DEEPER PI DETECTION DEPTH

[simonbaker]

***** Deleted at request of poster - sensitive information. *****

You should sell the secret, not the MD! That's the best pitch I've heard!

-SB

[Tinkerer]

You do have a point, micros emit a load of noise.

You have to be clever to get rid of it/reduce it to a tolerable level.

It can be weeks of work too. Months on complex radios with heavy processor power / data flying round.

I can do this stuff but Ive had years of fiddling with filters, screens, copper tape, conductive polymers, layout care and have ulcers and grey hair from the experience of doing it for customers like Tosh, HP, Siemens, etc.

S

Would you like to apply all this experience into a fun project? To design an extreme deep detecting machine?
They tell me that having a lot of fun heals the ulcers and prolongs life.

Tinkerer

[golfnut]

You do have a point, micros emit a load of noise.

You have to be clever to get rid of it/reduce it to a tolerable level.

It can be weeks of work too. Months on complex radios with heavy processor power / data flying round.

I can do this stuff but Ive had years of fiddling with filters, screens, copper tape, conductive polymers, layout care and have ulcers and grey hair from the experience of doing it for customers like Tosh, HP, Siemens, etc.

S

[Sean_Goddard]

OK, I've had my laugh now..

Will SOMEONE PLEASE tell me WHY people have an insistence of putting a micro in a PI?

You are looking for nV level signals and to be honest none of the PCB's I've seen here so far (with ALL due respect to the hard, excellent work done by those who post them) are what might be termed "low noise designs".

Try this, run a PIC in a SIMPLE timing loop for a PI and put it next to an AM radio receiver. Listen to how much noise there is! Well, ALL of that is getting coupled into your PI RX side too (as it's a broadband system by nature). Sooooooo, keep micros of ANY sort WELL away from PI designs, go for a low noise PCB layout (WATCH OUT for ground loops especially) and you wont go far wrong.

Oh and remember, DECOUPLE EVERYTHING!

[golfnut]

It may be possible to grow the rising edge slowly - to very short flat top, then abrupt shut off as normal. Easy with an DtoA from a micro


1)Rising edge introduces little or no target eddys (di/dt rising-edge = small)
2)Saves battery life (current/area under the curve)

Steve

[simonbaker]

It is true that the eddies from an unsettled turn-on will partially cancel the turn-off eddies, but I also doubt it is very significant. It would be interesting to test for this, by using a series resistor to flat-top the current on a wide TX pulse width, then removing the series R and reducing TX pulse width until the peak coil current at turn-off is the same as the flat-top current.

You could also argue that, although the unsettled current opposes the turn-off eddies, it is really the di/dt that matters, and reversing the unsettled current is part of the total di/dt and has the same sign (no reversal) -- potentially could even help, although my guess is it gives about the same di/dt function, just starting from a different initial condition.

-SB

[Carl-NC]

It is true that the eddies from an unsettled turn-on will partially cancel the turn-off eddies, but I also doubt it is very significant. It would be interesting to test for this, by using a series resistor to flat-top the current on a wide TX pulse width, then removing the series R and reducing TX pulse width until the peak coil current at turn-off is the same as the flat-top current.

[Tinkerer]

No, I don't think it does. The initial current ramp is creating eddy currents in the target that at switch off must effectively be cancelled before the current can begin to flow the other way. So your final eddy current amperage would actually be less than if let the current flat top allowing eddy currents in the target to settle before you commence switch off. I doubt it has that significant a negative effect, on account of the switch of di/dt being so much larger but I can't see it being a positive thing.

I doubt I'll be much help on creating a better LTspice target since I know very little about the program, but if I can help I will. I am curious as to the variables included in your current simulated targets. What do you have inductance, capacitance, resistance, surface area?

Yes, of course, the switch off eddy currents will cancel the switch on eddy currents. But how can we quantify the effort needed to do that?
What would be a good test method for quantifying the proportion of response loss due to the cancelling of switch on eddy currents?

I am only at the beginning of the learning curve for LTSpice. So far I am using resistance and inductance to simulate the target metal and target size.
The experiment with adding target capacitance has not yielded results. I also tried some magnetic response. This seems to give some kind of reasonably similar results to a real component response.

Volume? surface area? How to do that?

Tinkerer

[Midas]

The eddy currents generated by a linear pulse ramp penetrate about 2mm deep into a gold nugget, within about 250us.
When we switch off, the current inverses, we could say it comes out of the gold nugget. The rate of change of the magnetic field is now much higher that when it went in, therefore the eddy currents it generates, are of much higher amplitude.

Does this make sense?

