Patent Publication Number: US-7714718-B2

Title: Optical security sensor for a door

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to surveillance system sensors, and, more particularly, to surveillance system sensors for detecting the opening of a door or window. 
   2. Description of the Related Art 
   Surveillance systems, also known as security systems, are known to include door sensors for monitoring the opening and closing of a door. Door sensors are known to be in the form of a pushbutton that is held in a depressed state by the door when the door is in a closed position. When opening, the door moves away from the pushbutton, thereby releasing the pushbutton from the depressed state. A controller monitors the state of the pushbutton, and may issue an alarm signal if the door is opened without authorization. A problem with this type of sensor is that an intruder can defeat it by inserting a thin object, such as a piece of sheet metal, between the door and the pushbutton such that the object holds the pushbutton in a depressed state when the door is opened. Thus, the controller cannot detect that the door has been opened. 
   Another type of door sensor is the magnetic reed switch type that includes a reed switch sensor mounted on the door frame. The sensor detects and monitors the presence of a magnet that is mounted on the door at a location that is adjacent to the sensor when the door is in the closed position. Thus, the magnet may be detected by the sensor only when the door is closed. A problem with this type of sensor is that it too may be defeated by an intruder. For example, the intruder may attach another magnet adjacent to the reed switch sensor before opening the door such that the sensor&#39;s detection of the presence of a magnet is uninterrupted. Here too, the sensor, and a controller connected to the sensor, cannot detect that the door has been opened. 
   What is needed in the art is a door/window sensor that cannot be easily defeated by an intruder and that can be incorporated into a security system. 
   SUMMARY OF THE INVENTION 
   The present invention provides a door sensor having a first part that may be mounted on a door frame or on a door, and that includes an optical emitter and an optical receiver. A second part of the door sensor may be mounted on the other one of the door frame and the door, and includes a reflector arrangement that reflects an optical beam from the emitter back to the receiver. The reflected beam received by the receiver may be laterally offset from and substantially parallel to the beam as provided by the emitter. 
   The invention comprises, in one form thereof, a security assembly including a first building structure at least partially defining a building opening. The first building structure has a first surface. A movable building structure is movable between a closed position in which the movable building structure covers the opening and an open position in which the movable building structure uncovers the opening. The movable building structure has a perimeter with a second surface disposed in opposition to the first surface when the movable building structure is in the closed position. An optical sensor apparatus includes an electronics module mounted in association with the first surface or the second surface. The electronics module has an optical emitter and an optical receiver. The optical emitter emits a first beam. A reflector arrangement is mounted in association with the other of the first surface and the second surface. The reflector arrangement provides a plurality of sequential reflections of the first beam to thereby produce a second beam directed at the optical receiver. 
   The invention comprises, in another form thereof, a security sensor apparatus for sensing movement of an object. An electronics module includes an optical emitter and an optical receiver. The optical emitter emits a first beam. The electronics module is mounted in either a first surface of the object or a second surface of a fixed structure disposed in opposition to the first surface. A reflector arrangement includes at least one reflective surface and is mounted in the other one of the first surface and the second surface. The at least one reflective surface receives the first beam and produces a second beam directed at the optical receiver. The second beam is substantially parallel to and offset from the first beam. 
   The invention comprises, in yet another form thereof, a method of detecting a position of a movable building structure, including mounting at least one reflective surface along a perimeter of the movable building structure. A first optical beam is transmitted towards the at least one reflective surface while the movable building structure is in a closed position. The at least one reflective surface is used to receive the first optical beam and produce therefrom a second optical beam that is substantially parallel to and offset from the first optical beam. The second optical beam is received while the movable building structure is in the closed position. Whether the movable building structure is in the closed position is determined based upon a status of the second optical beam. 
   An advantage of the present invention is that it is difficult for a would-be intruder to defeat. For example, because the final reflected beam is offset from and substantially parallel to the beam as originally emitted, it would be difficult for an intruder to insert a single planar mirror or sheet of paper between the door and the door frame to thereby intercept the emitted beam and reflect it toward the optical receiver. 
