Abstract:
A device for detecting random objects that are being carried inside a container includes an excitation coil and a sensing coil with a gap therebetween. When the construction of the container includes a conducting loop, the container is positioned on a path between the two coils, with the plane of the loop substantially perpendicular to the path. The excitation coil is then activated to generate a magnetic field that is directed along the path. This magnetic field has both a sinusoidal component and a cosinusoidal component. Importantly, the cosinusoidal component is adjusted to match a characteristic dimension of the loop, to thereby cause zero mutual inductance with the loop in the magnetic field. On the other hand, conducting objects inside the loop will cause inductance perturbations in the magnetic field which can be detected to establish the presence of the objects in the container.

Description:
FIELD OF THE INVENTION 
     The present invention pertains generally to detection devices and their methods of operation. More particularly, the present invention pertains to devices and methods for removing unwanted interference from detection signals. The present invention is particularly, but not exclusively, useful as a device for the detection of flat conducting objects which are carried in suitcases that have a reinforcing metallic frame. 
     BACKGROUND OF THE INVENTION 
     Magnetic detection devices generally rely on the phenomenon which results when a conducting material, such as a metallic object, is positioned in a magnetic field. Specifically this phenomenon involves inductance coupling between the detection device and the conducting material (object) whereby a changing current in one (the detection device) induces a current in the other (the object). The inducement in this case is caused by a magnetic field (B) which is generated by the changing current in the detection device. In response, a current is induced in the object which will alter the magnetic field (B). Magnetic detection devices are useful for finding hidden or concealed objects because the alterations in the magnetic field that result from inductance coupling are detectable. 
     Although metal detection devices are efficacious in many circumstances, it happens that a conducting material which is configured as a sheet (i.e. the sheet is flat) can not be so easily detected when it is oriented edgewise with its flat surfaces substantially parallel to the magnetic flux lines in a magnetic field. Further, even when flat conducting materials are oriented with their surfaces perpendicular to the magnetic flux lines, interference from other conductors can effectively prevent detection of the target material. Specifically, it is known that the loop-like metallic frames which are used for reinforcing large suitcases will cause substantial interference in a magnetic field. Consequently, a flat, sheet-shaped electrically conducting object which is carried in a reinforced suitcase will not be detected by magnetic detection techniques. 
     Inductance coupling is effectively nullified if there is zero mutual inductance (M=0). As a practical matter this will occur even when a conducting material is located in a magnetic field if the flux into the object (Φ in ) is equal to the flux that is coming out of the object (Φ out ). Stated differently, M=0 when, Φ in =Φ out . Under this condition, the object will not cause an alteration of the magnetic field. Thus, no detectable signals will be generated and the object will be effectively invisible. 
     In light of the above, it is an object of the present invention to provide a device and method for detecting random conducting objects which are located inside a conducting loop which generates a magnetic field that exhibits substantially zero mutual inductance with the loop and thereby effectively eliminates the effect of the loop during the detection of the objects inside the loop. It is another object of the present invention to provide a device and method for detecting random conducting objects that are located inside a conducting loop wherein the mutual inductance between the detecting magnetic field and the loop can be adjusted to zero. Still another object of the present invention is to provide a device and method for detecting random conducting objects located inside a conducting loop regardless whether the objects are sheet-like or have a more three dimensional volumetric shape. It is also an object of the present invention to provide a device for detecting random conducting objects located inside a conducting loop which is easy to use, relatively simple to manufacture, and comparatively cost effective. 
