Ulllted States Patent
`
`[19]
`
`[11] Patent Number:
`
`5,883,582
`
`Bowers et al.
`
`[45] Date of Patent:
`
`Mar. 16, 1999
`
`US005883582A
`
`[54] ANTICOLLISION PROTOCOL FOR
`READING MULTIPLE RFID TAGS
`
`[75]
`
`Inventors: John H. Bowers, Clarksburg, N.J.;
`John Nelson Daigle, University, Miss.;
`Rade Petrovic, Woburn, Mass.
`
`[73] Assignee: Checkpoint Systems, Inc., Thorofare,
`Ni
`
`[2]] App]. No: 795,545
`[22]
`Filed:
`Feb. 7, 1997
`
`5,383,134
`5,430,441
`5,444,223
`
`5:446/I47
`5,448,230
`5,530,437
`5,530,702
`5,539,394
`5,550,547
`5,591,951
`5,627,544
`§,72g,630
`,74 ,036
`
`1/1995 Wrzesinski.
`7/1995 Bickley et al.
`8/1995 Blama.
`
`.
`
`.
`
`8/1995 Carney 6‘ a1~ ~
`9/1995 Schanker et al.
`6/1996 Goldberg .
`6/1996 Palmer et al.
`7/1996 Cato et al.
`.
`8/1996 Chan et al.
`1/1997 Doty.
`5/1997 Snodgrass et al.
`.
`3/1998 Mlarsh et al.
`....................... 340/822?
`4/1998 Care ................................ .. 340/82 . 4
`
`.
`
`.
`
`Int. Cl.5 .............................. H04Q 5/22, G08B 13/14
`[51]
`[52] U.s. Cl.
`....................................... 340/825.54; 340/572
`[58] Field of Search ............................ 340/825.54, 825.5,
`340/572, 505, 370/445, 447, 235/375
`_
`References Cited
`
`[56]
`
`Primary Exami{4er—Brian Zimmerman
`ASSISHW EWm"W—EdWard M641
`fiI”Z”1“e1y;CAge”’» 0’ F"’"—Pa““°h Schwarze Jacobs &
`3 e> ~
`~
`[57]
`
`ABSTRACT
`
`4,734,680
`4,799,059
`4,862,160
`5:103:30
`5,182,543
`5,189,397
`5,266,925
`5,276,431
`5,347,263
`
`,
`
`3/1988 Gehman er al.
`.
`1/1989 Grindahl et al.
`8/1989 Ekchian ........................... ..
`4/1992 Rode 6‘ 31-
`-
`.
`1/1993 Siegel et al.
`,
`.
`2/1993 Watkins et al.
`11/1993 Vercellotti et al.
`1/1994 Piccoli et al.
`.
`9/1994 Carroll et al.
`.
`
`.............. .. 340/825.54
`
`from the tags with large, non-transmission intervals between
`transmissions. The non-transmission intervals are fixed for a
`given tag, but are randgm between tags due [0 Inanufactul-_
`ing tolerances in electrical components from which the tag
`is constructed, such that no coordination of transmissions
`from the interrogating antenna is required.
`
`31 Claims, 6 Drawing Sheets
`
`T A G
`
`
`
`ANTEN NA
`ASSEMBLY
`
`
`
`TRANSMITTER
`
`RECEIVER
`
`24
`
`28
`
`
`
`
`
`
`
`
`
`DATA
`
`PROCESSING
`
`AND
`CONTROL
`
`30
`
`TIME
`CLOCK
`
`
`
`OUTPUT SIGNAL
`
`
`
`SCHRADER
`
`EXH. 1005
`
`Page 1005-1
`
`Page 1005-1
`
`

`
`U.S. Patent
`
`Mar. 16, 1999
`
`Sheet 1 of6
`
`5,883,582
`
`I9
`
`MOD TIMER
`
`GND MEMORY
`
`I8
`
`TAG
`
`ANTEN NA
`ASSEMBLY
`
`22
`
`24
`
`TRANSMITTER
`
`RECEIVER
`
` 26
` 28
`
`30
`
`TIME
`CLOCK
`
`
`
`
`
`OUTPUT SIGNAL
`
`
`
`FIG. 2
`
`Page 1005-2
`
`
`
`
`
`PROCESSING
`AND
`CONTROL
`
`DATA
`
`Page 1005-2
`
`

`
`U.S. Patent
`
`Mar. 16, 1999
`
`Sheet 2 of6
`
`5,883,582
`
`DATA STREAM
`
`DATA STREAM
`
`ourpur
`
`EMORY DATAA-‘
`-MEMORY DATA——‘-GA
`DATA
`FIG. 3:
`
`32
`
`34
`
`DATA STREAM
`
`DATA STREAM
`
`OUTPUT V
`
`DATA-[—~GAP
`DA fl >T*< S
`
`36-
`
`38
`
`38
`
`36
`
`FIG. 3b
`
`36
`
`Page 1005-3
`
`Page 1005-3
`
`

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`
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`
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`
`Page 1005-4
`
`

`
`U.S. Patent
`
`Mar. 16, 1999
`
`Sheet 4 of 6
`
`5,883,582
`
`Tag I
`
`Tx
`
`wmr
`
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`
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`
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`
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`
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`
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`
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`
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`
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`
`Page 1005-5
`
`Page 1005-5
`
`

`
`
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`
`8
`
`Page 1005-6
`
`
`

`
`U.S. Patent
`
`Mar. 16, 1999
`
`Sheet 6 of6
`
`5,883,582
`
`09
`
`8b
`
`9??
