Abstract:
A system including wireless tags that transmit information from fixed locations to nearby wireless tag readers possessed by moving persons also includes a wireless tag status inference apparatus to which the wireless tag readers send identifying information received from the wireless tags. The wireless tag status inference apparatus logs the information received from the wireless tag readers, and compares the logged information with a stored list of installed wireless tags to identify suspected inoperable wireless tags. Wireless tags requiring replacement or repair can thereby be identified promptly and inexpensively, without the need to dispatch personnel on periodic inspection tours of all areas in which the wireless tags are installed.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to apparatus for reading wireless tags, and to an apparatus, system, and program for inferring the status of the wireless tags. 
         [0003]    2. Description of the Related Art 
         [0004]    The technology of placing wireless tags in roadways and other public facilities to provide location-specific information to moving persons has reached the initial stage of deployment. In a system described in Japanese Patent Application Publication No. 2002-165825 and illustrated in the block diagram in  FIG. 1 , for example, wireless tags are embedded in guidance blocks provided in paved walkways to aid visually impaired pedestrians. The pedestrian has a wireless tag reader  20  including an electronic cane operable to communicate with the wireless tags  1301  and a data communication device operable to communicate with a guidance information database  1303  disposed at a central site. When the pedestrian&#39;s electronic cane reads an identification number from a wireless tag  1301 , the data communication device queries the guidance information database  1303  to obtain corresponding information, which is provided to the pedestrian by audible means. 
         [0005]    In an intelligent traffic system described in Japanese Patent Application Publication No. 2005-293348, wireless tags are embedded in roads to assist drivers of motor vehicles. A wireless tag reader mounted in a vehicle reads information stored in the wireless tags as the vehicle speeds past, and advises the driver about road conditions ahead. 
         [0006]    Since the wireless tags in these and other such systems are generally installed in heavily trafficked outdoor locations, they are subject to wear and tear and are exposed to the elements. These factors can cause tags to malfunction, denying needed information to the user. Japanese Patent Application Publication No. 2002-230109 therefore proposes an inspection system in which a vehicle equipped with wireless tag communication equipment is driven over a course along which wireless tags are installed to determine the operational status of the tags. 
         [0007]    The proposed inspection system is labor-intensive, however, as it requires inspection personnel to operate the vehicle and tour all sites where wireless tags are installed. Since labor-intensive inspection tours tend not to be made at frequent intervals, the proposed system is also slow in identifying inoperable tags. 
       SUMMARY OF THE INVENTION 
       [0008]    An object of the present invention is to enable the status of wireless tags to be inferred without the need to dispatch inspection personnel to the sites where the tags are installed. 
         [0009]    Another object of the invention is to enable the status of inoperable wireless tags to be determined promptly. 
         [0010]    The invention provides a wireless tag status inference system comprising a wireless tag status inference apparatus for inferring the status of wireless tags disposed along a route traversable by a moving person, and at least one wireless tag reader usable by a moving person to acquire information from wireless tags nearby. 
         [0011]    The wireless tag status inference apparatus includes a wireless tag information storage unit that stores information pertaining to the wireless tags, a log compiling unit that compiles wireless tag information acquired by the wireless tag reader from some or all of the wireless tags as the moving person traverses the route, and a tag status inference unit for identifying suspected inoperable wireless tags from the information stored in the wireless tag information storage unit and the information compiled by the log compiling unit. In one scheme, a wireless tag is inferred to be inoperable if it is logged infrequently or not at all, despite the logging of other wireless tags in its vicinity. 
         [0012]    The wireless tag reader includes a first communication device for acquiring information from nearby wireless tags, and a second communication device for transmitting the acquired information to the wireless tag status inference apparatus. The acquired information is preferably transmitted in a form that prevents an operator of the wireless tag status inference apparatus from tracing the movements of the moving person precisely. 
         [0013]    The invented system enables the wireless tags to be inspected on the basis of the information they provide during normal use, without requiring inspection tours by maintenance personnel. Suspected inoperable wireless tags can be identified at low cost. Maintenance personnel can proceed promptly to the suspected inoperable wireless tags and repair or replace them if necessary. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    In the attached drawings: 
           [0015]      FIG. 1  is a block diagram of a known system using wireless tags to aid visually impaired pedestrians; 
           [0016]      FIG. 2  is a block diagram of a wireless tag status inference system illustrating a first embodiment of the invention; 
           [0017]      FIG. 3  is an exemplary table of tags encountered by a pedestrian, listed in sequence of encounter; 
           [0018]      FIG. 4  is a corresponding table of information sent from the tag logger to the log reader in the first embodiment; 
           [0019]      FIG. 5  is a corresponding table of information stored in the outgoing ID number memory; 
           [0020]      FIG. 6  is an exemplary table of information stored in the tag information memory in the first embodiment; 
           [0021]      FIG. 7  is an exemplary table of information stored in the tag log in the wireless tag status inference apparatus in the first embodiment; 
           [0022]      FIG. 8  is an exemplary table of information, acquired from one wireless tag reader during one traverse of a route, stored in the tag log in the wireless tag status inference apparatus; 
           [0023]      FIG. 9  shows exemplary locations of wireless tags in the first embodiment and an exemplary route taken by a pedestrian; 
           [0024]      FIG. 10  is a flowchart illustrating the process by which a wireless tag reader reads the identification numbers of wireless tags and sends them to the wireless tag status inference apparatus in the first embodiment; 
           [0025]      FIG. 11  is a flowchart illustrating the process by which the wireless tag status inference apparatus infers the status of wireless tags from information supplied by wireless tag readers; 
           [0026]      FIG. 12  is an exemplary table of information, acquired from a plurality of wireless tag readers, stored in the tag log in the wireless tag status inference apparatus; 
           [0027]      FIG. 13  is an exemplary table of results obtained by the tag status inference unit in the first embodiment; 
           [0028]      FIGS. 14 ,  15 ,  16 , and  17  illustrate exemplary arrangements of wireless tags in a second embodiment; 
           [0029]      FIGS. 18 and 19  are exemplary tables of information stored in the tag log in the wireless tag status inference apparatus in the second embodiment; 
           [0030]      FIG. 20  shows an example of information stored in the tag information memory in a third embodiment. 
