DEEPER PI DETECTION DEPTH

[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]

-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

[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

[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.

[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.

[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

[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

[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

[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

[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

[simonbaker]

This is a running joke at work, especially with the software engineers... "all you have to do is..."

Eng. manager: "I want a PI that discriminates iron."
SW engineer: "How do I do that?"
Eng. manager: "All you have to do is write software to identify iron and eliminate it."
SW engineer: "Uh... OK."

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

[Carl-NC]

...then all you have to do is...

This is a running joke at work, especially with the software engineers... "all you have to do is..."

Eng. manager: "I want a PI that discriminates iron."
SW engineer: "How do I do that?"
Eng. manager: "All you have to do is write software to identify iron and eliminate it."
SW engineer: "Uh... OK."

[golfnut]

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

[Tinkerer]

Many thanks Steve,

yes, I have about 1000 questions left but I guess I must not overdo it. On the other side, there are probably many other members of the forum who dare not ask themselves but are glad to learn from your answers.

So please, just one more...
R34, is to adjust the offset caused by the shield of some types of cables.
hmm

I don't know why this happens. The pot solves the problem, but I would prefer to avoid the problem all together. The cables are also very sensitive to touching with the hand and moving /vibration.

What can be the cause for that?

We, the people, or forum members, greatly appreciate your help.

Ah, by the way, I use the TEM TX method above, where I recycle most of the power. I like a powerful TX that gives a high amplitude RX signal that is way above a lot of the pesky noise.

Tinkerer

[golfnut]

whoa, breathe dude..

What are the specs for the inductors?
A question about the impedance:
I have a variable voltage divider between R9, U7, R13. I use this to adjust both legs of the RX coil to the same signal amplitude.
The diodes should not conduct, they are there as a precaution only for when there is a problem with the induction balance.
R34, is to adjust the offset caused by the shield of some types of cables.

Increasing the impedance, reduces the amplitude of the signal, which allows for more amplification in the preamp, at the cost of more noise.

Eliminating more noise with the filters, before the input of the preamp is going to help.

I feel there is a sweet spot, where the S/N is best.
How do I determine that spot?

What are the specs for the inductors?

No constraints really, if the value is good, The R value will only be an ohm or so.
Layout in more important, dont have th coils close together as they will couple signal rather than conduct signal.

A question about the impedance:
I have a variable voltage divider between R9, U7, R13. I use this to adjust both legs of the RX coil to the same signal amplitude.

ok


The diodes should not conduct, they are there as a precaution only for when there is a problem with the induction balance.

yes

R34, is to adjust the offset caused by the shield of some types of cables.

hmm


Increasing the impedance, reduces the amplitude of the signal,

aha


which allows for more amplification in the preamp,

which would then require more amplification in the preamp to get the same signal level back.


at the cost of more noise.
smaller signals in - need to crank the gain, more noise.. S/N suffers.


Eliminating more noise with the filters, before the input of the preamp is going to help.

It will also reduce falses from pick up of any signals above say 0.5MHz as the attenuation here is getting real high, plus the usable gain window for wanted signals should be better as less blocking due to removal of the myriad of high power signals being filtered away. Keeps the dynamic range.


I feel there is a sweet spot, where the S/N is best.

That is an engineering question and a good one. All radio links have a system plan/design and a system budget.
You look at the signals and their amplitudes max and min and the noise powers and you look at the Reciever noise contribution (in the appropriate system BW)

This lot works if you have all of the puzzle pieces - We dont have this luxury.

All we know is we want to detect real deep and small gold etc.

For us this amounts to the smallest S/N we can get a reliable threshold detect.

How do I determine that spot?

As I eluded, there isnt a spot which is optimal the nature of the game is to have the highest S/N you can - to get the smallest and deepest you will have nothing spare in S/N.


A large Tx signal
A quiet Receiver
A detection threshold minutely above the ambient noise out of the Rx - in order to see small signals is the best u can hope for out of a simple machine like these.

If you picked a spot S/N - the axes of freedom are then Tx power, Rx NF and gain, depth, soil type and target size and material.

You would reliably be able to say then at this spot S/N for my system - If I use this Tx power, Rx NF and gain, depth, soil type and target size and material - I will get a target detected.

I can guess ur next question and I dont have the answer to hand

Steve