   Another advantage is that it is difficult for a would-be intruder to defeat by inserting an optical emitter between the door and the door frame to thereby emit an optical beam directly at the optical receiver. The emitted optical beam may carry a specific signal, and the electronic module may detect tampering by ascertaining that the beam received by the optical receiver does not carry a signal that has a certain relationship to the signal carried by the originally emitted beam. The signal may vary from electronic module to electronic module, or may vary with time, thereby making it difficult for a would-be intruder to reproduce the signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above mentioned and other features and objects of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a plan view of one embodiment of a door assembly including an optical sensor apparatus of the present invention. 
       FIG. 2  is a block diagram of the optical sensor apparatus of  FIG. 1 . 
       FIG. 3  is a block diagram of the electronic module of the optical sensor apparatus of  FIG. 2 . 
       FIG. 4   a  is a schematic view of one embodiment of the reflector arrangement of the optical sensor apparatus of  FIG. 2 . 
       FIG. 4   b  is a schematic view of another embodiment of the reflector arrangement of the optical sensor apparatus of  FIG. 2 . 
       FIG. 4   c  is a schematic view of yet another embodiment of the reflector arrangement of the optical sensor apparatus of  FIG. 2 . 
       FIG. 5  is a flow chart of one embodiment of a method of the present invention for detecting a position of a door. 
       FIG. 6  is a plan view of one embodiment of a window assembly including an optical sensor apparatus of the present invention. 
   

   Corresponding reference characters indicate corresponding parts throughout the several views. Although the exemplification set out herein illustrates embodiments of the invention, in several forms, the embodiments disclosed below are not intended to be exhaustive or to be construed as limiting the scope of the invention to the precise forms disclosed. 
   DESCRIPTION OF THE PRESENT INVENTION 
   Referring now to the drawings and particularly to  FIG. 1 , there is shown one embodiment of a security assembly, in particular a door assembly  10 , of the present invention for incorporation into a structure  12  such as a building, or, more particularly, a wall of a building. Door assembly  10  includes a movable building structure in the form of a door  14 , which is surrounded by portions of structure  12 , such as a door frame  16  and a floor surface  18 . Door frame  16  and a floor surface  18  define a building opening  19  in the form of a doorway that door  14  covers when door  14  is in a closed position and that door  14  uncovers when door  14  is in an open position. An optical sensor apparatus  20  is mounted partially within door  14  and partially within door frame  16 . More particularly, optical sensor apparatus  20  includes a reflector arrangement  22  and an electronics module  24  which may be mounted in opposing locations in association with door  14  and door frame  16 , respectively. 
   Door  14  may be opened by manually grasping knob  26  and rotating door  14  about hinges  28   a ,  28   b , i.e., about an axis  30  defined by hinges  28 , as is well known. If door  14  is locked, i.e., if a latch  32  of door  14  is locked in a coupled state with frame  16 , an intruder may nevertheless open door  14  by breaking hinges  28  and/or latch  32  away from frame  16 , thereby allowing door  14  to be moved away from frame  16 , as is also well known. 
   Reflector arrangement  22  may be mounted in a surface of door  14  at a location that is along a perimeter  34  of door  14 . Perimeter  34  may be defined as an outer section of door  14  that is between outer edges  36  of door  14  and locations indicated generally by dashed line  38 . Reflector arrangement  22  is shown mounted in a surface of perimeter  34  that is disposed opposite from hinges  28 . However, reflector arrangement  22  could alternatively be mounted in a surface of perimeter  34  that is adjacent to hinges  28 , as indicated at  40 . Moreover, reflector arrangement  22  could be mounted not in a jamb, but rather in a surface of an upper portion of perimeter  34 , as indicated at  42 . 
   Regardless of in which location in the surface of perimeter  34  reflector arrangement  22  is mounted, electronic module  24  may be mounted in a surface of door frame  16  at a location that opposes the mounting location of reflector arrangement  22 . Particularly, the relative mounting locations of reflector arrangement  22  and electronic module  24  may be such that an optical beam emitted by electronic module  24 , as indicated by arrow  44 , may be reflected back to an optical receiver of electronic module  24 , as indicated by arrow  46 . Reflector arrangement  22  may receive the emitted optical beam and reflect the beam a plurality of times such that the final beam directed back to the optical receiver is offset from and substantially parallel to the originally emitted beam, as indicated generally by the orientation and spacing of arrows  44 ,  46 , and as described in more detail hereinbelow. 
