Patent Publication Number: US-2015061362-A1

Title: Brake control apparatus

Description:
BACKGROUND OF THE INVENTION 
     Technical Field 
     The present invention relates to a brake control apparatus which is suitably used for applying a braking force to a vehicle. 
     BACKGROUND ART 
     As a brake apparatus to be mounted in a vehicle, the following one is known. Specifically, the brake apparatus includes an input member which is configured to move forward and backward in accordance with an operation of a brake pedal, a piston which is provided movably with respect to the input member to generate a hydraulic pressure in a master cylinder, and an electric booster including a drive motor which move the piston forward and backward based on the operation of the brake pedal to variably control the hydraulic pressure in the master cylinder (for example, see Japanese Patent Application Laid-open Nos. 2012-96649 and 2013-28273). 
     In the electric booster used in the brake apparatus described above, when the drive motor comes into a full-load state, a reaction force (pedal feeling) generated by the operation of the brake pedal changes to sometimes give a weird pedal feeling to a driver. In Japanese Patent Application Laid-open No. 2012-96649, in order to eliminate the weird pedal feeling, a spring for applying the reaction force when the drive motor comes into the full-load state is provided so as to adjust the change in reaction force. Moreover, as disclosed in Japanese Patent Application Laid-open No. 2013-28273, a hydraulic-pressure rise caused by the operation of the brake pedal is suppressed to suppress a change in the reaction force occurring when the drive motor comes into the full-load state. 
     According to the related art disclosed in Japanese Patent Application Laid-open No. 2012-96649, the spring for applying the reaction force is additionally provided. As a result, a mechanism of the electric booster becomes disadvantageously complex. On the other hand, the related art disclosed in Japanese Patent Application Laid-open No. 2013-28273 has a problem in that an output hydraulic pressure generated by the operation of the brake pedal, which is started with a predetermined stroke, is lowered. As a result, an operation amount of the brake pedal is disadvantageously increased to generate a necessary output hydraulic pressure. 
     SUMMARY OF INVENTION 
     The present invention has been made to solve the above-mentioned problems of the related art, and therefore has an object to provide a brake control apparatus which has a simple structure and is capable of suppressing a change in reaction force, which occurs when a drive motor comes into a full-load state, without lowering an output hydraulic pressure generated by a pedal operation. 
     In order to solve the problems described above, the brake control apparatus according to one embodiment of the present invention includes: a master-cylinder pressure control unit configured to control a drive motor configured to pressurize a hydraulic fluid in a master cylinder in accordance with an operation of a brake pedal to which a hydraulic reaction force is transmitted; and a wheel-cylinder fluid supply control unit provided between a wheel cylinder provided to a wheel and the master cylinder, which controls supply of the hydraulic fluid to the wheel cylinder. When a driving force of the drive motor becomes a maximum driving force in a period during which the brake pedal is operated, a hydraulic stiffness of the wheel cylinder has been already increased by the wheel-cylinder fluid supply control unit. 
     According to one embodiment of the present invention, a change in reaction force, which occurs when the drive motor comes into the full-load state, can be suppressed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an overall configuration diagram of a brake apparatus to which a brake control apparatus according to a first embodiment of the present invention is applied. 
         FIG. 2  is a circuit block diagram illustrating a circuit configuration of control devices including a first ECU and a second ECU illustrated in  FIG. 1 . 
         FIG. 3  is a front view illustrating an external structure of an ESC illustrated in  FIG. 1 . 
         FIG. 4  is a characteristic diagram showing the relationship between a pedaling force (F) on and a pedal stroke (S) of a brake pedal. 
         FIG. 5  is a flowchart illustrating control processing for adjusting a hydraulic stiffness on a downstream side, which is performed by the controller (second ECU) on the ESC side of the master cylinder. 
         FIG. 6  is a flowchart illustrating control processing for adjusting the hydraulic stiffness on the downstream side according to a second embodiment of the present invention. 
         FIG. 7  is an overall configuration diagram of a brake apparatus to which a brake control apparatus according to a third embodiment of the present invention is applied. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Now, a brake control apparatus according to embodiments of the present invention is specifically described referring to the accompanying drawings, taking a brake apparatus to be mounted in a four-wheeled automobile as an example. 
       FIGS. 1 to 5  illustrate a first embodiment of the present invention. In  FIG. 1 , a left front wheel  1 L, a right front wheel  1 R, a left rear wheel  2 L, and a right rear wheel  2 R are provided to a lower side of a vehicle body (not shown) constructing a body of a vehicle. A front-wheel side wheel cylinder  3 L is provided to the left front wheel  1 L, whereas a front-wheel side wheel cylinder  3 R is provided to the right front wheel  1 R. Similarly, a rear-wheel side wheel cylinder  4 L is provided to the left rear wheel  2 L, whereas a rear-wheel side wheel cylinder  4 R is provided to the right rear wheel  2 R. The wheel cylinders  3 L,  3 R,  4 L, and  4 R are cylinders of a hydraulic disc brake or drum brake. Each of the wheel cylinders  3 L,  3 R,  4 L, and  4 R applies a braking force to each of the wheels (front wheels  1 L and  1 R and rear wheels  2 L and  2 R). 
     A brake pedal  5  is provided on a front side portion of the driver&#39;s seat (not shown) of the vehicle body. The brake pedal  5  is operated by a driver to be pedaled in a direction indicated by the arrow A illustrated in  FIG. 1  at the time of a brake operation for the vehicle. The brake pedal  5  is provided with a brake switch  6  and a brake sensor  7 . 
     Here, the brake switch  6  detects whether or not the brake operation for the vehicle is performed, and outputs a signal for turning on and off a brake lamp (not shown), for example. In this case, the brake switch  6  is connected to a first ECU  26  described later and outputs a brake lamp switch signal (ON/OFF signal) for detecting the depression of the brake pedal  5  to the first ECU  26 . As described later, an ON signal (BSW signal) of the brake lamp switch signal is used as “another start signal” which activates (starts) a system of the first ECU  26 . 
     The brake sensor  7  as an operation-amount detection unit is a stroke sensor which detects a brake operation amount of the brake pedal  5  of the vehicle. Specifically, the brake sensor  7  detects the amount of a pedaling operation on the brake pedal  5  as a stroke amount and outputs the detection signal corresponding to the detected amount of the pedaling operation (the stroke amount) to the first ECU  26  described later. The pedaling operation on the brake pedal  5  is transmitted to a master cylinder  8  via an electric booster  16  described later. The operation-amount detection unit is not limited to the stroke sensor which detects the amount of pedaling operation on the brake pedal  5  as the stroke amount and may also be a pedaling-force sensor for detecting a pedaling force on the brake pedal  5 . Moreover, although the brake sensor  7  is provided to the brake pedal  5  as the stroke sensor, a stroke sensor which detects a stroke of an input piston  19  described later may be used instead. 
     The master cylinder  8  includes a cylinder main body  9  having a cylindrical shape with a closed end. Specifically, the cylinder main body  9  has an open end on one side and a bottom portion on the other side. The open-end side of the cylinder main body  9  is removably firmly fixed to a booster housing  17  of the electric booster  16  described later by using a plurality of mounting bolts (not shown) or the like. The master cylinder  8  includes the cylinder main body  9 , a first piston (including a booster piston  18  and an input piston  19  described later), a second piston  10 , a first hydraulic chamber  11 A, a second hydraulic chamber  11 B, a first return spring  12 , and a second return spring  13 . 
     Here, in the master cylinder %, the first piston includes the booster piston  18  and the input piston  19  described below. The first hydraulic chamber  11 A formed inside the cylinder main body  9  is defined between the second piston  10  and the booster piston  18  (and the input piston  19 ). The second hydraulic chamber  11 B is defined inside the cylinder main body  9  between the bottom portion of the cylinder main body  9  and the second piston  10 . 
     The first return spring  12  is located in the first hydraulic chamber  11 A, and is provided between the booster piston  18  and the second piston  10  to bias the booster piston  18  toward the open-end side of the cylinder main body  9 . The second return spring  13  is located in the second hydraulic chamber  11 B, and is provided between the bottom portion of the cylinder main body  9  and the second piston  10  to bias the second piston  10  toward the first hydraulic chamber  11 A. 
     When the booster piston  18  (input piston  19 ) and the second piston  10  are displaced toward the bottom portion of the cylinder main body  9  in accordance with the pedaling operation of the brake pedal  5 , the cylinder main body  9  of the master cylinder  8  generates a hydraulic pressure as a master-cylinder pressure by a hydraulic fluid (hereinafter referred to as brake fluid) in the first hydraulic chamber  11 A and the second hydraulic chamber  11 B. On the other hand, in the case where the operation of the brake pedal  5  is released, when the booster piston  18  (and the input piston  19 ) and the second piston  10  are displaced by the first return spring  12  and the second return spring  13  toward the opening portion of the cylinder main body  9  in a direction indicated by the arrow B, the cylinder main body  9  of the master cylinder  8  releases the hydraulic pressure in the first hydraulic chamber  11 A and the second hydraulic chamber  11 B while being supplied with the brake fluid from a reservoir  14  (described below). 
     The reservoir  14  which stores the brake fluid therein is provided as a hydraulic fluid tank to the cylinder main body  9  of the master cylinder  8 . The reservoir  14  supplies the brake fluid to the hydraulic chambers  11 A and  11 B inside the cylinder main body  9 . The hydraulic pressure as the master-cylinder pressure generated in the first hydraulic chamber  11 A and the second hydraulic chamber  11 B of the master cylinder  8  is transmitted to an ESC  30  described later, which is a hydraulic-pressure supply device (that is, a hydraulic-pressure control unit), through, for example, a pair of cylinder-side hydraulic pipes  15 A and  15 B. 
