Coil for PI metaldetector

[Bosco91]

Hi to everyone, i read several threads but i didn't find a discussion about this. I try to explain: i have the sniffer xr-71 as metal detector/ powerscan pro2 (i think is a version a bit modified from the original sniffer xr-71), and i'm using the coil 1x1mt. I saw several deep metaldetectors as Garrett and other brands that use a double coil, one front and one rear (one tx and one rx). Someone know if it could be better as coil in the xr-71? I'm pretty sure that could be absolutely easier for everyone to use because you can hold in one hand, is smaller and maybe with more performance.
I took this idea from this: http://www.geotechru.com/security/se...l_detector_mg1

Thanks
Nicola

[Funfinder]

2 coils for a PI? This will reduce sensitivity a lot if the RX coil is too far away from the TX coil.
If it works at all. But you could built some narrow and long elliptical hand-held mono-coil (35cm x 100cm).

The link shows a very stable setup but beware that in the middle sits already some larger amount of metal
itself which has to be filtered out and if the coils are 30-40cm above ground you'll lose depth, too.

[Bosco91]

I don't know exactly how it works, but i thought that for PI sniffer xr71 could be much more comfortable to use rather than coil like 100x100 etc....the problem is to know how it works, or if someone already tried

[Ferric Toes]

I don't know exactly how it works, but i thought that for PI sniffer xr71 could be much more comfortable to use rather than coil like 100x100 etc....the problem is to know how it works, or if someone already tried

The two coil horizontally separated configuration is valid for a PI, but only for certain conditions. The arrangement ignores small (coin size) metal objects and surface trash but comes into its own when looking for large deeply buried items. Examples are cannon balls, metal chests, modern buried bombs and shells, underground pipelines etc. The detection range, which depends on object size, really starts at a depth that is equal to the coil spacing.

Eric.

[Geo]

The two coil horizontally separated configuration is valid for a PI, but only for certain conditions. The arrangement ignores small (coin size) metal objects and surface trash but comes into its own when looking for large deeply buried items. Examples are cannon balls, metal chests, modern buried bombs and shells, underground pipelines etc. The detection range, which depends on object size, really starts at a depth that is equal to the coil spacing.

Eric.

What should be the distance between the coils in relation to their diameter ????

[Orbit]

Distance is the size of the spool times x2 +_ 10% !
Hi Eric ! Does coils should have the same inductance ?

[Orbit]

I personally think that pi detector can not have this discrimination ! It is possible that herein combination pi and bfo ?! I think the video !

[Ferric Toes]

Distance is the size of the spool times x2 +_ 10% !
Hi Eric ! Does coils should have the same inductance ?

The beauty of having separate TX and RX coils is that you can optimise each for best performance. This usually means having considerably more turns on the RX coil. Because it is loosely coupled to the TX and there are no high currents flowing, it can look into a relatively high impedance preamp.

Using PI you can have both coils in the same plane or have orthogonal coils as sine wave two box detectors do. With orthogonal mounting the TX/RX relationship can be finely adjusted so that there is no TX induced in the RX coil. Sampling in both the ON time and OFF time can then give ferrous/non-ferrous discrimination as it becomes a PI induction balance system.

By size of the spool, presumably you mean the radius? I can't remember the rule for spacing but I have seen it in the geophysical literature somewhere. The coil radius is important and the spacing is measured centre to centre.

Another method that works for detecting large objects deep is to lay a large TX coil on the ground. This could be 5m x 5m or even 10m x 10m and then search with a 1m roving coil inside it. This method is quite common in geophysical prospecting.

Eric.

[Ferric Toes]

I personally think that pi detector can not have this discrimination ! It is possible that herein combination pi and bfo ?! I think the video !

The was a PI in the 1970's called the Groundhawk which was a combination of PI and BFO. A PI mono coil makes a good basis for a BFO, so this detector was basically two detectors in one. You searched in PI mode and then switched to BFO for discrimination. The big problem is the difference in detection ranges between the two systems.

I had a pure PI discriminator in the 1980's that used an automatically balanced stacked coaxial RX1-TX-RX2 arrangement. It worked nearly perfectly in dry sand, fresh water and non-mineralised soil, but iron mineralised ground defeated the discrimination as did a salt wet beach. Done nowadays, if combined with a good GB system, it would be great detector. No need for the stacked coaxial coil either as a coplanar coaxial works just as well.

Eric.

[Teleno]

The beauty of having separate TX and RX coils is that you can optimise each for best performance. This usually means having considerably more turns on the RX coil. Because it is loosely coupled to the TX and there are no high currents flowing, it can look into a relatively high impedance preamp.

In all Pi designs the Rx coil is used in voltage mode, connected in parallel with a damping resistor of a value as high as possible. Because the decay of the coil's current is fast, targets with longer time constants are measurable. However, targets with short time constants are a challenge.
In this configuration the inductance has to be made as low as possible (few turns -> faster decay but also less sensitivity).

I'm considering using the Rx in current mode. The Rx coil would be shorted (in parallel with as low a resistance as possible) and the current - not the voltage - induced by the target would be measured (transconductance amp). The coil's time constant L/R would be set as high as possible (high inductance). In a balanced system it would be possible to sample at zero delay, however, targets with time constants near or above L/R would be lost.

[6666]

Another method that works for detecting large objects deep is to lay a large TX coil on the ground. This could be 5m x 5m or even 10m x 10m and then search with a 1m roving coil inside it. This method is quite common in geophysical prospecting.

Hi Eric , this is interesting , is this system called a ground loop ?

[Orbit]

Thanks Eric!
My great respect !

I tried coils diameter 32cm tx and rx coil inductance 450uh damped resistor 390 r 200us pulse width sampling 40us ,in the same plane placed I received about 10 percent and more lower results ,compared to when I coils placed in orthogonal level ,I got the best results when I from center to center coil a distance of 70 cm I had a very good range ?! Yes bfo provides less range but we have two in one with relatively good discrimination between ferrous and non-ferrous metals !But probably that we have the time to stop pulse tx and include bfo ??

[Orbit]

Another method that works for detecting large objects deep is to lay a large TX coil on the ground. This could be 5m x 5m or even 10m x 10m and then search with a 1m roving coil inside it. This method is quite common in geophysical prospecting.

Eric.


Does spool search from 1m x 1m should be included tx and rx, namely whether the transmitter turned on or used coil pinned to the input resistor ie rx ? Thank you !

[Bosco91]

The two coil horizontally separated configuration is valid for a PI, but only for certain conditions. The arrangement ignores small (coin size) metal objects and surface trash but comes into its own when looking for large deeply buried items. Examples are cannon balls, metal chests, modern buried bombs and shells, underground pipelines etc. The detection range, which depends on object size, really starts at a depth that is equal to the coil spacing.

Eric.

Hi Eric, i mainly search WW2 relics like helmets etc, then i'm not interested about coins. Do you know how to build a coil with 2 horizontal separated coild? I mean how many turns for TX and RX and resistance and inductance?

Thanks
Nicola

[Geo]

Another method that works for detecting large objects deep is to lay a large TX coil on the ground. This could be 5m x 5m or even 10m x 10m and then search with a 1m roving coil inside it. This method is quite common in geophysical prospecting.

Eric.

The bad is that we must move the 10x10m coil again and again.... not so easy especially at places with a lot of small trees and big stones.
If i remember good your Superscan has this ability.