You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to electronic door locks, and more particularly to an override mechanism for unlatching or opening an electronic door mortise lock from an electronically controlled locked side in instances when the electronic door lock cannot or should not be unlocked electronically.  
           [0003]    2. Description of Prior Art  
           [0004]    Electronic door mortise locks having an electronically controlled locked door side and an unlocked door side are generally used to lock doors of hotel rooms and the like. Such door locks generally comprise a lever handle on the locked door side which remains locked and a lever handle on the unlocked door side which remains unlocked. An electronic mechanism such as a magnetic card reader is generally mounted to the locked door side such that a user having access to a magnetic card programmed to be recognized by the magnetic card reader can unlatch the lock from the locked door side. Such a mechanism is often powered by a battery connected thereto and is not accessible from the locked door side, for safety reasons.  
           [0005]    A problem with such electronic door locks occurs when the battery goes dead or when the electronic mechanism becomes defective. In such cases, the electronic door lock fails to operate and the lock cannot be unlatched, preventing the door from being opened.  
         SUMMARY OF THE INVENTION  
         [0006]    One aim of the present invention is to provide an override mechanism which unlatches such an electronic door mortise lock from the electronically controlled locked side and which does not require modifications to the integrity of the lock.  
           [0007]    In accordance with a broad aspect of the present invention, there is provided in an electronic door mortise lock having an electronically controlled locked side and an unlocked side, an override mechanism for unlatching the electronic door mortise lock from the electronically controlled locked side. The override mechanism comprises an interior drive mechanism connected to a door handle input hub of the unlocked side for driving the door handle input hub from a closed position to an open position and a locking mechanism accessible from said electronically controlled locked side and adapted to actuate the interior drive mechanism.  
           [0008]    The locking mechanism may comprise an exterior drive mechanism connected to the interior drive mechanism for mechanically driving the interior drive mechanism from the electronically controlled locked side, and actuation of the exterior drive mechanism actuates the interior drive mechanism and drives the door handle input hub from the closed position to the open position, thereby unlatching the electronic door mortise lock from the electronically controlled locked side.  
           [0009]    The exterior drive mechanism may comprise a cylinder lock connected to a tailpiece, and the locking mechanism may comprise a key.  
           [0010]    The interior drive mechanism may comprise a gear mechanism having a spur gear connected to the tailpiece and a rotatable rack member connected to the lever handle input hub, and the gear mechanism may provide a gear ratio between the cylinder lock and the input hub of at least 3:1, and preferably of at least 5:1.  
           [0011]    The cylinder lock and spur gear may rotate less than one turn to unlatch the door lock. The spur gear may be free to rotate without the tailpiece when the door handle is turned from the inside, and the tailpiece may drive the spur gear when the cylinder lock is rotated.  
           [0012]    The gear mechanism may comprise a geared portion of a stop plate connected to a lever handle of the unlocked side.  
           [0013]    The override mechanism may further comprise a detector for detecting actuation of the interior drive mechanism and an output for connecting the detector to the electronic door lock. Such a detector may comprise a cam connected to the tailpiece and having an outside rim, and a switch disposed adjacent the cam such that the switch is activated when the tailpiece is actuated, whereby when the cam is rotated with the tailpiece, the outside rim contacts the switch, thereby generating a signal indicative of actuation of the interior drive mechanism.  
           [0014]    The interior drive mechanism may comprise a gear mechanism comprising a spur gear engaged with a geared portion of a lever handle stop plate connected to the lever handle input hub.  
           [0015]    The override mechanism of the present invention allows a user having privileged access to unlock the electronic door mortise lock from the electronically controlled unlocked door side when the electronic door mortise lock is unlockable electronically, such as when electronic components thereof fail to operate or when the battery goes dead.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration a preferred embodiment thereof, in which like numerals refer to like components, and in which:  
         [0017]    [0017]FIG. 1 is a side view of an electronic door mortise lock mounted to a door and comprising an embodiment of an override mechanism in accordance with the present invention, showing in housingly breakaway view internal components thereof;  
         [0018]    [0018]FIG. 2 is a plan view of the inside housing of the housing of the electronic door mortise lock and showing the override mechanism in accordance with the present invention;  
         [0019]    [0019]FIG. 3 is a front perspective view of a spur gear of the embodiment of the override mechanism shown in FIG. 2; and  
         [0020]    [0020]FIG. 4 is a rear perspective view of a cam of the embodiment of the override mechanism shown in FIG. 2.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    Referring now to FIG. 1, there is provided an electronic lock  10  for insertion in a mortise of a door  12  having an exterior door side  14  and an interior door side  15  and for electronically controlling the locking of the exterior door side  14 , while leaving the interior door side  15  unlocked. The electronic lock  10  comprises an override mechanism  16  in accordance with the present invention.  
         [0022]    The electronic lock  10  has a housing assembly  18  including an outside housing  20  mounted to the exterior door side  14  and an inside housing  22  mounted to the interior door side  15 . An outside lever handle  26  outwardly extending from the outside housing  20  and an inside lever handle  28  outwardly extending from the inside housing  22  are mounted to opposed ends of a square spindle  30  extending through the mortise of the door  12  and which is connected to a retractable latch bolt  31 . As mentioned above, the inside housing  22  remains unlocked so that when the inside lever handle  28  is rotated, the square spindle  30  rotates from a closed position to an open position and the latch bolt  31  is retracted inside the mortise of the door  12  from a closed position to an open position.  
