Patent Publication Number: US-2013241703-A1

Title: Rfid reader/writer and rfid tag system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims benefit of priority under 35 U.S.C. §119 to Japanese Application No. P2012-058698 filed on Mar. 15, 2012, which is expressly incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to an RFID reader/writer and an RFID tag system, particularly to an RFID reader/writer and an RFID tag system, which can prevent generation of a null point. 
     RELATED ART 
     Conventionally, an RFID tag system including an RFID tag and an RFID reader/writer is provided. In such a system, a radio wave transmitted from the RFID reader/writer is reflected by a floor or a wall, the reflected radio wave (a reflected wave) and a direct wave are combined to reach the RFID tag, and the RFID reader/writer receives a response signal of the RFID tag. 
     The conventional RFID tag system is configured as described above. Sometimes a phase difference between the direct wave and the reflected wave becomes 180 degrees when the radio wave from the RFID tag is reflected by the floor or the wall. At this point, when the direct wave and the reflected wave are incident to the RFID tag, unfortunately the direct wave and the reflected wave are combined and weakened to generate a point (the null point) where a reading error is generated. 
     SUMMARY 
     The present invention has been devised to solve the problems described above, and an object thereof is to provide an RFID reader/writer and an RFID tag system, in which the null point is not generated. 
     In accordance with one aspect of the present invention, an RFID reader/writer that conducts communication with an RFID tag includes an antenna that transmits and receives a linearly-polarized radio wave having a polarization plane leaned (i.e., slanted) at a predetermined angle with respect to a reflecting surface. 
     Preferably the predetermined angle is an angle at which a reflected wave from the reflecting surface and a direct wave from the antenna do not interfere with each other to be weakened. 
     Preferably the predetermined angle is about 45 degrees. 
     Preferably the antenna is one of a patch antenna, a dipole antenna, or a slot antenna. 
     In accordance with another aspect of the present invention, an RFID system includes an RFID tag and an RFID reader/writer, in which the RFID tag and the RFID reader/writer conduct communication with each other, wherein the RFID reader/writer includes an antenna that transmits and receives a linearly-polarized radio wave having a polarization plane leaned at a predetermined angle with respect to a reflecting surface, an antenna of the RFID tag is disposed so as to have a polarization plane identical to the polarization plane of the radio wave, and the antenna of the RFID tag receives and responds to the radio wave emitted from the antenna of the RFID reader/writer. 
     The RFID reader/writer of the present invention includes the antenna that transmits and receives the linearly-polarized radio wave having the polarization plane leaned at the predetermined angle with respect to the reflecting surface. Therefore, the RFID reader/writer is hardly influenced by a vertical wall surface or a horizontal floor surface, which are frequently provided in an environment where the RFID tag system is generally installed. 
     As a result, the null point can be reduced to establish the stable communication. An additional component is unnecessary for the RFID reader/writer of the present invention, and the RFID reader/writer can easily be constructed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram illustrating an entire configuration of an RFID tag system; 
         FIG. 2  is a perspective view illustrating a state in which an RFID tag according to a first embodiment is attached to an article; 
         FIGS. 3A and 3B  are perspective views illustrating the conventional case and the case that the RFID tag of the first embodiment is attached to the article; 
         FIG. 4  is a graph illustrating a difference of an effect between the conventional case and the case of the first embodiment; 
         FIGS. 5A and 5B  are perspective views illustrating configurations of a conventional RFID tag and an RFID tag according to a second embodiment; 
         FIG. 6  is a view illustrating a configuration of an RFID reader/writer according to a third embodiment; 
         FIG. 7  is a flowchart illustrating an operation of a controller of the RFID reader/writer of the third embodiment; 
         FIG. 8  is a block diagram illustrating a device that verifies an effect of the third embodiment; and 
         FIG. 9  is a graph illustrating the effect of the third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     (1) First Embodiment 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings.  FIG. 1  is a schematic block diagram illustrating an entire configuration of an RFID tag system  100  according to an embodiment of the present invention. The RFID tag system  100  includes an RFID reader/writer  10  and an RFID tag  50 . The RFID reader/writer  10  includes a controller  20  that controls the whole of the RFID reader/writer  10 , a transmitter  11  and a receiver  12 , which are connected to the controller  20 , and an antenna  13  that is connected to the transmitter  11  and the receiver  12 . 
     The controller  20  includes a CPU  23  that controls the controller  20 , a transmission data generator  21  that is controlled by the CPU  23  to generate such predetermined transmission data as a command, and a reception data processor  22  that processes received data received from the RFID tag  50 . The data generated by the transmission data generator  21  is transmitted to the RFID tag  50  through the transmitter  11  and the antenna  13 . A signal from the RFID tag  50  is processed by the reception data processor  22  through the antenna  13  and the receiver  12 . 
