Patent Publication Number: US-2021179039-A1

Title: Braking assistance apparatus for a vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. JP2019-223895 filed on Dec. 11, 2019, the content of which is hereby incorporated by reference in its entirety into this application. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a braking assistance apparatus for a vehicle such as an automobile. 
     2. Description of the Related Art 
     As a collision prevention apparatus for a vehicle such as an automobile, a braking assistance apparatus for applying a braking force to a vehicle is known in which, when an obstacle is detected in front of a host vehicle, a collision risk level at which the host vehicle collides with the obstacle is determined, and braking assistance control is performed when the collision risk level is high. In order to prevent the host vehicle from colliding with an obstacle, a braking force for braking assistance needs to be increased as the collision risk level is higher and as a braking operation amount of a driver is smaller. 
     For example, in Japanese Patent Application Laid-open Publication No. 2015-81075, there is described a braking assistance apparatus configured to calculate a threshold value that is smaller as a collision risk revel is higher, when a braking operation amount of a driver is equal to or more than the threshold value, calculate a braking assistance amount such that a ratio of the braking assistance amount to the braking operation amount of the driver is increased as the collision risk revel is higher, and generate a braking force corresponding to a sum of the braking operation amount of the driver and the braking assistance amount. 
     According to the braking assistance apparatus described in the above Publication, a braking force corresponding to the sum of the braking operation amount of the driver and the braking assistance amount is generated, and the braking assistance amount is increased such that the ratio of the braking assistance amount to the braking operation amount of the driver is increased as the collision risk revel is higher. Therefore, the higher the collision risk revel of the host vehicle colliding with an obstacle, the higher the braking force for braking assistance can be applied to the vehicle, so that the collision of the host vehicle with the obstacle can be effectively prevented as compared to where, for example, the braking force for braking assistance is constant. 
     However, in the braking assistance apparatus described in the above Publication, a braking force for the braking assistance is calculated to increase as a collision risk revel increases without considering a braking operation amount of the driver. Accordingly, in a situation where the collision risk revel is high and the braking force for the braking assistance is calculated to be a high value, and a braking operation amount of the driver is also large, it is inevitable that the driver feels uncomfortable because the braking force becomes excessive and a deceleration of the vehicle becomes excessive when a braking force corresponding to the sum of the braking operation amount of the driver and the braking assistance amount is generated. 
     If a braking force for the braking assistance is calculated to be smaller in order to avoid excessive deceleration of the vehicle due to excessive braking force, a braking force corresponding to the sum of the braking operation amount of the driver and the braking assistance amount is smaller and a deceleration of the vehicle is also smaller, so that the host vehicle cannot be effectively prevented from colliding with an obstacle. 
     SUMMARY 
     The present disclosure provides a braking assistance apparatus improved so as to prevent a host vehicle from colliding with an obstacle by applying a braking force for braking assistance to the host vehicle while preventing the braking force for the braking assistance from becoming excessive. 
     According to the present disclosure, a braking assistance apparatus for a vehicle is provided which has an obstacle information acquisition device configured to acquire information on a relative distance and a relative speed between an obstacle in front of a host vehicle and the host vehicle, a braking operation related quantity acquisition device configured to acquire a braking operation related quantity of a driver, and an electronic control unit for controlling a braking device of the host vehicle. 
     The control unit is configured to: 
     calculate a first target deceleration of the host vehicle for avoiding the host vehicle colliding with an obstacle based on the relative distance and the relative speed between the obstacle and the host vehicle acquired by the obstacle information acquisition device; 
     calculate, based on the relative distance and the relative speed, an assist level that increases as the risk of the host vehicle colliding with the obstacle increases; 
     calculate a second target deceleration of the host vehicle based on the assist level and the braking operation related quantity acquired by the braking operation related quantity acquisition device so that the second target deceleration increases as the assist level increases and as the braking operation related quantity increases; 
     set weights such that the weight of larger one of the first and second target decelerations is larger than the weight of smaller one of the first and second target decelerations; 
     calculate a final target deceleration of the host vehicle based on a weighted sum of the first and second target decelerations so as not to exceed the larger one of the first and second target decelerations; and 
     perform braking assistance by controlling the braking device so that a deceleration of the host vehicle becomes the final target deceleration. 
     According to the above configuration, a first target deceleration of the host vehicle for avoiding a collision is calculated based on a relative distance and a relative speed between the obstacle and the host vehicle, and a second target deceleration of the host vehicle is calculated based on an assist level and a braking operation related quantity. In addition, weights of the first and second target decelerations are set such that the weight of larger one of the first and second target decelerations is larger than the weight of smaller one of the first and second target decelerations; a final target deceleration of the host vehicle is calculated based on a weighted sum of the first and second target decelerations so as not to exceed the larger one of the first and second target decelerations; and the braking device is controlled so that a deceleration of the host vehicle becomes the final target deceleration. 
