Induction balanced spider coil development

[Tinkerer]

Joecoin,
good observation. Do you have more information about the "swingy thingy"? It might be a good idea to try a dedicated thread on several detector forums to collect opinions.
I believe that the present common detector ergonomics are inherited from the de-mining detectors. Not necessarily the best way to look for coins, relics or nuggets. However, it is difficult to change habits and expectations. Definitely not something that a major detector manufacturer wants to invest a few million to try.
Tinkerer

[Aziz]

Hi,

On the PI, the target response is defined/limited by the target's TC curve. This means that most of the target's eddy currents are gone after 2 TC's. Take a small piece of gold like a 0.1 gram nugget or a hollow gold ear ring, and the TC is about 3us. That means that with a delay of 10us to the first sample, very little signal amplitude is left.
Now, I have never worked with VLF detectors, but the few experiments I made showed that a VLF frequency of 30Khz or so is high enough to detect such targets.
Why?
Tinkerer

Very basic view of the target:
It is a coil with frequency dependend resistance. And the coil is more or less coupled with the primary search coil. It is coupled, when its nearby the search coil.

Then basic tranformator equations can be applied:
1. u1(t) = (L1 *di1)/dt - (M * di2)/dt (1. trafo equation)
i1 = current through primary coil
i2 = current through secondary coil (target)
di/dt = first deviation of current to time

if current flow through primary coil is harmonic (e.g. sine wave) then:
2. U1 = jw0*L1*I1 - jw0*M*I2
w0 = 2*pi*f

It is also harmonic, when the current is pulsed (PI).

On the simple trafo equations, you can see that the induced voltage is dependend to the frequency. The higher the frequency, the more energy can be transformed from the primary to secondary coil. The induced energy will then be consumed (current flow producing magnetic field and heat) and will cause secondary magnetic field. Because, the inductivity of the target is same (you change only the operation frequency f, but not changing the target, M, L1), it is direct proportional to operating frequency. The higher the frequency, the higher your SNR! Your signals getting better. On small objects, only the higher frequency can increase the SNR. That is the answer!

Please, look into the transformator prinziples. Also keep in mind, that all targets are coils (or materials) with having R (real resistance), L (inductivity), C (capacitance), permeability, defining frequency dependend complex resistance: RG = R + j(LC) .

One more question about the IB coil arrangement. I can attest that the coil arrangement you describe above does indeed work. I am using it with the TX and bucking coil in series. However, I get considerably less amplitude in target response with this IB arrangement than when I use the same coil, with the same coil current in the traditional way, with only TX and RX coils. This is why I believe that it is very important and useful to analyze the interacting fields to find the best possible combination of coil disposition, coil characteristics and coil currents.
Please continue analyzing.
Tinkerer

The arrangement above is not symmetrical. The response on the top side may differ from the bottom side. It depends, on which side the target is placed. So I need more information about the configuration. Where is the RX coil is placed. I am a little bit confused. Also, I need the diameters (radius 1, 2), resistance of the coils (or diameter of the cu-wire), number of windings, etc.
I it obvios, that the strength of the magnetic fields above the arrangement are by far less than on bottom side (magnetic field density). Remember the transformator principles. The inductive coupling is important. Place your target there, where the energy from the primary coil can be transfered to the secondary coil (target).

Aziz

[Aziz]

By the way:
An interesting concept is, switching the tx and the bucking coil anti-parallel. This could enable better TC (lower resistance and inductivity). The induction balance can be made through different current flows (different resistances of the transmit coils) and geometry of the coils.

Did you tried this yet?

