Abstract:
A sensor panel including a transmit coil for producing a magnetic field in response to a source signal, a plurality of receive elements, each responsive to changes in the magnetic field incident thereon to provide an output, and a cancellation circuit arranged to remove from the outputs any signal induced in each receive element by the transmit coil. The receive elements are arranged in a matrix to provide an indication of the shape of a detected object. The sensor panel can be incorporated as part of a detection apparatus for use in airports and the like, and can also be used to detect buried landmines and ordinances.

Description:
FIELD OF THE INVENTION 
     This invention relates to a sensor panel and to a detection apparatus incorporating the same. The sensor panel uses magnetic fields to detect the presence of particular materials. 
     BACKGROUND ART 
     Detection apparatuses are commonly used to detect the presence of metal objects on or about persons entering a secure area, such as an airport. 
     Existing detection apparatus consist of an archway through which pedestrians walk. A single coil is connected to an oscillator, and produces an alternating magnetic field in the archway. The coil is driven by the oscillator at a tapping that is a fraction of the total turns. The signal at the total turns is utilized to detect the change in the magnetic field caused by metallic objects passing through the magnetic field. 
     Detection apparatus of this type are useful in providing an indication that a metal object exists, but do not give any information about the location or shape of the object. Consequently, at present security guards with hand-held detectors are used to locate the object once the existence thereof has been detected. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention there is provided a magnetic detection apparatus for sensing an object in a sensing region, the apparatus including: 
     a magnetic field generating means for producing a magnetic field within said sensing region in response to a source signal; 
     a plurality of magnetic receiving elements arranged in a matrix, each magnetic receiving element being responsive to changes in the magnetic field within the sensing region to provide an output signal; 
     cancellation means for generating a feedback signal adapted to minimise spurious magnetic effects of the magnetic field generating means on the output signals of the magnetic receiving elements; and, 
     signal processing means for processing said output signals of the magnetic receiving elements and producing an image corresponding to variations produced in said output signals by the object in the sensing region whereby, in use, an indication of the location and approximate shape of the object in the sensing region can be obtained. 
     In one arrangement, the cancellation means comprises a feedback coil arranged to be excited by the source signal, the feedback coil inducing a feedback signal in each magnetic receiving element to negate the effects of the magnetic field generating means. 
     In an alternative arrangement, the cancellation means comprises a plurality of feedback coils, one for each magnetic receiving element, wherein each feedback coil is provided in close physical proximity to a corresponding magnetic receiving element, each feedback coil inducing a feedback signal in its corresponding receiving element to negate the effects of the magnetic field generating means. 
     In a further alternative arrangement, the cancellation means comprises a plurality of feedback coils, each coil contributing a portion of a feedback signal, first switching means arranged to selectively isolate each feedback coil so as to remove its contribution to the feedback signal, and second switching means arranged to combine said feedback signal with each output signal in turn. 
     Preferably, the cancellation means further comprises memory means for storing information concerning which feedback coils are to be isolated for each receiving element. 
     Preferably, each of said magnetic receiving elements are provided in a substantially planar configuration so as to form a sensor panel. 
     Preferably, each magnetic receiving element comprises a receive coil. 
     Preferably, each receive coil is wound on a bobbin, the bobbins being provided on the sensor panel. 
     Preferably, the position of the bobbins are adjustable in a direction transverse to the plane of the sensor panel whereby the output signal from each receive coil in the absence of any object in the sensing region can be minimised. 
     Preferably, each receive coil is provided on a printed circuit board as a spiral track thereon. 
     Preferably, wherein the printed circuit board is a multi-layer printed circuit board. 
     Preferably, the magnetic field generating means comprises a transmit coil. 
     Preferably, the transmit coil is provided around the periphery of the matrix of magnetic receiver elements. 
     In one arrangement, the magnetic field generating means comprises a first and a second transmit coil connected to an oscillator, said transmit coils being provided in a substantially parallel, spaced apart manner on opposing sides of a predetermined volume. 
     Preferably, the magnetic field generating means, the magnetic receiving elements and the cancellation means form a sensor panel. 
     Preferably, the signal processing means is arranged to further process the output signals from said receiving elements by one or more of the following methods: interpolation, Fourier analysis, edge detection, or boundary collapsing. 
     Preferably, the magnetic detection apparatus further comprises a camera arranged to take pictures of the volume, said image being superimposed on the pictures of said volume. 
