Design and realization of frequency agile piezoceramic transducers

Being high Q devices, low frequency underwater transducers often lack bandwidth. Variable resonance frequency transducers offer the double advantage of increased effective bandwidth and maximum response at all frequencies within the bandwidth. However the means to obtain this variable frequency differ depending on the type of transducer. This paper presents and evaluates a technique to vary the first resonance frequency of some widely used underwater acoustics transducers: flexural piezoceramic bars and disks. DC bias electric fields are added to the AC driving field and generate in-plane tensile or compressive loads. These loads modify the flexural rigidity of the transducer, which in turn affects its resonance frequencies. Theoretical investigations show that the increase in the effective bandwidth depends on the length to thickness ratio of the transducer, the type of piezoceramic material and coupling, and the piezoceramic thickness and coverage. Slender transducers, k₃₃ coupling and high piezoelectric constant d materials give higher frequency shift per unit DC field. Calculations also show that a transducer of reasonable size, using PZT5H material and k₃₁ coupling, can provide both a high frequency shift per unit DC field (about 15 % per DC kV/mm) and a high coupling coefficient (0.3 for a 0.39 material coupling. The corresponding increase in the effective -3 dB bandwidth was calculated to be 175 % over a -400 V/mm to +800 V/mm range of DC fields. One such transducer was built and tested. The experimental frequency shift was 5 % per kV/mm, leading to a 50 % increase in the effective bandwidth over the -400 V/mm to +800 V/mm field range. The experimental coupling coefficient was 0.2. Measurements show the existence of a polarization switch due to a combined compressive stress and field effect at -400 V/mm DC field. This polarization switch limits the range of negative DC fields, therefore limiting the potential frequency shift, and also results in a permanent reduction of the polarization level, therefore reducing the amount of frequency shift for a given DC field. In the case of k₃₁ coupling, the best strategy is to determine in advance the safe stress and field region for the piezoceramic material, and try to stay within that region during operation. In the case of k₃₃ coupling, the combined stress and field effect does not occur, and the available range of negative DC fields should be much higher.