Patent Publication Number: US-2023145916-A1

Title: Wireless tag communication device, image forming apparatus, and method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-184185, filed on Nov. 11, 2021, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate to a wireless tag communication device, an image forming apparatus, and a method. 
     BACKGROUND 
     There is a wireless tag communication device capable of communicating with a sheet attached with a wireless tag, using radio frequency identifier (RFID) technique. In addition, there is an image forming apparatus including the wireless tag communication device. 
     Some wireless tag communication devices are capable of transmitting radio waves including two types of polarized waves to a sheet. In addition, there is a sheet provided with a plurality of wireless tags. When the radio wave is transmitted to the sheet on which the plurality of wireless tags are provided, stable communication can be performed by transmitting a radio wave including an appropriate polarized wave of the two types of polarized waves. 
     However, the appropriate polarized wave cannot be unconditionally determined but depends on positions on a sheet provided with the wireless tags, which may disable stable communication. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is an external view showing an example of an overall configuration of an image forming apparatus according to an embodiment; 
         FIG.  2    is a block diagram showing a hardware configuration; 
         FIG.  3    is a diagram showing a specific example of an internal configuration; 
         FIG.  4 A  is a diagram showing a sheet A; 
         FIG.  4 B  is a diagram showing a sheet B; 
         FIG.  4 C  is a diagram showing a sheet C; 
         FIG.  4 D  is a diagram showing a sheet D; 
         FIG.  5    is a graph showing a sheet and a received power intensity; 
         FIG.  6    is a graph showing a sheet and a received power intensity; 
         FIG.  7    is a graph showing a sheet and a received power intensity; 
         FIG.  8    is a graph showing a sheet and a received power intensity; 
         FIG.  9    is a graph showing a sheet and a received power intensity; 
         FIG.  10    is a graph showing a sheet and a received power intensity; 
         FIG.  11    is a flowchart showing a flow of acquiring an appropriate polarized wave; 
         FIG.  12    is another flowchart showing the flow; and 
         FIG.  13    is still another flowchart showing the flow. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a wireless tag communication device, an image forming apparatus, and a method that are capable of performing stable communication with a plurality of wireless tags provided on one sheet are provided. 
     The wireless tag communication device according to the embodiment includes an antenna, a first maximum value acquiring unit, a second maximum value acquiring unit, a first minimum value acquiring unit, a second minimum value acquiring unit, and a polarization direction determining unit. The antenna can switch a polarization direction of a radio wave transmitted to a wireless tag. The first maximum value acquiring unit is configured to acquire, for each of n (n is an integer of 2 or more) wireless tags provided on a sheet, a maximum value M 1 ( k ) (k=1 to n) of a received power intensity of the wireless tag when the radio wave is transmitted from the antenna to the sheet in a first polarization direction. The second maximum value acquiring unit is configured to acquire, for each of the wireless tags, a maximum value M 2 ( k ) (k=1 to n) of a received power intensity of the wireless tag when the radio wave is transmitted from the antenna in a second polarization direction that is different from the first polarization direction. The first minimum value acquiring unit is configured to acquire a minimum value m 1  of M 1 ( k ) (k=1 to n). The second minimum value acquiring unit is configured to acquire a minimum value m 2  of M 2 ( k ) (k=1 to n). The polarization direction determining unit is configured to determine the polarization direction along which one of m 1  and m 2  that is not smaller is acquired as the polarization direction of the radio wave transmitted to the sheet. 
     In the wireless tag communication device according to the embodiment, stable communication can be performed with a plurality of wireless tags provided on one sheet. Hereinafter, the wireless tag communication device according to the embodiment and an image forming apparatus including the wireless tag communication device will be described in detail. 
       FIG.  1    is an external view showing an example of an overall configuration of an image forming apparatus  100  according to the embodiment. The image forming apparatus  100  is, for example, a multifunction device. The image forming apparatus  100  includes a display  110 , a control panel  120 , a printer unit  130 , a sheet storage unit  140 , and an image reading unit  200 . The printer unit  130  of the image forming apparatus  100  may be an electrophotographic device that fixes a toner image, or an inkjet device. 
