Method and system for interrogating RFID tags

Abstract

A transition system for interrogating UHF RFID tags and monitoring a passage between two separated spaces, and method for operating such a transition system, comprises generating an activation field (AF) inside a first field zone (FZ1) configured to activate said tags, and generating an interrogation field (IF) inside a second field zone (FZ2) configured to interrogate within said second field zone the activated tags. At least a part of the activation field and interrogation field overlap with each other in a third field zone (FZ3). The third field zone separates the activation and interrogation field from each other by including an exclusion field (EF), by simultaneously performing the steps of generating the activation and interrogation field, wherein the activation and interrogation field are spatially directed such that the third field zone extends across the passage.

Description

[0001] Title: METHOD AND SYSTEM FOR INTERROGATING RFID TAGS

[0002] The invention relates to a method and system for interrogating one or more UHF RFID tags.

[0003] UHF RFID is a powerful technology for uniquely identifying multiple items in a short period of time without having a direct line-of-sight, which makes it suitable for use in anti-theft detection systems or inventory tracking systems.

[0004] Antenna design for UHF RFID can be very challenging, since generally low cost and small form-factor are key requirements. Therefore, it is not uncommon for antennas to have relatively strong side-lobes and back- lobes, which may reduce the reliability of the system. Furthermore, a well- defined and narrow antenna beam is practically impossible at low cost price and small form-factor.

[0005] In commercial environments, such as retail environments, it may be desired that interrogation of tags is restricted to a well-defined region, for example to distinguish tags of objects that are being transported through a gate from one zone to another zone from tags of objects that are in the neighborhood of the gate. In a retail environment, for example, an anti-theft detection system may be installed to detect items that leave the store without having been paid for. It is desired that such an anti-theft detection system is not easily triggered to produce a false alarm, e.g. when an item is brought close to an exit without actually leaving the store.

[0006] SUMMARY

[0007] It is an object of the present invention to ensure reliable and accurate operation of a transition system, such as an anti-theft detection system or inventory tracking system, in which UHF RFID tags are movable between areas. In summary, the invention provides an improved method of operating a transition system for interrogating one or more UHF RFID tags, the transition system monitoring a passage between two separated spaces and comprising a first antenna and a second antenna. The method enables a rapid and accurate determining of a transition, i.e. movement, of an UHF RFID tag, e.g. from first space to the second space, and / or vice versa.

[0008] The method according to the invention comprises generating, using the first antenna, an activation field inside a first field zone (e.g. an activation zone) configured to activate said one or more UHF RFID tags within said first field zone (e.g. to an activated state that is distinct from a neutral state), and generating, using the second antenna, an interrogation field inside a second field zone (e.g. an interrogation zone) configured to interrogate within said second field zone the one or more UHF RFID tags that were activated by the activation command. At least a part of the activation field and at least a part of the interrogation field overlap with each other to thereby form a third field zone (e.g. an exclusion zone) between the first field zone and the second field zone.

[0009] The third field zone separates the activation field and the interrogation field from each other by including an exclusion field inside the third field zone, by simultaneously performing the steps of generating the activation field and the interrogation field. The steps of generating the activation field and the interrogation field are performed by spatially directing the fields such that the third field zone extends across the passage.

[0010] Accordingly, in the third field zone where the activation field and the interrogation field are both simultaneously present, the UHF RFID tags cannot interpret instructions in the activation field and the interrogation field because they interfere with each other, e.g. the interrogation field signal is received by the one or more UHF RFID tags while they are also receiving the activation field signal. In other words, the third field zone can be regarded as an exclusion zone in which the activation field and the interrogation field interfere with each other, thereby locally blocking responsiveness of the one or more UHF RFID tags.

[0011] Thus, by generating two different fields, three distinct zones are provided (e.g. an activation zone, an interrogation zone, and an exclusion zone), to perform different functionalities. In the activation zone, the tags are activated by an activation signal. In the interrogation zone, activated tags are interrogated, e.g. detected, by an interrogation signal. The activation and interrogation signals may be modulated or non-modulated. And in the exclusion zone, the overlapping region of the activation and interrogation zone, the tags are unable to understand the instructions in the activation and interrogation fields due to the interference caused by the activation and interrogation field signals being simultaneously present there. In this way any adverse effects caused by side or back lobes of the antennae, or reflections of the activation or interrogation field, which would normally trigger an undesired and / or unpredictable response from the UHF RFID tags, can be minimized by the exclusion zone extending between the first and second field zone.

[0012] For example, depending on a frequency difference between the activation field and the interrogation field, the interference at the one or more tags can cause a disruption in the wireless powering circuit that powers the tags, disturb the signal modulation, and / or create an decreased signal to noise ratio wherein the interfering field is interpreted as background noise.

[0013] In general, the present disclosure proposes to apply, shape and tune, in addition to the first and second field zones, a third field zone which separates the activation field and the interrogation field from each other, e.g. providing an exclusion or buffer zone. In this way, it can be prevented that tags are interrogated while located in the first field zone where the tags are to be activated, or (re)activated in the second field zone where the tags are to be interrogated. Also, in practice, irregularities in antenna characteristics and reflections on objects or persons may cause the interrogation field to be received by tags that are located in the activation zone, leading to incorrect observations. As a result, e.g. when applied in an anti-theft detection system, the number of false alarms can be reduced.

