Chance PI coil...

[baum7154]

A wider and modified version of this coil is what I am using on my Chance detector. I used 32 turns of 26 AWG silver plated, stranded 600 volt teflon insulated wire on a 1.050 wide, .093" thick 8" OD. Lexan form with the slots machined .400" deep from each inner and outer edge. The feed line is a continuation of the coil wire 33" long and twisted at a rate of 3 twists per inch. This results in a 335 uh coil.

The system ground is connected to the wire end of outer winding and this results in what I believe is a degree of self shielding due to the geometry of this coil construction. I used 4 each lengths of 3/8" urethane foam 'Backer Rod' and 1/4" thick x 1/2" wide strips of urethane packing foam to fill out the cross sectional profile of the coil. The Backer Rod and foam strips were over wrapped with Curity self adhesive cloth hospital tape, using a 50% tape overlap on the inner part of the form. Consistent tape wrap pressure was used to avoid crushing the profile. Then at least 2 layers of fiberglass mat and resin were applied and a Lexan cross support with Lexan mounting ears were glassed in across the lower third of the assembly.

Initial critical damping of this coil performed with the coil suspended on the boom in the air required 1040 ohms in order to get the unit to operate down to a Guard Interval setting of '10'. While detecting a 1/2" SQUARE aluminum target on the ground I noticed that a damping resistance of 980 ohms gave the best signal strength. However the unit would not operate in open air at a '10' GI setting with that resistance value, it was just on the threashold of operation. I re-ran the damping adjustment in air last night and ended up with 1110 ohms and good overall sensitivity while achieving good idle state quietness of the detector i.e. just a few occasional beeps. Will test with the foil target on soil this evening to see what the impact to signal strength has been.

All of this leads up to my perception that bringing the earth/capacitance into the picture appears to lower the self resonant frequency of the coil and results in reducing the optimum value of the damping resistance. Has anyone addressed this issue in a different process for determining critical damping resistance? Otherwise I believe that setting resistance in open air does result in some over-damping of the coil when it is in use close to the earth. I do recognize that 'Earth' effect on a coil is highly variable so maybe damping in open air is the best we can do.

Regards,

Dan

Pictures attached here with the description above should give a pretty good idea of this coil construction sequence. Unfortunately I do not have pix of the fiberglassing of the coil and the glassing in of the support member but the finished coil pix should give the general idea. The cross member is constructed of the same polycarbonate as the coil form in the first picture. I misspoke about the foam type in the description above…it is polyethylene foam and backer rod. The polycarbonate mounting ears were heat formed on a wooden profile. The polycarbonate was heated to 375 F and bent around the wooden form and allowed to cool in the shape of the ears.



More photos of this CHANCE PI build will be posted this weekend in the TECH FORUM under the title of CHANCE PI BUILD.

Dan

[baum7154]

I will post some detailed pictures of the construction of the 8" 335uh fast SPIDER coil used in the tests above tonight or tomorrow.

Dan

[baum7154]

Last night I did a bit more testing with the 8" fast coil and the same 4.5 grain 14K gold target. My detector has the capability to lower the main supply voltage in steps down to about 72% of the battery voltage. In the test performed I was able to extend first indication of detection to 3" by lowering voltage from 13.3VDC to 11.3VDC.

This makes me think I could do with a bit more coil damping resistance at full battery voltage. Does this make sense?

[Davor]

Not even the best VLFs can do that

[baum7154]

Thanks for your input! I have no experience in metal detecting in the field with anything but the CHANCE detector I have built so it is good to get opinions from those who do. I need to spend much more time in the gold bearing areas to get a better idea of what this detector is capable of doing overall. The last time I was out on the old dredge piles there was so much aluminum can debris in small pieces that it took most of my time digging useless holes. I need to choose a better location to minimize the presence of the more modern aluminum debris that permeates most of the easily accessible grounds. Anyone have a way to discriminate gold from aluminum bits? Now that would be useful!

Thanks again,

Dan

[WM6]

This is very good result for 0.27gram piece of gold using 8" mono coil.
To detect such small piece we usually need to use 4" or 5" coils.

[baum7154]

CHANCE Sensitivity test

I performed another sensitivity test on CHANCE with the 8" fast coil today. Using a 4.5 grain piece of 14K jewelry gold (a 'head', made to hold a stone in a pierced earring but with no earring post) CHANCE gave first indication of detection in air at 1.750" or at 44.5mm. I don't know if this is 'good sensitivity' compared to some other detectors. Does anyone have comparable test results?

Regards,

Dan

[baum7154]

Coil damping

Tested detector with 1110 ohm coil damping on 1/2' target tonight over wet grass and saw no degradation in signal strength from last night's test over wet grass. Got the same 8 signal strength units at maximum proximity to target. Coil is 8" diameter.

