Abstract:
The present invention provides a non-contact IC medium communication device ( 1 ) capable of detecting a non-contact IC medium ( 25 ) in a desired area with a simple configuration, by obtaining, from each of separate read areas, identification information of the non-contact IC medium that exists within the read area and a signal level of a reception signal received from the non-contact IC medium, the identification information and the signal level being associated with each other, performing difference operation or division with respect to respective signal levels of reception signals from the separate read areas, for each of the identification information, to obtain a composite signal level, and extracting the identification information whose composite signal level falls within a predetermined threshold range. The present invention further provides a method thereof, a program thereof, and a computer-readable storage medium storing the program.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a non-contact IC medium communication device, for example, which detects a non-contact IC medium within a desired area, a method thereof, a program thereof, and a computer-readable storage medium storing the program. 
       BACKGROUND ART 
       [0002]    Non-contact IC media capable of performing communication of information stored therein in a noncontact manner are conventionally utilized. Such non-contact IC media include a medium using a UHF band available for long-distance communication. In most cases, a single-element patch antenna is often used as an antenna for communicating with the non-contact IC medium using the UHF band. The single-element patch antenna has a half-power width (beamwidth) as broad as about 70 and therefore has the advantage of reading information from the non-contact IC media in a broad area. 
         [0003]    However, in some cases, only the non-contact IC medium within a specific area need to be read. Particularly, such cases include, for example, a case that, under the situation where articles are transported while undergoing a plurality of working stages, only the non-contact IC medium of the article in one working operation need to be read. 
         [0004]    In this case, it is considered to use an antenna with a narrow half-power width. However, such an antenna needs to be extremely large in size, like a multiple-element array antenna and a parabola antenna. 
         [0005]    Meanwhile, there has been proposed a tag communication device that calculates the position of a non-contact IC medium on the basis of estimated incoming direction of a radio wave from the non-contact IC medium (see Patent Literature 1). The tag communication device applies different weights to outputs of antenna elements to detect the intensity of the radio wave in a specific direction. 
         [0006]    However, the tag communication device requires complicated calculations in order to find the direction where the one non-contact IC medium exists. 
         [0007]    Patent Literature 1 
         [0008]    Japanese Patent Application Publication, Tokukai, No. 2006-10345 A (Publication Date: Jan. 12, 2006) 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0009]    The present invention has been made in view of the foregoing problems, and an object of the present invention is to provide a non-contact IC medium communication device capable of detecting, with a simple configuration, a non-contact IC medium in a desired area, a method thereof, a program thereof and a computer-readable storage medium storing the program. 
       Solution to Problem 
       [0010]    The present invention is characterized by a non-contact IC medium communication device comprising: receiving means for obtaining, from each of separate read areas, identification information of a non-contact IC medium that exists within the read area and a signal level of a reception signal received from the non-contact IC medium, the identification information and the signal level being associated with each other; operating means for performing difference operation or division with respect to respective signal levels of reception signals from the separate read areas, for each of the identification information, to obtain a composite signal level; and extracting means for extracting the identification information whose composite signal level falls within a predetermined threshold range, a method thereof, and a program thereof. 
         [0011]    The separate read areas differ from each other in their coverage areas, and read directions of an antenna having directivity in the separate read areas differ from each other. 
         [0012]    The non-contact IC medium is a medium, such as an RF-ID tag, which can store information and perform communication in a noncontact manner. The non-contact IC medium includes a passive type medium which is not provided with a power source and obtains induced electromotive force from an external entity to transmit a response signal, a semi-passive type medium which is provided with a power source and transmits the response signal in response to a request from an external entity, and an active type medium which is provided with a power source and transmits signals at regular intervals. 
         [0013]    The identification information is information by which the non-contact IC medium can be identified, such as an ID of an RF-ID tag. 
         [0014]    A signal received from the non-contact IC medium is a response signal for the non-contact IC medium transmitting the identification information or another signal transmitted by the non-contact IC medium. 
         [0015]    The receiving means can be realized by one or more array antennas whose directional patterns can be changed or by a plurality of antennas whose directional pattern cannot be changed. 
         [0016]    The operating means and extracting means can be realized by means for performing an operation, such as CPU or MPU. 
         [0017]    The non-contact IC medium communication device can be a reader/writer device which transmits/receives information to/from the non-contact IC medium or a reader device which reads information from the non-contact IC medium. Furthermore, the non-contact IC medium communication device also includes a device used as a detection device for detecting the non-contact IC medium. 
         [0018]    Note that the non-contact IC medium communication device may be realized by a computer. In this case, the present invention also includes a non-contact IC medium communication program of the non-contact IC medium communication device which realizes the non-contact IC medium communication device by causing the computer to operate as each of the means, and a computer-readable storage medium storing the program. 
       ADVANTAGEOUS EFFECTS OF INVENTION 
       [0019]    The present invention can provide a non-contact IC medium communication device capable of detecting a non-contact IC medium in a desired area with a simple configuration, a method thereof, a program thereof, and a computer-readable storage medium storing the program. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0020]    
         FIG. 1 
       
