DEEPER PI DETECTION DEPTH

[Tinkerer]

The simulation model of the target cannot "saturate", unless you add a core model.

The expression "saturate" is often used, it could be changed to another expression often used,"fully charged", which is just as wrong, to say the the eddy currents have reached a maximum level.

Tinkerer

[Mechanic]

Mick,

what is better:
Wasting high power for a long pulse duration (saturated TX coil current) or
wasting low power for multiple short pulses (collecting the signal response over multiple short pulses)?

This question applies to all members of course.

Cheers,
Aziz

G'day Aziz,

The method that finds big chunks of gold of course

For big gold I am leaning toward the longer slower pulse as it will take longer for the eddy currents to build and decay during the on time. The higher the current in the target after the coil current has reached 0 the better well thats my thoughts anyway.

Cheers Mick

[Aziz]

G'day Simon,

Well my thoughts on this are that although case 1 has a higher current change in the target, the problem is once the turn off current has reached 0 and we are able to sample the target only has 250uA compared to the 650uA in the target that had the flat top(case 2) We are only able to measure the signal during the off time conventionally but if something can be worked out to sample during the flyback and reject ground response well then case 1 would win.

Now what if we were able to make a current pulse that is square wave, rather than the current pulse that we end up with, with the resistor in series with the coil. We would have a fairly rapid rise at tx turn on and once the desired current is achieved(say 3A) we then hold it at that for(200uS) and then turn it off. This will give enough time for eddie currents in larger targets to settle before turn off. A switch-mode power supply would be ideal to drive this to minimize losses. Yes this will use more battery power, but who cares, li-ion batterys are cheap and light and if I'm finding more gold it will worry me not

Cheers Mick

Mick,

what is better:
Wasting high power for a long pulse duration (saturated TX coil current) or
wasting low power for multiple short pulses (collecting the signal response over multiple short pulses)?

This question applies to all members of course.

Cheers,
Aziz

[Qiaozhi]

Something to think about.

Glitch disappears when 100 ohms is added to the gate, which slows down the slew rate.

- Carl

The same thing happens in the simulation. Increasing the gate resistor to 100 ohms almost removes the glitch.

However ... just to throw a spanner in the works ... here's a simulation with the mosfets replaced by ideal switches. The glitch is still there.

[Qiaozhi]

Does this suggest that the target is "saturated"?

The simulation model of the target cannot "saturate", unless you add a core model.

[Mechanic]

Actually, case 1 has a larger delta I (change in current) in the target. Since magnetic field follows current, could this not actually create a larger signal in an RX coil? Or at least virtually as much? In other words, no penalty for established current?

-SB

G'day Simon,

Well my thoughts on this are that although case 1 has a higher current change in the target, the problem is once the turn off current has reached 0 and we are able to sample the target only has 250uA compared to the 650uA in the target that had the flat top(case 2) We are only able to measure the signal during the off time conventionally but if something can be worked out to sample during the flyback and reject ground response well then case 1 would win.

Now what if we were able to make a current pulse that is square wave, rather than the current pulse that we end up with, with the resistor in series with the coil. We would have a fairly rapid rise at tx turn on and once the desired current is achieved(say 3A) we then hold it at that for(200uS) and then turn it off. This will give enough time for eddie currents in larger targets to settle before turn off. A switch-mode power supply would be ideal to drive this to minimize losses. Yes this will use more battery power, but who cares, li-ion batterys are cheap and light and if I'm finding more gold it will worry me not

Cheers Mick

[Aziz]

..how big a jump

=Supply current/voltage

..how fast

=Supply frequency

A target (coil) has an impedance, which is frequency dependent.

Aziz

PS: To see the induced voltage behaviour at the receiver coil, just replace the I(L3) by d(I(L3)). d() is the first derivation of the term in brackets.

PPS:
A high stimulation frequency will lead to a lower target eddy current (due to impedance). On the other hand, the RX coil would see more induced voltage due to higher eddy current change dI/dt.
So, where is the free lunch?

[Carl-NC]

Unfortunately, Carl has not given the target TC's. The target TC is what makes the difference.

US nickel and US silver dollar, whatever they are.

Looks like Miller effect. Fast voltage changes on the drain capactively couple through to the gate turning it back on. Definitelty a real life effect I was getting it on my POS MD. I implemented the same solution.

Agree, below is a real oscope plot (using my nifty new Rigol) of my test circuit, upper trace is the coil current. Glitch disappears when 100 ohms is added to the gate, which slows down the slew rate.

- Carl

[Midas]

Edit on the above.
I just run it again, and it has now the same oscillation in both sims. The oscillation disappears when the Mosfet gate resistor is changed from 10R to 100R, or, in other words slowing down the switching a little bit.

Looks like Miller effect. Fast voltage changes on the drain capactively couple through to the gate turning it back on. Definitelty a real life effect I was getting it on my POS MD. I implemented the same solution.

[simonbaker]

I think we need to stick with Carl's numbers, 100us TX. Otherwise we compare apples and potatoes.

Unfortunately, Carl has not given the target TC's. The target TC is what makes the difference.

On the sawtooth, when we look at the eddy currents increasing during TX ON, we see that the 15us target eddy currents increase until about 50us, or roughly 3TC, then they start decaying, in spite of the current ramp still raising nearly linear. Does this suggest that the target is "saturated"?

The longer TC eddy currents are still rising at switch OFF.

When we look at the flattop, we can see the same behavior, but the decay is enhanced as the delta I diminishes and goes flat.
There is a current spike at switch ON, that distorts the behavior a little.

Could it be that it makes a difference if the eddy currents are still raising at switch OFF, or that they are already decaying?