[0014] The passage may for example comprise a detection gate comprising the first and second antenna. Such a detection gate can e.g. be provided at an entrance / exit of a retail store, or between an inventory area and store front area. In this way, it can be ensured that the separation between the activation field and interrogation field is applied at the passage, where the transition of UHF RFID tags between the activation zone and the interrogation zone occurs.

[0015] Having an exclusion zone as described herein provides the benefit of ensuring reliable and accurate operation of a transition system (e.g. antitheft detection system or inventory tracking system) in which UHF RFID tags are movable between an activation zone and an interrogation zone. The exclusion zone provides a clear separation, or distinction, between the activation and interrogation zones, without substantially increasing the distance between the activation and interrogation zones. For example, in retail environments, the total floor area for the display of products to be sold may be limited, thus imposing constraints on the floor area available for implementing a transition system.

[0016] In one embodiment, the activation field comprises an activation command configured to activate the one or more UHF RFID tags, the interrogation field comprises an interrogation command configured to interrogate the one or more activated UHF RFID tags, and the interrogation command and the activation command collide with each other in the third field zone.

[0017] The step of activating the one or more UHF RFID-tags within the first field zone, e.g. into an activated state, may involve broadcasting an activation command with the activation field. An activation zone may be designated wherein the activation field should have a sufficient power to configure each tag present therein, despite variations in the tag sensitivities and local effects like reflections and variations in tag orientation. A single activation command may suffice for an arbitrarily large set of tags, the act of activating UHF RFID-tags of items that are being transported is not hampered by the presence of other RFID-tags within the first field zone.

[0018] The step of interrogating UHF RFID-tags in the second field zone may involve broadcasting an interrogation command with the interrogation field. This may require a more complicated protocol, comprising a two-way communication between the activated UHF RFID tags and an interrogation transceiver arranged for broadcasting an interrogation command with the interrogation field. However, in this stage the number of UHF RFID-tags to be interrogated is limited to those which are present within the interrogation field after they were configured by the activation field.

[0019] Preferably, e.g. upon installing the system, the method further comprises the step of shaping the exclusion field by positioning the first and second antenna with respect to each other, and / or by adjusting a power difference and / or a frequency difference between the activation and interrogation field. In this way, the size and shape of the exclusion field can e.g. be adapted to adequately cover an area between the first and second field zone, e.g. an entrance or exit of a shop or storage facility, and to the type and properties of UHF RFID tags to be used.

[0020] The activation and interrogation commands may comprise a sequence of one or more symbols, e.g. representing a “0” and representing a “1”. Accordingly, each of the activation and interrogation command can have a command length ranging between 1 and 22 bits, or more, e.g. corresponding with a time duration between 6 and 750 microseconds, or more. To ensure separation between the activation and interrogation field, each of the activation and interrogation commands is preferably transmitted such that it is received by the tag while the tag is already receiving another command, such that neither of the commands can be interpreted by the tag. Two exactly synchronized readers transmitting the same command will not lead to confusion. The goal is that the activation and interrogation fields are interfering with each other, e.g. by having different frequencies, and not synchronized timing. Preferably, a signal of the activation field is transmitted with a first timing, a signal of the interrogation field is transmitted with a second timing, and a time difference between the first and second timing is more than symbol length but less than command length of the signals. Accordingly, it can be ensured that the activation command and the interrogation command collide, or interfere, with each other in the third field zone.

[0021] In practice the activation and interrogation readers may complete their own command routine. Although a routine generally starts with a query command, the rest of the sequence may depend on how many tags receive the query command and join the routine. An empty slot may result in a “queryrep” command, indicating the next tag's turn, while in an occupied slot, a return message (e.g. RN16) is received from the tag which is to be returned by the reader. The more the routines of the activation and interrogation readers collide, e.g. are mixed up, the less the tags are able to interpret the commands in the exclusion field.

[0022] For example, the activation reader may continuously broadcast activation commands, while the interrogation reader continuously attempts to interrogate tags. The corresponding activation and interrogation routines may be completely different, e.g. having different commands of different lengths. This makes it impossible for a tag to understand what is being asked, so that the tag is unable to respond.

[0023] Measurements have indicated that, in general, UHF RFID tags become non-responsive when a difference between signal power received from the activation field and the interrogation field is less than 6 decibel (dB). Conversely, when the power difference is larger than 6 dB, the tag(s) may still be able to process the higher power signal instructions while disregarding the lower power signal instructions as noise. The tag is preferably irresponsive whenever the power difference experienced by the tag is <6dB. How much power the tag obtains from either reader may be determined by the power and polarization of the field. In a range between 6 and 12 dB, some tags may either be responsive or non-responsive depending on other conditions, e.g. depending on specific implementations of the tags’ internal circuitry. Accordingly, to improve separation, the activation field may be generated to have a first signal power, while the interrogation field may be generated to comprise a second signal power. Preferably, in the third field zone, i.e. the overlapping area between the first and second zones, the power difference experienced by the tag is such that the tag is irresponsive. In general, the power difference experienced by the tag varies throughout the third field zone and may be different from location to location and depending on tag orientation. Preferably, the experienced power difference between the first and second signal power is smaller than 12 dB, preferably smaller than 6 dB, e.g. about 5 dB, 4 dB, 3 dB, or even less. In general, in the exclusion zone the relative field strengths experienced by the tag are smaller than a threshold value for a given polarization of the activation and interrogation field.

