Ground Balance Theory

[Davor]

Completely true, except maybe formulation of

2. Each delay in TD determines a frequency in FD.

...which in fact is a frequency band from DC to a frequency corner set by the Nyquist–Shannon sampling theorem.

Both CW and PI Tx-es suffer from poor PSRR, and your reasoning viz. the air signal error vector is very relevant so power supply stabilisation and filtering is essential. There is however one catch: air signal is reduced by IB coils, where both cyclo-stationary component due to the power supply DC component, and the AM error vector due to the power supply ripple get reduced proportionally. The AGC schemes are mostly a mild let down because vast majority of them are referred to a power rail, hence not helping with PSRR at all, and in the above Garrett example gloriously miss the whole point of regulation.

Otherwise there are little differences between PI and CW.
At the moment it seem as if PI has some advantage over CW, but that's only because of CW development stopped at about 1990, with only a few stray designs that did not help. Only recently CW Tx coils "discovered" low inductance as the approach to stronger field, and Rx-es are still catching their own tails.

Congratulations on your new job

[Aziz]

Now you guys can imagine, why I am sampling and processing the TX coil voltage as well (either PI or CW PI/VLF/LF).

Aziz

[Davor]

Sure, it makes sense. There is still much room for improvements in Tx layout and coil balance, because other approaches tend to fall into some of the contradictory traps. Division by Tx envelope would fix all the cyclo-dynamic components of Tx AM errors, but only if applied prior to motion compensation filtering, hence at full dynamic range with "air signal" and all. E.g. it would not help my QSD chopper approach with motion compensation, as cyclo-stationary component is not passed through. (I could devise some kind of signal sink that would perform such division ... worth a try)

Funny thing is that once you are finished observing PI and CW as different and separate systems many solutions become interchangeable

[sido]

Why is it i cannot get some sort of straight to the point answer to this question?.....Why...maybe no one knows.

How does the TX driver influence the ideal Magnetic Field generated in a particular soil? What parameters are important?

What determines the TX driver's characteristics in relation to a target's maximum magnetic field in so and so soils?

Please understand, I am only referring to the Research studies above in those documents.....If it's flawed, then we can all move on.

Cheer's Sid

[Davor]

And you are referring to PI Tx?

[sido]

And you are referring to PI Tx?

Yes

Regards Sid

[Davor]

What parameters are important?

with given L relevant is the L(di/dt) which equals to the coil voltage. With that in mind, the coil voltage spectrum should have something to do with frequency response of the fastest targets resolvable by the receiver (and not faster than that), which is not necessarily the case with typical PI.

[moodz]

with given L relevant is the L(di/dt) which equals to the coil voltage. With that in mind, the coil voltage spectrum should have something to do with frequency response of the fastest targets resolvable by the receiver (and not faster than that), which is not necessarily the case with typical PI.

...hint hint .... you might want to rethink that ... magnetic fields are generated by currents not voltages. The coil is not a resistor ... it is a reactive component .... coil voltage is not the same as current ... for instance it is possible to hold the coil voltage constant for a period of time whilst changing the current and thus the magnetic field. Measuring the voltage at this time might lead you to an incorrect analysis.

[Ferric Toes]

Why is it i cannot get some sort of straight to the point answer to this question?.....Why...maybe no one knows.

How does the TX driver influence the ideal Magnetic Field generated in a particular soil? What parameters are important?

What determines the TX driver's characteristics in relation to a target's maximum magnetic field in so and so soils?

Please understand, I am only referring to the Research studies above in those documents.....If it's flawed, then we can all move on.

Cheer's Sid

Hi Sid,

It appears that what is referred to is a single coil. Footnote 9 shows what needs to be done if a separate RX is added. I don't see a reference to the TX current waveform, which seems to me to be important as the shape of the generated magnetic field will be similar. Ideally the coil circuit time constant should be short compared to the pulse width, so that the final current is limited by the dc resistance of the coil circuit. Some PI's do not reach this point, so the current is still growing exponentially and has not reached its final value at the switch off. At the switch off, the drive circuit becomes a high impedance and the voltage across it rises to try and maintain the current and field as it was just prior to the switch off. In fact the TX coil now appears as a closely coupled target of high resistivity and to generate any current at all, 100's or even 1000's of volts are required; hence the high voltage spike. The important thing to remember is that it is the current waveform that determines the shape of the magnetic field pulse and that no high current spike appears at the end of it; just a ramp down, or a fast exponential decay down, depending on any voltage limiting circuit that may be in operation, and the method used to damp any oscillation due to parasitic capacitance.

