Patent Publication Number: US-2020276957-A1

Title: Seatbelt use determination device

Description:
TECHNICAL FIELD 
     The present invention relates to a seatbelt use determination device that determines use of a seatbelt device configured to restrain an occupant sitting in a vehicle seat with a webbing. 
     BACKGROUND ART 
     Seatbelt devices restrain an occupant when webbing has been put on by engaging a tongue, through which the webbing is passed, with a buckle fixed to a vehicle body. Generally, a buckle switch provided to the buckle is used to issue an alert regarding use of the seatbelt device. The alert regarding use of the seatbelt device is performed by detecting insertion of the tongue into the buckle. 
     However, in cases in which the seatbelt device is used improperly, for example by deliberately placing the webbing behind the body of the occupant or around the rear of a seatback and inserting the tongue into the buckle, insertion of the tongue into the buckle is detected, and so an alert regarding use of the seatbelt device is not issued. 
     Japanese Patent Application Laid-Open (JP-A) No. 2012-183942 proposes technology to detect when a seatbelt is being used improperly. 
     Specifically, the technology disclosed in JP-A No. 2012-183942 includes a door opening and closing detection device, a seatbelt use detection device, an engine start detection device to detect an operation to start an engine, an engine stop detection device to detect an operation to stop the engine, a control device to determine whether or not the seatbelt is in an improper use state, and an alert device to alert an occupant that the seatbelt is in the improper use state. An alert is issued in cases in which the seatbelt has been detected to be in an in-use state when an operation to stop the engine has been detected and when a door is detected to have transitioned from a closed state to an open state, and an operation to start the engine is then performed. 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in JP-A No. 2012-183942, after an alert has been issued, the alert can be disabled while the seatbelt device remains in an improper use state by removing and then reinserting the tongue. Therefore, there is room for improvement. 
     In consideration of the above circumstances, an object of the present invention is to provide a seatbelt use determination device capable of reliably detecting improper use of a seatbelt device. 
     Solution to Problem 
     In order to achieve the above object, a seatbelt use determination device of a first aspect of the present disclosure includes a sensor section, a detection section, and a notification section. The sensor section is configured to detect insertion of a seatbelt tongue into a seatbelt buckle. The detection section is configured to detect rotation of a take-up section for taking up a webbing. When insertion of the tongue into the buckle has been detected by the sensor section, the notification section is configured to use a detection result of the detection section to determine a predetermined improper use of the webbing and to provide notification of the improper use. 
     In the first aspect of the present disclosure, the sensor section is configured to detect insertion of the tongue into the buckle, and the detection section is configured to detect rotation of the take-up section for taking up the webbing. 
     When insertion of the tongue into the buckle has been detected by the sensor section, the notification section is configured to use a detection result of the detection section to determine the predetermined improper use of the webbing and to notify of the improper use. Namely, when the webbing is used improperly, for example by being passed behind the body of the occupant or to the rear of a seatback, there is less variation in the rotation of the take-up section than when the webbing is being used correctly, enabling the notification section to employ the detection results for the rotation of the take-up section in the determination and notification of improper use. Moreover, reliable detection and notification of improper use is possible due to detecting the rotation of the take-up section even if the tongue is removed and then reinserted. Accordingly, a seatbelt use determination device capable of reliably detecting improper use of a seatbelt device is capable of being provided. 
     In a second aspect of the present disclosure, configuration may be made wherein when insertion of the tongue into the buckle has been detected by the sensor section, the notification section is configured to determine the improper use and to provide notification of the improper use in cases in which a time period, during which a rotation change amount per unit time of the take-up section is continuously less than a predetermined value, is longer than a predetermined first threshold. Namely, when insertion of the tongue into the buckle is detected in a case in which the webbing is being used correctly, some degree of variation in the rotation of the take-up section arises due to acceleration during travel and movement of the occupant. When the webbing is being used improperly, there is less such variation in the rotation than when the webbing is being used correctly. Accordingly, determining whether or not the time period during which the rotation change amount per unit time is continuously below the predetermined value is less than the predetermined first threshold enables reliable detection and notification of improper use of the webbing. 
