Abstract:
A hydraulic main brake cylinder, preferably having an electro-mechanical brake booster for a brake assembly of a vehicle is disclosed. The main brake cylinder is provided with two pressure rod pistons, preferably nesting within each other, one of which is actuated by muscle power, and the other one is actuated by the brake booster. Furthermore, an energy store is provided, which stores energy when the main brake cylinder is released and then transferred to the main brake cylinder upon the actuation thereof, thus supporting the operation of the main brake cylinder.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2010/057290, filed on May 27, 2010, which claims the benefit of priority to Serial No. DE 10 2009 028 034.0, filed on Jul. 27, 2009 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     The disclosure relates to a hydraulic master brake cylinder for a vehicle brake system. 
     BACKGROUND 
     Master brake cylinders having a vacuum-type brake force booster are known and are common for vehicle brake systems in passenger motor vehicles. 
     The laid-open specification DE 103 27 553 A1 discloses an electromechanical brake force booster having an electric motor which acts via a rotation/translation conversion mechanism on a thrust rod piston of a hydraulic master brake cylinder. The electric motor of the known brake force booster is a hollow-shaft motor, the hollow shaft of which is formed as a nut of a spindle gearing. The spindle gearing forms a rotation/translation conversion mechanism which converts the rotational drive movement of the electric motor into a translatory drive output movement for displacing the thrust rod piston. An electromechanical brake force booster may be of some other design. A thrust rod piston refers here to the or a piston of the master brake cylinder which, for the actuation of the master brake cylinder, is displaced in the master brake cylinder by the brake force booster and/or by the muscle force of a vehicle driver. The thrust rod piston may also be referred to as primary piston or simply as piston or master brake cylinder piston. The displacement of the thrust rod piston by muscle force, that is to say an actuation of the master brake cylinder under muscle force, is conventionally carried out with a foot by means of a (foot-operated) brake pedal or by hand by means of a (hand-operated) brake lever. 
     SUMMARY 
     The master brake cylinder according to the disclosure has two thrust rod pistons which are displaceable relative to one another. One of the two thrust rod pistons is displaced by a vehicle driver by muscle force, and the other thrust rod piston is displaced by the brake force booster. In contrast to the prior art, coupling of the booster force from the brake force booster and of the muscle force of a vehicle driver takes place not mechanically outside the master brake cylinder but rather hydraulically in the master brake cylinder. An independent displacement of the two thrust rod pistons, and as a result an independent movement of the brake force booster and of the muscle force movement of the vehicle driver, is possible, wherein the independence of the two movements may be restricted in developments and refinements of the disclosure. The ratio of the piston surface areas of the two thrust rod pistons determines the boost factor of the brake force booster. 
     During a normal service braking operation, the brake force booster displaces the other thrust rod piston synchronously with respect to a thrust rod piston which is displaced by the vehicle driver by muscle force. During the synchronous displacement of the two thrust rod pistons, it is the case, as stated, that the ratio of the piston surfaces of the two thrust rod pistons determines the boost factor of the brake force booster. 
     A so-called springer function, that is to say a slightly increased brake force boost at the start of the brake actuation and at the start of the displacement of the thrust rod piston, is achieved by means of an initially greater displacement travel of the thrust rod piston which is displaced by the brake force booster. 
     The master brake cylinder may be actuated both exclusively by the brake force booster and also exclusively by muscle force, rather than both by the brake force booster and by muscle force as is the case during the normal service braking operation. The actuation both by means of the brake force booster and also by muscle force is a power-assisted braking operation, the actuation exclusively by the brake force boosting is an externally-powered braking operation, and the actuation exclusively by muscle force is a muscle-force-powered braking operation. The master brake cylinder according to the disclosure permits an exclusively muscle-force-powered braking operation for example in the event of failure of the brake force booster, wherein the brake force booster is not jointly moved and the actuation by muscle force is not made more difficult or hindered. 
     The master brake cylinder according to the disclosure is particularly suitable for hybrid vehicles with drive provided by an internal combustion engine and by an electric motor, and also for purely electric vehicles. In such vehicles, for braking the vehicle, the electric drive motor can be operated as a generator, such that a greater or lesser part of the brake force or brake power is imparted by the electric drive motor and the remaining part is imparted by the vehicle brake system. The fraction of the braking power provided by the electric drive motor during generator operation may lie between 100% and 0%; it varies as a function inter alia of the respective driving situation and for example also as a function of a state of charge of a battery which provides electrical current for driving the vehicle by means of the electric drive motor and which is charged with electrical current by the electric drive motor in the generator mode during braking. The recovery of kinetic energy of the vehicle is referred to as recuperation. 
