PI Power Output

[simonbaker]

Hi B^C,

Well, I tested it hundreds of times. I know what the results are. However, every time I open my mouth about it, I am being told that it is against the laws of physics.
Now, I am old enough to have learned a lot of things at school that have been proved different since then. I am also old enough to be suspected of senility, so I would really like for others to try and to confirm if what I say is right or to convince me to check in to the "Friendly Home"

All the best

Tinkerer

It would be really useful to state again precisely exactly what your theory or theories are and what any disagreeing theories are. It gets hard to follow this because we all use different language and analogies -- there might be more agreement than we think except our words make it seem different.

My feeling is that if we had a really decent math model of a target, we should be able to answer a lot of these interesting questions. I think the coil magnetic field is pretty well understood because we can measure the voltage and current fairly well and it's pretty linear. But the target is harder to measure.

First of all, is a target "linear" or non-linear? Is skin effect non-linear? I ask the question because non-linear systems are really hard to get a handle on -- hitting them hard is different from hitting them soft, they don't scale.

But my basic question is: can't we treat this discussion like any other electronic circuit -- write a differential equation for the response to a stimulus of our choice and run it out? We throw around ideas like "saturation" etc., but these should show up in our equations.

If targets are "linear" in terms of response, then a lot of stuff should be well understood. For instance, response to an impulse (PI TX pulse) and to a sine wave (VLF TX signal ) are completely related - you know one, you know the other (of course you need all sine waves...). And it's no big deal to take a specific pulse shape and run it through a computer program and see the response and answer the question of how best to sample the PI response.

So it seems like we need to get a really good model of some targets and get running some simulations. Aziz, are you available?

Now if it doesn't match real life, good. More work is need on the models. Once you can match reality, you can understand what's going on.

I suggest this because one man's saturation may be another man's steady state -- loose terms that are misleading unless we tie them to an equation.

I'm not saying stop talking with analogies, it's fun, I'll keep doing it too. But it seems we go in circles because we don't have a common language to nail down conclusions. Unfortunately, that language is mathematical models.

I'd like to start with the concept of "saturation" of targets, and ask what is really meant by that and what analogy is behind it?

Cheers,

-SB

[simonbaker]

I made a simple comparison between the time it takes for coils connected to IRF640 (200V breakdown) and IRF740 (400V breakdown) to discharge. The TX drive pulse width for each circuit was adjusted so that each coil had the same coil current 2.75A - within a few milliamps.

Now, everybody who reads here knows that I have no honest to god test equipment but do my work in a spice simulator. But, I am working on a machine or two. I compare my situation to the guy who wants a hamburger but is still growing the cow because the local burger joints don't serve Angus. (I'll try not to starve before the Angus is fully grown.)

Anyway, the IRF740 circuit took about 3us to discharge and the IRF640 took about 5us. From what the PI gurus tell us, a 40% increase in discharge speed will equate to more energy directed at the target.

That's too bad because I have about two dozen IRF640 on hand and only a few IRF740, but, facts are facts. I'm too old to waste time lugging around a PI detector running IRF640s now that I know darn well that the IRF740s should give considerably better performance, all other things being the same.

The main advantage I see in using IRF640 is that other components in the system don't need to be rated for the higher voltage, (and also a slight decrease in drive requirement), but I somehow don't think this is a good trade-off.

Perhaps it could be argued that my test is invalid because comparing IRF640 and IRF740 is like comparing apples and oranges, and all I can say to that is take it for what it's worth.

Hi Porkluvr: naive question - can you put two IRF640 in series to increase the breakdown voltage? If not, why?

Regards,

-SB

[simonbaker]

Thanks Carl,

so I am not crazy after all. What I have observed hundreds of times, is real.

Now, let's tackle the other persistent myth. Hey, I like the "Myth busters"

The myth says that the ON transient can not generate as much Eddy Currents as the OFF transient. Laws of physics are blamed for that. After all somebody or something has to be blamed for our mistakes no?

I keep insisting that this is a fallacy but my whining falls on deaf ears. (Didn't use the right language, dahh..)

So lets look at this explanation, does this make sense?

Hmm maybe I should make a sketch, they say a picture is worth a thousand words...

Looking at the sketch,

a) represents the TX coil current with about 1.5 TC, at switch ON.
b) and c), from post #43, the fast and slow Flyback.

