Effects of nonlinearities in power ultrasonic transducers using time reversal focalization

This paper presents the characterization of nonlinearities in a Langevin-type ultrasonic power transducer using pulse excitations and a time reversal focalization technique. The nonlinear behavior of this power transducer is evaluated analyzing the signal obtained after focalization in time reversal process. In a linear regime, time reversal produces a focused pulse which amplitude and width depends only on the transducer’s transfer function. When the supplied power is increased, three non-linear effects appear in the systems response. First, the focus shape loss symmetry respect to center; second, the focus amplitude increases without proportionality to input voltage, and finally, in the frequency spectrum appears harmonics of the thickness mode resonance frequency. The displacement at the end transducer surface was measured by an optical fiber vibrometer. Traditional frequency domain methods are also used to show phase variations close to each resonance frequency. The time reversal is implemented using the Frequency Domain Time Reversal (FDTR), that technique ensures the linear regime in the first step of the process.