No, I don't think it does. The initial current ramp is creating eddy currents in the target that at switch off must effectively be cancelled before the current can begin to flow the other way. So your final eddy current amperage would actually be less than if let the current flat top allowing eddy currents in the target to settle before you commence switch off. I doubt it has that significant a negative effect, on account of the switch of di/dt being so much larger but I can't see it being a positive thing.

I doubt I'll be much help on creating a better LTspice target since I know very little about the program, but if I can help I will. I am curious as to the variables included in your current simulated targets. What do you have inductance, capacitance, resistance, surface area?

[Tinkerer]

Hi All,

Well this has been somewhat enlightening for me. It would seem that a faster switch off is in fact allround beneficial. Intuitively equating inductance with momentum it seemed to make sense that superfast pulses might not be the best way to transfer a given quantity of energy. Kind of like trying to get a car moving by shooting bullets at it. At some very high frequency I'm sure that would prove to be true. ie. if you tried to pulse it faster than the resonant frequency of the target nugget. But since thats probably somewhere in the MHz (anyone ever tried to work it out?) it can probably be completely ignored. And since Aziz has quite rightly pointed out that increased skin effect is less detrimental than the benefits of higher frequency, bring on the near vertical current ramps...

Carl, I'm not sure alumimium foil is much good as a 'standard'. 0.022 millimetres is apparently typical thickness but I highly doubt its universal across all brands. Coins are an obvious choice for consistancy though I guess there not small enough for low TC nugget simulation.

Midas

I think we should keep looking at the TX pulses for a bit longer, from all different angles.

For a traditional PI TX pulse, we can consider each pulse as an individual transient event.

For example:

The eddy currents generated by a linear pulse ramp penetrate about 2mm deep into a gold nugget, within about 250us.
When we switch off, the current inverses, we could say it comes out of the gold nugget. The rate of change of the magnetic field is now much higher that when it went in, therefore the eddy currents it generates, are of much higher amplitude.

Does this make sense?

I am sure we can find a few more angles to look at it. Let's see which one we decide to be the most correct one.

It also would be very interesting to build a real accurate LTSpice model for simulating PI detectors. The spice model I use above, gives results that are roughly similar to the results obtained with a real part circuit, but it could be improved a lot.

Is somebody willing to help me with building a more perfect PI LTSpice model?

Tinkerer

[Midas]

Hi All,

Well this has been somewhat enlightening for me. It would seem that a faster switch off is in fact allround beneficial. Intuitively equating inductance with momentum it seemed to make sense that superfast pulses might not be the best way to transfer a given quantity of energy. Kind of like trying to get a car moving by shooting bullets at it. At some very high frequency I'm sure that would prove to be true. ie. if you tried to pulse it faster than the resonant frequency of the target nugget. But since thats probably somewhere in the MHz (anyone ever tried to work it out?) it can probably be completely ignored. And since Aziz has quite rightly pointed out that increased skin effect is less detrimental than the benefits of higher frequency, bring on the near vertical current ramps...

Carl, I'm not sure alumimium foil is much good as a 'standard'. 0.022 millimetres is apparently typical thickness but I highly doubt its universal across all brands. Coins are an obvious choice for consistancy though I guess there not small enough for low TC nugget simulation.

Midas

[Tinkerer]

[/QUOTE]
As above, I think you are looking at this backward, and your conclusion is only valid because a long dt flyback precludes the use of early sampling required for short TC targets. That is, if we could use a high current TX pulse and still use early sampling (perhaps by using an IB coil) then short TC targets would also have a higher response amplitude.


[/QUOTE]

You are right, I look at things from a different angle, so I perfectly agree with your comments.
I like to use a saw-tooth TX wave form that is not commonly used with traditional PI and sample at, what would be called extremely early, which is not possible with traditional PI.

And then I forget to mention all that and just talk about PI. So I cause all this confusion. Sorry about that.

Tinkerer

[Carl-NC]

I think you have 2 targets still with a TC of 10us. The TC doesn't depend on surface area, surface area just affects the strength (amplitude) of the response.

It has been some time since I made this test, but as I remember, each of the 1/2" squares had a TC of about 5us. Of course, there is the possibility that the lower target amplitude made the response fade into the noise sooner and that I mistook this for the TC. I did not actually calculate the TC from the decay curve.

That's probably the case.

I have a set of 1" square targets made of household aluminum foil, with thicknesses of 1x, 2x, 4x, 8x, 16x, and 32x, each laminated with clear tape. Makes a good standard.

With a thicker target I was thinking of a solid target of a thickness that corresponds to the skin depth. Aluminum foil has a coating of aluminum oxide, which is an excellent insulator. Laminating 32 layers of aluminum foil might have strange effects due the the the inter-laminate capacitance.

