Abstract:
An electro-mechanical transducer provides a very low frequency wide band response by using a quad configuration of piezoelectric cantilevers, providing additive output between the resonant frequencies of the cantilevers and achieves this at great depths under free flooded or oil filled conditions.

Description:
RELATED CASES 
     Priority for this application is hereby claimed under 35 U.S.C. §119(e) to commonly owned and co-pending U.S. Provisional Patent Application No. 61/539,018 which was filed on Sep. 26, 2011 and which is incorporated by reference herein in its entirety. 
    
    
     GOVERNMENT RIGHTS 
     The present invention was made, in part, with Government support under a Government contract. The Government may have certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates in general to transducers, and more particularly to underwater acoustic transducers. The present invention also relates to a transducer capable of radiating acoustic energy over a wide band of frequencies including very low frequencies. More particularly, the present invention relates to a multiply-resonant, piezoelectric-cantilever symmetrical-transducer with preferably four vibrating tines mounted from a common stiff rigid structure. Even more particularly, the present invention relates to such a transducer that operates in the bending mode. 
     BACKGROUND OF THE INVENTION 
     Low frequency underwater sound transducers require a large volume and a compliant structure in order to obtain a low resonant frequency, such as 15 Hz, along with a high output level. This can be difficult to accomplish within a fixed volume at very low frequencies, even at shallow depths, and it becomes extremely challenging at deep depths, such as at depths where a submarine may reside, and where the hydrostatic pressure is high. 
     Accordingly, it is an object of the present invention to provide an improved low frequency acoustic transducer. 
     Another object of the present invention is to provide a low resonant frequency transducer that is operable at low frequencies such as 15 Hz and that generates substantially high output levels. 
     Still another object of the present invention is to provide an acoustic transducer as mentioned above and that can be accomplish within a fixed volume at very low frequencies at both shallow and deep depths. 
     Still a further object of the present invention is to provide an acoustic transducer with the above objectives and that is further characterized by having a wide bandwidth. 
     SUMMARY OF THE INVENTION 
     To accomplish the foregoing and other objects, features and advantages of the invention there is provided an improved electro-mechanical transduction apparatus that employs a symmetrical system that preferably excites only odd bending modes, providing additive output between the successive resonant frequencies, yielding a very wide band response from very low frequencies to high frequencies, with additive output between the successive resonant frequencies, because of the excitation of preferably only the odd modes of vibration. In one embodiment there is provided a “quad” cantilever resonant structure providing one of the lowest flexural resonances for a given length and which there are no nulls between the modes of vibration that cannot be mitigated providing a wide band response The device may be comprised of four piezoelectric cantilever structures, each driven with opposite phase on opposite sides, creating bending motion, and mounted and driven symmetrically or anti-symmetrically on or from a common stiff base along with a pair of endplates. 
     In accordance with the invention there is provided an electro-mechanical transduction apparatus that employs at least four mechanical cantilever benders of which at least two are electro-mechanically drive and all four attached to a common central stiff mounting structure. In its most basic form, two of the cantilevers are mounted in the same plane on one side of the mounting structure with the free ends of the cantilevers on the opposite ends, and the other two cantilevers are mounted on the opposite side of the mounting structure. This quad arrangement may be used to form an array of these transducers by stacking additional quad units on the remaining surfaces of the support structure. Stiff plates can also be mounted on the remaining surfaces to reduce the out-of-phase cancellation of the interior and exterior radiation from the cantilever tines, leaving only acoustic radiation from the free open end of the interior cavity. Operation is in the free flooded mode with optional contained interior compliant fluid for improved low frequency performance and allowing operation at deep ocean depths. 
     For shallow water operation where the hydrostatic pressure is not as great, the open ends can be blocked and the interior filled with a gas or air for greater output. Air backing can also be used under greater depths by filling the interior with compressed air or a compliant fluid or fluid filled with compliant structures could also be used. Greater output from a single quad structure can be obtained by adding four more cantilever tines on the two remaining surfaces of the mounting structure with the interior filled with either a gas or fluid. 
     Although the invention described herein serves as a means for obtaining a significant very low frequency wide band underwater acoustic response, it could also be used in air as a source of sound and as an alternative to the common tuning fork. The structure could also serve as a receiver of sound and vibration, such as a microphone, hydrophone or accelerometer. 
