Frequency Controllable Multilayer Piezoelectric Transducers by the Control of Electrical Terminal Impedance

Abstract

A resonant frequency of piezoelectric vibrator with high electromechanical coupling varies remarkably depending on electrical impedance connected to the electrical terminals. In this paper, this effect has been applied to realize a frequency controllable ultrasonic transducer made of composite piezoelectric vibrator. The characteristics of the frequency controllable ultrasonic transducer is investigated theoretically by the new transmission-line model for a thickness mode piezoelectric vibrator which has been recently derived including the electrical impedance connected to the electrical terminals. It is further confirmed experimentally that the resonant frequencies can be controlled satisfactorily by a variable electrical impedance.
The results are summarized as follows:
(1) In the case of the two layered piezoelectric transducer, the resonant frequency has been controlled continually in wide frequency range (180kHz_??_580kHz). In this case, the effective attenuation for the variable resonant frequencies are less then 7dB for the frequency range of 330kHz_??_470kHz when the acoustic load is water.
(2) In the case of the bolt-clamped type piezoelectric transducer, the resonant frequency has been controlled continually from the fundamental resonant frequency (27kHz) to the second harmonic resonant frequency (60kHz) by varying the values of electrical inductance L_e. The characteristics of electro-acoustic efficiency have a minimum at 52kHz, where the minimum value was above 20%.