Service and emergency brake system having a master cylinder with a travel simulator gas-filled spring

A travel simulator having a progressive pedal characteristic to be in operation in a power-actuated service brake. The hydraulic brake system has a valve assembly for the switching of the brake system into operation as either a service brake or as an emergency brake. The master cylinder has a pressure fluid reservoir, two pressure chambers, and an intermediate piston as well. The intermediate piston is supported on a travel simulator spring. When the service brake is in operation, pressure fluid can be diverted from the second pressure chamber into the reservoir, and the intermediate piston is moved against the resistance of the spring. When the emergency brake is in operation, both pressure chambers communicate with one another so that the intermediate piston is pressure balanced. When the pedal is actuated, pressure fluid is expelled from the first pressure chamber. The intermediate piston is not subject to any motion during an emergency brake operation. The brake system is for use in road vehicles.

BACKGROUND OF THE INVENTION 
The invention is based on a hydraulic brake system for road vehicles. 
German Patent 40 29 793 A1 makes known a brake system of this kind, in 
which at the start of braking, the first stop valve in the brake circuit 
remains in the open position, so that by means of actuating the brake 
pedal, pressure fluid is displaced from the pressure chamber of the master 
cylinder, and brake pressure is increased in the wheel brake cylinder. The 
brake pedal executes a deflection, which is predominantly determined by 
the air play in the wheel brake, as well as by the elasticity of the wheel 
brake and the brake line. However, the generally desired progressive pedal 
characteristic, i.e. the resistance that at the onset of pedal travel is 
slight and progressively increases with greater deflection, is unsteady in 
the known brake system, in that from a predetermined pressure threshold, 
the first stop valve is switched to the closed position and the power 
actuation of the service brake is activated. As the load exerted by the 
driver increases, the brake pedal has no further significant travel. It 
is, however, desirable for approximately the same pedal characteristic to 
prevail, both upon actuation of the service brake as well as upon 
actuation of the emergency brake. 
OBJECT AND SUMMARY OF THE INVENTION 
The brake system according to the invention has an advantage over the prior 
art that the spring influences the pedal characteristic upon operation of 
the brake system as a power-actuated service brake; over the entire range 
of action, steady pedal travel and a progression of the spring tension is 
produced without abrupt changes in force. While the pressure fluid from 
the second pressure chamber of the master cylinder is displaced into the 
pressure fluid reservoir, the first pressure chamber remains separated 
from the brake circuit by means of the first stop valve so that an 
inadmissibly great increase in the pedal travel is avoided. When the brake 
is released, the energy absorbed by the spring is taken completely off the 
brake pedal. 
In comparison, if the emergency brake is activated due to a failure of the 
service brake, both pressure chambers of the master cylinder communicate 
with each other by means of the third and fourth line and the intermediate 
piston is pressure balanced, i. e. upon an actuation of the brake pedal, 
the intermediate piston maintains its starting position and pressure fluid 
is displaced only from the pressure chamber on the pedal side. Doing this, 
the pedal characteristic is not influenced by means of the spring so that 
an undesired increase in pedal travel does not occur. This is why the 
available pedal travel can be used entirely for displacing pressure fluid 
from the pressure chamber on the pedal side. This allows one to 
advantageously keep the diameter of the master cylinder small and, because 
of the thus reduced actuation forces, to reduce the diameter of a vacuum 
brake booster if one is necessary, or to do without it altogether. 
With the embodiment of a spring set forth herein, a pedal characteristic 
can be achieved in a simple, cost-effective manner; this pedal 
characteristic can be varied widely by varying both the properties of the 
elastomer and the contribution and size of the cells. 
The improvement of the invention distinguishes itself in that the 
installation chamber of the master cylinder provided for the spring can be 
used entirely. 
The invention will be better understood and further objects and advantages 
thereof will become more apparent from the ensuing detailed description of 
a preferred embodiment taken in conjunction with the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 indicates a hydraulic brake system 10, of which merely one brake 
circuit 11 is shown, which serves both as an externally actuated service 
brake and as manually-actuated emergency brake for a street vehicle, 
particularly a passenger vehicle. Essential elements of the brake system 
10 are a pedal actuatable master cylinder 12, wheel brakes 13 and 14 of 
the brake circuit 11, a servo pressure source 15, valve assemblies 16 and 
17 for pressure modulation in the wheel brake cylinders 13 and 14, a valve 
assembly 18 for reversing the manner of function of the brake system as 
service brake or emergency brake, and an electronic control unit 19. 
Throughout this disclosure the term "service brake" relates to a normal 
operation of the brake system by operation of the brake pedal and of the 
servo pressure source to control the braking. The term "manually-actuated 
emergency brake" is an operation of the brake system by movement of the 
brake pedal in the event the service brake system fails. 
The brake system 10 has a first line 21, which leads from a pressure fluid 
reservoir 22 of the master cylinder 12 and which branches into two line 
branches 21.1 and 21.2, which lead to the wheel brake cylinders 13 and 14. 
