One turn coil never crosses 0 while balancing

[samu]

Dear all,

I have tried a 5mm big coil for an induction balance metal detector. The coil is made out of 5mm copper wire (see geometry image, transmitter is outter coil). The problem is that it is impossible to balance it properly. If I move one of the receiving coils closer to the transmiting coil, the signal lowers until a limit (lets say 2 volts) and them starts growing again (while changing its phase). I have tried any single placement moving all the coils, but never goes under this limit. If I change the geometry a little, maybe the limit is shightly lower or higher, but that's all. As far as I know (and have experienced), there should be a point in which both coils are balanced and there is a 0 in the output. I assume that a perfect 0 is difficult to achive or to mantain, but you should see how the signal crosses 0 at any point while moving a coil. It is like if the two coils weren't 180º phased and there were always an overlap between the signals.

The coils are placed in a wodden board and the geometry is pretty accurate. The board is installed in a metallic enclosure, but this shouldn't be a problem, there should be always a way to balance the coils.

I'm pretty sure that there is something I am not taking into account. Any hint will be very much appreciated. <br>

Thanks!

[lucifer]

Hi,
The two RX coils must be exactly the same so each of them sees exactly the same amount of TX magnetic field. It's quite difficult to achieve with this configuration. It's very likely one of the RX coils sees more of the TX field than the other. Try to adjust its shape /area or its position slightly so they become identical.

[waltr]

There could also be cross-talk or coupling outside to the coil geometry. This could be in the cables or on the Circuit board.
If this is happening then No amount of coil tweaking will give you a Zero Amplitude Null.

[samu]

Hi,
The two RX coils must be exactly the same so each of them sees exactly the same amount of TX magnetic field. It's quite difficult to achieve with this configuration. It's very likely one of the RX coils sees more of the TX field than the other. Try to adjust its shape /area or its position slightly so they become identical.

Thanks for your reply.

Yes, I know that the magnetic field has to be the same, to achieve this, I can move the coils position or even change area, but the issue is that the signal never crosses 0. I tried using the same geometry in another board with thinner cable and it worked right.

[samu]

There could also be cross-talk or coupling outside to the coil geometry. This could be in the cables or on the Circuit board.
If this is happening then No amount of coil tweaking will give you a Zero Amplitude Null.

I think that this is more likely the problem here. The three coils leave the board by means of short straight pieces of the same wire, but the question is: shouldn't be that signal also balanceable by means of coils placement? I have tried moving the coils in every way possible, even changing its geometry, but it never goes under certain level.

[green]

Dear all,

I have tried a 5mm big coil for an induction balance metal detector. The coil is made out of 5mm copper wire (see geometry image, transmitter is outter coil). The problem is that it is impossible to balance it properly. If I move one of the receiving coils closer to the transmiting coil, the signal lowers until a limit (lets say 2 volts) and them starts growing again (while changing its phase). I have tried any single placement moving all the coils, but never goes under this limit. If I change the geometry a little, maybe the limit is shightly lower or higher, but that's all. As far as I know (and have experienced), there should be a point in which both coils are balanced and there is a 0 in the output. I assume that a perfect 0 is difficult to achive or to mantain, but you should see how the signal crosses 0 at any point while moving a coil. It is like if the two coils weren't 180º phased and there were always an overlap between the signals.

The coils are placed in a wodden board and the geometry is pretty accurate. The board is installed in a metallic enclosure, but this shouldn't be a problem, there should be always a way to balance the coils.

I'm pretty sure that there is something I am not taking into account. Any hint will be very much appreciated. <br>

Thanks!

[ATTACH]57165[/ATTACH]

What is the Tx amplitude and frequency? Is the 2 volt null at Rx out or amplifier out. Have you tried adjusting for null outside the metal enclosure?

I would try a ferrite core and or a piece of wire the coil is made of, large enough to cause a signal change when changing position on the coil. Position Rx coils for minimum signal, then try positioning either or both targets to reduce null (board not in metal enclosure).

[Carl-NC]

IB coils are just like targets, they can have a reactive response and a resistive response. Ideally we think of the resistive response as normally being zero so the only thing we have to deal with in nulling the IB is the reactive null (X-null). But you can have capacitive coupling that give rise to a resistive response that also needs to be nulled (the R-null). The only way to get a completely nulled IB is to have a perfect X-null and perfect R-null.