           [0031]      FIG. 21  shows an exemplary area divided into subareas in the third embodiment; and 
           [0032]      FIGS. 22 and 23  show further examples of information stored in the tag information memory in the third embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]    Embodiments of the invention will now be described with reference to the attached drawings, in which like elements are indicated by like reference characters. 
       First Embodiment 
       [0034]    Referring to  FIG. 2 , the first embodiment is a wireless tag status inference system  10  that adds a wireless tag status inference apparatus  30  to the system for aiding visually impaired pedestrians illustrated in  FIG. 1 . 
         [0035]    The wireless tags in this system include integrated circuits (ICs) and are operable to communicate by radio signals with a wireless tag reader  20 . So-called radio-frequency identification (RFID) tags are one example of such wireless tags. Each wireless tag has a unique identification (ID) number, which it transmits when interrogated by the wireless tag reader  20 . Although only one wireless tag  101  is shown in  FIG. 2 , the system normally includes a plurality of wireless tags. 
         [0036]    The wireless tag reader  20  communicates with the wireless tag  101  to obtain its ID number, and submits the ID number to the wireless tag status inference apparatus  30 . The wireless tag reader  20  comprises a wireless tag communication unit  21 , a tag logger  22 , an outgoing ID list assembler  23 , a reading commander  24 , and a transmitter  25 . Although only one wireless tag reader  20  is shown in  FIG. 2 , the system normally includes a plurality of wireless tag readers  20 . The wireless tag reader  20  may be incorporated in an electronic cane used by a visually impaired person, or in a portable information processing device equipped with an RFID reader. If the wireless tag reader  20  is used in a vehicle, it may be attached to the underside of the chassis of the vehicle. 
         [0037]    The wireless tag communication unit  21  serves as an interface for communication with the wireless tag  101 . When the wireless tag communication unit  21  detects a wireless tag  101  with which it can communicate, it interrogates the wireless tag  101  to obtain its ID number, and passes the obtained ID number to the tag logger  22 . 
         [0038]    The tag logger  22  stores ID numbers received from the wireless tag communication unit  21  in a tag log memory  22   a , and sends the ID numbers to the outgoing ID list assembler  23 . The tag logger  22  also monitors the times when the user of the wireless tag reader  20  enters and leaves the neighborhood of a wireless tag  101 . The time of entering may be defined as the time when the tag logger  22  first receives the ID number of the wireless tag  101 , and the time of leaving may be defined as the end of a predetermined duration of time during which the tag logger does not again receive the ID number of the wireless tag  101  from the wireless tag communication unit  21 . Alternatively, the time of leaving may be the time when the user stops receiving guidance service, or the time when the wireless tag reader  20  moves out of communication range of the wireless tag  101 . 
         [0039]      FIG. 3  is an exemplary table listing tags encountered by a pedestrian in their sequence of encounter. The tag log memory  22   a  temporarily stores the ID numbers received from the wireless tag communication unit  21 , storing both encounter numbers  201  and the ID numbers  202 . The encounter numbers  201  are recorded as an ascending sequence of integers starting from ‘1’. Each time the pedestrian enters the vicinity of a wireless tag  101 , the tag logger  22  writes its ID number  202  in the tag log memory  22   a , paired with the next encounter number  201  in the ascending sequence. 
         [0040]    The tag logger  22  sends the ID numbers  202  stored in the tag log memory  22   a  to the outgoing ID list assembler  23  in a sequence determined by the reading commander  24 , as described below. The tag logger  22  obtains this sequence from the reading commander  24  by sending the reading commander  24  the value of the last encounter number  201  stored in the tag log memory  22   a , that is, the maximum encounter number  201 . 
         [0041]    When supplied with a maximum encounter number  201  by the tag logger  22 , the reading commander  24  generates a random sending sequence for the ID numbers  202  stored in the tag log memory  22   a . The reading commander  24  includes a random number generator such as a pseudorandom number generator. The reading commander  24  uses the random numbers generated by the random number generator to shuffle the integers from ‘1’ to the maximum encounter number  201  supplied by the tag logger  22  into a random sequence of what will be referred to as lottery numbers, performing essentially the same operation as when lottery numbers are drawn at random from a box until the box is empty. The reading commander  24  sends the randomized lottery numbers to the reading commander  24  in the sequence in which they are ‘drawn’. A simple procedure is to have the random number generator generate random integers from ‘1’ to the maximum encounter number  201 , ignore each integer that has already been generated, and continue until no integers are left. Upon receiving each lottery number, the tag logger  22  reads the corresponding ID number  202  from the tag log memory  22   a  and sends it to the outgoing ID list assembler  23 . 
         [0042]      FIG. 4  is a table illustrating this procedure for the information shown in  FIG. 3 . If the sequence of lottery numbers generated by the reading commander  24  is ‘3’, ‘6’, ‘5’, ‘1’, ‘2’, ‘4’, ‘7’, then the tag logger  22  first sends the ID number corresponding to encounter number ‘3’, namely ID number ‘107’, to the outgoing ID list assembler  23 . Continuing in this way, the tag logger  22  sends the outgoing ID list assembler  23  the ID numbers  202  in the randomized sequence ‘107’, ‘109’, ‘107’, ‘111’, ‘105’, ‘108’, ‘110’. 
         [0043]    The reading commander  24  may also respond to a query from the outgoing ID list assembler  23  by informing it whether there is still an ID number left to be read, that is, whether there is still a lottery number left to be sent to the tag logger  22 . 
