Patent Publication Number: US-2020276956-A1

Title: Force limiter control system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 USC 119 from Japanese Patent Application No. 2019-036499, filed on Feb. 28, 2019, the disclosure of which is incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a force limiter control system. 
     Related Art 
     Japanese Patent Application Laid-Open (JP-A) No. 2018-131168 discloses a structure including a variable force limiter mechanism that can change a load (force limiter load) applied to a seat belt at a time of vehicle collision. In JP-A No. 2018-131168, the severity of a collision is predicted based on a detection result from a relative speed sensor, and the force limiter load is changed based on the severity of the collision and the detected load at the time of the collision. 
     Incidentally, in a collision safety system that detects an obstacle ahead using a sensor, a preceding vehicle cannot be detected when the distance to the preceding vehicle is small. For this reason, even if the host vehicle moves in a lateral direction (that is, the vehicle width direction) as a result of steering or the like and collides with another vehicle, the force limiter load of the variable force limiter mechanism is not changed. 
     SUMMARY 
     In the present disclosure, a force limiter control system is obtained that is capable of improving collision safety performance in a configuration including a variable force limiter mechanism. 
     A force limiter control system according to a first aspect of the present disclosure includes: a variable force limiter unit configured to change a force limiter load applied to a seatbelt at a time of vehicle collision; a relative speed detection unit that detects a relative speed between a host vehicle and a preceding vehicle; a force limiter load changing unit that sets a lower load as the force limiter load of the variable force limiter unit in a case in which the relative speed detected by the relative speed detection unit is lower than a predetermined threshold value, than in a case in which the relative speed is equal to or greater than the predetermined threshold value; and a low load invalidation unit that invalidates a change to the force limiter load by the force limiter load changing unit in a case in which a collision with a collision object other than the preceding vehicle has been predicted in an undetectable region in which an inter-vehicular distance to the preceding vehicle is smaller than a predetermined distance. 
     The force limiter control system according to the first aspect of the present disclosure is configured such that the force limiter load applied to a seatbelt at a time of a vehicle collision by the variable force limiter unit may be changed. In addition, a relative speed detector that detects the relative speed between the host vehicle and the preceding vehicle is provided. When the relative speed detected by the relative speed detection unit is smaller than a predetermined threshold, the force limiter load changing unit sets the force limiter load of the variable force limiter unit at a low load. Thereby, a situation in which, when a low collision load is input to an occupant, there is unnecessary restraint by a seatbelt, may be avoided. That is, the load on the occupant is reduced by changing the force limiter load according to the severity of the collision. 
     In addition, when a collision with a collision object other than the preceding vehicle is predicted in an undetectable region in which the distance between the host vehicle and the preceding vehicle is smaller than a predetermined distance, the change to the force limiter load by the limiter load changing unit is invalidated by the low load invalidation unit. Thereby, for example, even when the host vehicle avoids the preceding vehicle and collides with a large vehicle, the force limiter load is not changed to a low load, owing to the invalidation of the change to the force limiter load, and the occupant is favorably restrained. Here, the “preceding vehicle” refers to a vehicle traveling in front of the host vehicle in a state before the host vehicle enters the undetectable region. 
     The force limiter control system according to a second aspect of the present disclosure is that of the first aspect, in which the low load invalidation unit invalidates the change to the force limiter load by the force limiter load changing unit in a case in which an overlap amount, in a vehicle width direction between the host vehicle and the preceding vehicle at a time of entry into the undetectable region, is smaller than an estimated lateral movement amount in the vehicle width direction of the host vehicle after entry into the undetectable region. 
     In the force limiter control system according to the second aspect of the present disclosure, when the overlap amount in the vehicle width direction between the host vehicle and the preceding vehicle is smaller than the estimated lateral movement amount in the vehicle width direction of the host vehicle, the possibility of the host vehicle avoiding the preceding vehicle is increased. Therefore, in this case, by predicting that there will be a collision with a collision object other than the preceding vehicle, and invalidating the change to the force limiter load, the occupant is favorably restrained. 
     The force limiter control system according to a third aspect of the present disclosure is that of the second aspect, in which the estimated lateral movement amount is calculated based on the relative speed between the host vehicle and the preceding vehicle, and an angle of the host vehicle relative to a direction of travel, at the time of entry into the undetectable region, and based on a maximum lateral relative acceleration due to driver operation. 
     In the force limiter control system according to the third aspect of the present disclosure, in addition to the relative speed between the host vehicle and the preceding vehicle and the angle of the host vehicle with respect to the direction of travel, the estimated lateral movement amount is calculated in view of the maximum lateral relative acceleration due to driver operation. Thus, it is accurately determined whether or not the host vehicle may avoid the preceding vehicle. 
     The force limiter control system according to a fourth aspect of the present disclosure is that of the second or third aspects, in which a prevention sensor including at least one of an optical camera, a millimeter wave radar or a laser radar is provided at a front part of the host vehicle; and the overlap amount between the host vehicle and the preceding vehicle is detected by the prevention sensor. 
     In the force limiter control system according to the fourth aspect of the present disclosure, the prevention sensor includes at least one of an optical camera, a millimeter wave radar, or a laser radar. The prevention sensor detects the overlap amount between the host vehicle and the preceding vehicle. For this reason, the overlap amount is detected without separately providing a dedicated sensor. 
