[6A2] Automated contact-free spot-weld inspection with a novel laser-based ultrasound method
M Brauns1, W Rohringer1, F Schubert2, N Lehmann3 and B Fischer1
1XARION Laser Acoustics GmbH, Austria
2Porsche Leipzig GmbH, Germany
3Fraunhofer IKTS, Germany
Resistance spot welding is a safety-relevant process used for joining the parts of a car body in a fully automated way. The critical parameter for this process is the diameter of the weld nugget at the interface of the joint parts. Due to its penetration depth, ultrasonic testing is the state of the art for measuring the weld nugget diameter. However, despite the high degree of automation in the automotive industry, spot weld inspection is still performed manually using liquid-coupled piezoelectric sensors. Due to the delicate alignment and the liquid coupling agent, automation of this process has not yet been achieved to the level of speed and reliability necessary for automotive production environments. While air-coupled piezoelectric transducers eliminate the liquid coupling agent, the problem of alignment remains. In addition, another challenge is added due to the acoustic impedance mismatch between air and metal, which strongly reduces the measurement sensitivity so that it is not sufficient for spot-weld inspection in relevant materials like steel.
Together with an industrial partner, XARION has developed a novel method for spot-weld inspection with fundamental advantages:
• contact-free measurement without coupling gel contamination of the surface
• one-sided accessibility to the testing component
• automated measurement and imaging evaluation
In this single-sided approach, the sensor head comprises a fiber-coupled excitation laser as well as XARION’s proprietary fiber-coupled optical microphone for detection. The laser-based detection scheme of the optical microphone makes this unique technology unprecedented in its bandwidth and sensitivity for the detection of airborne ultrasound. The sensor head’s footprint of only 34 mm by 16 mm enables inspection even in very constrained locations, and the alignment needed is relaxed to several millimeters, thus making it compatible with mounting the sensor head on an industrial robot for automated inspection.
We present measurements on hundreds of spot welds in real car body parts, which we verify by simulations using a specially developed numerical software tool based on the elastodynamic finite-integration technique (EFIT). These simulations, which comprise both thermoelastic wave generation and wave propagation in air, also help us account for possible deviations from the ideal spot weld geometry occurring in real-life welding processes, and hence improve the robustness of our method under realistic conditions. Finally, we also compare the results to state-of-the-art methods in order to demonstrate the suitability of our novel technique for use in industrial production environments.
Together with an industrial partner, XARION has developed a novel method for spot-weld inspection with fundamental advantages:
• contact-free measurement without coupling gel contamination of the surface
• one-sided accessibility to the testing component
• automated measurement and imaging evaluation
In this single-sided approach, the sensor head comprises a fiber-coupled excitation laser as well as XARION’s proprietary fiber-coupled optical microphone for detection. The laser-based detection scheme of the optical microphone makes this unique technology unprecedented in its bandwidth and sensitivity for the detection of airborne ultrasound. The sensor head’s footprint of only 34 mm by 16 mm enables inspection even in very constrained locations, and the alignment needed is relaxed to several millimeters, thus making it compatible with mounting the sensor head on an industrial robot for automated inspection.
We present measurements on hundreds of spot welds in real car body parts, which we verify by simulations using a specially developed numerical software tool based on the elastodynamic finite-integration technique (EFIT). These simulations, which comprise both thermoelastic wave generation and wave propagation in air, also help us account for possible deviations from the ideal spot weld geometry occurring in real-life welding processes, and hence improve the robustness of our method under realistic conditions. Finally, we also compare the results to state-of-the-art methods in order to demonstrate the suitability of our novel technique for use in industrial production environments.
