GB2277850A - Intelligent radio frequency transponder system - Google Patents

Abstract

An identification and location system for persons or objects which have to be identified and located. A tag in the form of a programmable radio transponder is attached to each person or object. A local transmitter VCT broadcasts to the tags at VLF, while the tags respond to a central receiver 12 at UHF. Knowledge of the transmitter VCT responded to enables the approximate location of the tag, while the response carries the identity thereof. Tag battery life is prolonged by providing each tag with a high sensitivity receiver which is switched on by reception of a triggering signal by a low current receiver which is continuously on. The system has application to monitoring the location of persons liable to wander, safe evacuation of sites in emergencies, asset tracking, cargo management, personnel location and communication, wireless data gateways, and the like. <IMAGE>

Description

"Intelli9ent Radio Frequency Transponder System" This invention relates to an intelligent radio frequency transponder system, and relates more particularly but not exclusively to a system for identifying, locating and making interactive data exchanges with persons or objects by means of miniature radio frequency apparatus affixed thereto. For simplicity, such a system is referred to herein as a "tag system" and the apparatus as a "tag".

A variety of tag systems are known which provide identity and/or location information, but the known systems suffer from a number of disadvantages.

One known system uses very low frequency (VLF) radio frequency identification devices, commonly in the form of read-only passive tags, used in access control systems or as asset identification. Read ranges are dependent on the attitude of the tag relative to the reader, being typically 5 to 20 centimetres, with battery assisted versions giving up to a metre. Most systems have problems if there is more than one tag within range, reading only the nearest or giving misreads. Data rates are low, basically determined by the VLF energising field. If the tag is moving past the reader, there is no opportunity to retrieve a misread.

There is no location capability other than that implicit in the reading of a device which must be close to a reader of known position.

A second type of known system is beam powered radio frequency identification devices. These are usually passive and are energised by a microwave beam which is significantly directional. Passive read ranges are of the order of 3 to 10 metres in the case of installations licensed by radio regulatory authorities, and a little under a metre in the case of licenceexempt installations. Data rates can be high, allowing tags to be read while moving rapidly. Only the nearest tag is read. The cost of tags can be low, but interrogator costs tend to be high to very high.

Also known are tags for use in animal tracking. These are burst-transmitting devices which, in conjunction with an appropriate receiver, provide information such as a bearing to assist in location. Ranges may be many hundreds of metres to some kilometres, but there is usually little discrimination of identity. All such tags are active, with transmissions governed only by the tag's internal controls, and useful life is limited by battery capacity.

The present invention seeks to improve on such known systems by providing a tag system giving a better combination of some or all of the following features: - identification of each tag - ability to read plural tags in any area - capability of interactive data exchange - capability of data storage - capability of being re-programmed while in active use - compatible with licensing regulations - low cost of both tags and associated equipment - long operational life - tag orientation less critical - good combination of operating range and position discrimination The invention accordingly provides a tag system comprising one or more tags each attachable to a person or object, at least one VLF (Very Low Frequency) radio transmitter, and at least one UHF (Ultra High Frequency) radio receiver, said VLF radio transmitter operating to transmit VLF control signals, and the or each said tag including a respective VLF radio receiver and a respective UHF radio transmitter which operates under the control of the VLF radio receiver of that tag to transmit UHF radio signals for reception by said UHF radio receiver(s).

Preferably, a said UHF radio receiver provides a relatively large reception zone within which are positioned a plurality of VLF radio transmitters each having a relatively small zone of effective transmission. Preferably also, the VLF radio transmitters operate in near-field mode.

The location of a said tag within said relatively large zone may be deduced from the location(s) of the VLF radio transmitter(s) which cause said tag to transmit.

there more than one said tag is present within said relatively large zone, each said tag may be identified by transmitting a code unique to that tag, or by causing said tag uniquely to transmit in response to a unique code in the VLF control signal.

From another aspect, the invention provides a tag device for use in the foregoing system, said tag device comprising a VLF radio receiver, a UHF radio transmitter, and a control circuit.

Preferably, said control circuit is so arranged that the tag device operates in response to VLF control signals when these are received within predetermined time periods, and said tag device operates in response to internal controls when VLF control signals are not so received.

