Abstract:
A braking force control system includes: a brake device and at least one electronic control unit. The brake device is configured to generate a braking force commensurate with a brake operation amount of a driver. At least one electronic control unit is configured to execute vehicle speed control for controlling a speed of a vehicle to a target speed by controlling a driving force and a braking force. The electronic control unit is configured to cause the brake device to generate an actual braking force corresponding to a total value of an additional braking force and an operational braking force when brake operation is performed during execution of the vehicle speed control. The additional braking force corresponds to a controlled braking force required by the vehicle speed control. The operational braking force is required through the brake operation.

Description:
INCORPORATION BY REFERENCE 
       [0001]    The disclosure of Japanese Patent Application No. 2016-136265 filed on Jul. 8, 2016 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
       BACKGROUND 
     1. Technical Field 
       [0002]    This disclosure relates to a braking force control system. 
       2. Description of Related Art 
       [0003]    Japanese Patent Application Publication No. 2011-183983 (JP 2011-183983 A) describes a vehicle having the function of adaptive cruise control (ACC) including inter-vehicle distance control and constant speed control. The inter-vehicle distance control causes a host vehicle to follow a preceding vehicle while keeping a target inter-vehicle distance. On the other hand, the constant speed control causes a host vehicle to travel while keeping a target vehicle speed when there is no preceding vehicle to follow. 
         [0004]    The ACC is usually cancelled when a driver performs brake operation. By cancelling the ACC in this way, it is possible to immediately return the initiative of vehicle speed control to the driver at the time when the driver requires braking force. On the other hand, when the ACC is cancelled through driver&#39;s slight brake operation, braking force that has been generated before the cancellation disappears, with the result that the driver may experience a drop of braking force. 
         [0005]    In order to take measures against this problem, the system described in JP 2011-183983 A does not cancel the ACC when the driver performs slight brake operation during execution of the ACC. Moreover, this system causes the host vehicle to generate a larger one of braking force that is required from the ACC and braking force that is required through brake operation. 
       SUMMARY 
       [0006]    The system described in JP 2011-183983 A is able to prevent a large drop of braking force resulting from slight brake operation. For this reason, this system is able to reduce a feeling of strangeness that is experienced by a driver at the time when the driver performs brake operation under the ACC. 
         [0007]    Incidentally, a driver of a vehicle under the ACC performs brake operation in a situation that the driver feels a need for further braking force in addition to braking force generated by the ACC. However, in the system described in JP 2011-183983 A, if braking force that is required through brake operation does not exceed braking force generated by the ACC, braking force is just kept after the start of brake operation as well. Even when braking force generated through brake operation exceeds brake force generated by the ACC, the increment of braking force is slight. In any case, there is a concern that the driver experiences a feeling of strangeness from a change in braking force in response to brake operation performed by the driver. In this way, the system described in JP 2011-183983 A does not eliminate a feeling of strangeness that is experienced by a driver when the driver performs brake operation during execution of vehicle speed control accompanied by braking force. 
         [0008]    This disclosure provides a braking force control system that can achieve deceleration without a feeling of strangeness that is experienced by a driver when the driver performs brake operation during execution of vehicle speed control accompanied by braking force. 
         [0009]    An aspect of the disclosure provides a braking force control system. The braking force control system according to the aspect includes: a brake device and at least one electronic control unit. A brake device is configured to generate a braking force commensurate with a brake operation amount of a driver. At least one electronic control unit is configured to execute vehicle speed control for controlling a speed of a vehicle to a target speed by controlling a driving force and a braking force, and when brake operation is performed during execution of the vehicle speed control, cause the brake device to generate an actual braking force corresponding to a total value of an additional braking force and an operational braking force. The additional braking force corresponds to a controlled braking force required by the vehicle speed control. The operational braking force is required through the brake operation. 
         [0010]    With the above configuration, it is possible to control the speed of the vehicle to the target speed by executing the vehicle speed control. When the driver feels a need for braking force during execution of the vehicle speed control, the driver performs brake operation. At this time, the actual braking force that is a total of the additional braking force corresponding to the controlled braking force and the operational braking force corresponding to the brake operation is generated. In this case, the braking force for which the driver feels a need is added to the braking force generated under the vehicle speed control, so deceleration without a feeling of strangeness to the driver is achieved. 
