Ground Balance

[6666]

No worries I'll go find a few different magnets and do some more testing and report back shortly.

Hi Mick do you remember what types of magnets you used for testing the EF? where they ceramic ?

[revmaster]

Have tried this earlier, but with smaller target sample the S/N ratio is reduced a lot...

[Carl-NC]

To simplify, I would turn off the EMI sample and work with just Target & GB sample. Try reducing the Target sample to 10us. Also try increasing the GB delay.

[revmaster]

Hi Carl,
My timing is as under;
Tx=150us
Delay Period=16us (Have a fairly fast coil)
Target Sample=20us
Delay=10us (Did try to adjust this delay)
GB Sample=(variable)
After long delay=EMI sample (GB sample width-target sample width)

GB Sample width = Target sample width + EMI sample width.

[Carl-NC]

What is your timing without GB, and what is it with GB?

[revmaster]

Did anyone tried PWM based GB, i tried but has been unable to find sweet spot.... If i try to balance soil the sensitivity is reduces a lot.

I am using differential integrator and have taken 3 samples: Target(+), GB(-) and EF(+). Width of Target + EF = GB Width.
Any inputs as where am i going wrong....

[greylourie]

I don't know about "round to-its" or "round tuits", but there appears to be more than one "fundi" posting here. (I will just quietly go off, and mindlessly drool at the images posted up by Carl and Davor - it will sink in...gradually)

Thanks for the round to-it, Carl.

[Carl-NC]

Yep, that's true, and pretty easy to do when you use PWM for the GB sample instead of variable gain. When you move to a 3-sample GB scheme this makes even more sense.

[Davor]

I may have an addendum to this one:

Subtractive GB does not require diff integrators, you could use the non-diff integrator above for each channel (target & GB). You also don't have to use a variable gain stage. Instead, sample the ground with a variable sample width and now the width is the GB control. And since the ground channel no longer needs a different gain, there is no longer any need for a separate channel... you can do everything in the single non-diff integrator channel above using this timing:

[ATTACH]30591[/ATTACH]

You can also overlap Target and GB channels' EF samples, and notice that in a case of equal gain in positive and negative subchannel thea cancel each other while they overlap.So instead you may also use a single shorter EF to the same effect. That's basically what the scheme I proposed earlier would do, and the arithmetic is simple as all the samples are quantised.

[Carl-NC]

True, but generally you set the integrator tau to achieve a minimal target latency and then you get what you get in terms of how many cycles are integrated. At 3000 pps a 1500-cycle integration will probably give too much delay. By using bipolar pulsing and a short TX pulse width you could run the pulse rate upwards of 20,000 pps and, for a given integrator tau, integrate more samples. Since target signals correlate and noise does not, this could be a win-win solution.

[bbsailor]

OK, I found my round tooit so here are a few drawings to clarify. First, in a common non-GB PI (Hammerhead, SurfPI, etc) we often see the differential integrator:

[ATTACH]30586[/ATTACH]

Most of these designs have a target sample and an Earth-field sample:

[ATTACH]30587[/ATTACH]

The differential nature of the integrator subtracts the Earth field from the target signal. For best results you want good matching between the time constants, which usually means getting well-matched caps.

You can do the same thing with a non-diff integrator:

[ATTACH]30588[/ATTACH]

Now both samples use the same time constant. The gain of -1 is pretty easy to make sufficiently accurate. Timing is the same as above.

When we implement subtractive GB we normally sample shortly after the target, amplify the GB sample, and subtract from the target sample. This is usually done with a completely separate channel and its gain is the GB control. Again, matching the taus is critically important, and if the target sample has an Earth-field subtraction then so must the GB sample:

[ATTACH]30589[/ATTACH]

Timing is now:

[ATTACH]30590[/ATTACH]

Subtractive GB does not require diff integrators, you could use the non-diff integrator above for each channel (target & GB). You also don't have to use a variable gain stage. Instead, sample the ground with a variable sample width and now the width is the GB control. And since the ground channel no longer needs a different gain, there is no longer any need for a separate channel... you can do everything in the single non-diff integrator channel above using this timing:

[ATTACH]30591[/ATTACH]

Finally, I've mentioned before the benefits of bipolar pulsing, namely getting rid of the Earth-field samples:

[ATTACH]30592[/ATTACH]

This potentially allows for much faster pulse rates.

Carl,

One more thing that you may want to consider is the integration of from 500 to 1500 RX cycles such as what Eric Foster has introduced into the PI technology. This now puts the focus on methods to extract a meaningful signal from and within varying noise levels rather than increase the TX power. If you look at how lock-in amplifiers work you will see a similarity between the number of samples taken and Eric Foster 3,000 PPS PI machines with incresed RX sensitivity. Many PI machines have been using this technique but it is good to see how the lock-in technology has been adopted to PI machines beeing able to see very small target signals even in the presence of noise. Just do a web search on "lock-in amplifier" to begin to connect the dots.


bbsailor

[Qiaozhi]

OK, I found my round tooit ...

If anyone else needs a "round tuit", you can buy them on eBay and Amazon.
A very useful device to have around when you need to get something finished..

[Carl-NC]

OK, I found my round tooit so here are a few drawings to clarify. First, in a common non-GB PI (Hammerhead, SurfPI, etc) we often see the differential integrator:



Most of these designs have a target sample and an Earth-field sample:



The differential nature of the integrator subtracts the Earth field from the target signal. For best results you want good matching between the time constants, which usually means getting well-matched caps.

You can do the same thing with a non-diff integrator:



Now both samples use the same time constant. The gain of -1 is pretty easy to make sufficiently accurate. Timing is the same as above.

When we implement subtractive GB we normally sample shortly after the target, amplify the GB sample, and subtract from the target sample. This is usually done with a completely separate channel and its gain is the GB control. Again, matching the taus is critically important, and if the target sample has an Earth-field subtraction then so must the GB sample:



Timing is now:



Subtractive GB does not require diff integrators, you could use the non-diff integrator above for each channel (target & GB). You also don't have to use a variable gain stage. Instead, sample the ground with a variable sample width and now the width is the GB control. And since the ground channel no longer needs a different gain, there is no longer any need for a separate channel... you can do everything in the single non-diff integrator channel above using this timing:



Finally, I've mentioned before the benefits of bipolar pulsing, namely getting rid of the Earth-field samples:



This potentially allows for much faster pulse rates.

[Davor]

Yup!
With a single pair EF pulse is duration 1, with 2 pairs EF pulse is duration 5, with 3 pairs EF pulse is duration 21, and so forth. It is simple only for a single pair of target/GB samples. EF pulse also needs additional time to prevent it from spoiling the rest of the math, and the only natural answer to this would be going for alternating Tx pulses to avoid EF altogether. It could make a whole process highly simplified as all the timing would remain in simple power of 2 counters ... including Tx.

I'm also considering constant current Tx, as it prevents diminishing target response of large tau targets, and keeps coil currents sane. I guess classic unregulated Tx also influences the effective ground response exponent, as for the longer delays these responses converge ... provided a flyback does not "reset" the previous viscous response "memory".

[WM6]

Going to alternating Tx pulses may be a great answer to this problem.

What mean here "alternating"? Changing in polarity in pulses?