In a particularly preferred form, the VLF radio receiver of said tag device comprises a first, high sensitivity radio receiver circuit and a second, low sensitivity radio receiver circuit, the second receiver circuit being permanently energised but said first radio receiver circuit being energised only in response to receipt of a strong signal burst by said second radio receiver circuit.

Another aspect of the invention provides a method of controlling a tag system having at least one control transmitter, at least one control receiver, and a plurality of tags each having a tag receiver and a tag transmitter, said method comprising the steps of transmitting signals from the control transmitter to cause the tags to transmit signals each coded with the identity of the individual tag, scanning the signals received at the control receiver to distinguish tag identity codes, identifying any unambiguous tag identity code and in response thereto transmitting a control signal from the control transmitter causing that identified tag to cease transmission, and repeating this procedure for a predetermined period of time sufficient to achieve statistical probability of all tags having been unambiguously identified.

Still another aspect of the invention resides in a tag system comprising at least one control transmitter, at least one control receiver, and a plurality of tags, each tag having a tag receiver and a tag transmitter, in which each tag includes means responsive to predetermined signals from the control transmitter to cause the tag to adopt a quiescent or an active state.

Suitably, when in the quiescent state the tag transmitter is disabled and the tag does not respond to control transmitter signals other than the predetermined signal for changing to the active state.

This aspect of the invention is particularly suitable for use in a system where entry to an area covered by the VLF/UHF system can be gained only through a limited number of access points. These can be in the form of conventional electronic portals, which are used to switch the tags from quiescent to active on entering the area and vice versa.

From a further aspect, the invention resides in a tag device for use in a radio-frequency tagging system, said tag device comprising a first, high sensitivity receiver circuit, a second, low sensitivity receiver circuit, and control means responsive to receipt of a high intensity transmission by the second receiver circuit to enable the first receiver circuit for a predetermined period of time, or until further command.

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a schematic block diagram of one form of system in accordance with the invention; and Fig. 2 schematically illustrates the area coverage of the system.

The system of Fig. 1 comprises a central system control 10 connected to a number of VLF control transmitters VCT1-VCT4 and to a receiver 12, together with a number of tags one of which is shown at 14. The tag 14 comprises a VLF radio receiver 16, a UHF radio transmitter 18, and an internal control circuit 20, and is powered by an internal battery (not shown).

The control transmitters VCT1-VCT4 and the VLF receivers 16 operate in the near-field mode in which, as is known per se, energy circulates within the field but is not propagated to any great distance, with the effect of providing well defined signal zones around each transmitter. Fig. 2 illustrates this diagrammatically. (Fig. 2 is not to scale and shows field outlines which are distorted from reality). In the example shown, the fields of the four transmitters overlap, but this is not essential. In Fig. 2, tag 1 is within the field of VCT1, tag 2 is within the fields of both VCT2 and VCT4, and tag 3 is outside any transmitter field but within the range of UHF receiver 12. Tag 4 is beyond the range of the UHF receiver 12.

The use of VLF for transmission of control signals means that reception at the tag is relatively free from problems caused by antenna attitude, maintaining line of sight, and reflected signals.

The basic operation of the system is for the individual transmitters VCT1-VCT4 to transmit control signals at different times, which trigger any tag within range to carry out a commanded function: this may be to transmit, to refrain from transmitting, or to perform a data transaction such as writing in memory. In the simplest case, a UHF response is transmitted containing an identity code for that tag, plus data if desired.

This identifies the area within which the tag presently is.

The tags 1-4 transmit at UHF in conventional far field mode giving a range typically of a hundred metres, which allows a single receiver to survey a considerable area.

A particularly preferred feature of the invention is that the tag control circuit 20 contains instructions which can be set by the user either before installation or at any time while the tag 14 is within range of a control transmitter VCT. The instructions will normally be set such that, if no VLF control signal is received by a particular tag within a predetermined time period, that particular tag transmits a predetermined UHF signal. Receipt of this UHF signal by the UHF receiver 12 defines the tag as being within range of the UHF receiver 12 but outside the fields of the transmitters VCT1-VCT4.

A further preferred feature of the invention relates to the operation of the tag VLF receiver 16. It is desirable in a portable tag to minimise power consumption and hence battery size. A sensitive receiver requires in practical terms, a consumption of the order of 3mA (milli-amperes), but for the battery size and life desired, the maximum possible standing current is of the order of 5yA (micro amperes). This problem has been approached in the past by cycling receivers on and off until a signal is received. A pocket pager, for example, spends about 0.25 seconds on and 0.75 seconds off, requiring that a signal has to be repeated a number of times to ensure statistical probability of valid reception. As duty cycles are reduced to maximise battery life, transmission and response times become unacceptably long.