     In accordance with one embodiment of the present invention there is provided an electro-mechanical transduction apparatus that comprises: at least four symmetrically mounted piezoelectric driven cantilever tines with the greatest motion at its ends, along with isolated end plates to reduce acoustic cancellation, achieving very low frequency acoustic response because of the cantilever resonance operation and achieving wideband performance because of odd mode excitation yielding an additive output between modes. 
     In accordance with other aspects of the present invention there is provided the means for stacking these quad elements to form an array of elements or means for adding four more cantilever tines to create an eight tine dual quad structure. There is also means provided for operation with a free flooding or contained fluid within the interior cavity of the cantilever structure and pressure release means for increasing the interior compliance. In addition to this there is also provided a means for replacing the piezoelectric tines in one plane with passive non-piezoelectric tines, such as steel or aluminum, yielding a response similar to the all piezoelectric response. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Numerous other objects, features and advantages of the invention should now become apparent upon a reading of the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  schematically illustrates the quad cantilever transducer with four piezoelectric tines driven into the bending mode and mounted on a common base mounting structure; 
         FIG. 2  schematically illustrates, for one tine, the electrical connections for piezoelectric  31  mode tri-laminar operation with two plates of piezoelectric material mounted on a central substrate wired for bending mode operation; 
         FIG. 3   a  schematically illustrates the motion of the quad structure in a symmetric mode of vibration; 
         FIG. 3   b  schematically illustrates the motion of the quad structure in an anti-symmetric mode of vibration; 
         FIG. 4  illustrates the cantilever motion of the first three modes of vibration along with the initial case of no motion; 
         FIG. 5  illustrates a single cantilever of the quad structure with an end mass along with two compression rods and wired for piezoelectric operation in the 33 mode; 
         FIG. 6  illustrates a single piezoelectric element showing a margin separating the electrodes as well as two holes for the two compression tie rods; 
         FIG. 7  illustrates an array of quad units along with stiff end plates and an intermediate supporting bar; 
         FIG. 8   a  shows the response (output versus frequency) of an array of quad units with stiff end plates operating in the symmetric,  19 , and anti-symmetric mode,  20 ; 
         FIG. 8   b  shows the response of an array of quad units with stiff end plates operating in the symmetric mode,  19 , spliced with the anti-symmetric mode,  20 , response; 
         FIG. 8   c  shows the response of an array of quad units with stiff end plates operating with the back piezoelectric tine section replaced with a metal parasitic tine section; 
         FIG. 9  schematically illustrates a center supported dual bender transducer composed of two tri-laminar piezoelectric drive circular plates; and 
         FIG. 10  schematically illustrates a center supported dual quad bender transducer composed of eight tri-laminar piezoelectric drive bender bars. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with the present invention, there is now described a number of different embodiments for practicing the present invention. In the main aspect of the invention there is provided four piezoelectric cantilevers mounted on a central support providing very low frequency and wide band response even at great ocean depths. The central rigid mount is important to this cantilever invention as it provides the rigid boundary conditions for the cantilever tines and because of the design symmetry no additional masses or structures are needed for the cantilever central boundary condition on any of the tines operating in a symmetric mode of vibration. 
     A simplified diagram of the cantilever acoustic transducer is illustrated in the four tine,  1 , quad arrangement of  FIG. 1 . When operated in the water there would be top and bottom plates (not shown) attached to the central rigid mount,  2 , and mechanically isolated from the cantilever tines  1  with water filling the interior section. As illustrated in  FIG. 1 , the central support mounts two, spaced-apart tines, in opposed directions there-from. This free flooding would require a rubber boot and/or potting to electrically isolate the piezoelectric benders, tines  1 , from the exterior water and interior water, which allows for pressure equalization and operation at great ocean depths. Transducers could be stacked on top of each other and used together as an array to attain a greater source level. 
       FIG. 2  schematically illustrates the wiring arrangement for the piezoelectric plates or bars,  1 , driven by voltages,  5 , and,  6 , for operation in the piezoelectric  31  mode. Because of the polarization direction, arrows  10 , and the wired polarity shown as,  5 , and  6 , the top and bottom piezoelectric plates,  4 , of  FIG. 2  are driven out of phase causing bending motion of the cantilever tine,  1 , mounted on structural support base,  2 . In this particular tri-laminar arrangement a central inactive substrate,  3 , is used to obtain a higher electromechanical coupling coefficient. In  FIG. 2  the various layers or plates  3  and  4  may be attached together in any one of a number of ways, so that all plates bend together. 
