Hydraulic brake force amplifier

A hydraulic brake force amplifier for multiple-circuit brake systems provided with a control valve which is actuated by the brake pedal via a travel-limiting spring. Disposed parallel to the control valve is a main cylinder, which is actuatable by the brake pedal either in accordance with pressure or when there is a failure of the pressure supply. In order that the travel path of the pedal will not be unnecessarily long when the pressure supply is intact, an arresting piston member is used, which limits the stroke of the control valve. The arresting piston member is combined with means which, upon failure of the pressure supply, permit its use as a further main cylinder piston. In this manner, a compact and lightweight structure of the brake force amplifier is possible. The brake force amplifier is preferably used in passenger vehicles.

BACKGROUND OF THE INVENTION 
The invention relates to a hydraulic brake force amplifier. A brake force 
amplifier of this type is known from German Offenlegungsschrift 27 50 491 
which has a corresponding U.S. Pat. No. 4,197,710 which is incorporated 
herein by reference. 
Fundamental to this known design is the problem of travel path simulation, 
which is intended to be limited, when the supply of pressure is intact, in 
order to keep pedal travel at a minimum. 
As is well known, hydraulic brake force amplification systems can be 
classified as either auxiliary force or external force system. In this 
connection, the terms "open" and "closed" brake circuits are often used. 
The first, "open" brake circuits are associated with the external force 
systems. Intermediate forms are also known in the state of the art, where 
one circuit is open and one circuit is closed. Such systems have the 
disadvantage that if the pressure supply fails one brake circuit also 
fails at the same time. 
In auxiliary force amplification systems, a distinction is made for systems 
where the amplifier portion and the main brake cylinder are disposed 
together in a tandem arrangement. These systems function without travel 
path simulators. The pressure volume characteristic of the cooperating 
brake circuits determines the pedal travel at a particular time. The pedal 
characteristic is given according to the amount of brake force 
amplification. These systems are disadvantageous in the event of brake 
circuit failure, because in that case the pedal travel path is lengthened. 
This disadvantage is avoided in systems which utilize a travel path 
simulator. For human engineering reasons, a relatively short pedal travel 
path is desired for fully applying brake pressure. If the pressure supply 
fails, on the other hand, the entire pedal travel path should be utilized 
for generating brake pressure via the main brake cylinder piston. For 
these reasons, the state of the art, according to the cited German 
Offenlegungsschrift 27 50 491, makes use of an arresting piston member, 
which limits the pedal travel path, or the path simulator's travel, after 
full brake pressure has been applied. This arresting piston member is not 
permitted to function if the pressure supply fails. The switchover of the 
arresting piston member function is accomplished by means of switching 
valves. These interrelated factors mean that the basic expenditure for a 
hydraulic amplification system of this kind comprises a brake valve with a 
travel path simulator, two main brake cylinder pistons and one arresting 
piston member with switchover valves. The basic expenditure is thus 
relatively high. 
OBJECTS AND SUMMARY OF THE INVENTION 
The hydraulic brake force amplifier has the advantage over the prior art 
that because of a particular disposition of the arresting piston member in 
combination with a switchover device, the function of the arresting piston 
member can be utilized in two ways. In the event that the pressure supply 
is intact, the arresting piston member acts as a limitation for the travel 
path simulator. In the event of pressure supply failure, the arresting 
piston member assumes the function of a main brake cylinder. This spares 
expense for structural elements. 
When the pressure supply is intact, one brake circuit functions as an open 
circuit. A leak in the corresponding brake circuit results in failure of 
the energy supply. In accordance with further characteristics of the 
invention, it is attained that the spontaneous failure of the pressure 
supply in the event of a leaking brake circuit can be prevented by means 
of an appropriate logical combination of switching signals in an 
anti-locking apparatus. If a leak in the open brake circuit is recognized 
by this means, then during brake actuation the associated anti-locking 
adjustment member is actuated, which then permits no delivery of pressure 
medium to the outside, that is, to the main brake cylinder. A further 
opportunity is presented by the usage of a hydraulic linkage which can be 
disposed, from a three-dimensional standpoint, in any arbitrary manner. 
Further advantages result from features disclosed herein. 
The invention will be better understood and further objects and advantages 
thereof will become more apparent from the ensuing detailed description of 
preferred exemplary embodiments taken in conjunction with the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A hydraulic brake force amplifier 1 is disposed in a hydraulic brake system 
between a brake pedal 2 and two pairs of wheel brake cylinders 3, 3' and 
4, 4'. The first wheel cylinder pair 3, 3' is part of a brake circuit I 
and the other wheel cylinder pair 4, 4' is part of a brake circuit II. 
Both brake circuits I and II are monitored by a multiple-position 
adjusting member 5 of an automatic anti-locking apparatus, not shown. 
A pressure source comprising a pump 6 and a reservoir 7, attached to a 
housing 8 of the brake force amplifier 1, serves the purpose of brake 
force amplification. Two parallel bores 9 and 10 are provided in the 
housing 8, the first bore 9 being intended for one main cylinder 11 and 
the other bore 10 being intended for a control valve 12 and for an 
arresting piston member 13 disposed behind the control valve 12. 