In the preferred form of the present tag, the tag receiver 16 has a low sensitivity receiver which is active at all times, and a high sensitivity receiver which is activated only occasionally, typically for about 1/1000th of the time. The former draws only about 3yA to 5pA and the latter about 3mA. A very short burst of a strong signal can therefore be detected by the first receiver and used to switch on the high sensitivity receiver for valid reception of the weaker signal.

Another area addressed by the present invention is the problem of "data collision" which can occur where a number of tags are transmitting within range of a given receiver. In this situation it can be impossible for the system to distinguish the tags in the area of interest as tag identity signals become confused with each other. Therefore, in a preferred form of the invention the system control 10 scans incoming data for any received code which is clearly and unambiguously that of a specific tag 14. Whenever such identification is made, the system control 10 causes the appropriate control transmitter VCT to transmit a command signal causing that tag to cease transmission. As each tag is identified and turned off, the risk of data collision reduces and the speed of identification increases.For any given system, it is possible by statistical techniques, or by empirical methods, to define a time period which gives an acceptably high statistical probability of all tags within a given area having been identified in this way, after which the process can be repeated.

It is also preferred that the internal control circuit in each tag includes memory means which can be used to associate data with an individual tag. Such data can simply be an identity code, as discussed above, or can be variable data such as size, quantity or status, which can be written to and read from the tag by means of the VLF and UHF channels.

In the foregoing description, tag location is defined simply in terms of the transmitter zone within which the tag is located. This can be used to define position typically within a radius of a few metres or tens of metres. A more precise location can be derived, at the expense of some additional complexity. One method is to measure VLF signal strength at the tag, and transmit this to the system control. Another possibility is by measurement of UHF signal strength at the UHF receiver, which can be highly informative. A further possibility is to use a plurality of physically separated UHF receivers to define the tag position by triangulation once the tag is transmitting and its approximate location known.

In summary, the preferred arrangement of radio frequency tag technology in accordance with the invention is as follows: The tags consist of a low frequency radio receiver, a microprocessor and associated memory, and a UHF transmitter.

Tags have two sources of command - internal pre-programmed controls - radio signals transmitted to them Tags have three possible sources of data - pre-programmed, internally held data (such as an identity code) - radio signals transmitted to them - direct input (typically by wire) Tags have two ways of outputting data - through their UHF transmitter - direct output (typically by wire) Tags are active (battery powered) and typically thick credit card sized. They receive low frequency signals from a Control Transmitter (CTx), and transmit their UHF output to a Data Receiver (DRx). Typical systems consist of Tags, CTx's and DRx's.

The technology provides wireless capabilities in three key elements: - distributed intelligence with interactive communication - tracking (location and movement) - data gateway Sub-System Operation There are three elements in the sub-system - Control Transmitters (CTx), Tags and Data Receivers (DRx).

CTx's receive their commands from the main system, and transmit instructions to the tags. Tags transmit to the DRx's, which feed back to the main system. (See Figure 1). There may be any number of CTx's, tags and DRx's under the command of the main system. CTx's and DRx's are typically fixed, optionally mobile, while tags are typically mobile, optionally fixed.

Any tag within 15 metre radius of a Control Transmitter can be read, written or reconfigured, and read only within 100 metre radius of a Data Receiver. There is no need to know location to perform transaction.

Tags are active (battery powered) comprising a radio receiver, internal microprocessor control and memory, and a radio transmitter. Size is thick credit card.

Tags may be addressed by CTx's uniquely, in blocks, or globally to store data, to set up or modify the tag's internal controls, or to command transmission to the DRx's.

Low Frequency Command Link Typically, tags operate on a programmable sleep/wake cycle, with "always awake" an option. For a transaction to be made, the tag must be in its wakened state. Also typically, tags will waken unconditionally when in the immediate vicinity of a control transmitter, (Portal Mode) a very low power receiver detecting the relatively strong signal and turning on a more sensitive receiver to accept data and commands.

Identities and associated data may be read on passing through a portal, with tags commanded to shut down for a set period as soon as this transaction is complete to avoid data collisions with other tags in the field.

