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Going to give a oOo or DOD pi coil a try, does TX 300uH and two 180uH ish coils out of phase sound about right ?
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phisics is equal same doing you OOO or OO or O coil. we say about effective coil area, as for TX so for RX.
also, look Eric's Re: Question on coils
you CAN NOT do the depth MORE never if will do bigger coil. THE PHISICS.
Posted by: Eric Foster
Date: May 18, 2005
Here are the curves I have used for many years. The range reaches a maximum when it is equal to the radius of the coil. Coils larger or smaller than this optimum will result in less range. To show how this works, along the bottom axis you see coil diameter, which is obviously 2 x the radius. So for an 11in coil, if we go up the vertical scale to A, we have 5.5in. Also note the diagonal line and the series of ever increasing semicircles. Everything to the left of this line shows increasing detection range up to the maximum where it intersects the line, then decreasing range to the right, where the semicircles are shown dashed.
If a certain metal object is just detected at 5.5in with the 11in coil, then going larger in coil size will cause a reduction (going down the dashed side), and going smaller in coil size will have a similar effect. Initially, it won?t be much, i.e. going from an 11in to an 8in coil will only make 0.5in difference but below 4in diameter, the range will drop rapidly.
Now, suppose with the 11in coil, you can detect an object at about 12.5in (B on the vertical scale. This indicates that the coil is not an optimum size for that particular object. If we carry on up the curve (direction of arrow) we can see that by using a 20in coil, we could gain another 2.5in (C). The curve peaks at 15in with a 30in coil. But the extra inch gained hardly makes such an unwieldy coil worth while.
Other factors come into play of course. The curves assume that the number of turns and the coil current is the same in all cases; which it isn?t necessarily. For the same inductance value, a smaller coil has more turns, which counteracts to some degree the loss in range. Also a smaller coil will pick up less electromagnetic noise, earth?s field noise and ground effect, which make for a smoother threshold.
The end result is, that with a small nugget that can be detected at between 5 and 7in with the 11in coil, so that it is on the top part of the curve, an 8in coil may well give a similar range. That is not to say that smaller coils do not have other advantages. Small coils and probes are very useful in rocky areas or searching in undergrowth. They have less drag too for water hunting, and less pickup from mineralised soil or conductive sea water plus better signal separation on close or multiple objects.
One other point regarding PI, is that the small object sensitivity is largely determined by the sample pulse delay. If an object is so small, or thin, or made of high grade stainless steel, such that all the signal has decayed before sampling takes place, it would not matter how small a coil you made, it would never be picked up.
Eric. -
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Originally posted by Carl-NC View Post
Thanks I will try Mikebg coil idea he had some idea to use a piece of ferrite I think it was to fine tune the balance , but I have to stay within a 12 inch coil so I can 3D print a houseing. Pity he is no longer here to ask him about it.
KT315 I am experimenting just to see the characteristic's of the coil not going for depth etc.Comment
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What kind of response does a DOD give? Something like - + - ? ThanksComment
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Originally posted by Altra View Post
Just winding the coils now, so dont know yet, also got to modify a machine for DDComment
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It gives a + - response with a null in the middle.Comment
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Originally posted by Carl-NC View Post
Thanks, following this drawing for winding the coils not sure if its correct, will find out the hard way I guessComment
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https://www.geotech1.com/forums/show...505#post205505 I tried a DOD awhile back. Lot less signal when target at 12inches(not measurable)for Rx subtracting. Any thoughts what I did wrong? Think I nulled one Rx coil against Tx, then added the second Rx and nulled if it matters.Originally posted by Carl-NC View PostComment
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Both Rx coils are wound the same. Connect in series, end winding of each coil connected. Start winding of each coil for coil. Probably need a damping resistor across each Rx coil at coil, something higher than critical damped(maybe 1.5 to 2 times critical?).Originally posted by 6666 View PostComment
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Think I see what I've been doing wrong. Figure8 Rx I use is two round coils side by side connected figure8. I compare calculated signal vs distance for a mono coil against measured for the figure8 I use with same diameter(each coil). Should be using DOD Rx diameter not overall diameter when comparing? Maybe the reason they started with a 19inch diameter coil?Comment
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Got a bit distracted yesterday, my DOD has to fit inside a 12' housing so its a small scale project, I will probably try it on my PI4 modded to take a DD, damped and wired as per Carls drawing for first try, each coil damped with 390 Ohms and 1K series resistorsComment
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Well so far the TX measures 320uH and the two RX coils measure 333uH togetherComment
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On small gold nuggets that are shallow I get a squirrely detection noise as the irregular shape of the nugget effects the overlapping coils. On deeper nuggets the receiver coils become additive and sound just like a Mono coil response.
I recently cut a defective GPZ 7000 DOD 14 inch coil open and found the following;
GPZ 14 measurements with full cable
Receiver 6.2 Ω 427 uH
Transmit 0.38 Ω 312 uH
There is a small printed circuit board in the center of the coil where the coil cable is attached. It has a 1k resistor across each receiver coil.
The two receiver coils are wired in series (signal and inductance adding). They are connected to a small coax that goes to the electronics box.
The two ends of the transmit coil are not in a coax. They are a twisted pair going to the electronics box.
There is a rectangular Ferrite housing surrounding the printed circuit board, for EMI out/in?
The entire coil assembly is shielded with two paper sheets painted with graphite on one side only. The graphite coating has a resistance of 150 ohms when placing the probes 1 inch apart. At 6 inch separation it measures 240 ohms. One sheet is on top and one on the bottom. Each sheet has a small bare wire taped down across the graphite surface. Each sheet is over lapped around the perimeter by short 22mm cuts/tabs to allow easy bending and fitting then taped by masking tape to hold them in place. The clean side of one sheets tabs laps over the painted side of the other sheets tabs in a manner that the graphite tabs from one sheet does not short out to the other sheet. This allows the overall shield to act as a Faraday shield. The two bare wires are connected to a single ground wire that runs up the cable to the electronics box.
The transmit coil is approximately 4 mm above the receiver coils. The transmit coil is approximately 1 cm above the bottom of the coil housing. This spacing is to reduce the magnetizing of nanoparticles in the ground.
The transmit coil is wound with a large diameter Litz wire. The receiver coils are wound with very small Litz wire.
The entire shielded coil assembly is suspended at the top center by four soft rubber cushion mounts in a manner that prevents it from touching the inside of the entire coil housing. This prevents most false target affects from bumping into rocks and brush.
Have a good day,
ChetComment
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