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

Focal point position control is significant in deep penetration laser beam welding because it is apt to change during welding due to some fluctuating factors, such as the non-planar workpiece, welding distortion and thermal focusing. This work describes the design of a double closed-loop control of the focal point position for CO 2 laser beam welding. A photodiode sensor (PS) and a plasma charge sensor (PCS) are simultaneously 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 distance based 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 variation with focal point position. Therefore, a system for double closed-loop controlling the focal point position has been set up, using the PCS signal to control the nozzle–workpiece distance and the PS signal to control the focal point position. The experimental results with a non-planar workpiece show that the double closed-loop control of the focal point position not only automatically follows the contours of the workpiece surface, but also controls the focal point position constant by compensating the effect of thermal focusing.