Tags also waken under their internal timings, allowing transactions to be made anywhere within range of a control transmitter (Wide Area Mode). Unique intelligent data slicers and a novel data structure provide extremely robust wireless interconnect.

The link complies with the requirements of MPT 1337, operating in the near field mode with a range of over fifteen metres.

UHF Read Link The output of a tag and the circumstances under which it transmits are functions of how the tag is configured. Typically, tags will transmit when so commanded by a control transmitter, but may also transmit at the command of their internal timings whether in or out of range of a control transmitter.

This link complies with the requirements of MPT 1340, operating in the conventional far field mode with a free space range of one hundred metres.

Example 1 - Wandering Individuals Various forms of dementia may give rise to wandering of individuals.

The prime need is to know if a sufferer leaves their area of safety, while avoiding rigid constraints such as locked doors or continuous surveillance. Secondary needs are for a carer to be able to find a wanderer who has strayed and, in some case at least, for a periodic check on continued presence within the area of safety.

A case which frequently arises is that of someone prone to wandering and living in their own home, most commonly with the spouse as carer. It is possible to install access control antennae around all the external doors of the house, but this is costly, obtrusive, and by no means 100% successful in detecting passage of the wanderer through the door.

A VLF control transmitter plugged into some convenient 13A socket will provide all the surveillance which is required with no fixed installation around doorways while including the immediate vicinity of the house within the area of safety - it is highly desirable that a person should be able to go into the garden or answer the door bell without setting off alarms.

To conserve battery life, the tag operates on an internally controlled 'sleep-wake' cycle. In sleep, the tag is in a near-quiescent state draining virtually no current from the battery. At pre-programmed intervals, say once every thirty seconds, the tag switches on its VLF receiver and listens for a signal from the control transmitter, which transmits near-continuously. If a signal is received, the tag recognises this and goes back to sleep. If however no VLF signal is detected, the tag operates its UHF transmitter; this transmission is detected by the UHF receiver, triggering an alarm for the carer.In the case of the elderly, this is frequently all that is required, since wanderers are unlikely to move so fast that they are out of sight within the time in which a carer can respond, but with more agile sufferers it may be necessary to equip the carer with a direction finding receiver so that the wanderer can be found.

Various refinements can be incorporated.

It is possible that a VLF transmission is missed, say through some abnormal screening effect. The tag therefore does not transmit but keeps its VLF receiver switched on; if it detects successive misses, it outputs its UHF signal.

It is desirable periodically to check that the tag is still in its area of safety (as distinct from detecting the fact of its having left) and that the tag is fully operational. To deal with this, the VLF transmitter periodically transmits a different code which causes the tag to transmit, followed by a period of no VLF transmission, with the alarm temporarily disabled. The ability of the tag to respond to the VLF signal and also to respond to the absence of VLF signal are thereby checked.

Finally, the alarm system may be connected to a telephone so that if there is no response from a carer within a given time, a telephone alert arrangement is activated.

Example 2 - Industrial Safetv (Site Evacuation) In the field of industrial safety, in the event of evacuation from any site with more than a few people, it is essential to know where people are. The prime need is to know that there is no-one remaining in an area of danger. Of almost equal importance is knowing which areas are clear (so that safety teams are at no unnecessary risk) and that there is a capability to find anyone who, for whatever reason, is still within an area of danger.

The site is covered with a network of VLF transmitters.

A very large site, or one with large metal partitions, may also require a network of receivers.

Battery life is maximised through use of the dual sensitivity receiver described above. Tags are woken from a very low current state on arrival on site on passing through a portal which generates sufficient signal strength to activate the low sensitivity receiver and which also reads tag identity. Tags thereafter go into a sleep-wake cycle as in Example 1.

At the end of a normal period on site, the tag is returned to the quiescent state on exiting through a portal or by recognising the absence of VLF signal and turning itself off.

In the event of an evacuation, all tags on site are switched to transmit mode. If the sleep-wake cycle has a 30 second period, then all tags will be in transmit mode within 30 seconds of the alarm, with staff already responding to conventional alarms and moving towards muster points. On arrival at muster points, local transmitter-receiver stations identify tags in their vicinity and switch these tags into the 'off' state.

Any tags still transmitting are potentially in an area of danger and the appropriate actions can be taken.

