Induction balanced coils

[Tinkerer]

In recent times there seems to be growing interest in IB coils for PI detectors.
What are the properties of Induction Balanced Coils that are specific for PI detectors?

What is the best size? shape? amount of windings?

How to dispose the different windings?

DD coils? Figure 8 coils? Concentric coplanar coils.

How do the 3 or more coils interact?

What is the best relation in size, between TX, RX, Bucking coils?

Should the Bucking coil be of the minimal possible induction or should it be maximal? Why?

Is the IB PI method best for high power or low power detectors?

What are the advantages of the IB coils over the mono coils?

The PI IB coils are quite different from VLF IB coils, what exactly are these differences? Why?

Tinkerer

[simonbaker]

Some more questions I'd like to add:

- For IB VLF detectors, I assume we want as powerful a magnetic field as possible to overcome noise and go deeper. The magnetic field will be proportional to the total current around the coil loop I think. So that is why we make coils with many windings rather than a single winding, to increase the total current (# of windings X current per winding).

My question is, why do we not use as many windings as possible? What are the limiting factors that cause us to use about 100 turns? Why not many more? Shouldn't we also use the thickest wire possible to reduce resistance?

- Another questions is, to save battery energy, it would seem we want a very "high-Q" resonant coil system, so the current sloshes back and forth and the energy is conserved as much as possible. Things in the ground will steal some energy, but we should still want a very hi-Q coil tank circuit I think to save energy as much as possible. Wouldn't it be best to use thicker wire for that reason also, to reduce resistance and increase the Q factor? How much do the detector designs try to increase the Q-factor, and how do they do it?

- Also, why with concentric coils is the RX coil much smaller than the TX coil? Does the size of the RX coil affect depth? I would think it would due to some reciprocity laws.

[Aziz]

In recent times there seems to be growing interest in IB coils for PI detectors.
Tinkerer

Indeed,

some home-made problems could be avoided:

- power supply for the analog part (opamps) gets simplified due to galvanic decoupled of the RX from the TX coil. So the power supply can be realized without switching capacitor regulators (very noisy).

- higher analog amplification by higher number of windings for RX, without increasing the TX coil capacitance much. The TX coil can have very few windings to work very quick.

- less over-voltage problems of the RX side (inductively not coupled to TX)


Aziz

[porkluvr]

rambling on

More coil windings or larger gauge wire add more weight to the search head. But it is correct to assume an increase in depth due to increased field strength. Also, correct about saving energy with higher Q coil (particularly important in an IB detector). I just ran a simulation in LTspice, varying the resistance in a Bandito oscillator. Although as coil resistance was increased there was a small decrease in supply current, it was accompanied by a substantial decrease in transmitted power. This is not a good trade, imho!

Another aspect of increasing coil windings: you do not get a proportional increase in coil Q as you add more windings because there is added inter-winding capacitance to be considered. The law of diminishing returns applies. I think that optimum coils are often determined empirically - through trial and error - as opposed to their being calculated mathematically. Find what works, and stick with it.

“3W” (three windings) concentric coil configurations can be changed i.e., there is no reason why you could not add the bucking coil in the receiver section. Carl addresses this in his coil.pdf.
http://geotech.thunting.com/pages/metdet/info/coils.pdf

However, it is customary to buck the transmit coil and there is probably more information available for this approach. The general idea is that (almost) complete cancellation occurs between the TX and RX coils - except when a target is present to upset the balance. ANY geometry that works, can be used . If you want or need a balanced transmit coil (and using twisted pair wire instead of coax), then bucking the receive coil may be appropriate to minimize energy radiated from the TX cable.

Concerning PI detectors, as Aziz stated [ less over-voltage problems of the RX side (inductively not coupled to TX)], a properly nulled 3W concentric coil might be used advantageously in a PI detector because it may eliminate a high voltage that would otherwise be applied to the receiver (and this negates the need to use a noisy series resistor, or having to implement receiver blanking - else fry the receiver amplifier) but I think you may lose sensitivity using a 3W concentric coil in a PI. Ground effects are also minimized, but, again, at the expense of reduced depth capability. Also, we don't want a high PI transmit inductance because we desire fast turn-off of PI coil current. I think IB coils generally have about 500x~1000x more inductance than PI coils. IB and PI coils are different animals and have different considerations. In a PI detector, I would be careful about relying on coil null to protect the receiver input, but it is possible and may be more commonly used than I could know. But, PI coil inductance should be very low compared to an IB coil. Apples and oranges.

Although the math is not extremely complicated, 3W coil calculation can still be daunting. Besides the need for proper winding ratios and proper orientation to obtain a good null, because of an unknown "k" factor (coefficient of coupling) calculating total coil inductance is not so easy when a reverse-wound bucking coil is included. Total inductance, and also the correct orientation/turns ratios must be considered. Why re-invent the wheel, so to speak? At least with a DD coil, there is no need to calculate the bucking coil, because there ain't one. Also, there there is the omega coil - that when properly constructed only requires proper orientation of the RX coil in relation to the TX, to obtain a null. (Look at the Magnum IB for an example.)