     The electric booster  16  is provided as a booster mechanism for increasing an operation force on the brake pedal  5  between the brake pedal  5  of the vehicle and the master cylinder  8 . The electric booster  16  actuates the master cylinder  8  by an electric actuator  20  described later in accordance with the brake operation amount to supply a hydraulic pressure to the wheel cylinders  3 L,  3 P,  4 L, and  4 R. Specifically, the electric booster  16  controls the drive of the electric actuator  20  based on an output signal (a detected signal) from the brake sensor  7  to control the hydraulic pressure generated in the master cylinder  8  (that is, the master-cylinder pressure). 
     The electric booster  16  includes the booster housing  17 , the booster piston  18 , and the electric actuator  20  described later. The booster housing  17  is provided so as to be fixed to a front wall of a vehicle interior (not shown), which is the front board of the vehicle body. The booster piston  18  is provided as a driving piston to the booster housing  17  so as to be movable (that is, movable forward and backward in an axial direction of the master cylinder  8 ). The electric actuator  20  applies a booster thrust to the booster piston  18 . 
     The booster piston  18  is formed of a cylindrical member which is slidably inserted and fitted into the cylinder main body  9  of the master cylinder  8  from the open-end side in the axial direction. On the inner circumferential side of the booster piston  18 , the input piston  19  is slidably inserted and fitted. The input piston  19  is formed of a shaft member which is directly pressed in accordance with the operation of the brake pedal  5  to move forward and backward in the axial direction of the master cylinder  8  (that is, in directions indicated by the arrows A and B). The input piston  19  serves as the first piston of the master cylinder  8  together with the booster piston  18 . Inside the cylinder main body  9 , the first hydraulic chamber  11 A is defined between the second piston  10 , and the booster piston  18  and the input piston  19 . 
     The booster housing  17  includes a speed-reducer case  17 A having a cylindrical shape, a support case  17 B having a cylindrical shape, and a lid body  17 C having a cylindrical shape with a step. The speed-reducer case  17 A houses a speed-reduction mechanism  23  described later therein. The support case  17 B is provided between the speed-reducer case  17 A and the cylinder main body  9  of the master cylinder  8 , and supports the booster piston  18  so that the booster piston  18  is slidably displaceable in the axial direction. The lid body  17 C is provided on the side opposite to the support case  17 B in the axial direction (one axial side) across the speed-reducer case  17 A, and closes an opening of the speed-reducer case  17 A on one axial side. On the outer circumferential side of the speed-reducer case  17 A, a support plate  17  for fixedly supporting a drive motor  21  described later is provided. 
     The input piston  19  as an input member is inserted from the lid body  17 C side into the booster housing  17 , and extends inside the booster piston  18  in the axial direction toward the first hydraulic chamber  11 A. An end surface of the input piston  19  on a distal end side (the other axial side) receives the hydraulic pressure generated in the first hydraulic chamber  11 A at the time of the brake operation as a brake reaction force (hydraulic reaction force). The input piston  19  transmits the generated hydraulic pressure to the brake pedal  5 . As a result, an appropriate pedal feeling is provided to the driver of the vehicle through the brake pedal  5 . Thus, a good pedal feeling (good braking) can be obtained. As a result, an operation feeling for the brake pedal  5  can be improved and a good pedal feeling (firm pedal feeling) can thus be maintained. As described above, in this embodiment, the input piston  19  forms a transmission unit which transmits the hydraulic reaction force generated in accordance with the hydraulic pressure in the master cylinder  8  to the brake pedal  5 . The input piston  19  is included in the electric booster  16 , and has the end surface on the distal end side (the other axial side) which is exposed in the first hydraulic chamber  11 A of the master cylinder  8 . The transmission unit is not limited to the input piston  19 . The hydraulic pressure in the master cylinder  8  may be supplied to a hydraulic cylinder which is provided independently of the electric booster  16 . Beside the mechanism which directly transmits the hydraulic pressure of the master cylinder  8 , the transmission unit may also have a mechanism which applies the reaction force to the brake pedal  5  by the electric actuator  20  that is actuated based on a signal from a hydraulic-pressure sensor  30  described later, that is, transmits the hydraulic reaction force indirectly to the brake pedal  5 . 
     The electric actuator  20  of the electric booster  16  includes the drive motor  21 , the speed-reduction mechanism  23  such as a belt, and a linear-motion mechanism  24  such as a ball screw. The drive motor  21  including an electric motor is provided to the speed-reducer case  17 A of the booster housing  17  through intermediation of the support plate  17 D. The speed-reduction mechanism  23  transmits the rotation of the drive motor  21  to a cylindrical rotary body  22  provided in the speed-reducer case  17 A after reducing the speed of the rotation. The linear-motion mechanism  24  converts the rotation of the cylindrical rotary body  22  into an axial displacement (forward and backward movement) of the booster piston  18 . The booster piston  18  and the input piston  19  each have a front end (the other axial end) exposed in the first hydraulic chamber  11 A of the master cylinder  8 , and generates the brake fluid pressure in the master cylinder  8  by the pedaling force (thrust) transmitted from the brake pedal  5  to the input piston  19  and the booster thrust transmitted from the electric actuator  20  to the booster piston  18 . 
     Specifically, the booster piston  18  of the electric booster  16  forms a pump mechanism which is driven by the electric actuator  20  based on the output (power feeding) from the first ECU  26  described later to generate the brake fluid pressure (master-cylinder pressure) in the master cylinder  8 . A return spring  25  for constantly biasing the booster piston  18  in a direction in which the braking is released (direction indicated by the arrow B illustrated in  FIG. 1 ) is provided inside the support case  17 B of the booster housing  17 . When the drive motor  21  is rotated in a reverse direction at the time of release of the brake operation, the booster piston  18  is returned in the direction indicated by the arrow B to an initial position illustrated in  FIG. 1  and is also returned in the direction indicated by the arrow B by a biasing force of the return spring  25 . 
     The drive motor  21  is formed by using, for example, a DC brushless motor. A rotation sensor  21 A called “resolver” is provided to the drive motor  21 . The rotation sensor  21 A detects a position of rotation of the drive motor  21  (motor shaft), and outputs the detection signal to the first ECU  26  described later. The rotation sensor  21 A also has a function as a rotation detection unit to detect a rotational displacement of the drive motor  21  to detect an absolute displacement of the booster piston  18  with respect to the vehicle body based on the detected rotational displacement. 
     Further, together with the brake sensor  7 , the rotation sensor  21 A constitutes a displacement detection unit for detecting a relative displacement amount between the booster piston  18  and the input piston  19 . The detection signals of the rotation sensor  21 A and the brake sensor  7  are transmitted to the first ECU  26 . The rotation detection unit is not limited to the rotation sensor  21 A such as the resolver, but may also be a rotary potentiometer capable of detecting the absolute displacement (angle). The speed-reduction mechanism  23  is not limited to the belt or the like, and may also be formed by using, for example, a gear speed-reduction mechanism or the like. Further, the speed-reduction mechanism  23  may not be provided, and the cylindrical rotary body  22  may be directly rotated by the drive motor  21 . 
     The first ECU  26  as a master-cylinder pressure control unit includes a microcomputer (CPU)  26 A and a plurality of electronic circuits, as illustrated in  FIG. 2 . The first ECU  26  is a controller (control device) for the electric booster, which electrically controls the drive of the electric actuator  20  of the electric booster  16 . Specifically, as the master-cylinder pressure control unit, the first ECU  26  controls the drive motor  21  for pressurizing the hydraulic fluid in the master cylinder  8  by the operation of the brake pedal  5  to which the hydraulic reaction force is transmitted and thrusts the piston (booster piston  18 ) of the master cylinder  8  by a rotating force of the drive motor  21 . 
     In this case, the first ECU  26  includes an inverter circuit  26 B to be controlled by the CPU  26 A. By current supply from the inverter circuit  26 B, the drive motor  21  is controlled. The first ECU  26  also includes a memory  26 C. In the memory  26 C, a processing program for determining whether or not boost control is required and data for the control are stored. 
     The brake switch  6 , the brake sensor  7 , and the rotation sensor  21 A of the drive motor  21  are connected to the CPU  26 A of the first ECU  26 . The brake switch  6  detects whether or not the brake pedal  5  is operated through an interface circuit (not shown). The brake sensor  7  detects the brake operation amount (the pedaling operation amount of or the pedaling force on the brake pedal  5 ). Moreover, an in-vehicle communication line  27  called L-CAN, for example, through which communication can be performed, is connected to the CPU  26 A through a communication circuit  26 D. The CPU  26 A is also connected to a vehicle data bus  28  through a CAN circuit  26 E. The vehicle data bus  28  is a serial communication network called V-CAN mounted in the vehicle. 
     The first ECU  26  is supplied with power from an in-vehicle battery B through a power supply line  29 . As illustrated in  FIG. 2 , the power from the power supply line  29  is supplied to the inverter circuit  26 B through a fail safe relay  26 F which is subjected to OFF control by the CPU  26 A. The power from the power supply line  29  is supplied to a power supply circuit  26 J through an ECU power supply relay  26 H. The ECU power supply relay  26 H is subjected to ON/OFF control by an activation determination circuit  26 G which is an OR circuit. The power supply circuit  26 J converts a power supply voltage into a voltage for activating the CPU  26 A (for example, converts a 12V vehicle power supply to 5V). From the power supply circuit  26 J, the power is fed to the CPU  26 A, circuits, and sensors. 
