Abstract:
The present invention relates to an improved hydraulic brake system for controlled and comfortable braking operations which includes a hydraulic pump for build-up of a hydraulic pressure that effects braking and a valve assembly by way of which the hydraulic pressure can be reduced, the hydraulic system being in particular characterized in that the valve assembly is an analog valve, and in that the analog valve and the hydraulic pump are adapted to be actuated by a control device in such a manner that the brake pressure for comfort braking can be increased or decreased in an essentially continuously variable manner.

Description:
This application is a 371 of PCT/EP99/03221 filed May 11, 1999. 
     TECHNICAL FIELD 
     The present invention generally relates to vehicle braking systems and more particularly relates to a hydraulic brake system for controlled and comfortable braking operations. 
     BACKGROUND OF THE INVENTION 
     Hydraulic brake systems are increasingly equipped with devices which permit performing also independently controlled comfortable braking operations. In contrast to a controlled braking operation effected by a driver, which is generally related to traction and/or brake slip control, comfort braking is activated by a cruise control system, for example. One problem encountered in this matter, however, is that such a comfort braking operation must be carried out with a controlled brake force because otherwise the cruise control system cannot be operated with a sufficient rate of accuracy and continuity. 
     In view of the above, an object of the present invention is to provide a hydraulic brake system of the type mentioned hereinabove which allows applying a brake pressure that is continuously controllable in small steps for conducting comfortable braking operations. 
     The present invention includes an analog valve and a hydraulic pump both of which are adapted to be actuated by a control device in such a manner that the brake pressure for comfort braking can be increased or decreased in an essentially continuously variable manner. 
     In a preferred embodiment, the valve assembly is configured as an analog three-way valve provided for each wheel brake, the inlets of the valve being connected to the pressure side of the hydraulic pump, and the first outlet thereof being respectively connected to a wheel brake and the second outlets thereof being connected to a low-pressure side of the brake system, wherein the three-way valves and the hydraulic pump are actuatable by a control device to such effect that the brake pressure for a comfortable braking operation can be increased or decreased in an essentially continuously variable manner. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a basic circuit diagram showing the first embodiment. 
     FIG. 2 is a basic circuit diagram showing the second embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Only one brake circuit  100  which acts upon two wheel brakes  11 ,  14  of a vehicle is respectively shown in the two Figures. The respectively other brake circuit for the other two wheel brakes has an identical design and, therefore, was omitted for the sake of clarity. According to FIG. 1, the brake circuits are acted upon by a master cylinder  1  which is supplied with hydraulic fluid by way of a hydraulic supply reservoir  2 . The master cylinder  1  is actuated by a brake pedal  4  by way of a brake force booster  3 . 
     Connected to the master cylinder  1  is a brake line  5  of the brake circuit  100 , which line extends through an analog, normally open valve  6  and, at it&#39;s outlet, branches into a first brake branch  7  and a second, brake branch  8 . The first brake circuit extends over a first, normally open inlet valve  9  into the first wheel brake  11 , while the second brake branch  8  is connected to the second wheel brake  14  by way of a second, normally open inlet valve  12 . 
     A connection to a low-pressure side of the brake circuit can be established by way of a first, normally closed outlet valve  10  at the first wheel brake  11 . The same applies to a second, normally closed outlet valve  13  at the second wheel brake  14 . The low-pressure side is acted upon by a first low-pressure accumulator  15  and is connectable to the hydraulic supply reservoir  2  by way of a first non-return valve  15   a  and a first, normally closed separating valve  18 . 
     Additionally, the suction side of a hydraulic pump  16  is connected to the low-pressure side by way of a second non-return valve  16   a . The pressure outlet of this pump extends via a third non-return valve  16   b , a damping chamber  17 , and an orifice  17   a  into the two brake branches  7 ,  8 . 
     In view of a controlled braking operation (i.e., application of the brake pedal  4 ) effected by a driver, the function of the present embodiment corresponds to that of prior art brake circuits of this type, with the hydraulic pump and the related inlet and outlet valves being correspondingly actuated for traction or brake slip control purposes by means of sensors on each wheel brake. 