         [0044]    The outgoing ID list assembler  23  receives the ID numbers  202  stored in the tag log memory  22   a  from the tag logger  22 , assembles a list of information to be sent to the wireless tag status inference apparatus  30 , and sends this information to the transmitter  25 , as described below. The outgoing ID list assembler  23  includes an outgoing ID number memory  23   a.    
         [0045]    The outgoing ID number memory  23   a  is a temporary storage area for information that the outgoing ID list assembler  23  provides to the transmitter  25 . Referring to  FIG. 5 , the outgoing ID number memory  23   a  is organized as a table of transmission sequence numbers  401  and ID numbers  402 . The ID numbers  402  are identical to the ID numbers  202  stored in the tag log memory  22   a . The transmission sequence numbers  401  indicate the sequence in which the ID numbers  402  will be sent to the transmitter  25 . 
         [0046]    The outgoing ID list assembler  23  accordingly receives the ID numbers  202  stored in the tag log memory  22   a , and stores them in the outgoing ID number memory  23   a  together with transmission sequence numbers  401  indicating the sequence in which they have been received. If the value of the first ID number sent from the tag logger  22  is ‘107’ for example, the outgoing ID list assembler  23  writes ‘1’ in the outgoing ID number memory  23   a  as a transmission sequence number  401 , and writes ‘107’ as the corresponding ID number  402 , as shown in  FIG. 5 . If the next ID number sent from the tag logger  22  is ‘109’, the outgoing ID list assembler  23  increments the transmission sequence number  401  by ‘1’, writes ‘2’ as a new transmission sequence number  401 , and writes ‘109’ as the corresponding ID number  402 . 
         [0047]    The outgoing ID list assembler  23  does not know how many ID numbers it will receive from the tag logger  22 , so after receiving each ID number, it queries the reading commander  24  to find out whether there still is a lottery number to be sent to the tag logger  22 . When informed that there are no further lottery numbers, the outgoing ID list assembler  23  sends the information stored in the outgoing ID number memory  23   a  to the transmitter  25 . 
         [0048]    When storing information in the outgoing ID number memory  23   a , the outgoing ID list assembler  23  does not store an ID number  402  that is already stored in the outgoing ID number memory  23   a . For example, if the outgoing ID list assembler  23  receives ID numbers ‘107’, ‘109’, ‘107’, ‘111’ in this sequence from the tag logger  22 , it stores transmission sequence number ‘1’ and ID number ‘107’ as the first entry in the outgoing ID number memory  23   a , and transmission sequence number ‘2’ and ID number ‘109’ as the next entry. Next, the outgoing ID list assembler  23  receives ID number ‘107’ again, but as ID number ID number ‘107’ is already stored in the outgoing ID number memory  23   a  with transmission sequence number ‘1’, the outgoing ID list assembler  23  skips the second ‘107’ and stores transmission sequence number ‘3’ and ID number ‘111’ as the next entry in the outgoing ID number memory  23   a.    
         [0049]    The transmitter  25  receives the information sent from the outgoing ID list assembler  23 , and submits it to the wireless tag status inference apparatus  30  via the network  40 . 
         [0050]    The network  40  connects the wireless tag reader  20  to the wireless tag status inference apparatus  30 . Any connection protocol may be employed. Alternatively, the wireless tag reader  20  may be connected by a direct link to the wireless tag status inference apparatus  30 , without using the network  40 . 
         [0051]    The wireless tag status inference apparatus  30 , which receives information from the wireless tag reader  20  and infers the operational status of the wireless tags  101 , comprises a receiver  31 , a wireless tag information storage unit, referred to below as a tag information memory  32 , a log compiling unit  33 , and a tag status inference unit  34 . The wireless tag status inference apparatus  30  may be configured by installing a wireless tag status inference program in an information processing apparatus such as a computer. Either distributed processing or centralized processing may be employed in this configuration. 
         [0052]    The receiver  31  receives lists of outgoing ID numbers (e.g., the list in  FIG. 5 ) from the wireless tag reader  20  via the network  40 , and submits them to the log compiling unit  33 . 
         [0053]    The tag information memory  32  stores information such as the ID numbers of the wireless tags  101  to be maintained and the geographical coordinates of the locations at which they are installed.  FIG. 6  shows an example of this information. The ID numbers  501  are the ID numbers of the wireless tags to be maintained. The installation location information  502  represents the installation locations of the wireless tags with the corresponding ID numbers  501 . In the example shown, location is represented by latitude and longitude coordinates, but the installation location information  502  may be represented by any scheme that pinpoints the location of the wireless tags. 
         [0054]    The log compiling unit  33  compiles information on the usage of the wireless tags  101  based on the information sent from the receiver  31 , and submits it to the tag status inference unit  34 . The log compiling unit  33  includes a tag log  33   a.    
         [0055]    The tag log  33   a  is a temporary storage area for information that the log compiling unit  33  provides to the tag status inference unit  34 . Referring to  FIG. 7 , the tag log  33   a  is organized as a table of ID numbers  601  and log counts  602 . The ID numbers  601  are identical to the ID numbers  501  stored in the tag information memory  32 . The log counts  602  indicate the number of times that the ID numbers  601  have been submitted by the wireless tag reader  20  to report that the wireless tags have been read. Initially, all of the log counts  602  may be zero. 
         [0056]    The log compiling unit  33  initializes the information stored in the tag log  33   a  according to the information in the tag information memory  32 . Initialization may involve copying the ID numbers  501  from the tag information memory  32  into the ID number column in the tag log  33   a , and entering ‘0’ as the log count  602  of each ID number  601  as shown in the  FIG. 7 , for example. 
         [0057]    The log compiling unit  33  receives information of the type shown in the  FIG. 5  from the wireless tag reader  20 , and updates the information stored in the tag log  33   a  according to the received information. When an ID number ID number  402  received from the wireless tag reader  20  matches an ID number  601  in the tag log  33   a , the log compiling unit  33  updates the corresponding log count  602  by incrementing its value by ‘1’. From the initial state shown in  FIG. 7 , for example, if the ID numbers ‘107’, ‘109’, ‘111’, ‘105’, ‘108’ and ‘110’ are received by the receiver  31 , the log compiling unit  33  changes the log counts  602  corresponding to ID numbers ‘107’, ‘109’, ‘111’, ‘105’, ‘108’ and ‘110’, from ‘0’ to ‘1’ in the tag log  33   a , as shown in  FIG. 8 . 
