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Pulse induction metal detectors.Is more than 1 pulse necessary?

Hi i'm trying to experience about metal detectors based on eddy currents. The basics principle is that something remains after applying a varying magnetic field induced by a current pulse,eddy currents should go on for many tenths of us. I have built a circuit that sends a current pulse on a 30 turns coil from an existing broken metal detector. The steep variation of the current is only in one direction, during the falling edge. The current decreases from 2A to 0A in about 50us. This repeats not very often ,say once in 100 ms,eddy currents probably goes zero in such long time. The test object is a 10 Kg weight lifting barbell plate at only 10 cm. Checking on a 2nd coil i can see on oscilloscope the pulse of induced voltage at same time of the steep variation of current in 1st coil. I expect to see also something after the "strong" current variation,but i can't see no difference depending if the iron plate is present or not. There is something missing,but i dare ask you before guess too much, my impression is that it has to do with repetition of pulses,but why? Do eddy currents grows if induced by close enough pulses ?

Thanks for your opinion.

Diego

Reply to
blisca
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The signal due to the target object will be very very much smaller than the signal due to the transmit pulse. I think a typical oscilloscope might not be able to handle the overload of the transmit pulse without messing up its behaviour for a long time so that it could never see the tiny target signal. I think you will need a good electronic (MOSFET?) switch to disconnect the coil from the receiver during the transmit pulse, and then the receiver will need very high gain, quite likely more than an oscilloscope would normally provide, so you may need a pre-amplifier.

There may be a benefit in averaging the echoes from multiple transmit pulses, as a way to reduce the effect of interference and noise, especially if you test it near or in a building containing electrical wiring and appliances. Really you probably ought to test it outdoors, far from any building or cable.

Reply to
Chris Jones

In data luglio 2021 alle ore 08:15:57, Steve Wilson snipped-for-privacy@not.com ha scritto:

Thanks for your hints,links and for having to do with my poor english ,

going on with patience I just found that effect of eddy currents(or something after current pulses) are visible on oscilloscope(2 mV/div)! That was possible only arranging the 2 circular coils in a double D fashion,say as olympic rings. Before reading anything about the double D arrangement, it's advantages became evident: if the two coils centers are exactly aligned the effect of current pulse appears huge on 2nd coil,say it has positive sign,anything right after this huge pulse is hidden by ringing; if the coils are side by side the sign of the pulse on 2nd coil changes,there is still big ringing ; moving again the centers toward alignment,the negative pulse decreases,and when the entering field equals the outgoing field, the pulse disappears,so does the ringing,and effect of eddy currents appears from noise. In 2021 i just noticed what engineers probably got clear in the 50's, on tube oscilloscopes

Reply to
blisca

You may have found a new method of PI. None of the references I found mention double D coils. Please continue updating us on your progress.

Reply to
Steve Wilson

In data luglio 2021 alle ore 07:18:49, Steve Wilson snipped-for-privacy@not.com ha scritto:

It sound very strange to me,when i started gathering informations I met soon lot of references to double D coils.Anyway,I'm not shure that the only reason is suppression of the exciting magnetic field.

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Reply to
blisca

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