Abstract:
An apparatus has a sensor with an electrically conductive ground member, electrically conductive first and second parts spaced from and proximate to each other and the ground member, and an insulator disposed between the ground member and the first and second parts. A different configuration involves a tag having circuitry, and a sensor supported on the tag and having electrically conductive first and second parts that are spaced from and proximate to each other, the first and second parts each being electrically coupled to the circuitry.

Description:
This application claims the priority under 35 U.S.C. §119 of U.S. provisional application No. 60/732,240 filed Nov. 1, 2005, the disclosure of which is hereby incorporated herein by reference. 

   FIELD OF THE INVENTION 
   This invention relates in general to monitoring techniques and, more particularly, to techniques for monitoring a metal part such as a door of a shipping container. 
   BACKGROUND 
   A variety of different products are shipped in cargo containers. Products are packed into a container by a shipper, after which the container doors are closed and then secured with some type of lock or seal. The container is then transported to a destination, where a recipient removes the lock and unloads the container. 
   The shipper often finds it advantageous to have some form of monitoring while the container is being transported. For example, the cargo within the container may include relatively valuable products such as computers or other electronic devices, and thieves may attempt to break into the container and steal these products if the container is left unattended during transport. It is not cost-feasible to have a person watch a container at all times in order to provide security and/or monitoring. Accordingly, electronic systems have previously been developed to provide a degree of automated security and/or monitoring. Although these pre-existing systems have been generally adequate for their intended purposes, they have not been satisfactory in all respects. 
   As one example, mechanical door sensors have previously been used to monitor a door of a shipping container, in order to verify that the door remains closed during transport. Mechanical door sensors typically have at least one part (such as a shaft or plunger) that moves when a container door is opened or closed. In some applications, the moving part has to be hermetically sealed before it enters an enclosure containing sensing electronics. Vandals or terrorists may attempt to defeat a mechanical sensor by locking the moving part in place, for example with an epoxy adhesive, or a drill bit. If the movable part is no longer able to move, it cannot detect a situation where the door is opened. 
   SUMMARY OF THE INVENTION 
   One broad form of the invention involves a sensor that includes: an electrically conductive ground member; electrically conductive first and second parts spaced from and proximate to each other and the ground member; and an insulator disposed between the ground member and the first and second parts. 
   A different broad form of the invention involves a tag having circuitry; and a sensor supported on said tag and having electrically conductive first and second parts that are spaced from and proximate to each other, said first and second parts each being electrically coupled to said circuitry. 
   Another broad form of the invention involves monitoring an electrical characteristic between electrically conductive first and second parts that are spaced from and proximate to each other and an electrically conductive ground member, where an insulator is disposed between the ground member and the first and second parts. 
   Still another broad form of the invention relates to a tag having thereon a sensor with electrically conductive first and second parts that are spaced from and proximate to each other and that are electrically coupled to circuitry within the tag. This form of the invention involves monitoring an electrical characteristic between the electrically conductive first and second parts using the circuitry in the tag. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention will be realized from the detailed description that follows, taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a diagrammatic top view of an apparatus that embodies aspects of the invention, and that includes a radio frequency identification tag, a sensor supported on the tag, and two movable doors of a shipping container. 
       FIG. 2  is a diagrammatic view of the sensor of  FIG. 1 . 
       FIG. 3  is a diagrammatic sectional view of the sensor, taken along the section line  3 - 3  in  FIG. 2 . 
       FIG. 4  is a diagrammatic perspective view of the tag and sensor of  FIG. 1 . 
       FIG. 5  is a different diagrammatic perspective view of the tag and sensor of  FIG. 1 , with an outer housing of a control module omitted so that certain structure within the control module is visible. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a diagrammatic top view of an apparatus  10  that includes a radio frequency identification (RFID) tag  11 , a sensor  12  supported on the tag, and two movable doors  13  and  14 . In  FIG. 1 , the tag  11  is removably supported on an edge of the door  13 . The doors  13  and  14  can each move between open and closed positions.  FIG. 1  shows each of the doors  13  and  14  in the closed position. 
