New PI technology proposals.

[ZED]

Monolith my experiences with on time sampling have always shown that the target signal is always strongest at switch off,the pictured scope shot that i showed was from an enquire into how fast i could get the coil current to rise in a transmit coil.The reason for this was that i had an idea about sampling for a target signal at the start of the transmit while the ground signal was at its lowest amplitude,meaning "while the magnetic domains in the ground matrix were just starting to align themselves with the coil field".
If your getting a weaker signal at switch off then it sounds like the rate off coil field change is slower than when it was rising,how long does it take for the coil current in your setup to reach zero after switch off ?

Simonbaker
Yes the target signal after switch off was clipped,it went of the screen on the scope and the visual was uploaded to the pc and it showed it a little differently,cant remeber its peak value but it was in the volts range,also notice how the target signals have different decay rates from one another,interesting dont you think ?

Zed

[Monolith]

correction

In a traditional PI (mono coil, sampling the OFF decay), it is only the collapsing field we care about. During the ON time when the coil current is rising, yes, there can be a target signal but we cannot see it. - Carl

Agreed

However, with an IB coil you can look for a signal in the ON time. And, depending on the target conductivity and the current slew rate, that can be a usable signal. I won't readily agree with Monolith that it's 10x as big, but I don't know what his design looks like, so I'll leave it at that. - Carl[/quote]

I wrote that post "adhoc" and admit it is not totally correct. Depending on the way the IB coil is balanced, the ON time signal can be many times larger than the OFF time signal. I never did an actual precise measurement. So the number 10 times has to be considered as a subjective number.

But I disagree with Monolith's earlier post. There seems to be a continued misunderstanding with a number of folks about what is going on during the ON time of traditional PI. The concepts of "charging" the target and the need to have enough ON time to fully "saturate" the target are mistaken. Targets don't get "charged" or "saturated" during the ON time. Nor is there a need to allow the ON current to settle out... the reverse eddy currents are negligible, and running the current until its settled out just wastes power. - Carl[/quote]

This theory is often stated and there are many facts that seem to prove it.

However, from the point of view of the ON time sampling, this theory does not fit at all. Nor does the ON current need to settle out. Agreed on that.

What I still would like to see is a comparison. Which signal is of greater amplitude. The ON time signal or the OFF time signal.
Looking at dI\dt, the OFF time signal should be bigger.
It is also true that the fastest switch OFF and Flyback decay results in the greatest OFF time signal.


But, my test have shown me that the ON time signal is much larger.

I say this is caused by the Flyback going in both directions, canceling part of the signal.

So lets compare some results.
I post the signal amplitude as measured at the output of the Preamp, taken with a pulse that has no Flyback, as shown in the scope picture above.

Will somebody please post the signal amplitude for the OFF time sample?

TX oil about 300uH, 20 Ohm series resistance, 50us TX pulse at 12V, Gain 1000 on the preamp. Or something in the ball park. If the comparison looks interesting, we can always repeat with more precise parameters.

Lets see and compare the results.

Monolith

[Qiaozhi]

A PI transmitter circuit in some ways resembles the inductive spark system of a car. A current flows through a coil until the magnetic field is fully established, then the switch opens and the magnetic field collapses, creating a back-emf. In the case of a car, this is fed to the spark plugs via a distributor. In the PI it is this back-emf that is responsible for creating eddy currents in the target. Because this back-emf is so large compared to any target-induced voltage, it must be quenched as quickly as possible, before the target signal has a chance to die away. If you use an IB coil, then some interesting possibilities arise, such as sampling during the on-time, even though this is traditionally not where target sampling is performed. Such target signals are significantly smaller than those during the flyback period.

In a car ignition system there is also the possibility to use capacitive discharge. This tends to provide more energy in the spark than the normal inductive discharge system. I have wondered whether this technique could be applied to PIs. Capacitive discharge systems usually employ a flywheel diode that returns a lot of otherwise wasted energy to the capacitor, and partially charges it ready for the next pulse. Maybe this has already been tried before, but I've never heard of it. Might be worth a try.

