field test unit no 001 "model T"

[hdphilip]

Philip, you're combining the target tau with the coil inductance. You either need to combine the target tau and target L, or the coil tau and coil L.

ok i get it, thanks carl

Philip

[moodz]

Energy conservation .....

It's all a matter of balance ..... You want to store as much energy in the target as you can then stop storing and start measuring as fast as you can ... However if you are storing energy still ... No matter how small your measurement will be reduced. Radar is a very bad analogy for PI ... It implies that you send then wait for a recieve .... The target is never invisible to the coil not during transmit , flyback or immediately after in recieve. With radar until you get a reflection the target is invisible.
Moodz

[chudster]

So guys,

has the penny dropped?

Another penny to drop:
What is the source for such a high "target" response?

Claim: A big portion of the response energy comes from the coil itself.

Aziz

If you read my analysis of the waveforms (which may be right or wrong but no one has bettered publicly or commented on yet) the response without target shows some small decay presumed from the coil residual current itself. All other things being equal, for the waveform with a ferrous target it is assumed that the extra response comes from that target (putting some reliance on Moodz' circuit without seeing it). It may be that Moodz just has built an inductance meter and the extra signal has nothing to do with eddy currents but rather a change of inductance due to whacking an iron core into the coil. It may come from cosmic rays or some weird influence of little green men who are messing with our heads. But I have no evidence- especially of the latter two.

As Warren Buffet once paraphrased: "In God we Trust, all others need evidence."

I'd love to be more sure to make claims but without having seen Moodz' circuit I have to confess to ignorance. I have some trust that he has been looking at this for a (just a little while ) while and that the waveforms he has posted are indicative of detecting a target. Wonderful thing about ignorance though is that (as any guru will tell you), admission of ignorance is the path to wisdom.

So, no Aziz. No penny has dropped. I am still ignorant and excited about enlightenment.

I see your claim though and want to know more. Could you please give us your explanation of Moodz' waveforms?

Chudster

[Carl-NC]

Philip, you're combining the target tau with the coil inductance. You either need to combine the target tau and target L, or the coil tau and coil L.

[hdphilip]

hummm, ok

so what your saying is XL=2piFL isn't a real formula as i apply it to a mono search head coil in present of a target?

i'm not saying i'm correct, i just thought i'd throw it out for discussion,

I'll have to try it on the circuit i'm working on to see if their's an effect

Philip

[moodz]

here's how i look at it

XL = 2piFL

so a coil that 330uh and a 5 us tc

XL = 2piFL
= 2 x 3.141592653 x 200 kHz x 330 uH
= 415 ohms (to 3 significant digits)
= 415 ohms

or with a 10us tc
= 2 x 3.141592653 x 100 kHz x 330 uH
= 207 ohms (to 3 significant digits)
= 207 ohms

so the reactance (coil impedence) changes with the target time constant.

Philip

.....are you sure ? I think you are forgetting about mutual inductance between coil and target which is usually very small or very very very small or teensy weeny .... So the coil impedance changes bugger all for most real target situations .... Clue 6 ... My method would not work if coil impedance changed the way you describe

[Aziz]

So guys,

has the penny dropped?

Another penny to drop:
What is the source for such a high "target" response?

Claim: A big portion of the response energy comes from the coil itself.

Aziz

[hdphilip]

here's how i look at it

XL = 2piFL

so a coil that 330uh and a 5 us tc

XL = 2piFL
= 2 x 3.141592653 x 200 kHz x 330 uH
= 415 ohms (to 3 significant digits)
= 415 ohms

or with a 10us tc
= 2 x 3.141592653 x 100 kHz x 330 uH
= 207 ohms (to 3 significant digits)
= 207 ohms

so the reactance (coil impedence) changes with the target time constant.

Philip

[chudster]

Hmmmm what does a constant current source do if you remove the "load" suddenly.
PS ... I know someone who was nearly killed by a cable power system when this happened.

Fortunately it is not a constant current source. Its a short 3us pulse at 1nA. I'd happily put my tongue on it as I used to do as a child to test batteries.... Maybe that explains a lot.

Chudster

[chudster]

Hi Chudster,

you are still wrong. Change your current source into a voltage source and look at it again. Why is the coil a voltage source and not a current source?

Aziz

As stated I have no problem with being wrong as I expect to learn something and I am also not claiming that you are wrong. What I am claiming is there is interesting stuff to be understood.

From my limited understanding of target modeling it was natural to model an induced current in an inductor for an eddy current magnetic decay as a small current source injection. It approximates (more simply) the coupled inductor models that I have seen and would love to apply if I had some more background and understanding why they are accurate- but my jury is out. I note though that whether current or voltage sources are chosen to model, circuit impedances translate between voltage and current in the coil anyway so all other things being equal a 500 ohm vs 1 Mohm damping impedance is the critical change. I'd be happy to see your topology to model this with a voltage source.

