Abstract:
An inverse flextensional projector exhibits a low frequency flexural mode and a higher frequency “breathing” mode to defeat stealthy targets and to conduct short and long range detection and tracking in littoral waters. The device has much broader bandwidth than conventional flextensional transducers, slotted cylinders and conventional cylinder transducers. The device has a low frequency capability similar to slotted cylinder projectors (SCP) but is broader band and does not suffer from the unsupported gap of SCP projectors. The invention has a more uniform radiation velocity than both SCP and flextensional transducers, making it much less susceptible to cavitation limitations.

Description:
RELATED APPLICATION 
   This application claims priority to U.S. Provisional Application No. 60/626,032, filed Nov. 8, 2004, the subject matter thereof incorporated by reference herein in its entirety. 

   FIELD OF THE INVENTION 
   The invention in general relates to transducer devices, and more particularly, to a flextensional transducer device. 
   BACKGROUND OF THE INVENTION 
   Electroacoustical transducers are advantageous because they provide a conversion between electrical energy and acoustical energy. For example, when alternating current signals are introduced to an electroacoustical transducer, the transducer vibrates and produces acoustical energy in accordance with such vibrations. The conversion of electrical energy to acoustical energy has a number of different uses such as in loud speakers and in sonar applications, for example. 
   Piezoelectric elements, primarily crystals and ceramics, are employed in a variety of devices including crystal microphones, ultrasonic devices, accelerometers and oscillators. One of the most common uses of piezoelectric elements is in underwater sonar equipment in which a piezoelectric sonar transducer is stimulated by electrical signals to emit sonar signals which radiate out from the transducer. The sonar signals are reflected off of underwater objects and the reflected signals are then detected by the transducer, which in turn produces and delivers electrical signals carrying information about the underwater object. 
   Flextensional sonar transducers of the prior art may employ a stack of piezoelectric transducer elements interspersed with electrically conducting plates for stressing the elements and for picking up electrical current produced by the elements; a prestressed compression band, made for example of a filament wound material, wrapped about the piezoelectric stack; and an outer elliptically-shaped shell wrapped about the compression band. The stack of piezoelectric elements generally extends along the major axis of the ellipse defined by the outer shell. When an alternating voltage is applied to the conducting plates, the stack of piezoelectric elements is caused to be displaced in the direction of the major axis in proportion to the instantaneous value of the voltage. The vibration and displacement of the stack is transmitted to the shell which amplifies the vibration along the minor axis of the ellipse to produce the sonar signals. That is, as the stack expands to expand the major axis of the ellipse, the long walls of the ellipse perpendicular to its minor axis contract, and as the stack contracts to expand the long walls of the ellipse, vibration of the shell necessary to generate the sonar is produced. In an alternative arrangement of a flextensional transducer, a magnetostrictive element may replace the piezoelectric stack. 
   The elliptical shells used in flextensional transducers are typically preformed of filament-wound composites such as glass, reinforced plastic or aluminum. In order to incorporate the stack of piezoelectric elements in the shell, the shell is compressed along its minor axis by means of a press, and the piezoelectric stack is inserted into the shell to coincide with the major axis. Upon removal of the compressive force from along the minor axis, a residual force remains in the shell to retain the stack and apply a predetermined compressive stress thereto. Construction of the assembly in this fashion requires the piezoelectric stack and elliptical shell be prepared to close tolerances both to allow for easy insertion of the stack within the compressed shell, and to retain tight contact between the stack and the shell upon removal of the compressive forces. 
   Slotted Cylinder Projectors or SCPs, have been used to provide low frequency transducer devices capable of operating in the low frequency range (about 425 Hz and below). More particularly, compact SCPs having diameters less than or equal to T-size (i.e. 12.75 inch outer diameter) have been used for such low frequency range operation. However, these SCPs exhibit a very narrow bandwidth which limits the breadth of operation of such devices. In addition, high power SCPs require a great number of segmented 33-mode rings, each of which is formed from multiple wedges. This causes difficulty in both the initial manufacturing process (which is very labor intensive), as well as in the prestress portion and installation into the inert shell. Furthermore, such SCPs exhibit reliability problems resulting from the unsupported gap or slot therein.  FIG. 1  is an illustration of a prior art transducer device  10  having an inert tubular member  12  with a gap  14  and a plurality of sectionalized transducer elements  16  arrayed within the member  12  in abutting and progressive relationship to one another and in abutting relationship to the inner wall of the member  12 . The gap is typically covered with a thin boot to avoid suppressing motion. The unsupported gap causes high stress risers in the ceramic which results in ceramic failure and flooding failure into the gap region. Moreover, the high velocity near the gap region often results in undesirable cavitation. A cylindrical transducer which overcomes one or more of the aforementioned difficulties is highly desirable. 
