Deploying electric fields

Abstract

Electric fields are used to determine electrical characteristics of an object, such as permittivity, by energising selected electrodes and monitoring remaining electrodes. A dielectric substrate (101,

Description

The present invention relates to an apparatus for deploying electric fields to determine electrical characteristics of an object. The present invention also relates to a method of deploying electric fields to determine electrical characteristics of an object. The sensing of the electrical permittivity of an object is disclosed in US 8,994,383. A sensor is described that includes a dielectric layer that presents a surface defining the base of a volume in which a test object may be placed and an electrically active layer beneath the dielectric layer, comprising a first set of electrodes that extend in a first direction and a second set of electrodes that extend in a second direction that is perpendicular to the first direction. The electrodes are electrically isolated by deploying the first set on the first surface of the dielectric material and the second set on the second surface of the dielectric material. By selecting one of the sets, energising a selected electrode of that set and monitoring one of the remaining electrodes of that set, data may be obtained relating to the permittivity of the material. A similar operation may be repeated during which the same electrode is energized but a different electrode is selected to be monitored; such that resulting electric fields penetrate the object to a greater or a lesser degree compared to the first iteration. As described in US 10,753,898, an object can be scanned by an external electric field by placing the object on a support platform. A dielectric membrane in proximity to the support platform may include input lines and output lines and a strobing circuit may apply input voltages to the input lines while a sampling circuit receives output voltages from the output lines. A processing device compares selected output signals against a reference signal to produce voltage control data. A voltage adjustment circuit adjusts the input voltage from a first intensity to a second intensity in response to the voltage control data. Apparatus of this type may be deployed as a security device for scanning the shoes worn by passengers before they board an aeroplane for example. Scanning may be performed using the first set of electrodes on the first surface of the dielectric material, followed by using the second set of electrodes mounted on the second surface of the dielectric material. However, experiment has shown that the results obtained from the two scanning procedures are different. In applications such as shoe scanning, this can create difficulties in terms of identifying the data that is actually correct. Furthermore, problems may arise in terms of adjusting input voltages if different scanning procedures are producing different results. A known printed circuit board A1 of a dielectric material is shown in Figure A. On the first surface of the circuit board A1, material has been etched away to reveal a first array of electrodes A11 to A18. Electrodes are also etched on an underside second face of board A1, identified as A19. Surface A19 is also shown in Figure A. This includes a second array of electrodes A21 to A28. The second set of electrodes A21 to A28 is substantially orthogonal to the first set of electrodes A11 to A18. This arrangement may be used for two-dimensional scanning, during which a selected electrode of the first set is energized and a selected electrode of the second set is monitored. This procedure is repeated for all possible electrode combinations and the results obtained may be used to present an image of an object that is being scanned. As is known in the art, this technique may be used for scanning many different objects and the present inventor has performed experiments in relation to the scanning of shoes, as worn by passengers about to enter an aeroplane or other protected area, for example. In order to gain a better understanding of the material composition of the object under investigation, is also known to perform layering procedures with respect to a single array of electrodes. Thus, layering may be performed with respect to the first set of electrodes A11 to A18 and then repeated with respect to the second set of electrodes A21 to A28. A cross section of the first set of electrodes A11 to Al 8 is shown in Figure B, when performing a layering operation. The electrodes are mounted on the board A1 and a ground plane B1 is present to shield the device from external electrical noise. During a layering procedure, it is possible for the first electrode A1 to be energized and the second electrode A12 to be monitored. On the next cycle, the first electrode A11 is again energized but this time the third electrode A13 is monitored. This is followed by the first electrode A11 being energized again with the fourth electrode A14 being monitored. Thereafter, the first electrode A11 is energized and the fifth electrode A15 is monitored. As shown in Figure B, as the distance between the electrodes increases, the depth of penetration into the material under examination also