Abstract:
An electromechanical cable lock system comprising an electromechanical locking mechanism including a cable having at least one plug end receivable in the locking mechanism and a plurality of slidable locking plates engageable with the plug in said locking mechanism. At least one of the locking plates is capable of movement in a first direction different from the movement of a second of said locking plates in a second direction. The locking plates are driven by a motor-driven cam mechanism. The system includes a card reader or other credential verifying device and an electronic controller for activating the locking mechanism, The system also includes a barrel lock for over-riding the electronic controller. A plurality of features can detect and alarm in the event of tampering or unauthorized entry.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application No. 61/536,014, filed Sep. 18, 2011. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to lock systems employing a locking element including a flexible cable; more particularly, to an electromechanical lock system employing a cable; and most particularly, to such a system having a plurality of mode options for activating the locking mechanism and a plurality of different mechanisms for defeating attack. 
       BACKGROUND OF THE INVENTION 
       [0003]    Locking systems employing a flexible cable are well known. In such a system, a tamper-resistant cable is threadable through an opening in the item(s) to be secured and may be lockably attached at both cable ends to prevent unwanted removal of the item from the cable and the premises. Some prior art cable lock systems are as simple as having loops formed on each end of the cable secured by a keyed padlock or a combination lock. Other cable lock systems may include a simple lock case into which one or both ends of the cable may be inserted, the cable ends having lockable features formed thereon. The locking mechanism may be a barrel lock, openable by a key, or a slidable locking plate movable by correct rotation of a number combination, that engages with a detent in the cable end. 
         [0004]    The above mentioned systems are not significantly tamper-proof and lack electronic means for indicating by alarm any attempt at tampering. In some prior art locks of the lock-case type, the lock case may be opened relatively easily to gain access to the lock mechanism. Further, it is known that a slidable locking plate may be dislodged to the open position by a blow to the lock housing. 
         [0005]    What is needed in the art is an improved cable lock system that may be opened by a plurality of actuating means; that includes one or more tamper-defeating features; and that can provide an alarm in response to an attempt at tampering. 
         [0006]    It is a principal object of the present invention to provide secure retention of one or more items that may be strung on a locking cable. 
       SUMMARY OF THE INVENTION 
       [0007]    Briefly described, the present invention is directed to an electronic cable lock system capable of being unlocked using a card reader or other credential verifying apparatus, and preferably also being unlockable by a key. The system is suitable for use by, for example, hardware stores for locking merchandise together in an unsecured area to prevent theft. The system includes a housing containing a card reader, a circuit board, a motor, and a key operated, over-riding cylinder lock, and may be mounted on a wall or post. 
         [0008]    Key features of a cable lock system in accordance with the present invention are:
       Hardened sheet metal plates—the sides and top of the mechanism are shielded by hardened metal plates to prevent tampering.   Cylinder lock tamper detection—a micro-switch at the base of the cylinder lock detects the rotation of the cylinder to differentiate between key unlock and credential unlock.   Cable/end plug connections—the plugs are designed to mechanically pierce the end of the mating cable, the plug is then torqued down on the cable end to secure it in place. Flats are provided on the end plugs to facilitate torqueing. A set screw is used to temporarily hold the cable in place while the end plug is torqued down. No special tools are necessary to make the connection.   End plug o-ring—an o-ring is provided on the end plug to seal the connection against water entry.   End plug snap ring—a provision for including a snap ring on the end plug is provided so that the user may optionally prevent one end of the cable from being removed when the lock is released. The snap ring would be secured in place on the end plug after the end plug is inserted through the housing cover.   Tamper-proof end plugs—the openings in the housing for the end plugs are configured to prevent a forced separation of the end plug from the cable while the end plug is locked in place in the housing.   Multiple openings in housing—the design provides for additional pairs of sockets in the housing so that multiple cables can be secured to the housing at the same time.   