Douglas C. Markley

Design of and Fabrication Improvements to the Cymbal Transducer Aided by Finite Element Analysis

A miniature flextensional transducer, the “cymbal,” is an emerging underwater transducer technology for large area and restricted volume transmit and receive arrays. The performance of the device is being evaluated for a number of applications requiring a large number of elements. Simple design, low cost and the ability to tailor performance to the desired application are attractive features of the cymbal. Analysis of the fabrication and performance of the cymbal has revealed that the benefits of the cymbals flexible design also present production concerns. Asymmetry in the cavity depth or epoxy layer can result in unwanted resonances that can detract from the in-water performance. To avoid these spurious resonances, tolerances in cavity depth could be as low as 5 μm, resulting in low production yields. These and other fabrication concerns have been analyzed through experiment and finite element modeling software using the ATILA code. We were able to identify ways of improving manufacturing yields and minimizing variances to accommodate large array production. The origins of the spurious resonances are discussed along with the performance of a prototype array.

Size effects in capped ceramic underwater sound projectors

Cymbal transducers are small, thin Class V flextensional transducers. A single cymbal element consists of a piezoelectric disk sandwiched between two metal cymbal-shaped endcaps which serve as mechanical transformers, converting the low impedance, small extensional motion of the ceramic into high impedance, large flextensional motion of the endcap. A cymbal transducer element is very small, with a thin profile. A single element is characterized by high Q, low efficiency, and medium power capability. When comparing cymbals to other transducers, the designer should consider their use in large flexible, low cost arrays. This paper represents the work of eight researchers over a period of several years. The effects of changes in materials and dimensions on the cymbal-type flextensional transducers, in-air and water-loaded, were examined experimentally and through finite-element-analysis (FEA). Experimental and FEA calculated results matched quite well. After gaining experience and confidence in the FEA models, extensive parametric studies were performed using FEA to investigate the size and material effects on cymbal transducer characteristics. The scaling factor (i.e., overall size), endcap stiffness, and cavity design have the strongest influence on resonance frequency. Adjusting the dimensions and materials used for drivers and endcaps provides a range in the fundamental flexural frequency from about 5 to 150 kHz in water. These results also indicate trends that can be used to extend the ranges further. The scaling factor, cavity depth, and PZT thickness had the strongest effects on the projector/receiver performance (TVR/FFVS). Investigations are underway to combine and optimize some of these parameters for potentially significant improvements in bandwidth and efficiency.

Pressure Dependence of Cymbal Transducers

The hydrostatic pressure limit that a receiver can withstand without failure is of major importance in underwater sonar systems. In this paper, the hydrostatic pressure tolerance and sensitivity of cymbal receivers were investigated. The failure mode in cymbal transducers under hydrostatic pressure is described. Effects of cavity geometry and material selection on hydrostatic piezoelectric coefficients and pressure limits were evaluated using both experimental data and finite-element analysis (FEA). It was found that cavity depth has a very strong effect on the stability of underwater sensitivity and pressure tolerance of these devices. Cymbals made with soft piezoelectric transducers (PZTs) possess higher figures of merit and better pressure tolerance than those made with hard PZTs. Alternatively, the cymbal sensitivity and pressure tolerance can be improved by changing the cap material.

Performance of transducers with segmented piezoelectric stacks using materials with high electromechanical coupling coefficient

Tonpilz acoustic transducers for use underwater often include a stack of piezoelectric material pieces polarized along the length of the stack and having alternating polarity. The pieces are interspersed with electrodes, bonded together, and electrically connected in parallel. The stack is normally much shorter than a quarter wavelength at the fundamental resonance frequency so that the mechanical behavior of the transducer is not affected by the segmentation. When the transducer bandwidth is less than a half octave, as has conventionally been the case, for example, with lead zirconate titanate (PZT) material, stack segmentation has no significant effect on the mechanical behavior of the device in its normal operating band near the fundamental resonance. However, when a high coupling coefficient material such as lead magnesium niobate-lead titanate (PMN-PT) is used to achieve a wider bandwidth with the tonpilz, the performance difference between a segmented stack and a similar piezoelectric section with electrodes only at the two ends can be significant. This paper investigates the effects of stack segmentation on the performance of wideband underwater tonpilz acoustic transducers. Included is a discussion of a particular tonpilz transducer design using single crystal piezoelectric material with high coupling coefficient compared with a similar design using more traditional PZT ceramics.

Comparison of the properties of tonpilz transducers fabricated with 001 fiber-textured lead magnesium niobate-lead titanate ceramic and single crystals.

Tonpilz transducers are fabricated from 001 fiber-textured 0.72Pb(Mg(1/3)Nb(2/3))O(3)-0.28PbTiO(3) (PMN-28PT) ceramics, obtained by the templated grain growth process, and PMN-28PT ceramic and Bridgman grown single crystals of the same composition. In-water characterization of single element transducers shows higher source levels, higher in-water coupling, and more usable bandwidth for the 81 vol % textured PMN-28PT device than for the ceramic PMN-28PT element. The 81 vol % textured PMN-28PT tonpilz element measured under large signals shows linearity in sound pressure levels up to 0.23 MV/m drive field but undergoes a phase transition due to a lowered transition temperature from the SrTiO(3) template particles. Although the textured ceramic performs well in this application, it could be further improved with compositional tailoring to raise the transition temperature and better processing to improve the texture quality. With these improvements textured piezoelectric ceramics will be viable options for medical ultrasound, actuators, and sonar applications because of their ease of processing, compositional homogeneity, and potentially lower cost than single crystal.