     The image forming apparatus  100  forms an image on a sheet using a developer such as a toner. The sheet is, for example, paper or label paper. The sheet may be a sheet attached with a wireless tag. The sheet may be any object as long as the image forming apparatus  100  can form the image on a surface of the object. As the sheet attached with the wireless tag, the wireless tag may be attached to the surface of the sheet, or the wireless tag may be embedded inside the sheet. The wireless tag according to the present embodiment is a wireless tag using a radio frequency identifier (RFID) technique, and is also called an RF tag. 
     The display  110  is an image display device such as a liquid crystal display or an organic electroluminescence (EL) display. The display  110  displays various kinds of information related to the image forming apparatus  100 . 
     The control panel  120  includes a plurality of buttons. The control panel  120  receives an operation of a user. The control panel  120  transmits a signal corresponding to the operation performed by the user to a control unit of the image forming apparatus  100 . The display  110  and the control panel  120  may be configured as an integrated touch panel. 
     The printer unit  130  forms an image on the sheet based on image information generated by the image reading unit  200  or image information received via a communication path. The printer unit  130  forms the image by, for example, the following processes. The printer unit  130  forms an electrostatic latent image on a photoconductor drum based on the image information. The printer unit  130  forms a visible image by adhering the developer to the electrostatic latent image. The toner is a specific example of the developer. The printer unit  130  transfers the visible image onto the sheet. The printer unit  130  fixes the visible image on the sheet by heating and pressurizing the sheet. The sheet formed with the image may be a sheet stored in the sheet storage unit  140 , or a manually fed sheet. The sheet formed with the image is discharged to a paper discharge unit  210 . 
     The sheet storage unit  140  stores the sheet used for image formation in the printer unit  130 . In the present embodiment, the sheet storage unit  140  is provided with four paper feed cassettes. 
     The image reading unit  200  reads the image information to be read based on brightness of light. The image reading unit  200  records the read image information. The recorded image information may be transmitted to another information processing device via a network. The recorded image information may be used to form an image on the sheet by the printer unit  130 . 
       FIG.  2    is a block diagram showing a hardware configuration of the image forming apparatus  100  according to the embodiment. The image forming apparatus  100  includes the display  110 , the control panel  120 , the printer unit  130 , a paper feeding unit  205 , a storage device  151 , a memory  152 , a processor  153 , a wireless tag communication device  154 , an external interface  155 , and the image reading unit  200 . Since the display  110 , the control panel  120 , the printer unit  130 , and the image reading unit  200  are described above, description thereof will be omitted. The paper feeding unit  205  is a mechanism that feeds sheets placed in the sheet storage unit  140  and a manual feed tray described later to the printer unit  130 . Hereinafter, the storage device  151 , the memory  152 , the processor  153 , the wireless tag communication device  154 , and the external interface  155  will be described. The functional units are connected via a system bus  160  in a data-communicable manner. 
     The storage device  151  is, for example, a hard disk or a solid-state drive (SSD), and stores various kinds of data. The various kinds of data are a print job received from an external communication device and a software program for controlling an operation of each functional unit of the image forming apparatus  100 . The print job may be a job related to double-sided printing or a job related to printing of a plurality of sheets. The print job may include image information related to an image to be printed on the sheet. 
     The memory  152  temporarily stores data used by each functional unit provided in the image forming apparatus  100 . The memory  152  is, for example, a random-access memory (RAM). The memory  152  may store digital data generated by the image reading unit  200 . The memory  152  may temporarily store the print job of printing the sheet by the printer unit  130  and write information to be written to the wireless tag. 
     The processor  153  controls the operation of each functional unit of the image forming apparatus  100 . The processor  153  executes the process by loading the software program stored in the storage device  151  into the memory  152 , and executing the software program. Here, a specific process of the processor  153  will be described with an example. 
     The processor  153  controls printing on a sheet attached with a wireless tag based on a print job received via the external communication device or the control panel  120 . When the print job related to the sheet attached with the wireless tag is received, the processor  153  acquires write information specified in the print job and image information associated with the write information from, for example, a write information server (not shown). The image information associated with the write information is information related to the image to be formed on the sheet. The image information is not required to be associated with the write information. In this case, the image information may be included in the print job. The processor  153  controls the paper feeding unit  205 . The paper feeding unit  205  feeds the sheet attached with the wireless tag. 