[0024] The exclusion zone can also be shaped based on a frequency difference between the activation field and the interrogation field. For example, it has been found from experiments and simulations that reliable separation can be obtained by having a frequency difference of more than 500 kHz, e.g. more than 600 kHz, 750 kHz, or 900 kHz, preferably more than 1 MHz, e.g. 1.5, 2, 2.5, 3 MHz, or even higher. For example, the frequency difference may be up to 25 MHz for the US band. With smaller frequency differences, tags could already enter a non-responsive state with relatively large differences in field intensity. Most RFID tag antennas have a characteristic that is comparable to an electric dipole. As a result, the tags have a highest sensitivity to electric fields with a polarization that is parallel to the dipole axis. This implies that the entering of the non-responsive state is not only determined by the power or frequency of the fields, but also to the relative polarization between the activation and interrogation field. In other words, there is a dependency between the orientation and polarization of the activation field and interrogation field with respect to each other. For this reason, in preferred embodiments, the activation field and the interrogation field have the same polarization, e.g. circular polarization or substantially parallel to each other.

[0025] Some tags are passive, i.e. powered by the activation or interrogation field. Activation makes use of a persistent state of the tag. This state remains active at least for as long as the tag is powered. In the exclusion zone the tag still receives power from both fields, however it is unable to demodulate the commands transmitted by the readers.

[0026] In some jurisdictions, such as the European Union, regulation may require that antenna signals are regularly interrupted, e.g. for a period of 100 milliseconds at 4 second intervals, or that antenna signals are regularly switched to different transmitter frequencies or channels. To avoid that while in the exclusion zone, tags are suddenly able to respond to the activation or interrogation field, e.g. due to the activation field or interrogation field being temporarily dropped, it can be enforced that the readers start and stop at the same time. For this purpose, the method may comprise the step of interrupting generation of the activation field and the interrogation field at mutually congruent intervals. For example, the activation field and the interrogation field may both be interrupted for the same period at the same interval, while both the period as well as the interval of interruption are synchronized in time, such that the activation field and interrogation field, in particular the activation and interrogation commands, are either simultaneously present or simultaneously absent in the third field zone. Similarly, the activation and interrogation field may be simultaneously switched to different frequencies or channels.

[0027] Other aspects of the present invention pertain to a transition system for interrogating one or more UHF RFID tags. The transition system is arranged for monitoring a passage between two separated spaces and comprises a first antenna arranged for generating an activation field inside a first field zone, a second antenna arranged for generating an interrogation field inside a second field zone, and a controller arranged for controlling the first and second antennae. The activation field is configured to activate UHF RFID tags that are present in the activation field, while the interrogation field is configured to interrogate UHF RFID tags activated by the activation field and present in the interrogation field.

[0028] The first and second antennae are positioned with respect to each other such that the first field zone and the second field zone partially overlap to thereby form a third field zone.

[0029] The third field zone separates the activation field and the interrogation field from each other by including an exclusion field inside the third field zone, by the controller establishing interference between the activation field and the interrogation field in the third field zone by controlling the first and second antennae to simultaneously generate the activation field and the interrogation field, wherein the activation field and the interrogation field are spatially directed such that the third field zone extends across the passage such as to close the passage by the third field zone.

[0030] The system can e.g. be implemented in a parcel, such as a store or retailer where the one or more UHF RFID tags are coupled to respective items to be sold or tracked for inventory purposes. Alternatively, the system can be implemented in any other type of facility, e.g. in a distribution facility for tracking parcels to be delivered, or in a manufacturing or processing facility for tracking objects to be manufactured, assembled, inspected or packaged.

[0031] Preferably, the activation field comprises an activation command configured to activate the one or more UHF RFID tags, the interrogation field comprises an interrogation command configured to interrogate the one or more activated UHF RFID tags, and the interrogation command and the activation command collide with each other in the third field zone. The activation command and the interrogation command thus interfere with each other in the third field zone.

[0032] According to the invention, an activation field may be generated in both the first field zone as well as in the second field zone. Similarly, an interrogation field can be generated in both fields. This allows the system to be implemented for two-way transition reading. For example, in the first field zone activation signal A is provided to activate tags into an “activation A” state, and tags are interrogated that are in an “activation B” state, while in the second field zone activation signal B is provided to activate tags into the “activation B” state and tags in the “activation A” state are interrogated.

[0033] In some embodiments, the passage comprises a detection gate, wherein the first and second antennae are mounted to the detection gate. Alternatively, the first and second antennae can be mounted to any other structure, such as a ceiling, wall or floor. The detection gate may comprise one or more elements for mounting the first and second antennae, e.g. to the left and / or right side of the passage, in the middle of the passage, the floor and / or the ceiling.

