Android Phone Based Metal Detector - FCMD

[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.

[moodz]

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

https://youtu.be/pzm3iB40Adc

[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.

[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.

[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]

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]

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.

[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]

Hi Paul,



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]

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

https://youtu.be/pzm3iB40Adc

...and here is the salt response https://youtu.be/4JJUaO6ZHIk?si=HmHHzrQfoB62_ZM5

[JoyJo]

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.

You are absolutely right. Vdi is not of priority importance. But if, from possible factors, it can go beyond 0-180 degrees (or some types of targets will go beyond the scale), then they simply will not be voiced. And they won't be found.

[JoyJo]

That's all: homemade detectors have such a procedure: ferrite calibration. With the help of this calibration, the phase parameters of the detector are brought to the desired state: the vdi scale is calibrated. To do this, you should always have a piece of ferrite with you. It's not convenient. The whole question is to make this calibration automatic, without human and ferrite participation.

[Carl-NC]

4. Notch out things we dont want like rusty iron, foils, hotrocks etc ( ability to do task 1,2 and 3 ) without chasing crap.

Rusty iron can easily mask a good target, and I've dug small jewelry that ID'd as foil. So if you want to eliminate the possibility of missing a good target, you use AM mode and dig absolutely every signal. But -- depending on the amount of trash, the hardness of the ground, or the time available to hunt -- this may not be practical. Maybe today you want to cherry-pick silver coins, in which case discrimination and VDI become useful tools. There is no "absolutely right," it depends on the situation.

[moodz]

...I agree. If I am a bit more charitable .. VDI does have its use cases. If you are searching a well searched place like a beach then you are only interested in the stuff that was dropped in the last day or so, the good targets will all be nearer the surface ( ie close to the coil ) and you will have good VDI readings.
However my research at the moment is concentrated on deep targets and cancelling the ground / salt. I also want to eliminate all the tiny nuisance targets like lead shot and slivers of rust from decayed cans and wire. Although lead shot is similiar to small gold.

[moodz]

The big challenge for my work is to cancel conductive ground ( like salt ) AND ferrite simultaneously ....

Here is a video of the results ...

https://youtu.be/CxXo76ON9_Y