Dual-circuit hydraulic brake system

The present invention relates to a dual-circuit hydraulic brake system with slip control, in particular for automotive vehicles, wherein wheel brakes (26, 27, 41, 42) are connected via electromagnetically actuatable multiple-position valves (38, 39, 88, 40, 24, 89) to brake circuits which are hydraulically isolated from one another, wherein the brake circuits are adapted to be pressurized by a tandem master cylinder (1), on the one hand, and are connectible to a dual-circuit pump aggregate (76, 77) adapted to be driven electromotively, on the other hand. Both outlets of the pump aggregate (76, 77) are connected to normally closed brake valves (43, 44) which are governed by the master cylinder pressure and separating valves (11, 12) which are adapted to be closed by the outlet pressure of the brake valves (43, 44) are inserted into the connections between the tandem master cylinder (1) and the wheel brakes (26, 27, 41, 42).

BACKGROUND OF THE INVENTION 
The present invention relates to a dual-circuit hydraulic brake system with 
slip control, in particular for automotive vehicles, wherein wheel brakes 
are connected via electromagnetically actuatable multiple-position valves 
to brake circuits which are hydraulically isolated from one another, 
wherein the brake circuits are pressurizable by a tandem master cylinder, 
on the one hand, and connectible to a dual-circuit pump aggregate that is 
adapted to be driven electromotively, on the other hand. 
A hydraulic brake system comprising the preceding features is known for 
instance from the print "Bosch-Technische Berichte", volume 7, (1980), 
number 2. 
The brake system described in this print comprises substantially a tandem 
master cylinder with a vacuum brake booster connected upstream thereof, 
wherein a first working chamber of the tandem master cylinder pressurizes 
jointly the wheel brake cylinders at the rear axle of an automotive 
vehicle, while the second working chamber of the tandem master cylinder 
serves for the pressure supply of the wheel brakes at the front axle of 
the automotive vehicle. Inserted into the connection between the first 
working chamber of the tandem master cylinder and the wheel brakes of the 
rear axle of the automotive vehicle is a solenoid valve controllable by 
slip control electronics, which valve, in the de-energized state, 
establishes a free passage to the first working chamber of the tandem 
master cylinder, while it interrupts said connection in a second position 
and in a third position provides for a condition in which the first 
working chamber of the tandem master cylinder is isolated from the wheel 
brakes of the rear axle of the automotive vehicle and pressure fluid is 
taken from the wheel brakes fed by the first working chamber. 
The wheel brakes at the front axle of the automotive vehicle are supplied 
with pressure from the second working chamber of the tandem master 
cylinder, with each connection between the second working chamber of the 
tandem master cylinder and a wheel brake at the front axle of the 
automotive vehicle including an electromagnetically actuatable 
multiple-way valve which, in respect of its function, corresponds to the 
electromagnetically actuatable valve in the connection between the first 
working chamber of the tandem master cylinder and the wheel brakes at the 
rear axle of the automotive vehicle. A multiple-circuit brake system will 
be accomplished by a like combination, wherein the wheel brakes at the 
rear axle of the automotive vehicle will be controlled jointly in respect 
of slip, while the wheel brakes of the front axle are controllable 
individually. 
A change of the switching position of one of the electromagnetically 
actuatable valves results at the same time in the start-up of an 
electromotively driven dual-circuit pump aggregate which during slip 
control in the phase of pressure build-up supplies a corresponding 
auxiliary pressure to the wheel brakes. 
It has to be regarded as a disadvantage in the brake system described that 
in the phases of pressure build-up during slip control always the outlet 
pressure of the pump aggregate becomes effective in the wheel brake 
cylinders. This may have as a consequence that the braking pressure rises 
very quickly during the control action so that a new locked condition may 
occur in a correspondingly short period of time. Hence follows that 
relatively great pressure differences will be encountered during slip 
control. Besides, the outlet pressures of the dual-circuit pump aggregate 
will enter directly into the working chambers of the tandem master 
cylinder so that strong pulsation will be felt at the brake pedal during 
slip control what may under certain circumstances contribute to diverting 
the driver's attention from traffic in a braking situation which is 
critical anyway. 
It is therefore an object of the present invention to provide a 
multiple-circuit hydraulic brake system with slip control comprising the 
features initially referred to, wherein the operation of the pump 
aggregate has no influence on the position of the brake pedal. 
