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A ferromagnetic metal detector

graemeian

Oct 2, 2014
13
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Oct 2, 2014
Messages
13
Hi all,
We process plastics and I want to detect sub-micron ferromagnetic particles in the plastic. My first thought is a Helmholtz design where one coil can be driven by a switching circuit and the other coil a pick-up. The ferrite should increase coupling and my pick-up saw tooth amplitude would change. I could drive a MOSFET gate and divide the gate with a pot to set a threshold. When the amplitude was reached, to trigger the gate, a PIC that is driving this circuit would light an led.

A sine wave driving the primary 24VAC 60Hz may work well as I am only looking for a change in amplitude. I once used a PIC to "look" at a sinewave amplitude by measuring the "on" time of the upper sine wave. I only needed a series resistor to protect the PIC. I think I remember the PIC having a built-in diode.

I need a detection area with about 2"' diameter and 1" separation. One part size is about 1" x 1" and the other is a disc that could be rastered like a record player with a Helmholtz clamp. Any thoughts on what sensitivity I could expect? Is there a better design I am missing?
 

graemeian

Oct 2, 2014
13
Joined
Oct 2, 2014
Messages
13
I did some back of envelope calculations and it looks like I may need to take advantage of planar geometry. Concentric coils may be advantageous but not for the disc. It looks like like my sensitivity is the inverse ratio of the coil effective volume (inner bagel) to the particle volume but I do get the generous mu value for iron. This leads to large particle sizes which is not good unless at the plane, the effective coil volume is Area times the coil height or something close to it.
 

Alec_t

Jul 7, 2015
3,212
Joined
Jul 7, 2015
Messages
3,212
A sine wave driving the primary 24VAC 60Hz may work well
I'd be very surprised if it did. There's so much interference all around us at that frequency and its harmonics. Why not go for a much higher frequency where it would be easier to filter out interference?
 
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