         [0058]    The tag status inference unit  34  receives the information stored in the tag log  33   a  from the log compiling unit  33 , and infers the status of the wireless tags based on the received information. For example, the tag status inference unit  34  may search for ‘0’ values in the information stored in the tag log  33   a , which it receives from the log compiling unit  33 , and infer that the wireless tags  101  having corresponding ID numbers  601  may be inoperable because they have not been logged, i.e., they have not been confirmed to operate. 
         [0059]    The tag status inference unit  34  may output the results of its inferences via any output means (not shown in the drawing): for example, it may display the results on a display unit, store the results in a memory unit such as a disk memory unit, or print the results on a printing unit such as a printer. The results may be output as, for example, a list of unlogged wireless tag ID number and the corresponding installation location information  502  stored in the tag information memory  32 . 
         [0060]    After output of the results, the log counts in the tag log  33   a  may be cleared to zero in preparation for the next logging and inference process. Tag status inferences may then be made at periodic intervals, which may be fixed or variable. For example, the tag status inference unit  34  may determine the status of the wireless tags  101  after the passage of a predetermined interval of time from the clearing of the log counts, or from the first reception of log data by the log compiling unit  33  after the clearing of the log counts. Alternatively, the tag status inference may be made when the log compiling unit  33  has received a predetermined amount of log data. Any scheduling method that provides the tag status inference unit  34  with an adequate amount of data on which to make inferences may be used. 
         [0061]    The operation of the wireless tag status inference system in the first embodiment will now be described. 
         [0062]    It will be assumed that eleven wireless tags  101 - 111  have been installed at a railway station as shown in  FIG. 9 , and that wireless tags  104  and  106  are inoperable. The user of the wireless tag reader  20  will be assumed to follow the route  1201  indicated by the dark line, encountering wireless tags  111 ,  105 ,  104 ,  107 ,  108 ,  107 ,  109 ,  110  in this sequence. The wireless tag reader  20  acquires the ID numbers of the wireless tags in this sequence, except that it does not acquire the ID number of inoperable wireless tag  104 . The reference numerals of the wireless tags  101  to  111  will be used as their ID numbers. For example, the ID number of wireless tag  111  is ‘111’. 
         [0063]    At the start of the procedure in  FIG. 10 , the tag log memory  22   a  and outgoing ID number memory  23   a  are empty. 
         [0064]    Referring to the flowchart in  FIG. 10 , as the user of the wireless tag reader  20  traverses route  1201  (S 1001 ), the wireless tag communication unit  21  in the wireless tag reader  20  acquires the ID number of each encountered wireless tag and sends the ID number to the tag logger  22  (S 1002 ). In the first encounter, for example, the wireless tag communication unit  21  interrogates wireless tag  111  and the tag logger  22  receives its ID number ‘111’. 
         [0065]    The tag logger  22  logs each received ID number by writing it in the tag log memory  22   a , assigning it the lowest available encounter number (S 1003 ). First, for example, the tag logger  22  writes ID number ‘111’ in the tag log memory  22   a , assigning it encounter number ‘1’. 
         [0066]    The tag logger  22  keeps track of the value of the lowest available encounter number, and increments this value by one each time it writes an ID number in the tag log memory  22   a  (S 1004 ). When the tag logger writes the first ID number ‘111’, for example, it increments the value of the lowest available encounter from ‘1’ to ‘2’, so that the next ID number received will be assigned encounter number ‘2’. 
         [0067]    When the user of the wireless tag reader  20  leaves the neighborhood of a wireless tag, if the user proceeds to another wireless tag, the process returns to step S 1001  and another ID number is logged. If the user leaves the area in which the wireless tags are installed, the tag logger  22  detects this from the elapse of a predetermined interval of time in which the user does not receive further guidance and the wireless tag communication unit  21  does not supply any further tag ID numbers. The tag logger  22  then decides that the user has finished using the wireless tags (S 1005 ) and proceeds to the next step (S 1006 ). 
         [0068]    At this point the tag log memory  22   a  stores the data shown in  FIG. 3 . 
         [0069]    Following the decision that the user has finished using the wireless tags in step S 1005 , the tag logger  22  sends the maximum encounter number  201  to the reading commander  24  (S 1006 ). In this example, ID numbers have been acquired seven times, so the maximum encounter number is ‘7’. 
         [0070]    When the reading commander  24  receives the maximum encounter number  201 , it selects a random lottery number from ‘1’ to the maximum encounter number (‘7’ in this case), and sends the lottery number to the tag logger  22 . The tag logger  22  retrieves the ID number  202  having this lottery number as its encounter number  201  from the tag log memory  22   a , and sends the retrieved ID number  202  to the outgoing ID list assembler  23  (S 1007 ). The outgoing ID list assembler  23  receives the ID number  202  and writes it into the outgoing ID number memory  23   a , assigning it the lowest available transmission sequence number  401  (S 1008 ), then increments the value of the lowest available transmission sequence number  401  in preparation for reception of the next ID number (S 1009 ). 
         [0071]    The outgoing ID list assembler  23  now queries the reading commander  24  as to whether there is still a lottery number to be sent to the tag log memory  22   a  (S 1010 ). If the outgoing ID list assembler  23  receives an affirmative response, the process returns to step S 1007  and the above operations are repeated, the ID number having the next lottery number as its encounter being written in the outgoing ID number memory  23   a  together with the incremented transmission sequence number. When there are no more lottery numbers to be sent, the outgoing ID list assembler  23  proceeds to the next step (S 1011 ). At this point, in the present example the outgoing ID number memory  23   a  stores the data shown in  FIG. 5 . 
