Impedance plane analysis or phase analysis is used because the signals generated in an eddy current test are vector and not scalar quantities. That is to say, they are properly described by two quantities, their amplitude and phase rather than just by their amplitude. Phase analysis of the test signal using a cathode ray oscilloscope or LCD screen instead of mere amplitude measurements with a meter allows a greater level of differentiation between relevant signals and unwanted noise. The effect of eddy currents on the coil impedance is described on the impedance plane diagram.
Eddy currents have surprisingly well ordered effects on the amplitude and phase of coil voltages. Thanks mainly to the efforts of Dr Forster in the immediate post-war years, these effects have been rationalised using mathematics. Computers can now be used with a good level of accuracy to predict eddy current test responses to simulated defects.
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