Abstract:
A low-profile robust switch having low pre-travel and high overtravel is provided through the use of counteracting biasing elements that operate upon the actuating area of the switch in the absence of externally applied forces. These biasing elements set the switch very close to its actuation point. In the disclosed embodiments, these counteracting biasing elements include a lever and at least one spring which are arranged to provide a low-profile switch.

Description:
TECHNICAL FIELD 
   The present invention relates to a robust electrical switch having low pre-travel and high overtravel. In particular, this application is related to co-pending U.S. application Ser. No. 11/473,655 entitled “Device limit switch with low pre-travel and high overtravel,” filed on Jun. 23, 2006. 
   BACKGROUND OF THE INVENTION 
   Pre-travel and overtravel are two commonly specified requirements for a switch. Pre-travel is the amount of movement of the switch actuator mechanism before switch changes its electrical state, i.e., the electrical state between the terminals of the switch changes. The electrical state between a pair of switch terminals is typically either an electrical open circuit or a short circuit. Overtravel is the amount of movement that the switch actuator is designed to accommodate after the switch changes state. The sum of pre-travel and overtravel is the total travel of the switch actuator. 
   The rise of terrorism in the world has created the need to secure and protect activities of a general commercial nature. One such need is that of securing shipping containers against unauthorized opening after the container has been readied and sealed for shipment. Electronic systems that utilize electrical switches are being designed to track and monitor containers with respect to unauthorized opening as the containers are in transit to their respective destinations. As there is a large embedded base of shipping containers, the switches for this application must be compatible with existing container designs and must be capable of withstanding rough treatment and operate flawlessly with different containers having rather large part tolerances. In one proposed application, it is contemplated that a door switch be mounted on each shipping container between the small space between the door and door jamb. For this application, the switch must be robust and possess particular pre-travel and overtravel requirements. While electrical switches exist in a myriad of shapes, sizes, and designs, no existing switch exists that can met the requirements of the shipping container application. Accordingly, providing a door switch for a system designed to detect unauthorized opening of shipping containers would be desirable. 
   SUMMARY OF THE INVENTION 
   Broadly, the present invention relates to a low-profile switch having low pre-travel and high overtravel. Advantageously, this switch is suitable for use in systems for tracking and monitoring shipping containers. In order to meet rather stringent pre-travel and overtravel requirements, the switch utilizes an actuator that moves in response to an external force and, in turn, causes the switch to change its electrical state. The actuator engages with elements that operate on an internal switch element having a plurality of terminals and an actuating area. The application of forces to this actuation area causes the electrical states between the terminals of the switch to change. In accordance with the present invention, the switch incorporates elements that provide opposing biasing forces with respect to the actuation area so that the switch provides “hair trigger” actuation. Specifically, a first element incorporating a lever is disposed over the actuating area of the switch and provides a first biasing force sufficient to cause the switch to change state. In the absence of an external force applied to the switch actuator, the first biasing force supplied by the first element is opposed by a second biasing force provided by a second element. Preferably, to reduce the switch profile, the second element is offset from the first element. The application of external forces sufficient to provide very slight actuator movement, removes the opposing biasing force provided by the second element and the switch responds to the biasing provided by the first element and changes state. In one embodiment of the present invention, the lever of the first element is configured to act as a spring so as to supply the first biasing force. In another embodiment of the present invention, a spring element acts on the lever to provide the first biasing force. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of an assembled switch in accordance with an embodiment of the present invention; 
       FIG. 2  is an exploded perspective view of the internal elements of the switch shown in  FIG. 1  in accordance with a first embodiment of the present invention; and 
       FIG. 3  is an exploded perspective view of the internal elements of the switch shown in  FIG. 1  in accordance with a second embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A low profile, low pre-travel and high overtravel switch in accordance with the present invention is designed to preclude unauthorized tampering with a shipping container after it has been readied for shipment. In this application, the door switch is mounted to the container on its door frame and is actuated by the movement of the container door so that after the door is closed and the container is secured, even the slightest opening of the door causes the switch to change state and trigger an alarm. The alarm could be audible, visible, or a combination of both and the alarm could sound either proximate to the container that had been tampered with or at a monitoring location far removed from the container. 
     FIG. 1  shows a first embodiment of an assembled switch  100  in accordance with the present invention. The internal elements of this switch in accordance with the preferred embodiment of the present invention are shown in  FIG. 2 . Referring to  FIGS. 1 and 2 , switch  100  includes a housing  101 , switch element cover  120 , switch element  130 , lever  140  and cover  180 . In the contemplated shipping container application, the housing and cover of switch  100  are fabricated of stainless steel. However, in other less severe applications, other metals or plastics may be used. 
   Housing  101  includes an end wall  102 , sidewalls  103 , bottom  104 . Optionally, the housing also includes tabs  105  with holes  106 . Cover  180  includes a pair of holes  181  where each of these holes is aligned with a different one of holes  106 . Each hole  181  and its associated hole  106  is designed to receive a fastener, such as a screw or rivet, so as to secure cover  180  to housing  101 . Cover  180  also includes a hole  182  for receiving plunger  170 . 