Opposing movement of locking plates/low force motor—this design uses multiple locking plates that move in different releasing directions to help deter unwanted release. A motor, actuated by a card reader, rotates a double-lobed cam which, in turn, acts upon the opposing locking plates, preferably via opposing roller followers. The followers are attached to pins which move the locking plates in different directions. Roller followers are used to reduce friction so that a low force motor can be used.   Locking plate bias springs of different wire diameter or coil spacing to deter unauthorized release—the locking plate bias springs are wound to resonate at different vibration frequencies, to inhibit simultaneous unauthorized movement of the locking plates.   Cam position and locking plate unlocked sensors—one Hall-effect sensor is used to detect when the motor/cam is in its locked position and its unlocked position. When sensed to be in its unlocked position, the motor is stopped and held in that position for a time period and then it rotates in an opposite direction back to its locked position. A second set of Hall-effect sensors detect when the locking plates are in their unlocked position. Thus, when a signal is received that the locking plates are unlocked while another signal is received that the motor/cam is in its locked position, an unauthorized release of the lock may be detected.   Cable breach detection—several methods of detecting a breached cable are disclosed. A first method of detecting a breach is by detecting a change of electrical resistance in the cable itself. This method requires that the plug ends be electrically insulated from the lock plates by being plated by hard anodizing. The design contemplates “teaching” the circuitry the resistance of a non-breached cable each time the cable is inserted into the housing. Another method is by a vibration transmitter/receiver located at the ends of the cable. A vibration signal through the cable would change or not be received if the cable were cut or severed. Further methods include the measurement of a time-domain reflectometry (TDR) signal pulsed through the cable for detecting a cable breach or the use of an insulated wire embedded in the stranded cable to measure wire continuity for detecting a cable breach. In one aspect of the invention, more than one insulated wire may be embedded in the cable with only one of the embedded wires establishing electrical continuity between the cable ends. Thus, an intruder would not know which wire could be jumped to avoid detection of a cable separation.   Plug retention plate—a feature is provided to retain the plug in the housing even when the locking plates are in the unlocked position. In addition to the locking plates, a retention plate is provided that serves to hold the plug in the housing even when the locking plates are in their unlocked position. The profile of the end plug provides for a two-way taper to engage plug retention plate, and one-way tapers to engage the locking plates.   Reversible locking plates—the top and bottom locking plates may be identical and interchangeable for manufacturing purposes.   Housing breach detection—a Hall-effect sensor may be used to detect when the cover is removed from the housing without authorization.   Tamper resistant housing cover—two additional screws used to mount the cover to the housing are accessible only through access holes in the cover. Access to the screws is blocked unless the key is used first to turn the cylinder lock.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0025]      FIG. 1  is an isometric view of the electric cable lock system, in accordance with the invention; 
           [0026]      FIG. 2  is an isometric view of the system shown in  FIG. 1  with the housing cover removed, in accordance with the invention; 
           [0027]      FIG. 3  is a view of the plug assemblies and cable, with one end shown in phantom, in accordance with the invention; 
           [0028]      FIG. 3A  is a close-up view of the plug and cable connection, as shown in  FIG. 3 ; 
           [0029]      FIG. 4  is a partial view of cable lock mechanism, in accordance with the invention; 
           [0030]      FIG. 5  is a side view of the cable lock mechanism including the locking plates and plug assembly, in accordance with the invention; and 
           [0031]      FIG. 6  is an isometric view of the cable lock mechanism showing the locking plates and their surrounds, in accordance with the invention. 
       
    
    
       [0032]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0033]    Referring to  FIGS. 1 through 3  and  FIG. 3A , a first embodiment  10  of an electronic cable lock system in accordance with the present invention comprises a housing cover  12  having a card/credential reader module  14 , a barrel lock cylinder  16  for manual operation, and two openings  18  for receiving a cable plug assembly  20  attached to either end of a cable  22 . Cable  22  may be comprised of several individual strands of wire wrapped tightly together such as shown in  FIG. 3A  and commonly referred to as aircraft cable. In one aspect of the invention, the cable itself may be configured as a conductor covered with a high strength stainless steel flexible casing. 