     In a third aspect of the present disclosure, configuration may be made wherein the notification section is configured to determine the improper use and to provide notification of the improper use, in cases in which a rotation change amount of the take-up section, during a predetermined time period prior to insertion of the tongue into the buckle being detected by the sensor section, is less than a predetermined second threshold. When the webbing is being used correctly, the webbing is pulled out to some degree before being inserted into the buckle. Therefore, some degree of variation in the rotation of the take-up section in a pull-out direction arises during the predetermined time period prior to insertion of the tongue into the buckle being detected by the sensor section. When the webbing is being used improperly, there is less such variation in the rotation than when the webbing is being used correctly. Accordingly, determining whether or not the rotation change amount during the predetermined time period prior to insertion of the tongue into the buckle being detected is less than the second threshold enables reliable detection and notification of improper use of the webbing. 
     In a fourth aspect of the present disclosure, configuration may be made wherein the notification section is configured to determine the improper use and to provide notification of the improper use, in cases in which a rotation change amount of the take-up section, during a predetermined time period following detection of insertion of the tongue into the buckle by the sensor section, is less than a predetermined third threshold. When the webbing is being used correctly, the webbing is pulled out to some degree before being inserted into the buckle. Therefore, some degree of variation in the rotation of the take-up section in a take-up direction arises following the predetermined time period after insertion of the tongue into the buckle has been detected by the sensor section. When the webbing is being used improperly, there is less such variation in the rotation than when the webbing is being used correctly. Accordingly, determining whether or not the rotation change amount following the predetermined time period after insertion of the tongue into the buckle has been detected is less than the third threshold enables reliable detection and notification of improper use of the webbing. 
     Advantageous Effects of Invention 
     As described above, the present invention exhibits the advantageous effect of being able to provide a seatbelt use determination device capable of reliably detecting improper use of a seatbelt device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a face-on view illustrating an overall seatbelt device configuring a determination target for a seatbelt use determination device according to an exemplary embodiment. 
         FIG. 2  is a block diagram illustrating configuration of a seatbelt use determination device according to the present exemplary embodiment. 
         FIG. 3  is a diagram to explain a determination method for improper use of a seatbelt device, employed by a seatbelt use determination device according to the present exemplary embodiment. 
         FIG. 4  is a flowchart illustrating an example of a flow of processing performed by a controller of a seatbelt use determination device according to the present exemplary embodiment. 
         FIG. 5  is a diagram to explain a modified example of a determination method for improper use of a seatbelt device, employed by a seatbelt use determination device according to the present exemplary embodiment. 
         FIG. 6  is a flowchart illustrating a first modified example of a flow of processing performed by a controller of a seatbelt device according to the present exemplary embodiment. 
         FIG. 7  is a flowchart illustrating a second modified example of a flow of processing performed by a controller of a seatbelt device according to the present exemplary embodiment. 
         FIG. 8  is a flowchart illustrating a third modified example of a flow of processing performed by a controller of a seatbelt device according to the present exemplary embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Detailed explanation follows regarding an example of an exemplary embodiment of the present invention, with reference to the drawings.  FIG. 1  is a face-on view of an overall seatbelt device configuring a determination target for a seatbelt use determination device according to the present exemplary embodiment. In  FIG. 1 , the arrow OUT points toward a vehicle width direction outside (vehicle right side), and the arrow UP points upward. 
     As illustrated in  FIG. 1 , a seatbelt device  10  is provided to a vehicle seat  12 . The front, right, and upper side of the seat  12  are respectively aligned with the front, right, and upper side of the vehicle. A lower section of the seat  12  is provided with a seat cushion  12 A, and a rear section of the seat  12  is provided with a seatback  12 B. A vehicle occupant  14  is able to sit on the seat  12 . 