[0034] For example, the first antenna can be mounted to a first side of the detection gate (or other structure) facing the first field zone, and the second antenna can be mounted to a second side of the detection gate (or other structure) facing the second field zone. In this way, the third field zone extends across the passage between the first and second field zone, to minimize the effects of side or back lobes of the antennae as well as field reflections that enter the passage. The first and second antennae may be part of a single antenna device. For example, all three fields may be generated by a single antenna device that is capable of transmitting multiple beams, e.g. one beam toward room A, one beam toward room B and the overlap of both beams will be the exclusion zone.

[0035] An optimal size and shape of the three zones, i.e. the first (activation) field zone, the second (detection) field zone, and the third (exclusion) field zone, can be obtained by having the first and second antennae face away from each other, preferably at an angle with respect to each other between 45-180 degrees, more preferably between 90-120 degrees, e.g. about 100 degrees. In other words, the angle between the antenna beams or fields generated by the first and second antennae is between 45-180 degrees, more preferably between 90-120 degrees, e.g. about 100 degrees. Alternatively, it is also possible to use a different type of antenna or antenna arrangement that enables to generate field in different spatial directions. For example, in some implementations, the first and second antenna are provided by an antenna array comprising a plurality of antennas. In particular, it is possible to apply a multibeam antenna array that is configured for generating field in beams that are spatially oriented differently. The different spatial orientation is thereby achieved by suitably operating some of the antennas in the arrays, for example a subset of antennas wherein different spatial orientation is achieved by suitably controlling the power to each of the active antennas.

[0036] In some embodiments, each of the first and second antennae comprises a patch antenna. The relatively flat shape of such an antenna makes it easy to integrate in structures such as a detection gate, ceiling or floor, with a relatively small and unobtrusive installation volume.

[0037] To shape of the activation field or interrogation field generated by the first or second antenna, the patch antenna can be mounted on a ground plate provided with an upstanding edge, e.g. that at least partially circumferentially extends around the patch antenna, for shielding the generated activation or interrogation field.

[0038] To provide a compact solution, the first and second antennae can be provided by a single antenna unit device arranged for generating the activation field, the interrogation field, and the exclusion field. For example, the first and second antennae may be arranged at an angle with respect to each other, e.g. between 45-180 degrees or between 90-120 degrees, such that a part of the activation field overlaps with a part of the interrogation field in a third field zone, in which the exclusion field is provided by simultaneously generating the activation and interrogation field.

[0039] BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The invention will be further elucidated in the figures:

[0041] FIG. 1 illustrates an embodiment of a method and system for interrogating one or more UHF RFID tags;

[0042] FIG. 2 illustrates another or further embodiment of the method and system described herein;

[0043] FIGs. 3A and B provide graphs of respective situations without and with an exclusion field as described herein;

[0044] FIG. 4 illustrates an exemplary exclusion field as described herein;

[0045] FIGs. 5A and B provide other exemplary situations without and with an exclusion field as described herein;

[0046] FIG 6 illustrates yet another example of an exclusion field as described herein;

[0047] FIG. 7 provide a graph illustrating the responsiveness of a tag inside an activation field, an interrogation field, and an exclusion field as described herein.

[0048] FIGs. 8A-C illustrate exemplary embodiments of the system.

[0049] DETAILED DESCRIPTION The invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. In the drawings, the absolute and relative sizes of systems, components, layers, and regions may be exaggerated for clarity. Embodiments may be described with reference to schematic and / or cross-section illustrations of possibly idealized embodiments and intermediate structures of the invention. In the description and drawings, like numbers refer to like elements throughout. Relative terms as well as derivatives thereof should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the system be constructed or operated in a particular orientation unless stated otherwise.

[0050] FIG. 1 schematically illustrates a portion of a building, parcel or venue that comprises a first area Al and a second area A2. The first area Al may be an interior of a store, or the like. The second area A2 may be an exterior of the store. This, however, is just an example of a traditional setting implementing the explained concept. The separated spaces are in this case formed by the first area Al and the second area A2, but may as well be provided by different areas that are not physically separated, i.e. not by a wall. A virtual or effective separation between two spaces may likewise be applied. Examples thereof are a gate or railing of any kind, or even furniture such as a rack with shelves or other store furniture. In yet a different setting, the separation may be virtually formed by a light effect, a line on the floor (e.g. monitored by a camera or other security system) or other sort of barrier. Also a plurality of detection gates may define a large passage between different spaces in a shopping mall, warehouse or store-in - a-store concept.

[0051] Back to figure 1, a passage 30 allows access between the two areas Al and A2. The passage 30 may be a door, a gate or other separation (with reference to the paragraph above). The passage 30 may e.g. be provided in a wall 31 of the building. The passage may comprise a detection gate 200, for detecting tags 10 moving from the first area Al to the second area A2, and / or vice versa. In other words, the one or more UHF RFID tags are movable between the first field zone (FZ1) and the second field zone (FZ2) via the passage 30.

