Patent Publication Number: US-2019180062-A1

Title: Wireless tag reading apparatus and wireless tag reading method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2017-236585, filed on Dec. 11, 2017, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a wireless tag reading apparatus and a wireless tag reading method. 
     BACKGROUND 
     In the related art, a wireless tag reading apparatus for reading commodity information from a commodity provided with a wireless tag such as a radio frequency identification (RFID) tag or the like is known, which collectively reads the commodity information while the commodity is accommodated in a hermetically sealed storage chamber that shields radio waves. 
     However, since such a wireless tag reading apparatus requires a dedicated storage chamber for shielding radio waves, there is a problem that the size of the apparatus increases and the cost for constructing the system increases. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing an external appearance of a wireless tag reading system according to a first embodiment. 
         FIG. 2  is a view taken in the direction of arrow A in  FIG. 1 , and showing a schematic configuration of the wireless tag reading apparatus. 
         FIG. 3  is a hardware block diagram showing an example of a hardware configuration of the wireless tag reading system. 
         FIG. 4  is a functional block diagram showing an example of a functional configuration of the wireless tag reading apparatus. 
         FIG. 5  is a functional block diagram showing an example of a functional configuration of an RFID tag. 
         FIG. 6  is a view taken in the direction of the arrow A in  FIG. 1 , and provided for explaining a reading range of the RFID tag. 
         FIG. 7  is a flowchart showing an example of a flow of a process performed by the wireless tag reading apparatus. 
         FIG. 8  is a flowchart showing an example of a flow of a process performed by a wireless tag reading apparatus according to a modification example of the first embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An object of the embodiments is to provide a wireless tag reading apparatus capable of reading identification information registered in a wireless tag in a predetermined region without accommodating the wireless tag in a storage chamber that shields radio waves. 
     A wireless tag reading apparatus according to one embodiment reads identification information registered in a wireless tag, and includes radio wave transmitting means, radio wave receiving means, phase difference calculating means, information determining means, and tag information reading means. The radio wave transmitting means transmits signals of different frequencies from each other. The radio wave receiving means receives a response signal of the wireless tag which is activated by receiving the signal transmitted by the radio wave transmitting means. The phase difference calculating means calculates a phase difference between a plurality of the response signals received by the radio wave receiving means. The information determining means selects the wireless tag from which the identification information is to be read based on the phase difference calculated by the phase difference calculating means. The tag information reading means reads the identification information registered in the wireless tag selected by the information determining means. 
     First Embodiment 
     This embodiment is an example applied to a wireless tag reading system that reads identification information registered in a wireless tag. 
     Description of Schematic Configuration of Wireless Tag Reading Apparatus 
     A schematic configuration of the wireless tag reading system  100  will be described with reference to  FIG. 1 .  FIG. 1  is a perspective view showing an external appearance of a wireless tag reading system  100  according to a first embodiment. The wireless tag reading system  100  is installed at a shop such as a supermarket and used for registration processing of commodity information and settlement processing of commodity M purchased by a customer. In particular, the wireless tag reading system  100  reads information such as identification information which uniquely identifies the RFID tag T registered in an RFID tag T (see  FIG. 2 ) attached to commodity, in a non-contact manner by using radio waves, to perform the registration processing of the commodity information of commodity M purchased by the customer. The RFID tag T is an example of a wireless tag. 
     As shown in  FIG. 1 , the wireless tag reading system  100  includes a POS terminal  1  placed on a check-out counter  10  and a wireless tag reading apparatus  30 . The POS terminal  1  identifies the commodity M purchased by the customer based on the reading result of the RFID tag T read by the wireless tag reading apparatus  30  and performs registration processing for registering commodity information such as commodity code, quantity, price of the commodity M, and so on. Further, the POS terminal  1  performs settlement processing for a payment of the registered commodity M. In addition, the POS terminal  1  controls the operation of the wireless tag reading apparatus  30 . The POS terminal  1  includes a clerk-side display  3 , a customer-side display  4 , a keyboard  5 , a barcode reader  6 , a receipt printer  8 , and the like. 
     The clerk-side display  3  is attached to an upper part of the POS terminal  1  toward the inside of the check-out counter  10  (toward the clerk side). The clerk-side display  3  displays information such as name and price of the commodity M corresponding to the identification number I (see  FIG. 3 ) read from the RFID tag T. The identification number I is an example of the identification information described above. 
     The customer-side display  4  is attached to a back side of the clerk-side display  3  toward the outside of the check-out counter  10  (toward the customer-side). The customer-side display  4  displays the commodity information and the like of the commodity M corresponding to the identification number I registered in the RFID tag T read by the wireless tag reading system  100 . The identification number I is a commodity code of commodity M, for example. 
     The keyboard  5  includes a key such as a finish key for declaring the end of the commodity sales data processing of the commodity M purchased by the customer. 
     The barcode reader  6  optically reads a barcode attached to the commodity M and is arranged to be used at a central portion of the check-out counter  10 . The barcode includes information such as commodity code pre-assigned for each commodity M that identifies the type of each commodity M. It should be noted that the RFID tag T and the barcode are attached to each commodity M. Hereinafter, in order to describe the wireless tag reading function which is a characteristic function of the present embodiment, the barcode reading process by the barcode reader  6  will not be described. 
