Communication device and control method for communication device

Abstract

The present invention provides a communication device that can prevent misreading of wireless tags other than the target of communication without the need for reading restriction antennas or the like. [Solution] The communication device is a communication device that communicates with a wireless tag, and is a loop antenna that communicates with the wireless tag by outputting a magnetic field corresponding to a transmission signal relating to communication with the wireless tag, and comprises a plurality of loop antennas, each independently communicating with the wireless tag, a communication control means that outputs a transmission signal to the loop antenna to be communicated among the plurality of loop antennas, and a control means that controls a loop antenna that is not the target of communication but is different from the target of communication to generate a magnetic field that cancels out the leakage magnetic field from the target of communication loop antenna.

Description

Technical Field

[0001] The present invention relates to a communication device that communicates with a wireless tag and a control method thereof.

Background Art

[0002] A reader and a reader / writer that utilize a frequency band such as the HF (High Frequency) band compliant with the ISO15693 standard (hereinafter, the reader and the reader / writer are not particularly distinguished and are collectively referred to as the reader / writer including the reader) couple a loop antenna installed on the reader / writer side and a loop antenna mounted on the wireless tag side such as an RFID (Radio Frequency IDentification) tag or an RFID card by electromagnetic induction action to read and write data to / from the wireless tag in a non-contact manner.

[0003] For example, the reader / writer loads information such as a read command of the wireless tag using a predetermined modulation method on a high-frequency carrier signal (carrier wave) and outputs it as a magnetic field from the loop antenna. In the wireless tag, the magnetic field generated by the reader / writer is taken in as an electrical signal by the electromagnetic induction action of the mounted loop antenna to obtain operating power. Further, the wireless tag decodes the read command included in the taken-in electrical signal and returns the data stored in the internal memory to the reader / writer side by load switching or the like according to the decoded content. Then, the reader / writer reads the data by receiving and decoding the return signal from the wireless tag by the loop antenna.

[0004] Furthermore, as shown in Figure 11(A), a reader / writer has been proposed that uses multiple loop antennas to read and write data to wireless tags. In a reader / writer with multiple loop antennas, a switch circuit is provided between a communication control unit 1110 that performs communication control and the antenna unit 1120 which includes a loop antenna 1121 and a matching circuit 1122. By selecting the loop antenna 1121 to communicate with, data is read and written to the wireless tag 1123 located on the target loop antenna 1121.

[0005] In a reader / writer with multiple loop antennas, it is required to reduce the overall area occupied by the loop antennas by narrowing the spacing between them. However, narrowing the spacing between loop antennas can lead to accidental data writing or reading to wireless tags placed on loop antennas adjacent to the loop antenna being communicated with, making it impossible to independently read and write to wireless tags on each loop antenna. This is because, as shown in Figure 11(B), the magnetic field 1142 generated by the loop antenna 1141 being communicated with leaks outside of the loop antenna 1141, supplying the magnetic field 1142 to the wireless tag placed on the adjacent loop antenna 1143, making it possible to read and write data to it.

[0006] As an example of countermeasures against this, Patent Document 1 proposes a technique to prevent misreading of RF tags placed near RF tags on sample containers adjacent to the object to be read. Furthermore, Patent Document 2 proposes a technique to prevent misreading of RFID tags placed on adjacent antennas by providing a reading antenna for reading RFID tags and a reading restriction antenna that restricts reading by other adjacent reading antennas, and appropriately controlling them. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2011-75360 [Patent Document 2] Japanese Patent Publication No. 2021-49331 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, the technology described in Patent Document 1 has only one reading antenna in the system and cannot be applied to reader / writers with multiple antennas. Furthermore, as described in Patent Document 2, providing a read-limiting antenna to restrict reading leads to increased complexity of the antenna structure, which is undesirable.

