WO2010041173A1

WO2010041173A1 - Detection of separation between tag and associated item

Info

Publication number: WO2010041173A1
Application number: PCT/IB2009/054293
Authority: WO
Inventors: Jeremy Moss
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2008-10-09
Filing date: 2009-10-01
Publication date: 2010-04-15

Abstract

A method of detecting antenna detuning caused by the proximity of different materials is presented that require little or no hardware additions above those required for a standard low power radio. The detection of changes e.g. in antenna resonance can then be used for applications such as theft detection in asset tracking applications.

Description

DETECTION OF SEPARATION BETWEEN TAG AND ASSOCIATED ITEM

TECHNICAL FIELD

The present invention relates to a method of detecting removal of a tag from an item it is attached to. The invention equally relates to a corresponding tag, a master device and a computer program product comprising instructions for implementing the steps of said method.

BACKGROUND OF THE INVENTION

Asset tracking systems are becoming increasingly popular in many different industries thanks to improvements in a number of key technology areas. Asset tracking systems provide companies with a number of useful features including:

• Theft detection;

• Item location;

• Process analysis; and

• Stock optimisation. One of the key technology enablers has been passive radio frequency identification (RFID) tags. The technology allows devices to be read at short range via a radio -frequency link, enabling faster scanning of the tags than was previously possible with bar codes, or from manually reading labels.

Some applications require longer reading ranges than are currently feasible with passive RFID tags. For these applications, active RFID tags are required which have their own power source, enabling radio frequency communications to be conducted over a greater distance. The biggest disadvantages of such tags are currently their size and cost. This, however, is unlikely to remain the case forever, and it is envisaged that future devices will be much more suitable for affixing to smaller, awkwardly shaped objects. In hospitals, it has become apparent that the ability to track devices throughout the hospital is very valuable. Preferably, both high and low asset value items can be tracked. These items include objects such as blood bags that have a form factor that makes existing active tags difficult to attach. This is one of the key problems that need to be solved.

All RFID tags require carefully designed antennas to ensure efficient communication between tag and reader. The design of such antennas is dependent on a number of factors including operating frequency, passive, semi-passive or active operation, desired range, and form factor constraints. Antennas are also affected by the material onto which the tags are attached. The material cannot only reduce the efficiency of the antenna, it can also detune it, meaning that although the antenna gain has not changed (or only changed by an insignificant amount), it no longer operates in the desired frequency band, meaning that signals are heavily attenuated resulting in dramatically reduced range, if not complete inoperability. Figure 1 shows hypothetical frequency, gain and bandwidth effects on an antenna as a result of close-proximity materials. Sn indicates the reflected signal strength, so a minima on the diagram indicates maximum radiation at that point. These effects are a result of impedance changes to the antenna caused by close- proximity objects. The detuning effect can be combated in a number of ways.

1. An antenna could be designed to have a bandwidth far wider than is required by the radio link. Any detuning that then occurs, doesn't affect the radio's link budget as the desired operating frequency remains within the wide bandwidth of the antenna. A key disadvantage of this is that the wide bandwidth of the antenna results in more noise being passed through to the radio circuitry leading to poorer signal-to-noise ratios. This results in reduced range, poorer battery life, or increased bit errors.

2. If the material onto which the tag will be attached is known beforehand, the antenna can be designed so as to have the correct frequency and bandwidth of operation only once it is attached. The antenna is therefore purposely designed with an incorrect frequency so that the detuning effect of the material brings the frequency back to the correct value. This has the disadvantage that the material properties must be known beforehand. The tag can then only be attached to objects that have the same or very similar material properties.

3. A configurable tuning network can be used at the feed of the antenna. Once the antenna is attached to a device, a frequency calibration process is started that selects the correct tuning parameters to bring the operating frequency of the antenna back to its desired value. Switchable components can be used to allow programmable adjustment of such a network. An algorithm can then be implemented within the device that performs this tuning process automatically under certain conditions, by searching for maximum signal strengths (for example). This solution has the disadvantage of extra cost in terms of the extra hardware required to implement the solution, but has the advantage of allowing one tag design to be used on different types of objects with diverse material properties.

