How can tuning a coil with a capacitor help detect multiple metal objects?

[bldavis83]

Good Day All,
I would like to know if anyone can help with the following; I am running it a few problems and I am not able to find the formulas that will allow me to prove my idea.

I have a loop of 20 Gauge magnet wire; it's 24 inches by 2 inches. I'm using this as a detector, but i want to detect multiple items. I know that if I tune the coil loop with a RLC circuit that the coil will be detuned when a metal object is introduced inside the loop. but i can not find out what the change would be or what that change would be in relationship to multiples objects of the same size would be.

if someone would be willing to point me in the right direction that would be great; I understand the this is very simuliar to a metal detector or vehicle loop detector but i can't find the formula to allow me to see if this works the way that I think it will...

 

[vk6kro]

This is one of those areas where you need to just make it and see what happens. The situation around a large coil is so complex that a formula is just not going to be able to describe it.

Metal detectors can work on a frequency shift, an amplitude shift or a change in coupling between two coils, or a combination of these effects.

 

[skeptic2]

Tuning the coil with a capacitor is a good idea. Once tuned, if you bring a magnetic material near it, the magnetic material will increase the inductance of your circuit and lower its resonant frequency.

If you bring a non-magnetic metal near it, the current in your loop will induce eddy currents in the aluminum which have a magnetic field opposing the the magnetic field of your coil. Those opposing magnetic fields reduce the inductance of your circuit and raise the resonant frequency.

Either way the current in your loop will be reduced. Perhaps if you use your pickup coil in an audio oscillator, you'll be able to hear the change in pitch indicating if the metal is magnetic or nonmagnetic.