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This application claims the priority under 35 U.S.C. §119 of provisional application No. 60/906,051 filed Mar. 9, 2007. 

   FIELD OF THE INVENTION 
   This invention relates in general to security for containers that can hold one or more items and, more particularly, to a method and apparatus for sealing such containers. 
   BACKGROUND 
   One common use for containers is the shipment of goods from one location to another. Goods are packed into the container, and a door of the container is closed and latched. Then, the container is transported to a destination by one or more vehicles, such as trucks, planes, trains and/or ships. At the destination, the container door is unlatched and opened, and the goods are removed. 
   The transportation industry has recognized that it is important to provide security for the goods being transported in such containers. As one aspect of this, there is a need to prevent goods from being removed from a container while it is in transit to its destination, even if the container itself is not stolen, misrouted or misplaced. There is also a need to prevent someone from opening the container and inserting some additional item, such as a bomb. 
   For this purpose, there are existing seal devices that are used to seal or lock the latch mechanism for the door of the container. The most common type of seal device has a disposable bolt and a reusable housing. The bolt is inserted through the latching mechanism of the container, and the reusable housing is then pressed onto an end of the bolt. The bolt and housing have cooperating structure that completely prevents withdrawal of the end of the bolt from the housing in a direction opposite to its insertion direction. To remove this seal device from a container, the disposable bolt must be cut with a bolt cutter. 
   Some seal devices of this type also include radio frequency identification (RFID) tag circuitry. If the circuitry detects any form of tampering with the seal device, the circuitry transmits a wireless signal that contains information indicative of the tampering. While seal devices of this type have been generally adequate for their intended purposes, they have not been satisfactory in all respects. 

   
     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 side view of a seal device that embodies aspects of the invention, and that is used to seal or lock the latch mechanism for the door of a shipping container. 
       FIG. 2  is a diagrammatic sectional side view of a seal device that embodies aspects of the invention, and that is an alternative embodiment of the seal device of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a diagrammatic side view of an apparatus in the form of a seal device  10  that embodies aspects of the invention, and that is used to seal or lock the latch mechanism for the door of a shipping container. Two parts of a container latch mechanism are shown diagrammatically in broken lines at  13  and  14 . The container and its latch mechanism are entirely conventional. Therefore, the parts  13  and  14  of the latch mechanism are not illustrated and described here in detail, but instead are discussed only briefly, to facilitate an understanding of the invention. The parts  13  and  14  have respective cylindrical openings  17  and  18  therethrough, which are coaxially aligned with each other in  FIG. 1 . The seal device  10  prevents relative movement of the parts  13  and  14  in a horizontal direction in  FIG. 1 , as evident from the discussion that follows. 
   The seal device  10  includes two spaced metal parts  26  and  27  that have a high magnetic permeability, and that are fixed against movement with respect to each other. In the disclosed embodiment, the parts  26  and  27  are each made of steel, but they could alternatively be made of any other suitable material. The parts  26  and  27  each have approximately the shape of the letter “F”. In this regard, the parts  26  and  27  have respective main portions  31  and  32  that extend parallel to each other. The part  26  has two spaced and parallel projections  36  and  37  that extend outwardly from the main portion  31  approximately perpendicular thereto, in a direction toward the part  27 . Similarly, the part  27  has two spaced and parallel projections  38  and  39  that extend outwardly from the main portion  32  approximately perpendicular thereto, in a direction toward the part  26 . 
   The projection  37  is located at one end of the main portion  31 , and the projection  39  is located at one end of the main portion  32 . The projections  37  and  39  are aligned with each other, and have a space between their outer ends. The projection  36  is provided at a location approximately halfway along the length of the main portion  31 , and the projection  38  is provided at a location approximately halfway along the length of the main portion  32 . The projections  36  and  38  are aligned with each other, and have a space between their outer ends. The main portion  31  has a cylindrical opening  41  extending therethrough near an end remote from the projection  37 , in a direction approximately parallel to the projections  36  and  37 . The main portion  32  has a cylindrical opening  42  extending therethrough near an end remote from the projection  39 , in a direction approximately parallel to the projections  38  and  39 . The cylindrical openings  41  and  41  are coaxially aligned. 
   A permanent magnet  51  is disposed between and engages the outer ends of the projections  36  and  38 . The magnet  51  serves as magnetic field generator. A circuit board  61  is fixedly coupled to each of the parts  26  and  27  by several screws or bolts, one of which is identified by reference numeral  62 . A magnetic field detector  66  is supported on the circuit board  61 , at a location between the ends of the projections  37  and  39  on the parts  26  and  27 . In the disclosed embodiment, the detector  66  is a Hall effect sensor, but it could alternatively be any other type of suitable detector, one example of which is a magnetoresistive sensor. A radio frequency identification (RFID) tag circuit  68  is also provided on the circuit board  61 , and is responsive to the output of the Hall effect sensor  66 . The tag circuit  68  is a type of circuit that is well known in the art, and it is therefore not described here in detail. The tag circuit  68  includes a not-illustrated transceiver that can send and receive wireless signals. 
