Very loooong and narrow coil for metal detector...

Thread Starter

Externet

Joined Nov 29, 2005
1,448
Hi.
What should be considered to detect metals in soil using a ~6 metres long x 5cm narrow coil, like

(=================================================================)

to be dragged transversally (up or down on the above sketch) in such way that will sweep a wide trail, instead of typical 30cm diametre round ones ?

Such coil to be lodged inside a 4" PVC pipe, driven by some proper circuitry [if possible:rolleyes:]

Next Images just to show the concept, but much, much longer and narrowly shaped than :
1579824144458.png1579824298334.png
 

MisterBill2

Joined Jan 23, 2018
4,820
Why would you choose that shape for a metal detector coil? The detection is achieved by the metal affecting the magnetic field created by the coil, and such a skinny coil will not produce a large magnetic field.
So I am asking what is the reason for considering a coil of that shape?
 

Thread Starter

Externet

Joined Nov 29, 2005
1,448
Hi.
Speculating on how to make a coil that could sweep a very wide trail on a single pass... :oops:
It would still be a coil; it would still make/sense a magnetic field of nearly a single plane shape instead of a cylindrical shape, hopefully interacting with metal crossing the narrow gap. As analogy of an optical document scanner. o_O Also, a tape recording head is perhaps 3mm x 0.01mm.
Wikipedia mentions the gap width here ----> https://en.wikipedia.org/wiki/Tape_head
 

TeeKay6

Joined Apr 20, 2019
553
Hi.
What should be considered to detect metals in soil using a ~6 metres long x 5cm narrow coil, like

(=================================================================)

to be dragged transversally (up or down on the above sketch) in such way that will sweep a wide trail, instead of typical 30cm diametre round ones ?

Such coil to be lodged inside a 4" PVC pipe, driven by some proper circuitry [if possible:rolleyes:]

Next Images just to show the concept, but much, much longer and narrowly shaped than :
View attachment 197504View attachment 197505
Your loop is 6m long. The metal you hope to detect is 2cm(?) diameter. Even if your "metal" were a ferrite of high permeability sitting within the plane of the loop, it would have negligible effect on the loop inductance (compared to "metal" that completely filled the loop). For a fixed shape, a larger loop is less affected by a fixed size metal-to-be-detected. Multiple overlapping smaller loops would be more sensitive but would also require more circuitry to manage each loop.
 

Thread Starter

Externet

Joined Nov 29, 2005
1,448
Hmmmm... For manageability, perhaps like :
1579895540450.png
(Image borrowed from the web)

If inside the towed/dragged 6m PVC pipe, there is a couple of dozen overlapping oval loops/coils with their own circuitry instead of a single one, each with a showing LED to monitor which detected metal on/in the soil, well, that would be even better...
Could they be multiplexed ?
 
Last edited:

TeeKay6

Joined Apr 20, 2019
553
Hmmmm... For manageability, perhaps like :
View attachment 197557
(Image borrowed from the web)

If inside the towed/dragged 6m PVC pipe, there is a couple of dozen overlapping oval loops/coils with their own circuitry instead of a single one, each with a showing LED to monitor which detected metal on/in the soil, well, that would be even better...
Could they be multiplexed ?
First, the circular cross section of PVC pipe limits how close to the ground your coil(s) can get to the ground, assuming you are hoping to penetrate ground to some depth(?)--or are you looking for items in grass but not below earth surface?. A container having a flat bottom would allow coil(s) to be closer to soil. If you know what you are doing, coils can certainly be multiplexed--following the aphorism that (almost) anything is possible if you have enough money. Multiplexing would limit how fast the "bar" could be moved/dragged while still getting full coverage by detection area of each coil. Each coil is an inductor and a short, but finite, time will be required to process the output of each coil in turn. There are several schemes for metal detection; I will not attempt to assess which is best for your purpose. The different schemes differ in how long processing takes and how complex is the processing circuitry (and likely firmware) required. If you decide to proceed, you would be wise to start with one coil and one set of non-multiplexed electronics. Once that is working well (and detects what you want to detect) and you understand it, then you could consider whether multiplexing seems appropriate. This will not be a trivial project. Out of curiosity, what "metal" objects do you hope to detect?
 

