Latest Minelab Patent - 11th Feb 2016

[kt315]

KT, you continue to look at the BBS/FBS
---
this pic line 'a' is OK for you? has BBS coil current another form?

[Carl-NC]

What you show is for a traditional PI. BBS/FBS is entirely different.

[kt315]

are these pics OK for you? (voltage, NOT current)

Как мы видим подводимый к ТХ катухе сигнал представляет собой дискретную бинарную посылку.

Обратите внимание на картинку ниже, пачка начинается длинным импульсом затем идут 7 коротких с частотой 25 кгц которые заканчиваются опять же длинным импульсом, далее пауза и опять посылка.

Парамметры:

Размах сигнала- 10 вольт (+-)
Общая длительность бинарной посылки 470 мкск

Начальный импульс длительностью- 88 мкск
Далее подряд идут 7 иипульсов длительностью по 20 мкск, пауза 20 мкск = 25кгц
Длинный оконечный импульс 88 мкск.
Пауза перед следующей посылкой 176 мкск.

Несколько пачек подряд.



Одна пачка.



На данный момент я начинаю перечитывать около 20 патентов

[kt315]

and this

[moodz]

As we can see katuhe supplied to the TX signal is a discrete binary package.
Note the picture below, the pack begins long pulse followed by 7 short at a frequency of 25 kHz which end again a long pulse, followed by a pause, and again posting.
Parammetry:
Swipe SIGNAL- 10 volts (+ -)
The total duration of the binary packages 470 mksk
The initial pulse dlitelnostyu- 88 mksk
Then go straight 7 iipulsov duration of 20 mksk, pause 20 = 25kHz mksk
Long terminal 88 mksk momentum.
Pause before sending the next 176 mksk.

...it says voltage in the translation.

[Davor]

The voltage is irrelevant other than the mathematical relationship it has to current, due to the fact that an inductor is in play. What matters to the transmitted magnetic field is the coil current. Look at that, and you'll see a different picture.

I beg to differ. As the Rx is a voltage sensor, it is voltage that is mostly relevant. You could try a transimpedance preamplifier to get into the current ballpark, but let's not promote confusion.

Regarding targets, it is all the same since coupling is minuscule.

A diagram you posted earlier that looks as n odd FSK signal with wildly varying amplitude would have a constant voltage amplitude, and for a target that responds the same on both frequencies, it would have equal voltage response. That's why even multifreakers (thanks Dave) have holes, same as PI. That's precisely why BBS works well.

As for frequency/time, the relevant frequency span would be, say, an octave. The targets have very dull frequency responses. Therefore any kind of processing that collects the most energy from an octave of frequency span may also be regarded as frequency domain. It works for digital filters as well. Heck, even cochlea works that way. Mozart didn't complain.

[Carl-NC]

are these pics OK for you? (voltage, NOT current)

Yes, those are BBS/FBS TX voltage waveforms. Now, what does the TX current look like?

I beg to differ. As the Rx is a voltage sensor, it is voltage that is mostly relevant. You could try a transimpedance preamplifier to get into the current ballpark, but let's not promote confusion.

Transmitted magnetic field is directly proportional to the TX coil current. Everything else follows from that.

[Davor]

OK, so you like to calculate with current, I get it. And you calculate current from voltage supplied to the Tx coil. And next magnetic field, and when you go back to the Rx, you calculate flux density to get induced voltage. Duh. Faraday did not complicate that much.

Even for transformer calculation you need to know current and field only for the sake of core saturation limits. Otherwise it is Faraday through and through.

[Teleno]

OK, so you like to calculate with current, I get it. And you calculate current from voltage supplied to the Tx coil. OK, so you like to calculate with current, I get it. And you calculate current from voltage supplied to the Tx coil. And next magnetic field, and when you go back to the Rx, you calculate flux density to get induced voltage. Duh. Faraday did not complicate that much.

Once you get a current going in a coil you're stuck with a current problem, for it behaves as a current source. You got to remove that current quickly, not a voltage. Moodz uses a current sink as evidence that the problem can be treated efficiently in the current domain. You can even detect the target's decaying magnetic field as a current in a shorted Rx coil.

[Carl-NC]

OK, so you like to calculate with current, I get it. And you calculate current from voltage supplied to the Tx coil. And next magnetic field, and when you go back to the Rx, you calculate flux density to get induced voltage. Duh. Faraday did not complicate that much.

Even for transformer calculation you need to know current and field only for the sake of core saturation limits. Otherwise it is Faraday through and through.

Looking at the TX current instead of the voltage better reveals what the heck is going. Just look at the confusion over BBS, and how many people think that just because the drive voltage is a kind of square wave that it must be a type of PI. When looking at the current, it is obviously not. Yet in another design, a similar kind of voltage waveform might produce an obvious PI-style current.

Another good example is the GPZ, which a lot of people call a VLF. Yet the TX current suggests is it way closer to a PI.

[Carl-NC]

For those still interested in what BBS/FBS is, here is the voltage & current waveforms:



I've posted this a few times in the past but it doesn't seem to make much of a lasting impression. Look closely at the current waveform... see any 'off' time? Pretty continuous-looking, eh? This is why I posted that 2-frequency sinusoid and asked what is it? and how would you process it?

P.S. -- if you think my Visio art might not be an accurate representation, I've also measured the waveform with a current probe on an oscope. Same thing.

[Dave J.]

I get the impression that most of the people posting in this thread don't know enough high school physics to understand how metal detectors work.

1. The magnetic field produced by the transmitter is proportional to the transmit current.

2. The voltage induced in a metal target located in the field is proportional to the rate of change of the field; and therefore to the rate of change of the transmit current.

3. The voltage induced in the receiver coil resulting from induction imbalance is proportional to the rate of change of the field.

4. Knowing what the voltage on the transmitter coil is, does not tell you the amplitude of the current flowing in the transmitter coil or even its polarity, and therefore does not tell you what the magnetic field is.

* * * * *

If you don't even know what the magnetic field is doing, discussion of "VLF" versus PI is incomprehensible gobbledegook. The reason Carl keeps dragging the conversation back to current (and not voltage) is because the current is what produces the magnetic field. It's not the voltage.

[deemon]

For those still interested in what BBS/FBS is, here is the voltage & current waveforms:

I've posted this a few times in the past but it doesn't seem to make much of a lasting impression. Look closely at the current waveform... see any 'off' time? Pretty continuous-looking, eh? This is why I posted that 2-frequency sinusoid and asked what is it? and how would you process it?

P.S. -- if you think my Visio art might not be an accurate representation, I've also measured the waveform with a current probe on an oscope. Same thing.

But imagine now , Carl , that you add a differentiator stage somewhere in your TX block before the coil - and what should you get ? This operation ( taking a derivative ) must transform the triangle current wave in the coil to a square shape ( and voltage wave to the short spikes ) , isn't it ? So the signal should become a kind of "PI-like" , yeah ? And now - imagine that you insert this diff stage not in TX block but in RX one ( after RX coil ) ... so you'll get just the same "PI" signal on your receiver , without any change of transmitter - what a strange thing ... .

And now imagine that you perform just the same operation with any typical VLF device with a sine wave current - nothing will change , only the sine will transform to cosine ... this "mental experiment" must help us to understand better a fundamental difference between ULF and PI , as I think

[deemon]

You can even detect the target's decaying magnetic field as a current in a shorted Rx coil.

... or even in a shorted TX coil

[moodz]

...and theres a real eyepopper coming in application 2015903535 ... it does what some say is impossible. Cant say anymore obviously ...;-)