     The hydraulic pump  16  and the analog valve  6  are used in first place to carry out a comfortable braking operation, for example, by a cruise control system. The hydraulic pump  16  causes build-up of brake pressure in the brake branches  7 ,  8 , while the brake pressure can be reduced by way of the analog valve  6 . These variations of the brake pressure occur in an infinitely variable fashion because the hydraulic pump has a pulsed operation and/or the analog valve is opened gradually and controlled by an increase in cross-section. 
     In detail, the analog valve  6  is closed and the first separating valve  18  opened during pressure build-up. The hydraulic pump  16  may then aspirate hydraulic fluid from the hydraulic supply reservoir  2  connected to the master cylinder  1  and conduct it into the two brake branches  7 ,  8 . During pressure decrease, the first separating valve  18  is closed, the hydraulic pump  16  deactivated and the analog valve  6  opened gradually. 
     These two operations may also take place permanently and alternatingly so that a continuous pressure control is possible in small steps. 
     The analog valve is preferably a seat valve having a proportionated magnet drive. 
     FIG. 2 shows a second embodiment of the present invention. In this case, too, the two wheel brakes  11  and  14  are acted upon by way of the brake circuit  100  because the force exerted on the brake pedal  4  is boosted by the brake force booster  3  and transmitted into the hydraulic fluid by way of the master cylinder  1  which is connected to the hydraulic supply reservoir  2 . The brake circuit  100  is connected to the master cylinder  1  by way of the brake line  5 . The brake line  5  extends into the two brake branches  7 ,  8  by way of a second, normally open separating valve  20 . The first brake branch  7  is connected to the inlet of a first three-way valve  21 , to the first outlet of which the wheel brake  11  is connected and the second outlet of which is connected to the low-pressure side. The second brake branch  8  extends to a second three-way valve  22  having its first outlet connected to the wheel brake  14  and its second outlet connected to the low-pressure side. 
     The analog three-way valves are preferably slide valves with an integrated seat. 
     The low-pressure accumulator  15  acts upon the low-pressure side and is connected directly to the hydraulic supply reservoir  2  by way of the first non-return valve  15   a . In this embodiment also, the suction side of the hydraulic pump  16  is connected to the low-pressure side by way of the second non-return valve  16 a. The pressure side of the hydraulic pump  16  extends into the two brake branches  7 ,  8  by way of the third non-return valve  16   b  and the damping chamber  17  with an orifice  17   a  connected downstream thereof 
     In the event of a controlled braking operation performed by a driver, the brake pressure is conducted into the brake branches  7 ,  8  by way of the brake line  5  and the second, normally open separating valve  20  and acts upon the associated wheel brake  11  and  14 , respectively, by way of the first and second three-way valve  21 ,  22 . Traction and brake slip control in turn is effected by the per se known actuation of the hydraulic pump  16  and the second separating valve  20  and the respective three-way valve  21  or  22  in case slip is sensed on the associated wheel. In this case, the three-way valve performs the functions of the inlet and outlet valves  9 ,  10  and  12 ,  13 , respectively, as in FIG.  1 . 
     In the event of al comfortable braking operation, the second separating valve  20  is closed and the hydraulic pump  16  operated in a pulsed manner for pressure increase so that the brake pressure in the wheel brakes  11 ,  14  is built up analogously, that means gradually in small steps, by way of the three-way valves  21 ,  22 . The hydraulic pump  16  is deactivated for pressure reduction, and the three-way valves are actuated in steps continuously so that in each case the first outlet is connected analogously, that means with increasing cross-section, to the second outlet, with the result that the brake pressure is discharged gradually into the low-pressure side. 
     In this second embodiment, too, pressure increase and pressure reduction can take place permanently and alternatingly so that a continuous pressure control in small steps is possible. 
     In addition, it is possible with both embodiments to use pressure modulation to the end of assisting a controlled braking operation initiated by the driver, mainly above an operating point of a booster. 
     It should still be noted that the function of the analog valves which includes achieving a continuous and analog pressure reduction could also be brought about by a hydraulic pump which is operable with opposite rotation in order to return the brake fluid this way. In this case, the hydraulic pump would have to be controlled by an angular control (control mirror).