Coil Saturation in a PI?

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"The term "Atocha Coins" refers to silver and gold Spanish coins recovered from the shipwreck of the Nuestra Senora de Atocha."
I learn something new everyday, now only if I could remember the following day.

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It just occured to me that so called "target saturation" might largely depend on the physical orientation of the target with respect to the magnetic flux being presented to it.
With this in mind, a perfectly spherical target can be saturated(energized) differently depending on its orientation, if its molecular structure and composition were not uniform and perfect throughout.
The implication here is that so called "target saturation" is a rather ambiguous term.
The eddy currents can flat top based on just orientation of the target, which would give it, in theory, an infinite number of saturation points,

[ivconic]

Heh, no kidding. Geotech forums are now over 20 years old and so much has been discussed and re-discussed it's impossible to keep track of it all.
https://www.geotech1.com/forums/show...?7033-Welcome! I decided long ago it is what it is. Each thread is a conversation, often re-hashing the same information, but with different audiences. But perhaps some of the more re-hashed information could be compiled into white papers similar to Joe R's fast coil paper.

That would be nice.

[ivconic]

You can saturate
a magnetic core
a dielectric
a bipolar transistor
a MOSFET
an opamp
a CCD
and probably a whole lot more.

My preferred term for coil charging is flat-topping. Steady-state is a more common textbook term.

Target saturation is an unclear term to me. The eddies can flat-top (if the B-field is a linear ramp) but I don't think that's a good application of the term.

What if we consider the target as kind of a core?

It is not most happy choice of a term, though.
Can't disagree.

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It's now become crystal clear the importance of where along the current curve the tx is switched off. If the TC of the target is small, it is washed out by the collapsing field of the coil beyond a certain coil current(given a specific coil TC characteristic). So it becomes a trade off. So the maximum saturation point of the target is really an optimal/optimum point and not an absolute point.

[ivconic]

I propose the following defintion for target saturation:
"A target is said to be saturated when all the magnetic domains in the target are aligned and there are no more available."

Magnetic field from coil can not instantly penetrate the target due eddy currents.
So at first it hits the target surface.
But if pulses are of high current and long enough it will cause the skin effect depth to increase.
"Target saturation" would present state at target with 100% skin effect depth.
This is logical to me, yet i am not sure if it is possible case in real life.
At least that is my understanding of such state.
But since i have no means to practically check and prove this; it is only an assumption.

[dbanner]

Skin effect is dependent on the frequency of the pulses, right? So when we refer to target energization, saturation, we are refering to the eddy currents present at the skin effect depth for that paricular frequency of electromagnetic pulses. It is the absolute maximum saturation point(max. eddy current) for that specific skin depth at that specific frequency and ampiltude of electromagnetic waves and for that particular orientation of the target.
But this is not the point at which the tx is switched off. It is switched off at the optimal saturation point which, depending on the TC of the target, the coil and the current curve, is somewhere before it reaches maximum saturation(especially so for targets with shorter TC)
All this is just my speculation of course.

[dbanner]

The ideal scenario is to have the target at its maximum saturation point(where the optimal would be the same as maximum), but except for very large targets(or long TC), the collapsing magnetic field of the coil stands in the way.
In other vwords, the time taken for the coil current to return to zero would exceed the time taken for the eddy currents to dissipate, rendering the target invisible.

[green]

The ideal scenario is to have the target at its maximum saturation point(where the optimal would be the same as maximum), but except for very large targets(or long TC), the collapsing magnetic field of the coil stands in the way.
In other vwords, the time taken for the coil current to return to zero would exceed the time taken for the eddy currents to dissipate, rendering the target invisible.

Not clear what you are suggesting. I would think target saturation might mean an increase in peak Tx current at Tx off wouldn't increase the target or Rx signal. What is your meaning of target saturation?

Including a spice simulation(coil current and target signal) with different coil resistance, close to constant rate, charge TC=150us and charge TC=30us. Power supply volts adjusted for 2amps at 300us Tx on time.
Does the simulation make sense? Are there changes to the simulation that would make it better? Is there a way to simulate target saturation?

[Tinkerer]

Atocha silver bar

Yes, yes, and yes.

Most PI designs do not attempt to flat-top the current, but it's nice to have an understanding of what's going on if, for example, you want to detect Atocha bars.

An Atocha silver bar of about 90 pounds.

[ivconic]

Things are not that simple as i initially understood.
I don't mind really if you gonna call it "saturation" or "steady-state" or even "flat-topping" ... as long as i am able to understand the process.
And most important; whether or not any "saturation" is having any importance in detection and discrimination of target.

[ivconic]

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An Atocha silver bar of about 90 pounds.

Aha! So!
It becomes clear to me now what you did there so many years!
Where there is any silver in ore veins; there is a gold too.
And once you take it; what is the best place to keep&hide it.... CH of course!

[waltr]

Not clear what you are suggesting. I would think target saturation might mean an increase in peak Tx current at Tx off wouldn't increase the target or Rx signal. What is your meaning of target saturation?

Including a spice simulation(coil current and target signal) with different coil resistance, close to constant rate, charge TC=150us and charge TC=30us. Power supply volts adjusted for 2amps at 300us Tx on time.
Does the simulation make sense? Are there changes to the simulation that would make it better? Is there a way to simulate target saturation?

Interesting Spice simulation that does show Target response verse series resistance with coil.
The first sim, with coil R = 10mOhm, it takes a long time for the current to peak (flat top) and the target response is greatest with largest coil current.
The last sim, with coil R =10 Ohm, the current peaks (flat tops) much sooner so TX pulse time can be shorter to obtain the same response.

These sims were with a short TC target (US nickle) and as I have stated in above posts a short TX pulse works.
I would like to see these same sims done with a longer TC target, US Quarter.

Also, why are the coil inductance and damping R different in the three sims? They should all be the same then ONLY change the Series Resistance.

To throw in another factor: A larger series R gets the current to peak sooner. This allows shorter TX pulse times. This then allows high pulse rate (pps) which is known to improve over all detector sensitivity IF the integrator stage has the optional values of C & R. Read posts by Eric Foster about this.

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My point was that the target is never fully saturated, but is saturated to a point along the current curve which is optimum for the coil TC versus the target TC. The Tx is switched off at this optimal point. Otherwise there is a point of diminishing returns(depending on the target TC)

[waltr]

We seem to be in a bad place with this thread's title.
Saturation really does not apply to either the coil or the target and needs to be struck from being used when discussing PI detectors.

What is important is the coil current reaches a 'steady state' so that the eddy currents due to the increasing magnetic field have died out before switching OFF the coil current.