Coil Questions

[Tinkerer]

Coil questions:
A PI coil that has only one winding that serves as TX as well as RX will have an optimal detecting depth about equal as the diameter of the coil.
How does this work with a PI coil with 2 windings, the outer TX and the inner RX. Is the optimal detecting depth now about equal as the diameter of the RX winding?
This is important, because for the shortest delay, the RX winding should be as far as possible away from the TX winding, therefore the RX winding is usually of a smaller diameter than the TX coil.
Tinkerer

[Carl-NC]

Optimum depth also depends on target size/shape/conductivity. For a given target, keeping the TX coil the same size will result in the exact same target response field. Then, shrinking the RX coil could either improve or degrade the reception of the target field depending on its flux density.

- Carl

[Aziz]

Hi,

I am working on this at the moment. I hope, in one or two weeks, I am able to simulate such questions and able to proof this.
Damn. I have awful toothache! I have to drink cold coffee, beer, water, tee to ease my toothache.
Aziz

[Tinkerer]

Sizing RX and TX coils

Optimum depth also depends on target size/shape/conductivity. For a given target, keeping the TX coil the same size will result in the exact same target response field. Then, shrinking the RX coil could either improve or degrade the reception of the target field depending on its flux density.

- Carl

OK, so the key word is the field that is caused by the eddy currents in the target. So an optimal coil has to be designed for a specific purpose.
Spiral wound coils are good at pinpointing because the center of the coil gives a much stronger signal, the detecting field seems to be shaped like a funnel.
Would such a coil be good for finding gold nuggets? Relics? Coins?
Would a coil with a more cylinder like detecting field be better?
Should a multipurpose coil have the largest possible RX coil within the limits imposed by the TX coil?
Tinkerer

[Tinkerer]

Hi,

I am working on this at the moment. I hope, in one or two weeks, I am able to simulate such questions and able to proof this.
Damn. I have awful toothache! I have to drink cold coffee, beer, water, tee to ease my toothache.
Aziz

Please see to your teeth first. After you are well again, I have 100 more coil questions waiting for you.
I hope you get well soon.
Tinkerer

[Aurificus]

The more I learn, the less I know!

Thank-you gentlemen, for this discussion.

Aziz, please take care of yourself.....First, before coils even.


Tinkerer, thanks for the graphic with the narrowed coils, looks like a null position does still exists between and just outside the Tx coils. Excellent!

http://thunting.com/geotech/forums/a...1&d=1205690236

Regards, Aurificus

[Aurificus]

Is increasing coil field strength... a waste of energy??

Here's a coil question that has been disturbing my sleep for a couple of days!

In VLF type detectors (not PI) AC in the Tx coil creates an alternating magnetic field.The Tx magnetic field induces current flow in a target.
Changes in target current flow create a target magnetic field.
Magnetic lines of force cannot cross each other, so does the target field induce current in the Rx coil by distorting the Tx lines of force to make them cross the Rx coil windings and disturb the null?

Ok, thats not the real question, this is..

If we make the Tx field stronger then the target field needs to be stronger to deflect/distort the Tx field and generate a Rx response. Is this a dog chasing its tail? Is this why there has been negligible improvement in VLF detectors in 30 years? (yes, I know...that's 2 questions)


Answers: Yes, No, Maybe, All of the above, None of the above?


Or...does frequency shift (or something else) allow the target field to lead or chase the the Tx field back to the Rx coil. (another dog analogy). I'm way out of my depth here.

Help, my brain hurts...

Aurificus

[bbsailor]

Here's a coil question that has been disturbing my sleep for a couple of days!

In VLF type detectors (not PI) AC in the Tx coil creates an alternating magnetic field.The Tx magnetic field induces current flow in a target.
Changes in target current flow create a target magnetic field.
Magnetic lines of force cannot cross each other, so does the target field induce current in the Rx coil by distorting the Tx lines of force to make them cross the Rx coil windings and disturb the null?

Ok, thats not the real question, this is..

If we make the Tx field stronger then the target field needs to be stronger to deflect/distort the Tx field and generate a Rx response. Is this a dog chasing its tail? Is this why there has been negligible improvement in VLF detectors in 30 years? (yes, I know...that's 2 questions)


Answers: Yes, No, Maybe, All of the above, None of the above?


Or...does frequency shift (or something else) allow the target field to lead or chase the the Tx field back to the Rx coil. (another dog analogy). I'm way out of my depth here.

Help, my brain hurts...

