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PICList Thread
'Metal Detector'
1998\06\19@020228 by Hector Juan Golia

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Necesitaria informaci—n sobre detectores de metales usando PIC
Gracias a todos.
Escriban en ingles.
Hector

1998\06\19@044833 by tjaart

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Hector Juan Golia wrote:

> Necesitaria informaci—n sobre detectores de metales usando PIC
> Gracias a todos.
> Escriban en ingles.
> Hector

Usted conseguir‡ mucho m‡s ayuda si usted:

        1) escribe en espa–ol inglŽs es entendido solamente por algunos de los
        miembros de PIClist.

        2) env’a su email pues el texto (no HTML) algunos programas de lectura del email
        rechazar‡ el HTML

--
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Tjaart van der Walt
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1998\06\19@150948 by Martin McCormick

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Hector Juan Golia writes in Spanish and says that he will need information
about metal detectors using PIC's.

       That's a pretty general question, but here's an idea or two.

       Some metal detectors use a radio frequency oscillator in which the
inductor or coil is the search loop.  There is a second oscillator that
drives a chopper or product detector whose input is the first oscillator
that was connected to the search loop.

       One tunes one or the other oscillators until the frequencies are
close enough that the sum or difference signal is an audible frequency.

       This frequency may be amplified and fed to a speaker or pair of
headphones.  When the search loop passes over metal, the metal either adds
or subtracts some inductance, depending upon whether or not it is ferrous.
This changes the frequency of the search oscillator and causes the tone
that one hears to rise or fall.

       Other metal detectors have a pair of antennas fixed at right angles
such that it is not possible for the receiving antenna to pick up signal
from the transmitting antenna in free air.  When a piece of metal is near,
this changes the pattern of the antennas and causes signal to appear in the
receiver.

       A PIC could read the signal generated by the search apparatus and
possibly let the person searching know of smaller variations in the signal
than he or she could hear.  The PIC's clock could be used as either the
chopping source for the product detector or as the transmitted signal in
a detector that uses the nulled antenna approach.

       Bueno suerte in buscando para el oro.

Martin McCormick WB5AGZ  Stillwater, OK
OSU Center for Computing and Information Services Data Communications Group

1998\06\19@171614 by Sean Breheny

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On Fri, 19 Jun 1998, Martin McCormick wrote:

> Hector Juan Golia writes in Spanish and says that he will need information
> about metal detectors using PIC's.
>
>         That's a pretty general question, but here's an idea or two.
>
>         Some metal detectors use a radio frequency oscillator in which the
> inductor or coil is the search loop.  There is a second oscillator that
> drives a chopper or product detector whose input is the first oscillator
> that was connected to the search loop.
>
>         One tunes one or the other oscillators until the frequencies are
> close enough that the sum or difference signal is an audible frequency.
>
>         This frequency may be amplified and fed to a speaker or pair of
> headphones.  When the search loop passes over metal, the metal either adds
> or subtracts some inductance, depending upon whether or not it is ferrous.
> This changes the frequency of the search oscillator and causes the tone
> that one hears to rise or fall.

Maybe you could shed some light on this for me, I never understood why
non-ferromagnetic metals should have any effect on metal detectors.
Ferromagnetism is such a VERY MUCH larger effect than diamagnetism
(exhibited by all materials) or paramagnetism (exhibited my some
materials especially at low temperatures), that I would not think that
the same apparatus used to detect ferromagnetic materials would be
suitable to detect materials which exhibit either of the other two effects.

The only thing I can think of is that the conductivity of the metal
causes some of the signal to be coupled to it and thereby alters the
inductance by creating a mutual inductive effect, or causing resistive
losses in the signal, or by acting as a resonant object. But I would
again think that these effects would be TINY compared to the
ferromagnetic effect, especially at the RF frequencies usually used in
metal detectors (which, again, I am assuming are not much more than a
couple of MHz, considering that even soft iron should have a good
permeability at the freq. used by the detector).

{Quote hidden}

Sean

1998\06\19@172013 by John Tibbits

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I have a little experience with metal detectors, and I agree completely with
the info provided by Martin.  I have two additional notes that further
clarify his data.

1.  The detection range of a metal detector is partially determined by how
strong the field is.  The strength of the field I believe is proportional to
the number of turns in the coil that is driven, and the current that is
flowing in the coil.  In my experiments it worked better to have the coil
part of a resonant tank with the capacitor connected as close to the coil as
possible.   (The detection range diminishes as (1/d) raised to the fourth
power.  This is essentially one over "r" squared for the outbound signal and
one over "r" squared for the return signal)  So adding power is helpful if
you want more range.   In many industrial metal detectors the products being
scanned go right through the detector coil,  which is optimum.