`
`M7
`
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`
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`
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`
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`
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`
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`
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`
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`
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`
`Page 1005-7
`
`ProbabilityofReadingTagsin1Second
`
`098
`
`096
`
`0.94
`
`092
`
`0‘.
`0
`
`0.88
`
`0.86
`
`084
`
`0.82
`
`S601 fiuspoaa :0 Kuuqoqold‘
`
`Page 1005-7
`
`

`
`5,883,582
`
`1
`ANTICOLLISION PROTOCOL FOR
`READING MULTIPLE RFID TAGS
`
`BACKGROUND OF THE INVENTION
`
`Tagging of articles for identification and/or theft protec-
`tion is known. For instance, many articles are identified
`using a bar code comprising coded information which is
`read by passing the bar code within view of a scanner. Many
`articles also include a resonant tag for use in theft detection
`and prevention. More recently, passive resonant security
`tags which return unique or semi-unique identification codes
`have been developed. These security tags typically include
`an integrated circuit which stores the identification code.
`Such “intelligent” security tags provide information about
`an article to which the tag is affixed which is detected in the
`zone of an interrogator. The tags are desirable because they
`can be interrogated rapidly, and from a distance. U.S. Pat.
`No. 5,446,447 (Carney et al.), U.S. Pat. No. 5,430,441
`(Bickley et al.), and U.S. Pat. No. 5,347,263 (Carroll et al.)
`disclose three examples of intelligent tags.
`Intelligent tagging of articles provides substantial benefits
`at the point of manufacture, at the point of distribution, and
`at the point of sale. That is, any place where articles are
`stored, shelved, displayed or inventoried, intelligent tags can
`result in substantial cost savings. For example, one function
`of a distribution center is to take merchandise that has been
`
`packed and shipped in bulk, and repack the merchandise into
`smaller “tote” boxes. Often the tote box is packed with
`single units of a variety of products. Mistakes in inventory
`during this repacking process can be very costly and there is
`a possibility of shipping products to the wrong retailer. An
`intelligent tagging system can check the contents of tote
`boxes with an interrogator or point reader at high speeds and
`confirm exactly what is being shipped to individual retailers.
`Employees today spend many hours hand counting
`articles for inventory control and manually checking product
`expiration dates. Intelligent tags obviate the need for such
`hand counting and manual data checking. Rather than hand
`counting a plurality of items, an employee can point an
`intelligent
`tag reader at
`individual product clusters on
`shelves and scan entire product groups in minutes. Intelli-
`gent tags also allow employees to scan a product group to
`learn critical expiration dates to avoid spoilage, reduce stock
`and maintain continuous inventory counts.
`Another example of an environment in which the use of
`intelligent tags is desirable is a library. Manual taking of
`inventory of a library collection is an expensive and time
`consuming task. Currently inventory taking is such an
`expensive and time consuming task that most libraries do
`not conduct a full inventory check as frequently as they
`should, if at all. Accordingly, there is a need for systems
`which allow library employees to efficiently check their
`inventory. Intelligent tags fulfill such a need.
`One problem with attempting to read multiple RFID tags
`within an interrogation zone of a reader is that more than one
`tag could be activated by the reader or interrogator at about
`the same time, such that two or more tags may transmit their
`identification information to the reader at about the same
`
`time, thus causing the information to collide, which corrupts
`the information and prevents the reader from obtaining the
`desired information.