     The receipt printer  8  prints a receipt after the settlement processing is ended and issues the receipt from a receipt issue opening (not shown in  FIG. 1 ). 
     The wireless tag reading apparatus  30  reads the identification number I registered in the RFID tag T attached to the commodity M in response to an instruction from the POS terminal  1 . It is assumed that the commodity M is put in a shopping basket  12  and placed within a reading range  32  clearly indicated on a table of the check-out counter  10 . 
     Next, a schematic configuration of the wireless tag reading apparatus  30  will be described with reference to  FIG. 2 .  FIG. 2  is a view taken in the direction of the arrow A in  FIG. 1 , showing a schematic configuration of the wireless tag reading apparatus  30 . The wireless tag reading apparatus  30  includes an RFID antenna  22  below the reading range  32  of the check-out counter  10 . In response to receiving the instruction from the POS terminal  1 , the RFID antenna  22  radiates a radio wave toward the reading range  32 . That is, the RFID antenna  22  has directivity in the direction toward the reading range  32 . 
     The radiated radio wave activates the RFID tag T attached to the commodity M in the shopping basket  12 . The activated RFID tag T radiates a radio wave as a response signal, which carries an identification number I that uniquely identifies the commodity M to which the RFID tag T is attached. 
     The RFID antenna  22  receives the response signal radiated by the RFID tag T. Then, the wireless tag reading apparatus  30  reads the identification number I included in the response signal. 
     Description of Hardware Configuration of Wireless Tag Reading Apparatus 
     Next, the hardware configuration of the wireless tag reading system  100  will be described with reference to  FIG. 3 .  FIG. 3  is a hardware block diagram showing an example of a hardware configuration of the wireless tag reading system  100 . 
     As shown in  FIG. 3 , the wireless tag reading system  100  includes a POS terminal  1 , a wireless tag reading apparatus  30 , and an RFID tag T. 
     The POS terminal  1  includes a control unit  26  and a storage unit  27 . The control unit  26  controls the overall operation of the wireless tag reading system  100 . The control unit  26  includes a central processing unit (CPU) (not shown), a Read Only Memory (ROM), and a Random Access Memory (RAM). The CPU comprehensively controls the operation of the POS terminal  1 . The ROM is a storage medium that stores various programs and data. The RAM is a storage medium that temporarily stores various programs and rewrites various data. Then, the CPU executes a control program P 0  stored in the storage unit  27  with the RAM as a work area. In this manner, the control unit  26  has a general computer configuration. 
     The storage unit  27  is a storage device such as a hard disk drive (HDD) or a solid state drive (SSD). The storage unit  27  is connected to the control unit  26  through an internal bus. The storage unit  27  stores the control program P 0 , a commodity master (not shown), and the like. The control program P 0  includes implementing the functions of the operating system and the POS terminal  1 . The control program P 0  includes realizing the characteristic functions according to the present embodiment. In addition, the storage unit  27  stores the commodity master provided from a shop server (not shown) that manages the POS terminal  1 . The commodity master is a master file for each commodity M sold at the shop, which associates the identification number I, the commodity name, and the commodity unit price by which the commodity can be discerned from each other. 
     The control unit  26  is connected to the keyboard  5 , the clerk-side display  3 , the customer-side display  4 , the barcode reader  6 , the receipt printer  8 , and the communication I/F (interface)  28  through an internal bus. 
     Each of the clerk-side display  3 , the customer-side display  4 , the keyboard  5 , the barcode reader  6 , and the receipt printer  8  has the functions described above. 
     The communication I/F  28  communicates with a communication I/F  36  of the wireless tag reading apparatus  30  which will be described below. The communication I/F  28  instructs the wireless tag reading apparatus  30  from the POS terminal  1  to start reading the RFID tag T. In addition, the communication I/F  28  receives the identification number I of the RFID tag T read by the wireless tag reading apparatus  30 . 
     The wireless tag reading apparatus  30  includes a control unit  33 , a storage unit  34 , a communication I/F  36 , a reader and writer unit  38 , and an RFID antenna  22 . 
     The control unit  33  controls the operation of the wireless tag reading apparatus  30 . The control unit  33  includes a CPU, a ROM, and a RAM (not shown). The CPU comprehensively controls the operation of the wireless tag reading apparatus  30 . The ROM is a storage medium that stores various programs and data. The RAM is a storage medium that temporarily stores various programs and rewrites various data. Then, the CPU executes the control program P 1  stored in the storage unit  34  with the RAM as a work area. In this manner, the control unit  33  has a general computer configuration. 
     The storage unit  34  is a storage device such as HDD or SSD. The storage unit  34  is connected to the control unit  33  through an internal bus . The storage unit  34  stores the control program P 1  and the like. The control program P 1  includes implementing the functions of the operating system and the wireless tag reading apparatus  30 . The control program P 1  includes realizing the characteristic functions according to the present embodiment. 
     The control unit  33  is connected to the reader and writer unit  38  and the communication I/F  36  through the internal bus. Further, the reader and writer unit  38  is connected to the RFID antenna  22  described above. 
