Stacked Spiral Coil Windings

[Aureous]

22 hours ago, RONS DETECTORS MINELAB said:

Thanks Aureous for the additional information.

Do you know how wide and high the 11” and under coil windings are? Are they always two high and  however wide?

Yeah, always only 2 stacks because anything more than that is unnecessary. The more layers, the less benefits from having the flat-winding in the 1st place. The layers are separated  by only a very thin isolation to keep the firmness when applying the wire. 

Another method for applying flat windings (if the wire length is only 10% over the coil shell capacity) is to coil or bundle leftover wire in a suitable spot. Not sure thats what X Coils have done though coz Ive never seen their GPX coils nor any scans. But Ive seen other manufacturers build trial coils using that method. Narrower Ellip shaped coils have less ground signal per sweep and hence sensitivity is enhanced for tiny targets. Simon, that probably explains the extra sensitivity you've experienced I reckon.

[phrunt]

Nope, they didn't bunch up the wire in any spots on the 10", it's completely flat, the 10" is a round coil. 

The 12x6" is a semi spiral, but the 10" is more sensitive than it to smaller targets and of course deeper being a round coil, the sensitivity to tiny targets is most pronounced at the very centre of the coil.

[Chet]

Flat Spiral Wound Coils increase and concentrate the magnetic field near to the coil.

The following four ten inch coils are designed with 2 mm Litz wire with number of turns needed to approximately meet the industry standard inductance of 300 microhenrys (uH).

The flat spiral coil has 43 turns in a 3.39” band that leaves a 3.23” diameter center hole.

There is another flat spiral coil that has 0.1 mm spacing between turns. It has 55 turns that leaves a 1” diameter center hole.

The 5x5 stacked coil has 24 turns in a 0.4” square band that leaves a 9.2” diameter center hole.

The cylinder coil has 16.5 vertical stacked turns in a 0.2” band that leaves a 9.8” diameter center hole.

The magnetic unit of Tesla or milliteslas (mT) is named after Nikola Tesla.
Examples are (5 mT) – the strength of a typical refrigerator magnet; (.05 mT) – the strength of the earth’s magnetic field on a compass.

The attached chart shows that the magnetic field of spiral wound coils is strongest close to the coil and concentrates through the center hole of the flat windings. This provides exceptional results on small nuggets and sharp pinpointing within the center hole of the coil windings. 

Click on chart to expand it.

[geof_junk]

Good cross over indicated in the graph were the the features of coils come into play.

[Redz]

Since this is log scale however this would indicate large increase in sensitivity for flat wound close to the coil (about 2 miniteslas compared to cylindrical), and the cylindrical only has a marginal incease at depth of 0.005.

[GB_Amateur]

15 hours ago, Chet said:

The attached chart shows that the magnetic field of spiral wound coils...

Very nice plot, but more info would help.  E.g. pulse characteristics, how long after the pulse was the field measured, how was the field measured?  Could you provide a reference?  Was the pulse generated by an actual commercially available detector running in a mode, etc. available to owners?

[Chet]

I have over the years built a few detectors and a large number of coils of all sizes and configurations. I have spent many hours with oscilloscopes and test equipment looking at results of the different timings and settings of the GPX 5000 with different coils and different target sizes. I have generated a lot of Excel spreadsheets in the process. 

Some of what I have learned is that the best coils are constructed with Litz wire which reduces skin effect losses. 

Coils should have an overall coil resistance of less than 0.5 ohms. This is important to ensure enough current flow has occurred to fully magnetize the coil before discharging it. Otherwise it will result in a weaker transmit magnetic field. 

High coil frequency response is required to sample for the smallest nuggets right after the pulsed magnetic field collapses. To accomplish this the coil is constructed in a manner that reduces stray capacitance and when possible low proximity effect losses between adjacent windings. Spiral windings are a partial solution to reducing stray capacitance and proximity effect losses and providing a high frequency response.

The magnetic field pattern/lines of force around an air core coil are pretty much the same whether it is AC or DC or Pulsed currents. 

In order to save time I now rely on online calculators to simplify the process and design of coils.  I used the online calculator below to generate each one-inch magnetic value for input data to the previous Excel chart. 

https://www.accelinstruments.com/Magnetic/Magnetic-field-calculator.html
 

[jasong]

Interesting. The area under each curve should be proportional to the total energy contained in the magnetic field at a given time during the pulse (not equal to, just proportional). So integrating the area below each coil type's curve should give a rough idea of the overall efficiency of each type of coil, by some proportionate factor. 

IE - It seems to show your wire spacing model which reduces the stray capacitance improves the efficiency of the coil. More of the pulse energy is effectively stored in the magnetic field rather than lost to stray capacitance. It would be interesting to make an algorithmic FE program that brute forces a great number of different designs with the goal of maximizing the area under that curve. 

Have you experimented with gradient coil windings like they use in MRIs to produce specific linear variations in the field? Not exactly similar, but something like changing winding density along the Z axis? Or other variations... Potentially with the aim of making a really deep coil that doesn't need to be really large at the sacrifice of sensitivity to smaller shallow gold? I guess with these coils there are so few windings that options are kinda limited though. 

I'm just thinking in terms of shaping those curves to match specific use cases - putting as much of the energy as possible into the places it's needed for different uses, while minimizing energy in the places it's not needed.

[Chet]

The most deciding factor appears to be the smaller the center hole the more concentrated and stronger the magnetic force is across the coil face and through the center. 

I have built a few porotype coils modeled after different MRI coils and even modeled an airborne coil that is used for detecting oil pools trapped under the arctic ice. It used the earth’s magnetic field instead of a transmitter. Sadly they didn’t work very well for metal detecting. 

Some of the high energy coils used in nuclear fusion power experiments are appealing but scaled down are still just not practical for small light weight coils.  
 

[jasong]

I wonder if there is any benefit to be had by concentrating flux density via specifically placing ferrous components wound along with a coil (like a steel guitar string for instance). Or ferrite components maybe?

Or if that would affect both the ground balance as well as the inductance of the coil such that no (or negative) benefits are gained?