Abstract:
A method and apparatus are provided for detecting both an absence and a presence of a light beam moving in an optical fiber to determine the position of a component. The method is carried out by the apparatus which includes a plunger having first and second positions coordinated with first and second component positions respectively. The optical fiber is capable of having a light beam move in a first direction and of having the light beam move in a reverse direction in the optical fiber. A detector is provided for indicating the absence of the reverse direction of the light beam moving in the optical fiber and for indicating the presence of the light beam moving in the first direction in the optical fiber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE 
       [0001]    [Not Applicable] 
       FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    [Not Applicable] 
       MICROFICHE/COPYRIGHT REFERENCE 
       [0003]    [Not Applicable] 
       FIELD 
       [0004]    Certain embodiments of the disclosure relate generally to fiber optic position sensors for use in the control of mechanical systems and methods for using the sensors. More specifically, certain embodiments of the disclosure relate to fiber optic position sensors that are particularly useful for controlling mechanical systems in aircraft such as for indicating the open and closed positions of landing gear and of various aircraft door operating systems. 
       BACKGROUND 
       [0005]    Fiber optic technology is a relatively new form of technology particularly used in communication systems, such as for the transmission of telephone signals, internet communication systems and cable television signals. Simply stated, fiber optic technology comprises a system of transmitting information by using pulses of light through an optical fiber assembly. Fiber optic technology is a fairly recent development that started in the 1970s. 
         [0006]    One principal advantage of fiber optic communication systems is that there is virtually non-existent electromagnetic interference (EMI) when using optical fibers in an operating system. The lack of EMI issues is a very significant benefit as it results in a high level of reliability. Another distinct advantage of fiber optic usage to the aircraft industry is that fiber optic systems are much lighter in weight than copper and aluminum wiring used in aircraft. The reduction in weight of wiring used in aircraft is a significant benefit since any reduction in weight of an aircraft improves fuel efficiency levels. 
         [0007]    Position sensors are commonly used in aircraft for a variety of purposes. One broad area of use of position sensors involves indicator lights indicating whether any of the various doors used in aircraft are in the open or closed positions. Position sensors used in connection with commercial aircraft components include, but are not limited to passenger entry/exit doors, emergency doors, cockpit security doors, landing gear doors, and landing gear assemblies themselves; in military aircraft, position sensors are useful in indicating whether bomb bay doors are open or closed. 
         [0008]    Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with the present disclosure as set forth in the remainder of the present application with reference to the drawings. 
       BRIEF SUMMARY 
       [0009]    In one aspect of the disclosure, a position sensor is provided for detecting both an absence and a presence of a light beam moving in an optical fiber and detecting first and second component positions of a structure. The position sensor includes a frame and a plunger linearly and reciprocally mounted along an axis on the frame. The plunger has first and second plunger positions and is coordinated with the first and second component positions respectively. The optical fiber is mounted on the frame and is capable of having the light beam move in a first direction in the optical fiber at all times and is capable of having the light beam move in a reverse direction in the optical fiber member when the component position changes between the first and second component positions and when the plunger position changes between the first and second plunger positions in coordination with the first and second component positions. A detector indicates the absence of the reverse direction of the light beam moving in the optical fiber and indicates the presence of the light beam moving in the first direction in the optical fiber. 
         [0010]    In another aspect of the disclosure, a position sensor is provided for detecting the presence of a light beam and the absence of a light beam moving in an optical fiber for providing a position signal of a component of an operating system of a structure. The position sensor includes a frame with a plunger linearly and reciprocally mounted along an axis on the frame between first and second positions. The optical fiber is mounted on the frame and is capable of having the light beam move in a first direction and in a reverse direction in the optical fiber. A prism is mounted in the frame for redirecting the light beam moving in a first direction in the optical fiber to at least the axis of the plunger and for redirecting the light beam moving in a reverse direction from the axis to the optical fiber. A reflector is mounted in the frame and has both an inoperative position and an operative position. The reflector, when in the operative position, reflects the light beam from the first direction into the reverse direction to the prism and the prism directs the light beam in the reverse direction into the optical fiber while providing a position signal of the presence of the reverse direction of the light beam in the optical fiber. A light beam absorber is mounted in the frame and has an inoperative condition and an operative condition. When the light beam absorber is in the operative condition, the light beam is absorbed and prevents the reverse direction of the light beam in the optical fiber. The reflector is in the inoperative condition when the light beam is in the operative condition, while providing a position signal of the absence of the reverse direction of the light beam in the optical fiber. 
