Anti-skid braking system for automotive vehicle

An anti-skid braking system for an automotive vehicle having a left-front and right-front wheel, and a left-rear and a right-rear wheel, comprising:

• a first brake system including an operator-controlled brake operating member, a master cylinder responsive to an operation of said brake operating member for generating a fluid pressure which corresponds to an operation value of said brake operating member, a first brake provided for each of said left-front, right-front, left-rear and right-rear wheels of the vehicle, and having a brake cylinder which is actuated by the fluid pressure generated by said master cylinder, and first solenoid-operated control valve means disposed in a fluid passage between said master cylinder and said brake cylinder, and operable between a first position for communication of said brake cylinder with said master cylinder, and a second position for communication of said brake cylinder with a reservoir;
• a second brake system including a hydraulic pressure source operated by a power unit to generate a fluid pressure, a reservoir for storing a working fluid, a second brake provided for each wheel, and cooperating with said first brake to apply a brake to each wheel, and second solenoid-operated control valve means controlled by said anti-skid controller, between a first position for communication of a brake cylinder of said second brake with said hydraulic pressure source, and a second position for communication of said brake cylinder of said second brake with said reservoir;
• a speed sensor for generating a speed signal indicative of a rotating speed of said wheel;
• and
• an anti-skid controller connected to said first and second brake systems and said speed sensor, for estimating a slip ratio of said wheel of the vehicle based on said speed signal generated by said speed sensor, determining whether the estimated slip ratio exceeds a predetermined upper limit, and controlling said first solenoid-operated control valve means, if said estimated slip ratio of said wheel has exceeded said upper limit, so as to lower the fluid pressure in said brake cylinder of said first brake to a level lower than a point at which the slip ratio is in a predetermined optimum range, said anti-skid controller subsequently controlling a brake force of said second brake, for adjusting a total brake force of said first and second brakes, so as to maintain said slip ratio within said optimum range,
• said second solenoid-operated control valve means including front-wheel solenoid-operated control valve means connected to a pair of said second front-wheel brakes provided for said left-front and right-front wheels, and rear-wheel solenoid-operated control valve means connected to a pair of second rear-wheel brakes provided for said left-rear and right-rear wheels, said speed sensor being provided for each of said left-front and right-front wheels, and left-rear and right-rear wheels,
• said anti-skid controller controlling said front-wheel solenoid-operated control valve means based on one of two front-wheel signals of the speed sensors for said left-front and right-front wheels, which indicates a higher rotating speed than the other front-wheel speed signal, said anti-skid controller controlling said rear-wheel solenoid-operated control valve means based on one of two rear-wheel speed signals of the speed sensors for said left-rear and right-rear wheels, which indicates a lower rotating speed than the other rear-wheel speed signal.