Abstract:
A sensor diagnostic apparatus for diagnosing a sensor includes a moving object counter, a reference value storage, and a comparator. The moving object counter counts, in accordance with identification data acquired by a plurality of sensors in a predefined time period, a local number of moving objects moving between a sensing area of a first sensor and a sensing area of a second sensor near the first sensor. The reference value storage stores a preset reference value for the first sensor and the second sensor. The comparator compares a value derived from the local number of moving objects counted by the moving object counter with the preset reference value stored in the reference value storage to determine the first sensor to be in trouble when a difference between the value derived from the local number of moving objects and the preset reference value exceeds a predefined threshold value.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a sensor diagnostic method and apparatus for diagnosing each sensor of a sensor system. 
         [0003]    2. Description of the Related Art 
         [0004]    Nowadays, data about an amount of time (a travel time) required to move from one place to another is provided on major roads as road information. Japanese Laid-open Patent Publication No. 11-110684 discusses a sensor system for acquiring such data regarding a travel time, for example. 
         [0005]    In addition, Japanese Laid-open Patent Publication No. 2006-244338 discusses a sensor system for locating a specific product, for example. 
         [0006]    As shown in  FIG. 1 , the sensor system for acquiring data about a travel time includes sensors (vehicle license plate readers)  1   a,    1   b,  and  1   c  allocated in a plurality of locations on the road, and a center apparatus  3  connected to the sensors  1   a,    1   b,  and  1   c  through a network  2  and gathering number data (read vehicle license number and read time) output from the sensors  1   a,    1   b,  and  1   c.    
         [0007]    The center apparatus  3  determines a travel time of each section on the basis of number data sent from the sensors  1   a,    1   b,  and  1   c,  by obtaining a difference between the shot times of the same vehicle license number at respective locations where the sensors  1   a,    1   b,  and  1   c  are allocated. 
         [0008]    As shown in  FIG. 2 , the system for locating a specific product includes an RFID (radio frequency identification) tag  6  attached to a product  5  to be managed, sensors (RFID readers)  7   a,    7   b,  and  7   c  allocated in a plurality of locations within a roaming area of the product, and a center apparatus  9  connected to the sensors  7   a,    7   b,  and  7   c  through a network  8  and gathering ID data (read ID and read time) output from the sensors  7   a,    7   b,  and  7   c  through the network  8 . The center apparatus  9  grasps a current location and roaming history of each product by obtaining read times of a product with the same ID at respective locations where the sensors  7   a,    7   b,  and  7   c  are allocated. 
         [0009]    Conventional systems operate on the presumption that sensors operate normally as expected and identification data of a specific product within a predefined distance from a sensor can be surely acquired without any loss. However, in fact, a sensor may output wrong data or lose data due to aging, a change in installation environments, and the like. 
         [0010]    For example, a vehicle license plate reader, which reads a vehicle license plate from a video image captured with a camera, may not correctly read a vehicle license plate if a camera lens gets fogged or soiled during operation. In this case, an output result may involve an error or loss. 
         [0011]    In addition, an RFID reader may not correctly read an ID when some object shielding or reflecting a radio wave is allocated within a sensing area or a direction of an antenna is changed during operation. In this case, an output result may involve any loss. 
         [0012]    As discussed above, an abnormal operation of a sensor during operation of a system causes an abnormal operation of the system. Thus, it is necessary to check whether each sensor operates normally in order to normally operate the system. 
       SUMMARY 
       [0013]    One conceivable solution to this problem is to provide a self-diagnostic function for checking normal operations to each sensor, and get notification in case of trouble. The self-diagnostic function, however, may not be easily realized and may be expensive because all changes that would influence a sensor operation, including aging and environmental change, must be considered to detect a trouble. 
         [0014]    Accordingly, it is an object of the present invention to provide a sensor diagnostic method and apparatus capable of checking normal operations of each sensor without providing a self-diagnostic function to each sensor. 
