PI Power Output

[Qiaozhi]

I don't know how to represent a target for simulation. I only know how to find the target with a detector.

If you think there is some possibility of adding discrimination to a PI by comparing the response during the ON-time and the OFF-time, then you could try the following with your setup:

Take a limited number of common targets (coins, rings, pulltabs, etc.) and record the samples for each target at different distances from the coil. Then plot these on a graph for comparison.

Although this experiment appears relatively simple , it is complicated by the balancing of the IB coil. For anyone else to duplicate your results, they would need to have their coils set up in the exactly the same way.
This is why simulation is going to be tricky, to say the least. I've had this working successfully in SPICE for a VLF (with the correct phase-shifts) so that it matches reality, but it's a matter of working with both a prototype and the simulator at the same time to fine tune the models. In this particular case, we would also need to take measurements on the real circuit, and then add the findings to the simulator. Many times people get bogged down in simulations, and they forget that it's just another tool. SPICE simulations are extremely useful for getting you in the right ballpark, but eventually you have to build a real circuit. It's then that you find out which of your assumptions were correct, and which were not. Then you can go back and refine the models to match reality, and in the process make other discoveries.

So, what I'm trying to say, is that (in this case) we need a number of real world measurements, before exploring more deeply with the simulator, because of the added complication of using an IB coil, which also has an unknown setup ..... if that's clear?

[Qiaozhi]

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?

I suspect you are correct in your statements. That's why I suggested the experiment using a limited number of common targets at different distances from the coil. Any correlation between these targets and the actual target response will most likely vanish once the graphs are plotted.

The only way anyone will prove (one way or the other) if there is a correlation, is to build a prototype and test it in the real world. SPICE simulations are not going to add any benefit at this time.

[ZED]

G`day Robby_h

What you say is true,the reactive component of the ground signal during the on time does make the idea of sampling during the on time nigh impossible,....for now! ..well in Australia anyway but ground conditions vary through out the world as you may well know.
Do you think the reactive component of the ground response to be a problem world wide.
Lets not forget that the reactive ground signal will only ever be a problem while the coil field is in a state of change,with a static coil field we can kiss the reactive component goodbye and say hello magnetic component .
Now sampling during the ontime while the coil field is static.....thats an interesting place to go.

Zed

[B^C]

robby_h,

"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?"
----------------------------------------------

Your correct in that the time base has changed, this is due to a manually triggered pulse setup.

Not sure how it could be the same as with a balanced RX coil-IB, could you explain why it appears to be the same as the test setup has no similarity to this or anything else for that matter?

I would appreciate your input.

[robby_h]

Zed,
Sampling during the on-period isn't nigh on impossible as it is routinely done in all CW metal detectors transmitting sinusoidal and rectangular waveforms.

"Do you think the reactive component of the ground response to be a problem world wide".

It depends on what you mean by this. USA goldfields have a substantial reactive component but little R component as is evident when someone says they can set a 2100's GB pots anywhere, something which isn't possible here. This also applies to the TDI where the USA operator can set the GB to set the TC pivot point for iron and coins etc. My local park has areas that require critical ground balance on a GS5 with a ten turn pot so this wouldn't work here either. This USA ground though is often reported to give a ground balancing vlf fits so it's safe to assume these soils have a very substantial reactive component.

"Lets not forget that the reactive ground signal will only ever be a problem while the coil field is in a state of change,with a static coil field we can kiss the reactive component goodbye and say hello magnetic component .
Now sampling during the ontime while the coil field is static.....thats an interesting place to go".

But this then dictates when you can sample and you will miss a lot of targets. There are other ways around the problem if you have been reading the relevant patents.

[robby_h]

B^C,
Quite simple really, I just don't know what I'm looking at or where and in what part of a circuit.
Your first pic shows a flat line with small glitches. The third pic looks very similar to the three short pulses in a GP pulse train, ie, ~40usec on and ~80 off. This then gets more confusing in the 4th pic because of the close spacing.

How did you obtain these waveforms?

[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

I think Aziz is doing a fantastic job with the graphs. What we need to do is to compare the results with real targets and real TX pulses, to look for discrepancies that could give information for possible improvements.
There are a great many factors involved with target response.
Maybe the most evident at the beginning of the response is the surface area that the target presents to the B field.
Take a coin for example, on edge the surface it presents is very different from the surface it presents when lying flat.
The surface eddy currents or skin effect are the first. They influence mostly the shape of the response curve at the leading edge.
The core eddy currents from a minimal surface target, a sphere, will appear after the skin effect eddy currents have already mostly decayed.

We probably can divide all targets into a few broad categories with common characteristics.
Then try to simulate each category.
Then verify with real component tests if the responses correspond.

