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 
     The invention is concerned with electrically driven locks, and in particular is directed to solenoid driven deadbolt locks, a particular case being those driven by keep solenoids. 
     Electrically operated locks are often driven by a solenoid, with the reciprocable solenoid plunger driving a deadbolt or spring latch of the lock. In such a lock a spring can be positioned to push the deadbolt or spring latch to the extended position, while retraction is effected by powering the solenoid, to retract the plunger. 
     Electric locks have often employed a specific type of solenoid known as a keep solenoid, so that, especially in the case of a deadbolt lock, whether the lock bolt is extended or retracted, no power is required to maintain the position. A keep solenoid has both a permanent magnet and an electric coil. The spring loaded solenoid plunger is released outwardly when the coil is energized in a first polarity, creating an approximately equal but opposite magnetic force to that of the permanent magnet, effectively neutralizing the permanent magnet. This allows the spring to cause the outward movement of the plunger. When the solenoid coil is energized in the opposite, second polarity, it creates a magnetic force aligned with that of the permanent magnet, essentially doubling the magnet pulling power, overcoming the spring and pulling the plunger to the inward retracted position. 
     Thus, use of a keep solenoid in an electrically actuated lock is advantageous, due to the fact that the locking element can be moved from lock to unlock and from unlock to lock states with momentary use of power, with no additional power required to keep the lock in either the locked or unlocked state after the use of momentary power. 
     However, certain disadvantages arise from the use of a keep solenoid. First, the travel distance of the plunger is limited by the size of the magnet and coils, and the power source. Utilizing commercially available battery cells such as AA, AAA or 9 volt, only a small throw distance can be achieved for the lock element, approximately 7 mm or ¼ inch given practical considerations. Theoretically the throw could be longer from action of the spring, but if the throw is too long the solenoid and permanent magnet will not be able to retract the plunger and the lock element. This factor, along with battery and magnet requirements, must be balanced against desired throw distance in the lock design, and thus there is a practical limit on bolt throw distance. 
     A second important shortcoming of a keep solenoid-driven lock is that the locking element or bolt can be pushed back if someone can gain access to any part of the bolt and push it back using a finger or a tool. This is also a failing of simple solenoid driven locks, as opposed to keep solenoids, as outlined above. One need only overcome the relatively light force of a spring in order to push the locking element back and open the door or cabinet. 
     SUMMARY OF THE INVENTION 
     The electric lock of the invention overcomes both these problems, by use of an indirect drive mechanism between a solenoid plunger and a lock element, especially a bolt. 
     The lock unit has a housing, and within the housing is a reciprocating bolt and a solenoid, preferably, a keep solenoid. Interacting between the plunger of the keep solenoid and the bolt are two pivoted mechanical elements. A lever connected to the plunger is pivotally mounted at a pivot point on the housing. One arm of the lever is on the plunger side of the pivot point and is attached to the end of the plunger, while the second arm of the lever extends toward the lock element or bolt, but is not connected directly to the bolt. Between the second arm of the lever and the bolt is a link, pivotally connected both to the lever&#39;s second arm and to the base end of the bolt. The solenoid plunger is arranged relative to the first arm of the lever so that with the travel of the plunger, the first arm swings back and forth. The second arm of the lever is longer than the first, so that a greater travel distance is produced at the end of the second arm. The link is positioned such that when the lever swings one way, it causes, through the link, the bolt to be retracted. Rotation of the lever in the other direction (by the plunger), causes the lever, through the link, to fully extend the bolt. A greater throw distance for the bolt is produced by this mechanical arrangement, as compared to lock bolts directly driven by a solenoid plunger. 
     Moreover, and importantly, the link and pivoted lever are so arranged that the link is in alignment with the second arm of the lever when the bolt is fully extended. In other words, the pivot point (of the lever as pivoted on the housing) and the two pivoted connections of the link (to the second arm and to the bolt) form three points which are all in alignment. With this alignment, the bolt or lock element cannot be pushed back inwardly even if one could reach the bolt with a hand or a tool. The alignment of the link and the second arm of the lever will rigidly hold the extended bolt in place. 
     It is thus an object of the invention to improve the operation of an electrically operated lock, particularly a deadbolt lock, by providing a mechanical linkage between a solenoid plunger and locking element such that the components of the mechanical linkage provide a greater travel or throw of the locking element and such that the locking element cannot be forced back inwardly when it is in a fully extended state. These and other objects, advantages and features of the invention will be apparent from the following description of a preferred embodiment, considered along with the accompanying drawings. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an elevation view of an electrically operated lock in accordance with the invention, with a housing cover removed, and showing a locking element in fully extended (locked) position. 
         FIG. 2  is a view similar to  FIG. 1 , but showing the locking element in a retracted (unlocked) position. 
         FIG. 3  is a perspective view showing the lock unit of  FIGS. 1 and 2 , with the lock element extended, and with the cover and a wall of the housing removed for clarity. 
         FIG. 4  is a perspective view showing the solenoid, plunger and mechanical elements of the lock of the invention. 