`In order to overcome such data
`
`collisions, some interrogators include a means for control-
`ling the transmission of data from individual
`tags,
`for
`example, by shutting individual tags off for predetermined
`time periods after a response signal is transmitted. However,
`the transmission of a signal by the interrogator to an indi-
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`vidual tag to shut the tag off may require the generation of
`signals in excess of the levels allowed by regulatory
`authorities, such as the Federal Communications Commis-
`sion (FCC). Other systems include tags which include
`circuitry to detect the simultaneous transmission of data by
`multiple tags. Upon detection of such simultaneous
`transmissions, the tags abort their transmissions and wait for
`a prescribed time prior to retransmission, usually for a
`period of time that is set by a random number. However, this
`method requires the tags include detection circuitry and a
`battery, both of which excessively increase the cost of the
`tag. Accordingly, there is a need for a method of detecting
`substantially simultaneous transmission of data by multiple
`tags at the same frequency located within an interrogation
`zone and compensating for such multiple transmissions in
`order to accurately read the data transmitted by each tag.
`The present
`invention provides a method of simulta-
`neously reading multiple RFID tags located in a field of an
`interrogating antenna based on periodic transmissions from
`the tags with long non-transmission intervals between trans-
`missions. The non-transmission intervals are fixed for a
`
`given tag, but are random between tags, preferably due to
`manufacturing tolerances, such that no co-ordination of
`transmissions from the interrogating antenna is required.
`BRIEF SUMMARY OF THE INVENTION
`
`Briefly stated, the present invention comprises a method
`of reading data from a plurality of radio frequency intelli-
`gent devices located within an interrogation zone using a
`one sided protocol, with the devices never being turned off.
`In a first step, an interrogation device transmits a continuous
`interrogation signal. The interrogation signal comprises an
`electromagnetic field at
`a first predetermined radio
`frequency, wherein a strength of the electromagnetic field
`defines the interrogation zone. In a second step, a plurality
`of radio frequency intelligent devices located within the
`interrogation zone are acted upon by the electromagnetic
`field. The electromagnetic field induces a voltage in each
`intelligent device which provides power to the intelligent
`devices.
`
`In response to being powered by the induced voltage, each
`of the plurality of intelligent devices within the interrogation
`zone reads a respective prestored data field and repeatedly
`transmits a message stored therein at a second predeter-
`mined radio frequency at predetermined periodic intervals
`with a fixed length nontransmission interval between each
`transmission interval. A length of the non-transmission
`interval
`is much greater than a length of the message
`transmission interval. The interrogation device reads the
`message transmissions of each of the plurality of intelligent
`devices. A probability of two or more of the plurality of
`intelligent devices transmitting their respective messages
`simultaneously (i.e. having overlapping transmission
`intervals) is reduced due to variations among the intelligent
`devices in the fixed non-transmission time and by making
`the length of the non-transmission interval much greater
`than the length of the message transmission interval.
`The present invention also provides a radio frequency
`intelligent device comprising:
`an integrated circuit for storing data;
`an antenna connected to the integrated circuit, wherein
`exposure of the antenna to an electromagnetic field at
`a first predetermined radio frequency induces a voltage
`therein which provides power to the integrated circuit
`such that
`the data stored therein is read from the
`
`integrated circuit and repeatedly transmitted at a second
`predetermined radio frequency;
`
`Page 1005-8
`
`Page 1005-8
`
`

`
`5,883,582
`
`3
`a predetermined transmission period for repeatedly trans-
`mitting the integrated circuit data at the second prede-
`termined resonant frequency; and
`a fixed wait period between each predetermined transmis-
`sion period, wherein the wait period is much greater
`than the transmission period.
`The present invention is also a radio frequency identifi-
`cation device comprising:
`an integrated circuit for storing data;
`an antenna connected to the integrated circuit, the antenna
`comprising an inductor and a capacitor, wherein expo-
`sure of the antenna to an electromagnetic field at a first
`predetermined radio frequency induces a voltage in the
`inductor which provides power to the integrated circuit
`such that the data stored therein is read and provides a
`continuous data output signal;
`a transmitter for repeatedly transmitting the data output
`signal at a second predetermined radio frequency; and
`a timer for establishing a fixed non-transmission period
`between each data transmission period, wherein a
`length of the non-transmission period is much greater
`than a length of the transmission period.
