Hydraulic brake system with hydraulic brake force boosting

A brake system with hydraulic brake force boosting comprises a master cylinder (1), to which the wheel brakes (36 to 39, 36' to 39') are connected, as well as of an auxiliary-pressure supply system (11, 12) and of an auxiliary-pressure control valve (10) which causes an auxiliary pressure proportional to the pedal force (F). Inserted into the pressure fluid conduits from the master cylinder (1) to the wheel brakes (36 and 39, 36' to 39') are pressure-controlled multidirectional valves (15, 15', 16, 16') which, in their initial position, provide for hydraulic communication between the master cylinder (1) and the wheel brakes. After a second switch position has been assumed, the auxiliary-pressure source (11, 12) will be connected to the wheel brakes (36 to 39, 36' to 39'), whereby dynamic braking is effected instead of the master cylinder (1).

BACKGROUND OF THE INVENTION 
The present invention relates to a hydraulic brake system equipped with 
hydraulic brake force boosting. The system is substantially composed of a 
pedal-actuated master cylinder to which the wheel brakes are connected by 
way of pressure fluid lines of an auxiliary-pressure supply system, and of 
an auxiliary-pressure control valve which causes an auxiliary pressure 
proportional to the pedal force. 
A known brake system of this type consists of a single-type or tandem-type 
master cylinder with a hydraulic brake power booster connected upstream 
thereof as well as of an auxiliary-pressure supply system comprising a 
pump and a hydraulic accumulator. The hydraulic booster contains an 
auxiliary-pressure control valve which, on actuation of the brake pedal, 
brings about an auxiliary pressure that is proportional to the pedal force 
and that acts on the pistons in the master cylinder. The boosting factor 
of the brake system is selected by the ratio of the surfaces of a 
transmission piston in the interior of the brake power booster in relation 
to the surface of an actuating piston coupled mechanically to the brake 
pedal. As the brake circuits are designed as static circuits, the volume 
of the pressure chambers in the master cylinders must be adapted to the 
respective brake system. 
Furthermore, slip-controlled brake systems are known wherein the hydraulic 
braking pressure generator is likewise composed of a master cylinder and a 
hydraulic brake power booster connected upstream thereof, as disclosed in 
German printed and published patent applications Nos. 30 40 561 and 30 40 
562. During slip control, dynamic pressure out of the auxiliary-pressure 
supply system is introduced by way of the booster chamber into the brake 
circuits which are connected to the master cylinder and which are static 
until commencement of slip control. This way, the discharge or pressure 
fluid into the pressure supply reservoir during the phases of pressure 
reduction will be compensated. Such systems are rather complicated and 
costly. 
Likewise known are slip-controlled brake systems, the braking pressure 
generator of which is also composed of a hydraulic brake power booster 
with a master cylinder connected downstream thereof, and wherein on 
commencement of slip control dynamic pressure is metered out of the brake 
power booster directly into the wheel brake cylinders of the wheels 
connected to the master cylinder. To this end, the wheel brakes connected 
to the static brake circuit communicate by way of electromagnetically 
actuatable multidirectional control valves with the master cylinder so 
that change-over of these valves causes interruption of the hydraulic 
connection between the master cylinder and the wheel brakes and permits 
connection of the auxiliary-pressure source instead of the master 
cylinder. During normal braking operations (i.e., without slip control) or 
until change-over of the solenoid valves, respectively, the circuits 
concerned are strictly static brake circuits. 
It is an object of the present invention to develop a brake system with 
hydraulic boosting which is comparatively simple and entails little effort 
and which system likewise permits to be extended to a slip-controlled 
brake system by insertion of electromagnetically controllable inlet and 
outlet valves and by equipment with a measuring and control electronics. 
SUMMARY OF THE INVENTION 
This object is achieved in a technically advanced fashion by a hydraulic 
brake system of the type referred to wherein pressure-controlled 
multidirectional valves are inserted into the pressure-fluid conduits from 
the braking pressure generator to the wheel brakes, which valves, in their 
inactive or initial position, provide hydraulic connection between the 
master cylinder and the wheel brakes and which, after change-over into a 
second switch position, will connect the auxiliary-pressure source, 
instead of the master cylinder, to the pressure-fluid conduits leading to 
the wheel brakes. 
