Vallon VMH3CS Mine Detector

[Carolina]

Kingswood:
Will you be taking both coils I hope. Have you done any air test on depth with the stock coil and the 30cm? I am thinking the depth should be very close the same but a few more square inch's of coverage with the 30cm? I prefer to lay my coils flat on the ground and ground balance, then swing target above the coil rather than set up horizontal on a table say. I like the ground to play a part in the test. But this is with my VLF machines. Well, that's just me. I suppose either method would give an indication of any difference.


Thanks
Carolina

[Ferric Toes]

Hi Mate....I dont have an oscilloscope...plus I am not sure I would know how to use one if I did
I will ask around town and see if anyone knows...Failing that, I may post it to Eric for a month
Out with my friend on Wednesday coming (day off yay!!) so will report back.

What I do is measure the following at the 7 pin connector on the coil shaft; centre pin live and any outer pin, ground. 1) dc resistance using a good multimeter. 2) measure the inductance with digital inductance meter. 3) measure the resonant frequency with a sine wave signal generator, oscilloscope, and inserting series resistor of, say, 47k between sig gen and coil. Scope across coil, tune sig gen from 100 - 200kHz and look for a resonant peak.

The most likely thing for the detector to sense is the resonant frequency, although I have found when making my own coils for the Vallon, than any departure from the stock coil parameters results in the detector not working, or being very noisey.

Eric.

[kingswood]

Kingswood:
If you have the VALLON 30cm coil, can you compare it to the stock coil to see if there is a difference in the inductance, or the resistance or the resonant frequency. Eric say he needs to stay in these same perimeters to make all his coils work. I'm curious what it is the VALLON senses as everyone else is. Auto compensation should take care of any ground effects for any of the coils I would think? I wonder just what the adjustment of the sampling window is? Also, have you been out with your friend and his Minelab?

Carolina

Hi Mate....I dont have an oscilloscope...plus I am not sure I would know how to use one if I did
I will ask around town and see if anyone knows...Failing that, I may post it to Eric for a month
Out with my friend on Wednesday coming (day off yay!!) so will report back.

[Carolina]

Kingswood:
If you have the VALLON 30cm coil, can you compare it to the stock coil to see if there is a difference in the inductance, or the resistance or the resonant frequency. Eric say he needs to stay in these same perimeters to make all his coils work. I'm curious what it is the VALLON senses as everyone else is. Auto compensation should take care of any ground effects for any of the coils I would think? I wonder just what the adjustment of the sampling window is? Also, have you been out with your friend and his Minelab?

Carolina

[kingswood]

We still don't know what the detector is sensing to make it adjust the sampling windows.

Eric.

I dont know either
Digital sample processing is not my forte....I dont really have a forte when it comes to detectors at all !!
I am interested to know though!
Maybe the sampling changes to adjust for the differences in the ground signal with the larger head (????)....dont know...just trying to think it thru...
EDIT: After reading the above reply, Eric did you mean what the detector is actually sensing in regards something in the coil that lets the electronics know its a different coil??....

[Altra]

Hi Eric,

One scheme used by Garrett in their GTP series detectors was to embed different value resistors in their coils. Another resistor in the detector formed a voltage divider which the processor could measure and ID the coil. Maybe a resistor is potted in the coil connector of the Vallon?

Mark

[Ferric Toes]

I did email them and asked!....Helpful company is Vallon
Their reply: "Regarding your question the frequency stays the same we adjust the sampling windows for the evaluation of the receiving signal for the different search head sizes"......

We still don't know what the detector is sensing to make it adjust the sampling windows.

Eric.

[kingswood]

I have no idea on this one. The larger and smaller heads I have made all have the same inductance, resistance, and about the same resonant frequency, so as far as the electronics goes there is nothing substantially different to the stock coil.

You have to be a bit careful with the Vallon manual as not all features that it mentions are available. It depends on which version of firmware has been loaded. It would be interesting to measure the above three parameters on your 30cm coil to see if there is any difference to the stock coil.

Eric.

I did email them and asked!....Helpful company is Vallon
Their reply: "Regarding your question the frequency stays the same we adjust the sampling windows for the evaluation of the receiving signal for the different search head sizes"......

