Advanced PI coils

[Aziz]

I think that the main problem of all kinds of "complicated" coils is that they cannot give the maximum possible signal from the target . There are 2 conditions of the maximum target signal :

1. To excite the target with maximum signal from the transmitter coil .
2. To receive maximum signal from the target by our receiver coil .

To meet the first condition we need to place our transmitter coil as close to the target as we can . So the best place is the point on the ground surface just under the target . And the second condition will be satisfied if we place the receiver coil to the closest point too .... so we need to place both coils in the same place But we can do it only if we make the same coil to transmit and to receive ... so the mono coil rules anyhow

Let's invent the mono coil (like Apple(evil)(patent-troll)-style: it has round corners ...)
oooops, it's already invented long long before...

Yep, the mono coil rules. It has benefits and disadvantages. But we can't use it everywhere.
Aziz

[Ferric Toes]

Hi Eric, just wondering if you could post some spec's on the coil, I.E. the diameter and turns, please. When I was experimenting with the twisted pair coils, using cat 5 cable but had hassles getting the delay down, best I could do was about 8us as measured directly on the input of the diff amp. Maybe the cat 5 cable wasn't the best wire for the job.


Cheers
Mick

Hi Mick,

The only wire I have used so far in a bifilar coil is 0.25mm Teflon insulated. I tend to use this wire for low pulse current PI's and it is good for short delays. How short I can get with this one I have yet to find out, but its resonant frequency with no cable is much the same as if it had been a standard mono made with the same wire i.e. 1MHz. Diameter is 8in and it has 12 twisted pair turns. It measures 80uH from either end to the CT and 320uH end to end. The TX circuit should certainly be fast switching with only 80uH

Eric.

[mickstv]

Hi Mick,

The only wire I have used so far in a bifilar coil is 0.25mm Teflon insulated. I tend to use this wire for low pulse current PI's and it is good for short delays. How short I can get with this one I have yet to find out, but its resonant frequency with no cable is much the same as if it had been a standard mono made with the same wire i.e. 1MHz. Diameter is 8in and it has 12 twisted pair turns. It measures 80uH from either end to the CT and 320uH end to end. The TX circuit should certainly be fast switching with only 80uH

Eric.

Hi Eric, thanks for the reply. From memory the last coil I made was 10in with about 18turns, this is alot more turns compared to yours. This might explain why I couldn't get the sample delay down. Tomorrow I'll make another coil and wind the same spec's as yours and see what happens.


Might be time for me to purchase an inductance meter.

[Aziz]

Hi guys,

I'm back and back to the beauty of co-axial coils. No more fiddling with complex coils and the revival of the prior art and simple co-axial IB AI coil: RX+, TX, RX- (the symmetrical co-axial one with all same diameters and co-axial distances).

To achieve a good EMI noise rejection, RX+ and RX- should be very identical (same diameter, same turns count, same inductivity, same parasitic capacitance, same wire.., but opposite winding direction).

There is a co-axial distance between RX+ and TX and TX and RX- and should be at least 1/10 of the diameter of the coils (the more, the better of course).

Would you like to get shocked, when I'm going to compare it against an equivalent concentric-coplanar IB coil?

The KISS-principle rules.
Aziz

PS: I'm referring to the following coil in figure 7 in the following link:
http://www.geotech1.com/forums/showt...light=co-axial

[Ferric Toes]

Hi Eric, thanks for the reply. From memory the last coil I made was 10in with about 18turns, this is alot more turns compared to yours. This might explain why I couldn't get the sample delay down. Tomorrow I'll make another coil and wind the same spec's as yours and see what happens.


Might be time for me to purchase an inductance meter.

Mine only ended up at 12 turns because I was too lazy that day to calculate how much wire I would need to get 300uH end to tap. I just walked from one end of my workshop to the other and thought that would be enough. It wasn't. Also the other factor for me was to hold one pair of ends in the vise and the far ends in a hand drill to twist them up and stretch. Anyway, it is enough to try out the principle. Optimise later.

