Induction Balanced Coils, IB coils

[Tinkerer]

Since the conception of metal detectors, Induction Balanced coils have been used.

Then came the mono coil. Very simple, just a few turns of wire. The TX coil is also the RX coil. It worked just fine, why bother with tricky inductance balancing?

As detectors became more sophisticated, the mono coils became more trick to build too. Shorter delays, to make small targets detectable made the mono coil building a fine art. How to reduce the inter wire and wire to shield capacitance? What type of wire to use? How best to do the shielding? Tiny differences made notable differences in the coil performance.

When we look at the fine art of making a really fast mono coil, there is not much difference in the difficulty and amount of work between the 2 types of coils.

Yet, it seems that people are afraid to tackle the job of building an Induction Balanced coil. IB coil for short.

Why are people afraid of IB coils?

What makes IB coils more difficult to build?

Where lie the advantages of IB coils?

How to build an IB coil?

How to calculate an IB coil?

How to balance an IB coil?

Tinkerer

[Jose]

Hi Tinkerer
Thanks for sharing your projects forum.
I'm interested in making one of his projects Pi-IB.
I hope the development of this issue, and I do not know in detail the construction of this type of coil.
Regards, Jose

[Tinkerer]

http://www.geotech1.com/pages/metdet...planar_300.pdf

This excellent article, by Dave Emery, explains the construction of an IB coil for a PI detector very well.

I suggest we use this as the starting point. Then we can look at different coil sizes and coil shapes and how to fine tune the induction balance.

[Jose]

Thanks Tinkerer
Dave Vi article Emery, TX understand that the coil must have a compensation coil diameter equal to ½ of the TX, and ¼ of the number of turns wound in TX phase opposition.
My question is, as a bucking coil influences the total inductance TX.
The RX coil in your projects, you use two coils for differential amplifiers.
My question is if each coil is 300 uH example, that total is 600 uH.
Or is 150 +150, "uH".
Thanks for your attention
José

[Tinkerer]

Thanks Tinkerer
Dave Vi article Emery, TX understand that the coil must have a compensation coil diameter equal to ½ of the TX, and ¼ of the number of turns wound in TX phase opposition.
My question is, as a bucking coil influences the total inductance TX.
The RX coil in your projects, you use two coils for differential amplifiers.
My question is if each coil is 300 uH example, that total is 600 uH.
Or is 150 +150, "uH".
Thanks for your attention
José

For the TINKERERS designs, the coils must be a little different.

RX 150uH + 150uH is OK.

The TINKERERS TX has a resonant component that increases the target signal, eliminates the TX damping resistor and recycles the power.

For optimum power recycling, the TX winding must have minimum resistance. About 0.5 Ohm, for cable and coil is OK. This is not extremely critical, but any increase in resistance increases the power losses proportionally.

http://www.miscel.dk/MiscEl/miscel.html This is a very useful calculator for detector design.

On the picture you see the calculations for a 40cm diameter TX coil.
You see the wire thickness
The wire length in meters
The weight of the wire
The resistance of the wire.
The inductance of the coil with 16 turns

With this you can find all the parameters for any size coil.

When you change the wire bundle thickness, you see that the inductance changes. This tells you that the thickness of the wire insulation plays a role as well as the tightness of the winding or how tight you bind the bundle.

Note that the initial coil diameter is the internal diameter. The average diameter is the internal diameter plus the bundle thickness.

The final coil will be even thicker as you need to ad a spacer and shielding.

Enjoy

Tinkerer

[Jose]

Many Thanks Tinkerer, maybe you do not remember that long ago taught me to use MiSCeL to calculate the current in the coil, resistors based on associates and TX pulse width. I use it for my coils calculating PI mono.
Just one more thing, for 16 laps compensasion coil must be 4 laps if I remember correctly.
I know the RX coil diameter may be small, what would be the ideal diameter.
Thanks again
Jose

[Tinkerer]

Many Thanks Tinkerer, maybe you do not remember that long ago taught me to use MiSCeL to calculate the current in the coil, resistors based on associates and TX pulse width. I use it for my coils calculating PI mono.
Just one more thing, for 16 laps compensasion coil must be 4 laps if I remember correctly.
I know the RX coil diameter may be small, what would be the ideal diameter.
Thanks again
Jose

It is best to wind the RX coil on the BU coil. Same diameter.
If you change the RX+BU coil diameters, you must recalculate the number of turns.
Calculating the relative surface area gives an approximate relation of the coupling factor and coil turns.

Dave Emery suggests to leave a half turn open on the BU coil. This helps fine tuning the induction balance, but increases the residual signal on the RX.

[Tinkerer]

This IB coil is designed for the TINKERERS detectors. It is not ideal for traditional PI.

I only have #20 AWG available, so I twisted 4 wires together for the TX.

Why 4? we have about 1A average current running in the coil. We do not want the coil to get hot.
Inner diameter is 24cm. At 26 turns this should be about 350uH. Resistance should now be about 250mOhm for the TX and BU in series. The coil Flyback is resonant. Therefore, the Ohmic losses are to be avoided.

The BU coil is wound counter clockwise, 7 turns
The TX is wound clockwise 26 turns.

Picture #1 BU coil, 10cm inner diameter, 7 turns
#2 BU and TX wound, TX is 24cm inner diameter, 26 turns
#3 TX coil bundled
As you can see, all this is not very precise. The diameter of the bundle will be different with different wires.
With different bundle diameters, the inductance will be different too.
How do we correct for the differences?
How do we balance the coils?
We will get to that next time.