     A first embodiment of the present invention will be described below. In the first embodiment, the RFID reader/writer  10  and the RFID tag  50  communicate with each other using slant polarization. As used herein, the slant polarization means that an orientation of a composite wave of a horizontal electric field and a vertical electric field intersects a ground surface or a wall. For example, when being reflected by a floor or the wall, the slant polarization of obliquely right 45 degrees changes to the slant polarization of obliquely left 45 degrees. 
     At this point, horizontal polarization differs from vertical polarization in a phase when being reflected by the floor. In a metallic surface, because a phase difference between the horizontal polarization and the vertical polarization is about 180° at a reflection point, a reflected wave of the polarization of obliquely right 45 degrees becomes obliquely left 45 degrees. 
     In a radio wave, which is output from the RFID reader/writer  10  and incident to the floor or the wall in the slant polarization, a polarization direction changes by 90 degrees. Therefore, a direct wave and the reflected wave exist as a cross polarization component at a position of the RFID tag  50 . When the cross polarization component exists while the RFID tag is linear polarization, an influence of the reflected wave can be reduced by combining a polarization plane of the RFID tag and a polarization plane of the direct wave. 
     When both the antennas of the RFID reader/writer and RFID tag are slanted, the reflected wave is not matched with a polarization direction of the antenna of the RFID tag while the direct wave is matched with the polarization direction of the RFID tag, so that the influence of the reflected wave can be largely reduced. 
     As described in the related art, the generation of the null point can be prevented when the influence of the reflected wave is reduced. 
     A specific example in which the RFID tag  50  is obliquely placed on an article will be described below.  FIG. 2  is a view illustrating the case that an RFID tag  51  is provided on one end surface of such a rectangular-solid conveyance article (a conveyance medium)  60  as a pallet. The RFID tag  51  is provided on one end surface of the conveyance article  60 , and an antenna  52  (indicated by a dotted line in  FIG. 2 ) of the RFID tag  51  is obliquely placed with respect to a rectangular side constituting the end surface of the conveyance article  60 . When the RFID tag  51  is attached while slanted with respect to the rectangular side constituting the end surface of the conveyance article  60 , inevitably the antenna  52  of the RFID tag  51  is obliquely disposed with respect to a floor surface or a wall surface. 
     The antenna  52  of the RFID tag  51  is obliquely placed with respect to the floor surface or the wall surface by way of example. At this point, preferably an antenna of the RFID reader/writer is formed into a rod shape and the rod-shaped antenna is obliquely placed with respect to a horizontal surface and a vertical surface. The largest effect is obtained, when the slants of both the antenna  52  of the RFID tag  51  and the antenna of the RFID reader/writer are combined to obliquely place the antenna  52  of the RFID tag  51  and the antenna of the RFID reader/writer with respect to the floor surface or the wall surface. 
     The antenna of the RFID reader/writer may be a patch antenna or a slot antenna. For the patch antenna, a feeding point may be disposed such that the slant polarization is generated with respect to the floor surface or the wall surface. 
     The effect of this case will be described below.  FIGS. 3A and 3B  are views illustrating a device used in an experiment to detect a difference of the effect between the conventional configuration ( FIG. 3A ) and the configuration of the first embodiment ( FIG. 3B ). A metallic plate  61  was placed in a lateral (a side surface) direction instead of the ground surface, and a variation in reception level was detected in the case that the antenna  52  of an RFID tag  56  was placed in a direction parallel to the metallic plate  61  ( FIG. 3A ) and in the case that the antenna  52  was placed in an oblique 45-degree direction ( FIG. 3B ).  FIG. 4  is a graph illustrating an experimental result. At this point, the antenna (not illustrated) of an RFID reader/writer  62  is oriented toward the same direction as the antenna  52  of the RFID tag  56 . 
     In  FIG. 4 , a vertical axis indicates a reception level (dB) and a horizontal axis indicates a distance (m) from the RFID reader/writer  62 . A solid line indicates received data in the case that the antenna is vertically placed, and a dotted line indicates received data in the case that the antenna is obliquely placed by 45 degrees. 
     In an experimental environment, the null point is generated at a point of 2 m in the case that the antenna is vertically placed, while the influence can largely be reduced in the case that the antenna is obliquely placed. Accordingly, it is found that a large effect to prevent the falling out of the reading exists in the environment in which the slant polarization is dominantly reflected by the floor. 
