PI Power Output

[Aziz]

Hi B^C,

Gday Aziz, the simulations are good, thank you!

Couldn't help but ask & off the subject a little but maybe relevant.
Different diameter inductors-coils have a distinct magnetic field strength surrounding them, how can we add or subtract this into the spice sim to compensate for this?
Can we just drop the supply voltage to do this or change the coupling of the target?--calibrating is another thing i know.

The point i am getting at is that the transient spike voltage & characteristic would seem to be dependant on the strength of the magnetic field surrounding the inductor at the time of collapse. The diameter of the coil has an influence on this as well it would seem.
If i'm wrong just belt me over the head!
Eg:
If we have two coils with the same inductance etc & fed from the same power supply a 8" coil is going to have a more intense magnetic field surrounding it than say a 16" coil has so the transient spike must be affected?
It would seem as though this may be the reason why large coils won't detect small nuggets, the spike ain't big enough to induce them hence the use of smaller coils?

So one would think to look at things better we need a Test std especially if the transient spike voltage changes with different diameter coils. This way we can all look at & test the same things?

Or i may need a kick in the butt!

Or how much of a transient spike is measured from a 16" coil compared to an 8" with the same specs, inductance etc?

Indeed, the spice simulation can be weight up with some gold! (psst, almost nobody realized that ).

To simulate different TX coil diameter, the parameter TgtCoupling=0.01 must be changed. For a given target (smaller than the coil radius), bigger coils have less coupling to the target. Smaller coils have more coupling. But the distance, shape, size and orientation of the target has also an influence to the coupling factor. So it is difficult to calibrate a system. Therefore only relation of the response is important. Not the absolute values. If you know all of these parameters, than a more realistic simulation could be done for a specific configuration.

Supply voltage Ubat and ton (transmit pulse width) defines the exposed magnetic field energy and is not convenient to simulate different TX coil sizes.

An 8" and 16" coil with same inductivity L would give for the same target different response. The coupling of TX coil and target differs in this case. Small targets on bigger coils would therefore have less response due to less coupling. Coupling means: How many and how strength magnetic fields are going through the target?

The basic spice simulation indeed allows deep analysis of very interesting questions. I have taken an ideal switch for the model, to make a comparison between fast and slow damping as the flyback voltage shouldn't clipped due to mosfets avalanche breakdown voltage.

---

I have found, that the conductivity of the target matters much in respect to the target response! If you take a comparison for a fixed time constant target TC and different conductivity of the target (target inductivity differs then), then the target response differs quite much. Particularly long TC targets with low conductivity have less response.

We should all play a bit more with spice simulations.

Aziz

[ZED]

Aziz looking at your last graph i dont see anywhere where your faster flyback has any real advantage,sampling one to two u/s after switchoff is not realistic,five to six u/sec after switchoff is a more likely scenario in which case it would appear that the slower system has more to offer.

Zed

[B^C]

Gday Aziz, the simulations are good, thank you!

Couldn't help but ask & off the subject a little but maybe relevant.
Different diameter inductors-coils have a distinct magnetic field strength surrounding them, how can we add or subtract this into the spice sim to compensate for this?
Can we just drop the supply voltage to do this or change the coupling of the target?--calibrating is another thing i know.

The point i am getting at is that the transient spike voltage & characteristic would seem to be dependant on the strength of the magnetic field surrounding the inductor at the time of collapse. The diameter of the coil has an influence on this as well it would seem.
If i'm wrong just belt me over the head!
Eg:
If we have two coils with the same inductance etc & fed from the same power supply a 8" coil is going to have a more intense magnetic field surrounding it than say a 16" coil has so the transient spike must be affected?
It would seem as though this may be the reason why large coils won't detect small nuggets, the spike ain't big enough to induce them hence the use of smaller coils?

So one would think to look at things better we need a Test std especially if the transient spike voltage changes with different diameter coils. This way we can all look at & test the same things?

Or i may need a kick in the butt!

Or how much of a transient spike is measured from a 16" coil compared to an 8" with the same specs, inductance etc?

[ZED]

These simulations are really fascinating.

Now, why do you think that the 100uS target gives only a low amplitude response, while the 10us target gives a high amplitude response?
Maybe not enough time to fully excite it?

Tinkerer

Very good question Tinkerer ! consider this,why do m/lab pi`s for the most part only find nuggets smaller than 120 oz [there are a couple of exeptions]

Zed

These simulations are really fascinating.

Now, why do you think that the 100uS target gives only a low amplitude response, while the 10us target gives a high amplitude response?
Maybe not enough time to fully excite it?

Tinkerer

It is generally accepted that to fully energize a conductive target the tx pulse length should be at least 2xtarget TC.

