Methods of Controlling Lift - off in Conductivity Invariance Phenomenon
for Eddy Current Testing
Abstract
Previously, a conductivity invariance phenomena (CIP) has been
discovered – at a certain lift-off, the inductance change of the sensor
due to a test sample is immune to conductivity variations, i.e. the
inductance – lift-off curve passes through a common point at a certain
lift-off, termed as conductivity invariance lift-off. However, this
conductivity invariance lift-off is fixed for a particular sensor setup,
which is not convenient for various sample conditions. In this paper, we
propose using two parameters in the coil design – the horizontal and
vertical distances between the transmitter and the receiver to control
the conductivity invariance lift-off. The relationship between these two
parameters and the conductivity invariance lift-off is investigated by
simulation and experiments and it has been found that there is an
approximate linear relationship between these two parameters and the
conductivity invariance lift-off. This is useful for applications where
the measurements have restrictions on lift-off, e.g. uneven coating
thickness which limits the range of the lift-off of probe during the
measurements. Therefore, based on this relationship, it can be easier to
adjust the configuration of the probe for a better inspection of the
test samples.