Master brake cylinder for two-circuit braking system

A master cylinder for a two-circuit brake system which includes anti-skid control has two separate chambers and pistons. The main control piston is surrounded by an auxiliary piston which it engages after a predetermined amount of travel. The auxiliary piston has an extension which holds the brake control valve and which is opened by the control piston during its travel. Furthermore, the second circuit cylinder includes a pressure sensor which can activate a warning when the piston travel is not in a predetermined relation to the applied fluid pressure.

BACKGROUND OF THE INVENTION 
The invention relates to a two circuit braking system for vehicles, and 
more particularly to a master brake cylinder for such a system. The master 
cylinder includes an auxiliary piston within which slides a control 
piston. When suitably displaced, the control piston opens a control valve 
disposed between a source of brake pressure and a first braking circuit. 
The pressure within the first braking circuit reacts back onto the control 
piston and also onto the control surface of a piston functioning in the 
main cylinder intended for the second braking circuit. A master brake 
cylinder of this type is described in the German Offenlegungsschrift No. 2 
312 641. In the apparatus of that publication, after the control piston 
has been slightly displaced, a flat seat valve lying between a source of 
pressure and the first brake circuit is opened but this opening also 
requires the opening of a slide valve. The pressure fed to the second 
brake circuit also affects the control surface of the piston for the 
second brake circuit and displaces it. 
The known apparatus also includes an auxiliary piston which is displaced by 
the pedal when the source of pressure fails and which thus creates the 
brake pressure for both circuits. The apparatus further includes 
mechanical means passing through the auxiliary piston and providing a 
displacement of the piston for the second brake circuit when the pedal is 
actuated but when the first circuit leaks. 
OBJECT AND SUMMARY OF THE INVENTION 
It is a principal object of the present invention to improve the known 
master brake cylinder so as to make it simple in construction while 
maintaining its advantages. 
This object is attained according to the invention by providing that the 
surface of the auxiliary piston remote from the pedal is exposed to the 
pressure from the brake pressure source and also includes the braking 
control valve. it is further provided that the auxiliary piston has an 
extension protruding into the control chamber of the main cylinder and 
this extension engages the piston of the main brake cylinder when 
sufficiently near. 
The apparatus according to the invention provides the advantage of easier 
ventilation of the first circuit and a simplified manner of changing 
pressures, as will be shown below. 
When the source of pressure is intact and the first brake circuit is also 
intact, the auxiliary piston is exposed to the primary pressure of the 
pressure source and not, as in the known state-of-the-art, to the 
controlled pressure. Thus the requirement described in the 
state-of-the-art for making the friction of the auxiliary piston larger 
than the friction of the control piston so as to prevent the displacement 
of the former during normal operation, is not present here. Furthermore, 
the invention provides an extension of the auxiliary piston which 
mechanically displaces the operating piston of the second main brake 
cylinder when the source of pressure fails or the first circuit becomes 
inoperative. This arrangement is a substantial simplification. 
In the master brake cylinder according to the invention, just as in the 
cited Offenlegungsschrift, the control piston is preferably made in two 
parts and a coupler is interposed between the two members for the purpose 
of simulating piston travel and this coupler may preferably be a spring. 
The control piston and the parts cooperating therewith are so embodied 
that, in the initial position, there is a fluid communication between the 
storage container and the first brake circuit which permits the decay of 
pressure at the end of the braking cycle. When the brake is actuated, this 
communication is interrupted immediately. This interruption may take place 
by the control piston closing a relief aperture after a very small motion, 
but preferably there is also provided a seat valve, the movable part of 
which travels with the control piston and closes the valve for 
interruption of the above-mentioned communication. With appropriate 
dimensions of the end faces of the portions of the valve located on the 
control piston, the valve may be so embodied that it closes for a given 
actuating force and reopens at a smaller actuating force. In order to 
shorten the overall length of the apparatus, the path simulator between 
the two control piston parts at least partially surrounds the control 
piston. 
The extension of the auxiliary piston or the piston itself may be provided 
with a valve which is located in the supply line of pressure from the 
first brake circuit to the control chamber of the second circuit and which 
is held open by a rod attached to the piston of the second brake cylinder 
when the second brake cylinder is intact. As a result, this valve closes 
when the second brake circuit is defective and the second piston is 
displaced more than a permissible amount with respect to the auxiliary 
piston. 
