Antiskid control system for four-wheel-drive vehicle

1. An antiskid control system for a four-wheel-drive vehicle, comprising:

a master cylinder generating a braking hydraulic pressure;

a braking cylinder attached to each wheel of the four-wheel-drive vehicle, the braking cylinder generating a braking force by receiving the braking hydraulic pressure;

a switch control valve disposed between the master cylinder and the braking cylinder, the switch control valve switching between a pressure decreasing control condition of decreasing the hydraulic pressure of the braking cylinder and a pressure increasing control condition of increasing the hydraulic pressure;

a wheel speed detector attached to each wheel, the wheel speed detector detecting a wheel speed of each wheel and outputting a wheel speed indicative signal; and

a control unit coupled to the switch control valve and the wheel speed detector, the control unit being arranged,

to calculate a target control speed on the basis of the wheel speed of each wheel,

to calculate a wheel acceleration of each wheel from the wheel speed,

to set the switch control valve at the pressure decreasing control condition when the wheel speed reaches the target control speed,

to set the switch control valve at the pressure increasing control condition when the wheel acceleration is outside of a range from zero to a predetermined positive value after the switch control valve is set at the pressure decreasing control condition,

to calculate a difference between an average of the wheel accelerations of front wheels and an average of the wheel acceleration of rear wheels,

to determine that a driveline vibration is generated when the difference is greater than a vibration determination threshold, and

to execute a vibration convergence control for converging the driveline vibration when the driveline vibration is generated,

wherein the vibration determination threshold includes a vibration determination starting threshold for determining a starting point of the vibration convergence control and a vibration determination terminating threshold for determining a terminating point of the vibration convergence control.

2. An antiskid control system for a four-wheel-drive vehicle, comprising:

a master cylinder generating a braking hydraulic pressure:

a braking cylinder attached to each wheel of the four-wheel-drive vehicle, the braking cylinder generating a braking force by receiving the braking hydraulic pressure:

a switch control valve disposed between the master cylinder and the braking cylinder, the switch control valve switching between a pressure decreasing control condition of decreasing the hydraulic pressure of the braking cylinder and a pressure increasing control condition of increasing the hydraulic pressure:

a wheel speed detector attached to each wheel, the wheel speed detector detecting a wheel speed of each wheel and outputting a wheel speed indicative signal; and

a control unit coupled to the switch control valve and the wheel speed detector, the control unit being arranged,

to calculate a target control speed on the basis of the wheel speed of each wheel,

to calculate a wheel acceleration of each wheel from the wheel speed,

to set the switch control valve at the pressure decreasing control condition when the wheel speed reaches the target control speed,

to set the switch control valve at the pressure increasing control condition when the wheel acceleration is outside of a range from zero to a predetermined positive value after the switch control valve is set at the pressure decreasing control condition,

to calculate a difference between an average of the wheel accelerations of front wheels and an average of the wheel acceleration of rear wheels,

to determine that a driveline vibration is generated when the difference is greater than a vibration determination threshold, and

to execute a vibration convergence control for converging the driveline vibration when the driveline vibration is generated,

wherein the vibration convergence control includes a process for executing the pressure decreasing control for a predetermined time period and a process for prohibiting the pressure increasing control for a predetermined time period after the execution of the pressure decreasing control and for executing a pressure holding control.

3. An antiskid brake control system for a four-wheel-drive vehicle, comprising:

a master cylinder generating a braking hydraulic pressure;

a braking cylinder attached to each wheel of the four-wheel-drive vehicle, the braking cylinder generating a braking force by receiving the braking hydraulic pressure;

a switch control valve disposed between the master cylinder and the braking cylinder, the switch control valve switching between a pressure decreasing control condition of decreasing the hydraulic pressure of the braking cylinder and a pressure increasing control condition of increasing the hydraulic pressure;

a wheel speed detector attached to each wheel, the wheel speed detector detecting a wheel speed of each wheel and outputting a wheel speed indicative signal; and

a control unit coupled to the switch control valve and the wheel speed detector, the control unit being arranged,

to calculate a target control speed on the basis of the wheel speed of each wheel,

to calculate a wheel acceleration of each wheel from the wheel speed,

to set the switch control valve at the pressure decreasing control condition when the wheel speed reaches the target control speed,

to set the switch control valve at the pressure increasing control condition when the wheel acceleration is outside of a range from zero to a predetermined positive value after the switch control valve is set at the pressure decreasing control condition,

to calculate a difference between an average of the wheel accelerations of front wheels and an average of the wheel acceleration of rear wheels,

to determine that a driveline vibration is generated when the difference is greater than a vibration determination threshold, and to execute a vibration convergence control for converging the driveline vibration when the driveline vibration is generated,

wherein the control unit is further arranged to determine that a small vibration is generated when the difference is greater than the small vibration determination threshold, and to execute a small vibration convergence control for converging the small vibration when the small vibration is generated.