     The force limiter control system according to the fifth aspect of the present disclosure is that of any one of the first to the fourth aspects, further including an initialization unit that overrides invalidation, by the low load invalidation unit, of the change to the force limiter load in a case in which a collision is not detected after a predetermined time has elapsed since entry of the host vehicle into the undetectable region. 
     In the force limiter control system according to the fifth aspect of the present disclosure, the change to the force limiter load is made effective by overriding the invalidation of the change to the force limiter load. Thereby, when a collision of the host vehicle is avoided, the variable force limiter unit may be allowed to function. 
     The force limiter control system according to a sixth aspect of the present disclosure is that of the first aspect, further including a large vehicle detection unit that detects a large vehicle, which is larger than the preceding vehicle, traveling in a periphery of the preceding vehicle, wherein the low load invalidation unit invalidates the change to the force limiter load by the force limiter load changing unit in a case in which it is detected by the large vehicle detection unit that a large vehicle is traveling in the periphery of the preceding vehicle. 
     In the force limiter control system according to the sixth aspect of the present disclosure, when the large vehicle detection unit detects that a large vehicle is traveling in the periphery of the preceding vehicle, a collision with a collision object other than the preceding vehicle is predicted, and the change to the force limiter load by the force limiter load changing unit is invalidated. As a result, even if the host vehicle collides with a large vehicle after avoiding a collision with the preceding vehicle, the force limiter load is not changed to a low load, and therefore, the occupant is favorably restrained. 
     The force limiter control system according to a seventh aspect of the present disclosure is that of the first aspect, further including a traffic lane detection unit that detects a traffic lane boundary line, wherein the low load invalidation unit invalidates the change to the force limiter load by the force limiter load changing unit in a case in which it is detected by the traffic lane detection unit that the host vehicle has deviated from a traffic lane in the undetectable region. 
     In the force limiter control system according to the seventh aspect of the present disclosure, when the traffic lane detection unit detects that the host vehicle has deviated from a traffic lane, a collision with a collision object other than the preceding vehicle is predicted, and the change to the force limiter load by the force limiter load changing unit is invalidated. Thereby, for example, even when the host vehicle enters an adjacent lane and collides with another vehicle, the force limiter load is not changed to a low load, and therefore, the occupant is favorably restrained. 
     The force limiter control system according to an eighth aspect of the present disclosure is that of the first aspect, further including a steering angle detection unit that detects a steering angle, wherein the low load invalidation unit invalidates the change to the force limiter load by the force limiter load changing unit in a case in which it is detected by the steering angle detection unit that the steering angle is larger than a predetermined threshold value. 
     In the force limiter control system according to the eighth aspect of the present disclosure, when the steering angle detected by the steering angle detection unit is larger than a predetermined threshold value, a collision with a collision object other than the preceding vehicle is predicted, and the change to the force limiter load by the force limiter load changing unit is invalidated. As a result, even if the host vehicle collides with another vehicle or an obstacle different from the preceding vehicle, the force limiter load is not changed to a low load, and therefore, the occupant is favorably restrained. 
     According to the force limiter control system according to the present disclosure, it is possible to improve the collision safety performance in a configuration including a variable force limiter mechanism. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments will be described in detail based on the following figures, wherein: 
         FIG. 1  is a plan view schematically illustrating a vehicle in which a force limiter control system according to an embodiment is installed, and a preceding vehicle, and illustrates a state before the host vehicle enters an undetectable region; 
         FIG. 2  is a plan view illustrating a state when the host vehicle enters the undetectable region from the state of  FIG. 1 ; 
         FIG. 3  is a block diagram illustrating a hardware configuration of the force limiter control system according to the embodiment; 
         FIG. 4  is a block diagram illustrating a hardware configuration of an ECU configuring the force limiter control system according to the embodiment; 
         FIG. 5  is a block diagram illustrating an example of a functional configuration of the force limiter control system according to the embodiment; 
         FIG. 6  is a part of a flowchart illustrating a flow of force limiter control processing in the embodiment; 
         FIG. 7  is a part of a flowchart illustrating a flow of force limiter control processing in the embodiment; 
         FIG. 8  is a block diagram illustrating an example of a functional configuration of the force limiter control system according to a first variant example; 
         FIG. 9  is a part of a flowchart illustrating a flow of force limiter control processing in the first variant example; 
         FIG. 10  is a part of a flowchart illustrating a flow of force limiter control processing in the first variant example; 
         FIG. 11  is a block diagram illustrating an example of a functional configuration of the force limiter control system according to a second variant example; 
         FIG. 12  is a part of a flowchart illustrating a flow of force limiter control processing in the second variant example; 
         FIG. 13  is a block diagram illustrating an example of a functional configuration of the force limiter control system according to a third variant example; and 
         FIG. 14  is a part of a flowchart illustrating a flow of force limiter control processing in the third variant example. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a force limiter control system  10  according to an embodiment will be described with reference to the drawings. Hereinafter, when description is given using front-rear, left-right and vertical directions, these refer to the front-rear of the vehicle front-rear direction, the left and the right of the vehicle width direction and the vertical of the vehicle vertical direction, unless otherwise specified. 
