Patent Publication Number: US-2010109841-A1

Title: Reader and management system

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims benefit of priority from the Japanese Patent Application No. 2008-284108, filed on Nov. 5, 2008, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a reader and a management system. 
     In various fields, RFID (Radio Frequency Identification) systems have been introduced which perform non-contact data transmission using radio transmission between an RFID tag with IC chip and a reader. The RFID tags include an active tag and passive tag. The active tag is provided with a battery, and the IC is driven by electric power from the battery. Further, there is also an active tag which includes a transmission unit. Meanwhile, the passive tag is not provided with a battery, and the IC is driven by receiving electric power from a reader; electromagnetic wave transmitted from the reader is reflected by the tag using a technique called Back Scatter, whereby transmission is performed. 
     Recently, article management systems have been studied which use a UHF-band or microwave-band RFID having longer transmission distance than an electromagnetic induction type HF-band RFID. In the article management systems, in order to limit the transmission area, it is assumed that a reader antenna and RFID tag communicates in the inner side of metal shelves or boxes. 
     Inside metal shelves or boxes, however, there are some cases where reading cannot be performed due to the position of tag. For example, when the tag is placed close to the metal wall, since electromagnetic wave cannot propagate through the metal, current is induced on the metal wall so that incident electric field is cancelled, and thus tangential components of the electric field become zero. Consequently, of the electromagnetic wave radiated from the reader, the electric field of a component parallel to the metal wall is cancelled in the vicinity of the metal wall, so it is difficult to read the tag placed close to the metal wall. 
     Further, the interference of the electromagnetic wave radiated from the reader with reflected electromagnetic wave from the metal wall produces standing wave, and the intensity of electric field lowers at the position of nodes of the standing wave, so it is difficult to read the tag. Particularly, for a passive tag not including a power source, it is also difficult to get a drive power source for the tag. 
     Thus, a radio transmission system has been proposed in which a reflecting plate is arranged facing the antenna wherein the plate makes the polarization plane of reflected electromagnetic wave different from that of incident electromagnetic wave from the antenna, and the reflecting plate reflects the electromagnetic wave transmitted from the antenna over the wireless IC tag, so that standing wave is suppressed (for example, refer to JP-A 2007-116451 (KOKAI)). 
     However, according to the related art radio transmission system, while standing wave can be suppressed, the performance in reading the tag placed close to the metal wall cannot sufficiently be improved. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, there is provided a reader that performs wireless communication with a tag inside a metal housing including a plurality of metal walls, the reader comprising: 
     an antenna which transmits and receives electromagnetic wave; 
     a transmission control device that writes and/or reads information to/from the tag via the antenna; and 
     a reflecting plate, reflecting the electromagnetic wave, and having a reflection phase different from that of the metal wall. 
     According to one aspect of the present invention, there is provided a management system comprising: 
     a metal housing including a plurality of metal walls capable of containing an article with a tag attached thereto; 
     an antenna which transmits and receives electromagnetic wave; 
     a transmission control device that writes and/or reads information to/from the tag via the antenna; and 
     a reflecting plate, arranged on at least one of the metal walls inside the metal housing, reflecting the electromagnetic wave, and having a reflection phase different from that of the metal wall. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are schematic configuration diagrams of a management system according to a first embodiment of the present invention; 
         FIG. 2  is a view for describing space coordinates of opposite metal walls of a metal housing according to the first embodiment; 
         FIGS. 3A to 3C  are electric field intensity distribution diagrams of the metal housing according to the first embodiment; 
