PI Power Output

[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

[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.

[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.

[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

[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]

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

Zed,

I am going to start a new thread for this phase shift measuring idea. Would you please help me discussing this idea?

Tinkerer

As best i can,im not very bright.

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

[Tinkerer]

Phase shift measuring in PI

Zed,

I am going to start a new thread for this phase shift measuring idea. Would you please help me discussing this idea?

Tinkerer

[Tinkerer]

G`day Tinkerer

The delay in the target signal i imagine would be caused by the natural inductance of the target trying to resist eddie currents forming but with time eddies will grow.
Im with you in that the eddies appear to be stronger during on time as apposed to the left over eddies after the transmit.
The target signal you see at picture "E" will reflect the speed of the growing coil field.The growing coil field will be at its fastest initially and begin to slow as it approaches saturation,this will be reflected in the target signal where during the linear part of the coil field growth is where you will find the best eddie current growth,as the coil field growth slows the current eddies cannot be sustained because of the slower growth rate of the coil field and so the eddies will decay to a point reflecting the slowing growth rate of the coil field until when saturation is reached and then all you have left are very low level eddie currents that need a little more time to decay to zero.
Thats my interpretation anyway.

Zed

Hi ZED,

I think we agree on the general idea. Lets get into a bit more details on it.
The time of eddy current growth should be readable from the shape curve. (as shown in green and red in my post #85)

Maybe there is information to be gleaned by comparing the "Time of Growth" with the time of decay.

What I have observed so far, is the moment in time where the response has a "PIVOT". This is a dead spot, where a measurement produces 0. It is the point where the "relative polarity" changes.
The PIVOT for FE is at a very different time than for non-magnetic targets. With the right kind of decoding, it seems to me that the same phase shifts as in VLF can be read in this "PIVOT SINE WAVE"

Tinkerer

[Tinkerer]

Hi Tinkerer,

Thanks for the explanation.
I didn't associate Carl's response with your experiments, although he specifically answered to your observations. From what Carl said, even for VLF it is very difficult to discriminate against junk, since junk can be found all over the VDI sca;e.
The fact that iron can be pretty easy identified is already a major advantage, since a very significant portion of the junk is iron.
About the resistive and reactive response, if you measure them separately (I think I read about that on some patents), they won't cancel each other.


Regards,
Nicolae

Hi Nicolae,

You got that exactly right if you measure them separately the don't cancel each other.
It is all about separating the total lumped information into its individual branches and reading them separately.

Tinkerer

[ZED]

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

G`day Tinkerer

The delay in the target signal i imagine would be caused by the natural inductance of the target trying to resist eddie currents forming but with time eddies will grow.
Im with you in that the eddies appear to be stronger during on time as apposed to the left over eddies after the transmit.
The target signal you see at picture "E" will reflect the speed of the growing coil field.The growing coil field will be at its fastest initially and begin to slow as it approaches saturation,this will be reflected in the target signal where during the linear part of the coil field growth is where you will find the best eddie current growth,as the coil field growth slows the current eddies cannot be sustained because of the slower growth rate of the coil field and so the eddies will decay to a point reflecting the slowing growth rate of the coil field until when saturation is reached and then all you have left are very low level eddie currents that need a little more time to decay to zero.
Thats my interpretation anyway.

Zed

[ZED]

Now B^C if you want more signal try increasing the avalanch voltage on your coil switch [ fet ],say if its 200v go for 400v.
What i see is not a target saturating but a coil field collapsing at a steady rate for a period of 300 ns i think
This is caused by the diode in your coil switch clipping the high voltage fly back and this controls the speed of the collapsing coil field and this speed will be constant for the duration,the target signal will reflect this with a flat horizontal zone at its peak signal .

Zed

[ZED]

Gday Zed,

All targets are saturated with a magnetic field this is what creates the eddy currents, the extent of the saturation, well that's another story,
Old Boy!
These targets are being belted with a very strong magnetic pulse & they
only seem to produce so much even with a time variant so one would assume they can't be saturated any more?

Well its your universe dude,you can have any physics you like but to me saturation is the end of the road,no more,finished,maxed out !!!

Zed

[nick_f]

Hi Nicolae,

The way I see it and the way I understand Carl's quote, it should be possible to get somewhat similar information as with the sinusoidal response of the VLF.
I am not there yet. For the time being I have observed different responses for different metals, but there are so many factors, size, shape, volume distance to the coil etc. it will take a lot of work to unravel the maze.

For FE it is relatively simple. The response is negative for FE and positive for non-magnetic targets.
But, if a target, lets say a rusty thin piece of steel sheet, presented in its flat position to the coil has the same amount of reactive response as it has resistive response, they cancel each other.
Now, if you present this same target on edge to the coil, you see mostly the reactive response that is characteristic to FE.
Tinkerer

Hi Tinkerer,

Thanks for the explanation.
I didn't associate Carl's response with your experiments, although he specifically answered to your observations. From what Carl said, even for VLF it is very difficult to discriminate against junk, since junk can be found all over the VDI sca;e.
The fact that iron can be pretty easy identified is already a major advantage, since a very significant portion of the junk is iron.
About the resistive and reactive response, if you measure them separately (I think I read about that on some patents), they won't cancel each other.


Regards,
Nicolae