increases. With the object remaining in position, a similar procedure may be performed with respect to the second set of electrodes A21 to A28. Thus, again, the first electrode of the second set A21 is energized with the second electrode A22 being monitored. The procedure is repeated with a third electrode A23 being monitored, then the fourth electrode A24 being monitored and the fifth electrode A25 being monitored. Again, as the distance between the energized electrode and the monitored electrode increases, the depth of penetration also increases. However, a problem has been identified, in that electric fields generated as a result of energizing the second set of electrodes A21 to A28 are somewhat attenuated compared to the electric fields generated by the first set of electrodes A11 to A18. As can be seen from Figure B, this situation can be appreciated, given that the electric fields will be attracted more towards the ground plane B1 and the electric fields must also penetrate the width of the board Al. Experiments have also shown that this difference has a greater effect when the voltage of the energizing electrode increases. Thus, when scanning shoes for example, it may be necessary to deploy substantially higher voltages and, in some embodiments, voltages may be increased adaptively to optimize the level of penetration. However, given the configuration described with reference to Figure A and Figure B, this optimization can only occur with respect to the first set of electrodes A11 to A18 or with respect to the second set of electrodes A21 to A28. According to a first aspect of the present invention, there is provided an apparatus for deploying electric fields to determine electrical characteristics of an object, as set out in claim 1. In an embodiment, the dielectric substrate is a board and the electrodes are established on said board by an etching process. In an alternative embodiment, the dielectric substrate is flexible. In a further embodiment, each electrode track has a normal width defining a track footprint on the substrate; and each continuous electrode at each crossing has a reduced width, to reveal an uncovered region of the track footprint. The ends of discontinuous electrodes may extend into the uncovered regions of said track footprints and the reduced width may define uncovered regions of track footprints with trapezoidal shapes. According to second aspect of the present invention, there is provided a method of deploying electric fields to determine electrical characteristics of an object, as set out in claim 8. Embodiments of the invention will be described, byway of example only, with reference to the accompanying drawings. The detailed embodiments show the best mode known to the inventor and provide support for the invention as claimed. However, they are only exemplary and should not be used to interpret or limit the scope of the claims. Their purpose is to provide a teaching to those skilled in the art. Components and processes distinguished by ordinal phrases such as “first” and “second” do not necessarily define an order or ranking of any sort. In the drawings: Figure 1 shows an apparatus for deploying electric fields to determine electrical characteristics of an object; Figure 2 shows a first embodiment of electric bridges; Figure 3 shows an alternative embodiment for providing electric bridges; Figure 4 details the electric bridges identified in Figure 3; Figure 5 shows an alternative embodiment with electrodes of reduced width at the positions of discontinuities; Figure 6 shows an environment for deploying electric fields to determine electrical characteristics of an object; Figure 7 shows procedures performed within the environment of Figure 6; and Figure 8 shows an example of an output signal derived from a monitored electrode. Figure 1 An apparatus for deploying electric fields to determine electrical characteristics of an object is shown in Figure 1. The apparatus includes a dielectric substrate 101 that has a first surface, as shown in Figure 1 and a second surface obscured in Figure 1. The apparatus has a first set of substantially parallel electrodes on the first surface. The number of electrodes present will depend upon the particular application and may typically comprise four, eight or sixteen electrodes. In the embodiment shown in Figure 1, the first set of substantially parallel electrodes consists of eight electrodes 111 to 118. The apparatus also includes a second set of substantially parallel electrodes, in an arrangement similar to that described with reference to Figure A and Figure B. However, in the embodiment of Figure 1, the second set of substantially parallel electrodes is also on the first surface of the dielectric substrate 101. In this embodiment, the second set of substantially parallel electrodes again includes eight electrodes 121 to 128. As shown in Figure 1, the second set of substantially parallel electrodes (121 to 128) is substantially orthogonal to the first set of substantially parallel electrodes (111 to 118), thereby defining electrode crossings, such as crossing 129, where the first electrode 121 of the second set crosses the first electrode 111 of the first set. To allow this configuration to operate, discontinuities