         [0015]    According to an aspect of the present invention, provided is a sensor diagnostic apparatus for diagnosing a sensor among a plurality of sensors. Each of the plurality of sensors identifies an object and outputs acquired identification data. The sensor diagnostic apparatus includes a moving object counter, a reference value storage, and a comparator. The moving object counter counts, in accordance with identification data acquired by the plurality of sensors in a predefined time period, a local number of moving objects moving between a sensing area of a first sensor and a sensing area of a second sensor near the first sensor. The reference value storage stores a preset reference value for the first sensor and the second sensor. The comparator compares a value derived from the local number of moving objects counted by the moving object counter with the preset reference value stored in the reference value storage to determine the first sensor to be in trouble when a difference between the value derived from the local number of moving objects and the preset reference value exceeds a predefined threshold value. 
         [0016]    Each of the plurality of sensors may output, as well as the identification data, data of an acquired time of the identification data. The moving object counter of the sensor diagnostic apparatus may count the local number of moving objects of which the identification data were acquired by the first sensor and the second sensor and the acquired times of the identification data indicate times within the predefined time period. 
         [0017]    A dimension of the preset reference value may be identical to a dimension of the local number of moving objects. In such a configuration, the comparator compares the local number of moving objects, as the value derived from the local number of moving objects, with the preset reference value. 
         [0018]    A dimension of the preset reference value may be identical to a dimension of a ratio of the local number of moving objects against a whole number of moving objects identified by the first sensor. In such a configuration, the comparator compares the ratio of the local number of moving objects against the whole number of moving objects identified by the first sensor, as the value derived from the local number of moving objects, with the preset reference value. 
         [0019]    The reference value storage may store a plurality of preset reference values corresponding to different environmental conditions. In such a configuration, the comparator compares the value derived from the local number of moving objects with a preset reference value selected, in accordance with a current environmental condition, from among the plurality of preset reference values stored in the reference value storage. 
         [0020]    The comparator may determine the first sensor to be normal when the difference between the value derived from the local number of moving objects and the preset reference value is less than or equals to the predefined threshold value. In such a configuration, the sensor diagnostic apparatus may further include an updater for updating the preset reference value for the first sensor and the second sensor in accordance with the local number of moving objects when the comparator has determined the first sensor to be normal. 
         [0021]    According to another aspect of the present invention, provided is a sensor diagnostic method executed by a sensor diagnostic apparatus for diagnosing a sensor among a plurality of sensors. Each of the plurality of sensors identifies an object and outputs acquired identification data. The sensor diagnostic method includes: counting, in accordance with identification data acquired by the plurality of sensors in a predefined time period, a local number of moving objects moving between a sensing area of a first sensor and a sensing area of a second sensor near the first sensor, storing a preset reference value for the first sensor and the second sensor, and comparing a value derived from the local number of moving objects counted in the operation of counting a local number of moving objects with the preset reference value to determine the first sensor to be in trouble when a difference between the value derived from the local number of moving objects and the preset reference value exceeds a predefined threshold value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a diagram illustrating an example of conventional sensor systems; 
           [0023]      FIG. 2  is a diagram illustrating an example of conventional sensor systems; 
           [0024]      FIG. 3  is a diagram illustrating an example of an entire system configuration of a sensor system according to an embodiment of the present invention; 
           [0025]      FIG. 4  is a diagram illustrating an example of a function configuration of a sensor diagnostic function provided to a center apparatus according to an embodiment of the present invention; 
           [0026]      FIG. 5  is a diagram illustrating an example of data format of data stored in a sensor output storage part according to an embodiment of the present invention; 
