Slip-controlled brake system for automotive vehicles with driven front and rear axle

A slip-controlled brake system for automotive vehicles with driven front and rear axles is equipped with a pedal-actuated braking pressure generator (1) comprising a power brake booster (2) connected to an auxiliary pressure source (4). The booster communicates directly with one or several wheel brakes (HR, HL) and which acts upon a master cylinder (3), to the working chambers (6, 7) of which the other wheel brakes (VR, VL) are connected. Electromagnetically actuatable multi-directional control valves (EV, AV, 16) contained in the pressure fluid conduits leading to the wheel brakes and in the pressure fluid return lines to a supply reservoir as well as in a pressure fluid conduit (14) leading from the power brake booster (2) to the master cylinder (3) serve on control action to reduce the braking pressure at the individual vehicle wheels and to deliver pressure into the master cylinder (3).

BACKGROUND OF THE INVENTION 
The present invention relates to a slip-controlled brake system provided 
for automotive vehicles with driven front axle and rear axle and which is 
equipped with a pedal-actuated braking pressure generator comprising a 
power brake booster connected to an auxiliary pressure source. The booster 
communicates directly with one or several wheel brakes and which acts upon 
a master cylinder to the working chambers of which the other wheel brakes 
are connected. The brake system includes electromagnetically actuatable 
multi-directional control valves contained in the pressure fluid conduits 
leading to the wheel brakes and includes pressure fluid return lines which 
connect the wheel brakes with a pressure supply reservoir and which 
contain likewise electromagnetically actuatable multi-directional control 
valves. The system further comprises a normally closed pressure fluid 
conduit from the power brake booster to the master cylinder which can be 
switched to its opened position and additionally comprises wheel sensors 
and electronic circuits for the determination of the wheels' rotational 
behavior and for the generation of valve control signals. 
Slip-controlled brake systems of the type initially referred to are known 
and which prevent locking of the controlled wheels by virtue of control of 
the multi-directional control valves which are inserted in the pressure 
fluid conduit from the braking pressure generator to the controlled wheels 
and in the pressure fluid return lines from the wheels to the pressure 
supply reservoir. The risks involved when the wheels become unstable or 
lock, respectively, in particular the risk of skidding and loss of 
steerability, thereby will be eliminated or at least reduced considerably. 
However, such systems do not have any influence on the spinning of wheels 
due to excessive driving torque. 
Moreover, it has also been proposed to design hydraulic brake systems of 
the like such as to limit traction slip as well. To this end, a connecting 
valve was employed according to the proposal described in German patent 
application P No. 33 27 401.0, now U.S. Pat. No. 4,565,411 in which 
because of the valve the driven wheels will be isolated from the braking 
pressure generator and connected directly to the auxiliary pressure source 
as soon as traction slip becomes too high. This allows introduction of 
hydraulic pressure into the connected wheel brakes even without 
application of the brake. 
According to another solution pressure out of the auxiliary pressure source 
is supplied directly into the master cylinder for the reduction of 
traction slip. The pressure propagating by way of a prechamber and by way 
of check valves in the sleeves of the master cylinder pistons into the 
working chambers of the master cylinder and from there by way of the inlet 
valves to the wheel cylinders. The individual multi-directional valves in 
the pressure fluid conduits to the wheel brakes allow to dose the 
pressure, if necessary with the assistance of the outlet valves which can 
establish connection between the wheel brake cylinder and the pressure 
supply reservoir. The driven wheels must be connected to the master 
cylinder in a like system. 
It is an object of the present invention to improve upon a like brake 
system in a particularly simple manner and requiring minimal effort so 
that it can be employed for all-wheel-driven vehicles and which not only 
prevents locking of the wheels but also limits the traction slip of the 
wheels to an admissible value. 
SUMMARY OF THE INVENTION 
This object is achieved in a remarkably simple fashion with a 
slip-controlled brake system of the type referred to hereinabove, which is 
improved so that the auxiliary pressure source communicates with the wheel 
brakes directly connected to the power brake booster and with the working 
chambers in the master cylinder via electromagnetically actuatable 
multi-directional control valves which, for the purpose of traction slip 
control, allow to meter pressure into the wheel brakes directly connected 
to the power brake booster and into the working chambers in the master 
cylinder and thus into the wheel brakes connected to the master cylinder. 
According to a preferred embodiment of the brake system, the auxiliary 
pressure source, instead of the power brake booster, is for traction slip 
control connectable to the wheel brakes which latter are "normally" (i.e., 
except for in the traction slip control period) connected to the power 
brake booster, and is connectable to the pressure fluid conduit leading to 
the master cylinder, by means of two two-way/two-position directional 
control valves. That is one valve thereof being closed in its inactive 
position while the other one assumes its opened position when inactive. 
Within the scope of the present invention, two wheel brakes can be 
connected to the power brake booster via one joint two-way/two-position 
directional control valve which normally assumes its opened position, the 
said two wheel brakes communicating with the pressure fluid return line 
via one joint, normally closed two-way/two-position directional control 
valve, while for traction slip control and additional two-way/two-position 
directional control valve which normally is in its opened position is 
inserted into one or into both of the pressure fluid conduits leading to 
the wheel brakes. 
Another embodiment of the present invention provides for traction slip 
control wherein the auxiliary pressure source instead of the power brake 
booster is connectable to the pressure fluid conduit leading to the master 
cylinder by virtue of two two-way/two-position directional control valves, 
and that pressure fluid lines are provided leading from the master 
cylinder to the wheel brakes, which latter normally are in direct 
communication with the power brake booster, the said pressure fluid lines 
containing normally closed multi-directional control valves which can be 
switched to their opened position. In this brake system, the pressure 
fluid lines which extend from the master cylinder to the wheel brakes that 
normally communicate directly with the power brake booster can be 
connected to a chamber or a line in the master cylinder into which 
pressure out of the auxiliary pressure source can be introduced for the 
purpose of traction slip control. 
That is, in the manner proposed by this invention, the slip-controlled 
brake system mentioned before can be extended by use of only few 
additional multi-directional control valves to a system which enables to 
individually control the traction slip at the separate wheels even in an 
all-wheel-driven vehicle. The valves required to this end may likewise be 
retrofitted.

