Vehicle brake hydraulic pressure control apparatus

1. A vehicle brake hydraulic pressure control apparatus comprising:

a hydraulic pressure adjusting unit that individually adjusts brake fluid pressures acting on wheel brakes for right and left front wheels and right and left rear wheels;

a split road determining section that determines as to whether or not road surfaces which the right wheels and the left wheels are in contact with constitute a split road in which coefficients of friction of the road surfaces are different from each other;

a differential pressure control section that, in a state where the split road determining section determines during execution of antilock braking control that the road surfaces constitute the split road, determines command pressures for the right and left wheel brakes so that differential pressures between the brake fluid pressures of the right and left wheel brakes are equal to or less than a permissible differential pressure; and

a hydraulic pressure adjusting and driving section that controls the hydraulic pressure adjusting unit based on the command pressures determined by the differential pressure control section, wherein

the differential pressure control section sets a permissible differential pressure from start of differential pressure control until elapse of a first predetermined time period to be smaller than that after the elapse of the first predetermined time period, and

the permissible differential pressure is increased after the elapse of the first predetermined time period, and

the timing at which the differential pressure is increased is determined based on the first predetermined time period without depending on operating speed of a brake pedal and operating amount of the brake pedal.

2. The apparatus according to claim 1 , further comprising:

a vehicle body velocity acquiring section that acquires a velocity of a vehicle body, wherein

the differential pressure control section sets the permissible differential pressure from the start of the differential pressure control until the elapse of the first predetermined time period, based on the velocity of the vehicle body acquired by the vehicle body velocity acquiring section.

3. The apparatus according to claim 1 , further comprising:

a vehicle body velocity acquiring section that acquires a velocity of a vehicle body, wherein

the differential pressure control section sets the permissible differential pressure from the start of the differential pressure control until the elapse of the first predetermined time period so that the permissible differential pressure increases as the velocity of the vehicle body acquired by the vehicle body velocity acquiring section decreases.

4. The apparatus according to claim 2 , wherein the differential pressure control section searches for the permissible differential pressure from the start of the differential pressure control until the elapse of the first predetermined time period, based on a map in which the permissible differential pressure is determined in response to the velocity of the vehicle body in advance.

5. The apparatus according to claim 3 , wherein the differential pressure control section searches for the permissible differential pressure from the start of the differential pressure control until the elapse of the first predetermined time period, based on a map in which the permissible differential pressure is determined in response to the velocity of the vehicle body in advance.

6. The apparatus according to claim 1 , further comprising:

a steering angle sensor that detects a steering angle; and

a yaw rate sensor that detects a yaw rate of a vehicle, wherein

the differential pressure control section sets the permissible differential pressure after the elapse of the first predetermined time period since the start of the differential pressure control, based on the steering angle detected by the steering angle sensor and the yaw rate detected by the yaw rate sensor.

7. The apparatus according to claim 2 , further comprising:

a steering angle sensor that detects a steering angle; and

a yaw rate sensor that detects a yaw rate of a vehicle, wherein

the differential pressure control section sets the permissible differential pressure after the elapse of the first predetermined time period since the start of the differential pressure control, based on the steering angle detected by the steering angle sensor and the yaw rate detected by the yaw rate sensor.

8. The apparatus according to claim 3 , further comprising:

a steering angle sensor that detects a steering angle; and

a yaw rate sensor that detects a yaw rate of a vehicle, wherein

the differential pressure control section sets the permissible differential pressure after the elapse of the first predetermined time period since the start of the differential pressure control, based on the steering angle detected by the steering angle sensor and the yaw rate detected by the yaw rate sensor.

9. The apparatus according to claim 1 , further comprising:

an antilock braking control section that determines as to whether or not executing the antilock braking control, and that calculates a hydraulic pressure control amount during the execution of the antilock braking control, wherein

in the state where the split road determining section determines that the road surfaces constitute the split road, the differential pressure control section calculates the differential pressures between the brake fluid pressures of the right and left wheel brakes,

the hydraulic pressure adjusting and driving section controls the hydraulic pressure adjusting unit based on the hydraulic pressure control amount calculated by the antilock braking control section and the differential pressures calculated by the differential pressure control section,

