Patent Abstract:
A transducer assembly is provided for projecting acoustic signals into a medium. The assembly includes a support member having first and second layers of piezoelectric material mechanically linked to the support member. The first and second layers are joined to electrical drive circuitry such that one layer receives a driving voltage signal while the other layer receives the driving voltage with a stiffening voltage. The transducer assembly can use both the 3-1 and 3-3 drive modes. Multiple configurations are supported, and both bender bar and slotted cylinder configurations are shown.

Full 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. 
    
    
     CROSS REFERENCE TO OTHER PATENT APPLICATIONS 
     None. 
     BACKGROUND OF THE INVENTION 
     1) Field of the Invention 
     The invention relates generally to increasing the efficiency and frequency band of operation of all transducers/projectors, and particularly slotted cylinder projectors. 
     2) Description of the Prior Art 
     It is known to provide slotted cylinder projectors and piezoelectric transducer assemblies.  FIG. 1  shows a cross-sectional view of one such prior art transducer. A slotted cylinder transducer  10  features a hollow support member  12  in a cylindrical configuration and having an axial opening  14 . The diameter of the support member  12  is D. Transducer material  16  is supported concentrically within the support member and is typically of a piezoelectric material and provided with an axial opening. The outer support member  12  may be thinned at selected locations to facilitate control of vibrational frequency and frequency bandwidth of the transducer assembly. The thinned portions of the support member  12  may be adapted to retain a compliant material, such as urethane, to smooth the outer surface of the support member  12 . The hollow interior  14  of the transducer assembly  10  may be filled with a compliant material, such as urethane. The support member  12  is provided with an axially extending side opening  18 , and the transducer material  16  is similarly provided with an axially extending side opening  20 , the two side openings  18  and  20  being aligned with each other. Side openings  18  and  20  give a slot having dimension “g”. Bending nodes  22  and  24  occur in the transducer  10  opposite side openings  18  and  20 . Transducer  10  bends as shown at arrows  26 . The slotted cylinder projector operates in a bending mode in a manner analogous to other resonant objects forced by piezoelectric components. These include tuning forks and vibrating cantilevers. 
     Piezoelectric material must be polled before it can be used as a transducer. Polling involves raising the temperature of the material and putting an electric field across the material in the same direction that a field will be applied to the material in use. When the piezoelectric strain is desired in a different dimension from the direction of electric field application and polling, the transducer material is known as a 3-1 transducer material. In a 3-3 piezoelectric material, strain is produced in the same direction as the polling direction and application of the electric field. 
     When electrical signals are introduced to the transducer material  16 , the transducer material  16  vibrates. The outer support member  12  limits the amplitude of the vibrations of the transducer material  16 . Such transducers  10  are generally referred to as slotted cylinder projectors and are capable of providing low frequency acoustics. Slotted cylinder projectors are efficient and small in size, and provide sufficient power to find application in underwater sonar projectors. 
     The resonant frequency (F r ) of a slotted cylinder projector is proportional to the square root of Young&#39;s modulus, Y, of support member  12 : 
                       F   r     ≅     .0655   ×     ct     D   2           =     .0655   ×     t     D   2       ⁢       Y   p                 (   1   )               
wherein c is sound speed, t is thickness, D is the diameter of the inner ring, Y is the effective Young&#39;s modulus, and ρ is the effective density of support member  12 .
 
     An equivalent circuit model developed based upon kinetic and potential energies of a slotted cylinder of length L, effective density (ρ), effective Young&#39;s modulus (Y), length of the cylinder L, thickness t, diameter of the inner ring D 1  wherein M=dynamic mass and K E =stiffness, comprises:
 
 M= 5.4 ρLtD,  
 
and
 
 K   E =0.99 YL ( t/   D ) 3   (2)
 