     (2) Second Embodiment 
     A second embodiment of the present invention will be described below. In the conventional embodiment, as illustrated in  FIG. 5A , the RFID tag  51  including a horizontally extending antenna  52   a  is horizontally provided in a rectangular tag case  53   a  including a pair of horizontal side facing each other and a pair of vertical sides facing each other (see  FIG. 5A ). 
     On the other hand, in the second embodiment, the antenna of the RFID tag  54  is obliquely disposed by around 45 degrees with respect to a reflecting surface (the floor or the wall). Such a structure as the floor and the wall is horizontal or vertical to the ground surface, and the structure is rarely obliquely disposed. That is, usually the reflection point is horizontal or vertical to the ground surface. Because the RFID tag is placed on the pallet, frequently the RFID tag is used in the horizontal or vertical state. 
     Accordingly, in the case that the antenna of the RFID tag is obliquely placed with respect to the ground surface or the wall, the slant polarization is obtained when viewed from most reflection points. 
       FIG. 5B  is a perspective view illustrating the RFID tag of the second embodiment. As illustrated in  FIG. 5B , in the second embodiment, the RFID tag  54  includes a long antenna  52   b  provided in a rectangular surface. The antenna  52   b  is accommodated in a tag case  53   b  of the RFID tag  54 , and the antenna  52   b  extends in a direction oblique to sides  54   a  and  54   b  constituting the rectangular surface of the tag case  53   b . The tag case  53   b  may be a molding body including the antenna  52   b.    
     Usually, the RFID tag is attached to a tag attachment part provided in the article, and the tag attachment part is provided parallel or vertical to the side of the rectangular surface constituting the article. Accordingly, the polarization plane can be slanted only by attaching the RFID tag to the rectangular tag attachment part. 
     In the second embodiment, because the antenna  52   b  itself of the RFID tag  54  is obliquely provided, the antenna  52   b  is automatically obliquely placed when a user attaches the RFID tag  54  to the predetermined tag attachment part as usual. Therefore, the user can unintentionally use the slant polarization. 
     In the second embodiment, the tag case is formed into the rectangular shape by way of example. Alternatively, the tag case may have a polygonal shape including a side that becomes a base when the RFID tag is attached to the article, or an arc shape except the side that becomes the base. 
     In this case, the same effect as the second embodiment is also obtained. 
     (3) Third Embodiment 
     A third embodiment of the present invention will be described below. In the case that the pallet is used while the RFID tag is attached to the pallet, the pallet is placed in not only the horizontal direction but also the vertical direction. When the RFID tag is obliquely attached to the pallet by correct 45 degrees, the RFID tag can deal with both the horizontally-placed pallet and the vertically-placed pallet. However, actually the RFID tag is not always obliquely attached to the pallet by correct 45 degrees. In this case, in the first and second embodiments, a communication failure is generated, and the RFID tag cannot deal with both the horizontally-placed pallet and the vertically-placed pallet only when the slant polarization is used. For example, in the case that the RFID tag is placed at obliquely right 45 degrees while the pallet is horizontally placed, the RFID tag becomes obliquely left 45 degrees when the pallet is placed upright. In the case that the polarization plane of the RFID reader/writer is set to obliquely right 45 degrees according to the horizontally-placed pallet, the RFID reader/writer cannot read the RFID tag because the polarization plane of the RFID reader/writer is not matched with the polarization plane of the RFID tag. 
     The system of the third embodiment can deal with such a case. 
     In the communication field, sometimes transmitting and receiving antennas facing each other are operated in the circular polarization in order to reduce an influence of ground surface reflection. However, usually the antenna of the RFID tag is formed into a dipole shape, and operated as the linear polarization antenna. Accordingly, when attention is focused on the communication between the RFID reader/writer and the RFID tag, an effect of polarization diversity is small, and the maximum polarization plane is determined by the orientation of the RFID tag irrespective of the existence or non-existence of the reflection from the floor. For this reason, the RFID reader/writer transmits and receives the fixed polarization. 
     In the configuration of the RFID tag system of the third embodiment, the RFID tag conducts communication with the RFID reader/writer while the antenna of the RFID tag is obliquely placed, and the RFID reader/writer combines a maximum ratio using the polarization diversity while separating the vertical polarization and the horizontal polarization. 
     The patch antenna can be applied to the antenna of the RFID tag in order to transmit the circularly-polarized radio wave. 
       FIG. 6  is a block diagram illustrating a circuit configuration of the whole RFID reader/writer in the RFID tag system of the third embodiment. An RFID reader/writer  30  of the third embodiment includes an antenna  31 , a transmission circuit  32  that transmits desired data through the antenna  31 , a reception circuit  34   a  that receives the horizontal polarization component, a reception circuit  34   b  that receives the vertical polarization component, in which the phase of the received signal is changed by 90 degrees, using a 90-degree phase shifter  31   a , a combination circuit  36  that is connected to the reception circuits  34   a  and  34   b  to combine signals of the reception circuits  34   a  and  34   b , circulators  33   a  and  33   b  that switch between the reception circuits  34   a  and  34   b  and the transmission circuit  32 , and a controller  37  that controls all the circuits. 
