Shear force sensors for process monitoring in ultrasonic metal welding—design, calibration, and validation

Abstract Ultrasonic metal welding (USMW) is a widely used solid-state welding process for electrical components. Since the joining zone remains largely hidden during the welding process and high-frequency oscillations are used to form the joint, there is only a limited range of technologies available for process monitoring. The strategies and sensor technologies used so far are not always able to detect process disturbances or defective welds. Consequently, intensive research is currently being carried out to obtain significant process information through additional sensor technology. One solution is to install sensors on the tools (horn and anvil) in the closest distance to the joining zone. Due to their flexibility and contactless measurement, laser vibrometers are frequently used in USMW research to determine the working and interfering frequencies and their amplitudes. However, the integration of such systems into production is limited by the high system costs, the lack of installation space, and the failure of sensors due to contamination. An alternative solution for determining amplitudes and frequencies is the use of piezoelectric shear force sensors in the anvil. This overcomes the challenges of limited installation space and contamination. At the same time, however, sensor integration poses new challenges such as reduced anvil stiffness and possible resonant excitation of the sensor, resulting in a sensor defect. In this study, we demonstrate a design process for a shear force sensor system for USMW, including calibration and validation, which provides high-frequency process information on the anvil side. It is shown in detail that piezo-based force sensors can be used as a cost-effective and integrable alternative to laser-based sensors in USMW processes. The correlation of the measurement data from 175 welding tests results in a mean coefficient of determination R 2 ~ 0.986 with an error measure nRMSE of ~ 0.032 between signals of the shear force sensors and a vibrometer as a reference sensor.