Design Challenge

[rickb]

Which of those 29 threads are you referring to?

[Sean_Goddard]

It had the unique property that when GB was enabled, the detector got more depth.

Spoke to Dave Emery this week about just that very property. The pulse devil depth INCREASES when there is ferrous near the target and when the GEB is turned on.

Also US10,181,720 B1. I have been using this method in my PI design (with Daves permission) which is why I cannot go commercial with it. he refuses to grant license to use it for commercial designs even to me .

[eclipse]

Here's a link to Tinkerer's IB-PI thread

[Carl-NC]

Sure, there's just as much info during a PI transmit as there is during a VLF transmit. Including lots of ground signal. Quite a number of people, including here on Geotech, have explored this but either got nowhere with it, or they did and kept their mouth shut. At White's I figured out how to combine the TX-time response with the flyback response to cancel ground and discriminate. It had the unique property that when GB was enabled, the detector got more depth.

[Sean_Goddard]

I assumed so. You can't discriminate without a reactive component.

Definitely on the right track there Carl, I suspect Qiaozhi is too. It's one of those "OH FFS" moments when you realise there is a LOT more information in the Tx signal of a PI than was previously realised, and you wonder why no one spotted this before. I had a major face-palm moment when, whilst trying to achieve something entirely different, quite by accident I stumbled across this method.

I was trying to gate the Tx pulse to give maximum power with any coil fitted, but when I adjusted some timings, the steel washer I was using was no longer picked up but when I reached across to turn the machine off, my wedding ring gave a signal. I thought "No way", then tried some other targets and sure enough my suspicions about what was happening were confirmed. It's not perfected and the circuit is on 3 breadboards atm so I'm going to try to reduce it down. Dave Emery says the PD only has 4 op-amps in it and I'm trying to get down to less that the dozen or so I have atm.

[Skippy]

"The BLUE signal is the pulse that generates the slope"
So you know when the slope will occur. So, as per my previous suggestion, start a negatively sloping waveform generator at the correct rate, sum it with the actual rising slope, and you have a near-flat waveform. Use a later trigger to see if the 'flat' voltage is the same near the end of the ramp-time as it was at the beginning. Maybe a sample-hold is needed at the beginning, to compare against?

[Carl-NC]

One thing I WILL give away is that the disc method works ALSO uses information from the RISE portion of the Tx waveform.

I assumed so. You can't discriminate without a reactive component. I did a lot of work on this at White's, but the TDI doesn't use it.

The jitter is likely due to the ramp waveform itself, not the detection circuit.

[Sean_Goddard]

Ah I see where some have got the wrong end of the barge pole .

The BLUE signal is the pulse that generates the slope via the magic circuit, the GREEN (apt really) trace is the actual slope.

Logic "1" when the signal is rising then falling to logic "0" when the signal levels off. The logic signal is the derivative of the green analogue signal. What I need to do is use the signal to gate off the green signal or the discriminate part "folds back" on itself and I get a disc range from nail, though to pull tabs, then instead of rejecting everything up to silver coins, the disc somehow flip and accepts these "bad" targets and rejects the good ones which is, as you can imagine, not very useful.

Regarding the start level of the slop, yes it is around 25mV, this is due to the blanking circuit further up not reacting to the Tx pulse (+200V) no acting fast enough (plus a little shoot through) but, funnily enough, this gives a better nail knockout capability. I set the blanking to kill this and it becomes an all metal machine, with this small step it will reject nails as Disc zero.

As stated, I suspect my design is very similar to the TDi even though I have not seen one of those machine in the flesh or had chance to examine one.

One thing I WILL give away is that the disc method works ALSO uses information from the RISE portion of the Tx waveform. The wanted info is VERY subtle, but it's definitely there. It is how you process the decay signal in conjunction with this information which gives the discriminate signal. That is as much as I'm letting on . I believe the Pulse devil uses a VERY similar system, and that is one machine I HAVE played with (yes, it really does exist).