[0009] Other possible countermeasures include installing a shielding plate between the loop antenna being communicated and an adjacent loop antenna, or generating interference signals from an adjacent loop antenna. However, reducing the magnetic field leakage to the outside by installing a shielding plate between loop antennas requires placing the shielding plate as close to the loop antenna as possible and ensuring sufficient length of the shielding plate. However, this also reduces the magnetic field strength necessary for reading and writing, so it is not an appropriate countermeasure. Furthermore, reducing the leakage magnetic field by generating interference signals from an adjacent loop antenna requires generating interference signals of a sufficient level, but in that case, the interference signals will also wrap around to the loop antenna being communicated, leading to a deterioration of communication quality.

[0010] Thus, the measures described above were insufficient to prevent misreading of wireless tags placed on adjacent loop antennas due to leakage magnetic fields. When multiple wireless tags are placed on the target loop antenna and they are read in overlapping order, the target loop antenna is required to generate a strong magnetic field, making it difficult to prevent misreading of wireless tags placed on adjacent loop antennas.

[0011] This invention has been made in view of these circumstances, and aims to provide a communication device and a control method thereof that can prevent misreading of wireless tags other than the communication target without providing a read restriction antenna or the like. [Means for solving the problem]

[0012] The communication device according to the present invention is a communication device for communicating with a wireless tag, and is a loop antenna that outputs a magnetic field corresponding to a transmission signal relating to communication with the wireless tag and communicates with the wireless tag, comprising a plurality of loop antennas, each independently communicating with the wireless tag, a communication control means that outputs the transmission signal to the loop antenna that is the target of communication among the plurality of loop antennas, and a control means that controls a loop antenna that is not the target of communication but is different from the loop antenna that is the target of communication to generate a magnetic field that cancels out the leakage magnetic field from the loop antenna that is the target of communication. [Effects of the Invention]

[0013] According to the present invention, it is possible to provide a communication device and a control method thereof that can prevent misreading of wireless tags other than the communication target without providing a read-limiting antenna or the like. [Brief explanation of the drawing]

[0014] [Figure 1] This diagram illustrates the prevention of misreading of wireless tags in this embodiment. [Figure 2] This figure shows an example of the configuration of a communication device in the first embodiment. [Figure 3] This figure shows an example of a level adjustment circuit configuration. [Figure 4] This figure illustrates the prevention of misreading of wireless tags in a communication device in the first embodiment. [Figure 5] This figure shows another example of the configuration of the communication device in the first embodiment. [Figure 6] This figure shows another example of the configuration of the communication device in the second embodiment. [Figure 7] It is a diagram showing a configuration example of a horizontal reading prevention circuit. [Figure 8] It is a diagram for explaining prevention of misreading of a wireless tag in a communication device in the second embodiment. [Figure 9] It is a diagram for explaining generation of signals in adjacent loop antennas. [Figure 10] It is a diagram showing an example of the relationship between the resonance frequency in adjacent loop antennas and the operating voltage obtained by the wireless tag. [Figure 11] It is a diagram for explaining a reader / writer having a plurality of loop antennas.

Embodiments for Carrying Out the Invention

[0015] Hereinafter, embodiments of the present invention will be described based on the drawings. In the following description, a wireless tag arranged in a read / write area defined in a loop antenna, that is, a wireless tag arranged for the purpose of reading and writing data by the loop antenna, is also referred to as a "wireless tag placed on the loop antenna".

[0016] As shown in FIG. 1, without increasing the distance between the antennas, in order to prevent misreading of a wireless tag placed on an adjacent loop antenna 103 due to the leakage magnetic field 102 from the loop antenna 101 of the communication target, a magnetic field 104 in the same direction as the loop antenna 101 of the communication target may be generated in the adjacent loop antenna 103 to cancel the leakage magnetic flux. In each of the embodiments described below, such a magnetic field 104 is generated in the adjacent loop antenna 103 to prevent misreading of a wireless tag placed on the adjacent loop antenna 103. By flowing a current in the same direction and the same phase as the loop antenna 101 of the communication target through the adjacent loop antenna 103, such a magnetic field 104 can be generated. In FIG. 1, the arrows shown around the loop antenna indicate the direction of the current flowing through the loop antenna (the same applies hereinafter).