You have said: that Moodz' claim that no damping resistor leads to little signal improvement.

getting rid of the damping resistor across the coil does not help much to increase the target signal strength.

There is a lot of expectation of the improvements that Moodz has made. If we are to embrace his innovations we will need to appreciate the benefits achieved. If it is sixty times (as simmed) I'll hope to build circuits without a damping resistor as it pulls the signal out of the noise floor that everyone struggles with. If its a 1% improvement then maybe not.

As I said before if we differ it is probably due to assumptions on different topologies or other assumptions.

My calc from a sim (assuming circuit impedances are equivalent to 1 megaohm) is that it is worth about sixty times improvement in target signal. His posts of actual waveforms (with a long time constant target) seem to show a good signal without amplification.

Further, if my sims are right it also shows that the target signal will hang around for longer (see the long tail on the 1 Mohm sim vs 500 ohms) which would help with ADC sampling of the signal.

Maybe the answer is that other circuit impedances are much lower than 1 Mohm?

Given that there is at least common ground that there will be some increase in target signal strength, what figure can you quantify and put to that increase?

Chudster

[moodz]

Thanks a heap for this Moodz. It starts to get to the questions I have been hoping to answer.

So my interpretation:

(1)The initial undershoot is not so much a target feature is a control artifact (no criticism, its a fast circuit and I am amazed it settles that quickly) to get the circuit to a "critical damping" point. I think you said before that you "release" near that point.

This is the quick and dirty circuit ... It can be made to be near perfect.

(2) In the trace with no target, there is still a tiny bit of current in the coil so from t= +2uS, we see a (small) exponential decay of the type we would expect from an undamped inductive decay. You can see its a little noisy.

This is a mono coil. It is a "self target" as well. .. There will always be some residual energy from the self target and parasitic caps. For noise just substitute "radio antenna" for coil.

OK so now I think I see and can explain the target response (maybe).

(3) With the Fe target in range. We pull out the electrical energy and the "eddy current" response now manifests as a higher and larger decay current starting at t=+2uS. If you could remove more undershoot I could imagine Carl's impulse blip.

It's a non- fe target .... I don't bother with fe targets.

What still doesn't jive is that this is a large Fe target. If the theory was right its time coinstant should be large and I would have expected its peak would have bee more to the right.

See above


HOWEVER now we see some use in simulation as if you look at my simulation above you can see that the nice symmetrical current pulse gets skewed to a long trailing tail if there is no damping resistor.

BUT what still does not sit is that this target presumably (by the theory) would have a time constant more than shown or am I wrong?

OPEN is that if we could drop the flyback time more, would we see more of an impulse (t.e^)-t/tau) signal per Carl. It appears that t=0 is the FET turn off point/ start or coil current decay.

Moodz, thanks a heap for taking the time to put this together. Its getting closer to answers (well at least for me- maybe the gurus know this already).

How do others interpret Moodz waveforms vs the theory? Do I have it right?

My 2 cents is that you just concentrate on damping ASAP without sapping target energy an don't worry about target tau as there are very complex surface current and bulk magnetic effects that are almost impossible to model. Eg take 10 grams of gold and form it into a ring, a rod, a disc, a sheet, a wire, a random blob etc etc ect ad infinitum ...... They will all have diff target responses but still just 10 grams of gold.

Moodz

[moodz]

Hi Chudster,

you are still wrong. Change your current source into a voltage source and look at it again. Why is the coil a voltage source and not a current source?

Aziz

Hmmmm what does a constant current source do if you remove the "load" suddenly.
PS ... I know someone who was nearly killed by a cable power system when this happened.

[Aziz]

Hi Chudster,

you are still wrong. Change your current source into a voltage source and look at it again. Why is the coil a voltage source and not a current source?

Aziz

[Tinkerer]

Tinkerer,

Thanks for your reply. I am to start shortly to build a design and I will look at the circuit as it sounds impressive. Before I build a circuit though there are basic engineering steps. These are:
1) What is the characteristic signal that I am to process?
2) What other signal/ noise is it embedded in and how do I extract it?
3) How may it be characterised so that I can build strategies and algorithms capture its salient features in hardware and then to accurately represent that digitally?
4) Model (your model is useful here but steps 1,2 & 3 are best completed first)
5) Design
6) Build and test.
7) Iterate 4-6 and revisit 1,2,3

So I am at stage 1 to 2. Present best understanding is that the signal is an impulse function embedded in the flyback signal seemingly at the end of flyback decay (if flyback was quick enough for the target). If it wasn't quick enough then its info was lost when the signal was clipping at kilovolts or if it was a slow target hopefully I can see it.

That is what I see is likely from Moodz' waveforms posted here which by reputation are pretty much state of the art and from my extensive reading of the forums.

