Patent Publication Number: US-6985407-B1

Title: Multi-layer composite transducer array

Description:
STATEMENT OF GOVERNMENT INTEREST 
   The invention described herein may be manufactured and used by or for the Government of the United States of America for Governmental purposes without the payment of any royalties thereon or therefor. 
   BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   The present invention relates generally to transducer arrays, and more particularly to a multi-layer composite transducer array that provides a broadband frequency response. 
   (2) Description of the Prior Art 
   A variety of sonar applications such as vehicle homing require the steering of acoustic beams. Existing homing array technology uses numerous narrowband and high-power longitutdinal tonpilz resonators to form the aperture of an active transducer. Each tonpilz resonator consists of several active and inactive mechanical components that work together as a spring-mass, single degree-of-freedom system. Unfortunately, tonpilz resonators are expensive to fabricate and offer only a limited operational bandwidth above their first length mode resonance. 
   To address operational bandwidth limitations of tonpilz resonators, recent work has focused on constructing multi-resonance tonpilz elements using 1–3 piezocomposites as the active component. While this approach provides improved bandwidth when compared to that of the original single-mode tonpilz resonators, these devices are still limited to first order resonance. Furthermore, the fixed-size radiation head masses inherent to tonpilz resonators prevent them from being used to realize resonators that are “frequency agile”. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to provide a transducer array that can operate in a broadband frequency range. 
   Another object of the present invention is to provide a broadband transducer array that is inexpensive to fabricate. 
   Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings. 
   In accordance with the present invention, a multi-layer composite transducer array includes at least one pair of composite transducers with a layer of dielectric material segments interposed therebetween. Each composite transducer is defined by a piezoelectric polymer composite panel having opposing first and second surfaces with at least one common electrode coupled to the first surface and a plurality of electrode segments electrically isolated from one another and coupled to the second surface. Each pair of composite transducers is configured such that the electrode segments associated with a first composite transducer oppose and are aligned with the electrode segments associated with a second composite transducer. Each dielectric material segment in the layer thereof is sized, shaped and aligned in correspondence with opposing and aligned ones of the electrode segments associated with the first and second composite transducers. Spaces formed in the layer between the dielectric material segments are filled with a viscoelastic material. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will become apparent upon reference to the following description of the preferred embodiments and to the drawings, wherein corresponding reference characters indicate corresponding parts throughout the several views of the drawings and wherein: 
       FIG. 1  is an exploded perspective view of a pair of composite transducers and an isolation layer that forms a multi-layer composite transducer array in accordance with the present invention; 
       FIG. 2  is a side view of an assembled embodiment of the multi-layer composite transducer array; 
       FIG. 3  is a side view of one of the layers of the transducer array in which the piezoelectric polymer composite panel and the electrodes coupled thereto are shaped or curved; and 
       FIG. 4  is a cross-sectional view of a multi-layer composite transducer array assembly for use in an underwater environment in accordance with the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
   Referring now to the drawings, simultaneous reference will be made to  FIGS. 1 and 2  where a multi-layer composite transducer array is shown and referenced generally by numeral  100 . More specifically,  FIG. 1  is an exploded perspective view depicting a pair of composite transducers having a segmented electrode surface and isolation layer disposed between the composite transducers, and  FIG. 2  is a side view of an assembled embodiment of the multi-layer array. 
   Array  100  has a pair of composite transducers  102  and  104  with an electrical and mechanical isolation layer  106  disposed therebetween. Each of composite transducers  102  and  104  is identically constructed so that the following description of composite transducer  102  applies to composite transducer  104 . A plurality of electrode segments  12  are supported on a first major surface of a piezoelectric polymer composite panel  20 . The number, size and shape of electrode segments is not a limitation of the present invention. Details of a suitable composite panel are described in U.S. Pat. No. 6,255,761, the contents of which are hereby incorporated by reference. Briefly, composite panel  20  is constructed using spaced-apart piezoelectric (e.g., a ferroelectric material such as piezoceramic materials lead zirconate titanate or lead titanate) columns or rods  22  that span the thickness or height H of composite panel  20 . Filling the spaces between rods  22  for the full height thereof is a viscoelastic material  24  such as a thermoplastic epoxy. 