Cymbal and BB underwater transducers and arrays

Abstract.The cymbal is a miniaturized class V flextensional transducer that was developed for use as a shallow water sound projector and receiver. Single elements are characterized by high Q, low efficiency, and medium power output capability. Its low cost and thin profile allow the transducer to be assembled into large flexible arrays. Efforts were made to model both single elements and arrays using the ATILA code and the integral equation formulation (EQI).Millimeter size microprobe hydrophones (BBs) have been designed and fabricated from miniature piezoelectric hollow ceramic spheres for underwater applications such as mapping acoustic fields of projectors, and flow noise sensors for complex underwater structures. Green spheres are prepared from soft lead zirconate titanate powders using a coaxial nozzle slurry process. A compact hydrophone with a radially-poled sphere is investigated using inside and outside electrodes. Characterization of these hydrophones is done through measurement of hydrostatic piezoelectric charge coefficients, free field voltage sensitivities and directivity beam patterns.

Processing of Miniature Hollow Sphere Transducers

During the past two decades, we have designed and fabricated a large family of composite ferroelectrics for use as sensors, actuators, and transducers. These devices employ stress and strain amplification mechanisms and provide superior acoustic matching to seawater and the human body. Recently we have concentrated on flat-panel arrays thinner than 3 mm constructed from miniature flextensional transducers (Cymbals) or small hollow sphere transducers (BBs) embedded in polymers. This paper focuses on fabrication of BB hollow spheres. Millimeter size hollow spheres are produced using a coaxial nozzle slurry process or sacrificial core coating from 1-10 mm diameter with 10-200 w m wall-thickness. The principal resonance modes are the breathing mode (ranging from 100 kHz at 15 mm diameter to 1.5 MHz at 1 mm diameter) and the wall thickness vibration (10-100 MHz). We also discuss a method for providing the BBs with internal electrodes, a process which allows for even smaller diameters in the sub-millimeter range.

Underwater flat-panel transducer arrays

Flat-panel arrays less than 3 mm thick have been constructed from miniature flextensional transducers (cymbals) and from small hollow sphere transducers (BBs) embedded in polymer matrices. Both are intended for large area, volume restricted applications. Transmit voltage response (TVR) and free field voltage sensitivity (FFVS) measurements are reported on these structures along with some design variants. The basic cymbal transducer is a small class V flextensional transducer consisting of a PZT disk and two shaped metal caps which act as motion amplifiers. Originally designed as actuators and hydrophones, they are now being developed as shallow water sound projectors and receivers. Their low cost and thin profile allow the cymbal transducers to be assembled into large flexible flat-panel arrays. We have modeled and tested a number of modified cymbals and cymbal arrays. Mini-cymbals and maxi-cymbals ranging in diameter from 3 to 30 mm have extended the frequency range to 1-100 kHz. Cymbal arrays incorporating 10 to 100 transducers have given excellent results as underwater projectors and receivers in the 10-40 kHz range. BB hollow sphere arrays work best at higher frequencies near the breathing mode resonance, generally above 100 kHz. Millimeter size hollow spheres are produced using a coaxial nozzle slurry process and by a sacrificial core coating process in sizes ranging from 1-10 mm in diameter and 10-200 /spl mu/m in wall thickness. Two poling configurations have been studied: radial poling with inside and outside electrodes, and tangential poling with top and bottom outside electrodes. The principal modes of vibration are the breathing mode (100-800 kHz) and the wall thickness vibration (10-100 MHz). BBs are now used as miniature hydrophones and are being developed as high frequency biomedical transducers and as multi-element arrays.

High‐power single crystal based projectors.

Significant progress has been made at integrating single crystalline relaxor ferroelectrics into many types of SONAR transducers. For high‐power projectors, PMN‐28PT has offered efficient, broad band‐width capability. For higher duty cycles, thermal limitations are reached resulting in power and duty cycle tradeoffs. Electrical bias is also required for crystal implementation when high‐power operation is needed. Continuing research in compositional tailoring has resulted in several new modified relaxor crystal systems with improved temperature stability, lower loss, and higher coercive fields. PIN‐PMN‐PT ternary crystals are of particular interest. This ternary system offers 25–40 °C improvement in working temperature, reduced temperature sensitivity, and approximately two times increase in coercive field without sacrifice in electromechanical coupling or piezoelectric coefficients. To demonstrate the impact of this newer ternary crystal composition on high‐power transduction, planar tonpilz arrays were f...

Feasibility of miniature high-frequency piezoelectric ceramic hollow spheres for exposimetry and tissue ablation

Abstract Miniature, high frequency piezoelectric ceramic hollow spheres were evaluated for potential use as hydrophones for exposimetry of high intensity ultrasound fields and as minimally invasive tissue ablation devices. Spheres with diameters ranging from 0.7 to 1.0 mm, with resonance frequencies from 1.8 to 2.7 MHz were used as hydrophones. An almost constant sensitivity was reported for these hydrophones and an omni directional receive pattern was also demonstrated. The hollow sphere hydrophone exhibited twice the sensitivity of a needle hydrophone but with no pre-amplification stages and could withstand four times higher pressure. As a minimally invasive interstitial ablation device, the results demonstrated an increased necrosed tissue volume for increasing exposure time. For example, with a 1.0 mm diameter sphere (f = 1.87 MHz), the necrosed tissue diameter as a function of exposure times was 2.35 ± 0.34,3.00 ± 0.37 and4.61 ± 1.13 mm for 5,10 and 15 sec. sonications, respectively.