If the R-null is bad then as you move the coils around (which mostly affects the X-null) you will see the signal decrease, then roll over 180 deg, and increase again. One way to fix the R-null is to take a thin wire (35-40 awg) and make a small single-turn loop maybe 2-4cm diameter. This loop will have an almost purely resistive response and you can move it around the windings to suppress the R-null. Once you get the R-null suppressed you can try again with the X-null.

[green]

IB coils are just like targets, they can have a reactive response and a resistive response. Ideally we think of the resistive response as normally being zero so the only thing we have to deal with in nulling the IB is the reactive null (X-null). But you can have capacitive coupling that give rise to a resistive response that also needs to be nulled (the R-null). The only way to get a completely nulled IB is to have a perfect X-null and perfect R-null.

If the R-null is bad then as you move the coils around (which mostly affects the X-null) you will see the signal decrease, then roll over 180 deg, and increase again. One way to fix the R-null is to take a thin wire (35-40 awg) and make a small single-turn loop maybe 2-4cm diameter. This loop will have an almost purely resistive response and you can move it around the windings to suppress the R-null. Once you get the R-null suppressed you can try again with the X-null.

Interesting. I have thought, maybe all wrong. If I place a nickel on the center of one of the Rx coils, I would need to add a nickel to the center of the other Rx coil to cancel the signal. A quarter positioned anywhere on the second coil wouldn't cancel the nickel on the first coil. samu's coil is made with 5mm diameter wire(around 75us time constant). I'm guessing a single turn coil made with awg40 wire would have a time constant less than 1us. Thinking the adjusting R tau needs to be the same as the R tau causing the offset. Where is my thinking wrong?

I'm learning about PI, know nothing about VLF. Maybe VLF is different?

[samu]

What is the Tx amplitude and frequency? Is the 2 volt null at Rx out or amplifier out. Have you tried adjusting for null outside the metal enclosure?

I would try a ferrite core and or a piece of wire the coil is made of, large enough to cause a signal change when changing position on the coil. Position Rx coils for minimum signal, then try positioning either or both targets to reduce null (board not in metal enclosure).

Thank you Green!.
The coil is working at 16KHz and the TX amplitude is about 15Vpp. I use a transformer connected the RX coil to increase the voltage about x100, I'm balancing at the transformer's output. I have used that transformer with similar coils in the past and everything worked right, although sometimes, I had to use some copper or metal to fine balancing. I have tried on this coil with some metals but the unbalance is so big that I would have to put a too big metal to balance it (I achieved it with a ~2Kgs iron piece).

IB coils are just like targets, they can have a reactive response and a resistive response. Ideally we think of the resistive response as normally being zero so the only thing we have to deal with in nulling the IB is the reactive null (X-null). But you can have capacitive coupling that give rise to a resistive response that also needs to be nulled (the R-null). The only way to get a completely nulled IB is to have a perfect X-null and perfect R-null.

Thank you Carl!

If I understand it well, the R response from one of the coils is also 180? apart from the other. So, the R signal could be also balanced by placement of the coils. The capacitive coupling, could in fact be a problem due to it don't being cancelled by the turn orientation, however, at 16KHz, and with a separation of about 20mm the two wires are too far for a significant coupling (I have roughly calculate it and it is less than 1mV).

If the R-null is bad then as you move the coils around (which mostly affects the X-null) you will see the signal decrease, then roll over 180 deg, and increase again. One way to fix the R-null is to take a thin wire (35-40 awg) and make a small single-turn loop maybe 2-4cm diameter. This loop will have an almost purely resistive response and you can move it around the windings to suppress the R-null. Once you get the R-null suppressed you can try again with the X-null.

This is exactly what happens! In the past, I used the small single-turn solution with great success in balancing the coil but with the drawback that the sensibility was much bigger above the small coil than in the rest of the big coil, so it was impractical. I used a small coil in series with TX coil. Maybe I should use the coil in parallel with RX? I'm not sure here, any hint is welcome.

I also don't understand well how can I measure the R signal or the X signal while balancing. If I got it right, the R signal is the resulting signal at 180?, in the point where the phase change from both contrary turns cancels each other. Is it right? So you propose to place the coils to achieve the minimum at 180? (2 said volts) and then, tweak it with a single short turn?

Thank you all.

[Carl-NC]

Interesting. I have thought, maybe all wrong. If I place a nickel on the center of one of the Rx coils, I would need to add a nickel to the center of the other Rx coil to cancel the signal. A quarter positioned anywhere on the second coil wouldn't cancel the nickel on the first coil. samu's coil is made with 5mm diameter wire(around 75us time constant). I'm guessing a single turn coil made with awg40 wire would have a time constant less than 1us. Thinking the adjusting R tau needs to be the same as the R tau causing the offset. Where is my thinking wrong?