     The reader and writer unit  38  controls the operation of the RFID antenna  22  in response to receive an instruction from the control unit  33 . More specifically, the reader and writer unit  38  causes the RFID antenna  22  to transmit a high-frequency signal of a predetermined frequency generated by the control unit  33  as a transmit signal. In addition, the reader and writer unit  38  reads the high-frequency signal received by the RFID antenna  22  as a response signal to the transmit signal. 
     The communication I/F  36  communicates with the communication I/F  28 . The communication I/F  36  transmits the identification number I of the RFID tag T read by the wireless tag reading apparatus  30  from the wireless tag reading apparatus  30  to the POS terminal  1 . In addition, the communication I/F  36  receives an instruction to start reading the identification number I of the RFID tag T from the POS terminal  1 . 
     The RFID tag T includes a control unit  40 , a storage unit  42 , an RFID antenna  44 , and a reader and writer unit  46 . 
     The control unit  40  controls the operation of the RFID tag T. The control unit  40  includes a CPU, a ROM, and a RAM (not shown). The CPU centrally controls the operation of the RFID tag T. The ROM is a storage medium that stores various programs and data. The RAM is a storage medium that temporarily stores various programs and rewrites various data. Then, the CPU executes the control program P 2  stored in the storage unit  42  with the RAM as a work area. In this manner, the control unit  40  has a general computer configuration. 
     The storage unit  42  is a non-volatile memory device such as a flash memory. The storage unit  42  is connected to the control unit  40  through an internal bus. The storage unit  42  stores the control program P 2 , the identification number I of the RFID tag T, and the like. The control program P 2  includes implementing the function of the RFID tag T. The control program P 2  includes realizing the characteristic functions according to the present embodiment. 
     The control unit  40  is connected to the reader and writer unit  46  and the storage unit  42  through an internal bus. Further, the reader and writer unit  46  is connected to the RFID antenna  44  described above. 
     The RFID antenna  44  receives the transmit signal transmitted by the RFID antenna  22 . In addition, the RFID antenna  44  transmits a response signal including the identification number I of the RFID tag T. 
     The reader and writer unit  46  controls the operation of the RFID antenna  44  in response to receiving the instruction from the control unit  40 . More specifically, the reader and writer unit  46  reads the transmit signal from the wireless tag reading apparatus  30  received by the RFID antenna  44 . In addition, the reader and writer unit  46  transmits the response signal including the identification number I of the RFID tag T generated by the control unit  40  from the RFID antenna  44 . 
     Description of Functional Configuration of Wireless Tag Reading Apparatus 
     Next, a functional configuration of the wireless tag reading apparatus  30  will be described with reference to  FIG. 4 .  FIG. 4  is a functional block diagram showing an example of a functional configuration of the wireless tag reading apparatus  30 . 
     The control unit  33  of the wireless tag reading apparatus  30  deploys the control program P 1  stored in the storage unit  34  (see  FIG. 3 ) described above onto the RAM and operates according to the control program P 1 , whereby each functional unit shown in  FIG. 4  is generated in the RAM. Specifically, the control unit  33  includes a radio wave transmission unit  50 , a radio wave reception unit  52 , an oscillator  54 , a frequency control unit  56 , a phase detection unit  58 , a phase difference calculation unit  60 , a tag information reading unit  62 , and an information determination unit  64  as functional units. 
     The radio wave transmission unit  50  causes the RFID antenna  22  described above to radiate a transmit signal, that is, the high-frequency signal outputted from an amplifier  72  which will be described below, into the air . More specifically, the radio wave transmission unit  50  includes a modulator  70 , the amplifier  72 , and a radio wave transmission and reception unit  74 . The radio wave transmission unit  50  is an example of the radio wave transmitting means. In addition, the transmit signal transmitted by the radio wave transmission unit  50  includes an instruction to execute the RFID tag T receiving the transmit signal. 
     The modulator  70  converts digital data outputted from the frequency control unit  56 , which will be described below, into an analog signal, using for example, an encoding circuit and a filter, and mixes the analog signal and a sinusoidal wave outputted from the oscillator  54  in a mixer circuit, and outputs a high-frequency signal. 
     The amplifier  72  amplifies the high-frequency signal from the modulator  70  using the amplification circuit and outputs the amplified high-frequency signal. The amplification factor of the high-frequency signal is set according to setting of the reading range of the RFID tag T. Details will be described below. In addition, the amplifier  72  may not be necessarily installed when the power of the high-frequency signal output from the modulator  70  is sufficient to perform wireless communication within a predetermined range. 
     The radio wave transmission and reception unit  74  causes the RFID antenna  22  to transmit a transmit signal for reading the identification number I of the RFID tag T to the reader and writer unit  38 . 
     More specifically, the radio wave reception unit  52  includes a radio wave transmission and reception unit  74  and a demodulator  76 . The radio wave reception unit  52  receives the high-frequency signal propagating through the air as a response signal to the RFID antenna  22 , and outputs the received response signal to the demodulator  76 . The radio wave reception unit  52  is an example of radio wave receiving means. 
     The radio wave transmission and reception unit  74  causes the RFID antenna  22  to receive the response signal from the RFID tag T to the reader and writer unit  38 . Then, the radio wave transmission and reception unit  74  acquires the response signal received by the RFID antenna  22  through the reader and writer unit  38 . 
