Signal analysis VLF-Detektor

[SIMONN]

Hello,

Since one week I've worked with the VLF metal detection principle. Unfortunately, there are deficits for my part regaring to the signal evaluation.

During the last week I built up miscellaneous coils. (Double-D-Head and concentric coils [Tx (Transmit) and Rx (Receive)]).
To concretise my concern now, I've used a sinalgenerator to get sinodial signals to the Tx-Coil. Furthermore, the signal by the Rx-coil is strengthened by an amplifier and diagramed by an oscilloscope. With the oscilloscope I can recognise a minimum change of the amplitude, provided then metals are close to the coils.

To be honest, I don't understand how to use the signals. Therefore I need assistance, please. Does somebody have experience with data evaluation?Another question:
- Can I already use the signal (if phase difference is available) to discriminate between different metals?
Additionally, I'd like to digitize the whole process via DAQ.
I'm looking forward to your advices.

Best,
Simon

[kt315]

Simon,

if you have IB detector... find R and X channels outputs, connect your o-scope's probes on them. turn off an inside synchronization of your oscilloscope, connect the signals on o-scope's X and Y channels. you must see somethind you see in this video - an reaction on all types of metal.

http://www.youtube.com/watch?v=m9DuMekVwic

this video is NOT MINE (esp. for Mr. ivconic).

[SIMONN]

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!

[Qiaozhi]

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.

[SIMONN]

Hmm okay.... I thought it would be easier. I don't have experience with the lissajous pattern but I wanna try to find out more.
Primarily, I thought that I can use the phase difference from the transmitted signal to the received signal...
Do I really not have the chance to discriminate between targets by using just the sinodial signals?

Thanks a lot Qiaozhi

[dfbowers]

Simon,

if you have IB detector... find R and X channels outputs, connect your o-scope's probes on them. turn off an inside synchronization of your oscilloscope, connect the signals on o-scope's X and Y channels. you must see somethind you see in this video - an reaction on all types of metal.

http://www.youtube.com/watch?v=m9DuMekVwic

this video is NOT MINE (esp. for Mr. ivconic).

Very nice.. Why not take each channel and route their own VCO, each with a distict tone. Then mix them together. I can see a concept where the mixed tones could provide a wealth of information on target ID. Guess this would only be practical with absolutely no ground signal though. Very distinct signature for some items!

Or.. maybe route the disc channel to a VCO and the AM channel to modulate the volume, just so the sound from the disc channel doesn't drive ya nuts from swinging the coil over the ground.

[Qiaozhi]

Hmm okay.... I thought it would be easier. I don't have experience with the lissajous pattern but I wanna try to find out more.
Primarily, I thought that I can use the phase difference from the transmitted signal to the received signal...
Do I really not have the chance to discriminate between targets by using just the sinodial signals?

Thanks a lot Qiaozhi

Whatever approach you decide to use, you will need to add more circuitry than you have already. Synchronous demodulation is by far the simplest approach, especially if you just want to set a discrimination level. You could try measuring the phase-shift directly, but it will probably result in a more complicated circuit. Apart from using a PIC, one possible solution would be to feed the TX and RX signals into two separate comparators and measure the delay between the edges. Again, this would result in something more complicated than sync demod.

[SIMONN]

Whatever approach you decide to use, you will need to add more circuitry than you have already. Synchronous demodulation is by far the simplest approach, especially if you just want to set a discrimination level. You could try measuring the phase-shift directly, but it will probably result in a more complicated circuit. Apart from using a PIC, one possible solution would be to feed the TX and RX signals into two separate comparators and measure the delay between the edges. Again, this would result in something more complicated than sync demod.

Thank you a lot! Currently I need DAQ and LabVIEW to create signals digital. Therefor it shouldn't be a problem to "create" a demodulator.

The problem is I don't know which signal is expected on the oscilloscope. First I should think about the "right" coil. Do you know electrical properties to the VLF-coils? I can't find values for the resistance and inductance.

My basic set-up is: signal-generator --> transmitter coil --> receiver coil --> oscilloscope.

Thank you again!

Simon

[Qiaozhi]

Thank you a lot! Currently I need DAQ and LabVIEW to create signals digital. Therefor it shouldn't be a problem to "create" a demodulator.

The problem is I don't know which signal is expected on the oscilloscope. First I should think about the "right" coil. Do you know electrical properties to the VLF-coils? I can't find values for the resistance and inductance.

My basic set-up is: signal-generator --> transmitter coil --> receiver coil --> oscilloscope.

Thank you again!

Simon

It depends what TX frequency you intend to use. To calculate the number of turns you can use this calculator.

[SIMONN]

It depends what TX frequency you intend to use. To calculate the number of turns you can use this calculator.

Thank you! But what's with the resistance of the coil(s)? I thought it should be between 1.8 (Ohm) and 3.0 Ohm. But I don't know if both coils should have 3.0 (Ohm) or just one!? The coils aren't in serie, because I use just one coil as the transmitter coil.

[SIMONN]

It depends what TX frequency you intend to use. To calculate the number of turns you can use this calculator.

Hey, just another question. What's the "background" formula of your coil calculator? If you don't want to give this information then tell me the kind of coil please. Short air coil, single layer?
Thanks!

[Qiaozhi]

Hey, just another question. What's the "background" formula of your coil calculator? If you don't want to give this information then tell me the kind of coil please. Short air coil, single layer?
Thanks!

It's an air coil calculator and assumes a square cross-section. This is more accurate than the usual solenoid formula.

What transmit frequency are you planning to use?

[SIMONN]

Ah okay... Thx! I wanna use 10kHz. Hope this formulary works at this frequency?!

[Qiaozhi]

Ah okay... Thx! I wanna use 10kHz. Hope this formulary works at this frequency?!

If you require a transmit frequency of 10kHz, then let's assume a tuning capacitor of 47nF. In this case the coil inductance needs to be 5.4mH.

As an example, let's say that we have 30AWG (0.254mm) wire available, and we want to wind a 10" (254mm) diameter coil. Please see the calculator image below. Of course, this is just an illustration as you've not said which type of coil you're planning to use - DD, concentric, omega, OO, etc.

The result for this example is 83 turns, which gives an inductance of 5.433mH.

Then you need to perform the same calculations for the receive coil, based on the coil type you require. Don't forget that a concentric coil will also require a nulling coil in series with the TX coil, so you'll need to take this into account. If you want to build a DD, then you'll have to fudge the values to account for the change in shape as you form the D shape.

Good luck!

[SIMONN]

If you require a transmit frequency of 10kHz, then let's assume a tuning capacitor of 47nF. In this case the coil inductance needs to be 5.4mH.

As an example, let's say that we have 30AWG (0.254mm) wire available, and we want to wind a 10" (254mm) diameter coil. Please see the calculator image below. Of course, this is just an illustration as you've not said which type of coil you're planning to use - DD, concentric, omega, OO, etc.

The result for this example is 83 turns, which gives an inductance of 5.433mH.

Then you need to perform the same calculations for the receive coil, based on the coil type you require. Don't forget that a concentric coil will also require a nulling coil in series with the TX coil, so you'll need to take this into account. If you want to build a DD, then you'll have to fudge the values to account for the change in shape as you form the D shape.

Good luck!

Thx, that's even more accurate. I built up housings for three different coils. So I'll try the coplanar (concentric), and DD in three different sizes with varying frequency.
My problem is, how can i assume the capacity and at least the inductance? Isn't 5.433mH is much? I thought about 350uH...

Thank's for your help! I really apprechiate it