     The seatbelt device  10  further includes a flexible, elongated strap-shaped webbing  16  (belt). 
     The webbing  16  is taken up and stored by a take-up device  18 , serving as a take-up section, from a length direction base end side of the webbing  16 . The take-up device  18  is fixed to a vehicle body (or to the seat  12 ) at a lower portion at the vehicle width direction outside of the seat  12 . The webbing  16  is pulled out toward the vehicle front side from the take-up device  18 . 
     The take-up device  18  is provided with a lock mechanism, not illustrated in the drawings. In a vehicle emergency such as a collision (when the webbing  16  is pulled out sharply from the take-up device  18  or when the vehicle decelerates suddenly), the lock mechanism actuates, locking the webbing  16  against being pulled out from the take-up device  18 . 
     A shoulder anchor  20  is fixed to the vehicle body (or the seat  12 ) at the vehicle rear side of an upper portion of a vehicle width direction outside section of the seat  12 . An elongated, rectangular insertion hole is formed penetrating the shoulder anchor  20 . 
     The webbing  16  is inserted through the insertion hole in the shoulder anchor  20 , such that the webbing  16  folds back on itself and is supported by the insertion hole in the shoulder anchor  20 . 
     A length direction leading end of the webbing  16  is supported by an anchor  22 . The anchor  22  is fixed to the vehicle body (or to the seat  12 ) at the vehicle width direction outside and lower side of a rear section of the seat  12 . 
     A tongue  24  is provided to a portion of the webbing  16  located between the shoulder anchor  20  and the anchor  22 . An elongated, rectangular insertion hole is formed penetrating the tongue  24 . The webbing  16  is inserted through the insertion hole such that the webbing  16  is capable of moving in its length direction through the insertion hole. 
     A buckle  26  is fixed to the vehicle body (or to the seat  12 ) at the vehicle width direction inside and lower side of the rear section of the seat  12 . The tongue  24  is capable of being attached to and detached from the buckle  26 . The webbing  16  is worn at the front side of the occupant  14  sitting in the seat  12  by pulling the webbing  16  out from the take-up device  18  and fitting the tongue  24  into the buckle  26 . 
     The webbing  16  folds back on itself and is supported by the insertion hole in the tongue  24 , such that a portion of the webbing  16  located between the shoulder anchor  20  and the tongue  24  (a portion on one side of the tongue  24 ) configures a shoulder webbing  16 A (shoulder belt). The shoulder webbing  16 A is worn diagonally across a shoulder and the chest of the occupant  14 . 
     A portion of the webbing  16  located between the tongue  24  and the anchor  22  (a portion on the other side of the tongue  24 ) configures a lap webbing  16 B (lap belt). The lap webbing  16 B is worn horizontally across the lumbar region of the occupant  14 . 
     A tongue stopper  28  is attached to the lap webbing  16 B so as to project from both faces of the lap webbing  16 B. The tongue stopper  28  limits movement of the tongue  24  toward the length direction leading end side of the webbing  16 . 
     Explanation follows regarding configuration of the seatbelt use determination device according to the present exemplary embodiment.  FIG. 2  is a block diagram illustrating configuration of the seatbelt use determination device according to the present exemplary embodiment. 
     A seatbelt use determination device  30  includes a controller  50  that serves as a notification section that detects improper use of the seatbelt device  10  and performs notification control in order to perform notification. 
     The controller  50  is configured by a computer in which a CPU  50 A, ROM  50 B, RAM  50 C, and an input/output interface (I/O)  50 D are connected to a bus  50 E. 
     The ROM  50 B stores a program used to perform notification control in order to detect and notify of improper use of the seatbelt device  10 . The program stored in the ROM  50 B is expanded into the RAM  50 C and executed by the CPU  50 A in order to perform the notification control to detect and notify of improper use of the seatbelt device  10 . 
     A buckle switch  52  serving as a sensor section, a rotation angle sensor  54  serving as a detection section, and an alert notification section  56  serving as a notification section are connected to the I/O  50 D. 