2. Metal detectors detect a lot of things that are not metal, like water and
some minerals.  They are more immune to water, etc if the frequency is very
low.  Some of the popular metal detectors  use frequencies as low as 20kHz.
(Many of the older ones use frequencies in the 100kHz and higher range.)
Sensitivity is better at higher frequencies,  but there are usually more
false readings.

3.  One other note, the detectors that I made used the null-coil method and
a synchronous detector.  When the coils were nulled to minimum feedthrough,
the error signal could not be reduced to zero.  The waveform of the error
signal was not sinusoidal.   This inherent error signal was a limiting
factor in how small of a variation could be detected.  Perhaps a PIC could
be used to process this signal to achieve a better "noise" floor.

Best of luck-
John Tibbits

{Original Message removed}

1998\06\20@100742 by Martin McCormick

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many thanks to John Tibbits for his discussion of the operating principles
of metal detectors.  I was somewhat curious about the finer points, myself.

       My one experience with a metal detector was when a neighbor brought
one to our house in the early seventies and asked me to install the
add-on earphone jack which the manufacturer supplied.  They had a suitable
hole in the case of the detector and a little bag containing the jack and
the mounting nut and probably a little schematic and that was pretty much it.

       After getting the jack connected, I remember playing with it a bit.
It was one of the heterodyne-type metal detectors which probably operated
around 100 KHZ or so.  I remember tracing the reinforcing bars in the
concrete slab of our house.  The one thing that I think I recall that sort
of peaked my curiosity was that when I took it outside and swept it over the
grass, the oscillator pulled a little in the opposite direction that it had
gone for the iron bars and bolts I had been putting near the coil.  I wasn't
sure if this opposite pulling was due to the grass or the dirt underneath,
but one could definitely hear a little rise in the tone if ferrous metal
made it drop in pitch or vice versa.  If I sound a little vague, it is
because a heterodyne-type metal detector will give opposite results
depending upon which side of the center frequency one has tuned the search
oscillator in reference to the detector's oscillator.  This should be a hint
or slight warning to anybody who wants to use a PIC.  Be sure you know
the relationship between the two oscillators or design your frequency
counting routine to either read the search oscillator directly or
self-calibrate for a period of time to get a base-line reading before going
treasure-hunting.:-)

Martin McCormick WB5AGZ  Stillwater, OK
OSU Center for Computing and Information Services Data Communications Group

1998\06\20@231136 by paulb

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Sean Breheny wrote:

> I never understood why non-ferromagnetic metals should have any effect
> on metal detectors.  Ferromagnetism is such a VERY MUCH larger effect
> than diamagnetism (exhibited by all materials) or paramagnetism
> (exhibited my some materials especially at low temperatures),

 Well, if you are looking for iron bolts (or by the same token,
reinforcing rod), you may be right.  If you are looking for rust
(minerals) or trying to look *through* it, you are probably lucky these
have so little ferromagnetism.

> The only thing I can think of is that the conductivity of the metal
> causes some of the signal to be coupled to it and thereby alters the
> inductance by creating a mutual inductive effect, or causing resistive
> losses in the signal, or by acting as a resonant object.

 Now you«re getting it.  The "shorted turn" effect.  A gold wedding
ring should be *very* detectable (*if* it is broadside!).  Apart from
saturation generating harmonics as also mentioned in this thread, and
using twin coils with cancelled mutual inductance, you appear to have
ably summarised the effects.

 Remeber the "tuning wand" used to trim RF coils, including striplines?
A fiberglass wand with a ferrite slug (chosen for the frequency, one
hopes) at one end and a brass(, copper, silver) slug at the other?  A
shorted turn lowers the inductance.  Ground resistivity lowers the "Q",
but if you can actually compensate for that, so much the better.

 They say the good ones, using secret circuitry (ICs with the markings
ground off; supposedly may use microprocessors) really do discriminate.

 I«ve often thought however, that working in the sub-megahertz range, a
process similar to the capacitance measurement using the PIC«s input
hysteresis could quite likely be used to measure the inductance of a
coil which is after all, what you are doing metal detecting.  It could
perhaps by extension assess "Q" and balance.

 There is a really nifty "stud finder" (generally capacitive, though
some of these use both capacitive and inductive sensors, and include an
AC voltage detector as well) which has an "auto balance" button instead
of adjusting the knob till the light goes out.  I said "hey, a PIC could
do that for sure" (e.g. 12C508).

 Cheers,
       Paul B.

1998\06\21@181938 by Hector Juan Golia

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Friends:  
For a development of security I need information it has more than enough detection of such metals as weapons.  
The person portadora should go by a door that detects the presence of metallic elements.  
Thank you and a greeting to all.

Hector

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