High Power Electromechanical Characterization of Piezoceramics and Low Frequency Ultrasound Transducers by Using Algorithm for Tracking Changes in Resonant Frequency and Electrical Impedance

Abstract

Electromechanical and acoustical characterization of PZT piezoceramic elements and high-power ultrasound transducers has been done around resonance frequencies in order to determine the features of the models that describe the considered devices, both as equivalent circuits and as nonlinear oscillators. Several measurement methods have been compared, including the frequency-sweeping method with constant excitation voltage, the magnitude-sweeping method at a constant excitation frequency and the method that uses impulse excitation. The parameters that describe the losses in a coupled nonlinear electro-mechanical-acoustical system greatly depend on the type of excitation and its level. A novel algorithm is tested, designed for tracking the changes of the series resonance frequency and impedance magnitude and phase with the excitation level. It enables a more precise determination of the resonance frequency and impedance changes over different levels of excitation, which is especially important in high-power resonant applications. The characterization based on frequency sweeping at different excitation levels is influenced by changing electrical parameters (current, voltage, power) due to different complex loading of the excitation amplifier and the changes of thermodynamic conditions. The characterization of transducers in loaded condition through frequency sweeping at higher signal levels gives no results when cavitation in front of the horn appears, because the impedance magnitude changes at higher excitation voltages. Using the tracking algorithm, the impedance magnitude at resonance frequency has been determined in a range of different excitation levels up to 35 VRMS. Before the onset of nonlinear effects such as cavitation or acoustic streaming, the impedance magnitude increases due to viscosity effects.