   Each of electrode segments  12  can have a dedicated electrical lead coupled thereto. This can be accomplished by passing conductors (e.g., conductors  31  and  32  are illustrated in  FIG. 2 ) through a side of composite panel  20 . More specifically, each of conductors  31  and  32  is routed through viscoelastic material  24  and electrically coupled to one of electrode segments  12 . The second major surface of composite panel  20  has a single common electrode  40  that substantially spans and is coupled to composite panel  20 . Note, however, that the single common electrode  40  could be replaced with a plurality of common electrodes (i.e., at the same potential) without departing form the scope of the present invention. Typically, the height H of composite panel  20  is the same throughout so that planes defined by electrode segments  12  and common electrode  40  are parallel to one another. 
   Each layer of the multi-layer array can also be shaped to conform to simple or complex contours if viscoelastic material  24  comprises a thermoplastic material such as thermoplastic epoxy. For example, as illustrated in  FIG. 3 , composite panel  20  has been shaped (e.g., by heating) such that the planes defined by, electrode segments  12  and common electrode  40  are curved in correspondence with one another and composite panel  20 . 
   Composite transducers  102  and  104  are configured and positioned in array  100  such that electrode segments  12  on composite transducer  102  oppose and are aligned with electrode segments on composite transducer  104 . Separating composite transducers  102  and  104  is isolation layer  106  that consists of dielectric material segments  50  extending through layer  106  and a viscoelastic material  52  that can be the same material as that used for viscoelastic material  24 . Each of dielectric material segments  50  is sized, shaped and aligned with opposing and aligned ones of electrode segments  12  from composite transducers  102  and  104 . Since electrode segments  12  are electrically isolated from one another by spaces therebetween, similar spaces are formed between dielectric material segments  50 . The spaces between segments  50  (and regions surrounding segments  50  up to the edges of array  100 ) are filled with viscoelastic material  52 . In this way, dielectric material segments  50  provide the needed electrical isolation between opposing electrodes  12  on composite transducers  102  and  104 , while viscoelastic material  52  provides mechanical damping and isolation between composite transducers  102  and  104 . 
   Composite transducers  102  and  104  are typically bonded to isolation layer  106  by an adhesive  108  so that no external type of clamping is required to hold array  100  together. Any commercially-available structural adhesive can be used provided it is acoustically transparent and can withstand the rigors of the environment in which array  100  is to be deployed. 
   The multi-layer composite transducer array described herein can be used as part of an underwater array assembly such as assembly  200  illustrated in  FIG. 4  where like reference numerals are used to describe the elements incorporated into assembly  200 . A waterproof housing (e.g., a waterproof encapsulant)  202  has one or more arrays  100  (e.g., two are shown) fitted and sealed therein. An acoustic absorbing material  204  (e.g., a particle-filled epoxy) partially fills waterproof housing  202 . The lowermost composite transducer in the stack of multi-layer arrays  100  is coupled to acoustic absorbing material  204  by means of adhesive  108 . More specifically, common electrode  40  of the lowermost composite transducer is adhered to acoustic absorbing material  204 . At the other end of the stack of multi-layer arrays  100 , common electrode  40  of the uppermost composite transducer abuts waterproof housing  202 . Note that this portion of waterproof housing  202  must be acoustically transparent to facilitate the transmission of sound waves. Another isolation layer  106  is disposed between arrays  100  and is coupled to each of arrays  100  by adhesive  108 . 
   Signal electronics  206  can be located within and/or outside of housing  202  as illustrated. Conductors (not shown for clarity of illustration) coupling signal electronics  206  to the electrodes (i.e., electrode segments  12  and common electrodes  40 ) in multi-layer arrays  100  are passed through acoustic absorbing material  204  and through each composite transducer&#39;s composite panel as described above. 
   The advantages of the present invention are numerous. Broadband operation is achieved owing to the combination of: (i) the inherent broadband resonance of each composite transducer&#39;s piezoelectric polymer composite panel  20 , and (ii) the fact that the array&#39;s individual layers can be separately addressed/tuned to a different frequency range. The present invention also provides an improved spatial field-of-view since numerous elements may be formed by selectively applying electrodes over the array aperture to form elements having different (non-uniform) apertures. The invention teaches element apertures that can be varied in size by simply addressing electrode segments separately. High frequency responses are achieved using small sized electrode segments. The electrode segments can be combined for low frequency responses, or larger sized electrode segments could be used. 
   It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.