Process-control in laser welding utilising optical signal oscillations

We describe an optical sensor for process monitoring of Nd:YAG laser welding. This sensor detects the broadband radiation produced by the welding process, dividing it into broad spectral bands (designated as UV/visible and IR). Fourier analysis is used to investigate an oscillatory intensity modulation of the optical signals, believed to arise from a combination of keyhole and weld pool oscillations. The spectral content of the oscillations may be used to detect a fully open welding keyhole, and determine work-piece thickness in this welding regime. These oscillations have also been utilised in the construction of a seam tracking system which allows us to follow the seam of a lap-weld. Additional signal processing also allows optimum positioning of the laser spot.