     The buckle switch  52  is a switch provided to the buckle  26 , and is switched ON or OFF according to whether or not the tongue  24  is inserted into the buckle  26 . A detection result of the buckle switch  52  for whether or not the tongue  24  has been inserted is input to the controller  50 . 
     The rotation angle sensor  54  detects rotation of the take-up device  18  that takes up the webbing  16 , and inputs detection results to the controller  50 . The controller  50  is capable of detecting a pull-out amount or a take-up amount of the webbing  16  using the detection results of the rotation angle sensor  54 . Note that in the following explanation, pull-out amounts and take-up amounts of the webbing  16  are both referred to as belt extension amount. 
     The alert notification section  56  uses a warning lamp, warning sound, or the like to notify the occupant when improper use of the seatbelt device  10  has been detected by the controller  50 . 
     In the present exemplary embodiment, the seatbelt use determination device  30  detects improper use of the seatbelt device  10  based on the detection results of the rotation angle sensor  54 . 
     Specifically, when the seatbelt device  10  is being used correctly, as illustrated by the single-dotted dashed line in  FIG. 3 , the belt is pulled out or taken up to some degree in conjunction with acceleration of travel or movement of the occupant. By contrast, when the seatbelt device  10  is being used improperly, for example by passing the seatbelt device  10  behind the body of the occupant or to the rear side of the seatback  12 B, as illustrated by the solid line in  FIG. 3 , changes in the belt extension amount (changes in the rotation amount of the take-up device  18 ) are much smaller than when the seatbelt device  10  is being used correctly. Accordingly, in the present exemplary embodiment, the controller  50  detects the belt extension amount when insertion of the tongue  24  into the buckle  26  has been detected, and determines and notifies that the seatbelt device  10  is being used improperly in cases in which changes in the belt extension amount are smaller than those when the seatbelt device  10  is being used correctly. More specifically, detection results of the rotation angle sensor  54  are acquired per unit time (intervals of X seconds). A time period T 1  during which a belt extension change amount L is continuously less than a predetermined threshold L 1  during the X-second period is detected, and in cases in which the time period T 1  is longer than a predetermined threshold Ta, improper use is determined and the alert notification section  56  performs alert notification. 
     Next, explanation follows regarding specific processing performed by the controller  50  of the seatbelt use determination device  30  according to the present exemplary embodiment configured as described above.  FIG. 4  is a flowchart illustrating an example of a flow of processing performed by the controller  50  of the seatbelt use determination device  30  according to the present exemplary embodiment. The processing in  FIG. 4  is started when an ignition switch, not illustrated in the drawings, has been switched ON. 
     At step  100 , the controller  50  determines whether or not tongue insertion has been detected. This determination is made by determining whether or not insertion of the tongue  24  has been detected based on a signal from the buckle switch  52 . Processing stands by until determination is affirmative, and then transitions to step  102 . 
     At step  102 , the controller  50  monitors the detection results of the rotation angle sensor  54 , and detects the time period T 1  during which the belt extension change amount L is continuously less than L 1  (L 1  being a predetermined value) during the X-second period. 
     Processing then transitions to step  104 . 
     At step  104 , the controller  50  determines whether or not the time period T 1  is longer than the predetermined threshold Ta. Namely, in a state in which insertion of the tongue  24  into the buckle  26  has been detected, the controller  50  determines the presence or absence of a state in which the rotation change amount of the take-up device  18  per unit time is continuously small as a result of improper use. Processing transitions to step  106  in cases in which determination is affirmative, and processing returns to step  100  and the above processing is repeated in cases in which determination is negative. 
     At step  106 , the controller  50  controls the alert notification section  56  to notify of the improper use of the seatbelt device  10 , and then processing returns to step  100  and the above processing is repeated. 
