Electromagnetic acoustic resonance (EMAR) is a well-established non-contact method for ultrasonic thickness measurement, typically operated at frequencies above 1 MHz using an electromagnetic acoustic transducer (EMAT). This study successfully extends EMAR into the sub-MHz range, allowing supply voltages below 60 V and thus offering safer and more cost-effective operation. Experiments were conducted on copper blocks approximately 20 mm thick, where a relative thickness accuracy of better than 0.2% is obtained. Regarding this result, the research identifies a critical design principle: Stable thickness resonances and subsequently accurate thickness measurement are achieved when the ratio of ultrasonic wavelength to EMAT track width ($\lambda$/w) falls below 1. This minimizes the excitation and interactions with structural eigenmodes, ensuring consistent measurement reliability. To support this, the study introduces a system-based model to simulate the EMAR method. The model provides detailed insights into how wave propagation affects the accuracy of EMAR measurements. Experimental results align well with the simulation outcome and confirm the feasibility of EMAR in the sub-MHz regime without compromising precision. These findings highlight the potential of low-voltage EMAR as a safer, cost-effective, and highly accurate approach for industrial ultrasonic thickness measurements.

The demand for lithium-ion battery cell monitoring is growing rapidly as the number of cells used in various applications increases. A novel monitoring approach is based on the mechanical properties of the battery, which change during the charge/discharge process and during its lifetime. In this approach, an acoustic wave is generated inside the battery by a transmitter and propagates through the layered structure of the cell to a receiver. The propagation speed of the wave changes depending on the mechanical properties of the cell, and so does the measurable time of flight between the transmitter and receiver. To apply this monitoring approach, it is essential to understand how this wave actually forms and travels through the individual layers. To improve this understanding, a model is presented that simulates both the generation and propagation of the wave within the battery. The model is parametrized by performing reference measurements and a tear down analysis of the battery. The simulation result shows good agreement with the measurement results and also provides a visualization of the acoustic wave motion within the cell, which greatly improves the understanding of the propagating wave.

Acoustic/ultrasonic testing is now a common method in the field of nondestructive testing for detecting material defects or monitoring ongoing mechanical changes in a structure during operation. In many applications, piezoelectric transducers are used to generate mechanical waves inside the specimen. Their actual operating frequency is highly dependent on the dimensions of the transducer. Larger dimensions of the piezoelectric transducer allow for a lower operating frequency. However, these dimensions limit the use of piezoelectric transducers in certain applications where the size of the transducer is restricted due to limited installation space and when low-frequency excitation is required. One application that places these requirements on the transducer is the monitoring of mechanical seals. Here, the transducer must be mounted on the stationary ring of the seal. In this paper, a continuously operated electromagnetic acoustic transducer (EMAT) is presented as an alternative to piezoelectric transducers as a transmitter. The advantage of a EMAT is that it meets the requirements of limited sensor size (sensor area < 10 × 6 mm) and can excite mechanical waves with frequencies below 10 kHz. A structural analysis of the stationary ring shows that the first two mechanical resonances occur around 4 and 5.5 kHz. An experimental study meterologically demonstrates the ability of the EMAT to excite these first two mechanical resonances of the ring. A comparative simulation agrees well with the measurement.

Zusammenfassung: Winde und Luftströmungen stellen eine potentielle Bedrohung für Hochspannungsleitungen dar. Die Luftbewegungen lassen die Leiterseile in beliebige Richtungen schwingen und verursachen mechanischen Stress an den Aufhängepunkten und den Leiterseilen. Um die Amplitude der Seilauslenkung zu messen, kann ein Vibrationssensor, basierend auf einer Beschleunigungsmessung, an der Leitung angebracht werden. Damit der Sensor in einem solchen Umfeld korrekt arbeitet und die Auswertung der Seilauslenkung ordnungsgemäß erfolgt, müssen zuerst Überlegungen angestellt werden. In diesem Beitrag sind diese herausgestrichen und in einen Laborprototypen eingeflossen, welcher anhand von Laborexperimenten verifiziert und charakterisiert wurde. Die Leistungsaufnahme des Prototyps sowie dessen Messunsicherheit und der mittlere quadratische Fehler, bezogen auf die Auslenkung, werden präsentiert.