     It is desirable for the fact that a part of the braking force is generated not by the vehicle brake system but rather by the electric drive motor during generator operation to be as imperceptible as possible to a vehicle driver. The braking partially by means of the electric drive motor in the generator mode and otherwise by means of the vehicle brake system is referred to as “blending”. Blending which is as imperceptible as possible to the vehicle driver is difficult inter alia because the braking power of the electric drive motor may vary constantly. As a result of the possibility for independent displacement of the two thrust rod pistons by muscle force and by the brake force booster, the master brake cylinder according to the disclosure makes it possible to more easily realize blending which is as imperceptible as possible. As a result of the hydraulic coupling of the brake force booster and the muscle force actuation, imperceptible blending is more easily possible with the master brake cylinder according to the disclosure than with mechanical coupling. 
     For imperceptible blending, the boost factor of the brake force booster must be reduced, in accordance with the braking power of the electric drive motor in the generator mode, to such an extent that, for the same muscle force exerted on the thrust rod piston of the master brake cylinder, and preferably for the same muscle force travel, an overall braking power of the electric drive motor in the generator mode and of the vehicle brake system is obtained which is identical to that obtained in the case of a braking operation exclusively by means of the vehicle brake system. 
     Advantageous developments and refinements are set forth below in the disclosure. 
     One development of the disclosure provides a resilient connection between the two thrust rod pistons in the displacement direction. The resilient connection may act in one or both directions. The resilient connection provides mechanical coupling in addition to the hydraulic coupling. The resilient connection has the advantage that, in the event of a displacement of one of the two thrust piston rods, the other is likewise displaced, albeit to a lesser extent. 
     One development of the disclosure provides a limitation of the displacement of the two thrust rod pistons relative to one another in order to avoid an unlimited displacement of the two thrust rod pistons relative to one another. 
     One development of the disclosure provides a controllable brake force booster. “Controllable” means that a booster force of the brake force booster can be controlled independently of the muscle force exerted on the master brake cylinder, wherein within the context of the disclosure “control” should also be understood to mean “regulation”. An electromechanical brake force booster is, owing to its design, controllable, for which reason such an electromechanical brake force booster is preferably provided. A vacuum-type brake force booster may be designed to be controllable, for example by virtue of its working chambers being designed such that they can be aerated by means of a valve. As a valve, there is provided for example a solenoid valve, or for better controllability, a proportional valve. The disclosure is not restricted to the brake force boosters listed. 
     In a preferred development of the disclosure, an energy store is provided which stores energy upon the release of the master brake cylinder and which, upon an actuation of the master brake cylinder, transmits stored energy to the master brake cylinder and thereby assists the actuation of the master brake cylinder. The storing of energy in the energy store takes place, for example in the event of an active release of the master brake cylinder, by means of the brake force booster, which may be possible with an electromechanical brake force booster or generally a brake force booster which can impart a booster force counter to the actuating direction. Although energy must be imparted by the brake force booster both upon actuation and also upon release, it is however the case that, during a brake actuation, the booster force and the energy imparted thereby are lower than they would be without the assistance by the energy store. It is therefore possible to use a brake force booster of lower power. A further advantage of this development of the disclosure is that energy which is released upon the release of a vehicle brake system is stored in the energy store. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The disclosure will be explained in more detail below on the basis of an exemplary embodiment illustrated in the drawing, in which: 
         FIG. 1  shows a master brake cylinder according to the disclosure in axial section; and 
         FIG. 2  shows a cross-sectional illustration along the line II-II in  FIG. 1 . 
     
    
    
     The figures are simplified schematic illustrations for explaining and for understanding the disclosure. 
     DETAILED DESCRIPTION 
     The master brake cylinder  1  according to the disclosure illustrated in  FIG. 1  is a tandem master brake cylinder with two thrust rod pistons  2 ,  3  and a floating piston  4 . A thrust rod piston  2 ,  3  can also be referred to as primary piston or input piston, and the floating piston  4  can also be referred to as secondary piston. One of the two thrust rod pistons  2  is a tubular hollow piston in which the other thrust rod piston  3  is held in an axially displaceable manner. The two thrust rod pistons  2 ,  3  are sealed off with respect to one another and the tubular first thrust rod piston  2  is sealed off in the master brake cylinder  1 . The tubular first thrust rod piston  2  is actuated mechanically by muscle force by means of a (foot-operated brake) pedal  5  via a piston rod  6 , which is articulatedly connected to the pedal  5  and to the first thrust rod piston  2 . A muscle force actuation may also be realized for example by hand via a hand-operated brake lever (not illustrated). 