`
`BRIEF DESCRIPTION OF THE SEVERAL
`VIEWS OF THE DRAWINGS
`
`The foregoing summary, as well as the following detailed
`description of a preferred embodiment of the invention, will
`be better understood when read in conjunction with the
`appended drawings. For
`the purpose of illustrating the
`invention, there is shown in the drawings an embodiment
`which is presently preferred, it being understood, however,
`that the invention is not limited to the precise arrangement
`and instrumentalities disclosed. In the drawings:
`FIG. 1 is a schematic diagram of an equivalent electrical
`circuit of a resonant frequency identification (RFID) device
`in accordance with a preferred embodiment of the present
`invention;
`FIG. 2 is a schematic block diagram of an interrogator and
`an RFID tag in accordance with the present invention;
`FIG. 3a is a timing diagram of a protocol for transmitting
`data from the RFID device;
`FIG. 3b is a timing diagram of a preferred protocol for
`transmitting data from the RFID device;
`FIG. 4a is a timing diagram of a plurality of tags each
`outputting a data signal in response to an interrogation signal
`according to a preferred embodiment of the present inven-
`tion;
`FIG. 4b is a continuation of the timing diagram of FIG.
`4a;
`FIG. 5a is a graph of the probability of reading a plurality
`of RFID devices as a function of time; and
`FIG. 5b is a graph of the probability of reading a plurality
`of RFID devices within a predetermined time limit.
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`Certain terminology is used in the following description
`for convenience only and is not limiting. The words “top”,
`“bottom”, “lower” and “upper” designate directions in the
`drawings to which reference is made. The terminology
`includes the words above specifically mentioned, derivatives
`thereof and words of similar import.
`The present invention is directed to a method of reading
`multiple RFID tags or intelligent devices simultaneously.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`4
`The method is achieved by providing tags which continu-
`ously transmit their respective identification information for
`as long as the tag is within an interrogation zone. Each data
`transmission is followed by a fixed wait period or non-
`transmission interval. The non-transmission interval is pref-
`erably more than ten times greater than the data transmission
`period. Each tag is constructed such that electrical compo-
`nents within each tag include predetermined manufacturing
`tolerances, such that although the length of the non-
`transmission interval
`is fixed for each tag,
`the non-
`transmission interval varies between tags at least within the
`prescribed tolerances. The variation in the length of non-
`transmission intervals among tags causes the transmission
`intervals among the tags to be skewed, or not to overlap
`when the tag is within the interrogation zone. That is, there
`is a high probability that no two tags will simultaneously
`begin and thereafter continue data transmission at the same
`instant in time (and thus cause a data collision). Further, over
`a period of time, such as a period of time including four
`transmission intervals of a tag, the probability increases that
`the interrogator will successfully receive each tag’s identi-
`fication information. That is, over a period of time including
`several transmission intervals for each tag, an interrogation
`device is able to successfully read each tag’s identification
`information.
`
`Referring now to the drawings, wherein the same refer-
`ence numeral designations are applied to corresponding
`elements throughout the several figures, there is shown in
`FIG. 1 a schematic diagram of an equivalent electrical
`circuit of a resonant frequency identification tag or device 10
`in accordance with a preferred embodiment of the present
`invention. RFID tags are generally known and applicable to
`a wide variety of uses. U.S. Pat. No. 5,430,441 discloses a
`transponding tag which transmits a digitally encoded signal
`in response to an interrogation signal. The above disclosed
`tag comprises a rigid substrate constructed from a plurality
`of dielectric layers and conductive layers and includes an
`integrated circuit embedded entirely within a hole in the
`substrate and tab bonded to conductive foil
`traces. The
`
`device 10 comprises an antenna 12 electrically connected to
`an integrated circuit (IC) 14. Preferably,
`the antenna 12
`comprises a resonant circuit which resonants at a predeter-
`mined radio frequency (RF) corresponding to a radio fre-
`quency of an associated interrogation device, as discussed in
`more detail hereinafter.