[0017] (First Embodiment) Figure 2 shows an example configuration of a reader / writer 200 as a communication device in the first embodiment. The reader / writer 200 is a multi-antenna reader / writer having multiple loop antennas 221. The reader / writer 200 communicates with wireless tags 230 such as RFID (Radio Frequency Identification) tags and RFID cards using each loop antenna 221, and performs data reading and writing to the wireless tags 230. The reader / writer 200 has a communication control unit 210 and an antenna unit 220. The communication control unit 210 and the antenna unit 220 are connected via a connector such as an SMA connector.

[0018] The communication control unit 210 performs various controls for communicating with the wireless tag 230. For example, the communication control unit 210 transmits a transmission signal to the wireless tag 230 containing information such as processing commands for the wireless tag (read commands, write commands, etc.), and receives a reception signal regarding the response from the wireless tag 230. The communication control unit 210 includes an MPU (Microprocessor) 211, a signal processing unit (SPU) 212, a transmission circuit 213, a reception circuit 214, a switch circuit 215, and a switch control circuit 218. In the example shown in Figure 2, the configuration related to communication processing in the communication control unit 210 is shown, but the communication control unit 210 may have other components, and it goes without saying that it may have a power supply circuit, for example.

[0019] The MPU 211 comprehensively controls the reader / writer 200 (communication control unit 210). The signal processing unit 212 performs signal processing on signals transmitted and received with the wireless tag 230 via the loop antenna 221. The transmission circuit 213 modulates the signal processed by the signal processing unit 212 according to a predetermined modulation scheme to generate and output a transmission signal. The generated transmission signal is supplied to the wireless tag 230 via the loop antenna 221. The reception circuit 214 demodulates the received signal supplied from the wireless tag 230 via the loop antenna 221 according to the modulation scheme and outputs it to the signal processing unit 212.

[0020] Switch circuit 215 is a switch circuit for selecting the loop antenna 221 to be communicated with. Switch circuit 215 also supplies a signal to a loop antenna 221 other than the loop antenna 221 to generate a magnetic field (a magnetic field in the opposite direction to the leakage magnetic field) that cancels out the leakage magnetic field from the loop antenna 221 to be communicated with. Switch circuit 215 has a selection circuit 216 and level adjustment circuits 217A to 217D.

[0021] The selection circuit 216 is implemented, for example, by a 1:N switch (where N is the number of loop antennas 221), and selectively selects the loop antenna 221 to be communicated from among the multiple loop antennas 221A to 221D. That is, the selection circuit 216 controls the connection between the transmitting circuit 213 and the receiving circuit 214 and the loop antennas 221A to 221D so that signals are transmitted and received to the loop antenna 221 to be communicated. For example, when loop antenna 221A is the loop antenna to be communicated, the selection circuit 216 connects the transmitting circuit 213 and the receiving circuit 214 to loop antenna 221A so that it can communicate. Similarly, when loop antenna 221B is the loop antenna to be communicated, the selection circuit 216 connects the transmitting circuit 213 and the receiving circuit 214 to loop antenna 221B so that it can communicate. The same control is applied when the loop antennas to be communicated are loop antenna 221C and loop antenna 221D.

[0022] The level adjustment circuits 217A to 217D adjust the level of the signal transmitted to a different loop antenna 221 from the target loop antenna 221 in order to generate a magnetic field that cancels out the leakage magnetic field from the target loop antenna 221. In this example, the transmission signal (modulated wave) sent to the target loop antenna 221 is output to loop antennas 221A to 221D after the current level is adjusted by the level adjustment circuits 217A to 217D. Note that the level adjustment circuit 217 corresponding to the target loop antenna 221 has its output in a high impedance (Hi-Z) state, and no signal is output.

[0023] An example of level adjustment circuits 217A to 217D is shown in Figure 3. Level adjustment circuits 217A to 217D are variable attenuators (attenuators) connected to resistors 301 to 303 and a switch 304, as shown in Figure 3, and attenuate the current level of the input signal before outputting it. By appropriately selecting the resistance values ​​of resistors 301 to 303, the amount of level adjustment in level adjustment circuits 217A to 217D can be changed. In level adjustment circuit 217 corresponding to the loop antenna 221 to be communicated, the switch 304 is controlled to be in the off state (open state, non-conductive state), and in level adjustment circuits 217 corresponding to the other loop antennas 221, the switch 304 is controlled to be in the on state (closed state, conducting state), and a signal with an attenuated current level is output. Note that the level adjustment circuits 217A to 217D shown in Figure 3 are just examples and are not limited to them. Variable output amplifiers and the like can also be applied as level adjustment circuits 217A to 217D.