A second line 23, leading from these brake cylinders 13 and 14 via line 
branches 23.1 and 23.2, likewise communicates with the pressure fluid 
reservoir 22. A third line 24 extends between a first pressure chamber 25 
of the master cylinder 12 and the second line 23. A fourth line 26 is 
additionally provided, which extends between a second pressure chamber 27 
of the master cylinder 12 and the second line 23. 
A high pressure pump 30 of the servo pressure source 15 is disposed in the 
first line 21 and operated by a motor which is controlled by the 
electronic control unit 19. Pressure fluid can be sucked out of the 
pressure fluid reservoir 22 of the master cylinder 12 by means of the high 
pressure pump 30 and can be fed at a high pressure level into a pressure 
reservoir 31 connected to the first line 21 downstream of the pump. A 
pressure sensor 32 is provided connected to the first line 21 to monitor 
the pressure in the pressure reservoir 31. Downstream of the high pressure 
pump 30 and the pressure reservoir 31, a pressure holding valve 33 is 
disposed in the first line 21 in the form of a 2/2-way valve, having a 
spring-actuated closed position and an electromagnetically switchable open 
position. 
The valve assembly 16 is comprised of an inlet valve 36 disposed in the 
line branch 21.1 of the line 21 and an outlet valve 37 disposed in the 
line branch 23.1 of the second line 23. The inlet valve 36 and the outlet 
valve 37 are embodied as throttling directional control valves with two 
connection openings. In its one outer end position, the inlet valve 36 
assumes a spring-actuated closed position and in its other outer end 
position, assumes an electromagnetically switchable open position. As its 
outer end position, the outlet valve 37 has a spring-actuated open 
position and an electromagnetically switchable closed position. The valve 
assembly 17 associated with the brake cylinder 14 has the same valve 
construction. On the brake cylinder side, a pressure sensor 38 and 39 is 
additionally provided to monitor the pressure in each of the brake 
cylinders 13 or 14. 
The brake cylinders 13 and 14 disposed in the brake circuit 11 are 
associated with wheel brakes of the front axle of the road vehicle, which 
are not shown. A second brake circuit which includes the wheel brakes of 
the rear axle of the vehicle is not shown in FIG. 1. It can be connected 
on the one end to the servo pressure source 15 and on the other side with 
the pressure fluid reservoir 22 of the master cylinder 12 and can include 
valve assemblies for pressure modulation in the wheel brakes of the rear 
axle. 
A first stop valve 41 is disposed in the third line 24 of the brake system 
in the structural shape of the 2/2-way valve, having a spring-actuated 
open position and an electromagnetically actuated closed position. The 
first stop valve 41 is circumvented by a bypass line 42 having a check 
valve 43 disposed in it, which opens in the direction from the wheel brake 
cylinders 13, 14 to the master cylinder 12. In the third line 24, also on 
the master cylinder side, a pressure sensor 44 is connected for accepting 
the pressure generated in the first pressure chamber 25 of the master 
cylinder 12. 
A second stop valve 46 is disposed in the second line 23 between the 
pressure fluid reservoir 22 and the connection of the third line 24 to the 
second line 23. The second stop valve 46, just as the first stop valve 41, 
is a 2/2-way valve, but has a spring-actuated closed position and an 
electromagnetically actuated open position. The fourth line 26, which 
leads from the second pressure chamber 27 of the master cylinder 12 is 
connected to the second line 23 between the first stop valve 41 and the 
second stop valve 46. 
The master cylinder 12 can be actuated by means of a brake pedal 48. This 
acts, by means of an actuating rod 49, upon a piston 50, which defines the 
first pressure chamber 25 on the pedal side. By means of actuating the 
brake pedal 48, a brake light switch can be closed while the motion of the 
actuating rod 49 can be picked up by a travel sensor 52. As can be seen in 
the enlarged longitudinal section through the housing 53 of the master 
cylinder 12 shown in FIG. 2, an intermediate piston 55, which separates 
the two pressure chambers 25 and 27, is disposed in a cylinder bore 54, 
which is closed by means of the piston 50. This intermediate piston 55 is 
coupled in a known manner to the piston 50 so that it can slide 
longitudinally; when the brake pedal 48 is not actuated, a pressure spring 
56 received in and held in place by the pressure chamber 25 keeps both 
pistons 50 and 55 at a predetermined distance from each other. 
Roughly in the middle of its longitudinal span, the intermediate piston 55 
is provided with a collar 59, which almost reaches the inner circumference 
of a diametrical widening 60 of the cylinder bore 54. The diametrical 
widening 60 is defined against the second pressure chamber 27 by means of 
a bore step 61. The intermediate piston 55 and its collar 59 on the one 
side encompass the diametrical widening 60 of the cylinder bore 54 and the 
bore step 61 as well as on the other side encompassing an annular chamber 
62, in which a spring 63 is received. This spring has the shape of a 
straight, hollow, circular cylinder and fills the annular chamber 62 
completely. The spring 63 encompasses the intermediate piston 55 coaxially 
and is supported on the one side by the piston 55 and on the other side by 
the master cylinder housing 53. The spring 63 is comprised of an elastomer 
such as polyurethane, and is embodied having closed-pored, gas-filled 
cells, preferably microcells. The spring 63 determines the pedal 
characteristic in operation of the brake system 10 as a service brake. 