           [0027]      FIG. 6  is a diagram illustrating an example of sensor allocation according to an embodiment of the present invention; 
           [0028]      FIG. 7  is a diagram illustrating an example of data format of a sensor location data table stored in a sensor location storage part according to an embodiment of the present invention; 
           [0029]      FIG. 8  is a diagram illustrating an example of data format of data stored in a reference value storage part according to an embodiment of the present invention; 
           [0030]      FIG. 9  is a diagram illustrating an example of data format of a moving object number table according to an embodiment of the present invention; 
           [0031]      FIG. 10  is a diagram illustrating a flowchart of a sensor diagnostic process executed by a comparison part according to an embodiment of the present invention; 
           [0032]      FIG. 11  is a diagram illustrating a flowchart of an adjacent sensor selection process executed by a comparison part according to an embodiment of the present invention; 
           [0033]      FIG. 12  is a diagram illustrating a flowchart of a moving object count process executed by a comparison part according to an embodiment of the present invention; 
           [0034]      FIG. 13  is a diagram illustrating a flowchart of a determination process executed by a comparison part according to an embodiment of the present invention; 
           [0035]      FIG. 14  is a diagram illustrating an example of a travel time calculation system as a sensor system according to an embodiment of the present invention; 
           [0036]      FIG. 15  is a diagram illustrating an example of data format of data stored in a reference value storage part according to an embodiment of the present invention; 
           [0037]      FIGS. 16A and 16B  are diagrams illustrating examples of data format of data stored in a reference value storage part according to an embodiment of the present invention; 
           [0038]      FIG. 17  is a diagram illustrating examples of environmental conditions according to an embodiment of the present invention; 
           [0039]      FIG. 18  is a diagram illustrating examples of environmental conditions according to an embodiment of the present invention; and 
           [0040]      FIG. 19  is a diagram illustrating a flowchart of a determination process executed by a comparison part according to an embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0041]    Hereinafter, embodiments of the present invention will be discussed with reference to the accompanying drawings. 
         [0042]    In the following embodiments of the present invention, a trouble in a sensor is detected on the basis of the fact that a vehicle or other such objects moves following a fixed pattern. 
         [0043]    For example, sensors allocated on one road detect in turn most of vehicles running on the road. Then, IDs of the vehicles detected by each of the plurality of sensors are compared to one another. If many IDs are matched, the sensors may normally operate. If only a few IDs are matched, the sensors may not normally operate. The sensors are diagnosed on the basis of such an idea. 
         [0044]      FIG. 3  is a diagram illustrating an example of an entire system configuration of a sensor system according to an embodiment of the present invention. The sensor system according to the present embodiment includes n sensors  11 - 1  to  11 - n,  a center apparatus  13 , and an output unit  14 . The sensors  11 - 1  to  11 - n  are allocated along a route of an object and detect identification data of the object to output the detected identification data and the detected time. 
         [0045]    The center apparatus  13  is connected to the sensors  11 - 1  to  11 - n  through a network  12  and gathers data output from the sensors  11 - 1  to  11 - n.  Further, the center apparatus  13  has a sensor diagnostic function. The output unit  14  outputs a result of the sensor diagnosis by the center apparatus  13 . 
         [0046]    Vehicle license plate readers or wireless tag readers such as RFID readers are used as the sensors  11 - 1  to  11 - n,  but any other sensors may be used as long as being capable of detecting identification data of an object. 
         [0047]      FIG. 4  is a diagram illustrating an example of a function configuration of a sensor diagnostic function provided to a center apparatus according to an embodiment of the present invention. The sensor diagnostic function according to the present embodiment includes a sensor output collection part  21 , a sensor output storage part  22 , a sensor location storage part  23 , a reference value storage part  24 , a comparison part  25 , and a trouble notification part  26 . The sensor output collection part  21  receives data output from the sensors  11 - 1  to  11 - n  and stores a suit of data including an ID (vehicle license number or RFID) of an object read by each sensor and the read time in the sensor output storage part  22  for each sensor ID.  FIG. 5  is a diagram illustrating an example of data format of data stored in the sensor output storage part according to an embodiment of the present invention. 
