Chance PI coil...

[baum7154]

More pictures, this time of an experimental Teflon insulated 23 AWG solid wire wound spider coil form, and a different graphite shielded coil wound with 26AWG solid enameled wire. The teflon insulated 23AWG solid wire came from a plenum rated Cat6 cable. The graphite shielding was a mixture of varnish and powdered graphite and was applied at a rate of 1300 to 1500 ohms/inch. All coils are 8" diameter. The shielded coil was fiberglassed with two layers of woven fiberglass mat and a lexan cross support and mounting ears were 'glassed in' just like the coil in the prior posting.

Theses polycarbonate coil forms were machined on a milling machine using a rotary index table and a 1/8" cutter. Using a cutter with a larger diameter than the thickness of the material causes less bending stress on the wire/insulation because the wire is bent less than 45 degrees as it is passed through the radial slots and also minimizes parallelism of alternate wires within a slot resulting in less capacitance. This process is preferable to just slotting the form material with a hacksaw. That said,hacksaw slots can be hand filed to depth using a 1/8" fine round file to achieve similar results.

[6666]

Dan, thanks for taking the time to show us your coil construction.

[baum7154]

Thanks for taking the time to look at them! The rest of the CHANCE detector photos have just been posted in the TECH FORUM under the title of CHANCE PI BUILD.

Regards,

Dan

[baum7154]

Took this coil on my CHANCE out today and buried a 10.8 grain piece of dental gold in a hillside at my prospecting site. Was able to detect the piece at a depth of 4.5 inches with a signal strength of 5 units and a solid ID of 'Gold' on the left end of the scale.

Dan

[baum7154]

The following is a repost of a discussion on coil construction from the Tech Forum titled CHANCE PI CIRCUIT DIAGRAM NEEDED I thought it really belonged in the COILS forum…Dan:

Originally Posted by Waikiki_Sweep
Reporting broken off aluminum pull tab as gold is a legit result! Just OK!

Beer cap as gold is confusing but do not worry. You can find same failure for several very good metal detectors starting with popular Fisher CZ series.

Anyway your results with chance are amazing!!!!


There is your coil cooking receipt revealed:

insulated stranded wire = fast coil

basket shape = fast coil

insulated stranded wire X basket shape = SUPER fast coil




Yes you have the gist of what I have done! However there are a few more nuances to the coil.

1. Eliminate the coax by using a continuation of the coil wire to make a twisted pair feed line. This eliminates a lot of capacitance and there is no lead or tin from solder in the field of the coil.

2. Use the smallest stranded gauge wire with at least 600 volt (thick) PTFE AKA Teflon insulation to minimize capacitance. This coil was wound with 26AWG silver plated 600 volt teflon insulated wire. I am looking for some 28 AWG 600 volt teflon wire now. The earlier coil wound with 23 AWG solid conductor with thinner Teflon wire had much more capacitance. I know that the silver plating is a no-no due to eddy currents but as the wire gauge gets smaller it gets diminished.

3. Do not flood the coil with any insulating resin. I used rod and sheet polyethylene foam to fill out the profile as in the pictures posted. Then 2 layers minimum of fiberglass woven mat and resin for armor to protect the coil.

4. I did not do a shield on this coil, just tied the wire lead from outermost windings to ground. Done this way I believe the geometry of this coil does provide a degree of self shielding but it may not be enough in some situations. No problem! The additional shield, if needed, can be applied directly to the fiberglass shell in the form of a painted on mixture of graphite and varnish with a protective layer of fiberglass over that and a connection back to the ground end of the feed should do the trick. I would make the shield coating measure in the range of 2k at one inch separation of the probes. By the way, I have seen comments on testing a shield by putting your hand on the coil and monitoring to see if the detector reacts to your hand (no rings or jewelry). This detector with the self-shielded coil does not react to this hand test which indicates an adequate amount of shielding if I understand the test.

5. For the CHANCE PI detector you do not have to use the prescribed 400uh coil and in fact, if you seek small gold, a fast/low capacitance coil in the range of 300uh to 350uh with an optimized damping resistor is the way to go. The coil in the photos posted is 334uh and about 2.0 ohms with an 1150ohm damping resistor. Feed line is 33 inches long…shorter is better.

6. Coil slots in the coil form must be wider than the form material is thick. This will insure that the wires cross in the slots at about the optimum 90 degrees to each other minimizing capacitance. Slots made as narrow saw slits will put the wires parallel to each other as they pass through increasing capacitance.