         [0034]    A partially exposed internal structure of the cable lock mechanism  24  shown in  FIG. 2  includes three hardened sheet metal plates  26  to provide security against drilling. A micro-switch  28  or similar detector provides an indication whether a proper key  30  has been inserted into cylinder  16  and rotated. A microcontroller  32  is mounted to the underside of the card reader module  14 . A housing base  34  with mounting holes  36  allows the mechanism to be mounted to a plurality of structures such as electrical boxes, posts, walls, etc. 
         [0035]    Referring to  FIGS. 3 and 3A , and for clarity purposes, a transparent view  38  of plug assembly  20  shows the cable  22  inserted into plug assembly  20 . Cable plug assembly  20  includes plug  39  and plug collar  53 . As described in more detail below, plug  39  and collar  53  are joined together to mechanically secure cable  22  to first plug assembly  20 . 
         [0036]    Plug  39  includes first plug end  40  and second plug end  56 . First plug end  40  contains two slots  42 ,  44  for accepting locking plates (not shown). A third slot  46  may be included to receive a spring loaded detent plate (not shown). The operation of the locking plates and the optional detent plate in conjunction with plug assembly  20  is discussed in more detail below. 
         [0037]    An o-ring  48  seals the connector within the housing cover  12  to keep out elements such as dirt and moisture. A snap ring  52  and groove  50  may be included to allow plug assembly  20  to be optionally non-removably affixed to housing cover  12  if so desired. This is accomplished by inserting plug assembly  20  into housing cover  12  when removed from base  34  and installing a snap ring  52  into groove  50 . The outer diameter of installed snap ring  52  is slightly larger than the clearance diameter of openings  18  in housing cover  12  thereby preventing plug assembly  20  from being removed from the housing once the snap ring is in place. This allows the end user to permanently affix the right or left plug as desired, so that only one end of the cable is removably lockable to the housing. Alternatively, neither plug may be affixed with the snap ring, thus allowing the end user to remove the cable completely for convenience purposes such as, for example, for temporary storage, to easily replace the entire cable, or to change cable lengths. 
         [0038]    Plug collar  53  of plug assembly  20  has a tapped bore  54  for receiving mating threads of second plug end  56 ; a counter bore  58  to receive cable  22 ; a locking bore  60 ; and a tapped hole(s)  62  to receive a set screw (not shown). Second plug end  56  has a cone shaped feature  64  to bore into the end strands of cable  22  and to cause the end strands of cable  22  to be radially forced outwards against the locking bore  60 , pinching the end of the cable between feature  64  and bore  54 . Additionally, first plug end  40  has a tip  65  to make electrical contact with a circuit board (not shown) within the unit, if needed. 
         [0039]    Cable  22  is attached to plug assembly  20  by inserting an end of cable  22  into counter bore  58  of plug collar  53 , and by temporarily locking the cable in place to plug collar  53  with a set screw in tapped hole(s). Then second plug end  56  of plug  39  is threaded and tightened into collar  53  to a predetermined torque. First plug end  40  of plug  39  has flats  66  that allow plug  39  to be firmly torqued with a tool such as an open ended wrench against plug collar  53 . Plug collar  53  may also have flats  68  to hold the collar from rotating while a torque is applied to plug end  39 . After the predetermined torque is reached, and after the end of cable  22  is pinched between feature  64  and bore  60 , even with the removal of the set screw(s), the plug assembly grips the cable tightly. This method of attaching a cable to a fitting may be lower in cost and more convenient than prior art designs since no special tools are required as with prior art crimp-type connections. 
         [0040]    Note that, in one aspect of the invention, the general cross section of plug  39  and the cross section of openings  18  are circular. Further, the outside diameter of first plug end  40  is sized to fit snuggly into opening  18  in housing cover  12  and, when plug assembly  20  is fully inserted into opening  18  and locked in place, first plug end  40  is configured to be entirely below the outer surface of housing  12 . Thus, if an attempt were made to unscrew plug collar  53  from first plug end  40  after being locked in place, plug collar  53  and first plug end  40  would rotate together, thereby preventing collar  53  and plug end  40  from being separated. 
