PI Power Output

[Aziz]

Hi all,

be very carefull, when taking time constant of target (TC).

I have already mentioned this earlier and nobody showed an interest in this fact. The target response for one TC1 and another TC2, where TC1=TC2 but the target conductivity (resistance R) and inductance L differs, are not the same!!!

The target response, either signal magnitude or time to arrive maximum respose of signal (period of time) can be different, even the time constants are same.

Just play again with spice simulations!

Aziz

[Altra]

SB,

A text version is at this site. Do a number search

http://patft.uspto.gov/

The way I understand the FRD, is iron oxides have weak magnetics properties. When they are pulsed with a magnetic field, they become weakly magnitized and cause a DC offset in the RX. After a curtain amount of time they relax. Think of iron oxide as millions of microscopic magnets with N and S poles. Normally the poles are at equalibrium or randomly oriented. When a magnetic field is introduced they all align. A moving coil will cut the lines of flux created by these and introduce a field into the RX coil. Maybe a good visual is iron dust on a piece of paper and moving a magnet under. The bits stand up as it passes then fall after.

Regards
Mark

[Altra]

SB,

I left this out "Target Response" = (Target + FEo2 early sample) - FEo2 Late. Non-ferrous targets have a non-linear response. FEo2 has a very linear response. Lets say you sample at 8uS for gold it will cause a steap slope in the decay curve the FEo2 adds a curtain amount of DC offset. A later sample is taken long after the gold is no longer effecting the decay curve and is mostly FEo2. This sample is inverted and added to the oringinal signal, this cancels the iron oxides. Let the Ausies chime in they have iron oxides in their blood.

Mark

[simonbaker]

SB,

I left this out "Target Response" = (Target + FEo2 early sample) - FEo2 Late. Non-ferrous targets have a non-linear response. FEo2 has a very linear response. Lets say you sample at 8uS for gold it will cause a steap slope in the decay curve the FEo2 adds a curtain amount of DC offset. A later sample is taken long after the gold is no longer effecting the decay curve and is mostly FEo2. This sample is inverted and added to the oringinal signal, this cancels the iron oxides. Let the Ausies chime in they have iron oxides in their blood.

Mark

Thanks, makes it much clearer.

-SB

[ZED]

Hello,

Is anybody doing low side current sensing on the TX? For ground tracking or other information?

Thanks Mark

G`day Mark

Yes mate i am,doing the cct at the moment,having trouble with hot fets,using the coil switch to regulate the current,works good except for the hot fet,waiting for other fets to arrive.

Zed

[ZED]

I guess I need clarification on the term "ground X signal". Is that the coupling of the TX and RX coils through a ferrite-like magnetic medium in the ground? If so, is the assumption that such ferrite-like medium does not have a current and therefore no time-lag, but rather has an instantaneous effect on the magnetic field and has no effect once the mag field is gone? Is that what is meant by PI has a natural ability to discriminate out ferrite like objects (if you sample after TX field -- and ferrite effect -- gone)?

Regards,

-SB

Good question Simon

My understanding of the ground response doesn`t go much beyond the fundamentals,heres a report by bruce candy on the subject,see what you think.

http://www.minelab.com/consumer/xsta...hitePaper8.pdf

Zed

[ZED]

I think this touches on my question about "ground x signal" and ferrite. How fast do we assume the magnetic domains return to relaxed state. Or do they? Could they stay permanently magnetized by the pulse, therefore no affect after?

Three cases:
1. Return instantaneously -- we don't see them during sampling after TX.
2. Return with small delay -- they become part of post-TX sample.
3. Return very very slowly -- we don't see them during post-TX sample.

Which is most accurate?

Regards,

-SB

"The ground signal" now thats a very big subject and its way over my head,there are many papers writen on the subject.
The magnetic domains behavior can vary quite alot,its a function of the ground matrix [how mineralized it is]how quick the magnetic domains return to there relaxed state is often refered to as its viscosity or magnetic lag,the magnetic domains can relax very quickly or very slowly,slowly as in 300 u/secs.
The domains can have a random alignment as can be found in soils or clays and they can also be alighed in a particular direction. There alighnments can also be changed by lightning strikes and even magnets can have the same effects,generally the coil field will not have a permenent effect on the domains.
As for the reactive signal [X] i veiw it as eddy currents building up on the tiny minerals that exist in the ground matrix,these tiny minerals have extremly short T/C and so the eddys die within about a u/sec.

Zed

[ZED]

SB,

I left this out "Target Response" = (Target + FEo2 early sample) - FEo2 Late. Non-ferrous targets have a non-linear response. FEo2 has a very linear response. Lets say you sample at 8uS for gold it will cause a steap slope in the decay curve the FEo2 adds a curtain amount of DC offset. A later sample is taken long after the gold is no longer effecting the decay curve and is mostly FEo2. This sample is inverted and added to the oringinal signal, this cancels the iron oxides. Let the Ausies chime in they have iron oxides in their blood.