`
`The antenna 12 may comprise one or more inductive
`elements electrically connected to one or more capacitive
`elements. In a preferred embodiment,
`the antenna 12 is
`formed by the combination of a single inductive element,
`inductor, or coil L electrically connected with a capacitive
`element or capacitance CANT in a series loop. As is well
`known to those of ordinary skill in the art, the operational
`frequency of the antenna 12 depends upon the values of the
`inductor coil L and the capacitor CANT. The size of the
`inductor L and the value of the capacitor CANT are deter-
`mined based upon the desired resonant frequency of the
`antenna 12. In one embodiment of the invention, the device
`10 is constructed to operate at 13.56 MHZ. Although it is
`preferred that the device 10 resonates at about 13.56 MHZ,
`the device 10 could be constructed to resonate at other
`
`frequencies and the precise resonant frequency of the device
`10 is not meant to be a limitation of the present invention.
`Thus, it will be apparent to those of ordinary skill in the art
`that the antenna 12 may operate at radio frequencies other
`than 13.56 MHZ, and indeed at other frequencies, such as
`microwave frequencies. In addition, although the device 10
`includes a single inductive element L and a single capacitor
`
`Page 1005-9
`
`Page 1005-9
`
`

`
`5,883,582
`
`5
`element CANT, multiple inductor and capacitor elements
`could alteratively be employed. For instance, multiple ele-
`ment resonant circuits are well known in the electronic
`security and surveillance art, such as described in U.S. Pat.
`No. 5,103,210 entitled “Activatable/Deactivatable Security
`Tag for Use with an Electronic Security System”, which is
`incorporated herein by reference. Although a preferred
`antenna is described, it will be apparent to those of ordinary
`skill
`in the art from this disclosure that any means for
`coupling energy to/from the IC 14 may be used.
`The IC 14 preferably includes a programmable memory
`18, such as a 64 or 128 bit memory, for storing bits of
`identification data, although larger or smaller programmable
`memories could also be used. The IC 14 outputs a data
`stream comprised of the 64 (or 128) bits of data when
`sufficient power is applied thereto. The data bits or digital
`value stored in the programmable memory 18 can be used
`for a variety of purposes, such as to identify a particular
`object or person associated with the device 10. The memory
`18 may comprise one or more data fields for storing one or
`more digitally encoded messages. The stored digital value
`may be unique to each device 10, or in some instances, it
`may be desirable for two or more devices to have the same
`stored digital value. In addition to identifying an object, the
`data stored in the memory 18 could be used to store product
`identity information, product warranty information, as well
`as other information, such as when and where the product
`was manufactured, etc. Thus, when the device 10 is powered
`by an induced voltage, the IC 14 outputs the data stored in
`the programmable memory 18. The data is then transmitted
`at a predetermined radio frequency which is detectable by an
`interrogation device 20 (FIG. 2). The transmission of the
`data by the device 10 is termed herein a transmission period
`or interval.
`
`The IC 14 also preferably includes a timer circuit 19
`which establishes or defines a non-transmission period or
`interval, such that each data transmission interval is fol-
`lowed by a non-transmission interval. During a non-
`transmission interval,
`the device 10 does not transmit or
`radiate a signal. According to the present invention, a time
`length of the non-transmission interval is generally fixed.
`That
`is,
`the timer circuit 19 establishes a single, fixed,
`non-transmission interval. The timer circuit 19 requires the
`device 10 to wait for a fixed length or period of time after
`data is transmitted therefrom until data is again transmitted
`therefrom. Consequently, each data transmission interval is
`followed by a fixed length non-transmission interval. The
`non-transmission interval may be established by having the
`timer circuit 19 generate an enable signal which enables the
`device 10 to transmit data. Alternatively, the timer circuit 19
`could interact with the memory 18 such that the memory 18
`is only strobed or read at fixed intervals.
`The timer circuit 19 is constructed to time or count for a
`
`predetermined, fixed length of time, after each transmission
`interval or memory 18 access. The timer circuit 19 may be
`constructed using a variety of electrical components or
`devices, as is known by those of ordinary skill in the art. The
`specific manner in which the timer 19 is designed and the
`electrical components from which the timer 19 is con-
`structed is not important. That is, the timer circuit 19 could
`count up, count down, or be a simple delay circuit. Although
`it is preferred that the timer 19 be constructed as an integral
`part of the IC 14 and that the timer 19 interacts with the
`memory 18, the timer 19 could interact with an output of the
`IC 14, rather than the memory 18. Also, the timer 19 could
`be constructed external to the IC 14. It is to be understood
`
`that the importance of the timer 19 is that it functions to
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`assure that a data transmission interval is followed by a fixed
`length non-transmission interval.