SUMMARY OF THE INVENTION 
This object is achieved by the present invention in that both outlets of 
the pump aggregate are connected to normally closed brake valves which are 
governed by the master cylinder pressure, and in that separating valves 
which are adapted to be closed by the outlet pressure of the brake valves 
are inserted into the connections between the tandem master cylinder and 
the wheel brakes. It will be accomplished in a favourable manner by a like 
design that, in the initial phase of braking, the braking pressure will 
first be supplied via the opened separating valves to the wheel brakes, 
while a travel- or pressure-responsively acting switching member is 
provided at the master cylinder which starts the drive of the pump 
aggregate. As soon as a corresponding pressure is generated by the pump 
aggregate, the separating valves will close, thereby causing closure of 
the connections between the tandem master cylinder and the wheel brakes. 
The brake valves will now serve to modulate the outlet pressures of the 
pump aggregate in dependence on the pressures in the working chambers of 
the tandem master cylinder so that during brake slip control the pressure 
made available by the pump aggreagate will not be fed directly to the 
wheel brakes, but takes a course which is dependent upon the respective 
actuating force. In case of failure of the energy supply taken care of by 
the pump aggregate and the electric motor due to disturbance, the 
separating valves will remain opened so that the wheel brakes are 
actuatable directly by the tandem master cylinder. The split-up of the 
brake circuits is preferably chosen such that one of the brake valves 
governs the pressure supply to the wheel brakes of the rear axle, while 
the other brake valve governs the pressure supply to the wheel brakes of 
the front axle. The brake system described fulfils its function 
correspondingly, if a vacuum brake booster is connected upstream of the 
tandem master cylinder. It will thus be rendered possible in a simple and 
favourable manner to combine the brake system proposed retrospectively 
with already existing hydropneumatic brake systems. 
The brake valves and the separating valves of the inventive brake system 
are preferably designed as seat valves, each of the separating valves 
containing a valve piston provided with an axial bore. One end face of the 
valve piston is displaceable into an unpressurized chamber, while a valve 
closure member is shaped at the other end face of the valve piston 
preloaded towards a valve seat, and an annular chamber connected to a pump 
outlet is adapted to be closed by said valve closure member. It will be 
furthermore a particularly straightforward design feature to arrange 
coaxially to the valve piston a piston which is adapted to be acted upon 
by the pressure of the master cylinder, by which latter piston the axial 
bore of the valve piston is closable and the valve closure member shaped 
at the valve piston is adapted to lift from the valve seat subsequently. 
In a favourable fashion, normally unpressurized chambers are formed 
between the valve pistons and the pistons of the brake valves which 
chambers are each in hydraulic communication with a closure piston of a 
separating valve.

DETAILED DESCRIPTION 
In the drawing, reference numeral 1 designates a tandem master cylinder, 
upstream of which a vacuum brake booster 2 is connected which, in turn, is 
actuatable through a brake pedal 3. The tandem master cylinder 1 is in 
communication with an unpressurized supply reservoir 6 via pressure lines 
4, 5. The tandem master cylinder 1 contains in addition two housing ports 
7, 8 to which pressure lines 9, 10 are connected. The pressure line 9 is 
in hydraulic communication with a separating valve 11, while the pressure 
line 10 is hydraulically connected to a separating valve 12. 
The separating valve 11 disposes of an inlet chamber 13 accomodating an 
axially slidable piston 14 whose left-hand end, when viewing the drawing, 
is designed as valve closure member 15. Coaxially relative to the piston 
14, a bore 17 is designed in the housing 16 of the separating valve 11 
which bore terminates into the inlet chamber 13 and forms at this port a 
valve seat 18 adapted to be closed by the valve closure member 15. 
The piston 14 is designed as a stepped piston and projects in a sealed 
manner into a chamber 19 which is constantly unpressurized. The larger 
piston step 20 of the piston 14 is guided in a cylinder bore 21, and the 
right-hand end face, when viewing the drawing, of the larger piston step 
20 confines a chamber 22. A pressure line 23 leads from the bore 17 to an 
electromagnetically actuatable two-way/two-position valve 24 which, when 
de-energized, permits free hydraulic passage and is, in turn, connected 
via a pressure line 25 to wheel brakes 26, 27 at the rear axle of an 
automotive vehicle. 
