New Concentric Co-planar Coil with Strong MF.

[Monolith]

hello SB,

i also used the word "cancel" aswell as you on previous posts , we are using "laymans terms" , and it makes sense to us i think , but if i remember right a few folks used the terms deflect or bend , this was a few months ago , you asked the same question as me "do fields cancel" and i don't think we got a deff answer , did we ??

i think cancel , if we wind an electromagnet with same number of turns in one direction and then same number of turns in the other , we end up with power useage and nothing else , so i say cancel .

regarding the half diameter bucking coil , with 1/2 diameter and 1/4 of the turns , we have to loose 25% of the tx field doing this , don't we ??

a while ago , can't remember if it was aziz , tinkerer or moods , i saw a post where rx was the biggest and tx was 1/2 the size in the middle and the bucking coil was on the outside of the rx , even bigger diameter , can't remember if aziz did a field vis for that idea of not , would be interesting to see.

i remember about a year ago , when i first started looking into the whole metal detecting milarky , seeing a cartoon pic of a metal detector with rx and tx with the brief explination of sending a pulse and listening to the return , oh how easy it all sounded , why , i think i'll go off and make one of those i thought !!

i now look back and think of that pic , if only life was that simple.

Hi Dooley and SB,

The matter of IB coils is of great interest and definitely worth spending some time.
Do the fields cancel?
Or do the fields deflect?
Both are right, it only depends on the point of view. The fields do not cancel each other. They repel each other when they are of the same polarity and they aid or integrate into each other when they are of opposite polarity.
Now, looking at it from the point of view of the RX coil, the changing fields induce currents in the coil. Opposing fields induce opposing currents that cancel each other if they are of the same amplitude.
If the currents are not exactly at 180 degrees, or the amplitude is not exactly equal, there is a residual current. This is what happens in the IB coil.
The amplitude of this residual current or voltage controls the amount of gain we can add in the pre-amp.
With the coils well balanced, the residual voltage is about 0.0001%. ie. the Flyback of 500V is reduced to 50mV.
It seems that using high supply voltage for the pre-amp, like +/- 22V would give the possibility of higher pre-amp gain. Is it worth the trouble?

Monolith

[simonbaker]

hello SB,

i also used the word "cancel" aswell as you on previous posts , we are using "laymans terms" , and it makes sense to us i think , but if i remember right a few folks used the terms deflect or bend , this was a few months ago , you asked the same question as me "do fields cancel" and i don't think we got a deff answer , did we ??

i think cancel , if we wind an electromagnet with same number of turns in one direction and then same number of turns in the other , we end up with power useage and nothing else , so i say cancel .

regarding the half diameter bucking coil , with 1/2 diameter and 1/4 of the turns , we have to loose 25% of the tx field doing this , don't we ??

a while ago , can't remember if it was aziz , tinkerer or moods , i saw a post where rx was the biggest and tx was 1/2 the size in the middle and the bucking coil was on the outside of the rx , even bigger diameter , can't remember if aziz did a field vis for that idea of not , would be interesting to see.

i remember about a year ago , when i first started looking into the whole metal detecting milarky , seeing a cartoon pic of a metal detector with rx and tx with the brief explination of sending a pulse and listening to the return , oh how easy it all sounded , why , i think i'll go off and make one of those i thought !!

i now look back and think of that pic , if only life was that simple.

Electromagnetic fields are always hard to visualize for me, and their effects on surrounding currents, since they all interact simultaneously.

I look at the bucking coil this way: at zero depth, the TX field inside the coil circle is sort of constant I think. (Aziz has a nice program that will compute it.)

We can put a reverse current in the bucking coil just enough to "cancel" the TX field inside the bucking coil circle. Outside the bucking coil, but withing the TX coil, the two magnetic fields actually add together to make a stronger field.

So at zero depth, we have virtually no field under the center of the coil! Which seems weird since we would like to detect coins on the surface.

Well, even slightly below zero depth the TX field starts to overpower the bucking coil field, and the deeper you go, the bucking coil field dies off much quicker than the TX field so you get a stronger and stronger TX-to-bucking coil field ratio.