           [0021]      FIG. 1  is an explanatory view of working stages in a factory and a RFID detection device. 
           [0022]    
         FIG. 2 
       
           [0023]      FIG. 2  is a graph showing respective reception levels in different directional orientations. 
           [0024]    
         FIG. 3 
       
           [0025]      FIG. 3  is a graph of a reception level ratio. 
           [0026]    
         FIG. 4 
       
           [0027]      FIG. 4  is a flowchart of operations performed by a control section. 
           [0028]    
         FIG. 5 
       
           [0029]      FIG. 5  is an explanatory view of reception information stored in a storage section. 
           [0030]    
         FIG. 6 
       
           [0031]      FIG. 6  is an explanatory view of a coverage area of an antenna. 
           [0032]    
         FIG. 7 
       
           [0033]      FIG. 7  is a graph of leftward and rightward reception ratios in Example 2. 
           [0034]    
         FIG. 8 
       
           [0035]      FIG. 8  is a flowchart of operations performed by a control section in Example 2. 
       
    
    
     REFERENCE SIGNS LIST 
       [0036]      1  . . . RFID DETECTION DEVICE,  10  . . . ANTENNA,  13  . . . CONTROL SECTION,  25  . . . RFID TAG, A 1  and A 2  . . . THRESHOLD, R 2  . . . NORMAL AREA, Rx_L and Rx_R . . . RECEPTION LEVEL, Rx_DIF . . . RECEPTION LEVEL RATIO, Rx_DIF_L . . . LEFTWARD RECEPTION LEVEL RATIO, Rx_DIF_R . . . RIGHTWARD RECEPTION LEVEL RATIO 
       DESCRIPTION OF EMBODIMENTS 
       [0037]    The following will describe an embodiment of the present invention with reference to the drawings. 
       Embodiment 1 
       [0038]      FIG. 1  is an explanatory view of working stages in a factory and a RFID detection device  1 . 
         [0039]    In a factory, a conveyor belt  21  is placed along working stages A, B, and more stages. The conveyor belt  21  is driven by driving means (not shown) and transports articles  27  ( 27   a  through  27   c ) toward each of the working stages. 
         [0040]    In the working stage A, a worker Ma performs an operation for the article  27   a . In the working stage B, a worker Mb performs an operation for the article  27   b.    
         [0041]    The articles  27  have respective RFID tags  25  ( 25   a  through  25   c ) attached thereto. Each of the RFID tags  25  includes an antenna and an IC. The IC of the RFID tag  25  has a storage section that stores proper information such as an ID, which is identification information, and a name and specification of the article, which are information on the article. 
         [0042]    In the working stage B, the RFID detection device  1  is placed which can read the RFID tag  25  existing within a detection area R. The RFID detection device  1  includes an antenna  10 , a control section  13 , and a storage section  15 . 
         [0043]    The antenna  10  includes a three-element array antenna which can adjust its directional orientations in a range from 35° to −35°. 
         [0044]    The control section  13  includes a CPU, a ROM, and a RAM, and performs control operations and computations according to a program such as an RFID tag communication program. 
         [0045]    The storage section  15  includes a storage device such as a nonvolatile memory or a hard disk, and stores a program and information (data). 
         [0046]    Next, the following will describe the theory of a method for reading only the RFID tag  25  in a desired area with use of the RFID detection device. The present embodiment exemplifies that only the RFID tag  25  in a narrow area extending at angles from −15° to +15° from a front side of the antenna  10  is read, using the RFID detection device  1  as a pseudo-pencil beam antenna. 
         [0047]      FIG. 2  is a graph showing a directional pattern of the antenna  10 . In the graph, a longitudinal axis indicates a directional gain (Gain) of the antenna  10  in units of decibel (dBi). A lateral axis indicates an angle in units of degree (deg). 
         [0048]    A leftward directional gain D_L (θ) plotted in  FIG. 2  indicates directional gains in given directions where the front side of the antenna  10  is 0°, when the directional orientation (angle θ) of the antenna  10  is set to −35°. 
         [0049]    A centerward directional gain D_C (θ) plotted in  FIG. 2  indicates directional gains in given directions where the front side of the antenna  10  is 0°, when the directional orientation (angle θ) of the antenna  10  is set to 0°. 
         [0050]    A rightward directional gain D_R (θ) plotted in FIG.  2  indicates directional gains in given directions where the front side of the antenna  10  is 0°, when the directional orientation (angle θ) of the antenna  10  is set to 35°. 
         [0051]    Here, a reception level Rx_L can be expressed by the following equation (Equation 3). Note that relational equations in logarithmic form and antilogarithmic form are expressed by the following equations (Equations 1 and 2). 
         [0000]        Rx= 10×log 10 ( Rx ′)  (Equation 1) 
         [0000]        Rx′= 10 (Rx/10)   (Equation 2) 
         [0000]    where Rx (dBm) is expressed in logarithmic form, and Rx′ (mW) is expressed in antilogarithmic form.
 
