Tinkerer, Missed something on that coil balance thing

[GT Blocker]

Hey Tinkerer,
I decided to move the coil question/answer (http://thunting.com/geotech/forums/s...ad.php?t=14756 post #24) to here. I posted the following TP4 scope shot just before that post:



And you replied:

"Here is a drawing of the balanced coil output at TP4. I don't have a HH to do the actual picture.
The wave shape depends somewhat on the amount of power in the coil and the coil specs.
The coil can only be balanced for one specific coil current and\or pulse width.
Anytime you change the coil current you have to adjust the damping and the balance of the coil.
On the drawing I show the sampling point where the signal crosses the 0V line.
At this point, the signal goes positive for FE or negative for all non magnetic metals, or the opposite, depending if the coil or preamp is inverting.
I also show the effect of a FET transistor blocking the Flyback. In this case it is a shunt to ground. It can also be in series to block the signal totally.

Tinkerer"

I can get the trace to dip with that slight ringing at the start if I play with the bucking windings. However, your drawing shows a very sharp rise back to zero whereas I have a flat line for about 5uSec. Why is that? Is that sharper rise a result of the flyback blocking? If so, how do I implement that?

Especially interesting was "The coil can only be balanced for one specific coil current and\or pulse width". Could you elaborate on that?

Any insights that you or anyone else has would be appreciated. Thanks, GTB

[Tinkerer]

Hey Tinkerer,
I decided to move the coil question/answer (http://thunting.com/geotech/forums/s...ad.php?t=14756 post #24) to here. I posted the following TP4 scope shot just before that post:

[ATTACH]6992[/ATTACH]

And you replied:

"Here is a drawing of the balanced coil output at TP4. I don't have a HH to do the actual picture.
The wave shape depends somewhat on the amount of power in the coil and the coil specs.
The coil can only be balanced for one specific coil current and\or pulse width.
Anytime you change the coil current you have to adjust the damping and the balance of the coil.
On the drawing I show the sampling point where the signal crosses the 0V line.
At this point, the signal goes positive for FE or negative for all non magnetic metals, or the opposite, depending if the coil or preamp is inverting.
I also show the effect of a FET transistor blocking the Flyback. In this case it is a shunt to ground. It can also be in series to block the signal totally.

Tinkerer"

I can get the trace to dip with that slight ringing at the start if I play with the bucking windings. However, your drawing shows a very sharp rise back to zero whereas I have a flat line for about 5uSec. Why is that? Is that sharper rise a result of the flyback blocking? If so, how do I implement that?

Especially interesting was "The coil can only be balanced for one specific coil current and\or pulse width". Could you elaborate on that?

Any insights that you or anyone else has would be appreciated. Thanks, GTB

For the FE discrimination you have to get the TX pulse to OV at the output of the preamp. With high gain, the wave form will not be totally flat, but with a slight ringing. You get this by adjusting the Bucking coil.

Any time you change anything with any one of the three coils, all 3 coils are affected, since they are all transformer coupled. If you increase the TX time, you increase the coil current.
When I get home tonight, I will post the url of MISCEL, this is a small software, (free) that calculates all sort of very useful parameters, including the charge curve for inductance.
With the HH platform, the FE sample will have to be taken at the 0 crossing as shown.

The Wave form shown after TX OFF, shows the Flyback truncated. I achieve that by shunting to ground, using a FET instead of the diodes.
This eliminates the saturation of the opamp, thus reducing the delay to first sample.

Tinkerer

[Tinkerer]

miscel calculator

Here is the link to miscel.

http://www.miscel.dk/MiscEl/miscel.html

Download and install. It is free and probably the best tool for working with PI coils.

Now open "Charge curve" then fill in the inductance and resistance of your TX coil.
The click on the curve at the number of us of your TX pulse.
Now you see at the bottom the current that is flowing at this moment in the coil.

Play a bit with it, changing inductance and resistance and observe the changes.

Tinkerer

[Tinkerer]

DISCRIMINATING PI TEST

attached is an EXCEL sheet with a DISCRIMINATING PI TEST showing
ALL METAL SAMPLE and
FE DISCRIMINATING SAMPLE


Tinkerer

[Qiaozhi]

attached is an EXCEL sheet with a DISCRIMINATING PI TEST showing
ALL METAL SAMPLE and
FE DISCRIMINATING SAMPLE


Tinkerer

You forget to attach the spreadsheet.

[Tinkerer]

EXCEL FE test

Seems it did not upload. I try again.
Ah, I see EXCEl is not accepted lets see if I can change it into something else.
OK, Text works, it looses the colors but is readable
Tinkerer

[Qiaozhi]

Seems it did not upload. I try again.
Ah, I see EXCEl is not accepted lets see if I can change it into something else.
OK, Text works, it looses the colors but is readable
Tinkerer

Try compressing it into a zip file.

[Tinkerer]

PI FE test sheet

Qiaozhi,
thanks for the tip.
Here is the Zip file

Tinkerer

[GT Blocker]

Wave forms

Hey there Tinkerer,

I now have "The Beast" coming back up cleanly to 0v at 8.5uSec, but I can no longer get the 'sine-wave' shape before the blocking pulse.

"For the FE discrimination you have to get the TX pulse to OV at the output of the preamp. With high gain, the wave form will not be totally flat, but with a slight ringing. You get this by adjusting the Bucking coil."

I am wondering if the new two stage pre-amp config with it's lower gain stages has an effect on this. At one point I had the TP4 output rise or fall depending on ferrous/non-ferrous targets, no longer. All metal types now show a voltage drop. I spent two days 'tweaking' the bucking/damping to get down to 8.5uSec, with several iterations required.