         FIG. 5  is an elevation view showing a solenoid operated lock according to the prior art, shown with a housing cover removed and with a deadbolt in retracted position. 
         FIG. 6  is a view of a prior art lock unit as in  FIG. 5 , but with the deadbolt fully extended. 
         FIG. 7  is a view showing the prior art lock unit with a cover on the housing. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     A typical prior art solenoid driven electric locks is shown in  FIGS. 5, 6 and 7 . The lock  10  has a housing  12  with securing flanges  14 , the housing containing batteries  16  (two shown, e.g. 9 volt batteries) and a solenoid unit  18  having a plunger  20  driven outwardly or inwardly. The solenoid has a coil (not seen in the drawing) that creates a field when powered and exerts an electromagnetic force on the plunger  20 . In this case the solenoid  18  of the lock device  10  is a keep solenoid as described above. The solenoid has a spring  22  which urges the plunger  20 , and an attached reciprocating locking element or bolt  24 , outwardly. However, the force of the spring is normally overcome by a permanent magnet which is contained within the end  26  of the solenoid casing or unit  18 . As already described, when battery power at one polarity is put to the solenoid, this creates a magnetic field that will essentially neutralize that of the permanent magnet, allowing the spring  22  to push the bolt  24  out to its extended position as shown in  FIG. 6 . Only a momentary voltage need be applied to the solenoid coil, because once the plunger  20  (and the lock bolt  24 ) have reached the fully extended position, the permanent magnet can no longer pull the plunger back inward against the spring  22 .  FIG. 7  also shows the fully extended position of the lock bolt  24 , with the housing  12  closed by a cover  28 . 
     For retraction of the lock bolt  24 , the solenoid coil is momentarily powered at an opposite polarity, and in this state it produces a magnetic field that adds to that of the permanent magnet and is sufficient to overcome the force of the spring  22 , retracting the plunger  20  and lock bolt  24  to the retracted position shown in  FIG. 5 . Again, only momentary powering of the solenoid is required, since the permanent magnet will now keep the plunger and bolt in the retracted position.  FIGS. 5 and 6  show that the keep solenoid  18  in this typical prior art construction directly drives the bolt  24 , the plunger  20  being secured to the bolt with a fastener  29 . As explained earlier, if one could get access to the lock bolt  24  as with a tool or other device to frictionally engage the bolt  24  to enable pushing it back, this would unlock the lock, allowing access to the door or other closure. Further, the direct drive with the keep solenoid produces a limited throw for the bolt, limited as a practical matter to about 7 mm or ¼ inch as explained above. 
       FIGS. 1 through 4  show a form of the lock  30  of the invention, contained in a lock housing  31 . A greater throw of the lock bolt  24  is permitted by the invention, and, as shown in  FIG. 1 , when the bolt  24  is fully extended it cannot be pushed back against spring force but is securely and rigidly held in position. 
     Rather than directly driving the bolt, the plunger  20  of the solenoid  18  drives the bolt indirectly, through a mechanical linkage that includes a lever  32  and a link  34 . The lever  32  has two arms, a first arm  32   a  and a second arm  32   b . The lever is pivoted from a fixed pivot point position  36  on the housing, which can be via a machine screw as shown at  36 . When the lock is fully extended, as shown in  FIG. 1 , these mechanical components are so positioned that the second arm  32   b  of the lever and the link  34  are directly in alignment. Thus, pushing inwardly on the lock bolt  24  will not cause the solenoid plunger to retract and allow the bolt to be manually retracted. The link  34  is pivotally secured to the lock bolt at a pivot connection  40 , and to the outer end of the second arm  32   b  at a pivot connection  42 . The outer end of the plunger  20  is secured to the end of the first arm  32   a  of the lever at a free-pivoting connection  44 , a connection that also allows for a slight sliding translation. Comparing  FIGS. 1 and 2 , the latter showing the mechanical arrangement in the bolt-retracted position, it is seen that while the solenoid plunger  20  travels in a linear path, the end of the lever&#39;s second arm  32   a  actually moves in an arcuate path. For this reason, the end of the first lever arm  32   a  has an opening  46  that is slightly elongated as shown in  FIG. 4 . A fastener that makes the connection, seen at  44  in  FIGS. 1 and 2 , passes through a round bore  48  in the plunger (or two round bores of a split-end solenoid as seen in  FIG. 4 ), and this pivot fastener is allowed to travel along this oblong, slightly slotted opening  46  in the first lever arm as needed. Note that the angle of the first arm  32   a , in relation to the line of travel of the plunger  20 , preferably is selected so that first arm  32   a  is substantially perpendicular to plunger travel at a position that is halfway through the plunger&#39;s travel. Thus, the angles of the first arm  32   a  in  FIGS. 2 and 1  are essentially equally displaced from a right angle plunger position, aft and fore of that position. This minimizes sliding travel of the pivot member along the arm. In addition, positioning the arc of travel to straddle the right-angle position maximizes mechanical advantage of the plunger acting on the first lever arm  32   a.    
