Abstract:
A hydrophone deployment system including a hydrophone assembly, a container for enclosing the hydrophone assembly body in a coiled configuration, means for ejecting the hydrophone assembly from the container, a signal processing module for processing the electrical signals from the hydrophone units, and a transmitter module for converting said processed electrical signals and transmitting said converted signals to a remote receiver.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 11/548,006, filed Oct. 10, 2006, U.S. Pat. No. 7,719,925, issued May 18, 2010, which claims benefit of U.S. Provisional Application Ser. No. 60/726,774, filed Oct. 14, 2005, the entire contents of each being hereby 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 assembly and a system and method for deploying the assembly, particularly in hydrophone arrays for underwater acoustic sensing of subsurface marine vehicles. 
     2. Description of the Related Art 
     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 that operate by detecting acoustic signals and converting them to electrical impulses that 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. 
     It is desirable to detect not only the presence and magnitude of acoustic signals, but also the direction of the signals. Various directional acoustic sensors are known. Such vector acoustic sensors often employ an accelerometer, which can add to the cost of such equipment. What is needed is a simple and less costly acoustic sensing system, which can also provide directional information about the received acoustic signals. 
     SUMMARY OF THE INVENTION 
     A hydrophone assembly is provided herein, the hydrophone assembly comprising at least four hydrophone units for converting an acoustic signal to an electrical signal, the hydrophone units being parallel, in a cylindrically symmetric spatial relationship with each other, and at least one spacer or semi-rigid exoskeleton element to maintain the hydrophone units fixed in the spatial relationship to each other, wherein said hydrophone units and spacer element are embedded to form an elongated, flexible body. 
     Also provided herein is a system for deploying the hydrophone assembly. 
     The invention herein advantageously provides a means for deploying a directional hydrophone assembly with less chance of twisting the hydrophone wires. The assembly is collapsible into a size A or smaller sonobuoy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments are described below with reference to the drawings wherein: 
         FIG. 1  illustrates the hydrophone deployment system as deployed in a body of water; 
         FIG. 2  illustrates the hydrophone deployment system of the invention loaded in a sonobuoy; 
         FIG. 3  illustrates the hydrophone assembly; 
         FIG. 3   a  is a plan view illustrating the arrangement of the hydrophone units in the assembly; 
         FIG. 4  is a perspective sectional view of an individual hydrophone unit; 
         FIG. 5  is a circuit diagram of a sum and difference amplifier for use in conjunction with the hydrophone assembly according to the present invention; and 
         FIG. 6  are graphs illustrating the sum and difference amplifier response. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIGS. 1 and 2 , the system  100  is shown deployed in a body of water  10  in  FIG. 1  and packaged in a sonobuoy cannister  108  in  FIG. 2 . Deployment from the cannister  108  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 system  100  from the sonobuoy cannister  108  can be accomplished by various means such as the impact of the sonobuoy into the water. Alternatively, a battery  102  (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 assembly  110  to descend to the proper depth. Float  101  remains at the surface of the body of water and preferably includes a transmitter with an antenna and the appropriate battery-powered electronics package  102  for converting the electrical signals from the hydrophones into radio waves for wireless transmission to a remote receiver. Tether  103  of compliant material includes a conductive wire for transmitting electrical signals from preamp electronics package  104  to the float  101 . Typically, the tether  103  includes up to 250 feet of 1/16th inch diameter cable. Preamp electronics package  104  is discussed more fully below with reference to  FIG. 5 . Weight  106  serves to facilitate deployment of the hydrophone system  100  from the sonobuoy  108  and maintain the hydrophone assembly  110  in a vertical orientation once deployed. Hydrophone assembly  110  can be coiled around the preamp electronics package  104  when contained within the sonobuoy to make optimal use of the storage space available in the cannister  108 . The sonobuoy cannister  108  can be size A or smaller. 
     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 additionally now to  FIGS. 3 and 3   a , the hydrophone assembly  110  includes at least four, and optionally five or more, hydrophone units  120  in a parallel array, optionally a non-metallic central wire  112  extending axially through the hydrophone assembly for structural support, and one or more spacer elements  111  through which the hydrophone units  120  and preferably non-metallic central wire  112  are disposed. In the embodiment in  FIG. 3 , spacer element  111  maintains the hydrophone units  120  and central wire  112  in a fixed, parallel, cylindrically symmetric spatial relationship. The central wire  112 , hydrophone units  120  and spacer element  111  are preferably embedded in a pliable encapsulant  113 , which helps secure the hydrophone array against undesirable helical twisting of the hydrophone assembly  110  and torsional misalignment of the hydrophones  120 . Such misalignment causes errors in the directional information perceived and reported by the hydrophone assembly, hence the advantage of the encapsulant  113 , spacer element  111  and optional structural wire  112  as means to prevent such twisting of the hydrophone assembly  110 . Other means to accomplish this same end include fastening the hydrophones at regular intervals to an encompassing hose wall  115  ( FIG. 3   a ), typically fiber reinforced, as an exoskeleton to ensure a preferred shape when not acted upon by external forces. 
     The encapsulant  113 , when used, is preferably a polymeric potting/gelling agent which, when set, has a density-sound speed product similar to that of seawater so as to reduce the chance of modifying the acoustic signal passing through the assembly. The assembly is adapted to be potted as a unit with the hydrophone units  120  held under tension during the potting process until the potting agent is cured to a solid state from the liquid state. Suitable potting agents for use in the present invention include polyurethane “rho c” encapsulants available commercially from BF Goodrich Co. 
