MINIATURIZED FLEXTENSIONAL TRANSDUCERS AND ARRAYS

Open Access
Author:
Zhang, Jindong
Graduate Program:
Materials
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 15, 2000
Committee Members:
  • William Jack Hughes, Committee Member
  • Kenji Uchino, Committee Member
  • Leslie Eric Cross, Committee Member
  • Robert E Newnham, Committee Chair
Keywords:
  • metal-ceramic composite
  • transducer array
  • actuator
  • acoustic transducer
  • finite element analysis
  • transducer modeling
  • transducers
  • underwater transducers
  • cymbal
Abstract:
The cymbal transducer is a miniaturized Class V flextensional transducer that originated from the commercially successful "moonie" patent. It consists of a PZT disk sandwiched between two "cymbal" shaped metal end caps that provide the name. First proposed as an actuator, whose performance fills the gap between the multilayer and bimorph actuator, the cymbal was later developed by Newnham and coworkers to be used as an underwater transducer. This thesis describes the design, characterization, modeling and optimization of miniaturized flextensional transducers and arrays. The underwater behavior of the cymbal transducer was modeled using the finite element analysis code ATILA. It was found that the underwater response is strongly affected by the boundary conditions imposed by the testing fixtures. The PC boards used in the initial experiments partially clamp the motion of the transducer, leading to distorted beam patterns and spurious peaks in the measured TVR curve. Excellent agreement between the calculated and experimentally measured transmitting voltage response and directivity patterns was later obtained with a potting technique in which "free" mechanical boundary conditions were maintained for the transducer. The type of PZT, cap material and geometry on the underwater response of cymbal was investigated using finite element analysis code ATILA. In general, individual cymbal transducers have a high Qm and low electrical acoustic efficiency because of poor radiation efficiency, but much more promising results were obtained with cymbal arrays. Four different array assembly techniques were proposed and compared. The potting technique, which allows the transducers to vibrate freely, gives a higher and flatter TVR than the PC board technique. The resulting beam patterns are symmetric and in good agreement with a simple point source model. Two mounting techniques using pressure release materials, the copaco ring and corprene sheet, were also tested. Of the four techniques, the potting method appears to be the most successful. Therefore, a prototype 5x20 array was built by potting the cymbals in polyurethane. The array has a thin profile and low weight, making it superior to the 1-3 composites and tonpilz transducers for certain applications. The potted array is flexible and its conformability was demonstrated by mounting it on a curved shell. Two methods were used to analyze the acoustic loading effect in the array. The equivalent circuit model is straightforward and can provide a qualitative understanding of the array interactions. By coupling finite element analysis to the well developed boundary element method, a more accurate and quantitative prediction of the acoustic loading effect can be achieved. Even though the transducers in the array are driven with the same voltage and phase, they behave differently because of mutual interactions. In a 3x3 array, the central transducer has twice the displacement of the surrounding transducers. In addition to the displacement difference, the vibration velocities of the transducers show a significant phase difference. As a consequence, the resonance is strongly damped, resulting in a broader bandwidth. The TVR and beam pattern are affected by the element spacing in the array. The calculation has also shown that the detrimental effect of array interaction can be minimized by varying the resonance frequencies of the individual transducers in the array. This work has resulted in two new designs: the double-dipper and the double-driver. The double-dipper consists of a ceramic ring bonded between two inverted cap shaped caps, therefore also called "concave cymbal". The double-dipper significantly increases the pressure tolerance of the cymbal-type transducer while maintaining a high sensitivity. The use of a ceramic ring allows several poling configurations for the double-dipper that were investigated using ATILA. The double-driver consists of two ceramic disks glued back to back with a common ground electrode. The two disks are sandwiched between two metal end caps and are driven differently. While most flextensional transducers have an omnidirectional beam pattern, a directional beam pattern can be obtained from a double-driver by exciting the driving element into a bending mode.