     Preferably, said signal processing means is responsive to the phase and amplitude of the output signals from each receiving element to determine therefrom the type of material being detected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIG. 1 is a perspective view, partially broken away, of a first embodiment of the detection apparatus according to the present invention; 
     FIG. 2 is a schematic diagram of the electrical connection of the transmit coils of the first embodiment; 
     FIG. 3 is a side view of the first embodiment shown in FIG. 1, illustrating the lines of magnetic flux; 
     FIG. 4 is an enlarged view of the receive and transmit sections of the first embodiment shown in FIG. 1; 
     FIG. 5 a  is a front view of a receive bobbin and coil; 
     FIG. 5 b  is a side view of a receive bobbin and coil; 
     FIG. 6 is a side view of a bobbin attached to a frame illustrating the adjustment thereof; 
     FIG. 7 is a illustrative view of the output from the processing means; 
     FIG. 8 is a top view of the apparatus illustrating the position of a camera; 
     FIG. 9 is a perspective view of the sensor point of the second embodiment; and 
     FIG. 10 is a circuit showing the cancellation circuit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The first embodiment is shown in FIGS. 1-8 and is directed towards a detection apparatus  10  (see FIGS. 1 and 2) comprising first, second and third transmit coils  12 ,  14  and  16 , respectively. Each of the transmit coils  12 ,  14  and  16  are wound onto a nonconductive former and are provided substantially parallel with each other. The first and second transmit coils  12  and  14  are provided spaced apart to define a volume  18  therebetween. The transmit coils  12 ,  14  and  16  are of approximately human height. The volume  18  is sufficient for a person to carry hand luggage and walk comfortably therethrough. 
     The third feedback coil  16  is provided adjacent to the second transmit coil  14  remote from the first transmit coil  12 . 
     The transmit coils  12 ,  14  and  16  are connected in electrical series and are driven by an oscillator  20  (see FIG.  2 ). The first and second transmit coils  12  and  14  are wound onto their respective formers in the same direction and the third feedback coil  16  is wound onto its former in the opposite direction. The oscillator  20  causes the transmit coils  12 ,  14  and  16  to create alternating magnetic fields. The first and second transmit coils  12  and  14  are wound in the same direction and consequently act constructively. The magnetic field in the volume  18  consists of substantially parallel horizontal lines of magnetic flux. 
     The effect of the third transmit coil  16  being wound in the opposite direction to the first and second transmit coils  12  and  14  is that the magnetic field generated by the third transmit coil  16  acts to nullify the magnetic field in the region between the third transmit coil  16  and the second transmit coil  14 . As seen in FIG. 3, the third transmit coil  16  creates a vertical region  22  in which there is very little or no magnetic field. The position of the region  22  can be adjusted by altering the distance between the third transmit coil  16  and the second transmit coil  14 , and the relative number of windings between the transmit coils  12 ,  14  and  16 . The third transmit coil  16  acts as a feedback coil. 
     The detection apparatus  10  further comprises a plurality of receive elements  24  mounted in a matrix arrangement on a frame  26  (see FIGS.  1  and  4 ). The frame  26  and the receive elements  24  are provided adjacent the second and third transmit coils  14  and  16  such that the receive elements  24  are positioned in the region  22 . 
     Each receive element  24  comprises a receive coil  28  wound onto a bobbin  30  (see FIG. 5 a ). The bobbin  30  is relatively flat such that the receive coil  28  is substantially planar. Each bobbin  30  is mounted to the frame  26  via a spindle  32  (see FIG. 5 b ). The spindle  32  provides a mechanism for adjusting the position each bobbin  30  relative to the frame  26 , as indicated by the arrows labelled “A” in FIG.  6 . 
     The receive elements  24  and the transmit coils  14  and  16  form a sensor panel. 
     The detection apparatus  10  further comprises a processing means (not shown) responsive to the output from each receive element  24 . 
     In use, the position of each bobbin  30  is adjusted using the corresponding spindle  32  to minimise the output signal from the receive element  24  in the absence of any metallic objects in the volume  18 . 
     Non-metallic objects passing through the volume  18  will not greatly effect the magnetic field and consequently there will be little change in the output signals from the receive elements  24 . 
     The presence of a metallic object in the volume  18  will alter the magnetic fields, resulting in an imbalance at the region  22  (see FIG.  3 ). The imbalance will be greatest at the position in the region  22  perpendicular to the metallic object and the imbalance will gradually diminish as the distance from the object to the other parts of the region  22  increase. This imbalance in the magnetic field results in an increase in the signals detected by the receive elements  14 . The metal type influences the amplitude and/or phase output of the receive elements  24 . Further, the size and surface area of the object further influences the amplitude of the signals received by the receive elements  24 . 
