Patent Publication Number: US-2021171003-A1

Title: Brake control device

Description:
TECHNICAL FIELD 
     The present disclosure relates to a brake control device. 
     BACKGROUND ART 
     In recent years, in vehicles such as passenger cars, a brake hold function that automatically holds the brake force when the vehicle is stopped by the operation of the brake pedal by the driver has been widely adopted. This brake hold function is particularly convenient when the vehicle is stopped on a slope. 
     There have been roughly two methods to realize the brake hold function. The first method is a method of energizing the normally open differential pressure control valve in the hydraulic circuit to have it in a closed state when the brake pedal is operated by the driver and hydraulic pressure is applied to the wheels, thus maintaining the hydraulic pressure even after the brake pedal is no longer operated. The second method is a method of calculating and generating the electric brake force required to maintain the stationary state by Electric Parking Brake (EPB) when the brake pedal is operated by the driver and hydraulic pressure is applied to the wheels. 
     CITATIONS LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2006-306351 
     SUMMARY OF INVENTION 
     Technical Problems 
     In the first method, the differential pressure control valve must be continuously energized while the vehicle is maintained in a stationary state. As a result, power consumption is increased. Furthermore, in the second method, the electric brake force to maintain the stationary state is generated independently by the EPB separately from the hydraulic pressure, an extra braking force is generated, and therefore, an extra power consumption is generated. 
     Therefore, one of the problems of the present disclosure is to provide a brake control device capable of realizing a brake hold function with low power consumption by using the EPB. 
     Solutions to Problems 
     A brake control device according to the present disclosure relates to a brake control device applied to a vehicle, the brake control device comprising, a hydraulic braking device that makes a braking member press against a braked member rotating integrally with wheels by using hydraulic pressure so that hydraulic braking force applied to the front and rear wheels of the vehicle is generated, and an electric braking device that makes the braking member press against the braked member by driving a motor so that an electric braking force applied to an electric braking wheel that is either the front wheel and the rear wheel is generated, a control unit that, when execution of a brake hold control for maintaining a stationary state is permitted in a situation where the vehicle is in the stationary state by the hydraulic braking force, executes the brake hold control in which a propulsion shaft moves toward the braked member so that the propulsion shaft contacts with a piston by driving the electric braking device, in which a differential pressure control valve is controlled so that a required hydraulic braking force applies to a non-electric braking wheel, the hydraulic braking device having the differential pressure control valve connected to the non-electric braking wheel different from the electric braking wheel, the required hydraulic braking force calculated by subtracting a first braking force from a target braking force to maintain the stationary state, the first braking force that is the electric braking force applied to the electric braking wheel in the absence of hydraulic pressure after the propulsion shaft contacts with the piston. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view showing an overall outline of a vehicle braking device according to a first embodiment. 
         FIG. 2  is a schematic cross-sectional view of a wheel brake mechanism of a rear wheel system provided in the vehicle braking device of the first embodiment. 
         FIG. 3  is a configuration diagram showing a schematic configuration of a hydraulic braking device and an electric braking device according to the first embodiment. 
         FIGS. 4A-4G  are a timing chart showing the state of operation of each configuration when the brake hold function is executed in the vehicle braking device of the first embodiment. 
         FIG. 5  is a flowchart showing a process executed by the brake control device of the first embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Hereinafter, exemplary embodiments of the present disclosure, which is first embodiment and second embodiment, will be disclosed. The configurations of the embodiment shown below, and the operations and results (effects) provided by the configurations are merely examples. The present disclosure can also be realized with configurations other than the configurations disclosed in the following embodiments. Furthermore, according to the present disclosure, it is possible to obtain at least one of the various effects (including derivative effects) obtained by the following configuration. 
     First Embodiment 
     In a first embodiment, a vehicle braking device in which a disc brake EPB is applied to the rear wheel system will be described in this example.  FIG. 1  is a schematic view showing an overall outline of a vehicle braking device of the first embodiment.  FIG. 2  is a schematic cross-sectional view of a wheel brake mechanism of the rear wheel system provided in the vehicle braking device of the first embodiment. 
     As shown in  FIG. 1 , the vehicle braking device of the first embodiment includes a service brake  1  that generates a service brake force, which is hydraulic braking force, in response to the pedaling force of the driver, and an EPB  2  for regulating the movement of the vehicle when the vehicle is parked or the like. 
