TineFPGA-GOLD Metal detector

[dbanner]

Why not replicate the Al foil 1x 2x 3x 4x tests. See what your fpga does with those slopes.

[SaltyDog]

Why not replicate the Al foil 1x 2x 3x 4x tests. See what your fpga does with those slopes.

ok, could do, if I had any foil, I am in lockdown and am one of those that is immune compromised, but will see what I can do.

Then I will go out and find all those abundant Al squares found in nature .

[Qiaozhi]

Another interesting test, is regarding EF (Earth field). In practice I see no response from swinging my detector head at maximum sensitivity.
I do get a response when placing a large horse-shoe magnet next to the coil .... which is how I derived the LED number for it ..

I suspect the lack of response to the Earth's magnetic field is due to the reduced sensitivity of your current configuration. That is, direct sampling of the preamp output using a 10-bit ADC. A typical PI will have around 80dB more gain than you are using. This seems to be confirmed by the gold ring detection at only 6 inches, and the inability to detect a horseshoe magnet unless it's next to the coil.

[Mdtoday]

Saltydog, interesting project,
Have you managed to test the discrimination in real ground conditions?




cheers

Mdtoday

[green]

The coil Tx above was done with constant rate 160us on time with 1A peak. The coil is fast so mosfet does avalanche at about 560V. Not sure what you are calculating, I'm thinking di/dt=E/L. 1.18A/us at avalanche or 2.1ma/us at 1 coil Volt. Only time decay rate is constant is at avalanche. Maybe wrong?

Not sure what you are calculating

Maybe average current decay rate. Peak current/time to what lower current? Wondered how much decay rate effects target signal. Tried a spice simulation changing Rd to control decay rate, critical damped resistance and critical damped resistance/(2 or 4).


Two different targets, 1us TC and 10us TC. With 10us TC target, 3us after turnoff target signal is the same, 7us after turnoff Rx signal is the same and after 20us Tx receive signal is close to same with most damped being slightly higher.

With the 1us target, 1us after turnoff the more damped target signals are higher, 2us after the more damped Rx signals are higher, the critical damped is the only Tx decay the target is visible.

Tx damping has little effect on target and IB Rx signal. Big effect on how soon Tx signal can be looked at with a mono coil.

[green]

Decay charts with some different materials.

(third chart)1x1in targets cut from aluminum can, lead sheet and copper clad board have almost the same decay rate. Big difference between 1x1in aluminum foil and aluminum bar targets. Big difference between 1x1in copper clad and piece cut from copper tubing.

Second chart is an example of decay slopes with Carl's 1x1in aluminum foil targets(to 16 layers, didn't try 32 layers)

[Carl-NC]

Appreciate the comments. However they are "general" comments & as you say "theory", it's just not how my unit reacts "in practice" ...
.... and this is not a VDI machine, totally different machine & should not be used for comparison .... my 2c

If your design is induction based, then eddy currents work the same way regardless of how you are doing the induction. From what you've shown, it is a VDI machine, using LEDs to indicate target types. In the end, I think you're going to find that lead, aluminum, and stainless steel can all be gold. If I'm wrong, then you have a truly remarkable design!

Then I will go out and find all those abundant Al squares found in nature .

I use the aluminum squares to determine where target holes are in PI-GB designs. Assuming household foil is universal, anyone anywhere can make up a set and get the same results I get. They are also useful in demonstrating skin effect, whereby aluminum can mimic any target, even silver.

Another useful "universal" target is lead shot. I use a variety of shot sizes (down to #9) to emulate gold nuggets, and anyone anywhere can get the same shot sizes and therefore get the same results.

[WM6]

An update:

With the latest tweaks, I have managed to isolate stainless steel from gold and silver as well .... so currently I have.

LED1: Stainless STeel
LED2: Gold/Silver
LED3: Aluminium
LED4: Nickel coin etc
LED5: Iron
LED6: Lead
LED7: Magnets

Copper is still not detected, I need to try some other forms of copper ..

What about:

LED8: Diamant and Gemstones ? Interested in metallic colored gemstones.

Or at least LED9, if LED8 become Copper.

[green]

If your design is induction based, then eddy currents work the same way regardless of how you are doing the induction. From what you've shown, it is a VDI machine, using LEDs to indicate target types. In the end, I think you're going to find that lead, aluminum, and stainless steel can all be gold. If I'm wrong, then you have a truly remarkable design!