     The processor  153  controls the printer unit  130 . The printer unit  130  forms, on the sheet, an image indicated by the image information. The sheet formed with the image is discharged to the paper discharge unit  210 . The processor  153  communicates with the wireless tag communication device  154 . For example, the processor  153  notifies the wireless tag communication device  154  of start of conveying the sheet, and the like. 
     The wireless tag communication device  154  includes an arithmetic device  501 , a storage device  502 , a communication device  503 , and an antenna  504 . The arithmetic device  501  is, for example, a central processing unit (CPU) or an application-specific integrated circuit (ASIC). The storage device  502  is a read-only memory (ROM), a random-access memory (RAM), or the like. The storage device  502  stores a received power intensity (RSSI) of the wireless tag, a timing at which the received power intensity is acquired, identification information (for example, a unique identifier (UID)) for identifying the wireless tag, and the like. 
     The communication device  503  transmits radio waves in two different polarization directions (a first polarization direction and a second polarization direction). The communication device  503  acquires information from the wireless tag or writes the information to the wireless tag using the antenna  504 , which is capable of switching the polarization direction of the radio waves transmitted to the wireless tag. The wireless tag communication device  154  writes the information to the wireless tag provided on the sheet. Further, the wireless tag communication device  154  reads the above identification information from the wireless tag provided on the sheet, and the like. In the following description, the radio wave in the first polarization direction may be simply described as “first polarized wave”. Similarly, the radio wave in the second polarization direction may be simply described as “second polarized wave”. 
     Data transmission and reception with another device is performed via the external interface  155 . Here, the other device is, for example, an information processing device such as a personal computer, a tablet computer, or a smart device. The external interface  155  operates as an input interface to receive the data or an instruction transmitted from the other device. The instruction transmitted from the other device is the print job or the like. The data transmitted from the other device is the write information, the image information associated with the write information, and the like. Further, the external interface  155  operates as a transmission interface to transmit the data to the other device. 
       FIG.  3    is a diagram showing an internal configuration of the image forming apparatus  100 .  FIG.  3    additionally shows a manual feed tray  220 . The sheet is conveyed from the sheet storage unit  140  or the manual feed tray  220  along the conveyance path  250 , formed with an image by the printer unit  130 , and discharged to the paper discharge unit  210  by discharge rollers  156 . The wireless tag communication device  154  communicates with the wireless tag during the conveyance along the conveyance path  250 . 
     Next, an example of the sheet provided with the plurality of wireless tags will be described. The wireless tag communication device  154  can transmit the first polarized wave and the second polarized wave as described above. However, in many cases, the appropriate radio wave with respect to the sheet cannot be unconditionally determined but depends on positions on the sheet provided with the wireless tags. Therefore, a manufacturer of the wireless tag communication device  154  or the image forming apparatus  100  including the wireless tag communication device  154  may set recommended polarized waves for various sheets in advance. In order to determine the polarization direction, an appropriate polarized wave acquisition method described below will be performed. 
       FIGS.  4 A,  4 B,  4 C, and  4 D  are diagrams showing examples of four types of sheets provided with a plurality of wireless tags. For each of the sheets, a conveyance direction is an upward direction in the drawing. That is, the sheets are conveyed upward in the drawings. In  FIGS.  4 A,  4 B,  4 C, and  4 D , reference numerals  400 - 1 ,  400 - 2 , and  400 - 3  indicate the wireless tags.  FIG.  4 A  is a diagram showing an example of a sheet provided with two wireless tags  400 - 1  and  400 - 2 . In the sheet shown in  FIG.  4 A , the wireless tag  400 - 1  is provided parallel to the conveyance direction, and the wireless tag  400 - 2  is provided perpendicular to the conveyance direction. The type of the sheet shown in  FIG.  4 A  may be referred to as “sheet A” in the following description. 