     The service brake  1  is a hydraulic brake mechanism (also referred to as a hydraulic braking device) that generates brake hydraulic pressure based on the driver&#39;s depression of the brake pedal  3  and generates service brake force based on this brake hydraulic pressure. Specifically, the service brake  1  boosts the pedaling force corresponding to the depression of the brake pedal  3  by the driver with a booster  4 , and then generates a brake hydraulic pressure corresponding to the boosted pedaling force in a master cylinder (hereinafter referred to as M/C)  5 . Then, the brake hydraulic pressure is transmitted to a wheel cylinder (hereinafter, referred to as W/C)  6  provided in a wheel brake mechanism of each wheel to generate a service brake force. Furthermore, an actuator  7  for controlling brake hydraulic pressure is provided between the M/C  5  and the W/C  6 . The actuator  7  adjusts the service brake force generated by the service brake  1  and performs various controls (e.g., anti-skid control etc.) for improving the safety of the vehicle. 
     Various controls using the actuator  7  are executed by an electronic stability control—an electric control unit (ESC-ECU)  8  that controls the service brake force. For example, the ESC-ECU  8  outputs a control current for controlling various control valves provided in the actuator  7  and a motor for driving the pump to control the hydraulic circuit provided in the actuator  7 , and control the W/C pressure transmitted to the W/C 6 . Wheel slip is thereby avoided, for example, and the safety of the vehicle is improved. For example, the actuator  7  is configured to include, for each wheel, a pressure increasing control valve that controls the application of the brake hydraulic pressure generated in the M/C  5  or the brake hydraulic pressure generated by the pump drive with respect to the W/C  6 , a pressure decreasing control valve that decreases the W/C pressure by supplying brake fluid in each W/C  6  to a reservoir, and the like, and performs pressure increasing, maintaining, and decreasing control of the W/C pressure. Furthermore, the actuator  7  can realize the automatic pressurizing function of the service brake  1 , and based on the control of the pump drive and various control valves, can automatically pressurize the W/C  6  even when there is no brake operation. Details of the hydraulic braking device including the actuator  7  will be described later with reference to  FIG. 3 . 
     On the other hand, the EPB  2  generates electric parking brake force (hereinafter, also referred to as “electric braking force” and “electric brake force”) by driving the wheel brake mechanism by the EPB motor  10 , and is configured to have an EPB-ECU  9  that controls the drive of the EPB motor  10 . The EPB-ECU  9  and the ESC-ECU  8  transmit and receive information by, for example, Controller Area Network (CAN) communication. 
     The wheel brake mechanism is a mechanical structure that generates a brake force in the vehicle braking device of the first embodiment, and has a structure in which a wheel brake mechanism of the front wheel system first generates a service brake force by the operation of the service brake  1 . On the other hand, the wheel brake mechanism of the rear wheel system has a common structure that generates a brake force for both the operation of the service brake  1  and the operation of the EPB  2 . The wheel brake mechanism of the front wheel system is a wheel brake mechanism that has been conventionally used in general, in which a mechanism for generating the electric brake force based on the operation of the EPB  2  is omitted, as opposed to the wheel brake mechanism of the rear wheel system, and thus the description thereof will be omitted here, and the wheel brake mechanism of the rear wheel system will be described below. 
     In the wheel brake mechanism of the rear wheel system, not only when the service brake  1  is operated but also when the EPB  2  is operated, the brake pad  11 , which is the friction material shown in  FIG. 2 , is pressed to sandwich the brake disc  12  ( 12 RL,  12 RR,  12 FR,  12 FL) which is a friction object material by the brake pad  11 , thus generating a friction force between the brake pad  11  and the brake disc  12 , and generating a brake force. 
     Specifically, the wheel brake mechanism rotates the EPB motor  10  directly fixed to the body  14  of the W/C  6  for pressing the brake pad  11  as shown in  FIG. 2  in the caliper  13  shown in  FIG. 1  to rotate the spur gear  15  provided on a drive shaft  10   a  of the EPB motor  10 . Then, the brake pad  11  is moved by transmitting the rotational force (output) of the EPB motor  10  to a spur gear  16  engaged with the spur gear  15 , and the electric brake force by the EPB  2  is generated. 
     In the caliper  13 , in addition to the W/C  6  and the brake pad  11 , a part of the end face of the brake disc  12  is housed so as to be sandwiched by the brake pad  11 . The W/C  6  can generate the W/C pressure in a hollow portion  14   a,  which is the brake fluid storage chamber, by introducing the brake hydraulic pressure into the hollow portion  14   a  of the cylindrical body  14  through a passage  14   b,  and is configured to include a rotary shaft  17 , a propulsion shaft  18 , a piston  19 , and the like in the hollow portion  14   a.    