[Tinkerer]

Hi Tinkerer,



it is a misleading display. Note, that the shown display is the coil voltage difference of two systems at the clipping diodes (target/no target). If you scope the target voltage only (see below, represents target eddy current and it is direct proportional to target induction voltage), then you will see that fast decaying system has a higher target induction voltage, but which decays faster than the slow decaying system. The integral of the target induction voltage over time (after switch-off) should be equal for slow and fast decaying system (due to same transmit pulse energy and TC). So there is really no free lunch.

Somehow, the clamping diodes in the simulation give some advantage on the fast decaying system here, which allows a bit earlier sampling. That is quite interesting, which I didn't expect.

Aziz

These simulations are really fascinating.

Now, why do you think that the 100uS target gives only a low amplitude response, while the 10us target gives a high amplitude response?
Maybe not enough time to fully excite it?

Tinkerer

[Aziz]

Hi Tinkerer,

Interesting about the slow damping. So if we were to sample at 1.020mS, the 10uS target would actually have a higher amplitude?

Tinkerer

it is a misleading display. Note, that the shown display is the coil voltage difference of two systems at the clipping diodes (target/no target). If you scope the target voltage only (see below, represents target eddy current and it is direct proportional to target induction voltage), then you will see that fast decaying system has a higher target induction voltage, but which decays faster than the slow decaying system. The integral of the target induction voltage over time (after switch-off) should be equal for slow and fast decaying system (due to same transmit pulse energy and TC). So there is really no free lunch.

Somehow, the clamping diodes in the simulation give some advantage on the fast decaying system here, which allows a bit earlier sampling. That is quite interesting, which I didn't expect.

Aziz

[Tinkerer]

Hi Tinkerer,



See below: Ground is taken as TC=1µs and target with 10µs and 100 µs.
The TC=100µs signal will last very long at low signal level, whereas the others are almost zero.



I don't know. The switch off time is same for all systems. However, the higher coil capacitance will slow down the damping and decrease the flyback voltage. The critical coil damping resistor will be lower with increased coil capacitance.




I don't know.


Interesting fact on the fast damping system is, that the clamping diodes are recovering faster from the conductive state, which allows earlier sampling (4-5 µs). This gives a higher sampling integration possibility, which contains effectively more target response signals.

Aziz

Thanks Aziz, great job.

Now lets see if I understand right.

There is a time delay until the response of the targets reach their peak. The peak response is not at the same time for targets with a short TC and a long TC.
This is exactly what I am talking about. It makes it possible to read the TC from the response.

Interesting about the slow damping. So if we were to sample at 1.020mS, the 10uS target would actually have a higher amplitude?

Tinkerer

[Aziz]

Hi Tinkerer,

Could you show it with a target with a TC of 10uS and again with a target with a TC of 100uS?

See below: Ground is taken as TC=1µs and target with 10µs and 100 µs.
The TC=100µs signal will last very long at low signal level, whereas the others are almost zero.

Could there be a correlation between a fast switch OFF and a target with a short TC?

I don't know. The switch off time is same for all systems. However, the higher coil capacitance will slow down the damping and decrease the flyback voltage. The critical coil damping resistor will be lower with increased coil capacitance.

Or, could there be a correlation between a slow switch OFF and a target with a long TC?

I don't know.


Interesting fact on the fast damping system is, that the clamping diodes are recovering faster from the conductive state, which allows earlier sampling (4-5 µs). This gives a higher sampling integration possibility, which contains effectively more target response signals.

Aziz

[Tinkerer]

here is the basic circuit of the models...

Hi Aziz,

nice simulation.

Could you show it with a target with a TC of 10uS and again with a target with a TC of 100uS?

Could there be a correlation between a fast switch OFF and a target with a short TC?

Or, could there be a correlation between a slow switch OFF and a target with a long TC?

There are targets with low conductivity and others with high conductivity. We know that the low conductivity targets give a short TC relative to their size, while high conductivity give long TC's.
I don't think we know much about the inductivity and the capacity of the targets, maybe this is where the secrets are hidden?

Tinkerer

[Tinkerer]

Hi Tinker,

I have been following this thread and find it very interesting. I believe some of the questions you are asking are due to inductive coupling vs true pulse induction. Many of the great answers from the others are explaining what happens when the current is switched off and there after. Your observations are during switch on while the current is building. From what I have read you are using an induction balanced coil. It can be looked at as an air core transformer that has very low coupling. When a resistive target is placed into the changing magnetic flux, it creates an impeadance to the flow. This in turn upsets the balance in your transfomer and causes coupling. Ractive targets, in simplistic terms inhance the flow of flux and cause a disbalance in the transformer and coupling. A true PI is measures the decay of eddy currents induced into the target after the coil current collaspes. If you see how standard TR detectors operate it is the same as what you are doing during switch on. You are using a step function vs a sine wave. Time domain vs Frequency domain remember T=1/f and F=1/t. Keep up the interesting experiments

Regards
Mark

Hi Mark,

I think you come closest to what is actually happening.
OK it is not traditional PI. It is probably more of a hybrid.
I attach 2 pictures of the current wave form. If we look at the current wave form instead of the voltage wave form, we see that it is not all that different from a VLF.
Then maybe it is easier to understand where to look for the information that we can read in it.
This is TIME DOMAIN, the information is locked in a specific time frame.

