(12) United States Patent
`Ueda
`
`USOO6731199B1
`(10) Patent No.:
`US 6,731,199 B1
`(45) Date of Patent:
`May 4, 2004
`
`(54) NON-CONTACT COMMUNICATION
`SYSTEM
`
`(75) Inventor: Takashi Ueda, Kyoto (JP)
`s
`(73) Assignee: Rohm Co., Ltd., Kyoto (JP)
`(*) Notice:
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.: 09/359,651
`(22) Filed:
`Jul. 26, 1999
`(30)
`Foreign Application Priority Data
`Jul. 27, 1998
`(JP) ........................................ H10-210896
`(51) Int. Cl. .................................................. H04Q5/22
`(52) U.S. Cl. ................... 340/10.4; 340/10.3; 340/10.34
`(58) Field of Search ............................... 340/10.1, 10.3,
`340/10.4, 825, 572.2, 10.34, 572.1, 572.4;
`235/380
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`3.299,424 A * 1/1967 Vinding ...................... 235/439
`3,859,624 A * 1/1975 Kriofsky et al.
`... 187/391
`4,814,595 A * 3/1989 Gilboa ....................... 235/487
`4,899,036 A * 2/1990 McCrindle et al. ......... 235/380
`5,113,184 A 5/1992 Katayama ................ 340/10.51
`
`:
`
`
`
`4/1994 Turner et al. ................. 342/44
`5,305,008 A
`5,418,353 A * 5/1995 Katayama et al. .......... 235/380
`5,523,749 A * 6/1996 Cole et al. ............... 340/10.34
`5,689.239 A * 11/1997 Turner et al. ............... 235/383
`5,866,891. A * 2/1999 Fujimoto et al. ........... 235/380
`6,169.474 B1 * 1/2001 Greef et al. .............. 340/10.1
`* cited by examiner
`
`Primary Examiner Brian Zimmerman
`Assistant Examiner-Clara Yang
`(74) Attorney, Agent, or Firm-Arent Fox Kintner Plotkin
`& Kahn
`ABSTRACT
`(57)
`In a non-contact communication System, when an interro
`gator is attempting to detect that a responder is present in a
`range communicable there with, the interrogator transmits a
`weak radio wave from its tuning circuit. The responder
`receives this radio wave and acquires electric power by
`rectifying it, but, because the electric power thus obtained is
`insufficient, the responder is turned on and off repeatedly at
`regular intervals. This causes variation in the impedance
`with which the tuning circuit is loaded. The interrogator, by
`detecting this variation in the impedance occurring at regular
`intervals, recognizes that the responder is present within the
`range communicable there with. Only then, the interrogator
`transmits a radio wave Strong enough to permit the
`responder to acquire Sufficient electric power.
`
`7 Claims, 7 Drawing Sheets
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`U.S. Patent
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`May 4, 2004
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`Sheet 1 of 7
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`US 6,731,199 B1
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`FIG. 1
`
`
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`CONTROLLER
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`FIRST
`SIGNAL
`DETECTOR
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`SECOND
`SIGNAL
`DETECTOR
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`7
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`AEEE MODULATION ||
`CIRCUIT
`CIRCUIT
`ADJUSTER
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`|Earl
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`2
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`3
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`1
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`it is
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`4
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`5
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`U.S. Patent
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`May 4, 2004
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`Sheet 2 of 7
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`US 6,731,199 B1
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`Z
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`FIG. 3
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`-
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`Iril
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`(d)
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`1 A2 A2 A1
`1 O O 1 O
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`U.S. Patent
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`May 4, 2004
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`Sheet 4 of 7
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`US 6,731,199 B1
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`SN
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`3 d
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`- - - - - - - - - -
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`U.S. Patent
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`May 4, 2004
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`Sheet 5 of 7
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`US 6,731,199 B1
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`0NISSE OOHd
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`WIWQ
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`G
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`U.S. Patent
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`May 4, 2004
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`Sheet 6 of 7
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`US 6,731,199 B1
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`FIG. 6
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`CONTROLLER
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`SECOND
`SIGNAL
`DETECTOR
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`SENSOR-N-9
`UNIT
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`CARRIER
`MODULATION
`FEEDING
`CIRCUIT - CIRCUIT
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`TUNINGAAAA
`CIRCUIT
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`U.S. Patent
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`May 4, 2004
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`Sheet 7 of 7
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`US 6,731,199 B1
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`1
`NON-CONTACT COMMUNICATION
`SYSTEM
`
`BACKGROUND OF THE INVENTION
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`2
`unit for communicating with the first communication unit,
`the first communication unit generates a radio wave for
`detection weaker than the radio wave for communication So
`that the first communication unit, by detecting a predeter
`mined change in the radio wave for detection, recognizes
`that the Second communication unit is present within a range
`communicable with the first communication unit and then
`Starts transmitting the radio wave for communication.