[0052] For example, the building is provided with a system 100 for interrogating UHF RFID tags that are moved from the first area Al to the second area A2, and / or vice versa. The system 100 comprises a first reader or antenna 110, e.g. a patch antenna, arranged for generating an activation field AF inside a first field zone FZ1 that is at least partly within the first area Al. The activation field comprises an activation command configured to activate the one or more UHF RFID tags. For example, the first antenna is mounted to a first side of the passage 30 facing the first field zone FZ1, so that the activation field AF extends away from the passage 30 and into the first field zone FZ1. For example, the first antenna 110 is mounted to the ceiling, the floor, the wall 31 and / or to a frame. The first antenna 110 may be part of one or more detection units 210 of the detection gate 200, e.g. located on one or more sides of the passage, in or on the ceiling, or in or on the floor. The first field zone FZ1 e.g. extends into a retail space accommodating products to be sold, or into an inventory space where products are stored. Each product can be provided with an RFID tag to enable unique identification of the product. The activation field AF is at least strong enough to activate tags 10 that are in the immediate neighborhood of the passage 30 regardless of local effects such as reflections, the orientation and the sensitivity of the tag. In this way, any tags that are moved from the first area Al to the second area A2 pass through the activation field AF. The one or more UHF RFID-tags 10, that have received an activation message from the activation field AF assume an activated state deviating from a default state. The system 100 further comprises a second reader or antenna 120 arranged for generating an interrogation field IF inside a second field zone FZ2 that is at least partly within the second area A2. For example, the second antenna is mounted to a second side of the passage 30 facing the second field zone FZ2, so that the interrogation field IF extends away from the passage 30 and into the second field zone FZ2, on the opposite side of the passage 30. For example, the second antenna 120 is mounted to the ceiling, the floor, the wall 31 and / or to a frame. The second antenna 120 may e.g. be part of one or more detection units 210 of the detection gate 200, e.g. located at or near the passage. The second antenna 120 comprises an interrogation command that is configured to interrogate, e.g. detect, within said second field zone FZ2 the one or more UHF RFID tags 10 that were activated by the activation field AF and that have subsequently moved to the second area A2.

[0053] The system can e.g. be applied in an anti-theft detection system, for detecting products leaving the store (without having been paid for). Alternatively, the system can be applied for inventory tracking, e.g. by detecting which products are taken from an inventory, to facilitate restocking of the inventory. Transition reading may be a possible application as well, or monitoring items that are moved from one room to another, e.g. stock room to sales floor, truck to pallet / dock, etc.

[0054] A controller 130 is arranged for controlling the first and second antennae 110, 120, to generate the activation field AF and the interrogation field IF, respectively. However, as illustrated, the first field zone FZ1 and the second field zone FZ2 may partially overlap, e.g. in a third field zone FZ3. As a result, proper activation of the one or more tags 10 in the first field zone FZ1 may be not be guaranteed due to interference between the activation field AF and interrogation field IF. Conversely, in the second field zone FZ2, detection of the tags 10 may be suboptimal. The controller 130 is arranged for separating the activation field AF and the interrogation field IF from each other by providing an exclusion field EF inside the third field zone FZ3. To provide the exclusion field EF, the controller 130 controls the first and second antennae 110, 120 to simultaneously generate the activation field AF and the interrogation field IF in the third field zone FZ3. In particular, the activation field comprises an activation command configured to activate the one or more UHF RFID tags, the interrogation field comprises an interrogation command configured to interrogate the one or more UHF RFID tags that were activated by the activation command, and the activation field and the interrogation field are generated simultaneously such that the interrogation command and the activation command collide with each other in the third field zone. As a result, the one or more UHF RFID tags cannot understand and respond to either of the commands, effectively creating an exclusion field or zone between the activation field and the interrogation field where tags are non- responsive.

[0055] Preferably, the respective polarizations of the activation field and the interrogation field are equal, e.g. parallel to each other. In this way, the chance that the UHF RFID tags in the third field zone FZ3, receive the activation field and the interrogation field simultaneously can be maximized, regardless of the orientation of the tag(s) with respect to the fields.

[0056] The exclusion field EF provides a clear separation, or distinction, between the activation and interrogation fields AF, IF. In other words, the exclusion field EF provides a buffer zone between the activation field AF and the interrogation field IF. In this way, it can be prevented that tags 10 are already detected while still located in the first field zone FZ1, or (re)activated in the second field zone FZ2 where the tags are to be detected.

[0057] As illustrated in FIGs. 1 and 2, the exclusion field EF may be provided to extend across the passage 30, to separate the activation field and the interrogation field from each other. Hence, when a tag 10-1, 10-1’ is moved from the first area Al to the second area A2, it is first activated in the activation field AF, next it is passed through the exclusion field EF where the tag becomes non-responsive, after which it enters the interrogation field IF where the tag is interrogated, e.g. detected. Conversely, tags 10-2, 10-2’ that travel along path P2 extending only through the activation field AF are not inadvertently interrogated, e.g. by signal reflections, or back and / or side lobes of the interrogation field IF.

