United States Patent (19)
`Cole
`
`54 PRESENCE AND DATA LABELS
`
`75 Inventor: Peter Harold Cole, South Australia,
`Australia
`
`73 Assignee: Integrated Silicon Design Pty. Ltd.,
`Australia
`21 Appl. No.:
`09/214,313
`22 PCT Filed:
`Jul. 4, 1997
`86 PCT No.:
`PCT/AU97/00428
`S371 Date:
`Jan. 4, 1999
`S 102(e) Date: Jan. 4, 1999
`87 PCT Pub. No.: WO98/01837
`PCT Pub. Date:Jan. 15, 1998
`Foreign Application Priority Data
`30
`Jul. 5, 1996 AU
`Australia ............................... PO 0855
`(51) Int. Cl. ............................................... G08B 13/14
`52 U.S. Cl. ................... 340/572.1; 340/10.4; 340/10.42
`58 Field of Search .............................. 340/572.1, 572.2,
`340/572.7, 10.4, 10.42, 825.37,572.5
`
`
`
`56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`3,755,803 8/1973 Cole et al. ........................... 340/572.1
`3,990.065 11/1976 Purinton et al. ..................... 340/572.1
`
`US006144299A
`Patent Number:
`11
`(45) Date of Patent:
`
`6,144,299
`Nov. 7, 2000
`
`5,099,225 3/1992 Narlow et al. ....................... 340/572.1
`5,218,189 6/1993 Hutchison ...
`... 34.0/10.4
`5,477,210 12/1995 Belcher .......
`... 340/572.1
`5,604,486 2/1997 Lauro et al.
`... 34.0/10.42
`5,721.535 2/1998 Ikefuji .................................... 340/10.4
`FOREIGN PATENT DOCUMENTS
`
`45523/96 8/1996 Australia.
`O 324564 7/1989 European Pat. Off..
`90/O9707 8/1990 WIPO.
`Primary Examiner Daniel J. Wu
`ASSistant Examiner John Tweel, Jr.
`Attorney, Agent, or Firm-Larson & Taylor, PLC
`57
`ABSTRACT
`A System for detecting presence of and for communicating
`with an electronic coded label. The coded label includes a
`presence Signaling antenna, a label circuit and a data com
`munications antenna. The System includes presence detec
`tion means for creating a presence detection electromagnetic
`field and data communications means for creating an inter
`rogation electromagnetic field. The label circuit is adapted to
`operate in a first mode when the electromagnetic field to
`which the label is exposed is at a first power level, said first
`mode being that of a linear circuit with an impedance
`dependent upon frequency. The label circuit is adapted to
`operate in a Second mode when the electromagnetic field to
`which the label is exposed is at a Second power level, Said
`Second mode being that of a non-linear circuit with behavior
`dependent upon time.
`
`52 Claims, 11 Drawing Sheets
`
`LPO
`
`PDI
`
`DATA
`OUT
`
`DCI
`
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`U.S. Patent
`
`Nov. 7, 2000
`
`Sheet 1 of 11
`
`6,144.299
`
`LPO
`
`PDI
`
`FIG 1A
`
`DATA
`OUT
`
`DCI
`
`
`
`DIS
`
`disabling
`signal
`generator
`
`
`
`label disabling plate
`
`
`
`
`
`FIG 1B
`
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`U.S. Patent
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`Nov. 7, 2000
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`Sheet 2 of 11
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`6,144.299
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`FIG 2
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`
`
`FIG 3
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`U.S. Patent
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`Nov. 7, 2000
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`Sheet 3 of 11
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`6,144.299
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`
`
`Z (f)
`
`FIG 4
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`U.S. Patent
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`Nov. 7, 2000
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`Sheet 4 of 11
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`6,144.299
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`23
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`24
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`22
`
`C1
`
`C2
`
`FIG 5
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`
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`U.S. Patent
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`Nov. 7, 2000
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`Sheet 5 of 11
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`6,144.299
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`
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`FIG 7
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`U.S. Patent
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`Nov. 7, 2000
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`Sheet 6 of 11
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`6,144.299
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`23
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`24
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`22
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`A
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`C1
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`C2
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`Ll
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`B
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`FIG 8
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`
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`U.S. Patent
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`Nov. 7, 2000
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`Sheet 7 of 11
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`6,144.299
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`30
`
`VDD
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`PGC
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`NGC
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`VSS
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`
`
`
`
`
`
`
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`OOOOOOO
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`34
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`LPO
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`FIG 11
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`U.S. Patent
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`Nov. 7, 2000
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`Sheet 8 of 11
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`6,144.299
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`digital
`COVeter
`
`microcontroller
`
