INDUCTION BALANCED COILS FOR PI

[Reg]

Mike,

At first I was going to disagree with you, but you are correct, the factory coil can be considered a concentric design in the fact there are two windings positioned about a center axis.

The coil for the TDI PI is still a mono coil but built in a concentric design meaning there are two windings in series, a larger outer winding and a smaller inner winding. Whites calls this their dual field design. So, it is not a conventional concentric coil that has a transmit and a separate receive winding, but it is a modified mono coil that consists of two windings in series. The concept is such that the smaller series coil is more sensitive to smaller targets and the larger winding for bigger deeper stuff.

Now, the concentric dual field coil design used on the TDI will not reduce the ground response. In fact, in real bad ground such as that in OZ, I have been informed the dual field design doesn't work that well. On the other hand, a conventional concentric having a separate transmit and receive will have some limited ground signal reduction but the DD design is far superior in this aspect.

Reg

[WM6]

Mike,
The concept is such that the smaller series coil is more sensitive to smaller targets and the larger winding for bigger deeper stuff.

Now, the concentric dual field coil design used on the TDI will not reduce the ground response.
Reg

Thanks for info Reg.

Mean something like PCB spiral coil only separated in two sections?

What about shielding such dual field coil?

[Reg]

Hi WM6,

Whites uses two separate windings in their dual field as far as I know. I haven't taken a coil apart, but that is what I think they are doing.

I have built a dual coil like I think the Whites coil is made and it works fine. You do have to add a separate damping resistor across one of the coils though to minimize interactions.

I do need to experiment more to find the right combination of coil inductances and resistance for extra damping resistors though. So, I haven't optimized the design.

Whites uses a conductive paint inside their coils to shield them. I use a special shielding tape on each of the windings to accomplish the same goal.

The only manufacturer I know of that uses spiral coils for their PI is Tesoro. Unfortunately, I have not looked into one of their coils so I can't provide any details.

Now, quite some time back I built a different version of a dual field that is really quite easy to build. In this case, the coil winding is larger and is large enough that the coil can be formed into a lopsided figure 8 and the smaller part of the 8 folded back into the larger one to form the smaller coil. This worked quite well also.

Once again, the larger winding and the smaller winding are in series on the Whites, but are just part of the same winding on the one I built. In either case, all windings or coils are transmit coils, thus making them a mono version.

One of the things I mentioned before is I have made the inner coil a narrow elliptical shape that almost spans the distance front to back of the larger winding. This gives a nice wide response front to back while providing a good target response to very small objects. This design approaches the sensitivity of a DD on the real small stuff while maintaining close to the depth capability of the larger coil if it were the only mono coil. So, I have the best of both worlds.

Anyway, it is something to think about. Also, this discussion is to help people realize they shouldn't be afraid of trying different things.

Reg

[thedigger]

With a PI concentric coil,how critical is the nulling to the sensitivity of the coil?

[Tinkerer]

Nulling of PI-IB coil

With a PI concentric coil,how critical is the nulling to the sensitivity of the coil?

To get best results from an PI-IB coil, be it DD or concentric, the nulling has to be set to a precise point.

The exact nature of the precise point depends on the purpose of the detector and the region or area of search.

It is possible to enhance and to attenuate sensitivity to magnetic targets or soil, as well as to strong parasitic resistive response, like salt, by setting the null, or balance point.

In the post below, I show a circuit that helps to adjust and set the exact null or balance point after the coil has been terminated or in the field.

http://www.geotech1.com/forums/showp...11&postcount=8

Tinkerer

[thedigger]

Hi Tinkerer
I have just made a 18inch PI concentric coil
I gather that you are saying that it would be best to set the null with the coil on mineralized soil?

[Tinkerer]

IB-PI coil balance

Hi Tinkerer
I have just made a 18inch PI concentric coil
I gather that you are saying that it would be best to set the null with the coil on mineralized soil?

Congratulations to your 18" IB-PI coil. I take it that you have not epoxied or potted the coils yet?

With the Bucking coil control circuit you can adjust the balance of the coils a few hundred mV more positive or negative. But first of all you need to balance the TX and the RX coils as perfectly as possible.

Now, lets consider you are going to use the coil in ground that is highly magnetic, like they have in Australia, then you might want to compensate a bit for that when you balance the TX and RX coils, so that you still have the full range of the Bucking coil adjustment available for field adjustments.

Tell me a bit more about your coil and the detector circuit you want to use it for, so I can try to be more helpful.

Tinkerer

[thedigger]

Hi Tinkerer
Thanks for your reply.
I have made two small concentric for my GPX 4500, 6&8 inch,they both worked well.I set them by connecting the TX to a old 2000,and the RX to a meter,and then applied the bucking coil to the RX until the reading settled near zero.
But first of all you need to balance the TX and the RX coils as perfectly as possible.
Could explain what you mean by this,and also would I be better to do the setting with the coil on mineralized soil

[thedigger]

Hi Tinkerer
I forgot to mention,
Tx .35ohms L280uh
Rx 17 ohms L 480uh 9strands of 0.06mm

[Tinkerer]

Hi Tinkerer
Thanks for your reply.
I have made two small concentric for my GPX 4500, 6&8 inch,they both worked well.I set them by connecting the TX to a old 2000,and the RX to a meter,and then applied the bucking coil to the RX until the reading settled near zero.
But first of all you need to balance the TX and the RX coils as perfectly as possible.
Could explain what you mean by this,and also would I be better to do the setting with the coil on mineralized soil

I am not familiar with the electronics of ML detectors, so I can not help much.
If the procedure worked with the small coils, it probably also works with the larger coil.
With an IB coil it is very important that the coil is very rigid. Any flexing will produce false signals.

You could try to set the coil on mineralized soil and null it, then it will have more headroom for that type of soil.

Tinkerer