A key problem with existing asset tracking solutions is that unless the tag is physically part of the item it is attached to, a thief could remove the tag and then steal the item undetected. It is feasible that the system may start to notice "unusual behaviour" from the tag (e.g. the item has not moved for the past n hours which is very unusual), but the statistical nature of such a system is likely to generate some false positives and negatives. It may also be the case that the statistical change in behaviour cannot be detected for a number of hours by virtue of the normal behaviour of items within the asset tracking system. In these cases, the alarm would be raised long after the thief had made their escape.

It may be possible to arrange a physical or optical connection on the tag that is made or broken when the tag is removed from the device. Such a connection would need careful design to ensure that even if the thief had knowledge of its construction, it would still be very difficult to defeat the tamper mechanism so that the tag did not register its removal from the device. A key disadvantage of any of these solutions is the need to add extra hardware to the tag in order to detect tampering.

Before active tags are attached to their items, it is important that they are kept in a "sleep state". This ensures that their battery life is prolonged as much as possible.

Once the device is attached to its item, it must then be activated in some way for it to start operating in its normal mode. A mechanical switch or other such mechanism could be used to activate the label, but this requires extra hardware. An alternative could be to send a signal via the infrastructure network signalling that it should change to normal mode. This has the advantage of not requiring extra hardware, but the disadvantage of requiring the tag to periodically check for activation signals, increasing its sleep-state power consumption, and therefore reducing potential battery life.

It is thus the object of the present invention to overcome the above-identified difficulties and disadvantages.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a method of detecting removal of a tag from an item it is attached to, wherein the tag comprises an antenna for communication with at least another device over a radio channel, the method comprising detecting a change in the impedance characteristics and/or radiation pattern of the antenna as a result of its proximity to another item.

Thus, the present invention makes use of existing hardware within a potential radio design to detect removal of a tag from an object that it has been attached to. The hardware used to detect the removal needs to be present for this particular implementation of an active or passive tag, so the extra cost to the system to add this "tamper detection" feature would only be as a result of implementing the necessary algorithm and alarm mechanism. The hardware cost of the tag remains the same.

According to a second aspect of the invention, there is provided a computer program product comprising instructions for implementing the method according to the first aspect of the invention when loaded and run on computer means of the tag or a master device.

According to a third aspect of the invention, there is provided a tag arranged for detecting removal of the tag from an item it is attached to, wherein the tag comprises an antenna for communication with at least another device over a radio channel, the tag being arranged to detect a change in the impedance characteristics and/or radiation pattern of the antenna as a result of its proximity to another item. According to a fourth aspect of the invention, there is provided a master device arranged for detecting removal of a tag from an item it is attached to, wherein the master device comprises an antenna for communication with the tag over a radio channel, the master device being arranged to receive signals from the tag for deducing a change in the impedance characteristics and/or radiation pattern of an antenna of the tag as a result of its proximity to another item.

Other aspects of the invention are recited in the dependent claims attached hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent from the following description of non- limiting exemplary embodiments, with reference to the appended drawings, in which:

- Figure 1 shows a reflectivity curve of a tag antenna; - Figure 2 illustrates frequency sweep initiated by a master device and a received signal strength curve of a tag; and

- Figure 3 is a simplified block illustrating an antenna tuning network.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION The present invention is based on an idea that the tag or supporting network detects a change in the characteristics and/or radiation pattern of the antenna as a result of its proximity to an item. With reference to Figure 1, it would detect a change in part, or all, of the curve. Depending on the implementation, characteristics such as resonant frequency, bandwidth and gain, could be measured; or a subset of one or more. Next some embodiments of the present invention are explained in more detail.