[A] Rx_L in logarithmic form
 
         [0000]        Rx   —   L=Pt+Dt (θ)−Loss+ D   —   L (θ) 
         [0000]    where Pt is a transmission power of the tag, Loss is a free space loss, Dt(θ) is a directional gain of the tag, and D_L(θ) is a leftward directional gain, or
 
[B] Rx_L in antilogarithmic form
 
         [0000]        Rx   —   L′=Pt′×Dt ′(θ)×(λ/4 πD ) 2   ×D   —   L ′(θ)  (Equation 3) 
         [0000]    where Pt′ is a transmission power of the tag, D is a communication distance, Dt′(θ) is a directional gain of the tag, and D_L′(θ) is a leftward directional gain. 
         [0052]    Further, a reception level Rx_R can be expressed by the following equation: 
         [0000]    [A] Rx_R in logarithmic form 
         [0000]        Rx   —   R=Pt+Dt (θ)−Loss+ D   —   R (θ) 
         [0000]    where Pt is a transmission power of the tag, Loss is a free space loss, Dt(θ) is a directional gain of the tag, and D_R(θ) is a rightward directional gain, or
 
[B] Rx_R in antilogarithmic form
 
         [0000]        Rx   —   R′=Pt′×Dt ′(θ)×(λ/4 πD ) 2   ×D   —   R′ (θ)  (Equation 4) 
         [0000]    where Pt′ is a transmission power of the tag, D is a communication distance, Dt′(θ) is a directional gain of the tag, and D_R′(θ) is a rightward directional gain. 
         [0053]    Regarding the foregoing two equations, an operation for a reception level ratio (subtraction (difference) for the logarithmic form and division for the antilogarithmic form) is performed to determine a reception level ratio Rx_DIF. The reception level ratio Rx_DIF is expressed by the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       [ 
                       A 
                       ] 
                     
                      
                     
                         
                     
                      
                     Rx_DIF 
                      
                     
                         
                     
                      
                     in 
                      
                     
                         
                     
                      
                     logarithmic 
                      
                     
                         
                     
                      
                     form 
                   
                    
                   
                     
 
                   
                    
                   
                     
                       
                         
                           Rx_DIF 
                           = 
                             
                            
                           
                             Rx_R 
                             - 
                             Rx_L 
                           
                         
                       
                     
                     
                       
                         
                           = 
                             
                            
                           
                             
                               { 
                               
                                 Pt 
                                 + 
                                 
                                   Dt 
                                    
                                   
                                     ( 
                                     θ 
                                     ) 
                                   
                                 
                                 - 
                                 Loss 
                                 + 
                                 
                                   D_R 
                                    
                                   
                                     ( 
                                     θ 
                                     ) 
                                   
                                 
                               
                               } 
                             
                             - 
                           
                         
                       
                     
                     
                       
                         
                             
                            
                           
                             { 
                             
                               Pt 
                               + 
                               
                                 Dt 
                                  
                                 
                                   ( 
                                   θ 
                                   ) 
                                 
                               
                               - 
                               Loss 
                               + 
                               
                                 D_L 
                                  
                                 
                                   ( 
                                   θ 
                                   ) 
                                 
                               
                             
                             } 
                           
                         
                       
                     
                     
                       
                         
                           = 
                             
                            
                           
