coil would be good for shallow tiny nugget detection.

Another (lighter) possibility is to design an anti-Helmholtz coil, where an anti-phased bucking coil placed just above the TX coil to "focus" the field downward:

Unfortunately it tends to also reduce flux density at depth compared to just a single TX coil.

.

[jasong]

Thanks yeah that was what I was envisioning. I figured there would be a definite increase in flux density for near surface (small nugget) detection. Wasn't clear on if there might be deeper effects. The density increase also seems to be kinda "too near" the surface to be useful for general use coils, but for crumbing/dinking coils it might be interesting. 

That anti-Helmholtz configuration is interesting. Haven't looked at that before, but that seems kinda like achieving similar affects to the Halbach array. That's too bad the flux decreases at depth compared to a single TX though.

It definitely seems possible to increase the small/shallow target response. I wonder if there exists some strange or clever design that can trade decreased flux density towards the surface for increased flux density at depth? (well, a design that is actually usable by a human arm anyways 😅)

[Chet]

28 minutes ago, Geotech said:

Any time you place ferrite near a pancake coil it causes the flux lines to bend more. This is why mineralized ground reduces depth, it increases the flux curvature and reduces flux density at depth. Placing a ferrite shell on top of the coil certainly does what you envision (reduces the top-side field); at the same time it increases the bottom-side flux density very close to the coil but this advantage is quickly lost and at depth the flux density is reduced. Here is an FEM plot:

 

Those FEM plots are really informative. Thank you for posting them.

[Geotech]

Jason: Yes, the anti-Helmholtz is a bit like a Halbach array. I once looked into a Halbach array for a medical detector. I don't know any way to trade off near-field for far-field, as far-field always follows near-field. "There is no free lunch."

Chet: I'm using FEMM which is free and there are some Youtube instructional videos. After going through it a couple of times and setting up a few "starting point" coils, I can now do a sim in about 5 minutes.

[jasong]

One other design I'm curious about is a flat, single layer spiral coil wound with variable winding spacing. No spacing between windings at the center leading to more and more widely spaced windings towards the rim.

I might be envisioning it wrong, but it seems like denser winding spacing towards the center and less dense towards the rim may have some kind of flux density shaping effect leaning towards emphasizing more flux at depth (vs a standard coil) directly under the coil, at sacrifice of flux towards the edge of the coil and shallow? 

But yeah, I totally get the no free lunch. Most of physics can be derived from basic thermodynamic principals, I try to always look at systems from a conservation of energy viewpoint.

[Chet]

4 hours ago, Geotech said:

Jason: Yes, the anti-Helmholtz is a bit like a Halbach array. I once looked into a Halbach array for a medical detector. I don't know any way to trade off near-field for far-field, as far-field always follows near-field. "There is no free lunch."

Chet: I'm using FEMM which is free and there are some Youtube instructional videos. After going through it a couple of times and setting up a few "starting point" coils, I can now do a sim in about 5 minutes.

Carl; I will give FEMM a go. Can it model DD and DOD coils?

[Geotech]

So far, I've only done cross-sectional analyses of circular coils. I'm not sure it can do more complex shapes. It appears Elmer can, another free FEM tool, but I haven't found any idiot-training videos for Elmer as yet. I'm also to the point where I need to do some 3D analyses so if I find anything usable I'll post it here.