Patent Publication Number: US-11642702-B2

Title: Self-cleaning optical housing arrangement

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 371 of PCT/NO2020/050042 filed on Feb. 18, 2020, published on Sep. 24, 2020 under publication number WO 2020/190144 A1, which claims priority benefits from Norwegian Patent Application No. 20190363 filed Mar. 21, 2019, both of which are incorporated herein in their entirety by reference. 
     TECHNICAL FIELD 
     The invention relates to a self-cleaning optical housing arrangement for underwater optical systems that utilizes a mechanical cleaning process in order to avoid biofouling. The housing arrangement comprises a first cylindrical shell, a shaft, a motor, a second cylindrical shell element, a cleaning pad, a shaft magnet and a cylinder magnet. 
     BACKGROUND 
     Biofouling is considered a limiting factor when performing ocean monitoring using permanent or long time installations. Optical structures immersed in seawater will under normal conditions be rapidly covered by biofouling, making them useless until cleaned. Systems used for high resolution or high intensity monitoring is particularly sensitive to any optical disturbances, motivating the search for solutions for how to avoid biofouling. 
     The most common method for avoiding biofouling on optical systems underwater is to add copper material as close to the optical window as possible. This method limits the biofouling, at least for a limited time. Copper is considered potentially toxic, and is therefore prohibited in many situations. 
     Another more recently developed technique for avoiding biofouling is the use of a nanostructured surface on optical windows. Tear will however after some time remove the nanostructures, and the effect is reduced. The surface will then have to be recoated, a process that for many optical systems is either expensive or impossible to conduct. 
     It is the aim of the present invention to provide an improved solution for how to reduce the problem with biofouling on underwater optical installations. 
     SUMMARY OF THE INVENTION 
     The invention provides a self-cleaning optical housing arrangement comprising a first cylindrical shell, comprising a first transparent portion, a shaft, arranged with a shaft axis of rotation along the first cylindrical shell central axis, a motor, connected to the first cylindrical shell and to the shaft, where the motor is configured to rotate the shaft relative to the first cylindrical shell, a second cylindrical shell element, arranged at least in part around the first cylindrical shell and configured to rotate around the first cylindrical shell, the second cylindrical shell element is provided with a second transparent portion configured to be at least in part aligned with the first transparent portion, a cleaning pad, connected to the second cylindrical shell element, and arranged at least in part between the first cylindrical shell and the second cylindrical shell element so that it physically contacts the first cylindrical shell outer surface, a shaft magnet, connected to the shaft, a cylinder magnet, connected to the second cylindrical shell element, where the shaft magnet and the cylinder magnet are arranged in order to magnetically interact such that a rotation of the shaft causes the shaft magnet to exert a force on the cylinder magnet that further causes the second cylindrical shell element to rotate with the shaft, and where, the cleaning pad, upon rotation of the shaft and the second cylindrical shell element, sweeps across, and thereby cleans, at least a part of the outer surface of the first transparent portion. 
     The optical housing arrangement comprises according to one embodiment of the invention a first cap arranged on a first end of the first cylindrical shell such that it forms a watertight seal with the first cylindrical shell. 
     According to another embodiment of the invention the motor is connected with the first cylindrical shell via the first cap. 
     The optical housing arrangement comprises according to yet another embodiment of the invention a second cap, arranged on a second end of the first cylindrical shell such that it forms a watertight seal with the first cylindrical shell, and a bearing connected to the second cap and to the shaft, configured to secure the shaft to the second cap while allowing the shaft to rotate. 
     The second transparent portion is according to yet another embodiment of the invention an opening. 
     According to yet another embodiment of the invention the inner volume of the first cylindrical shell is partitioned by one or more essentially non-transparent walls. 
     The first cylindrical shell comprises according to yet another embodiment of the invention a plurality of cylindrical shell segments connected with each other, where the plurality of cylindrical shell segments are partitioned by one or more essentially non-transparent walls. 
     The optical housing arrangement further comprises according to yet another embodiment of the invention an optical sensor, configured to detect light from outside the optical housing arrangement, and a light emitting device, configured to illuminate light out of the optical housing arrangement, where the optical sensor and the light emitting device are connected to the shaft and are arranged in the inner volume of the first cylindrical shell. 
     The optical housing arrangement further comprises according to yet another embodiment of the invention an optical sensor, configured to detect light from outside the optical housing arrangement, and a light emitting device, configured to illuminate light out of the optical housing arrangement, where the optical sensor and the light emitting device are connected to the shaft, are arranged in the inner volume of the first cylindrical shell and are separated by an essentially non-transparent wall. 