         [0011]    In the aspect of the disclosure, the at least one electronic control unit may be configured to, when brake operation is performed during execution of the vehicle speed control, cancel a command to execute the vehicle speed control, store a controlled braking force at a point in time at which the brake operation is performed, as a braking force stored value, and compute the additional braking force based on the braking force stored value. 
         [0012]    With the above configuration, it is possible to return the initiative of control over braking force to the driver simultaneously with the start of brake operation. With this configuration, the additional braking force is computed on the basis of the controlled braking force that has been already generated at the point in time at which brake operation has been started. For this reason, with this configuration, when brake operation has been performed during execution of the vehicle speed control, it is possible to smoothly proceed with switching from the controlled braking force to the actual braking force. 
         [0013]    In the aspect of the disclosure, the at least one electronic control unit may be configured to, when brake operation is performed during execution of the vehicle speed control, store a brake operation amount at the point in time at which the brake operation is performed, as an operation amount stored value, and compute the additional braking force based on the braking force stored value and a ratio of a current brake operation amount to the operation amount stored value. 
         [0014]    With the above configuration, it is possible to store a brake operation amount that has been started under execution of the vehicle speed control, as an operation amount stored value. By using the ratio of the current brake operation amount to the operation amount stored value, it is possible to also cause the additional braking force to reflect driver&#39;s intention. For this reason, with this configuration, even when brake operation has been started through an interrupt in the middle of the vehicle speed control, it is possible to provide the driver with an operational feeling without a feeling of strangeness. 
         [0015]    In the aspect of the disclosure, the at least one electronic control unit may be configured to limit an upper limit value of the additional braking force to the braking force stored value. 
         [0016]    With the above configuration, by limiting the additional braking force to the braking force stored value, it is possible to prevent excessive response of the actual braking force to driver&#39;s brake operation. 
         [0017]    In the aspect of the disclosure, the at least one electronic control unit may be configured to, when brake operation started during execution of the vehicle speed control is cancelled, eliminate the additional braking force. 
         [0018]    With the above configuration, it is possible to eliminate the additional braking force at the time when brake operation is cancelled. Brake operation is cancelled at the time when the driver determines that there is no need for braking force. When the additional braking force is cancelled at this timing, it is possible to provide the driver with a braking feeling without a feeling of strangeness. 
         [0019]    In the aspect of the disclosure, the at least one electronic control unit may be configured to, when the vehicle stops after a start of brake operation during execution of the vehicle speed control, eliminate the additional braking force. 
         [0020]    With the above configuration, it is possible to eliminate the additional braking force at the time when the vehicle has stopped. When the vehicle has stopped, it is not necessary to generate a braking force for assisting driver&#39;s brake operation thereafter. With the above configuration, it is possible to prevent continuation of such useless assistance after a stop of the vehicle. 
         [0021]    In the aspect of the disclosure, the brake device may be configured to generate a braking force commensurate with a command value from the at least one electronic control unit. The at least one electronic control unit may be configured to, during execution of the vehicle speed control, provide the brake device with a command commensurate with the controlled braking force. The at least one electronic control unit may be configured to, when brake operation is started during execution of the vehicle speed control, provide the brake device with a command commensurate with the actual braking force. And the at least one electronic control unit may be configured to, when brake operation is started not during execution of the vehicle speed control, provide the brake device with a command commensurate with the operational braking force. 
         [0022]    With the above configuration, it is possible to achieve the mode of the disclosure in the brake by wire hardware configuration. 
         [0023]    In the aspect of the disclosure, the braking force control system may further include a hydraulic pressure circuit. The hydraulic pressure circuit may be configured to provide the brake device with a hydraulic pressure commensurate with the brake operation amount of the driver. The brake device may be configured to generate a braking force commensurate with the hydraulic pressure and a braking force commensurate with a command value from the at least one electronic control unit. The at least one electronic control unit may be configured to during execution of the vehicle speed control, provide the brake device with a command commensurate with the controlled braking force, and when brake operation is started during execution of the vehicle speed control, provide the brake device with a command commensurate with the additional braking force. 