     Therefore, since the final target deceleration does not become larger than the larger one of the first and second target decelerations, it is possible to prevent the final target deceleration from becoming an excessively large deceleration. In addition, since weights of the first and second target decelerations are set such that the weight of the larger one of the first and second target decelerations is larger than the weight of the smaller one of the first and second target decelerations, the final target deceleration can be calculated so that the larger one of the first and second target decelerations is preferentially reflected. Therefore, the final target deceleration can be prevented from becoming an excessively small deceleration. 
     Furthermore, even if one of the first and second target decelerations sharply decreases to be smaller than the other of the first and second target decelerations, the final target deceleration does not decrease sharply, which enables to reduce the possibility that an occupant or occupants of the vehicle feels uncomfortable. 
     In one aspect of the present disclosure, the electronic control unit is configured to set the weights of the larger and smaller ones of the first and second target decelerations to 1 and 0, respectively. 
     According to the above aspect, the final target deceleration can be set to the larger one of the first and second target decelerations. Therefore, the final target deceleration can be prevented from becoming an excessively large deceleration, and the deceleration of the vehicle can be controlled based on the larger one of the first and second target decelerations. Thus, even if one of the first and second target decelerations sharply decreases to be smaller than the other of the first and second target decelerations, the final target deceleration can effectively be prevented from sharply decreasing, which enables to effectively reduce the possibility that the occupant or occupants of the vehicle feels uncomfortable due to the decrease in the deceleration of the vehicle. 
     In another aspect of the present disclosure, the electronic control unit is configured to limit the second target deceleration by an upper limit value that increases as the assist level increases. 
     According to the above aspect, the second target deceleration is limited by an upper limit value that increases as the assist level increases. Therefore, as compared to where the second target deceleration is not limited by the upper limit value, the possibility that the second target deceleration is calculated to be an excessively large value is reduced, which enables to reduce effectively the risk that the final target deceleration is calculated to be extremely large and the deceleration of the vehicle becomes excessive. 
     Furthermore, the upper limit value is variably set according to the assist level so that the higher the assist level, the larger the upper limit value. Therefore, as compared to where the upper limit value is constant, in a situation where the assist level is low, the possibility that the limit of the second target deceleration by the upper limit value can be reduced, and in a situation where the assist level is high, the possibility that the second target deceleration is excessively limited by the upper limit value is insufficient can be reduced. 
     Other objects, other features and attendant advantages of the present disclosure will be readily understood from the description of the embodiments of the present disclosure described with reference to the following drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic configuration diagram showing a first embodiment of a vehicle braking assistance apparatus for a vehicle according to the present disclosure. 
         FIG. 2  is a flowchart showing a main routine of braking assistance control in the first embodiment. 
         FIG. 3  is a flowchart showing a subroutine executed in step  60  of the flowchart shown in  FIG. 2 , that is, a routine for calculating a target deceleration Gbt of the vehicle for braking assistance. 
         FIG. 4  is a flowchart showing a latter half of a routine for calculating a target deceleration Gbt of the vehicle in the second embodiment. 
         FIG. 5  is a map for calculating an assist level AL based on a collision margin time TTC. 
         FIG. 6  is a time chart for explaining a specific example of an operation of the braking assistance apparatus described in the above-mentioned Japanese Patent Application Laid-open Publication. 
         FIG. 7  is a time chart for explaining a specific example of an operation of the second embodiment. 
         FIG. 8  is a map for calculating an increase correction coefficient K based on a collision margin time TTC in a second modified example. 
         FIG. 9  is a map for calculating an upper limit value Gbtguard of a second target deceleration Gbt 2  based on a collision margin time TTC in a third modified example. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure will now be described in detail with reference to the accompanying drawings. 
     First Embodiment 
     The braking assistance apparatus  10  according to the first embodiment is applied to a vehicle (host vehicle)  14  including a braking device  12 , and performs braking assistance by intervention in order to prevent the host vehicle from colliding with an obstacle X. It is to be noted that the vehicle  14  may be any vehicle, for example, a vehicle having an engine as a driving force source, a hybrid vehicle, or an electric vehicle having only an electric motor or motors as a driving force source or sources. 
     The braking assistance apparatus  10  includes an obstacle information acquisition device  16 , a master cylinder pressure (referred to as “MC pressure”) sensor  18  that functions as a braking operation related quantity acquisition device, a collision prevention (pre-crash safety) electronic control unit  20 , and a braking electronic control unit  30 . As will be described in detail later, the collision prevention electronic control unit  20  and the braking electronic control unit  30  function as control units that cooperate with each other to control the braking device  12  when executing the braking assistance. The collision prevention electronic control unit is abbreviated as PCS ECU, and the braking electronic control unit is abbreviated as braking ECU. 
     The obstacle information acquisition device  16  detects an obstacle X (including another vehicle) in front of the host vehicle, and detects a relative distance and a relative speed between the host vehicle and the obstacle, and an azimuth of the obstacle relative to the host vehicle. In the embodiment, the obstacle information acquisition device  16  includes a millimeter wave radar  16   a  and a camera  16   b , but the obstacle detection may be performed only by the millimeter wave radar  16   a . A laser radar or the like may be used instead of the millimeter wave radar  16   a.    
     The millimeter-wave radar  16   a  detects an obstacle by transmitting a millimeter-wave band (for example, 60 GHz) radio wave forward and receiving a wave reflected by the obstacle X, thereby detecting a relative distance between the host vehicle and the obstacle, a relative speed between them and an azimuth of the obstacle. The millimeter wave radar  16   a  outputs information about the relative distance and relative speed between the host vehicle and the obstacle and the azimuth of the obstacle to the PCS ECU  20 . 