Tags may carry not only their own identity but that of the site, so that activities on one site are unaffected by those on an adjacent site.

The position of any tag still transmitting is immediately resolvable to the area of a specific VLF transmitter, with finer resolution available from signal strength comparisons and/or from direction finding techniques.

An important capability of the system is provision at any time of an inventory of tags on site or at a given location, allowing cross checks on tag status as required.

Exit through portals can be monitored to provide an earlier status than that available from muster points, whose validity depends inter alia on the time staff take to reach the muster point. The problems which occur in most tagging systems in handling the data collisions occasioned by passage through a portal of large numbers of tags in a short time are overcome by the present system, with the opportunity for subsequent cross-check of inventory by location.

Members of safety teams or any other members of staff who may have to be found quickly may have special codes, whether to call in block or to find in case of need, with this feature assigned or removed from individual tags at any time be VLF signal command.

Preferred details of operation of the industrial safety system are given below: 1. Site Arrival Tags awakened unconditionally on passing through portal. Tags transmit ID (identity) to register presence along with a flag which says "been off site".

(Resolves question "is tag coming or going?" Two stage portal to register direction of travel is an alternative). Tags put into sleep/wake cycle, selectively programmes according to priority of individual (first aiders or safety team members have to be located more quickly than the majority of workers).

2. Routine Operation Tags waken, listen for CTx (Control Transmitter) signals, typically go back to sleep. Optionally, tags may be programmed to transmit periodically, whether at command of internal controls or of CTX, so as to report location.

3. Site Departure Tags pass through any exit portal and are commanded to go to sleep until wakened by arrival at an entry portal, or are programmed to go to sleep if an "on site" signal from a CTx is not received an a specific time. (Precludes off site transmissions and conserves batteries).

4. Routine Location and Communication See Example 5 (Location and Communication) 5. Emergency CTx's issue "all tags stay awake on waking" command interleaved with any commands specific to the system.

Tags are read and closed down in Portal Mode as they pass through exits, or in Wide Area Mode as they enter safe zone. (Exploits ability to handle multiple tags in the field and to command set period or "until further notice" close-down). Tags may be addressed uniquely, in blocks globally according to the nature of the site, the nature of the emergency and the distribution of personnel.

6. Muster Point Tags read automatically at muster point by local portal (muster zone also a possibility) or by safety supervisors wand. Optionally, central system may radio to supervisor's unit to say which tags are on site and should be at muster point; unit radios status back to central system.

7. Staff at Risk At this time, any tags still transmitting have not reached their muster point. System provides identities and location of tags not at muster point.

Transmissions from tags on safety team members are appropriately handled, with opportunity for two-way communication if required.

8. Return to Normal Tags returned to normal sleep/wake cycle on passage through entry portal.

Example 3 - Asset Tracking and Protection This Example is presented as a series of elements, which, individually or in combination, are relevant to most Asset Protection situations.

1. Identification (ID) Tags are configured with fixed ID codes, variable codes, or part-variable codes. Variable codes and part-variable codes are altered on authorised command from the control transmitter (CTx), so that structured changes may be made each time a code is read to prevent illicit operations. Identities may be read unconditionally at a portal, or on tag wake-up in Wide Area mode. ID codes, data and commands may be encrypted.

2. Inventories A network of CTx's in an area allows a complete inventory of all tags in the area to be made within a single wake-up period. Data collision is overcome by structured tag reading and command.

3. Location 3.1 CTx transmission includes CTx ID, re-transmitted by tag to announce its location as a function of CTx location.

3.2 Tags contain additional circuitry to assess CTx signal strength (range data) 3.3 DRx's have capability to DF on tag output (bearing data) 4. Movement 4.1 Tags woken unconditionally at portals to detect fact and location of passage 4.2 Tags measure range as in 3.2 above and flag any changes.

4.3 Tags incorporate or are attached to motion detectors triggering alert.

5. Separation Dedicated CTx mounted near asset transmits periodic "stay quiet" command to tag. Separation signalled by tag's failure to receive this command at expected time.

This may be refined as in 3.2 above.