Look at Whites coplanar coil patent:
http://geotech.thunting.com/pages/metdet/patents/US4293816.pdf

Also Dave Emery’s article - I can’t find a link to Dave’s article, so I am posting it here.

Well, I’m not an expert but I needed a diversion from the final stages of a PCB layout that is giving me gray hair. Hope this has been worthwhile - I gotta go gray some more hair. Sorry about the rambling... it's hard not to ramble when trying to discuss PI and IB coils. In case I didn't make it plain enough, although they can have similarities, they have very different design consideration.

[simonbaker]

Excellent rambling by Porkluvr, very helpful thoughts.

Here are some more of my questions and thoughts:

Although as coil resistance was increased there was a small decrease in supply current, it was accompanied by a substantial decrease in transmitted power. This is not a good trade, imho!

How did you compute transmitted power? Was it the power needed to drive the oscillator? This may be misleading, consider this:

A high-Q LC oscillator uses less power than a low-Q oscillator, yet the current sloshing between the coil and capacitor is higher in the high-Q circuit (I think). Higher current means greater magnetic field, good for detecting depth! So the lower power circuit is better!

In other words, magnetic field is not directly related to oscillator input power but rather the current in the coil. If we used zero-resistance wire and lossless capacitors, then our Q would be huge (infinite) and the magnetic field would grow huge too. The only losses would be any radiated energy (which I think is small or zero for ideal coils) and losses into the ground and any targets.

So I think power (input power to the oscillator) is a misleading indicator. Instead, compute the magnetic field strength (due to total loop current). How does that change as you add more loops to the coil? Even though the current per loop decreases, you are adding loops. Where do you get the maximum "loop current" (current per loop X number of loops)?

Another aspect of increasing coil windings: you do not get a proportional increase in coil Q as you add more windings because there is added inter-winding capacitance to be considered. The law of diminishing returns applies. I think that optimum coils are often determined empirically - through trial and error - as opposed to their being calculated mathematically. Find what works, and stick with it.

That's pretty complicated to compute, yes. I was playing with a 100 turn coil, #32 wire, 155 mm diameter (6.125 inches). Using a signal generator, it seemed to have a very high-Q self-resonance at about 122 KHz. If I put a capacitor in parallel with it to try to make it resonant at about 10 Khz, it was very low Q, I don't know why. I was wondering if it would be good to just keep adding windings until the self-resonance was about 10 Khz, hopefully still high-Q, then drive it with a multivibrator to make a TX coil.

I'll keep researching the coil geometries, bucking coils, etc. My instincts keep telling me that having one coil smaller hurts depth, you need both big somehow (e.g. Double-D or Double-O; but then they are slightly out of line).

Always trade-offs. Keep rambling, it is good information and food for thought.

Thanks,

-SB

[simonbaker]

I think I remembered wrong about radiation from loops, I guess they do act as antennas and radiate, but there is some considerations for low frequencies I think, I'll keep researching... But doesn't change basic point about input power vs. mag field strength I was trying to make. No expert on this, just trying to figure it out.

-SB

[porkluvr]

I believe there is a direct 1:1 relationship between radiated power to coil current and/or number of turns. As you double either #turns OR coil current you double radiated power. Carl Moreland addressed this issue in one of his papers, I'll see if I can find it... but I seem to have lost it.

However, there is a disproportionate increase in a coil's inductance that results from adding turns. This disproportion depends on the particular geometry of the coil (flat spiral-wound [FSW], donut, helical, etc.) but I think it is most profound with FSW. This is because as more turns are added to the outside, each additional winding adds more inductance than the previous one did. Helical coil (don't have much use in metal detectors) or donut coil inductance will more than double with a doubling of turns because (in addition to the obvious inductance increase due to added turns) there is also mutual inductance between the windings. But, the inductance of a FSW coil will JUMP as more turns are added.

Here is a good Spiral Coil calculator:
http://www.deepfriedneon.com/tesla_f_calcspiral.html

The downside of this (as applies to PI coils) is that as more power is radiated from addional windings, the coil's response time may slow
dramatically. This is not even accounting for additional interwinding capacitance (which is, fortunately, minimized with the FSW geometry, I think).

Look at Don Lancaster's (ancient) paper for some technical info. PI detectors may not been invented (?) but this stuff is still valid.
http://geotech.thunting.com/pages/metdet/info/lancaster/lancaster_150.pdf

Maybe Carl will jump in with some more technical info.

On a side note, in my previous post I stated that there was a huge difference in radiated power related to a coil's resistance. This was based on a simple transmitter simulation. I added a receiver and a mutual inductance between TX and RX, and the resulting power difference was not so profound (as measured at the receiver output), an effect that I can't explain.
My LTspice simulation file is attached.

[simonbaker]

I believe there is a direct 1:1 relationship between radiated power to coil current and/or number of turns. As you double either #turns OR coil current you double radiated power. Carl Moreland addressed this issue in one of his papers, I'll see if I can find it... but I seem to have lost it.

