Triangular Wave Technology

[Ferric Toes]

Described in a 1975 thesis by Yves Lamontagne, Research in Applied Geophysics, University of Toronto.



I'm sure I have seen a logo somewhere recently, made of similar shapes.

Eric.

[Davor]

OK, there is a triangular current flowing through the Tx coil, but in all honesty, the step voltage is the force that produces triangular currents and that also excites the response from ground and targets.

[Tepco]

Yes, this is square wave voltage driven TX, current is then linear ramping triangular wave, some sort of balanced coil is then needed for RX signal recovery. Very useful, observe actual target wavefioms in this setup. (RX coil must be loaded with appropriate resistance too, non-resonant wide band circuit). Can be called “hybrid” approach, disc. is possible. Fisher Impulse used something similar, in energy recovery mode, but with monocoil, sampling after (way too long) recovery period, this design does not include flyback.

[Sergey_P]

and that also excites the response from ground and targets.

Yes, There there is all... http://www.md4u.ru/viewtopic.php?f=77&t=2037

[Davor]

But again, any other PI would perform just as good if only a short high voltage spike is applied to a Tx coil, or a monocoil. Charging current is here simply BECAUSE it produces a flyback spike, not the other way around.

And yes, I can see a potential for successful discrimination using step voltage excitation.

IMHO there is nothing so special about the coil charging period - you could as well charge some other coil with a proper core for a sole purpose of generating a flyback, and that charges while Rx does it's processing. When Rx is done - BANG!, a new spike is there. You could as well charge a capacitor (like X-ray machines or photographic flashes do) to avoid any magnetic coupling, and the moment you need it - BANG! a spike is there.

Given some modest sampling and processing time, and avoiding the charging period, you effectively improve S/N by keeping everything else as is. You can squeeze more Rx periods per time with all sampling timing as is. Furthermore, if you connect a coil in some bridge that enables you to reverse coil polarity, you lose the need for very late sampling, and thus gain even more time and S/N.

[Dave J.]

Fisher CZ (introduced in 1991 and still in production) transmits a triangular current waveform. The system architecture is VLF induction balance with demodulators running at the fundamental (5 kHz) and third harmonic (15 kHz). There is of course no second harmonic since the waveform is symmetrical.

--Dave J.

[Carl-NC]

Also the same method used in Minelab BBS/FBS. Flyback is intentionally suppressed so you are left with a more CW-type system. You can either demodulate in a traditional VLF-style CW demod (isn't that what CZ does Dave?) or in a time-domain-style demod like Minelab.

BTW, here is the Minelab TX voltage and current waveforms:

[Aziz]

This current wave form is working very nice on my sound card platform as well. But the demodulation and GB is way way more elegant. And you don't need the FBS/BBS thing.
Aziz

PS:

I'm sure I have seen a logo somewhere recently, made of similar shapes.

I'm sure, I have seen it somewhere recently too.

[Tepco]

But again, any other PI would perform just as good if only a short high voltage spike is applied to a Tx coil, or a monocoil. Charging current is here simply BECAUSE it produces a flyback spike, not the other way around.

Actually, it will not, situation is exactly opposite.


Linear ramp up of charging current is what excite target, and must be at least comparable or longer than target TC. Flyback is just undesired consequence of energy stored being abruptly released. Generating “power efficient” short pulses, like in class E style circuit is possible, or “recycling” energy, like in Impulse, but with some consequences. In “energy recovery” design, like Impulse, time to recover energy is approximately equal to time needed to ramp up TX current, sampling can take place only after, limiting design to shortest practical TX pulse, and then sampling century later, after similar period of time. Impulse design is about the best compromise in timing so far (only one, actually, so easy to be the best).With modern rechargeable batteries, few hundred milliamp consumption is not an issue, better to optimize something else instead of recovering energy lost in flyback.

[Dave J.]

Wrong on most of that, I'll leave it to others to explain if they care to do so.

--Dave J.

[Tepco]

Well i may be wrong on everything, and i will appreciate if someone can explain, if someone care.

[Dave J.]

Sorry to have had to put you off, Tepco, I didn't have time to explain earlier.

A PI receiver detects voltage induced in the receiver coil (which is most often the same physical coil as the transmitter coil.)

Voltage is induced in the receiver coil by a change (decay) of eddy current flowing in the metal target to be detected.

That eddy current flows because of voltage previously induced in the target.

That voltage is induced in the target by a change in current flowing in the transmitter coil. In a conventional PI, the change in current happens very rapidly (typically ramping from more than an ampere to zero in a few microseconds) during the flyback period.

In a conventional PI, the transmit period is many times the duration of the flyback event for the same amount of current change. And, it happens in time farther removed from the receive gate turn-on than does the flyback.

An argument can be made that the reactive voltage time product of the transmit period is (or can be) about equal to that of the reactive voltage time product of the flyback period, and therefore the transmit period is an important player in the response of high-conductivity targets. This is true, but unfortunate: the response to the transmit period mostly cancels the response to the flyback period, reducing sensitivity to high conductivity targets.

The (my) "power saving" PI design has reactive voltage time products which are (to a first approximation) equal for the transmit period and the flyback period. The product for the flyback period is actually less (due to resistive losses) but it happens a lot closer to receiver turn-on than does the transmit period so that's what dominates. Because there are no high voltages, this scheme allows you to run the receiver gate time fairly close to the end of flyback, close enough that salt water pickup is a noticeable problem despite the low dI/dt of the flyback period. However sensitivity to high conductivity targets is poor, for reasons I already explained.

It has long mystified me that my PI design never gained any traction among hobbyist experimenters. The thing is very easy to build-- everything runs on a 5 to 6 volt rail, no high voltages, earth field cancellation is intrinsic to the design, no requirement for induction balance, and in the receiver almost anything that looks like it should work, does. It's a very forgiving topology. I don't regard it as the basis for a 21st century high performance product, but just to build a PI to do it, it doesn't get any easier than the way I did it.

I'm pretty sure that someone has posted an Impulse schematic in the schematics forum. A hobbyist wouldn't necessarily do it that same way, but it provides a good starting place.

*****************

To summarize: the receiver of a PI detects the decay of the eddy current that was induced in the target during flyback. Currents flowing in the target which were induced during transmit flow in the opposite direction and reduce sensitivity.

--Dave J.

[mickstv]

It has long mystified me that my PI design never gained any traction among hobbyist experimenters.


--Dave J.

Hi Dave, did you actually have a thread going about this, with circuits etc ?

[Carl-NC]

Dave, it'll please you to know that your Impulse TX circuit inspired me to do a 6-month long (spare time, of course) investigation into ramp TX methods. The result is a 3-frequency ramp TX that looks nothing like your circuit but works a whole lot like it anyways. The goal is a next-gen version of BBS/FBS (I call it VBS as an internal joke, 129 frequencies!) that actually has excellent low-conductor sensitivity (also unlike BBS/FBS). I just got the PCBs and it is being built by my tech right now, though I don't know when I'll get to play with it as I have real product-projects that the boss insists I finish.

BTW, the reactive time difference between transmit and flyback is due to the flyback snubber diodes, they create a step-up voltage effect during flyback and increase the slew rate. Add more & more series diodes to see it approach a vertical, and watch your recycling efficiency drop to zilch.

- Carl

[Dave J.]

Hi Dave, did you actually have a thread going about this, with circuits etc ?

No, but I think I saw the circuit posted in the schematics forum.