Discrimination in a VLF detector

[xthack]

Hi everyone,

First, sorry if my English is bad at some moments...

For a school project, I'm currently working on the ability of a VLF detector to discriminate metals.
So I made some researches around, and if I understood well, the principle is that the signal received at the receiver coil has a phase shift with respect to the signal in the transmitter coil. This phase shift depends on the characteristics of the metal.

Then, my goal is to measure this phase shift in a "real" system.
So basically, what I intend to do is, create the search coil, then plug both, receiver and transmitter loops to an oscilloscope, and compare the raw signals. So my question is, will I get a result ? Or will I need to build the electronic circuit also ?
(I may recognize that my knowledge in electronics is near from zero...)

I also need to insist that, the goal of this project is not to build a complete and functionnal detector, but it's really based on the phase shift recognizing, so that's why I only need the necessary parts, just enough to read it... (No audio feedback for example...).

[Carl-NC]

Yes, you can put the oscope in XY mode and look at the Lissajous pattern to see the phase shifts. You may need an amp on the RX coil, or large targets.

[xthack]

I've seen another thread here :http://174.132.129.189/~moreland/for...ad.php?t=16956
On the 4th message,

Sorry, I don't have the whole IB detector... However, I just built up the coils for the detector. Are there other opportunities to create the "right" signal only with the coils and the signal generator?

Thank you!

You will need to use synchronous demodulators to extract the in-phase and quadrature components of the RX signal if you want to discriminate between different targets. This is what KT315 is showing in his video with the lissajous pattern on the scope.

That is the complete opposite answer... I'm a little bit confused here... -_-

[Qiaozhi]

I've seen another thread here :http://174.132.129.189/~moreland/forums/showthread.php?t=16956
On the 4th message,

That is the complete opposite answer... I'm a little bit confused here... -_-

I wouldn't say that it's a completely different answer. But ...

The discussion in the thread was concerning whether it was possible to "see" the phase-shift from various targets using just a coil and a signal generator. KT315 then posted a video from Ivconic showing a scope hooked to the R and X sync demod channels of a detector (probably a TGSL). Without some extra circuitry (at least an RX preamp, as Carl suggested) you won't really see anything useful.

Try adding a preamp and monitoring the output, along with the TX, with the scope in XY mode. Then you will get a more steady Lissajous pattern than the one shown in the video.

[Tepco]

Hi everyone,

First, sorry if my English is bad at some moments...

For a school project, I'm currently working on the ability of a VLF detector to discriminate metals.
So I made some researches around, and if I understood well, the principle is that the signal received at the receiver coil has a phase shift with respect to the signal in the transmitter coil. This phase shift depends on the characteristics of the metal.

Then, my goal is to measure this phase shift in a "real" system.
So basically, what I intend to do is, create the search coil, then plug both, receiver and transmitter loops to an oscilloscope, and compare the raw signals. So my question is, will I get a result ? Or will I need to build the electronic circuit also ?
(I may recognize that my knowledge in electronics is near from zero...)

I also need to insist that, the goal of this project is not to build a complete and functionnal detector, but it's really based on the phase shift recognizing, so that's why I only need the necessary parts, just enough to read it... (No audio feedback for example...).

What you want to do can be even more interesting and informative than actually taking measurements on complete, working MD and you can learn a lot this way. I can just give you few suggestions.

Considering you are not making actual detector, but just want to measure different target responses, skip anything related to ground balancing, sync detectors, phase shifts, filters etc, just make basic test setup. Unfortunately, some additional electronics may be needed.

First, make balanced coil system and adjust it , try DD and concentric coil configuration to see difference in response. Unlike “normal” VLF coil, this one will not be resonant so you don’t need to hit some exact parameters (inductance etc), eventually use some coil design with higher “transformer ratio”, less turns on TX, more on RX side, like in White’s IDX, to get more signal output. Balanced coil system does all the tricks here, electronics later just process this signal.

Typical test equipment, like signal\function generators etc, usually 50 or 600 Ohm output, or PC sound cards, are not the best way to drive coils directly in this configuration, some amplifier (old stereo will do the job) is needed. Maybe some low value (few tens of Ohm) resistor in parallel with TX coil will be needed for stability (amplifiers don’t like purely inductive loads).

On RX side, terminate coil with some medium value (few hundred ohm to few Kohm) resistor, signals will be at very low level here, in ten’s of mV range with target, so some amplifier may be a good idea, but not necessary.

In this setup, variable frequency sinewave can be used to characterize target response, PC sound card and some software, like Spectralab, can be very useful, but this is still classic amplitude\phase characterization, typical for VLF.

