Abstract:
A low-profile robust switch having low pre-travel and high overtravel is provided through the use of telescoping actuator and a housing having holes of different sizes, the larger hole being sized to completely receive a fastener within the housing and the smaller one of the holes being sized to retain a portion of the fastener within the housing while the rest of the fastener extends from the housing.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a robust electrical switch having low pre-travel and high overtravel and, more particularly, to a device limit switch, i.e., a switch designed to respond to the presence or absence of an associated device or apparatus, having such attributes. 
       BACKGROUND OF THE INVENTION 
       [0002]    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 the switch changes its electrical state, i.e., the electrical state between the switch&#39;s terminals 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. 
         [0003]    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. In a pending application, Ser. No. ______, filed Jun. 12, 2006, and assigned to the present assignee, a switch is disclosed that is intended to be mounted on the door of the shipping container to detect unauthorized opening of the container. In this application, a device limit switch is needed to accompany an electrical “black box” that would be disposed in shipping container and would communicate the location of the container to a predetermined monitoring location. The device limit switch would be mounted in each shipping container along with the electronic “black box” and would quickly respond to any attempt to tamper with or move the black box. As there is a large embedded base of shipping containers, a device limit switch for this application must be compatible with existing container designs, must be robust enough to withstand the rough treatment typically experienced by shipping containers, and must operate flawlessly. As the device limit switch must also operate with different containers and black boxes having rather large part tolerances, the switch must provide low pre-travel and high overtravel in a low height switch. While electrical switches exist in a myriad of shapes, sizes, and designs, no existing switch exists that can meet the requirements of this described shipping container application. Accordingly, providing a device limit switch for a system designed to detect unauthorized tampering or movement of the contents of a shipping container would be desirable. 
       SUMMARY OF THE INVENTION 
       [0004]    Broadly, the present invention relates to a low-profile switch having low pre-travel and high overtravel. Advantageously, this switch is suitable for use as a device limit switch in systems for tracking and monitoring shipping containers. The switch utilizes an actuator that, in response to an external force, engages with an actuation area of an internal switch element and causes the switch to change its electrical state. Preferably, the actuator is of a telescoping design that collapses upon itself so as to provide low pre-travel and high overtravel in a low height switch. In the disclosed embodiment, the switch also incorporates a housing having at least one pair of aligned, different sized mounting holes which receive a fastener. The larger hole is sized to complete receive the fastener. The smaller hole is sized to retain a portion of the fastener within the switch housing. As a result, it is more difficult to remove the fastener. Further, this hole arrangement precludes distortion of the switch housing from fastener over-tightening and protects the fastener. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a perspective view of an assembled switch in accordance with an embodiment of the present invention; 
           [0006]      FIG. 2  is an exploded perspective view of the internal elements of the switch shown in  FIG. 1 ; 
           [0007]      FIG. 3  is an exploded perspective view of the plunger assembly in the switch of  FIGS. 1 and 2 ; and 
           [0008]      FIG. 4  is an exploded cutaway perspective view of the assembled switch in  FIG. 1  showing its mounting arrangement. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0009]    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 switch is mounted beneath an item and responds immediately to any removal of the item by triggering 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. 
         [0010]    Refer now to  FIGS. 1 and 2 . Switch  100 , in accordance with the present invention, includes housing  110 , switch element  120 , switch element cover  130 , conical spring  140 , actuating assembly  150  and cover  160 . 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. 
         [0011]    Housing  110  includes end walls  111  and bottom  112 . Preferably, the housing, switch element cover and cover are configured for snap together assembly. To provide such assembly, each of the end walls  111  includes an extending tab  113  that engages with a corresponding slot  161  in cover  160 . Similarly, switch element cover  130  has a pair of extending tabs  131  (only one of which is shown in  FIG. 2 ). Each of these tabs engages with a corresponding slot  114  in sidewall  111 . Switch element cover  130  is designed to retain switch element  120  in its position as shown in  FIG. 2 . Finally, to complete the assembly, bottom  112  has four tabs  115 , two of which are shown in  FIG. 2 . Each of these tabs engages with a different slot  162  in cover  160 . 
