How does ground balance work in GPX 4500?

[green]

Ok. I have tested some ground balance with two red bricks tight together with insulation tape.

TX is 100 us. Peak current 7A. Coil resistance 1 Ohm. Ampere-turns 15*7=105 A*Turns
Ground balance sample is adjustable.

1) D1=10us S1=10us D2=5us S2=86.7us.
In this case I have full ground balance.
But I can not detect any targets - they just give negative value - small size (half gramm blob of solder, 1 gram of pure gold) and big size (coins). I am loosing almost 90 percent in detection distance.
But Green had no difference with detecting targets (GB_off vs GB_on). Strange. I think this is because your current-turns is many times smaller.


2) Later I have decided to move ground balance sample further away from the target sample - so to make sure all target currents are died away for sure.
So timings are, D1=8us S1=8us D1=90us S2=165.2us. But here I multiply ground sample S2 by a multiplier of 3.
(Signal is equal to X=S1-S2*3+static_fields.)
In this case detection distance is just inch of half less than in GB_off case.

Anyway I don't understand why math prediction doesn't work.
I mean
Integral of ~1/t from t1=10 to t2=20 (= ln(20/10)) is definitely not equal to the integral of 1/t from t1=25 to t2=111.7 (= ln (111.7/25)).


Ok.

Thanks to everybody for all information you gave to me. I need few days to digest all of it.
"Feedback" will be soon.

Do you read a target signal with above targets when not taking ground sample? With no ground sample all targets signal should be +. With ground sample, small targets + large coins probably -. I would be surprised you can sample at 10usec delay with Tx 7A peak. Try 15usec delay and ground balance, might lose small targets.

[belax2018]

Couple charts I posted in other threads. TRT_21, example of ground slope changing with Tx on time with constant current Tx vs constant rate Tx, each would require a different balance setting to cancel ground signal. target delay_1, some data taken awhile back showing detection distance and integrator out volts with some different targets. GEB on causes distance loss because noise increases when the 100usec GEB sample is added. I use a(Rx, two 8inch round connected figure eight)(Tx surrounds Rx)reduces noise and ground signal. Don't think it would have different timings vs mono coil. I did bury a target in back yard 10inches deep to see if integrator out voltage was the same as air with GEB on adjusted to cancel ground signal. Little or no difference.

Ok. I have tested some ground balance with two red bricks tight together with insulation tape.

TX is 100 us. Peak current 7A. Coil resistance 1 Ohm. Ampere-turns 15*7=105 A*Turns
Ground balance sample is adjustable.

1) D1=10us S1=10us D2=5us S2=86.7us.
In this case I have full ground balance.
But I can not detect any targets - they just give negative value - small size (half gramm blob of solder, 1 gram of pure gold) and big size (coins). I am loosing almost 90 percent in detection distance.
But Green had no difference with detecting targets (GB_off vs GB_on). Strange. I think this is because your current-turns is many times smaller.


2) Later I have decided to move ground balance sample further away from the target sample - so to make sure all target currents are died away for sure.
So timings are, D1=8us S1=8us D1=90us S2=165.2us. But here I multiply ground sample S2 by a multiplier of 3.
(Signal is equal to X=S1-S2*3+static_fields.)
In this case detection distance is just inch of half less than in GB_off case.

Anyway I don't understand why math prediction doesn't work.
I mean
Integral of ~1/t from t1=10 to t2=20 (= ln(20/10)) is definitely not equal to the integral of 1/t from t1=25 to t2=111.7 (= ln (111.7/25)).


Ok.

Thanks to everybody for all information you gave to me. I need few days to digest all of it.
"Feedback" will be soon.

[waltr]

No, that just subtracts out common-mode signals. The early sample usually has a DC component just from sampling the static flyback decay, whereas the late sample does not. So the output of the demod has a DC offset. Normally an RC high-pass filter follows the demod and knocks this out.

In the simpler PI detectors (Hammer Head, etc) the SAT stage is DC blocking. The Integrator stage output goes through a Cap (DC Block) to the Sat stage which then has a Zero offset output when No change happens under to coil.

Did I get this correct Carl?

[Carl-NC]

Could you please explain more on "dc blocking"?

As I understand almost all dc and ("dc-like") components are subtracted based on data received in a very late sample - we call EF sample.

Thank you

No, that just subtracts out common-mode signals. The early sample usually has a DC component just from sampling the static flyback decay, whereas the late sample does not. So the output of the demod has a DC offset. Normally an RC high-pass filter follows the demod and knocks this out.

[belax2018]

Could you please explain more on "dc blocking"?

As I understand almost all dc and ("dc-like") components are subtracted based on data received in a very late sample - we call EF sample.

