Metal detector (Garret AT Pro) target discrimination explained

Metal detector (Garret AT Pro) target discrimination explained


A few years ago I got myself a new hobby:
metal detection. For me, the fun is in finding things that
are interesting rather than valuable. So to find of a condom package from the 1980’s
is just as exciting as finding old silver dimes or an 18th century musket bullet. It is about imagining the story behind the
objects. What is it exactly and how did it get here? And sometimes it’s about being away from everything
and just enjoying the peace and quiet. The detector that I own is the Garrett AT
Pro. It is a pretty sturdy and versatile detector,
But the essential aspect of it, is that it can discriminate between different types of
objects and materials. Here in the Netherlands, there is a huge amount
of trash in the ground. So if you don’t want to be digging up rusty
nails all the time, it is vital to be able to disciminate before you start digging. In some areas there is a lot of military stuff
in the ground so discrimination can be a life saver if you’d like to avoid a specific type
of find, like for example this grenade. Anyway, it got me wondering how the discrimination
between different metals works, I mean technically. Viewed from the user interface, it is actually
very simple: the detector shows a specific ID, with values between 00 and 99. Now this ID is not like the ID in a database. A specific object might have one particular
ID, but a particular ID can refer to many different objects. So, let me illustrate: about 2 years ago,
I was at the beach and at one point I had dug up more than 50 pull tabs. Now these all showed an ID of around 76 – 77. So, after some time I was not even bothering
any more when that particular ID showed up. However, the next morning the first thing
I found was this heavy platinum wedding ring. The only reason I found it was, that I wasn’t
really paying attention to the ID. And the ID was, and I’m not kidding: 76. So the ID is not only dependent on the material
but also on the shape and the size of the object. But, there is definitely a trend when you
look at various materials. Like silver objects, they will generally have
a very high ID, lets say around 90 or even higher. And aluminum objects can also have very high
ID’s. But, there is a big difference between solid
aluminum objects and thin aluminum foil. Thin foil can have an ID as low as 40 and
a solid piece can have and ID of well over 90. And if you look at the metal Lead for example,
you see that it generally shows an ID between 55-80, and this value is also dependent on
the thickness and size. Iron on the other hand has and ID value generally
around 10-25 so way lower. As a special material I added Ferrite to the
table. This actually something that you will generally
not find in practice a lot and it is special in the sense that that it has magnetic properties,
but is completely non-conductive. Ferrite generally gives back an ID of around
00. So let’s find out how the detector measures
the ID in practice. Here is my test set-up: To the left the detection
coil of the metal detector positioned in the air. To the right of the coil, I placed the interface
unit of the detector and added an oscilloscope to do the measurements. I placed a small reference coil to pick up
the emitted magnetic field of the detector. As you can see in the scope, the detector
emits a high-frequency alternating magnetic field in a pure sine wave with a frequency
of 15.3kHz. The detector uses a double D coil, named after
the 2 D-shapes of the emision- and the reception coil. For the sake of simplicity I will display
the coils as circles or ellipses in the rest of this video. As you can see the two coils are partly overlapping. But why? Well it turns out that the detection coil
is positioned in the ideal place to achieve a very high detection sensitivity. Here you see a drawing of the emission and
reception coil far apart. The magnetic field lines are always closed
loops and the magnetic field detected in the reception coil is dependent on how much nett
magnetic flux passes through it. And if a metal object gets in the neighborhood,
the field in the reception coil will actually change only very slightly. So the object will be hard to detect in this
configuration. But if we place the reception coil partly
over the emission coil, we can choose the position such that the nett magnetic field
through the reception coil is exactly zero. So, the fields in areas A and B actually cancel
each other out because of the opposite polarity. And this means that, even though both coils
are very close together, the reception coil has suddenly become completely “blind” to
the high magnetic field emitted by of the emission coil. But of course it is not insensitive to changes
in the magnetic field distribution, and this is what the reception coil is looking for. So this principle is a nice trick to achieve
a high sensitivity, and it is used in many different metal detectors. High-end detectors can even have two of these
configurations in a single search coil. When a metal object gets in the vicinity of
this double D-coil, it slightly disturbs the magnetic field and at the same time the fine
balance between the magnetic fields in the areas A and B. This results in a nett magnetic flux through
the reception coil, which can then easily be detected. Back to our setup. To determine which one of the two D-shaped
coils is the emission coil, I used a second test coil. It turns out that the emission coil is the
left coil, which I indicated here by sticking tape on it. Next I placed this test coil in a position