                             
                               D_R 
                                
                               
                                 ( 
                                 θ 
                                 ) 
                               
                             
                             - 
                             
                               D_L 
                                
                               
                                 ( 
                                 θ 
                                 ) 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     5 
                   
                   ) 
                 
               
             
             
               
                 
                   
                     
                       ( 
                       B 
                       ) 
                     
                      
                     
                         
                     
                      
                     Rx_DIF 
                      
                     
                         
                     
                      
                     in 
                      
                     
                         
                     
                      
                     
                       anti 
                        
                       logarithmic 
                     
                      
                     
                         
                     
                      
                     form 
                   
                    
                   
                     
 
                   
                    
                   
                     
                       
                         
                           
                             Rx_DIF 
                             ′ 
                           
                           = 
                             
                            
                           
                             
                               Rx_R 
                               ′ 
                             
                             / 
                             
                               Rx_L 
                               ′ 
                             
                           
                         
                       
                     
                     
                       
                         
                           = 
                             
                            
                           
                             
                               ( 
                               
                                 
                                   Pt 
                                   ′ 
                                 
                                 × 
                                 
                                   
                                     Dt 
                                     ′ 
                                   
                                    
                                   
                                     ( 
                                     θ 
                                     ) 
                                   
                                 
                                 × 
                                 
                                   
                                     ( 
                                     
                                       
                                         λ 
                                         / 
                                         4 
                                       
                                        
                                       π 
                                        
                                       
                                           
                                       
                                        
                                       D 
                                     
                                     ) 
                                   
                                   2 
                                 
                                 × 
                                 
                                   D_R 
                                   ′ 
                                 
                                  
                                 
                                   ( 
                                   θ 
                                   ) 
                                 
                               
                               ) 
                             
                             / 
                           
                         
                       
                     
                     
                       
                         
                             
                            
                           
                             ( 
                             
                               
                                 Pt 
                                 ′ 
                               
                               × 
                               
                                 
                                   Dt 
                                   ′ 
                                 
                                  
                                 
                                   ( 
                                   θ 
                                   ) 
                                 
                               
                               × 
                               
                                 
                                   ( 
                                   
                                     
                                       λ 
                                       / 
                                       4 
                                     
                                      
                                     π 
                                      
                                     
                                         
                                     
                                      
                                     D 
                                   
                                   ) 
                                 
                                 2 
                               
                               × 
                               
                                 D_L 
                                 ′ 
                               
                                
                               
                                 ( 
                                 θ 
                                 ) 
                               
                             
                             ) 
                           
                         
                       
                     
                     
                       
                         
                           = 
                             
                            
                           
                             
                               
                                 D_R 
                                 ′ 
                               
                                
                               
                                 ( 
                                 θ 
                                 ) 
                               
                             
                             - 
                             
                               
                                 D_L 
                                 ′ 
                               
                                
                               
                                 ( 
                                 θ 
                                 ) 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                     
                 
               
             
           
         
       
     
