Patent Publication Number: US-2017369047-A1

Title: Electric brake system

Description:
TECHNICAL FIELD 
     The present invention relates to an electric brake system that applies a braking force to a vehicle. 
     BACKGROUND ART 
     There is known an electric brake system configured to activate a brake apparatus (a parking brake) as an auxiliary brake by advancing a piston with use of an electric mechanism, as an electric brake system mounted on a vehicle, such as an automobile (refer to PTL 1). 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent Application Public Disclosure No. H11-321599 
     SUMMARY OF INVENTION 
     Technical Problem 
     One possible operation to deal with such a situation that the electric mechanism of one of left and right brake apparatuses is in a failed state (an abnormality or a failure) is to totally prohibit control (activation) of the electric mechanisms of both the left and right brake apparatuses to, for example, prevent a braking force from being applied to only one wheel. However, carelessly prohibiting even the control while the vehicle is running may result in an incapability to apply the braking force according to a driver&#39;s intention even when, for example, the driver turns on a parking brake switch with an attempt to bring the vehicle to an emergency stop. 
     An object of the present invention is to provide an electric crake system capable of applying the braking force according to the driver&#39;s intention even when one of the left and right brake apparatuses is placed in the failed state. 
     Solution to Problem 
     To solve the above-described problem, according to one aspect of the present invention, an electric brake system includes at least a pair of braking apparatuses provided on a left side and a right side of a vehicle. Each of the brake apparatuses is configured to advance a piston for pressing a frictional member against a rotational member rotatable together with a wheel, and configured to press the frictional member against the rotational member with use of an electric mechanism according to a braking request signal and hold a pressing force of the frictional member. The electric brake system further includes a control apparatus configured to control the electric mechanism of each of the brake apparatuses and diagnose a failed state in which an abnormality has occurred in each of the brake apparatuses according to the braking request signal. This control apparatus is configured to, when any one of the brake apparatuses on the left side and the right side is diagnosed as being in the failed state, prohibit the electric mechanism of the one of the brake apparatuses from operating while the vehicle is running. 
     According to the electric brake system of the one aspect of the present invention, it is possible to apply the braking force according to the driver&#39;s intention even when one of the left and right brake apparatuses is placed in the failed state while the vehicle is running. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a conceptual drawing illustrating a vehicle on which an electric brake system according to an embodiment is mounted. 
         FIG. 2  is a vertical cross-sectional view illustrating, in an enlarged manner, a disk brake equipped with an electric parking brake function that is mounted on a rear wheel side illustrated in  FIG. 1   
         FIG. 3  is a block diagram illustrating a parking brake control apparatus illustrated in  FIG. 1 . 
         FIG. 4  is a flowchart illustrating control processing designed to be performed by the parking brake control apparatus before emergency stop control is performed. 
         FIG. 5  illustrates a relationship between control of a left disk brake and control of a right disk brake before the emergency stop control is performed, as a table. 
         FIG. 6  is a flowchart illustrating control processing designed to be performed while the emergency stop control is in progress. 
         FIG. 7  illustrates a relationship between control or the left disk brake and control of the right disk brake while the emergency stop control is in progress, as a table. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following description, an electric brake system according to an embodiment of the present invention will be described in detail based on an example in which the electric brake system is mounted on a four-wheeled automobile with reference to the accompanying drawings. Individual steps in flowcharts illustrated in  FIGS. 4 and 6  will be each expressed with use of the symbol “S”, and, for example, step  1  will be expressed as “S 1 ”. 
     Referring to  FIG. 1 , four wheels in total that include, for example, front left and right wheels  2  (FL and FR) and rear left and right wheels  3  (RL and RR) are mounted under a vehicle body  1  forming a main structure of a vehicle (on a road surface side). A disk rotor  4  is provided to each of these front wheels  2  and rear wheels  3  as a rotational member rotatable together with each of the wheels (each of the front wheels  2  and each of the rear wheels  3 ). The disk rotor  4  for the front wheel  2  is subjected to a braking force by a hydraulic disk brake  5 , and the disk rotor  4  for the rear wheel  3  is subjected to a braking force by a hydraulic disk brake  31  equipped with an electric parking brake function. Due to this configuration, a barking brake is applied to each of the wheels (each of the front wheels  2  and each of the rear wheels  3 ) independently of one another. 
     A brake pedal  6  is provided on a dash board side of the vehicle body  1 . The brake pedal  6  is operated by being pressed by a driver at the time of an operation of braking the vehicle, and the braking force as a regular brake (a service brake) is applied and released based on this operation. A brake operation detection sensor (a brake sensor)  6 A, such as a brake lamp switch, a pedal switch, and a pedal stroke sensor, is provided at the brake pedal  6 . The brake operation detection sensor  6 A detects presence or absence Of the operation of pressing the brake pedal  6  and an operation amount thereof, and outputs a detection signal thereof to a hydraulic supply apparatus controller  13 . The detection signal of the brake operation detection sensor  6 A is transmitted (output to a parking brake control apparatus  19 ) via, for example, a vehicle data bus  16  or a signal line (not illustrated) connecting the hydraulic supply apparatus controller  13  and the parking brake control apparatus  19  to each other. 
     The operation of pressing the brake pedal  6  is transmitted to a master cylinder  8  serving as a hydraulic source via a booster  7 . The booster  7  is configured as a negative pressure booster or an electric booster provided between the brake pedal  6  and the master cylinder  8 , and transmits a pressing force to the master cylinder  8  while boosting the pressing force at the time of the operation of pressing the brake pedal  6 . At this time, the master cylinder  8  generates a hydraulic pressure with the aid of brake fluid supplied form a master reservoir  9 . The master reservoir  9  includes a hydraulic fluid tank containing the brake fluid therein. The mechanism for generating the hydraulic pressure by the brake pedal  6  is not limited to the above-described configuration, and may be a mechanism that generates the hydraulic pressure according to the operation performed on the brake pedal  6 , such as a brake-by-wire type mechanism. 
     The hydraulic pressure generated in the master cylinder  8  is transmitted to a hydraulic supply apparatus  11  (hereinafter referred to as an ESC  11 ) via, for example, a pair of cylinder-side hydraulic pipes  10 A and  10 B. The ESC  11  is disposed between each of the disk brakes  5  and  31  and the master cylinder  8 , and distributes the hydraulic pressure from the master cylinder  8  to each of the disk brakes  5  and  31  via brake-side pipe portions  12 A,  12 B,  12 C, and  12 D. As a result, the braking force is applied to each of the wheels (each of the front wheels  2  and each of the rear wheels  3 ) independently of one another. 
     For achieving this function, the ESC  11  includes a dedicated control apparatus including, for example, a microcomputer, i.e., the hydraulic supply apparatus controller  13  (hereinafter referred to as the control unit  13 ). The control unit  13  performs control of increasing, reducing, or maintaining the brake hydraulic pressure to be supplied from the brake-side pipe portions  12 A to  12 D to each of the disk brakes  5  and  31 , by performing driving control of opening and closing each of control valves (not illustrated) of the ESC  11  and rotating and stopping an electric motor (not illustrated) for a hydraulic pump. This operation realizes execution of various kinds of brake control, such as boosting control, braking force distribution control, brake assist control, anti-lock brake control (ABS), traction control, vehicle stabilization control (including sideslip prevention), hill start aid control, and automatic driving control. 