[green]

Initial slope approximate 15mv/us. Lot less before levels of. What slope is levels of? 1mv/us, 100uv/us, 10uv/us? What changes with target change, 710mv amplitude, time to level off or both?

[Qiaozhi]

Now I'm a bit confused.

In your initial post you said: 'What I am trying generate a logic "1" for the rise period, but as soon as the signal levels off, I want a logic "0"., but then later on you've posted a diagram of your desired waveform which shows the generated output staying at logic "1" when the signal levels off.

Which is correct?

[Sean_Goddard]

Messing around in between call outs at work, I came up with the following;



It's NOT the most elegant solution, in fact it is cruder than Bernard Manning, but IT WORKS, it shouldn't but it does! OK, it has 6us delay and quite a bit of "follow-on" on the tail edge of the rise period. The other problem is the terrible jitter which, so far, I have not traced the source of (suggestions as to source / remedy please).

I'm going to play with some monostables as I think I have a cunning plan to use the RESET input to cut short the pulse and eliminate a lot of the tail edge jitter as I might try synchronising signals a la FPGA techniques to ensure all inputs are timed correctly. I'll have to think a bit more about how I can do this.

[Sean_Goddard]

When it flattens out. At that point, the magic with the other part of the circuit times (not mathematically, but spatially) another signal then, based on that time difference, these are the "X" and "Y" of the signal decay, a pot across the two provides virtually full range discrimination, from nail though to silver coins. The best part is that the sample time is <8us so this thing is insanely sensitive to the small gold flecks I'm looking for.

[green]

Thanks Green!!

For the rest of you, here are the waveforms (truncated as there is "secret" info in some parts)

[ATTACH]45060[/ATTACH]

[ATTACH]45061[/ATTACH]

For Green: Jitter is NOT a problem as I can average this out. I'd rather NOT have any delay, but the less the better. If predictable (repeatable) I can compensate in later stages by using subtraction from the DC voltage in later stages. Generally, the lower all "errors" the better, but this world isn't perfect ...YET.

Looking at Waveform 11. Ramp doesn't start at zero volts, maybe 25mv in zero time. 100 to 200mv in 7us(14000v/sec). 500 to 600mv in 10us(10000v/s). 600 to 710mv slope changes a lot. Are you looking for certain slope change or when it flattens out?

[Sean_Goddard]

Thanks Green!!

For the rest of you, here are the waveforms (truncated as there is "secret" info in some parts)





For Green: Jitter is NOT a problem as I can average this out. I'd rather NOT have any delay, but the less the better. If predictable (repeatable) I can compensate in later stages by using subtraction from the DC voltage in later stages. Generally, the lower all "errors" the better, but this world isn't perfect ...YET.

[green]

I am struggling to get a circuit to work despite all the calculations showing it SHOULD, so....Here is your mission;

I have a signal with an dv/dt of 15mV/us for 56us then the signal levels off at 710mV final value.

What I am trying generate a logic "1" for the rise period, but as soon as the signal levels off, I want a logic "0". Now I know a differentiator & schmitt trigger should do the trick, but I simply cannot get the circuit to work.

I have spotted something in the Rx waveform of a PI which gives some VERY consistent results and using some magic circuitry processing, I get this sloping output. By processing the slope in the manner above, I want to gate another signal which will provide REAL discrimination (yup Dave Emery isn't the only one who has cracked this at last). The only thing is that I can't post any schematics of the PREVIOUS part of the circuit which generates the slope, and I do not intend to patent this idea as there will be only TWO units made (one for me and one for a friend).

The theory all works out according to the simulations and calculations and I have but this one stumbling block so any ideas people? NO, why don't you try it a different way is NOT the solution, this is the ONLY way (unless I use a micro which I don't want to do for noise reasons).

Differentiator in spice. Set output=1.5v with 15mv/us in. Had to change time from 56us to 47.3us to get 710mv with 15mv/us in. Added sine input to show effect of noise in. Needed to add R2 and C1 to help filter noise. Causes a delay, how much delay is allowed? How much jitter is allowed? How much noise does your signal in have?