     The processing means samples the output of each receive element  24  and produces therefrom an image corresponding the scalar magnetic field at the region  22 . In the absence of any metallic objects in the volume  18 , the scalar field will be substantially uniform and have a minimal amplitude. 
     In the presence of a metallic object, the scalar field with be non-uniform as illustrated in FIG.  7 . The regions labelled “1” to “5” represent diminishing amplitudes of the scalar magnetic field in the region  22 . The processing means is arranged to alter the color and/or intensity of the image to represent different amplitude of the scalier magnetic field. One way of achieving this is to assign the regions ‘1’ to ‘5’ according to percentiles of the signals received. For instance, the region ‘1’ corresponds to the most significant 15% of the signals received, the region ‘2’ the next most significant 15% and so on.-Please 
     If desired, the processing means can perform signal processing on the image to refine the shape of the object. Such processing can include linear interpolation, edge detection, Fourier analysis or boundary collapsing. 
     A camera  34  is provided adjacent the apparatus  10  alongside the first transmit coil  12  (see FIG.  8 ). The transmit coil  12  is wound onto a substantially hollow former to provide an aperture  36  therein (see FIG.  1 ). The camera  34  is arranged to take visual images of the contents of the volume  18  through aperture  36 . 
     The processing means is arranged to overlay the image calculated from the output of the receive element  24  onto the visual image received from the camera  34 . This can be achieved using a genlock device. 
     As a person walks through the volume  18  carrying a metallic object, the magnetic field in the volume  18  will be altered by the presence of the metallic object. This alteration will result in the magnetic filed in the region  22  no longer being minimal near the metallic object. Consequently, signals will be induced on each receive coil  28 . The signals are received by the processing means and used to calculate an image corresponding to the magnetic field in the region  22 . The image is overlayed onto a visual image received from the camera  34  such that Security Personnel viewing the combined images will see both the person walking through the volume  18  and an indication of the presence and location of metallic objects. 
     The second embodiment is shown in FIGS. 9 and 10 and relates to a sensor panel  40 . The sensor panel  40  comprises a transmit coil  42  provided on a rectangular former. The transmit coil  42  extends around the periphery of a printed circuit board  44  which has a 6×9 matrix of receive coils  46  formed as spiral tracks thereon. In the embodiment, the printed circuit board is multi-layered and each receive coil  46  consists of spiral tracks provided on each layer so as to provide a greater inductance. 
     The sensor panel  40  of the embodiment is designed to be portable unit and is accordingly of a smaller dimensions that the corresponding panel shown in FIG.  1 . However, in other embodiments, the sensor panel may be provided in other sizes as required. 
     One end of each receive coil  46  is connected to ground. The other end of each receive coil  46  is connected via a cable (not shown), to an analogue multiplexer  48  which forms part of cancellation circuitry  50 . The cancellation circuitry  50  is provided remote from the sensor panel  40 . Processing circuitry  52  is also provided remote from the panel  40 . 
     The cancellation circuitry  50  comprises eight primary feedback coils  54   a - 54   h , each of which has a secondary feedback coil  56   a - 56   h  associated therewith. Each primary feedback coil  54  and its associated secondary feedback coil  56  are wound on a common core so as to provide mutual inductance therebetween. In the embodiment the core includes a ferrite slug (not shown) which can be adjusted into and out of the core so as to alter the degree of mutual inductance. The feedback coils  54   a  and  56   a  are configured so as to provided the least amount of mutual inductance therebetween, whilst the feedback coils  54   h  and  56   h  are configured to provide the greatest amount of mutual inductance therebetween. 
     Each of the coils  54   a - 54   h  are connected in series with each other and with the transmit coil  42  which in turn is connected to an oscillator  58 . As a result, when the transmit coil  52  is being powered by the oscillator  58 , each of the primary feedback coils  54   a - 54   h  are also being powered, and induce a portion of a feedback signal in the secondary feedback coils  56   a - 56   h  according to the degree of mutual inductance between the respective coils  54  and  56 . The feedback coils  54   b  and  56   b  have twice as much mutual inductance as the coils  54   a  and  56   a  and so on. Accordingly, the secondary feedback coils  56   a - 56   h  provide portions of a feedback signal, and by isolating each of the coils  56   a - 56   h , up to 256 degrees of feedback signal can be provided. 
     The isolation of each secondary feedback coil  56  is provided by the first switching circuitry, comprising for each secondary feedback coil  56  isolation switches  60 , bypass switch  62 , non-inverting buffers  64  and an inverting buffer  66 . The non-inverting buffers  64  activate the isolation switches  60 , and the inverting buffer  66  activates the bypass switch  62 . The inputs to the non-inverting buffer  64  and the inverting buffer  66  are connected together. 