     When the ECU power supply relay  26 H is brought into an energized state to start the energization of the CPU  26 A, the system of the first ECU  26  is activated (started). An ignition-ON signal (IGN signal) from an ignition switch, the ON signal of the brake lamp switch signal (BSW signal) from the brake switch  6 , and a wakeup signal from the CAN circuit  26 E are input to the activation determination circuit  26 G which controls the energization of the ECU power supply relay  26 H. By receiving input of any one of the signals described above, the activation determination circuit  26 G controls the ECU power supply relay  26 H so as to be brought into the energized state. 
     Here, the ignition-ON signal is transmitted as a start signal for the vehicle (enables energization) through the signal line when the vehicle is to be activated (started or powered ON). Specifically, when, for example, the driver operates a start button device or a start key device (both not shown) provided in the vicinity of the driver&#39;s seat so as to activate the vehicle, the ignition-ON signal is transmitted to the first ECU  26  and a second ECU  33  described later from the start button device or the start key device described above. As described later, the ignition-ON signal (IGN signal) is a start signal for activating (starting) the vehicle, that is, “one start signal” for activating the system of the first ECU  26  and the system of the second ECU  33 . 
     On the other hand, the ON signal (BSW signal) of the brake lamp signal is “another start signal” for activating (starting) the system of the first ECU  26 . In this case, the system of the first ECU  26  is activated (started) in accordance with the ignition-ON signal for the vehicle as the “one start signal” input through the signal line or the brake lamp switch signal (brake-ON signal) as the “another start signal” input from the brake switch  6  which detects the depression of the brake pedal  5 . 
     The hydraulic-pressure sensor  30  as a pressure detection unit detects the hydraulic pressure generated in the master cylinder  8 . Specifically, the hydraulic-pressure sensor  30  detects a hydraulic pressure in, for example, the cylinder-side hydraulic pipe  15 A and therefore detects a brake fluid pressure supplied from the master cylinder  8  through the cylinder-side hydraulic pipe  15 A to an ESC  31  (hydraulic-pressure control unit) described later. The hydraulic-pressure sensor  30  is supplied with the power from the second ECU  33  described later and is electrically connected to the second ECU  33  so that a detection signal of the hydraulic pressure is output to the second ECU  33 . The detection signal of the hydraulic pressure detected by the hydraulic-pressure sensor  30  is transmitted from the second ECU  33  through the communication line  27  to the first ECU  26  by the communication. 
     The first ECU  26  is connected to the drive motor  21 , the in-vehicle communication line  27 , and the vehicle data bus  28 . Then, the first ECU  26  controls the electric actuator  20  (the rotation of the drive motor  21 ) so as to generate the hydraulic pressure in the master cylinder  8  based on the detection signal from the brake sensor  7  (detection value of the operation of the brake). Specifically, the first ECU  26  variably controls the brake fluid pressure to be generated in the master cylinder  8  by the electric booster  16  in accordance with the detection signals from the brake sensor  7  and the hydraulic-pressure sensor  30 , and also determines whether or not the electric booster  16  is operating normally. 
     Here, in the electric booster  16 , when the brake pedal  5  is operated, the input piston  19  moves forward toward the cylinder main body  9  of the master cylinder  8  and the movement of the input piston  19  is detected by the brake sensor  7 . In response to the detection signal from the brake sensor  7 , the first ECU  26  feeds power to the drive motor  21  to rotationally drive the drive motor  21 . The rotation of the drive motor  21  is transmitted to the cylindrical rotary body  22  through an intermediation of the speed-reduction mechanism  23 . Then, the rotation of the cylindrical rotary body  22  is converted into the axial displacement of the booster piston  18  by the linear-motion mechanism  24 . 
     In this manner, the booster piston  18  displaces in the forward direction into the cylinder main body  9  of the master cylinder  8 . As a result, the brake fluid pressure in accordance with the pedaling force (thrust) applied to the input piston  19  from the brake pedal  5  and a booster thrust applied to the booster piston  18  from the electric actuator  20  is generated in the first hydraulic chamber  11 A and the second hydraulic chamber  11 B in the master cylinder  8 . By receiving the detection signal from the hydraulic-pressure sensor  30  via the signal line  27 , the first ECU  26  can monitor the hydraulic pressure generated in the master cylinder  8 , and therefore can determine whether or not the electric booster  16  is operating normally. 
     The hydraulic-pressure supply device  31  (also referred to as “ESC  31 ”) as the hydraulic-pressure control unit, which is provided between the wheel cylinders  3 L,  3 R,  4 L, and  4 R provided on the respective wheels (front wheels  1 L and  1 R and rear wheels  2 L and  2 R) of the vehicle, and the master cylinder  8  is now described. 
     The ESC  31  as the hydraulic-pressure control unit is provided between the master cylinder  8  and the wheel cylinders  3 L,  3 R,  4 L, and  4 R, and supplies and stops the brake fluid to the wheel cylinders  3 L,  3 R,  4 L, and  4 R. Specifically, the ESC  31  supplies the hydraulic pressure generated in the master cylinder  8  (the first hydraulic chamber  11 A and the second hydraulic chamber  11 B) as the master-cylinder pressure by the electric booster  16  individually to the wheel cylinders  3 L,  3 R,  4 L, and  4 R for the respective wheels. 
     More specifically, the ESC  31  constitutes a brake assist apparatus. When the brake fluid pressure to be supplied from the master cylinder  8  through the cylinder-side hydraulic pipes  15 A and  15 B to the wheel cylinders  3 L,  3 R,  4 L, and  4 R is insufficient or various types of brake control (for example, braking-force distribution control for distributing a braking force to the front wheels  1 L and  1 R and the rear wheels  2 L and  2 R, anti-lock brake control, vehicle stabilization control, and the like) are performed, the ESC  31  supplies a necessary brake fluid pressure obtained by compensation to the wheel cylinders  3 L,  3 R,  4 L, and  4 R. 
     The ESC  31  distributes and supplies the hydraulic pressure output from the master cylinder  8  (first hydraulic chamber  11 A and second hydraulic chamber  11 B) through the cylinder-side hydraulic pipes  15 A and  15 B to the wheels cylinders  3 L,  3 R,  4 L, and  4 R through brake-side pipe portions  32 A,  32 B,  32 C, and  32 D. In this manner, for the front wheels  1 L and  1 R and the rear wheels  2 L and  2 R, the independent braking force is applied to each of the wheels as described above. The ESC  31  includes control valves  39 ,  39 ′,  40 ,  40 ′,  41 ,  41 ′,  44 ,  44 ′,  45 ,  45 ′,  52 , and  52 ′, and an electric motor  47  for driving hydraulic pumps  46  and  46 ′. 
     A wheel-cylinder fluid supply control unit includes the ESC  31  and the second ECU  33 . The ESC  31  is provided between the master cylinder  8  and the wheel cylinders  3 L,  3 R,  4 L, and  4 R and is the hydraulic-pressure control unit for controlling the communication and interruption of fluid paths by electromagnetic valves (that is, controls valves  39 ,  39 ′,  40 ,  40 ′,  41 ,  41 ′,  44 ,  44 ′,  45 ,  45 ′,  52 , and  52 ′). The second ECU  33  is a controller for the ESC  31 . 
     The second ECU  33  as the wheel-cylinder fluid supply control unit controls the actuation of the ESC  31  as the hydraulic-pressure control unit. Specifically, similarly to the first ECU  26 , the second ECU  33  is the controller (control device) for the hydraulic-pressure supply device, for electrically controlling the drive of the ESC  31 . The second ECU  33  includes a microcomputer (CPU)  33 A and a plurality of electronic circuits as illustrated in  FIG. 2 . In this case, the second ECU  33  includes a memory  33 B. In the memory  33 B, a control processing program is stored, which is used for performing control for supplying and control for stopping the brake fluid to the wheel cylinders  3 L,  3 R,  4 L, and  4 R described later, which are illustrated in  FIG. 5 . 
     The hydraulic-pressure sensor  30 , wheel-speed sensors  34  described later, the control valves  39 ,  39 ′,  40 ,  40 ′,  41 ,  41 ′,  44 ,  44 ′,  45 ,  45 ′,  52 , and  52 ′, and the electric motor  47  are connected to the CPU  33 A of the second ECU  33  through an intermediation of an interface circuit (not shown). The communication line  27  (L-CAN) is connected through a communication circuit  33 C to the CPU  33 A of the second ECU  33 , while the vehicle data bus  28  (V-CAN) is connected through a CAN circuit  33 D thereto. 
     The second ECU  33  is connected to the power supply line  29  and fed with the power from the battery B through the power supply line  29 . More specifically, as illustrated in  FIG. 2 , the power from the power supply line  29  is supplied to a power supply circuit  33 F for converting the power supply voltage to a voltage for actuating the CPU  33 A (for example, a 12V vehicle power supply to 5V) through an ECU power supply relay  33 E. Then, the power is fed from the power supply circuit  33 F to the CPU  33 A, the circuits, the hydraulic-pressure sensor  30 , and other sensors. When the ECU power supply relay  33 E is brought into the energized state to start the energization of the CPU  33 A, the system of the second ECU  33  is activated (started). The ignition-ON signal (IGN signal) is input from the ignition switch to the ECU power supply relay  33 E. By receiving the input (energization) of the ignition-ON signal (IGN signal), the ECU power supply relay  33 E is placed in the energized state. 
     Here, the ignition-ON signal is transmitted as the start signal for the vehicle (enables energization) through the signal line when the vehicle is to be activated (started or powered ON). Specifically, when, for example, the driver operates the start button device or the start key device (both not shown) in the vicinity of the driver&#39;s seat so as to activate the vehicle, the ignition-ON signal is transmitted to (enables energization of) the first ECU  26  and the second ECU  33  from the start button device or the start key device described above. In this case, the ignition-ON signal (IGN signal) corresponds to the start signal for activating (starting) the vehicle, that is, the “one start signal” for activating the system of the first ECU  26  and the system of the second ECU  33 . 