In order to monitor the condition of the second piston, for example to 
sense inadequate ventilation, a further embodiment of the invention 
provides a pin extending radially into the brake cylinder of the second 
circuit. The degree of penetration of this pin into the cylinder is 
changed by a secondary piston which is actuated by the control pressure in 
the second circuit. The pin may also be displaced relative to this 
secondary piston in the sense of reducing its extension into the main 
cylinder and during such relative motion, an electric switch is actuated. 
The piston of the second brake circuit is provided with a taper which 
displaces the pin relative to the secondary piston when the main piston 
displacement is too large for the control pressure. The warning signal 
then produced by the closed electric switch indicates that this second 
circuit is faulty, for example due to inadequate ventilation. 
Inasmuch as it may be required by law that an auxiliary piston must move 
even during normal operation, the side of the auxiliary piston opposite 
that affected by brake source pressure is coupled by a spring, especially 
a pan-shaped spring, to the fixed member of the cylinder so that, when the 
pressure on the auxiliary piston changes, it undergoes small 
displacements. 
The source of pressure, which includes a pump and a hydraulic storage 
container, is preferably connected with a chamber ahead of the auxiliary 
piston by a check valve which opens in the direction of this chamber and 
thus prevents a pressure increase but not a pressure decrease. This 
insures that, if the source of pressure should fail, the pressure medium 
in the chamber ahead of the auxiliary piston may not escape into the 
hydraulic storage. 
The invention will be better understood as well as further objects thereof 
become more apparent from the ensuing detailed description of exemplary 
embodiments taken in conjunction with the drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning now to FIG. 1, there is illustrated the first embodiment of the 
invention showing an electromagnetic pump drive including a coil 2, an 
armature 3 and a spring 6. In one position of a piston 7, brake fluid 
flows from a reservoir 4 through a conduit 5 to the pump cylinder and is 
delivered by the driven pump piston 7 through a check valve 8 to an 
elastic storage container 9. The pump and the storage container represent 
the source of brake fluid pressure which is connected through a check 
valve 15 with a pressure chamber 16. The control piston 10 consists of two 
parts coupled together by means of a coupler 55 and the control piston 10 
is actuated by a brake pedal 1 via suitable linkage. The motion of the 
control piston 10 in turn displaces a pin 11 which pushes open the brake 
valve 12. Thus, pressure is admitted to the line 13. The same pressure 
acts backward onto the brake pedal via the piston 10 so that the operator 
receives information as to the magnitude of the applied pressure. An 
auxiliary piston 14 is provided as an emergency piston. In the illustrated 
initial position of the control piston, the line 13, and hence the first 
brake circuit, is connected to the reservoir 4. This connection is closed, 
however, by pedal actuation, firstly by the passage of a relief valve 17 
and, supplementarily, by a seat valve 18. The pressure admitted to the 
line 13 acts, firstly, through the open valve 20 and the valve 21 of the 
five position valve 22, on the rear axle brake circuit which would be 
connected at the location 23 and it also acts, through the open valve 20, 
on the piston 24. The displacement of this piston causes the pressure in 
the second brake circuit (line 25) to be built up. In order to control the 
second brake circuit, the piston 24 has an inclined surface 50, and a 
sensor pin 52 extends radially into a cylinder chamber. This pin is 
sliding within a piston 51; the relative position of the sensor pin 52 and 
the piston 51 may be altered against the force of the spring. When the pin 
52 is displaced with respect to the piston 51, it actuates a contact 53 in 
a warning circuit. The piston is exposed to the pressure within the 
control chamber 39. 
When the pedal 1 is actuated, the communication 13 to the fluid storage 
container is interrupted by the relief valve 17 and seal valve 18 and the 
valve 12 is opened so that the pressure in the first brake circuit 23 
increases directly, and is also admitted via control chamber 39 through 
the piston 24 to the second brake circuit. If the source of pressure 
becomes inoperative and the valve 12 is opened, because there is no 
pressure in chamber 16 -- the piston 14 is displaced to the left by means 
of piston 10, spring 55 and the stop 54; hereby a part of the fluid in 
chamber 16 is displaced into the brake circuit 23 and into chamber 39 so 
that a normal two-circuit brake system becomes operative. As is usually 
the case with brake force amplifiers, the characteristic curve of braking 
pressure as a function of pedal travel has a knee, as does the curve of 
pedal force versus pedal travel. This means that, at the end of the domain 
of power amplification, the path of the pedal may be increased only with 
very high pedal forces while the brake pressure increases accordingly. Due 
to the presence of the path simulator 55 and because of the control 
pressure reacting onto the pedal 1, the operator receives at all times 
sensations of customary brake actuation. If, in the embodiment of FIG. 1, 
the first brake circuit is defective, for example leaks, the auxiliary 
piston 14 is displaced also but the displaced fluid does not cause any 
braking action in one of the circuits. However, the pin 56 moves the 
piston 24, i.e., braking occurs in the second circuit. 
