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which forms part of a colpits oscillator circuit to determine different types of metal as they pass through the coil at a fixed speed. By analyzing the frequency and amplitude shift as the object passes through the coil I am able to compare these to my known control values and identify the material.

The coil itself is coreless - it is wound on a plastic former without any metallic content.

I am quite new to electro magnetics / coils and fields etc... so have a couple of question that I am helping someone could advise me further on.

1) What would happen if i replaced my plastic former / core with a stainless steel one? What would this do to the field, I am guess it would reduce it but would it also change its shape?

2) What would happen if I introduced a large piece of stainless steel into the proximity of the coil (think mounting bracket etc..) how would this effect the field strength and shape?

Is there a good reference source for this subject?

many thanks in advance!

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1) What would happen if i replaced my plastic former / core with a stainless steel one? What would this do to the field, I am guess it would reduce it but would it also change its shape?

Eddy currents would be induced in the SS former and probably render a colpitts oscillator to a state of uselessness. It will act like a shorted turn in your coil.

2) What would happen if I introduced a large piece of stainless steel into the proximity of the coil (think mounting bracket etc..) how would this effect the field strength and shape?

Providing the SS does not sap too much of the magnetic flux generated it should still work but, the SS would mask the field in its vicinity and make the oscillator's ability to discriminate various metals significantly harder.

Is there a good reference source for this subject?

I designed food and pharmaceutical metal detectors for many years and I never found anything really useful on my subject. Discrimination between various metals was important and operating frequency can switch things around a lot. Regarding Q2, the type of search head we used was fully enclosed in SS and this was useful for shaping the field for where it was intended to be - don't misunderstand, you can't focus a mag field!

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  • \$\begingroup\$ Thanks for your response its very useful. Out of curiosity, did you follow the same principle to discriminate metals? How did you measure the frequency change? In (2) would adding the SS change the shape of field? \$\endgroup\$ – Microkid Sep 2 '15 at 11:22
  • \$\begingroup\$ I measured the I and Q (in phase and quadrature phase) components of the change in oscillation using a separate receive coil. Frequency change is too insensitive for what I did because certain metals had no net inductive quality therefore they did not move the frequency one bit but, they still took energy from the field. \$\endgroup\$ – Andy aka Sep 2 '15 at 11:28

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