Embodiment 1

The first embodiment enables a frequency division multiple access mechanism. The first embodiment is illustrated by Figure 2. The master device 201 emits a frequency swept "ping" at regular intervals. The tag's 203 receiver sweeps its receiver in synchrony with the master. However, it is to be noted that a reverse situation is also possible, where the tag 203 transmits and the master 201 receives. The received signal strength is then measured throughout the sweep providing a number of signal strength measurements over a range of frequencies. These measurements are compared to a reference to detect a significant change. If such a change is found, the tag 203 signals this back to the master device 201 in accordance with one implementation possibility. A suitable action can then be taken either instantly, or after a number of such discrepancies have been reported. Thus, this embodiment is based on the fact that depending on the proximity of different materials to the tag, the received signal strength curve is different, or at least the location of the frequency peak is dependent on the environment where the antenna is located.

It is also possible that the tag 203 transmits the measured received signal strength measurements to the master device 201 which then decides whether the measurements are different from previous results. Thus, depending on the implementation details the comparison of different measurements can be done on the tag 203 or in the master 201 or alternatively it is also possible to invoke another network element to do the data processing.

In this embodiment, the master 201 and tag 203 must be capable of tuning themselves to the resonant frequency of the antenna to allow communication with optimal signal to noise performance if it is required. In Figure 2, the optimal frequency is the location of the frequency peak indicating the highest received signal strength.

Data can be superimposed over the swept carrier wave in a narrowband system. This could reduce the bandwidth overhead that such an implementation may have. It should be noted that the frequency sweep may be wide enough to detect the edges of antenna resonance. This will result in loss of data between master 201 and tag 203 at these points, i.e. where the signal strength is too low. Superimposing data on these swept signals is, therefore, only suitable for repeated or redundant data; or data that only has relevance at the received frequency.

The sweep could be used as a network beacon for devices. Repeated data within the sweep (grouped into packets) can be used for typical network management purposes. A field could be used in the transmitted packet that indicates the frequency bin that the current packet is being transmitted in. This could then be detected by the tag 203 and signalled back to the master 201 at a later point. This instructs the master 201 which frequency to use for future data communications to the tag 203. Such a system could form part of a multiple frequency network whereby the different amounts of detuning in the antennas enable simple and cheap multiple frequency communication, increasing network capacity (assuming different amounts of detuning across all devices). Such an implementation would then remove the need for any programmable matching networks (or perhaps reduce the need), thereby reducing the cost of the tag 203. Tags attached to similar items (e.g. blood bags) are likely to experience similar detuning effects. This will result in item groups naturally falling into unique frequency bins, providing a natural channelling mechanism for addressing these particular groups.

The intervals between frequency "pings" are dictated by the application and battery life constraints of the tags 203. It is desirable to average results over a number of measurements, before a decision is made. The sweeps therefore need to occur regularly so that theft can be detected before a thief has a chance to remove the item from the premises.

The master 201 needn't sweep the frequency band in a linear manner. It could hop to a number of different frequencies in any order. As long as the receiver is synchronised, the resultant antenna response measurements remain the same. This could therefore form part of a standard frequency hopping communications system.

In case the tag 203 transmits the frequency swept signals, the master 201 measures the signal strength received from the tag 203. In that case the master device can then deduce the impedance characteristics and/or radiation pattern of the tag antenna. For instance, the resonance frequency of the tag antenna can be determined in this way.

Embodiment 2

The second embodiment can be used in a single channel communication system whereby the antennas are all tuned to operate on a single RF frequency. Tags 203 equipped with a tuning network 301 similar to that shown in Figure 3 are able to compensate (to a certain extent) for the detuning effects of certain materials in close proximity. When the tag 203 is first attached to a new material, an algorithm is run that measures either the signal strength of a fixed frequency received signal; or transmits a fixed frequency signal, and a receiving device measures its signal strength. These signals are transmitted and measured for each of the available matching network combinations. The combination that provides the lowest antenna losses gives an indication as to the amount of detuning that the antenna is experiencing. This then becomes a reference against which all future measurements are made. If the tag 203 is removed from its intended item, this will be detected at the next measurement time as the combination needed to obtain the best antenna efficiency is most likely different and suitable action can be taken. For instance, with reference to Figure 3, the capacitors 303 can be turned on and off by use of the control switches 305. In this example, three capacitors 301 are shown, giving 2 =8 possible combinations. Thus, in accordance with this embodiment, by using the appropriate combination of capacitors

301, the antenna can be tuned to a desired frequency.

