A SENSOR PANEL AND A DETECTION APPARATUS INCORPORATING THE SAME

Abstract: A sensor panel comprising a transmit coil (14) for producing a magnetic field in response to a source signal; a plurality of receive elements (24), 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 (14). The receive elements (24) 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: TITLE "A Sensor Panel And A Detection Apparatus Incorporating The Same" 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 utilised 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 a first aspect of this invention, there is provided a sensor panel comprising: transmit means 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 cancellation means arranged to minimise the effects of the transmit means on the outputs of the receive elements. 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 receive element to negate the effects of the transmit coil. In an alternative arrangement, the cancellation means comprises a plurality of feedback coils, one for each receive element, wherein each feedback coil is provided in close physical proximity to a corresponding receive element. In a further alternative arrangement, the cancellation means comprises a plurality of feedback coils, each inducing 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 storing information on which feedback coils are to be isolated for each receive element. Preferably, said receive elements are provided in a substantially planar configuration. Preferably, said receive elements are provided in a matrix. Preferably, each receive element comprises a receive coil. Preferably, each receive coil is wound on a bobbin, the bobbins being provided on the panel. Preferably, the bobbins are movable transverse to the panel. Preferably, each receive coil is provided on a printed circuit board as a spiral track thereon. Preferably, the printed circuit board is a multi-layer printed circuit board. Preferably, the transmit means comprises a transmit coil. Preferably, the transmit coil is provided around the periphery of the matrix of receive elements. In accordance with a second aspect of this invention, there is provided a detection apparatus comprising: transmit means arranged to produce an alternating magnetic field in a predetermined volume; a plurality of receive elements responsive to changes in said magnetic field, said receive elements provided adjacent said volume; cancellation means arranged to minimise the effects of the transmit means on the receive elements; and processing means responsive to said receive elements. 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 receive element to negate the effects of the transmit coil. In an alternative arrangement, the cancellation means comprises a plurality of feedback coils, one for each receive element, wherein each feedback coil is provided in close physical proximity to a corresponding receive element. In a further alternative arrangement, the cancellation means comprises a plurality of feedback coils, each inducing 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 storing information on which feedback coils are to be isolated for each receive element. Preferably, said receive elements are provided in a substantially planar configuration. Preferably, said receive elements are provided in a matrix. Preferably, each receive element comprises a receive coil. Preferably, each receive coil is substantially planar. Preferably, each receive coil is provided on a bobbin, said bobbin being provided on a frame. Preferably, each bobbin is movable transverse to said frame. Preferably, each receive coil is provided on a printed circuit board as a spiral track thereon. Preferably, the printed circuit board is a multi-layer printed circuit board. Preferably, the transmit means comprises a transmit coil. Preferably, the transmit coil is provided around the periphery of the matrix of receive elements. Preferably, the transmit 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 said volume. Preferably, the processing means is arranged to produce an image from signals received from the receive elements. Preferably, the processing means is arranged to process signals from said receive elements by one of more of the following methods: interpolation, Fourier analysis, edge detection, or boundary collapsing. Preferably, said detection apparatus includes a camera arranged to take pictures of the volume, said image being superimposed on the pictures of said volume. Preferably, said processing means is responsive to the phase and amplitude of signals from each receive element to determine therefrom the type of metal being detected. Preferably, said processing means is arranged to activate an audible alarm if the amplitude and/or phase of said signals from the receive elements exceed a predetermined value. Two embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a perspective view, partially broken away, of a detection apparatus according to the first embodiment; Figure 2 is a schematic drawing of the electrical connection of the transmit coils of the first embodiment; Figure 3 is a side view of the first embodiment shown in figure 1 , illustrating the lines of magnetic flux; Figure 4 is an enlarged view of the receive and transmit sections of the first embodiment shown in figure 1 ; Figure 5a is a front view of a receive bobbin and coil; Figure 5b is a side view of a receive bobbin and coil; Figure 6 is a side view of a bobbin attached to a frame illustrating the adjustment thereof; Figure 7 is a illustrative view of the output from the processing means; Figure 8 is a top view of the apparatus illustrating the position of a camera; Figure 9 is a perspective view of the sensor point of the second embodiment; and Figure 10 is a circuit showing the cancellation circuit. The first embodiment is directed towards a detection apparatus 10 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. 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 figure 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. 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. 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. 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 figure 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. 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 will be non-uniform as illustrated in figure 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 colour 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 percentiies of the signals received. For instance, the region corresponds to the most significant 15% of signals received, the region '2' the next most significant 15% and so on. 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. The transmit coil 12 is wound onto a substantially hollow former to provide an aperture 36 therein. The camera 34 is arranged to take visual images of the contents of the volume 18 through the 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 overiayed 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 Figures 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 6x9 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 Figure 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 54a - 54h, each of which has a secondary feedback coil 56a - 56h 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 54a and 56a are configured so as to provided the least amount of mutual inductance therebetween, whilst the feedback coils 54h and 56h are configured to provide the greatest amount of mutual inductance therebetween. Each of the coils 54a - 54h 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 54a - 54h are also being powered, and induce a portion of a feedback signal in the secondary feedback coils 56a - 56h according to the degree of mutual inductance between the respective coils 54 and 56. The feedback coils 54b and 56b have twice as much mutual inductance as the coils 54a and 56a and so on. Accordingly, the secondary feedback coils 56a- 56h provide portions of a feedback signal, and by isolating each of the coils 56a -56h, 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 56a- 56h. For convenience, in Figure 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 analogue multiplexer 48. Using the address inputs of the analogue multiplexer 48 is it 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 analogue 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 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 analogue 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 64a and 66a to 64h and 66h 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.