Tinkerer

[Tinkerer]

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.

B^C,
I find your experiments very interesting. Did I understand right in that the responses you show have been generated by pulses at high voltage?

At what point in time were the responses sampled?

All the best

Tinkerer

[Tinkerer]

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,
It shows that your knowledge of PI is vast. I am honored by your participation in our conversation. Tomorrow I will answer on your post.

All the best

Tinkerer

[simonbaker]

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?

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.

Would you explain that some more, I don't understand the idea.

Regards,

-SB

[Unregistered]

Would you explain that some more, I don't understand the idea.

Regards,

-SB

"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."

He is correct about X during Tx on but WRONG about its effects during the Pi off period.High X particularly with frequency dependent susceptibility(and domains with very long relaxation times) increase the total R signal component during Tx off.
.

[ZED]

Zed,

Now sampling during the ontime while the coil field is static.....thats an interesting place to go".

But this then dictates when you can sample and you will miss a lot of targets. There are other ways around the problem if you have been reading the relevant patents.

Robby the idea of the static coil field was to get a ground sample just before switch off,the reason being is as you mentioned,"very poor target response"so the intent is to get a ground sample with little or no target signal left in it,the purpose of this is so it can be summed up against the first sample taken after the transmit,effectively removing the ground signal from the sample taken after the transmit and at the same time preserving the integrity of the target signal meaning no target signal is subtracted out of the ground balancing cct......earth field cancling will require seperate samples.
Now PI detectors have the natural ability to discriminate between ferrous and non ferrous targets but it all comes undone because of the second sample as in a conventional PI,the above ground cancling cct described preserves this because only one sample is required after the transmit,[earth field samples aside] so this sample will more accurately reflect wether a target is ferrous or non ferrous to full depth.
Personally i think there is a huge potential for information during the ontime,we just need to go in and have a look and try to understand what we see and give up on this nonsense about its all a waste of time because the reactive signal plays such a large roll and its nigh impossible[PI WISE].

Zed

[ZED]

B^C it helps if you can tell us what your doing,i know your sampling during the flyback because the target signal peaks with a flat top,this flat top also suggests that the flyback voltage is being regulated and not allowed to reach its peak plus you said so,also there is no flyback signal this would suggest an IB coil yet you say this is not the case,i see a number of indications that your using a standard tank cct with an IB coil of sorts but you say non of it is so.
So dude whats the story ?

Zed

[Tinkerer]

Robby the idea of the static coil field was to get a ground sample just before switch off,the reason being is as you mentioned,"very poor target response"so the intent is to get a ground sample with little or no target signal left in it,the purpose of this is so it can be summed up against the first sample taken after the transmit,effectively removing the ground signal from the sample taken after the transmit and at the same time preserving the integrity of the target signal meaning no target signal is subtracted out of the ground balancing cct......earth field cancling will require seperate samples.
Now PI detectors have the natural ability to discriminate between ferrous and non ferrous targets but it all comes undone because of the second sample as in a conventional PI,the above ground cancling cct described preserves this because only one sample is required after the transmit,[earth field samples aside] so this sample will more accurately reflect wether a target is ferrous or non ferrous to full depth.
Personally i think there is a huge potential for information during the ontime,we just need to go in and have a look and try to understand what we see and give up on this nonsense about its all a waste of time because the reactive signal plays such a large roll and its nigh impossible[PI WISE].

Zed

Zed,

I understand this differently.
A the end of the TX pulse, lets call it TX Omega, when it is "flat top" or more than 5 coil TC long, the coil current is at steady state.
No eddy currents are generated. Not in the ground and not in the target. (Unless, in a target with very long TC where the eddy current generation might continue a little due to the charge lag.)
With no eddy currents generated in the ground at TX Omega, it is not a good spot to measure the ground signal.
However, it is a good spot to receive the common mode noise, to subtract it then in the difference amplifier.

Tinkerer

[Tinkerer]

"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."

He is correct about X during Tx on but WRONG about its effects during the Pi off period.High X particularly with frequency dependent susceptibility(and domains with very long relaxation times) increase the total R signal component during Tx off.
.

Unregistered, hmm which one is it? Why don't you just add a handle so we know you are not that troll that we put on the ignore list?

Here is a simple experiment to demonstrate what you are saying.
Take a copper or brass washer and set it on a plastic spacer box on the coil of a PI in the flat position.
Note the amplitude of its response.

Now repeat with the washer in the vertical position. You will note immediately that the vertical position gives a very much fainter response.

Now repeat both tests with a steel washer of the same dimensions.

You will note that the steel washer on edge produces nearly the same response as when it is in the flat position.

This is because you are now seeing the X response of the steel.

Tinkerer