     As illustrated in  FIG. 1 , a prevention sensor  14  is provided at the front of a vehicle  12  to which the force limiter control system  10  according to the present embodiment is applied (hereinafter, sometimes referred to as “host vehicle  12 ” as appropriate). The prevention sensor  14  includes at least one of an optical camera, a millimeter wave radar, or a laser radar. The prevention sensor  14  may detect, for example, a preceding vehicle  100  traveling in front of the host vehicle  12 . The prevention sensor  14  is electrically connected to an electronic control unit (ECU)  16 , which is a control unit installed in the host vehicle  12 . 
       FIG. 3  is a block diagram illustrating a hardware configuration of the force limiter control system  10 . As illustrated in  FIG. 3 , the ECU  16  is electrically connected to the prevention sensor  14 , an airbag device  18 , and a seat belt device  20 . 
     The airbag device  18  is a device that inflates and deploys to restrain an occupant when the host vehicle  12  experiences a collision or a collision is predicted. For example, the airbag device  18  includes an airbag bag body that is folded and stored in a center pad of a steering wheel, and an inflator. Then, when the host vehicle  12  experiences a collision or a collision is predicted, the inflator is operated based on a signal from the ECU  16  to generate gas, and gas is supplied to the airbag bag body, such that the airbag bag body is inflated and deployed toward the vehicle rear side. The airbag device  18  may include, for example, in addition to the driver seat airbag device described above, a passenger seat airbag device that restrains an occupant in a passenger seat, or a side airbag device that is inflated and deployed at the side of a vehicle seat. Moreover, a curtain airbag device that inflates and deploys along a side door may be included, for example. 
     The seat belt device  20  is a device that restrains an occupant by a belt-like seat belt (that is, a webbing), and mainly includes a pretensioner unit  22  and a variable force limiter unit  24 . The pretensioner unit  22  is operated based on a signal from the ECU  16  at a time of vehicle collision, and forcibly takes up the seat belt that restrains the occupant, thereby applying tension to the seat belt. 
     The variable force limiter unit  24  is configured to be able to change the force limiter load applied to the seat belt when the host vehicle  12  collides. In the present embodiment, as an example, the variable force limiter unit  24  may be changed between two modes: a high load mode and a low load mode. As a specific configuration of the variable force limiter, the well-known configurations described in JP-A Nos. 2018-39336 and 2018-131168 may be applied. 
       FIG. 4  is a block diagram illustrating a hardware configuration of the ECU  16  configuring the force limiter control system  10 . As illustrated in  FIG. 4 , the ECU  16  includes a central processing unit (CPU: processor)  26 , a read-only memory (ROM)  28 , a random access memory (RAM)  32 , a storage  30  and a communication interface  34 . These respective components are connected via a bus  35  so as to be able to communicate with one another. 
     The CPU  26  is a central computation processing unit that executes various programs and controls the respective units. Namely, the CPU  26  reads a program from the ROM  28  or the storage  30 , and executes the program using the RAM  32  as a work space. The CPU  26  controls the various components and performs a variety of computation processing according to the program recorded in the ROM  28  or the storage  30 . 
     The ROM  28  stores various programs and various data. The RAM  32  temporarily stores programs or data as a work space. The storage  30  is configured by a Hard Disk Drive (HDD) or a Solid State Drive (SSD), and stores various data and various programs including an operating system. 
     The communication interface  34  is an interface for the ECU  16  to communicate with the prevention sensor  14 , the airbag device  18 , the seat belt device  20  and other devices, and utilizes standards such as the Ethernet®, FDDI, and Wi-Fi®. 
     The force limiter control system  10  of this embodiment implements various functions using the hardware resources illustrated in  FIGS. 3 and 4 . Functions realized by the force limiter control system  10  will be described. 
       FIG. 5  is a block diagram illustrating an example of a functional configuration of the force limiter control system  10 . 
     As illustrated in  FIG. 5 , the force limiter control system  10  includes, as functional configuration, a receiving unit  36 , a vehicle body detection unit  38 , a relative speed detection unit  40 , an overlap amount detection unit  42 , an inter-vehicle distance detection unit  44 , an angle detection unit  46 , a force limiter load changing unit  48 , a pretensioner activation unit  50 , a collision target prediction unit  52 , a low load invalidation unit  54 , and an initialization unit  56 . These respective functional configurations are realized by the CPU  26  reading and executing programs that are recorded in the ROM  28  or the storage  30 . 
     The receiving unit  36  receives information from sensors such as the prevention sensor  14  and information from other devices via the communication interface  34 . 
     The vehicle body detection unit  38  detects the preceding vehicle  100  (see  FIG. 1 ) based on the information from the prevention sensor  14  received by the receiving unit  36 . In the present embodiment, the size of the preceding vehicle  100  is also detected, and it is determined whether the preceding vehicle  100  is a normal vehicle or a large vehicle. 
     The relative speed detection unit  40  detects the relative speed between the host vehicle  12  and the preceding vehicle  100  based on information from the prevention sensor  14  received by the receiving unit  36  and information from a vehicle speed sensor (not illustrated) provided at the host vehicle  12 . 