if the brake fluid pressure of the wheel brake, among the wheel brakes for the right and left front wheels, being located on a higher coefficient-of-friction road side at a time when the differential pressure control between the wheel brakes for the right and left front wheels is started decreases in association with the antilock braking control after the differential pressure control is started, the differential pressure control section determines the brake fluid pressure of the wheel brake on the higher coefficient-of-friction road side at a time when the decrease of the brake fluid pressure is started, to be the command pressure for the wheel brake on the higher coefficient-of-friction road side,

the differential pressure control section continues to output the determined command pressure for the wheel brake on the higher coefficient-of-friction road side for a second predetermined time period, and

the hydraulic pressure adjusting and driving section controls the hydraulic pressure adjusting unit so that the brake fluid pressure of the wheel brake on the higher coefficient-of-friction road side becomes the determined command pressure for the wheel brake on the higher coefficient-of-friction road side.

10. The apparatus according to claim 2 , further comprising:

an antilock braking control section that determines as to whether or not executing the antilock braking control, and that calculates a hydraulic pressure control amount, wherein

in the state where the split road determining section determines that the road surfaces constitute the split road, the differential pressure control section calculates the differential pressure between the brake fluid pressures of the right and left wheel brakes,

the hydraulic pressure adjusting and driving section controls the hydraulic pressure adjusting unit based on the hydraulic pressure control amount calculated by the antilock braking control section and the differential pressure calculated by the differential pressure control section,

if the brake fluid pressure of the wheel brake, among the wheel brakes for the right and left front wheels, being located on a higher coefficient-of-friction road side at a time when the differential pressure control between the wheel brakes for the right and left front wheels is started decreases in association with the antilock braking control after the differential pressure control is started, the differential pressure control section determines the brake fluid pressure of the wheel brake on the higher coefficient-of-friction road side at a time when the decrease of the brake fluid pressure is started, to be the command pressure for the wheel brake on the higher coefficient-of-friction road side,

the differential pressure control section continues to output the determined command pressure for the wheel brake on the higher coefficient-of-friction road side for a second predetermined time period, and

the hydraulic pressure adjusting and driving section controls the hydraulic pressure adjusting unit so that the brake fluid pressure of the wheel brake on the higher coefficient-of-friction road side becomes the determined command pressure for the wheel brake on the higher coefficient-of-friction road side.

11. The apparatus according to claim 3 , further comprising:

an antilock braking control section that determines as to whether or not executing the antilock braking control, and that calculates a hydraulic pressure control amount, wherein

in the state where the split road determining section determines that the road surfaces constitute the split road, the differential pressure control section calculates the differential pressure between the brake fluid pressures of the right and left wheel brakes,

the hydraulic pressure adjusting and driving section controls the hydraulic pressure adjusting unit based on the hydraulic pressure control amount calculated by the antilock braking control section and the differential pressure calculated by the differential pressure control section,

if the brake fluid pressure of the wheel brake, among the wheel brakes for the right and left front wheels, being located on a higher coefficient-of-friction road side at a time when the differential pressure control between the wheel brakes for the right and left front wheels is started decreases in association with the antilock braking control after the differential pressure control is started, the differential pressure control section determines the brake fluid pressure of the wheel brake on the higher coefficient-of-friction road side at a time when the decrease of the brake fluid pressure is started, to be the command pressure for the wheel brake on the higher coefficient-of-friction road side,

the differential pressure control section continues to output the determined command pressure for the wheel brake on the higher coefficient-of-friction road side for a second predetermined time period, and

the hydraulic pressure adjusting and driving section controls the hydraulic pressure adjusting unit so that the brake fluid pressure of the wheel brake on the higher coefficient-of-friction road side becomes the determined command pressure for the wheel brake on the higher coefficient-of-friction road side.

12. The apparatus according to claim 9 , wherein after the second predetermined time period has been elapsed since the decrease of the brake fluid pressure by the antilock braking control is started after the differential pressure control is started, the differential pressure control section corrects the command pressure for the wheel brake on the higher coefficient-of-friction road side based on a command pressure correction amount so as to increase the command pressure for the wheel brake on the higher coefficient-of-friction road side.

13. The apparatus according to claim 9 , wherein after the second predetermined time period has been elapsed since the decrease of the brake fluid pressure by the antilock braking control is started after the differential pressure control is started, the differential pressure control section corrects the command pressure for the wheel brake on the higher coefficient-of-friction road side by adding a command pressure correction amount to a current command pressure for the wheel brake on the higher coefficient-of-friction road side.