         [0054]    As expressed in Equation 5, by performing subtraction or division with respect to the equations for reception levels in different directional orientations, a function having only θ irrelevant to distances and performance of the tag (reflected power and directional gains of the tag) is obtained. 
         [0055]    This function is shown in graph form in  FIG. 3 . In the graph, a longitudinal axis indicates the reception level ratio Rx_DIF (Gain) in units of decibel (dB). A lateral axis indicates an angle in units of degree (deg). 
         [0056]    As shown in the graph, the reception level ratio Rx_DIF is expressed in a graph form that appears to be a linear function at about 0°, which is near the front side of the antenna  10 . Note that the reception level ratio Rx_DIF at angles outside about ±10° is not expressed as a linear function because of a side lobe effect. 
         [0057]    Therefore, only the RFID tag  25  whose reception level ratio Rx_DIF satisfies a condition is extracted with use of thresholds A 1  and A 2 , which enables selection of only the RFID tag  25  that exists on the front side of the antenna  10 . That is, except for the target RFID tag  25 , the other RFID tags  25  can be eliminated even if they are read. The condition is expressed by the following equation: 
         [0000]      − A 1 &lt;Rx   —   DIF&lt;A 2  (Equation 6) 
         [0000]    where A 1  is a lower threshold and A 2  is an upper threshold. 
         [0058]    Next, the following will describe operations that the control section  13  in the RFID detection device  1  performs according to an RFID tag communication program to select only the RFID tag  25  in a target area and communicate with the selected RFID tag  25  on the basis of the foregoing theory, with reference to the flowchart shown in  FIG. 4 . 
         [0059]    The control section  13  requests the antenna  10  to transmit a signal with its directional orientation set to 35° (step S 1 ). The signal transmitted at this time is a signal to make the RFID tag  25  transmit the ID, which is the identification information for the RFID tag  25 , in response to the request. 
         [0060]    The control section  13  receives a response signal from the RFID tag  25  through the antenna  10  and then stores the received ID and the reception level Rx_R in the storage section  15  (step S 2 ). At this time, the storage section  15  stores the ID and a rightward directional reception level, as shown in  FIG. 5 , which is an explanatory view of reception information stored in the storage section  15 . 
         [0061]    The control section  13  requests the antenna  10  to transmit a signal with its directional orientation set to −35° (step S 3 ). As in the step S 1 , the signal transmitted at this time is a signal for making the RFID tag  25  transmit the ID, which is the identification information for the RFID tag  25 , in response to the request. 
         [0062]    The control section  13  receives a response signal from the RFID tag  25  through the antenna  10  and then stores the received ID and the reception level Rx_L in the storage section  15  (step S 4 ). At this time, as shown in  FIG. 5 , the storage section  15  additionally stores a leftward directional reception level as information associated with the ID. 
         [0063]    The control section  13  performs difference operation (subtraction for the logarithmic form and division for the antilogarithmic form) using the reception levels Rx_L and Rx_R for each ID to find the reception level ratio Rx_DIF, and then stores the reception level ratio Rx_DIF as information associated with the ID, as indicated in  FIG. 5  (step S 5 ). 
         [0064]    The control section  13  extracts an ID having the reception level ratio Rx_DIF that falls in a range from predetermined thresholds A 1  to A 2  (in a range from −A 1  to A 2 ), and then evaluates only the extracted ID as a valid ID (step S 6 ). This completes the operation. 
         [0065]    With the above configuration and operations, the RFID detection device  1  can select and read the RFID tag  25  within a target area narrower than a coverage area where the antenna  10  can intrinsically read the RFID tag  25 . 
         [0066]    More specifically, as shown in (A) through (C) in  FIG. 6 , the coverage area of the antenna  10  is originally vague. 
         [0067]    That is, when the directional orientation is 0° in (A) of  FIG. 6 , no clear boundaries exist between a strong area R 1  where the antenna  10  can receive a signal with high strength and a normal area R 2  where the antenna  10  can receive a signal to some extent and between the normal area R 2  and its surrounding area, and the all of these areas are smoothly continuous. Therefore, in one case the antenna  10  can read the RFID tag  25 , and in another case the antenna  10  cannot read the RFID tag  25 . This occurs depending on a surrounding environment of the antenna  10 . Moreover, due to the presence of a side lobe area R 3 , it is difficult to determine a clear coverage area of the antenna  10  from one received result. 
         [0068]    Similarly, in both cases of (B) of  FIG. 6  where the directional orientation is shifted by 35° rightward and (C) of  FIG. 6  where the directional orientation is shifted by −35° leftward, the coverage area of the antenna  10  is vague without boundaries. 
         [0069]    On the contrary, (D) of  FIG. 6  shows a threshold range obtained by synthesis of (B) of  FIG. 6  where the directional orientation is shifted by 35° rightward and (C) of  FIG. 6  where the directional orientation is shifted by −35° leftward by means of the foregoing method. In (D) of  FIG. 6 , the coverage area (target area for reading) is determined within a desired area (in (D) of  FIG. 6 , an area extending at angles from +15° to −15°, and the boundary between the coverage area and a non-coverage area is very clear. Therefore, in the RFID detection device  1 , the antenna  10  can be used in the same manner as a high-accuracy pencil beam antenna, which enables reading of only the RFID tag  25  in the desired area. 
         [0070]    Further, in the RFID detection device  1 , the desired area can be adjusted easily with only changes of the thresholds A 1  and A 2 . 
         [0071]    Moreover, the RFID detection device  1  performs difference operation for the logarithmic form and division for the antilogarithmic form, which enables cancellation of an error caused by distance and performance of the tag (reflected power and directional gain of the tag) and highly-accurate detection. 
         [0072]    Furthermore, the RFID detection device  1  performs reading in leftward and rightward directional orientations (in left and right directions) with a target direction set to be centerward. This makes it possible to reliably read the RFID tag  25  which exists in the target direction. 
       Embodiment 2 
       [0073]    Next, the RFID detection device  1  in Embodiment 2 is described. In this case, hardware in the RFID detection device  1  is the same as in Example 1, but only the program-based software processing in Example 2 is different from that in Example 1. Accordingly, only the different processing is described, and the other detailed description is omitted because it is the same as in Example 1. 
         [0074]    The reception level Rx_C described in Example 1 is expressed by the following equation: 
         [0000]    [A] Rx_C in logarithmic form 
         [0000]        Rx   —   C=Pt+Dt (θ)−Loss+ D   —   C (θ) 
         [0000]    where Pt is a transmission power of the tag, Loss is a free space loss, Dt(θ) is a directional gain of the tag, and D_C(θ) is a centerward directional gain; or
 
