DEEPER PI DETECTION DEPTH

[Tinkerer]

I would be speculating if I thrust an answer at you..

1)Having a Star ground point can have advantages. ie a single elected gnd/


It should be Low R and low L and then sheilds, bonds, battery -ve, pcb gnds all go to it.


2) If you are running a fair Tx signal the wrapped metalised polyester film cable shield may not be low enough L or R and will have signals resident on it.



Often if U have signals on braid it can be a sign that things are mismatched. I suspect this is partly the case. The characteristic impedance of the cable wont be the same as the impedance of the loop antenna system.

If U have a small amount of signal on Ur braid - which I guess U have - You burn these away in your pot. If U can live with it its quite an elegant solution.

U could try to work out the impedance of your loop to The Tx signals then either do a match cct so all of ur signal goes in, or pick a cable with a characteristic Z closer to you Tx loop at Tx frequencies.

Or fit a sleeve balun at the head end to trap braid currents

Or Use a ferrite clip on at the head end to absorb braid currents (ironically) due to eddy current losses in the ferrite.

S

Again, many thanks for the help and advice.
I have been living with these little problems. A bit of tinkering here and a bit of tinkering there and I always get it to work, but, the time has come to really dig down to the root of the problem to understand it.

The TX cables and the RX cables are separate.

So right now, I have 2 RG174/U cables from the center tapped RX coil to the RX board. The braid is the center tap and the 2 inners the 2 leads of the coil.
It takes 390 Ohms from the joined (at the board) braids to ground to get rid of the offset.

According to the attached specsheet of the cable, it would seem that the cable is OK for my signal frequency. I also see that it is a 50 Ohm cable. Hmm, 50 Ohms, should I do something with that?

Next thing to look at: The impedance of the RX coil x the impedance of the cable?

Impedance matching all the way from coil to preamp?

How do I do that?

Ah Steve, I warned you, I have never-ending questions.

Many thanks for you patience.

Tinkerer

[Aziz]

Feels familiar...

Seriously - detecting and estimating that signature is probably more work load than it is worth -- and would require digital processing for sure. In fact, really a "smoothing" problem, easier done after the fact. On top of that, the signatures Aziz refers to probably would be characterized by an average shape with a large stochastic variation, which you'd be left with anyway after "subtracting out". So a lot of work for one blip, even if you knew all the theory and had all the data to do it.

Probably easier to just dig.

-SB

The evil one blip problem:

Now imagine, the one blip comes several hundred times a minute from a motor-engine (motor bike/quad/car). Or from a PWM controlled e-motor or light (dimmer). The blip time (frequency) changes every time (to make it more complex now).

Eng. manager: "I want a PI that discriminates noise."
Software engineer: "Sir, I have an idea ..."
Eng. manager: "Shut up soft guy, I'm talking to the electronics engineer!"
Electronics engineer: "How do I do that?"
Software engineer: "All you have to do is write software to identify noise and eliminate it."
Electronics engineer: "Uh... OK."

Well, all the effort is worth to eliminate even one blip.

Aziz

[simonbaker]

The evil one blip problem:

Now imagine, the one blip comes several hundred times a minute from a motor-engine (motor bike/quad/car). Or from a PWM controlled e-motor or light (dimmer). The blip time (frequency) changes every time (to make it more complex now).

Eng. manager: "I want a PI that discriminates noise."
Software engineer: "Sir, I have an idea ..."
Eng. manager: "Shut up soft guy, I'm talking to the electronics engineer!"
Electronics engineer: "How do I do that?"
Software engineer: "All you have to do is write software to identify noise and eliminate it."
Electronics engineer: "Uh... OK."

Well, all the effort is worth to eliminate even one blip.

Aziz

You're right, it's worth trying, that's what tinkerers do!

And certainly if you have a noise environment that is quite endemic, analyzing and characterizing the noise is the smart thing to do.

I think PI detectors could especially benefit because the designs I've seen seem to use mainly time-domain detection processing and voltage levels, time-windows, etc.

If something like 50 or 60 Hz "mains" interference is present, I'd think that would be a good one to try to lock onto and subtract out.

In the end, some form of "noise" (including soil fluctuations) is what limits depth.

Regards,

-SB

[golfnut]

Hi tinkerer, sure I suggested some stuff based on my experience.


However as I mentioned, I can only take guided guesses from here



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.

Same with Rx coils.

What is the L of ur Tx coil? Is this a PI or VLF what frequency are U putting out if vlf. Or what is the rise time of spike if PI - into Ur coil.

S

[Tinkerer]

Hi tinkerer, sure I suggested some stuff based on my experience.


However as I mentioned, I can only take guided guesses from here



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.

Same with Rx coils.

What is the L of ur Tx coil? Is this a PI or VLF what frequency are U putting out if vlf. Or what is the rise time of spike if PI - into Ur coil.

S

Hi Steve,

thanks for the reply.

As reference we can use the TX circuit, including inductance etc, TEM.zip (LTSpice simulation) that I posted on post #104 above. On the screenshot of the simulation, you can see the TX current pulse as well as the response from 2 targets of different TC.

If you run the simulation, you can sum the TX coil current and the RX target response to see how the integrated signal changes with the targets.

In the simulation I have increased the scale of the target response, to fit on the same screen as the TX current, but of course, in a real circuit the response is several magnitudes lower.

For now, the TX cables, TXa, TXb, and Bucking coil, are a "twisted threesome" and separate from the RX cables.

The RX coil is about 300uH, center tapped.

Thanks for the help

Tinkerer

[Midas]

Is this a PI or VLF....?

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.

[Dr Vel]

I'm personally partial to dangerous voltage levels. - Carl

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.

[simonbaker]

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.

That's an interesting thought. Can you tell us more about the rotating field? Is the magnitude of the field sweeping across an area on the ground, or is it simply a field whose direction vector is rotating at each point in space?

-SB

[Tinkerer]

-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

[Dr Vel]

That's an interesting thought. Can you tell us more about the rotating field? Is the magnitude of the field sweeping across an area on the ground, or is it simply a field whose direction vector is rotating at each point in space?

-SB

It would depend upon what approach was used. It is just a thought I had which I have not spent a lot of time researching due to other priorities. More or less has anyone else thought about it and if so what ideas do they have on the subject.

[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

[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).

[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.

[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]

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