   As shown in  FIG. 2 , electronic module  24  may include a controller  48  that may be electrically connected to both optical emitter  50  and optical receiver  52 , such as through lines  54 ,  56 , respectively. Through line  58 , controller  48  may be electrically connected to a control panel (not shown) or some other centralized device that is capable of causing some type of alarm signal or tamper signal to be issued in response to controller  48  determining that door  14  has been opened without authorization. A determination that door  14  has been opened may be made by controller  48  as a result of sensing that receiver  52  is not receiving an optical beam that corresponds to or that is related to the optical beam that is being emitted by emitter  50 . 
   Emitter  50  may be in the form of a light-emitting diode (LED) that emits optical energy in the infrared range. Receiver  52  may be a photodiode or any other type of optical receiver that is capable of detecting optical energy of the frequency range emitted by emitter  50 . 
   As is best illustrated in  FIG. 2 , an advantage of the present invention is that it would be difficult to defeat sensor apparatus  20  by inserting a single planar mirror or a sheet of paper into a gap  60  between door  14  and door frame  16 . The difficulty of defeating sensor apparatus  20  in this way is at least partially attributable to originally emitted beam  44  and finally reflected beam  46  being substantially parallel, which makes it difficult for someone to replicate reflected beam  46  by inserting a single mirror or a sheet of paper into gap  60  at an orientation that is substantially perpendicular to emitted beam  44 . In order to take advantage of the tamper-inhibiting characteristics of substantially parallel beams  44 ,  46 , receiver  52  may be configured such that it may effectively receive beams only of the orientation of beam  46 , i.e., beams that are substantially perpendicular to surface  62  of door frame  16 . 
   In order to ensure that sensor apparatus  20  is operational despite receiver  52  effectively receiving beams only of the orientation of beam  46 , emitted  44  beam may be polarized. Further, a polarizing filter may be included in receiver  52  for receiving the polarized reflected beam. 
   Another attribute of beams  44 ,  46  that makes defeating sensor apparatus  20  difficult is a lateral offset  64  ( FIG. 3 ) between originally emitted beam  44  and finally reflected beam  46 . More particularly, if offset  64  were to be reduced to a degree that it is substantially eliminated, then the angle at which emitted beam  44  would need to be reflected to reach receiver  52  in a single reflection would approach zero. Thus, it would become more feasible to defeat the sensor apparatus by inserting into gap  60  a sheet of paper or a single planar mirror that is narrower than gap  60 , and by then orienting the mirror or paper slightly non-perpendicular to emitted beam  44  to thereby reflect beam  44  such that it may be received by receiver  52 . However, due to offset  64 , it may be practically impossible to insert paper or a small mirror into gap  60  and reflect emitted beam  44  such that it may be received by receiver  52 . 
   Although in one embodiment beams  44 ,  46  are substantially parallel, it is also possible within the scope of the invention for the emitted beam to diverge from the receiver such as at a direction indicated by dashed line  66  in  FIG. 3 . In this case, the reflector arrangement would have a slightly different configuration to thereby produce a reflected beam  46  that is substantially perpendicular to surface  62 . In addition, or alternatively, to the emitter producing a divergent beam such as at  66 , the receiver may be configured to receive a finally reflected beam from a divergent direction, such as indicated by dashed line  68 . In this case, the reflector arrangement would have another slightly different configuration to thereby produce a reflected beam that approaches receiver  52  at the angle indicated at  68 . Divergent beams such as indicated at  66  and  68  may have the advantage of making the optical sensor apparatus still harder to defeat by use of paper or a mirror inserted into gap  60 . That is, a divergent emitted beam  66  may be more difficult to reflect to the receiver than is emitted beam  44 ; and a divergent received beam  68  may be more difficult for a would-be intruder to produce than is beam  46 . 
