DEEPER PI DETECTION DEPTH

[simonbaker]

Thanks Simon,

the oscillation on the Flattop current wave form is due to the parasitic coil capacitance. This is a well known problem with traditional PI. This is the inter wire capacitance, the wire to shield capacitance and the cable capacitance. I have put it down as 200pf, but this is a very conservative figure. 500pf would not be exaggerated.

But why does it not appear on the saw-tooth? This seems to be something with the wire resistance that is not equal between the two sims, but I need to follow up on that.

Tinkerer

Edit on the above.
I just run it again, and it has now the same oscillation in both sims. The oscillation disappears when the Mosfet gate resistor is changed from 10R to 100R, or, in other words slowing down the switching a little bit.

How did that change your target response comparison?

-SB

[Qiaozhi]

Simon - I took your simulation and hacked it around to create a comparison between a PI without a resistor in series with the coil (case 1), and another with a 15 ohm resistor in series (case 2).

In case 1, the charging period is 18us.
In case 2, the charging period is 100us.
At the time the mosfet is ready to be switched off, they both have the same current flowing in the coil.
Plot "No-res" and "With_res" in one plot pane, and "Target1" and Target2" in a second pane.
As you will see in case1, the eddy currents are fairly well established in Target1 when the mosfet switches off. These eddy currents need to be overcome, resulting in a lower target response when compared to case2, where the eddy currents in Target2 have essentially decayed almost to zero.

These results appear to agree nicely with the practical test performed by Carl.

[Qiaozhi]

Thanks Simon,

the oscillation on the Flattop current wave form is due to the parasitic coil capacitance. This is a well known problem with traditional PI. This is the inter wire capacitance, the wire to shield capacitance and the cable capacitance. I have put it down as 200pf, but this is a very conservative figure. 500pf would not be exaggerated.

But why does it not appear on the saw-tooth? This seems to be something with the wire resistance that is not equal between the two sims, but I need to follow up on that.

Tinkerer

Edit on the above.
I just run it again, and it has now the same oscillation in both sims. The oscillation disappears when the Mosfet gate resistor is changed from 10R to 100R, or, in other words slowing down the switching a little bit.

It appears to be connected with the flyback voltage. Try plotting the coil voltage and coil current in different plot panes. You'll see that the kink in the current waveform follows the peak of the flyback. It might be a problem with the mosfet model, and that could be confirmed by replacing it with an ideal switch. Also, the mosfet model does not include the breakdown voltage. This can easily be fudged into the simulation by putting a large voltage zener diode in parallel with the mosfet.

[Tinkerer]

Hi Tinkerer:

If you zoom in on the TX currents during discharge, you can see something went wrong in the sim -- there is an oscillation in the "flattop" case that is probably skewing your results. Didn't analyze any further as to why...

Regards,

-SB

Thanks Simon,

the oscillation on the Flattop current wave form is due to the parasitic coil capacitance. This is a well known problem with traditional PI. This is the inter wire capacitance, the wire to shield capacitance and the cable capacitance. I have put it down as 200pf, but this is a very conservative figure. 500pf would not be exaggerated.

But why does it not appear on the saw-tooth? This seems to be something with the wire resistance that is not equal between the two sims, but I need to follow up on that.

Tinkerer

Edit on the above.
I just run it again, and it has now the same oscillation in both sims. The oscillation disappears when the Mosfet gate resistor is changed from 10R to 100R, or, in other words slowing down the switching a little bit.

[simonbaker]

Error in the Simulation?
I wonder.
So I re-did the simulation more precisely to Carl's numbers.
100us TX for both, 300uH for both 15 Ohm and 31V for flattop and 600 mOhm and 7V for sawtooth. Same damping at 500 Ohm. 2A coil current.

I attached the zipped LTSpice simulation files so you all can help me find the error.

3 different targets with TC's of 15us, 100us and 200us.

The 15us target comes out close with 13% more response for the flattop.
The 100us target has 41% more response.
The 200us target has 46% more response.

Where is the error?

Tinkerer

Hi Tinkerer:

If you zoom in on the TX currents during discharge, you can see something went wrong in the sim -- there is an oscillation in the "flattop" case that is probably skewing your results. Didn't analyze any further as to why...

Regards,

-SB

[Tinkerer]

So Carl, no sign of the effect that shows up in Tinkerers post #93, which seemed to indicate a detrimental effect of a flat top for one of the targets?

Thats kind of what I was getting at Tinkerer when I asked for real scope shots, something that could confirm or not whether that the strange effects in post #93 isn't just an error in the sim..

Error in the Simulation?
I wonder.
So I re-did the simulation more precisely to Carl's numbers.
100us TX for both, 300uH for both 15 Ohm and 31V for flattop and 600 mOhm and 7V for sawtooth. Same damping at 500 Ohm. 2A coil current.

I attached the zipped LTSpice simulation files so you all can help me find the error.

3 different targets with TC's of 15us, 100us and 200us.

The 15us target comes out close with 13% more response for the flattop.
The 100us target has 41% more response.
The 200us target has 46% more response.

Where is the error?

Tinkerer

[Midas]

So Carl, no sign of the effect that shows up in Tinkerers post #93, which seemed to indicate a detrimental effect of a flat top for one of the targets?

Thats kind of what I was getting at Tinkerer when I asked for real scope shots, something that could confirm or not whether that the strange effects in post #93 isn't just an error in the sim..

[Aziz]

To test the effect of turn-on settling, I ran a 100us pulse width with and without R=15 ohms in series with the coil. Without the R the VCC was 10V and with the R it was cranked to 38V to give the same peak current. Ergo, in both cases I had a 100us pulse width and about 2A peak current, the only difference was the slope of the current: flat top vs saw tooth.