   In the disclosed embodiment, the doors  13  and  14  are part of a conventional shipping container of a well-known type that conforms to standards set by the International Organization for Standardization (ISO). More specifically, the container complies with an industry-standard specification known as an ISO 668:1995(E) Series 1 freight container. The vast majority of containers that are currently in commercial use conform to this ISO standard. As is standard for this type of container, the doors  13  and  14  are each made of metal. The door  14  has a rubber door gasket with both conductive and polar properties. This door gasket and a metal strap are riveted to an edge of the door  14 . When the doors  13  and  14  are both closed, the gasket and metal strap are not readily accessible from outside the container. The ISO 668:1995(E) Series 1 container is mentioned by way of example. The present invention is not limited to this particular type of container, or containers in general. 
   The tag  11  includes a resilient metal support clip  21  that is a single integral part and that is bent to have approximately a C-shape. The inner surface of the clip  21  has several bosses  22 . The bosses  22  serve as gripping structure that helps resist movement of the support clip  21  relative to the edge of the door  13 . In particular, the bosses  22  resist detachment of the support clip  21  from the container door  13  due to horizontal movement in a rightward direction in  FIG. 1 , or due to vertical downward sliding movement of the support clip  21  along the edge of the door  13 . In addition to the bosses  22 , or in place of the bosses  22 , it would alternatively be possible to provide a gripping structure in the form of a non-slip sheet  23  that is securely mounted to one or more of the inner surfaces of the support clip  21 . The sheet  23  could, for example, be made of rubber or some other suitable non-slip material. 
   The tag  11  includes a wireless communication module  26  that is fixedly mounted to the outer end of one leg of the C-shaped support clip  21 . The module  26  includes a housing that has an antenna therein, and the antenna can be used to transmit and receive wireless signals, for example as shown diagrammatically at  27 . The wireless communication module  26  may also have within its housing some support circuitry for the antenna. When the tag  11  is removably supported on the container door  13 , the wireless communication module  26  is on the exterior side of the door  13 . 
   The tag  11  also includes a control module  31  that is fixedly mounted on the leg of the clip  21  opposite from the leg with the wireless communication module  26 . When the tag  11  is mounted on the container door  13 , and when the container door  13  is in its closed position, the control module  31  is disposed in the interior of the container. The control module  31  contains control circuitry of the tag  11 . The control circuitry within the control module  31  is electrically coupled to the antenna and any other circuitry within the wireless communication module  26 , in a manner discussed later. 
   The sensor  12  is fixedly mounted on the bight of the C-shaped support clip  21 . As mentioned above,  FIG. 1  shows the metal container doors  13  and  14  in their closed positions. It will be noted that the edge of the metal door  14  is disposed closely adjacent the sensor  12 . A gasket or seal on the door  14  may actually engage the sensor  12 . 
     FIG. 2  is a diagrammatic view of the sensor  12 . The sensor  12  is flexible, and is shown in  FIG. 2  in an approximately flat or planar state, in order to facilitate an understanding of the structure of the sensor  12 .  FIG. 3  is a diagrammatic sectional view of the sensor  12 , taken along the section line  3 - 3  in  FIG. 2 . The sensor  12  has a flexible casing  50  made of an insulating material. More specifically, the flexible casing  50  is defined by three layers  51 - 53  that are each made of the insulating material. In the disclosed embodiment, the insulating layers  51 - 53  are each made from a commercially-available tape that has an adhesive on one side thereof, which is the lower side of each layer in  FIG. 3 . This tape is commercially available under the trademark KAPTON® from E.I. DuPont De Nemours and Company Corporation of Wilmington, Del. This tape has a polyimide film, with a silicone adhesive on one side of the film. The polyimide film and silicone adhesive are heat resistant, and can be used over a wide operational temperature range, for example up to a temperature of 260° C. 