   One embodiment of controller  48  is shown in more detail in  FIG. 3 . Controller  48  may include a processor  70 , such as a microprocessor, electrically connected to a signal generator  72  and to a signal analyzer  74  via respective lines  76 ,  78 . Signal generator  72  may provide input to emitter  50  on line  54  specifying a unique identifying signal that is to be carried on emitted beam  44 . As a result, reflected beam  46  may carry a substantially equivalent signal, or at least reflected beam  46  may carry a signal that has a certain relationship to the signal carried by beam  44 . That is, the signal carried by beam  44  may undergo some transformation within reflector arrangement  22  before being carried by beam  46 , but it may be a somewhat predictable transformation. For example, the signal carried by reflected beam  46  may be reduced in amplitude, and/or shifted in phase, as compared to the signal carried by emitted beam  44 . Signal analyzer  74  may ascertain the signal carried by reflected beam  46  based upon communications that analyzer  74  receives from receiver  52 . Signal analyzer  74  and/or processor  70  may compare the received signal carried by reflected beam  46  to the emitted signal carried by emitted beam  44 . Signal analyzer  74  and/or processor  70  may thus determine, based upon a relationship between the received signal carried by reflected beam  46  and the emitted signal carried by emitted beam  44 , whether reflected beam  46  is a product of emitted beam  44  and reflector arrangement  22 . If it is determined that reflected beam  46  is a product of emitted beam  44  and reflector arrangement  22 , then it can also be determined that reflector arrangement  22  and electronic module  24  are disposed in opposition to each other and that door  14  is in a closed position within door frame  16 . 
   In order to prevent a would-be intruder from duplicating the reflected beam  46  and the signal carried thereby, the signal carried by emitted beam  44  may vary from electronic module to electronic module, or may vary with time, thereby making it difficult for the prospective intruder to determine what signal that processor  70  and/or signal analyzer  74  are expecting to receive at any point in time. It is further possible for emitted beam  44  to carry a signal having a security code that is embedded therein and that is randomly determined by processor  70  at any point in time. The would-be intruder would then need to ascertain and duplicate the security code in order to defeat the optical sensor apparatus. 
   In order to avoid interference from ambient light, such as from electric light bulbs, it is possible to oscillate emitted beam  44  at some particular frequency that gets passed on to reflected beam  46 . Thus, this characteristic frequency may be used by processor  70  and/or signal analyzer  74  to distinguish reflected beam  46  from ambient light. Household current may be typically oscillated at about 60 Hz. In one embodiment, emitted beam  44  is oscillated at a frequency of about 1000 Hz in order that reflected beam  46  may be more easily distinguished from ambient light. 
   Exemplary embodiments of reflector arrangement  22  mounted in a surface  80  of perimeter  34  of door  14  are illustrated in  FIGS. 4   a - c . In the first embodiment illustrated in  FIG. 4   a , reflector arrangement  22  is in the form of a light pipe. Emitted beam  44  may be channeled from a first end  82  of the light pipe to a second end  84  via a plurality of internal reflections within the light pipe. Reflected beam  46  may emanate from second end  84  as shown. The light pipe may be embodied by an optical fiber, for example. 
   In the embodiment of  FIG. 4   b , a reflector arrangement  122  is in the form of two planar mirrors  86   a ,  86   b . Mirror  86   a  may be oriented at an angle of about forty-five degrees relative to emitted beam  44  to thereby produce an intermediate reflected beam  45  that is oriented at an angle of forty-five degrees relative to mirror  86   a  and at an angle of ninety degrees relative to emitted beam  44 . Similarly, mirror  86   b  may be oriented at an angle of about forty-five degrees relative to intermediate reflected beam  45  to thereby produce a final reflected beam  46  that is oriented at an angle of forty-five degrees relative to mirror  86   b  and at an angle of ninety degrees relative to surface  80  of perimeter  34 . 
   In the third embodiment illustrated in  FIG. 4   c , a reflector arrangement  222  is in the form of a corner cube reflector. A corner cube reflector is characterized by three reflective planar surfaces, e.g., mirrors,  88   a ,  88   b ,  88   c , each of which is oriented at a right angle to each of the other two surfaces. The corner cube reflector has the unique property that optical energy directed at any one of the reflective surfaces is reflected off of each of the three surfaces and is directed back in a direction that is opposite to the direction of the incoming optical energy. In each of the three embodiments illustrated in  FIGS. 4   a - c , final emitted beam  46  is parallel to and offset from originally emitted beam  44  after a plurality of sequential reflections by a plurality of reflective surfaces. 