[Aziz]

Hi Carl,

Yes, critical damping is the fastest for a fixed RLC system. If it's underdamped, the decay will slew faster but then ring. But what if you switch in the damping R just before the decay crosses zero? You will get the initial fast underdamped decay, then switch to critical damping to avoid ringing. However, it is VERY timing sensitive.

- Carl

BBsailor's proposal gives an elegant way, to achieve the critical damping faster then the normal critical damping. There is an anomaly current flow just after switch-off (time >= toff+) observable through the damping resistor on the normal coil configuration. This anomaly will be removed by bbsailor's proposal and the damping current will not be disturbed and has higher slope (=more target stimulation). The reason for the anomaly isn't analysed yet. It may come perhaps from the coils capacitance.

I am working now on a new SPICE model to analyse the effects on target response whether they are suffering from the enhanced critical damping. Also the possibility of using the center-tapped coils in conjunction with the bbsailor's proposal.

Regards,
Aziz

[simonbaker]

Yes, critical damping is the fastest for a fixed RLC system. If it's underdamped, the decay will slew faster but then ring. But what if you switch in the damping R just before the decay crosses zero? You will get the initial fast underdamped decay, then switch to critical damping to avoid ringing. However, it is VERY timing sensitive.

- Carl

That was kind of what I was thinking, only somehow using diodes to couple-in a different resistor when it went negative. But even though you get to zero faster, you now have more energy to dissipate because it's stored somewhere (by definition of underdamping), so it seems in the end it would take longer because you lost some time during the underdamped initial portion where you weren't sloughing off energy as fast as possible.

I've heard of capacitors being used to capture the pulse -- without thinking about it, it seems appealing to have a capacitor you can switch in, put the genie in the bottle, then cork it up! (something doesn't make sense about that.... you want current off, not flowing...)

Regards,

-SB

[Carl-NC]

I was referring to Monolith's ideas that the growing magnetic field, not the collapsing one at turn-off, was causing the target signal that is detected.

In a traditional PI (mono coil, sampling the OFF decay), it is only the collapsing field we care about. During the ON time when the coil current is rising, yes, there can be a target signal but we cannot see it.

However, with an IB coil you can look for a signal in the ON time. And, depending on the target conductivity and the current slew rate, that can be a usable signal. I won't readily agree with Monolith that it's 10x as big, but I don't know what his design looks like, so I'll leave it at that.

But I disagree with Monolith's earlier post. There seems to be a continued misunderstanding with a number of folks about what is going on during the ON time of traditional PI. The concepts of "charging" the target and the need to have enough ON time to fully "saturate" the target are mistaken. Targets don't get "charged" or "saturated" during the ON time. Nor is there a need to allow the ON current to settle out... the reverse eddy currents are negligible, and running the current until its settled out just wastes power.

That is interesting. My original thought about "assymetric damping" maybe naive -- you have to dump the energy somehow, probably critical damping is the fastest.

Yes, critical damping is the fastest for a fixed RLC system. If it's underdamped, the decay will slew faster but then ring. But what if you switch in the damping R just before the decay crosses zero? You will get the initial fast underdamped decay, then switch to critical damping to avoid ringing. However, it is VERY timing sensitive.

- Carl

[simonbaker]

Hi Monolith,



I have achieved now a true compareable simulation system. I will add now the target response model, which requires a simulation model of 4 independend systems. Otherwise a true comparison is difficult to visualize.

But it shows, that SPICE simulations gives an enourmous potential to understand what is going on in the process. As the physical models are a typical second order functions (R,L,C second order of differential equations), it is quite difficult to solve the equations analytical (beyond my scope). SPICE tools are solving these equations with numerical methods and let show you the complex process.

I am really impressed about the SPICE simulation results, which will help to understand the PI technology better. So I was able to proof the idea in a different way and showed me the faulty of some thoughts.