     SUMMARY OF THE PREFERRED EMBODIMENTS 
     A device for detecting random conducting objects that are located inside a conducting loop, such as the reinforcing metallic frame of a suitcase, includes an array of excitation coils and an array of sensing coils. Both of these arrays are mounted on a base member and are separated from each other by a gap therebetween. Additionally, the present invention includes a computer which is electronically connected with both of the arrays. Specifically, the computer is used to electronically control the generation of a magnetic field with the array of excitation coils, and to analyze perturbations in the magnetic field as they are received by the array of sensing coils. As intended for the present invention, the magnetic field is respectively generated and sensed by an array of excitation coils and an array of sensing coils which are of substantially the same configuration. In accordance with the principle of reciprocity, those skilled in the art will recognize that the excitation coils and the sensing coils are interchangeable. 
     It is an important operational aspect of the present invention that in the operation of the device of the present invention, a conducting loop will be eliminated from detection and yet not interfere with the detection of other objects by the device. Specifically, any conducting objects that may be inside the loop, need to be detected. This is effectively accomplished by using the excitation coils to generate a magnetic field which will have no mutual inductance with the loop. For the present invention, this is done be generating a magnetic field whose spatial intensity pattern includes both a sinusoidal component and an adjustable cosinusoidal component. The result obtained by using both of these components is that the flux entering the loop (Φ in ) can be made equal to the flux coming out of the loop (Φ out ). As noted above, under these conditions the mutual inductance due to the loop is zero (Φ in =Φ out , and M=0). 
     In order to generate the sinusoidal and cosinusoidal components for the magnet field for the device of the present invention, the array of excitation coils is configured to include two cosine coils and one sine coil. Both of the cosine coils are connected with the computer so that the cosinusoidal component of the magnetic field can be adjusted to conform or match its spatial intensity pattern with a characteristic dimension of the loop. Consequently, when the matched magnetic field is directed along a path that is substantially perpendicular to the plane of the loop, Φ in  will equal Φ out  and there will be zero mutual inductance with the loop (M=0). On the other hand, any inductance caused by objects inside the loop will create perturbations in the magnetic field which can be detected. Because of the fundamental mathematical relationship, sin 2 +cos 2 =1, these perturbations are substantially uniform regardless of their location within the loop. 
     For the present invention, the sinusoidal component is preferably alternated with the cosinusoidal component somewhere between ten and one hundred times each second. The signals that are then respectively obtained in response to the alternated components are summed by the computer to obtain information about objects in the magnetic field. 
     In the operation of the present invention, a suitcase, luggage bag or any other type container which may be used for transporting objects is placed on a conveyor belt. Specifically, the suitcase, bag or container is placed on the conveyor belt so that any reinforcing metallic loops will be oriented substantially parallel to the conveyor belt. The suitcase, bag or container is then advanced on the belt through the base member of the device to a position between the excitation coils and the sensing coils. As it is so advanced, a device such as an optical encoder is used to measure a characteristic dimension, e.g. length, of the metallic loop is the suitcase, bag or container. With this characteristic dimension, the computer is then used to adjust either the excitation coils or the sensing coils, or both sets of coils, so that the cosinusoidal component of the magnetic field is matched with the characteristic dimension of the suitcase, bag or container. 
     When the suitcase, bag or container arrives on the conveyor belt at the predetermined position in the device where it is to be inspected, the excitation coils are activated and readings are taken by the sensing coils. In accordance with the above description of the present invention, any conducting loop(s) in the suitcase, bag or container will not cause an reading. On the other hand, objects inside the loop will provide readings which can be used by the computer to confirm detection of the object. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
     FIG. 1 is a perspective view of the device of the present invention with portions broken away for clarity; 
     FIG. 2A is a perspective view of a suitcase with objects carried therein shown in phantom; 
     FIG. 2B is a side elevational view of the suitcase shown in FIG. 2A; 
     FIG. 3 is an exploded perspective view of an array of coils as used for the present invention; 
     FIG. 4A is a schematic drawing showing the relationship between coils, the spatial intensity patterns of sinusoidal and cosinusoidal components of the magnetic field that is generated by these coils, and a suitcase being inspected by the device of the present invention; and 
     FIG. 4B is a schematic drawing as shown in FIG. 4A with an adjustment made to the cosinusoidal component. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to FIG. 1, a device for detecting objects in accordance with the present invention is shown and generally designated  10 . As shown, the device  10  includes an elongated base  12  which has open ends. The device  10  also includes excitation coils  14  which are mounted on the top of the base  12  and sensing coils  16  which are mounted on the bottom of the base  12 . There is also a dimension detector  18  that is mounted on the base  12 , and the device  10  further includes a computer  20  which is electronically connected to various of the components just disclosed. Specifically, the computer is electronically connected to the excitation coils  14  via a line  24 , to the dimension detector  18  via a line  26  and to the sensing coils  16  via a line  28 . Finally, the device  10  includes a conveyor belt  22  which extends between the ends of the base  12 , and which can be selectively activated to carry containers, bags, suitcases and the like through the device  10 . Specifically, such articles are carried through a detection chamber  30  that is formed in the base  12 , and through a gap  32  that is established in the detection chamber  30  between the excitation coils  14  and the sensing coils  16 . 
