Folded 8 PI coil question for Reg

[6666]

Hi Reg

I have been doing a bit of research into folded 8 mono PI coils and found that you are the inventor
back in about 2009, well done .

I was wondering if you have done much testing with the folded 8 coil, for example is there a ratio
of length and width of the folded section that maximises the field of sensitivity to small objects ?


Also I was reading a thread somewhere , were you said something to the effect of the total length of one turn of a folded 8 coil including the folded part should not exceed the length of the perimeter of a 11 inch coil, 34.55 inches, did I read that correctly ?

My experiments with a folded 8 coil will start with an oval shape of about 5x9, with a simple shield of graphite painted board above and below the coil, and would be interested in any comments thanks

 

[Reg]

I really didn't do a lot of testing, so that is something someone else can do. What little I did was written in a file on my old computer which I don't have set up now. The reason for the 11" max total diameter was because that was the max size that would work on a true 10 usec or less delay on a detector. Many detectors today can use a larger size coil to begin with.

The graphite shielding will have to be a low resistance to work well.

Instead, you might look into the DOD coil and try it. I was surprised just how well it worked.

Reg

 

[6666]

Hi Reg happy Easter
Tnx for the reply, if you ever find that file I would be interested in whatever it contains ,
RE DOD, you made one that works, good result, that deseign is on my long list of experiments
I was thinking along the lines of TX 300uH and 75/75uH for the two RX's.

 

[Reg]

The DOD coil I made I used parts I already had available and with them I made a modified DOD where the RX coil was made from a large winding and formed two separate D's and had the crossover between them. I drew up a quick pic of what I mean. This simple design allowed me to test the idea on a TDI without having to build windings. So, it doesn't look exactly like a typical DOD.

Now, my DOD had a 300uh TX and a 500Uuh RX when formed into 2 D's and would allow for sampling fine at 8 usec. The design did not require any GB so it would run on my TDI in straight PI mode. This allowed for much more depth. Close to a target only half of the coil worked right but farther away, it seemed to work well.

My design indicates even a RX with more inductance would work fine. So, I would recommend you try two separate windings between 250uh and 300uh to begin with. I suspect they will work right.

On my pic I exaggerated the openings in the D's and the crossover X design to better show what I mean. I apologize for the lousy pic but I am tired right now and only wanted to show what I mean and not draw a pretty pic.

Reg

 

[6666]

Hi Reg

Now, my DOD had a 300uh TX and a 500Uuh RX when formed into 2 D's and would allow for sampling fine at 8 usec. The design did not require any GB so it would run on my TDI in straight PI mode. This allowed for much more depth. Close to a target only half of the coil worked right but farther away, it seemed to work well.

Thats an interesting design thanks, I'm guessing it is starting to look like a figure 8 on its side, I'll gather some more wire and give it a try
not needing to be GB is a plus for me.
To make it simpleier to build do you think the wires at the Xing point can be just cris crossed or they need to be seperated into one coil winding bunch.

 

[PeterK]

Reg,

Isn't the "DOD" coil in your picture just another version of the standard AI or anti-interference coil? This type of coil cancels a percentage of the target's signal so is normally only used on highly mineralized damp ground that contains salt.

According to the GPZ7000 patent and white paper, the ML DODs are designed to cancel "linear motion" signals, which are only a problem for CW detectors that detect during a constant current TX period, this obviously doesn't apply to pulse induction metal detectors.

GPZ white paper: http://www.minelab.com/__files/f/266297/KBA_24-1%20ZVT%20Technology.pdf

The GPZ coils are shown in the attached picture from a patent. The dotted lines are the TX windings.

 

[Reg]

The only reason I built the design I did was because I already had windings available to try. The original DOD does the same thing except each D winding is built separately and then connected in series with one D flipped over. The original DOD design can be found on the Geotech1 forum and dates back to 2008 that I can find. The person who posted it went by the name of Mikebg. So, in my opinion this DOD design is not ML's design at all. They simply used it in their unit.

Also, I could care less what ML states on how it works and I doubt that "linear motion" only statement is true. . The original DOD design was laid out to be used on either a VLF or a PI years ago to take care of ground signals as well as noise. Personally, I wish I had tried the design when it was first introduced.

So, either two separate D windings or a larger winding split into two D's will work. Actually, two separate D's would be easier to calibrate because each winding would be wound on the same form using the same turns of wire. Then it would be just a matter of getting the best alignment. Don't forget to connect the two D's in series with one D flipped over to give the cancelling effect.

Yes, in my opinion, the DOD design is a version of the figure 8 ground cancelling concept. However unlike the standard figure 8 that genrally loses depth, the DOD will allow for increased depth because of the overall design and the ability to operate with little or no ground balance.

Reg

 

[6666]

Hi Reg
yes MikeBG published the DOD design many years ago, that's his drawing that I've been using.

Today I knocked up a simple 5x9 oval former and gently massaged a 11 inch 320uH mono into a folded 8
to fit the 5x9 oval shape former. It will be a slow coil as the wire is solid copper .47mm and has PVC insulation
none the less it will be an interesting test, I measured the inductance after folding it, and it remained at
320uH which I found interesting, that's one experiment crossed off the list
to compare the inductance before and after a coil bent into a folded 8 shape, still got to tack it down flat.

 

[PeterK]

Reg.
The DOD and OOO and DDs etc have been around since Adam was aboy but they don't always serve the same purpose.

The DOD you have made is a simple AI coil and will null any signal seen equally by the two reversed D windings and will lose depth for the same reason as a standard AI. This would be more obvious if the two Ds were made exactly the same and positioned equally either side of the TX coil.

Besides being very inneficient, it obviously can't work on the GPZ so there is nothing to compare. The DOD used by Minelab is effectively a DD with an additional but identical RX winding where current flows in the same direction in each D, not reversed as in your AI version. A normal DD minimises the ground signal because the RX doesn't see as much ground signal as the TX coil or a mono. The ML DOD doesn't do anything to help minimise this problem so it isn't being used for the reasons you suggested.

It is being used for reasons that are obviously not very well known.