     The overlap amount detection unit  42  detects an overlap amount between the host vehicle  12  and the preceding vehicle  100  based on the information from the prevention sensor  14  received by the receiving unit  36 . As illustrated in  FIG. 2 , in the present embodiment, the distance between a reference line extending from the left side end portion of the host vehicle  12  to the preceding vehicle  100  along the traveling direction of the host vehicle  12 , and parallel to this line, a reference line passing a right side end portion of the preceding vehicle, is defined as the overlap amount X. Here, the “traveling direction” refers to a direction along an imaginary line passing through the center in the vehicle width direction at the front end, and the center in the vehicle width direction at the rear end, of the host vehicle  12 . 
     The inter-vehicle distance detection unit  44  illustrated in  FIG. 5  detects the inter-vehicle distance between the host vehicle  12  and the preceding vehicle  100  based on the information from the prevention sensor  14  received by the receiving unit  36 . As illustrated in  FIG. 2 , in this embodiment, the distance along the traveling direction of the preceding vehicle  100  from the front end of the host vehicle  12  to the rear end of the preceding vehicle  100  is defined as an inter-vehicle distance L. 
     The angle detection unit  46  illustrated in  FIG. 5  detects the angle between the host vehicle  12  and the preceding vehicle  100  based on the information from the prevention sensor  14  received by the receiving unit  36 . As illustrated in  FIG. 2 , in the present embodiment, the angle θ between the traveling direction of the host vehicle  12  and the traveling direction of the preceding vehicle  100  is detected by the angle detection unit  46 . 
     The force limiter load changing unit  48  changes the force limiter load of the variable force limiter unit  24  when the relative speed between the host vehicle  12  and the preceding vehicle  100  detected by the relative speed detection unit  40  is smaller than a predetermined threshold value. Specifically, the force limiter load changing unit  48  is configured such that when the relative speed between the host vehicle  12  and the preceding vehicle  100  is smaller than a predetermined threshold value, the variable force limiter unit  24  is set to a lower load than when the relative speed is equal to or higher than the threshold value. The threshold value for changing the force limiter load is set in advance. 
     The pretensioner activation unit  50  operates the pretensioner unit  22  based on a signal from the ECU  16  at a time of collision of the host vehicle  12 . By actuating the pretensioner unit  22 , a spool (not illustrated), around which the seat belt is wound, is forcibly rotated and the seat belt is taken up. Thereby, tension is applied to the seat belt. A collision of the host vehicle  12  is detected using an acceleration sensor (not illustrated). 
     The collision target prediction unit  52  predicts a collision with a collision object that is different from the preceding vehicle  100 . Here, a collision target is predicted in an undetectable region in which the inter-vehicle distance between the host vehicle  12  and the preceding vehicle  100  is shorter than a predetermined distance. 
     The low load invalidation unit  54  invalidates the change of the force limiter load by the force limiter load changing unit  48  when the collision target prediction unit  52  predicts a collision with a collision object that is different from the preceding vehicle. In the present embodiment, the low load invalidation unit  54  determines whether or not to invalidate the change of the force limiter load in accordance with force limiter control processing described below. 
     The initialization unit  56  overrides the invalidation of the change of the force limiter load by the low load invalidation unit  54  when a collision is not detected even after a predetermined time has elapsed since the host vehicle  12  entered the undetectable region. 
       FIGS. 6 and 7  are flowcharts illustrating the flow of the force limiter control processing by the force limiter control system  10 . The CPU  26  reads a program from the ROM  28  or the storage  30 , deploys the program in the RAM  32 , and executes the program, whereby force limiter control processing is performed. 
     As illustrated in  FIGS. 6 and 7 , the CPU  26  sets an initial value in step S 102 . In the present embodiment, the variable force limiter unit  24  is set to the high load mode. In addition, the change in the force limiter load due to the force limiter load changing unit  48  is set to an effective state. 
     The CPU  26  determines whether or not a collision is inevitable in step S 104 . In the present embodiment, when it is predicted that the host vehicle  12  will collide with the preceding vehicle  100  based on the signal from the prevention sensor  14 , it is determined that the collision is inevitable, and the processing proceeds to step S 106 . However, if it is not determined in step S 104  that a collision is inevitable, since the host vehicle  12  will not collide, the force limiter control processing is terminated. 
     The CPU  26  starts a timer in step S 106 . That is, the time from the beginning of a state in which the collision becomes inevitable is counted. 
     In step S 108 , the CPU  26  determines whether or not the relative speed V between the host vehicle  12  and the preceding vehicle  100  is smaller than a predetermined threshold value VH. When the relative speed V is smaller than the threshold value VH, the processing proceeds to step S 110  and the mode is changed to the low load mode. That is, when the relative speed V is smaller than the threshold value VH, since the severity of the collision will be low, the seat belt is prevented from being imparted with more tension than necessary by setting the low load mode in accordance with the function of the force limiter load changing unit  48 . 
     However, when the relative speed V is larger than the threshold value VH in step S 108  (that is, in the case of “N”), the processing proceeds to step S 112  without passing through step S 110 . That is, the force limiter load is not changed by the force limiter load changing unit  48 , and the high load mode that is the initial setting value is maintained. 
     The CPU  26  determines whether or not there is an undetectable region in step S 112 . Specifically, it is determined whether or not the inter-vehicle distance L detected by the inter-vehicle distance detection unit  44  is shorter than a predetermined distance. The predetermined distance is calculated from the distance at which detection by the prevention sensor  14  becomes impossible and from a communication delay time. 
