Patent Publication Number: US-2019171922-A1

Title: Rfid tag

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. P2017-231796, filed on Dec. 1, 2017, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to an RFID tag and methods related thereto. 
     BACKGROUND 
     A tag (RFID tag) using radio frequency identification (RFID) technology receives and responds to radio waves from a reader device. The RFID tag sets a session time such that the reader device does not redundantly read the same RFID tag. For example, when the RFID tag responds to the reader device, responding to the reader device is prohibited until the session time elapsed. Therefore, the RFID tag prevents a useless response signal from overlapping a response signal of another RFID tag not to interfere with reading. 
     However, if a plurality of RFID tags are densely present, there are two interference phenomena of hindering reading of the RFID tag by the reader device. A first interference phenomenon is that a plurality of RFID tags share radio waves with finite power sent by the reader device, and each RFID tag has insufficient power. A second interference phenomenon is that the antennas of a plurality of RFID tags are electromagnetically coupled to each other to cause impedance mismatching between the antennas and an IC chip. In this case, once high-frequency power captured by the antenna is reflected at a connection point with the IC chip and is returned to the antenna to be re-radiated, thereby causing interference in reception by the reader device. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view illustrating a configuration example of an RFID tag according to a first embodiment; 
         FIG. 2  is a view for describing shift of a resonant frequency in an antenna of the RFID tag according to the first embodiment; 
         FIG. 3  is a block diagram illustrating the configuration example of the RFID tag; and 
         FIG. 4  is a view illustrating a configuration example of an RFID tag according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An exemplary embodiment provides an RFID tag capable of reducing deterioration of reading efficiency by a reader device. 
     In general, according to one embodiment, an RFID tag includes a plurality of antenna elements, a switch, and a control circuit. The switch is inserted between the plurality of antenna elements. The control circuit turns off the switch until a specified time elapsed after responding to radio waves from a reader device. In another embodiment, a method of mitigating interference when reading an RFID tag from a group of RFID tags densely arranged involves turning off a switch positioned between a plurality of antenna elements until a specified time elapses after responding to radio waves from a reader device. 
     Hereinafter, embodiments will be described with reference to the drawings. 
     It is assumed that the RFID tags according to first and second embodiments described below are attached to articles to be managed (such as books, products, parts, or the like). Information, such as an ID, of the RFID tag attached to the article is read by a reader device. In addition, the articles attached with the RFID tags are densely arranged and the reader device is used to read the RFID tags arranged at high density. A reader device used to read an RFID tag attached to each book placed in a library is exemplified. Since a plurality of books are aligned and arranged on shelves in the library, the RFID tags attached to the books are densely present. The RFID tags according to the first and second embodiments described below are reliably read by the reader device even when being densely present. 
     First Embodiment 
     First, an RFID tag according to a first embodiment will be described. 
       FIG. 1  is a view illustrating a configuration example of an RFID tag  1 A according to the first embodiment. 
     The RFID tag  1 A includes an IC chip  10 , a first antenna element  11 , a second antenna element  12 , and a high-frequency switch  13 . In the first embodiment, the RFID tag  1 A will be described as a passive-type tag. 
     The IC chip  10  includes various types of control circuits, a power supply circuit, a memory, and the like. The IC chip  10  includes a pair of antenna terminals connecting a balanced antenna. The IC chip  10  generates power for operation from radio waves received by the antenna connected to the antenna terminal. In addition, the IC chip  10  operates by the power generated from the received radio waves and performs wireless communication with a reader device through the antenna connected to the antenna terminal. That is, the IC chip  10  is a passive-type chip which operates by the power generated from the radio waves sent by the reader device and performs wireless communication with the reader device. 
     The first antenna element  11  and the second antenna element  12  are linked through the high-frequency switch  13 . In other words, the first antenna element  11  and the second antenna element  12  are divided by the high-frequency switch  13 . The first antenna element  11  is connected to each of the pair of antenna terminals of the IC chip  10 . The first antenna element  11  and the second antenna element  12  configure an antenna having a predetermined length when being connected through the high-frequency switch  13 . 
     The high-frequency switch  13  is a switch for switching the electrical connection state between the first antenna element  11  and the second antenna element  12 . For example, when the high-frequency switch  13  is turned on, the first antenna element  11  and the second antenna element  12  are electrically connected. When the high-frequency switch  13  is turned off, the first antenna element  11  and the second antenna element  12  are electrically disconnected. The high-frequency switch  13  may switch the electrical connection state between the first antenna element  11  and the second antenna element  12  according to a control signal from the IC chip  10 . For example, the high-frequency switch  13  is formed of a semiconductor chip such as a gallium arsenide FET having small insertion loss in a high-frequency region. 
