Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

5.3. Multi- frequency testing
5.3.1 Principles
In multi-frequency testing, two or more sinusoidal signals of different frequencies
are fed simultaneously to a single eddy current probe. Gain and phase of the
output signal from each frequency can be separately controlled. Multi-frequency
testing allows signals from undesirable variables to be eliminated. Usually, two
frequencies are mixed to suppress one variable in order to monitor a second
variable. The most common application is to eliminate unwanted signals, so that
signals of interest give clear, easy to interpret indications. Having obtained
signals from the condition to be eliminated at the test frequencies, the amplitude
and phase angle of the signal are adjusted and rotated to be nearly equal at the
selected test frequencies. These signals are then vectorially subtracted by the
instrument, resulting in a ‘mixed’ output which is insensitive to that condition.
The primary frequency used in multi-frequency testing is usually f 90 and, as a
general rule, the second frequency should be no greater than half the primary
frequency for external variables and no less than twice the primary frequency for
the internal variables.
Charlie Chong/ Fion Zhang