I got about what I expected, a 10-15% increase in preamp Vout deflection with the flat-top current, both with low and high conductance targets. This sounds good, until you realize that a 15% voltage increase is about a 2% depth increase. Ergo, in a std PI depth is not greatly affected by the di/dt just before turn-off.

- Carl

Thanks for sharing your test results Carl.

Indeed, it doesn't make sense with the flat-top current. What a waste of battery power to maintain the flat-top current for a negligible depth increase.

Can we say now, that the switch-off time period kicks the target in a PI detector? More precisely, the flyback period and provided that, that the flyback period is shorter than the pulse-on period. (High frequency stimulation matters.)

Cheers,
Aziz

[Tinkerer]

Looks interesting. Can you attach the LTSpice file?

Regards,

-SB

Here is the same simulation, but the target response is summed to the TX current (green trace) wave form.
Note the change in signal amplitude as well as the phase shift.

Tinkerer

[simonbaker]

-SB,

here is one for you. This is a sine wave simulation.

The green trace is the coil current.

The 3 other traces are the response signals of 3 different targets.

Tinkerer

Looks interesting. Can you attach the LTSpice file?

Regards,

-SB

[Carl-NC]

In any case, I am working on a circuit to measure this phenomenon. I'll post results next week.

To test the effect of turn-on settling, I ran a 100us pulse width with and without R=15 ohms in series with the coil. Without the R the VCC was 10V and with the R it was cranked to 38V to give the same peak current. Ergo, in both cases I had a 100us pulse width and about 2A peak current, the only difference was the slope of the current: flat top vs saw tooth.

I got about what I expected, a 10-15% increase in preamp Vout deflection with the flat-top current, both with low and high conductance targets. This sounds good, until you realize that a 15% voltage increase is about a 2% depth increase. Ergo, in a std PI depth is not greatly affected by the di/dt just before turn-off.

- Carl

[dfbowers]

Then you would have liked my front yard in Phoenix circa 1989. The older guy is Robert Golka of Project Tesla at Wendover AFB. Sadly that pic got a little messed up from a leaky roof years ago. You likely have seen him in one of those old Nova shows on PBS about Tesla. Yes those are pole transformers on the ground to the right side.

I have been wondering. Since one issue is consistency in the rate of sweeping the detector search coil, why has no one swept the field instead. Two TX coils set orthogonally fed by a quadrature drive signal would give you a precise rotating field while you were pinpointed over the target. You could pick the axis of rotation by coil placement. Curious as to why this idea has never been talked about anywhere I have found thus far. Another approach would be similar to the coils on the yoke of an old picture tube TV, just fed different frequencies than the vertical obviously. Using typical VLF frequencies in both coils as a starting point.

Ha! Interesting idea. Personally, I have not had good luck with mixing Tesla coils with modern devices. I was playing with mine inside the house, and I lost all the Ge diodes in my crystal sets and it somehow blew out the eletronics module in my water heater. Pictures are difficult to take with a digital camera because it keeps crashing the camera and TV sets go out to lunch and have to be power cycled to get them to come back. Not sure that it's good for your nuts either..
This one consumes about 450 Watts. Not sure how much of that energy makes it into the air but makes nice 15" steamers.

Don

[Carl-NC]

Id say the Tx loop and cabling would be the best place to start. Either end should be matched ie the OP Z of Tx cct shold be similar to cable Z and cable Z similar to Ant coil for maximum power transfer.

Impedance matching is not an issue with detectors.

[Carl-NC]

Originally Posted by Carl-NC
I'm personally partial to dangerous voltage levels. - Carl

Then you would have liked my front yard in Phoenix circa 1989.

Now you're talkin! However, I fear for portability, and battery life.

I have been wondering. Since one issue is consistency in the rate of sweeping the detector search coil, why has no one swept the field instead. Two TX coils set orthogonally fed by a quadrature drive signal would give you a precise rotating field while you were pinpointed over the target. You could pick the axis of rotation by coil placement. Curious as to why this idea has never been talked about anywhere I have found thus far. Another approach would be similar to the coils on the yoke of an old picture tube TV, just fed different frequencies than the vertical obviously. Using typical VLF frequencies in both coils as a starting point.

I don't think this would give you the results you think it would. Besides, loop motion is also useful for getting non-target information (ground).

[Tinkerer]

Good question!

Thanks for posting those sims Tinkererer, very interesting. It would seem that if you can fully saturate a target in one direction its all primed and ready to spring back actually assisting in the creation of eddy currents in the opposite direction. Quite counter intuative. Some questions I'd like to see answered are:
Where exactly is the point where reverse eddy currents become beneficial, is complete target saturation actually necessary, or is there some critical percentage ?

What happens with an isolateral triangular pulse, ie one the ramps up at the same rate as it ramps down?

Or what about a series of triangular pulses that at every point of inflection the ramp rate gets proggressively steeper and steeper?

I know you said the you got the sim as close as possilbe to your actual circuit but is there any chance you could post some real world scope shots for comparison ?

This will give people a better chance of helping you improve your simulation results as well.

Midas, thanks for the feedback.

I am not sure what you mean with "reverse eddy currents".
Maybe the SINEWAVE simulation gives you an idea of what the response looks like for a continuously changing current wave form.

I have posted many, many screen shots of real target response. Now I am trying to make simulations that fit the real target responses. One of the simulation problems I have, is to make different coupling factors (k) for different inductors within the same sim.

Would somebody be so kind and give me a tip of how to do that?

Triangular pulses and non linear triangular pulses. Or what about a series of triangular pulses that at every point of inflection the ramp rate gets proggressively steeper and steeper?

Could you give me a circuit for making such TX pulses? Say with a 500uH coil?

Tinkerer