     Power is supplied from a battery  14  to the control tip it  13  via a power source line  15 . As illustrated in  FIG. 1 , the control unit  13  is connected to the vehicle data bus  16 . Instead of the ESC  11 , a known ABS unit can also be used. Alternatively, the master cylinder  8  and the brake-side pipe portions  12 A to  12 D can also be directly connected to each other without the provision of the ESC  11  (i.e., with the ESC  11  omitted). 
     The vehicle data bus  16  includes a CAN (Controller Area Network) as a serial communication unit mounted on the vehicle body  1 , and performs in-vehicle multiplex communication between the vehicle data bus  16 , and a large number of electronic devices mounted on the vehicle, the control unit  13 , the parking brake control apparatus  19 , and the like. In this case, examples of vehicle information transmitted to the vehicle data bus  16  include information (vehicle information) based on detection signals from the brake operation detection sensor  6 A, a pressure sensor  17  that detects a master cylinder hydraulic pressure (the brake hydraulic pressure), an ignition switch, a safety belt sensor, a door lock sensor, a door opening sensor, a seat occupancy sensor, a vehicle speed sensor, a steering angle sensor, an accelerator sensor (an accelerator operation sensor), a throttle sensor, an engine rotation sensor, a stereo camera, a millimeter-wave radar, an inclination sensor, a shift sensor, an acceleration sensor, a wheel speed sensor, a pitch sensor that detects a motion of the vehicle in a pitch direction, and the like. 
     A parking brake switch (PKBSW)  18  is provided in the vehicle body  1  near a driver&#39;s seat (not illustrated). The parking brake switch  18  is operated by the driver. The parking brake switch  18  transmits a signal (an activation request signal) corresponding to a request to activate the parking brake (an application request or a release request) issued from the driver to the parking brake control apparatus  19 . In other words, the parking brake switch  18  outputs a signal (an application request signal or a release request signal) to activate brake pads  33  (refer to  FIG. 2 ) for the application or the release based on driving (a rotation) of an electric motor  43 B to the parking brake control apparatus  19 , which serves as a control unit (a controller). 
     When the parking brake switch  18  is operated by the driver toward a braking side (a parking brake ON side), i.e., when the application request (a holding request or a driving request) for applying the braking force to the vehicle is issued, the application request signal (a braking request signal) is output from the parking brake switch  18 . On the other hand, when the parking brake switch  18  is operated by the driver toward a braking release side (a parking brake OFF side), i.e., when the release request (a stop request) for releasing the braking force on the vehicle is issued, the release request signal is output from the parking brake switch  18 . In this case, the parking brake can be set into an application state when, for example, the driver pulls up the parking brake switch  18  (operates the parking brake switch  18  to switch on it), and a release state when the driver pushes down the parking brake switch  18  (operates the parking brake switch  18  to switch off it). 
     When the application request is issued while the vehicle is stopped, power for rotating the electric motor  43 B toward a braking side is supplied to the disk brake  31  for the rear wheel  3  via the parking brake control apparatus  19 . As a result, the disk brake  31  for the rear wheel  3  is set into a state where the braking force as the parking brake is applied thereto, i.e., an application state. 
     On the other hand, when the release request is issued while the vehicle is stopped, power for rotating the electric actuator  43  in an opposite direction from the braking side is supplied to the disk brake  31  via the parking brake control apparatus  19 . As a result, the disk brake  31  for the rear wheel  3  is set into a state where the application of the braking force as the parking brake is released, i.e., a release state. 
     The parking brake can be configured to be automatically applied (an automatic application) based on an automatic application request due to a logic for determining the application of the parking brake by the parking brake control apparatus  19 , for example, when the vehicle is kept stopped for a predetermined time period (the vehicle is determined to be stopped, for example, when the speed detected by the vehicle speed sensor is kept lower than 4 km/h for a predetermined time period according to deceleration while the vehicle is running), when the engine is stopped, when a shift lever is operated to P, when a door is opened, or when a seat belt is released. 
     Further, the parking brake can be configured to be automatically released (an automatic release) based on an automatic release request due to a logic for determining the release of the parking brake by the parking brake control apparatus  19 , for example, when the vehicle is running (the vehicle is determined to be running, for example, when the speed detected by the vehicle speed sensor is kept at 5 km/h or higher for a predetermined time period according to acceleration from a stopped state), when an accelerator pedal is operated, when a clutch pedal is operated, or when the shift lever is operated to a position other than P and N. 
     Further, when the application request is issued by the parking brake switch  18  while the vehicle is running, more specifically, when a dynamic parking brake (a dynamic application) using the parking brake as an auxiliary brake urgently while the vehicle is running is requested, power for rotating the electric motor  43 B toward the braking side is supplied to the disk brake  31  for the rear wheel  3  via the parking brake control apparatus  19 . As a result, the disk brake  31  for the rear wheel  3  is set into a state where the braking force as the auxiliary brake is applied, i.e., the application state. In this case, the parking brake can be set into the application state when, for example, the driver continuously pulls up the parking brake switch  18  (operates the parking brake switch  18  to turn on the braking), and the release state when the driver take his/her hand off the parking brake switch  18  (operates the parking brake switch  18  to turn off the braking). 
     The parking brake control apparatus  19  forms an electric brake system together with the left and right disk brakes  31 , which will be described below. As illustrated in  FIG. 3 , the parking brake control apparatus  19  includes a calculation circuit (CPU)  20  including a microcomputer and the like, and power is supplied from the battery  14  to the parking brake control apparatus  19  via the power source line  15 . The parking brake control apparatus  19  forms a control apparatus (a controller or a control unit), which is a component of the present invention, and functions to control the electric actuators  43  of the left and right disk brakes  31  to generate the braking force (the parking brake) when, for example, the vehicle is parked or stopped. In other words, the parking brake control apparatus  19  functions to activate (hold or release) the disk brake  31  as the parking brake or the auxiliary brake. 
     As illustrated in  FIGS. 1 to 3 , an input side of the parking brake control apparatus  19  is connected to the parking brake switch  18  and the like, and an output side of the parking brake control apparatus  19  is connected to the electric actuators  43  of the disk brakes  31  and the like. More specifically, as illustrated in  FIG. 3 , the parking brake switch  18 , the vehicle data bus (CAN)  16 , a voltage sensor unit  22 , motor driving circuits  23 , current sensor units  24 , and the like, in addition to a storage unit (a memory)  21 , are connected to the calculation circuit (CPU)  20  of the parking brake control apparatus  19 . The parking brake control apparatus  19  can acquire various kinds of state amounts of the vehicle that are required to control (activate) the parking brake, and various kinds of vehicle information from the vehicle data bus  16 . 
     The parking brake control apparatus  19  may be configured to acquire the vehicle information acquired from the vehicle data bus  16  due to a direct connection of the parking brake control apparatus  19  (the calculation circuit  20  thereof) to the sensors that detect these information pieces (for example, the accelerator sensor, the throttle sensor, the engine rotation sensor, the brake sensor, the wheel speed sensor, the vehicle speed sensor, a G sensor, and the like). Further, the calculation circuit  20  of the parking brake control apparatus  19  can be configured to receive the activation request signal from the parking brake switch  18  and another control apparatus (for example, the control unit  13 ) connected to the vehicle data bus  16 . 