   During use, after installation of optical sensor apparatus  20 , door  14  is moved to a closed position and sensor apparatus  20  is armed, such as by a user via a control panel (not shown). In the armed state, sensor apparatus  20  may continually monitor the status of door  14 . The user may disarm sensor apparatus  20  by entering a security code into the control panel, for example, perhaps within a grace time period after door  14  is opened. In the disarmed state, sensor apparatus  20  may no longer monitor door  14 , or may refrain from issuing an alarm signal or tamper signal if door  14  is opened. 
   In the armed state, if door  14  is opened, such as by an intruder, then receiver  52  will no longer be in position to receive reflected beam  46 . A determination that door  14  has been opened may be made by controller  48  based upon reflected beam  46  not being received by receiver  52  during a time period in which emitted beam  44  is still being emitted. Controller  48  may issue an alarm signal in response to the determination that door  14  has been opened without authorization. 
   If controller  48  determines that the signal being carried by the optical beam that is received by receiver does not have the expected relationship to the signal that is being carried by emitted beam  44 , then controller  48  may conclude that someone may be tampering with sensor apparatus  20 . That is, then controller  48  may conclude that someone may be unsuccessfully trying to defeat sensor apparatus  20  by attempting to simulate the reflected beam and accompanying signal that controller  48  expects to receive, and is directing the simulated beam and signal at receiver  52 . Controller  48  may then issue a tamper signal, which may be, for example, in the form of a beeping sound that indicates to the user that investigation or maintenance may be needed. 
     FIG. 5  illustrates one embodiment of a method  500  of the present invention for detecting the position of a door. However, it is to be understood that method  500  may be equally applicable for detecting the position of a window. In a first step  502 , at least one reflective surface is mounted along a perimeter of a door. For example, any embodiment of reflector arrangement  22  disclosed herein includes at least one reflective surface and may be mounted along perimeter  34  of door  14 . In a next step  504 , a first optical beam is transmitted towards the at least one reflective surface while the door is in a closed position. In the embodiments disclosed herein, originally emitted beam  44  may be transmitted towards the at least one reflective surface of reflector arrangement  22  while door  14  is in the closed position illustrated in  FIG. 1 . In step  506 , the at least one reflective surface is used to receive the first optical beam and produce therefrom a second optical beam, the second optical beam being substantially parallel to and offset from the first optical beam. For example, the at least one reflective surface of reflector arrangement  22  may receive originally emitted beam  44  and produce therefrom a final reflected beam  46  that is substantially parallel to and offset from beam  44 . The offset may be as indicated at  64  in  FIG. 3 , for example. In a next step  508 , the second optical beam is received while the door is in the closed position. That is, reflector arrangement  22  may be disposed opposite from electronics module  24  while door  14  is closed, and likewise receiver  52  may be in position to receive a final reflected beam  46  that may be produced by reflector arrangement  22  while door  14  is in the closed position. In a final step  510 , it is determined whether the door is in the closed position based upon a status of the second optical beam. In a particular example, controller  48  may ascertain the status of an optical beam to be received by receiver  52 . That is, controller  48  may ascertain whether receiver  52  is receiving and sensing an optical beam of any type. Further, if receiver  52  is indeed receiving and sensing an optical beam, controller  48  may ascertain whether the received optical beam carries a signal that has an expected relationship to a signal that may be carried by originally emitted beam  44 . For example, controller  48  may expect the signal carried by reflected beam  46  to be substantially equivalent to the signal carried by emitted beam  44 . As an alternative example, controller  48  may expect the signal carried by reflected beam  46  to have a certain drop in amplitude or a certain phase shift as compared to the signal carried by emitted beam  44 . If it is found that the received optical beam does indeed carry a signal that has an expected relationship to a signal that is carried by originally emitted beam  44 , then controller  48  may conclude that door  14  is in the closed position. 
   The present invention has been described herein as being applied to detecting the opening and closing of a hinged door that swings between an open position and a closed position. However, the present invention may be used to monitor any movable building structure that is movable between a closed position in which the movable building structure covers a building opening and an open position in which the movable building structure uncovers the building opening. 