     Comparing  FIGS. 1 and 2 , it is seen that the bolt  24  is retracted by extension of the solenoid plunger  20 . The extension of the plunger is by applying the appropriate voltage polarity to the solenoid coil. The lock is typically an electronic lock operated to unlock or lock by entry of a code on a keypad or by use of a hand-held electronic contact or proximity device, or via a network. The first arm  32   a  of the lever is shorter than the second arm  32   b , with the ratio of these two lengths being chosen appropriately to produce the desired throw distance for the bolt  24 . The ratio is about 2:1 in the example shown. As  FIG. 2  shows, the forward movement of the plunger and the end of the short arm  32   a  rotates the lever  32  about the housing-mounted pivot point  36 , swinging the end of the long arm  32   b  counterclockwise in these views. This swings the pivot connection  42  with the link, pulling the link  34  back while rotating the link somewhat about the bolt pivot connection  40 . 
     In a preferred embodiment the throw of the deadbolt  24  is increased by the invention from the approximately ¼ inch typical of the prior art to over ½ inch, approximately 13 to 14 mm. 
     Sensors can be included in the lock housing, to indicate lock status. Two sensors are shown at  50  and  52 . These two sensors, which can be electromagnetic or optical (or a limit switch) and sensitive to the presence of the adjacent deadbolt, confirm that the deadbolt  24  is fully extended as in  FIG. 1 , or fully retracted as in  FIG. 2 . A circuit board is shown at  53 , and the sensors can be mounted on the circuit board. 
     The perspective views of  FIGS. 3 and 4  show certain elements not visible in other views. These views show a solenoid coil  54  within the solenoid unit  18 , and also a permanent magnet  56  at the forward end of the solenoid unit, acting on the plunger  20  in the manner described previously. Also, the deadbolt  24  is shown in  FIG. 3  within its slide track  58 , and details of the pivot connections are better revealed. In addition,  FIG. 3  shows a stop post  60  that can provide a retraction limit for the bolt  24 . This is also seen in  FIGS. 1 and 2 , with  FIG. 2  showing the lever  32  engaged against the post  60  as a limit to movement of the mechanism and of the bolt. The retracted limit position of the bolt could be established by other means, such as a housing-connected stop directly in the path of the bolt  24 . 
     In a preferred embodiment as illustrated, the angle of the aligned link  34  and lever arm  32   b , as relates to the line of travel of the bolt  24 , is about 22° (or about 20° or 24°). The angle defined by the lever  32  itself, between the arms  32   b  and  32   a  (i.e. the change in direction from the arm  32   a  into the arm  32   b ), is about 57° (or about 55° to 60°). The obtuse angle defined at the pivot point  36  would accordingly be 120° to 125°. It should be understood that the solenoid could be positioned in any desired orientation within the housing, with a practical arrangement being shown in the drawings, and the angles above relating to that particular practical arrangement in the illustrated housing  31 . The lever  32  could have the short arm  32   a  at any reasonable angle relative to the long arm  32   b , and the solenoid would then be positioned so that its plunger travel line would relate to the arc of travel of the short arm  32   a  in the manner described above, for minimal sliding and to maximize mechanical advantage. The angle described above between the second arm  32   b /link  34  and the bolt line of travel could be 0° (alignment) if desired, the important factor being that the second arm  32   b  and the link  34  be in alignment when the bolt is fully extended. Further, the ratio of the long arm  32   b  to the short arm  32   a  could be different from the approximate 2:1 shown. 
     Still further, it is possible to eliminate the link  34 , and instead provide a sliding pivot connection between the outer end of the second arm  32   b  and the base end of the bolt  24 . This would simply be a slot in the inner end of the bolt, perpendicular to the direction of bolt travel, with a pin or fastener (such as  42 ) extending directly through that slot. The positioning of the components would then be such that the bolt is fully extended when the second arm is toward the bottom of the bolt slot as the bolt is seen in  FIG. 1 , with the second arm aligned with bolt travel when the bolt is fully extended. The second arm would be to the extreme top of the bolt slot when the bolt is retracted. The bolt base could be wider (taller as seen in the drawings) to provide a wide slot for adequate bolt throw. A ball bearing could be used in the slot for reduced friction. The embodiment depicted in  FIGS. 1 through 4  is preferred, but such a modified arrangement is possible. 
     The above described preferred embodiments are intended to illustrate the principles of the invention, but not to limit its scope. Other embodiments and variations to these preferred embodiments will be apparent to those skilled in the art and may be made without departing from the spirit and scope of the invention as defined in the following claims.

Summary:
An electronic lock driven by a keep solenoid has a non-direct mechanical connection between the solenoid plunger and the slidable deadbolt of the lock. The connection includes a pivoted lever connected to the plunger and a link between the lever and the deadbolt, these components being arranged in such a way that when the deadbolt is fully extended the pivot point of the lever and pivot points on both ends of the link, with the lever and with the deadbolt, are in alignment. The mechanical arrangement prevents retraction of the extended deadbolt by pushing inward on the deadbolt, which is a problem with a deadbolt driven directly by a solenoid plunger.