`A proximity reader or interrogator device 20 (FIG. 2) is
`used to detect and read the information transmitted by the
`device 10. In operation, the reader/interrogator 20 estab-
`lishes an electromagnetic field at or near the resonant
`frequency of the antenna 12. When the device 10 is close
`enough to the reader/interrogator 20 so as to be within the
`electromagnetic field, a voltage is induced on the inductive
`coil L, which provides power to the IC 14 at the ANT input
`of the IC 14. Preferably, the IC 14 internally rectifies the
`induced AC voltage at the ANT input to provide an internal
`DC voltage source. When the internal DC voltage reaches a
`level that assures proper operation of the IC 14, the IC 14
`functions to output the digital value stored in the program-
`mable memory at the MOD output of the IC 14.
`In the presently preferred embodiment, the antenna 12
`comprises a resonant circuit. A modulation capacitor CMOD
`is connected to the MOD output of the IC 14 and to the
`resonant circuit (antenna) 12. The data output pulses at the
`MOD output switch the capacitor CMOD into and out of the
`resonant circuit 12 by making and breaking ground connec-
`tions to change the overall capacitance of the resonant
`circuit 12 in accordance with the stored data, which in turn
`changes the resonant frequency of the resonant circuit 12,
`detuning it from a principal operational frequency to a
`predetermined higher frequency. Thus, data pulses of the
`device 10 are created by the tuning and detuning of the
`resonant circuit 12. The reader/interrogator 20 senses the
`changes in the consumption of energy within its electro-
`magnetic field to determine the digital data value output
`from the IC 14. Although a particular method and circuit for
`outputting or transmitting data to the interrogator 20 is
`disclosed, other means of transmitting stored data to the
`interrogator 20, such as other modulation techniques, are
`within the scope of the present invention.
`The IC 14 may also include a power return or GND output
`and one or more additional inputs 16 which are used for
`programming the IC 14 (i.e. storing or altering the digital
`value therein) in a conventional manner. In the presently
`preferred embodiment,
`the IC 14 comprises 128 bits of
`nonvolatile memory. Of course, it will be apparent to those
`of ordinary skill in the art that the programmable memory 18
`could have either a greater or smaller storage capacity.
`Referring now to FIG. 2, a schematic block diagram of the
`interrogator 20 suitable for use with the RFID tag or device
`10 described in FIG. 1 is shown. The interrogator 20 and the
`RFID device 10 communicate by inductive coupling, as is
`well known in the art. The interrogator 20 includes a
`transmitter 22, receiver 24, antenna assembly 26, and data
`processing and control circuitry 28, each having inputs and
`outputs. The transmitter 22 generates an interrogation signal
`which is provided to the antenna assembly 26 for generating
`an electromagnetic field at a first predetermined radio fre-
`quency. The strength of the electromagnetic field determines
`the size of the zone in which the RFID devices 10 will be
`
`the interrogation zone). The
`powered and detected (i.e.
`receiver 24 detects changes in the electromagnetic field
`caused by the transmission of a data output signal by an
`RFID device 10. The output of the transmitter 22 is con-
`nected to a first input of the receiver 24, and to the input of
`the antenna assembly 26. The output of the antenna assem-
`bly 26 is connected to a second input of the receiver 24. A
`first and a second output of the data processing and control
`circuitry 28 are connected to the input of the transmitter 22
`and to a third input of the receiver 24,
`respectively
`Furthermore, the output of the receiver 24 is connected to
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`5,883,582
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`7
`the input of the data processing and control circuitry 28.
`Interrogators having this general configuration may be built
`using circuitry described in U.S. Pat. Nos. 3,726,960, 3,816,
`708, 4,103,830 and 4,580,041, all of which are incorporated
`by reference in their entirety herein. Preferably however, the
`data processing and control circuitry of the interrogator
`described in these patents are modified to append date and
`time data thereto (e.g. a time-stamp). A time clock 30 is
`provided in the data processing and control circuitry 28 for
`appending the date and time data. The interrogator 20 is
`preferably a hand-held device. However, other physical
`manifestations of the interrogator 20 are within the scope of
`the invention, such as a pedestal structure. Moreover, the
`interrogator 20 may comprise a separate structure from the
`transmitter 22 and an associated antenna, and from the
`receiver 24 and an associated antenna, as is known in the art.
