PERFORMANCE OF AN OPTICAL STAND-OFF CONTROL-SYSTEM FOR LASER MATERIALS PROCESSING

This paper presents an experimental investigation into a high-bandwidt h optical range sensor for laser materials processing stand-off contro l. Radiation from a low-power laser beam is focused onto a workpiece s urface and light reflected from the surface is collected through a mai n lens and directed into an imaging lens which focuses the signal to t wo positions after being split by a beam splitter. The irradiances of the two beams are detected by photodiodes placed behind pinhole apertu res positioned fore and aft of the two focal positions. A differential amplifier is used to generate an output signal that determines the ma gnitude and direction of any workpiece displacement. The system facili tates a measuring range of +/-6mm. A set of of experiments are perform ed and results are analysed for different setup configurations. The ap proximate range of instrument linearity is +/-1 mm for the 75-mm focal length main lens and +/-2 mm for the 120-mm lens; in this linear rang e the optimal accuracy resolution is 1 mu m. The system's effectivenes s in controlling the stand-off distance of a laser cutting machine, an d hence cut quality, is assessed.