AU5694100A - Coil antenna device and method for generating a varying magnetic field - Google Patents

Description

WO 01/03237 PCT/GB00/02533 COIL ANTENNA DEVICE AND METHOD FOR GENERATING A VARYING MAGNETIC FIELD TITLE: ENERGISING APPARATUS Field of the Invention This invention relates to energising apparatus for energising an inductive pick-up element of a device to enable the device to operate, and to a method of operating such apparatus. The invention also relates to an identification system which uses such apparatus to energise identification transponders to enable the latter to transmit identification signals for reception and analysis by the system. Background to the Invention Identification systems are often used in situations where it is necessary to identify and/or locate moving objects. For example, such a system can be used in in-store security systems to raise an alarm if a product (fitted with a security tag) is being moved from a designated area. Alternatively, the apparatus can be used to identify livestock (carrying identification tags) entering, for example, a milking or feeding parlour or abattoir. Since the energy to operate the identification tags is provided by the energising means of the system, tags can be of a relatively compact, lightweight construction. Typically, the system uses the same set of antennas to both energise tags and receive the identification signals transmitted thereby. In general, the tags all carry inductive pick-up coils, and the systems energise those coils (and hence the tags) by creating a time varying magnetic field. Some systems have a full duplex mode of operation in which they are arranged to receive an identification signal from a given tag whilst energising the tag. In other examples, the magnetic field for energising the tags is periodically interrupted, and identification signals are received during said interrupts, thereby giving a half-duplex mode of operation. Generally, the energising magnetic fields are generated by means of antenna coils which in WO 01/03237 PCT/GB00/02533 2 some cases are also used to receive the identification signals. Other systems, however, have further antennas for receiving such signals. In any of these cases, however, there is a risk that the system will fail to detect a tag which is moving rapidly through the detection zone (so that it is in the energising field for insufficient time to be energised and to transmit its identification signal. Alternatively the orientation of the tag relative to the field generated by the energising apparatus may be such that no or insufficient voltage is induced in the tag's pick-up coil. The best magnetic coupling between an antenna coil of the energising apparatus and the pick-up coil is achieved if the coil axis is parallel to the magnetic field at the coil, but the quality of the coupling deteriorates for different orientations, and there is very little or no coupling if the pick-up coil axis is at 900 to the generated field. Consequently, it has not been possible for identification systems which use large "walk through" or portal antennas for identifying cattle passing therethrough to read identification tags (on the cattle) consistently every time an animal passes. Summary of the Invention According to a first aspect of the invention, there is provided energising apparatus for generating magnetic fields in a detection zone, for energising an inductive pick-up element in a device in said zone, the apparatus comprising a plurality of antenna coils situated in range of the detection zone, supply means for supplying current to said antenna coils, to cause said fields to be generated, and current control means for varying the pattern of current flow through the coils, thereby to alter the direction of said fields in the detection zone, so as to enable a pick-up element therein to be energised when in any one of a number of possible orientations. Thus, for example, a pick-up element comprising a coil might pass through the detection zone in an orientation which, initially, is such that the coil axis is perpendicular to the field line so that the coil does not cut any magnetic flux lines, and no appreciable current is WO 01/03237 PCT/GB00/02533 3 induced in the coil. However, when the control means then changes the orientation of the magnetic field, this situation will no longer apply and a current will be induced in the pick up coil. Thus, the invention enables, for example, identification tags to be correctly and consistently identified as they pass through the detection zone, even if different tags pass through the zone in different orientations. Preferably, the coils lie in different planes from each other. The coils may to advantage be arranged around said detection zone, which may, for example, lie in a passage, such as an entrance or exit to an enclosure. Preferably, the apparatus has three coils lying in different, non-parallel planes, preferably with the coil axes converging on said zone. Conveniently, the coil axes are mutually perpendicular and cross in the detection zone. Thus, where the coils are arranged around the passage, one of the coils may encircle the passage, the coil axis extending along the passage, one of the remaining two coils may be positioned to the side of the passage, with its axis extending horizontally thereacross, whilst a third coil is situated above or below the passage so that its axis extends vertically thereacross. Where three non co-planar, non parallel coils are used, the current control means may comprise switching means for the supply to be connected to each coil in turn. It will be appreciated that, in view of the different orientations of the coils, the direction of the magnetic field in the detection zone will be governed by the orientation of the coil currently being supplied with current and the direction of current flows through the coil. Preferably, each coil is situated opposite a respective further coil also connected to the supply means and control means, the apparatus therefore comprising six coils.

WO 01/03237 PCT/GB00/02533 4 Preferably, each coil is co-axial with its respective further coil, and is connected in parallel to the respective further coil. Preferably, at least one of the coils and its respective further coil are non-planar, each having a portion in a plane substantially parallel to the axis of the coils. Thus, each coil may be substantially U-shaped in a plane containing the coil axis. Preferably, each said portion of one of those coils is aligned with the corresponding portion of the other coil, said corresponding portions extending towards each other. In an alternative, simplified arrangement, the apparatus comprises a pair of opposed antenna coils, and the current control means is arranged periodically to reverse the direction of current flow through one of the coils, the current control means thus providing a first mode of operation in which current passes through the coils in the same sense, and a second mode of operation, in which current travels through the coils in opposite senses, said two modes alternating with each other. In either mode of operation, the two coils will generate magnetic fields which in some directions will tend to reinforce each other, whilst tending to cancel each other out in other directions. Changing the mode of operation causes the directions in which the fields are reinforced and cancelled out to be changed, so that changing from one mode to the other alters the direction of magnetic field in the detection zone. In addition, when currents flow through the coils in the same sense, the system is best able to energise a pick-up element situated on or near the axes of the coils. However, when current flows through the coils in opposite senses, the apparatus is better able to energise devices spaced from said axes. The coils may to advantage be co-axial.

WO 01/03237 PCT/GB00/02533 5 The invention also lies in an identification system for energising a remote transponder and detecting an identification signal transmitted thereby, the system comprising energising apparatus as hereinabove described and receiver means which is connected to the antenna coils of the energising apparatus and is operable to receive said transmitted signal via said coils. Preferably, the current supply means and receiver means are constituted by a common reader. An example of such a reader is the TIRIS 2000 reading head produced by Texas Instruments. According to a further aspect of the invention, there is provided a method of driving current through a pair of opposed coils so as to create a time varying magnetic field, in a detection zone, for energising an inducted pick-up element of a device in said zone, the method comprising supplying current to the coils in one direction and periodically reversing the direction of current through one of the coils. Brief Description of the Drawings Two embodiments of identification system in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of identification apparatus in accordance with the invention, the apparatus having three mutually perpendicular pairs of antenna coils; Figure 2 is an isometric view of a housing which contains said antennas and which defines a portal; Figure 3 is a diagrammatic view illustrating the relative orientations of the antenna coils; WO 01/03237 PCT/GB00/02533 6 Figures 4 - 7 each show, in plan, an antenna coil, and illustrate the areas around that coil in which an inductive pick-up coil can be energised at a given respective orientation relative to the antenna coil; and Figures 8 and 9 are plan views of antenna coils used by a modified version of the apparatus, each Figure illustrating the areas of coverage for a respective one of two possible modes of operation of the system. Detailed Description The system shown in Figure 1 is intended for use in identifying cattle passing through a gateway or portal into, for example, a feeding parlour. Each animal has a bolus or an ear tag which houses a respective transponder. The tags and transponders are of the type described in PCT Publication No. WO 98/08182. Each transponder has a pick-up coil in which currents may be induced by a time varying magnetic field so as to energise the pick-up coil and allow the transponder to transmit an RF identification signal which is picked up by the system. The field for energising the pick-up coil is generated by an arrangement of antennas in a generally parallelepipedal frame 1. The frame 1 surrounds a passage at the portal or gate and the direction in which the passage extends is indicated by the arrow 2. With reference to Figure 2, the frame 1 is formed from a number of hollow steel section in the form of a first rectangular end-piece formed from two opposed side uprights 4 and 6 and horizontal top and bottom members 8 and 10. This end-piece is attached to a similar end-piece 12 through twelve horizontal tubes extending in the direction of the passage. Six tubes, for example, the tubes 14 and 16, extend from the upright 4 to the corresponding upright on the end-piece 12, whilst a further six of the tubes extends from the upright 6 to the corresponding upright of the end-piece 12. Two diagonal members 18 and 20 extend between the uprights of the end-pieces and provide additional structural rigidity for the frame.

WO 01/03237 PCT/GB00/02533 7 All the ends of the tubes and the cross pieces at the top of the end sections are all connected together via electrically insulating components so as to be electrically insulated from one another. In practice any substantial framework could be used to support the cables, but steel offers the most practical solution. The diagonal members are an option for use when the unit is not fixed when extra stability can be achieved by a link between the uprights and for example metal posts set in concrete on each side of a passage some 100 mm away from the uprights. The antennas take the form of coils which are housed within the hollow members of the frame 1 and the orientation of the coils is shown more clearly in Figure 3. Each of the rectangular end-pieces contains a respective one of a pair of rectangular end antennas 20 and 22, both of which surround the passage. The uprights of the end-pieces also house vertical runs of two side antennas 24 and 26 in the form of rectangular coils. The horizontal portions of the coil 24 extend through the bars 14 and 28, whilst the corresponding portions of the coil 26 extend through the bars 30 and 32. As can be seen from Figure 3, the coils 24 and 26 are disposed one on either side of the passage. The remaining pair of antennas housed within the frame 1 are indicated by reference numerals 34 and 36 in Figure 3. These antennas take the form of coils which extend above and below the passage, but, unlike the other antennas, are non-planar. Thus, the coil forming the antenna 36 has a front horizontal portion 38 which extends through the member 10, a rear horizontal portion 40 extending through the lower cross-piece (referenced 42) of the end-piece 12, and two further horizontal portions 44 and 46 which extend in the direction of the passage and pass through corresponding bars 48. Similarly, the antenna 34 has horizontal portions 52 and 54 passing through the upper horizontal members of the end-pieces, and further horizontal portions 56 and 58 accommodated by the bars 16 and 60 respectively. Thus, the antennas 34 and 36 are of a generally U-shaped cross-section (in a plane perpendicular to the arrow 2).

WO 01/03237 PCT/GB00/02533 8 Each of the antenna coils has an inductance in the range 150 pH - 180H, and is formed from double insulated multiple core power cable. This type of cable is of a relatively low cost, but provides good resistance to the effects of water, thus allowing the antennas to be used in an exposed environment. The frame members may be sealed to prevent the ingress of water, or alternatively may include suitable drainage means to allow any water which has seeped into the members of the frame to drain away. For ease of manufacture the antenna coils are formed from 7 core cable. The number of turns actually used is varied between the coils in order to balance the inductances of the different sizes. For example the top and bottom have 6, one side has 6 the other 7, and the ends have 7 but this would alter if the relative sizes changed. The inductance and number of turns is also related to the value of the tuning inductor and type of reader. The height of the frame 1 is sufficient to enable the cattle to pass therethrough, and to that end is approximately 1.8 metres. The length of the frame (i.e. the distance between the front and rear of the frame) is such as to ensure that a cow passing through the passage is within range of the antenna coils for a sufficient length of time to enable its ear tag to be energised and then read. In the present example, the length of the frame 1 is of the order of 1.2 metres. Referring back to Figure 1, the current needed for driving the antenna coils is provided by a TIRIS 2000 reading head 60 connected to the antennas via a multiplexing circuit 62 and set of three co-axial cables 61. The power needed for operating the reading head 60 is provided by a low voltage power supply 64. The operation of the reading head 60 will not be described in detail since this piece of equipment is already well known in the field of identification systems. Briefly, the reading head 60 produces an alternating current at a frequency of 130.2 kHz in a series of bursts of approximately 50 mS duration. Each burst is followed by a pause of a further 50 mS during which the reading head "listens" for any identification signals picked up by the antenna coils. The head is provided with a data link 66 via which information on the WO 01/03237 PCT/GB00/02533 9 received identification signals can be relayed to, for example, a computer having a database relating to the tags and/or a log of identification signals previously received. A parallel inductor (not shown) is also connected in parallel between the head 60 and multiplexer 62. This inductor has the same function as the inductor described in WO 98/08182, and referenced 212 in Figure 4 of that document. The inductor is of the same inductance as the previously described inductor, 50 microHenrys. If the system were to use another type of reader, which operates with higher inductances, a parallel inductor having a different inductance, for example in the order of 300gH, might be used. Other methods of tuning could be used such as a series inductor. The object of both a parallel or a series inductor being to bring the inductance of antenna coils into the inductance range where the reader can operate. Therefore the use of the parallel inductor is not essential. The multiplexor 62 incorporates a switching device which connects each of the three cables 61 to the reading head in turn, so that at any one time, only one of the cables 61 is connected. One of the cables 61 is connected parallel to the pair of antennas 20 and 22, another to the antennas 24 and 26 and the third of the cables 61 is connected to the antennas 34 and 36. The multiplexer is arranged to change the connection of the cables 61 (and hence the pair of antennas connected to the reader 60) after the head 60 has sent a burst of alternating current to the selected pair of antennas and listened for an identification signal received by the antennas, i.e. once every reading cycle (in this case 100 mS). Alternatively, the multiplexer 62 can be arranged to change the selection of antennas at a period which is a multiple of 100 mS. Initially, therefore, one pair of opposed antennas, for example the antennas 20 and 22, are connected to the head 60 which causes an alternating current of a frequency of 134.2 kHz to flow through the antennas 20 and 22 for 50 mS. If a tag is in the range of either of the antennas 20 and 22 and is so orientated that its pick up coil is energised by the fields created by those antennas, the tag will then transmit an identification signal via its pick-up coil. In the 50 mS following the burst of current, the WO 01/03237 PCT/GB00/02533 10 head 60 remains connected to the antennas 20 and 22 and picks up any signal (appearing as an induced voltage in either of those antennas) transmitted by such a tag. The multiplexer unit 62 then alters the connections of the cable 61 so that a different pair of antennas, for example the antennas 24 and 26, are connected in parallel to the reading head 60. The reading head 60 therefore then causes current to flow through the second pair of antennas and again monitors those antennas for any received signal. The multiplexer 62 then connects the remaining pair of antennas, in this case the antennas 34 and 36, to the reading head 60 and the process is repeated. It will be appreciated that, by changing the pair of antennas through which the current is flowing, the direction(s) of magnetic field within the passage can be altered. Consequently, a tag whose pick-up coil is orientated in such a way that it does not link with the flux from one pair of antennas will almost likely link with the flux generated by either of the remaining two pairs, so that at least one of the pairs of antennas can energise the coil and receive the identification signal from the tag. By way of general illustration, Figures 4 - 7 show the results of an experiment conducted with a single coil antenna connected to a TIRIS 2000 reading head. In all four figures, the coil is shown in plan view and is represented by the line 70 which indicates the plane of the coil. The coil axis is therefore perpendicular to the line 70. In Figure 4, an identification tag is placed at various positions around the coil, with its pick-up coil being in a plane which is generally perpendicular to the line 70 (so that the axis of the pick-up coil is perpendicular to the axis of the coil connected to the reader). The zones referenced 72 and 74 are the areas in which the reader could pick up a signal from the tag when so orientated. Thus, when the tag lies on or near the axis of the antenna coil, the field generated by the antenna coil does not generate any (or sufficient) current in the pick-up coil to enable the tag to emit an identification signal which can be detected by the head. Figure 5 shows the results of the same experiment when the tag is orientated with the coil in a plane which is at approximately 600 to the line 70. In this case, signals can be WO 01/03237 PCT/GB00/02533 11 detected when the tag is in a zone 76 (which extends over part of the axis of the coil represented by line 70), and in a side lobe 78. Figure 6 shows the same experiment when the pick-up coil is at an angle of 300 to the line 70, giving rise to detection zones 80 and 82 whilst the results of the same experiment when the pick-up coil is parallel to the line 70 as shown in Figure 7 in which there is a large detection zone 84 extending along the axis of the antenna coil and two smaller side lobes 86 and 88. Because the fields generated by two opposed coils can in some places cancel each other out and in other places reinforce each other, the results illustrated in Figures 4 - 7 are not necessarily indicative of the detection zones that would be obtained from a pair of opposed coils. However, they do show how the orientation of the tag can have an effect on the zones in which a signal from the tag can be detected, and therefore illustrate the point that the use of differently oriented antenna coils (to provide the energising fields) can help to ensure that a tag is detected even if it is so orientated that (at a given position) it cannot be detected using one coil or a pair of coils. The height of the framework 1 (and hence the maximum distance between the antennas 34 and 36), is in fact greater than the read ranger of a tag. However, the portions of those two antennas which extend down and up the sides of the frame prevent this fact from giving rise to any dead zone in the passage which is out of the range of the antennas 34 and 36. Figures 8 and 9 show a simplified embodiment of the invention in which the three pairs of mutually perpendicular antennas are replaced by a simple pair of opposed co-axial antennas 90 and 92. As before, these are connected to a TIRIS reading head and power supply (of the same form as the supply 64 and the head 60). However, the antenna 90 is permanently connected to the head 60, whilst the antenna 92 is connected to a switching device (not shown) which controls the direction in which current flows through the coil 92.

WO 01/03237 PCT/GB00/02533 12 In this embodiment, the system has two modes of operation: a first mode in which the alternating current flows through the antenna coils 90 and 92 in phase and hence in the same sense, and a second in which it flows in antiphase, and hence in opposite senses. In Figure 8, the zone 94 represents the area, directly between the coils, in which tags can be energised and signals from them detected when energising current flows through the coils in the same sense. Figure 9 shows the corresponding detection zones 96 and 98 which are achieved if current flows through the coils 90 and 92 in opposite senses. Thus, when current flows through the coils in the same sense, the fields generated at or near the common axis of the coils are reinforced. It will be appreciated that those fields extend in the general direction of that axis. When the apparatus is operating in the mode illustrated in Figure 9, the fields at or near the common axis tend to cancel each other out, and the fields spaced from the axis will tend to reinforce each other, giving rise to the detection zones 96 and 98. It will be appreciated that the fields in the detection zones 96 and 98 tend to extend in the general direction of the passage, i.e. generally perpendicular to the common axis. The antenna shown in Figures 8 and 9 have a length (i.e. dimension in the direction of the passage) of 400 mm and a height of 1,500 mm, and it has been found that the combined fields from the antennas enable tags to be read at all angles except to within an angle of about 150 to the vertical over most of the space between the antenna. It will be appreciated that various modifications may be made to the embodiments described without departing from the spirit or scope of the invention. For example, the antenna structures could be of a longer length so as to cater for situations in which tags move through the passage at relatively high speeds. If the length is such that the central region of the passage is out of the range of the end antennas (of the six antenna designs) 20 and 22, then the system can include further antennas which are co-axial with the antennas 20 and 22 and are situated therebetween. The correct polarity of each adjacent antenna would have to be maintained by connecting each in phase with the next (i.e. the north/south, north/south format). This arrangement will enable accurate tag reading at speeds in excess of those required for cattle.

WO 01/03237 PCT/GB00/02533 13 The maximum height and width of the arrangement of antenna coils is related to the read range of the tags. Ideally, the tags are able to be read when at an angle of 450 to the axis of any of the antennas at the central point between all three pairs.

Claims (18)

1. Energising apparatus for generating magnetic fields in a detection zone, for energising an inductive pick-up element for a device in said zone, the apparatus comprising a plurality of antenna coils situated in range of the detection zone, supply means for supplying current to said antenna coils to cause said fields to be generated, and current control means for periodically varying the pattern of current flow through the coils, thereby to alter the directions of said fields in the detection zone, so as to enable a pick-up element therein to be energised when in any one of a number of possible orientations.

2. Apparatus according to claim 1, in which the coils lie in different planes from each other.

3. Apparatus according to claim 1 or claim 2, in which the coils are arranged around said detection zone.

4. Apparatus according to claim 3, in which the detection zone comprises a passage for said devices.

5. Apparatus according to any of claims 2 to 4, in which the apparatus has three coils lying in different, non parallel planes.

6. Apparatus according to claim 5, in which the axes of said coils converge on the zone.

7. Apparatus according to claim 6, in which the coil axes are mutually perpendicular.

8. Apparatus according to claim 4, in which at least one of the coils encircles the passage, the axis of the coil extending along the passage, another of the coils being WO 01/03237 PCT/GB00/02533 15 positioned to the side of the passage, with its axis extending therealong, and a third coil being situated above or below the passage with its axis extending vertically thereacross.

9. Apparatus according to any of claims 5 - 8, in which the current control means comprises switching means for causing current to flow through the coils in sequence such that current is supplied to one coil at a time.

10. Apparatus according to any of claims 5 - 9, in which each coil is situated opposite a respective further, co-axial coil also connected to the supply means and control means, the apparatus therefore comprising six coils.

11. Apparatus according to claim 10, in which the coils in one pair are non-planar, each having a portion in a plane substantially parallel to said axis.

12. Apparatus according to claim 11, in which each portion of said member of the pair is aligned with the corresponding portion of the other member.

13. Apparatus according to claim 12 when appended to claim 8 or claim 4, in which one member of said pair of non-planar coils is situated above the passage, the other underneath the passage, the said portions of each member being situated on either side of the passage.

14. Apparatus according to claim 1, in which the apparatus comprises a pair of opposed antenna coils, and the current control means is arranged periodically to reverse the direction of current flow through one of the coils relative to the other to provide a first mode of operation in which alternating current passes through the coils in phase, and the second mode in which current passes through the coils in anti-phase, said two modes alternating with each other.

15. Apparatus according to claim 14, in which the coils are co-axial. WO 01/03237 PCT/GB00/02533 16

16. Identification system for energising a remote transponder and detecting an identification signal transmitted thereby, the system comprising energising apparatus according to any of the preceding claims, and receiving means which is connected to the antenna coils and is operable to receive said transmitted signals via said coils.

17. A system according to claim 16, in which the current supply and receiver means are constituted by a common reader.

18. A method of driving currents through a pair of opposed coils so as to create a time bearing magnetic field in a detection zone for energising an inductive pick-up element of a device, the method comprising supplying alternating current to the coils in phase so that current moves round the coils in the same direction (at any given time), altering the phase of the current flow through one of the coils so that, at any time, the current flows through the coils in opposite senses.