I'll deal with the effect of the ground later. Busy day today.

Eric.

[Davor]

@moodz, you can observe it like that if you prefer calculus
Think of a transformer ... perfect power transfer happens below the saturation current. Transformers are calculated as per the voltage, and they are thus expressed in voltage ratio. Currents must be calculated only for losses and saturation estimation. Metal detectors are in fact a sort of transformer, so the whole logic behind transformers hold as well.

[Ferric Toes]

I wrote the following a couple of days ago, and it is relevant to this thread so here goes -

All PI mine detectors that I am aware of sample in the OFF time only, and this effectively decouples the RX signal from the TX. It is decoupled from the Air signal which is at its worst in the case of a mono coil, and it is decoupled from the Ground signal in the cases for normal ground conductivity and susceptibility. This is the case for a first sample delay of 10uS from the initiation of TX switch off. OK, you can sample much sooner with various techniques that have been covered in forum posts, but it is of limited value to do so in a detector for general location of nuggets, coins, rings and many mines. I have a Schiebel AN19/2 which was the primary NATO mine detector from 1980 – 1990. In the absence of detailed technical information, I estimate the sample delay to be about 15uS by observing the comparison in sensitivity to, say, the Deepstar of my design. The Schiebel detector has now been superceded by the Vallon VMH3CS and is often seen in current TV news reports from countries where conflicts are ongoing. I don’t have a complete VMH3CS, but I do have a few search coils that are in working order. It is a simple shielded monocoil in a truncated elliptical shape and has an inductance of 1.5mH. The best sample delay I can get with this coil is 20uS, so unless non-standard technology is being used the small object sensitivity is a bit less than the AN19/2.

Regarding the modulation of the TX current/field this does not seem to be an issue in the practical world, provided certain design steps are taken. Using a soldering iron and bench test equipment, I measured the inductance of the 30mm solenoid coil of my viscosity test gear. It came out at 182.7uH. I then inserted a 10gm sample of Australian ironstone with a susceptibility of 33,000 SI units as measured on a MS2B. The inductance increased to 189.00uH, or 3.4% increase. Provided the TC of the coil circuit is short compared to the width of the TX pulse any modulation of the TX field by susceptible ground will not be noticed, neither will the effects at switch off, provided switch off and damping occur well before sampling. When in Australia, I measured the change in inductance of a 11in 300uH coil when place on ironstone, and it was less than that on the small solenoid.

In some designs, and for other reasons, I have used a constant current source for the TX supply. This in itself would correct any changes in TX current due to magnetic ground effect. However, even in the worst ground conditions there is no evidence that there is a problem in this area.

In the final analysis, even if modulation of the TX field were to take place at higher levels, a good GB system will not respond to magnetic viscosity as the decay law stays the same irrespective of amplitude. The biggest modulation is simply swinging the coil.

Eric.

[moodz]

... Whoops ... Accidentally posted a secret.

[Ferric Toes]

... Whoops ... Accidentally posted a secret.

Who did?

[moodz]

Who did?

Me

[sido]

... Whoops ... Accidentally posted a secret.

Step up or down modulated current in TX on. In other words varying the current in steps between each TX on time cycle.

(instead of TX on and a induced 2 different state constant Voltage current TX (on).......like M.L hey)

So usual sampling at such given alternating TX (on) Currents in the RX for a better GB and elimination of common RF induced interference from ground, air and surrounding RFI.

This comes back to my initial question and thanks Eric for a detailed explanation above.
Sorry Moodz if i unintentionally reveiled your secret, if i have. I may have done so as its got me curious with TX Driver having a influence on ideal performance in soils as stated from the document posted by Eric.

Cheers Sid