Double closed-loop control of the focal point position in laser beam welding

Abstract

Focal point position control is significant in deep penetration laser beam weldingbecause it is apt to change during welding due to some fluctuating factors, such asthe non-planar workpiece, welding distortion and thermal focusing. This workdescribes the design of a double closed-loop control of the focal point position forCO2 laser beam welding. A photodiode sensor (PS) and a plasma charge sensor (PCS) aresimultaneously utilized to detect the optical emission and plasma charge current in welding.It is found that the PCS signal could be used to control the nozzle–workpiece distancebased on the decrease of PCS signal intensity with increasing nozzle–workpiece distance.The PS signal could be used to control the focal point position by using PS signal variationwith focal point position. Therefore, a system for double closed-loop controlling the focalpoint position has been set up, using the PCS signal to control the nozzle–workpiecedistance and the PS signal to control the focal point position. The experimental resultswith a non-planar workpiece show that the double closed-loop control of the focal pointposition not only automatically follows the contours of the workpiece surface, but alsocontrols the focal point position constant by compensating the effect of thermalfocusing.