Eddy current testing of broken wires in thickly insulated cables using planer-3D hybrid coils

As critical components in high-speed train systems, connecting cables endure harsh operating conditions—thermal cycling, overvoltage transients, and mechanical vibrations—which may induce wire breakage. Such failures present critical safety risks, potentially triggering operational interruptions, necessitating reliable non-destructive testing (NDT) methodologies for cable integrity evaluation. This paper proposes a flexible eddy current probe with a hybrid structure to detect broken wires in thickly insulated cables, such as those used in high-speed rail. The probe utilizes a flexible rectangular planar coil (FRPC) fabricated on a flexible printed circuit board (FPCB) as an exciter, integrated with differential three-dimensional (3D) race-track receiver coils. Combining the large field excitation capability of the planar coil and the high sensitivity of the 3D sensing coils significantly improves probe lift-off tolerance. Furthermore, an innovative asymmetric differential receiver structure effectively suppresses common-mode interference while overcoming dead zones and weak magnetic field gradients inherent in traditional symmetric differential methods. This enables reliable broken wire detection under significant lift-off conditions. Experimental results confirm the probe detects 0.18 mm diameter wire breaks at 8 mm lift-off with a 38.4 dB signal-to-noise ratio (SNR). Offering simplified system configuration, the probe provides an effective solution for NDT of in-service cables.