are formed in an electrode at each crossing, to isolate an electrode of the first set from an electrode of the second set. Thus, in the embodiment of Figure 1, the first electrode 121 of the second array has a first discontinuity 131, followed by a second discontinuity 132, a third discontinuity 133, a fourth discontinuity 134, a fifth discontinuity 135, a sixth discontinuity 136 a seventh discontinuity 137 and an eighth discontinuity 138. To allow electrical connectivity to be maintained, an electric bridge is created at each discontinuity that extends away from the plane of the first surface; as described with reference to Figure 2 and with reference to Figure 3 and Figure 4. The dielectric substrate 101 may take the form of a board and the electrodes may be established on the board by an etching process; in a manner substantially similar to preparing a conventional printed circuit. Alternatively, the dielectric substrate 101 may be made of a flexible acetate, having a thickness substantially less than that of a conventional printed circuit board. The choice of dielectric material will depend upon the mechanical constraints presented within a particular application. When scanning shoes, a solid circuit board is preferred given that, when deployed, the apparatus will have to withstand the application of significant forces. In these environments, the problem described with reference to Figure A and Figure B becomes more pronounced and the deployment of the invention creates significant advantages. Figure 2 The sixth electrode 116, the seventh electrode 117 and the eighth electrode 118 of the first set of substantially parallel electrodes are shown in Figure 2. These are continuous electrodes and, in an embodiment, all of the discontinuities are present in the second set of electrodes. However, this arrangement is not essential and each electrode crossing may be considered independently. In the example shown in Figure 2, the eighth electrode 128 of the second set has a first discontinuity 201 and a second discontinuity 202. An electric bridge is created at each discontinuity that extends away from the plane of the first surface to maintain electrical conductivity. In the embodiment of Figure 2, each bridge takes the form of a wire, such that a first wire 211 is present at the first discontinuity 201, with a second wire 212 being present at the second discontinuity 202. Thus, the wire physically passes over the continuous electrode at the crossing, with the first wire 211 passing over the seventh electrode 117 of the first set and the second wire 212 passing over the sixth electrode 116 of the first set. Figure 3 An alternative embodiment is shown in Figure 3, in which each bridge is formed by a respective conductor on the second surface of the dielectric board and electrical conductors pass through the dielectric substrate connecting ends of respective discontinuous electrodes. Thus, the eighth electrode 128 of the second array has a discontinuity 301. At discontinuity 301, a first conductor 311 and a second conductor 312 penetrate the board 101 so as to emerge on the second surface of the board. Conductive tracks, comprising a first conductive track 321 and a second conductive track 322, are etched on the second surface of the board; in a manner substantially similar to that described with reference to Figure A. Figure 4 The second surface 401 of the board 101 is shown in Figure 4. This shows the first conductive track 321 and the second conductive track 322 providing a bridge for the seventh electrode 127 and the eighth electrode 128 of the second set respectively. Figure 5 A further embodiment is illustrated in Figure 5, in which each electrode, such as the first electrode 111 of the first set, has a normal width 501 defining a track footprint on the substrate 101. However, at each discontinuity, such as at the first discontinuity 131, the continuous electrode 111 has a reduced width 502 to reveal an uncovered region 503 of the track footprint. In the embodiment of Figure 5, the ends of the discontinuous electrodes extend into the uncovered regions 503 of the track footprints. Thus, in the embodiment of Figure 5, the first discontinuous electrode 121 has a first end 511 and a second end 512. These ends (511, 512) extend into the uncovered regions 503 of the track footprints. In the embodiment of Figure 5, the reduced width of the continuous electrode defines uncovered regions of the track footprints that are substantially trapezoidal in shape, as shown at 513 and 514. Similarly, in the embodiment of Figure 5, the ends of the discontinuous electrodes (501, 512) also have a trapezoidal shape, as illustrated at 515 and 516. Figure 6 A method of deploying electric fields to determine electrical characteristics of an object may be achieved within an environment such as that shown in Figure 6. The method comprises the steps of energizing selected electrodes and monitoring remaining electrodes. An apparatus 601 is provided in a form substantially similar to that described with reference to Figure 1 to Figure 5. Thus, there is a dielectric substrate that has a first surface and a second surface, a first set of substantially parallel electrodes present on the first surface, and a second set