         [0024]    With the above configuration, it is possible to achieve the mode of the disclosure in the non-brake by wire hardware configuration in which driver&#39;s brake operation is directly transmitted to the brake device by hydraulic pressure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    Features, advantages, and technical and industrial significance of exemplary embodiments will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
           [0026]      FIG. 1  is a perspective view of a vehicle on which a braking force control system according to a first embodiment of the disclosure is mounted; 
           [0027]      FIG. 2  is a block diagram of the braking force control system according to the first embodiment of the disclosure; 
           [0028]      FIG. 3  is a timing chart for illustrating a problem to be solved by the braking force control system according to the first embodiment of the disclosure; 
           [0029]      FIG. 4  is a flowchart of a routine that is executed in the first embodiment of the disclosure; 
           [0030]      FIG. 5  is a timing chart for illustrating the operation of the first embodiment of the disclosure; 
           [0031]      FIG. 6  is a flowchart of a routine that is executed in a second embodiment of the disclosure; 
           [0032]      FIG. 7  is a timing chart for illustrating the operation of the second embodiment of the disclosure; 
           [0033]      FIG. 8  is a flowchart of a routine that is executed by an ACC-ECU in an alternative embodiment to the second embodiment of the disclosure; and 
           [0034]      FIG. 9  is a flowchart of a routine that is executed by a brake ECU in the alternative embodiment to the second embodiment of the disclosure. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     Configuration of First Embodiment 
       [0035]      FIG. 1  is a perspective view of a vehicle  10  on which a braking force control system according to a first embodiment of the disclosure is mounted. An electronic control unit (ACC-ECU)  12  is mounted on the vehicle  10 . The ACC-ECU  12  has the function of executing adaptive cruise control (ACC) over the vehicle  10 . The ACC includes inter-vehicle distance control and constant speed control. The inter-vehicle distance control is able to cause the vehicle  10  to follow a preceding vehicle while keeping a target inter-vehicle distance. The constant speed control is able to cause the vehicle  10  to travel while keeping a target vehicle speed when there is no preceding vehicle to follow. 
         [0036]    A brake unit  14  is mounted on the vehicle  10 . The brake unit  14  includes a brake ECU  30  and a brake actuator. The brake actuator further includes a stroke sensor and a master cylinder pressure sensor. The stroke sensor is able to detect the operation amount of a brake pedal (not shown). The master cylinder pressure sensor is able to detect hydraulic pressure generated inside the master cylinder that generates brake hydraulic pressure. 
         [0037]    A brake device  16  is installed in each wheel of the vehicle  10 . The vehicle  10  according to the present embodiment has a so-called brake by wire configuration. That is, a braking force command is supplied from the brake ECU of the brake unit  14  to each brake device  16 . Each brake device  16  generates a braking force commensurate with the supplied command in a corresponding one of the wheels. 
         [0038]      FIG. 2  is a block diagram of the braking force control system according to the present embodiment. As shown in  FIG. 2 , the braking force control system according to the present embodiment includes the above-described ACC-ECU  12 . A millimeter-wave radar  18  and a camera  20  are connected to the ACC-ECU  12 . The millimeter-wave radar  18  and the camera  20  are mounted on the vehicle  10  so as to be able to monitor a preceding vehicle. The ACC-ECU  12  is able to detect the presence or absence of a preceding vehicle, an inter-vehicle distance to the preceding vehicle, and the like, on the basis of information that is transmitted from the millimeter-wave radar  18  and the camera  20 . 
         [0039]    A vehicle speed sensor  22  and a brake pedal sensor  24  are connected to the ACC-ECU  12 . The ACC-ECU  12  is able to detect the vehicle speed of the vehicle  10  on the basis of output from the vehicle speed sensor  22 . The ACC-ECU  12  is able to detect the presence or absence of brake operation on the basis of output from the brake pedal sensor  24 . 
         [0040]    A global positioning system (GPS) receiving device  26  and a map database are connected to the ACC-ECU  12 . The ACC-ECU  12  is able to detect the current position of the vehicle  10  on the basis of information that is provided from the GPS receiving device  26 . The ACC-ECU  12  is able to detect the geographical traveling environment of the vehicle  10  on the basis of information that is provided from the map database. 
         [0041]    The brake ECU  30 , an engine ECU  32  and a human machine interface (HMI) device  34  are further connected to the ACC-ECU  12 . The brake ECU  30  is an ECU that is included in the brake unit  14  as described above. The brake ECU  30  is a unit that issues a braking force command to the brake devices  16  of the wheels. An ON/OFF signal of the ACC and a signal indicating an ACC braking force Facc are supplied from the ACC-ECU  12  to the brake ECU  30 . The ACC braking force Facc is a braking force that is required by the ACC. The brake ECU  30  is able to determine whether the ACC is being executed on the basis of those signals, and is able to detect the magnitude of braking force (Facc) to be generated in accordance with the ACC. 