     In this manner, in the present exemplary embodiment, insertion of the tongue  24  into the buckle  26  is detected and the detection results for rotation of the take-up device  18  are employed to determine improper use of the seatbelt device  10 . In cases in which the seatbelt device  10  is being used improperly, the rotation change in the take-up device  18  is smaller than when the seatbelt device  10  is being used correctly, thereby enabling improper use to be determined and notified by monitoring the rotation change. Moreover, even if the tongue  24  is removed from and then reinserted into the buckle  26  while the improper usage state continues even after improper use notification, since the change in the extension of the webbing  16  is less than it would be during correct use, improper use determination and notification can be repeated. Accordingly, insertion of the tongue  24  into the buckle  26  and rotation of the take-up device  18  are detected, and the detection results thereof are employed to determine improper use of the seatbelt device  10 . This thereby enables improper use of the seatbelt device  10  to be reliably detected. 
     Next, explanation follows regarding a first modified example of a determination method for improper use of the seatbelt device  10  by the seatbelt use determination device  30  according to the present exemplary embodiment.  FIG. 5  is a diagram to explain the modified example of the determination method for improper use of the seatbelt device by the seatbelt use determination device according to the present exemplary embodiment. 
     In the exemplary embodiment described above, explanation was given regarding an example in which insertion of the tongue  24  into the buckle  26  is detected and improper use is determined in cases in which the belt extension change amount is less than it would be during correct use. In the first modified example, a belt extension change amount is used to determine improper use prior to insertion of the tongue  24  into the buckle  26  being detected. 
     Specifically, when the seatbelt device  10  is being used correctly, the webbing  16  is pulled out by a length required in order to pass the webbing  16  to the front side of the occupant, and then the tongue  24  is inserted into the buckle  26 . Accordingly, as illustrated by the single-dotted dashed line in  FIG. 5 , the belt extension amount changes to some degree prior to insertion of the tongue  24  into the buckle  26 . However, when the seatbelt device  10  is used improperly, for example by using the seatbelt device  10  passed behind the body of the occupant or to the rear side of the seatback  12 B, as illustrated by the solid line in  FIG. 5 , the belt extension change amount is less than when the seatbelt device  10  is used correctly. Accordingly, in the first modified example, in cases in which the belt extension change amount is less than a predetermined threshold La in a predetermined time period before the buckle switch  52  is switched ON (before insertion of the tongue  24  into the buckle  26  is detected), improper use is determined and the alert notification section  56  performs alert notification. 
       FIG. 6  is a flowchart illustrating a first modified example of a flow of processing performed by the controller of the seatbelt device according to the present exemplary embodiment. Note that the processing in  FIG. 6  starts when an ignition switch, not illustrated in the drawings, has been switched ON. 
     At step  120 , the controller  50  starts monitoring the belt extension change amount L with the rotation angle sensor  54 , and processing transitions to step  122 . 
     At step  122 , the controller  50  determines whether or not tongue insertion has been detected. This determination is made by determining whether or not insertion of the tongue  24  has been detected based on a signal from the buckle switch  52 . Processing stands by until determination is affirmative, and then transitions to step  124 . 
     At step  124 , based on the detection results of the rotation angle sensor  54 , the controller  50  detects a belt extension change amount L 1  during the predetermined time period prior to the detection of tongue insertion, and processing transitions to step  126 . 
     At step  126 , the controller  50  determines whether or not the belt extension change amount L 1  is less than the predetermined threshold La. Processing returns to step  122  and the above processing is repeated in cases in which determination is negative, and processing transitions to step  128  in cases in which determination is affirmative. 
     At step  128 , the controller  50  controls the alert notification section  56  to notify of the improper use of the seatbelt device  10 , and then processing returns to step  120  and the above processing is repeated. 
