Abstract:
The design and method of fabrication for an acoustic sensor module for mounting on the hull of a submarine or surface ship is disclosed. The design involves the use of molded alignment structures within a two part rubber shell to reduce tooling requirements and eliminate a significant amount of assembly labor in fabricating hull mounted sonar arrays.

Description:
FIELD OF THE INVENTION 
     The field to which the invention relates is that of hull mounted acoustic sensor modules for submarines and surface ships. 
     BRIEF SUMMARY OF THE INVENTION 
     This invention relates to a design and fabrication method of a low cost acoustic sensor module for shipboard acoustic sensor arrays. The invention uses molded in alignment features within a rubber shell to align and position acoustic sensors during the assembly process. The use of molded in alignment features eliminates numerous labor intensive steps which would otherwise be required to fabricate the sensor module when positioning the acoustic sensors with external tooling fixtures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 A cutaway side elevation view of the acoustic sensor module. 
     FIG. 2 A side view of the acoustic sensor module of FIG. 1 placed on the hull of a submarine as part of a sensor array. 
     FIG. 3 An exploded cutaway side elevation view of the acoustic sensor module of FIG. 1 showing a configuration of upper and lower bosses used to position an acoustic sensor. 
     FIG. 4 An exploded cutaway side elevation view of the acoustic sensor module of FIG. 1 showing an acoustic sensor positioned on lower bosses. 
     FIG. 5 An exploded side elevation cutaway view of the acoustic sensor module of FIG. 1 showing the connection of an acoustic sensor by a telemetry line through a routing boss to the telemetry module. 
     FIG. 6 A perspective view of a routing boss. 
     FIG. 7 A perspective view of an alternative routing boss. 
     FIG. 8 A perspective view of the interior surface of the lower portion of the protective shell of the acoustic sensor module of FIG. 1 showing the telemetry module vulcanized into the lower portion. 
     FIG. 9 A side elevation perspective view of the acoustic sensor module of FIG. 1 showing the sensors and open channels through the sensor module. 
     FIG. 10 An exploded cross section of an open channel of the acoustic sensor module of FIG. 1 showing the rigid insert within the open channel. 
     FIG. 11 A perspective view of the exterior surface of the lower portion of the acoustic sensor module of FIG. 1 showing the grooves extending radially outward from the open channels. 
     FIG. 12 An exploded view of the lower portion of the acoustic sensor module of FIG.  1  and first fixture. 
     FIG. 13 An exploded view of the upper portion of the acoustic sensor module of FIG.  1  and second fixture. 
     FIG. 14 A side cross-sectional view of the upper portion secured to the second fixture of FIG. 13, and the lower portion secured to the first fixture of FIG. 12, being joined together during fabrication of the acoustic sensor module of FIG.  1 . 
     FIG. 15 A cutaway side elevation view of the acoustic sensor module of FIG. 1 showing the alignment of the molded lower channels with the corresponding upper channels in order to form an open channel from the exterior surface lower portion to the exterior surface of the upper portion. 
     FIG. 16. A perspective view of a telemetry module bottom portion and lid portion. 
     FIG. 17. A perspective view of a shoulder boss. 
     FIG. 18. A perspective view of a boss. 
     FIG. 19. A top view of a splice tray. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is a design and fabrication method for an acoustic sensor module. As shown in FIG. 1, acoustic sensor module  10  comprises a number of acoustic sensors  20  encapsulated within acoustically conductive medium  30  which is surrounded by outer protective shell  40 . Acoustic sensors  20  are connected by telemetry lines  50  to telemetry module  60  located within shell  40 , but which has connector  70  which extends outside of shell  40  allowing for connection between sensors  20  and an external system (not shown) for reading sensors  20 . Acoustic sensor module  10  is mounted on the hull of a submarine or surface ship, often as part of a larger sensor array, as shown in FIG.  2 . 
     A number of different materials can be used in fabricating the invention. In a preferred embodiment a tough rubber material which can be molded, such as nitrile rubber (Buna), is utilized for shell  40 . Acoustic medium  30  used to encapsulate sensors  20  in shell  40  may be a urethane, such as Cortauld&#39;s PRC-1547 or Thorodin Inc.&#39;s NGD-9. Both shell  40  and acoustic medium  30  should have a sound velocity that is less than that of sea water, a density comparable to sea water, be able to be molded, and capable of withstanding marine environments. 