         [0072]    In step S 1011 , the information stored in the outgoing ID number memory  23   a  is sent to the transmitter  25 , and the transmitter  25  sends the received information to the wireless tag status inference apparatus  30  via the network  40 . 
         [0073]      FIG. 11  is a flowchart illustrating the process by which the wireless tag status inference apparatus infers the status of wireless tags from information supplied by wireless tag readers. 
         [0074]    The ID numbers ‘101’, ‘102’, ‘103’, ‘104’, ‘105’, ‘106’, ‘107’, ‘108’, ‘109’, ‘110’, and ‘111’ are stored in advance in the tag information memory  32  of the wireless tag status inference apparatus  30 , as shown in  FIG. 6 . At periodic intervals, the log compiling unit  33  initializes the tag log  33   a  as shown in  FIG. 7 , based on the information stored in the tag information memory  32 . In the initial state, the tag log  33   a  stores ID number ‘101’ and log count ‘0’, ID number ‘102’ and log count ‘0’, ID number ‘103’ and log count ‘0’, and so on. 
         [0075]    The receiver  31  of the wireless tag status inference apparatus  30  receives the wireless tag ID numbers stored in the outgoing ID number memory  23   a  from the transmitter  25  of the wireless tag reader  20  as shown in  FIG. 5 , and submits this information to the log compiling unit  33  (S 1102 ). 
         [0076]    The log compiling unit  33  accordingly receives the ID numbers  402  shown in  FIG. 5 , and updates the log counts  602  of the corresponding ID numbers  601  in the tag log  33   a  (S 1103 ) by adding ‘1’. As the result, the log counts  602  corresponding to ID numbers ‘105’, ‘107’, ‘108’, ‘109’, ‘110’, and ‘111’ in the tag log  33   a  change from ‘0’ to ‘1’. The log counts  602  corresponding to the other ID numbers  601  remain ‘0’. 
         [0077]    The log compiling unit  33  now determines whether a predetermined monitoring interval has elapsed from the initialization of the tag log  33   a  (S 1104 ). If the predetermined monitoring interval has not elapsed, the log compiling unit  33  returns to step S 1102  and continues to receive log reports sent from wireless tag readers and to operate as described above. When the log compiling unit  33  determines that the predetermined monitoring interval has elapsed in step S 1104 , it proceeds to the next step (S 1105 ). 
         [0078]    In this next step (S 1105 ), the tag status inference unit  34  receives the information stored in the tag log  33   a  from the log compiling unit  33 , and determines the status of wireless tags  101  to  111 . In this case, the log counts  602  corresponding to ID numbers ‘104’ and ‘106’ in the tag log  33   a  have ‘0’ values and the log counts  602  corresponding to other ID numbers  601  have ‘1’ values, as shown in  FIG. 12 . The tag status inference unit  34  notes that the operation of wireless tags having ID numbers  601  corresponding to log counts ‘0’ has not been confirmed and infers that wireless tags  104  and  106  may be inoperable. The installation location information  502  stored in the tag information memory  32  is added to the ID numbers of the suspected inoperable wireless tags  104  and  106 , and the tag status inference unit  34  outputs the ID numbers  901  and the installation location information  902  as shown in  FIG. 13  via an output unit (S 1105 ). 
         [0079]    After the output of these inferred results, the log compiling unit  33  may repeat the entire procedure, starting from the initialization step (S 1101 ), to continue monitoring the status of the wireless tags. 
         [0080]    The first embodiment has the following effects. 
         [0081]    When the user of the wireless tag reader  20  uses the wireless tag reader  20  to read an arbitrary wireless tag  101  and acquire information from the guidance information center, the wireless tag status inference apparatus  30  logs the usage of the wireless tag  101  in the tag log  33   a . The tag status inference unit  34  extracts information from the tag log  33   a  to infer the status of each wireless tag  101 . If a wireless tag has not been logged, an inference is made that the wireless tag may be inoperable. 
         [0082]    Since the wireless tag communication unit  21  may fail to communicate with a wireless tag because the distance between the wireless tag reader  20  and the wireless tag is too great or because the wireless tag reader  20  was within range of the wireless tag for too short a time for communication to take place, even when a wireless tag is encountered but not been logged, that does not necessarily mean that the wireless tag  101  is inoperable; an inspection visit is necessary to determine whether the wireless tag is operable or not. On the other hand, when a wireless tag  101  is logged, it can be definitely concluded that the wireless tag is operable. Maintenance personnel therefore have to inspect only the unlogged wireless tags, instead of inspecting all of the wireless tags. This arrangement saves a great deal of maintenance time. 
         [0083]    The tag logger  22  and outgoing ID list assembler  23  send the wireless tag status inference apparatus  30  the ID numbers of the wireless tags  101  accessed by the wireless tag reader  20  in a sequence different from the sequence in which the ID numbers have actually been read. The user of the wireless tag status inference apparatus  30  accordingly cannot trace the course followed by the user of the wireless tag reader  20 . Further protection against tracing is provided because the wireless tag reader  20  sends each ID number acquired during a traverse to the wireless tag status inference apparatus  30  only once, even when the same wireless tag is visited more than once during the traverse. The user of the wireless tag reader  20  can accordingly provide information to the wireless tag status inference apparatus  30  without anxiety about loss of privacy. 
         [0084]    Since the status of the wireless tags can be monitored at a maintenance center by means of data obtained from pedestrians who encounter the wireless tags, maintenance personnel can keep track of the status of the wireless tags without making inspection tours. Since the data indicating the wireless tags read by the pedestrians&#39; tag readers is randomized before transmission to the maintenance center, the pedestrians&#39; privacy is protected. 
       Second Embodiment 
       [0085]    The wireless tag status inference system in the second embodiment is identical to the wireless tag status inference system in the first embodiment, except that the tag status inference unit  34 , instead of simply inferring that a wireless tag  101  may be inoperable if it fails to be logged, assigns a probability to each inference. 