That is, in the normal case (i.e., when the auxiliary-pressure supply 
system is intact) the brake system of the present invention provides for 
strictly dynamic braking. The master cylinder serves but to control the 
auxiliary-pressure control valve and to safeguard the brakes's function 
upon the occurrence of a defect and, more specifically, in the event of a 
malfunction in the auxiliary pressure source. The design of the system is 
extremely straightforward, since essentially there is only need for a 
master cylinder, a pressure-controlled auxiliary-pressure control valve, a 
hydraulic pump and some pressure-controlled multidirectional valves. Such 
valves are reliable in operation and allow low manufacturing cost, in 
particular in comparison to electromagnetically actuatable valves. 
Since the master cylinder substantially serves to control the brake system 
only and there is dynamic braking normally, one size of master cylinder 
can be utilized for differently dimensioned brake systems. As another 
advantage, the inventive brake system can be used as a hydraulic unit of a 
slip-controlled brake system, because there already is dynamic braking in 
the normal case of braking and, therefore, the hydraulic medium discharged 
in the phase of pressure reduction can be re-furnished by the 
auxiliary-pressure supply system. Hence there is no need for so-called 
main valves for the dynamic supply of pressure fluid into the static 
circuits, as they are required in known brake slip control systems. 
According to a further embodiment of the present invention, a 
three-way/two-position directional control valve is provided in each brake 
circuit as a pressure-controlled multidirectional valve which can be 
switched over by the controlled auxiliary pressure. Additionally, a second 
pressure-controlled multidirectional valve can be inserted into each brake 
circuit in the pressure-fluid conduit leading from the controlled 
auxiliary pressure or the auxiliary pressure source to the control port of 
the pressure-controlled multidirectional valve and to the 
auxiliary-pressure inlet of this valve, the second multidirectional valve 
closing in the inactive or initial position and being switchable to open 
by the action of the pressure developing in the respective circuit in the 
master cylinder when the brakes are applied. 
Furthermore, another embodiment of the present invention is arranged to 
connect the control inlet of the auxiliary-pressure control valve to a 
pressure chamber in the master cylinder in which there develops a pressure 
proportional to the pedal force. Further, the master cylinder is provided 
as a tandem-type master cylinder which is in communication with a control 
valve comprising two hydraulically isolated control chambers which are 
connected each to one of the two pressure chambers of the tandem master 
cylinder. On failure of one hydraulic circuit, the control valve will be 
controlled by the braking pressure in the pressure chamber of the intact 
circuit of the tandem master cylinder. 
According to a further embodiment of the inventive brake system, the 
auxiliary-pressure supply system contains an electromotively driven 
hydraulic pump, the drive motor of which can be switched on when the pedal 
is depressed. In still another embodiment, the inventive brake system 
advantageously disposes of a pedal travel simulator inserted downstream 
and admitting a displacement of the master cylinder piston proportional to 
the pedal force. Since the master cylinder solely controls the pressure 
control valve, a pedal travel is not required. 
According to another embodiment of this invention, the pedal travel 
simulator starts to function only after rise of the pressure in the 
auxiliary-pressure source in excess of a predetermined threshold value or, 
respectively, in the event of introduction of auxiliary pressure with the 
aid of the control valve. When the auxiliary-pressure supply system is 
defective, the pedal travel simulator is adapted to be locked. 
Suitably, the pedal travel simulator is essentially composed of a 
compression spring which is accommodated in a chamber inside the master 
cylinder and which is compressible by axial displacement of the master 
cylinder piston in the direction of the pedal force. The chamber is in 
communication with a pressure supply reservoir by way of a 
pressure-controlled two-way/two-position directional valve which is closed 
in the inactive position and which is switchable to open by virture of the 
auxiliary pressure.

DETAILED DESCRIPTION 
In the embodiment shown in FIG. 1, the braking pressure generator employed 
by the brake system is a tandem master cylinder 1, whose pistons 2 to 4 
are acted upon directly, that means without boosting of pedal force or 
brake force by the brake force F that is exerted by way of a brake pedal 5 
and symbolized by an arrow. Usually, piston 2 is referred to as 
intermediate piston, while piston 3 is termed as push-rod piston. The 
third master cylinder piston 4 belongs to a pedal travel simulator 
inserted downstream, the details and mode of function of which will be 
described hereinbelow. An auxiliary-pressure control valve 10 is connected 
to the two working or pressure chambers 6, 7 of the tandem master cylinder 
1 via pressure fluid lines 8,9. 