I use the aluminum squares to determine where target holes are in PI-GB designs. Assuming household foil is universal, anyone anywhere can make up a set and get the same results I get. They are also useful in demonstrating skin effect, whereby aluminum can mimic any target, even silver.

Another useful "universal" target is lead shot. I use a variety of shot sizes (down to #9) to emulate gold nuggets, and anyone anywhere can get the same shot sizes and therefore get the same results.

Assuming household foil is universal, anyone anywhere can make up a set and get the same results I get. They are also useful in demonstrating skin effect, whereby aluminum can mimic any target, even silver.

What is the definition of skin effect?

[Carl-NC]

What is the definition of skin effect?

https://en.wikipedia.org/wiki/Skin_effect

Wiki presents it in terms of AC current in a wire but the same thing happens with eddy currents in targets.

[Skippy]

Quote Carl:" Assuming household foil is universal ..."
Mr. Green and I have discussed this on one of the previous 'target testing' threads. The thickness is far from universal, and it needs to be accurately measured and stated. There appear to also be at least two different alloys, possibly three, used for 'kitchen foil' . The most common is 'pure aluminium', with just residual impurities. Another common type has a small percentage of iron in it, to ( I think ) improve ductility.
Obviously these details are not so important if you're simply comparing 1,2,3,4,.....32 thicknesses, as the thickness & alloy composition will impact all the test samples identically.

When I get back into the 'target testing' stuff again, one of the target types I will analyse is my large 'kitchen foil' collection , going up to nearly 0.2mm.

SaltyDog: good to see you're asking on DetectorProspector, I'm sure you'll soon be able to get your real gold samples. And politely ignore all those who suggest blobs of foil/lead/yak dung etc as substitutes.

[SaltyDog]

Thanks for all the suggestions guys .... but this "in-depth" testing is going to have to wait till when I actually finish the design, after all, I have only been on this project the two weeks of lock-down ...

I haven't even got my fast op-amps yet, or some other components I need .. things are a bit tough to get, especially in NZ with our strict lock-down ... but the governments aim here is "eradication" of the virus, not just "containment"

@Carl-NC I hear you, but regarding induction, induction by sine-wave and induction by fast pulse square-wave produce totally different eddy currents in the target .... it is comparing apples & oranges .. the VDI machine you refereed to is a multi freq sine wave VLF machine right?

[Carl-NC]

@Carl-NC I hear you, but regarding induction, induction by sine-wave and induction by fast pulse square-wave produce totally different eddy currents in the target .... it is comparing apples & oranges .. the VDI machine you refereed to is a multi freq sine wave VLF machine right?

Different waveforms, yes, but exactly the same physics. Exponential vs sinusoid, tau vs phase; it doesn't matter, eddies and skin effect work the same way. It's kinda like saying, "I have an RL circuit that behaves one way when driven with a sinusoid, but an entirely different way when driven with a square wave." Not really, it behaves like an RL circuit in both cases.

Yes, the V3 is a MF machine. A single freq machine will give similar results. So will a PI. All I'm saying is, don't get overly optimistic, you have a long way to go. It's a situation where I'd love to be wrong, but don't expect to.

[green]

https://en.wikipedia.org/wiki/Skin_effect

Wiki presents it in terms of AC current in a wire but the same thing happens with eddy currents in targets.

Thanks
Chart with stacked nickels and stacked quarters. Four quarters decay faster than 1 quarter at start of decay and slower latter in the decay with a PI. What happens when IDing stacked coins with a VLF?

Can a target thick enough to be effected by skin effect decay straight line on a linear X log Y chart?

[SaltyDog]

Different waveforms, yes, but exactly the same physics. Exponential vs sinusoid, tau vs phase; it doesn't matter, eddies and skin effect work the same way. It's kinda like saying, "I have an RL circuit that behaves one way when driven with a sinusoid, but an entirely different way when driven with a square wave." Not really, it behaves like an RL circuit in both cases.

Yes, the V3 is a MF machine. A single freq machine will give similar results. So will a PI. All I'm saying is, don't get overly optimistic, you have a long way to go. It's a situation where I'd love to be wrong, but don't expect to.

Let's just say we differ on that, and leave it at that ...