     The encapsulation of sensors  20  in acoustic medium  30  requires precise positioning tolerances (on the order of 0.025 inches in any direction). In the present invention, positioning of sensors  20  for encapsulation is accomplished by features molded onto interior surfaces of shell  40 . 
     As shown in FIG. 3, each sensor  20  is positioned within the inner volume  240  surrounded by shell  40  by a set of lower bosses  140  and upper bosses  130  molded on the interior surfaces of shell  40 . Shell  40  is composed of upper portion  120  and lower portion  110 . In fabricating lower portion  110  lower bosses  140  are molded into interior surface  90 . Likewise, upper bosses  130  are molded into interior surface  100  of upper portion  120 . Upper bosses  130  and lower bosses  140  are molded with high dimensional tolerances necessary to position sensors  20  within inner volume  240 . 
     A preferred embodiment of the invention is shown in FIG. 4, where each sensor  20  is positioned within inner volume  240  between three (3) upper bosses  130  and four (4) lower bosses  140 . In this preferred embodiment one of the four (4) lower bosses  140 , a shoulder boss  140 ′, is molded in a shape adapted to join with opening  160  in sensor  20  to form an interference fit. One possible shape of shoulder boss  140 ′ is shown in FIG.  17 . The interference fit may be facilitated by a threaded interior surface of opening  160 . The remaining six (6) upper bosses  130  and lower bosses  140  are preferably hemispherical in shape, as shown for example in FIG. 18, and press against upper surface  170  and lower surface  180  of sensor  20  to precisely locate sensor. An alternate embodiment includes hemispherical depressions  131  in upper surface  170  and/or lower surface  180  that mate with upper bosses  130  and lower bosses  140  respectively, as shown for example in FIG.  4 . Shoulder boss  140 ′ limits horizontal travel of sensor  20  prior to curing of acoustic medium  30 , with the other upper bosses  130  and lower bosses  140  limiting tilting of sensor  20 . The shoulder of shoulder boss  140 ′ is located below the final resting position of lower surface  180  of sensor  20  such that shoulder boss  140 ′ defines only the transverse location of sensor  20 , with remaining lower bosses  140  defining the vertical displacement of sensor  20  from interior surface  90  of lower portion  110 . An alternative embodiment of the invention would use three (3) lower bosses  140  and four (4) upper bosses  130 , with shoulder boss  140 ′ being molded into the interior surface  100  of upper portion  120 . 
     An additional feature of a preferred embodiment of the invention are routing bosses  190 , as shown for example in FIG.  5 . Routing bosses  190  are molded features in interior surface  90  of lower portion  110  of shell  40  used to route telemetry lines  50  between sensors  20  and telemetry module  60 . Routing bosses  190  may also be molded features in interior surface  100  of upper portion  120 . Telemetry lines  50  may be comprised of buffered or cabled optical fiber, copper wire cable, or a combination thereof, depending upon the transduction mechanism of the sensor. Telemetry module  60  is preferably a pressure-barrier enclosure containing optical couplers and/or optical amplifiers and optical fiber splices. It may also contain amplification and multiplexing electronics if electrical or piezoelectric sensors are used. FIG. 6 shows, for example, a preferred shape of a routing boss  190 . FIG. 7 shows an alternative shape of routing boss  190 . A number of shapes may be used for routing boss  190 , with the shape and size varying with the particular type, number and size of telemetry lines  50  being routed, and the details of the injection molding process used. 
     Telemetry module  60  has a bottom portion  62  and a lid portion  64 , as shown for example in FIG.  16 . Lid portion  64  and bottom portion  62  contain openings  61 , possibly threaded, for receiving fasteners  63  (such as screws) for joining bottom portion  62  to lid portion  64  after telemetry lines  50  have been connected. Bottom portion of  62  of telemetry module  60  is preferably vulcanized into interior surface  90  of lower portion  110  of the shell during fabrication of the lower portion, as shown for example in FIG.  8 . Alternatively, telemetry module  60  may be vulcanized or otherwise placed into interior surface  100  of upper portion  120 . Access hole  65  in telemetry module bottom portion  62  may be used for routing of telemetry lines  50 . Access hole  67  may be molded into lower portion  110  of shell  40  to facilitate location of multiple pin fiber optic connector as shown for example in FIG.  16 . However, alternative means of positioning telemetry module  60  within lower portion  110  may be used by those practicing the invention. One such example would be molding a slot into interior surface  90  of lower portion  110  for receiving bottom portion  62  of telemetry module  60 . In a preferred embodiment, telemetry module  60  contains a stack  66  of splice trays  71  and a coupler housing  68  within a cavity  69  contained within bottom portion  62 . The splice trays  66 , contain a series of clips  81  and overhanging projections  82  used to contain and organize splices, which may number around one hundred, and the associated optical fiber leads in a manner consistent with rapid replacement of failed splices and couplers following manufacture. The splice tray stack  66  has a lid  72  for protection of the assembly during handling. An example of a splice tray is shown in FIG.  19 . 