     The camera  16   b  is an imaging device that images the front of the vehicle. The camera  16   b  includes, for example, a pair of left and right imaging elements, and may be configured to detect a relative distance and a relative speed between the host vehicle and the obstacle based on the images captured by the imaging elements. The camera  16   b  outputs information about the relative distance and the relative speed between the host vehicle and the obstacle to the PCS ECU  20 . Notably, an arithmetic processing unit that calculates a relative distance and a relative speed between the host vehicle and the obstacle based on an image captured by the camera  16   b  may be included in the camera  16   b , or may be included in the PCS ECU  20  that receives an image information. 
     The braking device  12  includes a master cylinder device  34  driven by a brake pedal  32  being depressed by a driver, a brake actuator  36 , and braking force generation devices  40 FL,  40 FR,  40 RL and  40 RR provided on left and right front wheels  38 FL and  38 FR and left and right rear wheels  38 RL and  38 RR, respectively. As is well known, the braking force generation devices  40 FL to  40 RR increase or decrease braking forces of the corresponding wheels by increasing or decreasing pressures in wheel cylinders  42 FL to  42 RR, respectively, by the brake actuator  36 . 
     The brake actuator  36  includes a hydraulic circuit, which is not shown in the figure, including a pump that generates high pressure, various valve devices, and the like. The brake actuator  36  normally controls the pressures in the wheel cylinders  42 FL to  42 RR in accordance with a pressure in the master cylinder device  34 , that is, an MC pressure Pmc, thereby controlling braking forces of the wheels  38 FL to  38 RR according to an amount of braking operation by the driver. Further, the brake actuator  36  can individually control the pressures in the wheel cylinders  42 FL to  42 RR without depending on the MC pressure, and thereby individually controls the braking forces of the wheels  38 FL to  38 RR regardless of the braking operation amount of the driver. 
     The MC pressure sensor  18  is a device that detects an MC pressure Pmc as a braking operation related quantity in order to detect a braking operation amount and a braking operation speed of the driver. Since the MC pressure is generated in proportion to a braking operation amount of the driver (a pedal effort applied to the brake pedal  32 ), the braking operation amount can be detected by detecting the MC pressure, and a braking operation speed can be obtained by differentiating the MC pressure with respect to time. The MC pressure sensor  18  outputs information about the MC pressure to the braking ECU  30 . A braking operation amount of the driver may be detected by another device such as a pedal effort sensor provided on the brake pedal  32 , and the braking operation speed may be obtained as a time differential value of the braking operation amount detected by the pedal effort sensor. 
     The PCS ECU  20  and the braking ECU  30  each include a microcomputer, and each microcomputer includes a CPU that performs arithmetic processing, a ROM that stores a control program, a readable/writable RAM that stores arithmetic results, a timer, a counter, an input interface and an output interface. The PCS ECU  20  and the braking ECU  30  may be any arithmetic control device known in the art. Further, some of the functions of the PCS ECU  20  and the braking ECU  30  may be realized by another ECU. Further, some of the functions of the PCS ECU  20  and the braking ECU  30  may be realized by the other ECU. 
     The PCS ECU  20  loads a control program stored in the ROM into the CPU and executes the control program to execute processes such as calculation of a collision margin time TTC which will be described later, setting of an assist level AL, requests to the braking ECU  30  and a meter ECU (not shown). The PCS ECU  20  is communicatively connected to the obstacle information acquisition device  16  (the millimeter wave radar  16   a  and the camera  16   b ), the braking ECU  30 , and the like by an in-vehicle LAN such as a CAN (Controller Area Network) or a harness. 
     The PCS ECU  20  receives obstacle information output from the millimeter wave radar  16   a  and the camera  16   b , and obtains a relative distance Dr and a relative speed Vr of the host vehicle  14  relative to an obstacle X in front of the host vehicle  14  and an azimuth of the obstacle. A collision margin time TTC (Time To Collision) is calculated based on the relative distance, the relative speed, and the azimuth. The collision margin time TTC is a value obtained by dividing the relative distance Dr between the host vehicle and the obstacle by the relative speed Vr, and is a time until the host vehicle collides with the obstacle. The smaller the collision margin time TTC, the higher the risk of the host vehicle colliding with the obstacle. Therefore, the collision margin time TTC is also a collision risk level indicating a risk of the host vehicle colliding with the obstacle. 
     The relative distance between the host vehicle and the obstacle used when calculating the collision margin time TTC may be a distance detected by the millimeter wave radar  16   a , or a distance detected by the camera  16   b , and it may be an average value of the distance detected by the millimeter wave radar  16   a  and the distance detected by the camera  16   b . The relative distance and the relative speed between the host vehicle and the obstacle used when calculating the collision margin time TTC may be corrected to be a relative distance and a relative speed in the traveling direction of the host vehicle using the information on the azimuth of the obstacle detected by the millimeter wave radar  16   a.    
     The PCS ECU  20  sets an assist level AL as a collision risk level according to the calculated collision margin time TTC. The assist level AL is set in four stages of 0 to 3 according to the map shown in  FIG. 5 , for example. As shown in  FIG. 5 , the assist level AL increases as the collision allowance time TTC decreases, and thus the collision risk level increases as the assist level AL progresses from 0 to 3. The PCS ECU  20  outputs a signal indicating the assist level AL to the braking ECU  30 . 
     For example, when the assist level AL is 0, it is considered that the possibility of collision is low, and the braking assistance (brake assist) controlled by the braking ECU  30  described later is not performed, and the braking assistance may be performed when the assist level AL is 1 to 3. Therefore, when the assist level AL is 1 to 3, the PCS ECU  20  requests the braking ECU  30  to perform the braking assistance. 
