Abstract:
The invention herein pertains to a single-piece, multi-layer piezoelectric stack in a sonar transducer element utilized in acoustic arrays requiring many thousands of elements. A slurry formed by mixing ceramics, powders, and binders is filtered, dried and cast into a thin film on a moving substrate. When the film has dried, it is removed from the substrate and layered into piezoelectric stacks. Screening a pattern of conductive platinum ink onto a desired layer forms electrodes. Applied heat and pressure forms a unitary body with electrically accessible layers. Burning removes the binders and sintering produces a final density. Dicing the body exposes the desired electrode polarities. A strip of conductive material is applied to connect the electrodes of like polarity and the ceramic parts are polarized. The transducer elements may be arrayed to conform to the curved surfaces such as a ship&#39;s hull.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to sonar transducers, and more particularly to sonar transducers produced utilizing a multilayered fabrication and assembly process. 
       BACKGROUND 
       [0002]    Naval architects and electronic warfare designers have expressed interest in large area conformal sonar arrays for applications where the transducers are electronically scanned to transmit and receive acoustic beams. These arrays would require thousands of transducers each separately mounted in a plane, but until now constructing an array in this fashion would result in high production and maintenance costs, large mounting areas and an unreliable design. As such, except in a very limited sense, larger transmitting/receiving arrays such as found in phased array radar antenna technology generally or in conformal antennae technology particularly has not been possible for sonar applications. However, conformal acoustic arrays hold extraordinary possibilities for undersea sonar applications such as 3-dimensional mapping, mine hunting and mine avoidance. 
         [0003]    Large-scale array technology needs to solve several key problems before undersea operations can exploit its potential. The prior art includes arrays such as the bow array having transducers numbering in the hundreds and assembled using conventional technology. Bow arrays in cylindrical form are appendages to a ship and incapable of a fully streamlined integration into a hull design. Also, to conserve space and to minimize hull penetrations, the drive, and receive circuitry of large arrays would have had to be co-located in the proximity to the transducers and outside the vessel&#39;s hull requiring low voltages yet very high electric fields to exploit advanced transduction materials. Furthermore, array elements would have to be nearly identical in their input impedance characteristics in that the sheer numbers of elements contemplated would not permit individualized transformers/tuning circuits. 
         [0004]    The curvature of the hull surface, to which a conforming array would be mounted, typically presents the designer with challenges because of dimensional instability and size. For example, expansion and contraction of such an array under environmental influences changes element-to-element separation. These types of problems tend to degrade the shape, gain, and sidelobes of electronically scanned beams. Accurate beamforming and shaping is therefore difficult to achieve because a ship&#39;s surface expands and contracts significantly due to density and temperature variations and tends to flex under the force of required maneuvering. 
         [0005]    Sonar systems widely employ transmitting and receiving transducers utilizing the tonpilz configuration. These devices have a tail mass at a proximal end and a head mass at a distal end. Between these two ends piezoelectric ceramic element drivers extend longitudinally between and in physical contact with the head mass and the tail mass. A tie rod maintains the stack of drivers under a compressive stress. Excitation of the drivers at a frequency of resonance causes the head and tail masses to oscillate at a longitudinal frequency to provide a sonar signal. 
         [0006]    Conventional tonpilz configuration technology has not been sufficiently adaptable to large-scale array applications, at least in part because conventional manufacturing processes make it difficult to control the input impedance that in some instances requires individualized transformers/tuning circuits. Further, the technology does not facilitate close electrode coupling due at least in part to the use of cemented joints between piezoelectric elements. Finally, requirements for electrode foils, cementing, and soldering when applied to the thousands of transducers required for one array, make the conventional technology impractical for many applications such as conformal transducer array applications. A means for producing high electric fields from low voltage for high-power transduction in conformal array applications is desired. 