         [0042]    The HMI device  34  is arranged in a vehicle cabin. The ACC-ECU  12  is able to provide a driver with information, such as whether the ACC is being executed, with the use of the HMI device  34 . 
         [0000]    Problem Resulting from Execution of ACC 
         [0043]    The ACC executes speed control over the vehicle  10  on behalf of the driver. When the driver feels that the braking force is insufficient and performs brake operation during execution of the ACC, it is desirable to quickly return the initiative of speed control from the ACC-ECU  12  to the driver. However, if the ACC is simply finished together with the start of brake operation, there occurs an inconvenience as will be described below. 
         [0044]      FIG. 3  is a timing chart for illustrating a problem that can occur when brake operation has been performed under execution of the ACC. In  FIG. 3 , the solid-line waveform indicated by the reference numeral  36  represents an actual braking force that is generated in the vehicle  10 . The dashed-line waveform indicated by the reference numeral  38  represents a braking force that is required by the ACC (hereinafter, referred to as ACC braking force). The alternate long and short dashed-line waveform indicated by the reference numeral  40  in  FIG. 3  represents a braking force that is required through driver&#39;s brake operation (hereinafter, referred to as operational braking force). 
         [0045]    In the example shown in  FIG. 3 , the operational braking force  40  rises at time t 1 , and the operational braking force  40  disappears after time t 2 . This operation means that brake operation has been performed at substantially a constant operation amount from time t 1  to time t 2 . 
         [0046]    In the example shown in  FIG. 3 , the ACC is being executed at time t 0 . As brake operation is started at time t 1 , the ACC braking force  38  disappears in response to the start of the brake operation. This operation is achieved, for example, when execution of the ACC is cancelled along with the start of brake operation and the ACC braking force  38  is eliminated simultaneously with the cancellation of the ACC. 
         [0047]    After time t 1 , when the ACC braking force  38  disappears in this way, the actual braking force  36  temporarily increases after the start of brake operation; however, the actual braking force  36  turns to reduction immediately thereafter. Although a constant brake operation amount is kept, the actual braking force  36  converges to a value smaller than the ACC braking force  38  at time t 1 . 
         [0048]    At time t 1 , the driver feels a need for further braking force and starts brake operation. When the actual braking force  36  shows changes as described above, the driver feels a drop of braking force and tends to experience a feeling of strangeness. 
       Feature of First Embodiment 
       [0049]    In order to prevent occurrence of such a feeling of strangeness, when brake operation has been started during execution of the ACC, the braking force control system according to the present embodiment generates the operational braking force commensurate with a brake operation amount while the ACC braking force that has been already generated at that point in time remains unchanged. With the above control, it is possible to increase the actual braking force as intended by the driver, and it is possible to achieve deceleration without a feeling of strangeness. 
         [0050]      FIG. 4  is a flowchart of a routine that is executed by the brake ECU  30  in the present embodiment. The routine shown in  FIG. 4  is repeatedly executed at predetermined intervals after the start of the vehicle  10 . 
         [0051]    In the routine shown in  FIG. 4 , initially, it is determined whether the operation of the ACC has switched from an on state to an off state on the basis of information that is provided from the ACC-ECU  12  (step  100 ). The ACC-ECU  12  sets the ACC to the on state when an ACC switch is turned on by the driver. When brake operation is detected during execution of the ACC, the ACC-ECU  12  sets the ACC to the off state in order to return the initiative of vehicle speed control to the driver. Therefore, the condition of step  100  holds when brake operation has been started in a period from the last processing cycle to the current processing cycle under execution of the ACC. 
         [0052]    When it is determined in step  100  that the operation of the ACC has switched from the on state to the off state, it is determined that brake operation has been started during execution of the ACC. In this case, subsequently, the ACC braking force Facc generated at that point in time is stored, and an ACC end flag is set to an on state (step  102 ). Hereinafter, the ACC braking force stored in this step is referred to as Facc (stored value). 
         [0053]    On the other hand, when it is determined in step  100  that the ACC has not switched from the on state to the off state, the process of step  102  is skipped. That is, when the ACC remains in the on state or in the off state in the period from the last processing cycle to the current processing cycle, the process of step  102  is skipped. 