Tinkerer

Can you attach your latest simulation file if you updated? I'm not sure I'm looking at the exact same data. Does Qiaohzi file have all your latest changes in it? His is the easiest to use to compare for some questions.

I think we have to keep in mind the di/dt of it all. The current in the target I think should be driven the di/dt of the current in the TX coil. The current in our MD RX coil will be driven by the di/dt of the current in the target. We have double differentiation here the way I see it.

On the sawtooth, when we look at the eddy currents increasing during TX ON, we see that the 15us target eddy currents increase until about 50us, or roughly 3TC, then they start decaying, in spite of the current ramp still raising nearly linear. Does this suggest that the target is "saturated"?

Basically yes, but... The reason the slow target current is still increasing in magnitude is that it is far from saturated and the TX slope is almost constant.

However, the slope of the sawtooth is actually decreasing at a slow rate.

The reason the fast target starts decreasing when it does is because it is tracking the slope of the TX current much faster (than the slow target) and, although it does not completely "saturate" (of course it takes infinite time to completely saturate) to the initial slope of the TX current, it actually grows higher than for the slope of the TX current later on (sorry for that head-scratcher). So it starts tracking down to be proportional to the reduced slope of the TX current.

(Use photoshop and draw a line from the start to the end of the sawtooth ramp and you can see how it is decreasing in slope.)

If the TX current was a perfect linear ramp, even the fast target should be still increasing and approaching a constant level, and naturally way ahead of the slow target.

Pretty sure I got that right...

Could it be that it makes a difference if the eddy currents are still raising at switch OFF, or that they are already decaying?

My own personal intuition is that our RX coils will respond like a "secondary target", so they will be driven by the di/dt of the target current and respond according to their own dynamics (which is second-order, not first-order like normal targets).

Looking at the graphs, I think it really is more a matter of how big a jump and how fast the target current changes on its wild ride, which wouldn't seem to depend much on the direction the its eddy currents are flowing, more on the level they are currently at if anything (although it doesn't seem to depend very strongly on that either).

We really need to add the RX signal response to these simulations and look at that, not just the target signal.

-SB

[Tinkerer]

Actually, case 1 has a larger delta I (change in current) in the target. Since magnetic field follows current, could this not actually create a larger signal in an RX coil? Or at least virtually as much? In other words, no penalty for established current?

-SB

I think we need to stick with Carl's numbers, 100us TX. Otherwise we compare apples and potatoes.

Unfortunately, Carl has not given the target TC's. The target TC is what makes the difference.

On the sawtooth, when we look at the eddy currents increasing during TX ON, we see that the 15us target eddy currents increase until about 50us, or roughly 3TC, then they start decaying, in spite of the current ramp still raising nearly linear. Does this suggest that the target is "saturated"?

The longer TC eddy currents are still rising at switch OFF.

When we look at the flattop, we can see the same behavior, but the decay is enhanced as the delta I diminishes and goes flat.
There is a current spike at switch ON, that distorts the behavior a little.

Could it be that it makes a difference if the eddy currents are still raising at switch OFF, or that they are already decaying?

Tinkerer

[simonbaker]

Simon - I took your simulation and hacked it around to create a comparison between a PI without a resistor in series with the coil (case 1), and another with a 15 ohm resistor in series (case 2).

In case 1, the charging period is 18us.
In case 2, the charging period is 100us.
At the time the mosfet is ready to be switched off, they both have the same current flowing in the coil.
Plot "No-res" and "With_res" in one plot pane, and "Target1" and Target2" in a second pane.
As you will see in case1, the eddy currents are fairly well established in Target1 when the mosfet switches off. These eddy currents need to be overcome, resulting in a lower target response when compared to case2, where the eddy currents in Target2 have essentially decayed almost to zero.

These results appear to agree nicely with the practical test performed by Carl.

Actually, case 1 has a larger delta I (change in current) in the target. Since magnetic field follows current, could this not actually create a larger signal in an RX coil? Or at least virtually as much? In other words, no penalty for established current?

-SB

[Qiaozhi]

Good rearrangement for comparing the two.

BTW - That's Tinkerer's nice LTSpice simulation.

-SB

Oh yes - sorry, got lost in all the replies.

[Tinkerer]

How did that change your target response comparison?

-SB

OK, the fact of having R100 on the saw-tooth and R10 on the flattop, was a slip-up. Should be the same for comparison.
However, after correcting, the difference in signal response is insignificant.

To resume, for a short TC target, like TC=15us the result is similar to Carl's.

For longer TC's, the difference increases. This coincides with the opinion of many PI design aficionados and the theory of Eric Foster.

I am not happy with this result. Tests that I did in the past, were more in line with Carl's results.

So what is the cause of the difference?

Can we find something in the wave form of the eddy currents during TX ON, that could explain the result? Let's take a closer look.

Tinkerer

[simonbaker]

Simon - I took your simulation and hacked it around to create a comparison between a PI without a resistor in series with the coil (case 1), and another with a 15 ohm resistor in series (case 2).

In case 1, the charging period is 18us.
In case 2, the charging period is 100us.
At the time the mosfet is ready to be switched off, they both have the same current flowing in the coil.
Plot "No-res" and "With_res" in one plot pane, and "Target1" and Target2" in a second pane.
As you will see in case1, the eddy currents are fairly well established in Target1 when the mosfet switches off. These eddy currents need to be overcome, resulting in a lower target response when compared to case2, where the eddy currents in Target2 have essentially decayed almost to zero.

These results appear to agree nicely with the practical test performed by Carl.

Good rearrangement for comparing the two.

BTW - That's Tinkerer's nice LTSpice simulation.

-SB