       SUMMARY 
       [0007]    One embodiment of the invention is a process for fabricating a tape cast ceramic one-piece multi-layer piezoelectric stack utilized as a sonar transducer element driver. The process includes: (a) forming a slurry by mixing ceramics, powders, and binders; (b) filtering the slurry; (c); casting the slurry into a thin film tape (d) drying the cast; (e) removing the film when dried and (f) layering into stacks; (g) screening a pattern of conductive platinum ink onto a desired layer to form electrodes; (h) repeating the process f-h until a desired tape thickness is achieved; (i) indexing (j) heating and pressurizing to form layers into a green body of piezoelectric material with an internal electrode layer pattern; (k) burning the green body to remove the binder; (l) sintering the body to produce a final density; (m) dicing the body to expose the desired electrode polarities; (n) lapping to expose the electrodes (o) applying a strip of conductive material to connect the electrodes of like polarity; and (p) applying an electric field to achieve final polarization of the piezoelectric ceramic. 
         [0008]    Another embodiment is a tape cast ceramic one-piece multi-layer piezoelectric stack utilized as a sonar transducer element driver. 
         [0009]    According to yet another aspect, a tape cast ceramic sonar transducer array conforms to the shape of a ship hull and has multiple tape cast panels, each capable of operating as an electronically scanned sonar, and each capable of independently forming, steering, and shaping transmit and receive beams without the need for individualized tuning circuits. A signal switching distribution network allows transmit power and requisite sonar and control signals to be sent to and received from selected transducers or subsets of the panels. A processor coherently combines the return signals received from selected transducers or subsets of the panels for a wide range of undersea applications. The tape cast ceramic sonar transducer array provides a low voltage, yet very high electric field for high-power transduction in conformal array applications, for example on a ship&#39;s hull, which require low voltage outboard electronics. Advanced high-power drive materials, for example, Lead Magnesium Niobate, may be exploited by producing high electric field biasing and drive fields at low voltages. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The objects and advantages of the subject matter of this application will be apparent upon consideration of the following detailed description, taken in conjunction with accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
           [0011]      FIG. 1  is a process flow for producing a tape cast, one-piece multi-layered piezoelectric stack according to an exemplary embodiment; 
           [0012]      FIG. 2  is a semi-transparent isometric view showing a tape cast transducer element according to an exemplary embodiment; 
           [0013]      FIG. 3   a  is an isometric view of a 8×30 element transducer module according to an exemplary embodiment; 
           [0014]      FIG. 3   b  is an isometric, partial cut away view of an encapsulated 8×30 element transducer module according to an exemplary embodiment; 
           [0015]      FIG. 4  is an isometric partial cut away view of a single element tape cast transducer assembly according to an exemplary embodiment; 
           [0016]      FIG. 5  is a graph showing frequency response from a simulation of two different configurations of a tape cast transducer element according to an exemplary embodiment; 
           [0017]      FIG. 6   a  illustrates a conformal mounting of a sonar array on a doubly-curved surface of a ship according to an exemplary embodiment; 
           [0018]      FIG. 6   b  illustrates a conformal mounting of a sonar array on a singly-curved surface of a ship according to an exemplary embodiment; 
           [0019]      FIG. 7  illustrates a system for transmitting and receiving tape cast transducer signals according to an exemplary embodiment; 
           [0020]      FIG. 8  illustrates a circuit for receiving tape cast transducer signals according to an exemplary embodiment; 
           [0021]      FIG. 9  illustrates a circuit for transmitting tape cast transducer signals according to an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIG. 1  shows one embodiment of the inventive process  100  for making a one-piece multi-layer piezoelectric stack by forming a slurry  110  by mixing finely controlled piezoelectric ceramic powder, a solvent and binders. The second step requires filtering the slurry  115 , de-airing the mixture and then casting  120  a piezoelectric body several inches wide onto a moving substrate carrier producing a ceramic material having a thickness in a range of 0.002-0.020 inches. The piezoelectric material may comprise a PZT ceramic film, by way of example. The ceramic coated substrate carrier traverses through a low temperature oven, (e.g., typical range of 125-225 degree F.). Through a process of drying  125 , the volatile organic