     The rotary shaft  17  has one end connected to the spur gear  16  through an insertion hole  14   c  formed in the body  14 , so that when the spur gear  16  is rotated, the rotary shaft  17  is rotated with the rotation of the spur gear  16 . A male screw groove  17   a  is formed on the outer peripheral surface of the rotary shaft  17  at the end of the rotary shaft  17  opposite to the end connected to the spur gear  16 . On the other hand, the other end of the rotary shaft  17  is axially supported by being inserted into the insertion hole  14   c.  Specifically, the insertion hole  14   c  is provided with a bearing  21  together with an O-ring  20 , so that the O-ring  20  prevents the brake fluid from leaking out between the rotary shaft  17  and the inner wall surface of the insertion hole  14   c,  and the bearing  21  axially supports the other end of the rotary shaft  17 . 
     The propulsion shaft  18  is configured by a nut including a hollow tubular member, and has a female screw groove  18   a  to be screw fitted with the male screw groove  17   a  of the rotary shaft  17  formed on the inner wall surface. The propulsion shaft  18  is configured, for example, in a circular column shape or a polygonal column shape provided with a key for preventing rotation, so that even if the rotary shaft  17  is rotated, it cannot be rotated about the rotation center of the rotary shaft  17 . Therefore, when the rotary shaft  17  is rotated, the rotational force of the rotary shaft  17  is converted to a force for moving the propulsion shaft  18  in the axial direction of the rotary shaft  17  by the engagement between the male screw groove  17   a  and the female screw groove  18   a.  When the drive of the EPB motor  10  is stopped, the propulsion shaft  18  stops at the same position due to the frictional force from the engagement between the male screw groove  17   a  and the female screw groove  18   a,  where if the drive of the EPB motor  10  is stopped when the target electric brake force is obtained, the propulsion shaft  18  can be held at that position, desired electric brake force can be maintained and self-locking (hereinafter simply referred to as “lock”) can be performed. 
     The piston  19  is arranged so as to surround the outer periphery of the propulsion shaft  18 , and is formed by a bottomed cylindrical member or a polygonal cylindrical member and arranged such that the outer peripheral surface comes into contact with the inner wall surface of the hollow portion  14   a  formed in the body  14 . A structure is such that a seal member  22  is provided on the inner wall surface of the body  14  and W/C pressure can be applied to the end face of the piston  19  so that brake fluid does not leak out between the outer peripheral surface of the piston  19  and the inner wall surface of the body  14 . The seal member  22  is used to generate a reaction force for returning the piston  19  at the time of release control after the lock control. Since the seal member  22  is provided, basically, even if the brake pad  11  and the piston  19  are pushed in within a range not exceeding the elastic deformation amount of the seal member  22  by the tilted brake disc  12  during turning, they are pushed back toward the brake disc  12  so that the gap between the brake disc  12  and the brake pad  11  is held at a predetermined clearance. 
     In addition, to prevent the piston  19  from rotating about the rotation center of the rotary shaft  17  even if the rotary shaft  17  rotates, when the propulsion shaft  18  is provided with a rotation prevention key, the piston is provided with a key groove in which the key slides, and when the propulsion shaft  18  has a polygonal column shape, the piston has a polygonal cylindrical shape corresponding thereto. 
     The brake pad  11  is arranged at the distal end of the piston  19 , and the brake pad  11  is moved in the left-right direction in the plane of drawing accompanying the movement of the piston  19 . Specifically, the piston  19  is movable in the left direction in the plane of drawing accompanying the movement of the propulsion shaft  18 , and is movable in the left direction in the plane of drawing independently from the propulsion shaft  18  when the W/C pressure is applied to the end of the piston  19  (the end opposite to the end where the brake pad  11  is arranged). Then, if the brake hydraulic pressure in the hollow portion  14   a  is not applied (W/C pressure=0) when the propulsion shaft  18  is at the release position (the state before the EPB motor  10  is rotated), which is the standby position in the normal release, the piston  19  is moved in the right direction in the plane of drawing by the elastic force of the seal member  22  to be described later, and the brake pad  11  can be separated away from the brake disc  12 . Furthermore, when the EPB motor  10  is rotated and the propulsion shaft  18  is moved in the left direction in the plane of drawing from the initial position, even if the W/C pressure becomes 0, the movement of the piston  19  in the right direction in the plane of drawing is regulated by the moved propulsion shaft  18  and the brake pad  11  is held in place. 
