Displacement Mesurements During Laser Welding Using Self-Mixing Interferometry

Conclusions: In this thesis, a sensor based on the self-mixing effect has been introduced for displacement and vibration measurements during laser welding. Furthermore, the possibility of onlince detection of the penetration depth has been investigated by positioning the LD sensor beam into the keyhole.

The self-mixing interferometer was calibrated with the help of the comercial fiber distance sensor and sound speaker as the source of oscillations. Two signal processing methods have been applied in order to reconstruct the initial displacement of the workpiece. According to the calibration measurements, the fringe counting method and the phase-unwrapping approach have uncertainty of +/- 446 nm and +/- 298 nm respectively. During the calibration measurements, we studied the influence of the angle, between the sensor and a target, on the signal quality of the interferometer. The self-mixing signal was found to be still sufficient for the signal processing algorithys until 4 degrees tilt. The quality of the signal was also perturbed by the high amount of the back-reflected light which destabilizes the LD outpout power. This was overcome by using a gray filter. The electronics of the sensor determine the resolution of the interferometer, i.e. the sensor is band limited.

During the displacement and the vibration measurments of the robot arm several effects have been identified that complcate the application of the self-mixing interferometer for mointoring the laser weding process. 

First the self-mixing siganal suffers from the speckle effect which represents the intensitiy pattern of dark and bright spots on a diffusive surface of metal. Second, the complexity of the laser welding process fluctuations of the keyhole shape and also interaction of the LD beam with the metal vapor plume make it impossible to perform any displacement measurements. It was discovered that the influence of the vapor pllume emsission, also the reflections fo the disk laser are especcially strong at full penetration of the metal by the disk laser. Hence, these intensitis are not completely surpressed by the bandpass filter and interfere with the signal. The high temperature in the resgions close to the vapor plume changes the refractive index of the air that the LD beam is refracted.

The future improvement of the sensor might be the implementation of the speckle tracking technique which overcomes the amplitude modulation from the speckles by keeping the LD beam on the brightest spot on a target surface. The  usage of the sensor during the laser cutting might be promising since it does not have the complexity as the keyhole behaviour and the vapor plume influence which could give a sufficient signal for further signal processing. Faster electronics will provide better resolution.