I'm learning about PI, know nothing about VLF. Maybe VLF is different?

A traditional PI that only looks at the off-time decay does not care as much about the X-null because the target sampling is done when there is no reactive signal. You can literally stack an RX coil right on top of the TX coil and the PI will still work. In the case of the Figure-8, placing a coin on one lobe generates a resistive decay response and the only way to cancel it is to place the same tau coin on the other lobe.

A VLF looks at the signal all the time so it requires a good null, both X and R. Normally R is not much of a problem until (a) you are trying to get an exceptional null (>60db) or (b) you are dealing with high frequencies (>20-30kHz).

[Carl-NC]

I also don't understand well how can I measure the R signal or the X signal while balancing. If I got it right, the R signal is the resulting signal at 180?, in the point where the phase change from both contrary turns cancels each other. Is it right? So you propose to place the coils to achieve the minimum at 180? (2 said volts) and then, tweak it with a single short turn?

Correct, the R-null is what you see when the RX signal rolls over, right at the 90 deg point.

I originally read your post and then replied to it later, forgetting that you said you were using 5mm wire. That is likely where your R-null problem lies, the wire itself is creating a resistive signal via eddy currents that you cannot null out by moving the coils. The 40awg loop may still fix this but it may need to be much larger or even consist of more than 1 turn as I suspect your R signal is fairly strong.

[Skippy]

This is an interesting problem. I'm sure the previous diagnoses are correct - you are only adjusting the resistive null, and have no serious method of dealing with the reactive null.
I see no mention of screens/shields. Would an electrostatic shield between the TX and RX help deal with capacitive coupling ?
The lack of symmetry bothers me, particularly the difference between the RX at the 'toe' end, and the 'heel' end. It's worth considering making the RX coil truly symmetrical, with the X-over in the geometric centre. That is: one wire is like:
......... ______
_____/ .........

and the other wire can avoid the crossover by using those points as the two RX connections.

I wonder if copper wire is actually a poor choice? As it has the best conductivity, it therefore has the shallowest skin depth. If the coil(s) were brass, for example, would the greater skin depth ( due to the lower conductivity ), cause this nulling issue be less severe?

[Carl-NC]

This is an interesting problem. I'm sure the previous diagnoses are correct - you are only adjusting the resistive null, and have no serious method of dealing with the reactive null.

The opposite: he's adjusting the reactive null, with no way to deal with the resistive null.

I see no mention of screens/shields. Would an electrostatic shield between the TX and RX help deal with capacitive coupling ?

Often shielding increases R-null problems because of the capacitive effect. It's often necessary to add spacers between the coil windings and the shield.

The lack of symmetry bothers me, particularly the difference between the RX at the 'toe' end, and the 'heel' end. It's worth considering making the RX coil truly symmetrical, with the X-over in the geometric centre. That is: one wire is like:
......... ______
_____/ .........

and the other wire can avoid the crossover by using those points as the two RX connections.

The configuration shown should work just fine. I use something very similar in my walk-thru designs.

[samu]

Thank you for all the answers,

The lack of symmetry bothers me, particularly the difference between the RX at the 'toe' end, and the 'heel' end. It's worth considering making the RX coil truly symmetrical, with the X-over in the geometric centre. That is: one wire is like:
......... ______
_____/ .........

and the other wire can avoid the crossover by using those points as the two RX connections.

Its also bothers me... Of course the lack of symmetry is adding problems to the balancing. The geometry you propose looks much better, I will consider it for next coil.

I see no mention of screens/shields

No, I'm not using any shield.

I originally read your post and then replied to it later, forgetting that you said you were using 5mm wire. That is likely where your R-null problem lies, the wire itself is creating a resistive signal via eddy currents that you cannot null out by moving the coils. The 40awg loop may still fix this but it may need to be much larger or even consist of more than 1 turn as I suspect your R signal is fairly strong.

I used this wire before without much problem. I use it due to it lower resistance as weight is not an issue.

This is exactly what happens! In the past, I used the small single-turn solution with great success in balancing the coil but with the drawback that the sensibility was much bigger above the small coil than in the rest of the big coil, so it was impractical. I used a small coil in series with TX coil. Maybe I should use the coil in parallel with RX? I'm not sure here, any hint is welcome.

Could anyone clarify if the balancing short turn should be connected in series or in parallel and to which coil? In the past I used it a few times but the results were not good, the coils balanced but it only detected well over this short turn and much worse over the rest of the coils.

Thanks in advance

[kt315]

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