6. Authorised Movement Movement or separation validated by signal from tag on "mover" person or vehicle.

7. Tag Removal or Suppression 7.1 Tags equipped with tamper detecting mount to flag any attempted removal.

7.2 Tags transmit at set intervals. System raises alert if transmission not received on time.

Example 4 - Cargo Management At the Packing Station: Cases, boxes, crates are tagged. Package identities entered in tags, along with date codes, manifests etc.

as required. Data comes either from operator controlled keyboard or automatic data source.

Automatic checks validate data entry. (These procedures exploit capability to address any tag anywhere within 15 metre radius of Control Transmitter without need to locate or approach package).

Each exiting package automatically checked for valid identity, with fact of exit stored.

On Warehouse Entry: Lot arrival recorded automatically at point of entry (exploits capability to command tags to transmit unconditionally when they pass through a portal installation). Mobile Control Transmitter on forklift truck communicates with "fixed" tags permanently installed around warehouse to tell truck its location as it moves around the warehouse; location data entered into package tags from forklift at point of deposit.

System allows forklift drivers to move lots as required with no need to record movement or new location.

At Picking: Required lot addressed by its tag identity. Tag transmits its location to enable immediate finding.

Fact of move automatically recorded at moment of pick up by forklift.

Loading Bay Each case identified as it is deposited on lorry. All tags which should be on lorry addressed sequentially to check that they are on board and that individual manifests correspond to shipment manifest. Command issued "all live tags transmit" any case which should not be on lorry identified.

Leaving Yard: "All tags transmit" command issued. Entire shipment manifest checked - all aboard which should be, nothing aboard which should not be. (Exploits capability to handle multiple tags in the field).

At Delivery Point: Lorry-carried Control Transmitter issues "All tags transmit" command after down-load to check that all packages which should have been unloaded have been so, with none unloaded which should have remained on board.

Example 5 - Personnel Location and Communication Tags in accordance with the invention may be used to provide anything from a periodic update on the location of key personnel to a hierarchical two-way communication system.

Location The site is covered by a matrix of Control Transmitters (CTx's) to allow automatic location of any tag on the site to the area covered by a specific CTx, tags capturing CTx identity and transmitting that along with their own identity. CTx's have a range of 15 metres radius; a matrix of CTx's on a grid of 30 divided by root 2 metres (approximately 21 metres) gives seamless cover. (In some cases, it is sufficient to cover critical work or transit areas). Data with less resolution may be obtained from a matrix of Data Receivers (range 100 metres) on a 140 metre grid.

Refinements: CTx's may be more closely spaced to allow closer resolution; direction finding techniques may be applied to the UHF transmission of the tag to provide bearing information; tags can provide a statement of received low frequency signal strength, which is very closely related to the distance between tag and Control Transmitter. It is also possible to make progressive adjustments to Control Transmitter outputs to achieve the same effect without adding any way to tag complexity.

Communication A telephone call comes in, but the extension does not reply. The operator addresses the appropriate tag, commanding a specific type of response; the tag responds with an automatic location system to the operator (no action required from tag-carrier), plus an audible or visual signal to the tag carrier. In latter cases, the tag-carrier responds with appropriate signal from coded push-button set: "Do not disturb"; "Hold call until I reach extension"; "I will return call"; etc.

The level of sophistication depends on the extent of "keyboard" and display attached to the tag. Two-way priorities may be assigned; by tag-carrier to state level of willingness for interruptions, and by operator to state urgency of incoming call.

Group calls may be issued, eg to a safety team; one operator command instructs all members of team to assembly at muster point.

Application to Location and to Communication exploits the capability to address tags, uniquely or in blocks, irrespective of location on site, to identify location and to conduct interactive communication, coded or explicit.

Example 6 - Wireless Data Gatewavs In many applications, tags in accordance with the invention acquire their data from Control Transmitters, from pre-programmed information, or from the outcome of data processing conducted by the tag itself. On command, whether from a Control Transmitter or from the tag's internal controls, this data or a selected section of it is transmitted to a Data Receiver.

However, the tag is equally suited to taking data from other sources, typically but not necessarily hard-wired into the tag. This data may be accessed by the tag for immediate transmission, or may be stored in the tag itself.