Now, look at the "area under the curve" which one is the largest? Ooops, small mistake here, will have to come back on that later.

d) is the TC current in black and the target response in red. There is a lag between the target response and the stimulation. As the di/dt diminishes, the stimulation of the target and its response diminish. This slowdown is a bit exaggerated on the sketch.

e) is the TX charge curve with a "flat top", like a about 6 TC. The response of the 2 different TC targets (red and green) raises with a time lag, relative to their TC. The lower TC target (red) has reached its peak earlier and as the di/dt diminishes, the eddy currents start decaying.
The longer TC target (green) lags more, the time is too short for it to reach its maximum eddy currents.

OK, this is the first try at drawing the picture. It is not all that good, but maybe it gets the idea across and somebody with better skill and knowledge could improve on it.

Tinkerer

Hi Tinkerer - I'd like to understand better your diagrams.

First, I'm not sure what you are demonstrating. Are you demonstrating that TX turn-on can have stronger target response than TX turn-off? Or are you illustrating two different TC targets' response to TX turn-on? I'm confused because graphs b and c seem to be about TX turn-off, but I don't see how they tie in. Would you explain b and c a little more?

Looking at graph e, are you showing that a longer TX pulse allows the slower target current to grow more and therefore maybe there is an advantage to the slow turn-on phase compared to fast turn-off?

I have a couple of thoughts about that, which if I were smarter maybe I could show with equations. The first thought is that when we detect a target, our detector coil responds not to the peak current of the target but rather to the peak rate-of-change of the target current (same physics as the target responding to the rate-of-change of the TX current). So really in graph e it is the fast up slope and fast down slope that makes the received "signal". I'm not sure how that affects your point, but may be relevant.

The second thought is: is what you are getting at like the concept of "tuning", where the target responds better to a particular wave shape than others? For instance, suppose our target is underdamped and would "ring" in response to a pulse. Then it would make sense to create an on-off pulse that turns on and off at the ringing frequency so as the magnetic field reversal kicks the target in both directions synchronously with it's natural oscillation. I could buy that, although it would mean only certain targets would benefit from a given pulse width. Perhaps there is an analog to underdamped targets as well.

Ok, enough for today.

Cheers,

-SB

[hobbes_lives]

Hi Porkluvr: naive question - can you put two IRF640 in series to increase the breakdown voltage? If not, why?

I considered a similar design some time ago using stacked IRF740's. I did find some promising circuits on the web, but none that would work properly driving a PI coil, at least not for me.

The difficult part is not exceeding the maximum gate-source voltage on one of the FETs during turn-off/flyback. I'm sure the problem can be overcome, but I did not put enough time into the design before I moved on to other things.

I think the gains would be great, including lower capacitance across the coil and a higher avalanche voltage, overall.

[Qiaozhi]

It would be really useful to state again precisely exactly what your theory or theories are and what any disagreeing theories are. It gets hard to follow this because we all use different language and analogies -- there might be more agreement than we think except our words make it seem different.

This is a very good point. Unless we are all singing from the same hymn book (so to speak) we'll find ourselves just going round in circles.

[Tinkerer]

How do you diffirentiate the different inductances that targets have,wont this alone give you different pivot points ?
As for the delay in signal growth this could be used to get an estimate of target size,larger targets would have slower growth rates and so more phase where as smaller targets with less inductance would have a quicker signal response and less phase.

Where is this pivot point measured,at the coil or at the output of an opamp,your pivot scenario has me confused,im a dodo with vlf.

Zed

I am setting up a test platform to generate a scope picture of the PIVOT point. It is at the output of the preamp.

I think it represents very similar characteristics as the phase shift in VLF.
A fast signal growth shifts the phase to the left and a slow signal growth shifts it to the right.

Tinkerer

[Tinkerer]

It would be really useful to state again precisely exactly what your theory or theories are and what any disagreeing theories are. It gets hard to follow this because we all use different language and analogies -- there might be more agreement than we think except our words make it seem different.

My feeling is that if we had a really decent math model of a target, we should be able to answer a lot of these interesting questions. I think the coil magnetic field is pretty well understood because we can measure the voltage and current fairly well and it's pretty linear. But the target is harder to measure.

First of all, is a target "linear" or non-linear? Is skin effect non-linear? I ask the question because non-linear systems are really hard to get a handle on -- hitting them hard is different from hitting them soft, they don't scale.

But my basic question is: can't we treat this discussion like any other electronic circuit -- write a differential equation for the response to a stimulus of our choice and run it out? We throw around ideas like "saturation" etc., but these should show up in our equations.

If targets are "linear" in terms of response, then a lot of stuff should be well understood. For instance, response to an impulse (PI TX pulse) and to a sine wave (VLF TX signal ) are completely related - you know one, you know the other (of course you need all sine waves...). And it's no big deal to take a specific pulse shape and run it through a computer program and see the response and answer the question of how best to sample the PI response.