[0024] The switch control circuit 218 controls the selection circuit 216 and level adjustment circuits 217A to 217D of the switch circuit 215 based on the control by the MPU 211. The switch control circuit 218 controls the connection state in the selection circuit 216 and the adjustment amount in the level adjustment circuits 217A to 217D according to the loop antenna 221 to be communicated.

[0025] The antenna section 220 includes loop antennas 221A to 221D and matching circuits 222A to 222D. The loop antennas 221A to 221D output a magnetic field corresponding to the supplied transmission signal and communicate with the wireless tag 230 placed on the antenna, exchanging signals with the wireless tag 230 regarding data reading and writing, etc. Each of the loop antennas 221A to 221D can communicate with the wireless tag 230 independently. The matching circuits 222A to 222D are circuits for impedance matching.

[0026] A method for preventing misreading of wireless tags by the reader / writer 200 in the first embodiment will be explained with reference to Figure 4. Figure 4 shows an example in the configuration shown in Figure 2 where the loop antenna to be communicated is loop antenna 221A, and loop antenna 221B is shown as an adjacent loop antenna. The transmission circuit 213 outputs a transmission signal (modulated wave) modulated by a command to the wireless tag, and a magnetic field 401 corresponding to the transmission signal is generated from the loop antenna 221A to which it is supplied. At this time, by applying a transmission signal (modulated wave) of the same phase to the adjacent loop antenna 221B, it is possible to generate a magnetic field of the same phase as the loop antenna 221A to which it is supplied. Then, by adjusting the current level of the transmission signal (modulated wave) supplied to the adjacent loop antenna 221B with the level adjustment circuit 217, a magnetic field of appropriate strength 402 is generated at the adjacent loop antenna 221B, canceling out the magnetic field leaking from the loop antenna 221A to the adjacent loop antenna 221B. By canceling out the stray magnetic field from the loop antenna being communicated with in this way, it is possible to prevent misreading of wireless tags placed on adjacent loop antennas.

[0027] The reader / writer 200 shown in Figure 2 is configured to supply the transmission signal (modulated wave) output from the transmission circuit 213 to the loop antenna 221 that is to be communicated with, and to supply the transmission signal (modulated wave) output from the transmission circuit 213 to each of the loop antennas 221 that are not to be communicated with, via the level adjustment circuit 217.

[0028] The reader / writer 200 then selects the loop antenna 221 on which the wireless tag 230 to be read / written is placed as the loop antenna to be communicated with, and reads / writes data to the wireless tag 230 placed on the selected loop antenna 221. At this time, the reader / writer 200 also supplies a level-adjusted transmission signal to the unselected loop antennas 221 using the level adjustment circuit 217. For example, when loop antenna 221A is selected as the loop antenna to be communicated with by the switch circuit 215, the other loop antennas 221B, 221C, and 221D are supplied with a level-adjusted transmission signal using the level adjustment circuits 217B, 217C, and 217D. The adjustment levels of each of the level adjustment circuits 217 are pre-adjusted so that a magnetic field of the same strength as the leakage magnetic field leaking out from the loop antenna 221 to be communicated is generated.

[0029] In this way, according to the reader / writer 200 in the first embodiment, an adjacent loop antenna generates a magnetic field of the same strength as the leakage magnetic field leaked from the loop antenna being communicated to the adjacent loop antenna, thereby canceling out the leakage magnetic field from the loop antenna being communicated to prevent misreading of wireless tags placed on the adjacent loop antenna. Furthermore, even if the strength of the magnetic field output by the loop antenna being communicated is increased, the leaked magnetic field can be canceled out by the adjacent loop antenna, so that it is possible to read multiple wireless tags simultaneously while preventing misreading of wireless tags placed on the adjacent loop antenna.