[B] Rx_C in antilogarithmic form
 
         [0000]        Rx   —   C′=Pt′×Dt ′(θ)×(λ/4 πD ) 2   ×D   —   C ′(θ)  (Equation 7) 
         [0000]    where Pt′ is a transmission power of the tag, D is a communication distance, Dt′(θ) is a directional gain of the tag, and D_C′(θ) is a centerward directional gain. Then, subtraction (subtraction in logarithmic form, and division in antilogarithmic form) is performed using the reception levels Rx_L and Rx_C described in Example 1 to find a leftward reception level ratio Rx_DIF_L. The leftward reception level ratio Rx_DIF_L is expressed by the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             [ 
                             A 
                             ] 
                           
                            
                           
                               
                           
                            
                           Rx_DIF 
                            
                           _L 
                            
                           
                               
                           
                            
                           in 
                            
                           
                               
                           
                            
                           logarithmic 
                            
                           
                               
                           
                            
                           form 
                         
                          
                         
                           
 
                         
                          
                         
                           
                             
                               
                                 
                                   Rx_DIF 
                                    
                                   _L 
                                 
                                 = 
                                   
                                  
                                 
                                   Rx_C 
                                   - 
                                   Rx_L 
                                 
                               
                             
                           
                           
                             
                               
                                 
                                   = 
                                     
                                    
                                   
                                     
                                       D_C 
                                        
                                       
                                         ( 
                                         θ 
                                         ) 
                                       
                                     
                                     - 
                                     
                                       D_L 
                                        
                                       
                                         ( 
                                         θ 
                                         ) 
                                       
                                     
                                   
                                 
                                 ; 
                                 or 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         
                           
                             [ 
                             B 
                             ] 
                           
                            
                           
                               
                           
                            
                           Rx_DIF 
                            
                           _L 
                            
                           
                               
                           
                            
                           in 
                            
                           
                               
                           
                            
                           
                             anti 
                              
                             logarithmic 
                           
                            
                           
                               
                           
                            
                           form 
                         
                          
                         
                           
 
                         
                          
                         
                           
                             
                               
                                 
                                   Rx_DIF 
                                    
                                   
                                     _L 
                                     ′ 
                                   
                                 
                                 = 
                                   
                                  
                                 
                                   
                                     Rx_C 
                                     ′ 
                                   
                                   / 
                                   
                                     Rx_L 
                                     ′ 
                                   
                                 
                               
                             
                           
                           
                             
                               
                                 = 
                                   
                                  
                                 
                                   
                                     
                                       D_C 
                                       ′ 
                                     
                                      
                                     
                                       ( 
                                       θ 
                                       ) 
                                     
                                   
                                   - 
                                   
                                     
                                       D_L 
                                       ′ 
                                     
                                      
                                     
                                       
                                         ( 
                                         θ 
                                         ) 
                                       
                                       . 
                                     
                                   
                                 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     8 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    Further, subtraction (subtraction for the logarithmic form and division for the antilogarithmic form) is performed using the reception levels Rx_R and Rx_C described in Example 1 to find a rightward reception level ratio Rx_DIF_R. The rightward reception level ratio Rx_DIF_R is expressed by the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             [ 
                             A 
                             ] 
                           
                            
                           
                               
                           
                            
                           Rx_DIF 
                            
                           _R 
                            
                           
                               
                           
                            
                           in 
                            
                           
                               
                           
                            
                           logarithmic 
                            
                           
                               
                           
                            
                           form 
                         
                          
                         
                           
 
                         
                          
                         
                           
                             
                               
                                 
                                   Rx_DIF 
                                    
                                   _R 
                                 
                                 = 
                                   
                                  
                                 