[0058] Note that the activation field AF may be arranged for energizing the one or more UHF RFID tags, such that when outside the activation field, the one or more tags activated by the activation field remain in an activated state for at least 5 seconds, preferably for at least 10 seconds, more preferably for at least 30 seconds. Accordingly, when passing through the exclusion field EF the tags cannot understand what the readers are asking so they don’t respond. The tags remain in the activated state for as long as they are energized. As a result, the tags activated by the activation field AF can still be detected in the interrogation field IF.

[0059] Beneficially, the exclusion field EF does not substantially increase the distance between the activation and interrogation field AF, IF, since it is formed by overlapping the activation field AF and the interrogation field IF such that they are simultaneously present in the third field zone. When tags 10 are moved between the first field zone FZ1 and the second field zone FZ2 they pass through the exclusion field EF in the third field zone FZ3, where the tags simultaneously receive the activation field AF and the interrogation field IF. As a result, the tags 10 become ‘confused’ in that neither the activation field AF nor the interrogation field IF can be interpreted by the tags 10. As a result, the tags 10 become non-responsive while in the exclusion field.

[0060] FIGs. 3-6 provide various graphs illustrating the operation of the present invention. For example, FIG. 3A and B illustrate a generated activation field AF and defection field IF with and without providing an exclusion field EF therebetween, respectively. In FIG. 3A an arrangement with two opposing detection units 210 is illustrated, each comprising a first and second antennae that generate an activation field AF and interrogation field IF without an exclusion field EF. For example, the activation field AF and interrogation field are not generated simultaneously but instead are generated such that the tags are able to perceive and understand commands in both the interrogation field as well as the activation field. Since the activation field AF and the interrogation field IF may locally overlap, e.g. due to back lobes or reflections, any tags located in the activation field AF may also respond to the interrogation field IF. Vice versa, any tags located in the interrogation field IF may also respond to the activation field AF in areas where it (partially) enters the interrogation field IF. As a result, the performance of the system may be limited.

[0061] FIG. 3B illustrates a situation in which the activation field AF and the interrogation field IF are generated simultaneously. Hence, any tags located in areas where the activation and interrogation field (partially) overlap receive both signals simultaneously, which ‘confuses’ the tags so that they are unable to respond to either of the signals. In particular, the overlap area where the activation field AF and interrogation field IF are simultaneously present constitutes an exclusion field EF, illustrated in FIG. 4. The exclusion field EF separates the activation field AF and the interrogation field IF from each other, e.g. by forming a buffer zone there between. When comparing FIG. 3A and 3B with each other, it is clear that in FIG. 3B the size of both the activation field AF as well as the interrogation field IF is reduced with respect to the situation depicted in FIG. 3A, namely by the exclusion field EF as depicted in FIG. 4 (re)defining the boundaries of the activation field AF and the interrogation field IF.

[0062] FIGs 5A and 5B illustrate an embodiment of a system according to the invention comprising a single detection unit 210, comprising a first and second antennae for generating an activation field AF and an interrogation field IF. In FIG. 5A the activation and interrogation field are not generated simultaneously, and thus no exclusion field is provided therebetween. In FIG 5B the activation and interrogation fields are separated by an exclusion field EF as illustrated in FIG 6, due to the activation and interrogation field being simultaneously generated, making tags present in the exclusion field EF unable to understand any command signals that reach them.

[0063] FIG. 7 provides a graph illustrating the responsiveness of a tag as a function of the power difference between an activation field, e.g. generated by an ‘activation’ reader / antenna, and the interrogation field, e.g. generated by an ‘interrogation’ reader / antenna, as perceived by the tag. FIG. 5 shows that a power difference smaller than 6 dB cause the tag to enter a non- responsive state, thereby constituting an exclusion field EF, while power differences larger than 6 dB allow the tag to respond to either the interrogation field IF or the activation field AF, e.g. depending on the signal that is received strongest. However, depending on the type of tag, other power differences may be sufficient to render the tag unable to interpret either of the field signals. For example, it may be envisioned that in order to provide an exclusion field, for some tags the power difference may be about 5 dB, 4 dB, 3 dB, or even less.

[0064] By way of example, not intended to be limiting to the present invention, FIGs. 8A-C illustrate practical embodiments of the system 100. In each of these embodiments, the system 100 comprises a first antenna 110 and a second antenna 120 that are positioned with respect to each other such that the first field zone, in particular the activation field AF, and the second field zone, in particular the interrogation field IF, partially overlap in a third field zone, thereby providing an exclusion field EF between the activation field AF and the interrogation field IF. The exclusion field EF is provided by the activation field AF and the interrogation field IF being simultaneously present in the third field zone, such that the interrogation command and the activation command collide with each other in the third field zone. Accordingly, any UHF RFID tags located in the exclusion field EF receive the activation command while already receiving the interrogation command, or vice versa, which causes the UHF RFID tag to understand and respond to neither of these commands.

