field test unit no 001 "model T"

[moodz]

Hi Paul,

ground balance gets difficult, when the residual voltage is swinging at the coils self resonance frequency. It (frequency) will be changed due to ground conditions (coils inductance changes slightly).
It needs some refinement of the damping I think.
Aziz

Aziz you are right ... I can adjust the control loop to totally "neutralise" the "ringing" ... in fact there are twe parameters that can be adjusted to compensate .... the appropriate mathematical analysis is more Laplace / Heaviside step function than the critical damping analysis usually used.

Paul.

[moodz]

So is this the signal prior to the preamp?

Carl ... yes it is and we are on the 5 volts per division here so if I just applied amplification it would rail the amp. I let the circuit ring a little as it quite close to the optimally damped condition here but the ringing lets me know that I am not clamping the coil which can happen if one of the protective diodes starts conducting for example. (The circuit will still work without the protectives .. but would probably detonate if someone replugged a coil while the detector was operating)
If the coil clamps you appear to get a good damped condition but the circuit goes 'deaf' till the clamping action releases ... you have probably seen similiar behaviour yourself. The first dip in the signal at the second marker in the first picture posted above is the point at which the new damping method "releases" so the coil is open circuit after that point and the ring starts ... however the release point for the new method can be adjusted so that the coil does not ring at all post release by substituting a component...
Even though the circuit is relatively simple ... you will not kick yourself when you see it ... as I dont think it was an obvious solution ... took me long enough to find it.
I got these results by adding about eight main components to your standard PI frontend circuit ... a manufacturer could probably source them for 1 or 2 dollars. None of them are adjustable ... the circuit adjusts itself ... which is bee-utiful.
Paul.

[moodz]

back to sim city ....

the perfectly "equalised" PI pulse ....

... this is a sim but my real circuit can actually do this

...the maths that everyone has been using ( assuming ) for decades is wrong

[hdphilip]

no problem:

blue trace current thru coil

green trace across the in4148's

moodz ,

this line at the patent office is pretty long today, oh wait, their opening up another window...........


the "team"

[moodz]

no problem:

blue trace current thru coil

green trace across the in4148's

moodz ,

this line at the patent office is pretty long today, oh wait, their opening up another window...........


the "team"

hmm ... your coil current is not zero post decay ... make it ring or you cant prove the coil is unclamped and responding only to target fields.

nice snubber waveform

moodz.

[hdphilip]

hummm, ok

i had to adjust a few "things"

would be fun to revisit the differential front end with this,

[moodz]

hummm, ok

i had to adjust a few "things"

would be fun to revisit the differential front end with this,

OMG ....quick get down to the patent office before everyone discovers you can ground balance using the photoshop control ....


errr ... who's cat and bag is this ??

[chudster]

That is a bit defeatist ... there is always something to improve .. mostly because people look past the obvious stuff ...In PI technology everyone has assumed that the critical damping equation is the optimal solution and then proceeded to improve everything past the coil and damping resistor ...I offer some evidence below to show that if you want to make a chain stronger you have to look at the first link as well as the ones in the middle...

...I have used a minelab 18 inch coil which on a good day might get 8 microseconds to first sample .... the results below show that it can be done in 4 microseconds with very large target response.
The autodamping is functioning but the signal is not amplified ... note the signal response for a non ferrous target at the vertical marker ... I wont bother publishing a ferrous target

.....see it does work in real circuits .... not just sims.

The circuit can be used on any existing PI design IMHO. No manufacturing issues or part sourcing and it "tunes" the PI coil to have a perfect response so you coil hackers will have a wider window to play in with regard to coil parameters like inductance and resistance.

[ATTACH]20994[/ATTACH]
[ATTACH]20995[/ATTACH]

Firstly Moodz, fantastic work.


Got to jump in here though. I have scoured these forums looking for the magic trace that shows the target response from eddy currents. Can someone please point it out in theses traces?

I assume trace 1 is with the non ferrous target and trace 2 with air.

Why should I not simply interpret that the coil has higher inductance with a target meaning more energy is stored (yes eddy currents could be part of that) leading to a longer flyback time (to get the energy out)? Instead I keep hearing (the myth?) that there is this "magic" signal detected after flyback ends that no one has shown me in a sim or real life. To me it appears the two signals are more or less delayed or advanced flyback of each other due to energy/ inductance differences. We shorten and shorten the flyback and it does not seem to leap out.

Can someone please help me see what I am missing or has the Emperor got no clothes?

Chudster

[hdphilip]

chudster

those pictures don't actually show the "decay" as in the perfect sense of an example, due to moodz damping, but as you can see in the second picture, that increased "bump" at the marker would have been a "lost detection" if the flyback pulse was a microsecond longer.

so the importance of flyback control is very real,


moodz, photoshop..... i wish i could draw a straight line in photo shop

the metal detector line at patent office sure has been long lately


philip

[moodz]

Firstly Moodz, fantastic work.