       FIG.  4 B  is a diagram showing an example of a sheet provided with three wireless tags  400 - 1 ,  400 - 2 , and  400 - 3 . In the sheet shown in  FIG.  4 B , the wireless tags  400 - 1 ,  400 - 2 , and  400 - 3  are provided at equal intervals, and all of the wireless tags  400 - 1 ,  400 - 2 , and  400 - 3  are provided perpendicular to the conveyance direction. The type of the sheet shown in  FIG.  4 B  may be referred to as “sheet B” in the following description. 
       FIG.  4 C  is a diagram showing an example of the sheet provided with two wireless tags  400 - 1  and  400 - 2 . In the sheet shown in  FIG.  4 C , the wireless tags  400 - 1  and  400 - 2  are provided with an interval that is narrower than those in  FIG.  4 B , and both the wireless tags  400 - 1  and  400 - 2  are provided perpendicular to the conveyance direction. The type of the sheet shown in  FIG.  4 C  may be referred to as “sheet C” in the following description. 
       FIG.  4 D  is a diagram showing an example of a sheet provided with two wireless tags  400 - 1  and  400 - 2 . In the sheet shown in  FIG.  4 D , the wireless tags  400 - 1  and  400 - 2  are provided closer to a left side and in parallel to the conveyance direction. The type of the sheet shown in  FIG.  4 D  may be referred to as “sheet D” in the following description. 
     In the following  FIGS.  5  to  10   , one of the four types of sheets A to D described above and the received power intensity will be described.  FIGS.  5  to  10    show, in the form of graphs, results of measurement of the received power intensities by the appropriate polarized wave acquisition method described below. The sheet A is used in  FIGS.  5 ,  7   , and  8 , and the received power intensities are different in the drawings. The sheet A is used for the sake of convenience in order to simplify the description, and is described as sheets having different properties in  FIGS.  5 ,  7 , and  8   . 
     The appropriate polarized wave acquisition method will be described. In the following description, when no response is received from the wireless tag, the received power intensity of the wireless tag is set to 0. 
     First, the wireless tag communication device  154  acquires, for each of n (n is an integer of 2 or more) wireless tags provided on the sheet, a maximum value M 1 ( k ) (k=1 to n) of the received power intensity of the wireless tag when the radio wave is transmitted from the antenna  504  to the sheet in the first polarization direction. 
     Next, the wireless tag communication device  154  acquires, for each of the wireless tags, a maximum value M 2 ( k ) (k=1 to n) of the received power intensity of the wireless tag when the radio wave is transmitted from the antenna  504  in the second polarization direction. 
     The wireless tag communication device  154  acquires a minimum value m 1  of M 1 ( k ) (k=1 to n), acquires a minimum value m 2  of M 2 ( k ) (k=1 to n), and determines the polarization direction along which one of m 1  and m 2  that is not smaller is acquired as a polarization direction of the radio wave transmitted to the sheet. 
     The graphs shown in  FIGS.  5  to  10    described below are graphs when n=2 or n=3. A vertical axis represents the received power intensity of each wireless tag provided on the sheet, and a horizontal axis represents a time. Further, in a state in which the wireless tag communication device  154  is transmitting the first polarized wave, a sheet of one type is conveyed, and the wireless tag communication device  154  measures the received power intensity. Next, in a state in which the wireless tag communication device  154  is transmitting the second polarized wave, a sheet of the same type as that of the first conveyed sheet is conveyed again, and the wireless tag communication device  154  measures the received power intensity. The first conveyed sheet may be conveyed again, or a sheet that is a sheet of the same type but is different from the first conveyed sheet may be conveyed. 
     Since the wireless tag communication device  154  transmits the first polarized wave and the second polarized wave in this order as described above, the horizontal axis indicating the time indicates that the first polarized wave and the second polarized wave are transmitted in this order. In  FIGS.  5  to  10   , the received power intensity of the tag  400 - 1  provided on each sheet is indicated by a solid line, and the received power intensity of the tag  400 - 2  provided on the sheet is indicated by a broken line. In  FIG.  6   , the received power intensity of the tag  400 - 3  is indicated by a dot-and-dash line. 