     Furthermore, the PCS ECU  20  may perform driving assistance according to the assist level AL via a meter ECU (not shown) or the like. The meter ECU may be connected to a combination meter device (not shown) for notifying the driver by display, a notification sound generating device (not shown) for notifying the driver by voice, and the like. The meter ECU may control, in response to a request from the PCS ECU  20 , numerical values, characters, figures, indicator lamps, etc. displayed on the combination meter device, and also may control alarm sound and alarm voice notified by the notification sound generating device. For example, when the assist level AL is 1 to 3, the PCS ECU  20  may request the meter ECU to output an alarm sound for informing the driver of the possibility of a collision or turn on an indicator lamp. 
     The braking ECU  30  loads a control program stored in the ROM into the CPU and executes the control program to execute various processes relating to the braking assistance described later. Further, the braking ECU  30  is communicatively connected to the MC pressure sensor  18 , the PCS ECU  20 , the brake actuator  36 , and the like through an in-vehicle LAN such as CAN or a harness. 
     The braking ECU  30  controls the braking forces of the wheels  38 FL to  38 RR generated by the braking force generation devices  40 FL to  40 RR by controlling the brake actuator  36 . Particularly, when the assist level AL is 0, the braking ECU  30  controls the brake actuator  36  so as to control the braking forces in a normal manner. That is, the braking ECU  30  controls the brake actuator  36  to control the pressures in the wheel cylinders  42 FL to  42 RR according to the MC pressure Pmc so that the braking forces of the wheels  38 FL to  38 RR are controlled according to an amount of braking operation by the driver, and controls the brake actuator  36  so that the pressures are individually controlled as necessary regardless of the amount of braking operation by the driver. 
     On the other hand, when the assist level AL is 1 to 3, the braking ECU  30  controls the brake actuator  36  so that the braking assistance for preventing the collision of the vehicle is performed. Specifically, the braking ECU  30  performs the braking assistance when it can be determined that an emergency brake operation is performed by the driver based on the brake operation amount and the brake operation speed. The braking assistance is achieved by controlling the brake actuator  36  so that the deceleration of the vehicle becomes higher than the deceleration corresponding to the braking operation of the driver, and thus the pressures in the wheel cylinders  42 FL to  42 RR become higher than the MC pressure Pmc. In a hybrid vehicle or an electric vehicle, regenerative braking may be performed on the basis of a request from the PCS ECU  20  also in braking assistance. 
     In the first embodiment, the braking assistance is executed according to the flowcharts shown in  FIGS. 2 and 3 . A first target deceleration Gbt 1  of a PCS request is calculated based on the relative distance and the relative speed between the host vehicle and the obstacle, and a second target deceleration Gbt 2  is calculated so that the higher the MC pressure Pmc and the collision risk of collision with the obstacle, the higher the second target deceleration. Further, the target deceleration Gbt of the vehicle for the braking assistance is calculated as a weighted sum of the first target deceleration Gbt 1  and the second target deceleration Gbt 2 . A weight R is set so that a weight of the larger one of the first target deceleration Gbt 1  and the second target deceleration Gbt 2  is greater. 
     &lt;Braking Assistance Control in the First Embodiment&gt; 
       FIG. 2  is a flowchart showing a main routine of braking assistance control for collision prevention in the first embodiment, and the braking assistance control is achieved by the cooperation of the PCS ECU  20  and the braking ECU  30 . The braking assistance control according to the flowchart shown in  FIG. 2  is repeatedly executed at predetermined time intervals when an ignition switch (not shown) is ON. In the following description, the braking assistance control according to the flowchart shown in  FIG. 2  will be simply referred to as “the control”. In addition, in  FIG. 2 , the braking assistance is described as BA. 
     First, in step  10 , a collision margin time TTC is calculated based on a relative distance Dr and a relative speed Vr between the host vehicle and the obstacle, and an azimuth of the obstacle, and an assist level AL is determined by referring to the map shown in  FIG. 5  based on the collision margin time TTC. The assist level AL is an index value indicating a degree of need for the braking assistance, and as shown in  FIG. 5 , the assist level AL increases as the collision margin time TTC decreases. 
     Prior to step  10 , signals indicating the presence or absence of an obstacle X around the vehicle detected by the obstacle information acquisition device  16 , a relative distance Dr between the vehicle and the obstacle, a relative speed Vr, and an azimuth of the obstacle are read. Further, a signal indicating an MC pressure Pmc detected by the MC pressure sensor  18  is read. 
     In step  20 , a determination is made as to whether or not the assist level AL is 0, that is, whether or not the braking assistance for collision prevention is unnecessary. When an affirmative determination is made, the control proceeds to step  50 , and when a negative determination is made, the control proceeds to step  30 . 
     In step  30 , a determination is made as to whether a flag Fba is 1, that is, whether the braking assistance is being executed. When an affirmative determination is made, the control proceeds to step  60 , and when a negative determination is made, the control proceeds to step  40 . 
     In step  40 , a determination is made as to whether or not conditions for starting the braking assistance are satisfied. When an affirmative determination is made, the control proceeds to step  60 , and when a negative determination is made, the control proceeds to step  50 . 
     It should be noted that when the MC pressure Pmc is equal to or higher than a reference value Pmcc and a change rate Pmcd of the MC pressure is equal to or higher than a reference value Pmcdc, it may be determined that the conditions for starting the braking assistance are satisfied. As shown in Table 1 below, the reference values Pmcc and Pmcd are variably set according to the assist level AL. The reference values Pmcc 1  to Pmcc 3  are positive constants having a relationship of Pmcc 1 &gt;Pmcc 2 &gt;Pmcc 3 , and the reference values Pmcdc 1  to Pmcdc 3  are positive constants having a relationship of Pmcdc 1 &gt;Pmcdc 2 &gt;Pmcdc 3 . Therefore, the reference values Pmcc and Pmcdc are variably set according to the assist level AL so that the higher the assist level AL, the smaller the reference values Pmcc and Pmcdc. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Assist Level 
                 Reference 
                 Reference 
               