     In this case, the electric brake system can be configured in such a manner that the other control apparatus (for example, the control unit  13 ), for example, determines whether to hold/release the parking brake according to the above-described determination logic, instead of the parking brake control apparatus  19 . In other words, the control content of the parking brake control apparatus  19  can be integrated into the control unit  13 . 
     The parking brake control apparatus  19  includes the storage unit (memory)  21  (refer to  FIG. 3 ) including, for example, a flash memory, a ROM, a RAM, or an EEPROM. This storage unit  21  stores therein, for example, a program for realizing the above-described logic for determining whether to hold/release the parking brake, and programs for realizing processing illustrated in  FIGS. 4 and 6  that will be described below, i.e., processing programs for holding (applying the braking force) and releasing (stopping the braking) the disk brake  31  based on the signal from the parking brake switch  18  while the vehicle is running. 
     Further, the storage unit  21  stores therein information such as input information, intermediate processing, and an output before emergency stop control is performed while associating them with each other as a map, as indicated by tables illustrated in  FIGS. 5 and 7 . Then, when the emergency stop control is performed on the left and right disk brakes  31 , the left and right disk brakes  31  are controlled according to the processing programs (processing procedures) illustrated in  FIGS. 4 and 6  and the tables illustrated in  FIGS. 5 and 7 . 
     In the present embodiment, the parking brake control apparatus  19  is configured separately from the control unit  13  of the ESC  11 , but may be configured integrally with the control unit  13 . Further, the parking brake control apparatus  19  is configured to control the two left and right disk brakes  31 , but may be provided for each of the left and right disk brakes  31 . In this case, the parking brake control apparatus  19  can also be mounted integrally with the disk brake  31 . 
     The parking brake control apparatus  19  performs stationary application control and dynamic application control on the disk brake  31  depending on the state of the vehicle when the parking brake switch  18  is operated to turn on the braking. The stationary application control is performed when the parking brake switch  18  is operated to turn on the braking while the vehicle is stopped, and generates the braking force for stopping the vehicle on the disk brake  31  when, for example, the parking brake switch  18  is operated by being pulled up. 
     On the other hand, the dynamic application control is performed when the parking brake switch  18  is operated to turn on the braking while the vehicle is running, and, for example, gradually generates the braking force on the disk brake  31  when the parking brake switch  18  is continuously pulled up, and releases the braking force on the disk brake  31  when the driver takes his/her hand off the parking brake switch  18 . 
     As illustrated in  FIG. 3 , the parking brake control apparatus  19  includes, as built-in units thereof, the voltage sensor unit  22  that detects a voltage from the power source line  15 , the left and right motor driving circuits  23  and  23  that drive the left and right electric actuators  43  and  43 , respectively, the left and right current sensor units  24  and  24  that detect respective motor current values of the left and right electric actuators  43  and  43 , and the like. These voltage sensor unit  22 , motor driving circuits  23 , and current sensor units  24  are each connected to the calculation circuit  20 . 
     Due to this configuration, the calculation circuit  20  of the parking brake control apparatus  19  can stop driving the electric actuator  43  based on the motor current value of this electric actuator  43  when, for example, holding (applying) and stopping (releasing) the parking brake. In this case, the calculation circuit  20  determines that a state of the piston  39  established by a rotation-linear motion conversion mechanism  40  transitions to a holding state or a release state when, for example, the motor current value reaches a preset holding threshold value or release threshold value, and stops driving the electric actuator  43 . 
     Further, when the ignition switch is turned on, the parking brake control apparatus  19  regularly diagnoses an abnormality (a failure) in the left and right disk brakes  31  after that (until the ignition switch is turned off). In this case, the parking brake control apparatus  19  diagnoses at least whether the disk brake  31  subjected to the braking request signal is in an abnormal failed i.e., whether the operation of the disk brake  31  subjected to the broking request signal is normal. 
     More specifically, the calculation circuit  20  of the parking brake control apparatus  19  can detect the abnormality (the failure) in the disk brake  31 , in particular, the abnormality (the failure) in the rotation-linear motion conversion mechanism  40  and the electric actuators  43 , according to, for example, the voltage from the power source line  15  (the voltage input to the parking brake control apparatus  19 ) and the motor current value of the left or right electric actuator  43 . Further, the calculation circuit  20  of the parking brake control apparatus  19  can also detect a parameter required to activate the brake apparatus, such as a battery voltage, and a failure in a driving circuit and the like. Further, the parking brake control apparatus  19  (ECU) can also detect an abnormality in a battery voltage monitor, a motor terminal voltage monitor, a relay downstream voltage monitor, and a current monitor, besides each of the above-described detection of the abnormalities. 
     In this case, the calculation circuit  20  can determine, for example, whether any of the left and right disk brakes  31  is normal (for example, the present state is any of a state in which the left wheel has failed, a state in which the right wheel has failed, a state in which both the left and right wheels have failed, and a state in which both the left and right wheels are normal), whether this failure is a physical abnormality or controllable, based on, for example, a difference in the motor current value and a change therein. 
     Further, the parking brake control apparatus  19  also determines whether the parking brake control apparatus  19  itself is normal, i.e., makes a self-diagnosis. As this determination, the parking brake control apparatus  19  can make the self-diagnosis according to, for example, whether a predetermined signal, a predetermined operation, a predetermined calculation result, or the like can be acquired when a predetermined signal (a self-check signal) is fed to the calculation circuit  20 , the voltage sensor unit  22 , or the current sensor unit  24 , or when the calculation circuit  20  performs a predetermined calculation. 
     The failure information of the disk brake  31  and the parking brake control apparatus  19  is stored in the storage unit  21 , and required processing is performed according to this failure information. More specifically, the failure information is used to determine, based on the operation on the parking brake switch  18  (the braking request signal), whether to apply the braking force to the left and right disk brakes  31  (to perform the emergency stop control) while the vehicle is running. A result of the diagnosis made by the parking brake control apparatus  19 , i.e., the state in which the left wheel has failed, the state in which the right wheel has failed, and the state in which both the left and right wheels have failed are reported to the driver with use of a reporting means, such as a display device, an alarm, or a voice synthesizer. 
     Now, in the present embodiment, when the braking request to request the holding (the application) of the parking brake is issued while the vehicle is running, the parking brake control apparatus  19  activates the electric actuator  43  so as to advance the piston  39  on the normal wheel based on the failure states of the left and right disk brakes  31  that are stored in the storage unit  21 . In other words, for example, when the brake pedal  6  is stuck, the booster  7  has failed, or the fluid pressure (the hydraulic pressure) is lost while the vehicle is running, the driver may operate the parking brake switch  18  to turn on the braking (the application request signal may be output from the parking brake switch  18 ) to bring the vehicle to an emergency stop (acquire the braking force). 
     In this case, when the storage unit  21  stores therein that both the left and right disk brakes  31  are in the normal state, the parking brake control apparatus  19  activates the electric actuators  43  so as to advance the pistons  39  on both the left and right disk brakes  31 . 
