In eddy current testing, the impedance plane diagrams show that the phase angles between impedance planes vary with frequency. The phase angles between lift-off, cracks and permeability may be at one set of values at one frequency and at another set of values at a different frequency. If two test frequencies are used simultaneously in the test coil and displayed separately on the vector point display, then it may be possible to subtract unwanted signals.
This is the basis of multi-frequency tests. Tests with two frequencies are now common, but even five frequencies can be processed and analysed with the help of a computer.
Firstly two frequencies are selected to give good phase discrimination. One frequency is likely to be about ten times the other. The phase and sensitivity of the impedances at each frequency are then adjusted independently on the vector point display.
When the permeability plane at one frequency is 180° out of phase with the permeability plane at the other, they can be mixed to give a third vector point which has only lift-off and crack planes. This can then be adjusted in the usual way so that the lift-off plane is horizontal and off to the left of the screen.
In summary, in single frequency tests, one variable can be suppressed using phase analysis. With two frequencies, two variables can be suppressed. With three frequencies, three variables and so on. The more frequencies there are, the more complex the electronics and the greater the possibility of extraneous non-relevant signals due to 'cross-talk' between the frequencies.
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