So it seems like we need to get a really good model of some targets and get running some simulations. Aziz, are you available?

Now if it doesn't match real life, good. More work is need on the models. Once you can match reality, you can understand what's going on.

I suggest this because one man's saturation may be another man's steady state -- loose terms that are misleading unless we tie them to an equation.

I'm not saying stop talking with analogies, it's fun, I'll keep doing it too. But it seems we go in circles because we don't have a common language to nail down conclusions. Unfortunately, that language is mathematical models.

I'd like to start with the concept of "saturation" of targets, and ask what is really meant by that and what analogy is behind it?

Cheers,

-SB

You are right. To be able to work together on the subject, we need to agree on the terms.
To start with "saturation", lets banish this word. The meaning is the moment of the maximum response or the maximum generated eddy currents.

Is skin effect linear? I don't know. We need to research and dig up the information.

There is a wealth of information available relating to VLF. This is the same information that is applicable for PI. we just have to see where it fits in.

Here is an example:

Maybe you would be so kind to word it better for me.

With VLF we have a sine wave with a certain frequency. At a certain frequency the Tau has a certain value.

With PI we have a transient that has a Tau with a certain value. The effect is the same. The method of reading is different.

Tinkerer

[Tinkerer]

Hi Tinkerer - I'd like to understand better your diagrams.

First, I'm not sure what you are demonstrating. Are you demonstrating that TX turn-on can have stronger target response than TX turn-off? Or are you illustrating two different TC targets' response to TX turn-on? I'm confused because graphs b and c seem to be about TX turn-off, but I don't see how they tie in. Would you explain b and c a little more?

Looking at graph e, are you showing that a longer TX pulse allows the slower target current to grow more and therefore maybe there is an advantage to the slow turn-on phase compared to fast turn-off?

I have a couple of thoughts about that, which if I were smarter maybe I could show with equations. The first thought is that when we detect a target, our detector coil responds not to the peak current of the target but rather to the peak rate-of-change of the target current (same physics as the target responding to the rate-of-change of the TX current). So really in graph e it is the fast up slope and fast down slope that makes the received "signal". I'm not sure how that affects your point, but may be relevant.

The second thought is: is what you are getting at like the concept of "tuning", where the target responds better to a particular wave shape than others? For instance, suppose our target is underdamped and would "ring" in response to a pulse. Then it would make sense to create an on-off pulse that turns on and off at the ringing frequency so as the magnetic field reversal kicks the target in both directions synchronously with it's natural oscillation. I could buy that, although it would mean only certain targets would benefit from a given pulse width. Perhaps there is an analog to underdamped targets as well.

Ok, enough for today.

Cheers,

-SB

For certain targets, we can read a higher signal amplitude response during TX on than after TX OFF.
Consider a target with a TC of 5uS. Sampling at the very moment of maximum response, during TX you can read this maximum value.
Sampling the same target 15uS after switch OFF, you will get at best 5% of its maximum response.

For graphs b) and c) please refer to post #43. There you can see how the response of the target lags behind the excitation.

The same happens with the ON transient. The response lags behind the excitation.
I agree that the strongest excitation happens with the moment of highest di/dt, but there is a time lag between the "charge " of the target. Commonly we represent the target with LCR. So we can expect an exponential charge curve.
The unknown factor is the values for LCR, but we have the known factors of di/dt and the total time of the exposure of the target to this factor.

What we are trying to do with discrimination is to calculate the values of the target's LCR.

My graph e) shows the signal generated by 2 different targets, during TX only.
Now, if you use both the responses generated by the ON transient and the OFF transient, you have the information that would be generated by a low frequency and a high frequency VLF.

About "target ringing" I have no information yet. During my experiments I often found this "invisible sine wave" and was wondering if this represented a ringing target. After all, the eddy currents expand throughout the target, but where do they go when they have reached the far end? are they reflected back? If yes, ringing could be possible. Just speculating.

Now I believe that the PIVOT as a part of the "invisible sine wave" is simply a different Tau.

Tinkerer

[Tinkerer]

This is a very good point. Unless we are all singing from the same hymn book (so to speak) we'll find ourselves just going round in circles.

I would be very happy if you would write the hymn book for us. My vocabulary is very restricted in English as well as in Electronics.
The Google translator helps, but some things always get lost in translation, so we end up comparing apples with beets.

Tinkerer

[B^C]

Gday Zed,

Thanks for the replies, things will be more understood if there is more conversation.
The problem i have is that there are NO Fets, no MD circuit at all, all this was avoided for the reasons you mentioned.
The Target reading device is 1mtr away from the inductor.

