Optical Coherence Tomography for In-Situ Alignment Inspection of Hairpin Ends in EV Stator Laser Welding

Laser welding of hairpin ends is a key joining process for electric-vehicle (EV) traction motors, but pre-weld geometric deviations—including lateral gap variation, axial height mismatch, and focus offset—can compromise weld stability and long-term motor reliability. Conventional coaxial camera inspection provides reliable two-dimensional information but cannot directly measure axial mismatch or local end-face topography. To address this limitation, this study applies a coaxial in-line inspection system that integrates a spectral-domain optical coherence tomography (OCT) depth-sensing path and a CMOS visual imaging channel into a common optical axis. Unlike conventional OCT applications that aim for multilayer subsurface imaging in transparent media, the OCT channel in this work profiles the single reflective interface of opaque copper end faces and achieves axial linear residual errors within ±15 μm over a 4.5 mm working range, given an approximately 900 mm focus distance. The visual channel locates the weld seam and evaluates in-plane longitudinal stagger, whereas the OCT channel performs fixed-position B-scan profiling for lateral gap, axial height mismatch, end-face topography defects, and focus-offset assessment without requiring vision-guided scan repositioning. Experiments on 16 hairpin-end mock-up pairs, including 13 intentionally introduced defect samples, achieved correct classification for all tested samples. The OCT depth-profiling step was completed within the visual inspection time window, enabling parallel OCT–visual inspection without extending the original production takt time. These results demonstrate the feasibility of this non-contact method for pre-weld quality assurance in the EV stator laser welding scenario.