In-line monitoring of focus shift by kerf width detection with coaxial thermal imaging during laser cutting

Nowadays, industrial laser cutting systems employ a fixed set of process parameters throughout the cut of the same workpiece, which results in a good compromise between maximum productivity and surface quality. The process parameters are commonly set by trial-and-error experiments carried out on different materials and thicknesses or less frequently by physical modelling. However, the final cut quality is not constant even though the process parameters are kept fixed due to degradation of the initial status of the laser cutting system. One of the common issues in the laser cutting process is the local heating of the optical components due to contamination and/or high powers commonly employed, which cause shifting of the focus position. This can worsen the cutting-edge quality, and even result with loss of cut. Therefore, the online measurement of the position of focus is a requirement for a consistent process. An empirical method used in the industrial practice for initially setting and successively examining and adjusting the focus position is to measure the kerf width of a straight-line cut performed with constant process parameters. This paper proposes an algorithm to monitor the kerf width and yield the estimated focus position in real-time during the cutting process. The kerf width is observed during the process with a coaxial camera module mounted on the laser head which monitors the thermal interaction between the laser beam and the material. An image processing algorithm was developed for extracting the kerf width from the acquired images, and the algorithm parameters were experimentally calibrated such that the extracted value of the kerf width matches with its physical measure. To understand the influence of the focus position on the cutting kerf, an experimental campaign was conducted and subsequently a regression model was fitted. The real-time monitoring and computation of the kerf width and its correlation to the focus position give the opportunity for a closed-loop control of the focus shift, that would eventually lead to a gain of process stability and repeatability.