       FIG. 3   a  illustrates the motion of the cantilevers at the fundamental symmetric mode of vibration of a quad section. Here the two bottom tines  7  are wired in the same way of the two top tines  1  to create this symmetric vibration shown where the top two tines move in a direction opposite the bottom two tines.  FIG. 3   b  illustrates the motion of the cantilevers at the fundamental anti-symmetric mode of vibration of a quad section. Here the two bottom tines,  7 , are wired oppositely in phase from the top two tines creating all tine motions in the same direction with the center mount  2  moving in the opposite direction of the four tines. 
     The cantilever is an ideal component for very low frequency wide band performance. The fundamental resonance, f r , of a cantilever bender bar of length L and thickness t may be written as f r =0.1615 ct/L 2  where c is the bar sound speed in the material. The cantilever mode design has the advantage of achieving a low fundamental resonance frequency from a compact size. It has not only a size advantage, but a wideband response advantage, with additive motion between the overtones. The odd quarter wavelength multiples of the overtones are not harmonically related and the first few are at 6.27 f r  and 17.55 f r . The cantilever is excited by reversing the phase or direction of polarization of the electric field on opposite sides of the piezoelectric cantilever bender tines. 
     A finite element symmetry model of one tine  1 , of the cantilever quad design, with length approximately 30 inches, is shown in  FIG. 4  mounted on the central rigid support,  2 , with symmetry planes  8  and  9 . The motion of the first three modes of cantilever vibration at 15, 125 and 395 Hz are illustrated in  FIG. 4 . Although cantilever benders provide some of the lowest resonances for a given length, they are susceptible to excessive bending stress under deep operation and in these cases the design can benefit from pressure equalization. 
       FIG. 5  illustrates schematically a  33  mode arrangement, for a single tine, and which has approximately twice the coupling coefficient of the 31 mode arrangement of  FIG. 2 . Here, for simplicity,  FIG. 5  shows only four  33  mode piezoelectric plates  13 , with the direction of polarization indicated by the arrows  10 . The top and bottom part of the plates  13 , are connected out of phase to excite the bending mode of this single cantilever tine of a quad unit as illustrated in  FIG. 1 . Also illustrated is the mass  11 , which replaces part of the piezoelectric structure of the cantilever. The use of steel for mass  11 , allows a reduction in cost, approximately the same fundamental resonance frequency and yet an improved coupling coefficient. This is possible since there is little bending at this free end of the cantilever, and a piezoelectric section here would only add electrical capacity but little bending motion. Two high strength tie rods  12  are also provided to supply the needed compression on the piezoelectric material under high drive. Terminals  5  and  6  illustrate reversed phase operation between the upper and lower electrodes. 
     In the embodiment shown in  FIG. 5  a larger number of thinner piezoelectric pieces  13  may be used in actual practice allowing a lower voltage. One such piezoelectric piece is illustrated in  FIG. 6  with electrode surfaces  14 , inactive substrate  15 , and end pieces  16 . The end pieces  16  are provided with holes to accept the tie rods,  12 , such as illustrated in  FIG. 5 . An alternative improved performance arrangement would be to extend electrode polarized piezoelectric material to the end of the end pieces or, more simply, but with reduced performance, eliminate the end pieces  16  as illustrated in  FIG. 5 . In practice, the tines of the quad elements could be made in modular form, possibly 10 modules, each containing 14 PZT piezoelectric elements with a bar thickness on the order of 0.20 inches. 