`According to this configuration, when the Second com
`munication unit is not present in the range communicable
`with the first communication unit and thus neither of them
`is performing communication operation, the first communi
`cation unit transmits a radio wave weaker than the radio
`wave it transmits for ordinary communication. Thus, leSS
`electric power is consumed and accordingly higher energy
`efficiency is attained than in a conventional System that
`requires that a strong radio wave for communication be kept
`transmitted all the time.
`Moreover, when the Second communication unit is not
`present in the range communicable with the first communi
`cation unit and thus neither of them is performing commu
`nication operation, the first communication unit transmits a
`radio wave weaker than the radio wave it transmits for
`ordinary communication. Thus, as long as no communica
`tion operation takes place, there is less possibility of inter
`ference with other electric equipment or adverse effects on
`human bodies.
`Moreover, in the non-contact communication System
`according to the present invention, the first communication
`unit transmits alternatively the radio wave for communica
`tion or the radio wave for detection by using a single
`transmission means shared between those two radio waves.
`Using a Single transmission means to transmit the radio
`wave for communication and the radio wave for detection
`eliminates the need to provide Separate transmission means
`to transmit the radio wave for communication and the radio
`wave for detection. Thus, the first communication unit used
`here can be obtained simply by making a few modifications
`to a conventional configuration thereof, and thus does not
`require a larger-Scale configuration.
`Moreover, in the non-contact communication System
`according to the present invention, repeated activation and
`deactivation of the Second communication unit occur at
`regular intervals because the radio wave for detection trans
`mitted from the first communication unit is so feeble as to be
`equal to critical Strength that divides between activation and
`deactivation of the Second communication unit when the
`Second communication unit is present within the range
`communicable with the first communication unit. The non
`contact communication System is provided with a detecting
`means for detecting variation of amplitude in the radio
`waves transmitted from the first communication unit result
`ing from Such repeated activation and deactivation of the
`Second communication unit, a checking means for checking
`whether the Second communication unit is present within the
`range communicable with the first communication unit or
`not on a basis of an output from the detecting means, and a
`control means for controlling the first communication unit to
`transmit the radio wave for communication on a basis of an
`output from the checking means.
`According to this configuration, the radio wave for detec
`tion generated by the first communication unit when the
`Second communication unit is out of the range communi
`cable with the first communication unit is so feeble relative
`to the radio wave for communication as to generate electric
`power of critical Strength that divides activation and deac
`
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`1. Field of the Invention
`The present invention relates to a non-contact communi
`cation System in which a radio wave transmitted from an
`interrogator is received by a responder and the responder
`produces electric power from the received radio wave and in
`addition reproduces data from a modulated Signal carried by
`the received radio wave in order to transmit, in reply to the
`received data, reply data back to the interrogator.
`2. Description of the Prior Art
`Conventionally, as a non-contact communication System,
`radio-frequency tags (RF tags) and ID cards are known that
`acquire electric power from a radio wave transmitted from
`an antenna in order to transmit the data Stored in themselves.
`Such Systems are used, for example, with the lift facilities at
`a skiing ground, with the ticket examination equipment at a
`railway Station, and for Sorting of baggage in general.
