A design approach for longitudinal–torsional ultrasonic transducers

This study investigates combining longitudinal and torsional (L–T) vibration responses at the output face of a Langevin transducer. A mode degeneration method is adopted that converts the longitudinal response excited by the axially poled piezoceramic discs in the transducer into combined L–T vibration in the transducer front mass using geometric modifications of the wave path. The study uses three techniques; numerical, analytical, and experimental, in order to estimate and validate transducer electro-mechanical parameters for different geometric modifications, and also to evaluate transducer performance for different excitation levels in terms of the desired L–T response. A finite element (FE) analysis is used to optimise the mechanical structure of the transducer while the analytical model, which is based on an equivalent-circuit approach, is used to estimate the electrical impedance spectra and to confirm some of the FE calculations prior to fabrication. These models are then validated through an experimental characterization of the vibration response and impedance response and the results show that the operating frequency and torsionality can be tailored through modification of the transducer geometry.