         [0086]    As an example, it will be assumed that a plurality of wireless tags are installed in the vicinity of the wireless tag in question, and the maximum number of times any of these wireless tags has been logged (the maximum log count  602  in  FIG. 7 ) is ‘3’. ‘Vicinity’ is defined as described later. If the wireless tag reader  20  is an electronic cane possessed by a visually impaired user, it can be inferred that the user has encountered the wireless tag in question at most three times. Depending on how the cane is held, a tag may be read more than once at a single encounter, or may not be read at all, so the number of times a tag is logged is not necessarily equal to the number of times it has been encountered. The maximum count of ‘3’ accordingly may not have been produced by exactly three encounters; all that can be said with complete certainty is that a wireless tag in the vicinity of the tag in question has been encountered at least once. Nevertheless, the maximum count gives at least a rough idea of the number of times the wireless tag in question is likely to have been encountered. 
         [0087]    In the second embodiment, when a wireless tag is inferred to be inoperable, the frequency with which other wireless tags in its vicinity have been logged is used to assign a degree of reliability, referred to below as an ‘inferential probability’, to the inference. If the tags in the vicinity of the tag in question have been logged frequently, the tag in question should also have been logged frequently, so if it has not been logged at all, it is very likely to be inoperable. In general, the more often the wireless tags in the vicinity of the tag in question are logged, the more accurately the status of the tag in question can be determined. 
         [0088]    The difference between the first embodiment and the second embodiment is accordingly that the tag status inference unit  34  in the wireless tag status inference apparatus  30  in the second embodiment assigns levels of inferential probability to the unlogged wireless tags. 
         [0089]    Next, an exemplary configuration in which the tag status inference unit  34  assigns levels of inferential probability to wireless tags will be described. 
         [0090]    In the following description, inferential probabilities are calculated in terms of a basic unit ‘T’. This basic inferential probability T is related to the inherent likelihood of failure of the wireless tags, and can be measured by actual tests in the configuration of the wireless tag status inference system  10  described in the first embodiment, or can be calculated from the technical specifications of the wireless tags. Any method and any parameter values may be used in determining the basic inferential probability T. The inferential probability of an unlogged wireless tag is defined to be proportional to the basic inferential probability T and the maximum log count M (the maximum log count  602  in  FIG. 7 ) of the wireless tags in the vicinity of the unlogged wireless tag. The inferential probability can be represented in the following equation. 
         [0000]      Inferential probability of inoperability= M·T   (1) 
         [0091]    The above equation (1) is one exemplary formula for obtaining the inferential probability of an inoperability inference. Alternative methods are available, based on the mean log count of all the wireless tags except the unlogged tag in question in a given area, for example, or on other statistics of the log counts of the wireless tags in the vicinity of the tag in question. 
         [0092]    The vicinity on which the inferential probability is based should be an area in which all tags can be expected to be logged substantially equal numbers of times. In the arrangement shown in  FIG. 9 , for example, instead of individual wireless tags  101  to  111 , a group of wireless tags may be installed at each of the eleven indicated locations  101 - 111 , and each of these groups may constitute a vicinity. One such vicinity is shown in  FIG. 14 , comprising six wireless tags  106   a - 106   f  which may be installed together in place of wireless tag  106  in  FIG. 9 . A similar vicinity is shown in  FIG. 15 , comprising six wireless tags  109   a - 109   f  which may be installed in place of wireless tag  109  in  FIG. 9 . A smaller vicinity is shown in  FIG. 16 , comprising four wireless tags  108   a - 108   d  which may be installed in place of wireless tag  108  in  FIG. 9 . 
         [0093]    An unnecessarily wide area should not be defined as a vicinity, because different wireless tags in such an area are likely to be logged different numbers of times, and the maximum log count M will not be a good indicator of inferential probability for a wireless tag that has a tendency to be read less often. When the wireless tag reader  20  is an electronic cane for a visually impaired person, a vicinity may be limited to a range that can be covered by one swing of the cane. If a group of twenty-four wireless tags are installed in guidance blocks at a wide pedestrian crossing as shown in  FIG. 17 , for example, the group can be divided into several sub-groups, each sub-group constituting one vicinity group. Alternatively, vicinity groups may be defined according to the performance of the wireless tag reader  20  in the wireless tag status inference system  10  (for example, the range over which the wireless tag reader  20  can communicate with a wireless tag). 
         [0094]    The tag status inference unit  34  may identify the wireless tags in a vicinity group from the installation location information  502  stored in the tag information memory  32  in the tag status inference unit  34  for each wireless tag. The wireless tags located within a predetermined distance of an unlogged wireless tag may be defined as belonging to the same vicinity group. For example, the vicinity group of an unlogged wireless tag may be defined as the wireless tags located at the same latitude and longitude, expressed in seconds, or as the wireless tags having latitude and longitude values that differ from the latitude and longitude of the unlogged tag by five seconds or less. Alternatively, the wireless tag status inference apparatus  30  may have a memory that stores information (e.g., ID numbers) identifying the wireless tags in the vicinity group of each wireless tag. Any scheme may be used to define the vicinity groups. 
         [0095]    The wireless tag status inference apparatus  30  may output information giving the inferential probability of inoperability of each wireless tag, or it may output information listing the unlogged wireless tags arranged in order of their inferential probability of inoperability. This information may be output from the tag status inference unit  34  by any means (not shown): for example, the information may be output on a display unit, stored in a memory unit such as a disk drive, or printed by a printer. 
         [0096]    The calculation of inferential probabilities in the second embodiment will now be further described. 