       FIG. 2  shows a prior art transducer  30  known as a bender bar joined to a typical electrical driver  32  represented by an alternating current voltage source. Bender bar  30  includes a flexible bar  34  having a transducer member  36 A and  36 B positioned on either side of bar  34 . First electrodes  38 A and  38 B are positioned on a first side of each transducer member  36 A and  36 B, and second electrodes  40 A and  40 B are positioned on a second side of each transducer member  36 A and  36 B. Insulation  42  is provided to insulate flexible bar  34  from electrodes. As shown, transducer member  36 A is poled in the opposite direction from transducer member  36 B. The contraction and expansion of transducer members  36 A and  36 B causes flexible bar  34  to bend in response thereto. When subjected to a voltage from electrical driver  32 , this different poling causes transducer member  36 B to contract when transducer member  36 A expands resulting in bending shown at  44 B. When the voltage is reversed, bending reverses to that shown at  44 A. Rapidly changing the applied electrical signal causes vibrations in the bender bar  30 . 
     Acoustic transducers and more particularly slotted cylinder projectors are often used in high pressure environments and environments with varying temperatures. These environmental conditions change the resonance frequency of the transducer and cause the transducer to become inefficient and mismatched to its power amplifier. 
     SUMMARY OF THE INVENTION 
     There is provided herein a transducer assembly for projecting acoustic signals into a medium. The assembly includes a support member having first and second layers of piezoelectric material mechanically linked to the support member. The first and second layers are joined to electrical drive circuitry such that one layer receives a driving voltage signal while the other layer receives the driving voltage with a stiffening voltage. The transducer can use both the 3-1 and 3-3 drive modes. Multiple configurations are supported, and both bender bar and slotted cylinder configurations are shown. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is made to the accompanying drawings in which are shown illustrative comparative devices, as well as an illustrative embodiment of the invention, from which its novel features and advantages will be apparent, and wherein corresponding reference characters indicate corresponding parts throughout the several views of the drawings, and wherein: 
         FIG. 1  is a diagrammatic cross-sectional view of a prior art slotted cylinder projector; 
         FIG. 2  is a diagrammatic view of a prior art piezoelectric trilaminar bender bar; 
         FIG. 3  is a diagram of a trilaminar bender bar according to the current invention; 
         FIG. 4  is a diagrammatic cross-sectional view of a slotted cylinder projector using 3-1 drive mode according to the current invention; 
         FIG. 5  is a diagrammatic cross-sectional view of a slotted cylinder projector using 3-3 drive mode according to the current invention; 
         FIG. 6  is a detail view of one portion of the transducer provided in  FIG. 5 ; and 
         FIG. 7  is a detail view showing an alternate embodiment of one portion of the transducer. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 3  shows an embodiment of the current invention as applied to a bender bar  30 . The bender bar  30  has a flexible bar  34  joined to transducer member  36 A and  36 B positioned on either side of bar  34 . Electrodes  38 A and  38 B are positioned in electrical contact on a first side of each transducer member  36 A and  36 B, and second electrodes  40 A and  40 B are positioned in electrical contact on a second side of each transducer member  36 A and  36 B. Insulation  42  is provided to insulate flexible bar  34  from electrodes. Transducer member  36 A is poled in the opposite direction from transducer member  36 B. This embodiment gives a 3-1 mode of transducer material operation. In this embodiment the transducer members  36 A and  36 B and flexible bar  34  are operationally the same as used in the prior art. 
     Bender bar  30  is joined to a different electrical driver  48  that allows application of a direct current bias to transducer member  36 B. Electrical driver  48  has an alternating voltage signal generator  50  and a direct current bias voltage generator  52 . Direct current bias voltage generator  52  is joined to apply a bias voltage to transducer member  36 B. A ground  54  is also provided. 
     Applying a bias voltage to one of the transducer members changes the resonance frequency of the bender bar  30  by pre-stressing or de-stressing the bar. For example, curves  44 A and  44 B show bending of bender bar  30  before application of a bias voltage from direct current bias voltage generator  52 . After application of a direct current, bender bar  30  bends according to curves  56 A and  56 B. Direct current bias voltage can be changed in accordance with environmental or operational parameters to move the resonance frequency as necessary. 
       FIG. 4  shows a cross-sectional view of another embodiment of the current invention. This embodiment provides a slotted cylinder acoustic projector  60  that includes a cylindrical support member  62  having a hollow axial region  64 . Support member  62  has a longitudinal slot  66  formed therein. Transducer assembly  60  will have nodes  68 A and  68 B 180° around support member  62  from slot  66 . A slotted cylinder support member  62  can be made from steel, aluminum, graphite or other rigid material. For in-water applications, an outer water barrier, such as a rubber boot (not shown), can be used. 
     A first transducer material layer  70  is disposed on the interior surface of support member  62 . First transducer material layer  70  conforms to the interior surface of support member  62 . A second transducer material layer  72  is disposed on the interior surface of first transducer material layer  70 . The transducer material for both layers is preferably a piezoelectric material such as a piezoceramic composite. First transducer material layer  70  has electrical contacts  74 A and  74 B that are in contact with the transducer material layer  70  and insulated from electrical contact with other components. Second transducer material layer  72  has electrical contacts  76 A and  76 B in contact with second transducer material layer  72  and insulated to prevent electrical contact with other components. First transducer material layer  70  and second transducer material layer  72  are thus configured for 3-1 transducer mode operation because the electric field is provided in a different direction from the piezoelectric strain. 