     In this manner, in the first modified example, the belt extension change amount is detected during the predetermined time period prior to insertion of the tongue  24  into the buckle  26  being detected, and the detection results thereof are employed to determine improper use of the seatbelt device  10 . In cases in which the seatbelt device  10  is being used improperly, the rotation change in the take-up device  18  in the predetermined time period prior to insertion of the tongue  24  into the buckle  26  being detected is less than when the seatbelt device  10  is being used correctly, and therefore determination that compares this rotation change against a threshold can be employed in determination and notification of improper use. Moreover, even if the tongue  24  is removed from and then reinserted into the buckle  26  while the improper usage state continues even after improper use notification, since the change in the extension of the webbing  16  is less than it would be during correct use, improper use determination and notification can be repeated. Accordingly, in the first modified example, similarly to in the exemplary embodiment described above, improper use of the seatbelt device  10  can be reliably detected. 
     Next, explanation follows regarding a second modified example of a determination method for improper use of the seatbelt device  10  by the seatbelt use determination device  30  according to the present exemplary embodiment. 
     In the first modified example, improper use is determined employing the belt extension change amount prior to insertion of the tongue  24  into the buckle  26  being detected. However, in the second modified example, a belt extension change amount following a predetermined time period after detection of insertion of the tongue  24  into the buckle  26  is employed. 
     Specifically, when the seatbelt device  10  is being used correctly, the webbing  16  is pulled out by a length required in order to pass the webbing  16  to the front side of the occupant with some length to spare, and then the tongue  24  is inserted into the buckle  26 . Accordingly, as illustrated by the single-dotted dashed line in  FIG. 5 , the belt extension amount changes to some degree as a result of the excess in the webbing being taken up after inserting the tongue  24  to the buckle  26 . However, when the seatbelt device  10  is used improperly, for example by using the seatbelt device  10  passed behind the body of the occupant or to the rear side of the seatback  12 B, as illustrated by the solid line in  FIG. 5 , the belt extension change amount after insertion of the tongue  24  into the buckle  26  is smaller than when the seatbelt device  10  is being used correctly. Accordingly, in the second modified example, in cases in which the belt extension change amount is less than a predetermined threshold Lb following a predetermined time period after the buckle switch  52  is switched ON (after insertion of the tongue  24  into the buckle  26  is detected), improper use is determined and the alert notification section  56  performs alert notification. Note that the same value as the threshold La described above may be employed as the threshold Lb, or a different value may be employed. 
       FIG. 7  is a flowchart illustrating a second modified example of a flow of processing performed by the controller of the seatbelt device according to the present exemplary embodiment. Note that the processing in  FIG. 7  starts when an ignition switch, not illustrated in the drawings, has been switched ON. 
     At step  150 , the controller  50  starts monitoring the belt extension change amount L with the rotation angle sensor  54 , and processing transitions to step  152 . 
     At step  152 , the controller  50  determines whether or not tongue insertion has been detected. This determination is made by determining whether or not insertion of the tongue  24  has been detected based on a signal from the buckle switch  52 . Processing stands by until determination is affirmative, and then transitions to step  154 . 
     At step  154 , based on the detection results of the rotation angle sensor  54 , the controller  50  detects a belt extension change amount L 2  during the predetermined time period after the detection of tongue insertion, and processing transitions to step  156 . 
     At step  156 , the controller  50  determines whether or not the belt extension change amount L 2  is less than the predetermined threshold Lb. Processing returns to step  152  and the above processing is repeated in cases in which determination is negative, and processing transitions to step  158  in cases in which determination is affirmative. 
     At step  158 , the controller  50  controls the alert notification section  56  to notify of improper use of the seatbelt device  10 , and then processing returns to step  150  and the above processing is repeated. 
     In this manner, in the second modified example, the belt extension change amount is detected following the predetermined time period after insertion of the tongue  24  into the buckle  26  has been detected, and the detection results thereof are employed to determine improper use of the seatbelt device  10 . In cases in which the seatbelt device  10  is being used improperly, variation in the rotation of the take-up device  18  following the predetermined time period after detection of insertion of the tongue  24  into the buckle  26  is less than when the seatbelt device  10  is being used correctly, and therefore determination that compares variation in the rotation against a threshold can be employed in determination and notification of improper use. Moreover, even if the tongue  24  is removed from and then reinserted into the buckle  26  while the improper usage state continues even after improper use notification, since the change in the extension of the webbing  16  is less than it would be during correct use, improper use determination and notification can be repeated. Accordingly, in the second modified example, similarly to in the exemplary embodiment described above, improper use of the seatbelt device  10  can be reliably detected. 