     On the other hand, when the storage unit  21  stores therein that any one of the left and right disk brakes  31  is in the failed state, the parting brake control apparatus  19  prohibits the operation of this disk brake  31  in the failed state. Then, the parking brake control apparatus  19  activates the electric actuator  43  so as to advance the piston  39  of the other disk brake  31  stored as the normal state. 
     At this time, the parking brake control apparatus  19  can avoid a spin of the vehicle due to sudden braking by, for example, activating (driving) the electric actuator  43  so as to more gradually generate the braking force (the pressing force) than the braking force at the time of the application while the vehicle is stopped. Further, the parking brake control apparatus  19  can also perform ABS control of activating or releasing the electric actuator  43  based on the wheel speed information detected by the control unit  13  of the ESC  11  to prevent or reduce a slip of the wheel. Further, the driver may be notified that the emergency stop control is in progress, i.e., the electric actuator  43  is advancing the piston  39  while the vehicle is running, for example, with use of the report means, such as the display device, the alarm, or the voice synthesizer. 
     In the present embodiment, even when any one of the left and right disk brakes  31  is diagnosed as having the abnormality (the failure), the braking force can be generated on the disk brake  31  on the other normal wheel according to a driver&#39;s intention. As a result, even when any one of the left and right disk brakes  31  has the abnormality, the vehicle can be brought to the emergency stop when the driver operates the parking brake switch  18  to turn on the braking. 
     Next, a configuration of each of the disk brakes  31  and  31  equipped with the electric parking brake function that are mounted on the left and right rear wheels  3  and  3 , respectively, will be described with reference to  FIG. 2 .  FIG. 2  illustrates only one of the left and right disk brakes  31  and  31  respectively mounted in correspondence with the left and right rear wheels  3  and  3 . 
     The disk brake  31  as the brake apparatus is mounted on each of the left side and the right side of the vehicle. These disk brakes  31  are each configured as a hydraulic disk brake equipped with the electric parking brake function. The disk brake  31  forms a brake system (the brake apparatus) together with the parking brake control apparatus  19 . The disk brake  31  includes a mount member  32  attached to a non-rotatable portion on the rear wheel  3  side of the vehicle, the inner-side and outer-side brake pads  33  as a braking member (a fractional member), and a caliper  34  as a brake mechanism provided with the electric actuator  43 . 
     In this case, the disk brake  31  applies the braking force to the wheel (the rear wheel  3 ) and thus the vehicle by advancing the brake pads  33  by the piston  39  with the aid of the hydraulic pressure based on, for example, the operation performed on the brake pedal  6  to press the disk rotor  4  with the brake pads  33 . In addition thereto, the disk brake  31  applies the braking force to the wheel (the rear wheel  3 ) and thus the vehicle by advancing the piston  39  with use of the electric motor  43 B (via the rotation-linear motion conversion mechanism  40 ) to press the disk rotor  4  with the brake pads  33  according to the activation request based on the signal from the parking brake switch  18 , or the activation request based on the above-described logic for determining the application or the release or line parking brake, or the ABS control. 
     The mount member  32  includes a pair of arm portions (not illustrated), a thick support portion  32 A, and a reinforcement beam  32 B. The pair of arm portions extends in an axial direction of the disk rotor  4  (i.e., a disk axial direction) over an outer periphery of the disk rotor  4  and is spaced apart from each other in a disk circumferential direction. The support portion  32 A is provided so as to integrally couple respective proximal end sides of these arm portions with each other and is fixed to the non-rotatable portion of the vehicle at a position on an inner side of the disk rotor  4 . The reinforcement beam  32 B couples respective distal end sides of the arm portions with each other at a position on an outer side of the disk rotor  4 . 
     The inner-side and outer-side brake pads  33  axe disposed so as to be able to abut against both surfaces of the disk rotor  4 , and are supported so as to be movable in the disk axial direction by each of the arm portions of the mount member  32 . The inner-side and outer-side brake pads  33  are pressed against the both surface sides of the disk rotor  4  by the caliper  34  (a caliper main body  35  and the piston  39 ). Due to this configuration, the brake pads  33  apply the braking force to the vehicle by pressing the disk rotor  4  rotating together with the wheel (the rear wheel  3 ). 
     The caliper  34 , which serves as a wheel cylinder, is disposed at the mount member  32  so as to extend over the outer peripheral side of the disk rotor  4 . The caliper  34  includes the caliper main body  35 , the piston  39 , the rotation-linear motion conversion mechanism  40 , the electric actuator  43 , and the like. The caliper main body  35  is supported movably along the axial direction of the disk rotor  4  relative to the each of the arm portions of the mount member  32 . The piston  39  is provided in this caliper main body  35 . The caliper  34  advances the brake pads  33  with use of the piston  39  activated by the hydraulic pressure generated based on the operation performed on the brake pedal  6 . 
     The caliper main body  35  includes the cylinder portion  36 , a bridge portion  37 , and a claw portion  38 . The cylinder portion  36  is formed into a bottomed cylindrical shape having one axial side closed by a partition wall portion  36 A, and another axial side facing the disk rotor  4  that is opened. The bridge portion  37  is formed so as to extend from this cylinder portion in the disk axial direction as if straddling the outer peripheral side of the disk rotor  4 . The claw portion  38  is arranged so as to extend radially inwardly from the bridge portion  37  on an opposite side from the cylinder portion  36 , and abut against the outer-side brake pad  33  from a back surface side thereof. 
     The hydraulic pressure is supplied into the cylinder portion  36  of the caliper main body  35  via the brake-side pipe portion  12 C or  12 D illustrated in  FIG. 1  according to, for example, the operation of pressing the brake pedal  6 . This cylinder portion  36  is formed integrally with the partition wail portion  36 A. The partition wall portion  36 A is located between the cylinder portion  36  and the electric actuator  43 . The partition wall portion  36 A includes an axial through-hole, and an output shaft  43 C of the electric actuator  43  is rotatably inserted on an inner peripheral side of the partition wail portion  36 A. 
     The piston  39  as a pressing member (a movable member), and the rotation-linear motion conversion mechanism  40  are provided in the cylinder portion  36  of the caliper main body  35 . In the present embodiment, the rotation-linear motion conversion mechanism  40  is contained in the piston  39 . However, the rotation-linear motion conversion mechanism  40  does net necessarily have to be contained in the piston  39  as long as the rotation-linear motion conversion mechanism  40  is configured to advance the piston  39 . 
     The piston  39  displaces the brake pad  33  toward or away from the disk rotor  4 . One axial side of the piston  39  is opened, and the other axial side of the piston  39  that faces the inner-side brake pad  33  is closed by a cover portion  39 A. This piston  39  is inserted in the cylinder portion  36 . 
     The piston  39  is also displaced due to the supply of the hydraulic pressure into the cylinder portion  36  based on the pressing of the brake pedal  6  or the like, in addition to being displaced due to the supply of the current to the electric actuator  43  (the electric motor  43 B). In this case, the piston  39  is displaced by the electric actuator  43  (the electric motor  43 B) by being pressed by a linearly movable member  42 . Further, the rotation-linear motion conversion mechanism  40  is contained inside the piston  39 , and the piston  39  is configured to be advanced in art axial direction of the cylinder portion  36  by this rotation-linear motion conversion mechanism  40 . 