of substantially parallel electrodes also present on the first surface. The second set of substantially parallel electrodes is substantially orthogonal to the first set of substantially parallel electrodes, thereby defining electrode crossings. Discontinuities are formed in an electrode at each crossing, to isolate an electrode of the first set from an electrode of the second set. Furthermore, an electric bridge is created at each discontinuity that extends away from the plane of the first surface to maintain electrical conductivity. In the embodiment of Figure 6, a processing circuit 602 communicates with the apparatus 601 via a first multiplexer 611 and a second multiplexer 612. A first energizing line 621 supplies energizing signals to selected electrodes of the first set, and a second energizing line 622 supplies energizing signals to selected electrodes of the second set. Similarly, a first monitoring line 631 receives monitored output signals from a selected electrode of the first set, and a second monitoring line 632 receives monitored signals from a selected electrode of the second set. Thus, in this configuration, any electrode may be selected as an energized input electrode and any of the remaining electrodes may be selected as a monitored output electrode. A power line 633 supplies power to the processing circuit 602. Output signals from the processing circuit 602 are supplied on an output line 634, allowing data to be analysed further by a laptop computer 635 or similar data processing device. Figure 7 Procedures performed by the environment described with reference to Figure 6 are shown in Figure 7. After an object has been placed on the apparatus, a two-dimensional scanning operation is performed at step 701 involving both sets of electrodes. Thus, an electrode of the first set is energized and an electrode of the second set is monitored. This procedure is then repeated until all possible combinations of electrodes have been considered. In the embodiment of Figure 7, a question is asked at step 702 as to whether an image is identifiable. If this question is answered in the negative, it is assumed that the energizing input voltage is insufficient and therefore the input voltage is increased at step 703. The process is then repeated until the question asked at step 702 is answered in the affirmative. In response to identifying image data at step 702, positional data is displayed at step 704. Thus, in an embodiment, a graphical image of the shape of the object placed on the apparatus is displayed to an operative via the laptop computer 635. At step 705, first direction layering is performed of the type described with reference to Figure B. The resulting data is stored at step 706, whereafter second direction layering is performed at step 707. The second direction layering is substantially similar to that described with reference to Figure B but, on this occasion, using the second set of electrodes that are also present on the first upper surface of the dielectric substrate. In this way, the layering procedures performed at step 707 provides data that is substantially similar to the data obtained from the first direction layering procedures performed at step 705. At step 708, the data derived from the second direction layering is again stored and an analysis of the stored data is performed at step 709. This analysis may be performed by the processing circuit 602 or the analysis may be performed by the laptop computer 635. It will also be appreciated that a further degree of analysis may take place at a remote location. At step 710, information derived from the analysis step is displayed on the laptop computer 635. The apparatus may be deployed for scanning shoes to identify suspicious material. Steps 701 to 703 ensure that a sufficient degree of penetration is being achieved in order for any suspicious material to be detected if suspicious material is present. The two-dimensional scan performed at step 701 also provides a graphical image of the outline of the shoe, which can then be overlaid with the content information derived from the layering operations. Figure 8 An example of an output signal 801 derived from a monitored electrode is graphically represented in Figure 8, in which output voltage 802 is plotted against time 803. The output signal 801 has been derived from the first array of continuous electrodes 111 to 118. When a similar position is monitored by the second set of discontinuous electrodes 121 to 128, a similar response 801 is achieved. In the earlier known configuration of electrodes, described with reference to Figure A and Figure B, a similar output signal derived from the second set of electrodes would be attenuated, as illustrated by a notional response 811. To provide the first data stored at step 706 and the second data stored at step 708, multiple samples of the monitored output signal are digitized. Thus, in an embodiment, the processing system 602 includes an analogue-to-digital converter which, for each monitored output signal, produces a first sample 821 at time T1, a second sample 822 at time T2, a third sample 823 at time T3, a fourth sample 824 at time T4 and a fifth sample 825 at time T5.

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