         FIG. 4  is a top view of a reflecting plate according to the first embodiment; 
         FIG. 5A  is a cross-sectional view of the reflecting plate according to the first embodiment; 
         FIG. 5B  is a schematic view of the cross-section of the reflecting plate according to the first embodiment; 
         FIG. 6  is a graph illustrating reflection phase characteristics of the reflecting plate according to the first embodiment; 
         FIG. 7  is a top view of a reflecting plate according to a variation; 
         FIG. 8  is a cross-sectional view of the reflecting plate according to a variation; 
         FIG. 9  is a schematic configuration diagram of a management system according to a comparative example; 
         FIGS. 10A to 10C  are electric field intensity distribution diagrams of the metal housing according to the comparative example; 
         FIG. 11  is a schematic configuration diagram of a management system according to a variation; 
         FIG. 12  is a view illustrating an example of arranging a reflecting plate; 
         FIG. 13  is a view illustrating an example of arranging a reflecting plate; 
         FIG. 14  is a schematic configuration diagram of a management system according to a second embodiment of the present invention; and 
         FIG. 15  is a schematic configuration diagram of a management system according to a third embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present invention will be described below with reference to the drawings. 
     First embodiment 
       FIG. 1A  schematically illustrates a configuration of a management system according to a first embodiment of the present invention. The management system includes a metal housing  100  and a read device (reader). The read device includes reflecting plates  111  and  112  and a tag reader  120 . The metal housing  100  is composed of six metal walls  101  to  106 . Here, for illustrative purposes, the metal wall  105  positioned in the front side in  FIG. 1A  is not illustrated so that the inner side of the metal housing  100  can be seen; but actually the six sides of the metal housing are, as illustrated in  FIG. 1B , covered with the metal walls  101  to  106 . 
     The metal housing  100  contains an article  140  with a tag  130  attached thereto. Written in the tag  130  is identification data (ID) unique to the tag, information on the article  140 , and other types of information. The tag  130  is either an active tag or passive tag. 
     Inside the metal housing  100 , the reflecting plates  111  and  112  are arranged on the metal walls  101  and  102  facing each other. The reflecting plates  111  and  112  have a reflection phase different from the metal, implementing magnetic wall characteristics. The structure of the reflecting plates  111  and  112  will be described later. 
     The tag reader  120  includes a transmission control device  121  and antenna  122 . Inside the metal housing  100 , the transmission control device  121  transmits electromagnetic wave via the antenna  122  to the tag  130 , and also reads or writes information from/to the tag  130 . The frequency band of electromagnetic wave transmitted/received via the antenna is the UHF band or microwave band, for example. The UHF band and microwave band have longer transmission distance, but when a metal housing is used, the transmission area can be limited. 
     There will be described the electric field produced inside the metal housing  100  when electromagnetic wave is transmitted via the antenna  122 . As illustrated in  FIG. 2 , x-axis, y-axis and z-axis are set. More specifically, the metal walls  101  and  102  are parallel to the y-z plane; the metal walls  103  and  104  are parallel to the x-z plane; and the metal walls  105  and  106  are parallel to the x-y plane. Further, assume that the distance between the metal wall  101  and metal wall  102  is L, the x-axis coordinate of the metal wall  101  is Lmin, and the x-axis coordinate of the metal wall  102  is Lmax. The electric field on a plane parallel to the x-z plane indicated by the dotted line in  FIG. 2  will be described. Here, for simplicity, assume that the positions of the reflecting plates  111  and  112  are equal to those of the metal walls  101  and  102  and that the reflection phase is 0°. 
       FIG. 3  illustrates the distribution of electric field intensity E which appears inside the metal housing  100 . Here, the first to third modes are considered. 
       FIG. 3A  illustrates a case in which distance L satisfies Formula 1 and only the basic (first) mode exists. 
     