`Such RF tags and ID cards are formed as a non-contact
`card that has a nonvolatile memory and a transmitter/
`receiver unit incorporated therein but that has no power
`Source Such as a battery. Such a non-contact card operates on
`the electric power it produces from a radio wave (radio
`frequency modulated signal) it receives. Moreover, Such a
`non-contact card communicates data with its communica
`tion partner by using a radio wave, and thus offers the
`advantage of non-contact data communication.
`In Such a non-contact communication System, a non
`contact card is used, for example, as a responder. The
`responder receives a radio wave transmitted from an inter
`rogator and acquires electric power from the received radio
`wave. Therefore, conventionally, the interrogator needs to be
`kept transmitting all the time a radio wave for communica
`tion that carries electric power Sufficiently Strong to permit
`the responder to operate at a relatively remote location.
`40
`Thus, in this non-contact communication System, even when
`no responder is present within the range in which commu
`nication is possible, the interrogator needs to be kept trans
`mitting the radio wave for communication, and this clearly
`is a waste of electric power.
`Moreover, in this non-contact communication System, it is
`undesirable in the first place to keep the interrogator trans
`mitting all the time a radio wave that carries electric power
`Sufficiently Strong to permit the responder to operate,
`because this may cause interference with the operation of
`other electric equipment, or may have adverse effects on
`human bodies.
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`SUMMARY OF THE INVENTION
`An object of the present invention is to provide a non
`contact communication System in which an interrogator
`consumes less electric power to transmit a radio wave.
`Another object of the present invention is to provide a
`non-contact communication System in which the radio wave
`that an interrogator transmits when it is not communicating
`with a responder does not cause interference with the
`operation of other electric equipment nor have adverse
`effects on human bodies.
`To achieve the above objects, according to one aspect of
`the present invention, in a non-contact communication SyS
`tem provided with a first communication unit for radiating a
`radio wave for communication and a Second communication
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`US 6,731,199 B1
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`tivation of the Second communication unit, and thus the
`Second communication unit is activated and deactivated
`repeatedly at regular intervals. This causes variation to occur
`at regular intervals in the impedance with which the tuning
`circuit of the first communication unit is loaded. The first
`communication unit, by detecting this variation occurring at
`regular intervals, recognizes the presence of the Second
`communication unit. Moreover, the Second communication
`unit does not require any circuit other than the one which
`performs communication to make the first communication
`unit recognize the presence of the Second communication
`unit, and thus the Second communication unit having the
`Same configuration as in a conventional System can be used
`in the non-contact communication System according to the
`present invention.
`Moreover, in the non-contact communication System
`according to the present invention, the Second communica
`tion unit has a signal generating means for generating a reply
`Signal when the Second communication unit catches the
`radio wave for detection So as to make the first communi
`cation unit recognize that the Second communication unit is
`present within the range communicable with the first com
`munication unit.
`The Second communication unit has a circuit that can
`operate on the insufficient electric power obtained from a
`radio wave that is generated by the first communication unit
`So as to be weaker than the radio wave for communication.
`In addition, this circuit keeps the weaker radio wave modu
`lated at a fixed frequency all the time. Thus, when the first
`communication unit is made to recognize that the Second
`communication unit is present in the range communicable
`therewith, the first communication unit can be fed with a
`Signal that does not depend on the characteristics of the
`circuit provided within the Second communication unit for
`achieving communication.
`According to another aspect of the present invention, in a
`non-contact communication System provided with a first
`communication unit for radiating a radio wave for commu
`nication and a Second communication unit for communicat
`ing with the first communication unit, a mechanical or
`optical Sensor is provided within the first communication
`unit So that the first communication unit Starts transmitting
`the radio wave for communication when the first commu
`nication unit recognizes that the Second communication unit
`is present within the range communicable with the first
`communication unit. This makes it possible to reduce the
`electric power consumed when no communication takes
`place between the two communication units, and eliminate
`the possibility of serious interference with other electric
`equipment or adverse effects on human bodies.