   Sidewalls  103  of housing  101  include four apertures  107 . Each of these apertures receives a different one of four protrusions  121  on switch element cover  120 . Two such protrusions are shown in  FIG. 2  and two other protrusions, each aligned with a different one of the protrusions shown in  FIG. 2 , are in the background and hidden from view. Each protrusion extends through a different aperture  107  in housing  101  and is then bent over to retain the switch cover element within housing  101 . Switch element cover  120  is designed to receive and retain a waterproof single-pole, double-throw (SPDT) commercially available switch element  130  between the underside of switch cover plate  120  and the housing bottom  104 . Switch element  130  has three leads  131 ,  132  and  133  which are the conventional normally open, common and normally closed switch leads along with an actuating area  135 . While for purposes of this description, switch element  130  is an SPDT element, the present invention may be used with a single pole single throw (SPST) having two leads—common and either normally open or normally closed. Suitable low profile switch elements for use in the present invention include the B7000 series waterproof switches offered by Control Products, Inc. of East Hanover, N.J. Within the B7000 series, those designated as B7113 and B7112 are SPDT and SPST implementations with momentary contacts. 
   When actuating area  135  of switch element  130  is not depressed, there is an electrical open circuit exists between the common lead and the normally open lead and an electrical short circuit between the common lead and the normally closed lead. When actuating area  135  is depressed to what is referred to as the actuation point, the electrical states between leads  131  and  132  and between  132  and  133  are flipped, i.e., they are an electrical short circuit and an electrical open circuit respectively. As will be described, in an assembled switch, the switch element is maintained close to its actuation point so that a very small movement of plunger  170  causes the switch element to toggle and change the electrical state between leads  131  and  132  and between  132  and  133 . 
   Switch element cover  120  includes a hole in its top surface  122  for receiving actuator  135  of switch element  130 . Switch element cover  120  also incorporates lance or raised portion  123  on to which tab  141  of lever  140  is welded. Lever  140  further includes a spherical button  142  that extends from the underside of lever  140 . Lever  140  engages with conical bias spring  150  and counteracting coil spring  160 . In the assembled switch, each of these springs provides opposite forces on lever  140 . 
   In the assembled switch, after cover plate  180  is secured to housing  101  via a pair of fasteners that extend through holes  181  and holes  106  in tab  105 . When so assembled, conical bias spring  150  is compressed between lever  140  and the underside of cover plate  180  so as to exert a force on lever  140  that is downward with reference to  FIG. 2 . In this embodiment, conical bias spring  150  and lever  140  can be viewed as a first switch element. This spring force is sufficient to cause spherical button  142  extending from the underside of lever  140  to contact and depress switch actuating area  135  so that the switch would change its electrical state. The spring force provided by conical bias spring  150 , however, is opposed by a force provided by coil spring  160  which via skirt  171  of plunger  170  acts on tabs  145  of lever  140 . In this embodiment, coil spring  160  and skirt  171  can be viewed as a second switch element. Spring  160  extends between the bottom of housing  101  and a recess (not shown) in plunger  170 . To reduce the height or profile of the switch, the longitudinal axes of springs  150  and  160  are offset from one another. In an assembled switch with no external forces acting on plunger  170 , the plunger is in its fully extended position. This is the normal state of switch  100  and in this state circumferential skirt  171  on plunger  170  contacts the underside of fingers or pair of extending members  145  of lever  140  and exerts an upward force on the lever tabs. The opposing forces provided by springs  150  and  160  maintains the switch element  130  very close to its actuation point so that a very slight depression of plunger  170  compresses coil spring  150  downwardly in  FIG. 2  so that skirt  171  does not contact fingers  145  of lever  140 . As a result, the switch actuating area  135  is subject only to the downward force exerted by conical spring and the removal of the upward forces created coil spring  160  causes the switch element to toggle and change the electrical state existing between leads  131  and  132  and between  132  and  133 . Accordingly, in the contemplated shipping container application, the closure of the shipping container door depresses the plunger  170  so as to place the switch in a first state and the opening of the container door causes the switch to change from this first state to a second state. Switch  100  advantageously has a housing height of approximately 1.6 centimeters (cm) with a plunger free height above the housing of approximately 1.9 cm. The maximum pre-travel of switch  100  is 2 millimeters (mm) and the total travel of the plunger is 10 mm. 
   Refer now to  FIG. 3  which shows an alternate embodiment for the internal elements of switch  100 . As shown in  FIG. 3 , the conical bias spring  150  of  FIG. 2  has been eliminated by a modification of lever  140 . Modified lever  210 , as with lever  140 , is spot welded to lance  123 . Lever  210 , however, incorporates a knee  220  which depresses switch actuating area  135  in the assembled switch. This knee provides the effect of conical bias spring  150 . As in the embodiment of  FIGS. 1 and 2 , coil spring  160  opposes the downward force exerted by the knee as described in reference to  FIGS. 1 and 2 . The other elements of  FIG. 3  are identical to the identically numbered counterparts in  FIGS. 1 and 2 . The disadvantage of the embodiment of  FIG. 3  is that the formation of the knee and positioning of the lever  210  on lance  210  is critical and considered more difficult to manufacture and maintain over the life of the switch. The switch dimensions and pre-travel and travel characteristics in this embodiment are identical to that described in reference to  FIGS. 1 and 2 . 
   It should of course be understood that while the present invention has been described in reference to particular embodiments, other arrangements may be provided by those of ordinary skill in the art without departing from the spirit and scope of the present invention. For example, while the present invention utilizes a plunger element, other elements, including but not limited to a button, roller, or cantilever, can be substituted for the disclosed plunger. Or, for example, while the present invention relates to an electrical switch, the actuating mechanism disclosed could be uses to activate other types of switches, such as an optical switch.