New PI technology proposals.

[Tinkerer]

Target TC

G`day Simon

I must say i share the same thoughts as you on this point in regard to the relationship between the slew of the step response and the resulting eddies on targets.I think the amount of time that a target is exposed to a collapsing coil field is to important to be dismissive of it.
Slow targets by there very nature must tell you that they need more time exposed to a collapsing coil field in order to get maximium target stimulation,just like the inductance of a coil when one needs more time to build the current up.
The targets of interest with me are large nuggets from 1 oz to hundreds of ounces,these nuggets are full of surface irregularities and one would think that to some extent the eddies on the surface of these nuggets would cancle or oppose each other to some degree and there for resist the growth of eddies.


"inductance".


Zed

Round and round in circles, but finally back to square one.
It is all about the TC of the target.
There can be no accurate simulation if the Target is not accurately represented.
If the Target signal decay has a TC of 100uS, how long does it take to stimulate this Target to get full response?
What do we know about the Target TC?
Do we have comparable Targets with known TC's?

Tinkerer

[KingJL]

It is all about the TC of the target....What do we know about the Target TC?.....Do we have comparable Targets with known TC's?

Of this whole discussion, this the most important information. This would constitute knowledge, not theory; actual measured datas, not simulation results.

The silence is resounding!

[Monolith]

NO FLYBACK TEST

Gentlemen, allow me to disagree.
I say it is the expanding magnetic field produced by the coil current, that kicks the target. The Flyback is actually reducing the eddy currents in the target.
The Flyback is too short to "saturate" for want of a better word, the target.
The Flyback induces currents of opposing polarity since the magnetic field of the Flyback expands and then collapses, that is, the field cuts across the target conductor first the one way, the the other way.
The current raising in the coil during TX, generates an expanding magnetic field that cuts across the target \ conductor and induces currents within the target. The first currents are superficial skin effect currents. It takes time for the eddy currents to expand to the core of the target.
How much time? The time needed has been named the TC of the target. High conductive and large size targets have long TC's. Large silver coins can have a TC of hundreds of uS.
If the TX pulse is short, the eddy currents will not have the time to expand to the very core of the target, so they will not reach maximum amplitude.
Now look at the Flyback. It's duration is a matter of nano seconds for the one way and a few micro seconds on the return. Its highest power is at the maximum voltage point that is about even both ways and of a duration of nanoseconds. Just too short to induce much eddy currents. And going both ways, thus canceling the eddy currents induced by the going field, with the field collapsing.
However, the eddy currents induced by the TX coil current persist for some time.
How much time? about the same time it took to generate them The TC of the charge curve equals the TC of the discharge curve.

So, how will I prove my point? I will show you that I can read the eddy currents without having a Flyback.
Better than that, I will show you that, without the Flyback, the amplitude of the eddy currents is much higher.
I will pulse the coil with a usual square wave TX pulse but will modify the circuit to eliminate the Flyback.
Below is the scope picture of the coil current.
The pulse length is about 64us, you can see a slight kink and noise spike where the TX switches OFF. Forgive me for the noise spike, I don't think it can be called a Flyback.
The scope is set at 20us \div and 10mV\div.
Note, this is the coil current, not the TX voltage wave form.

Please allow me some time to setup an RX circuit to read the eddy current. Just the preamp.
I will use US$ 1c, 5c, 10c, 25c as test targets.
Monolith

So here it is, the NO FLYBACK samples.
Just a quick and dirty test to see if it works.

Would somebody be so kind and make a comparative test with a fast switch-off Flyback?
Monolith

[KingJL]

Do we have comparable Targets with known TC's?

Tinkerer,
I think we'll have to create a chart/table of our own. A good place to start would be to measure the half life time of a target and then divide by 0.693 (ln2) to get the TC. You'll have to make sure that your base line is 0 (or at least compensated for) at your measurement times. You can pick your initial measurement anywhere on the decay curve (as long as you have enough signal left to measure a point at 1/2 the value further down the curve). I would try to minimize the signal distortion caused by excessive amplification. When I get back from the coast, I will be making some measurements after my 10X pre-amp stage. I can't remember whether my signal right off the coil was enough to measure accurately. I would also try to use some 'standard' targets (standard coins, gold wedding band, specific wt. gold nugget, etc.). Positional consistency will also be required to have meaningful data.

It may even be beneficial to start another thread for work on this effort and let this thread follow it's pre-destined path.

Regards,
J. L. King