14. The apparatus according to claim 12 , wherein the command pressure correction amount is set so as to subsequently increase with time after the second predetermined time period has been elapsed.

15. The apparatus according to claim 1 , further comprising:

a master cylinder that outputs a hydraulic pressure in response to the operation amount of the brake pedal by a driver;

a pressure sensor that detects the output pressure of the master cylinder, and

a correction reference value setting section that sets a correction reference value based on the detected value by the pressure sensor, wherein

in the state where the split road determining section determines during the execution of the antilock braking control that the road surfaces constitute the split road, the differential pressure control section calculates the differential pressures between the brake fluid pressures of the right and left wheel brakes so that the brake fluid pressure of each wheel brake on a higher coefficient-of-friction road side is larger than that of the corresponding wheel brake on a lower coefficient-of-friction road side, and

after the elapse of the first predetermined time period, if the differential pressure control section determines based on the detected value by the pressure sensor that a further depression operation is performed for the brake pedal, the differential pressure control section adds a part of an amount by which the output pressure of the master cylinder detected by the pressure sensor exceeds the correction reference value set by the correction reference value setting section to command pressures, in a non-further depression operation state, for the wheel brakes on the higher coefficient-of-friction road side and determines brake fluid pressures obtained by the adding to be the command pressures, in a further depression operation state, for the wheel brakes on the higher coefficient-of-friction road side.

16. The apparatus according to claim 2 , further comprising:

a master cylinder that outputs a hydraulic pressure in response to the operation amount of the brake pedal by a driver;

a pressure sensor that detects the output pressure of the master cylinder, and

a correction reference value setting section that sets a correction reference value based on the detected value by the pressure sensor, wherein

in the state where the split road determining section determines during the execution of the antilock braking control that the road surfaces constitute the split road, the differential pressure control section calculates the differential pressures between the brake fluid pressures of the right and left wheel brakes so that the brake fluid pressure of each wheel brake on a higher coefficient-of-friction road side is larger than that of the corresponding wheel brake on a lower coefficient-of-friction road side, and

after the elapse of the first predetermined time period, if the differential pressure control section determines based on the detected value by the pressure sensor that a further depression operation is performed for the brake pedal, the differential pressure control section adds a part of an amount by which the output pressure of the master cylinder detected by the pressure sensor exceeds the correction reference value set by the correction reference value setting section to command pressures, in a non-further depression operation state, for the wheel brakes on the higher coefficient-of-friction road side and determines brake fluid pressures obtained by the adding to be the command pressures, in a further depression operation state, for the wheel brakes on the higher coefficient-of-friction road side.

17. The apparatus according to claim 3 , further comprising:

a master cylinder that outputs a hydraulic pressure in response to the operation amount of the brake pedal by a driver;

a pressure sensor that detects the output pressure of the master cylinder, and

a correction reference value setting section that sets a correction reference value based on the detected value by the pressure sensor, wherein

in the state where the split road determining section determines during the execution of the antilock braking control that the road surfaces constitute the split road, the differential pressure control section calculates the differential pressures between the brake fluid pressures of the right and left wheel brakes so that the brake fluid pressure of each wheel brake on a higher coefficient-of-friction road side is larger than that of the corresponding wheel brake on a lower coefficient-of-friction road side, and

after the elapse of the first predetermined time period, if the differential pressure control section determines based on the detected value by the pressure sensor that a further depression operation is performed for the brake pedal, the differential pressure control section adds a part of an amount by which the output pressure of the master cylinder detected by the pressure sensor exceeds the correction reference value set by the correction reference value setting section to command pressures, in a non-further depression operation state, for the wheel brakes on the higher coefficient-of-friction road side and determines brake fluid pressures obtained by the adding to be the command pressures, in a further depression operation state, for the wheel brakes on the higher coefficient-of-friction road side.

18. The apparatus according to claim 15 , wherein the correction reference value setting section performs a filter process for the detected value by the pressure sensor and sets the correction reference value by adding an offset value to a value obtained by the filter process.

19. The apparatus according to claim 1 , wherein:

regardless of the operating speed of the brake pedal and operating amount of the brake pedal, the timing at which the difference between the brake fluid pressure on the high coefficient-of-friction side and the brake fluid pressure on the low coefficient-of-friction side is increased is a point in time when the first predetermined time period elapses since a time at which the differential pressure control is started.