     The first antenna element  11  and the second antenna element  12  connected through the high-frequency switch  13  are designed depending on a frequency band (RFID communication band) used in communication with the reader device. For example, a wavelength at the center frequency of the RFID communication band is set to λ. The wavelength λ does not indicate a physical length in air but indicates the wavelength of the electrical length obtained by multiplying the physical length by a shortening rate according to a specific dielectric constant of a base material (for example, a PET film, a printed board, or the like) forming a conductor serving as an antenna. In this case, a λ/4 antenna is connected to each of the antenna terminals of the IC chip  10  to form a λ/2 dipole-type antenna with the both antennas. 
     That is, a total length from the first antenna element  11  connected to one antenna terminal to the second antenna element  12  through the high-frequency switch  13  is λ/4. Therefore, the RFID tag  1 A is formed such that the entire length of the antenna connected to the pair of antenna terminals of the IC chip  10  becomes λ/2. In the example illustrated in  FIG. 1 , the sum of the length L 1  of the first antenna element  11 , the length L 2  of the second antenna element L 2 , and the length L 3  of the high-frequency switch  13  is designed to be λ/4. 
       FIG. 1  schematically illustrates the electrical connections between the parts. The first antenna element  11 , the high-frequency switch  13 , and the second antenna element  12  are successively linked to be connected to the IC chip  10 . Accordingly, the total length of the antenna connected to the IC chip  10  is the total length (L 1 +L 2 +L 3 ) of the first antenna element  11 , the high-frequency switch  13 , and the second antenna element  12 . 
     As illustrated in  FIG. 1 , when the high-frequency switch  13  is turned on, the first antenna element  11  and the second antenna element  12  are electrically connected through the high-frequency switch  13 . Accordingly, when the high-frequency switch  13  is turned on, the length of the antenna connected to one antenna terminal of the IC chip  10  (the total length of the first antenna element  11 , the high-frequency switch  13 , and the second antenna element  12 ) becomes λ/4. Therefore, the entire length of the RFID tag  1 A becomes λ/2 as a whole antenna. 
     In contrast, when the high-frequency switch  13  is turned off, the first antenna element  11  and the second antenna element  12  are electrically disconnected by the high-frequency switch  13 . Accordingly, when the high-frequency switch  13  is turned off, the length of the antenna connected to one antenna terminal of the IC chip  10  becomes shorter than λ/4. Therefore, the entire length of the antenna of the RFID tag  1 A becomes shorter than λ/2. 
     Next, the design of the antenna used in the above-described RFID tag  1 A will be described. 
     In the RFID tag  1 A, when the high-frequency switch  13  is turned on, the length of the antenna connected to the IC chip  10  becomes λ/2 and communication with the reader device at the RFID communication band is performed. In contrast, when the high-frequency switch  13  is turned off, in the RFID tag  1 A, the length of the antenna connected to the IC chip  10  becomes shorter. When the length of the antenna becomes shorter than λ/2 due to the high-frequency switch  13  turned off, the resonant frequency band is shifted to a higher frequency band compared to the RFID communication band. 
     Here, when the center frequencies before and after the shifting are respectively set to F 1  and F 2 , a shift rate is (F 2 −F 1 )/F 1 ×100. The shift rate may be appropriately set, but may be set to 30% or more in order to prevent unnecessary reflected waves. When the shift rate is determined, the center frequency F 2  of the shift band of the resonant frequency may be set in relation to the center frequency F 1  of the RFID communication band used in communication with the reader device. 
       FIG. 2  is a view illustrating an example of the RFID communication band and the shift band of the resonant frequency. 
     For example, in the passive-type RFID of a UHF band, a 920-MHz band (in a range of 916.8 to 922.2 MHz) is used as the RFID communication band. When the RFID communication band is a 920-MHz band, if the shift band of the resonant frequency band is roughly a higher frequency band than a 1200-MHz band, the shift rate becomes 30% or more. In this case, the length of the first antenna element  11  is designed such that the shift band of the resonant frequency is roughly a higher frequency band than the 1200-MHz band. As a specific example, if the length L 1  of the first antenna element  11  illustrated in  FIG. 1  is 6.2 cm or less, the sum of L 1 +L 2 +L 3  may be about 8 cm. 