     The shaft comprises according to yet another embodiment of the invention a plurality of parts connected with each other. 
     The optical housing arrangement further comprises according to yet another embodiment of the invention a plurality of shaft magnets, each connected to the shaft, and a plurality of a cylinder magnets, each connected to the second cylindrical shell element, where the shaft magnets and the cylinder magnets are arranged in order to magnetically interact such that a rotation of the shaft causes the shaft magnets to exert a force on the cylinder magnets that further causes the second cylindrical shell element to rotate with the shaft. 
     The first cylindrical shell consists according to yet another embodiment of the invention a transparent material. 
     The cleaning pad has according to yet another embodiment of the invention an elongated shape and where the second cylindrical shell element has an elongated slot with a shape suitable for receiving the cleaning pad. 
     The optical housing arrangement further comprises according to yet another embodiment of the invention a first and a second cleaning pad magnet arranged in the cleaning pad, and a first and a second mounting magnet, arranged adjacent to the elongated slot, where the first and second mounting magnets are configured to exert a force on the first and a second cleaning pad magnets respectively such that the cleaning pad is pressed against the outer surface of the first cylindrical shell. 
     According to yet another embodiment of the invention the first and second cleaning pad magnets are respectively arranged in a first and a second distal end of the cleaning pad, where the first and second mounting magnet are respectively arranged adjacent to a first and second distal end of the elongated slot. 
     Other advantageous features will be apparent from the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to make the invention more readily understandable, the discussion that follows will refer to the accompanying drawings, in which: 
         FIG.  1   a    is a schematic representation of a self-cleaning optical housing arrangement comprising a first cylindrical shell, a shaft, a motor, a second cylindrical shell element and a cleaning pad, 
         FIG.  1   b    is a schematic representation of a self-cleaning optical housing arrangement comprising a first cylindrical shell, a shaft, a motor, a second cylindrical shell element and a cleaning pad, 
         FIG.  2    is a schematic representation of a self-cleaning optical housing arrangement comprising a first cylindrical shell, a shaft, a motor, a second cylindrical shell element and a cleaning pad, a shaft magnet and a cylinder magnet, 
         FIG.  3    is a schematic representation of a self-cleaning optical housing arrangement comprising a first cap, 
         FIG.  4    is a schematic representation of a self-cleaning optical housing arrangement comprising a second cap, and a bearing, 
         FIG.  5    is a schematic representation of a self-cleaning optical housing arrangement where the second transparent portion is an opening, 
         FIG.  6    is a schematic representation of a self-cleaning optical housing arrangement where the inner volume of the first cylindrical shell is partitioned by two essentially non-transparent walls, 
         FIG.  7    is a schematic representation of a self-cleaning optical housing arrangement where the first cylindrical shell comprises a plurality of cylindrical shell segments, 
         FIG.  8    is a schematic representation of a self-cleaning optical housing arrangement where the shaft comprises a plurality of parts, 
         FIGS.  9   a  and  9   b    are schematic representations of a self-cleaning optical housing arrangement comprising an optical sensor and a light emitting device, 
         FIG.  10    is a schematic representation of a self-cleaning optical housing arrangement where the first cylindrical shell consists of a transparent material, and 
         FIG.  11    is a schematic representation of a section of a self-cleaning optical housing arrangement where the cleaning pad has an elongated shape and where the second cylindrical shell element has an elongated slot. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following, general embodiments as well as particular exemplary embodiments of the invention will be described. References will be made to the accompanying drawings. It shall be noted, however, that the drawings are exemplary embodiments only, and that other features and embodiments may well be within the scope of the invention as claimed. 
     The present invention provides an optical housing arrangement suitable for use underwater and for housing various optical equipment, e.g. optical sensors, cameras, detectors, light sources, lenses, polarization filters, etc. 
     The housing arrangement  100  comprises as schematically illustrated in  FIGS.  1  and  2    a first cylindrical shell  110 , a shaft  130 , a motor  160 , a second cylindrical shell element  170 , a cleaning pad, a shaft magnet  210  and a cylinder magnet  220 . The housing arrangement  100  is sized such that it may house various optical equipment inside the inner volume  290  of the first cylindrical shell  110 . The first cylindrical shell  110  further comprises a first transparent portion  120  in order to enable electromagnetic communication, e.g. visual communication, between its inner volume  290  and its exterior. The first transparent portion  120  may for example be a window made from one or a combination of transparent materials, such as soda-lime glass, silica, a transparent plastic, or any other suitable material. The inner volume  290  of a cylindrical shell is according to the invention the cylindrical volume of the cylindrical shell. This should not be confused by the volume of the shell itself, i.e. the volume of the shell part of a cylindrical shell. 