     In the wheel brake mechanism configured as described above, when the service brake  1  is operated, the piston  19  is moved in the left direction in the plane of drawing based on the W/C pressure generated thereby so that the brake pad  11  is pressed against the brake disc  12  and the service brake force is generated. Furthermore, when the EPB  2  is operated, the spur gear  15  is rotated by driving the EPB motor  10 , and the spur gear  16  and the rotary shaft  17  are accordingly rotated, so that the propulsion shaft  18  is moved toward the brake disc  12  (left direction in the plane of drawing) based on the engagement between the male screw groove  17   a  and the female screw groove  18   a.  The distal end of the propulsion shaft  18  thereby comes into contact with the bottom surface of the piston  19  and presses the piston  19 , whereby the piston  19  is also moved in the same direction, so that the brake pad  11  is pressed against the brake disc  12  and an electric brake force is generated. Therefore, a shared wheel brake mechanism that generates a brake force for both the operation of the service brake  1  and the operation of the EPB  2  can be adopted. 
     Furthermore, it is possible to confirm the generation state of the electric braking force by the EPB 2  or recognize the current detection value by confirming the current detection value of the current sensor (not shown) for detecting the current through the EPB motor  10 . 
     A longitudinal acceleration sensor  25  detects acceleration in the longitudinal direction (traveling direction) of the vehicle and inputs a detection signal to the EPB-ECU  9 . 
     An M/C pressure sensor  26  detects the M/C pressure in the M/C  5  and inputs a detection signal to the EPB-ECU  9 . 
     A temperature sensor  28  detects the temperature of the wheel brake mechanism (e.g., a brake disc) and inputs a detection signal to the EPB-ECU  9 . 
     A wheel speed sensor  29  detects the rotation speed of each wheel and inputs a detection signal to the EPB-ECU  9 . Although the wheel speed sensor  29  is actually provided one for each wheel, detailed illustration and description thereof will be omitted here. 
     The EPB-ECU  9  is configured by a well-known microcomputer equipped with Central Processing Unit (CPU), Read Only Memory (ROM), Random Access Memory (RAM), I/O, and the like, and performs parking brake control by controlling the rotation of the EPB motor  10  following the program stored in the ROM and the like. 
     The EPB-ECU  9  inputs, for example, a signal corresponding to the operation state of an operation switch (SW)  23  provided on an instrumental panel (not shown) in the vehicle compartment, and drives the EPB motor  10  according to the operation state of the operation SW  23 . Furthermore, the EPB-ECU  9  executes lock control, release control, and the like based on the current detection value of the EPB motor  10 , and recognizes that the lock control is being performed based on the control state or that the wheel is in the lock state by the lock control, and that the release control is being performed or that the wheel is in the release state. or EPB release state, by the release control. Then, the EPB-ECU  9  outputs a signal for performing various displays to an indicator lamp  24  provided on the instrumental panel. 
     The vehicle braking device configured as described above basically performs an operation of generating a braking force in the vehicle by generating the service brake force by the service brake  1  when the vehicle is traveling. Moreover, when the vehicle is stopped by the service brake  1 , the driver performs operations such as pressing the operation SW  23  to operate the EPB  2  and generate the electric brake force thus maintaining the stationary state, and then releasing the electric brake force thereafter. That is, as the operation of the service brake  1 , when the driver operates the brake pedal  3  while the vehicle is traveling, the brake hydraulic pressure generated in the M/C  5  is transmitted to the W/C  6  thus generating the service brake force. Moreover, as the operation of the EPB  2 , the piston  19  is moved by driving the EPB motor  10 , and the electric brake force is generated by pressing the brake pad  11  against the brake disc  12  to have the wheels in the lock state, or the electric brake force is released by separating the brake pad  11  from the brake disc  12  to have the wheels in the release state. 
     Specifically, the electric brake force is generated or released by the lock/release control. In the lock control, the EPB  2  is operated by forward rotating the EPB motor  10 , the rotation of the EPB motor  10  is stopped at a position where a desired electric brake force can be generated by the EPB  2 , and this state is maintained. A desired electric brake force is thereby generated. In the release control, the EPB  2  is operated by reverse rotating the EPB motor  10 , and the electric brake force generated in the EPB  2  is released. 
       FIG. 3  is a configuration diagram showing a schematic configuration of the hydraulic braking device and the electric braking device according to the first embodiment. As illustrated in  FIG. 3 , the vehicle braking device according to the first embodiment includes a hydraulic braking device  60  configured to be able to apply a braking force (friction braking torque) to four wheels  50 RL,  50 RR,  50 FR, and  50 FL, and an EPB  2  ( FIG. 1 ) including the EPB motor  10  configured to be able to apply a braking force to two wheels  50 RL and  50 RR. 
     The hydraulic braking device  60  includes four wheel cylinders  6 , pressure adjusting units  34 RL,  34 RR,  34 FR and  34 FL, and a reflux mechanism  37 . Each of the four wheel cylinders  6  is a mechanism that pressurizes the brake pads ( FIG. 1 ) to apply braking force to the wheels  50 RL,  50 RR,  50 FR, and  50 FL. The pressure adjusting units  34 RL,  34 RR,  34 FR and  34 FL are mechanisms for adjusting the hydraulic pressure applied to the corresponding wheel cylinder  6 , respectively. The reflux mechanism  37  is a mechanism that returns the fluid (working fluid) serving as a medium for generating the hydraulic pressure toward the upstream side. The differential pressure control valves  33 R and  33 F open and close under the control of the ESC-ECU  8  (see  FIG. 1 ). 
