mono coil IB

[deemon]

Hi all,

the new ideas look quite easy to build. But they aren't IMHO! They can't achieve the mechanical induction balance quality. And they are limitted to few applications only (simple TR, single frequency VLF).

Sorry, but I'm not convinced at all after doing some spice simulations. But I do not want to prevent & discourage you to try it out. Please report your experience here.

Cheers,
Aziz

Ha-ha , as for me - it's much simpler to wind a simple round coil and a little additional transformer than to make a good and stable double D coil .... As I think - the main problem must be a need to screen this transformer , because the screen can affect the balance . So I wanna try the toroidal variant of the trans ( as I told before ) - because of its less leackage field . It seems to be a good solution ...

[Aziz]

Ha-ha , as for me - it's much simpler to wind a simple round coil and a little additional transformer than to make a good and stable double D coil .... As I think - the main problem must be a need to screen this transformer , because the screen can affect the balance . So I wanna try the toroidal variant of the trans ( as I told before ) - because of its less leackage field . It seems to be a good solution ...

Hi deemon,

regarding the stable double D coil:
Well, this isn't an issue anymore. I can recognize the instability and compensate it automatically.

It's easier to make the double D coil for me. The solution you have presented is fine but critical. It will introduce more problems.

Cheers,
Aziz

[Vague]

Hi Deemon
Your idea looks really interesting. Could you give me some idea of the number of turns you are using in your coils.
Thanks

[deemon]

Hi Deemon
Your idea looks really interesting. Could you give me some idea of the number of turns you are using in your coils.
Thanks

Number of turns depends on coil diameters and other factors and if you wind the coils with my numbers but with another wire , for instance - you won't have a balance . So the best idea is to download any coil calculator program and use it . Of course , you don't need a super precise calculations , because you can adjust the final balance changing the number of turns of L4 coil or inserting a little ferrite rod in the L3, L4 bobbin .

So you must begin with your L1,L2 coil . First you must decide what inductance and search coil diameter you need for your metal detector and calculate the coil using the program . In my experiments I used , for example , 3 coils from 0,27 mH to 1,5 mH . Then you wind it with 2 wires ( for example , I used 0,5 mm wire for L1 , L3 and 0,1 mm wire for L2 , L4 ) . After it you find a suitable bobbin for L3 , L4 coils , measure it diameter and length and calculate L3 coil - it must have inductance approximately L1/10 . Real inductance may have some deviation from the calculated value , so it's better to have L-meter to measure L1 and L3 after winding and finally calculate L4 with proper value according to the balance equation . When calculating L4 you must measure the outer diameter of L3 with paper isolation , and calculate L4 on this diameter .

Another method , more "practical" , is good when you have an oscilloscope ( or good voltmeter ) and signal generator . After winding your search coil you must connect L1 in series with L3 ( start with 20 turns L3 , for example ) , connect them to signal generator , set proper frequency , and connect 2 channels of the scope to the L3 and L2 coils . Changing the turns number of L3 you must set the voltage ratio of the scope channels about 1/10 ( it will be when L1/L3 =10 approx. ) . Then is only what you need is to multiply final L3 turns number on 10 - and you get approximately turn number of L4 . Wind it over L3 , then assemble the circuit ( don't forget about polarity ) and try to achieve the balance , adding or removing the turns of L4 and watching the balance on the scope .

By the way , when you turn it on the first time , the balance must be bad anyway ( it's very difficult to calculate all values so precisely ) . So a little ferrite stick will help you - if you insert the stick into the L3,L4 bobbin and balance becomes better ( less voltage on the receive chain ) - you must increase the L4 value ( add more turns ) , if the ferrite makes it worse - wind off some turns , of course .

[Vague]

Thank you Deemon that is a very helpful explanation. I will have a go when I can find the time.

[Tepco]

About the principle of compensation in this class of balance circuits - it's based not on subtraction the voltage drop on the two arms of the Wheatstone bridge or hybrid network , but on subtraction derivative of the coil current function ( proportional to the coil voltage according to Faraday's law ) from the real coil voltage that we can get from L2 winding . As we can see - this compensation transformer ( L3 , L4 ) is a real differentiator . Unlike the usual transformer that has voltage input and voltage output ( or current input and current output ) - this one has current input and voltage output ( proportional to the input current derivative ) , and this output voltage we subtract from the voltage on our bifilar-wound L2 . This subtraction cancels the input transmit voltage , but external magnetic field produces the induction current in L1 coil - and this current produces ( after differentiation ) the signal voltage on the L4 , that haven't been cancelled and appears on the output . And this output signal is proportional to derivative of the external magnetic field , just on the leads of a real unloaded receive coil , so this circuit after being balanced behaves itself like balanced 2-coil system , double D for instance , but without its drawbacks like corrupted directivity diagram and phase reverse on the sides of DD coil . And it of course must be good for metal discrimination purpose , so I decided to try my balance circuit in my new IB multi-frequency metal detector .