     If the CPU  26  determines that there is an undetectable region in step S 112 , the process proceeds to step S 116 . However, if it is not determined in step S 112  that the region is undetectable, that is, if the region is not undetectable, the processing proceeds to step S 114 . 
     In step S 114 , the CPU  26  determines whether or not a predetermined length of time has elapsed. Specifically, when a predetermined time has elapsed since the start of the timer in step S 106 , the CPU  26  determines that the collision of the host vehicle  12  has been avoided and ends the force limiter control processing. However, if the predetermined time has not elapsed since the timer was started in step S 106 , the CPU  26  returns to the processing of step S 108  to determine whether or not the speed V is smaller than the threshold value VH. 
     If the CPU  26  determines that there is an undetectable region in step S 112 , the processing proceeds to step S 116  of  FIG. 7 , and the inter-vehicle distance L, relative speed V, overlap amount X and angle θ are acquired. These parameters are acquired, for example, by a signal from the prevention sensor  14  when the host vehicle  12  enters the undetectable region. The acquired parameters are temporarily stored in the RAM  32  or the storage  30 . 
     The CPU  26  determines whether or not the pretensioner unit  22  has been activated in step S 118 . In the present embodiment, the pretensioner unit  22  is activated when it is determined that the host vehicle  12  has collided based on a signal from an acceleration sensor or the like. 
     If the CPU  26  determines in step S 118  that the pretensioner unit  22  has been activated, the processing proceeds to step S 122 . However, if the CPU  26  determines in step S 118  that the pretensioner unit  22  has not been activated, the processing proceeds to step S 120  and the CPU  26  determines whether or not a predetermined time has elapsed since the timer was started. 
     Specifically, when a predetermined time has elapsed since the start of the timer in step S 120 , the CPU  26  determines that the collision of the host vehicle  12  has been avoided and ends the force limiter control processing by means of the function of the initialization unit  56 . That is, even when the change of the force limiter load has been invalidated by the low load invalidation unit  54 , the invalidation is overridden. 
     However, if the predetermined time has not elapsed since the timer was started in step S 120 , the CPU  26  returns to the processing of step S 118 . That is, when the pretensioner unit  22  has not been activated, the processing of step S 118  and the processing of step S 120  are repeated until the predetermined time has elapsed. 
     In step S 122 , the CPU  26  determines whether or not the force limiter load is set to the low load mode. If the host vehicle  12  entered the undetectable region in the state changed to the low load mode in step S 110 , it is determined that the low load mode is set, and the processing proceeds to step S 124 . 
     However, if the CPU  26  determines in step S 122  that the force limiter load is not set to the low load mode, the CPU  26  ends the force limiter control processing. In this case, the force limiter load is the high load mode initially set in step S 102 . 
     In step S 124 , the CPU  26  determines whether or not the overlap amount X is larger than an estimated lateral movement amount S by the function of the collision target prediction unit  52 . Here, the overlap amount X is an overlap amount in the vehicle width direction between the host vehicle  12  and the preceding vehicle  100  when the vehicle enters the undetectable region. Further, as illustrated in  FIG. 2 , the estimated lateral movement amount S is an estimated lateral movement amount in the vehicle width direction of the host vehicle  12  after entering the undetectable region. An example of a method for calculating the estimated lateral movement amount S will be described below. 
     The estimated lateral movement amount S is calculated based on the relative speed V between the host vehicle  12  and the preceding vehicle  100  at the time of entering the undetectable region, the angle θ with respect to the traveling direction of the host vehicle  12 , and a maximum lateral relative acceleration a due to driver operation. 
     Specifically, the estimated lateral movement amount S is calculated by the following formula (1). V L  is a lateral component (that is, a component in the vehicle width direction) of the host vehicle  12  at the relative speed V. A preset numerical value is used for the maximum lateral relative acceleration a. 
         S=V   L ·( L/V )+( a ·( L/V ){circumflex over ( )}2)/2   [Expression 1]
 
     Here, the lateral component V L  of the host vehicle  12  at the relative speed V, and the relative speed V, satisfy the relationship of the following formula (2). 
         V   L   =V ·sin θ  [Expression 2]
 
     By substituting formula (2) into formula (1), the following formula (3) is obtained. 
         S=L ·sin θ+( a ·( L/V ){circumflex over ( )}2)/2   [Expression 3]
 
     The CPU  26  determines whether the overlap amount X is larger than the estimated lateral movement amount S in step S 124  of  FIG. 7 , using the estimated lateral movement amount S calculated by formula (3). When the overlap amount X is larger than the estimated lateral movement amount S, the processing proceeds to step S 126 . The CPU  26  ends the force limiter control processing in a state in which the low load mode is maintained in step S 126 . 
     However, when the overlap amount X is smaller than the estimated lateral movement amount S, the CPU  26  proceeds to the processing of step S 128 . In step S 128 , the CPU  26  invalidates the change of the force limiter load by the force limiter load changing unit  48 , by means of the function of the low load invalidation unit  54 . Then, the CPU  26  ends the force limiter control processing. 
     (Action) 
     Next, the action of the present exemplary embodiment will be described. 
     In the force limiter control system  10  according to the present embodiment, when the relative speed V between the host vehicle  12  and the preceding vehicle  100  is smaller than a predetermined threshold value VH, the force limiter load of the variable force limiter unit  24  is set to a low load by the force limiter load changing unit  48 . Thereby, when the collision load input to an occupant is low, a situation in which an occupant is unnecessarily restrained by a seatbelt may be avoided. That is, the load on the occupant may be reduced by changing the force limiter load according to the severity of the collision. 