`BRIEF DESCRIPTION OF THE DRAWINGS
`This and other objects and features of the present inven
`tion will become clear from the following description, taken
`in conjunction with the preferred embodiments with refer
`ence to the accompanying drawings in which:
`FIG. 1 is a block diagram showing the internal configu
`ration of the interrogator adopted in a first and a Second
`embodiment of the invention;
`FIG. 2 is a block diagram showing the internal configu
`ration of the responder adopted in the first and a third
`embodiment of the invention;
`FIG. 3 is a diagram showing the waveforms of the radio
`waves for responder detection and for communication;
`FIGS. 4A to 4D are diagrams showing equivalent circuits
`of the antenna circuit in a non-contact communication
`System;
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`4
`FIG. 5 is a block diagram showing the internal configu
`ration of the responder adopted in the Second embodiment of
`the invention;
`FIG. 6 is a block diagram showing the internal configu
`ration of the interrogator adopted in the third embodiment of
`the invention; and
`FIG. 7 is a block diagram showing the internal configu
`ration of the interrogator having two tuning circuits adopted
`in the invention.
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`A first embodiment of the present invention will be
`described with reference to FIGS. 1 to 3. FIG. 1 is a block
`diagram showing the internal configuration of the interro
`gator 1 employed in the first embodiment.
`The interrogator 1 has a controller 8, a carrier feeding
`circuit 2 for feeding a carrier having a predetermined fre
`quency f, a modulation circuit 3 for modulating the carrier
`in accordance with the data fed from the controller 8, an
`output power adjuster 4 for controlling the output power of
`the carrier, a tuning circuit 5 having functions of transmitting
`and receiving a radio wave S, and a first signal detector 6 and
`a Second Signal detector 7 for detecting a reply Signal from
`a responder 10, which will be described later. The controller
`8 is composed of a microcomputer or the like, and controls
`the carrier feeding circuit 2, the modulation circuit 3, the
`output power adjuster 4, the first Signal detection circuit 6,
`and the Second Signal detection circuit 7.
`FIG. 2 is a block diagram showing the internal configu
`ration of a responder 10 having an integrated circuit (I/C) 11.
`The responder 10 has a tuning circuit 12 that tunes in to the
`carrier having the frequency f transmitted from the interro
`gator 1, and also has, within the IC 11, a rectification circuit
`13, a Switch 14, a demodulation circuit 15, a regulator 16,
`and a data processing circuit 17. The tuning circuit 12 is
`composed of an inductor L and a capacitor C.
`In a non-contact communication System employing an
`interrogator 1 and a responder 10 that are configured as
`described above, when the responder 10 is not present in a
`range communicable with the interrogator 1, the controller
`8 controls the output power adjuster 4 So that a radio wave
`adjusted by the output power adjuster 4 to be 10 dB weaker
`than a radio wave used in ordinary communication
`(hereafter referred to as the radio wave for communication)
`is transmitted from the tuning circuit 5. In the following
`descriptions of the present Specification, this radio wave,
`which is transmitted to allow detection of whether the
`responder 10 is present in the communicable range or not,
`will be referred to as the radio wave for responder detection.
`The power of the radio wave for responder detection is so
`Set as to be weak enough to have no effects on other electric
`equipment and equal to critical Strength that is between the
`Strength that produces Sufficient electric power to activate
`the responder 10 within a predetermined range and the
`Strength that does not. The critical Strength differs according
`to the characteristics of the responder. The radio wave for
`responder detection, for which the controller 8 feeds no
`modulation Signal to the modulation circuit 3, has a wave
`form as shown at (a) in FIG. 3.
`Although no modulation signal is added to the radio wave
`for responder detection as described above in this
`embodiment, it is also possible to use as the radio wave for
`responder detection a radio wave having a modulation Signal
`added thereto.
`When the radio wave for responder detection is being
`transmitted, the controller 8 keepS active the carrier feeding
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`circuit 2, the output power adjuster 4, and the Second Signal
`detector 7, whereas it keeps the first Signal detector 6
`inactive. In this state, the controller 8 feeds no modulation
`signal to the modulation circuit 3. Hereafter, this state will
`be referred to as the detection mode. On the other hand, the
`State in which non-contact communication with the
`responder 10 is taking place will be referred to as the
`communication mode.
