Coil calibration

Sent my F75 in for a check up. It seems to have lost some depth and the VDI's were not as stable. I talked to the 1st Texas and they found the coil to be out of calibration. And to my ignorance I thought the coils on this unit were just windings, but it appears to have electronics in the coil that they can adjust. Anyway I'd like to know a little more about this calibration, just for my own knowledge.

There's no electronics, as such, just coils (which of course are inductors) and screening/shielding. There are in fact three coils, the main transmit and receive coils, and a small 'fine-tuning' coil, which connects to the transmit coil (I think), and this sits under the small cover at the 'heel' of the coil. The adjustment and fine-tuning of the search-coil is done by careful adjustment of the positioning of these three coils. They are normally bonded in place, but I guess it may be possible to 'unstick' the small coil and re-adjust it.

Hi,,,,I can't imagine them chipping away at the sealed epoxy trying to calibrate your coil..LoL..On the other hand contrary to what Pimento said, I have never seen a third small coil inside of a coil used to calibrate the coil by moving it around....However in Layman's Terms,,,, I have seen a variable capacitor inside of a coil on a board....This is used to fine tune the coils resonate frequency for peak performance....Once this is set it's sealed and never tweaked again!!....It would be almost impossible to tweak it again without damaging the internal circuitry....More than likely what they did was tweak or fine tune the Phase adjustment inside of your F75....JJ

I don't want to post a photo of the third coil, as I don't want to annoy anyone at Fisherlabs. But's it's not completely immersed in resin (maybe RTV silicone), so it is feasible to adjust it. As for variable capacitors - unlikely. It's viable to add/remove fixed capacitors, however, and variable resistors ARE used in some coils (Garretts, possibly the ACE machines).

Hi Pimento,,,,Here's the key words "I have never seen a third coil".....I never said that a third coil could not exist somewhere in a coil design....A Garrett Ace coil (the ones I have seen and tested) use a 5-25 PF variable capacitor....JJ

I do know Tesoro coils have to be tuned to their unit especially the ID units....also most are encased so if they do not tune them by taking them apart and Whites replaces them if out of tune. Don't have a prayer rel your situation...Perhaps a tech for the company involved can add further input.

"You can tune a piano but you can't tuna fish" Sorry no sleep and feeling silly.

There a capacitor under the door on the back of the coil.. that cap you see behind the rod mount....

Keith

Which model coil is that, Keith? The Fisher F2/F4 version has capacitor(s) inside (220nF+22nF), they probably fit them in the 'heel' compartment. The internal pic I have is of a Greek series coil, which is similar to the F75 in it's wiring.
Edit: NERDY STUFF ALERT!
I've just read your other post (on the coil-swapping thread) and had a thought...clunk...there it was. The F75's search-coil is DEFINITELY tuned to 13KHz operation. I have run it at a range of frequencies from 1KHz to 30+KHz as part of target ID testing, and the best null is at 13KHz, even at 12K or 14K, you can see it's not at it's best. But the Greek series coils ARE considered 'broadband', Dave J has stated it, and they operate at 8K and at G2 frequency. Having built a few search-coils, including one for the F75 (a work in progress) I know that tuning the null is a tricky operation, and there are several ways it can be achieved. I have a hunch that if the F75 does have a small adjustable cap in it, it's for tweeking the 'resistive' component of the null. It bleeds a tiny bit of signal from TX to RX. Adjustable capacitors are tiny in value, and serve no purpose in tuning the main coil operation, but could help in setting the null more accurately. This could account for why its null is optimal at 13KHz - a capacitor has characteristics (reactance) that vary with frequency. The broadband coils probably use a different method, my hunch is it's a secondary isolated coil with a resistor load on it. I was toying with using this on my homebrew F75 coil, at the moment I've settled on lead foil, strategically placed.

So apologies to JimmyJiver, a trimmer cap is possible on the F75. I guess I was thrown when you stated it was for fine-tuning the 13KHz operating frequency, that would need a golf-ball sized trim-cap.
However, if a coil isn't precisely nulled, the F75 will in fact still work fine. Only when it's so far out that it gives the 'overload' siren does it stop working. There doesn't seem to be much 'inbetween', no area where it's working badly, it's either normal, or silent / overloading. sub-standard performance is much more likely to come from screening problems.



Edited 2 time(s). Last edit at 02/08/2014 10:45PM by Pimento.

Hey Pimento...

Im nowhere tech savvy enough to talk on your level of expertise....

But I do feel the T2 and F75 are aligned to the 13khz freq and the greek and G2/GB and Eurotek's are able to handle the broadband coil's...

I feel the High gain circuitry requires a more aligned coil...

like on a Nautilus...the coils are very finnicky and to get the most out of them the R&C conrols are externally able to be adjusted..A tad off and the machine goes whacky..

The aligment can be done under the cover on the back of the Biaxial coil....on the other coils I believe they have to be opened up....I have a feeling alot of the noise that's being called EMI comes from misaligned coil's also...

Keith









...

You can actually fine-tune the null of a search-coil after it's been assembled, my stock 11 x 7 currently has such 'additions'. I experimented on the stock coil before I set about trying to get my home-brew one working. It would have been interesting to see what RubyDarlene's coil response to such tuning would have been.
Re; the biaxial coils covered rear space, I think it exists purely so the main windings DON'T need precise fitment. They can be inserted with modest care and precision, potted/glued in place, then after they are sealed up and the 'glue is dry', the fine-tuning is done via the rear space. I've seen it mentioned (on Geotech1) how potting resins/foams can change the adjustment of a coils' windings when they are applied, thermal expansion/movement, maybe also electrical effects (dielectric constant? who knows). So fine-tuning after everything is stuck down would get round these issues.