     Further, when a collision with a collision object that is different from the preceding vehicle  100  is predicted by the collision target prediction unit  52 , the change of the force limiter load is invalidated by the function of the low load invalidation unit  54 . Thus, for example, as illustrated in  FIG. 2 , even if the host vehicle  12  avoids the preceding vehicle  100  and collides with a large vehicle  102  traveling in the vicinity of the preceding vehicle  100 , by invalidating the change in the force limiter load, the force limiter load is not changed to a low load, and the occupant may be favorably restrained. That is, the collision safety performance may be improved. 
     In particular, in the present embodiment, when the overlap amount X is smaller than the estimated lateral movement amount S of the host vehicle  12  after entering the undetectable region, the collision target prediction unit  52  predicts a collision with a collision object that is different from the preceding vehicle  100 . As described above, when the possibility of the host vehicle  12  avoiding the preceding vehicle  100  is high, the change of the force limiter load is invalidated, so that the occupant may be favorably restrained in preparation for a collision. 
     Further, in the present embodiment, the estimated lateral movement amount S is calculated in consideration of the maximum lateral relative acceleration a due to driver operation, in addition to the relative speed V and the angle θ with respect to the traveling direction of the host vehicle  12 . Thereby, it is possible to accurately determine whether or not the host vehicle  12  may avoid the preceding vehicle  100 . 
     Furthermore, in this embodiment, the prevention sensor  14  includes at least one of an optical camera, a millimeter wave radar, or a laser radar. Since the prevention sensor  14  detects the overlap amount X between the host vehicle  12  and the preceding vehicle  100 , there is no need to separately provide a sensor exclusively for this purpose. 
     Furthermore, in the present embodiment, when a collision is not detected even after a predetermined time has elapsed since the host vehicle  12  entered the undetectable region, by enabling the change of the force limiter load, the variable force limiter unit  24  may function when a collision has been avoided. 
     In the present embodiment, when the overlap amount X between the host vehicle  12  and the preceding vehicle  100  is smaller than the estimated lateral movement amount S of the host vehicle  12 , the change of the force limiter load is invalidated; however, the present disclosure is not limited to this. The configurations described in the following first variant example, second variant example, and third variant example may be used. 
     FIRST VARIANT EXAMPLE 
     A force limiter control system  60  according to a first variant example is described with reference to  FIGS. 8 to 10 . 
       FIG. 8  is a block diagram illustrating an example of the functional configuration of the force limiter control system  60 . As illustrated in  FIG. 8 , the force limiter control system  60  includes, as functional configuration, a receiving unit  36 , a vehicle body detection unit  38 , a relative speed detection unit  40 , an overlap amount detection unit  42 , an inter-vehicle distance detection unit  44 , an angle detection unit  46 , a force limiter load changing unit  48 , a pretensioner activation unit  50 , a low load invalidation unit  54 , an initialization unit  56 , and a large vehicle detection unit  62 . These respective functional configurations are realized by the CPU  26  reading and executing programs that are recorded in the ROM  28  or the storage  30 . 
     This variant example differs from the foregoing embodiment in that a large vehicle detection unit  62  is provided instead of the collision target prediction unit  52 , as a functional configuration. The large vehicle detection unit  62  detects that a large vehicle larger than the preceding vehicle is traveling in the periphery of the preceding vehicle. As illustrated in  FIG. 2 , when a large vehicle  102  is traveling in the periphery of the preceding vehicle  100 , the large vehicle detection unit  62  detects the large vehicle  102  based on a signal from the prevention sensor  14 . 
       FIGS. 9 and 10  are flowcharts illustrating the flow of the force limiter control processing by the force limiter control system  60  according to the first variant example. The CPU  26  reads a program from the ROM  28  or the storage  30 , deploys the program in the RAM  32 , and executes the program, whereby force limiter control processing is performed. In the present variant example, step S 111  is added between step S 110  and step S 112 , as illustrated in  FIG. 9 . Further, as illustrated in  FIG. 10 , step S 124  is replaced with step S 123 . 
     In step S 111  of  FIG. 9 , the CPU  26  detects that the large vehicle  102  is traveling in the periphery of the preceding vehicle  100  by means of the function of the large vehicle detection unit  62 . The presence or absence of the large vehicle  102  is temporarily stored in the RAM  32  or the storage  30 . 
     If the CPU  26  determines that there is no undetectable region in step S 112  and if the predetermined time has not elapsed in step S 114 , the processing proceeds to step S 108 . Further, the processing proceeds to step S 111  once more and detects that the large vehicle  102  is traveling. The information regarding the presence or absence of the large vehicle  102  stored in the RAM  32  or the storage  30  is updated. 
     Next, when the processing has proceeded from step S 112  to step S 123  in  FIG. 10 , the CPU  26  determines whether or not a large vehicle  102  is present. At this time, the CPU  26  reads the information stored in the RAM  32  or the storage  30  in step S 111 , and determines whether or not the large vehicle  102  was traveling in the periphery of the preceding vehicle  100  immediately before the host vehicle  12  entered the undetectable region. 