I worry that I would have to sacrifice sampling time for discrimination. That would be unacceptable, as I dig ALL targets. In trashy areas in the hills here, the old timers were apparently adept at tossing their trash right on top of a nuggie. So I dig the trash and re-check the hole. Which makes me wonder if it is worth the trouble to continue looking for that elusive waveform. I wish that I had paid more attention to my sampling times when I did show similar results to your waveform.

Thoughts?

On another note, you said," I achieve that by shunting to ground, using a FET instead of the diodes."
Would you mind showing a schematic/sketch of how you replaced the diodes?



Thanks for your help,
GTB

[Tinkerer]

DISCRIMINATING WITH THE HH

[QUOTE=GT Blocker;81881]Hey there Tinkerer,

I now have "The Beast" coming back up cleanly to 0v at 8.5uSec, but I can no longer get the 'sine-wave' shape before the blocking pulse.
You are doing fine. The two stage pre-amp has a lot of different effects. I find that every different type behaves differently. I made a similar observation recently, when I changed the opamps for the preamp.
The flat shape works perfectly well too, as long as the balance point is right and you get back to 0V at TP4

"For the FE discrimination you have to get the TX pulse to OV at the output of the preamp. With high gain, the wave form will not be totally flat, but with a slight ringing. You get this by adjusting the Bucking coil."

I am wondering if the new two stage pre-amp config with it's lower gain stages has an effect on this. At one point I had the TP4 output rise or fall depending on ferrous/non-ferrous targets, no longer. All metal types now show a voltage drop. I spent two days 'tweaking' the bucking/damping to get down to 8.5uSec, with several iterations required.
Don't worry about the 8.5uSec. The signal amplitude during the TX ON, is about 10 times the amplitude after TX OFF. Take your first sample as soon as possible after TX ON, the second sample just before TX ON.

For adjusting the balance only look at the TX ON part. Adjust it to get the best discrimination. The signal amplitude is so high, you can easily see the positive and negative responses at TP4.

I found the best target for adjusting the discriminating balance is a steel bottle top. It is a very difficult target to discriminate because in the flat position the reactive response is nearly the same as the resistive response. The one being positive and the other being negative, they cancel each other.
However, if you present the bottle top on edge to the coil, it shows a strong FE reactive response. Use that for the adjusting. Since steel bottle tops seem to be going out of fashion in some parts of the world, you can also use any small piece of thin steel sheet.
I usually do the adjustment with low gain at the preamp and then increase the gain. I have used gains up to 3000 on the preamp, when using the discriminating TX ON sample.


I worry that I would have to sacrifice sampling time for discrimination. That would be unacceptable, as I dig ALL targets. In trashy areas in the hills here, the old timers were apparently adept at tossing their trash right on top of a nuggie. So I dig the trash and re-check the hole. Which makes me wonder if it is worth the trouble to continue looking for that elusive waveform. I wish that I had paid more attention to my sampling times when I did show similar results to your waveform.

Thoughts?

On another note, you said," I achieve that by shunting to ground, using a FET instead of the diodes."
Would you mind showing a schematic/sketch of how you replaced the diodes?

You mentioned a blocking pulse above. How do you implement that?
I will send you a pdf with the Mosfet shunt. This does not actually block the Flyback, but acts more like a controlled diode.

Tinkerer

[GT Blocker]

That discrimination thing...

Hey there Tinkerer;
I've been a bit busy (holidays, et al), sorry for the delay.

In your last post you said,"You mentioned a blocking pulse above. How do you implement that?
I will send you a pdf with the Mosfet shunt. This does not actually block the Flyback, but acts more like a controlled diode."

I have not tried to implement a blocking pulse as of yet. I was referencing a drawing from one of your posts above as it related to my current timings.
Sorry if I confused the discussion.

I still don't quite get the shunting with a mosfet, however.

On another note, is it possible to get similar results from a mono-coil configuration? Or does the waveform depend on the balance between two coils?

Thanks, GTB

[Tinkerer]

Hey there Tinkerer;
I've been a bit busy (holidays, et al), sorry for the delay.

In your last post you said,"You mentioned a blocking pulse above. How do you implement that?
I will send you a pdf with the Mosfet shunt. This does not actually block the Flyback, but acts more like a controlled diode."

I have not tried to implement a blocking pulse as of yet. I was referencing a drawing from one of your posts above as it related to my current timings.
Sorry if I confused the discussion.

I still don't quite get the shunting with a mosfet, however.

On another note, is it possible to get similar results from a mono-coil configuration? Or does the waveform depend on the balance between two coils?

Thanks, GTB

Attached is the front end, RX coil with damping, shunt Mosfet and single stage pre-amp. You can see a very high gain. This circuit was tested extensively and gave great results. The diodes are not needed anymore, but I left them in place on the breadboard to protect the preamp in case I goof up.
Since then I have changed to a 2 stage preamp that works even better.
On the Kingfisher from KingJL, you can see a Mosfet blocking that is probably more efficient.

The FE discrimination can only be done with an Induction Balanced coil. The wave form depends entirely on the balance of the coils.

Attached are 2 patents that use this principle.

Tinkerer

[GT Blocker]

one more question Tinkerer...

I think I get it...I'm starting a new coil then. One question though, does the Fe balance/waveform shift with the pulse width/frequency, or is it that once the coil is dialed in that it's fine across the range?

Thanks (again), GTB

[Tinkerer]

FE DISCRIMINATION

[quote=GT Blocker;82381]I think I get it...I'm starting a new coil then. One question though, does the Fe balance/waveform shift with the pulse width/frequency, or is it that once the coil is dialed in that it's fine across the range?

Thanks (again), GTB[/quote

It shifts the same way as the timing for the first sample after switch OFF shifts. So if you have a PIC control, you would program the whole sequence, or rater a series of different sequences and store them in the EEPROM.

Tinkerer