How does ground balance work in GPX 4500?

[waltr]

I've been using an Excel program also. Doesn't seem to balance ground with -1 slope with your settings. Wondering if I'm missing something. Not as easy to use as I would like. Do you know how to have Excel fill in the blanks after entering the timings or maybe your Excel program is easier to use.

Here is the spreadsheet I used.
The sampling sums for the 3 TC targets are in columns H,I,J. I shaded the target curves to help visualize where sampling is. All the samples are then Summed in H8 to K9, upper is with GEB sampling.
The target with 31us TC does lose depth.

GEB is obtained when the GEB sampling time sums = 0.

Sampling Integrations_GEB.zip

[Monolith]

Yes I confirm. That with GB_off all targets are plus and nice detectable.
At your and walter settings all targets loose 90% depth.
Only if I use huge D2 > 80 us and then take ground sample (whith multiplication coefficient of 3) I can detect all targets (up to a D=25mm coin) as before.
Also I confirm that GB works and timing tuned correctly - I simulated with my hands movments (up and down) of the bricks in front of the coil no noticeable change in the final signal.
I can sample targets after 6.5 us - 7 us. Even at Tx=200us I can sample at 7.5 us which ends up at around 18 Amps.
I use two AD8139 amplifiers.
First has gain 2400/600 ~4. Second 4700/150 = 31. Overall gain is around 120. Also After each AD8139 is a low pass filter network. But I don't think it can change a math behind GB very much.
After second AD8139 I have ADS5560 - 40msps ADC. After ADC I have Cyclone 4 FPGA. All integrations are inside FPGA. MCU reads only the results of integrations.
I will send you soon oscilloscope waveforms after second AD8139.


To Carl:
DC blocking worth a try. Thank you.

Also according to what you say GPX 4500 ground balance works as follows.

GPX-4500 has two power supplies. One is Vh (high voltage) and second is Vl (low voltage).
Waveform is: 7 short TX pulses with Vh where they take Target sample = Th. And one very long TX pulse with Vl where GPX takes only ground sample GBl after some big delay (to make sure all target response died away) . Then they do the math X = Th-GBl*Coeff + static_fields_account.



Also it has some type of DC blocking scheme in front of the amplifiers - simple CR high pass filter

Sorry for my English, but do I understand it right?
Thank you.

Could you give us some more information about your coil and TX voltage?
You say 15 turns, what is the diameter of the coil?
You say 7 Amps at 100us, what is the TX voltage?

[belax2018]

Could you give us some more information about your coil and TX voltage?
You say 15 turns, what is the diameter of the coil?
You say 7 Amps at 100us, what is the TX voltage?

It is in the first post. All units are in metric system.
TX coil: diameter 310 mm, 15 turns of Litz 49x0.1. Rtx=0.9 Ohm.
RX coil: diameter 150 mm, 47 turns of 0.03 sq.mm multythreded wire. Resistance I don't remember - can not measure now - but around 9 ohms.
Coil is shielded with acrylic electro insulation coating mixed with graphite powder in proportion 1:1 by volume.

[belax2018]

TX voltage is 4 LI-ION batteries. So TX voltage it is from 16.4 to 13.8 voltas.

Why do you ask?

[Monolith]

So your TX charge voltage would be around 16V, to achieve 7A in 100us and 26V to achieve 18A in 200us.
At 18A, the damping resistor would have to be about 55 Ohm to keep the Flyback under 1000V, unless there is heavy avalanche discharge.

This seems to be a very unusual PI TX which would explain the unusual behavior. We would love to get more information about it.

Your information about the 4S batteries was received while I posted. Thank you. It seems to confirm my estimate.

[belax2018]

Rdump is about 450 ohm.

Tx switch is CREE C2M0160120D. Which has Va=1200V in datasheet but in reality it is about 1500 V (if my resistive-divider network didn't lie).

And don't forget my amps have amplification of 120 and not >1000. That is why you are out of saturation faster.

[Monolith]

Rdump is about 450 ohm.

Tx switch is CREE C2M0160120D. Which has Va=1200V in datasheet but in reality it is about 1500 V (if my resistive-divider network didn't lie).

And don't forget my amps have amplification of 120 and not >1000. That is why you are out of saturation faster.

With Rdamp at 450 Ohm and 18A peak current, the Flyback would be around 10,000V. There must be something else that is different.