[joecoin]

Joecoin,
good observation. Do you have more information about the "swingy thingy"? It might be a good idea to try a dedicated thread on several detector forums to collect opinions.
I believe that the present common detector ergonomics are inherited from the de-mining detectors. Not necessarily the best way to look for coins, relics or nuggets. However, it is difficult to change habits and expectations. Definitely not something that a major detector manufacturer wants to invest a few million to try.
Tinkerer

Here it is:

http://www.informulation.on.ca/A5577...8!OpenDocument

[Tinkerer]

Joecoin,
thanks for the information. Have you given any thought of how to design an ergonomic metal detector to be used with a harness? I think it would probably be balanced differently if it were designed already with the harness in mind. Something to look at for a large loop, deep seeking detector.
Tinkerer

[Tinkerer]

Aziz,
anti-parallel, yes I have tried that. On first sight it did not seem to work better. However, the IB-PI setup, is extremely sensitive to minute changes, so only extended tests would show the full characteristics of every possible combination. It does take quite a bit of work and material to wind all different coil combinations and to test each one. I have done a few, many more could be tried.
By the way, very good observation of yours about the non-symmetrical coil arrangement discussed above.
In the traditional PI configuration, the field is essentially equal above and below the coils, so usually I suspend the target from the ceiling about 2.2 meters above the stationary coil. The coil lies atop a plastic box about 0.3m above the concrete floor. The concrete floor contains some kind of steel mesh. I consider this to represent a mineralized ground. I let the target swing as a pendulum over the coil.
Now, when I changed to the IB configuration, I just laid the bucking coil above the traditional TX-RX, PI coil. Obviously this distorts the field very much and this could well explain the diminished sensitivity. I will try with the bucking coil below, leaving the rest of the setup as is.
Tomorrow I will post the characteristics of the IB-PI coil configuration. In a few days I will post the comparison of the outputs of the 3 different coil configurations.
Tinkerer

[Aziz]

Hi Tinkerer,

I am looking forward to your results. So I would like to model your coils and will show you the calculated results.

Indeed, using stranded wire (HF=all isolated or normal=not isolated) for the tx and the bucking coil, it is very easy to balance the IB-coil by leaving some single wires on either tx or bucking coil unconnected. So the rx coil would not be changed and also totally decoupled from the electrical circuit (because of balancing matching network) of the transmit side. Even there is a very simple in phase electronic adjustment possible by using a simple R-Pot network (with single additional one or two turn coil). In case of thick crosssection of the total wire, a HF wire should be taken to lower the induced eddy currents. Also if the frequency domain signals exceed 30 kHz (skin effect).

I found a wire shop in Germany:
http://shop.spulen.com/default.php?cPath=72_81

So, I have still not decided which wire I should take. I have to do some impedance calculations and this defines the length of the wire and therefore the geometry of the coil arrangement. I will take most used stranded wires on the market (cheaper and better to get). My power amplifier has an impedance of 2-8 ohms. So the tx-coils will be at least totally 2 ohms for VLF-mode.

I would recommend you to ask me for a new IB-coil design to prevent the many makings of the coils. We must not waste to much time with many coils, that won't work. So, just ask me. I could give you the geometric specification of desired IB-coil. For single standard PI coils, it is obvious that they are very simple and need no balancing.


Indeed, I wound many dozens of coils for years. Then I decided to calculate (balancing) with the software before winding new coil. At the moment, I am coding the software for additional features. So, we could soon beat the standard coils on the market.

Aziz

[Aziz]

What the hell... it was to late..

I mean, the induction balance with stranded HF wire is only in anti-parallel mode possible. Unisolated wire would not work of course. In serial circuit of coils, this can not be applied.

[Tinkerer]

Aziz,
below are the characteristics of the coil arrangement that I am using now.

The next coil I want to make bundle wound, with the same outside diameter. TX outside.
RX same diameter than TX, placed below the TX at a distance of 30mm.
How should I place the Bucking coil?
Coil parameters:

This is a coplanar, flat wound coil with outer TX and inner RX. It is shielded with Graphite compound with an imbedded drain wire top and bottom

Housing outer diameter 290 mm

Outer coil TX outer diameter 275mm
Tx coil inner diameter 220mm
About 21 meters of wire
3 Ohm resistance
350uH inductance

Inner coil RX outer diameter 200mm
RX coil inner diameter 50mm
About 25 meters of wire
3,5 Ohm resistance
450uH inductance
Signal taken on inner wire +