   The seal device  10  further includes a seal bolt  81  that is magnetically permeable, that has an elongate cylindrical shank  82 , and that has a circular head  83  at one end of the shank, the head  83  having a diameter greater than the diameter of the shank  82 . A circumferential groove  84  is provided in the shank  82 , near an end remote from the head  83 . In the disclosed embodiment, the bolt is made of steel, but it could alternatively be made of any other suitable material(s) of high magnetic permeability. In  FIG. 1 , the shank  82  of the bolt  81  extends through the aligned openings  41  and  42  in the parts  26  and  27 , and also extends through the aligned openings  17  and  18  in the latch parts  13  and  14 . 
   The seal device  10  includes a retaining mechanism  88 . The retaining mechanism  88  is known in the art, and is therefore not described here in detail. When the shank  82  of the bolt  81  has been inserted successively through the openings  41 ,  17 ,  18  and  42 , and reaches the position shown in  FIG. 1 , the retaining mechanism  88  engages the circumferential groove  84 , and fixedly holds the bolt  81  against upward movement in  FIG. 1 . That is, the bolt cannot be withdrawn in an upward direction from the openings  41  and  42  in the parts  26  and  27  of the seal device  10 . The only way to disengage the seal device  10  from the latch parts  13  and  14  of a container is to intentionally cut the shank  82  of the bolt at a location between the parts  26  and  27 . 
   The seal device  10  has a housing  91  that is indicated diagrammatically by a broken line. The housing  91  encloses the retaining mechanism  88 , the circuit board  61 , the magnet  51 , and portions of the parts  26  and  27 . The permanent magnet  51  produces a magnetic field, and the magnetic flux of this field will follow the path of lowest reluctance. More specifically, when the bolt  81  is installed and intact, as shown in  FIG. 1 , the path of lowest reluctance for the magnetic flux is indicated diagrammatically by a broken line  93 . It extends from the upper end of the magnet  51  though the part  26  to the bolt  81 , through the shank  82  of the bolt to the part  27 , and through the part  27  to the lower end of the magnet  51 . On the other hand, if the bolt  81  is cut in the region of the latch parts  13  and  14 , and the portion thereof with the head  83  is withdrawn, the path  93  will no longer be the path of lowest reluctance. Instead, the path of lowest reluctance will be that indicated diagrammatically by a broken line  94 . This path extends from the upper end of the magnet  51  through the part  26  to the end of projection  37 , across the small gap between the projections  37  and  39  and thus past the Hall effect sensor  66 , and then through the part  27  to the lower end of the magnet  51 . 
   In essence, when the bolt  81  is installed and intact, as shown in  FIG. 1 , its shank  82  serves as a form of magnetic shunt for the flux from the magnet  51 , such that the flux is shunted through the bolt rather than being routed past the sensor  66 . In contrast, when the bolt  82  is cut and is no longer able to serve as a shunt, the magnetic flux is routed past the Hall effect sensor  66 . Thus, when the bolt  81  is installed and intact, as shown in  FIG. 1 , there will be a relatively low level of magnetic flux in the region of the Hall effect sensor  66 . In contrast, if the bolt is cut and a portion of the bolt is removed, there will be an increase in the level of magnetic flux at the Hall effect sensor  66 . The Hall effect sensor  66  can detect a change in magnetic flux, and then change its output signal. The change in the output signal of the sensor  66  will tell the tag circuit  68  that the bolt  82  has apparently been cut. If the container bearing the seal device  10  has reached its destination and is in the process of being opened, then this is normal. But if the container is still in transit and the seal device  10  should still be intact, then it is likely that a thief has cut the bolt  81  in order the remove the seal device  10  and gain unauthorized access to the interior of the container. Accordingly, the tag circuit  68  will transmit a wireless signal containing an indication that the seal device  10  has apparently experienced some form of tampering. 
     FIG. 2  is a diagrammatic sectional side view of a seal device  110  that embodies aspects of the invention, and that is an alternative embodiment of the seal device  10  of  FIG. 1 . Components in  FIG. 2  that are identical or equivalent to components in  FIG. 1  are identified with the same reference numerals in both drawing figures. For convenience and clarity, some portions of the seal device  110  have been omitted in  FIG. 2 . For example, the seal device  110  includes a housing and a retaining mechanism that are comparable to the housing  91  and retaining mechanism  88  in the seal device  10  of  FIG. 1 , but the housing and retaining mechanism of the seal device  110  have intentionally been omitted from  FIG. 2 . 
   The seal device  110  includes an L-shaped part  121  that is magnetically permeable, and that has two legs  122  and  123  extending approximately perpendicular to each other. In the disclosed embodiment, the part  121  is made of steel, but it could alternatively be made of any other suitable material. A cylindrical opening  124  extends through the leg  122 , near an outer end thereof. 
