Response Signal Test Proceedure

[gruder]

I am trying to test the target response signal from various metal objects using a pulse induction metal detector to induce the targets.

I have been using a O-scope probe connected across various ferrous & non ferrous samples with the metal detector a set height to try to read the response signal amplitude & decay etc from the metal targets--Air test only at this stage.

The problem i have is there seems to be very little change in signal amplitude & decay from test piece to test piece.

Like mentioned i only have the O-scope probe connected directly across the test pieces & maye this is not correct?

Am i doing this correctly & or is there a better way to test things?

Thank You

 

[zappedagain]

Response to what? What is your nominal response? Amplitude, frequency, pulse duration?

You might be seeing only minor changes because the majority of your pickup could be the inductive loop of your scope probe. If you connect probe tip directly to the ground wire on the probe do you see the same signal?

If you provide more information on your test setup and actual signals, there is a good chance you'll get a much better response out of this group.

Z

 

[IRstuff]

Have you even tried to determine what level of signal you should be expecting, and whether your scope can even see that signal?

Most scope probes can't see the signal from a thermocouple, yet, they work and are routinely used in many devices.

TTFN

FAQ731-376

 

[gruder]

No, i get no signal when connecting the ground wire to the probe, i tested that first up to make sure.

As mentioned i am trying to test the Amplitude & Decay also signal duration of metal targets once induced, the metal targets are induced by the magnetic pulse of the detector & create Eddy Currents in the targets.
The Eddy Current response signal from the metal targets are what i need to test, rise & fall times, Amplitude etc.

Thanks

 

[IRstuff]

Have you even tried to determine what level of signal you should be expecting, and whether your scope can even see that signal?

I didn't ask whether you checked out your scope, but rather, had you done the calculations to determine whether the eddy currents generate a large enough signal to seen by the oscilloscope.

TTFN

FAQ731-376

 

[gruder]

Yes, the signals are large enough to see, especially with an Air test setup.
The signals i am getting at the moment are around 10mV Amplitude from a small 1grm Gold Nugget.
The problem i seem to have is that metals with different conductivity seem to have the same Amplitude & Decay times as each other?

One would have thought that highly conductive metals such as annealed copper of the same size & shape would have a larger signal than say the gold nugget.

This is the reason i was questioning my test setup with the O-scope probe place directly across the test pieces.

 

[OperaHouse]

I would think the characteristics of the inducer coil would swamp what change is caused by the target. If size, distance, and material imperfections are not controlled, I don't see how you will be able to see just resistance changes.

 

[IRstuff]

Moreover, a two-point measurement cannot determine resistivity of a material. You need at least 4 points:

http://en.wikipedia.org/wiki/Van_der_Pauw_method

http://www.ecse.rpi.edu/~schubert/M...d Hall effect of discs of arbitrary shape.pdf

TTFN

FAQ731-376

 

[gruder]

Thanks for the replies although to be honest i am not sure we are on the right track?

Basically a pulse induction metal detector works by inducing a metal target with a magnetic pulse & then reads the eddy current return signal from the targets which is then amplified & makes the detector go off.
There is a short time delay in the circuit between the induction pulse & the recieve signal to make sure the detector only recieves the target signal & not any residual from the coil itself. The strength of target signal varies a lot & depends on size, shape depth & the grounds magnetic susceptibility & conductivity etc.
So trying to read the target signal in Air should be easy with the correct setup, whatever that may be?

Maybe i should not be trying a direct connection to the test pieces but instead try reading the target signal as the metal detector does itself with a coil at some set distance from the target?

Any ideas would be very helpful.

Thanks

 

[zappedagain]

If you already have a detector, why not monitor the output of the detector with the scope? That way you already have the built in detector filtering and you can reference known materials. That degree of separation will eliminate a lot of the concerns with accessing the part directly; Heisenberg's Uncertainty Principle comes in on the macro level too when measuring.

Or did I misinterpret your last post and you are trying to design a detector?

Z