     The demodulator  76  synthesizes the high-frequency signal received by the radio wave transmission and reception unit  74  and the output signal of the oscillator  54  using a mixer circuit, for example, and removes frequency components unnecessary for reception by a filter to extract an analog signal. The extracted analog signal is outputted to the phase detection unit  58 . In addition, the demodulator  76  digitizes the analog signal using a binarization circuit and outputs the digitized analog signal to the frequency control unit  56  described below. 
     The oscillator  54  outputs a sinusoidal wave having a stable frequency, using a phase locked loop (PLL) circuit for example, in accordance with an instruction from the frequency control unit  56 . The output signal of the oscillator  54  is a carrier wave of a high-frequency signal (radio wave) radiated to the RFID tag T. 
     The frequency control unit  56  operates when the phase detection unit  58  completes the detection of the phase after completing the demodulation of the high-frequency signal received by the demodulator  76 , and instructs the oscillator  54  on the frequency of the sinusoidal wave to be generated. Then, the oscillator  54  generates carrier waves having different frequencies according to the instruction of the frequency control unit  56 . The frequency selected by the frequency control unit  56  is set to a frequency within a range of 916.8 MHz to 922.2 MHz, which is a frequency range available based on the Radio Act in Japan, for example. 
     The phase detection unit  58  detects the phase of the response signal received by the radio wave reception unit  52 . In addition, the phase detection unit  58  outputs the detected phase to the frequency control unit  56  described above. The phase detection unit  58  detects the phase of the response signal by detecting, for example, an I signal that is in phase with the transmit signal and a Q signal that is a quadrature phase component with respect to the transmit signal from the response signal. This phase detection method is a known method called orthogonal demodulation (orthogonal detection). 
     The phase difference calculation unit  60  calculates the phase difference of a plurality of response signals received by the radio wave reception unit  52 . The phase difference calculation unit  60  is an example of the phase difference calculating means. For example, the phase difference calculation unit  60  calculates the phase difference between the phase of the first response signal received by the radio wave reception unit  52  with respect to the high-frequency signal having the first frequency transmitted by the radio wave transmission unit  50 , and the phase of the second response signal received by the radio wave reception unit  52  with respect to the high-frequency signal having the second frequency different from the first frequency transmitted by the radio wave transmission unit  50 . 
     The tag information reading unit  62  reads the identification number I of the RFID tag T, which is registered in the RFID tag T and included in the response signal received by the radio wave reception unit  52 . The tag information reading unit  62  is an example of the tag information reading means. 
     The information determination unit  64  selects the RFID tag T to be read by the tag information reading unit  62  based on the phase difference calculated by the phase difference calculation unit  60 . The information determination unit  64  is an example of the information determining means. 
     Specifically, the information determination unit  64  selects an RFID tag T as a target to be read, when the phase difference calculated by the phase difference calculation unit  60  for the corresponding response signal is equal to or less than a predetermined threshold. 
     Although not shown in  FIG. 4 , the wireless tag reading apparatus  30  also has a function of writing information in the RFID tag T as well as reading the identification number I registered in the RFID tag T. For example, the wireless tag reading apparatus  30  is capable of writing information indicating the completion of the reading of the identification number I to the RFID tag T from which the identification number I is read. 
     Description of Functional Configuration of RFID Tag 
     Next, the functional configuration of the RFID tag T will be described with reference to  FIG. 5 .  FIG. 5  is a functional block diagram showing an example of a functional configuration of an RFID tag T. 
     The control unit  40  of the RFID tag T deploys a control program P 2  stored in the storage unit  42  (see  FIG. 3 ) described above onto the RAM and operates according to the control program P 2 , whereby each functional unit shown in  FIG. 5  is generated in the RAM. More specifically, the control unit  40  realizes a radio wave transmission and reception unit  80 , an activation control unit  82 , an identification number reading unit  84 , and a reading completion information writing unit  86  as functional units. 
     The radio wave transmission and reception unit  80  causes the RFID antenna  44  to receive the transmit signal from the wireless tag reading apparatus  30  with respect to the reader and writer unit  46 . In addition, the radio wave transmission and reception unit  80  causes the RFID antenna  44  to transmit the response signal including the identification number I of the RFID tag T with respect to the reader and writer unit  46 . 
     Upon receipt of the transmit signal from the wireless tag reading apparatus  30 , the activation control unit  82  activates the RFID tag T. In addition, the activation control unit  82  stops the RFID tag T from being activated. 
     The identification number reading unit  84  reads the identification number I registered in the RFID tag T from the storage unit  42  described above. 
     The reading completion information writing unit  86  writes information indicating that the identification number I of the RFID tag T is read, on the RFID tag T in which the identification number I is registered. 
     Description of Setting Method of Reading Range 
     Next, a method of setting the reading range of the RFID tag T read by the wireless tag reading apparatus  30  will be described. 
