A high-sensitivity planar differential excitation eddy current probe with symmetric pickup coils

Planar eddy current (EC) probes are widely used for detecting defects in conductive components, and it is essential to enhance their detection sensitivity. Planar differential excitation EC probes have advantages in suppressing lift-off noise and enhancing the signal-to-noise ratio (SNR), but their limited layout space and strict symmetry requirements often restrict the number of turns of pickup coils, resulting in a lower output signal strength. To overcome this problem, a novel pickup coil structure is proposed in this study based on topological transformation, enabling more coil turns while maintaining perfect symmetry. Two multi-turn pickup coils of different shapes, derived from this topological structure, are integrated respectively with a differential Koch-curve-shaped excitation coil to form new probes. Both simulations and experiments are conducted on four different probes to compare their sensitivity and obtain the optimal probe configuration, in which the experimental probes are precisely fabricated using electrohydrodynamic printing. The results indicate that, compared to the single-turn pickup coil, the proposed multi-turn pickup coils significantly enhance output signal strength of the probes, with a maximum of 320%. The probe equipped with a multi-turn Koch-curve-shaped pickup coil and a magnetic core demonstrates the highest sensitivity and SNR, increasing output signal strength by up to 734% compared to the single-turn pickup coil probe. The results of further C-scan experiments on probe D for minor defects show that the probe has a sensitivity of 0.5 mm × 0.3 mm × 0.5 mm in detecting surface defects on aluminum.