[0030] In the example described above, a modulated transmission signal (modulated wave) is supplied to the adjacent loop antenna, but an unmodulated carrier signal (carrier wave) may also be supplied. Figure 5 shows an example configuration when a carrier signal (carrier wave) is supplied to the adjacent loop antenna. Figure 5 shows another example configuration of the reader / writer as a communication device in the first embodiment. In Figure 5, components having the same function as those shown in Figure 2 are denoted by the same reference numerals, and redundant explanations are omitted.

[0031] The reader / writer 500 shown in Figure 5 has a communication control unit 510 and an antenna unit 220. The communication control unit 510 corresponds to the communication control unit 210 shown in Figure 2. The communication control unit 510 has an MPU 211, a signal processing unit (SPU) 212, a transmitting circuit 213, a receiving circuit 214, a switching circuit 215, and a switching control circuit 218, in addition to an amplifier 514.

[0032] The transmitting circuit 213 specifically represents the transmitting circuit shown in Figure 2 and includes an oscillator 511, a modulation circuit 512, and an amplifier 513. The oscillator 511 is an oscillator for generating a carrier signal (carrier wave). The carrier wave generated by the oscillator 511 is modulated by the transmission signal from the signal processing unit 212 by the modulation circuit 512 to generate a modulated wave. This modulated wave is output by the amplifier 513 and then output via the switch circuit 215. Furthermore, the carrier wave generated by the oscillator 511 is amplified by the amplifier 514, and then its level is adjusted by the level adjustment circuits 217A to 217D of the switch circuit 215 before being output to the loop antennas 221A to 221D. The control of the selection circuit 216 and level adjustment circuits 217A to 217D of the switch 215 is the same as in the example described above.

[0033] Thus, even if an unmodulated carrier signal (carrier wave) is supplied to an adjacent loop antenna, the adjacent loop antenna can generate a magnetic field with the same phase as the magnetic field generated by the loop antenna being communicated. Therefore, similar to the example above, by generating a magnetic field in the adjacent loop antenna with the same strength as the leakage magnetic field leaking from the loop antenna being communicated to the adjacent loop antenna, the leakage magnetic field from the loop antenna being communicated is canceled out, thereby preventing misreading of wireless tags placed on the adjacent loop antenna.

[0034] Furthermore, in the example shown in Figure 2, the transmitting circuit 213 and the receiving circuit 214 are connected to the loop antenna 221 to be communicated via the selection circuit 216 of the switching circuit 215. However, it is also possible to connect only the receiving circuit 214 to the loop antenna 221 to be communicated via the selection circuit 216 of the switching circuit 215, and connect the transmitting circuit 213 to the loop antenna 221 via the level adjustment circuit 217. In this case, the level adjustment circuit 217 corresponding to the loop antenna 221 to be communicated should output the transmission signal (modulated wave) from the transmitting circuit 213 as is without level adjustment, while the level adjustment circuit 217 corresponding to the loop antenna 221 that is not to be communicated should output the transmission signal (modulated wave) from the transmitting circuit 213 after level adjustment.

[0035] (Second embodiment) Figure 6 shows an example configuration of a reader / writer 600 as a communication device in the second embodiment. In Figure 6, components having the same function as those shown in Figure 2 are denoted by the same reference numerals, and redundant explanations are omitted. The reader / writer 600 is a reader / writer (multi-antenna reader / writer) having multiple loop antennas 221. The reader / writer 600 communicates with wireless tags 230 such as RFID tags and RFID cards using each loop antenna 221, and performs tasks such as reading and writing data to the wireless tags 230. The reader / writer 600 has a communication control unit 610 and an antenna unit 620. The communication control unit 610 and the antenna unit 620 are connected via a connector such as an SMA connector.

[0036] The communication control unit 610 performs various controls for communicating with the wireless tag 230, similar to the communication control unit 210 shown in Figure 2. The communication control unit 610 includes 211, a signal processing unit (SPU) 212, a transmitting circuit 213, a receiving circuit 214, a switching circuit 215, and a switching control circuit 611.