DETAILED DESCRIPTION 
According to FIG. 1, the slip-controlled brake system comprises a braking 
pressure generator 1 which is substantially composed of a hydraulic power 
brake booster 2 and a master cylinder 3, herein a tandem master cylinder. 
There is also symbolic illustration of an auxiliary pressure source 4 at 
the inlet of the power brake booster 2 as well as a brake pedal 5 onto 
which the force F is exerted in direction of the arrow. 
This is direct connection of the rear wheels HR, HL to the power brake 
booster 2 by way of multi-directional control valves. The front wheels VR, 
VL communicate with the working chambers 6, 7 by way of hydraulically 
isolated pressure fluid circuits. The pressure generated in the power 
brake booster 2 that is proportional to the pedal force F will be 
transmitted in a known manner directly onto the working pistons 10, 11 in 
the master cylinder 3. 
Finally, there is still provision of a pressure supply reservoir 8 which is 
connected to the master cylinder 3 via a prechamber 9 illustrated only 
symbolically. The connection of this prechamber to the secondary sides of 
the working pistons 10, 11 as well as the conduit for flow of hydraulic 
energy out of the prechamber 9 into the working chambers 6, 7 when 
delivering pressure during brake slip control is designed in a known 
fashion and therefore has not been shown herein. 
The wheel brakes of the individual wheels are connected with the braking 
pressure generator by way of inlet valves EV which normally, i.e. in their 
inactive position, are switched to be opened, and they are connected with 
the pressure supply reservoir 8 via normally closed outlet valves AV. The 
return line via the valves AV to the reservoir 8 is not drawn, but is 
solely symbolized by the arrows at the exit of the outlet valves AV. 
In this arrangement, the rear wheels are connected in parallel as long as 
the additional two-way/two-position directional control valves 12, 13 are 
not energized and therefore are opened for the pressure medium, so that 
one joint inlet valve EV and one outlet valve AV are sufficient for both 
wheels. 
For the control of brake slip and, respectively, for the prevention of 
locking wheels, beside the inlet valves and outlet valves EV, AV there is 
still provision of a pressue fluid conduit 14 from the outlet of the power 
brake booster 2, at which pressure controlled proportionally to the pedal 
force F is prevailing, to the prechamber 9 via the two-way/two-position 
directional control valve 15 that is opened in its inactive position and 
via the three-way/two-position directional control valve 16. During 
anti-skid control, hydraulic energy will be supplied through this conduit 
14 into the working chamber 6, 7 of the master cylinder 3 for compensation 
of the pressure fluid discharging via the outlet valves AV. 
For the control of traction slip, the auxiliary pressure source 4 will be 
connected to the master cylinder 3 and to the wheel brakes of the front 
wheels VR, VL and besides directly to the rear wheels HR, HL by way of 
switching over of the valves 15, 16, 17 in respect of their illustrated 
switching position which represents the inactive position. The conduit 
from the auxiliary pressure source 4 to the outlet of the power brake 
booster 2 will thereby be closed by the valve 15; this valve 15 may be 
eliminated in some embodiments of the braking pressure generator 1 wherein 
the outlet of the power brake booster 2 and the prechamber 9 are 
pressure-balanced during this control period. 
The braking pressure for initiation of traction slip can be reduced by 
means of the inlet valves and outlet valves EV, AV in the embodiment 
according to FIG. 1, each one valve 12, 13 being inserted in the conduit 
to the rear wheels between the inlet valve EV and the wheel brakes, in one 
connecting line or, as is illustrated herein, in both connecting lines 
with a view to enabling individual control of the two wheels in this 
control period. 
In the system according to FIG. 2, in contrast to the brake system 
according to FIG. 1, the rear wheels which normally are connected directly 
to the power brake booster 2 are also in communication with the prechamber 
9 of the master cylinder 3 by way of two two-way/two-position directional 
control valves 18, 19 which are closed in their inactive position. For the 
control of traction slip at the rear wheels, energization of the valves 
18, 19 allows introduction of pressure through the associated conduit. The 
pressure variation at the rear wheels HR, HL can be de-coupled by means of 
an additional two-way/two-position directional control valve 21 which is 
opened in its inactive position. A two-way/two-position directional 
control valve 20, which is inserted into the pressure fluid conduit 14 
from the power brake booster 2 to the master cylinder 3 and which is 
normally switched to its opened position, likewise serves for de-coupling. 
Valve 20 is not required if in those control periods in which the valve 16 
responds the same pressure prevails at the outlet of the power brake 
booster 2 as in the prechamber 9.