     As illustrated in  FIGS.  1  and  2   , the optical housing arrangement  100  further comprises a shaft  130  and a motor  160 , where the shaft  130  is arranged with a shaft axis of rotation  140  along the first cylindrical shell central axis  150 . The motor  160  is connected to both the shaft  130  and to the first cylindrical shell  110  such that it can rotate the shaft  130  relative to the first cylindrical shell  110 . The shaft  130  does not have to be arranged exactly along the first cylindrical shell central axis  150 , and the term “arranged with a shaft axis of rotation  140  along” may thus in the context of the invention be interpreted as sufficiently along so as to allow the shaft  130  to rotate at least in part relative to the first cylindrical shell  110 . A motor may in the context of the present invention be considered as a motor assembly. A motor assembly may here comprise a motor, e.g. an electrical motor, and optionally one or more elements such as a gear mechanism, electrical power delivery coupling, transmission shaft, torque transfer mechanism, and/or other elements suitable for connecting the motor  160  to the shaft  130 . 
     The second cylindrical shell element  170  is as illustrated in  FIGS.  1  and  2    arranged at least in part around the first cylindrical shell  110  and configured to be rotated around the first cylindrical shell  110 . The second cylindrical shell element  170  may be interpreted as being arranged around the first cylindrical shell  110  and configured to being rotated at least in part around the first cylindrical shell  110 . The second cylindrical shell element may in the context of the invention be interpreted as a full cylindrical shell, such as illustrated in  FIG.  1     a , or a fraction of a cylindrical shell, e.g. as illustrated in  FIG.  1     b.    
     Any cylindrical shell or cylindrical shell element may in the context of the present invention be considered as essentially cylindrical. The structure of the optical housing arrangement will tolerate a deviation from a perfect cylindrical shapes as long as the second cylindrical shell element is shaped such that it may rotate at least in part around the first cylindrical shell. The first cylindrical shell and second cylindrical shell element may thus in the context of the present invention be considered as sufficiently cylindrically shaped so as to allow for the second cylindrical shell element to rotate around the first cylindrical shell. 
     The second cylindrical shell element  170  comprises as illustrated in  FIG.  2    a second transparent portion  180 . This second transparent portion  180  may be aligned with the first transparent portion  120  of the first cylindrical shell  110  in order to enable electromagnetic communication, e.g. visual communication, between the inner volume  290  of the first cylindrical shell  110  and the exterior of the second cylindrical shell element  170 . The alignment of the first transparent portion  120  and the second transparent portion  180  may be achieved e.g. upon rotation of the second cylindrical shell element  170  around the first cylindrical shell  110 . The second transparent portion  180  may be a window made from one or a combination of transparent materials, such as soda-lime glass, silica, a transparent plastic, or any other suitable material. 
     A cleaning pad is as illustrated in  FIGS.  1  and  2    connected to the second cylindrical shell element  170 . The cleaning pad can here be seen as arranged at least in part between the first and second cylindrical shell elements, such that it physically contacts the first cylindrical shell outer surface  200 . The cleaning pad will thus upon rotation of the second cylindrical shell element  170 , sweep across, and consequently clean, the outer surface  200  of the first cylindrical shell  110 . Any number of cleaning pads may in principle be used. The material of the cleaning pad may in principle be any type of material suitable for cleaning the outer surface of the first transparent portion when moved across this surface. The cleaning pad may for example comprise a sponge, fibre cloth, wiper or a textured rubber. 
       FIG.  2    illustrates a schematic illustration of the self-cleaning optical housing arrangement  100  that comprises a shaft magnet  210  connected to the shaft  130 , and a cylinder magnet  220  connected to the second cylindrical shell element  170 . A shaft magnet  210  may in the context of the invention be considered as any magnet connected to the shaft  130 , and may be e.g. a permanent magnet, a temporary magnet or an electromagnet. A cylinder magnet  220  may in the context of the present invention be considered as any magnet connected to the second cylindrical shell element  170 , and may be e.g. a permanent magnet, a temporary magnet or an electromagnet. 
     At least one shaft magnet  210  and at least one cylinder magnet  220  are according to the invention arranged such that they magnetically interact with each other, either through attractive or repulsive forces. Such interaction may be achieved as visualized in  FIG.  2   , where a shaft magnet  210  is positioned adjacent to a cylinder magnet  220  in a distance from the shaft  130 . The interaction between the shaft magnet  210  and the cylinder magnet  220  will according to one embodiment of the invention be such that a rotation of the shaft  130  causes the shaft magnet  210  to exert a force on the cylinder magnet  220  that further causes the second cylindrical shell element  170  to rotate with the shaft  130 . The magnetic interaction between the magnets is contactless, and enables the motor  160  to rotate the second cylindrical shell element around the first cylindrical shell  110  by rotating the shaft  130 . 