         [0011]    In a still further aspect of the disclosure, a method is provided for detecting both the absence and the presence of a light beam moving in an optical fiber for determining the position of a component. The optical fiber that is provided has the capability of having a light beam move in a first direction in the optical fiber and is capable of having the light beam move in a reverse direction in the optical fiber when the position of the component has changed. The absence of the light beam is detected when the light beam is not moving in the reverse direction in the optical fiber. 
         [0012]    The features, functions, and advantages that have been discussed can be achieved independently in various embodiments or may be combined in yet other embodiments, further details of which can be seen with reference to the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
         [0013]      FIG. 1  is pictorial view of an aircraft having multiple doors thereon wherein fiber optic position sensors can be utilized for detecting open and closed positions of the doors; 
           [0014]      FIG. 2  is pictorial view of the underside of an aircraft showing the lowered position of a landing gear and the open position of landing gear doors; wherein fiber optic position sensors may be utilized for indicating such positions; 
           [0015]      FIG. 3  is a view similar to  FIG. 2  showing the landing gear doors in the closed position; 
           [0016]      FIG. 4  is a view of a fiber optic position sensor of a normally open type of sensor; 
           [0017]      FIG. 5  is a view of a fiber optic position sensor of a normally closed type of sensor; 
           [0018]      FIG. 6  is a schematic view showing a door of an aircraft in a closed position operatively connected to a fiber optic position sensor of a normally open type interconnected to a transceiver and the transceiver is connected to a controller which is connected to indicators, such as lights, indicating that a particular door is in the closed position, wherein fiber optic members interconnect the position sensor, the transceiver and the controller; 
           [0019]      FIG. 7  is a view similar to  FIG. 6  except showing the particular door of the aircraft in the open position; 
           [0020]      FIG. 8  is a view similar to  FIG. 6  except the fiber optic position sensor is of the normally closed type; and 
           [0021]      FIG. 9  is a view similar to  FIG. 7  except the fiber optic position sensor is of the normally closed type. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    By combining the benefits of fiber optics with the range of possible uses for position sensors in aircraft, it would be highly desirable to provide a fiber optic position sensor in connection with systems related to indicators for open and closed positions of various moveable parts, such as doors and landing gear on an aircraft. Such moveable parts have operating systems The combining of fiber optics with position sensors in aircraft would provide light weight communications with high reliability and minimal or no electromagnetic interference (EMI) with the position sensors. The operating systems of the moveable parts, as will be described herein, are communicatively coupled to the position sensors. 
         [0023]    The following description and accompanying drawings provide details of a fiber optic position sensor for detecting the presence or absence of a light beam in an operating system. The description, as follows, is not to be considered in a limiting sense, but it is provided for the purpose of illustrating the general principle of the claims relating to the described fiber optic position sensor and method for its use. The scope of the disclosure will be defined by the claims following the detailed description. 
         [0024]    The description of the fiber optic position sensor is, for purposes of illustration, primarily directed to use with aircraft of the type that use a wide range of components, such as doors, that utilize indicator lights for indicating open and closed positions on any or all of the doors on the aircraft. The fiber optic position sensor being described herein is not limited for use with aircraft, but is intended to include a wide range of structures including fixed land based structures and vehicles, ships at sea and any type of aircraft or aerospace vehicles. Further, the components used in connection with the fiber optic sensors extend over a wide range. 
         [0025]    Referring to  FIG. 1 , the pictorial view shows an aircraft, generally  50 , of conventional design. The aircraft  50  is provided with a fuselage, generally  52 , wings  54 , fixedly mounted on the opposite sides of the fuselage  52 , a nose section  56  and a tail section, generally  58 . Of particular interest relative to the present disclosure are the passenger/exit door  60  and multiple emergency exit doors  62 , all of which are provided along the length of the fuselage  52 . 
         [0026]    Referring to  FIGS. 2 and 3 , there is shown the underside  64  of the fuselage  52 .  FIG. 2  shows a landing gear, generally  66 , in the lowered position and also shows a pair of landing gear doors  68  hingedly secured to the underside  64  of the fuselage  52  by hinge mechanisms (not shown) of conventional design. Referring to  FIG. 3 , there is a showing of the underside  64  of the fuselage  52  wherein the landing gear doors  68  are in the closed position and the landing gear  66  is in a retracted position in the aircraft  50 . 