This mono-coil recipe should work for just about any PI detector designed for finding small gold…not just CHANCE PI. The coil I use was derived by making a 40 turn/486uh coil and making measurements, removing 1 turn at a time, while testing sensitivity to small gold targets using the CHANCE PI detector. In this case 32 turns was best.

Happy Hunting

Dan

[baum7154]

I did find the quote referred to above on using the 'Hand Test" and it comes from the COILS forum in a thread on:

Rule of thumb for shield effectiveness for PI

Pi detectors with a delay to the first sample over 20us, do not need any shield.

If you are looking for targets with a TC of less than 10us you definitely need a shield.

4 to 8mm distance between coil winding and shield makes for faster coils.

Test your shield material with the detector. The detector should not "see" the shield material.

Whatever shield you use, always leave a small gap somewhere on the circumference, 2mm is enough.

Set your detector to maximum sensitivity and connect the coil without shield. Approach your hand to the coil. If the detector does not "see" your hand, you need no shield.

If the detector "sees" your hand without the shield connected, then it should not "see" your hand anymore after the shield is connected.

If the shield is graphite paint, just add layers until the "hand test" is OK.

The most important part of the shield, is to have a good connection to GND. A connection based on friction, (like aluminum foil not soldered) will make noise when the coil hits a rock or your shoe.

Tinkerer

Thanks to Tinkerer for the guidelines above.

The fast coil detailed in previous posts/pictures passes the 'Hand Test' with the detector set to a wide open minimum Guard Interval of 10 and Barrier 1, no MASK. There is no other shielding than the outermost coil winding connected to ground. I am tempted to do a graphite shield test on this coil to see if it will degrade small gold performance or improve quietness over mineralized ground.

I have some of the Scotch 24 shield tape but my detector 'sees' a 1" square piece of it when placed in close proximity to the coil shell. This stuff has been often proposed as a shield material but it is very expensive and I know it will not improve performance on my coil. That leaves graphite as the best choice for additional shielding of this coil.

I am also going to rerun the coil damping to try out slightly under-damping to see if further improvement can be had.

Dan

[sinclairuser]

i have had good results with graphite paint, applied to the inside of the coil shell, the best two coils (pi mono loops) that i made for my barracudas were simply paper tape applied to the inside of the coil shell(all over) then painted with graphite paint.
a thin uncoated copper wire is embedded into this paint where the coil wire enters the shell.
one of the two also has a round sheet of paper on the bottom of the coil painted with the paint also with a small wire.
but the two coils work more or less the same both are affectively sheilded so i'm not convinced that it needed the paper shield on the bottom.

[baum7154]

Is the copper wire you used detectable by the unit? Are you running less than 15 us delay? I used a very narrow strip of copper underr the graphite/varnish layers on a 407uh spider coil I built a couple years ago. I did critical damping on the coil again last night and it came out with a 1265 ohm optimum resistorvalue. Working the formula back puts the self resonant frequency at about 989 khz. Unfortunately the lowest Guard Interval attainable on my CHANCE PI with this coil is a '12' and that is only if the supply voltage is lowered to 11.8 volts. At full 12.5 supply voltage the lowest GI setting is '13'. I will post a picture of this shielded coil this weekend.

Dan

[sinclairuser]

Is the copper wire you used detectable by the unit? Are you running less than 15 us delay? I used a very narrow strip of copper underr the graphite/varnish layers on a 407uh spider coil I built a couple years ago. I did critical damping on the coil again last night and it came out with a 1265 ohm optimum resistorvalue. Working the formula back puts the self resonant frequency at about 989 khz. Unfortunately the lowest Guard Interval attainable on my CHANCE PI with this coil is a '12' and that is only if the supply voltage is lowered to 11.8 volts. At full 12.5 supply voltage the lowest GI setting is '13'. I will post a picture of this shielded coil this weekend.

Dan

short response to your questions, no the wire is not detected, but it is only about 2.5 inches long and super fine, just enough to connect the sheild, to reply to the second question no 18us is the fastest delay.
i've been following your posts as an 8" spider was the original coil used on my detector and i found it interesting.

[baum7154]

I see that the copper wire is not a problem then. On this 407uh coil the shield contact I used was .001" thick copper foil 1/8" wide under the graphite that was applied at 1300 to 1500 ohms per inch 6mm away from the coil windings. At a guard interval of 12 it is still able to see the 10.8grain dental gold at about 3" in air. This coil was flooded with urethane foam and was wound with 26 AWG enamel insulated magnet wire. I have since abandoned the urethane flooding and the magnet wire.