     When the high-frequency signal (radio wave) propagates through the air, the wavelength becomes shorter as the frequency becomes higher. Therefore, when the distance between the RFID antenna  22  and the RFID tag T is the same, the response signal is detected in a different phase when communicating with a high-frequency signal having a different wavelength. In other words, in the wireless tag reading apparatus  30 , the phase of the response signal detected when communicating with the RFID tag T using a first frequency f 1  and the phase of the response signal detected when communicating with the RFID tag T using a second frequency f 2  different from the first frequency f 1  are different from each other. 
     As the distance between the RFID antenna  22  and the RFID tag T increases, the phase difference changes. The degree of the change in the phase difference depends on the frequency difference between the first frequency f 1  and the second frequency f 2 . Specifically, when the frequency difference between the first frequency f 1  and the second frequency f 2  is small, that is, when the wavelength difference is small, a region having the same phase difference does not occur unless the RFID antenna  22  and the RFID tag T are separated by a certain distance. On the other hand, when the frequency difference between the first frequency f 1  and the second frequency f 2  is large, that is, when the wavelength difference is large, there occurs a region having the same phase difference with respect to a slight change in the distance between the RFID antenna  22  and the RFID tag T. 
     As such, by setting the frequency difference between the first frequency f 1  and the second frequency f 2  to be relatively small, when a response signal is received from the RFID tag T placed within the predetermined distance range from the RFID antenna  22 , the phase difference in a specific region between the phase of the response signal of the first frequency f 1  and the phase of the response signal of the second frequency f 2  is equal to or less than a predetermined threshold Δω. 
     How small the frequency difference between the first frequency f 1  and the second frequency f 2  is to be set may be determined based on the first frequency f 1 , the second frequency f 2 , and the read range of the RFID tag T to be used. More specifically, when the high-frequency signals of different frequencies f 1  and f 2  are transmitted to the RFID tag T attached to the commodity M in the shopping basket  12  placed within the reading range  32  (see  FIG. 1 ), the first frequency f 1  and the second frequency f 2  are selected so that the phase difference of the response signal acquired is equal to or less than a predetermined threshold Δω. In addition, the intensity of the transmitted radio wave is set to the intensity at which the RFID tag T is activated by the transmitted radio wave within a range where the phase difference equal to or less than the predetermined threshold Δω is detected. Specifically, the amplification factor of the amplifier  72  is set so that the RFID tag T is activated, within a range where a phase difference equal to or less than a predetermined threshold Δω is detected. 
     Next, the setting of the reading range of the RFID tag T will be described in more detail with reference to  FIG. 6 .  FIG. 6  is a view taken in the direction of the arrow A in  FIG. 1 , provided for explaining the reading range of the RFID tag T. As shown in  FIG. 6 , regions where the phase difference between the phase of the response signal of the first frequency f 1  and the phase of the response signal of the second frequency f 2  is equal to or less than the predetermined threshold Δω periodically appear as in the case of the regions R 1  and R 3 . Here, the regions R 1  and R 3  are regions surrounded by a substantially spherical surface having a constant distance from the RFID antenna  22 . Then, the wireless tag reading apparatus  30  sets only the region R 1  closest to the RFID antenna  22  as the reading range. In the region R 2  sandwiched between the region R 1  and the region R 3 , the phase difference between the phase of the response signal of the first frequency f 1  and the phase of the response signal of the second frequency f 2  is larger than a predetermined threshold Δω. Although not shown in  FIG. 6 , since the transmit signal radiated by the RFID antenna  22  is propagated as a wave, even in the region outside the region R 3 , the region where the phase difference is equal to or less than a predetermined threshold Δω appears periodically. 
     In addition, in the wireless tag reading apparatus  30 , the region where the intensity of the transmit signal radiated by the RFID antenna  22  is equal to or larger than a predetermined value, has a shape that expands toward the direction of the directivity of the RFID antenna  22  as the region Q 1  shown in  FIG. 6 , for example. That is, only the RFID tag T in the region Q 1  may be activated by receiving the transmit signal from the RFID antenna  22 . That is, the RFID tag T in the region S 1 , which is the region where the region R 1  and the region Q 1  overlap with each other, is activated by receiving the transmit signal radiated by the RFID antenna  22 . Further, in the RFID tag T in the region S 1 , the phase difference between the phase of the response signal of the first frequency f 1  and the phase of the response signal of the second frequency f 2  is equal to or less than a predetermined threshold Δω. As a result, it is possible to set the region S 1  to a range in which the identification number I of the RFID tag T can be read. As shown in  FIG. 6 , this region S 1  is set to include the shopping basket  12  placed within the reading range  32 . 
     That is, when the RFID tag T is located outside the region S 1 , the wireless tag reading apparatus  30  does not read the identification number I of the RFID tag T. For example, in  FIG. 6 , in the case where the RFID tag T is within the region R 2 , when the RFID tag T is located within the region Q 1 , the RFID tag T is activated, but the phase difference is larger than the predetermined threshold Δω. Therefore, the wireless tag reading apparatus  30  does not read the identification number I of the RFID tag T. In addition, when the RFID tag T is located outside the region Q 1 , the RFID tag T is not activated, so the wireless tag reading apparatus  30  does not read the identification number I of the RFID tag T. 
     In addition, in  FIG. 6 , in the case where the RFID tag T is within the region R 3 , the RFID tag T is not activated, so the wireless tag reading apparatus  30  does not read the identification number I of the RFID tag T. 