Anything above the Mosfet voltage would be dumped through the mosfet avalanche diode. The current that is dumped through the avalanche diode does nothing to excite the target.

[Mechanic]

Hi Belax,

Forget tx voltages for a minute, can we see the coil current waveform? Current is king!

As for timings, the gpx uses different timings depending what timing you select for the area you want to work.
The standard timing has 3 short tx pulses and one long tx pulse.
Enhance turns on the -15v for 12us then switches to -5v for ~220us for the first tx pulse and the second tx pulse is -15v for 70us.
Then the samples for each channel are taken after both tx pulses to obtain target and approximate ground balance.

In the sd2000 ch1's samples were only taken after the long pulse and ch2's were taken after the short pulses.
All the detectors after the 2000 take samples for each channel after both the long and short tx pulses.

Cheers Mick

[belax2018]

Hi Belax,

Forget tx voltages for a minute, can we see the coil current waveform? Current is king!

As for timings, the gpx uses different timings depending what timing you select for the area you want to work.
The standard timing has 3 short tx pulses and one long tx pulse.
Enhance turns on the -15v for 12us then switches to -5v for ~220us for the first tx pulse and the second tx pulse is -15v for 70us.
Then the samples for each channel are taken after both tx pulses to obtain target and approximate ground balance.

In the sd2000 ch1's samples were only taken after the long pulse and ch2's were taken after the short pulses.
All the detectors after the 2000 take samples for each channel after both the long and short tx pulses.

Cheers Mick

Thank you for answering directly to subject of the thread.

Could you please draw on paper timings of two modes of GPX-4500 - Enhanced and Standard? It is a confusion in my understanding of enhanced mode timings you explained above.

Below are my waveforms.



From the left to the right: Coil TX current, Flyback voltage, magnified bottom part of a flyback voltage. Ground contact of oscilloscope attached to Coil_VCC=15.5 V.
(R_d_coil=415 ohms. Coil current was measure across 0.3/4=0.075 ohm resistor. Coil voltage - along part of R_d_coil which is 84 ohms (415/84=4.95) )

[green]

Hi belax,

Have you found why you loose so much distance when ground balancing. Doesn't make sense to me with your timings unless targets are in GB hole.

[Teleno]

Hi belax,

Have you found why you loose so much distance when ground balancing. Doesn't make sense to me with your timings unless targets are in GB hole.

Because the signal linearly approaches zero on both sides of the hole.

[belax2018]

Hi belax,

Have you found why you loose so much distance when ground balancing. Doesn't make sense to me with your timings unless targets are in GB hole.

It is simply too much of a target signal left in the region of 20-100 us (I confirmed it by just setting target time start to 20 us and target sample width to 80 us). I think because my coil current is around 7A - is way bigger than yours.

I haven't tried this type of GB with 1.0A current.

Currently I am changing my programs (FPGA and MCU) for a different methos of GB - two TX pulses - short and long - GB will be (I hope so) similar to QED/GPX.

P.S.: I actually have found the oscilloscope waveforms for "standard" timings of GPX-4500. Will share soon. It is from one Australian man who does modding of Mlab machines.

[waltr]

P.S.: I actually have found the oscilloscope waveforms for "standard" timings of GPX-4500. Will share soon. It is from one Australian man who does modding of Mlab machines.

Very interested in seeing these.

[green]

It is simply too much of a target signal left in the region of 20-100 us (I confirmed it by just setting target time start to 20 us and target sample width to 80 us). I think because my coil current is around 7A - is way bigger than yours.

I haven't tried this type of GB with 1.0A current.

http://www.geotech1.com/forums/attac...7&d=1545255530 The US Quarter has 3 times the signal in the 100usec ground sample as it does in the 10usec target sample. Don't know why 7A would be different than 1A other than coil current decay time, slightly longer delay time needed. Maybe someone could explain if it would effect.

[belax2018]

http://www.geotech1.com/forums/attac...7&d=1545255530 The US Quarter has 3 times the signal in the 100usec ground sample as it does in the 10usec target sample. Don't know why 7A would be different than 1A other than coil current decay time, slightly longer delay time needed. Maybe someone could explain if it would effect.

You are right green. I think now I understand.

That is because I monitor the change of (short_average_of_signal - long_average_of_signal). If the difference is positive - I beep. If it is negative - I don't. In the case when target signal is higher in a ground region - my difference becomes negative - and I just ignore it. Need to rethink my detection scheme inside MCU.