How does ground balance work in GPX 4500?

[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.

[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.

[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) )

[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

[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.

[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]

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]

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

Why do you ask?

[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.

[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?

[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

[belax2018]

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.

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.

[green]

I use similar method to GB.
My timings are: D1 = 12 to 16us, S1 = 15us, D2 = 15us, S2 = 98us to GB conductive soil.

Still have very good sensitivity except to target that match the GB hole, like some iron.
Also this gives an increase of the integrator output Voltage for low conductive targets (nickles, pull tabs, brass shells), then the decrease in integrator Voltage for high conductive targets (silver and clad coins, copper).
This actually improves the sensitivity to high conductors versus with no GB sample (S2 = S1).

I have been able to simulate this in an Excel spreadsheet.
Target signal is calculated for time increments from: V = exp(-t/Tau)
Where:
Tau = target TC
t = time after TX Stops
V = received Voltage
Ground response simply calculated as -1/t
Fill columns in t, V & Gnd
Then SUM the V's during each sampling period. S1 is positive, S2 is negative (integrator inputs).
Gnd sum 0 zero.
For short target TCs (low conductive) the sum is positive.
For long TCs (high conductive) the sum is negative.

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.

[Carl-NC]

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?

Yes, the RC high-pass filter used for DC blocking is also part of the SAT circuit.

[waltr]

I use similar method to GB.
My timings are: D1 = 12 to 16us, S1 = 15us, D2 = 15us, S2 = 98us to GB conductive soil.

Still have very good sensitivity except to target that match the GB hole, like some iron.
Also this gives an increase of the integrator output Voltage for low conductive targets (nickles, pull tabs, brass shells), then the decrease in integrator Voltage for high conductive targets (silver and clad coins, copper).
This actually improves the sensitivity to high conductors versus with no GB sample (S2 = S1).

I have been able to simulate this in an Excel spreadsheet.
Target signal is calculated for time increments from: V = exp(-t/Tau)
Where:
Tau = target TC
t = time after TX Stops
V = received Voltage
Ground response simply calculated as -1/t
Fill columns in t, V & Gnd
Then SUM the V's during each sampling period. S1 is positive, S2 is negative (integrator inputs).
Gnd sum 0 zero.
For short target TCs (low conductive) the sum is positive.
For long TCs (high conductive) the sum is negative.