     If the CPU  26  determines in step S 123  that the large vehicle  102  is traveling in the periphery of the preceding vehicle  100 , the processing proceeds to step S 128 , and the change of the force limiter load by the force limiter load change unit  48  is invalidated by the function of the low load invalidation unit  54 . Then, the CPU  26  ends the force limiter control processing. However, if the CPU  26  determines in step S 123  that the large vehicle  102  is not traveling in the periphery of the preceding vehicle  100 , the CPU  26  proceeds to the processing in step S 126  and maintains the low load mode. 
     In this way, in this variant example, when the large vehicle detection unit  62  detects that the large vehicle  102  is traveling in the periphery of the preceding vehicle  100 , a collision with the large vehicle  102 , which is different from the preceding vehicle  100 , is predicted. Therefore, the change of the force limiter load by the force limiter load changing unit  48  is invalidated by the low load invalidation unit  54 . 
     According to this variant example, even if a collision with the preceding vehicle  100  is avoided and there is a collision with the large vehicle  102 , the force limiter load is not changed to a low load, and therefore, the variable force limiter unit  24  is in the high load mode. As a result, an appropriate tension is applied to the seat belt, and the occupant may be favorably restrained. 
     SECOND VARIANT EXAMPLE 
     A force limiter control system  70  according to a second variant example is described with reference to  FIGS. 11 and 12 . 
       FIG. 11  is a block diagram illustrating an example of the functional configuration of the force limiter control system  70 . As illustrated in  FIG. 11 , the force limiter control system  70  includes, as functional configuration, a receiving unit  36 , a vehicle body detection unit  38 , a relative speed detection unit  40 , an overlap amount detection unit  42 , an inter-vehicle distance detection unit  44 , an angle detection unit  46 , a force limiter load changing unit  48 , a pretensioner activation unit  50 , a low load invalidation unit  54 , an initialization unit  56 , and a traffic lane detection unit  72 . These respective functional configurations are realized by the CPU  26  reading and executing programs that are recorded in the ROM  28  or the storage  30 . 
     This variant example differs from the foregoing embodiment in that a traffic lane detection unit  72  is provided instead of the collision target prediction unit  52 , as a functional configuration. The traffic lane detection unit  72  detects a traffic lane boundary line using a sensor such as an optical camera. During normal travel, the traffic lane detection unit  72  alerts the driver when the vehicle  12  is about to deviate from a traffic lane. In this variant example, in addition to the function of providing an alert at a time of traffic lane deviation, the change in the force limiter load is invalidated when the host vehicle  12  deviates from the traffic lane after entering the undetectable region. 
       FIG. 12  is a flowchart illustrating the flow of the force limiter control processing by the force limiter control system  70  according to the second variant example. The CPU  26  reads a program from the ROM  28  or the storage  30 , deploys the program in the RAM  32 , and executes the program, whereby force limiter control processing is performed. In the force limiter control processing in the present variant example, the processing from step S 102  to step S 114  is the same as in  FIG. 6 . For this reason, with reference to  FIG. 12 , the processing from step S 116  is explained. 
     As illustrated in  FIG. 12 , in this variant example, step S 124  of the embodiment is replaced with step S 125 . In step S 125 , the CPU  26  determines whether or not there has been deviation from a traffic lane. When the CPU  26  determines that the host vehicle  12  has deviated from a traffic lane by means of the function of the traffic lane detection unit  72 , the CPU  26  proceeds to the processing of step S 128  and the change of the force limiter load by the force limiter load changing unit  48  is invalidated by means of the function of the low load invalidation unit  54 . Then, the CPU  26  ends the force limiter control processing. However, if the CPU  26  determines in step S 125  that there has been no deviation from the traffic lane, the CPU  26  proceeds to the processing in step S 126  and maintains the low load mode. 
     Thus, in this variant example, when the traffic lane detection unit  72  detects that the host vehicle  12  has deviated from a traffic lane, a collision with a collision object that is different from a vehicle is predicted. Therefore, the change of the force limiter load by the force limiter load changing unit  48  is invalidated by the low load invalidation unit  54 . 
     According to this variant example, even when the host vehicle  12  deviates from a traffic lane and enters an adjacent lane, the high load mode may be maintained in preparation for a collision with a vehicle traveling in the adjacent lane. Further, since the traffic lane deviation may be detected even after the host vehicle  12  enters the undetectable region, the possibility of collision with a different vehicle from the preceding vehicle  100  may be determined with high accuracy. 
     THIRD VARIANT EXAMPLE 
     A force limiter control system  80  according to a third variant example is described with reference to  FIGS. 13 and 14 . 
       FIG. 13  is a block diagram illustrating an example of the functional configuration of the force limiter control system  80 . As illustrated in  FIG. 13 , the force limiter control system  80  includes, as functional configuration, a receiving unit  36 , a vehicle body detection unit  38 , a relative speed detection unit  40 , an overlap amount detection unit  42 , an inter-vehicle distance detection unit  44 , an angle detection unit  46 , a force limiter load changing unit  48 , a pretensioner activation unit  50 , a low load invalidation unit  54 , an initialization unit  56 , and a steering angle detection unit  82 . These respective functional configurations are realized by the CPU  26  reading and executing programs that are recorded in the ROM  28  or the storage  30 . 
