Figure 8 coil

[Sergey_P]

Graphs show the vertical component of the received signal of the target. Multiplication diagrams Tx*Rx (vertical component) - get diagram sensor...

[Aziz]

Hi Aziz,

How do you think that the above series double D receiver compares with your magic top hat coils?

My expectation is that your top hat coils should cancel out noise and eternal EMF but would not cancel out ground noise as this one should.

I'm hoping to use the above without using any shielding.

I'll talk to you later about the PC/sound card project

Ken

Hi Ken,

sorry, I haven't been able to respond to you yet. I'll do this, when I have more time and get well soon too (influenza).
But the PC/sound card project is published now.
http://www.geotech1.com/forums/showt...885#post160885

Cheers,
Aziz

PS: Nice pictures Sergey.

[Aziz]

Hi Aziz,

How do you think that the above series double D receiver compares with your magic top hat coils?

My expectation is that your top hat coils should cancel out noise and eternal EMF but would not cancel out ground noise as this one should.
..
Ken

Hi Ken,

I don't know, which one would really perform better (regards to target signal response). I have to make some time consuming coil simulation comparisons to give you the correct answer. Pity, that the center axis isn't the hotspot (max. sensitivity region) so it makes the comparison very time consuming.
It would take some time as I am quite busy with the current projects.
Aziz

[Davor]

A centre region becomes a hot spot when both centre crossing wires are packed tightly together. True, you have a phase reversal there, but also a peak next to it.

[Aziz]

With Induction Balanced (IB) VLF design there is always cancellation going on so I don't see why the figure 8 can't potentially get a similar depth to other IB VLF designs. The current figure 8 coils don't maximize use of the magnetic field. I believe that by changing the design of the figure 8 we can get a better depth.

Below is a picture of a figure 8 modified to be more like a Dual double D receiver that I am hoping to build. Note that both the D s of this coil make up the receive coil.

[ATTACH]22005[/ATTACH]

There are some usability issues with the coil. You are only going to see an anomaly when it is over to one side of the coil or the other. An object directly under the middle of the coil will be canceled out.

I also expect that there may be problems trying to get a fully balanced version of this to be used on a current commercial detector that was not specifically designed for this coil. As the object goes from one side to the other, the Rx signal gets a 180 degree phase shift. The detector needs to work with both of these phases. It should also be able to tell the user which side the object is on.

I gather that a lot of commercial detectors are not really made for a fully balanced IB coil. Rather, they prefer at least a small signal to be present all the time.

These problems are worth persisting with as this coil has got some great advantages. These include:

- It automatically balances out any effects of the ground so long as the ground is reasonably consistent.
- It automatically balances out external EMI interference as long as the source of the EMI is a reasonable distance from the coil. In most situations in the field you will get this distance from the sources.
- It automatically balances out noise internally generated in the creation of the Tx signal.
- The symmetry of the coil makes it easy to manufacture. The Tx coil can use the Rx coil as a former or the Rx D subcoils can use the Tx coil as a former. It doesn't matter which one goes around the outside but they should be close together.
- It is easy to change the coil's sensitivity, regardless of size. To increase the sensitivity you just increase the number of windings on the Rx coil, that is equally on both the D subsections of the Rx coil.

I'm hoping to design my own IB VLF detector (PC + sound card etc) but this is the coil I am currently planning to use for testing it.

I would be interested in any comments you have on this version of coil.

Ken

Ok Ken,

I can give you the exact answer to your question now, whether which one performs better:
Either Tophat coil or your overlapped (crossed) figure-8 coil.

The answer: Tophat coil is performing much better.

Tophat coil: 10 inch diameter TX, 7 inch diameter RX, 5 inch distance between RX+ and RX-, TX = 300µH, RX = 300µH (total)
Overlapped figure-8 coil: 10 inch diameter TX, 9.6 inch RX (if round), RX cutting angle 140 ° , TX = 300µH, RX = 300µH (total)

As the comparison is made under same conditions, don't look at the absolute induced voltages (irrelevant).
Ok, here is something to see:

Performance comparison (log-view):



Sketch of the tophat coil:



Sketch of the overlapped figure-8 coil:



Cheers,
Aziz

[green]

New to forum. I've tried to design and build a couple of metal detectors in the past which didn't work. Just something I wanted to do. Lately I've been working on a figure 8 that works good on the bench. The Rx coils are wound on cardboard circles with the Tx coil wound around the two Rx coils. I need to wind the coils more rigid. Found your site and saw the thread on figure 8 coils while looking for winding coils. I bought two wood embroidery hoops, different diameters a while back thinking they might work for winding coils. Wondering if the wood hoops would work. Would need to buy two the same size. Like I said the detectors in the past didn't work so suggestions for the coils or the detector would be appreciated.
green

[Qiaozhi]

New to forum. I've tried to design and build a couple of metal detectors in the past which didn't work. Just something I wanted to do. Lately I've been working on a figure 8 that works good on the bench. The Rx coils are wound on cardboard circles with the Tx coil wound around the two Rx coils. I need to wind the coils more rigid. Found your site and saw the thread on figure 8 coils while looking for winding coils. I bought two wood embroidery hoops, different diameters a while back thinking they might work for winding coils. Wondering if the wood hoops would work. Would need to buy two the same size. Like I said the detectors in the past didn't work so suggestions for the coils or the detector would be appreciated.
green

Have you seen this? ->
http://www.geotech1.com/forums/conte...metal-detector

It might help you understand where you went wrong.

[green]

I know a little electronics. Not much about metal detectors. I get it working on the bench, then signal to noise kills me when I try to use it. I hope the figure 8 will cancel noise. This time I'm trying to make it simple. I unbalance about 20mv, then measure the voltage change. Signal looks like Surgey's post above. Overlapping or removing some of the circle appears to reduce signal. Works good as long as coil doesn't move. Would winding a more stable coil solve the problem or is concept the problem?
I might have to get a book.
green

[Qiaozhi]

I know a little electronics. Not much about metal detectors. I get it working on the bench, then signal to noise kills me when I try to use it. I hope the figure 8 will cancel noise. This time I'm trying to make it simple. I unbalance about 20mv, then measure the voltage change. Signal looks like Surgey's post above. Overlapping or removing some of the circle appears to reduce signal. Works good as long as coil doesn't move. Would winding a more stable coil solve the problem or is concept the problem?
I might have to get a book.
green

What metal detector circuit do you have attached to the coil?

[green]

What I'm using to play. Modify when I get the detector working.
green

[Qiaozhi]

What I'm using to play. Modify when I get the detector working.
green

The circuit you are using is a very simple TR detector. You should fix the coil overlap in a position where ferrous and non-ferrous targets change amplitude in different directions. If the coil windings are loose, or the TX and RX coils can move in relation to one another, then the received signal will be very noisy. I suspect you initially have a mechanical stability problem, but you later you will encounter lots of other issues, such as ground effect.

[green]

Thanks for the reply. I was hoping the figure 8 coil might solve the noise problems. Hope me being a novice didn't kill the thread.
green