The control valve 12 is a slide valve and has a slide 14, which, sealed off 
by means of an O-ring 15, protrudes out of the housing 8. At that point it 
is provided with an enlargement 16, which first acts as a contact surface 
for a travel-limiting spring 17 and second is grasped from behind by the 
inwardly bent end of a cylindrical body 18. In this manner, the 
travel-limiting spring 17 is inserted under initial stressing into the 
body 18. 
The body 18 is guided in a hollow cylindrical protrusion 19 of the housing 
8. Furthermore, the body 18 has an arm 21 penetrated by a longitudinal 
bore 20, and a piston 22 is inserted perpendicularly into the end of the 
arm 21. This piston is disposed opposite one end of a hollow push rod 23, 
which protrudes into the main cylinder 1 and there carries a main cylinder 
piston 24. 
The longitudinal passageway bore 20 of the arm 21 leads, in the area of the 
travel-limiting spring 17, to a travel-limiting spring urged piston 25, 
which acts as a spring plate for the travel-limiting spring. The pistons 
22 and 25, along with the fluid filling the longitudinal bore 20, comprise 
a hydraulic linkage, which functions as a means of both pressure 
equalization and failure prevention, with a corresponding stroke 
limitation. 
The arresting piston member 13 generally rests, as shown, on a shoulder 27 
of the housing and in this position limits the stroke of the control valve 
12 and of the travel-limiting spring 17. 
With its side oriented toward the control vavle 12, the arresting piston 
member 13 defines a primary pressure chamber 28, which communicates via a 
channel 29 with a primary pressure chamber 30 at the main cylinder piston 
24. A branch line 31 leads from the channel 29 and via two counteracting, 
controllable check valves 32 and 33 into a chamber 34, which can be 
connected to a pressure chamber 35 provided adjacent to the rear of the 
arresting piston member 13. The pressure chamber 35 has passing through it 
a piston rod 36 secured on the piston member 13, which can be sealed off 
by means of a sealing ring 13' and acts as the secondary pressure chamber 
for the brake circuit II connected to chamber 34. A corresponding 
secondary pressure chamber 37 for brake circuit I has a piston rod 38 
passing through it. The secondary pressure chamber 37 communicates through 
a line 52 to a supply chamber or relief location 54 and the fluid in said 
relief location is fed via a line 56 to the control valve 12. Line 56 also 
feeds fluid to the pump 6. 
Finally, a further reservoir pressure chamber 39 is provided in the brake 
force amplifier, communicating via an angled pressure channel 40 with the 
pressure source 6, 7. Two pressure monitoring pistons 41 and 42 are 
disposed in this reservoir pressure chamber 39 and are forced to the left 
due to pressure on piston 43 of a switchover device exerted by pressure in 
chamber 34 which is connected with the pressure sources 6 and 7. The 
pistons 41 and 42 are provided with rods that operate switches 41' and 
42'. With piston 43 under pressure, the pistons 41 and 42 will hold the 
switches 41' and 42' in their open position. If the pressure in pressure 
chamber 39 decreases due to a failure, pistons 41 and 42 move to the right 
which permits switches 41' and 42' to close. The switchover device 43 is 
provided with parallel fingers that control two check valves 32 and 33. 
With pressure applied in chamber 34, check valve 32 is in its open 
position which permits fluid under pressure to enter chamber 34. Check 
valve 33 is closed when there is pressure in chamber 34. As the switchover 
device moves to the right because of a pressure failure in sources 6 and 
7, the fingers close check valve 32 and opens check valve 33. This 
operation eliminates the hydraulic arrest for the arresting piston 13 so 
that it functions as a main cylinder piston. 
The switches 41' and 42' control electrical signals to an electronic 
switching device which evaluates the signals. The electronic switching 
device, not shown, controls magnetic valves which are arranged in the 
multiple-position adjusting member 5 which is well known in the art. 
A brake light switch 46 can be actuated via a ball 44 and oblique surfaces 
on the piston rod 36 and on a switch rod 45. 
MODE OF OPERATION 
When the brake pedal 2 is actuated, pressure is directed by the control 
valve 12 into the two primary pressure chambers 28 and 30. Via the check 
valve 32 which is still open, the pressure medium proceeds via the chamber 
34 into the brake circuit II, so that pressure is established in both 
brake circuits I and II. At the same time, the arresting pistion member 13 
moves toward the left, until the piston rod 36 attains a sealing contact 
in the seal 13'. The brake light switch 46 is actuated via the ball 44. 
If the pressure supply from the pressure source 6, 7 fails, then the 
pressure drop in the reservoir pressure chamber 39 permits valves 41 and 
42 to move to the right which effects the response of the switches 41' and 
42'. In addition, the switchover device 43 moves and reverses the position 
of the two check valves 32, 33 in such a manner that the valve 32 closes 
and the valve 33 opens. Now the force of the brake pedal 2 is exerted, on 
the one hand, directly via the arm 21 onto the main cylinder piston 24 
and, on the other hand, via the slide 14 onto the arresting piston member 
13. In both brake circuits I and II, a pressure is established, which 
pressure is derived from the secondary pressure chambers 35 and 37, which 
is proportional to the pedal force exerted. Due to the pressure exerted by 
pistons 13 and 24 braking occurs. 