The arrangement illustrated is equipped with an auxiliary-pressure supply 
system. The latter comprises as an essential component part an 
electromotively driven hydraulic pump 11, the pressure side of which is 
connected by way of the associated on-return valve 12 with the 
auxiliary-pressure control valve 10 and by way of the hydraulic connecting 
lines 13, 14 with the pressure-controlled multidirectional valves 15, 16 
which will be explained in more detail in the following. The suction side 
of the hydraulic pump 11 as well as a pedal-side annular chamber 17 at the 
push rod piston 3 of the master cylinder 1, a comparable chamber 18 at the 
intermediate piston 2 and a compensating chamber 19 in the control valve 
10 are in communication with a pressure-compensating and supply reservoir 
20. Finally, an annular chamber 21 is still connected to this reservoir 
20, the said chamber being arranged between two ring seals which 
hydraulically isolate two control chambers 22 and 23 of the control valve 
10. 
The pressure chambers 6, 7 of the tandem master cylinder 1 are connected in 
a known manner by way of central valves 24, 25 and channels 26, 27 with 
the annular chambers 17, 18 and by way of these with the 
pressure-compensating reservoir 20 as long as no brake force F is exerted 
on the pedal 5. Resetting springs 28, 29, 30 which return the pistons 2 to 
4 in the interior of the master cylinder 1 to their initial position 
illustrated when the brake pedal 5 is released are disposed in the 
pressure chambers 6 and 7 as well as in a chamber 31 pertaining to the 
pedal travel simulator. 
The pressure-controlled multidirectional valve 15, 16 allocated to the two 
static brake circuits I, II in the embodiment of the invention described 
herein each consist of two separate valves, namely of a 
pressure-controlled three-way/two-position directional valve 32, 33 and a 
pressure-controlled two-way/two-position direction valve 34, 35. In the 
inactive or the initial position which is illustrated, that is when the 
brake is not applied, the working or pressure chambers 6, 7 of the tandem 
master cylinder 1 communicate by way of the respective 
three-way/two-position directional valve 32 or 33 with the wheel brakes 
36, 37 and 38, 39, respectively. The pressure fluid lines 13, 14 leading 
to the auxiliary-pressure supply system and, respectively, to the pump 11 
and the control valve 10 are interrupted by the two-way/two-position 
directional valves 34, 35 as long as the brakes are not applied. 
The drive motor M first is not in operation, since the switch 40 will not 
close until depression of the pedal 5 and sets the motor M of the pump 11 
by way of the contact m into function. The pedal travel simulator, which 
is substantially composed of the simulator piston 4, the compression 
spring 30 and a pressure-controlled two-way/two-piston directional valve 
41, will be locked in consequence of the return line 42 being shut off by 
the valve 41, as long as the auxiliary pressure which can be transmitted 
by way of the pressure fluid line 43 is equal to, or less than, the 
pressure prevailing on the opposite side of valve 41. That is, because the 
pressure transmitted by way of control lines 44 and 45, which are 
connected to the pressure chambers 6, 7 in the master cylinder 1, is 
opposed to the auxiliary pressure transmitted by way of the line 43 in 
terms of its effect on the switch position valve 41. 
In another embodiment of the invention (not illustrated) the connecting 
lines 44, 45 are deleted so that the valve will be switched over to its 
opened condition in opposition to a resetting spring (not shown) as soon 
as the auxiliary pressure exceeds a predetermined threshold value. 
A non-return valve 46 which is connected in parallel to the valve 41 and 
which can be unitized with the valve ensures that, then the brake is 
released, the simulator piston 4 is allowed to slide back to its initial 
position under the pressure of the resetting spring 30, even after the 
valve 41 having been switched back to its closed position shown. 
The mode of operation of the brake system illustrated is as follows: 
On depression of the brake pedal, the master cylinder pistons 2, 3 are 
displaced to the left. The central valves 24, 25 will close so that 
pressure is subsequently allowed to develop in the pressure chambers 6, 7. 
Simultaneously, the hydraulic pump 11 is put into operation by closing of 
the switch 40. 
