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Shrad

Wed Jun 12 2013, 05:16PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

hi,

do someone have any knowledge about metal detection coil topology (apart from detector types and pancakes/star pattern)?

I'm searching for some resource like field pattern, field collimation, coil size versus field depth and shape

I've got several ideas involving TV deflection yokes to manipulate magnetic pulses generated at its focal point for either scanning or narrowing of the generated field, but have too little understanding of these phenomenons

can anyone direct me? I'll share designs if usable things come out of it

Conundrum

Thu Jun 13 2013, 07:23AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059

Hi, google "pulse induction"
I've made BFO (beat frequency) detectors before, its basically an AM radio receiver with two local oscillators.
The pulse induction variety use the effect of transfer of energy between as Tx and Rx coil, various people have tried the approach of hybridising a metglas based sensor with a PI detector to work around the sensitivity to iron that plagues all systems.

Did you try using FEMM? I've had a go on this, it is very useful and there is a lively online community.

Kind regards, -Andre

Shrad

Thu Jun 13 2013, 01:52PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

I know about topologies and techniques, I'm just trying to improve pulse selectivity over object types and maybe scanning to achieve sonar-like imaging

I'll have a look at FEMM I didn't remember this so many thanks for pointing this to me

I live over an ancient 1800's battleground where thousands of objects still lie underground, and I have always wanted to play a bit with metal detection

Apart from digging existing designs which is easy I want to design my own system under an original approach, starting from scratch as it usually produces the best results when you want to find new techniques

Dr. Slack

Fri Jun 14 2013, 03:37AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Never mind coil topologies, do you understand the many different modes by which 'metal detection' systems can work?

For induction type detectors, one or more test coils couple into a remote object, in order to detect some effect. The effect is complex (as in real + imag complex). Detectors that work by off-tuning a resonance are detecting mainly the imag part. Detectors that work by quenching an oscillator are detecting the real part. Single coil designs are essentially reflectometers, whereas two coil designs measure some aspect of transmission.

Pulses have a wider frequency range than single carrier types, which can improve contrast. In order to sort metal from soil, what you need is contrast. In the limit, ground penetrating radar is just very high frequency used for target illumination. Unfortunately you need to operate wideband to detect the broad range of frequencies from a pulse, which raises your background noise.

Having worked in the related industry of food and medicine metal detection (companies really don't like getting sued for bits of stainless sieve-wire ending up in their baby-food), choosing an imaging mode that improves the stability of the background signal (the food, or the soil) is as or more important than maximising the return of the target.

Then there are DC magnetometers, which detect only ferrous metals, won't do for lead musket balls, but would work really well for muskets. Proton precession is a very sensitive way to do that.

Then there is ground conductivity, which can detect not only bulk metal by its conductivity, but also the conductivity of soil moisture contaminated by leaching from buried solids.

Now here's the interesting bit for you. Almost all detectors were designed before modern electronics. The use of BFOs is a clumsy old way for detecting a change in frequency, you might do better today with ADC+DSP. Pulses are an old-fashioned way of getting a wider frequency range. If you use a DDS for the source and correlate the received signal with your transmitted signal, a chirp for instance, you can synthesise a wide frequency range and small noise bandwidth at the same time, to improve your SNR. Most proton precession designs I've seen use counters. I routinely use FFT methods orders of magnitude more sensitive. If you want to find new techniques, you might want to start from the coupling theory, assume the availability of cheap low noise RF amplifiers, ADCs, and say a raspberry PI for FFTs, presentation and GPS etc.

Amateur detectors are used in a 'I'll sweep it about in front of me until I get a signal' way, which really means you need detection thresholds suitable for a 'one hit' answer. Unless those thresholds are very high, you will be overwhelmed by false positives. What the pros do is multi-modal mapping. Using GPS, record a map of soil conductivity, of DC magnetic field, of real and imaginary part of coupling over a wide frequency range. Then back at the ranch, super-impose all of them on your computer. I won't labour the advantages of this approach.

Back to your original question, coil topology. Once you know what mode you are trying to couple into, and polarisation, and if high impedance electrostatically shielded (without a shorting turn), and if susceptible to external source pickup a balanced dipole or multipole design for the receive coil, all coil designs come down to area, orientation and impedance. The use of exotic geometries is just for people who don't understand the basics trying different things to see what happens, or posing with their fashionable new star-shaped design.

Conundrum

Fri Jun 14 2013, 05:18AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059

Interesting.
Add to that list measurement of ambient radiation, which should read Th232 and other primordial isotopes in the soil.
With a sufficiently sensitive detector it could act as an "X-ray" imager without the ionising radiation source normally needed

-A

Dr. Slack

Fri Jun 14 2013, 07:22AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Conundrum wrote ...

Interesting.
Add to that list measurement of ambient radiation, which should read Th232 and other primordial isotopes in the soil.
With a sufficiently sensitive detector it could act as an "X-ray" imager without the ionising radiation source normally needed

-A

Radiation, neat idea. Add to that surface temperature. At different times of the daily heating cycle, varying thermal conductivity and thermal capacity across the ground will give a usable contrast. However, with those two imaging methods, you're maybe more likely to pick up big structures rather than small metal artefacts. Unless the bit of metal itself is radioactive for some reason.

Shrad

Fri Jun 14 2013, 11:41AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

I was planning on sending a square chirp to generate enough harmonics to be able to DSP rich fast fouriers and match them to pre-recorded templates, which would also allow for easy ground noise suppression

Aside that, I was thinking about TV deflection yokes to either focus the field and modify covering, or raster scan

Conundrum

Fri Jun 14 2013, 06:30PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059

Yet another use for a quantum sensor, if my PG would turn up already.
Quantum sensor based detectors can be an order of magnitude more sensitive than the best end window tubes, and it is possible to enhance this still further by using PG coated with scintillator on the back and a conventional photodiode as the detector..

-A

Sulaiman

Sat Jun 15 2013, 02:02AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

or you could just get lucky

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