     It is assumed that the intensity of the transmit signal radiated by the RFID antenna  22  is increased and the region Q 1  described above overlaps with the region R 3 . In this case, when the RFID tag T is present at both positions belonging to the region Q 1  and the region R 3 , the RFID tag T is activated upon reception of the transmit signal. Then, the phase difference becomes equal to or less than a predetermined threshold Δω. Therefore, in this case, the wireless tag reading apparatus  30  reads the identification number I of the RFID tag T. In order to prevent this, it is necessary to adjust the intensity of the transmit signal radiated by the RFID antenna  22  so that the region Q 1  and the region R 3  do not overlap with each other. Conversely, the intensity of the transmit signal radiated by the RFID antenna  22  may increase to a range where the region Q 1  does not overlap with the region R 3 . As a result, the radio wave intensity in the region S 1  which is the original reading range can be increased, so that the reading accuracy of the RFID tag T can be improved. 
     Description of Flow of Process Performed by Wireless Tag Reading Apparatus 
     Next, the flow of process performed by the wireless tag reading apparatus  30  will be described with reference to  FIG. 7 .  FIG. 7  is a flowchart showing an example of a flow of a process performed by a wireless tag reading apparatus  30 . 
     The process at the wireless tag reading apparatus  30  is performed with a cooperation between the RFID tag T and the wireless tag reading apparatus  30  side. First, the flow of process on the wireless tag reading apparatus  30  side will be described. First, the radio wave transmission unit  50  causes the RFID antenna  22  to transmit radio waves having the first frequency f 1  (Act  10 ). 
     Next, the radio wave reception unit  52  determines whether or not the response signal is received (Act  12 ). When it is determined that the response signal is received (Act  12 : Yes), the process proceeds to Act  14 . On the other hand, when it is determined that the response signal is not received (Act  12 : No), the determination in Act  12  is repeated. Although not shown in  FIG. 7 , when the response signal is not received over a predetermined time, it is determined that the RFID tag T to be read is not present, and the process in  FIG. 7  is ended. 
     Next, in Act  14 , the phase detection unit  58  detects the phase of the response signal. Then, in Act  16 , the tag information reading unit  62  reads the identification number I included in the received response signal and stores the read identification number I in the storage unit  34  together with the phase. 
     Next, the radio wave transmission unit  50  causes the RFID antenna  22  to transmit radio waves having the second frequency f 2  (Act  18 ). 
     The radio wave reception unit  52  determines whether or not the response signal is received (Act  20 ). When it is determined that the response signal is received (Act  20 : Yes), the process proceeds to Act  22 . On the other hand, when it is determined that the response signal is not received (Act  20 : No), the determination in Act  20  is repeated. Although not shown in  FIG. 7 , when the response signal is not received over the predetermined time, it is determined that the RFID tag T to be read does not present, and the processing in  FIG. 7  is ended. 
     Next, in Act  22 , the phase detection unit  58  detects the phase of the response signal. Then, in Act  24 , the tag information reading unit  62  reads the identification number I included in the received the response signal and stores the read identification number I in the storage unit  34  together with the phase. 
     Although not shown in  FIG. 7 , since there are generally a plurality of RFID tags T, the process from Act  12  to Act  16  and the process from Act  20  to Act  24  are performed and repeatedly performed by the number of the RFID tags T present within the reading range  32 . 
     In Act  26 , the phase difference calculation unit  60  calculates the phase difference of the phase acquired from the RFID tag T having the same identification number I. That is, when the response signal having identification number I is acquired only as a response signal to the transmit signal of the first frequency f 1 , or only as a response signal to the transmit signal of the second frequency number f 2 , the RFID tag T having the identification number I is excluded from the reading target. 
     Subsequently, the phase difference calculation unit  60  determines whether the calculated phase difference is equal to or less than a predetermined threshold Δω (Act  28 ). When it is determined that the calculated phase difference is equal to or less than the predetermined threshold Δω (Act  28 : Yes), the process proceeds to Act  30 . On the other hand, when it is determined that the calculated phase difference is not equal to or less than the predetermined threshold Δω (Act  28 : No), the process proceeds to Act  34 . 
     Next, in Act  30 , the information determination unit  64  outputs the identification number I of the RFID tag T. 
     Subsequently, the radio wave transmission unit  50  causes the RFID antenna  22  to transmit the radio wave by designates the RFID tag T from which the identification number I is read as the reception destination (Act  32 ). At this time, the frequency of the radio waves to be transmitted does not have to be frequencies f 1  and f 2 , but may be any available frequency. 
     The phase difference calculation unit  60  determines whether or not the phase difference is calculated for all the received RFID tags T (Act  34 ). When it is determined that the phase difference is calculated for all the received RFID tags T (Act  34 : Yes), the process in  FIG. 7  is ended. On the other hand, when it is determined that the phase difference is not calculated for all the received RFID tags T (Act  34 : No), the process returns to Act  26 . 
     Next, the flow of process on the RFID tag T side will be described. First, the radio wave transmission and reception unit  80  determines whether or not the RFID antenna  44  receives radio waves having the first frequency f 1  (Act  40 ). When it is determined that the radio wave is received (Act  40 : Yes), the process proceeds to Act  42 . On the other hand, when it is determined that the radio wave is not received (Act  40 : No), Act  40  is repeated. 