A pressure proportional to the brake force F is transmitted by way of the 
hydraulic control line 9 into the control chamber 23 of the 
auxiliary-pressure control valve 10, whereupon pressure is exerted in the 
direction of closing of the spherical seat valve 48, which is a component 
part of the control valve, through the control piston 47 of the control 
valve 10. Now, auxiliary pressure can develop in the pressure fluid cycle 
of the hydraulic pump 11, the auxiliary pressure being thus proportional 
to the pressure in the control chamber 23, to that in the pressure chamber 
7 and to the pedal force F. 
The pressure caused by the braking action in the brake circuits I, II 
moreover has as a result change-over of the pressure-controlled 
two-way/two-position directional valves 34 and 35, so that now the 
auxiliary pressure also prevails at the control inlets 49, 50 and at the 
pressure fluid inlets 51, 52 of the three-way/two-position directional 
valves 32, 33. This has as a consequence that, after the valves 32, 33 
have been changed over, the auxiliary pressure source, instead of the 
tandem master cylinder 1 or the pressure chambers 6, 7 of this master 
cylinder, will be connected to the wheel brakes 36, 37, 38, 39. The 
pressure fluid volumes of the pressure chambers 6, 7 in the interior of 
the master cylinder 1 remain almost constant even in the event of the 
pedal force F continuing to increase. However, the brake pedal 5 is able 
to perform a travel proportional to the pedal force F because meanwhile 
the auxiliary pressure has exceeded the pressure in the chambers 6, 7 and, 
thereby the two-way/two-position directional valve 41 of the pedal travel 
simulator has switched to open. Therefore, the chamber 31 of the pedal 
travel simulator is in connection with the pressure-compensating reservoir 
20 so that the driver of the vehicle feels the resetting force of the 
spring 30 at the pedal 5. 
Upon the occurence of a pump defect or any other defect from which ensures 
pressure failure in the auxiliary-pressure supply system, the 
pressure-controlled valves 32, 33 will remain in their inactive position. 
As a result, the master cylinder 1 will perform its original task and 
transmit the pedal pressure onto the wheel brake cylinders 36 to 39, 
however, without brake power boosting. The pedal travel simulator remains 
locked in this situation. 
Upon failure of one brake circuit I or II, for instance due to leakage, the 
associated pressure-controlled two-way/two-position directional valve 34 
or 35 remains closed so that no pressure fluid loss may occur. However, 
dynamic braking is continued in the intact brake circuit II or I by 
change-over of the pressure-controlled multidirectional valves 33, 35 and, 
respectively, 32, 34 with the aid of the pressure introduced from the 
auxiliary-pressure supply system. 
On pressure failure in the brake circuit II, the actuation of the 
auxiliary-pressure control valve 10 by way of the control line 8 and the 
control chamber 22 is taken care of by the pressure in the pressure 
chamber 6. 
In the embodiment according to FIG. 2, the hydraulic brake system described 
in additionally equipped with electromagnetically actuatable valves 53 to 
60, which prevent locking of the wheels by way of limiting the slip of the 
individual wheels in dependence on pickups (not shown) for measuring data 
and electronic control circuits and by controlling the wheel slip to adopt 
an optimum value. To this end, in the embodiment shown, the pressure fluid 
conduits to the individual wheel brakes 36' to 39' contain so-called inlet 
valves 53, 54, 55 and 56 which normally adopt their opened position and 
which can be switched over electromagnetically to close in the phase of 
maintaining the pressure constant or in the phase of pressure reduction. 
For the purpose of pressure reduction, the outlet valves 57 to 60 are 
required which allow discharge of pressure fluid in a dosed quantity to 
the compensating reservoir 20' in the presence of excessive braking 
pressure. 
In contrast to known slip-controlled brake systems, the discharge of 
hydraulic medium into the compensating reservoir 20 and 20', respectively, 
for the purpose of decreasing the braking pressure at the wheel brake of 
the wheel tending to lock does not incur any difficulties. Due to the 
change-over to the auxiliary-pressure supply system which takes place 
during every braking operation anyway and due to the closing of the 
pressure fluid conduit to the master cylinder 1, the pressure in the wheel 
brakes 36' to 39' can be re-increased directly by renewed switching the 
inlet valves 53 to 56 back to their opened condition. The pressure fluid 
enclosed in the pressure chambers 6, 7 in the master cylinder 1 during 
braking is available as a reserve and enables an emergency braking, that 
means braking without boosting of the brake force, upon a possible failure 
of the auxiliary-pressure supply system and disconnection of the slip 
control.