     The two isolation switches  60 , the bypass switch  62 , the two non-inverting buffers  64  and the inverting buffer  66  are provided for each feedback coil  56   a - 56   h . For convenience, in FIG. 10 the isolating switch  60 , the bypass switch  62 , and the buffers  64  and  66  have the letter “a”-“h” appended thereto according to the corresponding secondary feedback coil  56  with which they are associated. 
     The secondary feedback coils  56  are provided in electrical series via the associated switching circuitry and are connected at one end to the processor circuitry  52  and at the other end to the output of the analog multiplexer  48 . Using the address inputs of the analog multiplexer  48  it is possible to connect each receive coil  46  to the processor circuitry  50  via the secondary feedback coils  56 . 
     By adjusting whether a high or low signal appears at the input to the buffers  64  and  66  associated with each secondary feedback coil  56 , it is possible to selectively isolate any combination of the secondary feedback coils  56 , so as to adjust the amount of feedback in order to more accurately compensate for any signal induced in the particular receive coils  46 . When the input to the buffers  64  and  66  is high, the non-inverting buffer  64  will present a high signal to the isolating switches  60 , which causes the switches  60  to operate as a closed circuit. Conversely, the inverting buffer  66  will present a low signal to the bypass switch  62  which will act as an open circuit. In this state, the corresponding secondary feedback coil  56  forms part of the connection between the analog multiplexer  48  and the processing circuitry  52  and the portion of the feedback signal induced in the secondary feedback coil  56  is combined with the output from the receive coil  46  which is being addressed. 
     When the input to the buffer  64  and  66  associated with a secondary feedback coil  56  are low, the non-inverting buffer  64  will present a low signal to the isolating switches  60 , which will act as open circuit, thereby isolating the corresponding secondary feedback coil  56  from the connection between the analogue multiplexer  48  and the processor circuitry  52 . Further, the inverting buffer  66  will present a high signal to the bypass switch  62  which will act as a short circuit in order to provide a path for the signal to flow from the analogue multiplexer to the processor circuitry  52 . 
     The embodiment also includes a non-volatile memory  100  (see FIG. 10) in which is stored, for each receive coil  46 , the combination of which secondary feedback coils  56  are to be isolated and which are to be included. As the address lines of the analog multiplexer  48  are varied so as to obtain an output from each of the receive coils  46  in turn, so the non-volatile memory is accessed to determine which feedback coil  56  are to be included to compensate for the transmit coil  42 , and the appropriate signals sent to the buffers  64   a  and  66   a  to  64   h  and  66   h  accordingly. 
     The processing circuitry  52  is arranged to produce a scalar image from the amplitude and phase of the signals received from each receive coil  46  having regard to the cancellation performed by the secondary feedback coils  56 . In this regard the sensor panel  40  of the embodiment can be arranged to perform as a metal detector in a similar manner to the first embodiment, however, in a more portable form. 
     However, it is has been discovered that the sensor panel  40  of the second embodiment can also be used in other applications. In particular, the sensor panel  40  and associated circuitry can be used in the detection of land mines and metal-cased ordinances. In this arrangement, the sensor panel  40  would be arranged parallel to the ground and the receive coils  46  would provide signals induced from the ground. In this regard, the presence of metallic compounds within the ground will produce some form of residual signal in each of the receive coils  46 . The presence of metal-cased land mines or ordinances will induce a distorted signal in some of the receive coils  46  in a similar manner to that described above in relation to the first embodiment with regard to metallic objects in the predefined volume. Accordingly, a land mine or ordinance with a metallic case will show up as increased activity by at least some of the sensors  46 . Further it has been found that plastic cased land mines also produce a variation of the signals received by some of the receive coils  46 , in that the plastic land mines contain little or no ferric or magnetic components, and accordingly the response received from the area surrounding the plastic land mine is reduced compared with the background response of the ground. Accordingly, a plastic land mine shows up as a reduction in the received signal in the receive coils  46 . By further processing the phase of received signals in each of their receive coils  46 , further discrimination of materials can be achieved. 
     It should be appreciated that other forms of switching circuitry can be provided without departing from the scope of the invention. 
     Further it is envisaged that the sensor panels of the second embodiment may be produced in larger scale and used to form part of the detection apparatus, replacing the transmits coils, feedback coils and receive coils. In particular, two such sensor panels can be provided on opposing sides of the predetermined volume. In such a system, it is preferred that the transmit coils of the sensor panels act cooperatively to produce parallel lines of magnetic flux. 
     It should be appreciated that the scope of this invention is not limited to the particular embodiment described above.