   In  FIG. 6 , there is shown another embodiment of a security assembly of the present invention in the form of a window assembly  110  for incorporation into a structure  112  such as a building, or, more particularly, a wall of a building. Window assembly  110  includes a movable building structure in the form of a movable window sash  114 , which is surrounded by portions of structure  112 , such as a wall, a window frame  116  and a fixed window sash  118 . Window frame  116  and a fixed window sash  118  define a building opening  119  in the form of a window opening that sash  114  covers when sash  114  is in a closed position and that sash  114  uncovers when sash  114  is in an open position. An optical sensor apparatus  120  is mounted partially within sash  114  and partially within window frame  116 . More particularly, optical sensor apparatus  120  includes a reflector arrangement  122  and an electronics module  124  which may be mounted in opposing locations within sash  114  and window frame  116 , respectively. 
   Sash  114  may be opened by manually grasping sash  114  and sliding sash  114  in an upward direction  125 , as is well known. Imaginary planes defined by sashes  114 ,  118  may be parallel to each other and displaced from each other in a direction into the page of  FIG. 6 . To at least partially open sash  114 , and thereby at least partially uncover opening  119 , sash  114  may be slid in direction  125  in tracks (not shown) in frame  116  such that sash  114  at least partially overlaps sash  118  in a direction into the page of  FIG. 6 , as is also well known. 
   Reflector arrangement  122  may be mounted in a surface of sash  114  at a location that is along a perimeter  134  of sash  114 . Perimeter  134  may be defined as an outer section of sash  114  that is between outer edges  136  of sash  114  and locations indicated generally by dashed line  138 . Reflector arrangement  122  is shown mounted in a vertically-oriented surface of perimeter  134 . However, reflector arrangement  122  could alternatively be mounted in the portion of the surface of perimeter  134  that is on the other end of sash  114 , as indicated at  140 . Moreover, reflector arrangement  122  could be mounted not in a vertically-oriented surface, but rather in a horizontally-oriented surface of perimeter  34  that is disposed opposite the window sill, as indicated at  142 . 
   Regardless of in which location in the surface of perimeter  134  reflector arrangement  122  is mounted, electronic module  124  may be mounted in a surface of window frame  116  at a location that opposes the mounting location of reflector arrangement  122 . Particularly, the relative mounting locations of reflector arrangement  122  and electronic module  124  may be such that an optical beam emitted by electronic module  124 , as indicated by arrow  144 , may be reflected back to an optical receiver of electronic module  124 , as indicated by arrow  146 . Reflector arrangement  122  may receive the emitted optical beam and reflect the beam a plurality of times such that the final beam directed back to the optical receiver is offset from and substantially parallel to the originally emitted beam, as indicated generally by the orientation and spacing of arrows  144 ,  146 , and as described in more detail hereinabove with regard to  FIGS. 1-5 . 
   The present invention has been primarily described herein in connection with sensing the opening of a hinged door that swings between an open position and a closed position. However, it is to be understood that the features of the present invention described herein may be equally applicable to sensing the opening of any movable building structure, such as a window or a sliding door, that translates between an open position and a closed position. 
   The present invention has been described herein as including a reflector arrangement and an electronic module mounted at opposing locations within the door and the door frame, respectively. However, it is to be understood that it is within the scope of the present invention for the reflector arrangement to be mounted within the door frame and the electronic module to be mounted within the door. Moreover, it is also within the scope of the present invention for one of the reflector arrangement and the electronic module to be mounted within a bottom edge of the door and the other to be mounted at an opposing location within the floor surface. 
   The reflector arrangement of the present invention has been described herein as being mounted in an outer edge of a door so as to receive and reflect optical signals that are oriented parallel to a plane defined by the door. However, it is also possible for the reflector arrangement to be mounted within one of the two large opposite surfaces of the door, albeit along the perimeter of the door such that the reflector arrangement is covered, when the door is closed, by a portion of the door frame that is parallel to the plane defined by the door. In this way, the reflector arrangement would receive and reflect optical signals that are oriented perpendicular to a plane defined by the door. 
   The electronics module of the present invention has been described herein as being disposed in a fixed building structure, such as a door frame or a window frame. However, it is to be understood that it is also possible within the scope of the invention for both the electronics module and the reflector arrangement to be disposed in opposing surfaces of two movable structures. For example, the electronics module and the reflector arrangement may be disposed in opposing surfaces of a pair of French doors or a pair of French windows, both of which are hinged at opposite outside edges, and which open in the middle between the two movable structures. 
   While this invention has been described as having an exemplary design, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles.