WO2007030004A1

WO2007030004A1 - System and method for detecting objects, and magnetic field generator for use in such a system or method

Info

Publication number: WO2007030004A1
Application number: PCT/NL2005/000653
Authority: WO
Inventors: Rob Maaskant; Jacobus Adrianus Kegel
Original assignee: Stichting Astron
Priority date: 2005-09-09
Filing date: 2005-09-09
Publication date: 2007-03-15

Abstract

A system for detecting objects in an area which is monitored by the system. The system has a magnetic field generator constructed to generate a magnetic field. The magnetic field has one or more null zones with a variable position in the monitored area. In the null zone at least one component of the magnetic field is substantially absent. The system further has a detector for detecting a signal transmitted by the object in response to an absorption of energy from the magnetic field by the object.

Description

Title: System and method for detecting objects, and magnetic field generator for use in such a system or method.

FIELD AND BACKGROUND OF THE INVENTION ,

The invention relates to a system for detecting objects, such as radio frequency identification (RFID) tags or transponders. The invention further relates to a method for detecting objects and to an antenna for use in such a system or method. Systems for detecting objects, such as transponders are known in the art, and are typically used in anti-theft systems in shops, tracking and accounting of livestock or otherwise. These known systems generate an energy field in an area monitored by the system, and are able to detect a transponder passing through the energy field. Typically, the energy field is a magnetic or electro-magnetic field. Transponders may be either active or passive. A passive transponder merely absorbs energy from the energy field generated by the detection system. This absorption may be detected, since it affects the energy field.

An active transponder absorbs energy from an energy field and uses the absorbed energy to transmit a signal. Examples of active transponders are RFID tags. In many circumstances, it is necessary to record data about a particular object, e.g. an article in a shop, an animal or otherwise, and to distinguish between objects, e.g. which type of article. Typically, an RFID tag absorbs energy from an electric, magnetic or electro-magnetic field and uses the absorbed energy to power or activate an electronic circuit on the RFID tag. The powered electronic circuit transmits a radio frequency signal which contains an identification or other data recorded on the, RFID tag.

However, a disadvantage of the known systems is that the transponder may pass the monitored area without being detected, since the system includes an antenna which generates an energy field with so called null zones. In those zones, the energy field has one or more components which have (almost) no strength. In most situations, e.g. anti-theft systems, the position and orientation of the transponder relative to the detection system maynot be controlled as it passes through area monitored by the detection system. Since one or more of the field components have (almost) no strength, at least for some orientations thereof, the transponder will not absorb energy from the energy field. Thus, a transponder will not be detected and pass the monitored area. To reduce the presence of null zones, several techniques are known.

United States patent 5 663 738 discloses a antenna device for detection of resonant tags. The antenna device includes an antenna wire loop which is formed into two electrically outer partial loops and one electrically central partial loop by twice twisting the antenna wire loop, in each case by 180 DEG. One of the two electrically outer partial loops is geometrically positioned between the electrically central partial loop and the other electrically outer partial loop. An impedance element is connected between two connecting points located at the entrance and exit of the geometrically central partial loop. The antenna wire loop combined with the impedance element forms a resonant circuit. At its resonance frequency, the resonant circuit acts as a barrier element which phase shifts the current running through the geometrically central partial loop by 90 degrees with respect to the geometrically outer partial loops. However, a disadvantage of the antenna device known from United States patent 5 663 738 is that for some orientations thereof, a transponder does not absorb energy from the electromagnetic field, since there are zones in which one or more of components of the electromagnetic field have (almost) no strength. The zones form a straight horizontal passage between opposites side of the monitored area. Accordingly, a transponder, and the object to which the transponder is attached, may pass the environment monitored by the antenna device without being detected, by moving the transponder along a straight horizontal line corresponding to the passage.

United States patent 6 720 930 discloses an antenna which comprises a pair of antenna coils that are placed in a crossing relationship with one another. A first antenna coil is provided in a closed loop configuration which extends along a first axis, and a second antenna coil is provided of a closed loop arrangement which extends along a second axis, the second antenna coil being placed perpendicular to and fields with opposite phases. Within the crossing pattern of the antenna itself, the magnetic fields have areas in which the total field strength is zero, the null zones or out of phase areas. However, a disadvantage of the antenna known from United States patent 6

720 930 is that a transponder may pass an area monitored by the antenna without being detected. Outside the null zones, the magnetic field has detection dead zones in which the magnetic field, is present but some components of the magnetic field have (almost) no strength. Thus, the transponder may not absorb energy from the magnetic fields generated by the antenna in the detection dead zones when not oriented properly. Together with the null zones, the detection dead zones form a straight, horizontal passage between opposites side of the monitored area. Accordingly, it is still possible that a transponder, and the object to which the transponder is attached, passes the environment monitored by the antenna without being detected by moving the transponder along a straight horizontal line corresponding to the passage.

SUMMARY OF THE INVENTION

It is one goal of the invention to provide a system which is able to monitor an area with a reduced risk that an object may pass the monitored area without being detected.

In order to achieve this goal, the invention provides an antenna system according to claim 1.

Such a system is able to monitor an area with a reduced risk of transponders passing through the monitored area undetected, because the magnetic field generator is constructed to generate a magnetic field with at least one null zone which has a variable position in the monitored area. Thus, it is less likely that a transponder passing through the monitored area will be in the null zone during the entire passage. Accordingly the risk of a transponder passing through the monitored area undetected is reduced.

Specific embodiments of the invention are set forth in the dependent claims.

BRIEF DESCRIPTION QF THE DRAWINGS

Further details, aspects and embodiments of the invention will be described, by way of example only, with reference to the attached drawings.

Fig. 1 shows a graph of the simulated strength of the components of the magnetic field of the prior art system disclosed in US patent 5663738. Fig. 2 shows a graph of the simulated strength of the components of the magnetic field of the prior art system disclosed in US patent 6720930. Fig. 3 shows schematically an example of an embodiment of a system according to the invention

Fig. 4 shows an example of a magnetic field generator suitable for the system of fig. 3. Fig. 5 shows a perspective view of an example of a construction element suitable for constructing the example of fig.4.

Fig. 6 shows a tilted, perspective view of a side of the example of fig. 5

Fig. 7 shows a second example of a magnetic field generator in a first state.

Fig. 8 shows the example of fig. 7 in a second state Fig. 9 shows a graph of the simulated strength of the components of the magnetic field of the example shown in fig. 3 in a second state of the example.

Fig. 10 shows schematically a third example of a magnetic field generator.

Fig. 11 shows schematically a fourth example of a magnetic field generator.

DETAILED DESCRIPTION

Every magnetic field H is composed of three perpendicular components. In this application, the components are respectively referred to as an x-component Hx in parallel with the horizontal x-axis of the coordinate system 7 shown in figs. 3, an y- component Hy in parallel with the horizontal y-axis of the coordinate system, and a vertical component Hz in parallel with the vertical z-axis of the coordinate system 7.

Fig. 1 schematically shows the results of a simulation of the components Hx₁Hy, Hz of the magnetic field H of the prior art antenna shown in fig. 3d of United States patent 5 663 738. Fig. IA shows the dimensions of the antenna used in the simulation in meters. In this simulation, an anti-clockwise current through the top loop with a strength of 1 Ampere was used. The current through the current loop in the middle was simulated to flow in a clockwise direction with a strength of IA. The current through the current loop at the bottom was simulated to flow in an anticlockwise direction with a strength of IA. Figs. IB- ID shown the simulated strength of the components Hx,Hy,Hz in a plane oriented in parallel with the antenna at a distance of 1 meter thereof. Fig. IE shows the simulated strength of the composed magnetic field H in this plane. Fig. 2 shows the result of a simulation of the components Hx,Hy,Hz of the magnetic field H of the prior art cross-shaped arrangement disclosed in United States patent 6720930. The dimensions of the horizontal and vertical loop of this arrangement are indicated in fig. 2A in meters. The current through the current loops was taken to be 1 Ampere in a clockwise direction. Figs. 2B-2D shown the simulated strength of the components Hx,Hy,Hz, in a plane oriented in parallel with the antenna at a distance of 1 meter thereof. Fig. 2E shows the simulated strength of the composed magnetic field H in this plane.

As shown in figs. IB-ID and 2B-2D, the prior art antennas generate a magnetic field which has one or more vertical null zones nx, caused by the absence of the x-component Hx. The magnetic field further has horizontal null zones ny,nyl,ny2,nz,nzl,nz2,nz3. These horizontal nulls zones are caused by the local absence of the y- and/or z-components Hy₁Hz of the magnetic field H.

The horizontal nulls zones ny,nyl,ny2,nz,nzl,nz2,nz3, as well as the vertical null zones nx provide a straight passage between opposite side of the monitored area 2. A transponder will only absorb energy from the magnetic field if oriented non- parallel to one or more of the components Hx, Hy and Hz. When a transponder is oriented perpendicular to the absent components, the transponder will therefore not absorb energy from the magnetic field in the null zone. Thus, in the prior art systems, a horizontal non-detection zone is present in which a transponder will not be detected if oriented properly.

Fig. 3 schematically shows an example of an embodiment of a detection system 1 according to the present invention. The detection system 1 is arranged to monitor an area 2, and to detect objects in the monitored area 2. In the example, the monitored area 2 is confined at the bottom 21 and at upright sides 22,23. At the bottom 21, the ground forms a boundary of the monitored area. At the upright sides 22,23, a a magnetic field generator 3 and the detector 4 form respective boundaries of the monitored area 2. The monitored area 2 is open at a top 20 and passage sides 24,25. Objects may enter and/or leave the monitored area 2 at the, open, passage sides 24,25 in a direction of passage P through the monitored area 2 or in a direction opposite to the direction of passage P. From hereon, the passage side at the left in fig. 3 is referred to as the entrance side 24, while the passage side at the right in fig. 3 is referred to as an exit side 25.

A system 1 with a passage-like monitored area 2 is especially suited for detecting the presence of objects which enter and/or leave a confined space, e.g. a shop. To that end, the system 1 may be positioned at an entrance or exit of the confined space, e.g. the door of a shop, and for example be positioned such that the monitored area 2 lies with its entrance side 24 or its exit side 25 adjacent to the entrance or exit of the confined space.

However, the invention is not limited to such applications, and the system 1 may also be present in an open area, and be used to detect the presence as well as the position of objects in the monitored area 2. For example, the system may be implemented such that the magnetic field generator generates a magnetic field in the entire space, e.g. a storage room or a plant. It is also possible that the monitored area 2 is open at one or more sides, for example by using a magnetic field generator 3, and optionally a detector 4, placed at a central position in the monitored area 2.

The system 1 shown in fig. 3 further includes a magnetic field generator 3 and a detector 4. The magnetic field generator 3 and the detector 4 are connected to a control device 5 via suitable connections 50-55. In the example of fig. 3, the magnetic field generator 3 is able to generate a magnetic field in the monitored area 2. The control device 5 can control the magnetic field generator, and send suitable control signals via the connections 50-54. The detector 4 may detect a signal transmitted by a transponder 6 in response to an absorption of energy from the magnetic field by the transponder 6. In response to the detection, the detector 4 transmits a detection signal to the control device 5 via the connection 50. The control device 5 processes the detection signal further, and e.g. outputs a warning signal in a for humans perceptible form.

In case a transponder 6 is of a passive type, its presence in the monitored area 2 may be detected by the detector 4, since a transponder 6 passing through the magnetic field H absorbs energy from the field. This absorption is monitored as an increment of the power that needs to be fed to the field generator. A passive transponder 6 thus passively transmits a signal in response to the magnetic field H. The amount of absorption may be constant of vary in time for example because of a movement of the transponder through the energy field.

In case a transponder 6 is of an active type, e.g. an RFID tag, its presence in the monitored area may be detected, since the transponder 6 will actively transmit an (electro) magnetic signal in response to an absorption of energy from the magnetic field H by the transponder 6. Typically, an active transponder uses the absorbed energy to drive an electronic circuit which outputs the signal. In case of an RFID tag, the signal will be an electromagnetic signal with a radio frequency. Depending on the specific implementation, any suitable type of detector may be used. Detectors for passive and/or active transponders are generally known in the art and for the sake of brevity not described in further detail.

The magnetic field generator 3 is constructed to generate a magnetic field H in the monitored area 2. At least in some states of the magnetic field generator 3, one or more of the components Hx,Hz and Hy of the magnetic field H are locally absent in the monitored area 2. In those states, the magnetic field H has one or more null zones nx,nz in the monitored area, i.e. a zone in which one, two or all components Hx,Hy,Hz of the magnetic field H are absent.

In this respect, without wishing to be bound to any theury, it is believed that every magnetic field has zones in which one or more of the components Hx,Hy,Hz are absent.

As explained below in more detail, in the example of fig. 3, the null zone nx,nz has a variable position in the monitored area. The chance that a transponder follows during the path of the null zone nx,nz the entire passage is small, since the position of the null zone varies in operation. Accordingly, the chance that a transponder 6, and the object to which the transponder 6 is attached, passes the monitored area undetected is reduced.

The magnetic field H generated by the magnetic field generator 3 may, for example, have at least one null zone nx,nz of which the position can be varied in time. To vary the position of the null zone in the magnetic field H, the magnetic field generator 3 may for example, include two or more magnetic sources with a different position and/or orientation. The control device 5 may, e.g., control the magnetic field generator 3 to switch between two or more different states. In a first state, for instance, one or more magnetic sources at a first position or orientation may be active, while in a second state magnetic sources at a second position or orientation different from the first position or orientation are active. Accordingly, the position and/or orientation of the magnetic field and hence of the null zone varies when the state of the magnetic field generator 3 is switched by the control device 5.

For instance, in the example shown in fig. 4, the control device 5 can activate in a first state a first current loop 330, and in a second state a second current loop 334 with a different orientation. Thereby, the orientation of the magnetic field and accordingly, the position of the null zone can be varied in the proximity of the magnetic field generator.

The at least two magnetic sources may for example each consist of a single current source. For instance, the example of a magnetic field generator 3 shown in fig. 4 includes a number of separate current loops 330-334 which differ in shape and/or orientation with respect to each other. The current loops 330-334 are indicated with the continuous, dashed, dotted, dash-dotted, and crossed-dashed lines in fig. 4. Each of the current loops 330-334 forms a controllable magnetic source. A first set of current loops 330-332, are mutually different in shape, is oriented with their longitudinal axis in the same direction. A second set of mutually different shaped current loops 333-334 is oriented with their longitudinal axis oriented in a second direction different from the first direction. In this example, the first and second direction are perpendicular, and the current loops form a cross-shaped arrangement. In the example of fig. 4, the position of the null zone can be varied in time.. For instance, in each state, a different selection of the current loops 330-334 may be active. (I.e. current loop 330 in a first state, current loop 331 in a second state etc.) Alternatively, a magnetic source may include two or more current loops. The example of a magnetic field generator 3 shown in figs. 7 and 8, for instance, includes a plurality of similar shaped current loops 320 connected to a current source 57' in a control device 5' via connections 56. The current source 57' can provide a current to each of the current loops 320. In case the current source provides a set 33 of current loops 320 with a current of same magnitude, phase and direction, but different in magnitude and/or direction and/or phase with respect to another set 34 of current loops 320, the individual fields of the current loops 320 in each set 33,34 combine, in accordance with Stokes' theorem, into a magnetic field of a single magnetic source.

(In the example of fig. 4, one or more of the current loops 330-334 differ in one or more of their shape, orientation and amount of current with respect to other current loops. Accordingly, the fields of the current loops 330- 334 do not combine into a magnetic field of a single magnetic source, and the respective current loops operate as separate magnetic sources .)

Due to the combination of the individual fields, each set 33,34 forms a single, controllable, magnetic source of which orientation and position can be varied by changing the current loops of the set in time, as shown in figs. 7 and 8 with the continuous and dashed lines. Thus, the position of the null zone can be varied in time in a flexible manner, and a large degree of variation of this position can be obtained. This further reduces the chance that a transponder passes the monitored area undetected. In the example of fig. 7 and 8, the current loops 320 are positioned in a matrix- shaped arrangement (in this example a 4 by 4 matrix). In fig. 7, for instance, the example of a magnetic field generator 3 is shown in a first state. In the shown first state, a first set 33 extends diagonally from the top-left to the bottom right of the matrix arrangement, e.g. includes the current loops 320 at matrix positions (1, 1), (2,2), (3,3), (4,4). A second set 34 extends diagonally from the bottom-left to the top right of the matrix arrangement, and includes the current loops 320 at matrix positions (1,4), (2,3), (2,3), (4,1) and (4,2). The first set 33 is provided with a current flowing in a clockwise direction, while the second set 34 is provided with a current flowing in an anti-clockwise direction. In fig. 8 the magnetic field generator 3 is shown in a second state. In the second state, the specific current loops 320to which a current is provided are changed with respect to the first state. In this example, the current through the current loops 320 at the corner positions in the matrix arrangement, e.g. at matrix positions (1,1), (1,4), (4,1), and (1,4), is stopped. Other current loops 320, which were without current in the first state, are provided with a current, e.g. in fig. 8 at matrix positions (1,3), (2,1), (3,4), and (4,2). Accordingly, the orientation of the magnetic source formed by each set 33,34 of current loops 320, is changed. Thus, the position of the null zone is varied in time as well.

It should be noted that the magnetic field generator 3 may simultaneously include one or more magnetic sources consisting of one current loop and at least one magnetic source including two or more current loops.

In addition to varying the position and./or orientation of the magnetic sources, the magnetic fields generated by the individual magnetic sources may be varied with respect to each other in time to change the position of the null zone.

The magnetic field generator 3 may for instance include two or more individually controllable magnetic sources, each of which may generate an individual magnetic field. The orientation and/or shape and/or strength and/or phase of these individual magnetic fields may be varied in time by a suitable control of the magnetic sources. The overall magnetic field H generated by the magnetic field generator 3 - results from the combined individual magnetic fields. Accordingly, the orientation and/or shape and/or strength and/or phase of the overall magnetic field H and hence the position of the null zone nx,nz may be varied in time.

For instance, in the examples of figs. 4, 7 and 8, the direction of the current provided to a current loop 320;330-334 can be varied in time, e.g. from clockwise to anti-clockwise. The orientation of the individual magnetic field generated by a current loop is dependent on the direction of the current flowing through the current loop

320;330-33. Hence, the orientation of the individual magnetic field varies in time, and the overall magnetic field is change. In addition or instead, e.g. the amount of current and/or phase of the current provided to a current loop can be varied with respect to the current provide to other current loops. This results in a variation of the orientation and/or shape and/or strength and/or phase of the individual magnetic fields generated by the separate magnetic sources. Accordingly, the orientation and/or shape and/or strength and/or phase of the overall magnetic field and hence the position of the null zone nx,nz may be varied in time as well.

It should be noted that the magnetic field may be varied in time in any suitable manner. The frequency with which the magnetic field varies may for instance be larger, such as at least two times and for example at least ten time larger, than the inverse of the time required to pass the monitored area. Thereby, the chance of detecting a transponder is increased. Also, the magnetic field may for instance be varied in a ( semi) random manner. Thereby the predictability of the shape of the null zone is reduced, as well as the chance that a transponder passes through the monitored area undetected. Instead of, or in addition to, varying in time, the null zone nx,nz may have a position which varies as a function of a distance of the null zone nx,nz from a boundary side of the monitored area, which boundary side forms a boundary of the monitored area in a direction transverse to the direction of passage through the monitored area 2. In the example of fig. 3, for instance, the magnetic field generator 3 can be implemented as shown in fig. 4. In such a case, the position of the null zone will vary as a function of the distance from the upright sides 22,32 of the monitored area, when all current loops are provided with a current, as explained below in more detail.

Thereby, the chance that a transponder 6 passes the monitored area undetected is reduced even further, since it is even less likely that the path of a transponder 6 passing through the monitored area 2 will show a noticeable variation in this distance. Accordingly, the transponder 6 will not follow a path corresponding to the null zone nx,nz. Furthermore, even if the transponder 6 is not detected by the system 1, the transponder 6 is likely to be detected visually by a human, since the movement of a transponder 6 along a path corresponding to the null zone nx,nz will be conspicuous, and therefore be noted easily by an operator or user of the system 1.

Fig. 9 shows the results of a simulation of the magnetic field of the example of a magnetic field generator according to fig. 4. The dimensions of the respective current loops 330-334 are indicated in fig. 9A in meters. The graphs of figs. 9B-9D show the components Hx,Hy and Hz of the magnetic field respectively, simulated at a distance in the direction y of 1 meter from the current loops 330-334, i.e. perpendicular to the x-z plane in which the current loops 330-334 extend. The current through the current loops 330,331 was taken to be 1 A in a clockwise direction. The current through the current loop 332 was taken to be .15 A in a clockwise direction. The current through loops 333,334 was taken to be I A in an anti-clockwise direction. As shown in figs. 9B-9D, the magnetic field H generated by the example of an embodiment of a magnetic field generator 3 has null zones nx,nz caused by the absence of the horizontal x-component Hx and the vertical z-component Hz respectively. As shown in fig. 9D, the magnetic field generator 3 shown in fig. 4 may for instance generate a magnetic field in the monitored area 3 having a null zone nz caused by the vertical component Hz being locally absent. In this example, the null zone nz varies in height as a function of the distance from the entrance side 24. In this example, the position of the null zone nx,nz varies along a continuous path between respective sides of the monitored area, which side define the monitored area parallel to the direction of passage P and face each other. As shown in fig. 9B, the null zone nx caused by the absence of the horizontal component Hx of the magnetic field varies along a continuous path between the top side 20 and the bottom side 21 of the monitored area 2.

The null zone nx,nz may vary in position as a function of the distance in any suitable manner. The null zones nx,nz may, e.g., be non-linear and be curved in any manner suitable for the specific implementation. For instance the null zone may be curved around an axis which is oriented at a non-parallel angle with respect to the direction of passage P. A curved null zone further reduces the chance that a transponder passes undetected since it is extremely unlikely that a transponder moving through the monitored area follows the curved path. To increase the likelihood of detection, the null zone may be curved around two or more axis or have another irregular shape. In the example of fig. 9A, for instance, the null zones nx,nz are curved around two, not shown, imaginary axis, at respective sides of the null zones nx,nz, as shown in fig. 9B-D.

A magnetic field with a null zone nx,nz which varies in distance from a side 20- 25 of the monitored area 2, may be obtained in any manner suitable for the specific implementation. The magnetic field generator 3 may for instance include one, two or more magnetic sources acting as two or more separate magnetic dipoles.

In this respect, it is noted that, without whishing to be bound to any theory, an arrangement of at least two magnetic dipoles with similar orientation of their poles is believed to act as a single magnetic dipole in case the dimensions of the arrangement , e.g. the distance between the magnetic dipoles, is much smaller than the distance between the magnetic dipoles and the observer of the magnetic field, i.e. the transponder. However, each of the separate magnetic dipoles in the arrangement, although acting as a single magnetic dipole, forms a magnetic source in case it operates independent from the other magnetic dipoles in the arrangement. For instance, two separate current loops form two separate magnetic source, but may form a single magnetic dipole when perceived at a sufficiently large distance. A substantially flat current loop is believed to act a single magnetic dipole in case the distance between the transponder and the current loop is much larger than the dimensions of the current loop. In such case, the magnetic field generated by the current loop will be sensed by the transponder as similar to a field generated by a single magnetic dipole. On the other hand, if the distance between the transponder and the current loop is in the order of the dimensions of the current loop, or smaller, the magnetic field generated by the current loop will be sensed by the transponder as being different from a field generated by a single magnetic dipole. In such case, each part of the current loop is believed to act as a separate magnetic dipole. However, the behaviour of the magnetic dipoles in de current loop is coupled because the current flowing through the current loop determines the strength and direction of the magnetic field generated by each part of the current loop (i.e. generated by each magnetic dipole).

The magnetic dipoles may be positioned in a mirror-symmetric arrangement of which an axis of symmetry is oriented at angle with respect to the direction of passages, i.e. non-parallel and not perpendicular to the direction of passage. In this respect, without wishing to be bound to any theory, it is believed that a mirror- symmetric arrangement of magnetic dipoles with its planes of symmetry oriented at a non-parallel and non- perpendicular angle with respect to the direction of passage, generates a magnetic field of which the null zones are curved around two axis. The distance between the magnetic dipoles in the arrangement may in such case be larger than de distance between the transponder and the arrangement.

Figs. 10 and 11 show examples of suitable shapes and orientations of the current loops 340,341 operated as magnetic sources in a mirror-symmetric arrangement. In figs. 10 and 11, the axis of symmetry are indicated by the dashed lines SA. The direction of passage P is supposed to be oriented in parallel with the horizontal x-axis of the coordinate system x,y,z schematically indicated in this figure. The distance between the magnetic sources and the transponder 6 is measured in the direction of the horizontal y-axis. The current loops 340,341 shown in figs. 10 and 11 have a non-circular and elongated shape, which in this example is rectangular but may be ellipsoid or shaped otherwise instead.

The example of fig. 10 includes a single, elongated current loop 340 which is oriented with its longitudinal axis at a non-parallel and non-perpendicular angle with respect to the direction of passage P. In the example of fig. 10, the current loop 340 has two perpendicular axis of symmetry SA, of which one is oriented parallel to the longitudinal axis of the current loop. It is found that in the area where the distance between the current loop and the transponder 6 is at least an order smaller than the length of the current loop, a suitable null zone present (and without whishing to be bound to any theory, the current loop 340 acts as a plurality of coupled magnetic dipoles). The ratio of this distance and the length of the current loop 340 may for instance be smaller than 1:5, such as 1:10 or less.

The example of fig. 11 includes two elongated current loops 340,341 which are oriented with their longitudinal axis at a non-paraEel and non-perpendicular angle with respect to each other. In this example, the current loops 340,341 are placed in a cross -shaped arrangement. A horizontal current loop has 340 its longitudinal axis in parallel with the direction of passage P. In the example of fig. 11, it is found that a suitable null zone is obtained in the area where the distance between the current loops 340,341 and the transponder 6 is in the order of the length of the current loops 340,341 or smaller. The ratio of this distance and the length of the current loops 340,341 may for instance be smaller than 1:1, such as 1:5 or less, and simulations show that a null zone with a larger amount of variation may be obtained if this ratio is about 1:10 or less. Instead of a mirror- symmetric arrangement, the magnetic field generator may for example include two or more identical or differing magnetic dipoles positioned in an arrangement without mirror-symmetry. Optionally, such a mirror-asymmetric arrangement may be rotationally symmetric, such as over 180 degrees. It is found that a mirror- asymmetric arrangement generates a magnetic field of which the null zone is curved even at a relatively large distance from the arrangement.

In the example of fig. 4, it is found that if the distance between the current loop and the transponder 6 is about an order of magnitude larger than the length of the current loop or smaller, still a suitable null zone is obtained. Without wishing to be bound to any theory, it is believed that the lack of mirror symmetry causes this advantageous ratio. The ratio of this distance and the length of the current loops may for instance be smaller than 10:1, such as 2:1 or less, and simulations show that a null zone with a large amount of variation may be obtained if this ratio is about 1:1 or less. To obtain an arrangement of magnetic dipoles which is without mirror symmetry, the magnetic field generator 3 may for instance include one or more current loops in an arrangement of mirror-symmetric geometry, to which the current is provided in such a manner such that the current flow pattern lacks mirror symmetry. To disturb the mirror symmetry, e.g., the geometry of the flow path, the amount of current, the phase of the current or the direction of the current may differ at respective side of an imaginary mirror plane.

For instance, the magnetic field generator 3 in the example of fig 4 includes a - first set current loops. The first set includes elongated loops 330,331 oriented with their longitudinal axis in a first direction. The first set further includes a square current loop 332. The magnetic field generator further has a second set of elongated loops 333,334 with a second longitudinal axis which is orient perpendicular with respect to the first longitudinal axis. In this example, the elongated current loops of both sets cross each other. The geometry of the arrangement of the current loops 330- 334 is mirror symmetric. The arrangement has a vertical plane of symmetry and a horizontal plane of symmetry, as indicated in fig. 9A with dashed lines SA.

During the simulation, as shown in fig. 9A, the current flow through the elongated loops 330,331;333,334 was mirror symmetric. However, due to the square loop 332, the overall current flow lacked mirror symmetry. The current through the square loop 332 perturbs the mirror symmetric current flow pattern through the elongated loops, but retains the rotational symmetry of the current loops. The overall current flow was rotationally symmetric over 180 degrees around a symmetry axis coinciding with the axial centre of the current loops.

In the simulations of fig. 9, a current was provided to the first set of current loops 330-332 flowing in a clockwise direction. To the second set of current loops

333,334 a current was provided flowing in a direction opposite to the current flow in the first set, e.g. in an anti-clockwise direction. The amount of current through the square current loop 332 was made different from the amount of current running through the other loops in the first set, e.g. 0.15 amperes instead 1 ampere, while flowing in the same direction.

The magnetic field generator 3 may be implemented in any suitable manner. The magnetic field generator 3 may for instance, like the examples of magnetic field generators 3 of figs. 4 resp.7 and 8 include at least two similar construction elements 31. In the examples, each of the construction elements 31 contains at least one current path. The construction elements are mechanically connected such that the current paths of at least two elements form at least one of the magnetic sources. The example of figs. 7 and 8 includes a number of similar construction elements 32, with current paths 320. The construction elements 32 are mechanically connected to each other. The current path of the construction elements are electrically separate, and the current path of each construction element forms a separate current loop. However, by controlling the current flowing through the current loops in a suitable manner a combination of the individual magnetic fields of the current paths resembles a magnetic field of a single magnetic source, as has been explained above.

Also, the construction elements may for example be mechanically connected such that the current paths of at least two elements form a current loop. In the example of fig. 4, the current path of the construction elements 31 are connected such that two or more closed current loops are obtained. Each of the closed current loops includes current paths of two or more construction elements .

Figs. 5 and 6 show the construction element 31 of the example of fig. 4 in more detail. As shown, the construction element 31 includes a carrier provided with parallel sets 316-317 of current paths. Near opposite surfaces the carrier 31, transmitter sets 316 are provided. In mounted position, the transmitter sets 316 form a part of the magnetic field generator 3, whereas the received set forms a receiving coil for detecting the signal transmitted by the transonder. Between the transmitter set a receiver set 317 is provided. As shown in fig. 6, a transmitter set can include current paths 310-313 in a diamond shape arrangement. One of the current paths 311 crosses at respective ends other current path.

In this example, the current paths 310-313 are electrically not connected, but have electrical contacts 314,315 projecting out of the carrier 31. The electrical contacts 315 of the crossing current paths are interchanged with respect to the electrical contacts 314. the current paths that do not cross each other. As shown in fig. 4, when a plurality of construction elements 31 is provided in a matrix-shaped arrangement, a arrangement of current loops 330-334 is obtained. In this example, the contacts 314- 315 at the sides of the matrix-shaped arrangement are connected to each other by means of connectors 319, to close the current loops. The contacts 314,315 at the bottom of the arrangement are connected to a current source by means of connectors 318.

It should be noted that the above-described embodiments illustrate rather than limit the invention, and various alternatives are possible without departing from the scope of the appended claims.. For example, the monitored area may have any suitable size and shape. The monitored area may for example be open or confined at one or more sides. Also, the magnetic field generator may be implemented as in any manner suitable for the specific implementation. Furthermore, in the example of fig. 3, the magnetic field generator and the detector are positioned at opposite sides of the monitored area in an upright position. The magnetic field generator and the detector are positioned in parallel with the direction of passage. The magnetic field generator and the detector thus confine the monitored area at upright sides , thereof. However, the magnetic field generator and/or detector may also be positioned in a horizontal arrangement. For instance, the magnetic field generator may be positioned lying on the ground, while the detector is oriented horizontally at a distance from the ground, facing the magnetic field generator . The detector and/or the magnetic field generator may also be placed in other orientations and/or positions. Also, although in the examples rectangular current loops are shown the current loops may have any shape and size suitable for the specific implementation.