     The other, inner thrust rod piston  3  is actuated, that is to say displaced in the master brake cylinder  1  or in or with the first tubular thrust rod piston  2 , by a brake force booster  7 . In the exemplary embodiment, the brake force booster  7  is an electromechanical brake force booster  7 , which is however not imperative for the disclosure. The brake force booster  7  has an electric motor  8  on which is flange-mounted a reduction gearing  9 , for example a planetary gear set, which via a rack and pinion gearing  10  displaces the other, inner thrust rod piston  3 . The rack and pinion gearing  10  has a pinion  11  on an output shaft of the reduction gearing  9 , which pinion meshes with a toothed rack  12  rigidly connected to the other, inner thrust rod piston  3 . 
     The tubular first thrust piston rod  2  has inwardly projecting flanges  13  at both ends. At a rear end of the tubular first thrust piston rod  2 , the toothed rack  12  extends through the flange  13 . The rear end is that end side of the thrust rod piston  2  which faces toward the pedal  5 . The master brake cylinder  1  has not one but two piston rods  6  which are arranged congruently adjacent to one another in longitudinal grooves of the toothed rack  12  (cf.  FIG. 2 ). The piston rods  6  are articulatedly mounted on the flange  13  at the rear end of the tubular first thrust rod piston  2 . 
     The inwardly projecting flanges  13 ,  14  of the tubular first thrust rod piston  2  form stops which limit a displacement travel of the two thrust rod pistons  2 ,  3  relative to one another. The flanges  13 ,  14  may also be regarded as a relative displacement travel limitation or relative displacement travel limitation means for the displacement travel of the two thrust rod pistons  2 ,  3  relative to one another. Arranged at both ends of the other, inner thrust rod piston  3  are spring elements  15 ,  16  which are supported on the flanges  13 ,  14  of the tubular first thrust rod piston  2 . The spring elements  15 ,  16  connect the two thrust rod pistons  2 ,  3  in a resilient manner in the displacement direction, that is to say in the axial direction. In the exemplary embodiment, the spring element  15  at the rear end, which faces toward the pedal  5 , of the other, inner thrust rod piston  3  is a helical compression spring. 
     The spring element  16  at the front end of the other, inner thrust rod piston  3  is, in the exemplary embodiment, a spring washer which is curved in the shape of a dome and the spring travel of which is a fraction, and the spring constant of which is a multiple, of the spring travel and the spring constant, respectively, of the spring element  15  at the rear end of the other, inner thrust rod piston  3 . Spring elements other than a helical compression spring and a spring washer are also possible; likewise, the arrangement of the short spring travel and the large spring constant on the front side of the other, inner thrust rod piston  3  is not imperative for the disclosure, but is preferable. 
     It is not imperative for the tubular first thrust rod piston  2  to be actuated by muscle force and the other, inner thrust rod piston  3  to be actuated by the brake force booster  7 . It is however considered to be advantageous for the tubular, outer first thrust rod piston  2  to be actuated by muscle force because, after a short displacement travel, it closes off a breather bore  17  via which a brake fluid reservoir  18  communicates with a pressure chamber  19  of the master brake cylinder  1 . Instead of two thrust rod pistons  2 ,  3  situated one inside the other, it is also conceivably possible to provide two thrust rod pistons which are arranged parallel or at an angle or in a skewed alignment with respect to one another, which thrust rod pistons are arranged in a correspondingly modified master brake cylinder  1 , or at least one of which even has its own cylinder (not illustrated). The two thrust rod pistons communicate hydraulically. 
     A normal service braking operation is realized by means of muscle force actuation, that is to say the brake pedal  5  is depressed and, via the piston rod  6 , displaces the tubular first thrust rod piston  2 . An electronic controller (not illustrated) controls the brake force booster  7  such that the other, inner thrust rod piston  3  is displaced synchronously with the tubular first thrust rod piston  2 , wherein “control” should also be understood to mean “regulation”. For the control of the brake force booster  7 , the master brake cylinder  1  has a travel sensor  20  and/or a force sensor (not illustrated) by means of which a pedal travel and/or a pedal force of the pedal  5  are measured. The displacement or position of the other, inner thrust rod piston  3  which is displaced by the brake force booster can be measured for example on the basis of an electronic commutation of the electric motor  8  of the brake force booster  7 . 
     The floating piston  4  is actuated, that is to say displaced in the master brake cylinder  1 , by hydraulic pressurization by the thrust rod pistons  2 ,  3 , in a manner known per se. 
