A complex response inductive method for improved gap measurement in laser welding

Laser welding needs precise measurement of weld gap position to avoid weld defects. Most often, optical measurement methods are used, but well-aligned narrow gaps can be difficult to detect. An improved inductive method capable of detecting zero gaps in square butt joints is proposed. The new method uses two eddy current coils, one on each side of the gap, and measures the complex response of the individual coils, i.e. both the inductive and resistive response. By combining the coil responses, both the position and the geometry of the weld gap can be estimated. The method was experimentally investigated by traversing a single coil over an adjustable gap between two plates and combining the measured coil responses into a simulated two-coil probe. The gap was adjusted in both misalignment and gap width up to 0.4 mm. Comparing the results to known settings and positions shows that gap position is measured to within 0.1 mm, if the probe is within a working area of 1 mm from the gap in both position and height. Results from the new method were compared to simulations, from the same experimental data, of a previously reported method where the coils were electrically combined by wiring them together. The previous method can give accurate results but has a much smaller working area and depends on servo actuation to position the probe above the gap. The improved method gives better tolerance to varying misalignment and gap width, which is an advantage over previous inductive methods.