Looking for a solution to improve slow coil electronically to let it decay fast and keep R-dump high

[Waikiki_Sweep]

I am looking for an advice.

Because of hydrodynamic shape limitations I can't add to much insulation and spacers into coil of my submersible PI detector.
I have to keep it slim. So coil will have too big internal capacitance anyway.

Is there any solution to improve slow coil electronically to let it decay faster and keep dumping resistor reasonably high?

For example using additional discharge circuit like you can see in Chance PI.

What you think?

[STAVROSNU]

I believe you should give it a try, as in Chance PI. Please do inform us of your results.

[green]

What is the inductance and resonance of your coil? What is the resonance frequency across the diodes with the damping resistor disconnected?

[baum7154]

I am looking for an advice.

Because of hydrodynamic shape limitations I can't add to much insulation and spacers into coil of my submersible PI detector.
I have to keep it slim. So coil will have too big internal capacitance anyway.

Is there any solution to improve slow coil electronically to let it decay faster and keep dumping resistor reasonably high?


For example using additional discharge circuit like you can see in Chance PI.

What you think?

If you short the coil with a mosfet as CHANCE PI does in the short pulse intervals, the short actually causes to coil to hold energy longer and to discharge more slowly.

I think you can do better with a spider wound toroidal coil with a coil form width of about 1.25" to 1.5". (See CHANCE PI COIL photos)The 'overwrap style' of spidercoil self shields (with the outside windings at system ground) and eliminates the need for a separate shield. This will help with the thin cross section you need for underwater work if you keep the windings flatter with a slightly wider form. Will the self shielding be enough for salt water? Only testing can tell. On the spider coil I use you can wrap my hand completely around it and with the CHANCE PI wide open and the self shielding is enough to keep the hand from being detected. If it does not work well in the water you can always apply a shield layer to the outside anyway.

By the way the (f.andy) original coil design of the CHANCE PI is quite fast but does not self shield and because it is so wide and flat it is very difficult to shield effectively and not lose a lot of it's speed, a similar problem to a spiral wound coil.

Good luck!

Dan

[Waikiki_Sweep]

Thank you guys for your reply. I will try to explain problem more clear.

After few years of detecting in salt water and building some coils and detectors I can say that

Every millimeter of coil thickness creates approximately one pound of drag in water.

Can't believe in it? Buy Whites Dualfield or Minelab Excalibur or Garrett Sea Mark II or Tesorro Sandshark or any detector so called "for water". Go to water and move it.

Better shape of coil to reduce that drag is "flat spiral". But flat spiral has huge coil to shield capacitance. If you add a spacer or insulation between coil and shield, sure you will have a good coil but again it will have huge drag in water.

So I am thinking how about to build a "bad" coil with high coil to shield capacitance but "good" for water (3-4mm. thin) and than use some smart dumping to accelerate decay of that slow coil for a moment right before to take sample.

This idea is not for Chance probably. Chance is good but we have no Chance source code to change software to improve it. But that damper schematic (second IRF9640) from Chance looks like exactly what I need.

Regarding Chance.
After some experiments.

With different coil inductance I can see some changes in target identification:

coil inductance --- 400 uH --- 300 uH
cooper coins --- high tone --- high tone
Au,Al,Ni --- middle tone --- middle tone
Ferric targets --- middle tone --- music box sound

Because no instructions I have no idea what that music "cannery bell" sound has to indicate? Is this Iron ID? Can you confirm?
If so, that is good because I want to get reed of iron trash. And lower inductance coil may help to adjust identification.
400 uH coil looks bad because gold and iron sound is same.

Those modes of iron rejection 2,3 reducing not only iron but gold rings signals significantly.

I have tried different coils. Inductance makes a difference in target ID.
Any shape can be used and for "slow" high capacitance coils guard interval has to be increased but sure platinum and some gold goes undetected if so.

Can't tell is my Chance works right or wrong. That is only what I can see.

[Waikiki_Sweep]

If you short the coil with a mosfet as CHANCE PI does in the short pulse intervals, the short actually causes to coil to hold energy longer and to discharge more slowly.

I think you can do better with a spider wound toroidal coil with a coil form width of about 1.25" to 1.5". (See CHANCE PI COIL photos)The 'overwrap style' of spidercoil self shields (with the outside windings at system ground) and eliminates the need for a separate shield. This will help with the thin cross section you need for underwater work if you keep the windings flatter with a slightly wider form. Will the self shielding be enough for salt water? Only testing can tell. On the spider coil I use you can wrap my hand completely around it and with the CHANCE PI wide open and the self shielding is enough to keep the hand from being detected. If it does not work well in the water you can always apply a shield layer to the outside anyway.

