Double D coil design

If you are designing this from scratch, the coils must be impedance matched to the frequency of operation of the master oscillator. If you are designing for an existing model, you have to measure that impedance and match it.

The wire size in the coil will determine how much current it can handle. Once you determine that, you calculate your turns based on impedance.

Of course you know that this is a very, very basic description. You need to pick up the book by Ram Publishing called The Totally Revised Modern Metal Detector by Charles Garrett. Be certain to get the 1998 edition. The older one is a waste of time for the cutting edge stuff.

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and search for patents on Induction Balance Metal Detectors. There's a wealth of information there with full theory for those in the know. Don't be afraid to use Google to search for more information using the same search terms I just suggested.

Help yourself to all the knowledge out there. Come back when you are ready to teach about what you just queried.

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http://www.thunting.com/smf/index.php/topic,13296.msg84062.html#msg84062

I've had a bone to pick with Analog ever since they bought Allegro and stopped making their parts. Allegro used to have the most robust parts for motor control ever designed and when Analog bought the company, everything was discontinued. It almost looked more like a mercy killing than an acquisition.

Did you check out the free patent site yet? I don't have much interest in IB machines but the search I did turned up a surprising number of hits.

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I don't think that current in the coil matters much. Unlike an antenna being used to send the signal as far as it can, the soil limits the depth the field can penetrate so there isn't much sense to going higher. The resistance, important to overall circuit function, is not a design function but a result of the impedance design of the coil. In other words, you select the coil impedance to match your circuit oscillator and determine wire size, coil size and windings based on all of those factors. The end result is a coil that is ready for the oscillator circuit with a resistance that can be measured. Using a standard formula for power, the output should not exceed 200mw in the USA. That's the law unless you have a license to transmit higher.

As it turns out, that value is nearly perfect for metal detector design in soil conditions anyway. There's just nothing to be gained from going higher and the reason brings us to soil geology. The nutrients, plant matter, insect matter, moisture and mineralization of soil limit how far a signal can penetrate. The physics of it all is discussed in a few of the patents but only obliquely. Sand is less of an issue but the silica in the sand and in the soil has a tendency to be coated with water making it an audio frequency absorption substance. The earth is against us, mate, but we make do.

All these things are the reason that we have several dozens of manufacturers with perhaps hundreds (if not thousands) of models of metal detector out there. Each believes they have the best design and aren't afraid to tell you about it.

Getting back to your coil, have you already got the machine that this coil will be using? If not, design the machine first with a single loop coil for simplicity and add the Double D designed to match your circuit parameters later. Naturally, when building it you allow for both coil types to be used.

In actual practice, the transmit coil develops a momentary field which has both an electrical and magnetic component. That field must be transmitted to a metal object or be absorbed in the soil. We don't want it to reflect back from the soil so absorption is important. When it strikes metal, the field enters the metal if it is ferrous and builds up on the surface if it is non-ferrous. Both result in a flux field being set up which in effect becomes a miniature transmitter coil sending a signal back to the receiver coils (or coil). The alignment of the target field in relationship to the transmitted field is the best way to analyze your target composition. But that's another topic.

I'll bet you never thought coil design could be so complicated. The trouble for us engineer types is we like to work it out on paper before we try it. The folks who start out buying wire and building a coil winder get good results sometimes without ever touching paper. This discussion will put those guys to sleep while firing up our neurons. Have fun, mate.

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http://www.thunting.com/smf/index.php/topic,13296.msg84336.html#msg84336

Hi Shakin97

Would you mind sending me that LC meter when you done with it 

Regards Xavier


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http://www.thunting.com/smf/index.php/topic,13296.msg84817.html#msg84817

Xavier

Don't forget that you might be missing the phase current changes which are not detectable using a scope. I usually clip a 10 meg (or greater) resistor to ground on the measurement point to stabilize the adjustments. Remove it when you're done and you can't really get any closer.

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