so that the nett magnetic field through the test coil is exactly zero, even with enhanced
sensitivity on the oscilloscope. The test coil does not show a signal. But if I now move a piece of aluminium over
the coils, a strong transient signal is observed. What we observe here is the disturbance that
the aluminum causes in the magnetic field distribution. Next is of course to see what kind of disturbance
we observe when we use different metals. In order to compare these signals we have
to decide on a specific location where we test. Because if we place the same object in area
A or area B, we will actually see opposite signals, because of the opposite external
field. I decided on area A as the location to measure
the samples because here you get the highest signals. Then I took samples of the materials from
the tabel and looked at the signal observed by the reception coil when placing the material
in the overlapping area of the coils. Now this is what I see: Starting with Silver, a phase shift of nearly
180 degrees is observed with respect to the original magnetic field. Next is the aluminum block and it also displays
a large phase shift, I measured it to be only slightly smaller than the silver, so 176 degrees. Now as I mentioned earlier there is a big
difference between aluminum foil and a block of aluminum in ID-value. So if I use a piece of aluminum foil, the
phase shift drops down to 82 degrees. Moving to the metal lead, we can clearly observe
a phase shift of 160 degrees and this is significantly lower than the values of silver and aluminum. The big jump in the phase shift is found when
we go to Iron: for Iron we observe a very small phase shift of around 20 degrees. The last graph I show is for ferrite, which
shows 0 degrees phase shift. Now it’s clear that the IDs presented by the
detector are directly derived from the phase shift. So where does this phase shift come from? Well it is not trivial question. Bottom line is that the presence of a metal
changes the inductance of the reception coil and I’ll try to explain this in a simplified
manner. I will do this by looking at how the inductance
is influenced by the magnetic and conductive properties of the material. Let’s start with the magnetic properties. There are actually 3 main types of magnetism:
diamagnetism, ferromagnetism and paramagnetism. We will leave ferromagnetism out of the discussion
for now, because it involves permanent magnetization and since we have a high frequency field,
it’s not playing a role in the detection scheme. So this leaves us with dia and paramagnetism. In the case of diamagnetism, the material
actually resists the penetration of magnetic fields into the material, and this is because
of the electronic structure of the atoms. So it is a material property. Example of a diamagnetic materials are silver
and Lead. In the case of paramagnetism, it is actually
the other way around, so the material is very inviting to magnetic fields. Strong paramagnetism is for example found
in Ferrite. So, if the magnetic field has the choice to
go through either air or ferrite, it will prefer going through the ferrite. And if you consider this effect in our setup,
the presence of ferrite in area A will actually increase the alternating magnetic field of
the emission coil in this area, which means that we detect an in-phase sigal. But of course there is more to it: aluminum
is slightly paramagnetic but still it gives a very high phase shift, so how should we
explain this? Well for aluminium, the inductive effect is
largely determined by it’s conductivity. So the emitted alternating magnetic field,
introduces electrical eddy-currents in conductive materials like aluminum. These eddy currents actually make their own
magnetic field and this field opposes changes in the external magnetic field. The better the conductivity of a metal, the
higher the eddy currents and the faster the build up of the opposing magnetic field. So even though Aluminum is (weakly) paramagnetic,
we still still see a huge phase shift due to it’s high conductivity. So let’s just look at our table again and
add the electrical conductivity and magnetic properties of the materials. I don’t want to go through the whole table
but I just want to discuss a few examples. With silver we can explain the highest phase
shift with the fact that it is the best conductor and in addition has the highest diamagnetic
constant. Both these properties cause a high phase shift. Now on the other hand, if we take for example
Iron, this is strongly paramagnetic, so a low phase shift. However, it is also conductive, although it
is not really very high. So this conductivity in iron is the reason
we see a phase shift relative to Ferrite, which is also strongly paramagnetic like iron,
but not conductive at all. Now there is one more subject I would like
to cover and that is triggering: the detector is actually only sensitive to metal objects
that pass the coil with a particular speed. So if the object passes the coil very slowly,
the detector will not give a signal. This shows that the detector is triggered
by the occurence of a fast transient rather than by the presence of a signal itself. So when a transient occurs, the detector will
measure the phase delay of the signal, convert this phase delay to an ID value in the range
of 0 and 99, and output this value to the display of the detector. Basically, that is all there is to it. Ok, that is it for now. Thanks for viewing to the end of this video. If you are into this kind of nerdy stuff,
you might want to consider subscribing to this channel because there is a more coming. So, maybe see you in the next video. Daag!

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