A bigger Target has no flat top as you put it on the signal but more peaks to a point & then decays but small targets do indeed have a flat top signal as mentioned, it does appear that they have reached there maximum.

Increases in voltage on the same smaller targets deliver the same signal results, but on larger targets there is a marked difference in the signal readings.

I get what your saying about saturation being the end etc, but this depends on how one looks at things i guess. Remember only yesterday when we all thought the world was flat?

I would explain more but i need a friendly response hahaha!

Any thought's would be appreciated.

[simonbaker]

You are right. To be able to work together on the subject, we need to agree on the terms.
To start with "saturation", lets banish this word. The meaning is the moment of the maximum response or the maximum generated eddy currents.

Is skin effect linear? I don't know. We need to research and dig up the information.

There is a wealth of information available relating to VLF. This is the same information that is applicable for PI. we just have to see where it fits in.

Here is an example:

Maybe you would be so kind to word it better for me.

With VLF we have a sine wave with a certain frequency. At a certain frequency the Tau has a certain value.

With PI we have a transient that has a Tau with a certain value. The effect is the same. The method of reading is different.

Tinkerer

Ok, Tau (time constant) is a good one. For example, just saying "Tau" implies we are typically talking about a "first order" linear system, because a second order system has two different time constants (unless they are identical). A first order system (if I haven't forgotten) is like an RC or RL circuit - it can't "ring"; a second order system is like an RLC circuit - it can ring, depending on the damping factor. First order systems can make low-pass or high-pass filters. Second order systems can make band-pass filters. Familiar stuff.

So basically Tau completely describes a first-order circuit, which is nice and simple. Now I wouldn't say with VLF detectors that the Tau has a certain value at a particular frequency because it really is just a property of the circuit. But the Tau will determine the phase shift at a particular frequency, which is probably what you are getting at.

So if we assume that different targets have different Taus, and we want to discriminate targets, then what we're really trying to do is estimate the Tau of our target by measuring something that Tau affects, like phase shift or the decay of the current in the target.

I'm not saying Tau really is a good way to discriminate targets, but we can try using it, and we can estimate it in various ways.

If we can model our targets as "linear" systems, we're home free because so much is available to predict exactly how linear systems respond to stimuli. There are the terms "forced response" and "natural response", and we really need to be aware of those concepts to discuss the pros and cons of using the TX turn-on versus the TX turn-off, because that's what were dealing with.

I mean it's really all there, the answers to how linear systems respond to any kind of stimulus. I'm just saying this because it seems we're beating around the bush about subjects that are extremely well known, such as whether a slower or faster forcing function will create this or that response etc.

The real question probably is just how to model a real target. For instance, is Aziz's linear RLC circuit realistic, or is something very important being left out.

Other than that, if we can assume the target is a first or second order linear system, then if anyone asks the question clearly enough, the answer should be available with a little calculation or LTSpice simulation. Getting those answers would be a good way to sharpen our intuition and get us all on the same page so to speak. I think Aziz made a couple of good stabs at it, we should keep pushing along those lines.

Regards,

-SB

[Tinkerer]

Ok, Tau (time constant) is a good one. For example, just saying "Tau" implies we are typically talking about a "first order" linear system, because a second order system has two different time constants (unless they are identical). A first order system (if I haven't forgotten) is like an RC or RL circuit - it can't "ring"; a second order system is like an RLC circuit - it can ring, depending on the damping factor. First order systems can make low-pass or high-pass filters. Second order systems can make band-pass filters. Familiar stuff.

So basically Tau completely describes a first-order circuit, which is nice and simple. Now I wouldn't say with VLF detectors that the Tau has a certain value at a particular frequency because it really is just a property of the circuit. But the Tau will determine the phase shift at a particular frequency, which is probably what you are getting at.

So if we assume that different targets have different Taus, and we want to discriminate targets, then what we're really trying to do is estimate the Tau of our target by measuring something that Tau affects, like phase shift or the decay of the current in the target.

I'm not saying Tau really is a good way to discriminate targets, but we can try using it, and we can estimate it in various ways.

If we can model our targets as "linear" systems, we're home free because so much is available to predict exactly how linear systems respond to stimuli. There are the terms "forced response" and "natural response", and we really need to be aware of those concepts to discuss the pros and cons of using the TX turn-on versus the TX turn-off, because that's what were dealing with.

I mean it's really all there, the answers to how linear systems respond to any kind of stimulus. I'm just saying this because it seems we're beating around the bush about subjects that are extremely well known, such as whether a slower or faster forcing function will create this or that response etc.