Got to jump in here though. I have scoured these forums looking for the magic trace that shows the target response from eddy currents. Can someone please point it out in theses traces?

I assume trace 1 is with the non ferrous target and trace 2 with air.

Why should I not simply interpret that the coil has higher inductance with a target meaning more energy is stored (yes eddy currents could be part of that) leading to a longer flyback time (to get the energy out)? Instead I keep hearing (the myth?) that there is this "magic" signal detected after flyback ends that no one has shown me in a sim or real life. To me it appears the two signals are more or less delayed or advanced flyback of each other due to energy/ inductance differences. We shorten and shorten the flyback and it does not seem to leap out.

Can someone please help me see what I am missing or has the Emperor got no clothes?

Chudster

Aziz might be able to assist you with a sim that shows target response .....

[chudster]

Thanks for your reply and input. Maybe I am still not seeing it but 1 just pretty much looks like a longer flyback time due to more energy to get out than 2.

Hdphilip, I hear what you are saying but the bumps I took to be Moodz' explanation that he has let it ring a little. However if that is the feature needed to be measured then at 500KHz fundamental sampling need be better than 1MSPS if sampling before demod.

Not to take away from Moodz' amazing achievements to terminate and shape flyback leaving essentially an open circuit coil without a resistive damped signal (which I am sure will revolutionise detectors because they do work already and it should make them work better), but I've read a lot of theory that says its there somewhere. That the time constant for non ferrous materials is short so they decay and show up just after flyback (provided its gotten to quickly enough and current is terminated fast enough) but I am still looking. If we could get flyback down to 2uS or 1uS would it show up? Can anyone point out that magic target signal in terms other than as longer/ shorter flyback less/more energy?

By the way I found this little reef known as "Lasseter's" not too far from my house http://en.wikipedia.org/wiki/Lasseter's_Reef . I'll share its location if someone can show me the magic eddy current blip If we don't know the signal we are searching for, it is a solution that needs no detector.

With hdphilip replied, Aziz? Moodz? Carl? Other gurus? Anyone? Can you help? Can anyone post that trace showing a discrete post flyback signal for us to hunt?

Chudster

[Carl-NC]

The ideal target eddy response has the form t*e^(-t/tau). You can easily see it in a real detector if you use Labview to subtract the instantaneous response from a held response. Early sampling only helps for fast-tau targets like tiny nuggets. Sampling at 5us does nothing for a 50us target that 10us sampling can't do. In fact, it could hurt.

[chudster]

The ideal target eddy response has the form t*e^(-t/tau). You can easily see it in a real detector if you use Labview to subtract the instantaneous response from a held response. Early sampling only helps for fast-tau targets like tiny nuggets. Sampling at 5us does nothing for a 50us target that 10us sampling can't do. In fact, it could hurt.

Carl,

Thanks for the reply. I believe you are saying that if we took Moodz two waveforms and subtracted then we would see an impulse like blip. I can visualise that this is so.

Assuming we are chasing small gold, doing a thought experiment, if we could terminate flyback within 100nS or less would we see a discrete blip for the eddy current because it seems not to be so in practice as we reduce flyback time so far?

Alternatively with a large ferrous target we terminate flyback quickly have some microseconds with no signal then bang we get the impulse. In other words the timing is not related to a physical time constant referenced to current termination. Its all bound up together as we remove energy.

The reason that I say this might be important since the metal detection problem could be turned into an energy timing problem. What energy went in when and where and what, where, when did it come out. Conservation of energy provides the basis for analysis. That is quite a different problem to a radar like approach where we excite/ reflect off targets and look for a signal later.

In consideration it seems we are removing as much electromagnetic energy as fast as possible so that the target energy can be measured as it is additive and more apparent as we take away excitation but always returns on the coat tails of removing the excitation energy so there is no discrete eddy current event. The reason it returns on the coat tails is that as we remove the current we also remove the excitation to the targets and they immediately start losing their energy according to their "time constant". If we remove energy too slowly we never see small gold which is why we aim to remove it fast.

Sorry if its old ground and understood by all but I have scoured the forums and not really been able to understand.

Am I making sense?

Chudster

[Aziz]

Aziz you are right ... I can adjust the control loop to totally "neutralise" the "ringing" ... in fact there are twe parameters that can be adjusted to compensate .... the appropriate mathematical analysis is more Laplace / Heaviside step function than the critical damping analysis usually used.

Paul.

Hi Paul,

do you convert the coil energy into heat or do you recycle it back?

I think you are using more or less a similar concept of my "interrupted TEM transmitter" with switched capacitor using the zero-current-switch.

Anyway, I'm interested what you have found and whether it is novel. But the most important thing is, that's not from Candy!!

Aziz

[Aziz]

BTW,

getting rid of the damping resistor across the coil does not help much to increase the target signal strength. The coil impedance is much much lower than the damping resistor and the signal won't be significantly attenuated.
Lets make a simple comparison:
Coils impedance: 1 Ohm
Damping resistor: 500 Ohm
See the relation of the impedances!!

Aziz