10:40
Session 8: Ultrasonic welding
10:40
20 mins
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Capacitive measurement system for ultrasonic welding – A proof of concept
David Lohuis, Markus Bellmann, Stefan Carosella, Michael May, Peter Middendorf
Abstract: Joining carbon fiber composites with ultrasonic welding can still be a challenge, due to unpredictable weld parameters. Parameters can vary even within the same part, emphasising the need for a prediction model, novel measurement techniques and improved control algorithms. The authors propose a capacitive measurement system for real time quality control and as a basis for future control algorithms.
The method exploits the dielectric properties of the polymer matrix and its correlation to temperature and viscosity. Quantitative data can be gathered about all three parameters by setting up an electric field across the bonding area. Since the sonotrode and the ambos of the welding equipment consist of metal, both can act as the electrodes, leading to a measurement system suitable for industrial application. As a proof of concept, a prototype with an ambos as the lower and an external steel sheet as the upper electrode is tested in a robotic welding station.
The experiment consists of a KUKA robot, welding equipment provided by MS Ultrasonic and an Agilent E4980A LCR-Meter. The LCR-Meter applies 500kHz AC voltage to two electrodes around the specimen to determine the amplitude and phase difference between the voltage and current signal. These differences are translated into a capacity value that changes during the weld. Specimen consist of two 25x25x3 𝑚𝑚3 carbon fiber/Elium pieces and a PMMA energy director.
Capacity curves can be well reproduced and show behaviour just as expected based on polymer physics. The data indicates a capacity rise due to a superposition of the part displacement and temperature changes. Maximum temperature is assumed to be reached at maximum capacity, where the hyperbolic decline correlates to the cool down phase. With further modelling it will most likely be possible to determine quantitative changes in temperature and viscosity, leading to a more accurate understanding of the process.
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11:00
20 mins
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Sensor based path-accuracy compensation for robotic continuous ultrasonic welding of thermoplastic composites
Sungi Han, Johan van Stuyvesant Meijen, Bram Jongbloed
Abstract: Thermoplastic composites offer high weight-specific properties and can be reprocessed with heat, enabling repairability, out-of-autoclave processing and joining. Continuous Ultrasonic Welding (CUW) is an especially fast and efficient thermoplastic joining process. However, achieving consistent and defect-free robotic CUW welds remains a challenge, in part due to the high forces being applied which deflect the robot and CUW end-effector. These deflections, especially tilt errors, negatively affect weld quality.
Building on previous work that characterized repeatable tilt errors during welding motion, this research integrates a laser line scanner to provide real-time feedback on the sonotrode orientation relative to the welding adherends. The laser line scanner measures the angle between the sonotrode and the adherends, enabling active compensation for tilt deviations caused by frictional and welding process forces. This work improves the reliability in automated CUW of thermoplastic composites, improving weld quality while removing the need for manual calibration work which helps industrialize the technique.
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