Exploration of lift-off effects reduction based on tilted coils for eddy current testing of planar structures

Compared with magnitude-based measurements, the phase response of an eddy current sensor is less sensitive to lift-off effects. This paper explores a tilted coil configuration to further reduce the lift-off effects in phase measurements. Although tilted coils have been used in eddy current testing previously and analytical models were established, the utilisation of its phase for lift-off effects reduction has not been explored. Furthermore, an intuitive physical interpretation of the utilisation of tilted coils is presented based on the interaction between magnetic flux and the material under test. An analytical model of tilted coils is employed to evaluate their impedance changes across different lift-offs in response to a planar structure. Numerical results demonstrate that the tilted coil effectively reduces lift-off effects, with optimal performance at a tilt angle of 90° (horizontal). A new quantitative criterion, Inter-Lift-off Phase Area (ILPA), is established in frequency-sweeping measurements to characterise lift-off effects, validated by experiments across multiple materials. The results show that the 90° coil (horizontal) provides an average improvement of 26.17% in phase stability compared to the regular 0° coil (vertical). A ferrite-core configuration was also evaluated and showed reductions in phase variation and ILPA, achieving an average improvement of 39.17% in phase stability across all tested materials, confirming the effect of tilt angle on phase stability.