I recommend all people to play with some SPICE simulations. The better the model is defined, the better will be the result to the real word. Now I will work on the topic to extract some benefits - if any - of the new findings.


Aziz

Can you post your recent spice ".asc" file so we can follow what you are doing?

Regards,

-SB

[Aziz]

Hi Monolith,

The only way to get this sorted out is for several people to post comparable results.
Same amount of power, same amount of amplification, same targets.

Monolith

I have achieved now a true compareable simulation system. I will add now the target response model, which requires a simulation model of 4 independend systems. Otherwise a true comparison is difficult to visualize.

But it shows, that SPICE simulations gives an enourmous potential to understand what is going on in the process. As the physical models are a typical second order functions (R,L,C second order of differential equations), it is quite difficult to solve the equations analytical (beyond my scope). SPICE tools are solving these equations with numerical methods and let show you the complex process.

I am really impressed about the SPICE simulation results, which will help to understand the PI technology better. So I was able to proof the idea in a different way and showed me the faulty of some thoughts.

I recommend all people to play with some SPICE simulations. The better the model is defined, the better will be the result to the real word. Now I will work on the topic to extract some benefits - if any - of the new findings.


Aziz

[Monolith]

TX ON sampling X TX OFF sampling

It sounds like your circuit is not able to switch off fast for some reason. Can you check that?

Regards,

-SB

The only way to get this sorted out is for several people to post comparable results.
Same amount of power, same amount of amplification, same targets.

Monolith

[Aziz]

Hello friends,

It seems, that the migration of bbsailors proposal and the use of the high flyback voltage in a center-tapped coil configuration gives new interesting possibilities, which has to be investigated further.

It could give the possibility of faster damping and higher target stimulation with the benefits of a differential signal line.

I am starting to make a better comparable SPICE simulation models which shows the result better.

Aziz

[simonbaker]

Aziz in an effort to see where your coming from in regard to the fly back i have a question,
The attatched scope pic is of a target signal taken straight of the receive coil.

Setup
1/2 inch round copper tubing bent into a circle of 18 inches diameter with ends not connected together,used as transmit coil.
Two receive coils residing within transmit coil,approx 30 turns of wire per receive coil,receive coils connected opposite in phase to cancle out flyback signal.
12 volts on transmit,45 amps running through coil,max current reached within 10u/s,pulse width 34u/s


Question
if i understand you correctly you say the flyback kick starts the eddy currents on the target.
on the scope pick,at the start of the transmit,while current is rising very fast i get a very good signal on the target,yet there is no flyback to generate this high target signal,are you able to explain why this is so???

Zed

I think it is relative -- there should be a signal while current rising, but a much larger one during flyback. Your flyback signal looks "clipped" in the picture. How far negative did it really go? Maybe much more amplitude than rising signal?

Regards,

-SB

[simonbaker]

This is just what I am talking about. My signal amplitude is about 10 times as high at the beginning of the TX pulse than after TX switch OFF. Using a common IB coil combination.
Not only that, it also DISCRIMINATES between FE and non magnetic targets.

What kind of relation of signal amplitude have you observed between sampling during TX ON time and TX OFF time?

Monolith

It sounds like your circuit is not able to switch off fast for some reason. Can you check that?

Regards,

-SB

[simonbaker]

Physically, it has to be so... induction is only caused by a changing magnetic field, not a static one. What other opinions are there?

I was referring to Monolith's ideas that the growing magnetic field, not the collapsing one at turn-off, was causing the target signal that is detected.

I've experimented with switched damping, that is, where the damping resistor is switched in during the flyback. I did this in my security wand, strictly for lower power.

- Carl

That is interesting. My original thought about "assymetric damping" maybe naive -- you have to dump the energy somehow, probably critical damping is the fastest.

If the problem with choosing a lower resistor for faster damping is "ringing", what about using a "pole-canceling" network to allow de-tune the coil and allow faster damping?