So I'm sure even the slight field below zero depth is sufficient to detect surface coins.

The nice thing is that it also helps prevent overloading the RX preamp because coins at shallow depth should not create much signal due to the weak net magnetic field there. This helps allow a high-gain front end without overloading.

So that is how I look at the basic concentric coil field. It would be nice to run Aziz's program and see a picture of the field strength vs. depth for different areas under the coil.

Now, what about reversing size of TX, RX/bucking coil like you suggest? It is possible, in my thinking, that it would work about the same because of symmetry arguments. It would take a lot of match to prove it! However, perhaps a small RX coil is desirable for picking up less EMI noise, maybe that is why that configuration is chosen. I think some experiments with such a "reversed size" coil would be really interesting to try.

I think the terms "cancel" or "deflect" don't matter -- as long as you use physics formulas to calculate fields, the math ultimately works out the effect of one set of moving charges on another. I tend to like the "cancel" picture because I feel that magnetic fields that are exactly opposing create a region of space that is indistinguishable from space with no magnetic field -- no experiment could detect the two fields that cancel. However, it may be actually virtually impossible to create two magnetic fields that exactly cancel in a finite region just because you can't position charges close enough to each other. Also, at a given point, there is no way to know what moving charges caused the field at that point -- you would need to survey a large portion of the magnetic field to deduce all the charges involved.

Well, that was a lot of wind... sorry!

-SB

[Monolith]

Attached is the user manual of a software called MAXWELL. This software is a great help in visualizing and calculating magnetic fields.
The link for a free Student 2D edition download is included.
Enjoy

Monolith

[Qiaozhi]

The link for a free Student 2D edition download is included.

The free student version no longer exists at that location.

[DOOLEY]

cheers SB ,

that cleared up some of my inacurate thought's regarding the "opposing" field.

i originaly had a picture in my mind of 25% of the tx field being attracted to the bucking field and canceling out , ony leaving 75% left to do the job , and i thought , NO , that's waistfull.

oh well , live and learn.

[DOOLEY]

hello monolith ,

regarding your +-22v suggestion , i think yes , a bigger window to play in , maybe not as high as that , poss a 30v "window" get the signal spread out , problem is everything that everything that follows the amp may not like it , we could split it into managable stages , would be tricky , overlap , or missed a bit , or we could wait till someone makes a 30v 4066 , or go back to using fets , hang on , i see it now , yes it's possible isn't it ??

[simonbaker]

cheers SB ,

that cleared up some of my inacurate thought's regarding the "opposing" field.

i originaly had a picture in my mind of 25% of the tx field being attracted to the bucking field and canceling out , ony leaving 75% left to do the job , and i thought , NO , that's waistfull.

oh well , live and learn.

well you're not totally off - the bucking coil does reduce the TX field (except where it adds to it), but fortunately the deeper you go, the less local cancellation there is (I believe). But there will always be some reduction along the center axis.

I think I need to run one of those mag field programs before I say much more.


-SB

P.S. Some typos in previous post -- I meant to say "it would take a lot of math to prove it.." (not match).

[Monolith]

hello monolith ,

regarding your +-22v suggestion , i think yes , a bigger window to play in , maybe not as high as that , poss a 30v "window" get the signal spread out , problem is everything that everything that follows the amp may not like it , we could split it into managable stages , would be tricky , overlap , or missed a bit , or we could wait till someone makes a 30v 4066 , or go back to using fets , hang on , i see it now , yes it's possible isn't it ??

Dooley,

look up the datasheets of ADG451, or ADG5434.
With the IB coils there are really many, many avenues to explore, countless different things to try, but life is too short for one person to try it all.
You are on the right track with your comment above. We do not send as signal and receive a signal.
We induce eddy currents in a remote conductor by transformer coupling and we measure the distortion of the magnetic field of the IB coil assembly, by the magnetic field of the eddy currents in the target.

And this is your answer for the question of repelling or aiding magnetic fields. If the fields were to cancel, then we could not sense the magnetic field of the target.

Now, to make things a bit more complicated, the TX field is within the Earth's magnetic field, so what you have, is many interacting magnetic fields and the balanced coils are capable of sensing the minute changes caused in these fields by the targets.

