Flat Spiral Coil Analysis

[Aziz]

Hello,

I found very interesting, to analyse the flat spiral coil. Especially the effects on the flattening and the weakening the magnetic fields.
The spiral coil has a starting radius (20 mm in this example) and an ending radius (80 mm in this example) with 50 windings.
Figure below shows the magnetic fields strength and directions on a half cross section of the coil. Red elements are the coil wire elements with showing the current direction. Black vectors are magnetic fields (H-Field).

http://imgbox.de/users/zet/spiral1.gif



By reducing the complexity of the coil for a better view (reducing the number of windings), you can see, why flat spiral coils are not effective. Between each two windings there occur canceling regions. The generated magnetic field from the outer winding will be slightly cancelled by the next inner winding. Also, this causes the flattening of the magnetic fields above the coil. See below picture for more details.

http://imgbox.de/users/zet/spiral4.gif




The detailed result is shown on the next picture:

http://imgbox.de/users/zet/spiral3.gif




The next picture shows two flat sprial coils with different diameters (same winding direction). The inner coil is driven with 0.4 A and the outer with 0.1 A current to show the canceling field better.

http://imgbox.de/users/zet/spiral2gif




to be continued..

[Aziz]

For induction balancing of this flat spiral coil, same copy of the coil (as RX-Coil) is positioned at a distance of 85.8 mm from the center point of TX-Coil. The RX-Coil has also a distance of 1.5 mm height from the copper layer of TX-Coil. Standard PCB boards have a thickness of 1.5 mm. So, if you want to balance this coils together, just put them together and move the RX-coil 85.8 mm from the center point of TX-coil.

Attached picture shows both coils in overlapping display:
PDF-File showing in a better resolution.

[Aziz]

3D-Display of the TX-Coil (red) and RX-Coil (green):

[Aziz]

The final PCB coil as high resolution PDF-File.
Final Coil Data:

Windings: 50 turns
Radius 1: 20 mm
Radius 2: 80 mm
Diameter: 16 cm
Cu Trace with: 0.6 mm
Cu Gap: 0.6 mm
Cu Length: 15.7 m
Coil Resistance: ca. 13.3 Ohms (of 35 micro meter thickness of Cu-layer)
Inductance: ??? (not computed yet, very time consuming and non trivial task)

Aziz

[Tinkerer]

Aziz,
thanks for posting the analyze of the spiral wound coils. I have made many spiral wire wound coils and have observed uneven fields which is perfectly explained by your visualizations. My reason for trying spiral wound coils, was the search for low capacitance coils. This is indeed possible with spiral wound coils, but as you pointed out, there are also disadvantages.
One advantage of the spiral coil, is the pinpointing capability.
So I moved on the flat wound coils, that is, instead of a single layer, I packed 2 or 3 layers of wire, giving the coil a flat rectangular cross section. I am still experimenting with this type of coils so I can not yet give a final opinion of it.
Next I will try bundle IB coils. I wonder what the best proportions are for the most effective TX and RX. The K factor favors distance between the TX and RX coils, but the RX coil should be of large diameter for good dept. Where lies the best compromise?
Tinkerer

[Aziz]

Hi,

Aziz,
thanks for posting the analyze of the spiral wound coils. I have made many spiral wire wound coils and have observed uneven fields which is perfectly explained by your visualizations. My reason for trying spiral wound coils, was the search for low capacitance coils. This is indeed possible with spiral wound coils, but as you pointed out, there are also disadvantages.
One advantage of the spiral coil, is the pinpointing capability.
So I moved on the flat wound coils, that is, instead of a single layer, I packed 2 or 3 layers of wire, giving the coil a flat rectangular cross section. I am still experimenting with this type of coils so I can not yet give a final opinion of it.
Tinkerer

I propose to take multi-layered flat spider coil arrangement (idea from VLF radios). So the height of the whole coil will grow and the with of the spider coil should be small as possible (or reasonalbe). We could try to use two or three spider coils together with some gap between them to mininize the overall capacity. One spider coil itself has two layers of wire, in which the windings are interleaved.
To lower the Q factor, we have to use thick HF wire with many very thin isolated wires, that are connected in the HF wire parallel. Also the RX-coil should be a HF wire but this could be thinner. So we can reduce the resistance of TX and RX coil. It will also reduce eddy currents on the coil wires.