[0065] More in particular, in FIG. 8A it is shown that the first antenna 110 and the second antenna 120 are positioned relatively close together on the same side of a passage 30, at or near the same wall segment 31a. The first antenna 110 may be arranged for generating the activation field AF towards a first area Al, e.g. on one side of the passage 30, while the second antenna 120 is arranged for generating the interrogation field IF towards a second area A2, e.g. on an opposing side of the passage 30. For example, the first and second antenna 110, 120 may both be mounted on a carrier element, such as a detection gate, or on a wall 31. The first and second antenna 110, 120 may be oriented at an angle with respect to each other, e.g. between 90 and 180 degrees, or between 120 and 180 degrees. In general, other angles between the first and second antenna 110, 120 may also establish an overlap between the activation and interrogation field AF, IF generated respectively by the first and second antennae 110, 120, to provide the exclusion field EF therebetween. For example, in FIG. 8 A, the angle is about 120 degrees. In FIGs 8B and C the first and second antenna 110, 120 are oriented substantially parallel to each other, e.g. at an angle of about 180 degrees. The angle may be dependent on the distance between the first and second antenna 110, 120. For example, when the first and second antenna 110, 120 are positioned farther away from each other, the angle A may be increased to ensure that the third field zone, where the activation field AF and the interrogation field IF overlap, is of a desired size and shape. In other words, the size and shape of the exclusion field EF can be adapted by modifying the relative position and orientation of the first and second antenna 110, 120 with respect to each other. FIG. 8B illustrates an embodiment where the first antenna is positioned at a first side of the passage 30, e.g. at or near the first wall segment 31a, while the second antenna is provided at a second side of the passage, e.g. at or near the second wall segment 31b. Accordingly, compared to the embodiment illustrated in FIG. 8A, the distance between the first and second antenna is increased, and their orientation with respect to each other is adapted to have the exclusion field EF extend across the passage 30.

[0066] FIG. 80 illustrates yet another embodiment of the system where the first antenna 110 is mounted to the floor 32, e.g. generating the activation field AF in an upward direction towards the ceiling 33, while the second antenna 120 is mounted to the ceiling 33, e.g. for generating the interrogation field in a downward direction towards the floor 32. The first and second antenna 110, 120 are offset from each other in the travel direction of the UHF RFID tags, e.g. from the first area Al to the second area A2, and / or angled with respect to each other, such that the exclusion field EF, i.e. the overlap between the activation field AF and the interrogation field IF, extends between the floor and the ceiling across the passage 30. In another example, both the first and second antennae 110, 120 may be mounted to the ceiling 33, or to the floor 32. Also, it can also be envisioned that the first and second antennae 110, 120 are mounted to any combination of structures, such as walls, doors, floors, ceilings, detection gates, and other carrier elements.

[0067] Besides the relative positioning of the first and second antenna 110, 120 with respect to each other, the type, shielding, and / or signal strength of the first and second antennae may also play a role in shaping the exclusion field EF. For example, by shielding the first and / or second antenna 110, 120 the shape of the activation or interrogation beam can be controlled, e.g. narrowed.

[0068] The first and second antennae 110, 120 may comprise antenna plates, or patch antennae, which provide a focused beam at each side of the antenna carrier 220. The system 100 according to the invention may comprise one or more detector units 220 as illustrated in FI s. 1 and 2. For example, a pair of detector units 220 may be arranged across the passage 30 between the first and second area Al, A2, such that the first antenna 110 is arranged for generating the activation field towards the first area while the second antenna is arranged for generating the interrogation field towards the second area. Instead of a pair of detector units, more than two detector units may be arranged across the passage, e.g. in case the passage is relatively wide.

[0069] It will be clear to the skilled person that the invention is not limited to any embodiment herein described and that modifications are possible which may be considered within the scope of the appended claims. The invention applies not only to retail applications where the method and system provide improved anti-theft detection and / or inventory tracking, but also to other technical, agricultural or industrial applications where tag interrogation is used. Also kinematic inversions are considered inherent to the invention disclosed herein. In the claims, any reference signs shall not be construed as limiting the claim.

[0070] The terms 'comprising' and ‘including’ when used in this description or the appended claims should not be construed in an exclusive or exhaustive sense but rather in an inclusive sense. Thus expression as 'including' or ‘comprising’ as used herein does not exclude the presence of other elements, additional structure or additional acts or steps in addition to those listed. Furthermore, the words ‘a’ and ‘an’ shall not be construed as limited to ‘only one’, but instead are used to mean ‘at least one’, and do not exclude a plurality. Features that are not specifically or explicitly described or claimed may additionally be included in the structure of the invention without departing from its scope.

[0071] Expressions such as: "means for ...” should be read as: "component configured for ..." or "member constructed to ..." and should be construed to include equivalents for the structures disclosed. The use of expressions like: "critical", "preferred", "especially preferred" etc. is not intended to limit the invention. To the extent that structure, material, or acts are considered to be essential they are inexpressively indicated as such. Additions, deletions, and modifications within the purview of the skilled person may generally be made without departing from the scope of the invention, as determined by the claims.

