Jindong Zhang

Modeling and underwater characterization of cymbal transducers and arrays

The cymbal is a miniaturized class V flextensional transducer that was developed for potential 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 element and transducer arrays by coupling finite element analysis (ATILA) and the integral equation formulation (EQI). The pressure and velocity distributions on the surface elements were calculated by ATILA and later used with EQI to calculate the far held properties of the transducer element and arrays. It eliminates the mesh of the fluid domain and makes the 3-D model of a transducer possible. Three-dimensional models of a cymbal transducer and a 3/spl times/3 cymbal array were developed in the modeling. Very good agreement was obtained between modeling and measurement for single element transducers. By coupling finite element analysis with the integral equation method using boundary elements, acoustic interaction effects were taken into account. Reasonable agreement was obtained between calculation and measurement for a 3/spl times/3 array.

A class V flextensional transducer: the cymbal

The cymbal is a miniaturized class V flextensional transducer which was developed for potential use as a shallow water sound projector. In underwater tests, the experimental fixture was found to have a pronounced effect on the performance of the transducer through modifications of the mechanical boundary conditions imposed on the device. A less restrictive setup was devised for the underwater test of a cymbal transducer and very good agreement between the calculations and the experimentally measured transmitting voltage response was then obtained.

Cymbal array: a broad band sound projector.

A prototype 3 x 3 planar cymbal transducer array was built and tested. The array has a radiating area of 5.5 cm x 5.5 cm and a thickness of less than 8 mm. The measured transmitting voltage response was above 134 dB re 1 microPa/V @ 1 m and flat over the frequency range of 16 and 100 kHz. Array interaction was analyzed using an equivalent circuit model. The array interaction leads to variations in radiation resistance and velocity of the transducers in the array according to their surroundings in the array. The effect is enhanced overall efficiency and a flat response.

Flexural traveling wave excitation based on shear-shear mode

This paper presents a new mechanism for circular flexture traveling wave excitation, which is based on a shear-shear mode in a single piezoelectric disc with a center hole was poled along its radial direction. Its bottom electrode was divided into four parts, and the top electrode acted as common ground. By use of a pair of alternating current (AC) voltage signals (sine and cosine), at the discs resonance frequency, a flexural traveling wave was generated circumferentially. Both finite element modeling (FEM) analysis of transducers by integration of Laplace equations (ATILA) analysis and experimental results demonstrate the concept. As an application example, a shear-shear mode prototype was constructed and characterized.

Cymbal transducers: a review

The cymbal transducers consist of a piezoelectric disk (poled in the thickness direction) sandwiched between two cymbal-shaped metal end caps. The metal caps serve as a mechanical transformer which transforms the high impedance, small extensional motion of the ceramic into low impedance, large flexural motion of the shell. The cymbal was originally designed as an actuator, which provides a sizable displacement as well as a large generative force, bridging the gap between the two most common actuators, the bimorph and the multilayer. It was later proposed as a shallow water sound projector and receiver. Single elements are characterized by high Q, low efficiency and medium power output capability. Their low cost and thin profile allow the transducer to be assembled into large flexible arrays. A 5/spl times/20 prototype array was built and tested. It has a broadband response in the frequency range between 17 kHz and 100 kHz. The array is flexible and easily conforms to a curved surface. Finite element code ATILA was used successfully in modeling and optimizing the single element transducer. Several modifications were investigated for various applications with the aid of computer modeling. Among them are the double-dipper (deep submergence), the double-driver (unidirective beam pattern) and the smart caps (adjustable resonance frequency).

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.

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.

Strain tunability of spontaneous polarization and enhanced ferroelectric properties in epitaxial (001) BiFeO3 thin films

We report the strain dependence of remanent polarization and coercive field of epitaxial (001)p BiFeO3 films. Our measurements reveal that the large spontanoues polarization of BiFeO3 is indeed intrinsic, the remanent polarization of (001)p BiFeO3 thin films has a strong strain dependence, even stronger than (001) PbTiO3 films, and the coercive field of BiFeO3 films is also tunable. In addition, the low coercive filed and the reduced leakage current in (001)p BiFeO3 membranes allows us to achieve a fatigue-free switching behavior to 1010 cycles. This experimental result strongly suggests that epitaxial (001)p BiFeO3 thin films are very promising materials for non-volatile memories and magnetoelectric devices.