If, on the other hand, it is the second brake circuit which leaks, the 
contact 53 senses that condition. Normally, the piston 51 is so influenced 
by the control pressure that it displaces the pin 52 and so varies its 
extension that the ramp 50 causes no relative displacement of the pin 52 
and the piston 51. On the other hand, when the second brake circuit 25 is 
leaking, the pressure admitted in the chamber 39 results in a more than 
proportional displacement of the piston 24 but not of piston 51 and the 
contact 53 thus is closed, causing a warning to be exhibited to the 
operator. This warning may also be used to actuate a shut-off valve which 
closes off the outlet of the main brake cylinder to the line 25 or else 
closes off the inlet to the control chamber 39 (not shown). In FIG. 1, a 
similar effect in case of a leaking is achieved in that the pressure ahead 
of the valve 20 and effective at the left of piston 76 displaces this 
piston 76 against the smaller counterpressure on the right (because of the 
leaking). The closure elements 41 and 42 are no longer lifted from their 
seats by the lifter 57 Serial No. 703,799 and therefore obturate their 
respective conduits. This displacement of the lifter 57 also takes place 
when wheel-lock prevention and consequent brake pressure control becomes 
operative and thus the brake pressure on the left of piston 76 is higher 
than on its right. During normal braking the closure elements 41 and 42 
are lifted by lifter 57. 
The warning switch 53 also indicates a condition of a poorly ventilated 
(bled) circuit 2. 
The leaf spring 58 is so dimensioned that during small variations of 
pressure control, associated minor movements of the auxiliary piston take 
place. 
A switch 59 is provided which indicates in one of its two positions whether 
the auxiliary piston 14 is in its initial position or whether it has been 
displaced due to a failure of pump power. 
The second brake circuit is divided in the five-stage valve 22 into two 
separate partial circuits 26 and 27 which include valves for separate 
pressure decay and which lead to the two front wheels. 
This five-stage valve 22 is moved into definite positions by means of the 
elements 2, 3 and 7. In unbraked operation, the armature 3 is peridically 
moved to the left by means of current pulses in the winding 2 and returned 
by the spring 6. The armature 3 also moves pump-piston 7 to the left 
whereby pressure fluid is pumped via valve 8 to the pressure reservoir 9. 
If in case of braking the control pressure is produced in line 13, this 
control pressure also determines the position of the piston 28 which, 
after the pressure reaches a predetermined but low value, so far displaces 
the lever 29 that a coupler, embodied here by way of example as a roller 
30, is placed between a part 31 moved by the elements 2, 3 and 6 and a 
movable valve member 32. The distance between the movable part 32 of 
five-position valve 22, forced to the right in the figure by the spring 
33, and the element 31 is chosen so that, during normal pump operation, 
the members 31 and 32 are not in engagement and that their coupling is 
insured only after the roller 30 has pivoted into the intervening position 
shown in dotted lines. Further connected to the lever 29 is a rod 34 which 
opens the valve 35 after the piston 28 has been displaced to the right and 
thus renders the pump ineffective due to the elastic property of the wall 
36. Thus in case of braking, the elements 2, 3 and 6 only are used to move 
member 32 in definite positions and hereby to close and open valves in the 
five-position valve 22 while the pump in this case is made ineffective. 
The displacement of the armature 3 is sensed during that movement into 
definite positions by contacts K.sub.1 to K.sub.4. In the illustrated 
initial position of the drive, the contact K.sub.4 makes contact with 
member 32; in this position of member 32 the pressure may increase at all 
brakes (connected to 23, 26 and 27). If member 32 must be moved into the 
second position (according to lock in a tendancy in which the valve 20 
will be closed, the armature 3 is moved until contact K.sub.1 makes 
contact with member 31 and thus signals the arrival in that position in 
which any further build-up of brake pressure is terminated at all wheels. 