   SUMMARY OF THE INVENTION 
   In accordance with an aspect of the present invention, there is described an inverse flextensional projector having a low frequency flexural mode and a higher frequency “breathing” mode. The device has much broader bandwidth than conventional flextensional transducers, slotted cylinders and conventional cylinder transducers. The device has a low frequency capability similar to slotted cylinder projectors (SCP) but is broader band and does not suffer from the unsupported gap of SCP projectors. The present invention provides for a more uniform radiation velocity than both SCP and conventional flextensional transducers, making it much less susceptible to cavitation limitations. 
   According to an aspect of the present invention, a flextensional apparatus for use in a flextensional transducer comprises a shell having an internal hollow bounded at a top surface and a bottom surface by a concavo-concave arm arrangement, each arm having a first and second end and each of a given thickness, with the top concave arm and the bottom concavo arm joined at the first end by a common thicker first end portion and each arm joined at the second end by corresponding common thicker second end portion. A plurality of vibratable elements are arranged in a stack from a first end to a second end, the stack positioned in the hollow of the shell and extending from one end of the hollow to the other end and positioned along an axis such that the first and second arms are symmetrically disposed with respect to the axis. A first radiator extends in a first direction relatively from the center of the first arm and is operably coupled thereto, and a second radiator extends in an opposite direction from the center of the second arm and is operably coupled thereto, whereby when the elements vibrate, the arms deform to cause the radiators to alter position according to the deformation. 
   According to another aspect, a flextensional transducer comprises a drive assembly comprising a stack of one or more vibratable elements responsive to an alternating power source; a flextensional shell having an internal hollow for accommodating the drive assembly, the shell having first and second bulbous end portions, each adapted to receive a corresponding end of the drive assembly, and a concavo-concave arm arrangement, each arm having a first and second end terminating at a respective one of the bulbous end portions, thereby defining the hollow; a first radiator extending in a first direction relatively from the center of the first arm and operably coupled thereto, and a second radiator extending in an opposite direction relatively from the center of the second arm and operably coupled thereto, whereby when the elements vibrate, the arms deform to cause the first and second radiators to alter position according to the deformation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Understanding of the present invention will be facilitated by consideration of the following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which like numerals refer to like parts, and: 
       FIG. 1  illustrates a view of a prior art transducer; 
       FIG. 2A  is a schematic perspective view of a flexural cylindrical projector transducer having a flextensional shell and radiator structure according to an embodiment of the invention; 
       FIG. 2B  is a schematic perspective view of a flexural cylindrical projector transducer comprising concatenated sets of flextensional shell and radiator structures according to an embodiment of the invention; 
       FIGS. 2C-2D  are schematic plan and side sectional views, respectively, of the transducer structure of  FIG. 2A ; 
       FIG. 2E  is a schematic perspective view showing the flextensional shell of  FIG. 2A ; 
       FIG. 2F  is a schematic plan view of the flextensional shell of  FIG. 2A ; 
       FIGS. 2G-2H  are schematic perspective and side views, respectively, of one of the radiator shells of  FIG. 2A ; 
       FIG. 3  is a schematic cross sectional view of a flexural cylindrical projector transducer according to an embodiment of the invention illustrating the shape of the device in an inactive or undeformed condition; 
       FIG. 4  is a schematic cross sectional view of a flexural cylindrical projector transducer according to an embodiment of the invention illustrating the shape of the device in an active or deformed condition; 
       FIG. 5  is a graph illustrating the first and second modes of operation associated with the flexural cylindrical projector transducer according to an aspect of the invention. 
   

   DETAILED DESCRIPTION 
   It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding, while eliminating, for the purpose of clarity, many other elements found in typical slotted cylinder transducers and drive assemblies and methods of making and using the same. Those of ordinary skill in the art may recognize that other elements and/or steps may be desirable in implementing the present invention. However, because such elements and steps are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements and steps is not provided herein. 