     The high-frequency switch  13  may be formed of a semiconductor chip such as a gallium arsenide FET having small insertion loss in a high-frequency region. The high-frequency switch  13  formed of the gallium arsenide FET or the like has a slight fixed length. In the actual antenna design, the total length of the antenna may be designed to be a desired length by adjusting the length L 2  of the second antenna element  12 . 
     Next, the configuration of a control system of the RFID tag  1 A according to the first embodiment will be described. 
       FIG. 3  is a block diagram illustrating the configuration example of the RFID tag  1 A according to the first embodiment. 
     As illustrated in  FIG. 3 , the IC chip  10  of the RFID tag  1 A includes a control circuit  21 , an RF front end  22 , a non-volatile memory  23 , a clock recovery circuit  24 , and a power supply circuit  25 . The RF front end  22  of the IC chip  10  is connected to the first antenna element  11 . The control circuit  21  of the IC chip  10  is connected to the high-frequency switch  13  provided between the first antenna element  11  and the second antenna element  12 . 
     The control circuit  21  performs communication control, data processing, or the like. For example, the control circuit  21  realizes command analysis, state machine, timing control, and the like. In the configuration illustrated in  FIG. 2 , the control circuit  21  operates by power supplied from the power supply circuit  25 . The control circuit  21  receives a clock from the clock recovery circuit  24  to operate. The control circuit  21  receives information indicating a signal received from the reader device from a demodulation circuit  27  and outputs, to a modulation circuit  28 , a signal indicating information to be output to the reader device. In addition, the control circuit  21  accesses the non-volatile memory  23 . 
     The control circuit  21  includes a register  21   a.  The register  21   a  sets a flag (an inventory flag) showing whether responding to the reader device is prohibited or whether responding to the reader device is possible. The inventory flag stored in the register  21   a  is in on state when responding to the reader device is prohibited and is in off state when responding to the reader device is possible. That is, the inventory flag stored in the register  21   a  is in the on state for a time (session time) according to session setting after responding to the inventory from the reader device. 
     The control circuit  21  outputs a signal instructing the high-frequency switch  13  to be turned on or off in response to the inventory flag stored in the register  21   a.  For example, when the inventory flag is in an on state, the control circuit  21  outputs a signal instructing the high-frequency switch  13  to be turned off. When the high-frequency switch  13  is turned on, the first antenna element  11  and the second antenna element  12  are electrically connected. In addition, the control circuit  21  outputs a signal instructing the high-frequency switch  13  to be turned on when the inventory flag is in an off state. When the high-frequency switch  13  is turned off, the first antenna element  11  and the second antenna element  12  are electrically disconnected. 
     The RF front end  22  processes the signal input or output through the antenna. In the configuration example illustrated in  FIG. 3 , the RF front end  22  includes a rectenna  26 , the demodulation circuit  27 , and the modulation circuit  28 . The rectenna  26  rectifies and converts radio waves received by the antenna into DC currents. The rectenna  26  supplies the generated DC currents to the power supply circuit  25 . The demodulation circuit  27  demodulates the radio waves received by the antenna. The demodulation circuit supplies the demodulated signal to the control circuit  21 . The modulation circuit  28  modulates a signal (for example, ID information) indicating information to be transmitted. The modulation circuit  28  modulates the signal from the control circuit  21  and outputs the modulated signal to the antenna. 
     The non-volatile memory  23  is formed of a non-volatile memory device. The nonvolatile memory stores identification information (ID) assigned to the RFID tag, for example. The clock recovery circuit  24  generates a clock for operation based on the signal from the demodulation circuit  27 . The clock recovery circuit  24  supplies the generated clock signal to the control circuit  21 . The power supply circuit  25  supplies power for operation based on the DC current supplied from the rectenna  26 . 
     Next, operation of the RFID tag  1 A having the above-described configuration will be described. 
     In a standby state, the first antenna element  11  and the second antenna element  12  are electrically connected through the high-frequency switch  13 . The first antenna element  11  and the second antenna element  12  electrically connected through the high-frequency switch  13  receive radio waves from the reader device as an antenna for communication. The radio waves received by the antenna are supplied to the RF front end  22 . The rectenna  26  of the RF front end  22  converts the received radio waves into DC currents and supplies the DC currents to the power supply circuit  25 . The power supply circuit  25  supplies the DC currents supplied from the rectenna  26  to the parts in the IC chip  10  as power for operation. The control circuit  21  in the IC chip  10  is activated by the power supplied from the power supply circuit  25 . 