`As shown in FIG. 4A, the tuning circuit 5 of the interro
`gator 1 and the IC 11 of the responder 10, together with the
`Space existing between them, can be regarded as forming an
`antenna circuit 100. Therefore, if it is assumed that the
`impedance with which the tuning circuit 5 of the interrogator
`1 is loaded equals Z, as shown in FIG. 4B when the
`responder 10 is inactive, equals Z, as shown in FIG. 4C
`when the responder 10 is active and the Switch 14 is open,
`and equals Z, as shown in FIG. 4D when the responder 10
`is active and the Switch 14 is closed, then these values of the
`impedance exhibit the relationship ZodZ>Z. In FIG. 4A,
`reference numeral 11a represents the internal circuit of the
`IC 11.
`In the responder 10, when the tuning circuit 12 tunes in to
`the radio wave for responder detection, the rectification
`circuit 13 rectifies the received radio wave and thereby
`produces the electric power to be supplied within the IC. By
`using this electric power, the responder 10 attempts to
`operate. At this time, since the Switch 14 in the responder 10
`is open, the impedance with which the tuning circuit 5 of the
`interrogator 1 is loaded changes from Z to Z. A variation
`in the impedance like this causes consumption of electric
`power within the responder 10, and thereby reduces the
`amplitude and thus the power of the radio wave for
`responder detection.
`The output power of the radio wave for responder detec
`tion is So controlled as to produce electric power that is
`insufficient for the operation of the responder 10 within a
`predetermined range. Therefore, when this radio wave
`shows a variation in amplitude as described above, it comes
`to produce leSS electric power than is necessary to make the
`responder 10 operate, and thus the responder 10 stops
`operating. This causes the above-mentioned impedance to
`return to Z, allowing the radio wave for responder detection
`to recover its original amplitude. As a result, the responder
`10 Starts operating again.
`In this way, the responder 10 is activated and deactivated
`repeatedly, and therefore the radio wave for responder
`detection now has a waveform of a signal modulated at a
`fixed frequency f1 as shown at (b) in FIG. 3. Moreover, as
`a result of repeated activation and deactivation of the
`responder 10 as described above, even if the interrogator 1
`transmits a radio wave carrying data, the data processing
`circuit 17 cannot process the data properly, and therefore the
`data processing circuit 17 never controls the Switch 14.
`Accordingly, when the responder 10 is active, the Switch 14
`remains open, and thus the above-mentioned impedance
`equals Z.
`The Signal, modulated as described above, output from
`the tuning circuit 5 of the interrogator 1 is detected by the
`Second Signal detector 7 by the use of a resonant circuit
`tuned to the frequency f1 of that Signal, and the detection
`output is fed to the controller 8.
`On detecting this signal having the frequency f1, the
`interrogator 1 recognizes, through the operation of the
`controller 8, that the responder 10 is present within a
`communicable range. Now, the controller 8 controls the
`output power adjuster 4 to increase the output of the trans
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`mitted Signal, deactivates the Second Signal detector 7, and
`activates the first Signal detector 6. Moreover,
`Simultaneously, the controller 8 starts feeding a modulation
`Signal to the modulation circuit 3. Thus, Switching from the
`detection mode to the communication mode is achieved.
`At this time, the carrier fed from the carrier feeding circuit
`2 is modulated in the modulation circuit 3 in accordance
`with the data Q fed from the controller 8, and thereafter a
`radio wave for communication that is amplified by the
`output power adjuster 4 So as to have electric power Suffi
`cient to make the responder 10 operate is transmitted from
`the tuning circuit 5. This radio wave for communication is
`a radio-frequency Signal in which modulated waves that are
`modulated in accordance with the data Q and non-modulated
`waves that are not modulated in accordance with the data Q
`appear alternately in periods T1 and T2, respectively, as
`shown at (c) in FIG. 3.
