Multiple coils inside a PI detector head?

[Janne]

Hello, first post to the forums. Sorry if this question has been discussed before but I couldn't find anything with search.

I've been thinking of building a PI metal detector. Something based on a micro controller, but without trying to do any analogue signal analyzing in software(at least yet..). Nothing too fancy for starters, and mainly just for the fun of it. First goal would be to scan around my garage for all the tools I've lost

When searching for information about the search coil a thought popped into my mind. Since smaller coils work better for small objects, but bigger coils (in diameter)can detect bigger objects deeper from the ground, why could not one include both in a search head? I mean, wind 2 - 3 coils inside each other. If thin wire is used there *SHOULD* not be too much eddy currents induced into the inner coils from the outer coil being used? Or is this one of the ideas best forgotten right from the start

[Carl-NC]

No, this is potentially a Good Idea. White's calls it a "Dual Field" coil. Trick is to get both coils properly damped.

[WM6]

I mean, wind 2 - 3 coils inside each other.

Welcome Jane.

Sorry, but your "2-3" already invented.
Even 10 or more coils inside each other.
Search for "spiral PI coil" ("dual field" too).

[kt315]

why could not one include both in a search head? I mean, wind 2 - 3 coils inside each other.

your vacancy in White's is free. feeling I have to discovery new continent up after Vesspuci... I did 2 coils inside many years ago before White's... believe me now.

[Janne]

Hi,

I wasn't really expecting this thing would have not been tried before, just didn't know the right search terms it seems. One thing I've learned with developement in general, is that almost everything has already been tried, if you can't find information you're not just looking hard enough. Or the idea is just too dumb even to concider

About dampening, I don't really understand why would both coils need damping? As far as I understand, it is enough to dampen the coil that is currently in use, as all the coils share the same magnetic flux inside them? So what I was thinking was to have some sort of switching mechanism to select one coil at a time for use.

For my first machine I think I will be going with a traditional monocoil, to keep things a bit more simple. I think I will have enough fun debugging it for starters. But it would be interesting to try a multicoil arrangement too.

[Tepco]

It is not quite clear what you intention is whit "multiple coils". Dividing single coil in 2 or more sections, or adding Rdump to each, or making some "flat spiral" design may have some benefit, considering coils are part of the single system. However, multiple coils whit some sort of selector switch is very bad idea. Coil in use then will "see" other coils just as a bunch of metal, and this will ruin detector performance (eddy current and all). Multiple coils can be made, but in perfectly balanced way, like on VLF machines. It is possible to use one TX coil (outer, for example), sample on TX side, and use two balanced coils (inner) to extract disc information on another channel, but they must be induction balanced, so no eddy current can be induced in RX coil. Otherwise, multiple coils and selector switch will not work at all.

[Janne]

Yeah multiple coils with a selector switch was what I was thinking. I'd have assumed thin wire would not have shown to the outer coils, as there would not be much eddy currents induced in there.

[Tepco]

Don't even try this, actual response of another coil, sampled whit main one, will produce ringing response, very specific, much worst compared to a piece of metal of similar size, dictated by coil RLC response and wire length (1/4 wavelength), producing complete chaos in response. Recepy for disaster, nothing else.