gold nugget simulation?

[Chet]

Charted the Ca nails with a 275mm DD coil. Orientation doesn't effect the slope as much as it does with the other coils I used.

Hi green

Looks like coil type and size makes a difference with these targets. Do you think the 38mm round coil results would change much with a larger round coil?

Could you plot these again using the same scales with log in the vertical axes only? I would like to compare them with some of your previous charts.

Thank you,
Chet

[green]

[Could you plot these again using the same scales with log in the vertical axes only? I would like to compare them with some of your previous charts.]

I'll try with a 200mm round coil latter.

[Chet]

Hi green

That helps, much easier to visualize the decay curve.

Thank you,
Chet

[green]

200 mm round coil, nail charts. Don't know why the DD coil slopes have less variation. Need to play with the DD coil some more to see if it repeats.

[Chet]

Hi green

Great chart as usual. I have a commercial Double D coil which I have used a number of times. It just doesn't work in the field as well as a good Mono coil.

Thank you,
Chet

[green]

More targets from Ca. I didn't expect the nails to plot a straight line on a log log graph. Since they did I thought maybe these targets would, except for V flat they didn't. The thin targets were rusted. Maybe someone could explain what target trait causes the slope to be straight on log log or linear log.(100 x 265 mm coil)

[Chet]

Hi green

My thoughts are that maybe the thicker ferrous materials keep an electromagnetic effect and eddy currents circulating longer than the thinner badly rusted can materials. The curved slope effect showed up in some of your previous charts. Since nonferrous materials are not magnetic this curved slope response effect does not appear to occur even in thick nonferrous objects.


This curved slope effect when combined with target amplitude might be useful for ferrous vs. nonferrous discrimination. On amplitude it seems that ferrous objects have a louder and sharper response than gold. Reviewing the Chance PI forum it appears that it may be using a combination of amplitude and time constant for ferrous discrimination.

Thank you for the chart,
Chet


Check your PM

[green]

More targets. At short delay times coil on time didn't effect signal amplitude. It does effect the decay slope.

Chet, check your PM

[green]

More targets. Didn't get what I expected, maybe I should have.

[Chet]

More targets. Didn't get what I expected, maybe I should have.

Hi green

Very interesting; if I understand this neither the size or distance made a difference in the time constant. What size coil did you use?

In this chart and the previous charted coins a common effect was apparent with the series resistor installed to widen the pulse width.

Could the difference in readings with the 260 us pulse width be accounted for by the parallel path of the damping resistor and the series resistor back to the stray circuit circuit capacitance?

Could the damping resistor be adjusted to compensate for this difference?

Anyway thank you for more good information,
Chet

Check your PM

[green]

Hi green

Very interesting; if I understand this neither the size or distance made a difference in the time constant. What size coil did you use?

In this chart and the previous charted coins a common effect was apparent with the series resistor installed to widen the pulse width.

Could the difference in readings with the 260 us pulse width be accounted for by the parallel path of the damping resistor and the series resistor back to the stray circuit circuit capacitance?

Could the damping resistor be adjusted to compensate for this difference?

Anyway thank you for more good information,
Chet

Check your PM

Replies #68, 69 and 71 I used the 100 x 265 mm rectangle. I don't know the answers to your other questions. It looks like if it plots straight on the linear log plot, on time doesn't make a difference. But then ground doesn't so I'm wondering if changing on time changes the slope. The nickel and $1 chip plotted close to straight on these charts.

[Chet]

Replies #68, 69 and 71 I used the 100 x 265 mm rectangle. I don't know the answers to your other questions. It looks like if it plots straight on the linear log plot, on time doesn't make a difference. But then ground doesn't so I'm wondering if changing on time changes the slope. The nickel and $1 chip plotted close to straight on these charts.

Hi green

Interesting results again. I think in theory the pulse width should not have this effect. If the current in the coil is the same at cutoff time it should generate the same flyback voltage and the same collapsing energy into the magnetic field. Did you happen to check the peak flyback voltage for each pulse width?

Thank you again,
Chet

[green]

Hi green

Interesting results again. I think in theory the pulse width should not have this effect. If the current in the coil is the same at cutoff time it should generate the same flyback voltage and the same collapsing energy into the magnetic field. Did you happen to check the peak flyback voltage for each pulse width?

Thank you again,
Chet

The peak flyback voltage is about 450 volts, the clamp for the IRF740.
More targets. Switched to the 110mm round coil to get more signal for the smaller steel and aluminum balls. The comparison of the 1 inch steel ball with each coil shows no difference in the decay.

[Chet]

The peak flyback voltage is about 450 volts, the clamp for the IRF740.
More targets. Switched to the 110mm round coil to get more signal for the smaller steel and aluminum balls. The comparison of the 1 inch steel ball with each coil shows no difference in the decay.

Hi green

Looks like the 260 us pulse provides fairly consistent data. Is it possible to determine if there is a critical pulse width between 62 us and 260 us at where the results abruptly change? Or is it a gradual change as pulse width changes?

Thanks again for some interesting data,
Chet

[Tinkerer]

For a target with longer than 62us TC, say 100us, the eddy currents are still increasing. In other words they have not yet reached their peak value. At switch OFF, these eddy currents must be eliminated before the new eddy currents, of opposite polarity are generated.

With 260us TX pulse, the eddy currents in a 100us target are already partly dissipated after the flat top TX current time. Less eddy currents to be eliminated, so more new eddy currents generated.