In addition, a shoulder 47 can be provided on the push rod 23 and a switch 
48 can be provided on the housing 8; the switch 48 is then actuated by the 
shoulder 47 whenever, under corresponding pedal force, the arm 21 has 
traveled a predetermined distance. 
As has already been noted at the outset, the disadvantage exists in brake 
systems having one open and one closed brake circuit that if the pressure 
supply fails one brake circuit also fails at the same time. The arresting 
piston member can then generate no further pressure. 
A leak in the open brake circuit can be reduced by means of an appropriate 
logical combination of switching signals. For instance, if after brake 
actuation the pressure in the reservoir 7 drops to an extent exceeding 
what would be proportional, then switchover device 43 moves to the right 
which actuates the switches 41' and 42'. Now if, during the same braking 
process, the switch 48 remains unactuated, then this signifies that no 
pressure builds up in the primary pressure chamber 30 which would force 
piston 24 to the left whereby the switch 48 would be actuated by movement 
of the rod 23. The switch 48 actuates an electronic switching device which 
operates specific magnetic valves in the adjusting member 5. 
The anti-locking multiple-position adjusting member 5 for brake circuit II 
is thereupon actuated via the electronic switching device. Now if the 
switch 48 responds and if there is, in some cases, also a pressure 
increase in the reservoir 7, then this is an indication of a leak in the 
corresponding brake circuit. 
Upon a subsequent brake actuation, a logical signal combination of this 
kind can be interrogated again; however, it can also be stored in memory. 
The latter action has the result that upon each braking procedure, the 
anti-locking multiple-position adjusting member 5 is additionally 
actuated. 
In FIG. 2, a further development of the structure shown in FIG. 1 is 
illustrated. Elements corresponding to the structure of FIG. 1 are given 
identical reference numerals. 
A supplementary step-up device 51 has a piston 52, which defines two work 
chambers 53 and 54, the first of which communicates with brake circuit I 
and the second of which communicates with brake circuit II. By means of an 
appropriate dimensioning of the effective surfaces, this step-up device 51 
can exhibit a pressure step-up property. A reservoir pressure switch 55 is 
provided on the side of the brake circuit II connection. 
With a device of this kind, the advantage is obtained that it is not the 
brake circuit having the highest pressure level which determines the 
dimensioning of the brake force amplifier and the height of the pressure 
level in the pressure source 6, 7, but rather that the structural design 
can be optimized specifically for the individual brake circuits and for 
the pressure supply. 
FIG. 3 shows that in addition to the check valves 33 and 34, a redundant 
switching valve 58 having a switching piston 59 can be used. This 
switching valve 58 is prestressed with a spring 60, so that the switching 
valve 58 does not open until a predetermined opening pressure, of 2 bar, 
for example, has been established. The spring 60 also has the purpose that 
the piston 59 which responds at conventional pressure closes the valve 58 
reliably. 
In this manner, it is attained that no pressure medium can flow out of the 
pressure chamber 35 at the arresting piston member 13, before the 
switching piston 59, exposed via a line 61 to the pressure directed by the 
control valve 12, still further increases the opening pressure. The 
dimensioning of the piston force of the switching piston 59 is designed 
such that even at the highest pedal force exerted on the arresting piston 
member 13, the appropriately high pressure is maintained by means of the 
switching valve 58. 
The advantage of the structural type is that if the pressure supply fails 
no control valve pressure is established either, and thus the arresting 
action of the arresting piston member 13 is reliably precluded in this 
event. 
In FIG. 4, finally, a device having two arresting piston members 63 and 63' 
in a tandem arrangement is shown. The brake valve pressure from the 
control valve 12 (see FIG. 1) proceeds via a line 62 and via a switchover 
device comprising two switchover valves 64 and 65 to the arresting piston 
members 63 and 63' and from thence to brake circuits I and II, 
respectively. The arresting piston members 63 and 63' act counter to a 
plate spring 66 and 66', so that a movement of the piston member occurs 
upon each actuation of the brake. 
The piston member movement is utilized for actuating a brake light switch 
67, while the arresting function is monitored by a switch 68. When the 
force at the pedal 2 is fully effective, this switch 68 is not permitted 
to respond. 
The switchover device 64 and 65 is exposed via a line 69 to the pressure of 
the pressure source 6, 7. It has the task of shutting off the control 
valve pressure after pressure falls below a predetermined level and 
connecting the line to the arresting piston members 63, 63' with a relief 
location (supply container). Then the two arresting piston members 63, 
63', just as in the exemplary embodiment of FIG. 1, act as a main cylinder 
piston. In order to establish the identical pressure level in both brake 
circuits I and II in this case, an equalizing piston member 70 is disposed 
between brake circuits I and II. 
The foregoing relates to preferred exemplary embodiments of the invention, 
it being understood that other embodiments and variants thereof are 
possible within the spirit and scope of the invention, the latter being 
defined by the appended claims.