Coil Saturation in a PI?

[bklein]

I tested some surf pi/DF/GP Extreme coils across machines, posted photos in the Coils section. It made me wonder: you can see the effects of coil saturation with a current probe. It seemed the saturated coils had more depth than on their original machines. Is this because of better rx channel circuits on the GP Extreme alone? Is optimized design accomplished where the coil almost saturates with the longest tx pulse - for a given coil, increase the tx voltage (and thus current) to the coil saturation point and thats the best you can do - or does going into saturation still increase depth a bit more?

[Carl-NC]

What do you mean by saturated? The only case I saw that looked to be near saturation was DFonExtreme, and only for the long pulse:

https://www.geotech1.com/forums/attachment.php?attachmentid=47617&d=1569390851

So in this case you got better depth than with the DF on a TDI? Or with a GPExtreme coil on a TDI? Or both? Not sure what is being compared.

[ivconic]

I think it is a wrong term.
How to saturate coil?
You can saturate core, RX front end opamp, any other opamp ... etc.
But simple stupid coil, few turns of wires... how to saturate it?

[Carl-NC]

Saturation means "can't get any higher." It's ok for this, because it's the point where the TX current can't go higher (5tau).

[ivconic]

Saturation means "can't get any higher." It's ok for this, because it's the point where the TX current can't go higher (5tau).

Ah ok! That's more like it.
Thanks!

[ivconic]

Probably totally out of topic...
Back in time; we used to call it "A class" working regime... in quite different kind of technology...

[6666]

I have a dim distant memory of Sean Goddard years ago posting about experimenting with PI coil saturation , I think he was saying that he was monitoring the coil peak voltage while increasing / decreasing the tx pulse width, and once the voltage peaked there was no point in increasing the tx pulse width further as the coil was saturated and you were just wasting current .
Which brings about another question can you use pulse width to enhance a certain type of target signal response.
And is saturating the coil a good thing.

[ivconic]

1) Yes. I think so.
2) No. Maybe yes; in case you don't mind longer first sample delay.

[waltr]

I have increase response of long TC targets by increasing the PI TX pulse time.
The general 'rule' is the pulse time to be 3 to 5 times the target TC. But in practice the pulse time needs to be only a bit longer than the target TC.
Example is on my HH2 increasing the pulse time from 100usec to 200usec increased the response of 'silver' coins in particular US quarters which have a TC of about 170us.

Theory also shows that it is the time of 'constant' current in the coil verse target TC to obtain the largest response.

[Carl-NC]

Which brings about another question can you use pulse width to enhance a certain type of target signal response.

Waltr is correct, a longer pulse width is better for long TC targets. The reason is it allows the "on-time" eddy currents to decay out, so they have less cancelation effect on the "off-time" eddy currents.

However, a longer pulse width also creates a higher flyback with longer decay settling, meaning you will need a later sample delay. That's OK if you are trying to enhance long TC targets anyway. So for short TC targets, you want a short pulse width but with still reasonably good flat-topping (add a series R) and lower flyback for faster settling. Minelab's solution is to use a wide TX pulse plus 3 narrow TX pulses.

[6666]

Thanks for replys from you both Waltr and Carl

Minelab's solution is to use a wide TX pulse plus 3 narrow TX pulses.

Carl do you recall when they take target and efe samples ? is it after all the long and short tx pulses finish ? thanks

[Carl-NC]

I don't recall, but I suspect it's in the patent.

[6666]

I don't recall, but I suspect it's in the patent.

Thanks

[bklein]

Turns out it looks like I was completely wrong in my basic supposition. After reading a bit on the subject, there will be no saturation point on a coil with an air core. So that strikes me down right away. Secondly, I had the theory completely wrong - the current will not roll off on saturation, it will rise linearly until saturation and then quickly increase - only limited by the resistance of the coil. This Maxim appnote describes some of the theory but what is still confusing to me is that it describes the current ramp through the inductor as exponential while discussions on saturation show a linear ramp until saturation starts.

There are some interesting inductor saturation tester projects on the net - they may be worth checking out.

The Beach Hog coil is for the TDI - I don't have a TDI unfortunately. I almost bought one just to see the TX waveform. The DF coil is a spare from a DF I have. Both this and the Beach Hog are higher resistance coils than the Mono for the GP Extreme. Both the Beach Hog and the DF coils work pretty well on the GP Extreme, and have increased depth over running them on the DF controller.

At least I started an interesting conversation, please continue :-)

[Carl-NC]

??? I think you're talking 2 different kinds of saturation. A ferrite core inductor can have core saturation. Wrong kind of saturation for us. In our case, we apply a voltage to a coil and, because of series resistance, the current exponentially rises and eventually (at ~5τ) flat-tops and we consider this to be "saturated." Yeah, now I'm thinking Ivica has a point.