       Normally Open Sensor 
       [0027]    Referring to  FIG. 4 , there is shown a fiber optic position sensor, generally  100 , of the normally open type which is one type of fiber optic position sensor to be described herein. The sensor  100  is schematically shown as having a frame, generally  102 . The frame  102  includes an outer cylindrical wall  104  which provides an enclosed space for the sensor components contained therein. The outer wall  104  is enclosed by an upper wall  106  and a lower wall  108 . The frame  102  further includes an upper platform  110  that is supported by the outer wall  104 . The upper platform  110  is spaced downwardly from the upper wall  106 . A lower platform  112  is spaced downwardly from the upper platform  110  and is supported on its outer edge by the outer wall  104 . 
         [0028]    A plunger assembly, generally  114  is mounted in an aperture provided in the upper wall  106  of the frame  102 . The plunger assembly  114  includes an upwardly projecting portion  116  which projects above the upper wall  106  of the outer frame  102 . The plunger assembly  114  includes a unitary shoulder  118 . The plunger assembly  114  is generally cylindrical in shape. The plunger assembly  114  also includes an interior lower portion  120 . A threaded boss  122  is schematically shown and is provided with a jam nut to secure the fiber optic position sensor  100  to the aircraft  50  operatively near to a movable component of the aircraft  50  such as the fuselage doors  60  and  62 , the landing gear doors  68  or the landing gear  64 . A seal  124  is provided in the boss  122  for slideably and sealably guiding the cylindrical plunger  116 . In a similar manner, the upper platform  110  and lower platform  112  of the frame  102  each includes seals  124  to provide similar guiding support for the reciprocal movement of the plunger assembly  114  within the frame  102 . The frame  102  provides an enclosure for the interior components of the sensor  100  to be described. The seals  124  cooperate with the enclosed frame to avoid any exterior contamination from entering the enclosed frame  102 . 
         [0029]    A biasing spring  126  bears against a unitary shoulder  118  of the plunger assembly  114  at its upper end. At the lower end, the spring  126  rests against the upper surface of the upper platform  110 . The biasing spring  126  acts to bias the plunger assembly  114  upwardly to position the upwardly projecting portion  116  above the threaded boss assembly  122 . In the position shown in  FIG. 4 , the plunger assembly  114  is in a rest position. As will be described hereinafter, the plunger assembly  114  has an activated position when the plunger assembly  114  is moved downwardly such as by a component of a door  60 , such as doors  60 ,  62  and  68 . One flat side  130  of the unitary shoulder  118  of the plunger assembly  114  is in close slideable proximity to a flat upright wall  128  that is fixed to the upper wall  106  and to the upper platform  110 . With the flat side  130  of the shoulder  118  of the plunger assembly  114  bearing against the flat wall  128 , during reciprocal movement of the plunger assembly  114 , rotation of the plunger assembly  114  is prevented. It is to be understood that other types of guides may be provided for preventing rotation of the plunger assembly  114 . 
         [0030]    The lower end  132  of the plunger assembly  114  fixedly carries a planar reflector  134  for reflecting a light beam as will be hereinafter described in further detail. Since the plunger assembly  114  is prevented from rotating, the flat reflector  134  faces laterally directly outwardly at 90 degrees to an imaginary central axis  136  of the plunger assembly  114  as shown in dotted view that extends downwardly and beyond the lower end  132  of the plunger assembly  114 . 
         [0031]    The lower wall  108  of the frame  102  has a fiber optic assembly  138  securely mounted in an aperture provided therein. The fiber optic assembly  138  has a multi-mode optical fiber  140  secured therein. A collimating lens  142  is secured at the upper exit end of the multi-mode optical fiber  140  and is rigidly secured within the fiber optic assembly  138 . The lens  142  is securely bonded within the fiber optic assembly  138  and is set securely in position against the upper surface of the multi-mode optical fiber  140 . Immediately above the collimating lens  142 , a prism  144  is rigidly secured to the upper surface of the fiber optic assembly  138 . The collimating lens  142  focuses or narrows a light beam as represented by lines  146 . The light beam  146  meets the hypotenuse  148  of the prism  144  and the prism  144  redirects the light beam at 90 degrees towards a light absorber  150 . The light absorber  150  is secured to the lower wall  108  of the frame and is fixed in position against a side of the fiber optic assembly  138 . The light absorber  150  is made of a known material, for example, a non-reflective material such as black foam. The light absorber  150  absorbs the light beam  146  so there is the absence of reverse movement of the light beam  146  in the optical fiber  140 . A threaded boss  152  is secured to the lower surface of the fiber optic assembly  138 . The multi-mode optical fiber  140  is a section that extends from the lower surface of the lens  142  to the lower surface of the threaded boss  152 . From the above description it is seen that the optical fiber assembly  138 , supports the optical fiber  140 , the lens  142 , and the prism  144 . 
