Multi-frequency search coil.

[ZBIQ]

Hello!
Its my first serious post here .
I'm building "Metal detector with DSP" for my engineering diploma. I'm form Poland and I'm studying at Warsaw University of Technology.

My goal is to build multi (probably 3) frequency detector.

I have many questions about building a search coil.
Right now I'm using half-bridge to drive transmitter coil with 3 frequency square signal (like in Whites V3i).

And there is another problem. How to calculate the coil ? I want to use about 300mm diameter DD coil. Spectrum: form about 2.5kHz to max 25kHz - in my opinion the most
reasonable.
I've heard that the self inductance of the coil should be around middle of my assumed spectrum. But how exactly can I calculate that ?
Or should I make standard resonant circuit (coil + capacitor) and eventually, try to lower the Q parameter to "fit" whole frequencies ?

I'm using forums calculator to get inductance of the coil. Also Ive heard about Wheeler`s formula which gives good effects.

Could you give me some examples from commercial detectors ? I saw somewhere disassembled coil from V3i but now link is gone.

Regards, Mateusz.

[Qiaozhi]

I'm using forums calculator to get inductance of the coil. Also Ive heard about Wheeler`s formula which gives good effects.

Wheeler's formula is good for solenoids, but Brooks' formula is more accurate for air-cored coils.

[ODM]

What sorts of references would you need for citation in your diploma? Could you use something from Geotech's list of patents which includes multi-frequency detectors? I believe there are also some MF search coil patents listed, which have some of the specific math included in the patent text.

[ZBIQ]

Yes, I can use patents to my work. But there is nothing (anyway I can`t find any) about MF search coils.
I`m using complex square wave signal to produce 3 main frequencies - based on Minelabs patent.

I need working stuff more than complex math.

Now I`m trying to produce LC parallel resonant circuit with lowered Q to transmit sine wave in wide spectrum.

[Carl-NC]

MF coils aren't all that special, just wideband. A coil of wire driven with a rectangular voltage waveform, no caps. You want a low enough L so that the self-resonance is well above your highest frequency, and a high enough L so that your lowest frequency current doesn't drain the batteries in an hour. Both White's & Minelab use a 500uH TX coil.

[ZBIQ]

Great ! Thats something.

Can you give me some details :
Why the self-resonance is above my band ? My guess - to have flat band in full spectrum ?
I planned to use 0.45mm diameter wire.

How much current goes through Minelabs/Whites coils ?
I know that Whites uses boost converter to raise voltage (to about 20V ?).

[ZBIQ]

Lets assume that the lowest frequency is 5kHz.
It gives about Z=15.7R. Great power consumption !

I will increase number of turns to get about 3mH inductance. I`m sure that self resonance frequency will be around 70-100kHz.

[sinclairuser]

"I need working stuff more than complex math."
But the maths comes first! any device is useless in your situation if you cannot explain its function!.

Building is not enough you also have to understand all variables .

[Tepco]

Lets assume that the lowest frequency is 5kHz.
It gives about Z=15.7R. Great power consumption !

I will increase number of turns to get about 3mH inductance. I`m sure that self resonance frequency will be around 70-100kHz.

You will not have too high current consumption this way, with 17.5 Ohm inductive reactance at 5 kHz and square wave drive. Just relatively large circulating current thru the coil and switches, but most energy is stored in coil field and recovered. Only loss is in ohmic resistance and switching, if total resistance is around 1.7Ohm, only 1\10 of that circulating current will be consumed from battery. No need for 3uH, large circulating current is good thing (large field). This is not audio amp-speaker connection when energy is lost on speaker resistance, square wave drive (half or full bridge or push-pull) can recover reactive part.

[Qiaozhi]

Lets assume that the lowest frequency is 5kHz.
It gives about Z=15.7R. Great power consumption !

I will increase number of turns to get about 3mH inductance. I`m sure that self resonance frequency will be around 70-100kHz.

Carl already gave you a massive hint: "Both White's & Minelab use a 500uH TX coil."

[Carl-NC]

Great ! Thats something.

Can you give me some details :
Why the self-resonance is above my band ? My guess - to have flat band in full spectrum ?
I planned to use 0.45mm diameter wire.

How much current goes through Minelabs/Whites coils ?
I know that Whites uses boost converter to raise voltage (to about 20V ?).

V3 nominally uses 10V, boost mode uses 30V.

Coil current is nominally di/dt = V/L so for a square wave drive with 10V and 500uH we get 20,000 amps/sec.
For 2.5kHz this comes to 4App, for 25kHz it's 400mApp. Tepco is right, you can get most of this back with a good-sized tank cap (low ESR) and low-R switches. But, in general, expect the low f to burn more current than the high f. You can see this on the V3 in the single frequency modes.

After thinking about it some more, self-resonance may not be such a big deal. May just lower efficiency slightly.