However, there is a disproportionate increase in a coil's inductance that results from adding turns. This disproportion depends on the particular geometry of the coil (flat spiral-wound [FSW], donut, helical, etc.) but I think it is most profound with FSW. This is because as more turns are added to the outside, each additional winding adds more inductance than the previous one did. Helical coil (don't have much use in metal detectors) or donut coil inductance will more than double with a doubling of turns because (in addition to the obvious inductance increase due to added turns) there is also mutual inductance between the windings. But, the inductance of a FSW coil will JUMP as more turns are added.

Here is a good Spiral Coil calculator:
http://www.deepfriedneon.com/tesla_f_calcspiral.html

The downside of this (as applies to PI coils) is that as more power is radiated from addional windings, the coil's response time may slow
dramatically. This is not even accounting for additional interwinding capacitance (which is, fortunately, minimized with the FSW geometry, I think).

Look at Don Lancaster's (ancient) paper for some technical info. PI detectors may not been invented (?) but this stuff is still valid.
http://geotech.thunting.com/pages/metdet/info/lancaster/lancaster_150.pdf

Maybe Carl will jump in with some more technical info.

On a side note, in my previous post I stated that there was a huge difference in radiated power related to a coil's resistance. This was based on a simple transmitter simulation. I added a receiver and a mutual inductance between TX and RX, and the resulting power difference was not so profound (as measured at the receiver output), an effect that I can't explain.
My LTspice simulation file is attached.

Maybe it has to do with where the power is going.

I'm looking at it more from IB standpoint than PI, so I'm thinking about coil as oscillator, or driven by oscillator.

With IB coils, if you take a theoretical case with infinite Q coil (negligable resistance compared to inductance) and very low frequency, then there is no "power" expended until a target (or the ground) enters the picture and starts responding to the magnetic field. There is actually a little radiated power I guess too that is always there, but at low frequencies not too bad. So for IB, I think we want to go for greatest magnetic flux, which should be proportional to N turns X current per turn. As we increase the turns, yes the current drops due to additional resistance but up to a point we still get more mag field because of the additional loop. Where that stops I don't know, an interesting calculation. As long as you can create a high Q tuned circuit with your coil, you can get big current, event though the inductance gets very large and the current taken from the battery is small - no problem, the current is still large in the coil, due to stored energy. But adding loops adds resistance which limits your Q and thus finally you can't achieve large current buildup. That's my thinking, but I need to really study and do calculations.

For PI, probably very different. You pulse the coil and put in energy each time you pulse. That energy gets used up, you can't "reuse" it like you can with an IB circuit. Well, maybe you can, that is an interesting area of research. Anyway, I agree with your analysis for PI, more input power to the coil means greater magnetic field, more depth, etc. Also, I see with more windings you slow response time and can have difficulties. I see what you are talking about.

For IB coils, I feel like I should keep adding turns as long as the magnetic field increases. I am thinking of a circuit where the coil is driven by another oscillator. If the coil is part of the oscillator, then I think you don't have as much freedom. In the end, I'm sure there are good reasons for the number of coil turns used in actual detectors, but I haven't confirmed it experimentally or with calculations. I hope to understand it better soon. You ideas are always interesting. Thanks for the references too.

[Aziz]

Hi to all,

I don't like the brute-force method! The increased power (current) through the coil brings not much as supposed. The opposite occurs:
- frequency + phase instability
- drifting problems
- EMI, EMF crosstalk
- high voltage on the coil

I did some interesting experiments with really high power LC oscillators having many hundred volts (>600 Vrms!!) on the coil. The coil gets warm and hot and has changed his resistance. I heard some permanent sound from the coil (operating frequency < 16 kHz) and this was disturbing. The capacitor on the LC osc gets damaged permanently and produced additional noise (like pop corn noise due to high voltage break-down). And I got an electric shock by touching the coil connection!

Driving a coil externally, you need a capacitor connected to coil to eliminate the impedance of the coil. Otherwise you can not force a high current through the coil.

The right answer lies in taming the inherent problems (noise, instability, drift, etc.). And you will even get more performance with very little coil current.

Aziz

[Tinkerer]

High performance with little coil current

Hi to all,

I don't like the brute-force method! The increased power (current) through the coil brings not much as supposed. The opposite occurs:
- frequency + phase instability
- drifting problems
- EMI, EMF crosstalk
- high voltage on the coil

I did some interesting experiments with really high power LC oscillators having many hundred volts (>600 Vrms!!) on the coil. The coil gets warm and hot and has changed his resistance. I heard some permanent sound from the coil (operating frequency < 16 kHz) and this was disturbing. The capacitor on the LC osc gets damaged permanently and produced additional noise (like pop corn noise due to high voltage break-down). And I got an electric shock by touching the coil connection!

Driving a coil externally, you need a capacitor connected to coil to eliminate the impedance of the coil. Otherwise you can not force a high current through the coil.

The right answer lies in taming the inherent problems (noise, instability, drift, etc.). And you will even get more performance with very little coil current.

Aziz

Well spoken!

Tinkerer