         [0032]    Referring to the lower area of  FIG. 4 , a connector  154  has a multi-mode optical fiber extension member  155  secured therein. The connector  154  is secured to the threaded boss  152 . The optical fiber member  155  has an end thereof securely pressed against the lower end of the section of the optical fiber  140  positioned in the optical fiber assembly  138 . The optical fiber extension member  155  is connected to an operating member to be hereinafter described. The optical fiber extension member  155  is the same type of multi-mode optical fiber as the multi-mode optical fiber  140 . In actual operation, the optical fiber  140  and the optical fiber extension member  155  function in a unitary and cooperative manner. 
       Method of Using Normally Open Sensor 
       [0033]    Referring to  FIG. 7 , there is a provided a schematic drawing of the fiber optic position sensor  100  in which the plunger assembly  114  is in the rest position.  FIG. 7  schematically shows a door  160  in the open position and with the sensor  100  in the rest position, that is, when the plunger assembly  114  is not depressed. The door  160  may be any type of door used on a wide variety of components and would include land based structures and vehicles, sea based ships at sea, and aircraft of all kinds, all of which have one or more doors that move between open and closed positions. For purpose of illustration, the door  160  is intended to reference landing gear doors  68  as shown in  FIG. 2 . 
         [0034]    As shown in  FIG. 7 , the fiber optic position sensor  100  is in the rest position and the door  160  is in the open position. The fiber optic position sensor  100  is connected by an optical fiber member  155  to an optical transceiver  162 . The optical transceiver  162  is connected by another optical fiber member  155  to a system controller, generally  164 . The system controller  164  is connected by an electrical wire  165 , such as copper wire, that is split into two electrical wires  165  with one wire  165  connected to a door open light  166  and with the other electrical wire  165  connected to a door closed light  168 . 
         [0035]    Referring to  FIG. 4 , in this position, the plunger assembly  114  is at rest. A light beam  146  passes through the multi-mode optical fiber  140 , and then is passed through the collimating lens  142 . The collimating lens  142  focuses the light beam  146  as it passes through the prism  144  and is reflected by the hypotenuse  148  of the prism  144  in a transverse direction through an open space. The light beam  146  is received by the light absorber  150 . At this time, the fiber optic position sensor  100  indicates the absence of light since there is no reverse movement of the light beam  146  in the multi-mode optical fiber  140 . In this way, an absence of the reverse movement of the light beam  146  in the optical fibers  140  and  155  is transmitted to the optical transceiver  162 . The indicator light  166  indicates then that the door  160  is in the open position. 
         [0036]    Referring to  FIG. 6 , when the door  160  is moved to the closed position, the plunger assembly  114  is depressed when the door  160  reaches the fully closed position. The lower end  132  of the plunger assembly  114  carries the reflector  134  which is positioned in the open space between the light absorber  150  and the prism  144 . The light beam  148  intercepts the imaginary center line  136  of the plunger assembly  114 . The plunger assembly  114  positions the reflector  134 , carried at the lower end  132  of the plunger assembly  114 , between the prism  144  and the light absorber  150 . At this time, the reflector  134  reflects the light beam  148  back to the prism  144  which directs the light beam in a reverse direction through the optical fiber  140  and the optical fiber member  155 . The reverse direction of the light beam  146  sends a presence of light signal back through the optical fiber  140  to the optical transceiver  162 . This reverse direction of the light beam  146  signals the optical transceiver  162  that the door  160  is in the closed position. The optical transceiver  162  sends a signal to the system controller  164 , through the optical fiber member  155 , which in turn sends a signal through the electrical wire  165  to the indicator light  168  to indicate that the door  160  is in the door closed position. 