     The activated control circuit  21  sets a session time according to a command from the reader device received through the demodulation circuit  27 . The control circuit  21  sets the inventory flag to the on state (a predetermined bit set in the inventory flag is changed from 0 to 1) which is stored in the register  21   a  when responding to the reader device through the modulation circuit  28  with information such as an ID. When the inventory flag is set to the on state, the control circuit outputs a signal (resonant frequency shift signal) instructing the high-frequency switch  13  to be turned off. Specifically, the control circuit  21  outputs the resonant frequency shift signal with a voltage capable of causing the high-frequency switch  13  to be turned off when the inventory flag is in the on state (a predetermined bit is 1). 
     The control circuit  21  monitors whether an elapsed time after responding to the reader device is passed the session time based on the clock supplied from the clock recovery circuit  24 . The control circuit  21  sets the inventory flag to the off state when the elapsed time after responding exceeds the session time. When the inventory flag is in the off state, the control circuit  21  outputs a signal for turning on the high-frequency switch  13 . 
     That is, the control circuit  21  sets the inventory flag according to session setting and outputs a signal (resonant frequency shift signal) in conjunction with the inventory flag to the high-frequency switch  13 . The on or off state of the high-frequency switch  13  is determined by the resonant frequency shift signal in conjunction with the inventory flag. When the inventory flag indicating a non-responsive state is in the on state, the high-frequency switch  13  is turned off by the resonant frequency shift signal from the control circuit  21 . When the high-frequency switch  13  is turned off, the first antenna element  11  and the second antenna element  12  are electrically disconnected. As a result, during a period in the non-responsive state, the resonant frequency of the antenna of the RFID tag  1 A is set to a shift band (for example, a 1200-MHz band). 
     According to the first embodiment, the RFID tag switches on or off state of the switch inserted between the divided antenna elements in conjunction with the event flag. The RFID tag sets the inventory flag to the on state and turns off the switch to electrically disconnect the antenna elements in the non-responsive state with respect to the reader device. Therefore, the RFID tag in the non-responsive state can shorten the antenna elements by dividing the antenna elements with the switch, thereby reducing an effective aperture. As a result, the RFID tag in the non-responsive state can reduce reflection of the radio waves arriving at the antenna element and reduce interference waves with respect to the reader device. 
     Second Embodiment 
     Next, a second embodiment will be described. 
     The RFID tag described in the first embodiment has a configuration in which the two-divided antenna elements are linked by the switch. In an RFID tag according to the second embodiment, an antenna element is divided into three or more antenna elements, and the divided antenna elements are linked by a plurality of switches. As the number of divided antenna elements is increased, the divided antenna elements may become shorter. As the antenna elements become shorter, the effective aperture may be made smaller so that reflection of radio waves arriving at each antenna element may be further reduced. 
       FIG. 4  is a view illustrating the configuration example of an RFID tag  1 B according to the second embodiment. 
     The RFID tag  1 B according to the second embodiment illustrated in  FIG. 4  includes an IC chip  10 , a first antenna element  31 , a second antenna element  32 , a third antenna element  33 , a first high-frequency switch  34 , and a second high-frequency switch  35 . 
     The IC chip  10  of the RFID tag  1 B illustrated in  FIG. 4  may be realized by a passive-type chip having the same configuration as the IC chip illustrated in  FIG. 1 or 3  described in the first embodiment. Accordingly, the detailed description of the IC chip  10  of the RFID tag  1 B will be omitted. However, the IC chip  10  of the RFID tag  1 B is connected to the first antenna element  31  as illustrated in  FIG. 4 . In addition, the control circuit  21  in the IC chip  10  of the RFID tag  1 B is connected to the high-frequency switches  34  and  35 . 
     The first antenna element  31 , the second antenna element  32 , and the third antenna element  33  are three-divided antenna elements. The high-frequency switches  34  and  35  link three antenna elements  31 ,  32 , and  33  in series. In the example illustrated in  FIG. 4 , the high-frequency switch  34  is provided between the first antenna element  31  and the second antenna element  32 . The high-frequency switch  35  is provided between the second antenna element  32  and the third antenna element  33 . 