         [0097]    Suppose, for example, that the tag log  33   a  stores the information shown in  FIG. 18  for the six wireless tags  106   a - 106   f  shown in  FIG. 14 . Since the log count  602  of wireless tag  106   d  is ‘0’, the tag status inference unit  34  recognizes wireless tag  106   d  as an unlogged wireless tag. The tag status inference unit  34  identifies wireless tags  106   a ,  106   b ,  106   c ,  106   e , and  106   f  as tags in the vicinity of the unlogged wireless tag  106   d . Of these tags, wireless tag  106   b  has been logged most frequently (three times), so the maximum log count M in the above equation (1) is ‘3’, and the inferential probability of inoperability of wireless tag  106   d  is calculated as M·T=3·T. 
         [0098]    Similarly, suppose that the tag log  33   a  stores the information shown in  FIG. 19  for the six wireless tags  109   a - 109   f  in  FIG. 15 . Since the log count  602  of wireless tag  109   d  is ‘0’, the tag status inference unit  34  recognizes wireless tag  109   d  as an unlogged wireless tag. The tag status inference unit  34  identifies wireless tags  109   a ,  109   b ,  109   c ,  109   e , and  109   f  as tags in the vicinity of the unlogged wireless tag  109   d . Since wireless tags  109   b  and  109   e  have been read out most often in this range (six times), the maximum log count M in the above equation (1) is ‘6’, and the inferential probability of inoperability of wireless tag  109   d  is calculated as M·T=6·T. 
         [0099]    Since the inferential probability of inoperability is 3·T for wireless tag  106   d  and  6 -T for wireless tag  109   d  wireless tag  109   d  has a higher likelihood of being inoperable than wireless tag  106   d.    
         [0100]    The second embodiment has the following effects. 
         [0101]    In the second embodiment, since the inferential probability of inoperability of an unlogged wireless tag depends on the log counts of the wireless tags in its vicinity (for example, on the maximum log count M), the inferential probability is a numerical value associated with the frequency with which the tags in the vicinity are used. Unlogged tags having a high inferential probability of inoperability thus also tend to be wireless tags that are heavily used. The wireless tags can therefore be efficiently maintained by making prompt inspections of these wireless tags, and dealing with wireless tags with lower inferential probabilities of inoperability as time permits. 
       Third Embodiment 
       [0102]    The third embodiment has the same general hardware configuration as the first and second embodiments, shown in  FIG. 2 . 
         [0103]    In the system for inferring the status of wireless tags in the second embodiment, the tag status inference unit  34  identified the wireless tags in the vicinity of an unlogged wireless tag on the basis of information such as geographical coordinates stored in the tag information memory  32  when the wireless tags are installed. In the third embodiment, the tag information memory  32  uses a hierarchical numbering system that facilitates vicinity identifications and other aspects of the management of the installed wireless tags. Aside from this hierarchical system, the third embodiment is identical to the second embodiment, so only the differences between the second embodiment and third embodiment will be described below. 
         [0104]    The third embodiment will be described in relation to the same wireless tags as the second embodiment, installed at the locations shown in  FIG. 9  and  FIGS. 14-16 . 
         [0105]      FIG. 20  shows an example of information stored in the tag information memory  32  in the third embodiment. The installation location information  502  has a hierarchical structure representing area, sub-area, wireless tag location, and sub-number. 
         [0106]    ‘Area’ designates the general location or facility where a wireless tag is installed. 
         [0107]    ‘Sub-area’ designates a more specific location in an area. For example, a sub-area may be a specific room in a building, or a specific part of some other type or area. The railroad station area in  FIG. 9 , for example, may be divided into one sub-area near the ticket counter and platform, another sub-area near the elevator, and yet another sub-area near the rest rooms, as shown in  FIG. 21 . 
         [0108]    ‘Wireless tag position’ is a number assigned to one wireless tag or a group of tags installed at the same place and providing the same guidance function. The wireless tags shown in  FIG. 14 , which are installed at a position  106  in  FIG. 21  where the path branches in three directions, are one example of such a group. When a large number of wireless tags are installed at one place, they may be divided into groups of appropriate size, as in  FIG. 17 , and a different wireless tag position number may be assigned to each group. A group of wireless tags having the same wireless tag position number corresponds to a vicinity group in the second embodiment. 
         [0109]    A different ‘sub-number’ is assigned to each wireless tag in the group of tags installed at the same wireless tag position, to identify the individual tags. 
         [0110]    The hierarchical numbering system in  FIG. 20  is exemplary; other hierarchical systems can be used instead. The hierarchy may have more than four levels, for example, or fewer than four levels. The vicinity group need not correspond to the penultimate level (‘wireless tag position’ in  FIG. 20 ); a higher level may used to designate vicinity groups. For example, there may be a ‘sub-position’ level between the ‘wireless tag position’ and ‘sub-number’ levels, with the wireless tags in the same wireless tag position still forming a single vicinity group. The area, sub-area, and position designations may also be replaced with different designations. 
         [0111]    The process by which wireless tags in the vicinity of an unlogged wireless tag are extracted in the third embodiment will now be described. It will be assumed that the wireless tags having the same wireless tag position designation form a vicinity group. 
         [0112]    The wireless tags  106   a - 106   f  listed in  FIG. 20  accordingly form a single vicinity group ( FIG. 14 ). Wireless tag  106   a , for example, is represented by area=‘Y station’, sub-area=‘near elevator’, wireless tag position=‘1’, and sub-number=‘1’. The other wireless tags in this vicinity group have the same area, sub-area, and wireless tag position designations, but different sub-numbers. Accordingly, when the tag status inference unit  34  selects the tags in the vicinity of wireless tag  106   a , it selects wireless tags  106   b - 106   f , which have the same wireless tag position (‘1’). 
         [0113]      FIG. 22  shows part of the information listed for wireless tags  109   a  to  109   f  ( FIG. 15 ) in the tag information memory  32 . All of these wireless tags have the same wireless tag position number (‘1’). Accordingly, when the tag status inference unit  34  selects the tags in the vicinity of wireless tag  109   a , it selects wireless tags  109   b - 109   f.    