       Normally Closed Sensor 
       [0037]    Referring to  FIG. 5 , there is shown a fiber optic position sensor, generally  250 , of the normally closed type. The fiber optic position sensor  250  as shown in  FIG. 5  is quite similar in both structure and method of use to the fiber optic position sensor  100 . Both the normally open position sensor  100  and the normally closed position sensor  250  operate in a system that detects the presence of a light beam moving in a reverse direction in an optical fiber and alternatively that detects the absence of a light beam moving in a reverse direction in an optical fiber. The presence or absence of such a light beam provides a signal to an indicator light that, for example, a door on an aircraft is open or closed. 
         [0038]    The normally closed sensor  250  of  FIG. 5  includes an outer frame, generally  252 . The outer frame  252  includes an outer wall  254 , preferably cylindrical, that provides an enclosed space for isolating the sensor components, to be described, from exterior contamination. Like the normally open sensor  100 , the outer wall  254  of the sensor  250  has an upper wall  256  and a lower wall  258  connected thereto. An upper platform  260  is spaced below the upper wall  256  and a lower platform  262  is spaced below the upper platform  260 . Both platforms  260  and  262  are secured at their outer edges to the outer wall  254 . 
         [0039]    The normally closed sensor  250  has a plunger assembly, generally  264 , mounted in an aperture provided in the upper wall  256  of the outer frame  252 . The plunger assembly  264  includes an upwardly projecting portion  266  which projects above the upper wall  256 . The plunger assembly  264  further includes a unitary shoulder  268 . The plunger assembly  264  is generally cylindrical in shape. The plunger assembly  264  includes a lower portion  270 . A threaded boss  272  is schematically shown and is secured to the upper wall  256  of the frame  252 . The threaded boss  272  is provided with a jam nut for securing the sensor  250  to the aircraft  50  in operative proximity to a door of the aircraft  50  for signaling that the door is in an open or a closed position. A seal  274  is provided in the boss  272  for providing a guide support and seal for the reciprocal movement of the plunger assembly  264  within the frame  252  to seal the enclosed frame  252  from any potential exterior contamination. 
         [0040]    A biasing spring  276  bears against the unitary shoulder  268  of the plunger assembly  264  at its upper end  266  and at its lower end, the biasing spring  276  rests against the upper surface of the upper platform  260 . The biasing spring  276  acts to bias the plunger assembly  264  upwardly to position the upwardly projecting portion  266  above the threaded boss  272 . As shown in  FIG. 5 , the plunger assembly  264  is shown in a rest position. The plunger assembly  264  has an activated position when the plunger assembly  264  is pushed downwardly as will be hereinafter described. 
         [0041]    The lower end of the plunger assembly  264  carries a preferably cylindrical light absorber  278  for absorbing a light beam in a manner to be hereinafter described. The plunger assembly  264  includes an imaginary central axis  280  shown in dotted view that extends downwardly and beyond the lower end of the plunger assembly  264 . 
         [0042]    The lower wall  258  of the frame  252  has a fiber optic assembly  282  securely mounted in an aperture provided in the lower wall  258 . The fiber optic assembly  282  has a multi-mode optical fiber  284  secured therein. A collimating lens  286  focuses or narrows a light beam as represented by lines  288 . The collimating lens  286  is rigidly secured within the fiber optic assembly  282  and rests on top of the multi-mode optical fiber  284 . The rapidly moving light beam  288  in the multi-mode optical fiber  284  passes through the lens  286  and the light beam  288  subsequently meets the hypotenuse  289  of a prism  290  which is in a fixed position on the fiber optic assembly  282  immediately above the collimating lens  286 . The prism  290  redirects the light beam at a 90 degree angle. A light beam reflector  292  is mounted on the fiber optic assembly  282  and is spaced laterally away from the upright wall of the prism  290 . The reflector  292  reflects the light beam  288  in a reverse direction back to the prism  290 . The prism  290  reflects the light beam  288  in the reverse direction back through the collimating lens  286  and back through the multi-mode optical fiber  284 . The multi-mode optical fiber  284  comprises a section that extends from the lower surface of the lens  286  to the lower surface of the threaded boss  285 . The optical fiber assembly  282  supports the optical fiber  284 , the lens  286 , the prism  290  and the reflector  292 . 