     The reception circuits  34   a  and  34   b  receive the horizontal and vertical polarization components, respectively, and the combination circuit  36  logically combines the horizontal and vertical polarization components. The controller  37  reconfigures the data using the combination circuit  36  such that the data always becomes maximum S/N. 
       FIG. 7  is a flowchart illustrating the operation of a controller  37  of the RFID reader/writer  30  of the third embodiment. The transmission circuit  32  transmits start-up data through the antenna  31  in order to start up the RFID tag (not illustrated) (S 11 ). The antenna  31  receives the signal from the RFID tag. At this point, the reception circuit  34   a  receives the horizontal polarization component and the reception circuit  34   b  receives the vertical polarization component (S 12 ). The controller  37  separately stores the received data in a memory (not illustrated) (S 13 ). The combination circuit  36  combines the maximum ratio based on the stored data (S 14 ). Specifically, the combination circuit  36  combines the horizontal polarization component and the vertical polarization component such that an S/N ratio (i.e., a signal-to-noise ratio) of the received signal is enhanced compared with the case that the horizontal polarization component and the vertical polarization component are separately received. Accordingly, the combination circuit acts as the combination part and the optimum combination part. The optimum combination part may be configured to select the received signal having the higher S/N ratio in the received signals of the horizontal polarization component and vertical polarization component. The horizontal polarization and the vertical polarization are described in the third embodiment. Alternatively, the same effect is obtained even if the obliquely-right-45-degree polarization component and the obliquely-left-45-degree polarization component are received. 
     A verification of the effect of the third embodiment will be described below. In order to verify the effect of the third embodiment, the horizontal and vertical antennas are manually switched instead of the circuit in  FIG. 6  to check a difference of the null point. 
       FIG. 8  is a block diagram illustrating an entire configuration used in the verification. In the verification, the RFID tag  50  and the RFID reader/writer  40  are disposed while facing each other, and the metallic wall surface  61  is provided as the reflecting surface in the side surfaces of the RFID tag  50  and RFID reader/writer  40  as illustrated in  FIG. 3 . In  FIG. 8 , the numeral  10   a  designates the direct wave and the numeral  10   b  designates the reflected wave. The RFID reader/writer  40  includes a transmission antenna  32 , a reception antenna  34 , a circulator  33  that switches between the transmission antenna  32  and the reception antenna  34 , and a controller  39  that is connected to the circulator  33 . 
       FIG. 9  is a graph illustrating a relationship between a communication range (a horizontal axis) and a reception level (dB) of the RFID reader/writer  40  in each of horizontal polarization H (a dotted line), vertical polarization V (an alternate long and two short dashes line), and the state in which the horizontal polarization H and the vertical polarization V are combined to increase the reception level to the maximum, when the reception antenna  34  is manually switched in the configuration in  FIG. 8 .  FIG. 9  also illustrates circular polarization CIR (an alternate long and short dash line) just for reference. 
     In each of the horizontal polarization H and the vertical polarization V, the reception level varies significantly according to the communication range. However, when the horizontal polarization H and the vertical polarization V are combined to connect the pieces of data having the highest reception levels (a solid line), the high reception level is obtained irrespective of the communication range. That is, it is checked that the null point can be reduced when the maximum ratio is combined with the combination circuit. In this case, the reception level higher than that of the circular polarization CIR for reference is obtained. 
     In the third embodiment, from the viewpoint of a placement area, the one antenna is used as the transmission and reception antennas. Alternatively, the transmission and reception antennas may be separated. 
     In the third embodiment, the two reception circuits are provided as illustrated in  FIG. 6 . Alternatively, the one reception circuit is provided, a switching circuit is provided to process the horizontal polarization and the vertical polarization in a time-sharing manner, and the maximum ratio may be calculated based on two-time communication result. 
     The method for processing the horizontal polarization and the vertical polarization in the time-sharing manner is well known as an adaptic array antenna control method by, for example, Japanese Patent No. 4581534. Therefore, the specific description is omitted. 
     The embodiments of the present invention are described above with reference to the drawings. However, the present invention is not limited to the above embodiments. Various modification and changes can be made without departing from the identical or equivalent scope of the present invention with respect to the illustrated embodiments. 
     The null point is not generated in the RFID reader/writer of the present invention, so that the RFID reader/writer can advantageously be used.