By the way the (f.andy) original coil design of the CHANCE PI is quite fast but does not self shield and because it is so wide and flat it is very difficult to shield effectively and not lose a lot of it's speed, a similar problem to a spiral wound coil.

Good luck!

Dan

Yes. Spider coils, basket coils are good but anyway if I have wire over wire design it will have more than 4 mm. thickness once coil will be finished with all insulation, waterproofing and fiberglass armoring.
Only flat spiral adding that small amount 0.5 mm of wire thickness to total coil thickness.

May be winding can selfshield in air but salt water much more conductive and can create capacitors even with very small unshielded areas. Practical test can show it works or not.

[Waikiki_Sweep]

Every millimeter of coil thickness creates approximately one pound of drag in water.

How I calculating it.

Go to the beach with Whites Dualfield.
Move it over dry sand with speed 3-5 ft. per second. That speed is pretty common for Surf PI and you can even find it in some manuals.

Than go to water and try to move coil with 3-5 ft. per second in water.

If you have not braked shaft (or your arms) instantly you may have to apply so many pounds of force if you want to move coil for at least part of that air speed.

One my friend is a very strong big man. Hi is using Minelab Excalibur and breaking a couple fiberglass Anderson shafts per month.

[Silver Dollar]

To speed it up you could try lowering the inductance. Some have gone to sampling during the TX pulse (no waiting
for the pesky delay). There is a time constant created by the inductance you cannot go beyond, though damping
individual windings might speed things up a bit.

I have some code I found online for the Clone PI it is very readable and looks very similar to the Chance code my
disassembler spit out. It's a 60k text file here it is in zip format.

[Ism]

What about redesigning shaft to be elliptical or similar low drag (side to side) shape?
I'm not underwater hunter so I might be off base but I have drug coil and shaft in surf and concluded he shaft is a good portion of the resistance.

[Waikiki_Sweep]

To speed it up you could try lowering the inductance. Some have gone to sampling during the TX pulse (no waiting
for the pesky delay). There is a time constant created by the inductance you cannot go beyond, though damping
individual windings might speed things up a bit.

I have some code I found online for the Clone PI it is very readable and looks very similar to the Chance code my
disassembler spit out. It's a 60k text file here it is in zip format.

Thank you! It is very interesting. I will try to read. I remember you told that code of Chance contains 60 pages should be mostly a "C" libraries linked to it.

[Waikiki_Sweep]

What about redesigning shaft to be elliptical or similar low drag (side to side) shape?
I'm not underwater hunter so I might be off base but I have drug coil and shaft in surf and concluded he shaft is a good portion of the resistance.

I am not only making shaft flat but hiding cables inside and making it sharp like two edges sword using sandpaper.

[Tinkerer]

Did you try to slow the "motion filter" for a coil speed of 1ft second? Best would be slowing the motion filter and changing the integration time for the same amount.

[Waikiki_Sweep]

Did you try to slow the "motion filter" for a coil speed of 1ft second? Best would be slowing the motion filter and changing the integration time for the same amount.

Thank you for an advise. Sure I can go slow. But my point is to go in water as fast as in air and machines I have already build allowing me to do so.
I just want to make coils even more thin to have less fatigue in arms but keep sensitivity to gold high.
There is a problem: thinner coil -> higher capacitance -> slow decay -> less sensitivity to gold.
And solution I am looking for is to improve high capacitance "slow" coil by alternating damping.
For example dump coil to several micro-volts fast just after fly-back than listening to samples with higher dumping resistor.
I hope it is doable.
Looks like I have to try if nobody objects yet.

Also if it is doable why not to speed-up coils for gold prospecting using same trick?
There should be something wrong or it has to be used in every detector 30 years ago.

P.S. I have interesting idea for that "integration time".
I saw cheap digital acceleration sensors on eBay. We can put one into coil
and let microcontroller to correct integration time depending on how fast operator swing the coil.

[Silver Dollar]

Have you read this thread?
http://www.geotech1.com/forums/showt...UIT-FOR-A-COIL

You can download the Patent here; http://ip.com/pat/AU2013101058A4

He talks about using constant current damping and not having to worry about capacitance.
This is something I was going to experiment with some time in the future...

[Waikiki_Sweep]

Have you read this thread?
http://www.geotech1.com/forums/showt...UIT-FOR-A-COIL

You can download the Patent here; http://ip.com/pat/AU2013101058A4

He talks about using constant current damping and not having to worry about capacitance.
This is something I was going to experiment with some time in the future...

Thank you for point me into right direction! Now I do not have to invent bicycle