University of Twente Student Theses


Penetration depth control using an optical spectroscopic sensor in ND:Yag laser welding

Pocorni, J.K. (2012) Penetration depth control using an optical spectroscopic sensor in ND:Yag laser welding.

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Abstract:In the welding industry weld defects such as lack of penetration are considered as problematic because they decrease the strength of the weld joint. Analysis of the process light coming out of the laser material interaction is used in industry to monitor quality parameters such as the penetration depth. This monitoring signal can then be used in a feedback system to steer the weld process to a desired condition or prevent a weld defect. Part of the process light is emitted by a plasma plume which is formed above the weld pool during high power Nd:YAG laser welding. When this plume is blown away with air, cross sections have shown that the penetration depth increases. The goal of this thesis is to implement a suitable controller which maintains constant penetration, and at the same time increases the weld efficiency by blowing the laser induced plume away. For the control system the availability of a reliable sensor is essential. For this purpose the radiation from the interaction zone was measured with a spectrometer, and from this radiation a signal was calculated which monitored the penetration depth. The monitoring signal was calculated by the ratio between light intensities at two distinct wavelengths. This signal was then fed to an integral controller which adjusted the laser power such that the penetration depth could be kept constant. First, characterization experiments were carried out to identify the weld process, and based on this identified process, the controller parameter was determined. The characterization experiments were conducted to relate the monitoring signal to the laser power and penetration depth. After this relation was known, the setpoint of the feedback system was set accordingly and the performance of the designed controller was evaluated in simulations. Finally, the controller was tested experimentally with two setpoints. The obtained results have shown that the measured spectral signal successfully monitored the penetration depth but the implemented controller did not steer the process to a constant penetration.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
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