Claims

CLAIMS1. A method of operating a transition system for interrogating one or more UHF RFID tags, the transition system monitoring a passage between two separated spaces and comprising a first antenna and a second antenna, the method comprising: generating, using the first antenna, an activation field inside a first field zone (FZ 1), configured to activate said one or more UHF RFID tags within said first field zone, generating, using the second antenna, an interrogation field inside a second field zone (FZ2), configured to interrogate within said second field zone the one or more UHF RFID tags that were activated by the activation field, wherein at least a part of the activation field and at least a part of the interrogation field overlap with each other to thereby form a third field zone (FZ3) between the first field zone (FZ1) and the second field zone (FZ2), the third field zone separating the activation field and the interrogation field from each other by including an exclusion field inside the third field zone (FZ3), wherein the exclusion field is provided by establishing mutual interference between the activation field and the interrogation field in the third field zone (FZ3) by simultaneously performing the steps of generating the activation field and the interrogation field, such that the one or more UHF RFID tags, when within said third field zone (FZ3) are unable to understand instructions in the activation and interrogation fields, wherein the steps of generating the activation field and the interrogation field are performed by spatially directing thefields such that the third field zone extends across the passage such as to close the passage by the third field zone.

2. The method according to claim 1, wherein the activation field comprises an activation command configured to activate the one or more UHF RFID tags, wherein the interrogation field comprises an interrogation command configured to interrogate the one or more activated UHF RFID tags, and wherein the interrogation command and the activation command collide with each other in the third field zone (FZ3).

3. The method according to any of the preceding claims, further comprising the step of shaping the exclusion field by positioning the first and second antenna with respect to each other, and / or by adjusting a power difference and / or a frequency difference between the activation and interrogation field.

4. The method according to any preceding claim, wherein the activation field is transmitted with a first signal power, wherein the interrogation field is transmitted with a second signal power, and wherein, in the third field zone (FZ3), a power difference between the first and second signal power experienced by the tag is smaller than a threshold value, preferably smaller than 6 dB, for a given polarization of the activation and interrogation fields.

5. The method according to any preceding claim, wherein the activation field is transmitted at a first signal frequency, wherein the interrogation field is transmitted at a second signal frequency, and wherein, in the third field zone (FZ3), afrequency difference between the first and second signal frequency is larger than 500 kHz, preferably larger than 1 MHz.

6. The method according to any preceding claim, wherein the activation field and the interrogation field have equal polarizations.

7. The method according to any preceding claim, comprising the step of simultaneously interrupting the steps of generating the activation field and the interrogation field at mutually congruent intervals.

8. The method according to any preceding claim, wherein the first antenna and the second antenna are provided by one of: two separate antennas provided in different entities; single antenna unit device wherein the first and second antennae are jointly mounted in a single housing; or an antenna array comprising a plurality of antennas, such as a multibeam antenna array.

9. The method according to any preceding claim, wherein the activation and interrogation field are generated at an angle with respect to each other between 45-180 degrees, more preferably between 90-120.

10. A transition system for interrogating one or more UHF RFID tags, the transition system monitoring a passage between two separated spaces and comprising:a first antenna arranged for generating an activation field inside a first field zone, configured to activate the one or more UHF RFID tags; a second antenna arranged for generating an interrogation field inside a second field zone, configured to interrogate within said second field zone the one or more UHF RFID tags that were activated by the activation field; a controller arranged for controlling the first and second antennae; wherein the first and second antennae are positioned with respect to each other such that the first field zone and the second field zone partially overlap in a third field zone; wherein the third field zone separates the activation field and the interrogation field from each other by including an exclusion field inside the third field zone, wherein the exclusion field is provided by the controller establishing mutual interference between the activation field and the interrogation field in the third field zone, by controlling the first and second antennae to simultaneously generate the activation field and the interrogation field, such that the one or more UHF RFID tags, when within said third field zone, are unable to understand instructions in the activation and interrogation fields, wherein the activation field and the interrogation field are spatially directed such that the third field zone extends across the passage such as to close the passage by the third field zone.

11. The system according to claim 10, wherein the activation field comprises an activation command configured to activate the one or more UHF RFID tags, wherein the interrogation fieldcomprises an interrogation command configured to interrogate the one or more activated UHF RFID tags, and wherein the interrogation command and the activation command collide with each other in the third field zone.

12. The system according to claim 10 or 11, wherein the passage comprises a detection gate, wherein the first and second antennae are mounted to the detection gate.

13. The system according to claim 12, wherein the first antenna is mounted to a first side of the detection gate facing the first field zone, and wherein the second antenna is mounted to a second side of the detection gate facing the second field zone.

14. The system according to any of claims 10-13, wherein the first and second antennae are arranged for generating the activation and interrogation field at an angle with respect to each other between 45-180 degrees, more preferably between 90-120.

15. The system according to any of claims 10-14, wherein each of the first and second antennae comprises a patch antenna.

16. The system according to claim 15, wherein the patch antenna is mounted on a ground plate provided with an upstanding edge for shielding the generated activation or interrogation field.

17. The system according to any of claims 10-16, wherein the first antenna and the second antenna are provided by an antenna array comprising a plurality of antennas, such as a multibeam antenna array configured for generating field in beams that are spatially oriented differently.

18. The system according to any of claims 10-17, wherein the first and second antennae are provided by a single antenna unit device arranged for generating the activation field, the interrogation field, and the exclusion field.

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