BFO, IB, PI, etc... Has everything really been thought of?

[Thomas Scarborough.]

Friends,

My son Matthew (16) said that there must be a metal detector principle that nobody has ever yet thought of. I did a simple electronic design recently (nothing to do with metal detecting) that filled a room with electromagnetic waves (just half a Watt). Then, when anybody entered the room, their body was "energised", and a coil picked up their presence. We tested it on a cat, too, and it worked just as well. Now supposing one were to insert probes into the ground, and "energise" all metal items in the ground within a demarcated area? A kind of pulse induction without the pulses? This might be a witless idea, but you might have some thoughts...

Thomas.

[Esteban Cabrera Grinok]

Re: BFO, IB, PI, etc... Has everything really been thought of?

I want to say all you that this type of stimulation to the metals already exist before, precisely during the First World War. A German radio technician wich surname is Haber to have made a "metal detector". This "metal locator" works generating a low frequency and good voltage in two separated electrodes. He was scanning the area with a "special" sensor and was managing to dig up all the unexploited ordnance, in order that the task of the sappers is facilitated in the construction of the trenches.

This is the reason why I work in other fields. In the latests 20 years I did many experiments with electronic long range locators with many different sensors, mainly coils and antennas. With this devices I found mainly good conductive items like copper, bronze, gold, silver and his alloys.

Thanks for your time and sorry my english.

[Dave Johnson]

Re: BFO, IB, PI, etc... Has everything really been thought of?

There is a variety of technologies related to metal detection: underground radar, Turam apparatus, resistivity apparatus, VLF apparatus using distant high power longwave radio signals (mostly from radionavigation beacons and military submarine communications), and of course magnetometry. Sonic and ultrasonic methods have been explored but as far as I know, no useful products have resulted other than for leak locating by utility companies and construction crews.

The military has used (and I suppose still uses) VHF mine detectors for locating both plastic and metallic mines; however these detectors don't have anything like the sensitivity needed for a decent metal detector due to the large and unpredictable effects of ground minerals and moisture-- effects which can be minimized in a regular metal detector.

So, back to your question-- has everything really been thought of in terms of basic technologies? Probably not. However, I'm not expecting anything to displace PI and IB (VLF/MF) technology in hand-held metal detectors in the near future. Within the next 5-10 years, R&D in military mine and UXO detection will probably result in practical hand-held underground radar type metal detectors which will carve out a special niche in nonmilitary markets.

--Dave J.

[Thomas Scarborough.]

Spiral BFO?

Dear Esteban,

This is fascinating. But now, has this principle been tried with new technology, or has it lain dormant since WWI?

On the original subject, I have been experimenting with BFO. A large coil is good for larger objects, a small coil does better with smaller objects than a large coil would ever do. It occurred to me, how about a spiral coil - concentric windings? Has this been done?

With kind regards,
Thomas.

[Sam]

Re: Spiral BFO?

I don't think the machines could be called BFOs, but a long time ago, perhaps in the early 70s a man with the last name of "Gardner" designed and marketed a line of metal detectors that contained spiral wound coils.

If memory serves me correctly, he received at least one maybe two US Patents on his detectors. I'm not certain if the patents had anything to do with the coil design but he did develop one of the best discrimination circuits for that time period.

The coils were heavy gauge copper wire and were potted in a spiral configuration in a routed groove in 1/4" plywood.

The machines were very heavy and hard to swing, but compared to what else was available at the time, they were really good coinhunting machines. He made at least two different coil sizes... a six and a 12 inch.

[Esteban Cabrera Grinok]

Re: Spiral BFO?






Hi,

I believe has it lain dormant since WWI, in part.

More about electrodes & company:

http://www.geocities.com/lemagicien_2000/elecpage/ugr/undr.html

This principle is valid to metal detection.



More:

See the two messages of S. Goddard under the title: MORE LRL stuff? NOOOOOoooo posted on the PI Technology Forum

http://www.findmall.com/metal/school1/config.pl

Date: Thursday, 28 March 2002.

*****

About spiral coil I send this image of one page. This pertain to italian PLL circuit.

I did experiment with spiral coil, includes PI circuits, and I believe it is better than the normal coil.

Other idea (I think): Many PI circuits are rely on BFO, because need the pulse frequency + sample frequency.

Chau

[Peter Jones]

How about different sized coils in parallel ?

Either switched or permanently parallel ? Thus one coil housing can be used for maybe several coil sizes , running individually or at the same time .

Worth thinking about ?

[Thomas Scarborough.]

This is how series worked.

Hello Peter, I'm working on a BFO design for a magazine. I now tried three concentric coils in SERIES. While this didn't improve sensitivity to large metal objects (greater than 30mm diameter), it VASTLY improved sensitivity to small metal objects (less than 20mm diameter). Look out for my IB design in EPE magazine in October 2002. www.epemag.wimborne.co.uk/issues.htm With kind regards, Thomas.

[Peter Jones]

Re: This is how series worked.

I'll keep an eye out for it.

[paul]

Re: Spiral BFO? - They work!!!!

Theese "pancake coils" work really!!!
At least on PI detectors
and with 20-30% more sensitivity.
paul

[Sean Goddard]

Re: BFO, IB, PI, etc... Has everything really been thought of?

How about an FM detector?

Using a LF to modulate an HF, sort of a "sweep frequency" machine. Any mileage in this idea Dave?

[Dave Johnson]

Re: BFO, IB, PI, etc... Has everything really been thought of?

If you mean sweeping a frequency through the VLF range in order to get the equivalent of multiple frequency operation, I suppose a number of engineers have thought of that, but I've never considered it to be an good approach, nor has anyone else as far as I know.

If you mean sweeping a "carrier" up in the range most people think of as being RF, say over 100 kHz, I don't know that anyone has thought of that other than in the context of underground radar, which is something entirely different from "metal detection" as we customarily think of it.

--Dave J.

[Roy]

Re: This is how series worked.

I found that if I put 5 coils in series I could get all sorts of target information but hadhave not the time to persue it any further.
For instance, by moving the line of coils around (X,Y,rotation) you can build up a pretty good idea of the target size and shape.
Easy
Roy

[Roy]

Re: BFO, IB, PI, etc... Has everything really been thought of?

What is wrong with sweeping through VLF?

[Dave Johnson]

because it adds complications without corresponding benefits. n/t