PI Power Output

[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/xstandard/files/ML_08_007WhitePaper8.pdf

Zed

[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

[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

[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

[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

[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

[simonbaker]

Hi Sb,

Read the "Background " section of this patent. It explains reactive X, resistive R, and instantanious/historical ground matrix. When he says salt is a 1mhz target invert it to time 1uS. R/L is used = frequency just convert to T= 1/(R/L). I am sure someone will explain in plain english.

Mark

Thanks, that is a good reference.

I read along a bit lazily so I could use some help understanding his idea.

I noted that most of his equations assumed the TX field was linearly growing to the point of turn-off, which seems quite different from our discussions where we routinely assume the TX pulse current stabilizes to a near constant before turn-off.

It seems he wants to estimate the portion of the RX signal due to "FRD" (ferrite relaxation decay) using a formula and a couple of signal samples. He distinguishes FRD effects from "ground eddy current" effects. He assumes that ground eddy current effects decay very fast and all sampling is done afterwards, but FRD effects do overlap the target signal.

The patent, as usual, is hard to read because of alternative designs interwoven. For example, he speaks of an alternate design with differing pulse intervals, not sure if that implies different TX pulse durations also. Other variations too.

It would be a lot clearer if somewhere he showed the total received signal (FRD + target) and how the math extracts the target from the combination.

Any help clarifying it would be appreciated.

The PDF file is a bit frustrating because it is an image and prevents text search or cut-and-paste.

Regards,

-SB

[Altra]

Hi Sb,

Read the "Background " section of this patent. It explains reactive X, resistive R, and instantanious/historical ground matrix. When he says salt is a 1mhz target invert it to time 1uS. R/L is used = frequency just convert to T= 1/(R/L). I am sure someone will explain in plain english.

Mark

[simonbaker]

Robby -Quote "You obviously aren't aware of this but if we transmit two wildly different pulse lengths and the tx waveform flat tops before the end of each pulse then the ground signal after each pulse will be identical!!!!!!!!"


Robby 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 ?
I would of thought the amplitude of the ground signal would of been the greatest after the 350 u/sec transmit.

Zed

Hi Zed:

Would you clarify your meaning of "ground signal" and how you measure it?

Regards,

-SB

[simonbaker]

Tinkerer your right about there being no eddy currents near the end of the transmit when the coil field is static just before switch off.
The ground signal i am referring to is the magnetic one,its when the magnetic domains begin to align themselves with the coil field during the transmit cycle and after switch off the magnetic domains slowly return to there relaxed state,this is the ground signal we hear when we sample after switch off,its also referred to as the resistive ground signal or R signal,why i dont now i haven`t figured it out yet.

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

[simonbaker]

Simon,
The ground X signal is only present during transmission so it obviously can't be present during a period of non-transmission regardless of what unregistered Doug says in post #131.

We could though use an example from his forum where they claim to sample earlier than Minelab by using a balanced coil even though they don't do anything to make this possible.

They claim to be able to sample at 11usecs when using a mono and have all GB samples out of the way 15usecs after switch off when using a balanced coil.

At the time they made this claim their sampling routine occupied 12usecs (2usec on, 2 off, 2 on, ~4 off, 2 on), so it would appear their first sample commenced at around 3usecs.

This means they would be sampling the spike in their first and second samples and sampling the true off period in their last sample.

Zed,
There is huge potential for information during the on time as has been disclosed in some patents.
I was only pointing out that the reactive signal is a problem for the novice who is still trying to get up to speed. This doesn't mean that others haven't a solution though. Remember ML brought out the Sovereign well before the SD2000 and obviously know the on time advantages and problems intimately and if you know the problems then you can work out the best way to address them.
I think you will find they have most bases covered if you spend time ploughing thru their patents.

I don't get what you mean here "Now PI detectors have the natural ability to discriminate between ferrous and non ferrous targets but it all comes undone because of the second sample as in a conventional PI".
Any objects such as ground material or metal objects give a signal of one polarity after pulse switch off in conventional single sample pi so I'm not sure what you mean when you say PI is a natural discriminator?
Have you tested your idea and do you have any pictures of the waveforms?

Tinkerer,
Enough of the vast knowledge bit. I wish!
I'm only interested in clearing up the simple basics. If we don’t then it just ends up a confusing mess.

I mentioned Dave Emery's patent because he uses a pi type pulse to generate a large back emf at switch off then causes this to ring with a cappy, something we avoid in pi. He then samples the resulting oscillations similar to vlf (IB) so this is actually a high powered vlf and not pi.
You do much the same but sample the spike itself so it is high powered IB or TR.

B^C,
I for one agree that your chart accurately depicts pi when looking at an ideal target provided the tx coil's spike has settled for a suitable period before sampling begins when using a mono or balanced coil.
I don't work for ML!
Take no notice of the troll. He is only interested in destroying Minelab, me, JP and anyone who thinks ML makes good detectors, He gets everything wrong as he clearly demonstrates here...
http://www.geotech1.com/forums/showthread.php?t=15417

The ground X signal is only present during transmission so it obviously can't be present during a period of non-transmission regardless of what unregistered Doug says in post #131.

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

[Altra]

Hello,

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

Thanks Mark

[ZED]

Robby -Quote "You obviously aren't aware of this but if we transmit two wildly different pulse lengths and the tx waveform flat tops before the end of each pulse then the ground signal after each pulse will be identical!!!!!!!!"


Robby 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 ?
I would of thought the amplitude of the ground signal would of been the greatest after the 350 u/sec transmit.

Zed

[ZED]

Zed,

I understand this differently.
A the end of the TX pulse, lets call it TX Omega, when it is "flat top" or more than 5 coil TC long, the coil current is at steady state.
No eddy currents are generated. Not in the ground and not in the target. (Unless, in a target with very long TC where the eddy current generation might continue a little due to the charge lag.)
With no eddy currents generated in the ground at TX Omega, it is not a good spot to measure the ground signal.
However, it is a good spot to receive the common mode noise, to subtract it then in the difference amplifier.

Tinkerer

Tinkerer your right about there being no eddy currents near the end of the transmit when the coil field is static just before switch off.
The ground signal i am referring to is the magnetic one,its when the magnetic domains begin to align themselves with the coil field during the transmit cycle and after switch off the magnetic domains slowly return to there relaxed state,this is the ground signal we hear when we sample after switch off,its also referred to as the resistive ground signal or R signal,why i dont now i haven`t figured it out yet.

Zed

[ZED]

I don't get what you mean here "Now PI detectors have the natural ability to discriminate between ferrous and non ferrous targets but it all comes undone because of the second sample as in a conventional PI".
Any objects such as ground material or metal objects give a signal of one polarity after pulse switch off in conventional single sample pi so I'm not sure what you mean when you say PI is a natural discriminator?
Have you tested your idea and do you have any pictures of the waveforms?

Sorry Robby i cocked up,i was thinking of the signal waveforms during the transmit period and how ferrous and non ferrous signals are generally opposite to each other,i was thinking it would continue to be the case after the flyback but on reflection it doesn`t.

Zed