         [0041]    Referring to  FIGS. 4 through 6 , mechanism  24  as depicted is further exposed. Twin locking plates  70   a,    70   b  are spring loaded in opposite directions and are normally locked into slots  42 ,  44 , respectively, on first plug end  40  ( FIG. 3 ). A key override bracket  72  is biased towards cylinder cam  74  by compression spring  76 . Override bracket  72  includes cam surfaces  78  that interact with two pins  80 ; each pin is pressed into locking plates  70   a,    70   b,  respectively. When cylinder  16  is rotated by key  30  ( FIG. 2 ), cylinder cam  74  pushes override bracket  72  against spring  76 , causing cam surfaces  78  to push pins  80  apart, thus pushing locking plates  70   a,    70   b  apart and releasing cable plug assembly  20  as shown in  FIG. 4 . 
         [0042]    Additionally, override bracket  72  has two notches  82  that block access to screws  84  that hold housing cover  12  to housing base  34  when override bracket  72  is in the key-removed position. Thus, removal of housing cover  12  from housing base  34  requires key  30  to be used to first move override bracket  72  to the unlocked position wherein access to screws  84  is then permitted through access holes  85  in cover  12  ( FIG. 1 ). 
         [0043]    It should be noted that locking plates  70   a,    70   b  have additional openings  86  ( FIG. 6 ) that, with the removal of knock-out plugs in housing cover  12 , can be used to accept two more plug assemblies  20 . This allows a user to have additional cables  22  that can be locked by cable lock system  10  and thus provide added utility. One skilled in the art can see how the design can be scaled up to accept any number of additional cable plugs  20  without deviating from the scope of the invention. 
         [0044]    It should also be noted that, in the particular design shown, locking plates  70   a,    70   b  are intentionally biased for movement in opposite directions to provide added security. One known method used to defeat locks is a technique known in the industry as “bumping”. A lock may be struck with a hammer for instance that causes the internal locking mechanism to move in reaction to the blow in such a way as to temporarily unlock, allowing a breach of the lock. By having a plurality of overlapping locking plates  70 , preferably two, moving in different, and preferably opposite, directions, it becomes unlikely that both locking plates  70   a,    70   b  will unlock simultaneously as a result of the lock mechanism being struck. At worst, one locking plate (i.e.  70   a ) will unlock while the second locking plate (i.e.  70   b ) will remain locked. While locking plates  70   a,    70   b  are shown as two plates moving in opposite direction to unlock, it is understood that more than two locking plates can be utilized to reduce the tendency of unlocking all plates by bumping and that plates can be designed to move in different directions other than opposite, such as for example at right angles, to further reduce the tendency. 
         [0045]    Further, the different locking plates can utilize springs having differing resonant frequencies that cannot vibrate in unison such that the probability of all plates unlocking simultaneously when the housing is struck is almost zero. 
         [0046]      FIGS. 5 and 6  show views of mechanism  24  in a locked state. An elliptically shaped cam  88  is mounted on a shaft (not shown). The shaft and cam  88  are rotated by a motor (not shown) between a locked position of the cam as shown and an unlocked position 90° from the locked position. Elliptically shaped cam  88  pushes on two roller cam followers  92   a,    92   b  journaled on pins  94  mounted to locking plates  70   a,    70   b.  Thus, when cam  88  rotates and pushes against roller cam followers  92   a,    92   b,  locking plates  70   a , 70   b  move from a locked position to an unlocked position. 
         [0047]    Two magnets  98   a,    98   b  are mounted on a circular shoulder  90  of cam  88 . The magnets may be of differing polarity orientation affixed to it 90° apart. Magnets  98   a,    98   b  interact with a first linear Hall-effect sensor  100  to serve as a cam rotary position sensor. At the locked position shown in  FIGS. 5 and 6 , magnet  98   a  points down (6 O&#39;clock) at Hall-effect sensor  100 . When cam  88  turns clockwise 90° to reach an unlocked position, opposite-polarity magnet  98   b  points down at Hall-effect sensor  100 . A signal is sent by sensor  100  to a controller circuit (not shown) of the microcontroller instructing the motor when cam  88  has reached either its unlocked position or its locked position. 