     An electrical drive circuit  78  is provided for transducer assembly  60 . Drive circuit  78  has an alternating voltage signal generator  80  and a direct current bias voltage generator  82 . Alternating voltage signal generator  80  is joined to electrodes  76 A and  76 B on second transducer material layer  74 . Direct current bias voltage generator  82  is joined to apply a bias voltage to transducer member  70  in addition to the voltage from signal generator  80 . A ground  84  is also provided. Bias voltage provided to transducer member  70  changes its stiffness and alters the resonant frequency of transducer assembly  60 . Other known circuitry can be provided to control bias voltage with respect to environmental conditions and resonance frequency. 
     In accordance with the present invention, first transducer material layer  70  has a maximum affect on the resonance frequency change of assembly  60  when located in the vicinity of 180° across from the slot  66  and extending slightly beyond the nodes ( 68 A and  68 B). There is no requirement that the entire interior surface of support member  62  be covered by or joined to transducer layer  70 . 
       FIG. 5  shows an alternate embodiment of the current invention having a slotted cylinder projector or transducer assembly  90  utilizing a 3-3 mode of transducer operation. A detail view of one portion of this embodiment is given in  FIG. 6 . Transducer assembly  90  has an outer shell or support member  92 . In this embodiment support member  92  is cylindrical having an axial hollow  94 . A slot  96  is formed in a portion of the support member  92 . When vibrating, nodes  98 A and  98 B will occur in the transducer assembly  90  opposite of slot  96 . Wedge shaped transducer portions  100  are distributed around the interior surface of support member  92 . Transducer portions  100  can be made from a single piece of piezoelectric material. 
     For purposes of reference, wedge shaped transducer portions can be referenced as arcuate wedges. These arcuate wedges have a major arcuate surface positioned against the interior of support member  92 . A minor arcuate surface is opposite the major arcuate surface in the support member hollow  94 . Each wedge portion has first and second radial surfaces adjacent to other wedge portions. First and second transverse surfaces of the wedge portions are provided perpendicular to the axis of the support member. 
     Each transducer portion  100  includes a first region  102  poled in a first direction and a second region  104  poled in a second direction. (The first direction and the second direction can be the same direction). For 3-3 operation it is preferred that the poling be from one radial surface to another. An inactive region  106  is positioned between the first region  102  and the second region  104 . Inactive region  106  is not poled. Transducer portions  100  are insulated from electrical contact with support member  92  by insulation  108 . Inactive region  106  can act as effective insulation between first region  102  and second region  104 . As an alternative, first region  102  can be formed separately from second region  104 , and inactive layer  106  can be a non-conducting adhesive. 
     As may best be seen in  FIG. 6 , one transducer portion  100  is shown. First region  102  has electrodes  110 A and  110 B positioned on the first radial surface and the second radial surface of portion  100 . Second region  104  has electrodes  112 A and  112 B disposed on the first and second radial surfaces of portion  100 . The first region electrodes  110 A and  110 B of each portion  100  are together joined to an electrical circuit much like that shown at  78  in  FIG. 4  in order to provide a driving voltage with a bias voltage. Electrodes  112 A and  112 B of each portion  100  are joined to the electrical circuit to provide a driving voltage to second regions  104 . Adjacent electrodes on different portions are insulated from each other. 
     In  FIG. 7 , there is shown an alternate embodiment of the transducer portion  100 . In this embodiment, a dielectric or insulating material  106 ′ is utilized between first region  102  and second region  104 . Insulating material  106 ′ has no piezoelectric properties. This embodiment could be easier to manufacture than that shown in  FIG. 6 . 
     In one embodiment, first region  102  is poled in an opposite direction from second region  104 . This allows opposite piezoelectric strain induction with a voltage having the same polarity on adjacent electrodes. In another embodiment, first region  102  and second region  104  are poled in the same direction. Magnitude of the piezoelectric strain induction can be controlled by providing different voltages to different electrodes. 
     There is thus provided an acoustic transducer wherein the stiffness thereof is variable, using at least two actively polled piezoelectric slotted cylinder projector layers within the slotted cylinder projector. Further, dynamic slotted cylinder projector nodes provide for active stiffness control of the split ring transducer by having the un-polled piezoelectric volume located between two active piezoelectric volumes, per  FIGS. 5 and 6 . Further, the dead piezoelectric volume offers a dynamic node region, the two piezoelectric volumes being voltage and phase controlled in order to achieve desired performance at various operating conditions and operating performances. Other benefits include the ability to drive the two polarized piezoelectric volumes in order to achieve the desired frequency operating bandwidth, the ability to shift the resonant frequency to the desired frequency of operation (operating at resonance allows maximum operating efficiency), the ability to drive the two polarized piezoelectric volumes in order to achieve the greatest efficiency at the optimal design frequency, resulting in decrease in operating bandwidth; and optimization of the two drive voltage magnitudes and phases at various ambient pressures to achieve the maximum frequency bandwidth, greatest efficiency, and desired performance. 
     Controlling the resonance frequency makes possible highly efficient transducer assembly operation obtained from operating close to, or at, resonance. The control of the resonance of the transducer assembly with the open and short circuit stiffness of the active piezoelectric material is used to drive the transducer assembly. Increasing the DC bias (V dc ) on the PZT driver stiffens the transducer assembly resulting increased resonance frequency. The resonance frequency is directly proportional to the Young&#39;s modulus of the assembly as seen in Equation 1. 
     It will be appreciated that this invention is applicable to all transducer/projectors and not limited to slotted cylinder projectors. Improved efficiency and band width can be realized on all transducers using this proposed active variable compliance, i.e. active stiffening. 
     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 principles and scope of the invention as expressed in the appended claims.

Technology Classification (CPC): 7