   As shown in  FIG. 2 , the casing  50  has a main portion  56  that is approximately rectangular, and has an extension portion  57  that projects outwardly from one end of the main portion  56 . The extension portion  57  has a width that is less than the width of the main portion  56 . An electrical connector  61  is mounted to the outer end of the extension portion  57  of the flexible casing  50 . The electrical connector  61  has three electrical contacts or terminals, which are shown diagrammatically at  62 - 64  in  FIG. 2 . 
   The sensor  12  includes a ground plane  66  in the form of a thin sheet of copper that is disposed between the insulating layers  51  and  52 . The ground plane  66  is thin and flexible. In the disclosed embodiment, the ground plane  66  has a thickness in the range of about 0.0007 inch to 0.0028 inch. Although the disclosed ground plane  66  is relatively thin, this is specifically to achieve its flexibility. In an alternative embodiment, the ground plane would not be flexible, and in that case the ground plane would not need to be thin, and could have any suitable and convenient thickness. 
   As shown in  FIG. 2 , the ground plane  66  of the disclosed embodiment has an overall shape similar to that of the casing  50 , including a rectangular main portion  67  and an extension portion  68 . However, the ground plane  66  has width and length dimensions that are slightly smaller than those of the casing  50 . The portions of the casing  50  that extend laterally beyond the edges of the ground plane  66  help to electrically isolate the ground plane  66  from structure external to the sensor  12 . At the outer end of the extension portion  68 , the ground plane  66  has a short, narrow strip that is electrically coupled to the electrical contact  62 . 
   The sensor  12  further includes two electrically conductive copper plates  76  and  77  that are generally rectangular, that are spaced a small distance from each other, and that are disposed between the insulating layers  52  and  53 . In the disclosed embodiment, the plates  76  and  77  each have a thickness in the range of about 0.0007 inch to 0.0028 inch. It is advantageous for the plates  76 - 77  to be relatively thin, because as the thickness of the plates is reduced, there is a reduction in the capacitance between the plates that is not related to the intended sample volume. The plates  76  and  77  are disposed approximately in a center region of the main portion  56  of the casing  50 . Each of the plates  76  and  77  has a narrow strip  78  or  79  that extends to the outer end of the extension portion  57  of the casing  50 . The strips  78  and  79  are respectively electrically coupled to the terminals  63  and  64  of the connector  61 . From an electrical perspective, the spaced plates  76  and  77  effectively define a capacitor. 
     FIG. 4  is a diagrammatic perspective view of the tag  11  and the sensor  12 .  FIG. 5  is a further diagrammatic perspective view of the tag  11  and sensor  12 , taken from a different direction, and with an outer housing of the control module omitted so that certain structure within the control module is visible. More specifically, there are four posts or standoffs  102  that each have one end fixedly coupled to the support clip  21 . A circuit board  101  is secured to the opposite ends of the posts  102 . 
   A ribbon cable  104  has one end coupled to the circuit board  102 , extends through an opening in one leg of the support clip  21 , and then extends along the inner surface of the support clip  21 . The ribbon cable  104  is adhesively secured to this inner surface, but could alternatively be held in place in any other suitable manner. The ribbon cable  104  then passes through an opening in a further leg of the support clip  21 , and into the wireless communication module  26 . Thus, the ribbon cable  104  electrically couples the control circuit on the circuit board  101  to the antenna and any other circuitry disposed within the wireless communication module  26 . 
   The circuit board  101  has control circuitry thereon, including an integrated circuit  106 . In the disclosed embodiment, the integrated circuit  106  is a 24-bit sigma-delta capacitance-to-digital converter that is available commercially as part number AD7745 from Analog Devices, Inc. of Norwood, Mass. An electrical connector  107  is mounted to the circuit board  101  at one edge thereof, and is electrically coupled to the integrated circuit  106  by several runs or traces on the circuit board  101 , as indicated diagrammatically by a broken line at  108 . 