     At Act  42 , the activation control unit  82  activates the RFID tag T. At Act  44 , the identification number reading unit  84  reads the identification number I of the RFID tag T. 
     Subsequently, at Act  46 , the radio wave transmission and reception unit  80  causes the RFID antenna  44  to transmit the response signal including the identification number I. Thereafter, at Act  48 , the activation control unit  82  stops the operation of the RFID tag T. 
     Next, the radio wave transmission and reception unit  80  determines whether or not the RFID antenna  44  receives radio waves having the second frequency f 2  (Act  50 ). When it is determined that the radio wave is received (Act  50 : Yes), the process proceeds to Act  52 . On the other hand, when it is determined that the radio wave is not received (Act  50 : No), Act  50  is repeated. 
     In Act  52 , the activation control unit  82  activates the RFID tag T. At Act  54 , the identification number reading unit  84  reads the identification number I of the RFID tag T. 
     Subsequently, at Act  56 , the radio wave transmission and reception unit  80  causes the RFID antenna  44  to transmit the response signal including the identification number I. Thereafter, at Act  58 , the activation control unit  82  stops the operation of the RFID tag T. 
     Next, the radio wave transmission and reception unit  80  determines whether or not the RFID antenna  44  receives radio waves (Act  60 ). When it is determined that the radio wave is received (Act  60 : Yes), the process proceeds to Act  62 . On the other hand, when it is determined that the radio wave is not received (Act  60 : No), Act  60  is repeated. 
     At Act  62 , the activation control unit  82  activates the RFID tag T. Then, the reading completion information writing unit  86  writes information indicating that reading of the identification number I is completed to the RFID tag T (Act  64 ). Thereafter, at Act  66 , the activation control unit  82  stops the operation of the RFID tag T, and ends the process in  FIG. 7 . 
     As described above, in the wireless tag reading apparatus  30  according to the first embodiment, the response signal of the RFID tag T (wireless tag) for signals of the first frequency f 1  and the second frequency f 2  different from each other transmitted by the radio wave transmission unit  50  (radio wave transmitting means) is received by the radio wave reception unit  52  (radio wave receiving means). Then, the phase difference calculation unit  60  (phase difference calculating means) calculates the phase difference of a plurality of response signals received by the radio wave reception unit  52 . The information determination unit  64  (information determining means) selects the RFID tag T from which the identification number I (identification information) is to be read based on the calculated phase difference. Then, the tag information reading unit  62  (tag information reading means) reads the identification number I (identification information) registered in the selected RFID tag T. Therefore, it is possible to read the identification number I registered in the RFID tag T in a predetermined region without accommodating the RFID tag T in a storage chamber that shields radio waves. 
     In addition, in the wireless tag reading apparatus  30  according to the first embodiment, the information determination unit  64  excludes the RFID tag T from the reading target, under a condition that in the tag information reading unit  62 , the response signals with respect to all the signals of a plurality of frequencies transmitted by the radio wave transmission unit  50  do not include a response signal acquired from the same RFID tag T. Therefore, it is possible to reliably read the identification number I registered in the RFID tag T. 
     In addition, in the wireless tag reading apparatus  30  according to the first embodiment, the information determination unit  64  selects an RFID tag T to be read by the tag information reading unit  62  under a condition that the phase difference calculated by the phase difference calculation unit  60  be equal to or less than a predetermined threshold value Δω. Therefore, it is possible to read only the identification number I registered in the RFID tag T within the predetermined region S 1 . 
     In the wireless tag reading apparatus  30  according to the first embodiment, the radio wave transmission unit  50  sets the intensity of the radio wave to be transmitted to at least the intensity at which the RFID tag T placed within the predetermined region S 1  is activated. Therefore, since it is possible to reliably activate the RFID tag T within at least the predetermined region S 1 , it is possible to reliably read the identification number I registered in the RFID tag T in the region S 1 . The intensity of the radio wave radiated by the RFID antenna  22  can be increased to a condition that the RFID tag T within the region R 3  (see  FIG. 6 ) is not activated. As a result, since the intensity of the radio wave in the region S 1  can be maximized, the reading accuracy of the identification number I registered in the RFID tag T can be improved. 
     Modification Example of the First Embodiment 
     Next, a wireless tag reading apparatus  30   a  (not shown) which is a modification example according to the first embodiment will be described. The wireless tag reading apparatus  30   a  has the same hardware configuration (see  FIG. 3 ) and the same functional configuration (see  FIGS. 4 and 5 ) as the wireless tag reading apparatus  30  described in the first embodiment. Then, in the first embodiment, a radio wave of the second frequency f 2  is transmitted to all the RFID tags T, and the modification example of the first embodiment is different from the first embodiment in that radio wave of the second frequency f 2  are transmitted only to the RFID tag T that returns the response signal to the transmit signal of the first frequency f 1 . In the following description, the constituent components of the wireless tag reading apparatus  30   a  are expressed using the same reference numerals as those of the wireless tag reading apparatus  30 . 