   Referring now generally to  FIG. 2A , there is shown a flextensional cylindrical projector transducer  20  comprising an inverse, flextensional shell structure  30  coupled to a pair of oppositely disposed radiators  51 ,  53 , for producing vibrational motion in response to a source of alternating signals applied to a drive assembly  40  positioned within flextensional shell  30 . In the configuration depicted in  FIG. 2A , the transducer  20  may comprise a single shell  30  (and corresponding radiator shells  51 ,  53 ), or alternatively, may include a plurality of flextensional shells  30   a ,  30   b , . . . ,  30   n  arranged in a stacked fashion and operably coupled to enable vibratory motion in response to a source of electrical signals, as depicted in  FIG. 2B . Throughout the drawings, like reference numerals are used to indicate like parts. As illustrated in  FIG. 2A , the shell structure  30  comprises first and second end portions  31 ,  33  integrally coupled with inwardly shaped (concave) arcuate central arm members  35  and  37  which are oppositely disposed with respect to one another. End portions  31 ,  33  are generally bulbous relative to the thinned central arm members  35 ,  37 . The inverse flextensional shell structure  30  includes a central portion  36  which is hollow and is bounded by the concavo-concave arm members  35  and  37 , which extend to the thicker end portions  31  and  33 . The hollow of the shell is configured to receive a drive assembly  40  such as a ceramic or magnetostrictive stack positioned therein and retained at first and second sides  60  and  61 . Each of sides  60 ,  61  includes a substantially planar central portion that terminates in arcuate extending portions  62   a ,  62   b , and  63   a ,  63   b , respectively, at opposing sides of the inner wall of the shell  30  within the hollow. 
   Referring now to  FIG. 2A  in conjunction with  FIG. 2E  and  FIG. 3 , the shell in an inactive or undeformed state resembles a hollow “dogbone” configuration. For the shell structure  30 , the flextensional cylindrical projector transducer includes a drive assembly comprising stack  40  of ceramic or magnetostrictive elements or crystals laid out in a linear array, with electrodes disposed between the elements. In an exemplary configuration, a magnetostrictive drive assembly may comprise one or more drive rods and biasing magnet surrounded by a drive coil subassembly of a substantially rectangular configuration, the magnetostrictive drive stack operably coupled between the first and second sides  60 ,  61  of respective end portions  31 ,  33 . The drive assembly may be coupled via one or more acoustic backing/matching layers  80 . The drive rod(s) may be formed of a material such as terbium dysprosium iron or Terfenol (e.g. Terfenol-D) and biasing magnet formed of a samarium cobalt material, for example. The shell structure  30  is preferably fabricated from a metal such as a high tensile strength, non-magnetic steel. Conductors carry electrical signals to the electrodes to stress the elements and cause them to vibrate along the axis of the stack  40 . The conductors also carry electrical signals produced by the stack  40  of piezoelectric elements when the elements intercept sonar signals, all in a well-known manner. 
   End portions  31  and  33  located at respective ends of the stack  40  are intimately coupled therewith. The end portions, together with arm members  35 ,  37  form a closed loop about stack  40 . The arm members are configured in symmetrical fashion and form an arcuate shape such that, in an inactive or undeformed position, the arms of the shell  30  extend in an inverse, arcuate manner toward the stack such that the distance D between the arm members and the stack is minimized at substantially the center or midpoint P of the stack, which is the midpoint of the transducer structure (see  FIG. 2C ). When alternating current signals are introduced to the sectionalized elements of the stack, the elements vibrate and produce vibrations in the shell at positions adjacent to the end portions, which cause flexure of the arm members in a direction normal to axis A as illustrated in  FIGS. 3-4 , such that each of the concavo-concave arms deform to a convexo-convex segment at the center or midpoint of the arm members, with adjacent concave segments offset from the midpoint along the arms and symmetrically oriented. The thickness and dimensions of the shell, including the varying thickness of the arm members and end portions, are selected to produce the vibrations at one or more preselected frequencies, such as in the 400 Hz-400 KHz range, by way of non-limiting example only. 
   Referring now to  FIG. 2A  in conjunction with  FIGS. 2C-2G , the inverse, flextensional shell  30  is monolithically formed and retained within the cylindrical projector  20  by oppositely disposed and symmetrically configured radiating shell structures  51 ,  53 , each having a mushroom or T-shaped configuration. Planar support members  51   a ,  53   a  extend from a bulbous head segment  51   b ,  53   b  for radiating in response to vibration of the ceramic or magnetostrictive stack  40  and subsequent flexure of flextensional shell  30 . Each radiating shell  51 ,  53  is preferably formed of a low density, high stiffness material such as a lightweight composite or plastic radiator. The top radiator shell portion  51  and bottom radiator shell portion  53  are symmetrically configured and oriented to accommodate the inverse flextensional shell  30 . Flextensional shell  30  includes corresponding tab projections  32 ,  34  extending outward in a substantially normal direction from the center of respective arm members  35 ,  37 , and along the entire longitudinal surface (z-axis) of shell  30  ( FIG. 2E ). Each tab portion  32 ,  34  is received in a corresponding aperture  51   c ,  53   c  ( FIG. 2A ) associated with support members  51   a  and  53   a  which extend from the top  51  and bottom  53  radiators, respectively, and abut the central portion of shell  30 . In this manner, the support members  51   a ,  53   a  receive the corresponding tab portions  32 ,  34  of arm members  35 ,  37  via the corresponding apertures or notches to provide for flexural response. It is also to be understood that each mushroom shaped radiator may be operably configured in a stacked manner to form the projector transducer as illustrated in  FIG. 2B . 