     A rotation of the second cylindrical shell element  170  around the first cylindrical shell  110  causes the cleaning pad to sweep across, and consequently clean the outer surface  200  of the first cylindrical shell  110 . The cleaning pad may as illustrated in  FIG.  1    be aligned such that it may sweep across, and consequently clean at least a part of the outer surface of the first transparent portion  120  of the first cylindrical shell  110 . 
     The second cylindrical shell element  170  is according to the invention arranged at least in part around the first cylindrical shell and configured to be rotated around the first cylindrical shell. The second cylindrical shell element  170  may be arranged adjoining, adjacent to, or at a non-zero distance from the first cylindrical shell, and may be held in place by magnets or by any suitable support structure. The second cylindrical shell element  170  may upon rotation around the first cylindrical shell simply slide over the first cylindrical shell, or alternatively be supported by some support structure as it rotates around the first cylindrical shell. 
       FIG.  2    illustrates an embodiment of the invention where the optical housing arrangement  100  comprises a plurality of shaft magnets  210  and a plurality of cylinder magnets  220 . The shaft magnets  210  and the cylinder magnets  220  are here arranged in order to magnetically interact such that a rotation of the shaft  130  causes the shaft magnets  210  to exert a force on the cylinder magnets  220  that further causes the second cylindrical shell element  170  to rotate with the shaft  130 . The use of a plurality of shaft magnets  210  and cylinder magnets  220  may be beneficial in order to achieve e.g. a more even rotation force, more torque, and a more precise alignment of the second cylindrical shell element  170  relative to the first cylindrical shell  110 . The plurality of shaft magnets  210  and cylinder magnets  220  may according to this embodiment of the invention be arranged in any suitable fashion. They may e.g. be arranged symmetrically around the shaft axis of rotation  140 . 
       FIG.  3    illustrates an embodiment of the invention where the optical housing arrangement  100  further comprises a first cap  230  arranged on a first end  240  of the first cylindrical shell  110 . The first cap  230  forms a watertight seal with the first cylindrical shell  110 , where the seal e.g. may be achieved by using a gasket, weld or any other suitable watertight sealing element/method. The first cap  230  may alternatively be an integral part of the cylindrical shell. The motor  160  may in this embodiment of the invention be connected with the first cylindrical shell  110  via the first cap  230 . This is illustrated in  FIG.  3   , where the motor  160  can be seen as located inside the first cylindrical shell  110  and as adjoining the first cap  230 . The first cap  230  may additionally support, and hold in place the second cylindrical shell element  170  while allowing it to rotate relative to the first cylindrical shell  110 . 
       FIG.  4    illustrates an embodiment of the invention where the optical housing arrangement  100  further comprises a second cap  250  arranged on a second end  260  of the first cylindrical shell  110 . The second cap  250  forms a watertight seal with the first cylindrical shell  110 , where the seal e.g. may be achieved by using a gasket, weld or any other suitable watertight sealing element/method. The second cap  250  may alternatively be an integral part of the cylindrical shell. The second cap  250  may further be fitted with a bearing  270  suitable for receiving the shaft  130  in a manner that allows the shaft  130  to rotate, such as e.g. a ball bearing. The bearing  270  here also acts as a fastening point that secures the shaft  130  to the second cap  250  while allowing the shaft  130  to rotate. The second cap  250  may additionally support, and hold in place the second cylindrical shell element  170  while allowing it to rotate relative to the first cylindrical shell  110 . 
       FIG.  5    illustrates the optical housing arrangement  100  where the second transparent portion  180  is an opening  280 . An opening  280  in the second cylindrical shell element  170  may be beneficial in order to allow, biofouling, dirt and other impurities to escape from the surface of the first cylindrical shell  110 , e.g. after having been cleaned of using the cleaning pad. The second cylindrical shell element  170  may additionally or alternatively be supplied with one or more openings in order to allow dirt to escape. 
     The optical housing arrangement  100  is according to the invention suitable for housing optical equipment, such as optical sensors, cameras, detectors, light sources, lenses, polarization filters, etc. The optical housing arrangement  100  is according to the invention intended for use underwater, and may therefore house at least one light emitting device for illuminating an object of interest outside the optical housing arrangement  100 , and at least one optical sensor in order to capture an image of the illuminated object. This setup results, however, in a problem with light pollution, i.e. where light goes straight from the light emitting device to the optical sensor inside the housing, or through the transparent portion of the first cylindrical shell without having gone via the object of interest outside the housing.  FIGS.  6  and  9    illustrates an embodiment of the present invention where potential problems with light pollution are limited by partitioning the inner volume  290  of the first cylindrical shell  110  by one or more essentially non-transparent walls  300 . Essentially non-transparent may in this context be interpreted as having a visible light transparency of less than 1%. The inner volume  290  of the first cylindrical shell  110  may alternatively be partitioned by fully non-transparent walls. The problem of light pollution may in this setup be limited e.g. by separating the light emitting device and the optical sensor by such a said wall. 