An inductance meter is a must in a PI workshop.

Eric.

[Tinkerer]

Hi guys,

I'm back and back to the beauty of co-axial coils. No more fiddling with complex coils and the revival of the prior art and simple co-axial IB AI coil: RX+, TX, RX- (the symmetrical co-axial one with all same diameters and co-axial distances).

To achieve a good EMI noise rejection, RX+ and RX- should be very identical (same diameter, same turns count, same inductivity, same parasitic capacitance, same wire.., but opposite winding direction).

There is a co-axial distance between RX+ and TX and TX and RX- and should be at least 1/10 of the diameter of the coils (the more, the better of course).

Would you like to get shocked, when I'm going to compare it against an equivalent concentric-coplanar IB coil?

The KISS-principle rules.
Aziz

PS: I'm referring to the following coil in figure 7 in the following link:
http://www.geotech1.com/forums/showt...light=co-axial

With IB coils it would be interesting to see the field vectors as arrows, since the coil fields are not uniform.

I would also call attention to a misconception. A smaller RX coil, for example 1/2 TX diameter, does not mean that the target signal reception is only under the RX coil. The IP coil configuration must be looked at as one unit. The interaction or mutual inductance, between the several coils is such that any change, anywhere within the total field of the configuration, changes the balance and therefore produces a signal.

This means also that any influence that changes the field vectors, not just the field strength, changes the balance of the coil configuration.

[Tinkerer]

Questions about EMI noise rejection.

What is the minimum distance needed between RX+ and RX-, to achieve good rejection.

With a twisted pair, what is the relationship between the number of twists per Inch and the frequency of the EMI, to get good common mode rejection.

[Ferric Toes]

Hi guys,

I'm back and back to the beauty of co-axial coils. No more fiddling with complex coils and the revival of the prior art and simple co-axial IB AI coil: RX+, TX, RX- (the symmetrical co-axial one with all same diameters and co-axial distances).

To achieve a good EMI noise rejection, RX+ and RX- should be very identical (same diameter, same turns count, same inductivity, same parasitic capacitance, same wire.., but opposite winding direction).

There is a co-axial distance between RX+ and TX and TX and RX- and should be at least 1/10 of the diameter of the coils (the more, the better of course).

Now there is a sensible thing to do.

RX+ and RX- can be the same direction and subtracted in the preamp.

Here is an extract from an Irish patent application of mine in 1981. Fine balance was done electronically and compensated for effect of temperature changes on coil. My configuration was coils 3 and 4 RX+ RX- for noise cancelling and discrim. Coil 2 TX/RX as straight PI. You could of course do this off the balance coils by sampling in off time as well as on time.

I never got around to combining the arrangement with GB, consequently iron mineralised ground was a big problem.

Eric.

PI Disc Coil003.pdf

[Sergey_P]

Fine balance was done electronically

My test bench to adjust http://www.md4u.ru/download/file.php?id=5910
…
8 for PI-IB with GB http://www.md4u.ru/viewtopic.php?f=77&t=2037 , http://www.md4u.ru/download/file.php?id=2845

[sido]

It seams mineralisation is the obstacle in any coil configuration.

I believe the differential coil is the goods.

Other coil configurations only complicate things ATM.

Sid

[Carl-NC]

A smaller RX coil, for example 1/2 TX diameter, does not mean that the target signal reception is only under the RX coil.

C&G (Tesoro predecessor) used to build coaxial coils, and Jack Gifford told me that he used RX=1/2 TX diameter coils to reduce ground pickup. It probably would also allow a tighter coil stack.

Questions about EMI noise rejection.

What is the minimum distance needed between RX+ and RX-, to achieve good rejection.

Ideally, zero, but zero is the worst for target detection. Empirical testing needed here.

- Carl

[Tinkerer]

C&G (Tesoro predecessor) used to build coaxial coils, and Jack Gifford told me that he used RX=1/2 TX diameter coils to reduce ground pickup. It probably would also allow a tighter coil stack.