     The pressure adjusting units  34 RL,  34 RR,  34 FR, and  34 FL each includes electromagnetic valves  35  and  36  capable of electrically switching between the open state and the closed state. The electromagnetic valves  35  and  36  are provided between the differential pressure control valve  33  and a reservoir  41 . The electromagnetic valve  35  is connected to the differential pressure control valves  33 R,  33 F, and the electromagnetic valve  36  is connected to the reservoir  41 . 
     The electromagnetic valves  35  and  36  open and close under the control of the ESC-ECU  8  to increase, maintain, or decrease the pressure generated by the wheel cylinder  6 . 
     The reflux mechanism  37  includes the reservoir  41  and a pump  39 , and a pump motor  40  that rotates the front-side and rear-side pumps  39  to transport the fluid toward the upstream side. One of each of the reservoir  41  and the pump  39  is provided in correspondence with the combination of the pressure adjusting units  34 RL and  34 RR and the combination of the pressure adjusting units  34 FR and  34 FL. 
     Here, in the first embodiment, the EPB motor  10  driven under the control of the EPB-ECU  9  ( FIG. 2 ) is connected to each of the two wheel cylinders  6  on the rear side. Thus, in the first embodiment, the brake pads  11  ( FIG. 2 ) of the two wheel cylinders  6  on the rear side are pressurized in response to the drive of the EPB motor  10 , so that the electric braking force is applied to the wheels  50 RL and  50 RR on the rear side. Therefore, in the first embodiment, the two wheel cylinders  6  on the rear side and the two EPB motors  10  connected to these two wheel cylinders  6  function as EPB  2  capable of generating a parking braking force separate from the hydraulic braking force by the hydraulic braking device  60 . 
     Here, the details of the control of the ESC-ECU  8  and the EPB-ECU  9  will be described. The ESC-ECU  8  and the EPB-ECU  9  are applied to a vehicle including a hydraulic braking device that presses the brake pad  11  (braking member) with hydraulic pressure toward the brake disc  12  (member to be braked) that rotates integrally with the wheels and generates a hydraulic braking force, for the front and rear wheels of the vehicle, and an electric braking device that presses the brake pad  11  by driving the EPB motor  10  toward the brake disc  12  and generates an electric braking force, for an electric braking wheel of either the front wheel and the rear wheel. 
     Then, when the execution of the brake hold control for maintaining a stationary state is permitted in a situation where the vehicle is maintained in the stationary state by the hydraulic braking force generated by the hydraulic braking device  60 , the ESC-ECU  8  and the EPB-ECU  9  execute the following brake hold control. 
     The EPB-ECU  9  drives the EPB 2  to move the propulsion shaft  18  toward the brake disc  12  and bring it into contact with the piston  19  of the rear wheels, which is electric braking wheels, and calculates a target braking force for maintaining the stationary state of the vehicle. In addition, the EPB-ECU  9  calculates a required hydraulic braking force applied to the front wheels, which is non-electric braking wheels, by subtracting a value of a first braking force that is the electric braking force is generated by the EPB 2  in the absence the hydraulic braking force after the propulsion shaft  18  contacts with the piston  19  from the target braking force. The ESC-ECU  8  controls the differential pressure control valve  33 F ( FIG. 3 ) in the hydraulic braking device  60  connected to the front wheels to generate the required hydraulic braking force. 
     Furthermore, when a value of the required hydraulic braking force obtained by subtracting the first braking force in the absence the hydraulic braking force after the propulsion shaft  18  contacts with the piston  19  from the target braking force is less than or equal to zero, the ESC-ECU  8  controls the differential pressure control valve  33 F in the hydraulic braking device  60  connected to the front wheels so that no hydraulic braking force is generated on the front wheels. 
     Moreover, when the hydraulic braking force generated by the hydraulic brake operation increases during the execution of the brake hold control, the ESC-ECU  8  and the EPB-ECU  9  again execute the brake hold control from the beginning. 