       
         
           
             
               
                 
                   
                     
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     where “f c ” is cut off frequency and “c” is the velocity of light. Therefore, c/f c  is free space wave length (cut off wave length) corresponding to cut off frequency. Formula 2 expresses the cut off wave length in the basic mode. 
     
       
         
           
             
               
                 
                   
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     Formula 3 expresses the cut off wave length in the second mode. 
     
       
         
           
             
               
                 
                   
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       FIG. 3B  illustrates a case in which distance “L” satisfies Formula 4 and the basic and second modes exist. 
     
       
         
           
             
               
                 
                   
                     
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     Formula 5 expresses the cut off wave length in the third mode. 
     
       
         
           
             
               
                 
                   
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     In this case, the electric field is composed of a combination of the basic mode and the second mode. 
       FIG. 3C  illustrates a case in which distance “L” satisfies Formula 6 and the first to third modes exist. The electric field is composed of a combination of the basic mode, second mode and third mode. 
     
       
         
           
             
               
                 
                   
                     
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     As evident from  FIGS. 3A to 3C , since the reflection phase of the reflecting plates  111  and  112  is zero, there occurs no cancellation of electric field at Lmin and Lmax (the positions of the metal walls  101  and  102 ) and thus the electric field intensity does not decrease. 
     The structure of the reflecting plate  111  ( 112 ) implementing the magnetic wall characteristics will be described.  FIG. 4  illustrates a top view of the reflecting plate  111 ; and  FIG. 5A  illustrates a sectional view of the reflecting plates  111  along the line A-A in  FIG. 4 . 
     The reflecting plate  111  includes a ground plane  151 , metal patch  152  arranged in a matrix shape on the ground plane  151 , dielectric material  153  arranged between the ground plane  151  and metal patch  152 , and via  154 , arranged substantially at the center of the metal patch  152 , and penetrating through the dielectric material  153 . The metal patch  152  is made of copper, for example. The dielectric material  153  is made of Teflon, for example. 
       FIG. 5B  is a schematic view partially illustrating the cross-section of the reflecting plate  111  illustrated in  FIG. 5A . As shown in  FIG. 5B , high frequency current readily accumulates electric charge in a section where the adjoining metal patches  152  are close to each other, and this section can be considered an equivalent circuit of capacitor C. 
     In a path that extends along the ground plane  151  facing the capacitor C, the change of phase occurs, and the path can be considered an equivalent circuit of inductance H. 
     Consequently, the reflecting plate  111  constitutes an LC (inductance-capacitance) resonator, and there exists a frequency band in which antiresonance occurs in a direction parallel to the ground plane  151 . In such frequency band, the impedance becomes significantly large, so the occurrence of high frequency current is suppressed and the reflection phase is close to 00. 
       FIG. 6  illustrates a result of simulating reflection phase characteristics on a mushroom surface when plane wave is vertically incident on an infinite periodic structure under a periodic boundary condition obtained by modeling an one-period structure of the periodic structure of the reflecting plate  111 . Here, the size (width) of the metal patch  152  is 36 mm, the gap between the metal patches is 2.0 mm, the permittivity of the dielectric material  153  is 10, the thickness of the dielectric material  153  is 5.0 mm, and the radius of the via is 0.25 mm. 