   As discussed above in association with  FIG. 3 , the insulating layer  51  is made from an electrically non-conductive tape that has an adhesive on one side, which is the bottom side thereof in  FIG. 3 . Referring again to  FIGS. 4 and 5 , this adhesive on the insulating layer  51  secures the sensor  12  to the C-shaped support clip  21 . The main portion  56  of the flexible casing  50  has a center region secured to the bight of the clip  21 , with opposite ends of the main portion  56  each extending around a curved portion of the clip  21  where the bight merges into the legs. 
   With reference to FIGS.  1  and  4 - 5 , the capacitive plates  76  and  77  are positioned so that, when the container doors  13  and  14  are both closed, an edge of the metal container door  14  will be closely adjacent the capacitive plates  76  and  77 . As best seen in  FIGS. 1 and 4 , the main portion  56  of the casing  50  has one edge that extends into the control module  31 , in particular by extending between the support clip  21  and an edge of the housing of the control module  31 . The extension portion  57  of the casing  50  then extends upwardly toward the circuit board  101 , where the electrical connector  61  on the extension is operatively engaged with the electrical connector  107  on the circuit board. Thus, the ground plane  66  and the capacitive plates  76  and  77  are each electrically coupled to the integrated circuit  106 . 
   In operation, the integrated circuit  106  supplies an electrical signal to the capacitive plate  76 , and this signal is then capacitively coupled from the plate  76  to the plate  77 . The integrated circuit  106  can measure the strength of the signal that is capacitively induced within the plate  77 . When the metal door  14  ( FIG. 1 ) is in its closed position adjacent the capacitive plates  76  and  77 , it influences the capacitive coupling between the plates  76  and  77  in a manner so that more energy is capacitively coupled from the plate  76  to the plate  77  than when the door  14  is in its open position spaced from the plates. Consequently, by monitoring the strength of the signal induced within the plate  77 , the integrated circuit  106  can determine whether the door  14  is closed or open. 
   In more detail, the integrated circuit  106  has a built-in excitation source. The capacitive plate  76  is electrically coupled to and driven by the excitation source, and the other capacitive plate  77  is coupled to an input of the sigma-delta converter. As mentioned earlier, the door  14  has a gasket secured to the edge thereof and, when both doors are closed, the gasket on the door  14  is in close proximity to both capacitive plates  76  and  77 . The combination of dielectric and conductive properties of the gasket and the metal of the door  14 , when located proximate to the two capacitive plates  76 - 77 , increases the capacitance between the plates. When the door  14  is opened, the gasket and metal of the door  14  move away from the two capacitive plates  76  and  77 , thereby decreasing the capacitance between these plates. 
   The sensing electronics in the integrated circuit  106  can measure small values of capacitance between the two conductive capacitor plates  76 - 77  (less than 1 picofarad), while tolerating larger shunt capacitances between either of the plates  76 - 77  and the ground plane  66 . The ground plane  66  effectively shields the capacitance measured between the capacitor plates  76 - 77  from all conductive or dielectric substances on the side of the ground plane opposite from the capacitive plates. The capacitance measured between the two capacitive plates  76 - 77  is thus indicative of the configuration of conductive and dielectric material currently located within a sample space or sample volume that is disposed on the same side of the ground plane  66  as the two capacitive plates. This facilitates use of the disclosed sensor  14  in applications where it is mounted on a metal object such as the door  13  of an ISO container, because this configuration minimizes any impact that the metal object might have on measurement of the capacitance between the two capacitor plates  76  and  77 . The effective capacitance between each of the plates  76 - 77  and the ground plane  66  shunts the desired capacitive effect produced within the intended sample volume on the other physical side of the plates  76 - 77 . An actual implementation exhibited a 3000:1 signal-to-noise ratio between the door open and door closed states, and was also able to reliably detect a state in which a door was partially open. 
