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

[Sergey_P]

Metal Detector - is the correlator! to extract weak signals of targets from noise. Noise >> signals of targets.

[Davor]

Yes, it is, but when interfacing a coil to a computer you have a problem of the non-existing sync which is on your way, otherwise you have to do all the sampling prior to the micro and supply it with the chewed up information, and Aziz obviously doesn't want that.

There is a more direct approach to establishing a phase. A Tx signal is mostly monotonous, and there is no need for some deep analysis in frequency domain. Given the Tx frequency as a fairly accurately known thing, you may expect that, given the sampling rate of ~4 times the frequency, your samples will walk the signal curve by a ~constant phase increment (BTW, that is exactly the principle of DDS). There is a method described in a Circuit Cellar article on a proton precession magnetometer that describes a "phase-slip" method of establishing exactly the phase increment between samples. This phase increment can be used for various purposes, including discrimination, phase locking, etc.
The best part is that you don't have to synchronise Tx with micro.

If you consider 4 successive time samples at ~4*f(Tx) as a single instantaneous phase sample, you'll have 4 voltages v0, v1, v2, and v3, and the phase sample is calculated as:
Φ=arctg((v0-v2)/(v1-v3))

Because of the sampling and Tx frequencies being out of sync, this Φ is increasing or decreasing by a constant rate according to the freq difference. But a bit more interesting thing is that if I do the same with Rx, I'll obtain Φ+φ, where φ is a Rx angle.

There is some staggering in φ, and also a possibility of phase reversal - due to the nature of arctg function and frequency difference, but it can be taken care of. A phase Φ can also be used as a reference for Φ+π/2, which is a next sample.

So, in any case you don't have to go for a grown up DSP.

[Aziz]

Metal Detector - is the correlator! to extract weak signals of targets from noise. Noise >> signals of targets.

Sure,

but how would you get the phase shift between TX and RX? Any ideas?

Well, I can detect phase shifts better than 1/1000 degree using the Lock-in amplifier. The Lock-in amplifier does not need to know the operating frequency of the detector as well.

Anyway, the digital (software) implementation of the simple VLF OSC detector gives incredible (!!!) results for less money (the Netbook not taken into account of course). I'm going to make the USB powered detector controller.

BTW, one can also use the commercial VLF coils . The LC oscillator need some modification (converting into a full-bridge version). But the center-tapped TX coil gives more bang to targets however.

Comeon Sergey, you and your Russian mates could do the "World's best VLF detector technology". I suggest, that you use the 24-bit sound cards. The high sample rate (96 kHz and more) isn't really required and it works even at 44/48 kHz sample rate too.

I'm looking forward to your suggestions.

Cheers,
Aziz

[Davor]

BTW, a phase increment Φ=arctg((v0-v2)/(v1-v3)) can be obtained as an average ... without averaging, but summing instead.

So I can write it as Φ=arctg((V0-V2)/(V1-V3)), where samples V0, V1, V2, and V3 are obtained as sums with depth N,
V0=sum(v(n)) ; V1=sum(v(n+1)) ; V2=sum(v(n+2)) ; V3=sum(v(n+3)) with n=(0,1,...N-1)

[Sergey_P]

<The high sample rate (96 kHz and more)> - required (to filter out the higher frequencies)
<24-bit sound cards> - isn't really required. Quite enough 16..18-bit, accumulated value of the digital signal – 22…24bit (I did layout detector with 1bit (sign) )

[Aziz]

Working on Universal USB Version

Hi all,

I realize, that an universal circuit is the best option for the USB version. So you can either use a center-tapped or a simple TX coil (or even commercial coils ).

Its in full implementation a full-bridge cross-coupled mosfet LC oscillator (for simple coils). If you leave the lower part of the transmitter unpopulated (n-mosfets parts) its a half-bridge for center-tapped TX coils (as usual).

Unfortunately, the full-bridge version requires a difference amplifier so I have to use a dual op-amp finally (NE5532/NJM2068 ). Anyway, the TX coil voltage is attenuated resistive in a difference amplifier (making the TX reference). And we have an active RX amplifier for more sensitivity and depth (therefore one dual op-amp required).

