TineFPGA-GOLD Metal detector

[SaltyDog]

Trying to understand conductivity. A chart of some targets made with copper wire. If conductivity of copper is a constant why does the decay slope change. Does a target have conductivity or does the material have conductivity?

Interesting thing about my project ... copper is invisible... I don't pick it up at all ..

[SaltyDog]

One thing I also learned with the Pickini project: try to make n samples x pulse interval = equal to or a multiple of the mains period.
Eg here in Europe 50 Hz = 20 ms. Suppose your pulse period is 2 ms. Accumulate 10 pulse samples, instead of the easier to calulate 8 or 16.

Thanks, but I seem to be quite immune to interference at the moment, that might change when I get more sensitivity, so good suggestion ..

[6666]

The 50/60 Hz mains frequency can be a problem in different parts of the world, having a TX pulse rate of 1200 pps and slightly variable can help because its dividable by both 50 and 60 Hz

[godigit1]

Thanks, I have already a LM4562 ordered ... taking awhile due to the covid-19 issue ...

Sorry about the spelling, was a fast text on my phone at lunch I should have double checked.

[SaltyDog]

Sorry about the spelling, was a fast text on my phone at lunch I should have double checked.

Yep, have no idea what thread you were referring to

[godigit1]

Sorry you probably already visited it though.
https://www.geotech1.com/forums/show...tly-improvable.
Cut and Paste I cant screw up.
Best

[6666]

Sorry you probably already visited it though.
https://www.geotech1.com/forums/show...tly-improvable.
Cut and Paste I cant screw up.
Best

Requires careful reading, I got the parts for it, but was too sick at the time, might have to revisit.

[SaltyDog]

Sorry you probably already visited it though.
https://www.geotech1.com/forums/show...tly-improvable.
Cut and Paste I cant screw up.
Best

Many thanks, I had forgotten about that thread... I will try some of the mods directly on the MPP board

[Tinkerer]

Trying to understand conductivity. A chart of some targets made with copper wire. If conductivity of copper is a constant why does the decay slope change. Does a target have conductivity or does the material have conductivity?

Very interesting chart.
Do we see a correlation of skin effect with AWG19, open and closed?

[green]

Very interesting chart.
Do we see a correlation of skin effect with AWG19, open and closed?

My thought, maybe correct maybe not. Skin effect causes the decay slope to be faster at start of decay. Time constant=L/R, R is higher at start of decay because current is just on skin. AWG19 is closer to straight line decay than AWG10(thicker wire) https://www.geotech1.com/forums/atta...3&d=1586531943. Was interested in thread and wasn't going to reply until SaltyDog replied The ADC specifics as well as the Slope detection module details I will be keeping close to my chest as they are unique idea's (IP) I want to keep . Couldn't see how to determine what the target material is based on decay slope so I posted the charts to show what I see as the problem. All targets where made of copper, many different slopes. My main interest in the thread was defining ferrous or non ferrous looking at signal during coil decay after Tx off . Tried looking at signal during Tx on with an IB coil. Worked on the bench but the ground signal made it difficult to impossible when trying to detect.

[flex]

As I understood this detector has ho issues with GB, even it use a mono coil. Which is impressive ;-)

[dbanner]

My thought, maybe correct maybe not. Skin effect causes the decay slope to be faster at start of decay. Time constant=L/R, R is higher at start of decay because current is just on skin. AWG19 is closer to straight line decay than AWG10(thicker wire) https://www.geotech1.com/forums/atta...3&d=1586531943. Was interested in thread and wasn't going to reply until SaltyDog replied The ADC specifics as well as the Slope detection module details I will be keeping close to my chest as they are unique idea's (IP) I want to keep . Couldn't see how to determine what the target material is based on decay slope so I posted the charts to show what I see as the problem. All targets where made of copper, many different slopes. My main interest in the thread was defining ferrous or non ferrous looking at signal during coil decay after Tx off . Tried looking at signal during Tx on with an IB coil. Worked on the bench but the ground signal made it difficult to impossible when trying to detect.

I'd be satisfied with a machine which can ignore iron and bottles caps, never mind discrimination.

The decay at the very early stages of the decay slopes look very much the same, not identical, but that may only be as a result of the resolution of the graph itself. I wouldn't want to be digging wire anyway.

On the white sand beaches, a set of preferred target slope characteristics such as US coins and typical gold rings could be stored in memory and compared to the slope of the actual target at the earliest possible part of the decay which allows for slicing.

Maybe this would be the best case scenario, try to look for targets that sort of approximate the desired targets slopes.

The variation of target material conductivity and geometry (just think of the myriad of objects found at the beach) would likely produce an endless number of decay slope possibilities, few of which may be distinct enough to allow for discrimination based in slope slicing.

Inland detecting poses the problem of soil mineralization.

[godigit1]

As dbanner is saying discrimination is problematic even with a Vlf many targets of different composition fall in the same Target Phase due to size or shape.
On a Fisher disc #79 is a zinc penny same 79 also turned out to be a 17 gram 10 k medallion.

I would be very happy with a Pi that could ignore Iron and still hit a picker. Even better just hit gold.

Not being able to hit copper at all is strange and interesting, some variation of copper should come out of the hole or I,m thinking about it wrong.

I've been working on a GB board ( finally finished the PCB) so this seems like a good time to take a break and get back to the op amp projects.

I'm still learning but will try to help where I can and try not to muddle things up.

[Tinkerer]

"My thought, maybe correct maybe not. Skin effect causes the decay slope to be faster at start of decay. Time constant=L/R, R is higher at start of decay because current is just on skin. AWG19 is closer to straight line decay than AWG10(thicker wire)"

I would agree with that.

How would you describe your TX transient?
How many Amps before switch off? What is the slope of the switch off transient? di/dt, or Amp/u second
How does this compare with the TC of the AWG19 closed wire ring?
Have you tried an iron wire ring of similar thickness wire?

[Qiaozhi]

Testing targets with a ring shape will give misleading results, as the shape itself will enhance eddy currents. Test targets are normally ball shaped for consistency, and to eliminate differences caused by orientation.