     The rotation-linear motion conversion mechanism  40  functions as a pressing member holding mechanism. More specifically, the rotation-linear motion conversion mechanism advances the piston  39  in the caliper  34  by an external force different from the force generated by the supply of the hydraulic pressure into the cylinder port ion  36 , i.e., the force generated by the electric motor  43 , and also holds the advanced piston  39  and line brake pads  33  there. As a result, the parking brake is set into the application state (the holding state). On the other hand, the rotation-linear motion conversion mechanism  40  retracts the piston  39  in an opposite direction from the advancing direction by the electric actuator  43 , thereby setting the parking brake into the release state (the stop state). Then, because the left and right disk brakes  31  are provided for the left and right rear wheels  3 , respectively, the rotation-linear motion conversion mechanisms  40  and the electric actuators  43  are also provided on the left and right sides of the vehicle, respectively. 
     The rotation-linear motion conversion mechanism  40  includes a screw member  41  and the linearly movable member  42  (as a spindle nut mechanism). The screw member  41  includes a rod-like body with a male screw such as a trapezoidal screw thread formed thereon. The linearly movable member  42  includes a female screw hole formed by a trapezoidal screw thread on an inner peripheral side thereof. The linearly movable member  42  serves as a driven member (an advancing member) displaced toward or away from the piston  39  by the electric actuator  43 . In other words, the screw member  41  threadably engaged with the inner peripheral side of the linearly movable member  42  forms a screw mechanism that converts a rotational motion by the electric actuator  43  into a linear motion of the linearly movable member  42 . In this case, the female screw of the linearly movable member  42  and the male screw of the screw member  41  are formed with use of highly irreversible screws, in particular, the trapezoidal screw threads in the present embodiment, thereby realizing the pressing member holding mechanism. 
     The rotation-linear motion conversion mechanism  40  is configured to hold the linearly movable member  42  (i.e., the piston  39 ) at an arbitrary position with the aid of a frictional force (a holding force) even when the power supply to the electric motor  43  is stopped. The rotation-linear motion conversion mechanism  40  may be any mechanism that can hold the piston  39  at a position to which the piston  33  is advanced by the electric actuator  43 . For example, the rotation-linear motion conversion mechanism  40  may be realized with use of another highly irreversible screw than the trapezoidal screw thread, such as a normal screw triangular in cross-section or a worm gear. 
     The screw member  41  provided while being threadably engaged with the inner circumferential side of the linearly movable member  42  includes a flange portion  41 A as a large-diameter flange portion on one axial side, and the other axial side of the screw member  41  extends toward the cover portion  39 A of the piston  39 . The screw member  41  is integrally coupled with the output shaft  43 C of the electric actuator  43  at the flange portion  41 A. Further, an engagement protrusion  42 A is provided on an outer peripheral side of the linearly movable member  42 . The engagement protrusion  42 A prohibits the linearly movable member  42  from rotating relative to the piston  39  (regulates a relative rotation) while allowing the linearly movable member  42  to axially move relative to the piston  39 . Due to this configuration, the linearly movable member  42  linearly moves by being driven by the electric motor  43 B, and contacts the piston  39  to displace this piston  39 . 
     The electric actuator  43  as an electric mechanism is fixed to the caliper main body  35  of the caliper  34 . The electric actuator  43  activates (holds/releases) the disk brake  31  according to the activation request signal of the parking brake switch  18 , the above-described logic for determining the application or she release of the parking brake, or the ABS control. The electric actuator  43  includes a casing  43 A, the electric motor  43 B, a speed reducer (not illustrated) and the output shaft  43 C. The casing  43 A is attached to an outside of the partition wall portion  36 A. The electric motor  43 B is located in this casing  43 A, includes: a stator, a rotor, and the like, and displaces the piston  39  due to supply of power (a current) thereto. The speed reducer amplifies a torque of this electric motor  43 B. The output shaft  43 C outputs the rotational torque after the torque is amplified fay this speed reducer. 
     The electric motor  43 B can be configured as, for example, a direct-current brushed motor. The output shaft  43 C extends axially through the partition wall, portion  36 A of the cylinder portion  36 , and is coupled with an end of the flange portion  41 A of the screw member  41  in the cylinder portion  36  so as to rotate integrally with the screw member  41 . 
     A coupling mechanism between the output shaft  43 C and the screw member  41  can be configured so as to, for example, allow them to move in the axial direction but prohibit them from rotating in the rotational direction. In this case, the output shaft  43 B and the screw member  41  are coupled with each other with use of a known technique such as spline fitting or fitting using a polygonal rod (non-circular fitting). The speed reducer may be embodied with use of, for example, a planetary gear reducer or a worm gear reducer. Further, in a case where the speed reducer is embodied with use of a known speed reducer unable to operate reversely (an irreversible speed reducer) such as the worm gear reducer, a known reversible mechanism such as a ball screw or a ball ramp mechanism can be used as the rotation-linear motion conversion mechanism  40 . In this case, the pressing member holding mechanism can be realized by, for example, the reversible rotation-linear motion conversion mechanism and the irreversible speed reducer. 
     When the driver operates the parking brake switch  18  illustrated in  FIGS. 1 to 3  toward the braking application side (operates the parking brake switch  18  to turn on the braking), power is supplied to the electric motor  43 B via the parking brake control apparatus  19 , and the output shaft  41 C of the electric actuator  43  is rotated. Therefore, the so rev member  41  of the rotation-linear motion conversion mechanism  40  is rotated integrally with the output shaft  43 C in one direction, and advances (drives) the piston  39  toward the disk rotor  4  side via the linearly movable member  42 . As a result, the disk brake  31  sandwiches the disk rotor  4  between the inner-side and outer-side brake pads  33 , thereby being set into the state applying the braking force as the electric parking brake, i.e., the application state (the holding state). 
     On the other hand, when the parking brake switch  18  is operated toward the braking release side (operated to turn off the braking), the screw member  41  of the rotation-linear motion conversion mechanism  40  is rotationally driven by the electric actuator  43  in the other direction (the reverse direction). As a result, the linearly movable member  42  (and the piston  39  if the hydraulic pressure is not supplied) is driven away from the disk rotor  4 , whereby the disk brake  31  is set into the state releasing the application of the braking force as the parking brake, i.e., the stop state (the release state). 
     In this case, in the rotation-linear motion conversion mechanism  40 , when the screw member  41  is rotated relative to the linearly movable member  42 , the rotation of the linearly movable member  42  in the piston  39  is regulated. Therefore, the linearly movable member  42  axially relatively moves according to a rotational angle of the screw member  41 . In this manner, the rotation-linear motion conversion mechanism  40  converts the rotational motion into the linear motion, thereby causing the linearly movable member  42  to advance the piston  39 . Further, along therewith, the rotation-linear motion conversion mechanism  40  holds the linearly movable member  42  at the arbitrary position with the aid of the frictional force with the screw member  41 , thereby holding the piston  39  and the brake pads  33  at the positions to which they ate advanced by the electric actuator  43 . 
