VARIABLE FREQUENCY FLEXURAL PIEZOELECTRIC TRANSDUCERS FOR HIGH POWER LINEAR CHIRP TRANSMISSION

摘要

To overcome low radiation resistance, low frequency underwater transducers must be operated in the vicinity of resonance. To provide maximum velocity at resonance, they are often designed as high Q narrowband devices. Various approaches have been used to increase the bandwidth of these transducers, including impedance matching layers and velocity feedback, among others. However, these approaches often result in reduced peak sensitivity (response level at the resonance frequency). In contrast, low frequency transducers with a variable and controllable resonance frequency, offer the double advantage of increased effective bandwidth and maximum sensitivity at almost all frequencies within the bandwidth. The responses of fixed frequency and variable frequency piezoelectric ceramic tonpilz transducers to a linear chirp were calculated and compared. The resonance frequency of the fixed frequency tonpilz was set to the center frequency of the linear chirp, while that of the variable frequency tonpilz was matched to the dominant frequency of the chirp at all times. The results show that the variable frequency transducer can efficiently transmit linear chirps of much larger bandwidth. A method of varying the first resonance frequency of some widely used underwater transducers, trilaminar flexural piezoceramic bars and disks, is also presented. Positive bias electric fields are added to the driving AC field to generate either a tensile in-plane load. This load modifies the effective flexural rigidity of the transducer, which in turn affects its resonance frequencies.