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Eddy current summation

A

Anonymous

Guest
Hi All,
I had an idea recently and it may have merits, I
 
IMHO
the TX overlapped with the flyback signal generate a shape that's not easy to analyze.
The great energy in the TX coil have a random-noise component that you cannot subtract to the target signal without affect the stability of the integrator (or DSP).
The TX and the sample must be separated.
 
Hi Cossaro,
Thanks for the feedback. Yes there would be noise and an increasing signal from the other coil as the pulse is growing, I'm under the impression though, that Minelab sample during TX on time. How do they overcome this limitation.
Sometime ago, I can't find the post now, one of the contributors (could have been Thomas Breuer, Robert Hoolko, Dave Emery or Dave Johnson?) produced some graphs depicting usable samples taken during pulse development.
I will in time give it a try, and maybe use a 3rd passive coil for a receiver. I just felt the need to post it anyway.
Cheers
Kev.
 
Hi Kev,
I would say that a balanced coil would be necessary, such as a DD. For two unbalanced coils, the TX that is on, will cause saturation of the other receiver, and no signal will be observed. With a balanced coil, you can sample during the on time and add the signal to that which occurs in the off time. You don't gain a lot however, as the eddy current signal at switch on is considerably less than that at switch off. This is due to the different rates of change of field. The added, complication (bonus), is that you also sample the unbalance signal caused by the presence of metal. One way for ferrous and the other way for non-ferrous. The summed signal after switch on is the derivative of the exponential growth of current in the coil and the eddy current signal from the object. Add this into the normal PI signal and things get interesting, particularly if you have a target in iron mineralised ground with both susceptibility and viscosity components.
Eric.
 
Hi Eric,
Thanks for your input. This should produce ferrous/non-ferrous discrimination info, much like George Payne's design (US4110679)
It would be good to get a good grasp of what he is doing, I find the patent difficult to unravel. I've got up to the point where the comparators trigger the sampling, but how he processes the samples has me stumped so far.
Cheers
Kev.
 
Kev, Go to this site from the link. Post a message with the heading: "Question for George Payne" and ask him your questions in your message. George is a great guy and he will answer you. Good luck, Dave. * * *
 
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