Thank you

[belax2018]

Dear Green.

Did three tests with different timing last weekend.
Will post it later today. Will be interesting.

[belax2018]

I have got it.
KT315 Thank you for the files.

[Carl-NC]

Good to know, thanks.

[Mechanic]

waltr is right, read the patents. The older ML designs used simple multiple pulse widths. The newer designs (all the GPX series, AFAIK) use "DVT" which means the short TX pulses are driven off a boosted supply. This matches their turn-on exponentials with the long pulse width exponential. I've not looked into the details of how the ground signal is extracted, though.

It just comes down to the sample timings.

Each of the target channels is near ground balanced with it's receive timings, and the ground channel is added/subtracted at the relevant % from the target channels.

The GB channel samples are not very complicated and are quite similar to the 2200 in operation.

Probably one of the biggest differences with the gpx series is the removal of the dc blocking stage from the signal chain.
Although the gpx is a motion detector, the "dc blocking" is done in the processor.

This allows for much higher dynamic range in the back end. The older detectors suffered from the dc blocking stage hitting the rails before the processor causing the gb to fail in bad conditions. Now that stage is digital and allows for working much more severe ground.

Cheers Mick

[Carl-NC]

waltr is right, read the patents. The older ML designs used simple multiple pulse widths. The newer designs (all the GPX series, AFAIK) use "DVT" which means the short TX pulses are driven off a boosted supply. This matches their turn-on exponentials with the long pulse width exponential. I've not looked into the details of how the ground signal is extracted, though.

[kt315]

direct link http://www.geotech1.com/forums/showt...ersal-PI-Micro

[kt315]

QED sent. check your box.

[waltr]

But anyway, does anybody know the exact method GPX implements for their GB.
I saw (youtube Woody DetectorMods) a GPX waveform.
It has 6 small peridos TX and one long T2. It looks like T2=6*T1.

Does it really nobody knows how GB works in GPX series?

Not sure if anyone really knows exact details of how GB is done on the GPX series.
The TX Wave forms you describe does sound like the way the SD2000 does GB. This is described in the MineLab patent US5506506.

Also Mine Lab has a list of patents per detector model on their web site:

https://www.minelab.com/usa/patents

These would be a good reads for how they may be doing GB. They also include other patents which are related so look these up.

Did notice that for the SDC, GPX & GPZ detector this patent is common.

https://patents.google.com/patent/US6636044

The SD2000 schematics are posted in this forum so a study will help show the weighing factors.

http://www.geotech1.com/forums/showthread.php?14001-Full-m-lab-SD2000-schematics

The GPX & GPZ use a processor instead of analog circuits for integration and weighting which no one has the code for.

[belax2018]

Thank you waltr and Qiaozhi.
I have read all these threads before.

Will try soon method from Qiaozhi link. As it looks promising.
But I honestly disagree with assumption Teleno does: That for T1>T2 (where T1 and T2 is a TX length) target signal is the same

But anyway, does anybody know the exact method GPX implements for their GB.
I saw (youtube Woody DetectorMods) a GPX waveform.
It has 6 small peridos TX and one long T2. It looks like T2=6*T1.

I guess they use the following math.

If the signal from soil governs by: G=T/t - ln((T+t)/t)

So integrating it we have: Integral of G = IntG = (T+t)*ln(t/(t+T))

Lets consider two cases.
1) T=T1, Sample is from t1 to t2
2) T=T2=6*T1, Sample is from t3 to t4, where t3=6*t1 and t2=6*t2

For the 1'st case: G1=IntG (from t1 to t2) = (T1+t2)*ln(t2/(T1+t2)) - (T1+t1)*ln(t1/(T1+t1))
For the 2'nd case: G2=IntG (from 6*t1 to 6*t2) = (6*T1+6*t2)*ln(6*t2/(6*T1+6*t2)) - (6*T1+6*t1)*ln(6*t1/(6*T1+6*t1))= 6*((T1+t2)*ln(t2/(T1+t2)) - (T1+t1)*ln(t1/(T1+t1))) = 6*G1

So we have G2=6*G1

Now we can eliminate ground signal from T1 periods without a "hole" based on ground signal from T2 periods.

But it is OK here on paper. How it works in practice is a big question.

Does it really nobody knows how GB works in GPX series?

[waltr]

All very good threads listed and what I read to learn how to GEB.

Also check my HH2 build thread from post #14.

http://www.geotech1.com/forums/showthread.php?24251-My-take-on-the-HH2

I use the three sample method and a dual (plus/minus) integrator like most of the simpler PI detectors use.
I fix the first sample time and second delay time then adjust the second sample width to GEB.
My thread has discussion, links to other threads and scope pictures.