                                   Rx_C 
                                   - 
                                   Rx_R 
                                 
                               
                             
                           
                           
                             
                               
                                 
                                   = 
                                     
                                    
                                   
                                     
                                       D_C 
                                        
                                       
                                         ( 
                                         θ 
                                         ) 
                                       
                                     
                                     - 
                                     
                                       D_R 
                                        
                                       
                                         ( 
                                         θ 
                                         ) 
                                       
                                     
                                   
                                 
                                 ; 
                                 or 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         
                           
                             [ 
                             B 
                             ] 
                           
                            
                           
                               
                           
                            
                           Rx_DIF 
                            
                           _R 
                            
                           
                               
                           
                            
                           in 
                            
                           
                               
                           
                            
                           
                             anti 
                              
                             logarithmic 
                           
                            
                           
                               
                           
                            
                           form 
                         
                          
                         
                           
 
                         
                          
                         
                           
                             
                               
                                 
                                   Rx_DIF 
                                    
                                   
                                     _R 
                                     ′ 
                                   
                                 
                                 = 
                                   
                                  
                                 
                                   
                                     Rx_C 
                                     ′ 
                                   
                                   / 
                                   
                                     Rx_R 
                                     ′ 
                                   
                                 
                               
                             
                           
                           
                             
                               
                                 = 
                                   
                                  
                                 
                                   
                                     
                                       D_C 
                                       ′ 
                                     
                                      
                                     
                                       ( 
                                       θ 
                                       ) 
                                     
                                   
                                   - 
                                   
                                     
                                       D_R 
                                       ′ 
                                     
                                      
                                     
                                       
                                         ( 
                                         θ 
                                         ) 
                                       
                                       . 
                                     
                                   
                                 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     9 
                   
                   ) 
                 
               
             
           
         
       
     