Bucking coil (not yet shielded) lying on top of the shield of the coil above, slightly off center to get the precise desired balance.
Outer diameter 170mm
Inner diameter 110 mm
1.5 Ohm resistance
140uH inductance
Inner wire +

The TX and the Bucking coil are driven with 12V with a total of 23.5 Ohm series resistance.
Tinkerer

[Aziz]

Hi Tinkerer,

I made some calculations on your next coil. The dimensions are displayed on the graphics below. This is a critical coil arrangement. The center point of the tx coils are on x=0, y=0, z=0. To get best balancing, the rx-coil must be exact -30 mm below the tx coils centered (as specified from you). If you place the rx-coil to -29 mm, than you can off-center it by round about 7 mm. If you place the rx-coil -31 mm or deeper, you can not balance the rx coil anymore (just minimizing on center point). So this is very critical part. The off-centering of the TX coils do not change much (except in z-direction = height). The inner tx coil has twice more windings than the outer one. I can do the inner tx coil smaller. But in this case, the number of windings on the inner tx coil will grow. I do not know, how the coil will work. Just try it out. The radius specification relate to the center of the coil (mean radius).

The other coils are coming soon...
Aziz

[Tinkerer]

IB coils

Aziz,
thanks for the coil analysis. I will study it.
Attached below, the reference samples with the coil that I am using now.
Tinkerer

[Aziz]

Aziz,
thanks for the coil analysis. I will study it.
Attached below, the reference samples with the coil that I am using now.
Tinkerer

Aziz,
thanks for the coil analysis. I will study it.
Attached below, the reference samples with the coil that I am using now.
Tinkerer

Tinkerer,
As I started to model your coils, I found lack of information. To prevent many questions in the future, I need to design a model the following specifications of each coil:
- Number of windings + winding direction (eg. 30 right, 28.5 left (half winding), etc. )
- starting radius (mean radius to center of the coil)
- ending radius (mean radius to center of the coil)
- for not spiral coils: Starting = Ending radius
- wire Diameter (for calculation very important: for points getting inside the wire)
- position: If any coil is displaced from one another, the exact coordinates related to base coil (e.g. x=2 cm, y=0, z=3 cm).
Without the exact specification, a balancing of the coil can not be made.
In the meantime I am coding a better 3D-Display of the software. At the moment, the display of vectors are confusing sometimes. So I have to code the z-sorting (distance of element to eye point) of the elements before displaying.

Aziz

[Tinkerer]

thanks Aziz,
The coil that I am using is already epoxied. It has been a long time since I made it, so I dont have much more information available. I never counted the turns on flat and spiral wound coils, rather I made them to a specified inductance.
I am very interested in IB coils, so I am going to setup a PCB drive board and a coil winding rig, to start testing a variety of different coil arrangements.
Maybe we could discuss possible arrangements for the first few trials.
My existing coil has 350uH inductance for TX and 450uH inductance on RX. This seems to work OK.
Maybe we should try something a bit more extreme, like 450uH TX and 900uH RX with various Bucking coil arrangements.
As a general idea I like to limit the outer wire coil diameter to 285mm, that would be the outside of the coil housing to be 300mm.
With a standard PI coil, the possible dept in very much related to the diameter of the coil. I suspect that this also applies to the IB coils, meaning that the dept is related to the diameter of the RX coil. This assumption has to be verified.
Now, the bucking coil distorts the TX field. What shape should the ideal (ground penetrating field have? The opinions seem to be divided. Some prefer the narrow conical shape, others would prefer a more cylindrical shape.
It would seem that, with the proper choice of the diameter and disposition of the bucking coil, we might be able to influence to shape of the ground penetrating field to some extent.
As a general idea, it would be preferable to have fairly uniform field. this does not seem to be the case with spiral coils, once the width of the spiral (r1-r2) exceeds a certain limit. This is why I am returning to bundle wound coils.
I would welcome opinions for criteria for a test series of coils as well as ideas for the drive board to test a variety of coil driving modes.
Tinkerer