     The solid line in  FIG. 6  represents the reflection phase characteristics of the reflecting plate  111  having the structure illustrated in  FIGS. 4 and 5 . As evident from  FIG. 6 , the reflecting plate  111  has a reflection phase of 0°, that is, AMC (Artificial Magnetic Conductor) characteristics. 
     The reflection phase characteristics can be implemented at any frequency by setting the metal patch size, the gap between adjoining metal patches, the permittivity and thickness of the dielectric material  153 , and the like. Consequently, the AMC characteristics can be implemented substantially at the operating frequency. 
     When the reflection phase is  00 , the reflecting plate  111  works as a perfect magnetic wall. However, in a range where −90°≦θ≦90°, substantially in-phase reflection occurs, so the cancellation of electric fields is suppressed in the vicinity of the metal wall  101  provided with the reflecting plate  111 . Consequently, the reflecting plate  111  is preferably designed so that the reflection phase falls into the range of +90° at the operating frequency. 
     The broken line in  FIG. 6  represents a result of simulating the reflection phase characteristics when the reflecting plate  111  does not have the via  154  as illustrated in  FIGS. 7 and 8 .  FIG. 8  is a cross-sectional view of the reflecting plate  111  along the line B-B illustrated in  FIG. 7 . As evident from  FIG. 6 , substantially the same reflection phase characteristics are provided regardless of the presence or absence of the via. Consequently, the reflecting plate  111  may not have the via  154 . 
     The reflecting plates  111  and  112  having the above structure are arranged on the opposite metal walls  101  and  102  inside the metal housing  100 . Consequently, the NULL positions of electric field intensity in each mode are, as illustrated in  FIG. 3 , different. Thus, sufficient electric field intensity is provided regardless of whether or not the tag  130  is close to the metal walls  101  and  102  in the metal housing  100 , so that the information of the tag  130  can be read. 
     COMPARATIVE EXAMPLE 
       FIG. 9  schematically illustrates a configuration of a management system according to a comparative example. The management system includes a metal housing  900  and tag reader  920 . The metal housing  900  is composed of six metal walls  901  to  906 . The metal housing  900  contains an article  940  with a tag  930  attached thereto. Differently from the first embodiment, the reflecting plate  111  is not provided on the metal wall. Similarly to  FIG. 1A , the metal wall  905  positioned in the front side in  FIG. 9  is not illustrated. 
     The tag reader  920  includes a transmission control device  921  and antenna  922 . Inside the metal housing  900 , the transmission control device  921  transmits electromagnetic wave via the antenna  922  to the tag  930 , and reads information from the tag  930 . 
     There will be described the electric field produced inside the metal housing  900  when electromagnetic wave is transmitted via the antenna  922 . As illustrated in  FIG. 9 , x-axis, y-axis and z-axis are set. More specifically, the metal walls  901  and  902  are parallel to the y-z plane; the metal walls  903  and  904  are parallel to the x-z plane; and the metal walls  905  and  906  are parallel to the x-y plane. 
     Similarly to the first embodiment, assume that the distance between the metal wall  901  and metal wall  902  is L, the x-axis coordinate of the metal wall  901  is Lmin, and the x-axis coordinate of the metal wall  902  is Lmax. The electric field on a plane parallel to the x-z plane indicated by the dotted line in  FIG. 9  will be described. 
       FIG. 10  illustrates the distribution of electric field intensity E which appears inside the metal housing  900 . Here, the first to third modes are considered.  FIG. 10A  illustrates a case in which distance “L” satisfies Formula 7 and only the basic (first) mode exists. 
     