     The self-cleaning optical housing arrangement may additionally be provided with electrical wiring in order to enable contact and/or power supply with any electrical and/or optical equipment within the optical housing arrangement. This wiring can for example go through any end cap of the housing arrangement, or alternatively be positioned in any other suitable spot. A slip ring may be used in order to transfer electricity to or in the optical housing arrangement. 
       FIG.  7    illustrates another embodiment of the invention where the first cylindrical shell  110  comprises a plurality of cylindrical shell segments  310  connected with each other. The plurality of cylindrical shell segments  310  may here further be partitioned from one another by one or more essentially non-transparent walls  300 . The plurality of cylindrical shell segments  310  may alternatively be partitioned from one another by one or more fully non-transparent walls  300 . Essentially non-transparent may in this context be interpreted as having a visible light transparency of less than 1%. The use of a plurality of cylindrical shell segments  310  may e.g. be beneficial when mounting the optical housing arrangement  100  together, and in order to avoid the first cylindrical shell acting as a waveguide. Each segment  310  may here be fabricated separately before being assembled into an optical housing arrangement  100 . 
     The shaft  130  can in the context of the present invention generally take any form as long as it can rotate at least partly inside the inner volume  290  of the first cylindrical shell  110  relative to the first cylindrical shell  110 .  FIG.  8    illustrates the optical housing arrangement  100  according to one embodiment of the invention where the shaft  130  comprises a plurality of parts  320  connected with each other. The shaft  130  may in other words be a structure made up from two or more parts  320  fastened together. 
       FIG.  9    illustrates an embodiment of the invention where the optical housing arrangement  100  further comprises an optical sensor  330  and a light emitting device  340 , both arranged in the inner volume  290  of the first cylindrical shell  110 . The optical sensor  330  is here aligned such that it can detect light from outside the optical housing arrangement  100 , e.g. by being arranged in front of the first transparent portion  120 . The light emitting device  340 , is in a similar manner aligned such that it may illuminate light out of the optical housing arrangement  100 , e.g. onto an object of interest positioned outside the optical housing arrangement  100 . The optical sensor  330  and the light emitting device  340  may be connected to the shaft  130  and may consequently be rotated by a rotation of the shaft  130 . The optical sensor may in this embodiment of the invention be incorporated in a camera. 
       FIG.  10    illustrates an embodiment of the invention where the first cylindrical shell  110  is transparent, i.e. that the first cylindrical shell  110  consists of one or more transparent materials  350 . The first cylindrical shell  110  may e.g. consist of one transparent material  350  such as soda-lime glass, silica, a transparent plastic, or any other suitable material. The first transparent portion  120  may thus make up the entire first cylindrical shell  110 . 
       FIG.  11    illustrates an embodiment of the invention where the cleaning pad  190 ,  360  has an elongated shape. An elongated shape may be beneficial for covering as large an area of the first cylindrical surface as possible without employing an unnecessarily large cleaning pad. The second cylindrical shell element  170  may in this embodiment of the invention have an elongated slot  370  shaped in as similar manner as the elongated cleaning pad  360  so that the cleaning pad  360  can be received in the slot  370 . The elongated cleaning pad  360  may as illustrated in  FIG.  11    be secured in the slot  370  by magnets. This can be performed by arranging a first and a second cleaning pad magnet  380  in the cleaning pad  360 , while arranging a corresponding first and second mounting magnet  390  adjacent to the elongated slot  370 . The magnets may here be positioned such that the first and second mounting magnets  390  exert a force on the first and a second cleaning pad magnets  380  respectively such that the cleaning pad  360  is pressed against the outer surface  200  of the first cylindrical shell  110 . The force between the mounting magnets  390  and the cleaning pad magnets  380  may be either attractive, or repulsive as in the optical housing arrangement in  FIG.  11   . In the case where the force between the mounting magnets  390  and the cleaning pad magnets  380  is attractive, the mounting magnets  390  would have to arranged closer to the shaft axis of rotation  140  than the cleaning pad magnets  380 .  FIG.  11    illustrates an embodiment of the invention where the first and second cleaning pad magnets  380  are respectively arranged in a first and a second distal end  400 ,  410  of the cleaning pad  360 . Here, the first and second mounting magnet are respectively arranged adjacent to a first and second distal end  420 ,  430  of the elongated slot  370 . 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Term: 
                 Figure reference 
               