Other targets were tested in air in the basement as follows:
TARGET DISTANCE
1 sq in x .001" thick aluminum foil 9.5"

1/2 sq in Aluminum foil 8"

1/4 sq in Alum foil 7"

1 cm sq Alum foil 4"

US Nickel 12"

US copper Penny 9"

US silver dime 7"

10.5 grain dental gold 6"

8.4 grain dental gold 4"

[baum7154]

A wider and modified version of this coil is what I am using on my Chance detector. I used 32 turns of 26 AWG silver plated, stranded 600 volt teflon insulated wire on a 1.050 wide, .093" thick 8" OD. Lexan form with the slots machined .400" deep from each inner and outer edge. The feed line is a continuation of the coil wire 33" long and twisted at a rate of 3 twists per inch. This results in a 335 uh coil.

The system ground is connected to the wire end of outer winding and this results in what I believe is a degree of self shielding due to the geometry of this coil construction. I used 4 each lengths of 3/8" urethane foam 'Backer Rod' and 1/4" thick x 1/2" wide strips of urethane packing foam to fill out the cross sectional profile of the coil. The Backer Rod and foam strips were over wrapped with Curity self adhesive cloth hospital tape, using a 50% tape overlap on the inner part of the form. Consistent tape wrap pressure was used to avoid crushing the profile. Then at least 2 layers of fiberglass mat and resin were applied and a Lexan cross support with Lexan mounting ears were glassed in across the lower third of the assembly.

Initial critical damping of this coil performed with the coil suspended on the boom in the air required 1040 ohms in order to get the unit to operate down to a Guard Interval setting of '10'. While detecting a 1/2" SQUARE aluminum target on the ground I noticed that a damping resistance of 980 ohms gave the best signal strength. However the unit would not operate in open air at a '10' GI setting with that resistance value, it was just on the threashold of operation. I re-ran the damping adjustment in air last night and ended up with 1110 ohms and good overall sensitivity while achieving good idle state quietness of the detector i.e. just a few occasional beeps. Will test with the foil target on soil this evening to see what the impact to signal strength has been.

All of this leads up to my perception that bringing the earth/capacitance into the picture appears to lower the self resonant frequency of the coil and results in reducing the optimum value of the damping resistance. Has anyone addressed this issue in a different process for determining critical damping resistance? Otherwise I believe that setting resistance in open air does result in some over-damping of the coil when it is in use close to the earth. I do recognize that 'Earth' effect on a coil is highly variable so maybe damping in open air is the best we can do.

Regards,

Dan

[bo198673]

Need to purchase CHANCE PI, any person to sell can contact me [email protected]

[golfnut]

couple of decent teaching links

http://www.electronics-tutorials.ws/...magnetism.html

http://www.electronics-tutorials.ws/...magnetism.html


http://www.electronics-tutorials.ws/...romagnets.html



http://www.electronics-tutorials.ws/...induction.html

[DOOLEY]

hello golfnut ,

regarding the spinning magnet idea , crossed my mind also a long time ago , could poss use many of those realy strong magnets , molidnyum ( bet iv'e spelt that word wrong! )

the timming would be the easy bit , hall sensor arangement as used in allot of brushless motors,

suspect the motor used to spin the arangement would deff use more power than the average vlf and poss more than most PI's though , even if on ball bearings , and we would have a life span problem , moving parts wearing and all that.

other thing is , as far as i'm aware , an efective shield for a magnet not yet invented.

as far as efficiency goes , coil would take some beating ,

sorry , some people say i'm quite negative.

[Al1g]

Built a coil according to these designed. The self resonance is ~1.2mhz. But when I connect it to the Hammerhead it rings terribly. Critical damping is 290ohm. Any ideas what the issue might be?

[johnandles]

Golfnut,I never thought outside of the square on this.
Your approach sounds feasible.
Carry on,I would like to see some results.
Regards
John

[golfnut]

Hi Dooley I dunno if mig wire being cooper caoted steel is a problem - I suspect not.

Maybe try one if I ever get spare time - you can get 0.4mm too for thin sheet welds.

But it could loose the return signal due to eddt current losses in the steel windings!!



John I know what u speak of Ive written assembly code for an ESC.

Jurys open here, I just think the indutry is missing a trick here - we want to induce eddy currents into targets,,,,,, A pokey magnet is just the job.

many ways of attempting this,, ally plate with slot rotating past magnets letting field out in bursts.

the fact that were simply sweeping, the target gets a decaying sinusoidal field into it...

or have bar magnets rotating, U could synch the Rx to rotation position,

surely mileage here somewhere - dunno if Ive the nouse

Steve