The real question probably is just how to model a real target. For instance, is Aziz's linear RLC circuit realistic, or is something very important being left out.

Other than that, if we can assume the target is a first or second order linear system, then if anyone asks the question clearly enough, the answer should be available with a little calculation or LTSpice simulation. Getting those answers would be a good way to sharpen our intuition and get us all on the same page so to speak. I think Aziz made a couple of good stabs at it, we should keep pushing along those lines.

Regards,

-SB

Simonbaker,

you lost me there.
I don't know how to represent a target for simulation. I only know how to find the target with a detector.
The Tau I was talking about is the Tau of the sine wave or the ON and OFF transients. Basically how steep the sine is. Like slew rate of an opamp.
Assuming, and I have no proof that this is so, a target with a TC of 5us would reach its peak eddy currents very quickly. A very steep charge curve. And it will discharge at the same rate.

Tinkerer

[simonbaker]

Simonbaker,

you lost me there.
I don't know how to represent a target for simulation. I only know how to find the target with a detector.
The Tau I was talking about is the Tau of the sine wave or the ON and OFF transients. Basically how steep the sine is. Like slew rate of an opamp.
Assuming, and I have no proof that this is so, a target with a TC of 5us would reach its peak eddy currents very quickly. A very steep charge curve. And it will discharge at the same rate.

Tinkerer

Right, I think we're talking about the same thing, it's in the equations but not everyone is familiar with the equations. Graphs work well. Seems like Aziz was making some headway with his graphs. I think if we keep making those graphs we'll at least get the answers for idealized targets on many of the questions.

Cheers,

-SB

[robby_h]

Hi Tinkerer,
I don't know who suggested we can sample earlier with a balanced coil arrangement but it is clearly wrong. We have to obey certain rules and one important rule is that we must delay sampling until after the TX coil's back emf spike has decayed. There is no way around this.

Your waveforms in post #30 are clearly IB, not pulse induction. You have actually built a high powered IB detector and also given a good example of why we can't sample earlier than at the tx coil when using a balanced rx coil in a pi design.

Your figures in the other thread are L= 325 uH, total R = 2.8 ohm, U = 12v, T = 41usec, therefore the coil current at switch off should be 12(1-exp(-41*2.8/325))/2.8 = 1.27 amps so with this current I think it's safe to assume the tx coil's spike will still be settling at the far right of your screen (~7 usecs) but your sample begins 2.5 usec after switch off and ends 5.2 usecs after switch off so you are looking at a portion of the spike's waveform using a balanced receive coil.
In post #59 on this thread, your conclusions are based on the spike's properties which will be almost identical for the 43 and 100usec pulses so your results are similar because you are sampling the same spike in both cases. Pulse induction would give different results.

The problem with sampling during the pulse on time (or the spike's on time) is that we have to deal with the reactive soil component whereas we don't during a true PI off period.

With pi, we aren't looking at the spike or how the target affects the spike's amplitude, decay, width or settling time, we are looking at the signal the target induces in the rx coil. The two might appear to blend together at the tx coil but they are separate. Some articles, papers and patents may say otherwise but they have it wrong, such as in this paper written by someone who supposedly has a PhD....

http://engnet.anu.edu.au/DEpeople/Sa...aching/TA5.pdf

Dave Emery's patent is also IB, not PI. He makes the spike resonate with a cappy and he samples the oscillations similar to vlf although he makes no mention of cancelling the ground R component or that it even exists.

Allan Westersten's patent US20080224704 is a mix of pi and IB but he doesn't include a means to separate the off-period small target trigger signal from the huge ground signals we experience here in the Oz gold fields.

B^C,
Your pictures appear to be what we would see with a balanced RX coil, which is just IB? The time base appears to have been changed in each picture along with the pulse train so it's hard to say what we are supposed to be looking at?

[robby_h]

This picture was originally drawn by Eric Foster and posted on the PI forum. The cupro nickel's waveform might look dramatically different to the silver ducat's and al ingot's, but note that it is just compressed time wise, ie, just a shorter TC. You might also see why disc is very difficult when ferrite is mixed with a coin, or even worse, a gold nugget.
Some proposed disc methods rely on the coils being critically balanced, such as we might have on the work bench, but this can be rather meaningless if the ground upsets the balance.

Eric's explanation for his waveforms here...

http://www.findmall.com/read.php?34,...778#msg-129778

I hope you realise that ML know these waveforms extremely well and you might eventually see why they favour square or rectangular transmitted waveforms over a PI on-period.