     The hydrophone assembly  110  is preferably continuous (i.e., unsegmented) along its entire length. 
     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, operating depth, and the vertical acoustic beamwidth desired. A preferred length for the hydrophone units  120 , depending on the design frequency and environmental conditions, is from about 128 to about 188, where 8 is the wavelength of the target acoustic signal. This length provides a balance between vertical beamwidth (given 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. 1 , L is the length of the hydrophone wires and in a preferred embodiment is 16 8. 
     The diametrical spacing D shown in  FIG. 3   a  between hydrophone units  120   a  and  120   b  in the array can preferably range from about 8/4 to about 8/20, more preferably about 8/10. D is also the spacing between hydrophones  120   c  and  120   d . The optimal spacing is determined to optimize signal strengths without the need for a pre-whitening filter. By way of example, for a design frequency of 10 kHz, L is preferably about 95 inches and D is preferably about 0.984 inches. For a design frequency of 15 kHz, L is preferably about 63 inches, and D is preferably about 0.656 inches. The dimensions given above are for illustrative purposes, and values outside of the range given above can be used when appropriate, for example when optimizing the design to cover a range of frequencies. 
     Various types of acoustic transducers can be used as hydrophones to detect acoustic waves transmitted through the water. For example, the acoustic transducer can comprise a cylindrical 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, 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 flexible piezoelectric polymeric material, such as polyvinylidene difluoride (PVDF), or a piezo-rubber composite material. Piezoresistive materials include, for example, conductive elastomeric polymeric foams or rubbers that 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 electric 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. 4 . 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  108  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 electric 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 electric material layer  124 . Optionally, the conductive layer  123  can be applied after the piezo electric 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 electric 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. 
       FIG. 5  is a schematic diagram of a sum and difference amplifier used in conjunction with the hydrophone assembly according to the present invention. Each hydrophone unit  120  is electrically connected to an electronics package  104  which amplifies the electronic signal from the hydrophones and also calculates differences and sums of the electrical signals, applies a 1/f function, where f is the frequency, to provide more amplification of lower frequency signals than higher frequency signals as part of a weighting function. 
     Referring to  FIGS. 3   a  and  5 , each hydrophone  120   a ,  120   b ,  120   c  and  120   d  is connected to the amplifier and represented in  FIG. 5  by the 500 pf capacitors C 2 , C 6 , C 11  and C 14 , respectively. The capacitors shown in  FIG. 5  are capacitive representations of the hydrophones. For example, the 500 pf capacitors C 2 , C 6 , C 11  and C 14  represent a hydrophone having a length of 48. When the piezoelectric material of the hydrophone elements  120   a - 120   d  is excited by a incident wave in the water, the amplifier of  FIG. 5  receives a signal from each of the hydrophone elements  120   a - 120   d .  FIG. 5  shows this signal as V 2  attached to hydrophone C 2 , and in operation, an incident wave produces a signal on each hydrophone. The amplifier then produces three differential signals, DIFF 1 , DIFF 2  and SUM, from the signals received from the hydrophone elements. DIFF 1  is a differential signal based on the difference between signals received from opposing hydrophones  120   a  and  120   b . DIFF 2  is a differential signal based on the difference between signals received from opposing hydrophones  120   c  and  120   d . SUM is a differential signal based on the summation of signals received from hydrophones  120   a - 120   d . These sum and difference signals are analogous to standard directional frequency and ranging (DIFAR) sonobuoy signals that when digitized and applied to the DIFAR processor&#39;s trigonometric equations, the direction a sound came from can be determined. 
       FIG. 6  are graphs illustrating the sum and difference amplifier response. The response curves are a standard Simulation Program with Integrated Circuits Emphasis (SPICE) model output that assumes either a swept constant amplitude sine wave or white noise. The x-axis on each graph is frequency and the y-axis is voltage in decibels. The upper graph is the difference signal DIFF 1  response and the lower graph is the summation signal SUM response from the sum and difference amplifier of  FIG. 5 . 
     The SUM output is equivalent to the prior art. Compatibility with conventional telemetry and processing is also provided for by the present invention. One main feature of the present invention that differs from a conventional DIFAR sonobuoy is in what is being sensed. A conventional DIFAR sonobuoy senses pressure at a single point and cross-axis motion via accelerometers, whereas the present invention senses virtual pressure at a single point via the SUM channel and cross-axis pressure gradient via the two DIFF channels. Further, the high frequency low-pass filter response corner can be modified by changing the simple capacitor-resistor shunt impedance of capacitor C 5  and resistor R 9 , and capacitor C 13  and resistor R 26 . 
     The electronics package  104  preferably also includes a compass for determining the alignment of the hydrophone assembly  110  by providing a directional magnetic reference signal. In a preferred embodiment of the present invention the compass is a flux gate compass that provides a magnetic North referencing system. 
     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 canister. The float remains on the water surface and the deployable array  110  drops to a predetermined depth, being drawn downward by weight  106 , which maintains the hydrophone assembly in a vertical orientation. 
     Acoustic waves which strike the array are converted by the hydrophone wires  120  into electrical signals which are then processed by the electronics package  104  and carried by a wire connection through the tether  104  to the float and electronics package  102 , finally converted to a radio signal and conveyed by wireless transmission to a remote receiver. 
     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.