Anti-lock brake system

Anti-lock brake system (ABS) for a road vehicle which is also equipped with a wheel slip control system (ASR) which operates on the principle of decelerating a vehicle wheel tending to spin by activating its wheel brake so that wheel slip remains held within a defined range of values compatible with adequate directional stability, wherein:

• the brake system is a hydraulic dual-circuit brake system having two static brake circuits, one brake circuit being allocated to the undriven wheels of the vehicle and the other brake circuit being allocated to the driven wheels of the vehicle;
• a brake unit is provided for supplying brake pressure to the two brake circuits, and has a primary and a secondary output-pressure space, one of which is allocated to each of the brake circuits respectively and from which brake unit, pressures can be applied to the brake circuits after being built up by a pedal-force-controlled displacement of piston means in said brake unit which movably delimits the output pressure spaces;
• a high-pressure pump means is provided for each of the two brake circuits to feed brake fluid into the output pressure spaces of the brake unit in control phases of the ABS system for the purpose of preventing the brake circuits from becoming empty as well as to obtain a warning as to the activation of the ABS system;
• the high-pressure pump means for the brake circuit of the driven vehicle wheels also provides an auxiliary pressure source for the ASR system to supply brake pressure to the wheel brake(s) of the driven wheels subjected to wheel slip under control of brake pressure control valve means individually allocated to the wheel brakes;
• means connecting and disconnecting the brake circuit of the driven vehicle wheels to the secondary output-pressure space, which is separated with respect to the primary output-pressure space of the brake unit by a floating piston of the piston means, in response to output signals from an electronic control unit means by an ASR control valve means for the duration of control cycles of the ASR system;
• the ASR control valve means assuming a throughflow position during normal braking mode and braking mode subjected to an anti-lock control, wherein the brake circuit of the driven vehicle wheels is connected to the secondary output-pressure space of the brake unit;
• the brake circuit of the undriven wheels of the vehicle operating on the feedback principle during ABS system control, wherein during pressure reduction phases of the anti-lock control, brake fluid is discharged from at least one of the wheel brakes of the undriven vehicle wheels and is pumped back into the primary output-pressure space of the brake unit;
• the brake circuit of the driven vehicle wheels, operating on the discharge principle during ABS system control wherein, during brake pressure reduction phases of the antilock control, brake fluid is discharged at one of the wheel brakes of the driven vehicle wheels and is channelled off towards a brake fluid reservoir of the brake system and wherein for a duration of the activation of the ABS system, brake fluid is pumped out of the reservoir into the secondary output-pressure space by a high-pressure pump;
• inlet pressure control valve means and an outlet pressure control valve means are provided for each wheel brake of the driven vehicle wheels;
• said inlet and outlet valve means being separately controllable by output signals from the electronic control unit means;
• a neutral position of the inlet valve means being their throughflow position;
• an excited position of the inlet valve means being their blocking position;
• a neutral position of the outlet valve means being their blocking position;
• an excited position of the outlet valve means being their throughflow position;
• wherein if a signal characteristic of activation of the ASR control is still present at the beginning of braking, the electronic control until means generates output signals for a specifiable retardation period t.sub.r, the duration of which is between 100 and 300 ms;
• and wherein the output signals hold the ASR control valve means in its blocking position switches the inlet valve means back into their neutral position sets the outlet valve means into their excited position and switches off the drive of the high-pressure pump.