Model based compensation of systematic errors in an inductive gap measurement method

A model-based correction of systematic errors to improve the measurement of gap dimensions in a recently presented method is described. Using one inductive coil on each side of the gap to measure distance and liftoff, the method detects zero width gaps and shows position error less than 0.1 mm. The correction model relies on observations of experimental data, and is calibrated to a small set of measurements. From the initial measurement of gap dimensions, the model estimates errors in each coil to calculate new values for gap width, alignment and height. The errors in the compensated results are within 0.1 mm except for gap width, which still suffers from the effect of combined gap width and misalignment. The method is intended for gap measurement in laser keyhole welding, where the laser beam and the resulting weld seam are very narrow, requiring high precision in alignment and gap preparation.