BTW, the full-bridge version makes very low inductivity TX coils possible (at the expense of more power consumption).

I'll build the full implementation and see what happens. If everything works as expected, then you can get the schematics (nothing to keep it secret - a very trivial work). A PCB version would make sense too.

Cheers,
Aziz

PS: Only the power supply of USB port is required. D+/D- not connected.

[Aziz]

Preliminary USB VLF OSC Detector Controller Schematics

Hi all,

this is the first version of the USB VLF OSC Detector Controller (subject to change). You know, I'm quite lazy and it will take some time until I can build the first prototype. But you will get the schematics first.

I'll put the LTspice files for your convenience too (including simple TX coil and center-tapped TX coil configuration).

This is the schematics:
(Yes, that's it!)



This is the LTspice file:
USB-VLF-OSC-Detector-Controller.zip


Enjoy and Cheers,

Aziz

[Aziz]

The Ultimate VLF Detector!!!

Hi all,

who is going to code the ultimate VLF detector software now?
( You know, I'm a retired(tm) software engineer. And I hate a lot of stuff. )

You finally have a very good VLF detector development hardware platform to make the "World's best VLF detector technology" now. It will easily knock out all the best and expensive VLF metal detectors.
(No kidding! Must see it's potential. )

BTW, I'm thinking of buying a very cheap VLF metal detector (50 - 100 EUR region) to re-use the coil and coil shaft. It's a bit risky with unknown commercial coils however. I know, that the K5000 coil (from CScope) would do it (it's a center-tapped TX coil). Pity, but I don't have my K5000 anymore.

It's time the smell the soldering iron again.
Cheers,
Aziz

[WM6]

BTW, I'm thinking of buying a very cheap VLF metal detector (50 - 100 EUR region) to re-use the coil and coil shaft. It's a bit risky with unknown commercial coils however. I know, that the K5000 coil (from CScope) would do it (it's a center-tapped TX coil). Pity, but I don't have my K5000 anymore.

Aziz, what are preferred data of those coil? Do you chose DD shape?

[Aziz]

Aziz, what are preferred data of those coil? Do you chose DD shape?

WM6, any induction balance (IB) VLF coil, which meets the following criterias:

Simple TX coil:
Extra shielding wire required (not single ended TX coil, shielding not connected to one end of the TX coil)
Either parallel capacitor connected (in the coil) or left free (TX tune capacitor in the controller box)
Should not have a damping resistor (not a PI coil)

Center-tapped TX coil:
Shield on center-tap or additional wire for shielding
Either parallel capacitor connected (in the coil) or left free (TX tune capacitor in the controller box)
Should not have a damping resistor (not a PI coil)

TX inductivity and resistance can be any. We can make it working. The LC oscillator is flexible.
RX coil can be any (inductivity & resistance). Preferably single ended.

I think that not every commercial coil is convenient.
Aziz

[WM6]

So something like this (why TX coil need to be shielded?):

[Aziz]

So something like this (why TX coil need to be shielded?):

Yep.

The shielding on the center-tapped TX coil could also be connected to the center-tap (usually COM). The center-tap in our circuit is the ground.

Shielding is good. If you don't have a shielding - ok, why not. It isn't much critical in the balanced version.
Aziz


PS: The IB coil of K5000 (CScope) would do. I have checked the schematics.

[WM6]

The shielding on the center-tapped TX coil could also be connected to the center-tap (usually COM).

As I understand your preferred design is centre tapped (as on drawing).

But you are onto build VLF, so to make coil, we need to know working frequency to tune it properly.
Or better working frequency and TX and RX tank data (to incorporate and tune LC tank in coil).
If you only purchase some in general suitable factory coil, your VLF will probably work, but with poor performance, cause of not precisely tuned (balanced).

[Davor]

Such coils can always be properly balanced with a small piece of ferrite that you may glue to the coil housing when you are done. It works.

[WM6]

Davor, be careful by those glued ferrite. It works, but fine sensitivity is gone and reactivity go worse, at least for me.