       
         
           
             
               
                 
                   
                     
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       FIG. 10B  illustrates a case in which distance “L” satisfies Formula 8 and the basic and second modes exist. 
     
       
         
           
             
               
                 
                   
                     
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     In this case, the electric field is composed of a combination of the basic mode and the second mode.  FIG. 10C  illustrates a case in which distance L satisfies Formula 9 and the first to third modes exist. The electric field is composed of a combination of the basic mode, second mode and third mode. 
     
       
         
           
             
               
                 
                   
                     
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     As evident from  FIGS. 10A to 10C , the electric field at Lmin and Lmax (the position of the metal walls  901  and  902 ) is zero in any of the modes. The reason for this is that the reflection phase is 180° on the metal wall, so there occurs cancellation of electric field in the vicinity of the metal wall. Thus, when the electric field of the tag antenna is parallel to the metal wall and the tag is disposed close to the metal wall, the tag reader  920  cannot communicate with the tag  930 . 
     According to the first embodiment, however, the reflecting plate (having a reflection phase different from the metal wall) implementing magnetic wall characteristics is provided on the metal wall of the metal housing. As a result, the cancellation of electric field in the vicinity of the metal wall is suppressed, so the tag reader can communicate with the tag which is disposed in the vicinity of the metal wall. 
     The structure of the reflecting plate  111  is not limited to those of  FIGS. 4 and 7  as long as the magnetic wall characteristics are implemented. For example, a magnetic material having a permeability greater than 1 can be used. 
     According to the first embodiment, the reflecting plates are arranged on the metal walls  101  and  102 . However, the reflecting plates may be arranged on any of the metal walls  101  to  106  depending on the orientation of the tag when the article is contained in the metal housing. 
     According to the first embodiment, the metal housing  100  is composed of the six metal walls  101  to  106 . However, as illustrated in  FIG. 11 , one metal wall, for example, the metal wall  105  may not be provided; in this case, a structure with one surface opened is formed. 
     With this structure, using electromagnetic wave radiated from the antenna  122  disposed outside the metal housing  100 , the information of the tag  130  attached to the article  140  contained in the metal housing  100  can be read. 
     Referring to  FIG. 11 , the reflecting plates  111  and  112  are arranged on the metal walls  101  and  102 . However, for example, when the metal housing  100  contains, as illustrated in  FIG. 12 , documents (or books)  141  placed side-by-side having attached thereto the tag  130  and the electric field of the tag  130  is parallel to the z-axis, the read device preferably further includes a reflecting plate  113  and the reflecting plate  113  is arranged on the metal wall  106 . 
     Further, when the orientation of the tag on containing the article can be substantially limited, the surface (metal wall) on which the reflecting plate is arranged can be limited to one parallel to the electric field of the tag. However, when the orientation of the tag is randomly set inside the metal housing, the reflecting plates are, as illustrated in  FIG. 13 , preferably provided on all the surfaces. That is, the read device includes the reflecting plates corresponding to all the surfaces of the metal housing. 
     Second Embodiment 
       FIG. 14  schematically illustrates a configuration of a management system according to a second embodiment of the present invention. The management system includes a metal housing  200  and a read device. The read device includes reflecting plates  211  and  212  and a tag reader  220 . The metal housing  200  is composed of six metal walls  201  to  206 . Here, similarly to  FIG. 1A , the metal wall  205  positioned in the front side in  FIG. 14  is not illustrated. 
     The metal housing  200  contains an article  240  with a tag  230  attached thereto. Written in the tag  230  is identification data (ID) unique to the tag, information on the article  240 , and other types of information. The tag  230  is either an active tag or passive tag. 
     Inside the metal housing  200 , the reflecting plates  211  and  212  are arranged on the metal walls  201  and  202  facing each other. The reflecting plates  211  and  212  have a structure similar to the reflecting plates  111  and  112  according to the first embodiment, and an explanation thereof is omitted. 
     The tag reader  220  includes a transmission control device  221  and antennas  222  and  223  arranged at different positions. Inside the metal housing  200 , the transmission control device  221  transmits electromagnetic wave via the antennas  222  and  223  to the tag  230  and reads the information. In this case, the transmission control device  221  performs diversity reception. More specifically, one of the antennas  222  and  223 , having better reception condition, is selected to receive data transmitted from the tag  230 . 
     In this way, two antennas are arranged at different positions, allowing more efficient communication with the tag. 
     The number of antennas is not limited to two, but may be three or more. Further, the reflecting plates may be provided not only on a pair of the opposite metal walls  211  and  212  but also on another metal wall. Further, the transmission control device  221  may be arranged inside the metal housing  200  or outside the metal housing  200 . Further, when two or more modes occurs in the electric field between the reflecting plates facing each other inside the metal housing, the antennas may be disposed at positions where NULL appears in the respective modes. 
     Third Embodiment 
       FIG. 15  schematically illustrates a configuration of a management system according to a third embodiment of the present invention. The management system is composed of metal shelves having multiple stages of containers and includes multiple shelf plates  301 ,  302 , . . . , reflecting plates  311  arranged on side walls of each stage, transmission control device  321  provided on the upper portion of the shelf and read devices with antennas  322  disposed on each reflecting plate  311 . Contained in the containers of each stage are articles (for example, documents)  340  having attached thereto a tag  330 . The transmission control device  321  is connected to the antennas  322  of each stage. 
     The reflecting plate  311  is similar to the reflecting plate  111  of the first embodiment. Similarly to the second embodiment, diversity reception is performed between the transmission control device  321  and the antennas  322  in each shelf stage. 
     Since the reflecting plate  311  is arranged on the side wall, the cancellation of electric field is suppressed in the vicinity of the side wall. Consequently, the information of the tag disposed close to the side wall can be read. Further, the antennas are provided on both side walls in respective stages to perform diversity reception, allowing more efficient communication with the tags. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.