         [0054]    As the above-described process ends, it is subsequently determined whether brake operation is in an on state, that is, brake operation is being performed (step  104 ). When brake operation is in the on state, it is further determined whether brake operation has switched from an off state to the on state in the period from the last processing cycle to the current processing cycle (step  106 ). 
         [0055]    When affirmative determination is made in step  106 , it is determined that it is the timing just after brake operation has been started. In this case, an operational braking force Fdr that is required through a brake operation amount at that point in time is stored (step  108 ). Hereinafter, the operational braking force stored in this step is referred to as Fdr (stored value). On the other hand, when negative determination is made in step  106 , it is determined that brake operation has been already started at the point in time of the current processing cycle. In this case, the process of step  108  is skipped. 
         [0056]    In the brake ECU  30 , subsequently, it is determined whether the vehicle  10  is traveling (step  110 ). When it is determined that the vehicle  10  is traveling, it is further determined whether the ACC end flag is in the on state (step  112 ). 
         [0057]    As described above, the ACC end flag is set to the on state at the time when the operation of the ACC has been switched from the on state to the off state, that is, at the time when brake operation has been started under execution of the ACC. The process of step  112  is executed only when affirmative determination is made in both step  104  and step  110 . For this reason, when affirmative determination is made in step  112 , it is determined that the vehicle  10  is placed in the following situation.
   (a) Brake operation has been started under execution of the ACC.   (b) Brake operation is being continued.   (c) The vehicle  10  has not stopped yet.   
 
         [0061]    When affirmative determination is made in step  112 , the brake ECU  30  computes the actual braking force in accordance with the following mathematical expression (step  114 ). 
         [0000]      Actual braking force= Fdr+Facc  (stored value)* Fdr/Fdr  (stored value)   (1)
 
         [0062]    The first term “Fdr” on the right-hand side in the above-described mathematical expression (1) is the operational braking force Fdr in the current processing cycle. That is, the value Fdr is a braking force that is actually required by the driver through brake operation. In the above-described mathematical expression (1), Facc (stored value) and Fdr (stored value) of the second term on the right-hand side are respectively an ACC braking force Facc and an operational braking force Fdr at the start of brake operation (see step  102  and step  108 ). According to this second term, it is possible to calculate a value obtained by increasing or reducing the ACC braking force Facc (stored value) at the start of brake operation with an increase or reduction in the operational braking force Fdr after the start of brake operation. For this reason, with the above-described mathematical expression (1), it is possible to calculate the actual braking force that achieves the following two advantageous effects. 
         [0063]    (a) At the time when brake operation has been started under the ACC, it is possible to generate the actual braking force obtained by adding the operational braking force Fdr required by the driver to the ACC braking force Facc that had been generated till then. With this actual braking force, the driver does not feel a drop of braking force at the start of brake operation. 
         [0064]    (b) It is possible to change the actual braking force in response to an increase or reduction in brake operation amount after the start of the brake operation. If the actual braking force changes in this way, when brake control has been started through an interrupt in the middle of the ACC, it is possible to provide the driver with an operational feeling without a feeling of strangeness while providing assistance commensurate with the ACC braking force Facc. 
         [0065]    The actual braking force calculated in step  114  is provided to the brake actuator and the brake devices  16  of the wheels as a command value. As a result, the actual braking force that achieves the above-described advantageous effects is generated in the vehicle  10 . As the above-described process ends, the routine shown in  FIG. 4  ends the current processing cycle. 
         [0066]    After brake operation has been started under the ACC, the driver ends brake operation at the point in time at which the driver does not feel a need for braking force. When brake operation is ended in this way, it is determined in step  104  in the routine shown in  FIG. 4  that brake operation is not in the on state. In this case, the brake ECU  30  clears Facc (stored value), clears Fdr (stored value), and sets the ACC end flag to the off state (step  116 ). 
         [0067]    Subsequently, the actual braking force is set to the operational braking force Fdr detected in the current processing cycle (step  118 ). Here, brake operation is in the off state, and the operational braking force Fdr is zero, so the actual braking force is substantially set to zero. 
         [0068]    In this way, with the routine shown in  FIG. 4 , it is possible to quickly set the actual braking force to zero at the point in time at which the driver ends brake operation. For this reason, with the braking force control system according to the present embodiment, it is possible to provide the driver with an operational feeling without a feeling of strangeness at the end of brake operation as well. 
