Taking a look at GROUND BALANCE on the bench

[mikebg]

.....We could also try to add a short TC resistive target. I can see some change with a 1" square alu foil, that has a TC of about 10us, however, at the gain of 10, the change is very little and probably difficult to see on the picture.

Suggestions??????????? Tinkerer

I have a suggestion:
Let somebody explain two figures.
The first one is a block diagram of principle on which received signals are formed. It explains what means "difficult ground".

http://www.geotech1.com/forums/showt...793#post163793

The second figure explains why the conductivity generates frequency dependent resistive and reactive components, and when magnetic permeability contains resistive component.

http://www.geotech1.com/forums/showt...886#post163886

These two figures contradict your statements in post # 1:

I open this thread to discuss the matter of GROUND BALANCE as seen during bench testing and simulations.
Difficult ground is the one that gives a resistive response and also a reactive response.

The resistive response is due to the conductivity of the ground.
The reactive response is due to the magnetic permeability of the ground.
The combination of the 2 responses in a wide variety of proportions, amounts to the generalized HOT GROUND.
....Tinkerer

[Tinkerer]

mikebg,

I don't understand what you mean. Could you explain?
I posted pictures of the wave forms, so that you can explain on these wave forms how to extract the target signal and how to remove the ground signal.

This is a time domain detector. If you know how to extract the target signal out of the signal shown, by using frequency analyzing, please explain how to do it directly, not with theoretical graphs and mathematical formulas, because I do not understand what you are saying.

My statement above is based on the following observation:

A gold target, (pure resistive, non permeable) and a ferrite (non conductive, permeable) target gibe a response that is 180 degrees apart.

An iron target has conductivity and has permeability. Sometimes, depending on the shape and orientation of the target, the response is very near equal from the conductivity as from the permeability, then the net signal is nearly 0.

I appreciate your help

Tinkerer

[Tinkerer]

Here are a few more pictures.

All have the ground, as the earlier ones, but then I added a target also.

1) Ground only
2) Ground and ferrite
3) Ground and 1"square alu foil that has a TC of 10us
4) Ground and silver dollar

Remember this is at the output of the preamp, that has a gain of only about 10. therefore the change in the signal wave form is very small for the alu foil target. Look at the tips of the sinusoid at the transient.

I find that I am showing the full dynamic range that this screen shot can show, the minimal, just barely visible deflection for the alu foil and the maximum of the silver dollar that just hits the edge of saturation on the negative rail.

Taking samples at several spots along the time line, we can obtain information about some characteristics of the targets.

We can also add or subtract or multiply or divide the results of the different samples we can obtain additional information. DSP would be good for that.

Tinkerer

[green]

Tinkerer
The first 30 usec of your traces look different than what I would expect. Have you tried reducing the amplifier gain to maybe 4? I think the no target signal should start at .6 volts times the gain and decay from there. Maybe I'm wrong.

[Tinkerer]

Tinkerer
The first 30 usec of your traces look different than what I would expect. Have you tried reducing the amplifier gain to maybe 4? I think the no target signal should start at .6 volts times the gain and decay from there. Maybe I'm wrong.

You are right, the wave form looks different from the traditional PI wave form. The traditional PI uses clipping diodes at about 0.6V and high gain in the preamp.

The TINKERERS uses an induction balanced coil to get rid of the Flyback voltage spike of several hundred V and uses no clipping diodes.

This is why, with the TINKERERS, you see the part that is normally clipped and the part that is usually lost in the preamp saturation.

You would need the usual 1k gain in a fast traditional PI preamp to see the response of the 1" square alu foil.

Tinkerer

[mikebg]

PHASE DIFFERENCE GND - TGT

....This is a time domain detector. If you know how to extract the target signal out of the signal shown, by using frequency analyzing, please explain how to do it directly, not with theoretical graphs and mathematical formulas, because I do not understand what you are saying.

A gold target, (pure resistive, non permeable) and a ferrite (non conductive, permeable) target gibe a response that is 180 degrees apart.....

Thinker, this is not true. Your oscilloscope shows that all metal detectors are time domain instruments. We all live in Time domain, but you are using a term "phase", attributable to Frequency domain. Can you explain how the phase angle appears on your oscilloscope? Please don't use your oscilloscope and Time domain for analysis.

The most suitable tool is Frequency domain because it removes the mist of math operation "convolution". Another tool is the DSP, but it uses terms as "deconvolution" and "impulse response" (see patent US
The visual Frequency analysis shows that if we usе suitable frequencies, we can make the phase difference between gold nugget and ferrite halfspace less than 180 deg. See how we can obtain phase difference almost 90 deg or 135 deg.
At frequency A we have ferrite response 0A1 and gold response 0A3. The phase difference is almost 90 deg. This is ideal for synchronous demolator.
At frequency B we have ferrite response 0B1 and gold response 0B3. The phase difference is 135 deg. This not ideal for synchronous demodulator, but visual analysis shows that at 45 deg and 135 deg it operates well.

http://www.geotech1.com/forums/showt...634#post164634

[Tinkerer]

Would you be so kind and show me an analog circuit to do that?