     FIG.  9 . shows, for example, another preferred embodiment of the invention which utilizes open channels  200  through sensor module  10  to facilitate installation of sensor module  10  onto the hull of a ship. Open channels  200  are comprised of lower channels  310  molded into lower portion  110  and upper channels  320  molded into upper portion  120 , as shown for example in FIG.  10 . Open channels  200  provide a means to attach sensor module  10  to a hull with fasteners (such as bolts) which pass through open channels  200 . Open channels  200  are molded into shell  40  and may have an inner reinforcement piece  210  (such as a titanium tube), as shown, for example, in FIG.  10 . 
     Open channels  200  may also be used for installing sensor module  20  to a hull with an adhesive. Bonding to the hull is accomplished by applying an adhesive coating to exterior surface  230  of lower portion  110  and the hull. Lower portion  110  of sensor module  10  is positioned against the hull, and a vacuum is drawn through open channels  200 . This will result in sensor module  10  being securely “pressed” or “drawn” against the hull while the adhesive cures. When using such a vacuum method of installation it may be desirable to have grooves  220  in exterior surface  230  of lower portion  110  which extend radially outward from channels  200 , as illustrated, for example, in FIG.  11 . Grooves  220  extending outward from channels  200  increase the surface area between sensor module  10  and the hull of the ship, thereby distributing the vacuum over a wider area to hold module  10  against the hull. Pressure injection of adhesive may also be used to attach module  10  to hull with the use of periodic standoffs between the module  10  and hull. Open channels  200  can be used to facilitate this method by serving as conduits or vents for acoustic medium  30 . A combination of fasteners and adhesive may also be used, with some open channels  200  being occupied by fasteners (such as bolts), and the remainder being used as vacuum lines. 
     One of the primary advantages of this invention is that it may be fabricated inexpensively and with minimum labor. A first step in fabricating the invention is the fabrication of lower portion  110  and upper portion  120  of shell  40 . As stated above, the upper portion  120  and lower portion  110  may be made of a tough rubber material capable of being molded. Upper bosses  130 , lower bosses  140 , routing bosses  190 , open channels  200 , grooves  220  and telemetry module  60  may all be molded into the interior and exterior surfaces of upper portion  120  and/or lower portion  110  of shell  140 . The result will be upper portion  120  and lower portion  110  with the desired, or necessary, molded features used for (1) positioning sensors  20  within inner volume  240  of shell  40 , (2) routing telemetry lines  50 , (3) injecting acoustic medium  30  into inner volume  240  to encapsulate sensors  20 , and (4) installing finished sensor module  10  onto the ship hull. By having all of these features molded into upper portion  120  and lower portion  110  of shell  40 , the need for multiple tooling sets is eliminated, greatly reducing cost. Fabrication labor is also greatly reduced. 
     Once lower portion  110  of shell  40  is fabricated, lower portion  110  is secured by its exterior surface  230  onto first fixture  250 , as shown for example in FIG.  12 . First fixture  250  may be flat, however in a preferred embodiment of the invention the surface of first fixture  250 , upon which lower portion  110  is placed, is curved as shown in FIG.  12 . The curvature of the first fixture  250  surface should match the curvature of the hull section to which sensor module  10  will be attached. Integral to first fixture  250  are vacuum lines  252  through which a vacuum is drawn to secure lower portion  110  to first fixture  250  during assembly and the injection molding process. Conductive heating elements  254  may be included in the interior of the fixture  250  to provide elevated temperature to acoustic medium  30  during curing. 
