PI discrimination between Ferro and NonFerro

[Great_Alex]

Hi to all

i have a question about how can a PI metal detector discriminate between Ferro and NonFerro metal ?
i found some diagram like this :


i had many tests and see the results in oscilloscope
but i found there is no differ between Ferro and NonFerro in RX wave shape specially in coin size object !!! ???

is the above diagram really true ?

i use 45cm 400 uh 1.5ohm MonoCoil f = 200 Hz amp-gain about 1000

[Teleno]

My intuition is the Rx response of a ferromagnetic object is a combination of a conducting material (e^(t/tau)) and a magnetic material (1/t). Some form of regression would provide discrimination.

The effect during the transmit period (Tx) is a slower rise of the coil current due to the magnetic material increasing the inductance. There would be a slightly longer transient for the same reason.

If your Tx current waveform is a ramp of fixed duration, measuring variations of the terminal current (I) would indicate the nature of the target (smaller I => ferrous, unchanged or larger I => non-ferrous.)

Just my 2 cents.

[Great_Alex]

Thanx Teleno
but i think it need 2 coil for check your solution . in mono coil when tx is on is hard to measure current I .
and small non ferro and ferro magnetic items response the same .
what's your opinion about above diagram ?

[sinko1]

PI disc. with mono coil is possibly.
https://www.youtube.com/watch?v=7aY4qrxMxgg

[Teleno]

Thanx Teleno
but i think it need 2 coil for check your solution . in mono coil when tx is on is hard to measure current I

It is very easy though I prefer to measure voltage. This is how I did this monocoil experiment:

[green]

The problem is subtracting the ground signal(ferrous) from the receive signal while looking at the target signal when coil current is changing. No ground signal, no problem. I think it's easier to look at the signal when the coil is on(constant rate)with an IB coil, but don't know how to subtract ground signal while coil is on(changing) or off(decaying).

[Great_Alex]

Thanks
Teleno please tell me where is your sample point , in Tx time or on RX
if it's in tx time does it need another opamp for Tx

please explain me more about the way that you discriminate


green I only want use one mono coil and you're right about IB coil

[Teleno]

Thanks
Teleno please tell me where is your sample point , in Tx time or on RX

Tx

if it's in tx time does it need another opamp for Tx

No, just a comparator.

please explain me more about the way that you discriminate

I count the time between turn-on and the current in the coil reaching a predetermined level. Fe: longer than average, non-Fe: otherwise.
I use an AVR chip at 16Mhz which gives a 60ns resolution.