It does not matter, what you use:
- Coils with less windings allow you higher currents (low resistance)
or
- Coils with more windings (strengther magnetic fields), the resistance will increase and limits therefore the current flow.

The magnetic field strength is direct proportional to the current flow of the TX coil and the number of windings. For the RX coil, it is also very important, to increase the Q factor (Resistance, Capacity).

Best performance should be obtained from decopling the RX coil from the TX-coil (TX coil is not for receiving switched). So enabling high current flow through TX coil with less windings. On the other side, you need RX coil with much more windings. This is typically used in transformators and will act as a simple amplifier (without power supply of course ;-) ).

Aziz,
Next I will try bundle IB coils. I wonder what the best proportions are for the most effective TX and RX. The K factor favors distance between the TX and RX coils, but the RX coil should be of large diameter for good dept. Where lies the best compromise?
Tinkerer

It depends on what object size do you want to search. All metal objects nearby the search coil are also producing its response magnetic fields (secondary magnetic field). The question is, are the induced magnetic fields of the object going through the covering area of the RX-coil? And at which strength and direction (because of inductive coupling)? The relation of the RX-coil size and the object size is becomming very important. So if the relation differs too much, you have to process the rx signal with more SNR (signal to noise ratio). If you increase the RX-coil area, you should also increase the number of windings, to compensate the size relation. This is also a problem of handling noises.

That is the main reason, why you can not detect small objects with large coils. The coupling factor (area of rx to object size) gets important.
Of course, one could combine two different RX coil sizes. Small one for small objects and big one for other. The spiral coils indeed realizing this feature.

I should make more coil analysis. May be, I can find more important things.
;-)
Aziz

[Aziz]

Fortunately, I can also simulate the secondary magnetic field. The target nearby the search coil acts herein as a small coil with less windings. This could be very interesting, to analyse the effects of different size of coils and targets. Especially the induction coupling factor of target to coil size could be of interest, to learn more about coil design and the need for the signal amplification factor (decibels, signal-to-noise ratio, etc.).

To realize this, I have to revise my software a little bit. This also could be an another topic here. Give me some time for this.

Aziz

[chemelec]

I make a Lot of Spiral Coils, for use on my PI Detector.
Most are Radial Wound with 20 AWG Wire.

Typical coils are about 400uH, With Typical Dimensions Below.
5.5 inch ID, 8 inch OD.
Or 8.9" ID and 10" OD
Or 10.5" ID and 12" OD

Free Air Resonance on these coils is uaually around 2 Mhz.

I Find Best Sensitivity to be near the Inner or Outer Edges of the winding, making Pin-Pointing Quite easy with any size of coil.

Can you do a Simulation of a Coil like above?
I would be curious to see your results and do Real Comparative testing.

Off Topic: In February I'm going to Chile for two weeks, Than to Hawaii for another week.
I have arranged to meet up with one person on this forum.

Anyone Else Living In Chile?
Or in "Maui".
If so, Email Me and see if it will work out to meet.

Its Always nice to Meet people.

(In 2006, I went to Cario Egypt and Switzerland where I met 2 persons that had built my metal detector.
Had a Great Time.)

Take care....Gary

[Aziz]

Hi chemelec,

ok, I will show you your first coil configuration. The magnetic field vectors are linear scaled. The length is the strength, the direction of the vectors are direction of the magnetic fields (H-Field). The magnetic field vectors are computed on the crosssection y=0 (half). The coil is placed on 0/0/0 (x,y,z). The view point is from your possible target. The other coil configurations may look similar, if they have same proportions. Have a nice trip.

[Aziz]

And this is 3D-view. You need a 3D-eye glasses (red/green one).

[Aziz]

This is a crosssection on x=0. Also including the 3D-view. Have fun. Aziz.

[chemelec]

Thanks Aziz.

I'll run some tests when I get some free time and see how detection compares to those plots.

Gary

[Aziz]

I made some improvements on software graphics. Exact magnetic strength values coming soon.
Aziz

[Aziz]

Aziz

[chemelec]

Aziz

Looks very interesting.

However, presently I am busy Preparing for my holiday. I'm going to Chile and Hawaii.
So I won't be able to do any tesing on this until I return in March.

Take care.........Gary