Tinkerer

[mikebg]

WE NEED DSP !

Would you be so kind and show me an analog circuit to do that?

Tinkerer

The gold prospecting machines operate at different frequencies depending on nugget size.
For large nuggets, the operators uses low TX frequency. See such machines of MINELAB.
However we live in 21st century and even in the second decade. We should use DSP. Note that patent pending for US4506225 is written in 1981.

[Tinkerer]

The gold prospecting machines operate at different frequencies depending on nugget size.
For large nuggets, the operators uses low TX frequency. See such machines of MINELAB.
However we live in 21st century and even in the second decade. We should use DSP. Note that patent pending for US4506225 is written in 1981.

No, I do not want to copy a Minelab machine. I want you to show me a working analog circuit of what you say. Show me a PI circuit with frequency domain target signal extraction.

[Davor]

Point is that each subsequent sample in a PI system has more and more reduced frequency content, so in fact you do have a kind of frequency domain processing whether you want it or not. Reality check... to detect small gold you need short delays, e.g. high frequency content.
With enough samples, and some proper weighting function, you could process samples by comb filtering method and learn more about target tau... which is not unlike nowadays rigs with several samples.

[Qiaozhi]

No, I do not want to copy a Minelab machine. I want you to show me a working analog circuit of what you say. Show me a PI circuit with frequency domain target signal extraction.

I get the distinct impression that you're going to have a long wait.

[mikebg]

No, I do not want to copy a Minelab machine. I want you to show me a working analog circuit of what you say. Show me a PI circuit with frequency domain target signal extraction.

"PI technology" is term of Time domain. The PI falls in WIDEBAND or BROADBAND technology in Frequency domain. Each sample at PI signal processing represents a frequency. This was discussed in this thread

http://www.geotech1.com/forums/showt...398#post163398

Even an ancient circuit diagram operating in the whole ELF radio (audible) frequency band was shown in the thread "PI History ..."
(I cant't find the link)
NOTE: The PI circuit is discrete (not analog) operating processor, even when it uses only a sample (integrating window).

[mikebg]

I get the distinct impression that you're going to have a long wait.

Qiaozhi, your alias shows that you are a legal agent of CHINELAB Unlimited in this forum.
I know that your company needs cicuit diagrams for free, but now we need money for something serious.
Let your company buy a PDF file for $2:

http://www.geotech1.com/forums/showt...343#post167343

This is serious. Let we know what was made for humanitarian demining in 1915.

Thinker, here's the link for mentioned above ancient analog operating PI circuit:
http://www.geotech1.com/forums/showt...704#post164704
Note that there is no samples. TX generates wide band audible frequencies. The timbre sensed by human ear is used as spectral analyzer.

[Carl-NC]

The "time-domain" vs "frequency-domain" debate is, IMO, much ado about nothing. Metal detectors are inherently time domain processors and I tend to do my design thinking (and simulations) in the time domain. But FD is also useful in some cases, such as the fundamental frequency response of targets. But once you've selected the frequency you want to use, it's TD pretty much to the end.

In my former life designing opamps, I primarily used FD sims for cases when the opamp was in a happy linear state; but when it came time to analyze large signal, overvoltage, and slew performance, an FD sim was pretty useless (I designed some fantastic opamps in the FD, that were total flops in the TD!). FD simulation is pretty useless for VCOs, PLLs, SHAs, and mixers. The point is, use the right tool for the right job.

Also, in traditional PI a single sample doesn't tell you squat about the tau of the target. It doesn't matter whether you sample at 5us or 50us, you have no idea what the heck is in the ground, except that at 50us it probably isn't a sub-gram nugget. Theoretically, you can take 2 or more sample points and get a good idea about tau. One of my engineers built up a bench test using Labview and got a PI system with target ID working. And it worked pretty good, until you added ground, and then it went to hell in a handbasket. Like my opamp designs, a lot of ideas work great, until...

[Tinkerer]

Point is that each subsequent sample in a PI system has more and more reduced frequency content, so in fact you do have a kind of frequency domain processing whether you want it or not. Reality check... to detect small gold you need short delays, e.g. high frequency content.
With enough samples, and some proper weighting function, you could process samples by comb filtering method and learn more about target tau... which is not unlike nowadays rigs with several samples.

Mickstv, sorry about the picture quality, if it is not good enough to do the superposition, I will redo tomorrow.
Davor, this one is for you. Could you help me measuring the time of when this target signal merges with the Ground signal? Maybe just using a comparator to start with?
I am going to start a new thread for that, as it is a different subject from the ground balance.
Below is the picture:

Ground and large target of short TC. The actual target is a piece of alu foil of about 2"x2". It is a large target of high amplitude response, but of short TC. I think we should see the difference in response between the silver dollar.

Tinkerer

Earlier we had the ground and a silver coin. The silver dollar has an estimated TC of over 300us. (just a guess)