Abstract:
The present invention is directed to an acoustic vector sensor, specifically an underwater acoustic vector sensor. The acoustic vector sensor contains three piezoelectric sensors orthogonally mounted inside a rigid housing, where each of the piezoelectric sensors measures acoustic energy from one of the three different, orthogonal, axial directions (X, Y, and Z). The piezoelectric sensor contains a proof mass, a base, and a piezoelectric crystal sandwiched therebetween. The bonding surfaces of the proof mass and the base are preferably castellated; and the piezoelectric crystal is preferably a shear mode (d 15 ) relaxor single crystal.

Description:
STATEMENT OF GOVERNMENT INTEREST  
       [[0001]]     The work leading to the present invention was supported in part by Navy ONR, Contract No. N66604-03-C-1309. The United States Government has certain rights in the invention. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention is directed to an acoustic vector sensor, specifically an underwater acoustic vector sensor.  
       DESCRIPTION OF RELATED ART  
       [0003]     Inertial accelerometers have been used as the sensing elements in underwater acoustic pressure gradient (vector) sensors. Conventionally, PZT piezoelectric ceramics were used as the sensing material. Due to reduced piezoelectric properties compared to a relaxor single crystal, vector sensors using PZT resulted in low signal-to-noise ratios and narrow bandwidths.  
         [0004]     It has recently been attempted to exploit a relaxor single crystal in a vector sensor design. Without good solutions to mitigate the adverse effects of the relaxor crystal, such as lateral constraint effect and large transverse responses, these efforts have so far ended in a vector sensor having little improvement and/or very fragile sensing structures.  
         [0005]     U.S. Pat. No. 6,038,924 to Lee et al. discloses a seismic accelerometer comprising three piezoelectric sensing elements, with each element sensing inertial forces applied to the accelerometer from a particular direction, X, Y, or Z. Each sensing element also has a stacking construction having a base, a piezoelectric sensing element, and a mass.  
         [0006]     U.S. Pat. No. 4,928,263 to Armstrong et al. discloses an accelerometer-based hydrophone using piezoelectric ceramic as the active material. The hydrophone comprises orthogonally arranged accelerometers that share an inertial mass and housed in a pressure vessel.  
         [0007]     U.S. Pat. No. 6,370,084 to Cray discloses an acoustic vector sensor comprising an acoustics sensing tri-axial accelerometer, a body of syntactic foam encasing the accelerometer, a body of viscoelastic rubber encasing the body of syntactic foam, and a rigid plastic coating encasing the body of viscoelastic rubber. The viscoelastic body is acoustically transparent and isolates the accelerometer and syntactic foam sphere from the mount, and insulates the accelerometer from structure-borne flexural vibrations from supporting structures near the sensor.  
         [0008]     U.S. Pat. No. 6,697,302 to Cray et al. discloses an underwater acoustic sensor that measures up to seven quantities of acoustic field at a collocated point. The quantities measured by the acoustic receiver sensor are acoustic pressure, three orthogonal components of acoustic particle acceleration, and three spatial gradients of the acceleration vector. These quantities are then combined to provide improved directivity of the acoustic receiver sensor.  
         [0009]     All previous solutions suffer from high noise level and narrow frequency bandwidth.  
       SUMMARY OF THE INVENTION  
       [0010]     It is therefore an object of the invention to provide a sensor having the advantages of a relaxor crystal without the disadvantages of high noise levels and narrow bandwidths. To achieve the above and other objects, the present invention uses a shear mode (d15) relaxor single crystal. Other improvements also include castellated mounting surfaces to alleviate the clamping effect, and an orientation of the crystal given by a rotation angle  
       α   =     arctan   ⁡     (       ⅆ   16       ⅆ   15       )           
 
 to eliminate the transverse piezoelectric responses. 
 