               
                 AL 
                 Value Pmcc 
                 Value Pmcdc 
               
               
                   
               
             
            
               
                 1 
                 Pmcc1 
                 Pmcdc1 
               
               
                 2 
                 Pmcc2 
                 Pmcdc2 
               
               
                 3 
                 Pmcc3 
                 Pmcdc3 
               
               
                   
               
            
           
         
       
     
     In step  50 , the normal control of the braking forces without the braking assistance is executed. That is, by controlling the pressures in the wheel cylinders  42 FL to  42 RR according to the MC pressure Pmc, the braking forces of the wheels  38 FL to  38 RR are controlled according to the braking operation amount of the driver. 
     In step  60 , a target deceleration Gbt of the vehicle for the braking assistance is calculated as described later according to the subroutine shown in  FIG. 3 . 
     In step  90 , target braking forces Fbtfl to Fbtrr of the wheels  38 FL to  38 RR are calculated based on the target deceleration Gbt in a manner known in the art. Further, by controlling the brake actuator  36  so that the braking forces of the wheels  38 FL to  38 RR become the target braking forces Fbtfl to Fbtrr, respectively, the pressures of the wheel cylinders  42 FL to  42 RR are controlled, and thereby the braking assistance is executed. 
     In step  100 , a determination is made as to whether or not a condition for ending the braking assistance is satisfied. When a negative determination is made, the control returns to step  10 , and when an affirmative determination is made, the flag Fba is reset to 0 in step  110 , and then the control returns to step  10 . 
     It should be noted that when any one of the following conditions is satisfied, it may be determined that the condition for ending the braking assistance is satisfied. 
     (1) The MC pressure Pmc is less than or equal to an end reference value Pmce (a positive constant).
 
(2) The vehicle speed is below an end reference value (a positive constant).
 
(3) An equipment necessary for executing the braking assistance, such as the obstacle information acquisition device  16 , is abnormal.
 
(4) A time more than an end reference time (a positive constant) has elapsed since the application of braking forces by executing the braking assistance was started.
 
     As shown in  FIG. 3 , the calculation of the target deceleration Gbt of the vehicle in the above step  60  is performed by executing the following steps  62  to  80 . 
     In step  62 , a first target deceleration Gbt 1  of a PCS (pre-crash safety) request is calculated based on the relative distance Dr and the relative speed Vr between the host vehicle and the obstacle detected by the obstacle information acquisition device  16 . For example, if a target relative distance when the vehicle is stopped by braking is represented by Drt and an elapsed time is represented by t, the following equations (1) and (2) are established. Therefore, the first target deceleration Gbt 1  may be calculated according to the following equation (3). It is to be noted that an azimuth of the obstacle may be considered when the first target deceleration Gbt 1  is calculated. 
         Drt=Dr−Gbt 1· t   2 /2  (1)
 
         Dr−Drt=Vr·t   (2)
 
         Gbt 1= Vr   2 /{2( Dr−Drt )}  (3)
 
     In step  64 , a basic target deceleration Gbt 0  based on the MC pressure Pmc is calculated to a value proportional to the MC pressure Pmc. That is, the basic target deceleration Gbt 0  is calculated according to the MC pressure Pmc so that the higher the MC pressure, the larger the target deceleration. 
     In step  66 , as shown in Table 2 below, an increase correction coefficient K for the basic target deceleration Gbt 0  is calculated based on the assist level AL. In Table 2, K1, K2 and K3 are positive constants having a relationship of K1&lt;K2&lt;K3. Therefore, the increase correction coefficient K is variably set according to the assist level AL such that the increase correction coefficient K has a larger value as the assist level AL is higher. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Assist Level 
                 Increase Correction 
               
               
                   
                 AL 
                 Coefficient K 
               
               
                   
                   
               
             
            
               
                   
                 1 
                 K1 
               
               
                   
                 2 
                 K2 
               
               
                   
                 3 
                 K3 
               
               
                   
                   
               
            
           
         
       