     Further, the wheel-speed sensors  34  (four sensors in total in  FIG. 1 ) for individually detecting rotation speeds (wheel speeds) of the front wheels  1 L and  1 R and the rear wheels  2 L and  2 R are connected to the second ECU  33 . The second ECU  33  performs necessary control such as anti-lock brake control for preventing each of the front wheels  1 L and  1 R and the rear wheels  2 L and  2 R from being locked in accordance with detection values (detection signals) from the respective wheel-speed sensors  34 . 
     In the first embodiment, the hydraulic-pressure sensor  30  as the pressure detection unit is connected to the second ECU  33 , as illustrated in  FIG. 1 . However, the configuration of this embodiment is not limited thereto. The brake sensor  7  as the operation-amount detection unit may alternatively be connected to the second ECU  33 , as indicated by the dotted line L in  FIG. 1 . In this case, the brake sensor  7  can be connected to the second ECU  33  directly or through a controller (not shown) different from the first ECU  26 . In any of the cases, the hydraulic-pressure sensor  30  as the pressure detection unit and the brake sensor  7  as the operation-amount detection unit are connected to the second ECU  33 . 
     The second ECU  33  individually controls the drive of the control valves  39 ,  39 ′,  40 ,  40 ′,  41 ,  41 ′,  44 ,  44 ′,  45 ,  45 ′,  52 , and  52 ′ and the electric motor  47  of the ESC  31  as described later. In this manner, the second ECU  33  performs control of reducing, maintaining, boosting, or pressurizing the brake fluid pressures to be supplied from the brake-side pipe portions  32 A to  32 D to the wheel cylinders  3 L,  3 R,  4 L, and  4 R individually for the wheel cylinders  3 L,  3 R,  4 L, and  4 R. 
     Specifically, by controlling the actuation of the ESC  31 , the second ECU  33  can perform, for example, control (1) to (8) described below. More specifically, the second ECU  33  can perform (1) braking-force distribution control for appropriately distributing a braking force to the respective wheels ( 1 L,  1 R,  2 L, and  2 R) in accordance with a vertical load at the wheel when the vehicle is to be braked; (2) anti-lock brake control for automatically adjusting the braking force to be applied to each of the wheels ( 1 L,  1 R,  2 L, and  2 R) at the time of braking to prevent the front wheels  1 L and  1 R and the rear wheels  2 L and  2 R from being locked; (3) vehicle stabilization control for suppressing understeering and oversteering while detecting a skid of each of the wheels ( 1 L,  1 R,  2 L, and  2 R) during running to automatically control appropriately the braking force to be applied to each of the wheels ( 1 L,  1 R,  2 L, and  2 F) regardless of the operation amount of the brake pedal  5  so as to stabilize a behavior of the vehicle; (4) hill start aid control for retaining a braked state on a hill (in particular, an uphill) to assist the vehicle in starting; (5) traction control for preventing each of the wheels ( 1 L,  1 R,  2 L, and  2 R) from idling at the start of the vehicle or the like; (6) vehicle tracking control for maintaining a certain distance from a vehicle in front; (7) lane departure avoiding control for keeping the vehicle in a driving lane; and (8) obstacle avoidance control for avoiding a collision against an obstacle in front of or behind the vehicle. 
     The ESC  31  as the hydraulic-pressure control unit includes a housing  56  described below ( FIG. 3 ) which forms an outer shell therefor. In the housing  56 , a dual-system hydraulic circuit including a first hydraulic system  35  and a second hydraulic system  35 ′ is provided. The first hydraulic system  35  is connected to one (that is, the cylinder-side hydraulic pipe  15 A) of output ports of the master cylinder  8  to supply the hydraulic pressure to the wheel cylinder  3 L for the left front wheel (FL) and the wheel cylinder  4 R for the right rear wheel (RR). The second hydraulic system  35 ′ is connected to another (that is, the cylinder-side hydraulic pipe  15 B) of the output ports to supply the hydraulic pressure to the wheel cylinder  3 R for the right front wheel (FR) and the wheel cylinder  4 L for the left rear wheel (RL). 
     Here, the first hydraulic system  35  and the second hydraulic system  35 ′ have the same configuration. Therefore, only the first hydraulic system  35  is described below. For the second hydraulic system  35 ′, the reference symbols of the respective components are followed by the apostrophe “′”, and the description thereof is herein omitted. 
     The first hydraulic system  35  of the ESC  31  includes a brake pipeline  36  connected to a distal end of the cylinder-side hydraulic pipe  15 A. The brake pipeline  36  branches into a first pipeline portion  37  and a second pipeline portion  38 , which are respectively connected to the wheel cylinders  3 L and  4 R. The brake pipeline  36  and the first pipeline portion  37  constitute a pipeline for supplying the hydraulic pressure to the wheel cylinder  3 L together with the brake-side pipeline portion  32 A, whereas the brake pipeline  36  and the second pipeline portion  38  constitute a pipeline for supplying the hydraulic pressure to the wheel cylinder  4 R together with the brake-side pipeline portion  32 D. 
     The brake fluid-pressure supply control valve  39  (hereinafter referred to simply as “supply control valve  39 ”) is provided to the brake pipeline  36  so as to be parallel to a check valve  53  described later. The supply control valve  39  is a normally-open electromagnetic selector valve for opening and closing the brake pipeline  36 . A boost control, valve  40  is provided to the first pipeline portion  37 . The boost control valve  40  is a normally-open electromagnetic selector valve for opening and closing the first pipeline portion  37 . A boost control valve  41  is provided to the second pipeline portion  38 . The boost control valve  41  is a normally-open electromagnetic valve for opening and closing the second pipeline portion  38  as well. 
     On the other hand, the first hydraulic system  35  of the ESC  31  includes a first pressure-reduction pipeline  42  for connecting the wheel cylinder  3 L side and a reservoir  51  for hydraulic-pressure control and a second pressure-reduction pipeline  43  for connecting the wheel cylinder  4 R side and the reservoir  51 . A first pressure-reduction control valve  44  is provided to the first pressure-reduction pipeline  42 , whereas a second pressure-reduction control valve  45  is provided to the second pressure-reduction pipeline  43 . The first pressure-reduction control valve  44  is a normally-closed electromagnetic selector valve for opening and closing the first pressure-reduction pipeline  42 . Similarly, the second pressure-reduction control valve  45  is a normally-closed electromagnetic selector valve for opening and closing the second pressure-reduction pipeline  43 . 
     The ESC  31  includes the hydraulic pump  46  including a plunger pump as a hydraulic-pressure generation unit which is a hydraulic-pressure source. The hydraulic pump  46  is rotationally driven by the electric motor  47 . The electric motor  47  is driven by power fed from the second ECU  33 . When the power feeding is stopped, the rotation of the electric motor  47  is stopped with the stop of the rotation of the hydraulic pump  46 . A discharge side of the hydraulic pump  46  is connected to a portion of the brake pipeline  36 , which is located on the downstream side of the supply control valve  39  (that is, at a position at which the first pipeline portion  37  and the second pipeline portion  38  branch) through a check valve  48 . An intake side of the hydraulic pump  46  is connected to the reservoir  51  for hydraulic-pressure control through check valves  49  and  50 . 
     The reservoir  51  for hydraulic-pressure control is provided to temporarily store an excessive brake fluid. The reservoir  51  for hydraulic-pressure control temporarily stores the excessive brake fluid flowing out from cylinder chambers (not shown) of the wheel cylinders  3 L and  4 R not only at the time of ABS control for the brake system (ESC  31 ) but also at the time of other brake control. The intake side of the hydraulic pump  46  is connected to the cylinder-side hydraulic pipe  15 A of the master cylinder  8  (that is, to a portion of the brake pipeline  36 , which is located on the upstream side of the supply control valve  39 ) through the check valve  49  and a pressurization control valve  52  which is a normally-closed electromagnetic selector valve. 
     The check valve  53  is provided in the middle of the brake pipeline  36  so as to be parallel to the supply control valve  39 . The check valve  53  allows the brake fluid to flow from the master cylinder  8  side into the brake pipeline  36  and inhibits a flow in the opposite direction. A check valve  54  is provided to the first pipeline portion  37  so as to be parallel to the boost control valve  40 . The check valve  54  allows the brake fluid to flow from the wheel cylinder  3 L side into the first pipeline portion  37  and inhibits a flow in the opposite direction. Further, a check valve  55  is provided to the second pipeline portion  38  so as to be parallel to the boost control valve  41 . The check valve  55  allows the brake fluid to flow from the wheel cylinder  4 R side into the second pipeline portion  38  and inhibits a flow in the opposite direction. 
     For each of the control valves  39 ,  39 ′,  40 ,  40 ′,  41 ,  41 ′,  44 ,  44 ′,  45 ,  45 ′,  52 , and  52 ′ and the electric motor  47  (motor for driving the hydraulic pumps  46  and  46 ′) that constitute the ESC  31 , operation control is performed in a predetermined procedure in accordance with power fed from the second ECU  33 . 
     Specifically, the first hydraulic system  35  of the ESC  31  directly supplies the hydraulic pressure generated in the master cylinder  8  by the electric booster  16  to the wheel cylinders  3 L and  4 R through the brake pipeline  36 , the first pipeline portion  37 , and the second pipeline portion  38  at the time of a normal operation based on the brake operation performed by the driver. For example, when antiskid control is to be executed, the boost control valves  40  and  41  are closed to maintain the hydraulic pressure in the wheel cylinders  3 L and  4 R. When the hydraulic pressure in the wheel cylinders  3 L and  4 R is to be reduced, the pressure-reduction control valves  44  and  45  are opened so that the hydraulic pressure in the wheel cylinders  3 L and  4 R is exhausted to be released to the reservoir  51  for hydraulic-pressure control. 