`
`
`
`
`digital
`ConVerter
`
`base band
`amplifier
`
`balanced
`mixer
`
`LPO
`
`FIG 12
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`U.S. Patent
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`Nov. 7, 2000
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`Sheet 9 of 11
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`6,144.299
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`
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`54
`
`FIG 13
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`U.S. Patent
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`Nov. 7, 2000
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`Sheet 10 of 11
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`6,144.299
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`
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`59
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`58
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`57
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`55
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`56
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`FIG 14
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`U.S. Patent
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`Nov. 7, 2000
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`Sheet 11 of 11
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`6,144.299
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`66
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`SST
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`62
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`63
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`64
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`65
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`balanced
`mixer
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`68
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`controllable
`phase shifter
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`
`
`
`
`
`
`
`
`microcontroller
`
`digital
`COn Veter
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`balanced
`mixer
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`70
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`69
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`71
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`DSO
`
`FIG 15
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`1
`PRESENCE AND DATA LABELS
`
`6,144.299
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`15
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`2
`changed. In this way reading of the label's data may be
`performed to signal to or to confirm that the presence
`Signalling Section of label 2 has been legitimately disabled.
`In the construction of presence detection and data carry
`ing labels, an important design parameter is the physical size
`of the antenna Section of the label, as both presence detec
`tion range and data interrogation range depend upon the size
`of that antenna. It is therefore desirable that the antenna for
`either of these operations be made as large as possible. One
`way in which this may be achieved is to make a single
`antenna connected to appropriate circuit elements fulfill both
`of the presence detection and data communication functions.
`Another approach is the employment of Separate antennas,
`together with ensuring that their size and relative positioning
`is Such as to permit a maximum antenna Size consistent with
`any necessary Separation of the functions of presence detec
`tion and data communication.
`In the achievement of sufficient range for both data
`communication operations and presence detection
`operations, note should be taken of electromagnetic com
`patibility regulations which limit the value of electromag
`netic field which may be legally established for each of these
`two functions. In many jurisdictions a CISPR quasi-peak
`detector is the instrument Specified for making measure
`ments of fields which are to be compared against regula
`tions. Because the response of that measuring instrument
`depends on a complex and in Some aspects non-linear way
`upon both the field amplitude and its time dependence, it is
`appropriate in the achievement of long range for both of the
`functions fulfilled by the label, that the electromagnetic
`Signals which perform these functions, and the response of
`the label to those Signals, should be shaped to achieve the
`largest possible range achievable within the regulations.
`This requirement needs to be taken into account in both the
`design of the label and the design of possibly combined or
`possibly Separate interrogator Systems for functions of the
`label.
`In the practical operation of Systems as described herein
`it may be necessary to disable the presence detection Section
`of a label to indicate that the attached item has been paid for.
`One method by means of which such disabling may be
`carried out is to Subject the presence Signalling Section of the
`label to an intense electromagnetic field at a frequency of
`resonance of that Section So as to produce failure of one of
`the presence Signalling circuit elements. It is normally
`desired that the data communication Section of the labels
`Survive this operation, and the latter Section should therefore
`be designed accordingly.
`In the design of systems which have the capability for
`both presence detection and data transmission, it is neces
`Sary to take into account the frequencies normally used for
`exploitation of these functions. In respect of presence
`detection, resonance of the label within the frequency band
`8.0-8.4 MHZ is frequently employed, but other frequencies
`Seem to be equally useful under the regulations, although
`Spectral occupancy by other users may be a relevant con
`sideration. For data communication, the frequencies 27.12
`MHZ, 13.56 MHZ and 6.78 MHZ are useful in that electro
`magnetic compatibility regulations allow, within a narrow
`band Surrounding these frequencies, transmissions of Sig
`nificantly greater amplitude (by 20 dB and Sometimes by a
`much greater amount) than in the immediately adjacent
`bands. The greatest range can be achieved if it can be
`arranged that the label antenna in the data communication
`mode of the label is resonant at one or other of these
`frequencies.