     Hereinafter, the flow of the process performed by the wireless tag reading apparatus  30   a  will be described with reference to  FIG. 8 .  FIG. 8  is a flowchart showing an example of a flow of a process performed by a wireless tag reading apparatus  30   a  according to a modification example of the first embodiment. 
     First, the flow of the process at the wireless tag reading apparatus  30   a  side will be described. The same process as in the first embodiment is performed until the transmit signal of the first frequency f 1  is transmitted to the RFID tag T and the response signal from the RFID tag T is received (Acts  70 ,  72 ,  74 , and  76 ). 
     Next, at Act  78 , the radio wave transmission unit  50  designates the RFID tag T storing the identification number I as a destination of the transmission, and causes the RFID antenna  22  to transmit the radio wave of the second frequency f 2 . 
     Subsequently, at Act  80 , the radio wave reception unit  52  determines whether or not the response signal is received. When it is determined that the response signal is received (Act  80 : Yes), the process proceeds to Act  82 . On the other hand, when it is determined that the response signal is not received (Act  80 : No), the process proceeds to Act  92 . Although not shown in  FIG. 8 , when the response signal is not received over a predetermined time at Act  80 , it is determined that there is no RFID tag T to be read, and the process in  FIG. 8  is ended. 
     Next, at Act  82 , the phase detection unit  58  detects the phase of the response signal. At Act  84 , the phase difference calculation unit  60  calculates the phase difference of the phase acquired from the RFID tag T having the same identification number I. 
     Subsequently, the phase difference calculation unit  60  determines whether the calculated phase difference is equal to or less than a predetermined threshold Δω (Act  86 ). When it is determined that the calculated phase difference is equal to or less than the predetermined threshold Δω (Act  86 : Yes), the process proceeds to Act  88 . On the other hand, when it is determined that the calculated phase difference is not equal to or less than the predetermined threshold Δω (Act  86 : No), the process proceeds to Act  92 . 
     Next, at Act  88 , the information determination unit  64  outputs the identification number I of the RFID tag T. 
     Subsequently, the radio wave transmission unit  50  causes the RFID antenna  22  to transmit the radio wave by designating the RFID tag T from which the identification number I is read as the reception destination (Act  90 ). At this time, the frequency of the radio waves to be transmitted need not to be the frequencies f 1  and f 2 , but may be any usable frequency. 
     The phase difference calculation unit  60  determines whether or not process for all the RFID tags T in response to the radio wave of the first frequency f 1  is completed (Act  92 ). When it is determined that the process for all the RFID tags T in response to the radio wave of the first frequency f 1  is completed (Act  92 : Yes), the process in  FIG. 8  is ended. On the other hand, when it is determined that the processing for all RFID tags T in response to the radio wave of the first frequency f 1  is not completed (Act  92 : No), the process returns to Act  78 . 
     Next, the flow of the process on the RFID tag T side will be described. The RFID tag T is activated by receiving the radio wave of the first frequency f 1  transmitted by the wireless tag reading apparatus  30   a  at Act  70  and stops the operation after transmitting the response signal carrying the identification number I registered in each RF ID tag T. The flow of the processing of this part is the same as described in the first embodiment (Acts  100 ,  102 ,  104 ,  106 , and  108 ). 
     Then, the radio wave transmission and reception unit  80  of the RFID tag T determines whether or not the RFID antenna  44  receives radio waves having the second frequency f 2  (Act  110 ). When it is determined that the radio wave is received (Act  110 : Yes), the process proceeds to Act  112 . On the other hand, when it is determined that the radio wave is not received (Act  110 : No), Act  110  is repeated. 
     At Act  112 , the activation control unit  82  activates the RFID tag T. At Act  114 , the radio wave transmission and reception unit  80  causes the RFID antenna  44  to transmit the response signal. Thereafter, at Act  116 , the activation control unit  82  stops the operation of the RFID tag T. 
     Subsequently, the RFID tag T is activated by receiving the radio wave transmitted by the wireless tag reading apparatus  30   a  at Act  90 , stops the operation after writing information indicating completion of reading, stops the operation, and ends the processing in  FIG. 8 . The flow of the process in this part will be referenced to the description of the first embodiment (Acts  118 ,  120 ,  122 , and  124 ). 
     As described above, in the wireless tag reading apparatus  30   a  according to the modification example of the first embodiment, the radio wave transmission unit  50  transmits the signal of the second frequency f 2  only to the RFID tag T that has the identification information I, based on the identification number I of the RFID tag T included in the response signal with respect to the transmitted signal of the first frequency f 1 . Therefore, it is possible to identify the RFID tag T to be read with a smaller calculation amount. 
     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. 
     For example, the frequency of radio waves to be used is not limited to two kinds only. That is, each of the embodiments described above may be realized by using transmit signals of three or more different frequencies f 1 , f 2 , and f 3 . In this case, when all three phase differences acquired by comparing two optional response signals among the response signals of the three frequencies are equal to or less than a predetermined threshold Δω, the identification number I registered in the RFID tag T may be read. 
     In addition, in the embodiment and its modification example described above, there is no restriction on the magnitude relationship between the first frequency f 1  and the second frequency f 2 . In other words, either of f 1 &lt;f 2  or f 1 &gt;f 2  may be applied.