         [0048]    Referring again to  FIG. 5 , at the bottom ends of pins  80  are two more magnets  104  that interact with two more Hall-effect sensors  106 . As shown, the locking plates are in their locked position and magnets  104  are aligned with Hall-effect sensors  106 . If one or both locking plates  70   a,    70   b  are moved to an unlocked position without cam  88  being in its unlocked position (magnet  98   b  aligned with Hall-effect sensor  100 ), an alarm code may be sent by the microcontroller and an alarm sounded. Note that magnets  104  may become not aligned with Hall-effect sensors  106  without cam  88  being in its unlocked position when key override bracket  72  moves pins  80  by turning the barrel lock cylinder  16  with key  30 . In that situation, an erroneous alarm code could be sent. However, when lock cylinder  16  rotates to move the locking plates, a micro-switch  28  ( FIG. 2 ) under cylinder  16  also opens. When microcontroller  32  senses a change of state of both the micro-switch  28  and both Hall-effect sensors  106 , it recognizes that a key is being used and the locking plate override is proper. If the processor senses one or both Hall-effect sensors have changed state without the micro-switch changing state or a proper credential applied (such as the cam drive motor being activated), the microcontroller signals that an unauthorized entry is being attempted. Note that, when the lock is accessed properly with a credential or key, both pins  80  move simultaneously. Since both locking plate pins have their own Hall-effect sensors, as shown in  FIG. 5 , if the microcontroller ever senses that only one of the two switches has changed state, an alarm signal may be sent. Thus, the microcontroller and circuitry may be designed to send an alarm code in the event of only one of magnets  104  being aligned with its associated Hall-effect sensor  106  and the other one of magnets  104  being not aligned with its associated Hall-effect sensor  106 . 
         [0049]    Note that, in an alternate embodiment, Hall-effect sensor  100  may be removed and, in its place, sensors  106  and magnets  104  could serve indirectly as a cam rotary position sensor. That is, when the magnets  104  are aligned with sensors  106 , as shown in  FIG. 5 , it would sense, directly, the positions of the locking plates and, indirectly, the position of cam  88 , assuming that cam  88  had rotated to position the locking plates as shown. When the magnets  104  are not aligned with sensors  106 , it would conclude that cam  88  is in an unlocked position. However, in such an embodiment, if the cable plug were partially inserted in the housing, thereby holding the locking plates in their unlocked position, and the motor were to begin to rotate the cam in a direction to lock the locking plates, the microcontroller could lose track of the angular position of the cam. To alleviate this condition, the microcontroller may be programmed such that, if movement of the locking plates is not sensed upon an initial rotation of the cam toward the locking position, power to the motor is turned off then turned back on after some preprogrammed time period to assure that the plug is no longer partially inserted. 
         [0050]    In another aspect of the invention, Hall-effect sensor  100  is designed to provide an analog signal to the microcontroller as opposed to a digital signal. Hall-effect sensors  106  may be designed to provide either analog or digital signals to the microcontroller, although an analog signal may be preferred to more accurately detect the precise position of the locking plates. 
         [0051]    Referring again to  FIG. 5 , an additional magnet/Hall-effect sensor is used to detect unwanted entry into the system should the housing cover be removed without authorization. As shown, cover pin  108  is pressed into housing cover  12 . Magnet  110  is affixed to the end of pin  108  and is aligned with a third Hall-effect sensor  112  when the housing cover is in place. Thus, an alarm code is sent when housing cover  12  is removed without authorization. 