`SUMMARY OF THE INVENTION
`According to one aspect of the present invention there is
`provided a System for detecting presence of and for com
`
`25
`
`BACKGROUND OF THE INVENTION
`The present invention relates to a System for both detec
`tion of the presence of, and for remote identification of or
`telemetry of data to or from, objects using electronically
`interrogatable coded labels. In particular the invention
`relates to a System for automated detection of the presence
`of labels attached to merchandise, and automated input to or
`extraction of data from those labels.
`An electronic label, the presence of which may be
`detected by field disturbance principles without the particu
`lar label being then identified, but which is also capable of
`carrying electronically coded data, may be attached to an
`article of merchandise.
`An electronic System called a presence detection interro
`gator and containing an antenna which creates an electro
`magnetic field in an exit region from a shop is able to detect
`the occurrence of an article tagged with one of the labels
`passing through the field. When the presence of the label is
`So detected an alarm is raised, as it is presumed that the
`article has not been paid for. However, if the article has been
`properly paid for, the label can at the point of Sale be
`removed or have its presence indicating function disabled,
`So that no alarm is raised when that label passes through a
`Scanned area.
`AS well as having a capacity to Signal its presence through
`field disturbance, the label may be electronically encoded
`with data which may be read either by presence detection
`means or data communications means. The presence detec
`tion means may include a presence detection Signal
`generator, a presence detection antenna for creating a pres
`ence detection electromagnetic field, and an analyser of
`impedance associated with the presence detection antenna.
`The data communications means may include a generator of
`an interrogation signal, an interrogation antenna for creating
`an interrogation electromagnetic field, a data receiver
`antenna, and a receiver and decoder of data communication
`Signals. The Signals received and decoded by the receiver
`and decoder may be presented as an output signal to a data
`processing System.
`Although the present invention is herein described with
`reference to a merchandising operation, it is to be appreci
`ated that it is not thereby limited to Such application. Thus
`the presence and data label can be applied to object identi
`fication operations generally, and has obvious uses in librar
`ies and in document control environments.
`A simplified diagram of the system is shown in FIG. 1A
`wherein a labelled item 1 carries an electronic label 2, the
`50
`presence of which (assuming that function is not disabled)
`can be detected by a presence Sensing antenna 3 connected
`to a label presence detector 4 and providing, when a label is
`detected, a label presence output signal LPO. Data which is
`present within label 2 may be extracted therefrom by a data
`interrogation antenna (DIA) 5 which is connected to a data
`communications interrogator (DCI) 6. Referring to FIG. 1B
`when it is desired that label 2 should produce no signal from
`presence detector 4 (ie. item 1 has been paid for), label 2 is
`placed over a disabling plate 7 which upon receipt of a
`disabling input Signal DIS to a disabling Signal generator 8,
`generates a disabling electromagnetic field. The latter
`changes the nature of a circuit contained within label 2 in a
`manner to be described herein.
`The disabling System may also contain means to change
`or to read the data within label 2 and also to provide a
`programming Signal to label 2 So that the data may be
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`3
`municating with one or more electronic coded labels, the or
`each coded label including a presence Signalling antenna, a
`label circuit and a data communications antenna, Said SyS
`tem including:
`presence detection means for creating a presence detec
`tion electromagnetic field;
`data communications means for creating an interrogation
`electromagnetic field;
`wherein the or each label circuit is adapted to operate in
`a first mode when the electromagnetic field to which
`the associated label is exposed is at a first power level,
`Said first mode being that of a linear circuit with an
`impedance dependent upon frequency, and the or each
`label circuit is adapted to operate in a Second mode
`when the electromagnetic field to which the associated
`label is exposed is at a Second power level, Said Second
`mode being that of a non-linear circuit with behaviour
`dependent upon time.