`The interrogator 20 can detect
`transmissions from a
`plurality of devices 10 (and thus their associated articles)
`located within the interrogation zone. In most instances,
`each of the devices 10 receive and respond to the interro-
`gation signal at a different instant in time, even when the
`devices 10 are physically close together. The string of
`returned signals is processed to sort out
`the individual
`signals from each of the devices 10. However, if two devices
`10 transmit a data signal at exactly the same time or at
`partially overlapping times, the interrogator 20 can detect
`this event and discard the data signals. Such detection
`circuitry is conventional and known by those of ordinary
`skill
`in the art. According to the present invention,
`the
`interrogation signal generated by the interrogator 20 is a
`generally continuous signal, as opposed to a periodic or
`pulsed signal. As previously discussed,
`the interrogation
`signal is internal to the interrogator 20 and is provided to the
`antenna assembly 26 to generate an electromagnetic field.
`The interrogation zone is the area within the electromagnetic
`field in which a voltage is induced in the intelligent device
`10 sufficient to power the IC 14. Thus,
`the size of the
`interrogation zone is defined by the strength of the electro-
`magnetic field.
`As long as a device is within the interrogation zone, the
`device 10 continually transmits or outputs its data. In order
`to allow the interrogator 20 to detect and receive data from
`a plurality of devices 10 located within the interrogation
`zone, after transmission of data by a particular device 10, as
`previously discussed, the device 10 waits for a fixed length
`of time before again transmitting its data.
`Referring now to FIG. 3a, a timing diagram of a protocol
`for transmitting data from the RFID device 10 is shown. The
`RFID device 10 output data stream comprises memory data
`32 followed by a fixed length gap or period where no data
`is transmitted 34. The memory data portion 32 comprises the
`message being transmitted from the device 10. The message
`may comprise all of the bits of information stored in the
`programmable memory 18 or a selected number of the data
`bits stored in the memory 18. Note that the RFID device 10
`continues to transmit its output data stream as long as the
`RFID device 10 is within the interrogation zone and the
`induced voltage from the electromagnetic field is high
`enough. The message may further comprise additional bits
`of information not stored in the data memory 18, such as for
`error detection and correction, or other control purposes, as
`will be apparent to those of skill in the art.
`As illustrated in FIG. 3a, the memory data portion 32 of
`the output data stream is longer than the gap or non-
`transmission interval 34. FIG. 3b is a timing diagram of a
`preferred protocol for transmitting data stored in the RFID
`device 10. Similar to the protocol shown in FIG. 3a, the
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`output data stream comprises a memory data portion 36
`followed by a fixed length gap or non-transmission interval
`38, and the RFID device 10 continuously outputs the data
`stream as long as the device 10 is within the interrogation
`zone and the induced voltage from the electromagnetic field
`is high enough. The difference between the protocol of FIG.
`3a and the protocol of FIG. 3b is that the length of the
`non-transmission interval 38 is greater than the length of the
`memory data portion 36 in FIG. 3b. Preferably, the length of
`the non-transmission interval 38 is much greater than the
`length of the memory data portion 36, such as about 100
`times longer. The timing circuit 19 establishes the length of
`the non-transmission interval. For instance, the data trans-
`mission interval 38 could comprise about 1 millisecond and
`the non-transmission interval could comprise about 100
`milliseconds.
`
`As previously discussed, the timing circuit 19 establishes
`the length of the non-transmission interval 38, which is
`preferably of generally fixed length. However, it has been
`determined that by constructing the timing circuit 19 using
`electrical components of a predetermined tolerance level,
`such as a +/-20% tolerance,
`that although the non-
`transmission interval 38 is a fixed length for a particular
`device, the length of the non-transmission interval varies
`among a plurality devices due solely to the manufacturing
`tolerance, which decreases the probability that two or more
`devices will transmit their memory data 36 at the same
`instant
`in time. That
`is, varying the length of the non-
`transmission interval 38 among various devices 10 desyn-
`chronizes transmissions between devices 10. In contrast, if
`the timing circuit 19 is constructed using electrical compo-
`nents with a tighter tolerance level, such as +/—5%, then the
`timing circuits in different devices are more likely to have
`the same length non-transmission interval and consequently,
`it is more likely that two or more devices within an inter-
`rogation zone will simultaneously transmit their memory
`data 36, thus causing a data collision. Thus, in operation,
`each device 10 within the interrogation zone theoretically
`transmits its memory data 36 at the same time, in reality,
`variations in the electrical components comprising the tim-
`ing circuit 19 cause the devices to transmit their memory
`data 36 at least slightly different times. In addition, even
`should two or more devices 10 initially transmit
`their
`memory data 36