`When the tuning circuit 12 of the responder 10 receives
`the radio wave for communication, the radio wave for
`communication is rectified by the rectification circuit 13 to
`produce electric power, and, by using this electric power, the
`responder 10 Starts operating. At this time, the data Q
`obtained by demodulating the modulated waves appearing in
`the periods T1 of the radio wave for communication by the
`use of the demodulation circuit 15 is processed by the data
`processing circuit 17, and the data processing circuit 17
`outputs reply data A. The reply data A is composed of a train
`of pulses, by which the Switch 14 is turned on and off.
`Turning on and off the Switch 14 in this way causes the
`impedance with which the tuning circuit 5 of the interrogator
`1 is loaded to vary in Such a way as to be equal to Z when
`the Switch 14 is off and equal to Z when the Switch 14 is on.
`AS the above-mentioned impedance with which the tuning
`circuit 5 is loaded varies, the non-modulated waves in the
`periods T2 are amplitude-modulated in accordance with the
`reply data Aas shown at (d) in FIG.3. This signal is detected
`by the first signal detector 6, and the detected Signal is fed
`to the controller 8.
`On completion of communication between the interroga
`tor 1 and the responder 10 as described above, Switching
`from the communication mode back to the detection mode
`is achieved. At this time, the controller 8 stops feeding the
`modulation Signal to the modulation circuit 3, and adjusts
`the Setting of the output power adjuster 4 So as to reduce the
`output from the output power adjuster 4. In addition, the
`Second Signal detector 7 is activated, and the first Signal
`detector 6 is deactivated.
`In the embodiment under discussion, the interrogator 1
`uses only one tuning circuit 5 to transmit the radio wave for
`responder detection and the radio wave for communication,
`and uses one of two separate Signal detectors 6 and 7,
`according to whether it is operating in the detection or
`communication mode. However, it is also possible, as shown
`in FIG. 7, to provide the interrogator 1A with a tuning circuit
`5a for transmitting a radio wave for responder detection, a
`tuning circuit 5b for transmitting a radio wave for
`communication, and an output Selector 20 between these
`tuning circuits 5a and 5b and the output power adjuster 4,
`with the tuning circuits 5a and 5b connected to the signal
`detectors 6 and 7, respectively.
`When this interrogator 1A is used, as a means for recog
`nizing presence of the responder 10 within a communicable
`range and as a means for achieving communication with the
`responder 10, the same means as described above can be
`used. However, on Switching from the detection mode to the
`communication mode and Vice versa, the controller 8 needs
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`to Switch not only the Setting of the output power of the
`output power adjuster 4 but also the output selector 20 to
`Switch the destination of the output between the tuning
`circuits 5a and 5b. Specifically, the controller 8 Switches the
`output Selector 20 in Such a way that, in the detection mode,
`a feeble radio wave S1 is transmitted from the tuning circuit
`5a and, in the communication mode, an amplified radio
`wave S2 is transmitted from the tuning circuit 5b.
`When a responder 10A (FIG. 5) having a specialized
`circuit that operates on the radio wave for responder detec
`tion as will be described later in connection with the second
`embodiment is used, the impedance Z shown at (b) in FIG.
`3 may be different from the impedance Z shown at (d) in
`FIG. 3.
`A second embodiment of the present invention will be
`described with reference to FIGS. 1 and 5. The interrogator
`employed in this embodiment has the same configuration as
`the integrator shown in FIG. 1 employed in the first embodi
`ment.
`FIG. 5 is a block diagram showing the internal configu
`ration of the responder 10A employed in this embodiment.
`This responder 10A, like the responder 10 employed in the
`first embodiment, has a tuning circuit 12 that tunes in to the
`carrier having the frequency f transmitted from the interro
`gator 1, and has, within an IC 11, a rectification circuit 13,
`a Switch 14, a demodulation circuit 15, and a regulator 16.
`In addition, within IC 11, the responder 10A further has a
`first data processing circuit 17a that can operate n compara
`tively low power, a Second data processing circuit 17b for
`performing Signal processing in the communication mode, a
`Switch 18, and Switching controller 19 for controlling the
`Switch 18.