`According to a further aspect of the present invention
`there is provided an electronic coded label for use with a
`System for detecting presence thereof and for communicat
`ing therewith, Said label including a presence Signalling
`antenna, a label circuit and a data communications antenna,
`wherein Said label circuit is adapted to operate in a first
`mode when an electromagnetic field to which Said label is
`exposed by Said System is at a first power level, Said first
`mode being that of a linear circuit with an impedance
`dependent upon frequency, and Said label circuit is adapted
`to operate in a Second mode when the electromagnetic field
`to which said label is exposed by Said System is at a Second
`power level, Said Second mode being that of a non-linear
`circuit with behaviour dependent upon time.
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT
`Preferred embodiments of the invention will now be
`described with reference to the accompanying drawings
`wherein:
`FIG. 1A shows major Sub units of a System performing
`both presence detection and data communication to and
`from an electronic label;
`FIG. 1B shows a system for disabling the presence
`Signalling Section of a label;
`FIG. 2 is a block diagram of one variety of presence
`detecting and data communicating label;
`FIG. 3 is a circuit diagram showing Significant elements
`of a data communicating microcircuit;
`FIG. 4 shows resonant curves of circuits within a presence
`detecting and data communicating label;
`FIG. 5 is a circuit diagram showing a different form of
`presence detecting and data communicating label;
`FIG. 6 shows an arrangement of antennae in a presence
`detecting and data communicating label;
`FIG. 7 shows resonant curves in another form of presence
`detecting and data communicating label;
`FIG. 8 shows circuit elements useful in another form of
`presence detecting and data communication label;
`FIG. 9 shows an arrangement of printed windings useful
`in the label of FIG.8;
`FIG. 10 shows a block diagram of a form of controller and
`modulator circuit;
`FIG. 11 shows a relationship between a label and a
`presence detector for presence detection;
`FIG. 12 shows functional blocks present in one form of a
`presence detector;
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`FIG. 13 shows an antenna configuration useful for label
`presence detection;
`FIG. 14 shows an antenna configuration useful for data
`communication with a label; and
`FIG. 15 shows functional blocks present in one form of
`data communication interrogator.
`In the system of which an outline is shown in FIG. 1A,
`labelled item 1 carries presence and data label 2 which is
`capable of modifying the field created by presence Sensing
`antenna 3. The latter may receive signals from and may
`transmit Signals to presence detector 4. Presence of label 2
`is indicated by means of a high quality factor resonant circuit
`which, when it is exposed to the field of presence Sensing
`antenna 3, produces a frequency Sensitive modification of
`the impedance of antenna 3. The change in impedance of
`antenna 3 is Signalled by presence detector 4, which explores
`over a frequency range within which label resonance is
`expected to occur, the impedance of presence Sensing
`antenna 3.
`When an impedance change in presence Sensing antenna
`3 greater than a threshold value is observed, a label presence
`output signal LPO emerges from presence detector 4.
`AS an alternative to detecting the changed impedance of
`a single antenna, label presence may be Sensed by detecting
`over a narrow frequency band, a change in coupling between
`a plurality of antennae or antenna parts forming with the
`presence detector 4 the presence Sensing System. In Such a
`case the antennae or antenna parts in the presence Sensing
`System may be arranged to be Substantially uncoupled in the
`absence of a label, and the label may provide Some fre
`quency Sensitive coupling therebetween.
`As indicated in FIG. 1A label 2 has, as well as the capacity
`of Signalling its presence through disturbance of the field
`created by presence Sensing antenna 3, a capacity to
`communicate, over an electromagnetic coupling link
`between label 2 and a data interrogator antenna (DIA) 5,
`with a data communications interrogator (DCI) 6. Label 2
`may contain no energy Source and receive energy for opera
`tion of its data communication function from the electro
`magnetic field created by DIA5. AS well as providing energy
`to label 2, the signal from DIA5 may contain information
`which may be programmed into label 2. Label 2 may also
`emit a reply signal which may be received by DCI 6 via DIA
`5.
`As is indicated in FIG. 1B, the system may also contain
`label presence disabling plate 7 which receives a label
`disabling Signal from disabling Signal generator 8, the later
`Signal being generated when disabling Signal generator 8
`receives a disabling input signal DIS. For the label disabling
`operation to occur, label 2 is placed close to label disabling
`plate 7 which generates a label disabling electromagnetic
`field. The label disabling field may take the form of a strong
`electromagnetic field at the resonance frequency of the tuned
`circuit which provides the label presence Signalling func
`tion.
