Penetration control in laser welding of sheet metal using optical sensors

For economical reasons it is desirable to apply the highest possible speed during laser welding. Increasing the welding speed at a certain laser power might result in insufficient penetration of the weld. This work describes the design of a feedback controller, which is able to maintain full penetration in mild steel sheets. An optical detector, which is placed inside the Nd:YAG laser source, measures the intensity of the weld-pool radiation through the optical fibre. This sensor signal is used as input of a feedback control system. A model, which describes the dynamic response of the welding process, including the sensor and laser source dynamics, has been obtained, using system identification techniques. The input of this model is the laser power and the output is the modelled sensor signal. Based on this dynamic model a feedback controller is designed and implemented. The laser power is used as an actuator. The controller maintains full penetration during welding of tracks with disturbances like sudden speed changes. This opens the possibility to optimise the welding speed without risking lack of penetration.