     This variant example differs from the foregoing embodiment in that a steering angle detection unit  82  is provided instead of the collision target prediction unit  52 , as a functional configuration. The steering angle detection unit  82  detects the steering angle of the host vehicle  12 . In this variant example, the change in the force limiter load is invalidated when the steering angle is equal to or greater than a predetermined threshold value after the host vehicle  12  enters the undetectable region. 
       FIG. 14  is a flowchart illustrating the flow of the force limiter control processing by the force limiter control system  80  according to the third variant example. The CPU  26  reads a program from the ROM  28  or the storage  30 , deploys the program in the RAM  32 , and executes the program, whereby force limiter control processing is performed. In the force limiter control processing in the present variant example, the processing from step S 102  to step S 114  is the same as in  FIG. 6 . For this reason, with reference to  FIG. 14 , the processing from step S 116  is explained. 
     As illustrated in  FIG. 14 , in this variant example, step S 124  of the embodiment is replaced with step S 127 . In step S 127 , the CPU  26  determines whether or not the steering angle of the host vehicle  12  is greater than a predetermined threshold value. The steering angle is detected based on information from the steering angle detection unit  82 . If the CPU  26  determines in step S 127  that the steering angle is equal to or greater than the threshold value, the processing proceeds to step S 128 , and the change of the force limiter load by the force limiter load change unit  48  is invalidated by the function of the low load invalidation unit  54 . Then, the CPU  26  ends the force limiter control processing. 
     However, if the CPU  26  determines in step S 127  that the steering angle is smaller than the threshold value, the CPU  26  proceeds to the processing of step S 126  and maintains the low load mode. 
     Thus, in this variant example, when the steering angle detected by the steering angle detection unit  82  is larger than the predetermined threshold value, a collision with a different collision object from the preceding vehicle  100  is predicted. Therefore, the change of the force limiter load by the force limiter load changing unit  48  is invalidated by the low load invalidation unit  54 . 
     According to this variant example, even when the host vehicle  12  collides with another vehicle or an obstacle different from the preceding vehicle  100 , the high load mode may be maintained. 
     Although the foregoing explanation has been made regarding a force limiter control system of an exemplary embodiment and variant examples, various aspects may, of course, be implemented within a range that does not depart from the gist of the present disclosure. For example, in the above-described embodiment and variant examples, a predetermined constant threshold value is used as the threshold value for changing the force limiter load; however, the present disclosure is not limited to this. 
     That is, the threshold value may be changed according to conditions; for example, the threshold value may be changed according to the size of the preceding vehicle  100  traveling in front of the host vehicle  12 . As illustrated in  FIG. 1 , the threshold value may be increased when the preceding vehicle is a large vehicle based, as a reference, on a case in which the preceding vehicle  100  traveling in front of the host vehicle  12  is a normal vehicle. In this case, by increasing the threshold value, it is possible to make it more difficult for the force limiter load to change to the low load mode. 
     Moreover, the first variant example, the second variant example, and the third variant example may be combined. For example, when the second variant example and the third variant example are combined, the functional configuration of the force limiter control system is a configuration including both a traffic lane detection unit and a steering angle detection unit. Further, control may be effected such that the change in the force limiter load is invalidated in at least one of a case in which traffic lane deviation is detected by the function of the traffic lane detection unit or a case in which the steering angle is detected to be equal to or greater than the threshold value by the function of the steering angle detection unit. 
     Furthermore, at least one of the first variant example, the second variant example, and the third variant example may be combined with respect to the force limiter control processing demonstrated in  FIG. 6  and  FIG. 7 . For example, when the first variant example is incorporated, the processing of step S 123  of  FIG. 10  may be added between step S 122  and step S 124  of  FIG. 7 . In this way, the process moves to step S 128  and the change in the force limiter load is invalidated in at least one of a case in which a large vehicle  102  is traveling in the periphery of the preceding vehicle  100  or a case in which the overlap amount X is smaller than the estimated lateral movement amount S. 
     Furthermore, in the above-described embodiment and variant examples, while the configuration is such that that the variable force limiter unit  24  may be changed between the two modes of a high load mode and a low load mode, the present disclosure is not limited to this. For example, the configuration may be such that it is possible to change between three modes, or such that it is possible to change between four or more modes. 
     Further, various types of processors other than a CPU may execute the respective processing that the CPU  26  executes by reading software (programs) in the above-described embodiment. Examples of such processors include programmable logic devices (PLD) with circuit configurations that are reconfigurable after manufacture, such as field-programmable gate arrays (FPGA), and dedicated electronic circuits that are processors including circuit configurations custom-designed to execute specific processing, such as application specific integrated circuits (ASIC). Moreover, each processing may be executed by one of such processors, or may be executed by a combination of two or more processors of the same type or different types (for example, plural FPGAs, or a combination of a CPU and an FPGA). More specific examples of hardware structures of such processors include electric circuits configured by combining circuit elements, such as semiconductor devices. 
     Further, in the above-described embodiment, an aspect has been explained in which each program is stored (installed) in advance in a computer-readable non-transitory recording medium, but the present disclosure is not limited to this, and each program may be provided in a format recorded in a non-transitory recording medium such as a compact disc read only memory (CD-ROM), a digital versatile disc read only memory (DVD-ROM), and a universal serial bus (USB) memory. Moreover, each of the programs may be provided in a format for downloading from an external device over a network.