     
     In step  68 , a second target deceleration Gbt 2  based on the MC pressure Pmc and the assist level AL is calculated based on the increase correction coefficient K and the basic target deceleration Gbt 0  according to the following equation (4). Therefore, the second target deceleration Gbt 2  is calculated so that the higher the assist level AL, the larger the target deceleration, and the higher the MC pressure Pmc, the larger the target deceleration. 
         Gbt 2=(1+ K ) Gbt 0  (4)
 
     In step  70 , an upper limit value Gbtguard of the second target deceleration Gbt 2  is calculated based on the assist level AL so that the higher the assist level AL is, the larger the upper limit value Gbtguard is. Further, a determination is made as to whether or not the second target deceleration Gbt 2  is larger than the upper limit value Gbtguard. When a negative determination is made, the control proceeds to step  74 . When an affirmative determination is made, the second target deceleration Gbt 2  is corrected to the upper limit Gbtguard in step  72 , and then the control proceeds to step  74 . 
     In step  74 , a determination is made as to whether or not the first target deceleration Gbt 1  is greater than or equal to the second target deceleration Gbt 2 . When an affirmative determination is made, a weight R of the second target deceleration is set to 0.1 in step  76 , and then the control proceeds to step  80 . On the other hand, when a negative determination is made, the weight R is set to 1 in step  78 , and then the control proceeds to step  80 . 
     It is to be noted that the weight R set in step  76  may be greater than 0 and smaller than 0.5 because the weight  1 -R of the first target deceleration Gbt 1  needs to be greater than the weight R of the second target deceleration Gbt 2 . However, the greater the weight R, the lower the degree of reflection of the first target deceleration Gbt 1  on the target deceleration Gbt so that the weight R is preferably a value close to 0, for example, a value larger than 0 and smaller than 0.15. 
     In step  80 , a target deceleration (final target deceleration) Gbt of the vehicle for the braking assistance is calculated as a weighted sum of the first target deceleration Gbt 1  and the second target deceleration Gbt 2  according to the following equation (5). 
         Gbt =(1− R ) Gbt 1+ R·Gbt 2  (5)
 