         [0012]    Cover  160  includes a hole  163  for receiving actuator assembly  150 . Cover  160  also includes a pair of holes  164  in the sidewall  165  of cover  160 . Each of the holes  164  is aligned with a corresponding smaller diameter hole  166  in sidewall  167  of cover  160  (only one such hole  166  is visible in  FIG. 2 ). As will be described below, holes  164  and  166  are designed to enable fastening of switch  100  in a manner that conceals the fastener, and precludes distortion of housing  110  and cover  160  that could arise from fastener over-tightening. This hole arrangement also serves to protect the portion of the fastener that would otherwise be exposed if the holes  164  and  166  were of the same size. 
         [0013]    Switch element cover  130  is designed to receive and retain a waterproof single-pole, double-throw (SPDT) commercially available switch element  120  on the bottom  112  of housing. Switch element  120  has three leads  121 ,  122  and  123  which are the conventional normally open, common and normally closed switch leads along with an actuating area  125 . While for purposes of this description, switch element  120  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. 
         [0014]    When actuating area  125  of switch element  120  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  125  is depressed to what is referred to as the actuation point, the electrical states between leads  121  and  122  and between  122  and  123  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 actuator  150  causes the switch element to toggle and change the electrical state between leads  121  and  122  and between  122  and  123 . 
         [0015]    Refer now to  FIG. 3  which shows further details about actuator  150 . As shown, actuator is designed to be “telescoping”. As shown, the actuator includes a hollow outer member  151 , an inner member  152 , an overtravel spring  153 , and clip  154 . When a slight amount of force is applied is applied to the end of outer member  151  it moves downwardly by a small amount sufficient to depresses actuation area  125  and change the electrical state between the switch element&#39;s terminals. As greater force is applied, overtravel spring  153  is compressed and the outer member telescopes over the inner member. Spring  153  has a higher spring constant than that of return spring  140  and the operation of spring  153  is designed to provide the high overtravel required in the contemplated shipping application. The telescoping action of the actuator assembly facilitates providing this overtravel while reducing the overall height of the switch that would otherwise be required if the actuator assembly was not telescoping. 
         [0016]    In an assembled actuator  150 , spring  153  is retained between seat  155  of inner member  152  and the walls defining the recess within outer member  151 . Clip  154  maintains the actuator in its assembled state by engaging with slots  156  in outer member  151  so that portion  157  of the clip protrudes into the outer member and engages with the underside  158  of shoulder  159  on inner member  152 . Referring to  FIG. 2 , clip  154  also serves as a stop for return spring  140  and engages with the walls of cover  160  surrounding hole  163  to retain all but a predetermined portion of outer member  151  within switch  100 . 
         [0017]    Refer now to  FIGS. 2 and 4 . Switch  100  is designed to be mounted to a stationary structure using a pair of conventional fasteners. These fasteners are inserted completed through holes  164  in cover  160  and bottom out against the cover walls surrounding each of the holes  166 . In this manner inadvertent over-torquing of these fasteners will not distort and possibly break the cover  160  or the housing  110 . In addition, these holes serve to protect the head of the fastener, which would otherwise be exposed if hole  166  were of the same diameter as hole  164 , and hamper unauthorized removal of the switch from its mounting site. 
         [0018]    Switch  100  advantageously has a housing height of approximately 2.5 centimeters (cm) with a plunger free height above the housing of approximately 1.5 cm. The maximum pre-travel of switch  100  is 2 millimeters (mm) and the total travel of the plunger is 10 mm. 
         [0019]    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, 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. Finally, while in the described switch application, the disclosed switch functions as a device limit switch, the present invention is not restricted to only such application but, indeed, may be utilized in other types of switch applications as well.