Study On The Frequency Characteristic Of Large Dimension Piezoelectric Transducer

Sandwich piezoelectric ultrasonic transducers are widely used in underwater acoustics and ultrasonic. According to traditional design theory of this transducer, the vibration of sandwich transducer is considered as one-dimensional with the Poisson effect and the radial vibration being ignored. Therefore, it is required that the radial dimensions of the transducer must be far less than the longitudinal dimension. Generally speaking, when the radial dimensions are less than a quarter of the longitudinal wavelength, one-dimensional theory can be used and the error between the measured and theoretical resonance frequencies is negligible. However, along with the development of ultrasonic technology, ultrasonic transducers are used in more and more new applications, such as high frequency ultrasonic metal and plastic welding and some practical applications concerning very large ultrasonic power. In these cases, radial dimensions are usually larger than quarter of a longitudinal wavelength. Therefore, one-dimensional design theory of the sandwich transducer is no longer applicable; Otherwise, large frequency error will be caused. Specifically speaking, one-dimensional design theory is not applicable for the following two types of sandwich piezoelectric ultrasonic transducers. (1)High frequency sandwich transducer such as those used in ultrasonic metal welding. When the resonance frequency of the sandwich transducer is increased, the longitudinal wavelength and geometrical dimension will accordingly decrease. According to the assumptions introduced in one-dimensional longitudinal theory, the radial dimension of the transducer must also be decreased. Therefore, the cross-section of the transducer is small and the mechanical strength and the power capacity will be lowered. To raise the mechanical strength and the power capacity, the radial dimensions of the high frequency sandwich transducer must be increased; thus, the radial vibration in this kind of transducers must be considered in order not to bring about appreciable frequency design error. (2)Higher power sandwich transducers. In some new ultrasonic applications, such as ultrasonic metal and plastic welding, very large ultrasonic power is needed; therefore, the radial dimensions exceed a quarter of longitudinal wavelength and one-dimensions exceed a quarter of longitudinal wavelength and one-dimensional theory is also not applicable. In the above-mentioned two cases, the vibration of the transducer is a coupled one of longitudinal and radial vibrations. Therefore, new design theory must be developed in order to study the coupled vibration of the sandwich transducer with a large cross-section or high resonance frequency.For the coupled vibration of sandwich piezoelectric ultrasonic transducers, numerical methods have been widely used to study the frequency characteristics and vibration modes. Among the numerical methods, the finite element method seems to be the most promising. Nowadays, some commercial software is available in the analysis of vibrational systems, such as ANSYS software. Although numerical methods are widely used in engineering problems, the preprocessing of these methods is complex; the physical meaning of the results is not as concise as that in one-dimensional vibrational theory.In this paper, based on the apparent elasticity method, the couple vibration characteristic of large dimension metal cylinder, piezoelectric ceramic will be analyzed respectively. The results show that we can design a new double-frequency or multi-frequency transducer by means of the intense radial vibration mode of the large dimension piezoelectric transducer. All of the conclusions will be testified by experiment and ANSYS result.