Embodiment 3

This embodiment enables channelised (i.e. multiple frequency channels) communication within a network. The advantage of this embodiment is that an accurate timing reference is achieved. For a multiple frequency network, it is necessary to have a tunable receiver, and an antenna that is suitable for all channels. As discussed above, a wideband antenna could be used, but this increases out of channel interference. A narrowband antenna needs to be tuned to the desired channel. The matching network shown in the second embodiment can be used to enable this tuning. The labelled frequency bins provide a mechanism for gaining time synchronisation. Taking an example of a tag that is able to tune its antenna (i.e. has a programmable tuning network), a possible synchronisation sequence could be:

1) The tag 203 sets up the antenna with a default tuning setting, perhaps one that is likely to ensure a central tuning. 2) It tunes its receiver to a central or most probable frequency.

3) If the antenna and receiver tuning are close enough, part of the swept beacon is received, and the frequency bin values are recorded. 4) From the recorded bin values, the tag extrapolates (by knowing the length of a frequency sweep) the start of the frequency sweep in the time domain. At the next swept beacon, the tag 203 can start its receiver sweep in synchrony with the master 201. The measured received frequency strengths (e.g. by using the information about the location of the peak of the received signal strength as shown in Figure 2) indicate to the tag 203 the amount of detuning currently being experienced, and the tuning network can then be adjusted if required. This is more efficient than searching for the start of a beacon in the time domain, as in this case, the device knows exactly when to start receiving.

A change in detuning indicates a potential theft. It is to be noted that the reverse situation is also possible, where the master 201 receives signals from the tag 203 for detecting the antenna characteristics and/or radiation pattern of the tag antenna. Above some embodiments of the present invention were described, however it is to be noted that antenna tuning changes need not be used just to detect theft. They could be used to wake up a tag 203 from a sleep mode (where it performs a detuning check only very occasionally), or to detect a deliberate change of item (if a tag is deliberately moved between items as part of a process flow for example), or any other event where the physical environment immediately surrounding the antenna changes. Thus, the present invention could be used for detecting legitimate device changes, not just theft situations. Triggers generated from changes in the surrounding physical environment, can be combined with information from other sources to provide increased functionality. For example, when a device is replaced, if the tag 203 detects a change (via its antenna measurements), while in the "goods-in" area, the system could conclude that this isn't a theft situation, but a device replacement situation, and act accordingly (for example by reminding the asset manager to enter the details of the new item into the database etc.).

The embodiments described above concentrate mainly on frequency shifts, i.e. the minima on the Sn curve. For these, it is important that the sweep is performed rapidly to minimise external influences on signal strength. Bandwidth measurements could also be made on the antenna to detect certain changes in physical circumstance.

In scenarios where all items in the environment have the same or similar physical properties, a thief could remove the label from one item and attach it to another very similar item. If this were done quickly enough, the system may not respond. Patches with unique electrical properties, designed to detune antennas by varying amounts could be used to provide unique detune signatures for each item. A thief would then also need to remove this patch (which could be placed inside the device, or affixed with an adhesive designed to destroy the electrical properties of the patch if it is removed) adding delay and or difficulty to its removal. The unique patches could also provide unique antenna tuning to aid in device identification (by detecting its transmit frequency), and also antenna optimisation by increasing the distance between the antenna and the item it is attached to.

The teachings of the present invention are applicable to all wireless network devices.