Thanks for more detailed explanation, this is much clearer now. Coil is actually part of air core “current transformer”, L2 produce voltage proportional to current thru L1, not voltage across it. This voltage is then substracted from L4 voltage, proportional to L3 current identical to L1, L3 having significantly (say 10 times) less inductance than coil, and L3\L4 ratio set to achieve balance condition, if i get it right?

This is still bridge type circuit, able to eliminate coil resistive (also skin effect etc) component from balance equation. From your description in later posts, L3\L4 combination appears to be air core coil . Maybe balancing can be achieved using additional tap on L4, few turns above and below null point, and additional trimmer of suitable value to adjust balance.

Anyway, air cored L3\L4 seems to be a must, i tried this circuit using ring core ferrite transformer and run into trouble with balancing. Namely, it is very hard to mach air core response (coil) and ferrite core response (L3\L4) even at single frequency due to ferrite characteristics, additional compensation is needed. Even more complicated for multifrequency or square, not to mention pulse signals. Will play wit this circuit when time allows. Thank you for sharing this interesting and original idea with us.

[deemon]

Thanks for more detailed explanation, this is much clearer now. Coil is actually part of air core “current transformer”, L2 produce voltage proportional to current thru L1, not voltage across it. This voltage is then substracted from L4 voltage, proportional to L3 current identical to L1, L3 having significantly (say 10 times) less inductance than coil, and L3\L4 ratio set to achieve balance condition, if i get it right

L2 ( and L4 ) voltage is proportional not to the L1 current itself , but to its derivative , of course . And this is true , when we speaking about current which produced by transmitter voltage . But what is interesting - the signal current , being induced by external magnetic field in L1 coil , makes a signal voltage on the L4 coil ( because of the current ) , but not on the L2 . It's the main point of the whole idea . Just because this we can subtract L4 and L2 voltages and suppress the transmit signal on the output , but the receive signal is still present there .

This is still bridge type circuit, able to eliminate coil resistive (also skin effect etc) component from balance equation. From your description in later posts, L3\L4 combination appears to be air core coil . Maybe balancing can be achieved using additional tap on L4, few turns above and below null point, and additional trimmer of suitable value to adjust balance.

As for me , I don't like the idea with the trimmer between the taps , because L4 must be completely unloaded for correct operation , as I think . So we can really make a taps , and connect the tap that is nearest to balance , and after this - make a fine tuning with a little thin ferrite rod trimmer . Another idea to fine balance tuning is to make a little part of L4 movable , and when we adjust it position precisely , we can permanently fix it .

Anyway, air cored L3\L4 seems to be a must, i tried this circuit using ring core ferrite transformer and run into trouble with balancing. Namely, it is very hard to mach air core response (coil) and ferrite core response (L3\L4) even at single frequency due to ferrite characteristics, additional compensation is needed. Even more complicated for multifrequency or square, not to mention pulse signals. Will play wit this circuit when time allows. Thank you for sharing this interesting and original idea with us.

Yes , of course , ferrite does have a problems here . This is why I advised to use ferrite-core L3,L4 transformer only if we have a search coil ( L1,L2 ) on the ferrite too . But due to non-perfect balance it can be useful only for a simple pinpointer ( with rod antenna as a search coil ) , but not for a big and serious device .

But when we use all coils with an air core , we can achieve a very good balance , suitable even for multifrequency metal detector . As I noticed in my experiments , it's quite real to get a good balance from 3 khz to 10 khz ( optimal frequency range with one of my experimental coils ) , or from 5 khz to 16 khz with another coil , for example . But when we wanna go further to high frequencies - it's better to use Litz wire for the search coil , of course .

[Tepco]

This is about the same conclusion i came to. (well, derivative, naturally, unless someone invents DC transformer). Ferrites are problematic, air core apparently can be fine tuned with trimmer without excessive circuit loading (just very fine tuning around balance point, few turns). Not measured in details, (null depth etc), fine tuning coil mechanically can be better solution, but then equally messy as tuning balanced coil. Can be wideband, but not enough for square or PI operation.

My idea is to try this concept using pure solid-state circuit to perform L3\4 function, hopefully fixing adjustment and BW issues, will see…any attempt so far?

[deemon]

This is about the same conclusion i came to. (well, derivative, naturally, unless someone invents DC transformer). Ferrites are problematic, air core apparently can be fine tuned with trimmer without excessive circuit loading (just very fine tuning around balance point, few turns). Not measured in details, (null depth etc), fine tuning coil mechanically can be better solution, but then equally messy as tuning balanced coil. Can be wideband, but not enough for square or PI operation.

