1. Field of the Invention
The present invention relates to an anti-skid controlling apparatus which controls the braking force applied to the wheels of a vehicle so that the wheels are not locked when braked.
2. Description of the Prior Art
When a running vehicle is abruptly braked and its wheels are locked thereby, the vehicle's purchase on the road surface may become unstable depending on the road surface condition. To prevent the wheels from getting locked in such a situation, the anti-skid controlling apparatus has been conventionally used on board the vehicle. The controlling apparatus controls the brake fluid pressure in the wheel cylinders attached to the wheels so as to optimally decrease or increase the braking force applied to the wheels.
As the brake fluid pressure in the wheel cylinders is raised, the wheel velocity abruptly drops just before the friction coefficient .mu. of the wheels reaches its maximum. This characteristic is utilized by prior art anti-skid controlling apparatus that control the brake fluid pressure in keeping with changes in the wheel velocity and wheel acceleration. The braking force is controlled in this manner so that the slip ratio of the wheels settles at around 20% where a maximum friction coefficient is acquired.
The representative prior art anti-skid controlling apparatus works as follows. The rotating speed of each of the wheels, i.e., the wheel velocity, is first detected. The vehicle velocity is deducted from the wheel velocity. In turn, the vehicle velocity is used to obtain a reference velocity for comparison with the velocity of the individual wheels. The result of the comparison is used to control the brake fluid pressure in the wheel cylinder of each wheel. Japanese Patent Laid-open No. 64-52568 discloses one such anti-skid controlling apparatus. This apparatus distinguishes a "pressure decreasing region" from a "pressure increasing region" in its pressure control setup depending on the slip ratio and wheel acceleration of the vehicle on which it is mounted, each region being variable as required. The disclosure also includes an apparatus capable of lowering the memory size required for skid control.
One disadvantages of the prior art above is that simply separating the pressure decreasing region from the pressure increasing region fails to address the diverse control conditions that vary depending on many factors: different pressure changing characteristics of the pressure controlling means for controlling the brake fluid pressure on different vehicles; different tires; different characteristics of the braking system as a whole from vehicle to vehicle; and different friction coefficients relative to different road surfaces. The above Japanese Patent Laid-open No. 64-52568 discloses that in order to control brake fluid pressure decreases, the pressure decreasing action and pressure holding action are alternated by pulse-based pressure decreasing controls. In this setup, the ratio of pressure decreasing time to pressure holding time is predetermined for the pressure decreasing region. This results in another disadvantage: predestined difficulty in flexibly achieving optimal pressure control.