Dan

[green]

[QUOTE=baum7154;183636]Is the copper wire you used detectable by the unit? Are you running less than 15 us delay? I used a very narrow strip of copper underr the graphite/varnish layers on a 407uh spider coil I built a couple years ago. I did critical damping on the coil again last night and it came out with a 1265 ohm optimum resistorvalue. Working the formula back puts the self resonant frequency at about 989 khz. Unfortunately the lowest Guard Interval attainable on my CHANCE PI with this coil is a '12' and that is only if the supply voltage is lowered to 11.8 volts. At full 12.5 supply voltage the lowest GI setting is '13'. I will post a picture of this shielded coil this weekend.Dan[/QUOTE

Not saying your resonant frequency calculation is right or wrong. I think the formula [Fr = Rd/(pi*L)] is correct for critical damping, knowing when the coil is critical damped might be a problem. Calculating Rd from the formula might be all right, calculating Fr probably isn't. If the detector is adjusted for best response the coil might not be critically damped. Under damping is better with a low amplifier Fc. I think if you disconnect the damping resistor and look across the diodes with a scope you should see a resonant decay after the coil volts drop below .6 volts. Calculate resonance from scope display. I'm just learning so if you think disconnecting Rd is a problem don't try it. Sorry about suggesting calculating Fr with the formula.

[baum7154]

I 'm always learning too and this is good input. I did do BB Sailor's signal gen / scope resonant procedure on the coil feedline a long time ago and I think it measures somewhat higher than this calculation with the coil in circuit. CHANCE has a complex transmit signal and I'll have to see if I can see the ringing/decay.

Thanks for the help!

Dan

[bhowiebkr]

I was messing around with this coil design in a program called Maya. This python code might be useful for some of you.

import math

sin = lambda angle: math.sin(math.radians(angle))
cos = lambda angle: math.cos(math.radians(angle))
blend = lambda start, end, weight: (start * (1-weight)) + (end*weight)

def makeSpline():
    radiusData = [9.5, 11.5]
    thickness = 0.1
    totalTurns = 6
    notches = 37 * 2
    profileData = [1, 7, 8, 9, 8, 7, 1]
    notchSkip = 5

    offsetScale = thickness
    notchDegree = 360.0 / notches
    numProfilePoints = len(profileData)
    pointRot = notchDegree / (numProfilePoints - 1)

    allPoints = []
    accRot = flipFlop = count = 0

    for i in range(totalTurns * notches):
        flipFlop = i % 2

        if count>notches:
            count=0
            offsetScale+=thickness

            if radiusData[0]<radiusData[1]:
                radiusData[0]-=thickness
                radiusData[1]+=thickness
            else:
                radiusData[0]+=thickness
                radiusData[1]-=thickness

            accRot+=notchDegree*2
            allPoints.append([sin(accRot)*radius, 0, cos(accRot)*radius])

            radiusData = [radiusData[1], radiusData[0]]
                    
        else:
            for j in range(numProfilePoints):
                accRot+=pointRot*notchSkip
                if j==0: accRot-=pointRot*notchSkip
                weight = j/float(numProfilePoints-1)
                offset = profileData[j]*offsetScale/9

                if flipFlop:
                    radius = blend(radiusData[0], radiusData[1], weight)
                else:
                    radius = blend(radiusData[1], radiusData[0], weight)
                    offset*=-1

                allPoints.append([sin(accRot)*radius, offset, cos(accRot)*radius])
            count+=1

    #import pymel.all as pm
    #circle = pm.circle(radius=thickness/2.0)[0]
    #curve = pm.curve(d=2, p=allPoints)
    #pm.extrude(circle, curve, extrudeType=2, fixedPath=True, useProfileNormal=True, useComponentPivot=True)
    return allPoints

allPoints = makeSpline()
print allPoints

[bhowiebkr]

By changing the inner and outer radius you can get a more perpendicular crossing alignment of the wires that could help with the capacitance of the coil.

[A.TTACH]28122[/ATTACH]

[baum7154]

slot correction

By changing the inner and outer radius you can get a more perpendicular crossing alignment of the wires that could help with the capacitance of the coil.

[A.TTACH]28122[/ATTACH]

I agree! The original designer of this coil was constrained by the coil housing he was trying to fit this coil into and as a result was not able to really optimize the final product for low capacitance. The ideal is probably 90 degree crossings of all wires but getting closer to 90 is good enough.

I wonder if your software can modify this coil shape to a 6" X 8" ellipse with 37 slots equally spaced around the inside and the outside edges? I would make the slots ..400" deep and the form 1.00" wide to start. I like to get a visual on how the windings come out.

Thanks,

Dan