     Once lower portion  110  is secured to first fixture  250  sensors  20  are positioned onto molded lower bosses  140 . It is anticipated that positioning of sensors  20  is done manually, although this does not preclude the use of automation to position sensors  20  onto lower bosses  140  if this is desired or necessary. As shown, for example, in FIG. 3, in a preferred embodiment of the invention there is at least one molded shoulder boss  140 ′ for each sensor  20  adapted for joining with sensor  20  by an interference fit. The interference fit may be achieved by providing a suitable threaded opening  160  in sensor  20  and inserting at least a portion of shoulder boss  140 ′ into opening  160 . The portion of shoulder boss  140 ′ inserted into opening  160  should be of a diameter to fill opening  160  such that sufficient friction forces will exist between shoulder boss  140 ′ and the walls of opening  160  to prevent accidental removal of sensor  20 . The interference fit between shoulder boss  20  and opening  160  is preferably located near the centerline of sensor  20 , with the periphery of sensor  20  being supported by appropriately positioned molded lower bosses  140  to prevent the tilting of sensor  20 . In a preferred embodiment shown in FIG. 4, three (3) lower bosses  140  are located at 120° angles from one another, with shoulder boss  140 ′ being located at the center of sensor  20 . Other configurations may be used as needed depending upon the particular sensor  20  being used. 
     Once the desired number of sensors  20  are positioned within lower portion  110 , sensors  20  are connected to telemetry module  60  by telemetry lines  50 . The particular number and type of telemetry lines  50  between each sensor  20  will vary depending upon the type of sensors  20  and telemetry being used. In a preferred embodiment of the invention the telemetry lines  50  are optical fibers surrounded by a protective jacket, with an input fiber and an output fiber for each sensor. Other types of telemetry lines, such as electrical, may also be used either alone or in combination. 
     In a preferred embodiment of the invention the telemetry lines  50  between sensors  20  and telemetry module  60  are routed using molded routing bosses  190  in interior surface  90  of lower portion  110 . The use of routing bosses  190  helps to prevent movement of telemetry lines  50  during injection of acoustic medium  30  into inner volume  240 , by an interference fit between telemetry lines  50  and routing boss  190 , and thus helps to ensure uniform encapsulation. There is no particular shape required for routing bosses  190 , the shape varying with the application. Some preferred examples are shown in FIG.  6  and FIG.  7 . It is anticipated that telemetry lines  50  will be manually placed in, on, or through routing bosses  190  as the case may be, although this does not preclude the use of automation. 
     After telemetry lines  50  have been connected to telemetry module  60 , telemetry module  60  is sealed to protect the internal workings. The sealing of telemetry module  60  may be accomplished by bottom portion  62  having been machined with sufficient flatness such that fastening lid portion  64  to bottom portion  62  using common fasteners  63  such as screws will provide a seal during injection molding with acoustic medium  30 . Alternatively, sealing may be accomplished by compressing rubber o-ring seals into glands, application of an adhesive bond joint, or a combination thereof. Telemetry line port  65  and connector port  67  are both sealed by potting with an adhesive prior to fastening lid portion  64  to bottom portion  62  of telemetry module  60 . 
     Once sensors  20  have all been positioned within lower portion  110 , connected to the telemetry module  60  by telemetry lines  50 , and telemetry module  60  has been sealed, upper portion  120  of shell  40  is secured by its exterior surface  270  onto second fixture  280 , as shown in FIG.  13 . Second fixture  280  may be flat, however in a preferred embodiment of the invention the interior surface  290  of second fixture  280 , upon which upper portion  120  is placed, is curved as shown in FIG.  13 . The curvature of second fixture  280  surface should match the curvature of first fixture  250  which as mentioned above corresponds to the curvature of the hull to which the sensor module  10  will be attached. Integral to second fixture  280  are vacuum lines  285  through which a vacuum is pulled to secure upper portion  120  to second fixture  280 . Conductive heating elements may be included within fixture  280  to facilitate elevated temperature during curing. 