`In a non-contact communication System employing an
`interrogator 1 and a responder 10A that are configured as
`described above, when the responder 10A is not in a range
`communicable with the interrogator 1, the interrogator 1
`operates in the detection mode as in the first embodiment,
`and therefore the output power adjuster 4 controls the output
`power of the radio wave for communication So that a radio
`wave 10 dB weaker than the radio wave for communication
`is transmitted as the radio wave for responder detection from
`the tuning circuit 5.
`Also in this embodiment, as in the first embodiment, no
`modulation Signal is added to the radio wave for responder
`detection. However, it is also possible to use as the radio
`wave for responder detection a radio wave having a modu
`lation Signal added thereto.
`When the tuning circuit 12 of the responder 10A receives
`the radio wave for responder detection, the radio wave for
`responder detection is rectified by the rectification circuit 13
`to produce electric power, and, by using this electric power,
`the responder 10A Starts operating. At this time, the Switch
`18 is in the contact “a” position, and therefore the electric
`power produced by the rectification circuit 13 is Supplied to
`the first data processing circuit 17a, which is thereby acti
`Vated. On the other hand, no electric power is Supplied to the
`Second data processing circuit 17b, which therefore remains
`inactive.
`When the first data processing circuit 17a Starts operating,
`it outputs data composed of a train of pulses having a fixed
`frequency f2. In synchronism with this data, the Switch 14 is
`turned on and off repeatedly to vary the impedance of the
`tuning circuit 12.
`By varying the impedance of the tuning circuit 12 as
`described above, the radio wave for responder detection is
`amplitude-modulated by the frequency f2. The resulting
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`modulated Signal causes the impedance acroSS the tuning
`circuit 5 of the interrogator 1 to vary. This variation is
`detected by the second signal detector 7 by the use of a
`resonant circuit tuned to the frequency f2 of that Signal, and
`the detection output is fed to the controller 8.
`On detecting the above-mentioned signal, the interrogator
`1 recognizes, through the operation of the controller 8, that
`the responder 10A is present within a communicable range.
`Now, the controller 8 controls the output power adjuster 4 to
`increase the output of the transmitted Signal, deactivates the
`Second Signal detector 7, and activates the first Signal
`detector 6. Thus, Switching from the detection mode to the
`communication mode is achieved. In the communication
`mode, the interrogator 1 operates in the same manner as in
`the first embodiment to transmit the radio wave for com
`munication while feeding a modulation signal to the modu
`lation circuit 3. When the tuning circuit 12 of the responder
`10A tunes in to the radio wave for communication, an abrupt
`increase in the electric power that is producing the radio
`wave is detected by the Switching controller 19, which then
`Switches the Switch 18 to the contact “b’ position.
`When the responder 10A becomes ready to communicate
`with the, interrogator 1 in this way, communication as
`performed in the first embodiment is started. On completion
`of communication, or when the electric power Supplied to
`the responder 10A weakens, the interrogator 1 is brought
`back into the detection mode, and thus the Switching con
`troller 19 Switches the Switch 18 back to the contact “a”
`position. Note that the Second data processing circuit 17b in
`this embodiment corresponds to the data processing circuit
`17 in the first embodiment, and thus the former operates in
`the same manner as the latter. Moreover, also in this
`embodiment, as in the first embodiment, it is possible to use
`an interrogator 1A having Separate tuning circuits. 5a and 5b
`for the detection and communication modes as shown in
`FIG. 7.
`Next, a third embodiment of the present invention will be
`described with reference to FIGS. 2 and 6. The responder
`employed in this embodiment has the same configuration as
`the responder shown in FIG.2 employed in the first embodi
`ment.
`FIG. 6 is a block diagram showing the internal configu
`ration of the interrogator 1B employed in this embodiment.
`This interrogator 1B has a controller 8B, a carrier feeding
`circuit 2 for feeding a carrier having a predetermined fre
`quency f, a modulation circuit 3 for modulating the carrier
`in accordance with the data fed from the controller 8B, a
`