   The seal device  110  includes a block  144  that is made from an electrically insulating material. In the disclosed embodiment, the block  144  is made from a rigid and durable plastic material, but it could alternatively be made from any other suitable material. The block  144  is fixedly coupled to an outer end of the leg  123  of the part  121 , for example by a plurality of screws or bolts that are not visible in  FIG. 2 . However, the block  144  could be coupled to the part  121  in any other suitable manner. The block  144  has a cylindrical opening  145  extending therethrough, at a location spaced outwardly from the leg  123  of the part  121 . The opening  145  is coaxially aligned with the opening  124  through the leg  122  of the part  121 . The block  144  also has a recess  146  in one side thereof. The recess  146  extends from the opening  145  to the leg  123  of the part  121 . 
   A cylindrical metal sleeve  148  is disposed within the opening  145  in the block  144 . The outside diameter of the sleeve  148  is approximately equal to the inside diameter of the opening  145 , such that the sleeve  148  is held within the opening  145  by a force fit. The sleeve  148  is also fixedly held in the opening  145  by a suitable adhesive, such as a commercially-available epoxy adhesive. The sleeve  148  could alternatively be held against axial movement in any other suitable manner. The sleeve  148  is made of a magnetically permeable material. In the disclosed embodiment, the sleeve  148  is made of steel, but it could alternatively be made of any other suitable material. The central cylindrical opening  149  through the sleeve is coaxially aligned with the opening  124  in the leg  122  of the part  121 . 
   A permanent magnet  152  is disposed within the recess  146 . In the disclosed embodiment, the magnet  152  is held in place by a known epoxy adhesive, but it could alternatively be held in place in any other suitable manner. One end of the magnet  152  contacts the sleeve  148 , and the other end of magnet  152  contacts the leg  123  of the part  121 . The circuit board  61  with the Hall effect sensor  66  thereon is fixedly supported on the leg  123  of the part  121 , for example by two or more bolts that are not visible in  FIG. 2 . The sensor  66  is disposed at a location where, in  FIG. 2 , it is approximately vertically aligned with the lower end of the sleeve  148 . 
   The shank  82  of the bolt  81  can be inserted through the central opening  149  in the sleeve  148 , through the aligned openings  17  and  18  in the latch parts  13  and  14 , and through the opening  124  in the leg  122  of part  121 , until the head  83  of the bolt is engaging the upper end of the sleeve  148 . In this position of the bolt, the not-illustrated retaining mechanism cooperates with the groove  84  to prevent withdrawal of the bolt in an upward direction. 
   When the bolt  81  is installed and intact, as shown in  FIG. 2 , the path of lowest reluctance for the flux generated by the magnet  152  is the path indicated diagrammatically by a broken line  193 . This path extends from the magnet  152  through the sleeve  148  to the shank  82  of bolt  81 , along the shank to the part  121 , and then through the legs  122  and  123  of part  121  to the magnet  152 . On the other hand, if the bolt  82  is cut in the region of the latch parts  13  and  14 , and if the upper portion of the bolt is removed, then the path of least reluctance for the magnetic flux would be that indicated diagrammatically by a broken line  194 . This path extends from the magnet  152  through the sleeve  148  to the lower end of the sleeve, then across the gap between the sleeve  148  and the leg  123  past the Hall effect sensor  66 , and then through the leg  123  to the magnet  152 . Thus, in the event the bolt is cut and its upper part is removed, the magnetic flux in the region of the Hall effect sensor  66  will change. The Hall effect sensor  66  can detect this change in flux, and then change its output signal. The change in the output signal of the sensor  66  will tell the not-illustrated tag circuit that the bolt  82  has apparently been cut. 
   In each of the disclosed embodiments, the static magnetic field produced by the permanent magnet is polarized. This increases the difficulty of defeating the seal device, because one would need to know the polarity of the magnetic field in order to attempt to introduce an external magnetic field that is properly polarized so as to mask the magnetic effect of cutting the bolt. Also, in each embodiment, portions of the flux paths that are not within the magnet, the bolt or the detector are virtually completely disposed within material having a high magnetic permeability. This reduces sensitivity of the seal device to external metal objects such as a container, as well as sensitivity to external magnetic fields. 
   Although selected embodiments have 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 claims that follow.

Summary:
A seal device includes a locking member with a magnetically permeable material portion, and structure that can receive the locking member. The structure supports a magnetic field generator and detector at locations spaced from each other and from a region that is occupied by the portion of the locking member when the locking member is received by the structure. The structure defines a main flux path as a loop having a first portion, a second portion and a remainder that are mutually exclusive, and that collectively define the entirety of the flux path. The first and second portions are respectively within the magnetic field generator and the region, and most of the remainder extends through magnetically permeable material of the structure. The detector is located where the magnetic field has different characteristics when the portion of the locking member is respectively present in and absent from the region.