         [0114]      FIG. 23  shows an example of information stored in the tag information memory for wireless tags  112   a - 112   n  and wireless tags  113   a - 113   n  in  FIG. 17 . All of these tags have the same area and sub-area designations. Wireless tags  112   a - 112   d  and  113   a - 113   d  have wireless tag position number ‘1’ and sub-numbers ‘1’ to ‘8’. Wireless tags  112   e - 112   h  and  113   e - 113   h  have wireless tag position number ‘2’ and sub-numbers ‘1’ to ‘8’. Wireless tags  112   j ,  112   k ,  112   m ,  112   n ,  113   j ,  113   k ,  113   m , and  113   n  have wireless tag position number ‘3’ and sub-numbers ‘1’ to ‘8’. Accordingly, when the tag status inference unit  34  selects the tags in the vicinity of wireless tag  112   a , it selects wireless tags  112   b - 112   d  and  113   a - 113   d.    
         [0115]    The third embodiment has the following effect. 
         [0116]    In the third embodiment, since the tag information memory  32  stores the installation location information  502  in a hierarchical numbering system such as the area, sub-area, wireless tag position, and sub-number system shown in  FIG. 20 , if the tag arrangement is changed by adding tags, removing tags, or moving tags to new locations), the information about the tags can be updated by altering only the relevant items, without disrupting the entire numbering system. Thus, this configuration facilitates the operation of the tag status inference unit  34  because despite changes in the tag information memory  32 , the numbering information continues to represent the vicinities of the wireless tags accurately. 
         [0117]    The invention is not limited to the foregoing embodiments. Some of the possible variations are described below. 
         [0118]    In the above embodiments, in order to remove evidence of the movements of the user of the wireless tag reader  20 , the reading commander  24  randomizes the order of the ID numbers acquired from the wireless tags with which the wireless tag reader  20  has communicated, and sends the wireless tag status inference apparatus  30  a data set in which the ID number of each logged wireless tag  101  appears only once, but other methods can be used instead. For example, the ID numbers can be re-arranged in a fixed order, such as ascending order or descending order; all ID numbers read from the wireless tags may be sent to the wireless tag status inference apparatus  30  the same number of times (not necessarily just once); or the number of times an ID number is sent may be arbitrarily varied. For example, if the ID numbers are represented in binary code, an ID number may be sent once if its least significant digit is ‘0’, and twice if its least significant digit is ‘1’. The wireless tag status inference apparatus  30  should then process the ID number in the same way regardless of whether it has been sent once or twice. 
         [0119]    In the above embodiments, information representing the movement of the user of a wireless tag reader  20  is randomized by a lottery number scheme involving the tag logger  22 , outgoing ID list assembler  23 , and reading commander  24  in the wireless tag reader  20 , but the randomization may be performed by a separate apparatus provided between the wireless tag reader  20  and the wireless tag status inference apparatus  30 . In this case, the separate apparatus may receive non-randomized information from the wireless tag reader  20  and remove any information that might identify the wireless tag reader  20 , as well as randomizing the sequence of the ID numbers of the wireless tags  101 , before sending the information to the log compiling unit  33 . 
         [0120]    In the above embodiments, the wireless tag status inference apparatus  30  uses the tag status inference unit  34  to infer the operational status of the wireless tags  101 , but the data stored in the tag log  33   a  may be mined for other purposes. For example, a system administrator may use the wireless tag status inference apparatus  30  to find out how frequently different wireless tags  101  are used, and then use this information to design a more effective arrangement of the wireless tags  101 , in order to provide more effective guidance to the user of the wireless tag reader  20 . Frequency-of-usage data may also be used to plan efficient routine maintenance. For example, wireless tags that are used more frequently tend to be stepped on more frequently and thus to have shorter service lives, so they can be replaced on a shorter schedule than other wireless tags. Wireless tags that are rarely or never used can be allowed to remain inoperable instead of being replaced. The data in the tag log  33   a  can also be used for reference when new systems are being planned and designed. 
         [0121]    In the above embodiments, the tag status inference unit  34  infers that the wireless tag  101  may be inoperable when its log count  602  in the tag log  33   a  is ‘0’. However, the tag status inference unit  34  may infer that the wireless tag  101  may be inoperable when its log count  602  is below a predetermined value. 
         [0122]    In the above embodiments, when the wireless tag reader  20  communicates with a wireless tag  101 , it acquires only the ID number of the wireless tag  101 , but the wireless tag reader  20  may also acquire other information from the wireless tag  101  and send this other information to the wireless tag status inference apparatus  30  as well. 
         [0123]    For example, if the wireless tag  101  is an ‘active’ tag that operates on an internal battery, the wireless tag reader  20  may acquire information indicating the amount of charge left in the battery and send this information to the wireless tag status inference apparatus  30 . The wireless tag status inference apparatus  30  can then infer the remaining life of the battery in the wireless tag  101  and identify wireless tags with low batteries, so that their batteries can be replaced or recharged before they run down. The battery charge information helps the user of the wireless tag status inference apparatus  30  plan efficient maintenance, and helps the user of the wireless tag reader  20  by reducing the occurrence of wireless tags that have gone silent because they have run out of power. 
         [0124]    In the above embodiments, the transmitter  25  sends information to the wireless tag status inference apparatus  30  as soon as the information is received from the outgoing ID list assembler  23 , but the information may be stored and sent later. For example, the information may be sent with a random delay, making it impossible for the user of the wireless tag status inference apparatus  30  to know when the user of the wireless tag reader  20  visited a particular wireless tag  101 . The privacy of the user of the wireless tag reader  20  is then further protected, so the user can provide information to the wireless tag status inference apparatus  30  with even less anxiety. 
         [0125]    The wireless tags shown in the above embodiments were used for the purpose of aiding visually impaired persons, but the wireless tag status inferring system of the present invention is applicable to systems in which wireless tags are used for other purposes. 
         [0126]    Those skilled in the art will recognize that further variations are possible within the scope of the invention, which is defined in the appended claims.