     Next, explanation follows regarding a third modified example of a determination method for improper use of the seatbelt device  10  by the seatbelt use determination device  30  according to the present exemplary embodiment. 
     The third modified example combines the exemplary embodiment, the first modified example, and the second modified example described above. 
       FIG. 8  is a flowchart illustrating a third modified example of the flow of processing performed by the controller of the seatbelt device according to the present exemplary embodiment. Note that the processing in  FIG. 8  starts when an ignition switch, not illustrated in the drawings, has been switched ON. 
     At step  200 , the controller  50  starts monitoring the belt extension change amount L with the rotation angle sensor  54 , and processing transitions to step  202 . 
     At step  202 , the controller  50  determines whether or not tongue insertion has been detected. This determination is made by determining whether or not insertion of the tongue  24  has been detected based on a signal from the buckle switch  52 . Processing stands by until determination is affirmative, and then transitions to step  204 . 
     At step  204 , based on the detection results of the rotation angle sensor  54 , the controller  50  detects the belt extension change amount L 1  during a predetermined time period prior to the detection of tongue insertion, and processing transitions to step  206 . 
     At step  206 , the controller  50  determines whether or not the belt extension change amount L 1  is less than the predetermined threshold La. Processing transitions to step  208  in cases in which determination is negative, and processing transitions to step  216  in cases in which determination is affirmative. 
     At step  208 , based on the detection results of the rotation angle sensor  54 , the controller  50  detects a belt extension change amount L 2  during the predetermined time period after the detection of tongue insertion, and processing transitions to step  210 . 
     At step  210 , the controller  50  determines whether or not the belt extension change amount L 2  is less than the predetermined threshold Lb. Processing transitions to step  212  in cases in which determination is negative, and processing transitions to step  216  in cases in which determination is affirmative. 
     At step  212 , the controller  50  monitors the detection results of the rotation angle sensor  54 , and detects the time period T 1  during which the belt extension change amount L is continuously less than L 1  (L 1  being a predetermined value) during the X-second period. Processing then transitions to step  214 . 
     At step  214 , the controller  50  determines whether or not the time period T 1  is longer than the predetermined threshold Ta. Namely, the controller  50  determines the presence or absence of a state in which the belt extension change amount is small due to improper use. 
     Processing transitions to step  216  in cases in which determination is affirmative, and processing returns to step  202  and the above processing is repeated in cases in which determination is negative. 
     At step  216 , the controller  50  controls the alert notification section  56  to notify of improper use of the seatbelt device  10 , and then processing returns to step  202  and the above processing is repeated. 
     By combining the exemplary embodiment and the respective modified examples in this manner, improper use of the seatbelt device  10  can be even more reliably detected by using the extension change amounts of the webbing  16  before and after fitting the tongue  24  into the buckle  26  and while the webbing  16  is in use. 
     Note that in the third modified example, explanation has been given regarding an example in which the exemplary embodiment, the first modified example, and the second modified example described above are all combined. However, there is no limitation to such a combination, and any two of the above may be combined. 
     Moreover, explanation has been given in which the processing performed by the controller  50  in the exemplary embodiment and the modified examples described above is implemented by software. However, there is no limitation thereto. For example, the processing may be implemented by hardware, or the processing may be implemented by a combination of both hardware and software. 
     The processing performed by the controller  50  in the exemplary embodiment and the modified examples described above may also be stored and distributed as a program on a storage medium. 
     The present invention is not limited to the above description, and obviously various other modifications may be implemented within a range not departing from the spirit of the present invention. 
     The disclosure of Japanese Patent Application No. 2016-103544, filed on May 24, 2016, is incorporated in its entirety by reference herein.