[0037] The switch circuit 215 selects the loop antenna 221 to be communicated. Unlike the first embodiment, the switch circuit 215 has a selection circuit 216 and does not have level adjustment circuits 217A to 217D. The switch control circuit 611 controls the selection circuit 216 and the side-reading prevention circuits 621A to 621D, which will be described later, of the switch circuit 215 based on the control by the MPU 211. The switch control circuit 218 controls the connection state of switches, etc., of the selection circuit 216 and the side-reading prevention circuits 621A to 621D according to the loop antenna 221 to be communicated.

[0038] The antenna section 620 includes loop antennas 221A to 221D and matching circuits 222A to 222D, as well as side-reading prevention circuits 621A to 621D. Each of the side-reading prevention circuits 621A to 621D is connected between the corresponding loop antennas 221A to 221D and matching circuits 222A to 222D. The side-reading prevention circuits 621A to 621D have capacitance (capacitors) and control the resonant frequency (tuned frequency) of the circuit including the loop antenna and capacitance. When the connected loop antenna 221 is not a loop antenna targeted for communication, the side-reading prevention circuits 621A to 621D control the resonant frequency (tuned frequency) of the circuit including the loop antenna and capacitance to tune to a frequency lower than the carrier signal (carrier wave).

[0039] An example of a horizontal reading prevention circuit 621A to 621D is shown in Figure 7. A horizontal reading prevention circuit 621A to 621D has, for example, a series-connected capacitor 701 and resistor 702, and two switches 703 and 704, as shown in Figure 7. Switches 703 and 704 are switches that switch between connecting the matching circuit 222 to the loop antenna 221 or connecting the series circuit of capacitor 701 and resistor 702. Specifically, if the corresponding loop antenna 221 is the loop antenna to be communicated, switches 703 and 704 are controlled to connect the loop antenna 221 to the matching circuit 222. If the corresponding loop antenna 221 is not the loop antenna to be communicated, switches 703 and 704 are controlled to connect the loop antenna 221 to the series circuit of capacitor 701 and resistor 702. In other words, if the corresponding loop antenna 221 is not the loop antenna being communicated with, a so-called series LCR circuit is formed by the loop antenna 221 and the capacitance (capacitor) 701 and resistor 702 of the cross-reading prevention circuit 621. If the required resistance value for resistor 702 is sufficiently small, it may be implemented using parasitic components without providing resistor 702.

[0040] A method for preventing misreading of wireless tags by the reader / writer 600 in the second embodiment will be explained with reference to Figure 8. Figure 8 shows an example in the configuration shown in Figure 6 where the loop antenna to be communicated is loop antenna 221A, and loop antenna 221B is shown as an example of an adjacent loop antenna. A magnetic field 801 corresponding to the transmitted signal is generated from the loop antenna 221A to which the transmitted signal (modulated wave) output from the transmitting circuit 213 is supplied. The magnetic field 801 output from the loop antenna 221A leaks out of the loop antenna 221A and links within the adjacent loop antenna 221B. As a result, a voltage is generated in the adjacent loop antenna 221B due to electromagnetic induction, and current flows in the adjacent loop antenna 221B due to the series-connected capacitor 802 and resistor 803 of the side-reading prevention circuit 621B. At this time, by tuning the resonant frequency of the circuit including the loop antenna 221B and capacitor 802 to a frequency lower than the carrier signal frequency, it is possible to generate a signal with the same phase as the transmitted signal (modulated wave) supplied to the target loop antenna 221A for communication with the adjacent loop antenna 221B. This signal with the same phase as the transmitted signal (modulated wave) generates a magnetic field 804 at the adjacent loop antenna 221B, canceling out the leakage magnetic field 801 from the target loop antenna 221A, thereby preventing misreading of wireless tags placed on the adjacent loop antenna.

[0041] The following explanation, with reference to Figure 9, describes how tuning the resonant frequency to a frequency lower than the carrier signal (carrier wave) makes it possible to generate a signal with the same phase as the transmitted signal (modulated wave) supplied to the target loop antenna for communication with an adjacent loop antenna.