         [0069]    When brake operation is started under execution of the ACC and, after that, the vehicle  10  has stopped, it is determined in step  110  that the vehicle is not traveling. In this case as well, the processes of step  116  and step  118  are executed. After the vehicle  10  has stopped, the actual braking force no longer needs to reflect Facc (stored value). With the above-described process, it is possible to leave only the operational braking force Fdr intended by the driver as the actual braking force by clearing Facc (stored value) and Fdr (stored value). In this respect as well, the braking force control system according to the present embodiment is able to provide the driver with an operational feeling without a feeling of strangeness. 
         [0070]    As described above, the ACC end flag is set to the off state at the point in time at which brake operation started under the ACC has been cancelled or at the point in time at which the vehicle  10  has stopped through that operation (see step  116 )From then on, as the routine shown in  FIG. 4  is executed, it is determined in step  112  that the ACC end flag is not in the on state. In this case, the operational braking force Fdr detected in the current processing cycle is set for the actual braking force (step  120 ). With this process, it is possible to provide the driver with an ordinary brake function. 
         [0071]      FIG. 5  is a timing chart for illustrating the operation of the braking force control system according to the present embodiment. In  FIG. 5 , the ACC braking force  38  and the operational braking force  40  are the same as those shown in  FIG. 3 . In  FIG. 5 , the actual braking force that is raised in the process of step  114  is indicated by the dashed line  42 . In  FIG. 5 , the actual braking force indicated by the reference numeral  36  represents a variation in actual braking force that is generated in the vehicle  10  continuously from the time when the ACC is being executed. 
         [0072]    As shown in  FIG. 5 , according to the present embodiment, while the brake operation amount is kept at a constant value after brake operation has been started, it is possible to generate the stable actual braking force  36 . In this way, with the braking force control system according to the present embodiment, it is possible to effectively solve the problem regarding a drop of braking force, which has been described with reference to  FIG. 3 . 
       Alternative Embodiment to First Embodiment 
       [0073]    Incidentally, in the above-described first embodiment, the ACC is used as control that is executed in the vehicle  10  for vehicle speed control; however, the disclosure does not limit the control to the ACC. That is, the ACC in the first embodiment may be replaced with simple speed control that does not include inter-vehicle distance control. 
         [0074]    In the above-described first embodiment, at the time when the actual braking force is computed in step  114 , a variation in operational braking force Fdr is incorporated together with Facc (stored value); however, the disclosure is not limited to this configuration. For example, in step  114 , the actual braking force may be calculated by simply adding Facc (stored value) to the operational braking force Fdr. 
         [0075]    In the above-described first embodiment, the ratio of Fdr to Fdr (stored value) is multiplied by Facc (stored value) in step  114 . With this method, if Fdr is a value larger than Fdr (stored value), the multiplied result is larger than Facc (stored value). In this case, the actual braking force can be larger than a braking force intended by the driver. For this reason, in step  114 , the multiplied result may be limited to Facc (stored value) or below. With this process, it is possible to effectively avoid excessive actual braking force when brake operation has been started through an interrupt in the middle of the ACC. 
       Second Embodiment 
     Configuration of Second Embodiment 
       [0076]    Next, a braking force control system according to a second embodiment of the disclosure will be described with reference to  FIG. 6  and  FIG. 7  together with  FIG. 1  and  FIG. 2 . The braking force control system according to the present embodiment includes the components shown in  FIG. 1  and  FIG. 2  as in the case of the first embodiment. However, the vehicle  10  has a brake by wire configuration in the first embodiment, whereas the vehicle  10  has a non-brake by wire configuration in the present embodiment. 
         [0077]    That is, in the vehicle  10  according to the present embodiment, a hydraulic pressure commensurate with depression force exerted on the brake pedal is provided to the brake devices  16  of the wheels shown in  FIG. 1  without intervening an electrical signal. The brake devices  16  are able to generate an operational braking force commensurate with the provided hydraulic pressure. 
         [0078]    On the other hand, the brake unit  14  shown in  FIG. 1  converts a braking force that is required from the ACC-ECU  12  or the brake ECU  30  to a command value and then provides the command value to the brake devices  16 . The brake devices  16  are able to generate a braking force commensurate with the provided command value solely or in combination with the above-described operational braking force. 