     Next, a maximum value of the received power intensity of the tag  400 - 1  is M 1 ( 1 ), a maximum value of the received power intensity of the tag  400 - 2  is M 1 ( 2 ), and a maximum value of the received power intensity of the tag  400 - 3  is M 1 ( 3 ) when the first polarized wave is transmitted from the antenna  504  of the wireless tag communication device  154 . A maximum value of the received power intensity of the tag  400 - 1  is M 2 ( 1 ), a maximum value of the received power intensity of the tag  400 - 2  is M 2 ( 2 ), and a maximum value of the received power intensity of the tag  400 - 3  is M 2 ( 3 ) when the second polarized wave is transmitted from the antenna  504  of the wireless tag communication device  154 . 
     m 1  represents a minimum value of M 1 ( 1 ) and M 1 ( 2 ), or a minimum value of M 1 ( 1 ), M 1 ( 2 ), and M 1 ( 3 ). m 2  represents a minimum value of M 2 ( 1 ) and M 2 ( 2 ), or a minimum value of M 2 ( 1 ), M 2 ( 2 ), and M 2 ( 3 ). Further, the polarization direction along which the one of m 1  and m 2  that is not smaller is acquired is determined as the polarization direction of the radio wave transmitted to the sheet. As indicated by symbols M 1  and m 2 , a numerical value (1 or 2) following M or m corresponds to the polarization direction. That is, M 1  and m 1  are values related to the first polarization direction, and M 2  and m 2  are values related to the second polarization direction. Based on the above,  FIGS.  5  to  10    will be described below. 
       FIG.  5    is a graph showing the sheet A and the received power intensity. Since the sheet A is provided with two wireless tags, n=2. As shown in  FIG.  5   , m 1  is M 1 ( 2 ), and m 2  is M 2 ( 1 ). The one of m 1  and m 2  that is not smaller is M 2 ( 1 ). Since the polarization direction along which M 2 ( 1 ) is acquired is the second polarization direction, the second polarization direction is an appropriate polarization direction with respect to the sheet A. Accordingly, the wireless tag communication device  154  determines the second polarization direction as a polarization direction of a radio wave transmitted to the sheet A. 
       FIG.  6    is a graph showing a sheet B and the received power intensity. Since the sheet B is provided with three wireless tags, n=3. As shown in  FIG.  6   , m 1  is M 1 ( 3 ), and m 2  is M 2 ( 1 ). The one of m 1  and m 2  that is not smaller is M 1 ( 3 ). Since the polarization direction along which M 1 ( 3 ) is acquired is the first polarization direction, the first polarization direction is an appropriate polarization direction with respect to the sheet B. Accordingly, the wireless tag communication device  154  determines the first polarization direction as a polarization direction of a radio wave transmitted to the sheet B. 
     Before description of the following  FIG.  7   , a case in which both m 1  and m 2  are not 0 and no significant difference is between m 1  and m 2  will be described. When there is no significant difference between m 1  and m 2 , the appropriate polarization direction may be set to either one of the first polarization direction and the second polarization direction, but may also be set as follows. 
     The wireless tag communication device  154  sorts M 1 ( k ) in ascending order into m 1 ( k ) and sorts M 2 ( k ) in ascending order into m 2 ( k ). Further, the wireless tag communication device  154  increments k from 1, and acquires first k for which |m 1 ( k )−m 2 ( k )| is no less than a threshold value m. That is, the wireless tag communication device  154  finds k having a large difference (a difference no less than the threshold value) in order from a smallest maximum value. Further, the wireless tag communication device  154  determines the polarization direction along which one of m 1 ( k ) and m 2 ( k ) that is not smaller is acquired as the polarization direction of the radio wave transmitted to the sheet. 
     When |m 1 ( k )−m 2 ( k )| is less than m even at the maximum value of k, that is, when k=n, the wireless tag communication device  154  acquires the polarization direction along which one of m 1 ( 1 ) and m 2 ( 1 ) that is not smaller is acquired as the appropriate polarization direction. This is because that it is preferable that the minimum received power intensities (m 1 ( 1 ), m 2 ( 1 )) is not smaller. 