The separating valve 12 is designed alike the separating valve 11 and 
disposes of a housing 28, an inlet chamber 29 and a piston 30 which 
carries a valve closure member 31 at its right-hand end, when viewing the 
drawing, and is guided with a larger piston step 32 in a cylinder bore 33. 
The separating valve 12 contains furthermore a chamber 34, which is 
constantly unpressurized, and a chamber 35 which is confined by the end 
face of the larger piston step 32. In addition, the separating valve 12 
contains a bore 36 which terminates into the inlet chamber 29 and forms a 
valve seat 37 opposite to the valve closure member 31. A pressure line 38 
leads from the bore 36 to electromagnetically actuatable 
two-way/two-position valves 39, 40 which, in the inactive position 
illustrated in the drawing, establish a hydraulic connection to wheel 
brakes 41, 42 at the front axle of the automotive vehicle. 
Further components of the brake system illustrated in the drawing are brake 
valves 43, 44, to which the pressure prevailing at the housing ports 7, 8 
of the tandem master cylinder is supplied via pressure lines 45, 46. The 
brake valve 43 comprises an inlet chamber 47 in which a piston 48 is 
axially slidably guided and loaded by a compression spring 49. The 
right-hand end of the piston 48, when viewing the drawing, projects into a 
control chamber 50 and is designed as valve closure member 51. The control 
chamber 50, on the other hand, is confined by a bore 52 which enlarges 
radially outwardly and constitutes a valve seat 53. Movable into abutment 
on the valve seat 53 is a valve closure member 54 which is part of a 
piston 56 provided with an axial bore 55 and biased against the valve seat 
53 by a compression spring 57. The right-hand end of the piston 56, when 
viewing the drawing, projects in a sealed manner into a chamber 58 wherein 
there is constantly prevailing atmospheric pressure. 
The brake valves 43, 44 are of identical design, an inlet chamber 59 of the 
brake valve 44 being in communication with the housing port 8 of the 
tandem master cylinder 1 via the pressure lines 46, 9. Projecting into the 
inlet chamber 59 of the separating valve 44 is a piston 60 which is loaded 
by a compression spring 61. The right-hand end of the piston 60, when 
viewing the drawing, projects in a sealed manner into a control chamber 62 
and is likewise designed as valve closure member 63. Projecting 
furthermore into the control chamber 62 is a piston 65 provided with a 
valve closure member 64, the said piston disposing of an axial bore 66 and 
extending with its right-hand end, when viewing the drawing, into a 
constantly unpressurized chamber 67. The valve closure member 64 of the 
piston 65 cooperates likewise with a valve seat 68 of the separating valve 
44 and is biased against the valve seat 68 by a compression spring 69. 
Pressure lines 70, 71 lead from the control chambers 50, 62 of the brake 
valves 63, 64 to the chambers 22, 35 of the separating valves 11, 12. 
Furthermore, the pressure lines 70, 71 have connections 72, 73 to the 
electromagnetically actuatable two-way/two-position valves 39, 40, 24, 
with check valves 74, 75 being inserted in said connections 72, 73. 
In the brake system illustrated in the drawing, a dual-circuit pump 
aggregate 76, 77 which is adapted to be driven by an electric motor 78 is 
provided for making available auxiliary hydraulic energy. Said electric 
motor 78 will for instance be put into operation after the brake pedal 3 
has performed an adjustable travel. Connected in parallel to the 
dual-circuit pump aggregate 76, 77 are pressure-relief valves 79, 80 which 
limit the outlet pressures of the pump aggregate 76, 77. The suction sides 
of the pump aggregate 76, 77 are connected to the unpressurized supply 
reservoir 6, while the pressure sides communicate via lines 81, 82 with 
the brake valves 43, 44. Besides, pressure accumulators 83, 84 are 
connected to the suction sides of the pump aggregate 76, 77 which, in 
turn, are connected via pressure lines 85, 86 with electromagnetically 
actuatable two-way/two-position valves 87, 88, 89. Said 
electromagnetically actuatable two-way/two-position valves 87, 88, 89 are 
closed in the de-energized state, thus preventing escape of the pressure 
fluid supplied to the wheel brakes 41, 42, 26, 27. 