         [0043]    The light beam interrupter or light absorber  278  is made of a known structure of a non-reflective material, such as black foam material. When the light absorber  278  is moved downwardly, in a manner to be hereinafter described, the light absorber  278  absorbs the light beam  288  before the light beam  288  can be reflected in a reverse direction by the reflector  292 . When the plunger assembly  264  and light absorber  278  are raised to the rest position, the light beam  288  is reflected by the reflector  292  back to the prism  290  and then the prism  290  directs the light beam downwardly from the hypotenuse  291  of the prism  290  in a reverse direction through the section of multi-mode optical fiber  284  in the fiber optic assembly  282 . 
         [0044]    Referring to the lower area of  FIG. 5 , a connector  294  has a multi-mode optical fiber extension member  296  secured therein. The connector  294  is secured to the threaded boss  285 . The optical fiber extension member  296  has one end thereof secured against the lower end of the optical fiber  284  which is secured in the optical fiber assembly  282 . The optical fiber member  296  is connected to an operating member to be hereinafter described. The optical fiber member  296  is the same type of multi-mode optical fiber as the multi-mode optical fiber  284 . In actual operation, the optical fiber  284  and the optical fiber member function in a unitary and cooperative manner. 
       Method of Using Normally Closed Sensor 
       [0045]    The method of using the normally closed sensor  250  will be described with reference to  FIGS. 5 ,  8  and  9 . 
         [0046]    Referring to  FIG. 9 , a door  160  is schematically shown and represents any of the doors  60 ,  62 , and  68  on the aircraft  50 . The door  160  is in the open position and the fiber optic position sensor  250  is in the rest position. The fiber optic position sensor  250  is connected to the optical transceiver  162  by an optical fiber extension member  296  and the optical transceiver  162  is connected to the system controller  164  by an optical fiber extension member  296 . The system controller  164  is connected to the door open indicator light  166  and the door closed indicator light  168  by electrical wires  286 , such as copper wires. 
         [0047]    Referring to  FIG. 9 , the door  160  is in the open position and the fiber optic position sensor  250  is shown in the rest position. The sensor  250  is in the rest position and the plunger assembly  264  is in the rest position. The biasing spring  276  raises the plunger assembly  264  to the rest position. The light beam absorber  278  is cylindrical in shape and rests above the space between the reflector  292  and the prism  290 . The optical transceiver  162  sends a light beam  288  through the optical fiber  284  and the collimating lens  286  focuses or narrows the light beam  288  to project against the hypotenuse  289  of the prism  290 . The light beam  288  is reflected at approximately a 90 degree angle directly towards the reflector  292 . The reflector  292  reflects the light beam  288  back to the prism  290  and the prism  290  then reflects the light beam  288  in a reverse direction back through the collimating lens  286  and through the multi-mode optical fiber  284  and the optical fiber member  296 . The reverse direction of the light beam  288  sends a signal to the optical transceiver  162  that a reverse light beam  288  is present. The optical transceiver  162  transmits the signal back to the system controller  164  through the optical fiber extension member  296 . The system controller  164  then sends a signal through the electrical wires  286  to the door open light indicator  168  signaling that the door  160  is open. 
         [0048]    Referring to  FIG. 8 , the door  160  is shown in the closed position. The plunger assembly  264  has been depressed in response to the closed door  160  and is moved downwardly within the frame  252 . Referring to  FIG. 5 , the light beam absorber  278 , which is preferably cylindrical in shape, is moved into the space between the reflector  292  and the prism  290 . The light absorber  278  absorbs the light beam  288  and prevents the reverse direction of the light beam in the optical fiber  284  since the light beam  288  has been intercepted by the light absorber  278 , the plunger assembly  264  having moved the light absorber  278  into the active position. In this way, there is no reverse movement of the light beam  288  from the light beam absorber  278 . Since the light beam  288  has not moved in a reverse direction, a signal is received by the optical transceiver  162  that there is an absence of a reverse movement of a light beam  288  in the optical fiber  284 . This information is transmitted from the transceiver  162  to the system controller  164  through an optical fiber extension member  296 . The system controller  164  sends a signal through the electrical wires  286  to the door closed indicator light  168  advising that the door  160  is in the closed position. 
         [0049]    While the disclosure has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the spirit and scope of the present disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from its scope. Therefore, it is intended that the present disclosure not be limited to the particular embodiments disclosed, but that the present disclosure will include all embodiments falling within the scope of the appended claims.