Vallon VMH3CS Mine Detector

[KingJL]

O.K., is this a DSP trick where you can sample into the flyback recovery period? Then there is the ground cancelling, how can that be done? Over to the experts in the digital domain, as I am not one.
Nothing changes in the above pictures if I switch to the ground cancelling position. What we need of course is a schematic, but I doubt very much if that is possible.

Eric, what are you using for the FE (I keep exploring for the better mousetrap). As for the DSP trick for sampling into the flyback, I have no idea. I can see how they could cancel out the flyback, but I can't conceive how they can sample into the high voltage and still discern minute changes. For mineralization/ground cancelation you can take your filtered input, feed it into a let's say 0.05 HZ decimating filter and subtract that output from your filtered input. This ground/mineralization filter needs to stop updating if there is no motion, again easy to do digitally, with todays accelerometers IC's.

[Ferric Toes]

I wired up a front end amplifier, the same as I have used many times in the past, to give some front end gain. The standard input configuration is via a resistor and a couple of diodes back to back to clip TX pulse and flyback transients. What we see is a standard PI response waveform. Amplifier saturation to plus and minus rails, both 5V; amplifier recovery to flat baseline. Scope trace starts at TX switchoff.

A metal can is then placed on the coil and standard decay observed for both pulse polarities.

The scope sensitivity is increased and the above repeated.

As above with metal can.


What is puzzling is the glitch at 103uS on both polarities. Is this a gating glitch or just an unimportant artifact appearing across the coil? The amplifier recovery with this high inductance coil also seems much too late to detect, say, a 0.3gm nugget; which the Vallon unit does quite easily. O.K., is this a DSP trick where you can sample into the flyback recovery period? Then there is the ground cancelling, how can that be done? Over to the experts in the digital domain, as I am not one.
Nothing changes in the above pictures if I switch to the ground cancelling position. What we need of course is a schematic, but I doubt very much if that is possible.

[Ferric Toes]

I think that's exactly what they are doing. It's not 100% efficient, ergo the "tilt" during the on-time.

Presumably, the tilt is the remainder of the the normal L/R charge up time until switch off?

[Carl-NC]

Hi Davor, I wonder if they are using the high voltage spike from one pulse to kick start the next opposite polarity pulse in order to get the fast rise time. A diode capacitor snubber would store the 250V which would be in the correct polarity to do this.

I think that's exactly what they are doing. It's not 100% efficient, ergo the "tilt" during the on-time.

[Davor]

Hi Davor, I wonder if they are using the high voltage spike from one pulse to kick start the next opposite polarity pulse in order to get the fast rise time. A diode capacitor snubber would store the 250V which would be in the correct polarity to do this.

My thought exactly. You may observe that current continues rising past the initial kick start. Seem like a perfect way to employ a snubber. In a unipolar machine it would require lots of effort to reverse polarity, but here it fits perfectly. You need to burn some energy stored in a snubber capacitor between cycles for it to run well, so there you have it.

I wonder if there is a difference in timing when ground balance is on. I'd expect to see some if Paltaglou principle is involved.

[Ferric Toes]

Hi Mick, Your waveform looks the cleanest. There are some odd splurges on the Vallon one, so I suspect there are other things going on in the background.

[Ferric Toes]

Eric,

I want to thank you for sharing your technical knowledge on various forums over the years. It was your forum that taught me about PI detectors and motivated me to write my "Making a Fast PI Mono Coil" article with the encouragement of Carl who runs this forum. It is good to see you exploring different PI designs again.

Thanks

Joseph J. Rogowski

Thanks Joe, I'm enjoying coming back and it stimulates the mind. Good to see that many past friends are still on this excellent forum.

[Ferric Toes]

It appears there are both acceleration and deceleration mechanism involved. There are steep, but near-linear current slopes on both sides. Interesting.

Hi Davor, I wonder if they are using the high voltage spike from one pulse to kick start the next opposite polarity pulse in order to get the fast rise time. A diode capacitor snubber would store the 250V which would be in the correct polarity to do this.

[Davor]

Here are a couple of shots of the coil current on the Vallon detector with 0.1 ohm series resistor.

It appears there are both acceleration and deceleration mechanism involved. There are steep, but near-linear current slopes on both sides. Interesting.

[mickstv]

Hi Eric, the picture below is what I got when I built my CCPI TX.

Top waveform current pulse measured across 0.1 ohm resistor.

[Ferric Toes]

Here are a couple of shots of the coil current on the Vallon detector with 0.1 ohm series resistor.

Bipolar pulses.

Negative pulse expanded.

[bbsailor]

Eric,

I want to thank you for sharing your technical knowledge on various forums over the years. It was your forum that taught me about PI detectors and motivated me to write my "Making a Fast PI Mono Coil" article with the encouragement of Carl who runs this forum. It is good to see you exploring different PI designs again.

Thanks

Joseph J. Rogowski

[Ferric Toes]

The Tektronix 2445A does have a trigger hold-off function, so that's a possible solution.

Thank you for those suggesting the HOLD OFF control, it works great. Now I can see some meaningful waveforms over positive and negative TX pulses. This is also confirmed in the manual for the scope, which is something that usually gets read as a last resort.
There is no indication so far that it is anything but pure PI. If I hold a chunk of steel close to the coil, I can even see the normal decay. Now to build a diode clipping network to get rid of the flyback transients and see what is going on at lower levels.

[nickel_n]

I take it easy now and don't look for work where I am under any pressure. When you reach 77, the years go by much quicker and sometimes I will go under the radar for a while doing different things and then something fires up the interest in hobby detectors with slightly different technology and I'll be back, like now.
Eric.

I would like to take the opportunity and say THANK YOU Erick

For all that your work offer to people like me over the years.

Thanks again

[ivconic]

Again; an inductive coupling is the way to go.
I would make same diameter, same inductance and same resistance copy coil and hook it on scope.
Than place in same plane as original coil and perform measurements.
Most probably it will need copy coil to move relatively to original slightly offset, but it is easy observable on scope display.
That's for measurements when you don't want to mix up into original circuit nor affect origin signal.
...
Ferric.. 77 is just number, meaning nothing.
Keep up the good spirit and you'll easily see many more numbers.