     When the hydraulic pressure to be supplied to the wheel cylinders  3 L and  4 R is to be boosted for stabilization control (antiskid control) during running of the vehicle, the hydraulic pump  46  is actuated by the electric motor  47  in a state in which the supply control valve  39  is closed. In this manner, the brake fluid discharged from the hydraulic pump  46  is supplied to the wheel cylinders  3 L and  4 R through the first pipeline portion  37  and the second pipeline portion  38 , respectively. At this time, the pressurization control valve  52  is opened. As a result, the brake fluid stored in the reservoir  14  is supplied from the master cylinder  8  side to the intake side of the hydraulic pump  46 . 
     As described above, the second ECU  33  controls the actuation of the supply control valve  39 , the boost control valves  40  and  41 , the pressure-reduction control valves  44  and  45 , the pressurization control valve  52 , and the electric motor  47  (that is, the hydraulic pump  46 ) based on vehicle operation information so as to appropriately maintain, reduce, or boost the hydraulic pressure to be supplied to the wheel cylinders  3 L and  4 R. As a result, the above-mentioned brake control such as the braking-force distribution control, the vehicle stabilization control, the brake assist control, the antiskid control, the traction control, and the hill start aid control is executed. 
     On the other hand, in a normal braking mode which is executed in a state in which the electric motor  47  (that is, the hydraulic pump  46 ) is stopped, the supply control valve  39  and the boost control valves  40  and  41  are opened, whereas the pressure-reduction valves  44  and  45  and the pressurization control valve  52  are closed. In this state, when the first piston (that is, the booster piston  18  and the input piston  19 ) and the second piston  10  of the master cylinder  8  displace in the axial direction inside the cylinder main body  9  in accordance with the pedaling operation of the brake pedal  5 , the brake fluid pressure generated in the first hydraulic chamber  11 A is supplied from the cylinder-side hydraulic pipe  15 A side through the first hydraulic system  35  and the brake-side pipe portions  32 A and  32 D of the ESC  31  to the wheel cylinders  3 L and  4 R. The brake fluid pressure generated in the second hydraulic chamber  11 B is supplied from the cylinder-side hydraulic pipe  15 B side through the second hydraulic system  35 ′ and the brake-side pipe portions  32 B and  32 C to the wheel cylinders  3 R and  4 L. 
     In a brake assist mode which is executed when the brake fluid pressure generated in the first hydraulic chamber  11 A and the second hydraulic chamber  11 B (that is, the hydraulic pressure in the cylinder-side hydraulic pipe  15 A, which is detected by the hydraulic-pressure sensor  30 ) is insufficient, the pressurization control valve  52  and the boost control valves  40  and  41  are opened, while the supply control valve  39  and the pressure-reduction control valves  44  and  45  are appropriately opened and closed. In this state, the hydraulic pump  46  is actuated by the electric motor  47  so that the brake fluid discharged from the hydraulic pump  46  is supplied to the wheel cylinders  3 L and  4 R through the first pipeline portion  37  and the second pipeline portion  38 , respectively, in this manner, together with the brake fluid pressure generated on the master cylinder  8  side, the braking force by the wheel cylinders  3 L and  4 R can be generated by the brake fluid discharged from the hydraulic pump  46 . 
     Further, in the case of failure of the electric booster  16 , the hydraulic pump  46  can be actuated by the electric motor  47  based on the detection signal from the hydraulic-pressure sensor  30  (or the detection signal from the brake sensor  7  when the brake sensor  7  is connected to the second ECU  33 ), which changes in accordance with the operation of the brake by the driver. By the brake fluid discharged from the hydraulic pumps  46  and  46 ′, the wheel cylinders  3 L,  3 R,  4 L, and  4 R can be pressurized (hereinafter described as “wheel cylinders are boosted” for the purpose of illustration). 
     A known hydraulic pump, such as a plunger pump, a trochoid pump, and a gear pump can be used as the hydraulic pump  46 . In the first embodiment, the plunger pump is used as illustrated in  FIG. 3 , for example. A known motor, such as a DC motor, a DC brushless motor, and an AC motor can be used as the electric motor  47 . In this embodiment, the DC motor is used in view of adaptability to vehicle installation. 
     Characteristics of the control valves  39 ,  40 ,  41 ,  44 ,  45 , and  52  of the ESC  31  can be appropriately set in accordance with a mode of use of each of the control valves. Among the above-mentioned control valves, the supply control valve  39  and the boost control valves  40  and  41  are configured as the normally-open valves, whereas the pressure-reduction control valves  44  and  45  and the pressurization control valve  52  are configured as the normally-closed valves. As a result, even when no power is fed from the second ECU  33 , the hydraulic pressure can be supplied from the master cylinder  8  to the wheel cylinders  3 L,  3 R,  4 L, and  4 R. Therefore, in view of fail safe and control efficiency of the brake apparatus, the use of the above-mentioned configuration is desired. 
     As illustrated in  FIG. 3 , the housing  56 , which forms the outer shell for the hydraulic-pressure control unit (ESC  31 ), is formed to have a cuboidal block structure by a molding unit such as aluminum die casting. The housing  56  includes an upper side surface  56 A, a lower side surface  56 B, a right side surface  56 C, and a left side surface  56 D. In order to reduce the housing  56  in size, the electromagnetic valves (that is, the control valves  39 ,  39 ′,  40 ,  40 ′,  41 ,  41 ′,  44 ,  44 ′,  45 ,  45 ′,  52 , and  52 ′) are arranged in a distributed manner with the hydraulic pumps  46  and  46 ′ which are plunger pumps being provided thereamong. 
     Specifically, in the housing  56 , the boost control valves  40 ,  40 ′,  41 , and  41 ′ and the pressure-reduction control valves  44 ,  44 ′,  45 , and  45 ′ are provided above the plunger pumps (hydraulic pumps  46  and  46 ′), whereas the supply control valves  39  and  39 ′ and the pressurization control valves  52  and  52 ′ are provided below the hydraulic pumps  46  and  46 ′. The boost control valve  40 , which is connected to the wheel cylinder  3 L for the front wheel  1 L through the brake-side pipe portion  32 A, is provided at a position close to the side surface  56 C which is an outer side surface of the housing  56 . Similarly, the boost control valve  40 ′, which is connected to the wheel cylinder  3 P for the front wheel  1 R through the brake-side pipe portion  32 B, is provided at a position close to the side surface  56 D which is an outer side surface of the housing  56 . 
     On the other hand, as illustrated in  FIG. 1 , a regenerative cooperation control device  57  for power charge is connected to the vehicle data bus  28  mounted in the vehicle. The regenerative cooperation control device  57  is a microcomputer or the like as in the case of the first ECU  26  and the second ECU  33 . The regenerative cooperation control device  57  uses an inertial force generated by the rotation of the wheels to control a drive motor (not shown) for driving the vehicle when the vehicle decelerates or is braked, thereby obtaining the braking force while recovering kinetic energy as power. The regenerative cooperation control device  57  is connected to the first ECU  26  and the second ECU  33  through the vehicle data bus  28 . The regenerative cooperation control device  57  is connected to the power supply line  29  to be supplied with the power from the battery B (see  FIG. 2 ) through the power supply line  29 . 
     The brake apparatus including the brake control apparatus according to the first embodiment has the configuration described above. The actuation of the brake apparatus is now described. 
     First, when the driver of the vehicle performs the pedaling operation of the brake pedal  5 , the input piston  19  is pressed in the direction indicated by the arrow A. At the same time, the detection signal from the brake sensor  7  is input to the first ECU  26 . The first ECU  26  controls the actuation of the electric actuator  20  of the electric booster  16  in accordance with the detection value of the detection signal from the brake sensor  7 . Specifically, the first ECU  26  feeds the power to the drive motor  21  based on the detection signal from the brake sensor  7 , thereby rotationally driving the drive motor  21 . 
     The rotation of the drive motor  21  is transmitted to the cylindrical rotary body  22  through an intermediation of the speed-reduction mechanism  23 , and the rotation of the cylindrical rotary body  22  is converted into an axial displacement of the booster piston  18  by the linear-motion mechanism  24 . As a result, the booster piston  18  of the electric booster  16  is displaced in the forward direction to move into the cylinder main body  9  of the master cylinder  8 . As a result, the brake fluid pressure in accordance with the pedaling force (thrust) applied from the brake pedal  5  to the input piston  19  and the booster thrust applied from the electric actuator  20  to the booster piston  18  is generated in the first hydraulic chamber  11 A and the second hydraulic chamber  11 B of the master cylinder  8 . 
     Next, the ESC  31 , which is provided between the wheel cylinders  3 L,  3 R,  4 L, and  4 P for the respective wheels (the front wheels  1 L and  1 R and the rear wheels  2 L and  2 R) and the master cylinder  8 , variably controls the hydraulic pressure from the cylinder-side hydraulic pipes  15 A and  15 B through the hydraulic systems  35  and  35 ′ and the brake-side pipe portions  32 A,  32 B,  32 C, and  32 D included in the ESC  31  to the wheel cylinders  3 L,  3 R,  4 L, and  4 R. At the same time, the ESC  31  distributes the hydraulic pressure as the master cylinder pressure generated in the master cylinder  8  (the first hydraulic chamber  11 A and the second hydraulic chamber  11 B) by the electric booster  16  into wheel-cylinder pressures for the respective wheels to be supplied thereto. In this manner, appropriate braking forces are individually applied to the wheels (the front wheels  1 L and  1 R and the rear wheels  2 L and  2 R) of the vehicle through the wheel cylinders  3 L,  3 R,  4 L, and  4 R. 
