New PI technology proposals.

[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

[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.

[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

[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

[simonbaker]

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

It would be surprising to see different decay rates if both signals represented the "natural response" (stimulating signal gone, looking at natural decay) of the target. However, because everything is happening so fast, I believe we are looking at various "forced responses" (stimulating signal still present to some degree).

It is tricky because we sort of have a natural response (the coil being turned on or off) of one system (the coil) being the forcing signal to another system (the target coin). Especially at current shutoff in the coil, we have two systems with very fast response, so the signal in the target coin is hard to interpret by looking at it.

Also, each time we go from magnetic field to current we take a derivitive, including on the way back to the receive coil -- I need to think about that a little. I'm now wondering if the target coin can make a nice signal as its current reverses direction from the up pulse to the down pulse -- in other words, do they tend to cancel each other, or could they enhance each other.

Need to think....

Cheers,

-SB

[Aziz]

Hello friends,

it seems, we could have some real breakthrough in the PI technology!!!
Provided that, the simulation results can be verified with the real measurements.

Aziz

[ZED]

SB the thing i find intreaging is the different decay curves ,if we took a sample 10 u/s after the transmit starts we get a 100 milli volt signal and if we took a sample 10 u/s after the transmit stops we get a signal probably in the u/volts,the scope pic is certainly suggestive of the possibility of sampling during the on time for a better target responce.

Zed

[1843]

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.

I think that establishing the magnetic field, requires a coil on-time at least 3 TC... Doesn't it?


Regards,
1843

[1843]

SB the thing i find intreaging is the different decay curves ,if we took a sample 10 u/s after the transmit starts we get a 100 milli volt signal and if we took a sample 10 u/s after the transmit stops we get a signal probably in the u/volts,the scope pic is certainly suggestive of the possibility of sampling during the on time for a better target responce.

Zed

During the on-time the dB/dt is less in compared to the transmit stop time, so the target response must be less, too.

I think there's something wrong with your experiment.


Regards,
1843

[Carl-NC]

Aziz,

I've built & tested the CT coil, plus an otherwise identical mono coil. Specs are:

Mono:

• 20 turns of 24awg
• 290mm diameter
• 299uH / 4 ohms

CT:

• 10 + 10 turns of 24awg
• 290mm diameter
• 77uH / 1.1 ohms and 76uH / 1.1 ohms

If I damp the CT coil per your schematic, it rings horribly. Not in the manner of an underdamped coil, rather I get heavy oscillations all the way down the decay on both coils. I am pretty sure this is due to interwinding parasitics between the two coils, as they were wound together as a single winding.

These oscillations are common-mode in-phase so that when you look differentially, they mostly cancel but not completely. This is likely sensitive to coil matching & parasitic matching, which means that in reality it will be all over the place. So you end up with a faster decay but with oscillations.

I can, however, change the damping to that of a traditional two-coil PI front-end, where each coil is individually parallel-damped. In that case, I get rid of the oscillations and end up with a clean response both individually & differentially. It is about 1us faster that the mono response, which crosses 1v at a 5us delay.

So in this mode, we have the advantage of a faster di/dt due to the 77uH TX coil and we still get the full 300uH RX coil. Sounds good so far. But one thing I didn't mention before is that, yes, dB/dt field strength is proportional to di/dt, but it is also proportional to N, the number of coil turns. For the CT coil N is 1/2, so we better get at least a 2x di/dt to make up for this. I'm clipping my MOSFET so it's hard to say. Also, I don't have this hooked up to a full detector so I can't do target comparisons.

I would like to next wind up a CT coil using separate 10T windings so there is less interwinding coupling. Also need to rig up a differential receiver so I can run the whole metal detector.

- Carl

[Carl-NC]

SB the thing i find intreaging is the different decay curves ,if we took a sample 10 u/s after the transmit starts we get a 100 milli volt signal and if we took a sample 10 u/s after the transmit stops we get a signal probably in the u/volts,the scope pic is certainly suggestive of the possibility of sampling during the on time for a better target responce.

Zed

Are you doing this with an IB coil system? If yes, the the 100mv signal you see is likely due to TX-RX induction and not from any target.