     Since no significant difference is between m 1 ( j ) and m 2 ( j ) with respect to j&lt;k, the appropriate polarization direction may be set to any one of the first polarization direction and the second polarization direction with respect to m 1 ( j ) and m 2 ( j ). On the other hand, since a significant difference is between m 1 ( k ) and m 2 ( k ), the polarization direction along which the maximum value of the one of m 1  and m 2  that is not smaller described above is acquired may be determined as the polarization direction of the radio wave transmitted to the sheet. In this way, more stable communication can be performed. The “significant difference”, that is, the threshold value m, is appropriately determined depending on performance or a design situation of the wireless tag communication device  154  or the wireless tags, or the like.  FIG.  7    will be described based on the above. 
       FIG.  7    is a graph showing a sheet A and the received power intensity. Since the sheet A is provided with two wireless tags, n=2. As shown in  FIG.  7   , when M 1 ( k ) is sorted in ascending order into m 1 ( k ), m 1 ( 1 )=M 1 ( 2 ) and m 1 ( 2 )=M 1 ( 1 ). When M 2 ( k ) is sorted in ascending order into m 2 ( k ), m 2 ( 1 )=M 2 ( 1 ) and m 2 ( 2 )=M 2 ( 2 ). 
     In  FIG.  7   , |m 1 ( 1 )−m 2 ( 1 )|&lt;m and |m 1 ( 2 )−m 2 ( 2 )|≥m. In this case, as described above, the wireless tag communication device  154  acquires the polarization direction along which one of m 1 ( 2 ) and m 2 ( 2 ) that is not smaller is acquired as the polarization direction of the radio wave transmitted to the sheet. The one of m 1 ( 2 ) (=M 1 ( 1 )) and m 2 ( 2 ) (=M 2 ( 2 )) that is not smaller is M 2 ( 2 ). Since the polarization direction along which M 2 ( 2 ) is acquired is the second polarization direction, the second polarization direction is the appropriate polarization direction with respect to the sheet A. Accordingly, the wireless tag communication device  154  determines the second polarization direction as the polarization direction of the radio wave transmitted to the sheet A. 
     Next, a case in which m 1 =m 2 =0 will be described with reference to  FIG.  8   .  FIG.  8    is a graph showing a sheet A and the received power intensity. Since the sheet A is provided with two wireless tags, n=2. As shown in  FIGS.  8   , M 1 ( 1 )≠0 and M 1 ( 2 )=0. In addition, M 2 ( 1 )=0 and M 2 ( 2 )≠0. Thus, m 1 =m 2 =0. In such a case, when only the first polarized wave or only the second polarized wave is acquired as the appropriate polarization direction, the wireless tag communication device  154  cannot communicate with one of the wireless tags. Thus, both the first polarization direction and the second polarization direction are the appropriate polarization direction with respect to the sheet A. Accordingly, the wireless tag communication device  154  determines both the first polarization direction and the second polarization direction as the polarization direction of the radio wave transmitted to the sheet A. 
     When both the first polarization direction and the second polarization direction are determined as the polarization direction, the wireless tag communication device  154  transmits the first polarized wave, and after writing to the wireless tags, then immediately transmits the second polarized wave to the sheet A. 
     In order to distinguish between the method of determining the polarization direction along which the one of m 1  and m 2  that is not smaller is acquired as shown in  FIGS.  5  and  6    and the method of using the significant difference as shown in  FIG.  7   , the former is referred to as “simple minimum value acquisition method”. 
     Next, a case in which either one of m 1  and m 2  is 0 will be described with reference to  FIGS.  9  and  10   .  FIG.  9    is a graph showing a sheet C and the received power intensity. Since the sheet C is provided with two wireless tags, n=2. As shown in  FIGS.  9   , M 1 ( 1 )≠0 and M 1 ( 2 )≠0. In addition, M 2 ( 1 )=0 and M 2 ( 2 )=0. Thus, m 1 ≠0 and m 2 =0. In such a case, the first polarization direction is an appropriate polarization direction with respect to the sheet C. Accordingly, the wireless tag communication device  154  determines the first polarization direction as a polarization direction of a radio wave transmitted to the sheet C. 