So it appears its not like radar or sonar where we ping, measure delay and then look for the reflected signal using techniques to pull a small signal out of noise but this is basically how I see it being talked about in the forums. I am wondering if its just lore and everyone else can see the Emperor's clothes that I cannot see.

To my knowledge, while it may match the theory, no one has posted a scope trace of the discrete "eddy current collapse" signal that we seek and it does not seem to be leaping out even as flyback time is shortened. I have seen it in simulations but that is step 4. As such no disclosure of Lasseter's reef location yet . Lasseter himself was sure that it was there and lots of people went along with his thinking but its never been found because they all looked in the wrong place out in the desert. Its actually near my house .

To be devil's advocate, existing designs are detecting something and they are successful. Moodz will disclose his breakthrough in due course. It may well be the end of PI improvement as we know it or TINKERERS_SB_MO THE MOTION VERSION might be the last word.

Call me an "old fashioned engineer" but before I build a signal processing circuit I'd like to be sure of the signal I am looking for. Alternatively I will just need to build something and find out for myself (because I am speaking from practical ignorance) and clearly need to suffer multiple soldering burns like the gurus have to attain my enlightenment and right of passage from a junior. I can do that but was hoping to poll (and challenge) the gurus before that was done hence the posts.

Hope this is on topic but it is Moodz' work that has got me thinking.

Chudster

Chudster,

Call me an "old fashioned engineer" but before I build a signal processing circuit I'd like to be sure of the signal I am looking for.

attached is a simulation of the IB version of the signal that you are looking for.

The yellow trace is the TX wave form as seen by the RX coil. No target.

The blue trace is the signal response of a 1us size gold nugget.

The red trace is the target response of a 10us size FE target.

Note 1: the target responses have been scaled vertically to enhance their characteristic, but their horizontal scale is correct.

Note 2: the yellow no target wave form changes with the presence of the ground response as well as all other environmental influences, so we always look at the delta or difference.

These simulations are as close to the real thing as I have come so far. I can also show you the oscilloscope pictures of the same wave forms, but the real wave forms are filtered and changed somewhat by the processing.

Once you understand the simulations, you can easily recognize the target signal on the real wave form.

On the top of the picture, you see the mathematical formula of the processing of each trace.

I do the processing in analog but I would love to do it in digital if I knew how.

Tinkerer

[chudster]

...The Chudster is asking the right questions but not all the questions that need to be asked yet ...I cannot say if the answers he is getting are right or wrong at this point in time.

I have prepared a circuit that pretty much is the production version .... see below shots from the QED front end .... the coil is an 18 inch 0.4 ohm 300u Minelab type.

Below is the damped UN-AMPLIFIED coil signal ...( ie what the first amp sees ). The baseline is ground. The flyback is damped in around 4 us ... 6 us if you are a pessimist LOL. The target responses are excellent IMHO for a PI.

This is the simplest and lowest cost version of the circuit ... the DSP version is much better ..but not off the testbench yet.

[ATTACH]21018[/ATTACH]



Now we zoom up the CRO gain to 500mv / div so we can see a little more detail ....

[ATTACH]21019[/ATTACH]

Now we add a largish non ferrous target .... remember ... this not amplified yet
verrry niiice.

[ATTACH]21020[/ATTACH]

Now we superimpose the non target and target response ..... I was holding the target slightly closer to the coil so the response is a little larger ... LOL.

[ATTACH]21021[/ATTACH]


moodz

Thanks a heap for this Moodz. It starts to get to the questions I have been hoping to answer.

So my interpretation:

(1)The initial undershoot is not so much a target feature is a control artifact (no criticism, its a fast circuit and I am amazed it settles that quickly) to get the circuit to a "critical damping" point. I think you said before that you "release" near that point.

(2) In the trace with no target, there is still a tiny bit of current in the coil so from t= +2uS, we see a (small) exponential decay of the type we would expect from an undamped inductive decay. You can see its a little noisy.

OK so now I think I see and can explain the target response (maybe).

(3) With the Fe target in range. We pull out the electrical energy and the "eddy current" response now manifests as a higher and larger decay current starting at t=+2uS. If you could remove more undershoot I could imagine Carl's impulse blip.

What still doesn't jive is that this is a large Fe target. If the theory was right its time coinstant should be large and I would have expected its peak would have bee more to the right.

HOWEVER now we see some use in simulation as if you look at my simulation above you can see that the nice symmetrical current pulse gets skewed to a long trailing tail if there is no damping resistor.

BUT what still does not sit is that this target presumably (by the theory) would have a time constant more than shown or am I wrong?

OPEN is that if we could drop the flyback time more, would we see more of an impulse (t.e^)-t/tau) signal per Carl. It appears that t=0 is the FET turn off point/ start or coil current decay.

Moodz, thanks a heap for taking the time to put this together. Its getting closer to answers (well at least for me- maybe the gurus know this already).

How do others interpret Moodz waveforms vs the theory? Do I have it right?

Chudster