materials are evaporated or burned off. When the film drying  125  is complete removing  130  the tape strips it from the substrate carrier. Layering  135  stacks the film atop one another. After layering  135  creates a specified stack thickness, selected layers have electrodes applied by screening  140  a pattern of conductive noble material such as silver, gold alloy or a platinum ink onto the desired layer. In one non-limiting embodiment one pattern produces a conductive film over a portion of a contiguous plane surface that covers three of the four edges of one layer. The pattern is alternately flipped about the vertical axis of a stack to produce a conductive coating representing corresponding alternate polarities. Repeating  138  the process of stacking layers by layering  135  strips of tape atop one another results in a final desired stack thickness. Indexing  145  the electrode pattern exposes the desired polarity, after dicing  165  the body into separate stacks. Heating (e.g., 150-300 degrees F. (Fahrenheit)) and pressurizing (1,000-5,000 p.s.i.)  150  the stack-up of layers consolidates them to form a contiguous body with an internal pattern of electrodes. Burning at 800-1500 degrees F.  155  removes the binder and sintering at 2102 to 2417 degrees F.  160  produces a final density and desired mechanical strength. Dicing  165  the solid body using a precision dicing saw produces individualized sonar transducer tape cast stacks. The process of dicing  165  marginally exposes the desired electrode polarities on two of the four sides of the stack. Lapping  168  individual stacks clearly exposes the desired electrodes. Applying  170  a strip of fired silver soldered wire, silver termination ink or conductive epoxy connects the electrodes of like polarity (positive on one side and negative on the other) to wire the transducer element stack. Polarizing of the piezoelectric element  175  is performed at 212-257 degrees F. under 50-80 volts per thousandth of an inch of ceramic thickness between electrode layers. 
         [0023]    In production, the process  100  has the ability to generate hundreds or thousands of individualized finished stacks each requiring very little labor. Furthermore, each may be produced at a cost of approximately one tenth of the cost of producing a conventional stack. Once assembled, the stacks provide approximately a 10%-15% improvement in electromechanical coupling efficiency over a conventional stacked element transducer due at least in part to the absence of cement joints. Furthermore, the stacks have nearly identical impedance with respect to the neighboring stacks without the need for individualized tuning circuits. 
         [0024]      FIG. 2  shows finished tape cast stacks such as  205   a  and  205   b  and not the individual layers of tape. The tape cast stacks  205   a  and  205   b , are placed into a sonar transducer element assembly  200 , each separated by insulating layer  230 . Each stack  205   a ,  205  may contain by way of a non-limiting example 20-30 active layers. In one non-limiting embodiment of the invention, the element assembly  200  comprises a folded low-density high stiffness magnesium aluminum alloy head mass  210 . A tail mass  220  includes a folded high-density tungsten alloy. Tie-rods  215  serve to connect the head mass  210  and the tail mass  220  to contain the stacks  205   a ,  205   b  in a rigid assembly under compression. In the embodiment described, electrodes  207   a ,  207   b  of like polarity are clad  225  using silver ceramic wiring to effectively connect the elements in each stack  205   a ,  205   b  in parallel. The positive electrodes  207   a  on stack  205   a  are all electrically joined and the negative electrodes  207   b  on stack  205   b  are likewise electrically joined; each of the positive and negative electrodes are electrically isolated from one another. On the rear side of the stacked assembly (not shown) positive electrodes  207   a  are electrically coupled together for stack  205   b  and the negative electrodes  207   b  are likewise electrically coupled to stack  205   a ; the positive and negative electrodes are also electrically isolated from one another. The stacks  205   a ,  205   b  electrical signals interface through the electrodes  207   a ,  207   b  via leads  235  having four conductors that attach clad  225  to an input/output port  240  formed into an isolation layer  237 . The transducer element  200  may be fastened to an array or module using array plate fasteners  241  and isolating washer  243 . 
         [0025]      FIG. 3   a  shows a plurality of finished sonar transducer element assemblies  200 , such as  200   a  and  200   b  assembled into a module  300 . Each module  300  may by way of a non-limiting example contain 30 to 36 element assemblies  200  in the vertical direction.  FIG. 3   a  shows module  300  as containing 8 element assemblies  200  in the horizontal direction and 30 element assemblies  200  in the vertical direction. The module  300  contains isolation washers  243  and fasteners  241 . 