       FIG.  10    is a graph showing a sheet D and the received power intensity. Since the sheet D is provided with two wireless tags, n=2. As shown in  FIGS.  10   , M 1 ( 1 )=0 and M 1 ( 2 )=0. In addition, M 2 ( 1 )≠0 and M 2 ( 2 )≠0. Thus, m 1 =0 and m 2 ≠0. In such a case, the second polarization direction is an appropriate polarization direction with respect to the sheet D. Accordingly, the wireless tag communication device  154  determines the second polarization direction as a polarization direction of a radio wave transmitted to the sheet D. 
     Next, the appropriate polarized wave acquisition method will be described using a flowchart.  FIG.  11    is a flowchart showing a flow of a process of acquiring an appropriate polarized wave. In the process described below, for ease of understanding the description, the UID for identifying n wireless tags provided on a sheet to be conveyed is set as 1 to n. For example, the UID of the wireless tag  400 - 1  is set as 1 and the UID of the wireless tag  400 - 2  is set as 2. 
     In  FIG.  11   , the wireless tag communication device  154  receives conveyance start, indicating start of conveyance of the sheet from the processor  153  that controls the paper feeding unit  205  (ACT  101 ). The wireless tag communication device  154  transmits the first polarized wave (ACT  102 ). The wireless tag communication device  154  acquires the received power intensity from each wireless tag (ACT  103 ), and stores the acquired received power intensity and the UID for identifying a wireless tag of an acquisition source in the storage device  502  (ACT  104 ). For example, the wireless tag communication device  154  stores in a form of (first polarized wave, UID, received power intensity) such that correspondence of the polarized wave, the wireless tag, and the received power intensity can be understood. 
     In this way, a plurality of (first polarized wave, UID, received power intensity) are acquired for each UID. For example, a plurality of received power intensities are acquired for each UID, such as (first polarized wave, 1, r 1 ), (first polarized wave, 1, r 2 ), . . . , and (first polarized wave, 1, rs). s is an integer of 0 or more, and is a different value for each UID depending on a state of communication with a wireless tag corresponding to the UID. 
     The wireless tag communication device  154  determines whether conveyance end indicating end of the conveyance of the sheet from the processor  153  is received (ACT  105 ). When the conveyance end is not received (ACT  105 : NO), the process returns to ACT  103 . Therefore, the wireless tag communication device  154  intermittently stores combinations of (first polarized wave, UID, received power intensity) when the sheet is being conveyed. 
     When the conveyance end is received (ACT  105 : YES) and a second conveyance start is received (ACT  106 ), the wireless tag communication device  154  transmits the second polarized wave (ACT  107 ). The wireless tag communication device  154  acquires the received power intensity from each wireless tag (ACT  108 ), and stores the acquired received power intensity and the UID for identifying the wireless tag of the acquisition source in the storage device  502  (ACT  109 ). For example, the wireless tag communication device  154  stores in a form of (second polarized wave, UID, received power intensity) such that correspondence of the polarized wave, the wireless tag, and the received power intensity can be understood. 
     In this way, similar to the case of the first polarized wave, a plurality of (second polarized wave, UID, received power intensity) are acquired for each UID. For example, a plurality of received power intensities are acquired for each UID, such as (second polarized wave, 1, r 1 ), (second polarized wave, 1, r 2 ), . . . , and (second polarized wave, 1, rs). s is an integer of 0 or more, and is a different value for each UID depending on a state of communication with the wireless tag corresponding to the UID. 
     The wireless tag communication device  154  determines whether the conveyance end indicating the end of the conveyance of the sheet from the processor  153  is received (ACT  110 ). When the conveyance end is not received (ACT  110 : NO), the process returns to ACT  108 . Therefore, the wireless tag communication device  154  intermittently stores combinations of (second polarized wave, UID, received power intensity) when the sheet is being conveyed. The process will be continually described with reference to  FIG.  12   . 
     In  FIG.  12   , the wireless tag communication device  154  acquires M 1 ( k ) (k=1 to n) (ACT  201 ). The wireless tag communication device  154  sets M 1 ( k ) to a maximum rq in (first polarized wave, k, rq) (1≤q≤s) when s is 1 or more. When s is 0, M 1 ( k )=0. Similarly, the wireless tag communication device  154  acquires M 2 ( k ) (k=1 to n) (ACT  201 ). The wireless tag communication device  154  sets M 2 ( k ) to a maximum rq in (second polarized wave, k, rq) (1≤q≤s) when s is 1 or more. When s is 0, M 2 ( k )=0. 
