Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
2001-03-01
2003-06-10
Wu, Daniel J. (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S573100, C340S693900, C340S572800, C340S568100, C340S870030, C340S010100, C342S042000, C342S047000, C342S095000, C342S106000, C342S107000, C342S109000, C342S113000, C342S114000, C342S139000, C342S146000, C342S118000
Reexamination Certificate
active
06577238
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to radio frequency identification (RFID) tagging and, more specifically, to the use of RFID transponders or “tags” in position monitoring and location.
2. Description of Prior Art
Current tag systems detect the movement or presence of a tag as it comes into range of a detector and have been used for access control (eg in the form of personal access cards) or for security control (eg being attached to articles for sale in retail outlets). However, such systems provide only a relatively crude or simple measure of security and rely on passage of the tag past a related detector to monitor movement.
RFID transponders, such as the Supertag™ transponders developed by CSIR & BTG are known for providing automatic data capture and dissemination of ID codes from tagged objects and provide advantages over conventional tagging systems such as bar codes, magnetic stripes and the like, particularly as they utilise so-called anti-collision technology which enables identification of individual tags among groups of tags without requiring singulation of the tags.
Such tags may comprise a single, integrated circuit chip bonded to an antenna which may be a flat, printed antenna, and can be incorporated in labels and the like and attached to objects to provide identification of the objects. The Supertag™ technology employs a reflection tag principle in which radio waves incident on the tag are modulated with the data content of the tag and re-broadcast for a given period of time to the interrogator. Following reading of the tag data, the tag is muted for another predetermined period of time to allow other tags in the range of the interrogator to be identified or counted. Such tags receive their operating energy by means of radiated electromagnetic energy and are known as so-called “passive radio tags” and therefore require no attached power source such as battery. The energy required to operate from RF energy is provided from a suitable “interrogator” or reader, referred to hereafter as a “detector”.
SUMMARY OF THE INVENTION
The present invention recognises that RFID tags can be utilised to provide location information and hence may be used to provide a simple security system for objects.
According to the present invention therefore, there is provided a system for monitoring the position of one or more RFID tags, the system comprising:
one or more detectors incorporating means for receiving signals from the RFID tag for detecting changes in the range of an RFID tag from the detector or detectors; and
control means comparing the signals received from the RFID tag at different times to detect a change in range of the RFID tag and triggering an alarm if a detected change in range exceeds a predetermined threshold.
The detector or detectors preferably measure the time of a returned radio signal from a tag to determine the range of the tag, but, alternatively, the strength of a returned radio signal or its waveform shape may be determined by the detector(s) to determine the range of a tag.
The system may also have means for recording the time of a change in range of a tag and may include plural detectors may also have means for calculating the location of one or more radio tags having individual codes.
The system may include tags which have circuitry arranged to emit short bursts of RF energy at periodic intervals, and the or each detector including circuitry for detecting changes in the periodic interval at which energy is transmitted by the or each tag. The or each detector preferably includes circuitry for predicting the time of receipt of a burst of energy from a tag and for triggering an alarm if the time of actual receipt varies from the predicted time of receipt by more than a predetermined interval and/or if the rate of change in the periodic interval at which energy is transmitted by a tag is outside a predetermined range. Alternatively, or additionally, the detector(s) may include circuitry for analysing changes in the rate of receipt of bursts of energy from a tag and for triggering an alarm if the rate of change is more than a predetermined level.
A system according to the invention may be utilised, for example in the home, for ensuring the security of components such as valuable equipment such as televisions, personal computers and the like, or other valuable items such as paintings, furniture and the like which may be relatively easily stolen by removal from their normal location, movement out of range of a detector being recognised by the central controller and an appropriate alarm signal given.
One example of such a system comprises one or more transmitter-receivers (or detectors) and one or more RFID tags, the transmitter-receiver transmitting radio frequencies coded so that the RFID tag can recognise a point in time at which to respond. In the case of a single transmitter-receiver and a single RFID tag, the time delay between the transmission of the time code and reception by the transmitter-receiver of the signal from the RFID tag is a function of both the time delays in the equipment and the propagation time of the signals travelling to and from the RFID tag. The total time is measured and stored within the transmitter-receiver. It is not necessary to know the value of any of the components of the total time.
This process is repeated and successive times are compared. The difference in times can be due to either changes in the time delay in the equipment or changes in the propagation time of the radio frequency signal. As the time delay in the equipment will change only slowly, time differences will be due to change in the propagation delay, which in turn will be due to changes in distance.
The equipment can thus sense a change in distance between the transmitter-receiver and the RFID tag which can be used to initiate an alarm or for some other purpose.
In the case of a single transmitter-receiver and two or more RFID tags the transmitter-receiver transmits and each RFID tag responds in the way previously described. The signal received by the transmitter-receiver from the RFID tags will have time and amplitude characteristics which are a function of the relative positions of the transmitter-receiver and the RFID tags. The transmitter-receiver captures the time-amplitude ‘picture’ of the returned signal. The process is repeated and successive time-amplitude ‘pictures’ are compared. As previously described the variations will be due to changes in the relative positions of the transmitter-receiver and RFID tags. The equipment can thus sense a change in distance between the transmitter-receiver and by two or more RFID tags which can be used to initiate an alarm or for some other purpose.
However, it is possible that an RFID tag could be moved in an arc without changing the distance between the transmitter-receiver and the RFID tag and without changing the time or time-amplitude ‘picture’ captured by the transmitter-receiver.
This can be circumvented by using two or more transmitter-receivers each operating as previously described with one or more RFID tags, but displaced in such a way that movement of an RFID tag in two or three dimensions can be sensed. The transmitter-receivers operate in a synchronised manner so that changes in relative distance between each transmitter-receiver and the RFID tags can be separately compared.
While the example described above may be operable with relatively small numbers of tags operating in a confined area, it may be advantageous to utilise other methods of range monitoring with larger numbers of tags and areas of operation.
In a further example of a system according to the invention, the tag is arranged to transmit a short burst, or “chirp”, of RF energy at periodic intervals. This is achieved by providing the tag with, for example, an on-board capacitor arranged to store electromagnetic energy received by the tag's antenna from an RF transmitter forming part of a detector in the system. Although the exact frequency of RF energy within a chirp is determined by the ta
Ball Alan Edward
Palmer Timothy John
Whitesmith Howard William
Hancock & Estabrook, LLP
McGuire George R.
Nguyen Tai T.
Tagtec Limited
Wu Daniel J.
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