     A thrust bearing  44  is provided on the partition wail portion  36 A of the cylinder portion  36  between this partition wall portion  36 A and the flange portion  41 A of the screw member  41 . This thrust bearing  44  receives a thrust load from the screw member  41  together with the partition wail portion  36 A, and facilitates a smooth rotation of the screw member  41  relative to the partition wall portion  36 A. Further, a seal member  45  is provided on the partition wall portion  36 A of the cylinder portion  36  between the partition wall portion  36 A and the output shaft  43 C of the electric actuator  43 . This seal member  45  seals between the partition wall portion  36 A and the output shaft  43 C so as to prevent the brake fluid in the cylinder portion  36  from leaking toward the electric actuator  43  side. 
     Further, a piston seal  46  and a dust boot  47  are provided on the opening end side of the cylinder portion  36 . The piston seal  46  serves as an elastic seal for sealing between this cylinder portion  36  and the piston  39 . The dust boot  47  prevents a foreign object from entering the cylinder portion  36 . The dust boot  47  is a flexible bellows-like seal member, and is attached between the opening end of the cylinder portion  36  and an outer periphery of the cover portion  39 A side of the piston  39 . 
     The disk brake  5  for the front wheel  2  is configured in an approximately similar manner to the disk brake  31  for the rear wheel  3 , except for the provision of the parking brake mechanism. In other words, the disk brake  5  for the front wheel  2  does not include the rotation-linear motion conversion mechanism  40 , the electric actuator  43 , and the like chat are activated as the parking brake, which the disk brakes  31  for the rear wheel  3  includes. However, the disk brake  31  equipped with the electric parking brake function may be provided for the front wheel  2  side, instead of the disk brake  5 . 
     The present embodiment has been described based on the hydraulic disk brake  31  including the electric actuator  43  by way of example. However, the configuration thereof does not necessarily have to be the brake mechanism according to the above-described embodiment, as long as this configuration is a brake mechanism that can press (advance) the braking member (the pad or the shoe) against the braking target member (the disk rotor or the drum) based on the driving by the electric actuator (the electric motor) and hold this pressing force, such as an electric disk Crake including an electric caliper, an electric drum brake that applies the braking force by pressing a show against a drum with use of an electric actuator, a disk brake equipped with an electric drum-type parking brake, and a configuration that activates the parking brake for applying the braking by pulling a cable with use of an electric actuator. 
     The brake apparatus of the four-wheeled automobile according to the present embodiment is configured in the above-described manner, and an operation thereof will be described next. 
     When the driver of the vehicle operates the brake pedal  6  by pressing it, this pressing force is transmitted to the master cylinder  8  via the booster  7 , and the brake hydraulic pressure is generated by the master cylinder  8 . The hydraulic pressure generated in the master cylinder  8  is distributed to each of the disk brakes  5  and  31  via the cylinder-side hydraulic pipes  10 A and  10 B, the ESC  11 , and the brake-side pipe portions  12 A,  12 B,  12 C, and  12 D, thereby applying the braking force to each of the front left and right wheels  2  and the rear left and right wheels  3 . 
     In this case, the disk brake  31  on the rear wheel  3  side operates in the following manner. The hydraulic pressure is supplied into the cylinder portion  36  of the caliper  34  via the brake-side pipe portion  12 C or  12 D, and the piston  39  is slidably displaced toward the inner-side brake pad  33  according to an increase in the hydraulic pressure in the cylinder portion  36 . As a result, the piston  39  presses the inner-side brake pad  33  against one side surface of the disk rotor  4 . A reaction force at this time causes the whole caliper  34  to be slidably displaced toward the inner side relative to each of the above-descried arm portions of the mount member  32 . 
     As a result, the outer leg portion (the claw portion  38 ) of the caliper  34  operates so as to press the outer-side brake pad  33  against the disk rotor  4 , and the disk rotor  4  is sandwiched from axial both sides by the pair of brake pads  33 . As a result, the braking force based on the hydraulic pressure is generated. On the other hand, when the brake operation is released, the supply of the hydraulic pressure into the cylinder portion  36  is stopped, which causes the piston  39  to be displaced so as to be retracted into the cylinder portion  36 . As a result, the inner-side and outer-side brake pads  33  are each separated from the disk rotor  4 , whereby the vehicle is returned into a non-braked state. 
     Next, when the driver operates the parking brake switch  18  toward the braking side (operates the parking brake switch  18  to turn on the braking) while the vehicle is stopped, the application request signal is output. In this case, the parking brake control apparatus  19  performs the stationary application control. Then, power is supplied from the parking brake control apparatus  19  to the electric actuator  43  of the disk brake  31 , whereby the output shaft  43 C of the electric actuator  43  is rotationally driven. The disk brake  31  equipped with the electric parking brake function converts the rotational motion of the electric actuator  43  into the linear motion of the linearly movable member  42  via the screw member  41  of the rotation-linear motion conversion mechanism  40  to axially move the linearly movable member  42  to advance the piston  39 . As a result, the pair of brake pads  33  is pressed against the both surfaces of the disk rotor  4 . 
     At this time, the linearly movable member  42  is maintained in the braking state with the aid or the frictional force (the holding force) generated between the linearly movable member  42  and the screw member  41  with a pressing reaction force transmitted from the piston  39  serving as a normal force, whereby the disk brake  31  for the rear wheel  3  is activated (applied) as the parking brake. In other words, even after the power supply to the electric motor  43 B is stopped, the linearly movable member  42  (thus, the piston  39 ) is held at the braking position by the female screw of the linearly movable member  42  and the male screw of the screw member  41 . 
     On the other hand, when the driver operates the parking brake switch  18  toward the braking release side (operates the parking brake switch  18  to turn off the braking), power is supplied from the parking brake control apparatus  19  to the electric motor  43 B for rotating the motor in the reverse direction, whereby the output shaft  43 C of the electric actuator  43  is rotated in the direction opposite to that at the time of the activation (application) of the parking brake. At this time, the holding of the braking force by the screw member  41  and the linearly movable member  42  is released, and the rotation-linear motion conversion mechanism  40  moves the linearly movable member  42  in a return direction, i.e., into the cylinder portion  36  by a movement amount corresponding to an amount of the reverse rotation of the electric actuator  43 , thereby releasing the braking force of the parking brake (the disk brake  31 ). 
     Then, one possible operation to deal with such a situation that one electric actuator  43  of one of left and right disk brakes  31  is in the failed state (the abnormality or the failure) is to totally prohibit the control (the activation) of the electric actuators  43  of both the left and right disk brakes  31  to, for example, prevent the braking force from being applied to only one wheel. On the other hand, when, for example, the brake pedal  6  is stuck, the booster  7  has failed, or the fluid pressure (the hydraulic pressure) is lost while the vehicle is running, the driver may operate the parking brake switch  18  to turn on the braking so as to activate the parking brake as the auxiliary brake (an emergency brake) with an attempt to stop the vehicle (the emergency stop). Therefore, carelessly prohibiting even the control while the vehicle is running may result in an incapability to apply the braking force according to a driver&#39;s intention even when, for example, the driver operates the parking brake switch  18  to turn on the braking with an attempt to bring the vehicle to the emergency stop. 
     Therefore, in the present embodiment, the parking brake control apparatus  19  is configured to, even when any one of the left and right disk brakes  31  is diagnosed as having the abnormality (the failure), be able to generate the braking force on the disk brake  31  for the other normal wheel according to the driver&#39;s intention (the driver&#39;s operating the parking brake switch  18  to turn on the braking). 
     In other words, the parking brake control apparatus  19  is configured to, when any one of the left and right disk brakes  31  is diagnosed as being in the failed state, prohibit the electric actuator  43  of the one of the disk brakes  31  from operating while the vehicle is running and cause the electric actuator  43  of the other of the disk brakes  31  to operate according to the braking request signal. 
     In the following description, the control processing performed by the calculation circuit  20  of the parking brake control apparatus  19  will be described with reference to  FIG. 4 . 
     When the processing operation illustrated in  FIG. 4  is started, in S 1  (step  1 ), the calculation circuit  20  determines whether the vehicle is running. The calculation circuit  20  can make this determination, about, whether the vehicle is running based on, for example, the wheel speed and/or the vehicle speed acquired via the vehicle data bus  16 . If the calculation circuit  20  determines “YES” in S 1 , i.e., determines that the vehicle is running, the operation proceeds to S 2 . On the other hand, if the calculation circuit  20  determines “NO” in S 1 , i.e., determines that the vehicle is not running (the vehicle is stopped), the operation proceeds to RETURN. 
     In S 2 , the calculation circuit  20  determines whether tooth the wheels in the left and right directions are in an unlocked state. The calculation circuit  20  can make this determination based on, for example, the signal from the wheel speed sensor that is acquired via the vehicle data has  16 . If the calculation circuit  20  determines “YES” in S 2 , i.e., determines that both the wheels are unlocked, the operation proceeds to S 3 . On the other hand, if the calculation circuit  20  determines “NO” in S 2 , i.e., determines that at least one of the wheels is in a locked state, the operation proceeds to RETURN. The determination about whether both the wheels are in the unlocked state may be omitted. 
     In S 3 , the calculation circuit  20  determines whether only any one wheel of the left and right disk brakes  31  is in the failed state. The calculation circuit  20  makes this determination based on the failure information for each of the left and right disk brakes  31  that is stored in the storage unit  21  of the parking brake control apparatus  19  as described above. In other words, the parting brake apparatus  19  regularly diagnoses the abnormality (the failure) in the left and right disk brakes  31  when the ignition switch is turned on, and such failure information is stored in an updatable manner in the storage unit  21 , for example, as indicated by the table designed to be used before the emergency stop control is performed illustrated in  FIG. 5 . 
     More specifically, the storage unit  21  stores therein whether the failure state of the brake apparatus is the state in which only one wheel has failed, the state in which the left wheel has failed, the state in which the right wheel has failed, the state in which both the left and right wheels have failed, or the state in which both the left and right wheels are normal, as the input information before the emergency stop control is performed illustrated in  FIG. 5 . Further, the failure information is information regarding whether the activation of the disk brake  31  due to the braking request signal is normal. In other words, the failure information does not include a failure (a failed state) in the activation of the disk brake  31  due to, for example, a loss of the hydraulic pressure. 
     If the calculation circuit  20  determines “YES” in S 3 , i.e., determines that the failure state of the brake apparatus is the state in which only one wheel has failed, the operation proceeds to S 4 . On the other hand, if the calculation circuit  20  determines “NO” in S 3 , i.e., determines that the failure state of the brake apparatus is not the state in which only one wheel has failed, the operation proceeds to step S 7 . In the next step, S 4 , the calculation circuit  20  determines whether the left brake apparatus (the disk brake  31  on the left side) is in the failed state. The calculation circuit  20  makes this determination based on the state in which the left wheel has failed and the state in which the right wheel has failed that are stored in the storage unit  21 . 
     If the calculation circuit  20  determines “YES” in S 4 , i.e., determines that the failure state of the brake apparatus is the state in which the left wheel has failed, the operation proceeds to the next step, step S 5 , where the calculation circuit  20  prohibits the emergency stop control on the left brake apparatus (the disk brake  31  on the left side). Then, the operation proceeds to the next step, S 9 . On the other hand, if the calculation circuit  20  determines “NO” in S 4 , i.e., determines that the failure state of the brake apparatus is the state in which the right wheel has failed, the operation proceeds to the next step, step S 6 , where the calculation circuit  20  prohibits the emergency stop control on the right brake apparatus (the disk brake  31  on the right side). Then, the operation proceeds to the next step, S 9 . 
     In S 7 , the calculation circuit  20  determines whether the failure state of the brake apparatus is the state in which both the wheels have failed. The calculation circuit  20  makes this determination based on the state in which both the left and right wheels have failed and the state in which both the left and right wheels are normal that axe stored in the storage unit  21 . If the calculation circuit  20  determines “YES” in S 7 , i.e., determines that the failure state of the brake apparatus is the state in which both the left and right wheels have failed, the operation proceeds to the next step, S 8 , where the calculation circuit  20  prohibits the emergency stop control on the brake apparatuses on the both sides (the left and right disk brakes  31 ). Then, the operation proceeds to the next step, S 9 . On the other hand, if the calculation circuit  20  determines “NO” in S 7 , i.e., determines that the failure state of the brake apparatus is the state in which both the left and right wheels are normal, the operation proceeds to the next step, S 9 . 
     In S 9 , the calculation circuit  20  determines whether the request for the emergency stop control (the dynamic application request) is issued. The calculation circuit  20  makes this determination based on whether the driver operates the parking brake switch  18  to turn on the braking. If the calculation circuit  20  determines “YES” in S 9 , i.e., the driver operates the parking brake switch  18  to turn on the braking, the operation proceeds to the next step, S 10 . On the other hand, if the calculation circuit  20  determines “NO” in S 9 , i.e., the driver does not operate the parking brake switch  18  to turn on the braking, the operation proceeds to RETURN. 
     In S 10 , the calculation circuit  20  starts the emergency stop control on the brake apparatus on the normal side. More specifically, the calculation circuit  20  prohibits the operation of the electric actuator  43  of the disk brake  31  for which the emergency stop control has been prohibited in S 5 , S 8 , or S 8 , and activates the electric actuator  43  of the disk brake  31  for which the emergency stop control has not been prohibited in S 5 , S 6 , or S 8  to advance the piston  39 . At this time, the calculation circuit  20  can avoid the spin of the vehicle due to the sudden braking by activating (driving) the electric actuator  43  so as to more gradually increase the braking force than, for example, the braking force at the time of the application when the vehicle is stopped. Further, the calculation circuit  20  may perform the ABS control of preventing or reducing a slip of the wheel by activating or releasing the electric actuator  43  based on the wheel speed information detected by the control unit  13  of the ESC  11 . 
     Next, after performing the control processing designed to be performed before the emergency stop control is performed as illustrated in  FIG. 4 , the calculation circuit  20  performs processing designed to be performed while the emergency stop control is in progress as illustrated in  FIG. 6 . More specifically, when the processing operation illustrated in  FIG. 6  is started, in S 11 , the calculation circuit  20  determines whether the vehicle is running. The calculation circuit  20  makes this determination in a similar manner to the determination processing in above-described S 1  illustrated in  FIG. 4 . If the calculation circuit  20  determines “YES” in S 11 , i.e., determines that the vehicle is running, the operation proceeds to the next step, S 12 . On the other hand, if the calculation circuit  20  determines “NO” in S 11 , i.e., determines that the vehicle is not running (the vehicle is stopped), the operation proceeds to RETURN. 
     In S 12 , the calculation circuit  20  determines whether the emergency stop control is in progress. The calculation circuit  20  can make this determination based on whether the driver is operating the parking brake switch  18  to turn on the braking. If the calculation circuit  20  determines “YES” in S 12 , i.e., determines that the emergency stop control is in progress, the operation, proceeds to S 13 . On the other hand, if the calculation circuit  20  determines “NO” in S 12 , i.e., determines that the emergency stop control is not in progress, the operation proceeds to RETURN. 
     In S 13  to S 18 , the calculation circuit  20  performs similar processing to S 3  to S 8  illustrated in  FIG. 4 , and then the operation proceeds to the next step, step S 19 . result of the processing in S 13  to S 18  is reported to the driver as the failure information (the state in which the left wheel has failed, the state in which the right wheel has failed, the state in which both the left and right wheels have failed, or the state in which both the left and right wheels are normal). 
     Then, in S 19 , the calculation circuit  20  determines whether the request for the emergency stop control is issued. The calculation circuit  20  can make this determination based on whether the driver is operating the parking brake switch  18  to turn oh the braking. In this case, the driver has been already in the middle of performing the emergency stop control in S 12 , so that the determination in S 19  is substantially equivalent to determining whether the driver has switched the parking brake switch  18  from operating it to turn on the braking to operating it to turn off the braking. 
     If the calculation circuit  20  determines “YES” in S 19 , i.e., determines that the driver continues operating the parking brake switch  18  to turn on the braking and the request for the emergency stop control is issued, the operation proceeds to step S 20 , where the calculation circuit  20  continues the emergency stop control on the disk brake  31  for which the emergency stop control has not been prohibited in S 15 , S 16 , or S 18 . 
     On the other hand, if the calculation circuit  20  determines “NO” in S 19 , i.e., determines that the driver is not operating the parking brake switch  18  to turn on the braking (the driver is operating the parking brake switch  18  to turn off the braking) and the request for the emergency stop control is released, the operation proceeds to S 21 , where the calculation circuit  20  releases the emergency stop control on the disk brake  31  on the normal side for which the emergency stop control has not been prohibited in S 15 , S 16 , or S 18 . As a result, the emergency stop control (i.e., the braking state) by all of the disk brakes  31  is released. 
     Therefore, according to the present embodiment, when one of the left and right disk brakes  31  is in the failed state, the parking brake control apparatus  19  prohibits the emergency stop control on this disk brake  31  in the failed state. Then, the parking brake control apparatus  19  can generate the braking force on the disk brake  31  on the normal side by the driver&#39;s operating the parking brake switch  18  to turn on the braking, when activating the parking brake so as to bring the vehicle to the emergency stop while the vehicle is running. 
     In other words, the parking brake control apparatus  19  is configured to activate the electric actuator  43  on the normal side of the left and right disk brakes  31  to advance the piston  39 , when the parking brake switch  18  is operated toward the braking side while the vehicle is running. Due to this configuration, the parking brake control apparatus  19  can generate the braking force on the disk brake  31  on the normal side according to the driver&#39;s intention to stop the vehicle even when one of the left and right disk brakes  31  is in the failed state. 
     Further, the parking brake control apparatus  19  prohibits the emergency stop control on the disk brake  31  diagnosed as being in the failed state. Due to this prohibition, the parking brake control apparatus  19  can prevent or reduce a malfunction of the disk brake  31  in the failed state, thereby ensuring stability of the vehicle. 
     Further, the parking brake control apparatus  19  prohibits the emergency stop control on the disk brake  31  in the failed state, and thereby can cut off the supply of the current from the battery  14  to this disk brake  31 . This cutoff can reduce a load on the battery  14 . 
     Further, even when any one of the left and right disk brakes  31  is placed in the failed state while the emergency stop control is in progress, the parking brake control apparatus  19  can generate the braking force on the disk brake  31  on the normal wheel side by prohibiting the emergency stop control on the disk brake  31  in the failed state and the driver&#39;s continuing operating the parking brake switch  18  to turn on the braking. 
     In other words, the parking brake control apparatus  19  is configured to, even when one of the left and right disk brakes  31  is placed in the failed state with, for example, the running speed of the vehicle slowed down to some degree during the execution of the emergency stop control while the vehicle is running, activate the electric actuator  43  on the other normal wheel side to advance the piston  39 . Due to this configuration, even when one of the left and right brake apparatuses is in the failed state, the parking brake control apparatus  19  can stop the vehicle further safely, i.e., while preventing or reducing the spin or the like of the vehicle, by generating the braking force on the brake apparatus on the normal wheel side according to the driver&#39;s intention. 
     The above-described embodiment has been described assuming that the parking brake control apparatus  19  diagnoses the failed state in which the left or right disk brake  31  is abnormal by way of example. However, the present invention is not limited thereto. For example, the parking brake apparatus may be configured to diagnose the operation of the left or right disk brake  31  as being normal, and permit the operation of the emergency stop control on the disk brake diagnosed as being normal. 
     Further, the above-described embodiment has been described assuming that the disk brake  31  equipped with the electric parking brake function is used as each of the brakes on the rear left and right rear wheel sides by way of example. However, the present invention is not limited thereto. For example, the disk brake equipped with the electric parking brake function may be used as each of the brakes on the all the wheels (ail of the four wheels). In other words, the disk brake equipped with the electric parking brake function can be used as the brake apparatuses on at least the pair of left and right wheels of the vehicle. 
     Further, for the normal wheel, the control may foe changed according to the speed. For example, the parking brake control apparatus  19  may perform the control so as to reduce the braking force when the vehicle is running at a high speed, and perform the control so as to increase the braking force when the vehicle is running at a low speed. 
     Having described several embodiments of the present invention, the above-described embodiments of the present invention are intended to only facilitate the understanding of the present invention, and are not intended to limit the present invention thereto. Needless to say, the present invention can be modified or improved without departing from the spirit of the present invention, and includes equivalents thereof. Further, the individual components described in the claims and the specification can be arbitrarily combined or omitted within a range that allows them to remain capable of achieving at least a part of the above-described objects or producing at least a part of the above-described advantageous effects. 
     The present application claims priority to Japanese Patent Application No. 2014-266811 filed on Dec. 27, 2014. The entire disclosure of Japanese Patent Application No. 2014-266811 filed on Dec. 27, 2014 including the specification, the claims, the drawings, and the abstract is incorporated herein by reference in its entirety. 
     REFERENCE SIGH LIST 
       2  front wheel (wheel) 
       3  rear wheel (wheel) 
       4  disk rotor (rotational member) 
       6  brake pedal 
       18  parking brake switch 
       19  parking brake control apparatus (control apparatus) 
       31  disk brake (brake apparatus) 
       33  brake pad (frictional member) 
       39  piston 
       43  electric actuator (electric mechanism)