         [0048]    The sensor location storage part  23  stores, in advance, data of positional relationships between the sensors  11 - 1  to  11 - n  allocated along the route on which an object moves.  FIG. 6  is a diagram illustrating an example of sensor allocation according to an embodiment of the present invention. In the example shown in  FIG. 6 , the sensors  11 - 1  to  11 - n  are allocated along a moving route  16  of an object  15  and have sensing areas  11 - 1   a  to  11 - na,  respectively.  FIG. 7  is a diagram illustrating an example of data format of a sensor location data table stored in a sensor location storage part according to an embodiment of the present invention. For example, if the sensors  11 - 1  to  11 - n  are allocated in line along the moving route  16  of the object  15  (target object) as shown in  FIG. 6 , IDs of adjacent sensors along the moving route  16  are stored for each sensor to obtain the sensor location data table as shown in  FIG. 7 . 
         [0049]    The sensor  11 - 1  is allocated at an end of the moving route  16 . Thus, “ 2 ” representing the sensor  11 - 2  is stored alone in the field of adjacent sensor ID for the sensor  11 - 1 . In contrast, “ 1 ” and “ 3 ” representing the sensors  11 - 1  and  11 - 3 , respectively, are stored in the field of adjacent sensor ID for the sensor  11 - 2 . 
         [0050]    The reference value storage part  24  stores, in advance, reference values for a moving pattern of an object. A reference value is defined as the number Sij of objects that move from one sensor location to another during a predefined time period T, for example.  FIG. 8  is a diagram illustrating an example of data format of data stored in a reference value storage part according to an embodiment of the present invention. A value of Sij may be set with a counted value that is obtained under such a condition that all sensors normally operate or with an empirically-derived value. 
         [0051]    The comparison part  25  reads data from the sensor output storage part  22  at regular time intervals or at a predefined date and time, and diagnoses the sensors  11 - 1  to  11 - n  with reference to the data stored in the sensor location storage part  23  and the reference value storage part  24 . If a trouble is found in the sensor as a result of the diagnosis, a trouble notification is output from the output unit  14  by way of the trouble notification part  26 . 
         [0052]    During operation of the system, the sensor output collection part  21  first receives data output from the sensors  11 - 1  to  11 - n  and stores, for an ID of each sensor, an ID of an object detected by a pertinent sensor and the read time in the sensor output storage part  22  as shown in  FIG. 5 . 
         [0053]    Then, the comparison part  25  diagnoses each sensor after the data output from the sensors is accumulated for a predefined time period. 
         [0054]      FIG. 10  is a diagram illustrating a flowchart of a sensor diagnostic process executed by a comparison part according to an embodiment of the present invention. A flow of the sensor diagnostic process will be discussed with reference to  FIG. 10 . 
         [0055]    In operation S 1 , it is determined whether all of the sensors  11 - 1  to  11 - n  have been checked. 
         [0056]    In operation S 2 , if any sensor is left to be checked (operation S 1 : No), a sensor i to be checked is selected. 
         [0057]    In operation S 3 , another two sensors j and k necessary for checking the sensor i are selected. Here, the sensor j is adjacent to the sensor i and the sensor k is adjacent to the sensor i or j. 
         [0058]    To elaborate, the two sensors j and k adjacent to the sensor i are selected with reference to the sensor location data table shown in  FIG. 7 , which is stored in the sensor location storage part  23 . If only the sensor j is adjacent to the sensor i, e.g. i=1 and j=2, the sensor k adjacent to the sensor j other than the sensor i, i.e. k=3, is selected. 
         [0059]    In operation S 4 , sensor diagnosis is carried out. First, output data of the sensors i, j, and k for a time period corresponding to a predefined time period T from time T 1  to time T 2  are read from the sensor output storage part  22 . Then, the output data of the sensor i is compared with that of the sensor j to calculate the numbers Tij and Tji of moving objects whose IDs are matched. As a result, a moving object number table shown in  FIG. 9  is obtained. 