     The high-frequency switches  34  and  35  are switched on or off in response to a signal (resonant frequency shift signal) from the IC chip  10 . When the high-frequency switches  34  and  35  are turned on, the first antenna element  31 , the second antenna element  32 , and the third antenna element  33  are electrically connected to form the entire antenna. When the high-frequency switches  34  and  35  are turned off, the first antenna element  31 , the second antenna element  32 , and the third antenna element  33  are electrically disconnected. 
     In the example illustrated in  FIG. 4 , the lengths of the first antenna element  31 , the second antenna element  32 , the third antenna element  33 , the first high-frequency switch  34 , and the second high-frequency switch  35  are L 31 , L 32 , L 33 , L 34 , and L 35 , respectively. In this case, the total length of L 31 , L 32 , L 33 , L 34 , and L 35  may be designed to be about 8 cm corresponding to the RFID communication band. If the high-frequency switches  34  and  35  are semiconductor chips having slight lengths, the entire length of the antenna can be designed to a desired length by adjusting the lengths of the antenna elements  31 ,  32 , and  33 . 
     In the RFID tag  1 B illustrated in  FIG. 4 , the control circuit  21  in the IC chip  10  outputs the resonant frequency shift signal to the high-frequency switches  34  and  35 . The control circuit  21  outputs the resonant frequency shift signal in conjunction with the inventory flag stored in the register  21   a,  similarly to the first embodiment. That is, the control circuit  21  sets the inventory flag to the on state as a non-responsive state while the session time is elapsed after responding to the reader device. The control circuit  21  outputs the resonant frequency shift signal for turning off the high-frequency switches  34  and  35  when the inventory flag is in the on state (in the non-responsive state). 
     Accordingly, when the RFID tag  1 B is in the non-responsive state, the high-frequency switches  34  and  35  are turned off according to the resonant frequency shift signal from the control circuit  21 . When the high-frequency switches  34  and  35  are turned off, the antenna elements  31 ,  32 , and  33  are electrically disconnected. The antenna elements  31 ,  32 , and  33  are obtained by dividing the antenna element having a length corresponding to the RFID communication band into three elements. Since the antenna elements  31 ,  32 , and  33  are shortened due to the three-division, the effective aperture becomes smaller. 
     As described above, the second embodiment is exemplified on the antenna used in the RFID tag having the configuration in which the plurality of switches are inserted and the antenna element is divided into three or more antenna elements. In the RFID tag according to the second embodiment, as the number of antenna elements divided through the plurality of switches is increased, the divided antenna elements become shorter. As a result, as each antenna element becomes shorter, the effective aperture becomes smaller, and the effect of reducing reflection of radio waves arriving at the antenna element can be enhanced. 
     Although the first and second embodiments are described, these are merely exemplary and do not limit the scope of the invention. For example, although the antenna element is divided into three elements in the second embodiment, the antenna element may be divided into four or more elements by increasing the number of high-frequency switches. In addition, each antenna element does not need to be linear as illustrated in  FIG. 1 or 4  and may be bent or curved. 
     The RFID tag according to the above-described embodiment includes a passive-type IC chip, an antenna element divided into a plurality of elements, and a switch inserted between the antenna elements. The IC chip controls the switches in conjunction with flag information indicating that the RFID tag is in a non-responsive state for a specified time as a result of responding to a telegraphic message from the reader device. 
     In addition, in the RFID tag according to the embodiment, if the divided antenna elements are connected through the switch, the total length of the antenna connected to the IC chip is included in the wavelength of the communication frequency band with the reader device. Further, in the RFID tag according to the embodiment, if all or some of the switches are in a disconnection state, the resonant frequency of the total length of the antenna connected to the IC chip is included in a higher frequency band compared to the above-described communication frequency band. 
     According to the above-described embodiments, even if RFID tags are arranged at high density, the antenna aperture area of the RFID tag after responding becomes small and an overlapping area is reduced. Therefore, each RFID tag can receive necessary power from radio waves sent from the reader device, and the response of each RFID tag becomes reliable. Since the antenna element of the RFID tag after responding is shortened due to division, unnecessary reflected power from each antenna element is reduced. As a result, interference waves with respect to the reader device from the RFID tag, which already responded to the reader device, can be reduced and reception operation by the reader device becomes reliable. 
     In other words, even if the RFID tags according to the embodiments are arranged at high density, the overlooking by the reader device can be reduced. Since the reader device is able to efficiently recognize the RFID tag with little error, it is possible to improve the operational efficiency for such as inventory or inspection. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.