In applications of this kind, power supply capacity is often sufficient to allow tags to be kept constantly awake so that tag data can be accessed immediately on Control Transmitter command, or transmitted as soon as some preset condition is reached. However, if power supply capacity is limited, tags can operate on a fully programmable sleep/wake cycle, with near zero current drain in the sleep state. In this mode, tags accept commands from Control Transmitters as soon as they wake up; if the wake period is very short and there is a possibility of having to queue for a data transmission slot, awakened tags are commanded to stay awake until further notice.Alternatively, tags can be awakened unconditionally by bringing a high level Control Signal to within a metre or so of the tag (as for example, from a hand-held unit carried by a patrolling operator) The tag's intelligence and its ability to accept remote commands allows selection on the fly of which sections of data are to be transmitted.

Tags may also transmit data independent of the influence of a Control Transmitter, commanded either by the tag's internal timing or by the fact of the input data reaching a specific level. (The transmission may include a statement that the tag is indeed out of Controller range). In this case, the tag's 100 metre read-only range may be of great value.

Application to Wireless Data Gateways exploits the dual frequency operation of the tags, the pervasive nature of the Control Signal and the robust technique of signal coding to permit very high standards of transmission integrity, reinforced by opportunity for handshakes, cross-checks, and if required, retransmissions.

While certain modifications of the invention have been described above, the invention is not restricted thereto and other modifications and variations can be adopted without departing from the scope of the invention as defined in the appended Claims.

Claims (14)

1. A tag system comprising one or more tags each attachable to a person or object, at least one VLF (very low frequency) radio transmitter, and at least one UHF (ultra high frequency) radio receiver, said VLF radio transmitter operating to transmit VLF control signals, and the or each said tag including a respective VLF radio receiver and a respective UHF radio transmitter which operates under the control of the VLF radio receiver of that tag to transmit UHF radio signals for reception by said UHF radio receiver(s).

2. A tag system as claimed in claim 1, wherein a said UHF radio receiver provides a relatively large reception zone within which are positioned a plurality of VLF radio transmitters each having a relatively small zone of effective transmission.

3. A tag system as claimed in claim 2 wherein said VLF radio transmitters operate in near-field mode.

4. A tag system as claimed in claim 2 or claim 3 wherein the location of a said tag within said relatively large zone can be deduced from the location(s) of the VLF radio transmitter(s) which cause said tag to transmit.

5. A tag system as claimed in claim 4 wherein in the event that more than one said tag is present within said relatively large zone, each said tag is identified by transmitting a code unique to that tag, or by causing said tag uniquely to transmit in response to a unique code in the VLF control signal.

6. A tag device for use in a tag system as claimed in any preceding claim, said tag device comprising a VLF radio receiver, a UHF radio transmitter, and a control circuit.

7. A tag device as claimed in claim 6 wherein said control circuit is so arranged that the tag device operates in response to VLF control signals when these are received within predetermined time periods, and said tag device operates in response to internal controls when VLF control signals are not so received.

8. A tag device as claimed in claim 6 or claim 7, wherein the VLF radio receiver of said tag device comprises a first, high sensitivity radio receiver circuit and a second, low sensitivity radio receiver circuit, said second radio receiver circuit being permanently energised but said first radio receiver circuit being energised only in response to receipt of a strong signal burst by said second radio receiver circuit.

9. A method of controlling a tag system having at least one control transmitter, at least one control receiver, and a plurality of tags each having a tag receiver and a tag transmitter, said method comprising the steps of transmitting signals from the control transmitter to cause the tags to transmit signals each coded with the identity of the individual tag, scanning the signals received at the control receiver to distinguish tag identity codes, identifying any unambiguous tag identity code and in response thereto transmitting a control signal from the control transmitter causing that identified tag to cease transmission, and repeating this procedure for a predetermined period of time sufficient to achieve statistical probability of all tags having been unambiguously identified.

10. A tag system comprising at least one control transmitter, at least one control receiver, and a plurality of tags, each said tag having a respective tag receiver and a respective tag transmitter, in which each said tag includes a respective signal-responsive means responsive to predetermined signals from the control transmitter to cause that tag to adopt either a quiescent state or an active state.

11. A tag device for use in a radio-frequency tagging system, said tag device comprising a first, high sensitivity receiver circuit, a second, low sensitivity receiver circuit, and control means responsive to receipt of a high intensity transmission by the second receiver circuit to enable the first receiver circuit for a predetermined period of time, or until further command.

12. A tag system substantially as hereinbefore described with reference to the accompanying drawings.

13. A tag device substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

14. A method of controlling a tag system, substantially as hereinbefore described with reference to any one of the Examples.