     In FIG. 2A, the suitcase  34  that is shown is of a type which is constructed to include a loop(s)  36 . Typically, the loop(s)  36  are for purposes of reinforcing the suitcase  34  and will be made of a metallic material. Thus, loop  36  will be a conductor. Further, FIG. 2A shows that the suitcase  34  has a characteristic dimension  38 . In this particular case the characteristic dimension  38  is the length of the suitcase  34 . Importantly, in all cases, the characteristic dimension  38  of any container will typically be the longest dimension of the loop  36 . As shown for the suitcase  34 , the loop  36  is generally planar in its configuration and, for the purposes of the present invention, the suitcase  34  (or other container) can include a plurality of loops  36 . 
     FIG. 2A also shows an object  40  being carried inside the suitcase  34 . For purposes of disclosure, it will be assumed that the object  40  is flat or sheet-like, and that it is made of a conductive material, such as a metallic foil. When inside the container  34 , the object  40  is also inside the loop  36 . 
     In accordance with well known metal detecting practices, it is known that when the x component of a magnetic field, B x , is directed toward the edge  44  of the object  40  and substantially parallel to its flat surface  42  (i.e. into the page in FIG.  2 B), the object  40  is not so easily detected. On the other hand, when the z component of a magnetic field, B z  is directed perpendicularly toward the flat surface  42  of the object  40  as shown in FIG. 2A, the object  40  will be rather easily detected. Nevertheless, the presence of the loop(s)  36  can easily interfere with the z component of the magnetic field, B z  (FIG.  2 A), and make what would otherwise be an relatively easy detection of the object  40  a near impossibility. In order to overcome this difficulty, the excitation coils  14  and the sensing coils  16  need to be specifically designed to avoid interference of B z  that may be caused by the loop(s)  36 . 
     When referring to FIG. 3 it will understood that the excitation coils  14  and the sensing coils  16  are substantially the same and that they are effectively interchangeable. Stated differently, there is reciprocity between the excitation coils  14  and the sensing coils  16 . As shown in FIG. 3 the excitation coils  14  and the sensing coils  16  each comprise an array of individual coils. Specifically, these individual coils are a sine coil  46 , a first cosine coil  48  and a second cosine coil  50 . More specifically, the sine coil  46  is used to generate or receive a magnetic field whose z component, B z , exhibits a spatial intensity pattern that is represented by the sine wave  52  in FIG.  4 A. Similarly, the first cosine coil  48  and the second cosine coil  50  will function together to generate or receive a magnetic field whose z component, B z , exhibits a spatial intensity pattern that is represented by the cosine wave  54  in FIG.  4 A. Thus, the z component of the magnetic field which is respectively generated or received by the excitation coils  14  and the sensing coils  16  will be alternated between a sinusoidal component and a cosinusoidal component. 