         [0012]     The piezoelectric sensor of the present invention comprises a proof mass, a base, and a piezoelectric crystal sandwiched therebetween. The bonding surfaces of the proof mass and the base are preferably castellated. And the piezoelectric crystal is preferably a shear mode (d 15 ) relaxor single crystal.  
         [0013]     The acoustic vector sensor of the present invention comprises three piezoelectric sensors orthogonally mounted on a rigid housing. Each of the three piezoelectric sensor measures acoustic energy from one of the three different, orthogonal, axial directions (X, Y, and Z).  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  shows a shear mode inertial sensor stack.  
         [0015]      FIG. 2  shows a shear mode inertial sensor stack with castellated bonding surfaces and an optimal cut relaxor crystal plate.  
         [0016]      FIG. 3  shows three such sensor stacks incorporated into a vector sensor.  
         [0017]      FIG. 4  shows the coordinate system of the crystal.  
         [0018]      FIG. 5  shows the optimal rotation of the crystal.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]     In an embodiment of the present invention, by utilizing a shear mode (d 15 ) relaxor single crystal which possesses the highest piezoelectric coefficient, coupled with the lateral constraint (clamping effect) alleviation mechanism and the identified optimal orientation of the relaxor crystal of the present invention, a vector sensor with much higher signal to noise ratio and wider frequency band can be realized.  
         [0020]     The present invention exploits the exceptionally high piezoelectric shear coefficient (d 15 ) of a relaxor single crystal, and it employs unique approaches to eliminate the adverse effects, i.e. lateral constraint (clamping) effect and transverse response effect. Consequently, a miniature vector sensor with high signal-to-noise ratio and wide bandwidth is achieved.  
         [0021]     A simple shear mode, piezoelectric, inertial stack is selected as the basic sensing element. This stack, which is shown in  FIG. 1  as  100 , includes a proof mass  102 , a shear mode relaxor crystal plate  104  and a base  106 . Two unique features are incorporated in the stack design, which are illustrated in  FIG. 2 , which shows a stack  200  including a proof mass  202 , a shear mode relaxor crystal plate  204  and a base  206 . The first feature is that the bonding surfaces  208 ,  210  are castellated, i.e., raised protrusions are made on the mass and base gluing surfaces. Castellated patterns can be fabricated on the bonding surface in accordance with U.S. Pat. No. 6,715,363 to Deng et al., which is incorporated herein by reference. This feature can greatly reduce the bonding area on the crystal, and doing so will effectively mitigate the lateral constraint adverse effect of the relaxor crystal. As a consequence, the piezoelectric output of the sensing structure is significantly improved. The second feature is that the relaxor shear crystal plate  204  is cut in a special orientation so that it only produces a maximum piezoelectric response in one direction (Y axis in  FIG. 2 ) and minimum piezoelectric outputs in the other two orthogonal directions (X and Z axes in  FIG. 2 ). The detailed information about this special orientation will be set forth below.  
         [0022]     Three of the aforementioned stacks  200  are mounted orthogonally to each other onto a rigid housing (base)  302 , which is depicted in  FIG. 3 , to form a vector sensor  300 . Each of the stacks  200  measures acoustic energy from one of the three different, orthogonal, axial directions (X, Y, and Z). Moreover, the sensor housing  302  is preferably more rigid than the rest of the stacks  200  (or has higher resonance than that of the stacks). In an embodiment of the present invention, as shown in  FIG. 3 , the housing  302  can also serve as a base for the stacks  200 . Alternatively, however, a separate base can be constructed for each stack and mounted onto the housing  302 . The enclosed housing  302  is preferably designed to maintain approximate neutral buoyancy in water. In a preferred embodiment, the feature size of the enclosed sensor  300  is equal to or smaller than the half wavelength of the highest frequency of interest. Once submerged in water, the sensor  300  can freely float, and thereby follows the acoustical particle movement. Consequently, the pressure gradient (acceleration) of the acoustical wave can be measured by the three correspondent stacks (X, Y, Z sensing directions in  FIG. 3 ).  
         [0023]     The old methods employed three independent piezoelectric inertial sensors and potted them together in the material which is much lighter than water. By contrast, the current invention mounts three piezoelectric sensing structures directly on a rigid, enclosed house. The resonance of the mounting structure and housing, which are integrated as one piece, is designed to be higher than the sensing structure itself. More importantly, there are two important features that are incorporated in the shear mode sensing stack: first is the unique castellated bonding surface, and the second is the optimal cut crystal plate. Both these features can be employed together or independently depending on the design of the sensor.  
         [0024]     The simple, compact sensing structure enables the most weight efficient design. The castellated bonding surfaces effectively alleviate the lateral constraint effect of relaxor crystal and still ensure a good bonding integrity. The optimal cut of the relaxor crystal plate guarantees a good sensing directivity which is crucial to a vector sensor.  
         [0025]     The following describes the particular crystal orientation to produce a maximum piezoelectric response in one direction and minimum piezoelectric outputs in the other two orthogonal directions:  
         [0026]     The exceptional piezoelectric property of the shear mode PMN-PT crystal is shown in the following Table 1:  
                                                                                                     TABLE 1                           Typical Piezoelectric Properties of the Three Modes                Material   Piezoelectric Charge   Piezoelectric           Properties   d Coefficients (pC/N)   Coupling Factor                Symbol   d 31     d 33     d 15     k 31     k 33     k 15                              PZT-4   −130   300   500   33%   68%   71%           PMN-PT   −900   2000   4600   53%   90%   97%                      
 