     Next, with reference to  FIG. 4 , the state of operation of each configuration when the brake hold function is executed in the first embodiment will be described.  FIG. 4  is a timing chart showing the state of operation of each configuration when the brake hold function is executed by the vehicle braking device of the first embodiment. In  FIGS. 4A to 4G , the horizontal axis represents time. In  FIG. 4( a ) , the vertical axis represents the hydraulic brake operation amount (the amount of depression of the brake pedal  3 ). In  FIG. 4( b ) , the vertical axis represents the presence/absence (ON/OFF) of the brake hold (BH) instruction (BH start operation by the operation SW  23 ). In  FIG. 4C , the vertical axis represents the energized state (energized/de-energized) of the differential pressure control valve  33 F connected to the front wheels. In  FIG. 4D , the vertical axis represents the energized state (energized/de-energized) of the differential pressure control valve  33 R connected to the rear wheels. In  FIG. 4E , the vertical axis represents the current value (current detection value) through the EPB motor  10 . In  FIG. 4F , the vertical axis represents the braking force applied to the front wheels. In  FIG. 4G , the vertical axis represents the braking force applied to the rear wheels. 
     It is assumed that while the vehicle is in the stationary state, as shown in  FIG. 4A , the driver performs the first hydraulic brake operation from time t 1  to time t 7 , and then performs the second hydraulic brake operation stronger than the first time from time t 8  to time t 14 . Furthermore, as shown in  FIG. 4B , it is assumed that the driver gives a BH instruction (BH start operation by the operation SW  23 ) at time t 3 , and then the ON state of the BH instruction continues. 
     In that case, the EPB-ECU  9  drives the EPB 2  to operate the EPB motor  10  to move the propulsion shaft  18  ( FIG. 2 ) toward the brake disc  12  and bring it into contact with the piston  19  provided with the rear wheels (time t 3  to t 4  in  FIG. 4E ). Thus, for the rear wheels, even if the hydraulic pressure applied to the rear wheels decreases thereafter (after time t 5  in  FIG. 4  A), an electric braking force substantially the same as the hydraulic braking force up to that point can be generated ( FIG. 4G ). 
     Next, the EPB-ECU  9  calculates the target braking force for maintaining the stationary state of the vehicle. For example, the target braking force is determined by using the road gradient calculated from the detection values of the longitudinal acceleration sensors. In addition, the EPB-ECU  9  calculates the required hydraulic braking force applied to the front wheels by subtracting the first braking force that is the electric braking force is generated by EPB 2  in the absence the hydraulic braking force after the propulsion shaft  18  contacts with the piston  19  from the target braking force. 
     Then, the ESC-ECU  8  controls the differential pressure control valve  33 F ( FIG. 3 ) in the hydraulic braking device connected to the front wheels  60  so that the required hydraulic braking force is applied to the front wheels. Thus, even if the hydraulic brake operation amount starts to decrease from time t 5  and becomes zero at t 7  ( FIG. 4A ), the braking force applied to the front wheels starts to decrease from time t 5  and becomes the required hydraulic braking force at time t 6 , and thereafter, maintains the required hydraulic braking force ( FIG. 4F ). 
     Moreover, when the driver performs the second hydraulic brake operation stronger than the first time from time t 8  to time t 14  during the execution of the brake hold control, the ESC-ECU  8  and the EPB-ECU  9  again execute the brake hold control from the beginning. That is, the EPB-ECU  9  first drives the EPB 2  to operate the EPB motor  10  to move the propulsion shaft  18  ( FIG. 2 ) toward the brake disc  12  and bring it into contact with the piston  19  for the rear wheels (time t 11  to t 12  in  FIG. 4E ). Thus, for the rear wheels, even if the hydraulic pressure applied to the rear wheels decreases thereafter (after time t 13  in  FIG. 4  A), an electric braking force substantially the same as the hydraulic braking force up to that point can be generated ( FIG. 4G ). That is, the electric braking force after time t 13  can be made larger than the electric braking force from time t 5  to time t 10  ( FIG. 4G ). 
     In addition, the EPB-ECU  9  calculates the required hydraulic braking force applied to the front wheels by subtracting the first braking force generated by EPB 2  in the absence of hydraulic pressure after the propulsion shaft  18  contacts with the piston  19  from the target braking force. At this time, when the calculated required hydraulic braking force is less than or equal to zero, the ESC-ECU  8  controls the differential pressure control valve  33 F in the hydraulic braking device  60  connected to the front wheels so that no hydraulic braking force is applied to the front wheels. As a result, as shown in  FIG. 4F , the braking force applied to the front wheels starts to decrease from time t 13  and becomes zero at time t 14 . However, after time t 14 , even if the braking force applied to the front wheels is zero ( FIG. 4F ), the braking force applied to the rear wheels is large ( FIG. 4G ), and the braking force for maintaining the stationary state is secured for the vehicle as a whole. 
     Next, the process executed by the brake control device will be described with reference to  FIG. 5 .  FIG. 5  is a flowchart showing a process executed by the brake control device of the first embodiment. 