         [0060]    For example, if the sensor  11 - 1  is in trouble, a deviation of the numbers T 12 , T 21 , T 13 , and T 31  of moving objects shaded in  FIG. 9  from reference values S 12 , S 21 , S 13 , and S 31  shown in  FIG. 8 , respectively, is large. Further, if the sensor  11 - 2  is in trouble, a deviation of the numbers T 12 , T 21 , T 23 , and T 32  of moving objects from reference values S 12 , S 21 , S 23 , and S 32  shown in  FIG. 8 , respectively, is large. 
         [0061]    In operation S 5 , the numbers Tij, Tji, Tik, and Tki of moving objects are compared with reference values Sij, Sji, Sik, and Ski corresponding to the numbers of moving objects, which are stored in the reference value storage part  24  to diagnose the sensor i. If the deviation therebetween reaches a predefined value or more, the sensor i is determined to be in trouble. 
         [0062]    In operation S 6 , if the sensor i is in trouble (operation S 5 : No), the trouble notification part  26  notifies the output unit  14  of the trouble of the sensor i. Then, the process returns to operation S 1  to check a next sensor. 
         [0063]    In operation S 7 , if the sensor i is normal (operation S 5 : Yes), the reference values Sij, Sji, Sik, and Ski are updated based on Expressions (1) to (4). 
         [0000]        Sij=α*Tij +(1−α)* Sij    (1) 
         [0000]        Sji=α*Tji +(1−α)* Sji    (2) 
         [0000]        Sik=α*Tik +(1−α)* Sik    (3) 
         [0000]        Ski=α*Tki +(1−α)* Ski    (4) 
         [0064]    Here, α is a fixed value of, for example, about 0.05 to 0.4. After that, the process returns to operation S 1  to diagnose a next sensor. 
         [0065]      FIG. 11  is a diagram illustrating a flowchart of an adjacent sensor selection process in operation S 3  executed by a comparison part according to an embodiment of the present invention. A flow of the adjacent sensor selection process in operation S 3  will be discussed with reference to  FIG. 11 . 
         [0066]    In operation S 11 , a row with the sensor ID “i” is selected from the sensor location data table. 
         [0067]    In operation S 12 , the value of the parameter j is set with a value of a sensor ID in a first column among adjacent sensor IDs. 
         [0068]    In operation S 13 , it is determined whether a second column among adjacent sensor IDs has a value of a sensor ID. 
         [0069]    In operation S 14 , if a value of a sensor ID is registered in the second column (operation S 13 : Yes), the value of the parameter k is set with the value of the sensor ID registered in the second column. 
         [0070]    In operation S 15 , If any value of a sensor ID is not registered in the second column (operation S 13 : No), a row with the sensor ID “j” is selected from the sensor location data table. 
         [0071]    In operation S 16 , a value of a parameter kδ is set with a value of a sensor ID in a first column among adjacent sensor IDs. 
         [0072]    In operation S 17 , it is determined whether the value of the parameter kδ equals to the value of the parameter i. 
         [0073]    In operation S 18 , If the value of the parameter kδ does not equal to the value of the parameter i (operation S 17 : No), the value of the parameter k is set with the value of the parameter kδ. 
         [0074]    In operation S 19 , if the value of the parameter kδ equals to the value of the parameter i (operation S 17 : Yes), the value of the parameter k is set with a value of a sensor ID in the second column among adjacent sensor IDs. 
         [0075]      FIG. 12  is a diagram illustrating a flowchart of a moving object count process in operation S 4  executed by a comparison part according to an embodiment of the present invention. A flow of the moving object count process in operation S 4  will be discussed with reference to  FIG. 12 . 
         [0076]    In operation S 21 , values of Tij and Tji are reset to 0. Moreover, the number m of extracted output data of the sensor i is reset to 0. 