     The interior surface  100  of upper portion  120  is next coated with acoustic medium  30 . Upper portion  120  is placed over and in contact with lower portion  110  such that shell  40  forms and surrounds inner volume  240  as shown in FIG.  14 . The placement of the upper portion  120  over lower portion  110  is also such that sensors  20  within inner volume  240  are located between upper bosses  130  and lower bosses  140  as shown in FIG.  14 . As shown in FIG. 4, the preferred embodiment has three (3) upper bosses  130  disposed around the upper periphery of sensor  20  at locations approximately 120° apart. Other configurations may be used as needed depending upon the particular sensor  20  being used. 
     If open channels  200  in sensor module  10  are used (and have therefore been molded into lower portion  110  and upper portion  120 ), placement of upper portion  120  over lower portion  110  also requires that each lower channel  310  (molded into lower portion  110 ) be aligned to join coextensively with its corresponding upper channel  320  (molded into upper portion  120 ), such that there exists an open channel  200  between exterior surface  270  of upper portion  120  and exterior surface  230  of lower portion  110 . This is illustrated in FIG.  15 . 
     After upper portion  120  and lower portion  110  are joined to form shell  40 , acoustic medium  30  is injected into inner volume  240  using standard injection molding techniques until acoustic medium  30  occupies all empty space within inner volume  240 . In a preferred embodiment of the invention vacuum lines  252  and pressure lines  285 , forming part of first fixture  250  and second fixture  280  respectively, are used to first evacuate inner volume  240  and then to inject acoustic medium  30  into inner volume  240  under pressure to minimize the size of bubbles created by any entrapped air within inner volume  240 . In order to ensure that upper portion  120  and lower portion  110  do not become detached from their respective fixtures it is desirable that the vacuum within inner volume  240  not exceed the vacuum used to secure upper portion  120  and lower  110  to their respective fixtures. 
     Once acoustic medium  30  has completely occupied inner volume  240 , encapsulating sensors  20 , telemetry lines  50  and sealed telemetry module  60 , acoustic medium  30  is cured under pressure so as to form a solid and bond upper portion  120  to lower portion  110 . In a preferred embodiment of the invention the curing process is accomplished by the application of heat from heating elements which are an integral part of first fixture  250  and second fixture  280 . An alternative embodiment utilizes an oven in which the module is placed with its fixtures to cure acoustic medium  30 . Upper portion  120  may be bonded to lower portion  110  first by allowing acoustic medium  30  to cure, followed by injection of acoustic medium  30  into inner volume  240 . 
     In an alternative process of fabricating the invention, interior surface  290  of second fixture  280  is provided with an inert non-stick coating (such as Teflon(®) which tends not to bond with acoustic medium  30  during the curing process. The second fixture  280  is placed on the first fixture  250  forming a temporary “upper portion” of shell  40  with lower portion  110 , as shown in FIG.  16 . Standoffs of a cured material similar to acoustic medium  30  may be placed between sensors  20  and second fixture  280  to ensure proper location of sensors  20  between second fixture  280  and lower portion  110 . Acoustic medium  30  is injected into inner volume  240 , contained between lower portion  110  and second fixture  280 , by way of vacuum port  252  in first fixture  250  and  285  in second fixture  280 . Acoustic medium  30  is then cured. After the curing process is complete, and acoustic medium  30  has solidified within inner volume  240 , second fixture  280  is removed from sensor module  10 . This facilitates inspection of the now solidified acoustic medium  30  to ensure quality (i.e. uniformity of fill, no air bubbles which will adversely affect sensor module performance etc). Upper portion  120  is then installed onto second fixture  280 . Interior surface  100  of upper portion  120  is then coated with acoustic medium  30 , and upper portion  120  attached to second fixture  280  is placed back onto cured acoustic medium  30  and lower portion  110  attached to first fixture  250 . The curing process is then repeated so as to bond upper portion  120  to acoustic medium  30  and lower portion  110  to form a completed acoustic sensor module  10 . 
     After curing, sensor module  10  is complete and may be removed from first fixture  250  and second fixture  280 . However, the present invention contemplates that sensor module  10  may remain in the fixtures for transportation, handling, storage and even installation. What follows is a glossary of terms to be used as an aid in the understanding of the disclosure and claims. 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 GLOSSARY OF TERMS 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Shell 
                 Any object, which at least partially 
               