`At approximately the same time as the label disabling
`field is generated, the data Section of label 2 may receive via
`a further electromagnetic field, Similar to that generated by
`DIA5, a signal, which may be coded, indicating that label
`disabling is to be, or has been, performed, and the data
`within label 2 may change in response to receipt of this
`Signal.
`A block diagram of a basic form of presence and data
`label2 is shown in FIG. 2. Presence and data label 2 contains
`a tuned circuit including inductor 9, a first capacitor 10, a
`Second (optional) capacitor 11 and a data communications
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`microcircuit (DCM) 12 which is connected at terminals A
`and B to the tuned circuit provided by inductor 9 and
`capacitors 10 and 11. The input impedance of DCM 12 is
`under certain conditions, expected to be reasonably repre
`Sented by a capacitance in parallel with a resistance, but the
`validity of this representation and the values of both of these
`elements depend upon the Signal level generated within the
`overall tuned circuit, and upon operations which take place
`within DCM 12.
`Some internal circuits of one preferred embodiment of
`DCM 12 are shown in FIG. 3. DCM 12 may contain a
`capacitor 13 which makes a contribution to establishing a
`resonant frequency of the circuit including inductor 9 at both
`low and high power. Abridge rectifier circuit 14 can provide
`supply voltages VDD and VSS to a controller and modulator
`(CAM) circuit 15.
`Signals PGC and NGC emerge from CAM circuit 15 to
`control transmission gate 16 which when closed introduces,
`in Series, capacitors 17 and 18. The latter are placed acroSS
`the input terminals marked A and B and therefore modify the
`resonance frequency of the tuned circuit including inductor
`9. Resistors 19 and 20, which may be merely representations
`of the on-resistance of the transmission gate 16, have the
`effect that the quality factor of the aforementioned resonance
`is reduced at the same time as the frequency is lowered.
`Capacitor 21 is simply a reservoir capacitor for bridge
`rectifier 14.
`It will be clear from the above description that label 2 acts
`as a linear frequency dependent device in a presence detec
`tion mode, and acts as a nonlinear time dependent device in
`a data communications mode. The factors which cause it to
`change its mode of operation may include a change of
`frequency or a change of amplitude of the electromagnetic
`field to which label 2 is exposed. In both cases it is the
`amplitude of the Signals internal to label 2, in relation to
`threshold Voltages within Semiconductor junctions or in field
`effect transistors, which determine in which mode label 2
`operates from time to time.
`Anotable feature of the invention is that both the presence
`detection signal and the data communications signal cause
`currents to flow in the entire label circuit, but the amount of
`the current, and its distribution in amplitude within the
`various parts of the label circuit, are each dependent upon
`both the frequency and the power level of the excitation
`electromagnetic field.
`Many of the design features and operations of CAM
`45
`circuit 15, including its capacity to receive programming
`information through modulation of the radio frequency
`Signal which excites the label; its capacity to Store in
`non-volatile memory that information and when appropriate
`to produce modulation of transmission gate 16 in the form
`of binary frequency shift coding of a Sub-carrier frequency
`generated from an oscillator within the controller and modu
`lator section; are described in applicant’s PCT AU/92
`000143 and PCTAU/92 000477 the disclosures of which are
`incorporated herein by croSS reference.
`Adding to those ideas, CAM circuit 15 may incorporate in
`addition a time delay, the function of which is to be
`described herein, between the application of the VDD and
`VSS Supply Voltages and the occurrence of any modulation
`via transmission gate 16, Prior to the occurrence of Such
`modulation, transmission gate 16 is constrained to remain
`Open.
`The effect of operation of DCM 12 on the resonance
`curve-of the tuned circuit-it including inductor 9 are shown
`in FIG. 4 wherein the magnitude of the impedance of that
`tuned circuit is plotted against frequency for various condi
`tions.