         [0077]    In operation S 22 , it is determined whether all output data of the sensor i has been read from the sensor output storage part  22 . If all output data of the sensor i has been read (operation S 22 : Yes), the process is terminated. 
         [0078]    In operation S 23 , if any output data of the sensor i is left to be read (operation S 22 : No), one suit of output data (time t, IDm) of the sensor i during a time period from the time T 1  to the time T 2  is extracted and the value of m is incremented by 1. 
         [0079]    In operation S 24 , the number mδ of extracted output data of the sensor j is reset to 0. 
         [0080]    In operation S 25 , it is determined whether all output data of the sensor j has been read from the sensor output storage part  22 . If all output data of the sensor j has been read (operation S 25 : Yes), the process returns to operation S 22 . 
         [0081]    In operation S 26 , if any output data of the sensor j is left to be read (operation S 25 : No), one suit of output data (time tδ, IDmδ) of the sensor j during a time period from the time T 1  to the time T 2  is extracted and the value of mδ is incremented by 1. 
         [0082]    In operation S 27 , it is determined whether the value of IDm equals to the value of IDmδ. If the value of IDm does not equal to the value of IDmδ, the process returns to operation S 25 . 
         [0083]    In operation S 28 , if the value of IDm equals to the value of IDmδ, it is determined whether the value of t is less than the value of tδ. 
         [0084]    In operation S 29 , if the value of t is less than the value of tδ (operation S 28 : Yes), Tij is incremented by 1 and the process returns to operation S 22 . 
         [0085]    In operation S 30 , if the value of t is more than or equals to the value of tδ (operation S 28 : No), Tji is incremented by 1 and the process returns to operation S 22 . 
         [0086]      FIG. 13  is a diagram illustrating a flowchart of a determination process in operation S 5  executed by a comparison part according to an embodiment of the present invention. A flow of the determination process in operation S 5  will be discussed with reference to  FIG. 13 . 
         [0087]    In operation S 41 , it is determined whether a deviation, i.e., an absolute value of a difference, between the number Tij of moving objects and the reference value Sij exceeds a predefined value (a first fixed value). 
         [0088]    In operation S 42 , if the deviation between the number Tij of moving objects and the reference value Sij exceeds the first fixed value (operation S 41 : Yes), it is determined whether a deviation between the number Tji of moving objects and the reference value Sji exceeds the first fixed value. 
         [0089]    In operation S 43 , if the deviation between the number Tji of moving objects and the reference value Sji exceeds the first fixed value(operation S 42 : Yes), it is determined whether a deviation between the number Tik of moving objects and the reference value Sik exceeds the first fixed value. 
         [0090]    In operation S 44 , if the deviation between the number Tik of moving objects and the reference value Sik exceeds the first fixed value (operation S 43 : Yes), it is determined whether a deviation between the number Tki of moving objects and the reference value Ski exceeds the first fixed value. 
         [0091]    In operation S 45 , if all conditions in operations S 41  to S 44  are satisfied, the sensor i is determined to be in trouble. 
         [0092]    In operation S 46 , if any of the conditions in operations S 41  to S 44  is not satisfied, the sensor i is determined to be normal. 
         [0093]      FIG. 14  is a diagram illustrating an example of a travel time calculation system as a sensor system according to an embodiment of the present invention. This system calculates an amount of time required to move from one place to another and includes a plurality of sensors (vehicle license plate readers)  31 - 1  to  31 - n  allocated along a road  30 , a center apparatus  33 , and an output unit  34 . The center apparatus  33  is connected to the sensors  31 - 1  to  31 - n  through a network  32  and gathers number data (read vehicle license number and read time) output from each of the sensors  31 - 1  to  31 - n.    
         [0094]    The sensors  31 - 1  to  31 - n  are not limited to the vehicle license plate reader but may be any other devices capable of uniquely identifying a target vehicle, more specifically, detecting an identification number of a vehicle  36 . For example, a DSRC (dedicated short range communication) device that reads a vehicle identification number by wireless may be used. 