Claims: CLAIMS 1. A sensor panel comprising: transmit means 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 cancellation means arranged to minimise the effects of the transmit means on the outputs of each receive element. 2. A sensor panel as claimed in claim 1 , wherein the cancellation means comprises a feedback coil arranged to be excited by the source signal, the feedback coil inducing a feedback signal in each receive element to negate the effects of the transmit coil. 3. A sensor panel as claimed in claim 1 , wherein the cancellation means comprises a plurality of feedback coils, one for each receive element, wherein each feedback coil is provided in close physical proximity to a corresponding receive element. 4. A sensor panel as claimed in claim 1 , wherein the cancellation means comprises a plurality of feedback coils, each inducing 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. 5. A sensor panel as claimed in claim 4, wherein the cancellation means further comprises memory means storing information on which feedback coils are to be isolated for each receive element. 6. A sensor panel as claimed in any one of the preceding claims, wherein said receive elements are provided in a substantially planar configuration. 7. A sensor panel as claimed in any one of the preceding claims, wherein said receive elements are provided in a matrix. 8. A sensor panel as claimed in any one of the preceding claims, wherein each receive element comprises a receive coil. 9. A sensor panel as claimed in any one of the preceding claims, wherein each receive coil is wound on a bobbin, the bobbins being provided on the panel. 10.A sensor panel as claimed in claim 9, wherein the bobbins are movable transverse to the panel. 11. A sensor panel as claimed in any one of the claims 1 to 8, wherein each receive coil is provided on a printed circuit board as a spiral track thereon. 12.A sensor panel as claimed in claim 11 , wherein the printed circuit board is a multi-layer printed circuit board. 13.A sensor panel as claimed in any one of the preceding claims, wherein the transmit means comprises a transmit coil. 14.A sensor panel as claimed in claim 13, wherein the transmit coil is provided around the periphery of the matrix of receive elements. 15.A detection apparatus comprising: transmit means arranged to produce an alternating magnetic field in a predetermined volume; a plurality of receive elements responsive to changes in said magnetic field, said receive elements provided adjacent said volume; cancellation means arranged to minimise the effects of the transmit means on the receive elements; and processing means responsive to said receive elements. 16.A detection apparatus as claimed in claim 15, wherein the cancellation means comprises a feedback coil arranged to be excited by the source signal, the feedback coil inducing a feedback signal in each receive element to negate the effects of the transmit coil. 17.A detection apparatus as claimed in claim 15, wherein the cancellation means comprises a plurality of feedback coils, one for each receive element, wherein each feedback coil is provided in close physical proximity to a corresponding receive element. 18.A detection apparatus as claimed in claim 15, wherein the cancellation means comprises a plurality of feedback coils, each inducing 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. 19.A detection apparatus as claimed in claim 18, wherein the cancellation means further comprises memory means storing information on which feedback coils are to be isolated for each receive element. 20.A detection apparatus as claimed in any one of claims 15 to 19, wherein said receive elements are provided in a substantially planar configuration. 21. A detection apparatus as claimed in any one of claims 15 to 20, wherein said receive elements are provided in a matrix. 22.A detection apparatus as claimed in any one of claims 15 to 21 , wherein each receive element comprises a receive coil. 23.A detection apparatus as claimed in any one of claims 15 to 22, wherein each receive coil is substantially planar. 24.A detection apparatus as claimed in any one of claims 15 to 23, wherein each receive coil is provided on a bobbin, said bobbin being provided on a frame. 25.A detection apparatus as claimed in any one of claims 15 to 24, wherein each bobbin is movable transverse to said frame. 26.A detection apparatus as claimed in any one of claims 15 to 23, wherein each receive coil is provided on a printed circuit board as a spiral track thereon. 27.A detection apparatus as claimed in claim 26, wherein the printed circuit board is a multi-layer printed circuit board. 28.A detection apparatus as claimed in any one of claims 15 to 27, wherein the transmit means comprises a transmit coil. 29.A detection apparatus as claimed in claim 28, wherein the transmit coil is provided around the periphery of the matrix of receive elements. 30.A detection apparatus as claimed in any one of claims 15 to 27, wherein the transmit 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 said volume. 31. A detection apparatus as claimed in claim 15, wherein the transmit means, the receive elements and the cancellation means form a sensor panel as defined in any one of claims 1 to 14. 32.A detection apparatus as claimed in claim 15 including a second sensor panel as defined in any one of claims 1 to 14, the sensor panel and the second sensor panel provided on opposite sides of the predetermined volume. 33.A detection apparatus as claimed in any one of claims 15 to 32, wherein the processing means is arranged to produce an image from signals received from the receive elements. 34.A detection apparatus as claimed in any one of claims 15 to 33, wherein the processing means is arranged to process signals from said receive elements by one of more of the following methods: interpolation, Fourier analysis, edge detection, or boundary collapsing. 35.A detection apparatus as claimed in any one of claims 15 to 34, wherein said detection apparatus includes a camera arranged to take pictures of the volume, said image being superimposed on the pictures of said volume. 36.A detection apparatus as claimed in any one of claims 15 to 35, wherein said processing means is responsive to the phase and amplitude of signals from each receive element to determine therefrom the type of metal being detected. 37.A detection apparatus as claimed in any one of claims 15 to 36, wherein said processing means is arranged to activate an audible alarm if the amplitude and/or phase of said signals from the receive elements exceed a predetermined value.

Inventors: NELSON-WHITE IAN DAVID, NELSON-WHITE, IAN, DAVID
Applicants: NELSON WHITE IAN DAVID, NELSON-WHITE, IAN, DAVID
Application year: 1998
Main classification: G01V3
IPC codes: G01V3, G08B13