Experimental investigations and modelling of a composite piezoceramic disc with different modes of vibrations

This contribution deals with the experimental investigations and modelling of the mechanical vibrations of the piezoelectric disc-shaped resonator, attached to a mechanically clamped metal membrane, which creates the composite resonator structure, the so called ultrasound therapy transducer. This transducer vibrates with a certain acoustic intensity at the typical frequency of 1 MHz, and the ultrasound waves are propagated into biologic tissue. The piezoelectric element in the composite structure is made from PZT ceramics; the clamped titanium membrane is tuned to the frequency of the resonator. Since a precise adjustment of the frequency of the transducer (which is exposed to the acoustical milieu of the air and other substances) is required, it is necessary to know the frequency spectrum of the vibrating composite resonator. This is influenced by a number of parameters. Tuning this composite structure is not a simple step in the manufacturing process, because the machining of the membrane is irreversible. In this paper, the completed investigations of the approach to the analytical solution of coupled vibrations of the piezoelectric resonators are presented, including the frequency spectra of the PZT disc-shaped resonator with a defined ratio of the diameter/thickness. However, this approach fails in the case of the real ultrasound transducer resonant structure (i.e., the disc-shaped resonator attached to the mechanically clamped metal membrane). Therefore the behavior of the vibrating structure has to be solved by some FEM method. In the described case the standard IEEE impedance/admittance measurement method and FEM simulation were used. The results allow an assessment of the suitability of the composite piezoceramic resonator in the terms of resonant frequencies, and tuning of the metal membrane to the appropriate resonant frequency.