How does phase shift work?

[UncleVinnys]

I understand the GENERAL principles of phase shift, but not the particulars.

Like how does the composition of the target (ferromagnetic versus conductive)
get translated into the phase shift? Sure, I can see eddy current being
induced in the target, and that iron targets would be more resistive,
but how does the conductivity of silver, for example interact with the wave from
the transmitter coil?

If anything, I would think the silver would keep on "ringing"
and therefore produce a lagging wave in the pickup coil.
What am I missing.

Here is my understanding so far, though I've been told
it's "WRONG!".
Not to worry, I don't have a sensitive ego, so let 'er rip.

http://www.unclevinnys.com/vlfphaseshift.html

 

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[sniffer]

I'm not an electronics whiz, but a former teacher of mine is, he was an electronics engineer on the apollo missions. this is the way he explained it to me.

the magnetic field generated by a metal detector is similar to trans echo location (dolphin sonar) except instead of sound it uses a magnetic field, which is sent out by one loop in the coil and the other loop picks up the reflection or mirror of the object it encounters.
the tricky part, here's where he lost me, the frequencies used in the metal detector, determine what the MD
will be strongest at finding. the higher the frequency, the better it is for finding gold, the lower the frequency, the better it is at finding ferous metals.
now the way it can tell the difference between different metals, is because of the amount of reflection of the magnetic field reacting with the metal object.
after all this, I just said great, I'll get a good all around machine.
I don't know if this helps, just repeating what I was told

Sniffer

 

[Reg]

Here is a simple link to show what phase shift does to a signal.

http://id.mind.net/~zona/mstm/physics/waves/introduction/introductionWaves.html

When you operate the controls, you will be able to see the signal shift left or right when the phase shift control is moved. The amplitude causes the signal to increase or decrease in amplitude. The wave length stretches or shrinks the time between waves.

Now, when a target enters the field of the transmit coil, all of these factors occur to the signal in the receive coil or may occur depending upon the type of transmitter used. Normally, the wave length change is quite small if a free running oscillator is used, but the amplitude and phase shift can be quite large.

Each target, whether it be silver, gold, iron, etc, will cause a certain phase shift to occur. This phase shift can be measured using different techniques, thus giving an idea of the target itself.

This is a very compact simplified explanation since a thorough one would take a book to define.

Reg

 

[GpSnoopy]

I certainly do not know the engineering behind the theory, but this site helped me with some of the basics. Phase shift description is near the bottom, and probably much less detailed than you were hoping for. I hope it helps a little. For what it's worth, and I could be absolutely totally wrong, I think that what may be incorrect is that you mention that the inductance or resistance of the target changes the phase shift of the signal. What it should be is that the transmitted signal from the detector induces a brand new field (eddy currents) around the target which has a different phase angle or phase shift because of the different inductances or resistances of the target. This new field is what the receive coil is picking up and what the comparator and phase demodulators circuits use to determine the targets ID.


http://www.arizonaoutback.com/detectortheory.html

 

[UncleVinnys]

Those are good articles - Thanks guys!

Still, I'm trying to understand the mechanism for the phase shift.
Silver has one easily moved electron in its outer shell, making it a conductor.
It produces a positive phase shift.
I am wondering if the silver, being sensitive to the approaching wave form
pushes these loose electrons away from the approaching wave, and whether or
not that is what causes the phase shift.
Dense, less conductive metals are more likely to produce heat than eddy currents,
and I don't know if that is what slows the wave down.

I understand phase shift.
I understand how it is detected.
I don't understand how the different targets CAUSE that phase shift.

 

[Reg]

Hi,

The phase shift mechanism of a target is based upon the target itself. Each non ferrous target can be simulated by a simple inductor and resistor. Pure silver approaches a pure resistor. So, any high conductor approaches a pure low value resistor, while a low conductor approaches a pure high value resistor or closer to 90 degrees. So, non ferrous objects fall in a 90 degree quadrant based upon their conductivity.

In theory, ferrous objects fall in the next quadrant or from 90 degrees to 180 degrees because of the magnetic qualities.

So, over the 180 degree area, pure ferrite, which has no resistive qualities, will be at 180 degrees, pure silver at 0 degrees. One can reference this differently and make the ferrite the 0 deg ref and the silver a minus 180, but it is the same.

This becomes quite complicated to try to understand but in simple terms, target separation is done by the conductivity of the non ferrous object and the fact that ferrous (iron) objects end up in the second quadrant because of their ferrous or magnetic features.

Now, in theory, this all sounds fine except, as the coil passes over iron junk, the coil sees this type of target differently at times and at times, only sees the resistive element, thus making it look much like a non ferrous object very briefly. So, at this time, the ferrous junk can give a non ferrous splatter of a signal. In other words, it sort of very briefly acts like a non ferrous object. This odd response is what makes it so difficult to totally eliminate the signal from ferrous junk.

You might want to read some of the patents associated with TID which try to explain this phase shift characteristic. They are confusing but that is the nature of the subject.

Please keep in mind, my recollection of what is happening is fuzzy since I haven't studied the topic for many years.

Here is a link to a patent that sort of displays the phase shift and a phasing diagram of what targets do.

http://www.eudem.vub.ac.be/publications/files/MDPatentSearch/docs/US4486713.pdf

As I said, the topic is a tough one to explain in simple terms.


Reg

 

[Carl-NC]

Still, I'm trying to understand the mechanism for the phase shift.

I don't understand how the different targets CAUSE that phase shift.

Basically, it's the response of the induced eddy currents due to a combination of the target's permeability, conductivity, thickness, and shape.

Consider an ideal superconductor... if you place it in an AC magnetic field, it will develop eddy currents which produce a perfect cancellation of the magnetic field right at the surface (this is a classic boundary-value problem in electromagnetics), with a skin depth of zero. The superconductor's counter-magnetic field is therefore 180-degrees from the source field, exactly the same magnitude as the source field, and independent of the AC frequency... it looks like a perfect mirror.

Now take instead a non-ideal conductor, like silver. It also develops eddy currents, but the resistance causes the eddy currents to have a non-uniform density vs depth, called skin effect. There is variable phase of the eddies vs depth, and this results in a composite target surface phase that is less than 180 degrees. Skin effect is dependent not only on resistance, but also frequency and permeability, so all this plays into the phase result. So does shape.

- Carl

 

[UncleVinnys]

BIG, BIG, THANK YOU, Carl!
Yeah, that is the kind of response I was looking for, and explains
what was not clear to me.
It also provides the key words - the identifying of the processes involved
that I have been seeking.

Now, to carry that further, the ferro-magnetic elements being even less
conductive, highly resistive response, so the "mirror" is even more
defective as a reflector. I am wondering if too the molecular structure
being chaotic in iron that the multiple eddy currents don't just cancel
each other out to a large extent, and generate heat, as opposed to a
reflective wave in the opposite direction.
Thanks again - I will look up some references skin effect and
counter-magnetic fields.

UV