Ground balance

[Bayu]

does anyone have a ground balance schematic for pi

[waltr]

Many in this forum.
First check the TDI schematic.
Then there are mods for the MPP and maybe the Surf PI.

Last is the firmware approach I used in my HH2. Easiest if all timing is done with a ucontroller but could be done with hardware timing (just messy).

[Bayu]

is there no separate schematic specifically pi? because I modified pi polones in the frequency section to be sensitive to gold, and the results are satisfactory, I just need to add GB for my device

[waltr]

As I posted, there are many GB schematics and methods in the forums here.

Is the PI Polones circuit ucomtroller based?
If so then check my HH2 thread on the 3-sample GB method.

The other common method is another integrator and two more sampling switches to create a GB 'channel'.
This is then subtracted from the 'main' detection' channel. TDI, SD2000, etc use this method.

[Bayu]

ok thank you, master, I will try to thank you for the advice

[eclipse]

How about automatic ground balance ? Is not something that I believe has been discussed.

[waltr]

How about automatic ground balance ? Is not something that I believe has been discussed.

I have read and on how newer VLF detectors do auto GB but never for a PI.
Can you please describe how this circuit would auto GB?

One of the key and tricky parts of auto GB is the Time constant for the GB adjust. If too fast then it adjusts out target response. if too slow then doesn't track changing ground conditions. I don't see any timing constant in your circuit.

[eclipse]

No this is just answer to the question of the topic (just a regular TDI/GoldScan ground balance with manual adjustment).
I'm curious about doing auto GB and I don't see this being discussed anywhere.

[eclipse]

You can use this circuit - just add 2 more delayed samples (GB1/GB2) to your regular circuit (MPP for example). op amp 5.2 is your regular intergrator (s1/s2); and op amp 5.1 is going to be connected to the delay samples integrator (GB).

GB1 = 10us after S1
GB2 = 10us after S2

[waltr]

No this is just answer to the question of the topic (just a regular TDI/GoldScan ground balance with manual adjustment).
I'm curious about doing auto GB and I don't see this being discussed anywhere.

Ahh.understand.I will try thinking about how to do auto GB with a PI.
Off hand nothing comes to mind as to what 'signal' to obtain and use.

I do have GB on mu HH2 PI but of course it is manual adjust which is done by 'pumping' the coil up and down on 'bad' ground then adjust until no audio response.
Possibly this could be done with a switch to enable 'auto-gb' then 'pump' coil.

[green]

METAL DETECTOR BASICS AND THE0RY by Bruce Candy suggests ground X signal varies, ground R signal doesn't. Maybe PI doesn't need auto ground balance. Needs manual GB adjustment if delay time or sample time is changed.

[6666]

Often thought about a auto ground balance, the highly mineralized ground in the gold fields here in Australia can vary quickly over a short distance

[eclipse]

So maybe Auto Ground balance for a PI with adjustment (Slow/Normal/Fast) switch would be nice to have.
Also will help accommodate different sweep speed. Only thing left is to figure out how to do it ...

[Bayu]

can auto zero using ic lm13600 on sd2000 be applied to the pi scheme ?? I am still formulating the scheme!

[eclipse]

The LM13600 in the SD2000 is for the audio circuit and not related to the ground balance. Look in the previous stages how the GB is done, right before the channel selector switch, but if you are talking about absolute value audio circuit then perhaps you can use ideas from this audio to apply it to your design.