       FIG. 6  schematically illustrates a possible six inch height of each quad element of this sample design, which contains 4.5 inches of piezoelectric ceramic PZT-8, active piezoelectric material, sandwiched between two 0.75″ plates of G-10 (or possibly PVC or a cast composition) or better yet, extended active piezoelectric material. Each of these plastic or ceramic plates has holes large enough for high strength steel compression tie rods. These two tie rods provide the necessary compressive bias on the PZT piezoelectric material. A cross section of this arrangement is illustrated in  FIG. 6  showing the PZT with the split electrodes, allowing oppositely phased voltages or reversed polarization for obtaining operation in the bending mode. This design is referred to as a 33 mode of operation as the electric field and useful displacements are in the same direction and parallel to the direction of polarization of the piezoelectric ceramic. This direction is perpendicular to the electrode surfaces and through the thickness of the bar. Alternatively, the two end pieces may also be piezoelectric with electrodes and margin of 0.40 inches and two holes for the tie rods, increasing the fully active size to, in this case, 6 inches. 
     A specific array of quad elements is illustrated in  FIG. 7  and shows, approximately, an overall length of 66″, thickness of 24″ and 48″ height for this particular transducer example.  FIG. 7  also shows the central rigid mounting structure  2 , along with a central support bar  21 , which together with structure  2 , support the end pates  17 , on the top and bottom of the array. 
     The finite element calculated sound pressure level (SPL) response for the array of  FIG. 7  with piezoelectric PZT pieces of  FIG. 6  is shown in  FIGS. 8   a ,  8   b  and  8   c  for a silicone oil filled condition for improved very low frequency response. The first three flexural cantilever resonances and additive wide band response are seen in  FIG. 8   a  which shows the case for symmetric drive,  19 , and anti-symmetric drive,  20 . The greater reduction in the vicinity of 50 Hz of the symmetric drive,  19 , condition can be improved by splicing in the response of the symmetric drive,  20 , as illustrated in  FIG. 8   b . Direct synthesis of the two motions driven at the same voltage amplitude, illustrated in  FIGS. 3   a  and  3   b , provide a condition where the bottom two tines,  7 , cancel each other as if they were not driven at all. This condition can also be achieved by simply not driving the bottom two tines and replacing them with a passive material such as aluminum or steel, creating a passive parasitic radiating resonator. The response for this case, where only the tines,  1 , are piezoelectric driven and the bottom tines,  7 , are steel is illustrated in  FIG. 8   c.    
     Although the focus of this invention is on cantilever bender bars, the same principles apply to a pair of flexural disc transducers with a center mount between the two, yielding a fundamental resonance frequency that is nearly twice as high as the quad cantilever construction, but with greater output because of the larger radiating area. This alternative configuration is schematically illustrated in  FIG. 9  showing a center post  18 , tri-laminar planar-mode piezoelectric discs  4  and center substrates  3 . Center post  18  may be constructed of a stiff metal material such as steel. Although not illustrated, the piezoelectric discs may also be replaced with piezoelectric square plates. An additional alternative eight tine arrangement, instead of the four tine quad structure of  FIG. 1 , is illustrated in  FIG. 10 . This eight tine structure allows greater output from the piezoelectric tines  1  and  7 , with substrates  3 , and the square supporting stiff structure  2 , preferably steel. Although not shown, a structure composed of three tine pairs  1 ,  7 , of piezoelectric tines may be used with an equilateral triangular supporting structure instead of the square member  2  shown in  FIG. 10 . 
     The above principles of this invention may be applied to transducers which transmit or receive acoustic waves in a fluid or gas. The principles can also be applied to accelerometers. Moreover, the electromechanical material may be single crystal material, piezoelectric ceramic, electrostrictive, magnetostrictive or electromagnetic. Common electromechanical transduction material such as PZT, PMN-PT, terfenol-D and galfenol could be used with this invention. 
     The following are a list of reference numbers associated with the specification and drawings.
           1 . Top piezoelectric tines     2 . Central rigid mount     3 . Inactive substrate     4 . Piezoelectric element     5 . Electrodes     6 . Electrodes     7 . Bottom piezoelectric tines     8 . Symmetry plane     9 . Symmetry plane     10 . Polarization arrow     11 . Tine end mass     12 . Tie rod     13 . Piezoelectric section     14 . Electrode surface     15 . Margin between electrodes     16 . End piece     17 . End plate     18 . Center support rod     19 . Symmetric response     20 . Anti-symmetric response     21 . End plate support beam       

     Having now described a limited number of embodiments of the present invention, it should now become apparent to those skilled in the art that numerous other embodiments and modifications thereof are contemplated as falling within the scope of the present invention, as defined in the appended claims.