Let's made a PC-base metal detector with usb interface !!!

[simonbaker]

G'day everybody,

I did some fundamental coil tests during last days. Produced another dozens of coils again. I found now a basic source for balancing problem (residual RX coil voltage, which can not be minimized further).

The reason lies in the inter-wire coil capacitance. This is the capacitance between each coil windings. The higher the inter-wire capacitance, the higher the ground (and human body) will affect the measured result (capacitance coupling between ground and coil).

To lower the inter-wire coil capacitance, I took an insulated wire for the coils. The copper core of the wire diameter is 0.5 mm and the total wire plus the insulation diameter is 1 mm. No shielding is applied to RX and TX coil. Insulation consists of a standard polymere.

I will analyse now the shielding effects deeper. Also the thickness of the coil wire (litz vs. wire).

Best wishes,

Aziz

Interesting! I had thought the residual voltage might be partly due to the capacitance between the TX and RX coils where they almost touch. You are saying it is the capacitance in each coil itself (parasitic)?

Also interesting you say more capacitance between windings means more sensitivity to nearby conductive objects. Why would that be?

It also brings up the subject of winding. If windings at one end of the coil are near windings at the other, there is more voltage between them, and thus more effect you would think. Do you have any special ideas on winding layout for minimum capacitive effect?

Regards,

-SB

[Götz von Berlichingen]

A Idea about coil Wire (not tested) :
How about Satelite-Receiver Type Coax Cable (Cell-PE Dielectric).
The Shielding has to be removed.
This should be a high-Q and cheap Coil. (?)
Inter-wire coil capacitance cant get much lower than this, except
you etch a PCB Coil, which is not cheap.

[Aziz]

G'day everybody,

I did some fundamental coil tests during last days. Produced another dozens of coils again. I found now a basic source for balancing problem (residual RX coil voltage, which can not be minimized further).

The reason lies in the inter-wire coil capacitance. This is the capacitance between each coil windings. The higher the inter-wire capacitance, the higher the ground (and human body) will affect the measured result (capacitance coupling between ground and coil).

To lower the inter-wire coil capacitance, I took an insulated wire for the coils. The copper core of the wire diameter is 0.5 mm and the total wire plus the insulation diameter is 1 mm. No shielding is applied to RX and TX coil. Insulation consists of a standard polymere.

I will analyse now the shielding effects deeper. Also the thickness of the coil wire (litz vs. wire).

Best wishes,

Aziz

[Aziz]

Hi all,

I have found interesting behavior on balancing. Neither the electrical balancing network nor the software controlled (inductively controlled) balancing will correct the signal phase shift and signal level behavior. I came to the conclusion that only the additional winding (on RX or TX coil) will do a much better adjustment. Any balancing networks will be discarded. The last correction of the coil can be made purely in the software by transforming the not linear behaviour of signal level and phase into more convenient form.

Concentric co-planar coils (CC) are showing a much better coil stability. So the mechanical stability will no longer an issue. Due to the less RX coil flux area on CC coils, the sensitivity will be lower than on DD coils.

DD coils needs some extra stability and have to decoupled from the coil housing.

Attached picture: experimental concentric co-planar coil assembly.
I have now an idea of how to make a good coil. Some tests must be done yet.

Aziz

[Aziz]

I am getting rid of DD coils at the moment. Due to overlapping of TX and RX coil, the coil housing gets quite thick and there is more sensitivity for mechanical instability.

For experimental coil tests, the concentric co-planar coils offer a better coil stability, software controlled balancing and less thickness of the coil housing (less weight). The most advantage of the coil assembly is that the coils are lying on the same plane and can be fixed better.

I will make a 21 cm diameter coil (wound on a 20 cm diameter pot) and will make a depth performance comparison to my old DD coils.
The coil will be sure much easier to make (software controlled balancing). Now looking forward to have more success and real field testing..

Aziz

[Aziz]

Interesting finding:

The reason for not able to balance to zero volt in the RX coil is not the shielding but the wire itself used in the TX and RX coil. I have used a 0.4 mm wire instead of litz wire (30 enamelled thin wires). I have removed all AL foil shieldings from the coils and there is still much residual voltage on the RX coil which can not be nulled.

Now I am getting really sick of nulling IB coils. I will drive the TX coil in single channel mode. Despite of half of transmit power, the second channel is reserved for software controlled balancing. This will make coil development really much much easier. Single channel driving mode has ~12 % less depth detection.

I am also getting sick of different sound-card specifications: Too low driving voltage, too much impedance, too low input amplification (line-input), too much noise (mic-input), etc...
For best performance, an external low-noise amplifier for TX and RX (booster) will solve this problem. I will focus first still on passive solution: no active external booster.

Now focusing on software controlled balancing of the damn coil..

Aziz

[simonbaker]

Hi bbsailor,



Thanks for your hint. I did not try the scotch 24 yet. I will look tomorrow for this shielding here. Anyway, shielding is not much an issue. AL foil will do this at least when I can not obtain the scotch 24.

The most difficult part is mechanical stability of the coil and coil adjustment.

Regards,
Aziz

It would be interesting to do a spectral analysis of the noise produced by "mechanical instability". I believe in a motion detector most of it is not too important unless you knock it against something. As the joke says, "don't do that..."

-SB

(patient: doctor, it hurts when I do this..." doctor: "don't do that...")

[Aziz]

Hi bbsailor,

Aziz,

What is the capacitance between the shield and each lead of your coil?
What is the individual equivalent round coil diameter?

Scotch 24 mesh shielding may offer a better alternative as eddy currents cannot be generated in the thin wire mesh. A gap is still needed with Scotch 24.

bbsailor

Thanks for your hint. I did not try the scotch 24 yet. I will look tomorrow for this shielding here. Anyway, shielding is not much an issue. AL foil will do this at least when I can not obtain the scotch 24.