Ideally, zero, but zero is the worst for target detection. Empirical testing needed here.

- Carl

Thank you for the feedback.
I am a little confused. The coil configuration I have been using, is a concentric, coaxial IB. The TX is the outer coil. The RX is bundled with the Bucking coil at 1/2r of TX. Both Rx coils are wound in the same direction and center tapped, so the signal of one half is RX+ and the other half is RX-. RX+ and RX- enter the Instrumentation Amplifier. The - input is inverted and added to the +input. This subtracts the equal signed signals. Common mode signals are therefore nulled.

This coil configuration is very easy to balance, but, like all IB coils, once potted, it is final.
There is a tap between the TX and BU coils, that allows for fine re-adjustment of the balance, by bypassing some of the coil current, after the coil has been potted.

This coil configuration works fine in the very noisy lab. The residual is low. The sensitivity is excellent. The sensitivity field is similar to the field of a mono coil, but with the added FE discrimination.

The only problem is the cable. 6 legs. The TX cable is thick, so it does not twist equal as the thin RX cables. Careless cable leading, can cause capacitive coupling between the cables and unbalance the differential input.

If I now want to build a EMI rejecting coil, I need to wind the RX+ coil in opposite direction than the RX- coil. Adding the signals + and -, the result is null. Using the differential input INA, that inverts the -input, I have again the same result as above. (forget the BU for now) The only difference is the distance between the R+ and R-. The target signal will be reduced to the difference in amplitude between the 2 coils, caused by the difference in distance from the target.

EMI, coming from a large distance will be received by both coils equal = common mode.
The target signal, coming from, say 100cm distance will be seen at a higher amplitude by the nearer coil, say 10cm distance between the coils. Not 1/10th times higher amplitude, but the cube root of it.

Do I have that right?

[Aziz]

Ok guys,

it seems, I have to do several coil configurations to show you the comparison. I need an EMI noise induction related normalizing reference. That will be the number of turns of RX * flux area of the RX coil. So all (individual) receive coils would induce the same EMI noise level or would have the same N(RX)*A(RX) product. That would be a fair comparison and we are limitted by EMI noise either.
In case of mono coil: TX = RX of course. TX will be normalized to total 300 µH. 10 inch mono coil as reference.
N(RX): number of turns count of RX coil
A(RX) = pi*R², R=radius of RX coil, A(RX)=the RX coil flux area
Anti-interference RX coil: Each RX coil part normalized to N(RX)*A(RX)

I hope to get some results before the end of days (dec-21-2012).

Let's start...
Aziz

[Carl-NC]

Tinkerer,

Sounds like you're talking about a coplanar concentric coaxial coil. I don't think you can get very effective EMI cancelation and still get good sensitivity. I was addressing the stacked concentric coaxial coil, which is what Aziz is talking about. In that case, best EMI rejection is when both RX coils are tightly spaced (which would then be coplanar); as you spread the coils apart sensitivity will increase up to some point, then begin to decrease as it forces the target farther away from the TX coil. The optimum point can be calculated mathematically (Aziz, are up to the challenge?) or it can be determined empirically.

- Carl

[Tinkerer]

Tinkerer,

Sounds like you're talking about a coplanar concentric coaxial coil. I don't think you can get very effective EMI cancelation and still get good sensitivity. I was addressing the stacked concentric coaxial coil, which is what Aziz is talking about. In that case, best EMI rejection is when both RX coils are tightly spaced (which would then be coplanar); as you spread the coils apart sensitivity will increase up to some point, then begin to decrease as it forces the target farther away from the TX coil. The optimum point can be calculated mathematically (Aziz, are up to the challenge?) or it can be determined empiracally.

- Carl

Right, my existing coil is a coplanar concentric coaxial coil.

As I am thinking of building another coil, stacked, concentric coaxial, I am trying to understand the difference. Difference in coil configuration and if a difference in signal processing needed.