Mark

Altra the above is true except for one thing,here in OZ the ground signal in the second sample is no way strong enough to cancle out the ground signal in the first sample,and i would hazard a guess and say this would also be true globally

Zed

[robby_h]

Zed,
Sorry but I have been a busy boy and I think the next week or so will be worse.
“.... if i have a 150 u/sec wide transmit pulse and then a little latter a 350 u/sec wide transmit pulse and both transmits have the coil field at a static state before switch off are you saying the ground signal will be of the same amplitude at the end of both transmits ?”.

This is correct. The signal will be identical in both shape and amplitude. Eric Foster posted pictures clearly showing this on the PI forum but they are now gone. This is always a problem when pictures are linked to a host site.

Simon,
Yes the purely magnetic effect follows a transmitted waveform instantly so it can’t exist in a period of non-transmission.

“PI has a natural ability to discriminate out ferrite like objects (if you sample after TX field -- and ferrite effect -- gone)?” I'm afraid pi detectors aren't natural discriminators.

[Altra]

Hello Zed,

On the second sample not being strong enough to cancel out the oxides, makes sense in your ground. If you look at the demodulators on your SD2000 schematic. There are two subtractions for each pulse width. First an early sample, then a later. These occur between each pulse. This is how Mr. Fosters circuits operate. This works fine in most soils. On the SD, a second subtraction takes place on each the short and long demodulators by adjusting the ground balance pots. If you note the sample times are late and longer on the third demoduator. This allows more ground signal to be intregrated. I believe this is how they balance the first and second samples?

What do you hope to do with your TX current sensing?

Mark

[Tinkerer]

Zed,
Sorry but I have been a busy boy and I think the next week or so will be worse.
“.... if i have a 150 u/sec wide transmit pulse and then a little latter a 350 u/sec wide transmit pulse and both transmits have the coil field at a static state before switch off are you saying the ground signal will be of the same amplitude at the end of both transmits ?”.

This is correct. The signal will be identical in both shape and amplitude. Eric Foster posted pictures clearly showing this on the PI forum but they are now gone. This is always a problem when pictures are linked to a host site.

Simon,
Yes the purely magnetic effect follows a transmitted waveform instantly so it can’t exist in a period of non-transmission.

“PI has a natural ability to discriminate out ferrite like objects (if you sample after TX field -- and ferrite effect -- gone)?” I'm afraid pi detectors aren't natural discriminators.

It would be helpful specifying the actual coil current at switch OFF. 150uS and 350uS TX will produce very different coil currents unless there is some kind of regulation.

Tinkerer

[simonbaker]

Zed,
...
Simon,
Yes the purely magnetic effect follows a transmitted waveform instantly so it can’t exist in a period of non-transmission.

“PI has a natural ability to discriminate out ferrite like objects (if you sample after TX field -- and ferrite effect -- gone)?” I'm afraid pi detectors aren't natural discriminators.

Yet from your answer it seems by sampling after the TX magnetic pulse dies out you automatically are immune from any purely magnetic effect - so at least it discriminates against that, correct? Whereas a VLF MD is constantly seeing the purely magnetic effect? Or is the purely magnetic effect not the problem anyway?

Regards,

-SB

[ZED]

It would be helpful specifying the actual coil current at switch OFF. 150uS and 350uS TX will produce very different coil currents unless there is some kind of regulation.

Tinkerer

Tinkerer lets say there is current regulation and the coil current reaches 2 amps at 100 u/sec and it is held there at 2 amps until 150 u/sec then the transmit is switched off then at a later point we start the same transmit again but this time we switch it of at 350 u/sec [still 2amps coil current] apparently the ground signal will have the same amplitude after each transmit.

Zed

[ZED]

Hello Zed,

On the second sample not being strong enough to cancel out the oxides, makes sense in your ground. If you look at the demodulators on your SD2000 schematic. There are two subtractions for each pulse width. First an early sample, then a later. These occur between each pulse. This is how Mr. Fosters circuits operate. This works fine in most soils. On the SD, a second subtraction takes place on each the short and long demodulators by adjusting the ground balance pots. If you note the sample times are late and longer on the third demoduator. This allows more ground signal to be intregrated. I believe this is how they balance the first and second samples?

What do you hope to do with your TX current sensing?

Mark

Mark you are aware of course that erics detector amplifys the second sample till the ground signal in it is of the same value as the first sample.

The long and late 240 u/sec sample is there for the purpose of taking an earth field sample,not a ground sample,that is why it is so late,most if not all of the ground signal is gone by then.

My current sensing cct is to be used with an experimental ground cancling cct,see post 132 of this thread,ignore the discrimination idea,the idea is to take a sample during the on time while using a mono coil.

Zed

[ZED]

My argument for a PI detector being a natural discrimintor is because of what goes on during the on time,refering to the picture robby posted in post 120 of this thread it can be seen that ferrous and non ferrous targets give opposite responses while the coil field is in a state of growth,i have also seen the same thing with my own experiments but unfortunately the same information is not there after the transmit .

Zed