[green]

Basically, with standard resistive damping, the rate at which the TX current decays in the coil slows down as you approach zero current. For small objects with short eddy current decay times you need a fast switch-off, even when the current is approaching zero. The dI/dt, or rate of switch-off with time must always be faster than the time constant of the object, otherwise signal is lost. With a half cosine TX pulse cut-off shape, the rate of switch-off speeds up as the current approaches zero which gives the signal from a small target a last microsecond boost.

Eric.

Played with other damping methods, mostly with spice. No success. Have you used anything but resistive damping with your detectors?

[stefan-s]

Thanks eric

[Ferric Toes]

Eric,

Thanks, and now i also know why some people are using the graphite powder paint in there coils

So when im correct the two yellow wires are from the coil winding and the green one is connected to the ground?

http://www.geotech1.com/forums/attachment.php?attachmentid=36408&d=1465639238

Oh yes btw the detector is using (when i remember correct) a 7 pin connector.. Why is this a 7 pins connector when there are only 3 wires coming from the coil?

Stefan

Hi Stefan,
This picture is of a ferrite core wound for a stick probe. Here, I am using lead tape instead of copper fabric tape, but you could use either. The yellow wires are the ends of the coil and the green wire is a wire from the ferrite core and also from the lead tape. The green wire goes to one end of the coil wire and which also joins to the coax braid. The other yellow wire goes to the middle conductor of the coax. You then have just the two far ends of the coax to connect to the 7 pin plug. Middle pin is live and all the outer pins are ground. this is to make the plug look like a coaxial connector.

Eric.

[stefan-s]

Eric,

Thanks, and now i also know why some people are using the graphite powder paint in there coils

So when im correct the two yellow wires are from the coil winding and the green one is connected to the ground?

http://www.geotech1.com/forums/attachment.php?attachmentid=36408&d=1465639238

Oh yes btw the detector is using (when i remember correct) a 7 pin connector.. Why is this a 7 pins connector when there are only 3 wires coming from the coil?

Stefan

[Ferric Toes]

Erik,


You are telling in your post #843 you are using "copper fabric tape as a shield" what is the reason tho use this?

Sorry for the many questions but i' am new to this kind of electronics.

Best regards
Stefan

Hi Stefan,

Without a grounded conductive shield between the coil and the ground surface, the detector could become unuseable from false signals generated by changes in the ground to coil capacitance. This problem gets worse if the ground is moist or worse still on a salt water beach.
Some manufactured coils use graphite or nickel loaded paint on the coil shell for a shield, but I prefer conductive woven tape wound right on the coil. Solid metal conductive tape generates too much signal of itself and swamps the detector, while the copper fabric tape gives no signal but is conductive enough, even when wrapped around the coil, to make a very effective shield without generating its own metal signal. A second benefit is that it attenuates electromagnetic interference. The conductive tape has to be connected to the electronics ground; usually the outer braid of the coax cable to the coil.

Unfortunately the woven copper fabric tape is getting very difficult to obtain. There is a nickel coated copper fabric tape which appears to work OK but you can't solder directly to it, and you need a drain wire.

Eric.

[kingswood]

I have no idea on this one. The larger and smaller heads I have made all have the same inductance, resistance, and about the same resonant frequency, so as far as the electronics goes there is nothing substantially different to the stock coil.

You have to be a bit careful with the Vallon manual as not all features that it mentions are available. It depends on which version of firmware has been loaded. It would be interesting to measure the above three parameters on your 30cm coil to see if there is any difference to the stock coil.

Eric.

Thanks Eric...I wonder if the "advance digital pulse induction" reduces the pulse rate or something similar when the bigger coil is applied....
I am going to email Vallon and ask them

[Ferric Toes]

One thing in the manual that has interested me is where vallon state: "Upon switching ON the detector electronics recognizes which search head has been connected and switches over the pulse control automatically".....I dont have the expertise or the equipment to work this out, but have you tested what exactly happens to the pulses when the larger heads are connected??

I have no idea on this one. The larger and smaller heads I have made all have the same inductance, resistance, and about the same resonant frequency, so as far as the electronics goes there is nothing substantially different to the stock coil.

You have to be a bit careful with the Vallon manual as not all features that it mentions are available. It depends on which version of firmware has been loaded. It would be interesting to measure the above three parameters on your 30cm coil to see if there is any difference to the stock coil.

Eric.