4. The antiskid control system as claimed in claim 3 , wherein the small vibration convergence control includes a process for prohibiting the pressure increasing control for a predetermined time period.

5. The antiskid control system as claimed in claim 3 , wherein the control unit is further arranged to detect the generating condition of the small vibration when the number of times that the difference becomes greater than or equal to the predetermined small vibration determination threshold is continuously counted up to a predetermined number of times.

6. The antiskid control system as claimed in claim 3 , wherein the small vibration includes a vibration generated when the vehicle travels a bad road.

7. An antiskid control system for a four-wheel-drive vehicle, comprising:

a master cylinder generating a braking hydraulic pressure;

a braking cylinder attached to each wheel of the four-wheel-drive vehicle, the braking cylinder generating a braking force by receiving the braking hydraulic pressure;

a switch control device switching between a pressure decreasing control condition for decreasing the hydraulic pressure of the braking cylinder and a pressure increasing control condition for increasing the hydraulic pressure;

a wheel speed detecting means for detecting a wheel speed of each wheel;

a pseudo-speed calculating means for calculating a pseudo-speed on the basis of the wheel speed of each wheel;

a target control speed calculating means for calculating a target control speed from the pseudo-speed upon taking account of a slipping of the wheel;

a wheel acceleration calculating means for calculating an acceleration of each wheel from the wheel speed;

a brake hydraulic pressure control means for controlling the switch control device at the pressure decreasing control condition when the wheel speed reaches the target control speed, the brake hydraulic pressure control means controlling the switch control device at the pressure increasing control condition when the wheel acceleration is outside of a range from zero to a predetermined positive value after the execution of the pressure decreasing control;

a difference calculating means for calculating an acceleration difference between an average of the wheel accelerations of front wheels and an average of the wheel acceleration of rear wheels;

a driveline vibration detecting means for detecting the generation of a driveline vibration by comparing the acceleration difference with a vibration determination threshold; and

a vibration convergence controlling means for executing a driveline vibration convergence control when the driveline vibration is generating,

wherein the driveline vibration is executed by the vibration convergence controlling means only when the acceleration difference is greater than the vibration determination threshold, and

wherein the driveline vibration is halted by the vibration convergence controlling means when the acceleration difference becomes less than a vibration termination threshold and the vibration termination threshold is slightly less than the vibration determination threshold.

8. An antiskid control system for a four-wheel-drive vehicle, the antiskid control system executing an antiskid control of each wheel of the vehicle by controlling a switch control valve disposed between a master cylinder and a wheel cylinder of each wheel of the vehicle, the antiskid control system comprising:

a control unit programmed

to detect a wheel speed of each wheel,

to calculate a target control speed on the basis of the wheel speed of each wheel,

to calculate a wheel acceleration of each wheel from the wheel speed,

to command the switch control valve to execute a pressure decreasing control of decreasing a hydraulic pressure of the wheel cylinder when the wheel speed reaches the target control speed,

to command the switch control valve to execute a pressure increasing control of increasing the hydraulic pressure of the wheel cylinder when the wheel acceleration is smaller than zero or greater than a predetermined positive value after the pressure decreasing control is executed,

to calculate an acceleration difference between an average of the wheel accelerations of front wheels and an average of the wheel acceleration of rear wheels,

to determine that a driveline vibration is generated when the acceleration difference is greater than or equal to the vibration determination threshold, and

to execute a control for converging the driveline vibration when the driveline vibration is generated,

wherein the driveline vibration converging operation is executed only when the acceleration difference is greater than the vibration determination threshold, and

wherein the control unit is further programmed

to halt the vibration convergence control when the acceleration difference becomes less than a vibration termination threshold and the vibration termination threshold is slightly less than the vibration determination threshold.