Aurificus

Aurificus,

Thi is a good question and should stimulate some interesting discussion.

Maybe a better question to ask about VLF TX power is to compare the potential improvement in increasing detecting distance in the air and then again when the coil is placed on the ground.

I think you will see that in the air and on some rather neutral soils, increased TX power will improve detecting distance but on any ground with some mineralization, the increased TX power will overload the RX circuit. This is sort of like driving in the fog (the analogy for mineralized ground) with your headlights on and then putting on your high beams making the visibility worse.

bbsailor

[Carl-NC]

Mathematically, magnetic fields are vector fields, so two magnetic fields that interact do so mathematically by vector addition. Conceptually, it's the same as superposition in circuit analysis, so you can either view a target response as being a distortion of the TX field, or as a separate superimposed RX field. Either way works.

On the second question, imagine an IB coil that is perfectly balanced; no TX couples to the RX. So if we double the TX field strength, we still get no leakage but will get double the target response field, everything else equal. That sounds good, eh? But if the coil is not perfectly balanced, then doubling the TX field strength will double the RX leakage, so now the doubled target response doesn't look so great. But RX leakage looks no different than ground response, and is nulled out by the GB scheme, so depending on the GB scheme you can still get an improvement.

The real problem is that increasing the TX field also increases the ground response, so the stronger TX field gives a diminishing return on target-to-ground signal ratio. Also, because GB schemes typically rely on sampling the zero-crossings instead of actually removing the ground signal, the front-end can run out of dynamic range as the ground signal gets bigger & bigger.

- Carl

[Aziz]

Hi Aurificus,

Magnetic lines of force cannot cross each other, so does the target field induce current in the Rx coil by distorting the Tx lines of force to make them cross the Rx coil windings and disturb the null?

The magnetic strength field vector H(P) of a Point P (anywhere x,y,z) is defined:
H(P) = Hp(P) + Hs(P), where
Hp(P) is the magnetic strength field vector of the primary field (caused by TX coils) and
Hs(P) is the magnetic strength field vector of the secondary (caused by target) field.
H(P), Hp(P), Hs(P), P are vectors with direction and length.
These magnetic force vectors are added together (vector addition). So this effect will change the magnetic flux of RX-coil and therefore balance of the coil arrangement.

If we make the Tx field stronger then the target field needs to be stronger to deflect/distort the Tx field and generate a Rx response. Is this a dog chasing its tail? Is this why there has been negligible improvement in VLF detectors in 30 years? (yes, I know...that's 2 questions)

No, no dog chasing its tail. It is a question of the stability of the LC-oscillator. Most common LC-oscillators do not deliver much more drive current for the TX coils. The stronger the magnetic Field (MF) at a target position P, the stronger the induced flyback magnetic field in a target. You can have more depth sensitivity if you increase the TX-coil current by using a stable power LC-oscillator.

Or...does frequency shift (or something else) allow the target field to lead or chase the the Tx field back to the Rx coil. (another dog analogy). I'm way out of my depth here.

Help, my brain hurts...

Aurificus

The higher the search frequency, the higher the induced voltage in the target and the higher the flyback magnetic fields caused by the target. This is the reason, why some gold nugget detectors work at 70 kHz or above. This also enables detecting small nuggets.

Regards,
Aziz

[Ultimate_Haze]

Hi

Hi,
Aziz!!!
Now its time to show is your softwere working at all!?!?!?
You have been asked to compare DD 19cm former size coil with 135 TX 0,25mm bare wire and RX 145w.And lets say DD 25,5 100/107 ratio.And 32,37,45!To see what are the numbers talking also did your sofwere see the differances in ground penetration acording the frequency and etc?
Tks
O and I forgot you post here some link to patents and there I saw this coil that I have aweys waned to try!Can you simulate it?It were posted as project somewhere O with two D inside!!

[Aziz]

Hi,

Hi,
Aziz!!!
Now its time to show is your softwere working at all!?!?!?
You have been asked to compare DD 19cm former size coil with 135 TX 0,25mm bare wire and RX 145w.And lets say DD 25,5 100/107 ratio.And 32,37,45!To see what are the numbers talking also did your sofwere see the differances in ground penetration acording the frequency and etc?
Tks
O and I forgot you post here some link to patents and there I saw this coil that I have aweys waned to try!Can you simulate it?It were posted as project somewhere O with two D inside!!