     FIGS. 2G-2H  provide a more detailed view of one of the mushroom or T-Shaped radiators  51  according to an exemplary embodiment of the present invention. As shown therein, the support member  51   a  is configured to have planar sides  52 ,  54  defining an aperture or channel  51   c  there between for accommodating the corresponding tab portion of shell  30  (not shown). One or more through holes  55  may be formed in each of sides  52  and  54  and aligned with corresponding holes formed in the flextensional shell tab portions so as to operably couple thereto using various fastening or securing means, including but not limited to rivets, bolts, screws, welds, adhesives or other fastening mechanisms. The various dimensions and geometries associated with the radiator and flextensional shell structures are a function of the particular application and may be influenced by various characteristics, including frequency (e.g. resonant frequency), bandwidth, coupling efficiencies, and the like. It is understood that the geometry associated with the transducer and flextensional shell and radiator assembly of the present invention may be symmetrical about the x and y axes, as depicted in the embodiments illustrated in  FIG. 2 . In a particular embodiment, the transducer comprises a magnetostrictive stack arrangement with flextensional shell and radiator structures having the following dimensions (in inches) with reference to the drawings: D 10 =12.5; D 11 =12.5; D 12 =6.84 ( FIG. 2C ); D 13 =4.0 ( FIG. 2D ); D 14 =8.0; D 15 =7.0; D 16 =0.3; D 17 =3.6; D 18 =1.1; radius R 11 =0.45; ( FIG. 2F ); D 19 =2.5; D 20 =1.55; D 21 =0.25; radius R 12 =0.12; R 13 =0.12 ( FIG. 2H ). The above dimensions represent merely one embodiment of the present invention and are provided for non-limiting purposes of explanation only. 
     FIGS. 3 and 4  illustrate various shapes associated with the cylindrical flextensional projector in both the inactive or undeformed shape ( FIG. 3 ) and the active or deformed shape ( FIG. 4 ). As shown, the inactive mode exhibits a minimum distance D from the center of the stack (and hence minimum radiator shell displacement). The arms  35  and  37  in the inactive state are in a concave-concavo arrangement. As seen, the concave upper arm  35  and the concavo lower arm  37  are joined at their first and second ends by the thicker end portions  31  and  32 . The stack  40  is symmetrically disposed between the arms  35  and  37  and there is symmetry of the unit about axis x (and y). The stack  40  is positioned at a center axis within the hollow of the shell  30 . The arms  35  and  37  are symmetrically disposed about that axis. In the active mode, the arm members are in flexure such that different segments of the arm members are now closer to the stack  40  (i.e. P 2 , P 3 , P 4 , P 5  in  FIG. 4 ) while the central portion of each arm is now further away from the stack (i.e. P 1 , P 6 ). This in turn causes the central portions of the flextensional shell to urge against respective support members  52 ,  54  to move each of the corresponding radiator shells  51 ,  53  in order to radiate acoustic energy from the projector device. As shown in  FIG. 4 , the central portions of the arms symmetrically deform to cause them to assume during the active state a convexo-convex configuration. The radiating members  51  and  53  coupled to the center of arms  35  and  37  by support members  52  and  53  move accordingly. It is understood that the deflection of the arms is a function of the magnitude of vibration and hence an infinite number of positions between the inactive ( FIG. 3 ) and active states ( FIG. 4 ) can be accommodated. 
   Referring now to the graphical illustration of  FIG. 5 , the projector  20  according to an aspect of the present invention is operative in a first fundamental vibration mode (i.e. flexural mode) and in a second vibration mode (i.e. breathing mode). For a 12.75 outer diameter, 24 inch length projector device  20 , the first mode  510  operates at about 425 Hz, while the second mode  520  operates at about 1300 Hz. The present invention thus provides substantially greater bandwidth than current transducer devices while providing an additional higher band at a center frequency of 2.5 times higher than the fundamental vibration mode. 
   The flextensional cylindrical projector of the present invention thus provides for a low frequency multi-band, transducer which is essentially omnidirectional and which provides greater flexibility for multiple environments. The present transducer structure is devoid of the stress and reliability concerns associated with conventional SCP devices while providing a low frequency projector at significantly lower cost than SCPs currently in use. 
   Those of ordinary skill in the art may recognize that many modifications and variations of the present invention may be implemented without departing from the spirit or scope of the invention.