Return Signals from Target

[Anonymous]

This question is for any engineer out there, but for Eric Foster mostly. I would like to know if there is a difference in the slope of the returned pulses from gold and iron. In other words, if you have pulses from gold and iron with a given height, does the gold decay faster than the iron? If this is the case, could this be looked at by circuitry to give metal ID?

 

[Anonymous]

Dale
The target decay slope can certainly be used to generate an ID value, but the meaning of that value is ambiguous (even more ambiguous than it is in a VLF detector). Gold does not have a particular decay rate. A handful of thin gold rings might all have similar decay rates, but a gold class ring will have a much slower decay. The size shape and orientation of the target can have more affect on the decay rate than the composition.
Having said that, I still plan to try this in my digital PI project. But I think iron ID is the most important, and I don't have much confidence that I will be able to reliably tell the difference between iron and everything else.
Robert

 

[Anonymous]

Thanks for the info. Thought that might be the case but wasn't sure. However, I do not worry to much about the iron --- it just gives me a little bit more needed exercise!

 

[Anonymous]

Did you see Don's "The Junk" picture on the Beach and Water hunting forum? As well as the excercise, just think of the good that is being done cleaning up the environment. Also, next time down on the beach, there is that much less to mask the goodies.
Eric.

 

[Anonymous]

Hi Dale,
This is a very complex issue as there are many variables. Gold is a non-ferrous metal and therefore has an exponential decay at times longer than one time constant. Iron with both conductive and magnetic properties has a mixed decay, which depends on its orientation and position relative to the search coil. For a gold object, the slope of the decay, or more accurately, the time constant, depends on the alloy of the gold, its mass and cross sectional area. As Robert says, this is why a thin ring will have a faster decay than a class ring, or heavy gent

 

[Anonymous]

I always wondered why log amps were not used. Now, as you surmise, with DSPs and good ADCs it ought to be possible to do true video integration, which is widely used in radar signal processing. Anyway, the following approach suggests itself to me:
1. Use a random pattern of interpulse intervals, to decorrelate all manner of background noise. This would eliminate the need to vary the pulse frequency, and is easy if one uses computer control, which is likely if one also uses DSPs.
2. Sum the received signal of at least 100 identical successive pulses. The analog-digital converter should be set up to start converting at a fixed offset from the pulse end (coil-current turnoff), and to convert at a fixed sample rate, so the resulting time series can be summed sample by sample. If wide pulses and narrow pulses are used, sum wide pulses only with wide pulses, and narrow with narrow. Timing from turn-off allows the resulting summed time series for wide pulses and for narrow pulses to be more easily compared, as the turn-off is the same for all pulse widths.
This process will cause most noise to average out. The signal voltage will grow linearly with the number of pulses included in the sum, while uncorrelated noise will grow with the square root of the number of included pulses. So, the signal to noise ratio will grow as the square root of the number of pulses, increasing by 10:1 for 100 pulses.
3. Take the logarithm of the sums, yielding a vector of logarithms. The noise-reduction from the prior step of summing multiple successive pulses will greatly reduce the noise.
4. Form the reverse cumulative sum of the vector of logarithms. This will preserve the shape of exponential-decay slopes, while further reducing the noise. (I posted a long-winded description of the reverse cumulative sum approach some months ago.)
Joe Gwinn