Now, I have found where to look, that is I know at what spot in time I can read some information, but I do not yet fully understand what I am reading.

Much less I can explain exactly what is happening.

All I know, is that the information is there.
I find it every time.
It tells every time the same story, but I don't understand all of it.

The one picture shows 10k PPS of coil current pulse. I don't remember the TC , but it looks like somewhere around 3 TC.

The other picture shows just two coil current wave forms. The TC is around 1 TC.
You can see that the switch OFF is fast and it produces a spike in the opposite polarity.
I think it is this spike that kills a good part of the OFF time signal.

You can also see on the ON slope 2 small switching noise spikes close together. This is one of the sample pulses.

Several samples along this slope give different information. At a certain time the FE signal is higher, at another time the FE signal all but disappears.
It is like an invisible sine wave is riding on the slope, the line of the slope being the 0V line.
Sampling where the sine wave crosses the 0V line gives no signal.
Non magnetic metals give a peak response at a different spot.
The TC of the target moves the peak spot to a different time.

Tinkerer

[Aziz]

here is the basic circuit of the models...

[Aziz]

Now, everybody can get crazy!

Hi all,

I made some circuit simulations. A four equivalent systems were analyzed:
- Fast coil damping, with target (left/top)
- Slow coil damping, with target (right/top)
- Fast coil damping, no target (left/bottom)
- Slow coil damping, no target (right/bottom)

The slow damping is achieved with higher coil capacitance. The flyback voltage is quite lower than on fast response configuration. Same transmit pulse energy on every system. You can see, that only few mA are missing on the slow damping. Where the hell is the missing energy gone? -> it is stored in the higher capacitance.

The signals are named systematically:
F for fast, S for slow, 0 means no target, xxclip.. means the coil voltage at the clipping diode. The Rin resistor is set to 10k to avoid damping resistor inbalance (for critical coil damping).
Please play with the circuit simulation (LTSpice model) and make different sweep analysis for:
- different pulse width
- different target inductivity (target size)
- different target resistance (target conductivity)

You can scope the signals, make the comparison between the simulation models. You will find interesting facts.

Now see the following response comparison at the diode clipping:
The fast response system gives tremendous big target signal response for bigger targets (higher inductivity). That is quite interesting.

The simulation models are very basic but quite accurate. The coil is splitted into three parts: series coil resistance, coil inductor and coil capacitor.

Have fun with circuit analysis.

Aziz

[Tinkerer]

Tinkerer

I understand that ampere turns is important but only if we can take full advantage of it, it's hard to explain. Let me test something out & i'll try to answer this differently. I tested some things & found that sometimes a little less is more so to speak. It's like have a 500hp engine but you can only get half throttle, i found something interesting & putting it words is not easy.


Yes short TX is the only way to go.

I guess the figures above are calculated from your operating frequency which is important.

What calculation are you using for your current rise vs time vs voltage?

I think I know what you mean. I have found something similar, but am not ready to go public with it yet. I need to find the right terms to present it first.

I find Misc.EL a great tool to calculate all sorts of things related to PI.

Tinkerer

[Tinkerer]

Aziz i beg to differ,your not considering the inductance of targets,targets need time to build up eddy currents,your suggesting to reduce this time in favour of increasing the speed of the coil field,it is true that the faster a field moves the stronger the eddy currents but what about the inductance of targets that resist the growth of eddy currents,increasing the speed and reducing the window for which targets are exposed to a moving field in my opinion produces a negative improvement or zero improvement for targets with high inductance.
Speed of the collapsing field and the exposure time to this field are both important criteria.

Zed

Zed,

I agree with you.
Lets look at the sampling DURING TX. There we can read the signal at the time it is generated, not some undefined time later like the OFF transient signal.
Carl states above. "The maximum di/dt is right at turn ON" (for the ON transient)
There is no doubt that this is right.
However, the maximum response from the target is not at that moment.
Also, the moment of maximum response from the target varies in time with the TC of the target and the material of the target.

THIS IS THE KEY TO DISCRIMINATION AND TARGET IDENTIFICATION

Once we fully understand why the signal amplitude is higher than expected and why there is a difference with the TC and target material at the ON transient, we will be able to fully discriminate and identify the target.

Simulations give the wrong results if the target is not realistically represented with a TC.

Tinkerer