     The second ECU  33  for controlling the ESC  31  feeds the power to the electric motor  47  to actuate the hydraulic pumps  46  and  46 ′ so as to selectively open and close the control valves  39 ,  39 ′,  40 ,  40 ′,  41 ,  41 ′,  44 ,  44 ′,  45 ,  45 ′,  52 , and  52 ′. In this manner, the braking-force distribution control, the anti-lock brake control, the vehicle stabilization control, the hill start aid control, the traction control, the vehicle tracking control, the lane departure avoiding control, and the obstacle avoidance control can be executed. 
     The following problem sometimes occurs in the brake apparatus including the electric booster  16 . Specifically, when the driver depresses the brake pedal  5 , the drive motor  21  thrusts the booster piston  18  in the direction indicated by the arrow A in  FIG. 1  as a result of the forward movement of the input piston  19 . Then, the hydraulic pressure in the master cylinder  8  increases at an approximately constant boost ratio in accordance with the operation amount of the brake pedal  5 . In this case, the relationship between an operation amount S of the brake pedal  5  and a pedaling force F (that is, a pedal reaction force) thereon can be represented as a characteristic line  58  indicated by the solid line in  FIG. 4 . 
     When the driving force (output) of the drive motor  21  reaches a maximum driving force and hence, the thrust of the booster piston  18  and the reaction force generated by the hydraulic pressure in the master cylinder  8  are balanced with each other, the drive motor  21  comes into a full-load state to stop the booster piston  18 . As a result, the booster piston  18  cannot move forward any more (in a state where the operation amount S of the brake pedal  5  becomes an operation amount S1 and the pedaling force F becomes a pedaling force F1 in  FIG. 4 ). When the vehicle is running, the driver does not perform a large amount of the pedaling operation on the brake pedal  5  in practice to achieve a deceleration to bring about the full-load state in which the driving force of the drive motor  21  becomes maximum. For example, when ABS control is actuated by the ESC  31 , the ABS control is started before the driving force of the drive motor  21  becomes maximum. Therefore, the drive motor  21  does not come into the full-load state. 
     However, when the pedaling operation of the brake pedal  5  is performed when the vehicle is in the stopped state, the ABS control is not actuated by the ESC  31  and therefore no deceleration is generated. Thus, an excessive pedaling operation of the brake pedal  5  can be performed beyond a position at which the drive motor  21  comes into the full-load state. Therefore, when the driver further depresses the brake pedal  5  by an operation amount equal to or larger than the operation amount S1 although the booster piston  18  is stopped under the full-load state, only the input piston  19  moves forward. Therefore, the input piston  19  comes into contact with the booster piston  18  which is in a stopped state. In this case, the relationship between the operation amount S of the brake pedal  5  and the pedaling force F (that is, the pedal reaction force) abruptly changes with a so-called “spongy pedal, feeling” as represented by a characteristic line  58 A indicated by the chain double-dashed line in  FIG. 4 . The “spongy pedal feeling” refers to a state in which a pedal stroke is made even with a small change in pedaling force. When the operation amount S becomes an operation amount S2 with which the input piston  19  comes into contact with the booster piston  18  in the stopped state, the driver has a weird pedal feeling as if the brake pedal  5  were suddenly blocked. 
     Therefore, in order to solve the problem described above, control processing illustrated in  FIG. 5  is performed by using the second ECU  33  which is the controller for the hydraulic-pressure control unit (ESC  31 ) in the first embodiment. By the control processing, it is possible to suppress a reaction-force change which is caused when the drive motor  21  comes into the full-load state without lowering the output hydraulic pressure generated by the operation of the pedal. 
     Specifically, after the control processing illustrated in  FIG. 5  starts, whether or not the pedaling operation of the brake pedal  5  is being performed is determined based on the detection signal from the brake sensor  7  (or the brake switch  6 ) in Step  1 . While it is determined as “NO” in Step  1 , the pedaling operation of the brake pedal  5  is not being performed. Therefore, the processing remains in Step  1  in a waiting state. When it is determined as “YES” in Step  1 , the pedaling operation of the brake pedal  5  is being operated. Therefore, the processing proceeds to subsequent Step  2  where the pedaling operation amount S of the brake pedal  5  is calculated based on the detection signal from the brake sensor  7 . 
     In subsequent Step  3 , a necessary motor current is calculated based on the pedaling operation amount S calculated in Step  2 . Specifically, the current value necessary for rotationally driving the drive motor  21  is calculated so that a movement amount of the booster piston  18  becomes a movement amount corresponding to the pedaling operation amount S of the brake pedal  5  when the drive motor  21  is rotationally driven to move the booster piston  18  into the cylinder main body  9  of the master cylinder  8 . 
     In subsequent Step  4 , whether or not the calculated value of the necessary motor current is larger than a predetermined value (for example, a current value at which the driving force of the drive motor  21  reaches the maximum driving force) is determined. The predetermined value in this case is set to a magnitude (value) at which, for example, the booster thrust to be applied from the electric actuator  20  to the booster piston  18  by the drive motor  21  rotationally driven to become a force corresponding to the pedaling force F1 shown in  FIG. 4 . The predetermined value cannot be achieved while the vehicle is running in the case where the drive motor  21  normally operates. In other words, when the brake pedal  5  is depressed by the amount corresponding to the predetermined value or larger, the ABS control works to stop the rotation of the drive motor  21 . Therefore, while the vehicle is running on a road, the motor current does not become as large as the predetermined value. 
     When it is determined as “NO” in Step  4 , the driving force of the drive motor  21  does not reach the maximum driving force yet (the driving motor  21  does not come into the full-load state shown in  FIG. 4  yet). Thus, the processing proceeds to subsequent Step  5  where it is determined whether or not a valve-closing command has been output to the boost control valves  40  and  40 ′ on the wheels FL and FR (front wheels  1 L and  1 R) among the boost control valves  40 ,  40 ′,  41 , and  41 ′ of the ESC  31 . Whether or not the valve-closing command has been output may also be determined based on the hydraulic pressure or the pedal stroke in place of the current value output to the boost control valves  40  and  40 ′. 
     When it is determined that the valve-closing command has not been output in Step  5 , the processing proceeds to subsequent Step  6  where normal brake control is performed. Specifically, in Step  6 , the electric booster  16  is actuated in accordance with the pedaling operation performed on the brake pedal  5  so as to increase or reduce the hydraulic pressure in the master cylinder  8  at a predetermined boost ratio in accordance with the operation amount of the brake pedal  5 . In this manner, the braking force is applied to the vehicle by the wheel cylinders  3 L,  3 R,  41 , and  4 R for the respective wheels. At this time, the relationship between the operation amount S of the brake pedal  5  and the pedaling force F (that is, the pedal reaction force) can be represented as the characteristic line  58  indicated by the solid line in  FIG. 4 . 
     Moreover, by controlling the actuation of the ESC  31  as needed, the braking-force distribution control, the anti-lock brake control, and the like can be executed. At this time, the second ECU  33  feeds the power to the electric motor  47  to actuate the hydraulic pumps  46  and  46 ′. As a result, the control valves  39 ,  39 ′,  40 ,  40 ′,  41 ,  41 ′,  44 ,  44 ′,  45 ,  45 ′,  52 , and  52 ′ can be selectively opened and closed. Then, the processing returns in Step  7  to perform the control processing which starts in Step  1  again. 
     On the other hand, the case where it is determined that the valve-closing commands has been output in Step  5  corresponds to the following case. Specifically, for example, in a state where the valve-closing command is output in Step  10  described later, the processing returns in Step  7 . Then, after the processing in Steps  1  to  5  is performed, the processing proceeds to Step  8 . Therefore, in Step  8 , a valve-opening command (specifically, a command to open the boost control valves  40  and  40 ′) is output after the output of the above-mentioned valve-closing command is stopped. Thereafter, the processing in Step  6  and subsequent steps is performed. 
     When it is determined as “YES” in Step  4 , the driving force of the drive motor  21  has reached the maximum driving force (the drive motor  21  is in the full-load state shown in  FIG. 4 ). Therefore, the processing proceeds to subsequent Step  9  where whether or not the vehicle is in a stopped state is determined. For example, based on the detection signals output from the wheel-speed sensors  34  (four sensors in total are illustrated in  FIG. 1 ), whether or not the vehicle is in the stopped state can be determined. 
     When it is determined as “YES” in Step  9 , the vehicle is in the stopped state. Therefore, the processing proceeds to subsequent Step  10  where the valve-closing command is output to, for example, the boost control valve  40  for the left front wheel FL (front wheel  1 L) and the boost control valve  40 ′ for the right front wheel FR (front wheel  1 R). In this manner, the hydraulic pressure is not supplied to the wheel cylinder  3 L for the front wheel  1 L and the wheel cylinder  3 R for the front wheel  1 R among the wheel cylinders  3 L,  3 R,  4 L, and  4 R for the respective wheels (the front wheels  1 L and  1 R and the rear wheels  2 L and  2 R) of the vehicle. Thus, the hydraulic pressure is supplied only to the wheel cylinder  4 L for the rear wheel  2 L and the wheel cylinder  4 R for the rear wheel  2 R. 
     Therefore, when the brake pedal  5  is depressed by a large amount while the vehicle is in the stopped state, that is, when the brake pedal  5  is depressed by the amount equal to or larger than the operation amount S1 although the drive motor  21  is in the full-load state to keep the booster piston  18  in the stopped state as represented by the characteristic line  58  shown in  FIG. 4 , the relationship between the operation amount S of the brake pedal  5  and the pedaling force F (that is, the pedal reaction force) changes as represented by a characteristic line  58 B indicated by the solid line in  FIG. 4 . Thus, an abrupt change as represented by the characteristic line  58 A indicated by the chain double-dashed line in  FIG. 4  can be suppressed. Thus, the so-called spongy pedal feeling can be suppressed. 