[0042] Figure 9(A) shows an example of an electrical circuit model of the loop antenna 901 being communicated and the adjacent loop antenna 902. L1 represents the inductance of the loop antenna being communicated, and R1 represents the series resistance of the loop antenna being communicated. V3 represents the source of the carrier signal (carrier wave). L2 represents the inductance of the adjacent loop antenna, and R2 represents the series resistance of the adjacent loop antenna. C1 represents the capacitance for forming a series resonant circuit between the adjacent loop antennas. K is the coupling coefficient between inductances L1 and L2.

[0043] Here, the carrier frequency of the carrier signal (carrier wave) generated by the oscillation source V3 was set to 13.56 MHz, the inductances L1 and L2 to 1 μH, the coupling coefficient K of inductances L1 and L2 to (-0.1), the resistors R1 and R2 to 1 Ω, and the capacitance C1 was initially set to 138 pF (the resonant circuit is tuned to the carrier frequency) for the simulation. As a result, it was confirmed that by increasing the capacitance C1 and adjusting the resonant frequency of the adjacent loop antenna 902 to a frequency lower than the carrier signal (carrier wave), a signal 912 in phase with the signal 911 supplied to the target loop antenna is generated in the adjacent loop antenna, as shown in Figure 9(B) as an example. Figure 9(B) schematically shows the signal waveforms in the target loop antenna and the adjacent loop antenna. Furthermore, by changing the value of capacitance C1, it is possible to adjust the level as shown in Figure 9(C). Figure 9(C) shows the relationship between the value of capacitance C1 and the current flowing through inductance L2. As shown in Figure 9(C), increasing the value of capacitance C1 (lowering the resonant frequency) reduces the current flowing through inductance L2. In other words, increasing the value of capacitance C1 (lowering the resonant frequency) reduces the combined magnetic field of the leakage magnetic field from the loop antenna being communicated with and the adjacent loop antenna. Therefore, the magnetic field supplied to a wireless tag placed on an adjacent loop antenna becomes smaller, preventing misreading and other errors. For example, as shown in Figure 10, if the resonant frequency (tuned frequency) of an adjacent loop antenna is lowered below the carrier signal frequency of 13.56 MHz, the voltage obtained by the wireless tag as the operating voltage decreases, and it can be seen that it becomes very low around 12.6 MHz.

[0044] The reader / writer 600 shown in Figure 6 is equipped with a horizontal reading prevention circuit 621 for each of the multiple loop antennas 221, and an LCR circuit can be configured for each of the loop antennas 221.

[0045] The reader / writer 600 then selects the loop antenna 221 on which the wireless tag 230 on which data is to be read or written is placed as the loop antenna to be communicated, and controls switches 703 and 704 of the lateral reading prevention circuit 621 so that the loop antenna 221 and the matching circuit 222 are connected, and reads or writes data to the wireless tag 230 placed on the selected loop antenna 221. At this time, in the lateral reading prevention circuit 621 corresponding to the unselected loop antenna 221, the reader / writer 600 controls switches 703 and 704 so that the loop antenna 221 and the series circuit of the capacitor 701 and resistor 702 of the lateral reading prevention circuit 621 are connected, and controls the circuit including the loop antenna 221 and capacitor 701 so that it is tuned to a frequency that is appropriately set lower than the frequency of the carrier signal (carrier wave).

[0046] Specifically, for example, if loop antenna 221A is selected as the loop antenna to be communicated, the anti-side-reading circuits 621B, 621C, and 621D of loop antennas 221B, 221C, and 221D that are not to be communicated are controlled to tune to a frequency lower than the frequency of the carrier signal (carrier wave). Similarly, if loop antenna 221B is selected as the loop antenna to be communicated, the anti-side-reading circuits 621A, 621C, and 621D of loop antennas 221A, 221C, and 221D that are not to be communicated are controlled to tune to a frequency lower than the frequency of the carrier signal (carrier wave). The same applies if loop antennas 221C and 221D are selected as the loop antennas to be communicated.