         [0075]    The leftward reception level ratio Rx_DIF_L and the rightward reception level ratio Rx_DIF_R are plotted in a graph form in  FIG. 7 . Therefore, if the RFID tag  25  which satisfies a condition is selected, an angle range (area) is limited, which enables extraction of the RFID tag  25  in the desired area. Note that in the example of  FIG. 7 , the thresholds A 1  and A 2  are identical. The condition is expressed by the following equation: 
         [0000]      Rx_DIF_L&gt;A1 AND Rx_DIF_R&gt;A2  (Equation 10) 
         [0000]    where A 1  and A 2  are thresholds. 
         [0076]    Note that a remainder (or a quotient) from the directional gains is conclusively required by the equations described in the present embodiment, and the remainder (or the quotient) is obtained as a ratio. Therefore, the present embodiment is described in units of mW or dBm. However, this is not the only possibility. The same result can be obtained even if dBW is used as the unit. Similarly, even with a conversion factor calculated with respect to an isotropic antenna using dBi as the unit as described in the present embodiment, or even with a conversion factor calculated with respect to a dipole antenna using dBd as the unit, which is different from the unit used in the present embodiment, the same result can be obtained. 
         [0077]    Next, the following will describe operations that the control section  13  in the RFID detection device  1  performs according to an RFID tag communication program to select only the RFID tag  25  in a target area and communicate with the selected RFID tag  25  on the basis of the foregoing theory, with reference to the flowchart shown in  FIG. 8 . 
         [0078]    The control section  13  requests the antenna  10  to transmit a signal with its directional orientation set to 35° (step S 11 ). The signal transmitted at this time is a signal for making the RFID tag  25  transmit the ID, which is the identification information for the RFID tag  25 , in response to the request. 
         [0079]    The control section  13  receives a response signal from the RFID tag  25  through the antenna  10 , and then stores the received ID and the reception level Rx_R in the storage section  15  (step S 12 ). 
         [0080]    The control section  13  requests the antenna  10  to transmit a signal with its directional orientation set to 0° (step S 13 ). As in the step S 1 , the signal transmitted at this time is a signal for making the RFID tag  25  transmit the ID, which is the identification information for the RFID tag  25 , in response to the request. 
         [0081]    The control section  13  receives a response signal from the RFID tag  25  through the antenna  10 , and then stores the received ID and the reception level Rx_C in the storage section  15  (step S 14 ). 
         [0082]    The control section  13  requests the antenna  10  to transmit a signal with its direction orientation set to −35° (step S 15 ). As in the step S 1 , the signal transmitted at this time is a signal for making the RFID tag  25  transmit the ID, which is the identification information for the RFID tag  25 , in response to the request. 
         [0083]    The control section  13  receives a response signal from the RFID tag  25  through the antenna  10 , and then stores the received ID and the reception level Rx_L in the storage section  15  (step S 16 ). 
         [0084]    The control section  13  performs difference operation or division using the reception levels Rx_R and Rx_C for each ID to find the rightward reception level ratio Rx_DIF_R (step S 17 ). 
         [0085]    The control section  13  performs difference operation or division using the reception levels Rx_L and Rx_C for each ID to find the leftward reception level ratio Rx_DIF_L (step S 18 ). 
         [0086]    The control section  13  performs difference operation or division using the rightward reception level ratio Rx_DIF_R and the leftward reception level ratio Rx_DIF_L for each ID to find the reception level ratio Rx_DIF (step S 19 ). 
         [0087]    The control section  13  extracts an ID having the reception level ratio Rx_DIF that falls in a range from predetermined thresholds A 1  to A 2  (in a range where the Rx_DIF_R is larger than A 1  and the Rx_DIF_L is larger than A 2 ), and then evaluates only the extracted ID as a valid ID (step S 20 ). This completes the operation. 
         [0088]    With the above configuration and operations, the same result as in Example 1 can be obtained, and the RFID tag  25  in the target area (within an area extending at target angles) can be detected. 
         [0089]    Particularly, in the Example 2, since the centerward reception level Rx_C is used, a length of the coverage area in the centerward direction, which is the target area, can be made adequately longer than the range in Example 1. That is, in Example 1, the centerward reception level Rx_C is not used. Therefore, the length of the coverage area in the centerward direction, which is the target area, decreases with increase of the difference in directional orientation angle between the reception level Rx_L and the reception level Rx_R. 
         [0090]    On the contrary, in Example 2, the centerward reception level Rx_C is used. Therefore, the length of the coverage area in the centerward direction, which is the target area, does not decrease, and the coverage area with a long length and in a narrow width can be arranged. 
         [0091]    This makes it possible to make the conveyor belt separated well away from the antenna  10 , which enables an easier design for a series of working stages than in Example 1. 
         [0092]    Note that in each of the foregoing embodiments, if the absence of an ID is confirmed corresponding to an ID which has been read in a first directional orientation in steps S 4 , n 14  and n 16  which are the steps of performing reading after the directional orientation is shifted, information associated with the ID may be discarded without being stored. Furthermore, in the steps S 2  and S 4  (the steps S 12 , n 14 , and n 16  in Embodiment 2), the information may be stored independently, and only if the presence of the same ID is confirmed in both of the steps (those three steps in Embodiment 2), the information corresponding to the ID may be extracted and subjected to operation. This makes it possible to easily eliminate the RFID tag  25  which has been received only in one of the directional orientations from target tags to be detected. 
         [0093]    A non-contact IC medium communication device of the present invention corresponds to the RFID detection device  1  in the embodiment. Similarly, the following will describe correspondences between components in the present invention and components in the foregoing embodiments: 
         [0094]    receiving means corresponds to the antenna  10 ; 
         [0095]    operating means corresponds to the control section  13  which executes step S 5 : 
         [0096]    extracting means corresponds to the control section  13  which executes step S 6 ; 
         [0097]    a non-contact IC medium corresponds to the RFID tag  25 ; 
         [0098]    a threshold range corresponds to a range from threshold −A 1  to threshold A 2 ; 
         [0099]    separate read areas correspond to normal areas R 2 ; 
         [0100]    signal levels corresponds to reception levels Rx_L and Rx_R; 
         [0101]    a composite signal level corresponds to a reception level ratio Rx_DIF; 
         [0102]    a first composite level corresponds to a leftward reception level ratio Rx_DIF_L; 
         [0103]    a second composite level corresponds to a rightward reception level ratio Rx_DIF_R; 
         [0104]    a receiving process corresponds to steps S 2 , S 4 , S 12 , S 14 , and S 16 ; 
         [0105]    an operating process corresponds to steps S 5  and S 19 ; 
         [0106]    an extracting process corresponds to steps S 6  and S 20 ; 
         [0107]    a non-contact IC medium communication program corresponds to the RFID tag communication program; 
         [0108]    identification information corresponds to the ID; and 
         [0109]    a signal received from the non-contact IC medium corresponds to a response signal from the RFID tag  25 . However, the present invention is not limited to the configurations of the foregoing embodiments, and many embodiments can be obtained.