         [0027]     Among all of the modes, the shear mode piezoelectric coefficient d 15  has the highest value. It could benefit many sensor and actuator applications.  
         [0028]     Table 2 below shows the piezoelectric matrix of a &lt;111&gt; poled PMN-PT crystal:  
                                                                   TABLE 2                           Piezoelectric Matrix of the 3 m Crystal Class                Output   Compression Axis   Shear Axis                Mode   1(X)   2(Y)   3(Z)   4(X)   5(Y)   6(Z)                       1(X)   0   0   0   0   d 15     −2d 22             2(Y)   −d 22     d 22     0   d 15     0   0           3(Z)     d 31     d 31     d 33     0   0   0                      
 
         [0029]     Significant transverse piezoelectric effects exist in the standard cuts. The standard reference coordinates of the &lt;111&gt; poled relaxor crystal are shown in  FIG. 4 .  
         [0030]     The transformation of the piezoelectric tensor in different orientations is as follows. The apparent piezoelectric matrix (d ij ) is actually a rank 3 tensor (d imn ), and the index transformations between them are shown in Table 3 below.  
                                                 TABLE 3                       Index Transformation Scheme for d imn  and d ij                                      Tensor   11   22   33   23   13   12           Index mn               32   31   21           Matrix   1   2   3   4   5   6           Index j                      
 
         [0031]     The new piezoelectric tensor after rotation is calculated by
 
 d   ijk   ′   =a   il   ·a   jm   ·a   kn   ·d   lmn , 
 
 where: 
        a ij  are the direction cosines;     index i,j,k,l,m,n=1,2,3;     d′ ijk =the piezoelectric coefficients after rotation; and     d lmn =the piezoelectric coefficients before rotation.        
 
         [0037]     The optimal orientation for zero transverse piezoelectric effects is given, as shown in  FIG. 5 , by rotating around the X axis by an angle  
       α   =       arctan   ⁡     (       ⅆ   16       ⅆ   15       )       .         
 
 The d matrix before and after that rotation will be given, where * indicate elements having non-zero values.  
               d   ⁢           ⁢   matrix   ⁢           ⁢   before   ⁢           ⁢   rotation     =     (         0       0       0       0       *       *           *       *       0       *       0       0           *       *       *       0       0       0         )                   d   ⁢           ⁢   matrix   ⁢           ⁢   after   ⁢           ⁢   rotation     =     (         0       0       0       0       *       0           *       *       *       *       0       0           *       *       *       *       0       0         )               
 
         [0039]     The concepts of the present invention have been experimentally verified using a testing structure (inertial shear stack) having the configuration shown in  FIG. 1 . The results are shown in Table 4 below.  
                                                                         TABLE 4                           Test Results of the Inertial Stacks Using Conventional       Cut and the New Cut PMN-PT Shear Plates                Normalized Outputs   Piezoelectric Output                Along 3 Orthogonal   Sensing   Transverse   Transverse           Directions   Direction   Direction 1   Direction 2                            Conventional   100%   18.0%   3.0%           Cut Shear Plate           Optimal New   100%   4.5%   3.0%           Cut Shear Plate                      
 
         [0040]     While a preferred embodiment of the present invention has been set forth above, those skilled in the art will recognize that other embodiments can be realized within the scope of the invention, which should therefore be construed as limited only by the claims to be set forth in the non-provisional application. What is claimed is: