Abstract:
A hydrophone unit comprising a resilient central wire; a conductive wire coiled around the resilient central wire, said conductive wire being coated with a piezo material for generating an electrical signal in response to the presence of an acoustic vibration, wherein the resilient central wire is fabricated from spring steel, the conductive wire is a copper wire, the piezo material includes polyvinylidene difluoride; a layer of conductive material deposited on the piezo material-coated conductive wire, wherein the layer of conductive material comprises a silver ink; and a jacket of polyurethane.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 11/539,948, filed Oct. 10, 2006, U.S. Pat. No. 7,697,374, issued Apr. 13, 2010, which claims benefit of U.S. Provisional Application No. 60/726,772, filed Oct. 14, 2005, the entire contents of each being hereby incorporated herein by reference. U.S. Provisional Application No. 60/610,342 filed on Sep. 16, 2004, is also incorporated by reference herein in its entirety. 
    
    
     STATEMENT OF GOVERNMENTAL INTEREST 
     This invention is made with Government support under NAVSEA Contract No. N00024-03-D-6606, awarded by the U.S. Navy. The Government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a hydrophone array and a system and method for deploying the array, particularly hydrophone arrays for underwater acoustic sensing of subsurface marine vehicles. 
     2. Background of the Invention 
     Hydrophone arrays are used militarily to detect the presence of submarines and to provide information about their movements. Because modern submarines have the ability with cruise missiles to attack surface ships at great distances, the protection of surface shipping requires the ability to detect and track submerged submarines over vast areas of ocean. Hydrophone arrays have typically been used for this purpose. 
     Hydrophones are acoustic transducers which operate by detecting acoustic signals and converting them to electrical impulses which can then be transmitted by radio waves to a distant receiver. Typically, an array of hydrophones is loaded into a sonobuoy which can then be dropped by airplane into the ocean. The array is then deployed while a float containing a transmitter remains at the surface of the water. 
     What is yet needed is a collapsible hydrophone array support system which maintains the array in a predetermined configuration when deployed with reduced chance of twisting the wires, and which can be fit into a sonobuoy. 
     SUMMARY OF THE INVENTION 
     A hydrophone array is provided herein, the hydrophone array comprising an inflatable shaped housing enclosing an interior space and formable between a collapsed configuration and an expanded configuration, a framework of compliant material disposed within the interior of the inflatable housing, and a plurality of hydrophone units attached to the compliant material at respective positions, wherein said hydrophone units are arranged in a predetermined geometric array when the shaped housing is in the expanded configuration. Also provided herein is a system and method for deploying the hydrophone array. 
     The invention herein advantageously provides a means for deploying an array of hydrophones in a predetermined configuration with less chance of twisting the wires with multiple support members. The array is collapsible into a size A or smaller sonobuoy and the inflatable support structure uses no metal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments are described below with reference to the drawings wherein: 
         FIG. 1  illustrates the hydrophone deployment system of the invention loaded in a sonobuoy; 
         FIG. 2  illustrates the deployed system in a body of water; 
         FIG. 3  illustrates the hydrophone array support; 
         FIG. 4  is a perspective view of the hydrophone array; 
         FIG. 5  is a partly sectional perspective view of an individual hydrophone unit; and, 
         FIG. 6  is a diagram for an amplifier circuit for processing signals from the hydrophone array. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 1 , the hydrophone array deployment system  100  includes a float  102 , battery  108 , tether line  104 , hydrophone array with collapsed (uninflated) support system  110 , and pump  106  loaded into a sonobuoy cannister  101 . The sonobuoy cannister  101  can be size A or smaller. 
     Referring also now to  FIG. 2 , the system  100  is shown deployed in a body of water  10 . Deployment can be accomplished by, for example, dropping system  100  into the water  10  by airplane, surface vessel, or any other suitable means. Jettison of the contents of the sonobuoy cannister  101  can be accomplished by various means such as the impact of the sonobuoy into the water. Alternatively, a battery  108  (e.g., seawater activated battery) can power the ejection of the contents by, for example, firing a squib. Also, back plate  109  can be jettisoned (by water impact, firing a squib, etc.) to allow the hydrophone array to descend to the proper depth. Float  102  remains at the surface of the body of water and preferably includes a transmitter with antenna  103  and the appropriate battery-powered electronics for converting the electrical signals from the hydrophones into radio waves for wireless transmission to a remote receiver. The transmitter  103  is preferably powered by battery  108 . Tether  104  preferably includes a harness  105  for holding the supported hydrophone array  110  and a conductive wire for conducting electrical pulses from hydrophone units to the transmitter  103 . Tether  104  can be of any length suitable for deploying the hydrophone array  110  at a desired depth and can typically range up to 250 feet in length. Amplifier  130  increases the power of the electrical signal from the hydrophones for transmission to the transmitter. Suitable amplifiers are known in the art and commonly available. Pump  106  serves as both a weight and an inflation means to introduce water into the inflatable housing  111 , as discussed more fully below, and can be powered by, for example, a falling weight, a spring, or other mechanical means, or may optionally include a battery for electrical power. 
     The system  100  can be “active” or “passive.” Active sonobuoy systems emit acoustic signals into the water and listen for the return echo. Passive systems merely listen for sounds made by underwater craft, e.g., power-plant, propellers, door closings or other mechanically generated or human generated noise. 
     Referring now to  FIGS. 3 and 4 , the deployable supported hydrophone array  110  includes a flexible waterproof inflatable housing which, in an uninflated collapsed configuration, can be fit within a size A or smaller sonobuoy. Upon being immersed in a body of water it is inflated with water by means of pump  106 . Valve  113  at the upper portion of the housing  111  is a release valve for air or excess pressure. Valve  114  at the bottom of housing  111  is a water inlet valve, receiving a flow of water via water feed line  107  from pump  106 . The inflatable housing  111  defines an interior space in which patterned compliant supports  112  are enclosed, and can be fabricated from any flexible, non-metallic waterproof material such as synthetic polymer or natural or synthetic rubber sheet (e.g., polyolefin, polyester, PVC, etc.). The housing  111  and compliant supports  112  preferably contain no metal or dense material which might cause reflections of sound waves within the hydrophone array. Optionally, one or more fins  116  can be attached to the exterior of the housing  111  to permit alignment with the ocean current and to inhibit unwanted rotation. As mentioned above, an amplifier  130  is employed to augment the electrical signals from the hydrophones  120  to which it is electrically connected. 
     The compliant supports  112  are preferably sheets of fabric or netting attached together and to the interior of the housing  111  and are so constructed such that when the housing  111  is inflated the supports  112  of compliant fabric are arranged in a three dimensional pattern ( FIG. 4 ). Acoustic transducers such as hydrophone units  120  are affixed to the supports  112  of compliant fabric sheets such that when fully deployed the hydrophone units  120  are oriented in vertical parallel lines. The hydrophone units  120  can be sewn into the compliant fabric  112 , bonded by adhesive, secured in pockets in the fabric, or affixed to the compliant fabric by any suitable means. Hydrophone units suitable for use in the invention are commercially available from Argotech Inc. of Fort Lauderdale, Fla. 
     Referring now to  FIG. 4 , the hydrophone units  120  when deployed are arranged as horizontally spaced-apart, vertically extending linear units so as to define, with the compliant fabric supports  112 , radially oriented planes  116  extending from a central line  117 . Preferably, the array includes planes  118  extending laterally between the radially oriented planes  116 . As shown in  FIG. 4 , three radial planar extensions  116  are included in a tripodal array containing 20-30 hydrophone units  120 . The hydrophones are preferably spaced apart from each other by a distance of from about λ/2 to about λ/4 wherein λ is the wavelength of the target acoustic signal. However, other configurations can alternatively be employed, such as 5-membered or 7-membered planar extensions  116 , or other multi-podal arrangements. 
     The optimal dimensions of the hydrophone array depend upon the target frequency of the acoustic signals, expected tilt of the array due to relative current between the surface float and the hydrophone array, and operating depth. A preferred length for the hydrophone wires is 48 or 58, where 8 is the wavelength of the target acoustic signal. This length provides a balance between vertical bandwidth (given the expected array tilt) and end-fire notch depth (to suppress any target frequency band noise originating from the surface float given the operating depth). For example, referring to  FIG. 3 , L is the length of the hydrophone wires and D is the diameter of the array. For a design frequency of 10 kHz, L is preferably about 26 inches and D is preferably about 28 inches. The inflatable housing  111  holds a volume of less than about 40 gallons of water and in a collapsed state would fit in a size A sonobuoy canister. For a design frequency of 15 kHz, L is preferably about 18 inches, D is preferably about 24 inches and the inflatable housing  111  holds less than about 20 gallons of water and would fit in a size A/2 sonobuoy canister. 
     Various types of acoustic transducers can be used to detect acoustic waves transmitted through the water. For example, the acoustic transducer can comprise a tube formed at least in part of a piezo material. Piezo materials can be piezoelectric, which generate an electrical pulse or current upon receiving a mechanical impulse such as from an acoustic vibration, or piezoresistive, which change resistance upon receiving a mechanical impulse. Piezoelectric material can comprise an active polarized ceramic material, such as barium titanate or lead zirconate titanate (PZT). The piezoelectric material can, in another embodiment, be a piezoelectric polymer material, such as polyvinylidene difluoride (PVDF), or a piezo-rubber composite material. Piezoresistive materials include, for example, conductive elastomeric polymeric foams or rubbers which become more conductive when compressed. 
     For purposes of the present invention the hydrophone needs to be sufficiently flexible to be folded into a sonobuoy canister. Typically, hydrophones include a central or core conductor, an outer conductor, and a layer of piezo material disposed between, and in contact with, the core conductor and outer conductor in a coaxial configuration. When subjected to mechanical force the piezo material, such as polyvinylidene difluoride (PVDF) generates an electrical current which is carried by the conductors. However, in a new, preferred embodiment, hydrophone  120  includes a “piano” wire type construction as shown in  FIG. 5 . In particular, a central wire  121  fabricated from a resilient metallic or non-metallic material, preferably spring steel, provides a resilient core which gives the hydrophone a shape memory and a biasing force such that when the hydrophone  120  is released from the canister  101  into the seawater it automatically returns to a straight configuration. A conductive wire coil  122  of smaller diameter wire than the central spring steel wire  121 , is first coated with a layer  124  of piezo material (e.g., PVDF) and then, in one embodiment of the invention, is coated with a conductive layer  123  of copper, aluminum, silver, gold or the like. Preferably, conductive layer  123  is applied (e.g., by spraying, dipping, painting, etc.) as a silver ink. The conductive wire coil  122  is then tightly wound around the central wire  121 . The conductive wire coil  122  is preferably fabricated from copper, aluminum, silver, gold, or alloys thereof, or of any other highly conductive ductile and flexible material. In an especially preferred embodiment, the wire coil  122  comprises a copper wire. In this embodiment the conductive wire coil  122  acts as an inner conductor and the conductive coating  123  acts as an outer conductor, both being in contact with the piezo material layer  124 . Optionally, the conductive layer  123  can be applied after the piezo material-coated conductive wire  122  is coiled around the resilient central wire  121 , preferably under pressure. In yet another embodiment the conductive layer (e.g., silver ink) is applied both before and after the piezo material-coated conductive wire  122  is coiled around the resilient central wire  121 . Preferably, hydrophone  120  can include an outer jacket  125  of polyurethane or other waterproof, electrically insulative flexible material to prevent electrical signals from the hydrophone from being dissipated in the seawater. The copper coil  122  provides significantly greater flexibility than a comparably sized hydrophone using a coaxial configuration with a solid copper core. The hydrophone wire  120  typically has a diameter ranging from about ⅛ to about 1/10 inches. 
     The system  100  is deployed, for example, by launch from an airplane. When the sonobuoy enters the water the contents of the sonobuoy are ejected from the cannister. The float remains on the water surface and the deployable array  110  drops to a predetermined depth. Pump  106  acts as a ballast. A falling weight or other suitable mechanism powers pump  106  to deliver water through feed line  107  and water inlet valve  114  into the flexible housing  110  which then expands to deploy the array of hydrophones  120  and sheets of compliant material  112 . Air or excess pressure is released through valve  113 . 
     Acoustic waves which strike the array are converted by the hydrophone array  110  are converted by the hydrophone units  120  into electrical signals which are processed by an amplifier  130 . A circuit diagram for an amplifier suitable for use in the present invention is illustrated in  FIG. 6 . The circuit can be embodied in a printed circuit board as small as 0.325 inches by 0.800 inches. The circuit shown in  FIG. 6  has the following parameters: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Output voltage swing 
                 5 volts p-p (10 volt differential) 
               
               
                   
                 Gain 
                 686.68 dB to each output (2718 
               
               
                   
                   
                 V/V), 74.68 dB to differential 
               
               
                   
                   
                 output, −3 dB at 114 Hz and well 
               
               
                   
                   
                 above 50 kHz. The gain can be 
               
               
                   
                   
                 altered by replacing two resistors. 
               
               
                   
                 Input impedance 
                 22 Megaohms, single ended 
               
               
                   
                 Input noise 
                 20 nV per root Hz at 10 kHz 
               
               
                   
                   
                 (estimated) 
               
               
                   
                 Power 
                 6 to 16 V DC, estimated 2.5 mA 
               
               
                   
                   
               
             
          
         
       
     
     While the above description contains many specifics, these specifics should not be construed as limitations of the invention, but merely as exemplifications of preferred embodiments thereof. For example, while the invention herein is particularly advantageous for military applications and has been described in terms of detection of submarines, it can clearly be employed in any situation wherein acoustic detection is needed, such as oceanographic or other scientific studies, rescue operations, and the like. Those skilled in the art will envision many other embodiments within the scope and spirit of the invention as defined by the claims appended hereto.