     Specifically, in this case, by the processing in Step  10 , the hydraulic pressure is not supplied to the wheel cylinder  3 L for the front wheel  1 L and to the wheel cylinder  3 R for the front wheel  1 R. Thus, the hydraulic pressure is supplied only to the wheel cylinder  4 L for the rear wheel.  2 L and the wheel cylinder  4 R for the rear wheel  2 R. Therefore, a hydraulic stiffness on the downstream side of the master cylinder can be increased. In other words, the driver who is depressing the brake pedal  5  can have a sufficiently firm pedal feeling (that is, a sufficiently large pedal reaction force generated by the pedaling force F) over a period in which the input piston  19  is moved by the operation amount S2 to reach a position at which the input piston  19  comes into contact with the booster piston  18  in the stopped state. As a result, the driver does not have a weird feeling for the pedal operation. 
     In Step  9 , the determination as “NO” is hardly made in practice. However, when it is determined as “NO” in Step  9 , the determination as “NO” means that the vehicle is not in the stopped state. Therefore, the processing proceeds to subsequent Step  6  where the normal brake control can be performed as described above. Therefore, an appropriate braking force as needed can be applied by the wheel cylinders  3 L,  3 R,  4 L, and  4 R for the respective wheels. 
     As described above, according to the first embodiment, even in the case where the brake pedal  5  is depressed by the amount equal to or larger than the operation amount S1 while the vehicle is in the stopped state, when the driving force of the drive motor  21  becomes the maximum driving force (that is, when the pedaling force F becomes the pedaling force F1 shown in  FIG. 4  to stop the booster piston  18 ), the second ECU  33  outputs the valve-closing command to the boost control valve  40  for the left front wheel FL (the front wheel  1 L) and the boost control valve  40 ′ for the right front wheel FR (the front wheel  1 R), which are included in the ESC  31 . 
     In this manner, the hydraulic pressure supplied from the master cylinder  8  through the ESC  31  toward each of the wheels is only supplied to the wheel cylinder  4 L for the rear wheel  2 L and the wheel cylinder  4 R for the rear wheel  2 P without being supplied to the wheel cylinder  3 L for the front wheel  1 L and the wheel cylinder  3 R for the front wheel  1 R. Therefore, the hydraulic stiffness on the downstream side can be increased. Specifically, the hydraulic stiffness of the wheel cylinders  3 L,  3 R,  4 L, and  4 R is changed by stopping the supply of the hydraulic fluid (brake fluid) to the wheel cylinders  3 L and  3 R or reducing the amount of supply of the hydraulic fluid. 
     As a result, even when the driver who is depressing the brake pedal  5  while the vehicle is in the stopped state depresses the brake pedal  5  by the large amount equal to or larger than the operation amount S1 shown in  FIG. 4 , the driver can have a sufficiently firm pedal feeling (that is, a sufficiently large pedal reaction force generated by the pedaling force F) as represented by the characteristic line  58 B indicated by the solid line shown in  FIG. 4  over a period in which the input piston  19  is moved by the operation amount S2 to reach a position at which the input piston  19  comes into contact with the booster piston  18  in the stopped state. Therefore, the driver does not have a weird pedal feeling for the operation of the pedal. 
     Moreover, inside the housing  56  which forms the outer shell for the hydraulic-pressure control unit (ESC  31 ), the boost control valve  40  for the left front wheel FL and the boost control valve  40 ′ for the right front wheel FR, to which the valve-closing commands are output from the wheel-cylinder fluid supply control unit (second ECU  33 ) as described above, are provided at the positions close to the side surfaces  56 C and  56 D which are the outer side surfaces of the housing  56 . Therefore, heat from solenoids, which is generated when the boost control valves  40  and  40 ′ of normally-open electromagnetic valves are closed by the energization (excitation), can be released to outside air. Therefore, heat-releasing performance from the outer wall surfaces (side surfaces  56 C and  56 D) of the housing  56  can be enhanced. 
     Therefore, a structure of the brake control apparatus according to the first embodiment can be simplified. Further, the change in the reaction force (that is, the pedaling force F) when the drive motor  21  comes into the full-load state can be suppressed without lowering the output hydraulic pressure (hydraulic stiffness on the downstream side) generated by the operation of the brake pedal  5 . In addition, the heat generated from the solenoids of the boost control valves  40  and  40 ′ can be easily released from the output wall surface (side surfaces  56 C and  56 D) of the housing  56 . 
     In the first embodiment described above, the case where the boost control valve  40  for the left front wheel FL (front wheel  1 L) and the boost control valve  40 ′ for the right front wheel FR (front wheel  1 R) are closed to suppress a change in the reaction force occurring when the drive motor  21  comes into the full-load state has been described as an example. However, the present invention is not limited to the embodiment described above. For example, the boost control valve  41  for the left rear wheel RL (rear wheel  2 L), the boost control valve  41 ′ for the right rear wheel RR (rear wheel  2 R), and the boost control valve  40  for the left front wheel FL (front wheel  1 L) or the boost control valve  40 ′ for the right front wheel FR (front wheel  1 R), that is, the boost control valves for three wheels in total may be closed, whereas the boost control valve may be opened for the remaining one wheel. 
     For example, a characteristic line  58 C indicated by the alternate long and short dash line in  FIG. 4  represents the relationship between the operation amount S of the brake pedal  5  and the pedaling force F (that is, the pedal reaction force) in a state in which the brake pedal  5  is depressed by the amount equal to or larger than the operation amount S1 when the boost control valves for the three wheels in total, that is, the boost control valve  41  for the rear wheel  2 L, the boost control valve  41 ′ for the rear wheel  2 R, and the boost control, valve  40  for the front wheel  1 L (or the boost control valve  40 ′ for the front wheel  1 R) are closed and the boost control valve for the remaining one wheel is opened. Even in the case with the characteristic line  58 C indicated by the alternate long and short dash line in  FIG. 4 , an abrupt characteristic change as represented by the characteristic line  58 A indicated by the chain double-dashed line in  FIG. 4  can be suppressed. 
     A characteristic line  58 D indicated by the dotted line in  FIG. 4  represents a characteristic in the case where all the boost control valves  40 ,  40 ′,  41 , and  41 ′ for all the four wheels FL, FR, RL, and RR are closed. In the case represented by the characteristic line  58 D) indicated by the dotted line, an abrupt characteristic change as represented by the characteristic line  58 A indicated by the chain double-dashed line can be suppressed. On the other hand, however, the hydraulic stiffness tends to be too high. 
     Alternatively, in the present invention, the boost control valves for any two of the four wheels FL, FR, RL, and RR may be closed, whereas the boost control valves for the remaining two wheels may be opened. For example, the boost control valves at a side of any two of the right and left set of wheels may be closed, whereas the boost control valves for remaining two wheels may be opened. Further, the boost control valves for two cater-cornered wheels may be closed, and the boost control valves for the remaining two wheels may be opened. Furthermore, alternatively, any one of the supply control valves  39  and  39 ′ illustrated in  FIG. 1  may be closed, whereas another thereof may be opened. On the other hand, each of the boost control valves or the supply control valves may be a flow-rate adjustable control valve. In this case, by reducing the amount of supply of the hydraulic fluid (brake fluid) to the wheel cylinders by appropriately reducing an opening degree of each of the valves, the hydraulic stiffness on the downstream side can be changed. 
     Further, in the present invention, when the necessary motor current (detection value) increases even after it is determined in Step  4  of  FIG. 5  that “the necessary motor current is larger than the predetermined value”, the number of control valves to be closed may be increased in accordance with the increase in the necessary motor current. Even in this manner, the hydraulic stiffness of the wheel cylinders can be changed by stopping the supply of the hydraulic fluid to any of the plurality of wheel cylinders or reducing the amount of supply thereto. 
     Next,  FIG. 6  illustrates a second embodiment of the present invention. In the second embodiment, the same components as those of the first embodiment described above are denoted by the same reference symbols, and the description thereof is herein omitted. The feature of the second embodiment resides in that the hydraulic pumps  46  and  46 ′ are driven by the electric motor  47  of the ESC  31  to change the hydraulic stiffness of the wheel cylinders  3 L,  3 R,  4 L, and  4 R in order to suppress a change in the reaction force (that is, the pedaling force F) when the drive motor comes into the full-load state. 
     The second embodiment is to be applied to the electric booster  16  having a characteristic different from that of the first embodiment. Specifically, the electric booster  16  to which the second embodiment is to be applied presupposes the following configuration. More specifically, in order to delay the time to reach a full-load point so as to prevent the so-called spongy pedal feeling, (delay) control for reducing the amount of actuation of the primary piston (that is, the booster piston  18 ) to be smaller than the stroke amount of the input member (that is, the input piston  19 ) is performed. 
     Therefore, in the second embodiment, control processing illustrated in  FIG. 6  is performed using the second ECU  33  which is the controller for the hydraulic-pressure control unit (ESC  31 ). Specifically, the hydraulic pumps  46  and  46 ′ are driven by the electric motor  47  of the ESC  31  to increase the hydraulic stiffness of the wheel cylinders  31 ,  3 R,  4 L, and  4 R so that a ratio of the operation amount (pedal stroke) to the pedaling force at the time of the operation of the pedal is not reduced. In this manner, a change in the reaction force, which is generated when the drive motor comes into the full-load state, can be suppressed. 