               
                   
                   
                 surrounds an inner volume, and which 
               
               
                   
                   
                 may be comprised of a plurality of 
               
               
                   
                   
                 parts. 
               
               
                   
                 Inner volume 
                 A region substantially or completely 
               
               
                   
                   
                 surrounded by a shell. 
               
               
                   
                 Sensor 
                 A device that responds to a physical 
               
               
                   
                   
                 stimulus (for example heat, light, sound, 
               
               
                   
                   
                 pressure, magnetism or a particular 
               
               
                   
                   
                 motion) and transmits a resulting signal 
               
               
                   
                   
                 (as for measurement or control), or a 
               
               
                   
                   
                 device for telemetry, signal conduction, 
               
               
                   
                   
                 signal processing, signal amplification, 
               
               
                   
                   
                 or the like. 
               
               
                   
                 Boss 
                 A solid extension or protrusion from a 
               
               
                   
                   
                 surface, such as an interior surface of a 
               
               
                   
                   
                 shell. 
               
               
                   
                 Telemetry line 
                 An object of a fixed length made of a 
               
               
                   
                   
                 material, or materials, which can carry 
               
               
                   
                   
                 power and energy signals to and from a 
               
               
                   
                   
                 sensor device, and which may include, 
               
               
                   
                   
                 for example, electrical wires or optical 
               
               
                   
                   
                 flbers. 
               
               
                   
                 Telemetry Module 
                 A device to which telemetry lines may 
               
               
                   
                   
                 be connected and through which signals 
               
               
                   
                   
                 from telemetry lines may pass to other 
               
               
                   
                   
                 telemetry lines. It may include optical 
               
               
                   
                   
                 couplers and fiber splices, optical 
               
               
                   
                   
                 amplifiers, electronic signal 
               
               
                   
                   
                 conditioning and/or multiplexing 
               
               
                   
                   
                 circuitry. 
               
               
                   
                 Connector 
                 Any device used to receive an electrical 
               
               
                   
                   
                 or optical signal and to transmit the 
               
               
                   
                   
                 signal with, or without, amplification or 
               
               
                   
                   
                 modification. 
               
               
                   
                 Routing Boss 
                 A boss of a configuration adapted to 
               
               
                   
                   
                 support at least one telemetry line. 
               
               
                   
                 Interference Fit 
                 A joint between two objects where the 
               
               
                   
                   
                 objects are prevented from moving in 
               
               
                   
                   
                 relation to one another by forces of 
               
               
                   
                   
                 friction. 
               
               
                   
                 Upper channel 
                 An opening or perforation in the upper 
               
               
                   
                   
                 portion of a shell. 
               
               
                   
                 Lower channel 
                 A hollow protrusion which extends 
               
               
                   
                   
                 from an opening or perforation in the 
               
               
                   
                   
                 lower portion of the shell to an open 
               
               
                   
                   
                 end above the interior surface of the 
               
               
                   
                   
                 lower portion. 
               
               
                   
                 Open channel 
                 An unobstructed passage between the 
               
               
                   
                   
                 exterior surface of the upper portion of 
               
               
                   
                   
                 the shell and the exterior surface of the 
               
               
                   
                   
                 lower portion of the shell. 
               
               
                   
                 Groove 
                 A depression in an exterior surface of 
               
               
                   
                   
                 the shell. 
               
               
                   
                 Upper boss 
                 A solid extension or protrusion from the 
               
               
                   
                   
                 interior surface of the upper portion of 
               
               
                   
                   
                 the shell. 
               
               
                   
                 Lower boss 
                 A solid extension or protrusion from the 
               
               
                   
                   
                 interior surface of the lower portion of 
               
               
                   
                   
                 the shell. 
               
               
                   
                 Acoustic medium 
                 Any material with an acoustic 
               
               
                   
                   
                 impedance. 
               
               
                   
                 Fixture 
                 A device to which something may be 
               
               
                   
                   
                 attached. 
               
               
                   
                 Non-stick coating 
                 Any material or substance which tends 
               
               
                   
                   
                 not to form a bond with an adhesive or 
               
               
                   
                   
                 the acoustic medium. 
               
               
                   
                 Standoff 
                 A solid element used to control spacing 
               
               
                   
                   
                 between at least two objects, such as 
               
               
                   
                   
                 between second fixture 280 and sensors 
               
               
                   
                   
                 20. 
               
               
                   
                 Surrounds 
                 To at least partially bound a volume.