The invention also relates to a computer program product that is able to implement any of the method steps as described above when loaded and run on computer means of the tag (203) or the master (201). The computer program may be stored/distributed on a suitable medium supplied together with or as a part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

The invention also relates to an integrated circuit that is arranged to perform any of the method steps in accordance with the embodiments of the invention.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive, the invention being not restricted to the disclosed embodiments. For instance, in the first embodiment it is possible to apply the tuning network shown in Figure 3 and described in connection with the second embodiment. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. The mere fact that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be advantageously used. Any reference signs in the claims should not be construed as limiting the scope of the invention.

Claims

C L A I M S

1. A method of detecting removal of a tag (203) from an item it is attached to, wherein the tag (203) comprises an antenna for communication with at least another device (201) over a radio channel, the method comprising detecting a change in the impedance characteristics and/or radiation pattern of the antenna as a result of its proximity to another item.

2. The method according to claim 1, wherein the impedance characteristics comprise at least one of the following: resonant frequency, frequency bandwidth and gain.

3. The method according to any one of claims 1 to 2, wherein the method further comprising providing a tuning network (301) for tuning signal reception characteristics of the tag (203).

4. The method according to claim 3, further comprising finding an optimal setting for the tuning network (301) for signal reception and/or transmission purposes, and when performing a new tuning and determining that new tuning setting is needed for optimal signal reception, then concluding that the tag (203) has been removed.

5. The method according to any one of the preceding claims, further comprising the tag (203) receiving frequency swept signals and measuring the received signal strength of the tag (203) throughout the sweep and comparing the measurement results to a given reference value and if the measurement results are different from the reference value by a given threshold, determining that the tag (203) has been removed from the item.

6. The method according to any one of claims 1 to 4, further comprising the tag (203) transmitting frequency swept signals to the other device (201) and the other device (201) measuring the signal strength received from the tag (203) for deducing the impedance characteristics and/or radiation pattern of the antenna.

7. The method according to any one of claims 5 to 6, wherein the swept signals comprise superimposed data.

8. The method according to claim 7, wherein the data comprise indication of the frequency bin the signal is transmitted in.

9. The method according to any one of claims 5 to 8, wherein the frequency swept signals follow the principles of frequency hopping.

10. The method according to any one of claims 1 to 4, further comprising: the tag 203 receiving frequency swept signals divided into frequency bins; tuning the antenna of the tag (203) to receive at a certain frequency; recording the received frequency bin values; extrapolating the start of the frequency sweep in time domain by knowing the sweep period; measuring at least one received signal strength characteristic and/or radiation pattern of the antenna.

11. The method according to claim 10, wherein the received signal strength characteristic and/or radiation pattern indicate(s) to the tag (203) the amount of detuning currently experienced; and a change in detuning indicating the removal of the tag (203).

12. The method according to any one of the preceding claims, wherein patches with unique electrical properties, designed to detune the antenna by varying amount is used in proximity with the item to provide unique detune signature for the item.

13. A computer program product comprising instructions for implementing the steps of a method according to any one of claims 1 through 12 when loaded and run on computer means of a radio communication device (201; 203).

14. A tag (203) arranged for detecting removal of the tag (203) from an item it is attached to, wherein the tag (203) comprises an antenna for communication with at least another device (201) over a radio channel, the tag (203) being arranged to detect a change in the impedance characteristics and/or radiation pattern of the antenna as a result of its proximity to another item.

15. The tag (203) further comprising a tuning network (301) for tuning signal reception characteristics of the tag (203).

16. A master device (201) arranged for detecting removal of a tag (203) from an item it is attached to, wherein the master device (201) comprises an antenna for communication with the tag (203) over a radio channel, the master device (201) being arranged to receive signals from the tag (203) for deducing a change in the impedance characteristics and/or radiation pattern of an antenna of the tag (203) as a result of its proximity to another item.

17. A radio frequency identification system comprising the tag (203) of any one of claims 14 to 15, and further comprising the master device (201) of claim 16.