         [0026]    In the example of  FIG. 3   b , the module  300  contains 240 tape cast sonar transducer element assemblies  200  arranged as 8 element assemblies in the horizontal direction and 30 element assemblies in the vertical direction. The module  300  measures 30 inches (30 in.) in the vertical direction, 8 inches in the horizontal direction, and 3.5 inches in depth. A titanium moisture barrier  335  covered by a vulcanized rubber seal  340  permits the module  300  to be employed in an underwater application without degrading performance. 
         [0027]    The stacks may be assembled into the transducer element assembly  200  as one contiguous stack or a plurality of stacks. The embodiment illustrated in  FIG. 2 , depicts two stacks  205   a  and  205   b . In a non limiting embodiment illustrated in  FIG. 4  a transducer element assembly  400  has a vertical body length of 2.3 inches containing one stack  405  having electrodes  407  with a nominal 50 mil separation. The head mass  415  is composed of magnesium-aluminum alloy having a square top dimension of 0.9×0.9 inches. A tail mass  430  also measures 0.9×0.9, in a square dimension. The stack  405  uses a conductive epoxy to form the positive electrode wire  420  and a conductive epoxy to form a negative electrode wire  435 . Two washers  410 ,  425  electrically isolate the stack  405  from the transducer element assembly  400 . 
         [0028]      FIG. 5  shows a comparison in the performance between a double stack transducer element assembly as shown in  FIG. 2  and a single stack transducer element assembly as for example shown in  FIG. 4 . As utilized in a transmit mode, the double stacks in  FIG. 2  were connected in parallel. In a receive mode the double stacks were connected in a series mode. The graph in  FIG. 5  shows the partitioning of the stack into 2 half stacks, referred to as the 4-wire design as having a 6 db improvement over the one stack  2  wire design. 
         [0029]      FIG. 6   a  and  FIG. 6   b  show ships  605   a  and  605   b  having conic section surface curvatures  602   a  and  602   b  that change continuously over surface  625 . Tape cast sonar transducer array formations  610   a  and  610   b , as illustrated, provide virtually instantaneous scan capability over a maximum 180° azimuth and elevation without degrading inertial effects and without mechanical scan losses. According to an exemplary embodiment, array  610   a  on the doubly-curved surface  602   a  continuously changes its radiating and receive element-to-element orientation in two dimensions to maintain conformality. According to another exemplary embodiment, array  610   b  on the singly curved surface  602   b  continuously changes its radiating and receive element-to-element orientation in one dimension to maintain conformality. Sonar transducer array formations  610   a ,  610   b  may be mounted either internal to the ship surface  605   a ,  605   b  respectively or upon the exterior surface  605   a ,  605   b  hull. 
         [0030]    The manufacturing and construction costs associated with conformal approaches are generally high, at least in part due to the variable surface curvature that requires the sub-panels constituting an array to conform. However, the encapsulated module illustrated in  FIG. 3   b  would reduce the cost associated with the surface curvature since each module would form a discrete and small chordal section along the curve. The plurality of finished sonar transducer element assemblies  200 , such as  200   a  and  200   b  assembled into a module  300  are applied end to end such that the module to module linear dimension is relatively small (e.g. approximately one-inch linear dimension for each tape cast sonar transducer element produces a 30 inch by 8 inch module) compared to radius of curvature as of the hull of, for example a Virginia Class Submarine (Length: 377 ft., Beam: 34 ft.) as illustrated by subtended angle  615 , thus rendering insignificant any curvature anomaly. 
         [0031]      FIG. 7  discloses a system  700  that integrates tape cast sonar transducers elements  724  arrayed in a conformal application so as to transmit and receive sonar signals from underwater targets. A tape cast ceramic sonar transducer array  735  conforms to the shape of a ship hull and has multiple tape cast transducer element module panels  737 , each capable of operating as an electronically scanned sonar, and each capable of independently forming, steering, and shaping transmit and receive beams. A signal switching distribution network  722  allows transmit power and requisite sonar and control signals to be sent to and received from selected transducer elements  724  or subsets of the panels  737 . A processor coherently combines the return signals received from selected transducers  724  or subsets of the panels  737  for a wide range of undersea applications. A waveform generator  712  produces stable waveforms with unique frequencies and phase characteristics for each one of the tape cast sonar transducer elements  724 . The transmitter signal driver  718  receives its power from energy storage device  720  and outputs a transmit signal to transmit/receive function  722 . Function  722  serves to switch or direct inputs and outputs from a tape cast sonar transducer element  724  module  737  or array  735 . 