     Specifically, after the control processing illustrated in  FIG. 6  is started, processing in Steps  11  to  14  is performed in the same manner as in Steps  1  to  4  illustrated in  FIG. 5 , which is described above in the first embodiment. When it is determined as “NO” in Step  14 , however, the driving force of the drive motor  21  does not reach the maximum driving force yet (the drive motor  21  does not come into the full-load state shown in  FIG. 4  yet). Therefore, the processing proceeds to subsequent Step  15  where it is determined whether or not the drive command has been output to the hydraulic pumps  46  and  46 ′ (specifically, the electric motor  47 ) of the ESC  31 . 
     When it is determined that the drive command has not been output to the hydraulic pumps  46  and  46 ′ (that is, the electric motor  47 ) in Step  15 , the processing proceeds to Step  16  where the normal brake control is performed. For the normal brake control, the same processing as that performed in Step  6  illustrated in  FIG. 5 , which is described above in the first embodiment, is performed. 
     On the other hand, the case where it is determined that the drive command has been output in Step  15  corresponds to, for example, the following case. Specifically, the processing returns in Step  17  in a state in which the drive command is output to the hydraulic pumps  46  and  46 ′ (electric motor  47 ) in Step  20  described later. After the processing in Steps  11  to  15  is performed, the processing proceeds to Step  18 . Therefore, in Step  18 , after the above-mentioned drive command to the electric motor  47  is stopped, the processing in next Step  16  and subsequent steps is executed. 
     Next, when it is determined as “YES” in Step  14 , the driving force of the drive motor  21  reaches the maximum driving force (the drive motor  21  comes into the full-load state shown in  FIG. 4 ). Thus, the processing proceeds to subsequent Step  19  where it is determined whether or not the vehicle is in the stopped state. When it is determined as “YES” in Step  19 , the vehicle is in the stopped state. Therefore, the processing proceeds to subsequent Step  20  where the drive command is output to the hydraulic pumps  46  and  46 ′ (electric motor  47 ) of the ESC  31 . 
     In the above-mentioned manner, the electric motor  47  of the ESC  31  rotationally drives the hydraulic pumps  46  and  46 ′. As a result, for example, the hydraulic pumps  46  and  46 ′ discharge the brake fluid, which is pumped into from the reservoirs  51  and  51 ′ for hydraulic-pressure control, to the brake pipeline  36  and  36 ′, the first pipeline portions  37  and  37 ′, and the second pipeline portions  38  and  38 ′ while supplying the hydraulic pressure to the wheel cylinders  3 L,  3 R,  4 L, and  4 R through the boost control valves  40 ,  40 ′,  41 , and  41 ′ and the brake-side pipeline portions  32 A,  32 B,  32 C, and  32 D. 
     As a result, even when the driver who is depressing the brake pedal  5  while the vehicle is in the stopped state depresses the brake pedal  5  by the large amount equal to or larger than the operation amount S1 shown in  FIG. 4 , the hydraulic stiffness on the downstream side can be increased by the hydraulic pressure supplied from the hydraulic pumps  46  and  46 ′ to the wheel cylinders  3 L,  3 R,  4 L, and  4 R. As a result, a change in the reaction force, which is generated when the drive motor  21  comes into the full-load state, can be suppressed. When it is determined as “NO” in Step  19 , the vehicle is not in the stopped state. Thus, the processing proceeds to subsequent Step  16  where the normal brake control can be performed as described above. Thus, an appropriate braking force as needed can be applied to the wheel cylinders  3 L,  3 R,  4 L, and  4 R for the respective wheels. 
     In the above-mentioned manner, even in the second embodiment having the configuration described above, when the driver depresses the brake pedal by a large amount to increase the driving force of the drive motor  21  of the electric booster  16  to the maximum driving force while the vehicle is in the stopped state, the hydraulic pumps  46  and  46 ′ can be driven by the electric motor  47  of the ESC  31  to change the hydraulic stiffness of the wheel cylinders  3 L,  3 R,  4 L, and  4 R. As a result, a change in the reaction force (that is, the pedaling force F), which is generated when the drive motor  21  comes into the full-load state, can be suppressed. 
     Next,  FIG. 7  illustrates a third embodiment of the present invention. In the third embodiment, the same components as those of the first embodiment described above are denoted by the same reference symbols, and the description thereof is herein omitted. The feature of the third embodiment resides in that, in order to suppress a change in the reaction force (that is, the pedaling force F), which is generated when the drive motor comes into the full-load state, the hydraulic stiffness of the wheel cylinders  3 L,  3 R,  4 L, and  4 R is changed by variably controlling the brake fluid pressure by using pressure control valves  61 A and  61 B as the wheel-cylinder fluid supply control unit. 
     Here, the pressure control valves  61 A and  61 B are generally referred to as proportioning valves. The proportioning valve controls a pressure so that a discharge pressure toward the downstream side is reduced at a constant rate with respect to an input pressure. The pressure control valve  61 A is provided in the cylinder-side hydraulic pipe  15 A which connects the first hydraulic chamber  11 A of the master cylinder  8  and the ESC  31  (hydraulic-control unit). Similarly, the pressure control valve  61 B is provided in the cylinder-side hydraulic pipe  15 B which connects the second hydraulic chamber  11 B and the ESC  31 . The pressure control valves  61 A and  61 B constitute the wheel-cylinder fluid supply control unit. By the control signal output from a first ECU  62 , the pressure control valve  61 A variably controls the hydraulic pressure in the cylinder-side hydraulic pipe  15 A, while the pressure control valve  61 B variably controls the hydraulic pressure in the cylinder-side hydraulic pipe  15 B. 
     The first ECU  62  is configured in the same manner as in the case of the first ECU  26  described in the first embodiment and functions as a controller (control device) for the electric booster, which electrically controls the drive of the electric actuator  20  (drive motor  21 ) of the electric booster  16 . However, an output side of the first ECU  62  is connected to the pressure control valves  61 A and  61 B in addition to the drive motor  21  so that the first ECU  62  has a function of outputting the control signal for increasing the hydraulic stiffness to the pressure control valves  61 A and  61 B. 
     Therefore, when the driving force of the drive motor  21  of the electric booster  16  becomes the maximum driving force (that is, the hydraulic pressure to be applied become a full-load hydraulic pressure) while the brake pedal  5  is being operated, the pressure control valves  61 A and  61 B perform pressure-reduction control (control for reducing an opening degree of each of the valves) on the hydraulic pressure to be supplied to the downstream side of the cylinder-side hydraulic pipes  15 A and  15 B in accordance with the control signal from the first ECU  62 . In this manner, the hydraulic stiffness of the wheel cylinders  3 L,  3 R,  4 L, and  4 R can be increased to be larger than that of the master cylinder  8 . 
     As described above, even in the third embodiment having the configuration described above, when the driver depresses the brake pedal  5  by a large amount while the vehicle is in the stopped state to increase the driving force of the drive motor  21  of the electric booster  16  to the maximum driving force, the hydraulic pressure to be supplied to the downstream side of the cylinder-side hydraulic pipes  15 A and  25 B is controlled by the pressure control valves  61 A and  61 B to change the hydraulic stiffness of the wheel cylinders  3 L,  3 R,  4 L, and  4 R. As a result, a change in the reaction force (that is, the pedaling force F) when the drive motor  21  comes into the full-load state can be suppressed. 
     In the third embodiment described above, the case where the pressure control valves  61 A and  61 B called “proportioning valves” are provided in the middle of the cylinder-side hydraulic pipes  15 A and  15 B is described as an example. However, the present invention is not limited to the above-mentioned embodiment. For example, on-off valves such as electromagnetic valves to be controlled to be opened or closed may be provided in the middle of the cylinder-side hydraulic pipes  15 A and  15 B. 
     Next, the invention encompassed in each of the embodiments described above is described. According to the present invention, the hydraulic stiffness of the wheel cylinder is increased by reducing the supply of the hydraulic fluid to the wheel cylinder. Moreover, the hydraulic stiffness of the wheel cylinder is changed by stopping the supply of the hydraulic fluid to any of the plurality of wheel cylinders. 
     On the other hand, the brake control apparatus of the present invention includes the master-cylinder pressure control unit which controls the drive motor configured to pressurize the hydraulic fluid of the master cylinder by the operation of the brake pedal to which the hydraulic reaction force is transmitted, and the wheel-cylinder fluid supply control unit provided between the wheel cylinder provided to the wheel and the master cylinder, which controls the supply of the hydraulic fluid to the wheel cylinder. During the operation of the brake pedal while the vehicle is in the stopped state, the hydraulic stiffness of the wheel cylinder is changed by the wheel-cylinder fluid control unit. 
     In this case, the hydraulic stiffness of the wheel cylinder is changed at least when the output of the drive motor becomes the maximum output while the vehicle is in the stopped state. Moreover, the hydraulic stiffness of the wheel cylinder is changed by reducing the supply of the hydraulic fluid to the wheel cylinder. Moreover, the hydraulic stiffness of the wheel cylinder is changed by stopping the supply of the hydraulic fluid to any of the plurality of wheel cylinders. 
     According to the brake control apparatus of the present invention, the wheel cylinders to which the supply of the hydraulic fluid is stopped are the wheel cylinders for the front wheels. Moreover, the master-cylinder pressure control unit is the controller for the electric booster which thrusts the piston of the master cylinder by the rotating force of the drive motor. Further, the wheel-cylinder fluid supply control unit is the controller for the hydraulic-pressure control unit, which is provided between the master cylinder and the wheel cylinders and controls the communication and interruption of the fluid paths by the electromagnetic valves. 
     Although only some exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teaching and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention. 
     The present application claims priority to Japanese Patent Applications No. 2013-180389 filed on Aug. 30, 2013. The entire disclosures of No. 2013-180389 filed on Aug. 30, 2013 including specification, claims, drawings and summary are incorporated herein by reference in its entirety.