By the way, your "switched damping" is interesting to me because I'm working on something similar, I think, for the TGS. I want to make a slower response version, but I notice that the double-differentiation of the two "bandpass" stages causes a single "up pulse" to turn into a "down - up - down" triple pulse. The TGS detects the middle pulse which is nice and stable. But if you slow the response, the middle pulse is quite delayed from the original target pulse, and not always as large as the first "down" pulse at the output. I am trying to use JFet switches to cancel the trailing "up-down" parts of the pulse and just use the initial "down" pulse for detection. It may be impractical in the long run, but your idea sounded similar in a way.

Regards,

-SB

[Monolith]

The Flyback question

Aziz in an effort to see where your coming from in regard to the fly back i have a question,
The attatched scope pic is of a target signal taken straight of the receive coil.

Setup
1/2 inch round copper tubing bent into a circle of 18 inches diameter with ends not connected together,used as transmit coil.
Two receive coils residing within transmit coil,approx 30 turns of wire per receive coil,receive coils connected opposite in phase to cancle out flyback signal.
12 volts on transmit,45 amps running through coil,max current reached within 10u/s,pulse width 34u/s


Question
if i understand you correctly you say the flyback kick starts the eddy currents on the target.
on the scope pick,at the start of the transmit,while current is rising very fast i get a very good signal on the target,yet there is no flyback to generate this high target signal,are you able to explain why this is so???

Zed

This is just what I am talking about. My signal amplitude is about 10 times as high at the beginning of the TX pulse than after TX switch OFF. Using a common IB coil combination.
Not only that, it also DISCRIMINATES between FE and non magnetic targets.

What kind of relation of signal amplitude have you observed between sampling during TX ON time and TX OFF time?

Monolith

[Tinkerer]

Does the Flyback produce the eddy current?

Gentlemen, allow me to disagree.
I say it is the expanding magnetic field produced by the coil current, that kicks the target. The Flyback is actually reducing the eddy currents in the target.
The Flyback is too short to "saturate" for want of a better word, the target.
The Flyback induces currents of opposing polarity since the magnetic field of the Flyback expands and then collapses, that is, the field cuts across the target conductor first the one way, the the other way.
The current raising in the coil during TX, generates an expanding magnetic field that cuts across the target \ conductor and induces currents within the target. The first currents are superficial skin effect currents. It takes time for the eddy currents to expand to the core of the target.
How much time? The time needed has been named the TC of the target. High conductive and large size targets have long TC's. Large silver coins can have a TC of hundreds of uS.
If the TX pulse is short, the eddy currents will not have the time to expand to the very core of the target, so they will not reach maximum amplitude.
Now look at the Flyback. It's duration is a matter of nano seconds for the one way and a few micro seconds on the return. Its highest power is at the maximum voltage point that is about even both ways and of a duration of nanoseconds. Just too short to induce much eddy currents. And going both ways, thus canceling the eddy currents induced by the going field, with the field collapsing.
However, the eddy currents induced by the TX coil current persist for some time.
How much time? about the same time it took to generate them The TC of the charge curve equals the TC of the discharge curve.

So, how will I prove my point? I will show you that I can read the eddy currents without having a Flyback.
Better than that, I will show you that, without the Flyback, the amplitude of the eddy currents is much higher.
I will pulse the coil with a usual square wave TX pulse but will modify the circuit to eliminate the Flyback.
Below is the scope picture of the coil current.
The pulse length is about 64us, you can see a slight kink and noise spike where the TX switches OFF. Forgive me for the noise spike, I don't think it can be called a Flyback.
The scope is set at 20us \div and 10mV\div.
Note, this is the coil current, not the TX voltage wave form.

Please allow me some time to setup an RX circuit to read the eddy current. Just the preamp.
I will use US$ 1c, 5c, 10c, 25c as test targets.
Monolith

I want to see these results.

Aziz,

Could you simulate a real target? Maybe a silver coin, that is a low resistance target, that has a long TC.

The simulation could maybe be a coil with some resistance, some capacitance and some inductance, such as to have a TC of 100us.
It would be interesting to see how this target works with your Spice simulation.

Tinkerer