I am working now on the coil software. It is not easy to test it. I found many different coil inductance formulas in documents with containing errors and misunderstandings described therein. So it is boring and time consuming.
But I have improved the coil inductance calculator: faster and accurate.
Till now, I did not take eddy currents effects, frequency effects, changing permeability into consideration (tensor of permeablity). These effects are low on VLF frequencies and can be neglected at the moment.
I have implemented basic principles which covers the most of the relevant effects. Simulations are planned from next week. I need some days of implementing and testing the coil software.

In a few weeks/month, I am able to model a search coil for a particular metal detector on the market. I also found a novel coil design with perfect penetration cross section of magnetic fields (named "The Coil").

So it is work for month with many surprises and fundamental coil analysis.
Aziz

[Aurificus]

Thank-you all for the great information and the clearest, most accessible explanations of the Tx-Rx interactions ever.

The effects of "the ground" on the Tx field and the best ways to compensate, account for and/or balance them shall be my next focus. All contributions gratefully accepted!

In the mean time, Aziz raised the subject of coil current. What are the "usual" current/voltage combinations for successful coils (better than say, 18v & 200mA)?

If we design resonance into the coil and boost current (or voltage) I expect we will have insulation problems and mechanical issues with heat & distortion. How do we manage this?

Is it possible to generate resonance with a 50% square wave DC pulse?
Will it get "stretched" into a DC sine wave or can we maintain the steep rise and fall of the pulses?

My brain still hurts

Aurificus

[Aziz]

Thank-you all for the great information and the clearest, most accessible explanations of the Tx-Rx interactions ever.

The effects of "the ground" on the Tx field and the best ways to compensate, account for and/or balance them shall be my next focus. All contributions gratefully accepted!

In the mean time, Aziz raised the subject of coil current. What are the "usual" current/voltage combinations for successful coils (better than say, 18v & 200mA)?

If we design resonance into the coil and boost current (or voltage) I expect we will have insulation problems and mechanical issues with heat & distortion. How do we manage this?

Is it possible to generate resonance with a 50% square wave DC pulse?
Will it get "stretched" into a DC sine wave or can we maintain the steep rise and fall of the pulses?

My brain still hurts

Aurificus

I can provide a schematics of a really high power VLF LC-oscillator (dozends of Watts). But this type of LC-oscillator is not well-tolerated with cardiac pacemaker. Too much for the electronics as well (>110 Vpp sine wave on the coil). You can also "hear" the LC-oscillator when working. It is realized with fast OPamp + booster. But I have to re-engineer some PCB-boards (schematics lost). This LC-osciallator can also be limited to a particular output power.
Aziz

[Aurificus]

Ground Balance - My instability is showing!

quote=Carl-NC;69402]

On the second question, imagine an IB coil that is perfectly balanced; no TX couples to the RX. So if we double the TX field strength, we still get no leakage but will get double the target response field, everything else equal. That sounds good, eh? But if the coil is not perfectly balanced, then doubling the TX field strength will double the RX leakage, so now the doubled target response doesn't look so great. But RX leakage looks no different than ground response, and is nulled out by the GB scheme, so depending on the GB scheme you can still get an improvement.

The real problem is that increasing the TX field also increases the ground response, so the stronger TX field gives a diminishing return on target-to-ground signal ratio. Also, because GB schemes typically rely on sampling the zero-crossings instead of actually removing the ground signal, the front-end can run out of dynamic range as the ground signal gets bigger & bigger.

- Carl[/quote]

I wasn't quite ready to work on this part but with a little knowledge and no experience (a most dangerous combination) here goes...

Effects of ground include basic distortion of the TX field upsetting the "perfect" null'.

As a GB system, I propose to run the flat Helmholtz coils in parallel and adjust the field the upper coil by increasing /decreasing the voltage to maintain the null position for the Rx coil.

There are inductance and capacitance effects between the ground and the Tx coils decreasing Tx power by shifting the resonant frequency.

I propose to maximum Tx power by adjusting the Tx FREQUENCY to keep the system in resonance (most systems seem to fix a frequency and then try to adjust coil inductance and/or capacitance to restore it).
A pot in the timer circuit will easily vary frequency, a row of leds or a meter will indicate max. V or A. ie peak resonance)
(I haven't decided on serial or parallel capacitance yet)

Does this look like it has merit or am I way of base, or just re-inventing the wheel?

I am still working my way back through the older posts, (so many posts, so little time) if I'm repeating previous work & questions, apologies in advance.

Cheers, Aurificus.