         [0095]    In this system, each sensor sends a detected vehicle ID and detected time to the center apparatus  33 . The center apparatus  33  retrieves the same ID from output data of the sensors  31 - 1  to  31 - n  and estimates an amount of time required to move between locations where the sensors are allocated according to a difference between the detected times. 
         [0096]    In this system, the center apparatus  33  diagnoses the sensors  31 - 1  to  31 - n.  The function configuration for the diagnosis is as shown in  FIG. 4 . 
         [0097]    The sensor location storage part  23  stores, in advance, data of positional relationships among the sensors  31 - 1  to  31 - n  allocated on the road. If the sensors  31 - 1  to  31 - n  are allocated as shown in  FIG. 14 , IDs of adjacent sensors  31 - 1  to  31 - n  on the road are stored for each sensor as shown in  FIG. 7 . 
         [0098]    Further, the reference value storage part  24  stores, in advance, reference values for a moving pattern of a vehicle. The reference value is defined as the number Sij of vehicles moving from a location of a sensor i to a location of another sensor j during a predefined time period T. The data is stored in the reference value storage part  24  as shown in  FIG. 8 . A value of Sij may be set with a counted value that is obtained under such a condition that all sensors normally operate or with an empirically-derived value. 
         [0099]      FIG. 15  is a diagram illustrating an example of data format of data stored in a reference value storage part according to an embodiment of the present invention. As shown in  FIG. 15 , the reference value may be set with a reference value Si_ij whose dimension is identical to a dimension of a ratio of the number Tij of vehicles (moving objects) moving from a location of the sensor i to a location of another sensor j against the total number Ni of vehicles detected by the sensor i during a predefined time period T. 
         [0100]      FIGS. 16A and 16B  are diagrams illustrating examples of data format of data stored in a reference value storage part according to an embodiment of the present invention. As shown in  FIGS. 16A and 16B , a plurality of reference values may be set in accordance with environmental conditions. The environmental conditions differ between the examples shown in  FIGS. 16A and 16B . Thus, the reference value Sij shown in  FIG. 16A  is different from the reference value Tij shown in  FIG. 16B  (of course, these values may happen to match with each other). 
         [0101]      FIGS. 17 and 18  are diagrams illustrating examples of environmental conditions according to an embodiment of the present invention. As for the environmental condition, a time zone may be employed as shown in  FIG. 17 , or various conditions may be employed as long as the conditions are quantifiable, e.g., a weather condition as shown in  FIG. 18 . According to the examples shown in  FIG. 17 , reference values shown in  FIG. 16A  are used during a time period from 8:00 to 17:00 and reference values shown in  FIG. 16B  are used during a time period from 17:00 to 8:00. According to the examples shown in  FIG. 18 , reference values shown in  FIG. 16A  are used in such an environment that the precipitation reaches 5 mm or more, and reference values shown in  FIG. 16B  are used in such an environment that the precipitation is less than 5 mm. 
         [0102]    During operation of the sensor diagnostic system, the sensor output collection part  21  receives output data of the sensors  31 - 1  to  31 - n  and stores, for each sensor ID, vehicle IDs read by the sensor and read time in the sensor output storage part  22  as shown in  FIG. 5 . The sensor diagnostic function is started after output data of the sensors is accumulated during a predefined time period T. 
         [0103]    The sensor diagnosis is carried out by the comparison part  25  through the process shown in  FIG. 10 . First, a sensor i to be checked is determined and another two sensors j and k necessary for checking the sensor i are selected. The sensors j and k are selected through the selection process shown in  FIG. 11 . The comparison part  25  references the sensor location data table shown in  FIG. 7  stored in the sensor location storage part  23  to select the two sensors j and k adjacent to the sensor i. If the sensor j is only adjacent to the sensor i, the sensor k adjacent to the sensor j other than the sensor i is selected. 