   Assume that the container is in transit, and that its doors  13  and  14  are supposed to remain closed throughout the trip. Further, assume that the sensor  12  detects that one of the doors  13  and  14  has been opened. In response to detection by the sensor  12  that one of the doors has been opened, the tag  11  can transmit a radio signal  27  to a not-illustrated receiver of a known type that is disposed at a remote location. The radio signal  27  would indicate that one of the doors  13  and  14  was opened at a time when it was supposed to be closed. Appropriate action can then promptly be taken. 
   The disclosed door sensor  12  has no moving parts, and this offers certain advantages in comparison to pre-existing mechanical door sensors. For example, as mentioned earlier, mechanical door sensors typically have at least one part (such as a shaft or plunger) that moves when a container door is opened or closed. In some applications, the moving part has to be hermetically sealed where it enters an enclosure containing sensing electronics. Vandals or terrorists may attempt to defeat a mechanical sensor by locking the moving part in place, for example with an epoxy adhesive, or a drill bit. If the movable part is no longer able to move, it cannot detect a situation where a door is opened. 
   In contrast, the disclosed capacitive door sensor  12  has no moving parts, and is more difficult to defeat. The capacitive sensor  12  measures the bulk properties of material within a sample space on the active side of the sensor ground plane  66 , or in other words the side with the two capacitor plates  76 - 77 . Any tampering within this sample space will necessarily affect the measured capacitance value. Consequently, attempts to mechanically defeat the capacitive door sensor  12  can change the measured capacitance, and thus result in detection of the tampering. In theory, one way to open the container door without detection by the capacitive sensor  12  would be to duplicate the bulk volumetric properties of the door gasket and the metal door  14  with something that remains in place when the door is opened. However, as noted above, the ISO door gasket is riveted to an edge of the door  14  with a metal strap that is not readily accessible from outside the container when both doors are closed. Even assuming that the gasket and strap could somehow be detached from the door  14  and then held in place near the sensor  12  while the door  14  was opened, the metal of the door  14  itself would move out of the sample space, and the sensor  12  would detect this. Any object or material slid between the door gasket and the capacitive plates  76 - 77  would change volumetric properties in very close proximity to the capacitive plates (i.e. the most sensitive region of the sample space), and would thus be readily detected by the sensor  12 . Consequently, the disclosed capacitive door sensor  12  provides a high degree of tamper detection. 
   As explained above, the sensing electronics for the door sensor  12  can be implemented with an integrated circuit  106 . Consequently, the disclosed door sensor  12  and associated circuitry can operate with very low power consumption, and can be manufactured with a lower cost than traditional mechanical door sensors. Although the foregoing discussion describes how the disclosed sensor  12  can be used to monitor the open or closed status of the doors of an ISO container, the disclosed sensor is not limited to this particular application, and could alternatively be used in any of a variety of other applications. 
   In the disclosed embodiment, the sensor  12  is implemented with several insulating layers  51 - 53  made of tape, with electrically conductive elements such as the ground plane  66  and plates  76 - 77  disposed between the insulating layers. However, it would alternatively be possible to implement the sensor  12  using technology known in the art as a flat flexible cable (FFC). Such a FFC would have thin layers of a conductive material such as copper laminated between insulating layers of an insulating material such as a polyimide. An suitable adhesive of a type known in the art could be provided on one side of the FFC to secure it to the tag  11 . 
   Also, in the disclosed embodiment, the C-shaped clip  21  is made of metal and the sensor  12  is mounted on the outer side of the clip. However, it would alternatively be possible to make the clip  21  of a non-conductive material that is not significantly polar, such as a suitable plastic, and in that case the sensor  12  could be mounted on the inner side of the clip. In that configuration, the plates  76 - 77  would be located between the clip and the ground plane  66 . Stated differently, the ground plane  66  would be between the plates  76 - 77  and the metal door on which the clip  21  is mounted. 
   Although a selected embodiment has been illustrated and described in detail, it should be understood that a variety of substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the following claims.