     First, the driver starts the first hydraulic brake operation (step S 1  in  FIG. 5 : time t 1  in  FIG. 4A ). 
     Next, when the operation unit (operation SW  23 ) is operated by the driver to instruct the execution of the brake hold function (step S 2  in  FIG. 5 : time t 3  in  FIG. 4B ), the EPB-ECU  9  drives the EPB 2  to operate the EPB motor  10  to move the propulsion shaft  18  toward the brake disc  12  and bring it into contact with the piston  19  connected to the rear wheels (step S 3  in  FIG. 5 : time t 3  to t 4  in  FIG. 4E ). 
     Next, the EPB-ECU  9  calculates the target braking force for maintaining the stationary state of the vehicle (step S 4  in  FIG. 5 ). Next, the EPB-ECU  9  calculates the first braking force generated by the EPB 2  in the absence of hydraulic pressure (step S 5  in  FIG. 5 ). For example, the first braking force is the same as the hydraulic braking force after the propulsion shaft  18  contacts with the piston  19 . 
     Next, the EPB-ECU  9  calculates the required hydraulic braking force by subtracting the electric braking force calculated in step S 5  from the target braking force calculated in step S 4  (step S 6  in  FIG. 5 ). Next, the ESC-ECU  8  controls the differential pressure control valve  33 F ( FIG. 3 ) in the hydraulic braking device  60  connected the front wheels so that the required hydraulic braking force calculated in step S 6  is applied to the front wheels (step S 7  in  FIG. 5 ). 
     Next, the driver terminates the first hydraulic brake operation (step S 8  in  FIG. 5 : time t 7  in  FIG. 4A ). According to the above control, as shown in  FIG. 4G , the braking force applied to the rear wheels is maintained even after time t 5  when the driver starts to loosen the hydraulic brake operation. Furthermore, as shown in  FIG. 4F , the braking force applied to the front wheels is maintained by the required hydraulic braking force from time t 6  to time t 9 . 
     The driver then initiates a second hydraulic brake operation (step S 9  in  FIG. 5 : time t 9  in  FIG. 4A ). 
     Next, when the hydraulic brake operation amount reaches its peak (time t 11  in  FIG. 4A ), the EPB-ECU  9  drives the EPB 2  to operate EPB motor  10  to move the propulsion shaft  18  toward the brake disc  12  and bring it into contact with the piston  19  for the rear wheels (step S 10  in  FIG. 5 : time t 11  to t 12  in  FIG. 4E ). 
     Next, the EPB-ECU  9  calculates the first braking force that is the electric braking force, which can be identical with the original hydraulic braking force, generated by the EPB 2  in the absence of hydraulic pressure (step S 11  in  FIG. 5 ). 
     Next, the EPB-ECU  9  calculates the required hydraulic braking force applied to the front wheels by subtracting the first braking force calculated in step S 11  from the target braking force calculated in step S 4  (step S 12  in  FIG. 5 ). Next, the ESC-ECU  8  controls the differential pressure control valve  33 F ( FIG. 3 ) in the hydraulic braking device  60  connected to the front wheels so that the required hydraulic braking force calculated in step S 12  is applied to the front wheels (step S 13  in  FIG. 5 ). 
     Next, the driver terminates the second hydraulic brake operation (step S 14  in  FIG. 5 : time t 14  in  FIG. 4A ). According to the above control, as shown in  FIG. 4G , the braking force applied to the rear wheels is maintained even after time t 13  when the driver starts to loosen the hydraulic brake operation. Furthermore, as shown in  FIG. 4F , the braking force applied to the front wheels becomes zero after time t 14 . 
     As described above, according to the brake control device of the first embodiment, the brake hold function can be realized with low power consumption by using the EPB 2 . That is, for example, as shown in  FIG. 4C , the differential pressure control valve  33 F connected to the front wheels can be energized from time t 3  to time t 12 . On the other hand, in the first method of the prior art described above, the differential pressure control valve connected to the front wheels had to be energized while the BH instruction is ON ( FIG. 4B ), resulting in large power consumption. 
     Furthermore, as shown in  FIG. 4D , it is not necessary to energize the differential pressure control valve  33 R connected to the rear wheels. On the other hand, in the first method, the differential pressure control valve connected to the rear wheels had to be energized while the BH instruction was ON ( FIG. 4B ), resulting in large power consumption. 