- Carl

[Carl-NC]

I think that establishing the magnetic field, requires a coil on-time at least 3 TC... Doesn't it?

Only if you want it settled out to a steady-state level. At 3τ the coil current (and the B-field) will be 95% settled. But you can shut off the TX at any time... at 1τ the current will be 63% of the peak value. The drawback is that the coil current is still changing and can induce an undesired target reverse eddy current that subtracts from the (desired) turn-off eddies. But it's a negligible effect, and ramping the current up to several τ's just wastes power.

- Carl

[Aziz]

Hi Carl,

thanks for your practical efforts. The center-tapped coil should be damped on each half with individual damping resistors instead of my proposal. This will give a faster decay.
The coil wire should have an insulation to keep the interwire capacitance low. Then the capacitive coupling between the half coils will be low.

As long as the ringing is totally in-phase, this shouldn't be a problem with differential signalling. But best would be to avoid any ringings.

If you are interested on some amazing results, please visit the Oz-forum. I just posted there.

Regards,
Aziz

[simonbaker]

Hi Carl,

thanks for your practical efforts. The center-tapped coil should be damped on each half with individual damping resistors instead of my proposal. This will give a faster decay.
The coil wire should have an insulation to keep the interwire capacitance low. Then the capacitive coupling between the half coils will be low.

As long as the ringing is totally in-phase, this shouldn't be a problem with differential signalling. But best would be to avoid any ringings.

If you are interested on some amazing results, please visit the Oz-forum. I just posted there.

Regards,
Aziz

Hey Aziz:

If flyback can be damped faster with lots of resistors in coil, why not make a coil with flat spiral windings, use wire with no insulation, embedded in a fairly conductive layer of graphite? Infinite number of resistors!

Maybe already done....

Cheers,

-SB

[bbsailor]

Aziz,

I've built & tested the CT coil, plus an otherwise identical mono coil. Specs are:

Mono:

• 20 turns of 24awg
• 290mm diameter
• 299uH / 4 ohms

CT:

• 10 + 10 turns of 24awg
• 290mm diameter
• 77uH / 1.1 ohms and 76uH / 1.1 ohms

If I damp the CT coil per your schematic, it rings horribly. Not in the manner of an underdamped coil, rather I get heavy oscillations all the way down the decay on both coils. I am pretty sure this is due to interwinding parasitics between the two coils, as they were wound together as a single winding.

These oscillations are common-mode in-phase so that when you look differentially, they mostly cancel but not completely. This is likely sensitive to coil matching & parasitic matching, which means that in reality it will be all over the place. So you end up with a faster decay but with oscillations.

I can, however, change the damping to that of a traditional two-coil PI front-end, where each coil is individually parallel-damped. In that case, I get rid of the oscillations and end up with a clean response both individually & differentially. It is about 1us faster that the mono response, which crosses 1v at a 5us delay.

So in this mode, we have the advantage of a faster di/dt due to the 77uH TX coil and we still get the full 300uH RX coil. Sounds good so far. But one thing I didn't mention before is that, yes, dB/dt field strength is proportional to di/dt, but it is also proportional to N, the number of coil turns. For the CT coil N is 1/2, so we better get at least a 2x di/dt to make up for this. I'm clipping my MOSFET so it's hard to say. Also, I don't have this hooked up to a full detector so I can't do target comparisons.

I would like to next wind up a CT coil using separate 10T windings so there is less interwinding coupling. Also need to rig up a differential receiver so I can run the whole metal detector.

- Carl

Carl

Since you were able to obtain a 1 us improvement in discharge time using two damping resistors, I suspect that the effective value of these two resistors in series across 10 turns each was a little higher than a single damping resistor across 20 turns. What were the values of the single Rd and the dual Rds?

What if you damped the coils at 5 turns each effectivly having the coil taps connected to 4 series damping resistors? I suspect that the combined value of this 4-resistor set might be a little higher than with 2 resistors in series thus further improving the discharge time.

I guess if you extended this to it's logical conclusion, you could have a damping resistor on each turn. That might really damp the flyback?

Then we could know the true role of the flyback pulse in (1) stimulating the target, (2) being a necessary artifact that must be quickly dispensed with or (3) if it produces some other measurable result related to detecting the target?

What do you think?

bbsailor