         [0052]    Upon presentation of proper credentials/authorization, microcontroller  32  is programmed to cause the motor (not shown) in system  10  to rotate cam  88  ninety degrees from a parked, locked position to an unlocked position. Microcontroller  32  is further programmed to hold cam  88  in its unlocked position for a predetermined period of time and then return cam  88  in an opposite direction to its parked, locked position. The predetermined period of time would be selected as appropriate for the application. 
         [0053]    During the predetermined period of time in which the locking plates remain held in their unlocked position by cam  88 , it is desirable that plug assembly  20  be held in place in its mating opening  18  without falling out. This is especially desirable when system  10  is mounted so that housing cover  12  faces downward. Referring again to  FIG. 5 , detent plate  118  is spring loaded in a direction to become seated in third slot  16  ( FIG. 3 ) of cable plug assembly  20  when plug assembly  20  is fully inserted into housing cover  12 . Since detent plate  118  works independent of locking plates  70   a,    70   b,  that is, detent plate  118  is always biased in a direction toward an inserted cable plug assembly regardless of the positions of the locking plates, the cable plug assembly will be held in place in its mating opening without falling out, even when the locking plates are in their unlocked position. Thus, even with the device unlocked (and without snap ring  52  in use), a force must be applied to cable plug assembly  20  to remove it from housing cover  12 . 
         [0054]    As best seen in  FIG. 5 , when elliptical cam  88  is rotated 90 degrees as shown, the locking plates  70   a,    70   b  are biased by springs  114  into a locked state. At the theoretical fully open position of the locking plates, the normal force between cam followers  92   a,    92   b  and cam  88  acts directly through the camshaft center, hence there is no leverage for the springs to rotate the cam away from the unlocked, fully open position and the motor does not need to exert any force to hold the cam in its unlocked position. In one aspect of the invention, the cam profile may have a larger “dwell” period at the nose of the cam so that even if the cam doesn&#39;t stop at the exact center of its fully unlocked position, the normal force will still act through the camshaft center. Further, in another aspect of the invention, the motor may be coupled to a transmission having a very large gear reduction (for example, 297:1) so it takes a relatively large amount of torque to back-drive the motor through the gearing. 
         [0055]    Note that even when locking plates  70   a,    70   b  are in a locked state, cable plug assembly  20  may be inserted into mechanism  24 . One-way cam surfaces  116  ( FIGS. 3 ,  3 A and  5 ) disposed at the tip of second end  56  of plug assembly  20  and adjacent one side of slots  42  and  44 , push the locking plates  70   a ,  70   b  and detent plate  118  temporarily out of the way until slots  44 ,  42  line up with locking plates  70   a,    70   b  and compression springs  114  can bias the locking plates into their respective slots. This allows a user to unlock with proper credentials and re-lock at his convenience. Note that an opposite one-way cam surface  120  is disposed adjacent one-way cam surface  116  at the tip of plug assembly  20  so that detent plate  118  can be moved out of the way when plug assembly  20  is pulled out of cable lock mechanism  24  while locking plates  70   a  and  70   b  are in their unlocked position. Note also that opposing one-way cam surfaces are not similarly provided adjacent slots  42 ,  44  to assure that plug assembly  20  cannot be pulled out of cable lock mechanism  24  when locking plates  70   a  and  70   b  are in their locked position. 