         [0023]    As shown in FIG. 2, a lever stop plate  32  is mounted to the square spindle  30  and connected to the inside lever handle  28 . The lever stop plate  32  has a curved rack  34  with eight teeth  36 . A circular spur gear  38  has an opening  39  at the center thereof and twelve spaced apart spurs  40  outwardly extending with respect to an axis going through the opening  39 . The spur gear  38  has its spurs  40  meshed with the teeth  36  of the curved rack  34 . Rotation of the spur gear  38  in the direction indicated by the arrow A moves the lever stop plate  32  from the closed position shown in FIG. 2 to an open position in the direction shown by the arrow B. As shown in FIG. 3, the spur gear  38  has a front surface  42  which defines an annular recess  44  about the opening  39 . An abutment  46  extends radially inwardly in the recess  44  for purposes to be later explained. As seen in FIG. 2, the opening  39  of the spur gear  38  receives a tailpiece  60  extending from a rotatable cylinder lock  62  (FIG. 1) for movement therewith. The cylinder lock  62  is actuatable with a key  64  (FIG. 1) insertable through an opening  66  in the outside housing  20 . The key rotates ⅔ of a turn for retracting the latch bolt  31 . The cylinder lock  62  may be a 6-pin “key-in-knob” such as the cylinder lock from Lori Lock and Rocky Mount, and the key  64  may be a Lockwood  1004  reverse. Mushroom pins (not shown) may be used for at least three pins for increased safety. The torque required on the key  64  will not exceed 9 in-lbs when the latch bolt  31  is projected or when the required torque on the inside lever is 50 in-lbs or less because of warped doors. The cylinder lock  62  may be incorporated in the die of the outside housing  20 . A medallion (not shown) can be used to cover the opening  66  for the keyway.  
         [0024]    The override mechanism  16  is operated when the key  64  is inserted in the opening  66  in the outside housing  20 . Rotation of the key  64  is transmitted to the tailpiece  60  rigidly connected to the cylinder lock  62  and rotates the spur gear  38 , which moves the inside lever stop plate  32  from the closed position as shown in FIG. 2 to the open position as indicated by the arrow B. This causes the latch bolt  31  to retract inside the mortise of the door  12 .  
         [0025]    A detector  48  is included for detecting the actuation of the spur gear  38 . The detector  48  comprises a cam  50  drivingly connected to the tailpiece  60  and having an outside rim  51 . As shown in FIG. 4, the cam  50  has a front surface  52  and a back surface  54  defining a rectangular slot opening  56  at the center thereof for receiving the tailpiece  60  so that a torque can be transferred from the tailpiece  60  to the cam  50 . The back surface  54  thereof is disposed adjacent the front surface  42  of the spur gear  38 . The back surface  54  of the cam  50  defines a circular projection  58  outwardly extending from the plan of the back surface  54 . An abutment  61  projects outwardly from the circular projection  58  with respect to an axis going through the slot opening  56 . As shown in FIG. 2, the abutment  61  is received in the recess  44  on one side of the abutment  46 . When a key is inserted into the cylinder lock  62  and rotated in a clockwise direction, as indicated by arrow A, the tailpiece  60  will rotate the cam  50  in the same direction, thereby causing the abutment  61  to push on the abutment  46 . This will cause the spur gear  38  and, thus, the stop plate  32  to rotate with the cam  50 . The movement communicated to the stop plate  32  will cause the inside lever handle  20  to rotate to retract the latch bolt  31 . However, when the inside lever handle  28  is operated to displace the latch bolt  31  to an open position, the induced counter clockwise rotation of the stop plate  32  will cause the spur gear  38  to rotate in a clockwise direction, thereby displacing the abutment  46  away from the abutment  61  of the cam  50 . Therefore, no motion will be communicated to the cam  50 .  
         [0026]    It is understood that once the override mechanism  16  has been actuated to retract the latch bolt  31 , a biasing force acting, for instance, on the stop plate  32  will bring back the spur gear  38  to its rest position (FIG. 2), thereby causing the abutment  46  to push on the abutment  61  so as to cause the cam  50  to return to its idle position (FIG. 2).  
         [0027]    A switch  70  is disposed adjacent the outside rim  51  of the cam  50 . An output (not shown) is connected to the switch  70  for connecting to the electronic door lock  10 . When the cam  50  is rotated by the tailpiece  60  from its idle position to an operative position, the outside rim  51  or cam surface thereof will trigger the switch  70 , thereby generating a signal indicative of actuation of the override mechanism  16 . Alternatively, the tailpiece  60  can be directly drivingly connected to the spur gear  38  if the detector  48  is not needed.  
         [0028]    While the invention has been described with particular reference to the illustrated embodiment, it will be understood that numerous modifications thereto will appear to those skilled in the art. Accordingly, the above description and accompanying drawings should be taken as illustrative of the invention and not in a limiting sense.

Summary:
The present invention relates to electronic door locks, and more particularly to an override mechanism for unlatching an electronic door mortise lock from an electronically controlled locked side when it cannot be unlocked electronically. The override mechanism comprises an interior drive mechanism connected to a door handle input hub of the unlocked side, for driving the door handle input hub from a closed position to an open position, and a locking mechanism adapted to actuate the interior drive mechanism.