     Next, the wireless tag communication device  154  determines whether an appropriate polarized wave is acquired by the simple minimum value acquisition method described in  FIGS.  5  and  6    (ACT  203 ). Here, for example, the wireless tag communication device  154  may store, in advance, a settable flag indicating whether the appropriate polarized wave is acquired by the simple minimum value acquisition method, and may use the flag for determination. In addition, the wireless tag communication device  154  may display an inquiry screen on the control panel  120 , and may perform determination according to a content input in response to an inquiry. 
     When the appropriate polarized wave is acquired by the simple minimum value acquisition method (ACT  203 : YES), the wireless tag communication device  154  substitutes the minimum value of M 1 ( k ) into m 1  (ACT  213 ) and substitutes the minimum value of M 2 ( k ) into m 2  (ACT  214 ), and the process proceeds to ACT  301  in  FIG.  13   . 
     When the appropriate polarized wave is not acquired by the simple minimum value acquisition method (ACT  203 : NO), the method described in  FIG.  7    is used. As described in  FIG.  7   , the wireless tag communication device  154  acquires m 1 ( k ) by sorting M 1 ( k ) in ascending order (ACT  204 ). Similarly, as described in  FIG.  7   , the wireless tag communication device  154  acquires m 2 ( k ) by sorting M 2 ( k ) in ascending order (ACT  205 ). The wireless tag communication device  154  substitutes 1 into k (ACT  206 ). 
     The wireless tag communication device  154  determines whether |m 1 ( 1 )−m 2 ( 1 )|≥m (ACT  207 ). Since the difference is significant when |m 1 ( 1 )−m 2 ( 1 )|≥m (ACT  207 : YES), the wireless tag communication device  154  substitutes m 1 ( k ) into m 1  (ACT  211 ) and substitutes m 2 ( k ) into m 2  (ACT  212 ), and the process proceeds to ACT  301  in  FIG.  13   . 
     Since there is a significant difference when |m 1 ( 1 )−m 2 ( 1 )|&lt;m (ACT  207 : NO), the wireless tag communication device  154  increments k by 1 (ACT  208 ), and determines whether k&gt;n (ACT  209 ). That is, whether k exceeds an upper limit is determined. When k≤n (ACT  209 : NO), the wireless tag communication device  154  determines whether |m 1 ( 1 )−m 2 ( 1 )|≥m again (ACT  207 ). When k&gt;n (ACT  209 : YES), the wireless tag communication device  154  substitutes 1 into k (ACT  210 ), and the process proceeds to ACT  211  described above. The process will be continually described with reference to  FIG.  13   . 
     In  FIG.  13   , the wireless tag communication device  154  determines whether m 1 =m 2 =0 (ACT  301 ). When m 1 =m 2 =0 (ACT  301 : YES), the wireless tag communication device  154  determines both the first polarization direction and the second polarization direction as the polarization direction of the radio wave transmitted to the sheet A (ACT  302 ), and the process is ended. 
     When m 1 =m 2 =0 is not satisfied (ACT  301 : NO), the wireless tag communication device  154  determines whether m 1 ≥m 2  (ACT  303 ). When m 1 ≥m 2  (ACT  303 : YES), the wireless tag communication device  154  determines the first polarized wave direction as the polarization direction of the radio wave transmitted to the sheet (ACT  304 ), and the process is ended. When m 1 &lt;m 2  (ACT  303 : NO), the wireless tag communication device  154  determines the second polarized wave direction as the polarization direction of the radio wave transmitted to the sheet (ACT  305 ), and the process is ended. 
     According to the embodiment described above, stable communication with the plurality of wireless tags provided on one sheet can be performed by determining the radio wave in the polarization direction along which the minimum value of the maximum value of the received power intensity is not smaller. 
     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. These embodiments can be implemented in various other forms, and various omissions, substitutions, and changes can be made without departing from the gist of the disclosure. These embodiments and modifications thereof fall within the scope and spirit of the invention and are included in the scope of the invention recited in the claims and the equivalent thereof.