Good info Pimento...

Those foam filled type coils do seem to get out of alignment more than the epoxy filled...those old fisher spider coils that were heavy and epoxy filled seemed like they ran for ever.. guess they still are running arent they......but know when the they went bad you tossed them...



Keith

Pimento Wrote:
-------------------------------------------------------
> You can actually fine-tune the null of a
> search-coil after it's been assembled, my stock 11
> x 7 currently has such 'additions'. I experimented
> on the stock coil before I set about trying to get
> my home-brew one working. It would have been
> interesting to see what RubyDarlene's coil
> response to such tuning would have been.
> Re; the biaxial coils covered rear space, I think
> it exists purely so the main windings DON'T need
> precise fitment. They can be inserted with modest
> care and precision, potted/glued in place, then
> after they are sealed up and the 'glue is dry',
> the fine-tuning is done via the rear space. I've
> seen it mentioned (on Geotech1) how potting
> resins/foams can change the adjustment of a coils'
> windings when they are applied, thermal
> expansion/movement, maybe also electrical effects
> (dielectric constant? who knows). So fine-tuning
> after everything is stuck down would get round
> these issues.

How do you go about checking the response? This seems to be a very progressive thread.

I'm not a stranger to electronics, but if anyone has ever disassembled one of these coils I'd like more information as to the inner workings if possible. I enjoy detecting and love to have a deeper understanding as to how something works.

Quote MarkG:"How do you go about checking the response?"
Could you be a little more specific? If you were referring to this line:
"It would have been interesting to see what RubyDarlene's coil response to such tuning would have been"
-I meant would RubyD notice any change, before/after better nulling of the coil.

And RubyD: I recommend taking a look on Geotech1, there are projects (many from hobbyist electronics magazines), technical articles, lists of patents, and a discussion forum, definitely plenty of talk about DD search-coil construction.
[www.geotech1.com]

Anymore information on this subject?

Didi you have any specific information in mind...?

I just wanted to obtain a better understanding of coil calibration. But the actual information might be more along the secret type of information.

Well, here's a likely procedure, based on the previous F75 coil discussion.....

The two main coils are assembled into place in the coil housing, and sealed in place with some resin/foam/glue. When that's cured, and they can be considered stable, the precise induction balance needs to be obtained, using 'components' inside the cavity on the rear 'heel' of the coil housing. It would seem that the F75 uses a small-value adjustable capacitor, likely to be no more than 50 picofarads maximum, and a small loop of enamelled copper wire. The loop is likely to connect in series with the transmit coil winding. The size of the third coil is no doubt determined experimentally during product development, but for starters, lets say it's a 1 inch diameter loop, of 3 turns. It can be moved around in the cavity, so that it is closer to the TX coil, or to the RX. It could also be flipped over, so its turns either add to, or subtract from the main TX windings, magnetically speaking.
Now I assume you've seen on Geotech1 etc how the precise overlap of the two coils is what determines the 'null', where there is no output from the receive coil. In the F75 coil, the two main windings are fixed, and the overlap is effectively varied by varying the position of the small third coil. Move it around, and you transition from under-balanced to over-balanced, get it just right and there's virtually no output from the coil. Tack the loop in place with tape, then glue it more permanently afterwards. How do you know the coil output? You use a detector, most likely, a 'reference' F75 with voltmeters and oscilloscopes connected to it. The actual output level from the coil is probably 2 mV RMS or less, so you need the amplifiers inside the detector, at least, to give you 100's mV, that you can easily see on a 'scope.
However, if you do this, you'll almost certainly find that you can't actually achieve zero output, there will be some residual signal, below which you can't adjust. This signal is (probably) caused by capacitive coupling between RX and TX, and as a result, magnetic variations like the third coil do very little. You need to introduce something that at least partially affects this '90 degrees' coupling. One way is to use a real metal target, eg. small pieces of lead, cupro-nickel. Another is to use something that simulates a real target - a loop of wire (inductor) with a resistor joined on its ends. This L-R combo can be tweeked to mimic any target, though a very low conductivity target would be my choice, this is likely what's inside the Greek series coils. Or...you can directly allow a tiny amount of the TX voltage to get onto the RX winding. A strategically connected capacitor will do this, make it adjustable, and the residual null that can't be eliminated magnetically can be reduced, to zero. There's certain to be some interaction, I imagine "move the coil, adjust the trim-capacitor, re-move the coil etc" is in order. Then seal it up.
Obviously, all this is done away from metal objects, or possibly an allowance is made for what metal ther may be around, such that, when used out in the open, the null is good.
I hope that made sense.



Edited 2 time(s). Last edit at 02/16/2014 06:46PM by Pimento.

That was a very good, understandable explanation. I understand some of it and it makes very good sense to me. Thanks for explaining in such detail, I just like to know as much as possible about a detector.

That was well written, Pimento.

HH
Mike

My head hurts

Hi,,,,Some manufactures of metal detectors are doing their coil matching internally now....I'm generally speaking about frequency matching....A prime example and debunking one companies claim of retuning the coil....is....A Garrett AT Pro runs at a frequency of 15kHz & the Gold at 18kHz..This is a whopping 3kHz span but yet they use the same coil...A Tesoro Tejon runs at 17kHz & the Vaquero runs at 14kHz another 3kHz span but yet they use the same coil....See a pattern forming here?..:..Think harder and harder and a light will slowly start to turn on about any possible performance improvements....Right now the law of Physics is holding back any major technological advances in the VLF detection field..:..JJ

Pimento, I can say, that is a very well prepared description of the subject of coil calibration. I love this site.

Thanks for the explanation I have been wondering.
Jim