Foil layers work fine as long as they are tightly pressed together. There is no vertical current flow so the oxide doesn't matter. Gaps between layers do matter which you can see if you let the stack relax, but the oxide thickness isn't enough affect this.

Well, actually, eddy currents reach about 5 skin depths deep.
Agreed, but the skin depth follows the classical exponential curve, where the 1TC is roughly 2/3 and the other 4 skins have rapidly diminishing little influence.

Be careful equating TC with skin depth. I think in this case you are simply comparing exponentials. In any case, a metal with a thickness of 5 skin depths will have a fairly different response than a same metal with a thickness of 1 skin depth, but not different than one with 10 skin depths.

With a slower switch off, I meant that the same di/dt applied over a longer time will generate a stronger target response in long TC targets.

A same di/dt with a "slower switch-off" (higher dt) implies a proportionally higher current, and I think this would be the only reason you would get a stronger response. This would also produce a stronger response in short TC targets, but the slower switch-off might prevent the early sampling required to see the target.

Yes, I mean eddy current penetration depth into the target. In general, the pulse width corresponds to the amount of energy stored in the coil magnetic field. With little energy, or short TX pulse, we can obtain a high di/dt for a short time. With a lot of energy stored in the coil magnetic field, we can obtain the same di/dt for a longer time. A longer time for a long TC target, gives a higher response amplitude.

Be careful! TX pulse width only corresponds to the amount of energy stored in the magnetic field if the current is still on the rise at the turn-off point. If my coil current flat-tops at 50us, then a pulse width of 100us produces the same TX field.

As above, I think you are looking at this backward, and your conclusion is only valid because a long dt flyback precludes the use of early sampling required for short TC targets. That is, if we could use a high current TX pulse and still use early sampling (perhaps by using an IB coil) then short TC targets would also have a higher response amplitude.

[Tinkerer]

Hi Carl, thanks for the feedback. I will add my comments between your lines.

I think you have 2 targets still with a TC of 10us. The TC doesn't depend on surface area, surface area just affects the strength (amplitude) of the response.
It has been some time since I made this test, but as I remember, each of the 1/2" squares had a TC of about 5us. Of course, there is the possibility that the lower target amplitude made the response fade into the noise sooner and that I mistook this for the TC. I did not actually calculate the TC from the decay curve.

I have a set of 1" square targets made of household aluminum foil, with thicknesses of 1x, 2x, 4x, 8x, 16x, and 32x, each laminated with clear tape. Makes a good standard.

With a thicker target I was thinking of a solid target of a thickness that corresponds to the skin depth. Aluminum foil has a coating of aluminum oxide, which is an excellent insulator. Laminating 32 layers of aluminum foil might have strange effects due the the the inter-laminate capacitance.


Well, actually, eddy currents reach about 5 skin depths deep.
Agreed, but the skin depth follows the classical exponential curve, where the 1TC is roughly 2/3 and the other 4 skins have rapidly diminishing little influence.

I don't understand the first half of this.

With a slower switch off, I meant that the same di/dt applied over a longer time will generate a stronger target response in long TC targets.


While the characteristic skin depth of a metal affects TC, so does thickness. So saying "lesser skin depth" == "shorter TC" isn't necessarily true, unless the targets are otherwise identical. Ferinstance, a 58-cal lead Minie has a longer TC than a 1 grain pure gold nugget.

"Skin depth" is a characteristic value that doesn't change with pulse width; I think what you want is a deep "penetration depth."

Yes, I mean eddy current penetration depth into the target. In general, the pulse width corresponds to the amount of energy stored in the coil magnetic field. With little energy, or short TX pulse, we can obtain a high di/dt for a short time. With a lot of energy stored in the coil magnetic field, we can obtain the same di/dt for a longer time. A longer time for a long TC target, gives a higher response amplitude.


- Carl

[Aziz]

What is better?
A higher or a lower stimulation frequency in respect to skin depth?


A lower stimulation frequency will increase the skin depth, which will increase the targets time constant due to lower current path resistance.
If we double the frequency, the skin depth will decrease by sqrt(2) and the resistance will increase by sqrt(2). The eddy current will decrease by sqrt(2).

On the other hand, if we double the stimulation frequency, the induced voltage in the target will double (direct proportional to frequency), which the eddy current doubles at the end if we don't take the skin effect into account.

Putting both together, if we double the stimulation frequency, the induced target voltage will double, the resistance will increase by sqrt(2):
I=U/R, f=2*f -> I = (2U)/(sqrt(2)*R)
Eddy current I will increase by 2/sqrt(2) = sqrt(2) = 1.41

The target can be energized with more eddy current by higher stimulation frequency.

So increasing the stimulation frequency wins. That's fine.

Aziz