The mode of operation of the brake system described will be explained in 
more detail hereinbelow, starting from the brake's release condition in 
which no actuating force is applied to the brake pedal 3 and all movable 
parts assume their position to be seen in the drawing. When an actuating 
force is applied to the brake pedal 3, pressure will develop in the 
working chambers of the tandem master cylinder 1 assisted by the vacuum 
brake booster. This pressure is supplied via the housing ports 7, 8 and 
the pressure lines 9, 10 to the separating valves 11, 12 which are opened 
at first. Via the pressure lines 23, 38, the pressure generated in the 
tandem master cylinder 1 will thus propagate to the wheel brakes 26, 27, 
41, 42. The pressure prevailing in the tandem master cylinder 1 will 
furthermore be supplied via the pressure lines 45, 46 to the inlet 
chambers 47, 59 of the brake valves 43, 44 so that the pistons 48, 60 will 
be displaced to the right, when viewing the drawing, in opposition to the 
force of the compression springs 49, 61, as a result whereof the valve 
closure members 51, 63 shaped at the pistons 48, 60 will move into 
abutment on the ports of the axial bores 55, 66 of the pistons 56, 65 and 
isolate the control chambers 50, 62 from the unpressurized supply 
reservoir 6. 
Simultaneously with the application of the brakes, the electric motor 78 
will be started and cause the dual-circuit pump aggregate 76, 77 to 
generate hydraulic pressure which is supplied via the pressure lines 81, 
82 to the brake valves 43, 44. In this arrangement, pressure limitation of 
the outlet pressures of the pump aggregate 76, 77 is effected by the 
pressure-relief valves 79, 80 which return the pressure fluid delivered by 
the pump aggregate 76, 77 to the unpressurized supply reservoir 6 in the 
event of adjustable pressure values being exceeded. When the actuating 
force applied to the brake pedal 3 is increased, finally a pressure will 
be accomplished in the inlet chambers 47, 49, upon attainment of which the 
pistons 56, 65 will be displaced to the right, when viewing the drawing, 
in opposition to the compression springs 57, 69 so that the valve closure 
members 54, 64 lift from the valve seats 53, 68 and pressurized fluid is 
fed to the control chambers 50, 62. The pressure brought about this way in 
the control chambers 50, 62 is proportional to the actuating force exerted 
on the brake pedal and will be supplied via the pressure lines 70, 71 to 
the chambers 22, 35 of the separating valves 11, 12, thus causing 
displacement of the pistons 14, 30 of the separating valves 11, 12 to the 
left, when viewing the drawing, and abutment of the valve closure members 
15, 31 on the valve seats 18, 37. The connections between the tandem 
master cylinder 1 and the wheel brakes 26, 27, 41, 42 will be interrupted 
in this switching position. Pressurization of the wheel brakes will now be 
performed by the hydraulically actuated brake valves 43, 44 which supply 
the pressure prevailing in the control chambers 50, 62 via the connections 
72, 73 to the wheel brakes 26, 27, 41, 42. 
When slip control electronics not shown recognize a critical condition at 
one or at several of the vehicle wheels, the two-way/two-position valves 
39, 40, 24 assigned to the vehicle wheels will be switched over so that 
the pressure in the wheel brakes 41, 42, 26, 27 remains constant. In case 
it is necessary for preventing a locked condition to decrease the pressure 
in one or in several of the wheel brakes 26, 27, 41, 42, the 
electromagnetically actuatable two-way/two-position valves 87, 88, 89, 
too, will be switched over so that pressure fluid is taken from the wheel 
brakes 26, 27, 41, 42 and flows into the pressure lines 85, 86. The 
pressure fluid taken from the wheel brakes 26, 27, 41, 42 will be partly 
received by the pressure accumulators 83, 84. In the process of release of 
the brake, the course of motion described will be reversed until finally, 
when the brake is released, the movable parts have re-assumed their 
position to be seen from the drawing. 
If on account of a disturbance of the pump aggregate 76, 77 no pressure 
develops in the pressure lines 81, 82, the separating valves 11, 12 will 
remain in an opened position in which the wheel brakes 26, 27, 41, 42 are 
pressurized by the tandem master cylinder 1 directly.