         [0104]    The comparison part  25  reads, from the sensor output storage part  22 , data output from the thus-selected sensors i, j, and k during a time period from time T 1  to time T 2  corresponding to a predefined time period T. Then, the output data of the sensor i is compared with that of the sensor j through the moving object count process shown in  FIG. 12  to calculate the numbers Tij and Tji of moving objects whose IDs are matched. Likewise, the output data of the sensor i is compared with that of the sensor k through the moving object count process shown in  FIG. 12  to calculate the numbers Tik and Tki of moving objects whose IDs are matched. 
         [0105]    Moreover, the comparison part  25  compares the numbers Tij, Tji, Tik, and Tki of moving objects with the reference values Sij, Sji, Sik, and Ski stored in the reference value storage part  24 , respectively, through the determination process shown in  FIG. 13 . If each deviation therebetween exceeds a predefined value, the sensor i is determined to be in trouble. 
         [0106]      FIG. 19  is a diagram illustrating a flowchart of a determination process executed by a comparison part according to an embodiment of the present invention. If the reference value Si_ij shown in  FIG. 15  is stored in the reference value storage part  24 , the determination process shown in  FIG. 19  is performed in place of the determination process shown in  FIG. 13 . A flow of the determination process will be discussed with reference to  FIG. 19 . 
         [0107]    In operation S 51 , it is determined whether a deviation, i.e., an absolute value of a difference, between a ratio Tij/Nj of the number Tij against the number Nj and the reference value Sj_ij exceeds a predefined value (a second fixed value). Here, the number Tij is defined as the number of vehicles moving from a location of the sensor i to a location of another sensor j during the predefined time period T. The number Nj is defined as the total number of vehicles detected by the sensor j during the predefined time period T. The dimension of the reference value Sj_ij is identical to the dimension of the ratio Tij/Nj. 
         [0108]    In operation S 52 , if the deviation between the ratio Tij/Nj and the reference value Sj_ij exceeds the second fixed value (operation S 51 : Yes), it is determined whether a deviation between a ratio Tji/Nj and the reference value Sj_ji exceeds the second fixed value. Here, the number Tji is defined as the number of vehicles moving from a location of the sensor j to a location of another sensor i during the predefined time period T. 
         [0109]    In operation S 53 , if the deviation between the ratio Tji/Nj and the reference value Sj_ji exceeds the second fixed value (operation S 52 : Yes), it is determined whether a deviation between a ratio Tik/Nk and the reference value Sk_ik exceeds the second fixed value. Here, the number Tik is defined as the number of vehicles moving from a location of the sensor i to a location of another sensor k during the predefined time period T. The number Nk is defined as the total number of vehicles detected by the sensor k during the predefined time period T. 
         [0110]    In operation S 54 , if the deviation between the ratio Tik/Nk and the reference value Sk_ik exceeds the second fixed value (operation S 53 : Yes), it is determined whether a deviation between a ratio Tki/Nk and the reference value Sk_ki exceeds the second fixed value. Here, the number Tki is defined as the number of vehicles moving from a location of the sensor k to a location of another sensor i during the predefined time period T. 
         [0111]    In operation S 55 , if all conditions in operations S 51  to S 54  are satisfied, the sensor i is determined to be in trouble. 
         [0112]    In operation S 56 , if any of the conditions in operations S 51  to S 54  is not satisfied, the sensor i is determined to be normal. 
         [0113]    If a plurality of reference values are set in accordance with various environmental conditions, the comparison part  25  may use reference values corresponding to an environmental condition for current determination process. 
         [0114]    According to the above described embodiments, it is possible to determine whether each sensor operates normally without providing a self-diagnostic function to each sensor. Thus, even in a sensor system using an inexpensive sensor having no self-diagnostic function or using an existing sensor, a sensor in trouble may be automatically detected, so a high-reliability system may be configured at a low cost.