Somehow I seem to remember something about a unified field theory......

Monolith

[Monolith]

well you're not totally off - the bucking coil does reduce the TX field (except where it adds to it), but fortunately the deeper you go, the less local cancellation there is (I believe). But there will always be some reduction along the center axis.

I think I need to run one of those mag field programs before I say much more.


-SB

P.S. Some typos in previous post -- I meant to say "it would take a lot of math to prove it.." (not match).

The TX field is slightly reduced because some of the power is used for the bucking coil. Since the field of the Bucking coil is of opposite polarity to the TX coil, the fields are deflected and loose some of the depth capability.
Increasing the overall TX field strength accordingly helps to mitigate the problem.

The cancellation is only in the RX coil and only if the RX coil is exactly positioned to receive equal amounts of field strength of the positive and negative fields. Any slightest change in the RX coil position within the TX coil, Bucking coil and RX coil assembly upsets the induction balance.

Monolith

Monolith

[simonbaker]

The TX field is slightly reduced because some of the power is used for the bucking coil. Since the field of the Bucking coil is of opposite polarity to the TX coil, the fields are deflected and loose some of the depth capability.
Increasing the overall TX field strength accordingly helps to mitigate the problem.

The TX field reduction depends highly on where in the field you look. With conventional design (RX and bucking coil have same size and position), the field "inside" the bucking coil perimeter is very small at the plane of the coil, since it creates equal and opposite total flux to the TX coil in that region. This creates the "balance" where the RX coil sees almost no signal. However, as you go deeper, the TX magnetic field is stronger than the bucking coil field -- which is how metal objects can be stimulated and detected.

The cancellation is only in the RX coil and only if the RX coil is exactly positioned to receive equal amounts of field strength of the positive and negative fields. Any slightest change in the RX coil position within the TX coil, Bucking coil and RX coil assembly upsets the induction balance.

Monolith

Monolith

Yes, we're probably saying the same thing.

Since the RX coil and bucking coil can be wound on the same form, there is not much chance of shifting relative position to upset the balance. And since the magnetic field within the TX perimeter is not highly variable with position, the balance is not too sensitive to slight shifts there either -- at least a lot less sensitive than with double-D coils, where the tiniest shift changes the balance.

Regards,

-SB

[6666]

Hi Aziz
thank you for giving the details on the graphite shielding paint
can I ask what are you polishing the graphite with or how ?
thanks

Some years ago when I was playing with rockets
I was experimenting with low resistance igniters
useing graphite mixed in with acetone and pingpong balls
they are made from nitrocellulose.

You bared the end of two twin wires and dipped it in the mix
but you had to be quick as the acetone dried out very quick
and the mix increased in resistance as it dried out
and then the mix was ruined and you had to stop .

When dried the resistance of the first few good igniters was very low ~ 2-3 ohms.
but it would make a good shield if you could paint it on quickly.

[DOOLEY]

HELLO MONOLITH,

just got round to looking up those part numbers you suggested to look at,

cant find anything on the ADG5434

BUT THE ADG451 , whoh ! 30v window , max 44v supply , then it says input can be from 0v to vdd , of 44v max

yes opens up the possibility of a big window / signal to sample from , so no more playing around with small signal changes .

think we would have to step it down a bit before it hits a 555 though it might not like 30v up its input !!!

[Aziz]

Hi Aziz
thank you for giving the details on the graphite shielding paint
can I ask what are you polishing the graphite with or how ?
thanks

Just take a stripe of paper or paper tape and roll it around your finger and rub the graphite shielding gently. After the paper has taken some graphite, it begins to polish the graphite shielding. So your are polishing graphite with graphite.

You can also take metal for polishing. But you have to be careful not to scratch the sensitive surface.

Aziz

[WM6]

Just take a stripe of paper or paper tape and roll it around your finger and rub the graphite shielding gently. After the paper has taken some graphite, it begins to polish the graphite shielding. So your are polishing graphite with graphite.

You can also take metal for polishing. But you have to be careful not to scratch the sensitive surface.

Aziz

Piece of cloth do the same even better.

[6666]

Thanks for the tips on polishing

cheers