     L 2  shown in  FIG. 4G  is a target value of the braking force applied to the rear wheels in the first method. According to the brake control device of the first embodiment, the braking force applied to the rear wheels larger than L 2  can be maintained even after the time t 5  when the driver starts to loosen the hydraulic brake operation by simply driving the EPB motor  10  from time t 3  to time t 4 , and thus it is efficient. Furthermore, larger braking force applied to the rear wheels can be maintained even after the time t 13  when the driver starts to loosen the hydraulic brake operation by simply driving the EPB motor  10  from time t 11  to time t 12 , and thus it is efficient. 
     L 1  shown in  FIG. 4F  is a target value of the braking force applied to the front wheels in the first method. According to the brake control device of the first embodiment, the braking force applied to the rear wheels can be maintained larger than L 2  from time t 6  to time t 9  ( FIG. 4G ), and hence the braking force applied to the front wheels ( FIG. 4F ) can be made smaller than L 1 , whereby power consumption can be reduced accordingly. 
     Furthermore, in the second method of the prior art described above, there is a problem that since the electric brake force required to maintain the stationary state is calculated and generated independently by the EPB separately from the hydraulic pressure, an extra braking force is generated, and therefore, an extra power consumption is generated. However, according to the brake control device of the first embodiment, when the brake hold function is executed, the hydraulic braking force applied to the front wheels is reduced by the amount of the large electric braking force applied to the rear wheels, so that such extra braking force and power consumption are not generated. 
     Furthermore, in the prior art, there is a method of holding the hydraulic braking force for a predetermined time and then switching to the electric braking force when executing the brake hold function, but the differential pressure control valve needs to be continuously energized during a predetermined time for holding the hydraulic braking force, resulting in large power consumption. On the other hand, according to the brake control device of the first embodiment, when executing the brake hold function, the differential pressure control valve connected to the rear wheels does not need to be energized at all, and the differential pressure control valve connected to the front wheels also does not need to be energized after time t 12  in the example of  FIG. 4 , and thus the power consumption can be prevented small. 
     Moreover, in the first method, even if the differential pressure control valve is continuously energized in order to maintain the stationary state, due to structural reasons, the fluid gradually may pass through the differential pressure control valve, and as the hydraulic pressure gradually decreases, the braking force decreases, and it may not be possible to maintain the stationary state for a long time. On the other hand, according to the brake control device of the first embodiment, since such a decrease does not occur with the electric braking force by the EPB 2 , the stationary state can be maintained even for a long time. 
     Second Embodiment 
     Next, a brake control device of the second embodiment will be described. The description on the matters same as in the first embodiment will be omitted as appropriate. 
     In the brake control device of the second embodiment, when the service brake  1  fails during the execution of the brake hold control, the EPB-ECU  9  controls the EPB 2  so that the electric braking force becomes the target braking force for the rear wheels. 
     Thus, even when the service brake  1  fails, stable braking control can be realized by controlling the EPB 2  so that the electric braking force becomes the target braking force. 
     The embodiment and modified examples of the present disclosure have been described above, but the above-described embodiments and modified examples are merely examples, and they are not intended to limit the scope of the disclosure. The novel embodiments and modified examples described above can be implemented in various forms, and various omissions, substitutions, or modifications can be made without departing from the gist of the disclosure. Furthermore, the embodiments and modified examples described above are included in the scope and gist of the disclosure, and are included in the disclosure described in the Claims and the equivalent scope thereof. 
     For example, in the embodiments described above, the rear wheels are electric braking wheels, but this is not the sole case, and the front wheels may be electric braking wheels. 
     Moreover, the hydraulic circuit is a so-called front-rear piping as shown in  FIG. 3  (piping configuration in which the output from the M/C  5  is divided into two systems, two front wheels and two rear wheels), but this is not the sole case, and the hydraulic circuit may be a so-called X piping (piping configuration in which the output from the M/C  5  is divided into two systems of front and rear wheels on a diagonal line). When the X piping is adopted, both of the two differential pressure control valves need to be energized when executing the brake hold function, but similar to the example of  FIG. 4  where the differential pressure control valve of the front wheels is de-energized after time t 12 , the two differential pressure control valves can also be de-energized from the middle of the execution of the brake hold function, and hence the power consumption can be suppressed to be small. 
     The present disclosure can also be applied at the time of execution of the brake hold function in an autonomous vehicle. 
     Furthermore, in the embodiments described above, a mode in which the presence/absence of the instruction of the brake hold control is switched by the driver operating the operation unit is shown. Instead, the brake hold control may be executed when the control unit determines that the brake hold control is necessary regardless of the driver&#39;s intention or operation. 
     Furthermore, in the embodiments described above, a mode in which the electric braking device is operated to execute the brake hold control in a situation where the hydraulic braking force is generated by operating the brake pedal is shown. Instead, the brake hold control may be executed in a situation where the hydraulic braking force is automatically generated by the control unit when the brake pedal is not operated.