Electricity: measuring and testing – Magnetic – With means to create magnetic field to test material
Reexamination Certificate
1999-06-21
2001-07-31
Snow, Walter E. (Department: 2862)
Electricity: measuring and testing
Magnetic
With means to create magnetic field to test material
C324S225000, C324S239000, C340S551000
Reexamination Certificate
active
06268724
ABSTRACT:
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 (&PHgr;
in
) is equal to the flux that is coming out of the object (&PHgr;
out
). Stated differently, M=0 when, &PHgr;
in
=&PHgr;
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 (&PHgr;
in
) can be made equal to the flux coming out of the loop (&PHgr;
out
). As noted above, under these conditions the mutual inductance due to the loop is zero (&PHgr;
in
=&PHgr;
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, &PHgr;
in
will equal &PHgr;
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 con
Nydegger & Associates
Quantum Magnetics
Snow Walter E.
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