Abstract
In eddy current testing, lift-off variations between sensor and sample plate affect sensor inductance and consequently inferred thickness. In this paper, a novel combined inductive and capacitive sensor is designed in which the output of the capacitive sensor can be used to infer the lift-off. An algorithm is proposed to combine the inferred lift-off and the inductance measurement for predicting thickness. The effect of lift-off variations is significantly reduced using this combined sensor approach. Compared to the multi-frequency thickness measurements in our previous work, this only requires a single frequency measurement for each of the capacitive and inductive modalities, leading to a relaxation of the instrument and measurement requirements.
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