About square pulse or PI - why not ? Frequency range that I achieved in my experiments , I got without any special measures , just "as is" , but if I wind the search coil with Litz wire , it will help to kill eddy currents in the coil wire , and eddy current is the main problem for wide-band balance system . As I know , Minelab uses Litz wires in their wideband double-D coils .

Another tip about the balance tuning - if we make balance of the coil on low frequencies , and find some disbalance on the highs , we can use a very simple compensation RC networks in parallel of the L2 and L4 coils . This networks must be calculated in order to leave the coils unloaded on the LF , but on the HF they will correct our balance .... of course , such a tricks seems to be important only if we wanna make a true multifrequency device , like Minelab does . But in the simple ULF or 2- or 3-frequency detectors we can use only the properly tuned basic circuit , without any special complications .

My idea is to try this concept using pure solid-state circuit to perform L3\4 function, hopefully fixing adjustment and BW issues, will see…any attempt so far?

I have some thought about it too , but decided not to go this way . You see , properly made inductance coil is a very stable and reliable part , but what about all this "electronics" ? Electron device can bring to the circuit some noise , distortions , thermal drift , etc . So if we can avoid any active device in the "narrow place" where we need precision - it's better to do it anyhow ...

[Anker]

mono coil IB is not the optimum gauge concerning classical balancing systems for the reason that the reception coil should not be strongly connected with generating is not optimum configuration. It approximately how to listen to a watch lying on the motor of a working tractor... From personal experiments mono coil IB considerably concedes on sensitivity DD and "ring".
Probably, therefore serious firms do not do balancing devices with one coil

[deemon]

mono coil IB is not the optimum gauge concerning classical balancing systems for the reason that the reception coil should not be strongly connected with generating is not optimum configuration. It approximately how to listen to a watch lying on the motor of a working tractor... From personal experiments mono coil IB considerably concedes on sensitivity DD and "ring".
Probably, therefore serious firms do not do balancing devices with one coil

Of course it would have some reason , if we make something like usual bridge circuit or some kind of "electronic balance" ... but what we really have here ? We have the same old inductive balance , although made in a different way . The same physical law works here - the Faraday's law . For example , any kind of instability of the coil shape in classic double D search head will cause disbalance , isn't it ? But just the same thing will occur in my circuit if I make a weak construction of the L4 coil , for instance . So what we need in every kind of induction balance system is a robust and mechanically stable construction , and if we do this we'll have a good balance of course . But when we have a good balance , we can increase the amplification and rise the sensitivity of our device , and it's true either for the classic or this new approach .

[Anker]

All is true, there is no difference how to make balance - geometry, the external transformer or electronic way. However.
I did such experience: took 2 identical coils one near to another, on one axis. Has made the scheme of balance with one coil, has remembered sensitivity, and then has included two coils - the result was much better. Why?
Probably, the matter is that for effective work of the device the reception coil should have high good quality, in it there should be a sharp resonance. In a case with one general coil low target resistance of the generator "kills" a resonance and system effectiveness falls. Here there is a conflict.
As consequence of it, in system with one general coil the increase in a current of the generator does not lead to increase in sensitivity of the device, unlike "classical" IB the device.

[deemon]

I did such experience: took 2 identical coils one near to another, on one axis. Has made the scheme of balance with one coil, has remembered sensitivity, and then has included two coils - the result was much better. Why?

It's a kind of a strange thing .... I really don't know why . It theory , signal power received from the metal object depends on transmit power and the distance from the transmit coil to metal and from metal to the receive coil ( both distances in cubic degree ) . And for example , in real double D search head both coils are standing quite near , so the distances there and back are equal and both equal to distance from a single coil in the same conditions ... so I can't explain the result of your experiment What I think - maybe your bridge circuit inserted some additional loss to the signal path ?

[Anker]

I suspect that in a case mono coil IB at any circuit decision of a chain of the generator will strongly disturb to receiver chains, and it will negatively affect on result of work of the device...

[deemon]

I suspect that in a case mono coil IB at any circuit decision of a chain of the generator will strongly disturb to receiver chains, and it will negatively affect on result of work of the device...

I have a strong doubt about it , because any power loss may be caused by resistance in the chain ... but for example in my circuit are no resistors at all . And what about the inherent wire resistance of the search coil , it don't affect the received signal , being well decoupled from the receiver chain due to balance principle . But on the other hand , if we have a classic bridge circuit , it really contains a resistors , and they can dissipate some portion of the signal power , of course .