         [0032]    As indicated transmit/receive function  722  directs the tape cast sonar transducer elements  724  received acoustic signals from the synchronized transmissions produced by transmitter signal driver  718 . The return signal also provides data to transmitter feedback conditioning and processing function  726  to adjust the waveform generator output  712  to condition subsequent wave form transmissions. 
         [0033]    The receptions from the tape cast sonar transducer elements  724  are processed and uplinked through an uplink interface  716  to telemetry equipment  710 . In its broad operational aspect, radio telemetry equipment  710  serves as a communication link between the underwater acoustic transmitting/receiving portion of the system  700  and a remote central station such as a surface ship via a transmission antenna not shown. The telemetry  710  houses a transmission device operably coupled to a transponder system not shown. The telemetry  710  transmission device transmits data received from the transponder system as electromagnetic energy in a particular frequency range exchanging digital control signals between the surface central station and the ship that has thereon installed system  700 . Telemetry  710  transmits a received signal digitizer  733  data over the airwaves. In certain applications, the ship-to-telemetry  710  communication downlinks  714  data to serve as a remote control of both telemetry and system  700 . In the embodiment shown in  FIG. 7  the downlink  714  controls the waveform generator  712  and the receiver scaling and equalization  732 . 
         [0034]    A digital receive sub system comprised of a preamplifier  728 , a scaling &amp; equalization module  732  and the receiver digitizer  733  provides amplification of the sound signal received at each tape cast sonar transducer element of an array as for example described in reference to  FIG. 6   a ,  FIG. 6   b . The digital receive sub system may also provide for a direct per channel analog-to-digital conversion of the sound signal; a digital memory to provide delays for focusing; and digital summation of the focused signals from all the channels. Other processing features of the digital receive system include phase rotation of a receive signal on a channel-by-channel basis to provide fine focusing, amplitude scaling (apodization) to control the beam sidelobes, and digital filtering to control the bandwidth of the signal. 
         [0035]      FIG. 8  shows a pre amplifier circuit  738  that serves to receive the sonar transducer tape cast sonar transducer elements  724 . The preamplifier tracks DC voltage changes out of the transducer over a specified rate of ascend and descend. Two series half stack receive networks  810  are tuned to receive a bi polar signal (e.g. 482 Hz transducer signal) for low noise amplification. A high pass filter having a 6 db break point corresponding to the signal (e.g. a break point of 482 Hz) is provided by RC network  815   a ,  815   b . Diodes  813  serve to provide differential and common mode over voltage protection. Low pass RC network  818   a ,  818   b  protect against RF rectification by filtering correlated noise. RC network  820  serves as an equalizer to balance the inputs to output linear amplifying system  830 . Resistors network  825  comprised of resistors R 10  and R 11  provide a differential calibration input. 
         [0036]      FIG. 9  illustrates an exemplary embodiment of a circuit for transmitting a tape cast transducer signal. The embodiment of transmitter  900  uses diodes to provide load compensation and load switching between transmit and receive operational modes. A tape cast multi-layer transducer element comprises an element driver having two piezoelectric half stacks  801   a  and  801   b  coupled by diode arrangements  901   a  and  901   b . Diode arrangements  901   a ,  901   b  compensate for load imbalances between the two half stacks  901   a ,  901   b . Diode arrangements  903   a  and  903   b  are also coupled between half stacks  801   a ,  801   b  to provide switching capability between transmit and receive modes for half stacks  810   a  and  810   b , respectively. Diodes  903   a  are configured in opposite polarities such that biasing diodes  903   a  cause the circuit flow to proceed through diode  903   a  in a direction dictated by the biasing voltage. Diodes  903   b  operate in a similar manner with respect to circuit flow control for half stack  810   b . The transmitter  900  is powered by a voltage controlled voltage source  905 . 
         [0037]    While the foregoing invention has been described with reference to the above described embodiment, various modifications and changes can be made without departing from the spirit of the invention. Accordingly, all such modifications and changes are considered to be within the scope of the invention.