     &lt;Operation of the First Embodiment&gt; 
     Next, the operation of the first embodiment will be described for a case where the braking assistance is unnecessary and a case where the braking assistance is required. The operation in the case where the braking assistance is unnecessary is the same in the second embodiment described later. 
     &lt;When the Braking Assistance is not Required&gt; 
     When the braking assistance is unnecessary because the possibility of collision is low, the assist level AL is determined to be 0 in step  10 , and an affirmative determination is made in step  20 . Further, even though there is a possibility of collision, when the conditions for staring the braking assistance are not satisfied and thus the braking assistance is not required, negative determinations are made in steps  20  and  40 . Therefore, in step  50 , the normal braking force control is performed without executing the braking assistance for collision prevention. 
     &lt;When the Braking Assistance is Required&gt; 
     When there is a possibility of collision and the braking assistance is required, in step  10 , the assist level AL is determined to be one of 1-3, and in step  20 , a negative determination is made. Step  30  or steps  30  and  40  are executed, and step  60 , and thus steps  62  to  80 , are executed. 
     In step  62 , a first target deceleration Gbt 1  of the PCS request is calculated based on the relative distance and the relative speed between the host vehicle and the obstacle. In steps  64  to  72 , a second target deceleration Gbt 2  based on the MC pressure Pmc and the assist level AL is calculated so that the higher the MC pressure Pmc and the assist level AL, the higher the second target deceleration Gbt 2  but does not exceed the upper limit value Gbtguard. Further, in steps  74  to  80 , a target deceleration Gbt of the vehicle for the braking assistance is calculated as a weighted sum of the first target deceleration Gbt 1  and the second target deceleration Gbt 2 . In this case, when the first target deceleration Gbt 1  is greater than or equal to the second target deceleration Gbt 2 , the weight R is set to 0.1, and when the first target deceleration Gbt 1  is smaller than the second target deceleration Gbt 2 , the weight R is set to 1. 
     According to the first embodiment, the first target deceleration Gbt 1  of the host vehicle for avoiding a collision is calculated based on the relative distance Dr and the relative speed Vr between the obstacle and the host vehicle  14 . The second target deceleration Gbt 2  of the host vehicle is calculated based on the assist level AL and the MC pressure Pmc which is a braking operation related quantity. In addition, the weights  1 -R and R of the first and second target decelerations, respectively, are set so that the weight of the larger one of the first and second target deceleration is greater than that of the smaller one. Further, the final target deceleration Gbt of the host vehicle is calculated according to the equation (5) as a weighted sum of the first and second target decelerations calculated so as not to exceed the larger one of the first and second target decelerations. The braking device  12  is controlled so that a deceleration Gb of the host vehicle becomes the final target deceleration Gbt. 
     Therefore, the final target deceleration Gbt never becomes larger than the larger one of the first target deceleration Gbt 1  and the second target deceleration Gbt 2 , so that it is possible to prevent the final target deceleration from becoming an excessively large deceleration. In addition, the weights are set so that the weight of the larger one of the first and second target decelerations is greater than the other weight, so that the final target deceleration can be calculated by preferentially reflecting the larger one of the first and second target decelerations. Therefore, it is possible to prevent the final target deceleration from becoming an excessively small deceleration, that is, it can be prevented that the collision of the vehicle cannot be effectively prevented. 
     Furthermore, even if one of the first and second target decelerations sharply decreases, the final target deceleration does not decrease sharply, so that it is possible to reduce the possibility that an occupant or occupants of the vehicle feels uncomfortable. 
     Second Embodiment 
     The braking assistance apparatus  10  of the second embodiment is configured similarly to the braking assistance apparatus of the first embodiment, and the braking assistance control of the second embodiment is the same as that of the first embodiment except step  60 . Step  60  in the second embodiment is executed according to the subroutine shown in  FIG. 4 , thereby a target deceleration Gbt of the vehicle for the braking assistance is calculated. 
     Although steps  62 - 68  are not shown in  FIG. 4 , steps  62 - 74  are performed similarly to steps  62 - 74  in the first embodiment. When an affirmative determination is made in step  74 , the target deceleration (final target deceleration) Gbt of the vehicle for the braking assistance is set to the first target deceleration Gbt 1  in step  82 , and then the control proceeds to step  90 . On the other hand, when a negative determination is made in step  74 , the target deceleration Gbt of the vehicle is set to the second target deceleration Gbt 2  in step  84 , and then the control proceeds to step  90 . 
     As can be seen from the comparison between  FIG. 3  and  FIG. 4 , setting the target deceleration Gbt of the vehicle in the second embodiment is the same as executing the step  80  by setting the weight R to 0 in the step  76  in the first embodiment. Therefore, the target deceleration Gbt of the vehicle is set to the value of the larger one of the first target deceleration Gbt 1  and the second target deceleration Gbt 2 . 
     &lt;Operation of the Second Embodiment when the Braking Assistance is Required&gt; 
     The target deceleration Gbt of the vehicle for the braking assistance is set to the first target deceleration Gbt 1  when the first target deceleration Gbt 1  is equal to or greater than the second target deceleration Gbt 2 , and is set to the second target deceleration Gbt 2  when the first target deceleration Gbt 1  is smaller than the second target deceleration Gbt 2 . The other operations are the same as those in the first embodiment. 
     According to the second embodiment, the final target deceleration Gbt can be set to the value of the larger one of the first target deceleration Gbt 1  and the second target deceleration Gbt 2 . Therefore, the final target deceleration can be prevented from becoming an excessively large deceleration, and a deceleration Gb of the vehicle can be controlled based on the value of the larger one of the first and second target decelerations. Thus, even if one of the first and second target decelerations sharply decreases, the final target deceleration Gbt can effectively be prevented from sharply decreasing, so that it is possible to effectively reduce the possibility that an occupant or occupants of the vehicle feels uncomfortable due to the sudden decrease. 
     It is to be noted that according to the first and second embodiments, the second target deceleration Gbt 2  is limited by an upper limit value Gbtguard calculated based on the assist level AL. Therefore, as compared to where the second target deceleration Gbt 2  is not limited by the upper limit value Gbtguard, the possibility that the second target deceleration is calculated to be an excessively large value is reduced, and thus, it is possible to effectively reduce the risk that the final target deceleration Gbt is calculated to be an excessively large deceleration and the vehicle deceleration Gb becomes excessive. 
     Further, the upper limit value Gbtguard is variably set according to the assist level AL such that the upper limit value Gbtguard increases as the assist level AL increases. Therefore, as compared to where the upper limit value Gbtguard is constant, in a situation where the assist level AL is low, it is possible to reduce the possibility that the second target deceleration Gbt 2  is insufficiently limited by the upper limit value, and in a situation where the assist level AL is high, it is possible to reduce the possibility that the second target deceleration Gbt 2  is excessively limited by the upper limit value. 
     &lt;Specific Example of the Operation of the Second Embodiment&gt; 
     Next, referring to  FIGS. 6 and 7 , a specific example ( FIG. 7 ) of the operation of the second embodiment will be described in comparison with a specific example ( FIG. 6 ) of the operation of the braking assistance device described in Japanese Patent Application Laid-open Publication No. 2015-81075 described above. 
     As shown in  FIG. 6 , it is assumed that the assist level AL becomes a value other than 0 from time t 1  to time t 7 , and a driver performs a braking operation from time t 2  to time t 7 , whereby the MC pressure Pmc increases from 0 at time t 2  to the maximum value at time t 4  and gradually decreases to 0 at time t 7 . 
     A braking assistance amount Pass corresponding to the MC pressure Pmc increases/decreases in accordance with the increase/decrease in the MC pressure Pmc. Therefore, a deceleration Gb of the vehicle when the braking assistance is performed becomes a value corresponding to the sum Pmc+Pass of the MC pressure Pmc and the braking assistance amount Pass. 
     Therefore, when an amount of braking operation by the driver is high and the MC pressure Pmc has a high value, the braking assistance amount Pass also has a high value, and as a result, an occupant or occupants of the vehicle may feel uncomfortable due to the cause that the deceleration Gb of the vehicle becomes excessive at, for example, time t 4  and before and after that. 
     On the other hand, according to the second embodiment, since the final target deceleration Gbt is set to the value of the larger one of the first target deceleration Gbt 1  and the second target deceleration Gbt 2 , the final target deceleration does not become an excessively large deceleration. Therefore, it is possible to prevent the occupant or occupants of the vehicle from feeling uncomfortable due to the excessive deceleration Gb of the vehicle. 
     For example, as shown in  FIG. 7 , it is assumed that the assist level AL and the MC pressure Pmc change as in  FIG. 6 . It is also assumed that the first target deceleration Gbt 1  of the PCS request occurs from time t 3  to time t 6  and increases and decreases as shown in the figure. It is also assumed that the second target deceleration Gbt 2  based on the MC pressure Pmc and the assist level AL gradually increases from 0 at time t 2  and then gradually decreases to 0 at time t 7 , and is corrected to the upper limit value Gbtguard from time t 3 ′ to time t 4 ′. Further, it is assumed that the first target deceleration Gbt 1  becomes smaller than the second target deceleration Gbt 2  at time t 5  immediately before the time t 6 . 
     Since the target deceleration (final target deceleration) Gbt of the vehicle is set to the value of the larger one of the first target deceleration Gbt 1  and the second target deceleration Gbt 2 , it changes as shown at the bottom of  FIG. 7 . That is, the target deceleration Gbt is the second target deceleration Gbt 2  in the sections from the time t 2  to the time t 3  and from the time t 5  to the time t 7 , and is the first target deceleration Gbt 1  in the section from the time t 3  to the time t 5 . 
     Therefore, since the target deceleration Gbt is not set to the sum of the first target deceleration Gbt 1  and the second target deceleration Gbt 2 , it does not become an excessive value even when the MC pressure Pmc has a high value. In particular, the second target deceleration Gbt 2  is corrected to the upper limit value Gbtguard when it exceeds the upper limit value. Therefore, even when the MC pressure Pmc is high and the second target deceleration Gbt 2  is high, the target deceleration Gbt can be reliably prevented from becoming excessive. 
     Further, the target deceleration Gbt is set to the second target deceleration Gbt 2  in the sections from the time t 2  to the time t 3  and from the time t 6  to the time t 7  where the first target deceleration Gbt 1  is 0. Therefore, it is possible to prevent the occupant or occupants of the vehicle from feeling uncomfortable due to insufficient deceleration of the vehicle in these sections. 
     Although the present disclosure has been described in detail with reference to specific embodiments, it will be apparent to those skilled in the art that the present disclosure is not limited to the above-described embodiments, and various other embodiments are possible within the scope of the present disclosure. 
     For example, in the above-described first and second embodiments, in step  66 , the increase correction coefficient K is set so that the higher the assist level AL is, the larger the correction value K is set, and in step  68 , the second target deceleration Gbt 2  is calculated as a product of the coefficient (1+K) and the basic target deceleration Gbt 0 . 
     However, as shown in Table 3 below, a braking assistance amount ΔGbt 2  of the deceleration may be calculated such that the higher the assist level AL, the larger the braking assistance amount, and the second target deceleration Gbt 2  may be calculated as a sum of the basic target deceleration Gbt 0  and the braking assistance amount ΔGbt 2  (a first modified example). In Table 3, ΔGbt 21 , ΔGbt 22 , and ΔGbt 23  are positive constants having a relationship of ΔGbt 21 &lt;ΔGbt 22 &lt;ΔGbt 23 . 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Assist Level 
                 Braking Assistance 
               