[Jose]

Hi Tinkerer
I have a question
The RX coil, is wound on the coil BU.
If so, do not lose the coil RX sensitivity, with such a small diameter of only 10 cm.?
thanks

[Tinkerer]

Hi Tinkerer
I have a question
The RX coil, is wound on the coil BU.
If so, do not lose the coil RX sensitivity, with such a small diameter of only 10 cm.?
thanks

Large coils are for deep targets.

Small coils are for shallow targets.

Smaller coils give higher sensitivity for small targets.

Where lies the best compromise?

To tell the truth, this is the smallest coil with the smallest RX and BU that I am building now. I am curious myself as to the results.

As you notice, the relation between the TX and RX coils is a weird one. According to my calculations it should work. Will it?

If nobody tries, we will never know. So I am trying.

To be a pioneer, to search for innovation, to break through the walls, one needs to be bold.

[Jose]

Thanks Tinkerer
I was struck by the relationship that is somewhat lower, which should be 12 cm.
I see also that compensates, increasing the turns.
Well, we hope to see as follows .....
Jose

[Tinkerer]

Experimenting with IB coils

Below is the 26cm outer diameter TINKERERS IB coil windings. This is an experimental coil. The TX-BU-RX proportions are a bit different for experimenting.

The TX coil was wound on a 24cm diameter nail bed. I used 4 #20 AWG magnet wires in parallel, slightly twisted.
This should be OK for 1.5 to 3A average coil current.

26 turns for about 360uH inductance.

The Bucking coil, BU, is wound on a 10cm diameter nail bed, 7 turns.

The mutual inductance is about 17%. The ideal mutual inductance is supposed to be about 33%. So, why so small?

The RX coil is wound the same diameter than the BU coil, right on top of the BU coil.

10cm inner diameter seems small for a RX coil.

The reasoning (please correct me if I am wrong) is as follows:

A low coupling factor means that only few turns are needed for the BU coil to null the flyback current. This means that the TX field is stronger.

It also means a lower amplitude residual (air) signal. This residual signal is a limiting factor with the gain of the pre-amp.

What about the RX? So small? Will it pick up the target response?

Experimenting with IB coils, I find that the sensitivity is of the total assembly, not just of the RX coil. I even once tried a 10cm diameter RX coil with a 150cm TX rectangular coil and it still worked.

There are several things different with the TINKERERS coils than with traditional PI coils:

Traditional PI coils are very sensitive to stray capacitance caused by the wire to wire and wire to shielding and cable capacitance. This reduces the possibility of sensing targets with TC's less than about 15us.

The TINKERERS coils do not have this problem, since the capacitance is used to form a resonant tank that recycles the power instead of burning it up in a damping resistor.

This is why you see me using ordinary magnet wire.

[Aziz]

Hi Tinkerer,

you're intending to drive the TX coil with high current obviously (due to thick coil wire bundle).
I might have an interesting new TX for you. It supports boost-mode.

It will be published as soon as I have more time.

Cheers,
Aziz

[mickstv]

Hi Tinkerer, If you wave a coin over this coil setup at say 30cm, when does the coin start to be detected, is it at the outer edge of the TX winding or more towards the middle receive winding ?


Cheers
Mick

[Pete the Builder]

Below is the 26cm outer diameter TINKERERS IB coil windings. This is an experimental coil. The TX-BU-RX proportions are a bit different for experimenting.

The TX coil was wound on a 24cm diameter nail bed. I used 4 #20 AWG magnet wires in parallel, slightly twisted.
This should be OK for 1.5 to 3A average coil current.

26 turns for about 360uH inductance.

The Bucking coil, BU, is wound on a 10cm diameter nail bed, 7 turns.

The mutual inductance is about 17%. The ideal mutual inductance is supposed to be about 33%. So, why so small?

The RX coil is wound the same diameter than the BU coil, right on top of the BU coil.

10cm inner diameter seems small for a RX coil.

The reasoning (please correct me if I am wrong) is as follows:

A low coupling factor means that only few turns are needed for the BU coil to null the flyback current. This means that the TX field is stronger.

It also means a lower amplitude residual (air) signal. This residual signal is a limiting factor with the gain of the pre-amp.

What about the RX? So small? Will it pick up the target response?

Experimenting with IB coils, I find that the sensitivity is of the total assembly, not just of the RX coil. I even once tried a 10cm diameter RX coil with a 150cm TX rectangular coil and it still worked.

There are several things different with the TINKERERS coils than with traditional PI coils:

Traditional PI coils are very sensitive to stray capacitance caused by the wire to wire and wire to shielding and cable capacitance. This reduces the possibility of sensing targets with TC's less than about 15us.

The TINKERERS coils do not have this problem, since the capacitance is used to form a resonant tank that recycles the power instead of burning it up in a damping resistor.

This is why you see me using ordinary magnet wire.

Tink, I just found this thread... Wow. Thank you so much for the excellent photos and descriptions. I understand much better now about this coil set, and coil winding in general.
This is just the thing for new people to see, especially if they want to build "Tinkerer's Tiger" . But it's a great way to help explain coil winding in general.
Wow. No wonder you have a good rep for coils! They are a thing of beauty and a joy, well, maybe not forever, but for a long time. Until the next design!
Cheers,
PtB