     A so-called springer function, that is to say a slightly increased brake force boost at the start of a brake actuation, can be achieved by virtue of the other, inner thrust rod piston  3  which is displaced by the brake force booster  7  being displaced further at the start of the displacement than the tubular first thrust rod piston  2  which is displaced by muscle force by means of the pedal  5 . 
     Force boosting, that is to say the boosting of the muscle force by the brake force booster  7 , that is to say a boost factor of the brake force booster  7 , is determined, during synchronous displacement of the two thrust rod pistons  2 ,  3 , by the ratio of the piston surface areas thereof. The piston surface area of the tubular first thrust rod piston  2  is a circular ring-shaped surface. For greater force boosting, the other, inner thrust rod piston  3  which is displaced by the brake force booster  7  is displaced further than the tubular first thrust rod piston  2  which is actuated by muscle force, and for lesser force boosting, the situation is reversed. The two thrust rod pistons  2 ,  3  are hydraulically coupled by means of brake fluid in the pressure chamber  19 , which they act on, of the master brake cylinder  1 , and are additionally mechanically and resiliently coupled by means of the spring elements  15 ,  16 . In the case of an unequal displacement of the two thrust rod pistons  2 ,  3 , the spring elements  15 ,  16  transmit a force from the thrust rod piston  2 ,  3  which has been displaced to a greater extent to the thrust rod piston  2 ,  3  which has been displaced to a lesser extent. 
     Autonomous braking without muscle force actuation may be realized by displacement of the other, inner thrust rod piston  3  by means of the brake force booster  7 . By means of the spring element  16  on the front side of the other, inner thrust rod piston  3 , the tubular first thrust rod piston  2  is driven along resiliently and in a damped manner. 
     In the event of failure of the electromechanical brake force booster  7 , exclusively muscle-force-powered actuation by means of the brake pedal  5  is possible. As a result of its small, circular-ring-shaped piston surface, the tubular first thrust rod piston  2  which is actuated by muscle force has a large hydraulic transmission ratio, which is advantageous in the case of an exclusively muscle-force-powered actuation of the master brake cylinder  1  in the event of failure of the brake force booster  7 . The spring element  15  at the rear end of the other, inner thrust rod piston  3  transmits a force from the tubular first thrust rod piston  2  to the other, inner thrust rod piston  3 , such that in the case of a muscle-force-powered actuation of the master brake cylinder  1 , the other, inner thrust rod piston  3  is also displaced in the master brake cylinder  1 , but the displacement travel of the other, inner thrust rod piston  3  is in this case shorter than the displacement travel of the tubular first thrust rod piston  2 . The hydraulic transmission ratio of the tubular first thrust rod piston  2  is decreased as a result of the other, inner thrust rod piston  3  being driven along. The spring element  15  at the rear end of the other, inner thrust rod piston  3  has been selected with a spring constant smaller than that of the spring element  16  at the front end of said other, inner thrust piston rod  3 , such that the other, inner thrust piston rod  3  is driven along more smoothly in the event of an exclusively muscle-force-powered actuation of the master brake cylinder  1 . If the spring element  15  at the rear end of the other, inner thrust rod piston  3  assumes a “blocked” state, the other, inner thrust rod piston  3  moves synchronously with the tubular first thrust rod piston  2 . 
     The master brake cylinder  1  according to the disclosure has an energy store  21  which is situated in front of the plane of the drawing in  FIG. 1  and which is therefore visible only in  FIG. 2 . Upon the release of the master brake cylinder  1 , that is to say in the event of a displacement of the thrust rod pistons  2 ,  3  out of the cylinder  1 , the energy store  21  stores energy, which energy is output again by the energy store  21  upon an actuation of the master brake cylinder  1 , that is to say in the event of a displacement of the thrust rod piston  2 ,  3  into the master brake cylinder  1 , and thereby assists the actuation of the master brake cylinder  1 . In the exemplary embodiment, the energy store  21  is a spring energy store, which is however not imperative for the disclosure. The energy store  21  has a spindle gearing  22  with a spindle  23  and a nut  24 . The spindle gearing  22  is of non-self-locking design. A spring element  25  is supported in a housing of the energy store  21  and presses axially against the nut  24 . A helical compression spring is illustrated, though other springs may also be used, in particular a plate spring pack (not illustrated). The spindle  23  has a pinion  26  which meshes with the toothed rack  12 , which toothed rack is rigidly connected to the other, inner thrust rod piston  3  and forms the piston rod of said thrust rod piston  3 . The pinion  26  of the energy store  21  engages on a side of the toothed rack  12  situated opposite the side engaged on by the pinion  11  of the brake force booster  7 , and the toothed rack  12  therefore has two mutually opposite toothings. The two pinions  11 ,  26  support the toothed rack  12  at alternate sides against transverse forces or transverse force components which act when the toothed rack  12  is driven by means of the pinions  11 ,  26 . A separate support of the toothed rack  12  against transverse loading is thereby dispensed with, and the other, inner thrust rod piston  3  is not acted on with a torque about a transverse axis. 