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`The narrow resonance curve A of large amplitude and
`small bandwidth represents the condition when the tuned
`circuit is operating at a power level too low for the devel
`opment of any significant rectified Voltage via bridge recti
`fier 14, and also approximately the condition when a Voltage
`sufficient to operate CAM circuit 15 has been developed, but
`a Sufficient time delay has not elapsed for transmission gate
`16 to have closed or modulation to have commenced. In
`consequence, Significant loading of the tuned circuit includ
`ing inductor 9 may not yet have occurred, and the effective
`input capacitance of bridge rectifier 14 may remain approxi
`mately at the value it has for small signals. Thus both the
`high quality factor and the resonant frequency of the reso
`nance including inductor 9 will be maintained approxi
`mately at their Small signal values, and that resonance
`continues to provide the previously discussed field distur
`bance of the impedance of the presence Sensing antenna 3
`upon which label presence detection is based.
`Also shown in FIG. 4 are resonance curves B and C of
`lesser amplitude and lower resonance frequency. Curves B
`and C Show the conditions obtained when a higher ampli
`tude and longer duration excitation signal is applied So that
`supply voltages VDD and VSS, sufficient for normal opera
`tion of CAM circuit 15, are developed, but for which the
`time delay which may be built into that circuit has elapsed
`So that modulation of transmission gate 16 has commenced.
`Curve B having the larger amplitude and higher resonant
`frequency represents the condition when transmission gate
`16 is open, and curve C having the lower amplitude and
`lower resonance frequency represents the condition when
`transmission gate 16 is closed.
`In the presence detection mode of label2, it is appropriate
`that it be exposed to a field which explores, over the
`frequency range F., to F, shown in FIG. 4, the nature of
`non-operating resonance curve A. AS will be described
`herein, this exploration may preferably be via a Signal which
`is Swept rapidly at an appropriately controlled rate over that
`band.
`In one preferred embodiment, for the data communication
`mode of operation of label 2, the label may be interrogated
`by a Substantially continuous wave Signal at a data interro
`gation frequency F positioned as shown in FIG. 4. During
`that interrogation, the tuned circuit resonance Switches
`between the two curves B and C shown in FIG. 4, with the
`result that the signal amplitude in inductor 9 varies, in both
`amplitude and phase, between two values which are defined
`by the operating points X and Y shown in FIG. 4. The
`amplitude and phase varying oscillation appears as a reply
`Signal Sub-carrier generated by modulation of the interro
`gation and energising Signal at the frequency F, and
`appears as Sidebands of the data interrogation frequency.
`The actual reply signal is modulated on to that Sub-carrier,
`as the result of CAM circuit 15 varying the pattern with
`which transmission gate 16 opens and closes.
`In another preferred embodiment the positions of reso
`nance curves B and C for the Switch open and the Switch
`closed positions are both shifted to the right but by different
`amounts So that points X and Y have the same amplitude.
`The benefit of this arrangement is that the amplitude of
`excitation of the data communications circuit is similar for
`both the Switch open and Switch closed positions. Because
`of the different phases, relative to the excitation Signal in the
`data interrogator, of the current in inductor 9 for the two
`conditions, a reply electromagnetic field is still generated.
`The preferred embodiment of the invention described so
`far has the benefit that a single inductor antenna 9 is used for
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`7
`both the presence detection and data communication func
`tion of the label, and that the antenna may conveniently be
`shaped to occupy almost all of the area of the label, with the
`result that the sensitivity of the label to both the presence
`detecting field and the data communication field is maxi
`mised.
`In this embodiment a number of techniques are available
`for changing the nature of the label to disable, if desired, its
`presence detection function. In a preferred embodiment this
`can be accomplished by the removal of capacitor 11, poS
`sibly by a punching operation, with capacitors 10, and 11,
`and the input capacitance of DCM 12 being proportioned So
`that the result of removal of capacitor 11 is a substantial
`rightward shift of the three resonance curves A, B and C
`shown in FIG. 4.
`The result of this shift may be that the high quality factor
`resonance which is observed at low power and prior to the
`operation of the time delay built into DCM 12 is shifted well
`to the right of the frequency Span over which the impedance
`characteristics of the label are explored in the presence
`detection mode. Although resonance curves B and C char
`acteristic of label operation in the data communications
`mode, will also be shifted to the right, they may still be
`wellpositioned in relation to the data communication inter
`rogation frequency F, perhaps So that the point of inter
`Section of those curves takes place at the data communica
`tions interrogation frequency F, So that good extraction of
`power from the communication field, and Substantial modu
`lation of the oscillation as transmission gate 16 opens and
`closes, if not in amplitude, then at least in phase, can occur.