Finally, the intermediate piston 55 is further sealed against the second 
pressure chamber 27 by means of a seal 64. 
The pump 30, the valves 33, 36, 37, 41, 46, the sensors 32, 38, 39, 44, 52, 
and the brake light switch 51 communicate electrically with the control 
unit 19. It analyzes the signals of the switch and of the sensors and 
switches or controls the pump and the valves according to a predetermined 
algorithm. 
The hydraulic brake system 10 functions as follows: 
When the service brake is in operation, the pressure reservoir 31 is loaded 
by operation of the pump 30 and the valves of the servo pressure source 
15, of the valve assemblies 16, 17, and 18 assume the indicated position. 
By means of actuating the brake pedal 48, the brake light switch 51 is 
closed so that the control unit 19 recognizes the desire to brake. The 
control unit 19 switches the first stop valve 41 into the closed position 
and the second stop valve 46 into the open position. The control unit 19 
also transfers the outlet valves 37 of the valve assemblies 16 and 17 into 
the closed position. The pressing down of the brake pedal 48 is monitored 
by means of the travel sensor 52. The actuating rod 49 effects a shift of 
the piston 50, which, due to the closed stop valve 41 and the stopping 
check valve 43, is transferred to the intermediate piston 55 by the 
hydraulic column in the pressure chamber 25. This displaces pressure fluid 
from the second pressure chamber 27 by means of the open second stop valve 
46 into the pressure fluid reservoir 22. At the same time the travel 
simulator spring 63 is compressed by the intermediate piston 55, which 
leads to an increase of the pressure in the first pressure chamber 25. 
This pressure is captured by the pressure sensor 44 and is changed by the 
control unit 19 into switching signals or control signals for the pressure 
holding valve 33 as well as the inlet valves 36 of the valve assemblies 16 
and 17. According to the signal of the pressure sensor 44, brake pressure 
is built up in the wheel brake cylinders 13 and 14 of the brake circuit 11 
and is monitored by the pressure sensors 38 and 39. If the pressure in the 
first pressure chamber 25 is reduced, the control unit 19 diverts pressure 
fluid from the brake cylinders 13 and 14 into the pressure fluid reservoir 
22 by means of the outlet valves 37. The second brake circuit of the brake 
system 10 is not shown, but its brakes are actuated in the same manner. 
In the previously described manner of function of the brake system 10 as a 
service brake, the travel simulator spring 63 determines the pedal 
characteristic because the pressure fluid enclosed in the first pressure 
chamber 25 is incompressible and therefore the pressure spring 56 is not 
stressed and furthermore because the pressure fluid can be displaced from 
the second pressure chamber 27 by means of the intermediate piston 55 
without resistance of any consequence. The spring 63 enclosed in the 
annular chamber 62 opposes the intermediate piston 55 with a resistance, 
which is due on the one hand to the deforming of the elastomer frame and 
is due on the other hand to the compression of the gas in the cells. By 
means of this, a progressively increasing movement of the actuation force 
with increasing actuation travel ensues. When the brake pedal 48 is 
relieved, the energy stored in the spring 63 is given up completely, and 
the spring assumes its original volume once again. The pressure fluid 
displaced from the pressure chamber 27 is replaced from the reservoir 22. 
If the service brake fails, the valves of the brake system 10 remain in the 
indicated position and the emergency brake is operational. The servo 
pressure source 15 cannot inject any pressure fluid into the brake 
cylinder 13 and 14. Instead, pressure fluid is expelled from the first 
pressure chamber 25 of the master cylinder 12 by means of the actuation of 
the brake pedal 48. By means of the open first stop valve 41 and the open 
outlet valves 37 of the valve assemblies 16 and 17, partial quantities of 
pressure fluid from the second line 23 reach the brake cylinder 13 and 14, 
where they cause brake pressure to become operative. The pressure created 
in the first pressure chamber 25 is also transmitted into the second 
pressure chamber 27 by means of the fourth line 26. Since a uniformity of 
pressure consequently prevails on both diametrically equal face ends of 
the intermediate piston 55, this does not lead to any movement. Thus, 
pressure fluid is merely displaced from the first pressure chamber 25. 
Upon operation of the emergency brake, the pedal characteristic is 
determined on the one hand by means of the stressing of the pressure 
spring 56 and on the other hand by means of the counterforce with which 
the brake circuit 11 opposes the pedal force. Also in this connection a 
progressiveness is given to the increase in force over the pedal travel. 
In the above described exemplary embodiment, this emergency brake acts only 
on the brake circuit 11 associated with the front axle of the vehicle. 
With safe operation, this is sufficient to transport the vehicle to a 
workshop for overhauling the brake system in the event of a failure of the 
service brake. 
The foregoing relates to a preferred exemplary embodiment of the invention, 
it being understood that other variants and embodiments thereof are 
possible within the spirit and scope of the invention, the latter being 
defined by the appended claims.