Android Phone Based Metal Detector - FCMD

[Aziz]

Hi Paul,

.. anyway the bottom line is that in engineering a good detector .. VDI accuracy is way down the list in priority from an engineering viewpoint.

yes, I think so too. Forget VDI and Disc. It is not reliable.

At the end, we are discriminating the targets to their own time constant (TC). This is dependent on target size, orientation and conductivity.
So make the detector to work on heavy mineralized ground (=good ground balance). And dig every target signal.
Aziz

[moodz]

That's what I'm talking about: the parameters of coils can be very different. The physical dimension can explicitly provide better stability and adjustment.

I am not sure why VDI should be the focus.
The priority in detection tasks is as follows :

1. Target sensitivity ( ability to detect targets small and big, deep and shallow )
2. Ground balance / removal ( ability to do task 1 with minimal ground effect )
3. Salt balance ( ability to do task 1 and 2 ) in salty conditions not just at the beach but where soils are conductive
4. Notch out things we dont want like rusty iron, foils, hotrocks etc ( ability to do task 1,2 and 3 ) without chasing crap.
5. If you can do tasks 1 - 4 ... you dont need VDI because there is only good targets left.

Pro searchers do task 1 to 4 as a combination of experience and learning their machine really well.
VDI is like a slot machine the pro searcher looks at the target after they have dug it up and says to themselves "waddya know ... the VDI said this gold sovreign was a ring pull ... LOL"

anyway the bottom line is that in engineering a good detector .. VDI accuracy is way down the list in priority from an engineering viewpoint.

[JoyJo]

That's what I'm talking about: the parameters of coils can be very different. The physical dimension can explicitly provide better stability and adjustment.

[moodz]

As far as I understand, such "zeroing" will solve the problem of temperature drift of the search sensor imbalance, but will not affect the stabilization (calibration) of the VDI scale.

The coil consists of inductance and capacitance and resistance ... you could have two different coils with different L C and R which give the same phase response at a fixed frequency.
Only frequency sweeping a coil will reveal the difference since thier resonances and Q will be different. So build a bode plotter into your detector.

[JoyJo]

Hmmm well would not you just adjust the TX phase till the signal at the demod ( RX ) is at some point like 0 degrees then you dont have to worrry about the coil phase shift.

As far as I understand, such "zeroing" will solve the problem of temperature drift of the search sensor imbalance, but will not affect the stabilization (calibration) of the VDI scale.

[Aziz]

... here is a short clip demonstrating Instant Ground Balance which seems like it might be a good thing.

https://youtu.be/pzm3iB40Adc

Great demo.

[moodz]

... here is a short clip demonstrating Instant Ground Balance which seems like it might be a good thing.

https://youtu.be/pzm3iB40Adc

[moodz]

I probably didn't describe the situation correctly. The resistor is located in the processing unit. And it sometimes connects instead of the TX coil: for some time necessary to measure the signal phase. It's embedded in the algorithm. And this measurement is a reference, for further calculation. This measurement takes into account only the parameters of the metal detector processing unit. And during the search process, for example, when entering the settings menu, the phase of the TX signal is measured: at the same time, the search coil is connected to the processing unit as usual. This is the advantage: you can measure the phase shift of any coil.

Hmmm well would not you just adjust the TX phase till the signal at the demod ( RX ) is at some point like 0 degrees then you dont have to worrry about the coil phase shift.

[JoyJo]

unfortunately measuring the phase in a resistor wont work .... The coils are manufactured to a certain mechanical and electrical standard so the manufacturer can "preset" or calibrate the characteristics in the code. They could read a resistor to identify the coil but nowadays its more likely to be a chip. I dont think any manufacturer really wants foreign coils on their detector.
In America the detector manufacturers have got together and formed the Induction Coil Enforcement agency ( ICE ) to make sure no-one is using foriegn coils on their detectors. You dont want to mess with those guys.

I probably didn't describe the situation correctly. The resistor is located in the processing unit. And it sometimes connects instead of the TX coil: for some time necessary to measure the signal phase. It's embedded in the algorithm. And this measurement is a reference, for further calculation. This measurement takes into account only the parameters of the metal detector processing unit. And during the search process, for example, when entering the settings menu, the phase of the TX signal is measured: at the same time, the search coil is connected to the processing unit as usual. This is the advantage: you can measure the phase shift of any coil.

[moodz]

Nice. I am looking forward to see your fancy math.

BTW, make your sweep tests with ferrite (or hot rock) and gold together. Just imagine the gold is surrounded in a high mineralized pocket.
Aziz

Here is coil sweep across large ferrite then crossing 1 gram gold nugget .. this particular nugget is a weak target and was found by a GPZ7000.
However the nugget signal clearly is extracted from the background signal.