MisterBill2

Joined Jan 23, 2018
4,820
Hi.
Speculating on how to make a coil that could sweep a very wide trail on a single pass... :oops:
It would still be a coil; it would still make/sense a magnetic field of nearly a single plane shape instead of a cylindrical shape, hopefully interacting with metal crossing the narrow gap. As analogy of an optical document scanner. o_O Also, a tape recording head is perhaps 3mm x 0.01mm.
Wikipedia mentions the gap width here ----> https://en.wikipedia.org/wiki/Tape_head
A tape head has the tape surface with the magnetic material in direct contact, that is a fair difference. And it is relying on an external magnetic field. So a tape head is not a similar application at all. An array of coils optimized for a section of the area, and arranged so that any metallic material would alter the received field could work well and be sensitive.
The key concept is "effective aperture", which relates fairly directly to coil center area. There was a military mine detector that used one coil to create a magnetic field and an array of several to sense that field. The sense coils were arranged to null the return signal when no metal was present. Thus my suggestion.
 

MisterBill2

Joined Jan 23, 2018
4,820
attempting the wide area scan also reduces your response signal ratio to the area of the coil elements that react to it
e.g. your signal goes very weak . . . that , unless you manage to beam your EMF down to earth as a curtain ???
https://www.google.com/search?q="wide+thin+beam"+radar
The generation of radar signals usually does not involve coils. And ground penetrating radar is not simple. So now we have a question of what sort of materials and what size of materials and how deep would those materials likely be? Even the shape of the items will matter. And a coil with a narrow aperture will have a short narrow magnetic field. So really that coil shape is not optimum, except that it may be easy to build.
 

Thread Starter

Externet

Joined Nov 29, 2005
1,448
Thanks, fellows.
I like the wording 'courtain' for a beam... :)
The roundness of the pipe or flatness of it using a square pipe are manageable to bring the gap closer to soil. Anyway, plain metal detectors are swept typically 2-3-4 inches above soil and somehow still work without being dragged.
Metal in question mostly iron based, but am not against the contraption to work on other non-magnetic metals. For soil surface or shallow buried, to be fine-tuned later. Expected target shape and size likely from peanuts to tennis balls.

1579908394036.png
Ignore "+N -S" , not on DC. The image above can be inside a square or round PVC pipe; the ends of the 'horseshoe' can be machined or spread or nearer to 'focus' the field shallower or deeper...
The red coiled core form in the image could be a 6 metres long trough, or many shorter overlapped.

More digestion of the idea needed, core materials or air core...
 

MisterBill2

Joined Jan 23, 2018
4,820
I do not see any similarity between the horseshoe electromagnet in the post and any metal detector coils that I have seen. Those coils generally have one axis pointing into the ground.
 

MisterBill2

Joined Jan 23, 2018
4,820
switching sense arrangements would add a huge amount of complexity. But as already mentioned, having one excitation coil and several pickup coils arranged so that the pickup signals cancel unless the field is disturbed is a method already used in some mine detection schemes.

Another scheme that is more reasonable is several detection systems sharing a common carrier tube. Each could then have it's own indicator light to show the segment where the item was sensed.

I am guessing that the purpose of this system is to find dropped items along a trail after a large crowd of humans has passed through in a hurry.
 

ci139

Joined Jul 11, 2016
1,084
. . . the horse shoe thing is like tape pickup head - i doubt it can have any significant range . . . if you use the ΘE pot core you just - a bit - get more depth to your vector + if you attach the receive thing around the Θ and x-mit thing around the ( • ) . . . ??? or the way around (then it's still a wild guess if it makes any "Sense")
 
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