     As intended for the present invention, the spatial intensity patterns of the sinusoidal and cosinusoidal components will be alternated approximately between 10 and 100 times per second. Further, the sinusoidal and cosinusoidal components of the magnetic field will preferably be respectively generated by the sine coil  46  and the cosine coils  48 , 50  using 10 volts (peak to peak) at a frequency in the range of one to ten megahertz (1-10 Mz). Additionally, in order to accommodate suitcases  34  having characteristic dimensions in the range of from fifty to eighty centimeters (50 cm-80 cm), the sine coil  46  will have a dimension of approximately 80 cm, the first cosine coil  48  will likewise have a dimension of approximately 80 cm, and the second cosine  50  will have a dimension of approximately 50 cm. 
     Operation 
     In the operation of the device  10 , the suitcase  34  is placed lengthwise on the conveyor belt  22  and advanced into the chamber  30  substantially as shown. As the suitcase  34  passes the dimension detector  18 , its characteristic dimension  38  (length) is recorded by the computer  20 . For the present invention, the characteristic dimension  38  will typically be between 50 cm and 80 cm. Accordingly, as disclosed above, the sine coil  46  and cosine coil  50  will both be approximately 80 cm in length. The cosine coil  52 , on the other hand, will be approximately 50 cm in length. 
     Inside the chamber  30  of base  12 , when the suitcase  34  is positioned substantially as shown on the path  56  between the excitation coils  14  and the sensing coils  16 , the excitation coils  14  are activated. With this activation, a magnetic field is generated which will have a z component, B z , that is directed substantially along the path  56  from the excitation coils  14  to the sensing coils  16 . As disclosed above, B z  has a sinusoidal component which is alternated with a cosinusoidal component. For the present invention, the cosinusoidal component of B z  is adjustable. 
     Consider the situation wherein the characteristic dimension  38  of the suitcase  34  is 80 cm. In this case, B z  of the magnetic field will be alternately generated by the sine coil  46  (80 cm) and only the cosine coil  48  (80 cm). The cosine coil  50  (50 cm) will not be needed. The result is alternating spatial intensity patterns represented by the sine wave  52  and the cosine wave  54  in FIG.  4 A. For reasons set forth above, because the loop  36  of suitcase  34  has a characteristic dimension  38  that is equal to approximately 80 cm, the loop  36  will not interfere with B z . The object  40 , however, will interfere with B z , and will cause perturbations in the magnetic field which will be detected by the sensing coils  16 . Signals of these perturbations will then be sent via line  28  to the computer  20  where an alarm can be initiated that will indicate the presence of the article  40  inside the suitcase  34 . 
     Next, consider the situation wherein the suitcase  34  has a characteristic dimension  38  which is less than 80 cm. In this case, the cosinusoidal component of B z  will need to be adjusted to comport with the shorter dimension. Specifically, using the characteristic dimension  38  measured by the dimension detector  18 , the computer  20  will appropriately adjust both the cosine coil  48  and the cosine coil  50 . It should be noted that if the characteristic dimension  38  happens to be 50 cm, this will comport directly with the dimension of cosine coil  50 , and only the cosine coil  50  is needed. In this particular instance, the cosine coil  48  would not be used. However, when the characteristic dimension  38  is between 50 cm and 80 cm, then the cosine coils  48  and  50  both need to be proportionately activated. The result in all cases wherein the characteristic dimension is less than 80 cm will be a spatial intensity pattern such as represented by the cosine wave  54 ′ in FIG.  4 B. Again, for reasons set forth above, because the cosine loops  48  and  50  are adjusted to the characteristic dimension  38 , the loop  36  will not interfere with B z . The object  40 , however, will interfere with B z , and will cause perturbations in the magnetic field which will be detected by the sensing coils  16 . Signals of these perturbations will then be sent via line  28  to the computer  20  where an alarm can be initiated that will indicate the presence of the article  40  inside the suitcase  34 . 
     While the particular Baggage Metal Detector as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.