However, other alternatives and modifications are also possible. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word 'a' is used as equivalent to the term 'at least one'. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures maynot be used to advantage.

Claims

1. A system for detecting objects in an area which is monitored by the system, including: a magnetic field generator constructed to generate a magnetic field with at least one null zone having a variable position in the monitored area, in which null zone at least one component of the magnetic field is substantially absent; said system further including: a detector for detecting a signal transmitted by the object in response to an absorption of energy from the magnetic field by the object.

2. A system according to claim 1, wherein the magnetic field generator is constructed to generate a magnetic field with at least one null zone which has a position which varies in time.

3. A system according to claim 2, wherein the magnetic field generator includes at least two magnetic sources, which are separately controllable, and further including a control device for controlling the magnetic field generated by individual magnetic sources.

4. A system according to any one of claims 3, wherein at least one of the magnetic sources includes at least one current loop and the system further includes a current source for controlling in an operational state of the system through which at least one current loop a current flows and/or the amount of current flowing and/or the direction in which the current flows and/or the phase of the current

5. A system according to claim. 4, wherein at least one magnetic source includes at least two current loops.

6. A system according to any one of the preceding claims, wherein the magnetic field generator is constructed to generate a magnetic field with at least one null zone which has a varying distance from a bounding side of the monitored area, which side forms a boundary of the monitored area in a direction transverse to a direction of passage through the monitored area.

7. A system according to claim 6, wherein the at least one null zone is oriented at a non-perpendicular and non-parallel angle with respect to the direction of passage.

8. A system according to claim 7, wherein the null zone is curved around at least one axis, which axis is oriented at a non-parallel angle with respect to the direction of passage.

9. A system according to any one of claims 1-8, wherein the magnetic field generator includes an arrangement of at least two magnetic dipoles, which arrangement is mirror-symmetric in at least one plane of symmetry, and of which arrangement the at least one plane of symmetry is oriented at a non-parallel and non- perpendicular angle with respect to a direction of passage through the monitored area

10. A system according to any one of the preceding claims, wherein the magnetic field generator includes an arrangement of at least two magnetic dipoles, which arrangement is mirror-asymmetric, and optionally which arrangement is rotationally symmetric around a symmetry axis, which symmetry axis is oriented at a non-parallel angle with respect to the direction of passage.

11. A system according to any one of the preceding claims, wherein the magnetic field generator includes an arrangement of at least two magnetic dipoles, which arrangement has an elongated shape with a longitudinal direction, which longitudinal direction is oriented at a non-parallel and non-perpendicular angle with respect to a direction of passage through the monitored area.

12. A system according to any one of the preceding claims, wherein the magnetic field generator includes at least one first current loop and at least one second current loop which differs with respect to the first current loop in at least one aspect, such as shape or orientation.

13. A system according to any one of claims 12-12, wherein the first and second current loops extend in substantially parallel planes and/or are positioned coaxially and/or concentrically with respect to each other.

14. A system according to any one of claims 12-14, wherein the at least one first current loop has a first longitudinal axis, and the at least one second current loop has a second longitudinal axis which is oriented non-parallel with respect to the first longitudinal axis.

15. A system according to any one of claims 12-13, wherein at least one of the first current loops crosses at least one of the second current loops, and optionally the magnetic field generator includes at least two first current loops which enclose at least partially the same area and, optionally, which cross each other.

16. A system according to any one of claims 12-15, wherein at least two of the first and/or second current loops are connectable to a current source which, in use, provides a first current to at least one of said current loops and a second current to at least one other of said current loops, which second current differs from the first current in at least one aspect, such as direction of flow, strength or phase.

17. A system according to any one of the preceding claims, wherein the magnetic field generator includes at least two similar construction elements, each of said construction elements containing at least one current path, the construction elements being mechanically connected such that the current paths of at least two elements are form at least one of the magnetic sources

18. A system according to claim 17, wherein the construction elements are mechanically connected such that the current paths of at least two elements form a current loop.

19. A method for detecting an object in a monitored area, including generating a magnetic field with at least one null zone which has a variable position in the monitored area, in which null zone at least one component of the magnetic field is substantially absent; and detecting a signal transmitted by the object in response to an absorption of energy from the magnetic field by the object.

20. A magnetic field generator constructed to generate a magnetic field with at least one variable null zone.

21. A magnetic field generator for use in a system according to any one of claims 1- 18.

22. A monitored area, including a system according to any one of claims 1-18.

23. Use of a system according to any one of claims 1-18 in a method according to claim 19.