The most difficult part is mechanical stability of the coil and coil adjustment.

Regards,
Aziz

[Aziz]

New sound card I assume means you don't need USB card? When you get a chance, more photos welcome!

Regards,

-SB

I need only one USB port. So it is a real PC-based metal detector with USB interface. The sound-card is plugged into this USB port.

I am thinking of making coil development much easier. It is really too difficult to meet the specifications. Particularly the right balancing and phase adjustment seems to be a big challenge. A two dimensional software mapping of the signal magnitude and phase could make things easy and could correct the coil properties. So every coil would have its own correction parameters.

Well, I will try first to make an another coil without software corrections. This coil will be cheap and a quick & dirty variant for some field tests. My first coil can't be reused or recycled... lot's of effort is gone but lot's of experience is gained.

Aziz

[bbsailor]

Hand detection test: direct comparison between graphite shielded coils vs. AL foil shielded coils. AL foil shielded coils do not detect my hand.

Aziz

Aziz,

What is the capacitance between the shield and each lead of your coil?
What is the individual equivalent round coil diameter?

Scotch 24 mesh shielding may offer a better alternative as eddy currents cannot be generated in the thin wire mesh. A gap is still needed with Scotch 24.

bbsailor

[simonbaker]

Hi Simonbaker,



When you see the TX coil as a stand-alone coil with AL shield, the shielding itself has less effects. The magnetic field lines of the TX coil goes parallel to the shielding surface area and therefore inducing nothing. No eddy currents are flowing on the shielding due to TX signal (shielding with open gap). It shields only external EMF.
The affect comes from the position of the shielded RX coil. The shield of the RX coil on the balanced position will have some flux area (perpendicular to TX magnetic field lines) wherein eddy currents are induced and flowed through the shield. The total flux area of the RX coil shield has to be lowered therefore. A thin strip of AL foil with 1 mm gap wound as a spiral to the RX coil could lower the shield flux area.
I don't know, whether your proposal would work well. Just try out.

As I am measuring just above the ground noise level, I will totally shield everything of the coil with reduced total shield flux area. Shield closed loops should be avoided.
Maybe a different shielding could work well: TX with AL foil, RX with graphite. I have to try it out some time.


My new Creative Xmod sound-card is arrived. It is working well and it seams to be a low noise one. I have to test this device now...

Aziz

Ok I'll think about that, Rx shield geometry. But I was thinking about small charge movement even with gap -- during oscillations, magnetic field moves charge from one side of gap to other (capacitive effect). The longer the shield, the closer you get to 1/4 wave length resonance. So I thought, maybe break it up into smaller pieces. But probably too negligible effect to mention.

Good to see your progress. New sound card I assume means you don't need USB card? When you get a chance, more photos welcome!

Regards,

-SB

[Aziz]

Hi Simonbaker,

Aziz: do you think the length of the shield matters for reducing depth? Would a multi-segment shield have any advantage? I had an idea for multi-segment shield with central connections by very fine wires. See image.

Just another crazy idea...

Regards,

-SB

Just realized... only good for concentric coils...

When you see the TX coil as a stand-alone coil with AL shield, the shielding itself has less effects. The magnetic field lines of the TX coil goes parallel to the shielding surface area and therefore inducing nothing. No eddy currents are flowing on the shielding due to TX signal (shielding with open gap). It shields only external EMF.
The affect comes from the position of the shielded RX coil. The shield of the RX coil on the balanced position will have some flux area (perpendicular to TX magnetic field lines) wherein eddy currents are induced and flowed through the shield. The total flux area of the RX coil shield has to be lowered therefore. A thin strip of AL foil with 1 mm gap wound as a spiral to the RX coil could lower the shield flux area.
I don't know, whether your proposal would work well. Just try out.

As I am measuring just above the ground noise level, I will totally shield everything of the coil with reduced total shield flux area. Shield closed loops should be avoided.
Maybe a different shielding could work well: TX with AL foil, RX with graphite. I have to try it out some time.


My new Creative Xmod sound-card is arrived. It is working well and it seams to be a low noise one. I have to test this device now...

Aziz

[simonbaker]

Aziz: do you think the length of the shield matters for reducing depth? Would a multi-segment shield have any advantage? I had an idea for multi-segment shield with central connections by very fine wires. See image.

Just another crazy idea...

Regards,

-SB

Just realized... only good for concentric coils...

[Aziz]

Aziz:

How do you know hand and ground detection is capacitive instead of inductive?

Regards,

-SB

Hand detection test: direct comparison between graphite shielded coils vs. AL foil shielded coils. AL foil shielded coils do not detect my hand.

Aziz

[simonbaker]

Hi Simonbaker,



AL foil decreases the coil Q. The losses of the coils are increased (producing heat on the AL shielding). The seen parallel coil resistance Rp is lowered. Coil Q affects the sensitivity of the system. The higher the coil Q, the better the sensitivity will be. Remember: Signal drop on the RX coil will be converted to heat due to coil resistances (series coil resistance Rs, parallel coil resistance Rp) and cannot be used for signal processing then.

Graphite shielding is quite good but not good enough for this application. I have observed a high sensitivity for capacitance and ground effects (hand detection, ground detection, etc.) . I will test next time AL shielded coils in the field and expect much lower capacitance and ground effects. Hand detection tests with AL shieldings are passed successfully so far (not detected). But there should not be any unshielded wire or components in the coil anymore. I will also shield the passive electronic components totally to eliminate such problems in the future. The additional balancing winding will be a thin coax cable (RG174).


Aziz

Aziz:

How do you know hand and ground detection is capacitive instead of inductive?

Regards,

-SB