               
                   
                 AL 
                 Aamount Δ Gbt2 
               
               
                   
                   
               
             
            
               
                   
                 1 
                 Δ Gbt21 
               
               
                   
                 2 
                 Δ Gbt22 
               
               
                   
                 3 
                 Δ Gbt23 
               
               
                   
                   
               
            
           
         
       
     
     Further, in the above-described first and second embodiments, the increase correction coefficient K is set to three levels so that the higher the assist level AL is, the larger the correction value K becomes, and the second target deceleration Gbt 2  is calculated as a product of the increase correction coefficient 1+K and the basic target deceleration Gbt 0 . Therefore, when the assist level AL changes stepwise with the increase or decrease of the collision margin time TTC, the second target deceleration Gbt 2  also changes stepwise. 
     Therefore, the increase correction coefficient K may be calculated by referring to, for example, the map shown in  FIG. 8  so that the smaller the collision margin time TTC is, that is, the higher the assist level AL is, the larger the correction coefficient K is, whereby the second target deceleration Gbt 2  may be continuously changed as the collision margin time TTC is increased and decreased (a second modified example). 
     Similarly, in the first and second embodiments described above, in step  70 , the upper limit value Gbtguard of the second target deceleration Gbt 2  is calculated based on the assist level AL so that the higher the assist level AL is, the larger the upper limit value is. Therefore, when the assist level AL changes stepwise as the collision margin time TTC increases and decreases, the upper limit value Gbtguard also changes stepwise. 
     Therefore, the upper limit value Gbtguard may be calculated by referring to, for example, the map shown in  FIG. 9  so that it continuously increases as the collision margin time TTC decreases, that is, as the assist level AL increases, whereby, the upper limit value Gbtguard may be continuously changed as the collision margin time TTC is increased and decreased (a third modification example). 
     Although in the first and second embodiments described above, braking operation related quantity indicating a braking operation amount of a driver is an MC pressure Pmc, it may be a brake pedal effort or a brake pedal stroke or any combination of an MC pressure, a brake pedal effort and a brake pedal stroke.