Much more important and interesting is “time domain” characterization, this way you can extract more target information compared to freq. domain only. To do this, drive coil with square wave, 5kHz (100uS half period) is good starting value, null the coil well and if needed, add some provisional shielding between coils, to reduce capacitive coupling. (maybe some dedicated MOSFET driver will be needed instead of audio amp, but just for square wave drive, this is not PI circuit) Then observe coil imbalance waveform response to different targets, results will be self-explanatory, most interesting measurement you can take.

I hope this is useful to some degree, good luck.
(Sorry for my English too)

[Skippy]

It IS possible to see the phase lag on an oscilloscope, BUT.... you will need a VERY well nulled search-coil, as the signal from your sample is likely to be comparable to the continuous signal from the coil. You will almost certainly need a pre-amplifier with gain of 10X to 100X (add some low-pass filtering too, to get rid of some of the noise) in order to see the signal. A modern scope that can do signal averaging of multiple samples will be useful too.
You will also need to allow for the following two effects: (1) the transmit coil will have some series resistance. If you measure the voltage across it, you are actually measuring the voltage across the L and the R. Hence the voltage on the coil is NOT in phase with that on L. (2) If you have a load resistor on the receive coil, allow for phase shifts, too, though they can be minimised by using a high enough load resistor.
It would also help understand if your apparatus was working correctly if you had some samples with 'known' phase shifts to test with. A large copper or silver coin, such as a US silver Dollar has a 'natural frequency ' of less than 1000Hz, so if your test apparatus runs at 10KHz, for example, you will see almost 180 degrees lag, as you are so much above the natural frequency.

[Rudy]

Yes, you can put the oscope in XY mode and look at the Lissajous pattern to see the phase shifts. You may need an amp on the RX coil, or large targets.

Wow! Carl I hadn't heard the name "Lissajous" in eons. Not since I worked on magnetic memory devices back in the seventies.

[Carl-NC]

I used to show students how to read Lissajous patterns when I taught Circuits Labs in college. Some years ago I was in one of those hands-on science museums where they had a Lissajous display, and they had completely botched it up. The kids didn't know the difference, though.

[osslo]

I used to show students how to read Lissajous patterns when I taught Circuits Labs in college. Some years ago I was in one of those hands-on science museums where they had a Lissajous display, and they had completely botched it up. The kids didn't know the difference, though.

Hi Carl-NC
I have seen your comment's about Lissajous pattern and phase shifts for question of Discrimination in a VLF detector.
I am teaching principles of working metal detectors. I can not to find literature about

interaction between radio waves and metals.
If you have any information about those books - give me please name of those books or web-adress for downloading those books.
I hope on your helping.

[Carl-NC]

Books on metal detector principles are rare... the best sources are the various magazine articles that I've posted on the main web site. There may be a new book coming out soon but I don't know exactly when.

[Rudy]

I used to show students how to read Lissajous patterns when I taught Circuits Labs in college. Some years ago I was in one of those hands-on science museums where they had a Lissajous display, and they had completely botched it up. The kids didn't know the difference, though.

That was back in the days when colleges were teaching more than the now typical digital engineering curriculum. Back then you learned the fundamentals and got a strong dose of analog.

[osslo]

Hi for every one!
If you have a free variant of that book:
Dan, Harvey E., Eddy Current Testing Theory and Practice, (1995).
may be for reading online
give me please some information.
I hope that you will be able to help me.

[osslo]

That was back in the days when colleges were teaching more than the now typical digital engineering curriculum. Back then you learned the fundamentals and got a strong dose of analog.

Hello!
Have you worked with metal detectors that can to discriminate one metal from others?
I search schematics of metal detectors that can to discriminate silver from other metals such as aluminum, zinc, copper and others.
thanks.

[Rudy]

Hello!
Have you worked with metal detectors that can to discriminate one metal from others?
I search schematics of metal detectors that can to discriminate silver from other metals such as aluminum, zinc, copper and others.
thanks.

As detectorists we used the term "conductance", as in a target is a high or a low conductor (ie. silver or gold). In reality, the target discrimination depends not on the conductance of the target, but its admittance, a combination of resistance and reactance. What some would correctly call a complex number.

Using current discrimination techniques, we can not separate arbitrarily shaped targets and resolve them to gold, silver, or other valuable metals simply because less valuable targets can have the same admittance..

[ashkelon]

i try the use same tecnique discriminate metals , mostly my problem is silver and steel has same value.