     Upon the release of the master brake cylinder  1 , the other, inner thrust rod piston  3  is displaced out of the cylinder  1  and displaces the toothed rack  12 , which forms the piston rod of said other, inner thrust rod piston  3 , in the direction of the pedal  5 . In so doing, the toothed rack  12  sets the pinion  26 , and with it the spindle  23  of the energy store  21 , in rotation. The nut  24  is displaced axially and stresses the spring element  25 , such that energy is stored in the energy store  21 . 
     To be able to stress the spring element  25  of the energy store  21 , the toothed rack  12  must be driven by the brake force booster  7 , that is to say the master brake cylinder  1  must be actively released by means of the brake force booster  7 . For this purpose, the energy store  21  assists the actuation of the master brake cylinder  1 : the spindle gearing  22  of the energy store  21  is of non-self-locking design and converts the axial force of the spring element  25  into a torque which loads the toothed rack  12  in the direction of the master brake cylinder  1  and thereby assists an actuation of the master brake cylinder  1 . The force and the energy which must be imparted by the brake force booster  7  for the actuation of the master brake cylinder  1  is correspondingly reduced as a result of the action of the energy store  21 . Because the actuating force is distributed between the brake force booster  7  and the energy store  21 , a mechanical loading of the toothed rack gearing  10  of the brake force booster  7  is correspondingly reduced, and said toothed rack gearing can be dimensioned to be of correspondingly lower strength. 
     The forces exerted by the pinions  11 ,  26  of the brake force booster  7  and of the energy store  21  on the two opposite toothings of the toothed rack  12  act perpendicularly to the tooth flanks, which bear against one another, of the teeth, which mesh with the toothings of the toothed racks  12 , of the pinions  11 ,  26 . The forces thus act in the longitudinal direction of the toothed rack  12  and are directed slightly obliquely inward, as indicated by the force arrows  27 , in  FIG. 1 . The forces  27 ,  28  therefore have an inwardly directed force component perpendicular to the toothed rack  12 ; said transverse force components are compensated if the forces acting on the toothings are of equal magnitude, specifically also if the pinions  11 ,  26  of the brake force booster  7  and of the energy store  21  are arranged with an axial offset. 
     Upon the release of the master brake cylinder  1 , the force which is exerted by the pinion  11  of the brake force booster  7  on the toothed rack  12  is reversed; said force, as indicated by the force arrow  29  in  FIG. 1 , is directed away from the master brake cylinder  1  and, in an unchanged manner, obliquely inward, whereas the force exerted by the pinion  26  of the energy store  21  on the toothed rack  12  is directed, in an unchanged manner, in the direction of the master brake cylinder  1  and obliquely inward. As a result, there is exerted on the toothed rack  12  a torque which acts counterclockwise in  FIG. 1 . Said torque increases with increasing spacing  30  of the opposite toothings of the toothed rack  12  and decreases with increasing offset  31  of the pinions  11 ,  26  of the brake force booster  7  and of the energy store  21  in the axial direction. The pinion  26  of the energy store  21  is therefore a greater distance from the master brake cylinder  1  than the pinion  11  of the brake force booster  7 , such that the two pinions  11 ,  26  have the offset  31 . 
     The energy store  21  is mounted in a pivotable manner by means of a joint  32 , enabling said energy store to pivot such that its pinion  26  passes out of engagement with the toothed rack  12 . A support  33  supports the energy store  21  so as to prevent it from pivoting out of engagement, which support  33  has a bar  34  which can be released by means of an electromagnet  35 . The release of the bar  34  causes the supporting action of the support  33  to be eliminated, as a result of which the energy store  21  pivots such that its pinion  26  passes out of engagement with the toothed rack  12 . A spring element  36  ensures reliable pivoting out of engagement when the support  33  is released. The pivoting of the pinion  26  of the energy store  21  out of engagement with the toothed rack  12  is provided in the event of a blockage or some other defect of the energy store  21  or in the event of a failure of the brake force booster  7 . The joint  32  and the releasable support  33  of the energy store  21  form a decoupling means with which the energy store can be decoupled from the actuation of the master brake cylinder  1 , that is to say can, as described, be placed out of engagement with the toothed rack  12 .