`Two-way data communication to and from DCM 12 is still
`then eminently possible without undue loSS of Sensitivity.
`Although these resonance curves may then lie within the
`frequency range explored by the presence detector, the
`amplitude and quality factor of both those resonances will be
`too low to activate the presence Sensing mechanism pro
`Vided by presence Sensing antenna 3 and presence detector
`4.
`In an alternative embodiment of the invention which
`allows deactivation of the presence detecting mechanism to
`be safely accomplished by non-contact means is shown in
`FIG. 5. In this embodiment, presence detection is accom
`plished through establishment of a high quality factor reso
`nance at an appropriate frequency between inductor 22 and
`capacitorS 23 and 24, while data communications operations
`are accomplished at generally different but appropriate fre
`quency through the action of inductor 25, capacitor 26 and
`DCM 12. So that large amplitude signals may be applied to
`one circuit without introducing excessive amplitude Signals
`in the other, the mutual inductance M between inductors 22
`and 25 is kept Small, and may be set to Zero with an
`appropriate design. An arrangement of conductors which
`shows the relationship between a two-turn planar winding
`for inductor 22 and a Single-turn planar winding for inductor
`25, in which arrangement the Overlap may be adjusted to
`achieve Zero mutual inductance, is shown in FIG. 6. Node
`labels C, D, E and F in FIG. 5 correspond to similar labels
`in FIG. 6.
`In this embodiment capacitor 24 shown in FIG. 5 can be
`made with thin dielectric having a low breakdown Voltage,
`while capacitor 23 is a low loSS capacitor which is relatively
`robust and is not Subject to Such breakdown. Capacitor 24
`Subject to breakdown may be made Significantly Smaller
`than capacitor 23, So that different manufacturing tolerances
`may be applied to each in case the manufacture of a
`capacitor with low breakdown Voltage accidentally or inci
`dentally involves a greater variation in capacitance value.
`
`45
`
`50
`
`55
`
`60
`
`65
`
`8
`In this embodiment, disabling the presence detection
`function may be achieved by applying through label dis
`abling plate 7 a large oscillating magnetic field at the
`resonance frequency formed by inductor 22 and capacitors
`23 and 24, with the result that voltages which exceed the
`breakdown voltage of capacitor 24 are generated, and
`capacitor 24 breaks down to become a short circuit. The
`result is the resonance of that tuned circuit is extinguished
`despite the intact nature of inductor 22 and capacitor 23.
`In another embodiment capacitors 23 and 24 shown in
`FIG. 8 might be in series. In that embodiment disabling of
`the presence detection function can be achieved, while
`leaving the data communications function intact, albeit at a
`possibly different operating frequency, by placing a short
`circuit in parallel with one of the capacitorS 23, 24. It is also
`possible to disable the presence indicating function by the
`introduction of an open circuit within a part of the presence
`Signalling resonant circuit.
`In the disabling operation it is important that damaging
`voltages are not placed across DCM 12. This may be
`accomplished by firstly insuring that mutual inductance
`between inductors 22 and 25 is small, and secondly that the
`resonant frequency of inductor 25 and the resulting capaci
`tance across terminals E and F is substantially different from
`the resonant frequency of the presence detection Section of
`the label.
`A Substantial Separation of the relevant frequencies and
`the resonant curves applying to the circuit of FIG. 5 is shown
`in FIG. 7. The presence detection circuit has a resonant
`frequency shown as F associated with curve A in FIG. 7,
`and presence detection is carried out within the frequency
`band F, to F. The data interrogation frequency is shown
`as F and may for example be 13.5 MHZ, whereas presence
`detection might be carried out in the vicinity of 8.2 MHz.
`The Switch-open and Switch-closed resonance curves
`formed by inductor 25 and capacitors connected thereto are
`marked B and C respectively in FIG. 7. It may be
`ascertained, by Studying these resonance curves in relation
`to the data interrogation frequency, that good transfer of
`power to the label from the data interrogation signal may be
`achieved at both open and closed positions of the Switch
`within DCM 12, and that there will be significant phase shift
`in the oscillation produced at inductor 25 between the
`Switch-open and Switch-closed positions, although there
`may be little or no amplitude modulation. Thus good power
`transfer to t