       Operation of Second Embodiment 
       [0079]      FIG. 6  is a flowchart of a routine that is executed by the brake ECU  30  in order to implement functions similar to those in the case of the first embodiment in the vehicle  10  having the above-described configuration. Hereinafter, in  FIG. 6 , like step numbers denote the same steps as those shown in  FIG. 4 , and the description thereof is omitted or simplified. 
         [0080]    The routine shown in  FIG. 6  is similar to the routine shown in  FIG. 4  except the following points.
   (a) Step  114 , step  118  and step  120  are respectively replaced with step  122 , step  124  and step  126 .   (b) Step  128  is added upstream of the end step.
 
However, step  128  is added in order to visualize the addition of the operational braking force Fdr to the actual braking force, and does not represent a process that is executed by the brake ECU  30 .
   
 
         [0083]    In the routine shown in  FIG. 6 , an ACC actual braking force is computed in accordance with the following mathematical expression in step  122 . 
         [0000]      ACC actual braking force= Facc  (stored value)* Fdr/Fdr  (stored value)   (2)
 
         [0000]    A command value of the ACC actual braking force, calculated in this way, is provided to the brake devices  16  of the wheels. The brake devices  16  of the wheels generate a value obtained by adding the ACC actual braking force to the operational braking force Fdr as the actual braking force. 
         [0084]    The right-hand side of the above-described mathematical expression (2) is equivalent to a mathematical expression obtained by excluding Fdr from the right-hand side of the mathematical expression (1) in the first embodiment. In the present embodiment, since depression force on the brake pedal directly generates the operational braking force Fdr, a command value that is provided to the brake devices  16  does not need to include Fdr. For this reason, once the ACC actual braking force expressed by the mathematical expression (2) is provided to the brake devices  16 , an actual braking force similar to that in the case of the first embodiment is ensured in the vehicle  10 . 
         [0085]    In each of step  124  and step  126  shown in  FIG. 6 , the ACC braking force 
         [0086]    Facc is set as the ACC actual braking force. The ACC braking force Facc is a braking force that is required by the ACC. In principle, the ACC is executed under the condition that brake operation is not being performed. With the processes of step  124  and step  126 , it is possible to cause the brake devices  16  to appropriately generate Facc that is required by the ACC. 
         [0087]      FIG. 7  is a timing chart for illustrating the operation of the braking force control system according to the present embodiment. In  FIG. 7 , the operational braking force  40  is similar to that shown in  FIG. 3  or  FIG. 5 . The dashed-line waveform indicated by the reference numeral  44  in  FIG. 7  represents an ACC actual braking force that is computed in the process of step  122 , step  124  or step  126 . In the present embodiment, since the operational braking force  40  is added to the ACC actual braking force  44 , the actual braking force  36  is as shown in the timing chart. In this way, with the above-described routine shown in  FIG. 6 , it is possible to implement functions similar to those in the case of the first embodiment in the vehicle  10  having the non-brake by wire configuration. 
       Alternative Embodiment to Second Embodiment 
       [0088]    In the above-described second embodiment, various computations at the time when brake operation has been performed under the ACC are executed in the brake ECU  30 . However, the unit that executes those processes is not limited to the brake ECU  30 . For example, an ACC braking force to be generated in accordance with the ACC may be computed in the ACC-ECU  12 , and only the process of generating a braking force in accordance with the computed result may be executed in the brake ECU  30 . 
         [0089]      FIG. 8  and  FIG. 9  show an example of flowcharts of routines that are executed by the ACC-ECU  12  and the brake ECU  30  in order to achieve the above-described alternative embodiment. The processes that are executed in these routines are substantially similar to the processes that are executed by the routine shown in  FIG. 6 , so the detailed description thereof is omitted. 
         [0090]    Incidentally, in the above-described first and second embodiments, the ACC serves as vehicle speed control, the ACC braking force serves as controlled braking force, and the second term on the right-hand side of the mathematical expression (1) or the right-hand side of the mathematical expression (2) serves as additional braking force. 
         [0091]    In the above-described first and second embodiments, Facc (stored value) serves as braking force stored value, and Fdr (stored value) serves as operation amount stored value. 
         [0092]    In the above-described second embodiment, the mechanism that provides a hydraulic pressure commensurate with depression force on the brake pedal to the brake devices  16  of the wheels serves as a hydraulic pressure circuit.