In the third position of the valve, brought about by the additional making 
contact of contact K.sub.2 to member 31, a pin 37 lifts a closure body 38 
from its seat so that, subsequently, a control chamber 39 and the rear 
axle brake circuit connected at the location 23 are connected to a return 
line 40 and a reduction of pressure results. Due to the presence of check 
valves 41 and 42, which are not lifted by lifter 57 during pressure 
control, however, there is no decrease of the brake pressure at the front 
wheels of the vehicle. The fourth position of the apparatus is reached, 
when the contact K.sub.3 also contacts member 31; in this position valve 
21 is closed and a pin 43 lifts closure body of the valve 41, resulting in 
a pressure decrease in the partial circuit 26 and nowhere else. The 
decrease is possible because control member 39 is still connected to 
return line 40 and thus piston 24 can move to the right. 
In the last position of the armature (not equipped with a special contact) 
and member 32, the closure member of valve 41 is pressed onto a second 
valve seat (at the left) while the closure member of valve 42 is lifted 
from its valve seat so that now the pressure decay takes place only in the 
circuit 27. 
As will be seen from the figure, the valves contained within the 
five-position valve 22 are seat valves in which the closure members move 
against spring pressure relative to the independently movable valve member 
of the five position valve. The forces of the springs, together with that 
of the spring 33, constitute the total spring force which acts on the 
armature in stepwise manner. 
The closure of the individual contacts K.sub.1 to K.sub.4 indicates to the 
control circuit for the coil 2 whether the armature has reached its 
intended position. In that case, the power is reduced. For example, if the 
five-position valve is to be brought into the position which would reduce 
the pressure in the partial circuit 26, the control circuit supplies the 
full control power to the coil 2 until such time as the contact K.sub.3 
indicates that the intended position has been reached and a power 
reduction is initiated. 
FIG. 2 illustrates in greater detail the left portion of the main brake 
cylinder in FIG. 1 and just indicates the leftmost portion of the 
auxiliary piston 14 in addition to the piston 24 of the second circuit and 
the extension 56' of the auxiliary piston 14. This extension includes a 
valve 60a and there is provided a further valve 60b. During normal 
braking, the piston 24 is displaced to the left and the valve 60a is 
closed, while valve 60b remains open. If, however, the pressure in the 
cylinder 16 decreases, the extension 56' travels to the left and closes 
off the valve 60b. If the circuit 25 is also leaking, the excursion of the 
piston then also closes the valve 60a. 
FIG. 3 shows a portion of the auxiliary piston 14, the control piston 10, 
the valve 12 and the pin 11, which opens the valve 12, in greater detail. 
This figure illustrates a possibility of embodiment of the seat valve 18 
in FIG. 1. The control piston 10 includes a member 62 which can be 
displaced against the force of the leaf spring 63. If the control piston 
is moved to the left, adjacent surfaces come into contact at the location 
64 just prior to the opening of valve 12, thus closing the connection 
between the valve 18, connected at the location 65 and the opening 66, 
which leads to the reservoir 4. By suitable dimensions of those surfaces 
of the part 62 which are exposed to the control pressure after the 
closure, one may obtain a hysteresis effect in this valve. This will be 
the case if the diameter D.sub.0 of the attachment ring at the location 64 
is smaller than the diameter D.sub.1 since a differential pressure force 
will then act on the piston 10 toward the right, as seen in the figure. If 
the two diameters D.sub.0 and D.sub.1 were identical, the differential 
force would be zero. 
FIG. 4 illustrates the control piston including two parts 71 and 72 between 
which is located a spring 73 acting as a path simulator. Embedded in an 
elastic portion 75 of the first part 71 is an actuating rod 74 which is 
limitedly pivotable and which is connected to the brake pedal. When the 
brake pedal is actuated, the part 71 is displaced within the part 72 and 
displaces part 72 via the spring 73, thereby opening the valve 12. The 
embodiment illustrated in FIG. 4 substantially reduces the length of the 
required construction. 
The foregoing represents preferred embodiments of the invention, it being 
understood that numerous other variants and embodiments are possible 
within the spirit and scope of the invention, the latter being defined by 
the appended claims.