         [0056]    Another security measure provided in accordance with the invention is to monitor the integrity of the cable through electronic means, such that a cut or severed cable may be detected. A first method requires the cable plug to be plated with a dielectric material such as hard anodizing, except plug tip  122  of second end  56  that makes electrical contact with circuit board  124  and a portion of cone shaped feature  64  that makes electrical contact with cable  22 . After both cable plug assemblies  20  have been inserted into mechanism  24  and both assemblies have made electrical contact with their respective contacts on circuit board  124 , the microcontroller will make note that continuity through cable  22  has been made. After a short duration, and at predetermined intervals thereafter, the microcontroller measures the electrical resistance through the cable. An alarm code will be sent by the microcontroller if a predetermined change in resistance (e.g., +/−30%) in the cable is measured, signifying that the cable has been cut into or severed or, in any fashion, removed from mechanism  24  without proper authorization. In one aspect of the invention, a base resistance measurement is taken after continuity has been established each time the lock is locked. This method of measuring a baseline resistance first can therefore accommodate the use of different lengths of cable with noticeably different resistances. This method also makes it more difficult for cable  22  to be “jumpered” and then cut between the shorted section of the cable without setting off the alarm. Note that, in a currently preferred method, the base resistance measurement may be taken automatically. In the alternative, a base measurement can be initiated manually, for example, by turning lock cylinder  16  in a direction opposite the direction to unlock the mechanism to trip a micro-switch and to initiate the base measurement event. In a variation of this embodiment, rather than the cable strands themselves serving as the conductor for measuring continuity, a smaller sized insulated wire  126  may be intertwined within the cable wires and electrically connected to the end plugs, as shown in  FIG. 3A . Thus, if an attempt is made to sever the cable, the insulated wire would be severed or damaged as well, either indicating a short or an open, and thereby detecting a breached cable. In an enhancement of this variation, multiple insulated wires  126  may be embedded in the cable strands with only one or a few of the wires being electrically connected to the end plugs. Thus, it would be nearly impossible to determine the correct wire to jumper in order to bypass the detection system. 
         [0057]    A second method utilizes a sound transmitter (not shown) on one end of the cable and a receiver on the other end. Knowing that an acoustic pulse takes a predictable amount of time to travel through any medium, the density of the medium is the primary factor in determining the speed at which the pulse itself can travel. In a consistent medium, the speed will be more or less constant so one can determine the length based on how long the pulse takes to travel from one end to the other. Alternately, when a pulse hits a change in medium, a portion of it will reflect back towards the origin. Using this concept, one can generate a pulse and measure it either at the other end or at the original end after it has been reflected. Thus, an alarm may be triggered if a sound signal is sent into the cable and not received or if a sound signal is received outside a predetermined acceptable level or time limit. As a further embodiment in accordance with the invention, the transmitter may send a signal of a predetermined frequency that can be randomly varied by the microcontroller. Various filtering techniques can also be employed to make false alarms unlikely. 
         [0058]    In another method, a piezoelectric buzzer (not shown) may be installed on spring-loaded receptacles on each end of the cable. The spring-loaded receptacles can allow for the necessary contact between the buzzer and the cable end while protecting them from shock during insertion. An acoustic pulse is then produced through the buzzer (for example, by using a capacitor to provide enough current to make the amplitude of the pulse greater) at one end of the cable. A second piezoelectric buzzer at the other end of the cable end receives the pulse (through vibration) and generates a current, which in turn creates a measurable voltage. The time between when the pulse was produced by the first buzzer and when it is received by the second buzzer can be considered constant assuming that all other variables remain more or less constant. Thus, when a pulse is sent by the first buzzer and not received by the second buzzer or received by the second buzzer outside a predetermined predicted range, the microcontroller can send an alarm code that the cable has been cut or severed. 
         [0059]    In yet another method, measurement of the inductance of the cable is used to monitor whether the cable has been cut or severed. Each time the cable is inserted into the housing and locked, the inductance of the cable is measured and stored into memory. The cable inductance is then continuously measured and compared to the stored value. The alarm will be triggered whenever the measured inductance deviates a predetermined amount from the initially stored amount, signaling that the cable has been cut or severed. This method can also detect if a substitute cable has been used to possibly defeat the alarm by comparing the inductance of the cable being used to some predetermined level of inductance representing the inductance level of a previously used cable. 
         [0060]    In yet another embodiment, time-domain reflectometry may be used to verify the length and consistancy of a pre-measured base cable and thus detect whether the cable has been damaged or severed. 
         [0061]    Note that more than one of the mentioned methods of detecting a cut or severed cable may be used to provide redundancy or to minimize inconvenient false alarms. For example, the mechanism may also be programmed such that two or more sensing methods must signal an alarm condition before an official alarm is sent. 
         [0062]    While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.