               
                   
                   
               
             
            
               
                   
                 Self-cleaning optical housing arrangement 
                 100 
               
               
                   
                 First cylindrical shell 
                 110 
               
               
                   
                 First transparent portion 
                 120 
               
               
                   
                 Shaft 
                 130 
               
               
                   
                 Shaft axis of rotation 
                 140 
               
               
                   
                 First cylindrical shell central axis 
                 150 
               
               
                   
                 Motor 
                 160 
               
               
                   
                 Second cylindrical shell element 
                 170 
               
               
                   
                 Second transparent portion 
                 180 
               
               
                   
                 Cleaning pad 
                 190 
               
               
                   
                 First cylindrical shell outer surface 
                 200 
               
               
                   
                 Shaft magnet 
                 210 
               
               
                   
                 Cylinder magnet 
                 220 
               
               
                   
                 First cap 
                 230 
               
               
                   
                 First end of the first cylindrical shell 
                 240 
               
               
                   
                 A second cap 
                 250 
               
               
                   
                 Second end of the first cylindrical shell 
                 260 
               
               
                   
                 A bearing 
                 270 
               
               
                   
                 Opening 
                 280 
               
               
                   
                 Inner volume of the first cylindrical shell 
                 290 
               
               
                   
                 Non-transparent walls 
                 300 
               
               
                   
                 Cylindrical shell segments 
                 310 
               
               
                   
                 Shaft Part 
                 320 
               
               
                   
                 Optical sensor 
                 330 
               
               
                   
                 Light emitting device 
                 340 
               
               
                   
                 Transparent material 
                 350 
               
               
                   
                 Elongated cleaning pad 
                 360 
               
               
                   
                 Elongated slot 
                 370 
               
               
                   
                 Cleaning pad magnet 
                 380 
               
               
                   
                 Mounting magnet 
                 390 
               
               
                   
                 First distal end of the cleaning pad 
                 400 
               
               
                   
                 Second distal end of the cleaning pad 
                 410 
               
               
                   
                 First distal end of the slot 
                 420 
               
               
                   
                 Second distal end of the slot 
                 430