[0047] In this way, the reader / writer 600 in the second embodiment generates a magnetic field in an adjacent loop antenna that cancels out the leaked magnetic field that has leaked from the loop antenna being communicated to the adjacent loop antenna, thereby preventing misreading of wireless tags placed on adjacent loop antennas. Furthermore, even if the strength of the magnetic field output by the loop antenna being communicated is increased, the leaked magnetic field can be canceled out in the adjacent loop antenna, so it is possible to prevent misreading of wireless tags placed on adjacent loop antennas while enabling the overlapping reading of many wireless tags.

[0048] It should be noted that the embodiments described above are merely examples of how the present invention can be implemented, and the technical scope of the present invention should not be interpreted as being limited by them. In other words, the present invention can be implemented in various forms without departing from its technical concept or its main features. [Explanation of Symbols]

[0049] 200, 500, 600 Reader / Writer 210, 510, 610 Communication Control Unit 211 MPU 212 Signal Processing Unit 213 Transmitter Circuit 214 Receiving Circuit 215 Switch Circuit 216 Selection Circuit 217 Level adjustment circuit 218, 611 Switch control circuit 220, 620 Antenna section 221 Loop Antenna 222 Matching circuit 230 Wireless Tags 621 Anti-horizontal reading circuit

Claims

1. A communication device that communicates with wireless tags, A loop antenna that outputs a magnetic field corresponding to a transmission signal for communication with the wireless tag and communicates with the wireless tag, comprising a plurality of loop antennas, each independently communicating with the wireless tag, A communication control means that outputs the transmission signal to the loop antenna to be communicated among the plurality of loop antennas, A communication device characterized by having a control means for controlling a loop antenna that is not the target of communication but is different from the loop antenna that is the target of communication, to generate a magnetic field that cancels out the leakage magnetic field from the loop antenna that is the target of communication.

2. The communication device according to claim 1, characterized in that the control means controls the loop antenna, which is not the communication target, to generate a magnetic field in the opposite direction to the magnetic field outside the loop antenna of the communication target.

3. The communication device according to claim 1 or 2, characterized in that the control means has a capacitance connected to the loop antenna when it is not the loop antenna to be communicated, and controls the resonant frequency of the circuit including the connected loop antenna and the capacitance to be tuned to a frequency different from the carrier frequency of the transmitted signal.

4. The communication device according to claim 3, characterized in that the control means controls the resonant frequency of the circuit including the connected loop antenna and the capacitor to be tuned to a frequency lower than the carrier frequency of the transmitted signal.

5. The communication device according to claim 3, characterized in that the control means has a switch that connects the communication control means to the loop antenna when it is the loop antenna to be communicated, and connects the capacitance to the loop antenna when it is not the loop antenna to be communicated.

6. The communication device according to claim 1 or 2, characterized in that the control means outputs a signal having the same phase as the transmission signal output to the loop antenna to be communicated to a loop antenna that is not the target of communication.

7. The communication device according to claim 6, characterized in that the control means outputs the transmission signal, whose level has been adjusted according to the strength of the magnetic field from the loop antenna to be communicated, to a loop antenna that is not the target of communication.

8. The communication device according to claim 6, characterized in that the control means outputs the transmission signal, whose current level has been attenuated according to the strength of the magnetic field from the loop antenna to be communicated, to a loop antenna that is not the target of communication.

9. The communication device according to claim 6, characterized in that the control means outputs a carrier wave of the transmission signal, whose level is adjusted according to the strength of the magnetic field from the loop antenna to be communicated, to a loop antenna that is not the target of communication.

10. A control method for a communication device having a loop antenna that outputs a magnetic field corresponding to a transmission signal related to communication with a wireless tag and communicates with the wireless tag, and having a plurality of loop antennas, each independently communicating with the wireless tag, A communication control step of outputting the transmission signal to the loop antenna that is the target of communication among the plurality of loop antennas, A control method for a communication device, characterized by comprising a control step of controlling a loop antenna that is not a communication target but is different from the loop antenna that is the communication target, to generate a magnetic field that cancels out the leakage magnetic field from the loop antenna that is the communication target.

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