Abstract:
The invention relates to a braking-force generator for a hydraulic vehicle braking system, comprising a force input element which can be coupled, or is coupled, to a brake pedal and is displaceable in a basic casing of the braking-force generator, a master brake cylinder, in which a primary piston is displaceably guided, the primary piston delimiting, with the master brake cylinder, a primary pressure chamber for generating a hydraulic braking pressure, a pedal-counterforce simulation means that can be coupled to the force input element, a pedal-actuation detection means for detecting a pedal actuation, and an actuating-force generation means for exerting an actuating force on the primary piston. In the case of this braking-force generator, provision is made whereby the pedal-counterforce simulation means can be coupled to the force input element via a hydraulic system, the hydraulic system being realized with a throttle valve which is provided in the hydraulic connection to the pedal-counterforce simulation means and can be optionally switched into different throttle valve positions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to German Patent Application No. 10 2004 041 924.8 filed Aug. 30, 2004, the disclosures of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a braking-force generator for a hydraulic vehicle braking system, comprising a force input element which can be coupled, or is coupled, to a brake pedal and is displaceable in a basic casing of the braking-force generator, a master brake cylinder, in which a primary piston is displaceably guided, the primary piston delimiting, with the master brake cylinder, a primary pressure chamber for generating a hydraulic braking pressure, a pedal-counterforce simulation means that can be coupled to the force input element, a pedal-actuation detection means for detecting a pedal actuation, and an actuating-force generation means for exerting an actuating force on the primary piston.  
         [0003]     In the case of currently common braking systems, the hydraulic braking pressure necessary for loading of the wheel brake on the vehicle is generated, predominantly, by means of a master brake cylinder. This requires an actuating force to be applied to the said master brake cylinder, which actuating force is generated in response to an actuation of the brake pedal by the vehicle driver. In order to improve the actuation comfort, the actual brake-pedal force is usually increased by a predetermined percentage by means of a brake booster, enabling the necessary brake-pedal actuating forces for a desired vehicle deceleration to be kept sufficiently small to permit adequate braking of the vehicle by any driver without exertion. Such a braking system having a brake booster is known, for example, from DE 44 05 092, and corresponding U.S. Pat. No. 5,493,946, both of which are incorporated by reference herein.  
         [0004]     A disadvantage of these braking systems is that, through his actuating action on the brake pedal, the driver influences the hydraulic pressure on the wheel brakes in each case. This is unproblematic as long as this supports the braking situation. As soon as the driver reacts incorrectly to the actual braking situation, however, for example by injecting too much or too little braking pressure, the braking behaviour, particularly the brake travel and the directional stability, of the vehicle may be impaired, which, in the worst case, may result in an accident.  
         [0005]     Nowadays, modern vehicle feedback-control systems (ABS, ESP, TC, etc.) are capable of using the instantaneous drive status of the vehicle to determine the optimum braking power required in the physical limits, and of thus optimising a braking operation. This, however, requires that the aforementioned direct influence of the driver on the braking pressure be prevented. Furthermore, it is now also considered to be a matter of discomfort if the driver is aware of the action of the vehicle feedback-control system on the brake pedal, such as, for example, a repeated vibration on the brake pedal upon activation of the ABS.  
         [0006]     In order to take account of these requirements associated with vehicle feedback-control systems, in the case of modern braking systems the brake pedal is already decoupled from the braking-force generation, the actuation of the brake pedal then serving only to detect the deceleration intention of the driver. The actual braking-force generation, for example for actuation of the master brake cylinder, is then effected by a separate braking-force generator, and is even then based only on control data of an electronic controller. It can thereby be checked in advance whether, for example, the desired vehicle deceleration would not exceed the instantaneously pertaining physical limits, in respect of brake travel and directional stability, determined by the vehicle feedback-control systems (ABS, ESP, TC, etc.). At the same time, obviously, the injection of an insufficient deceleration by the driver can also be compensated by the controller, through the injection of a greater braking pressure in order to minimize the stopping distance in emergency situations. Such a system is described, for example, in the prior art, of the generic type, according to EP 1 070 006, and corresponding U.S. Pat. No. 6,494,546, both of which are incorporated by reference herein. It has been found, however, that the production of such braking systems is relatively cost-intensive and that they require the application of a substantial amount of equipment in order that reliable braking operation can be guaranteed even in the event of failure of the braking-force generation means. A further disadvantage of such systems is that, in the case of emergency operation, in which the braking-force generation fails, they have a relatively large idle motion until the braking system exhibits any braking action resulting from a direct mechanical coupling then occurring between the brake pedal and the primary piston.  
         [0007]     An essential aspect in the provision of pedal-counterforce simulation means is the so-called “pedal feel”. This is understood to be the nature of the deployment and the level of the forces by which a brake actuation is opposed by the pedal-counterforce simulation means in relation to the braking that is achieved. Normally, the pedal feel has to be set according to vehicle manufacturers&#39; specifications. This makes it necessary to take precautions for a possible influencing of the operating behaviour of the pedal-counterforce simulation means.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     An object of the present invention is to provide a braking-force generator, of the type designated at the outset, which has a high reliability with a relatively simple and inexpensive structure, and in which the behaviour of the pedal-counterforce simulation means can be modified.  
         [0009]     This object is achieved by a braking-force generator, having the features designed at the outset, in which the pedal-counterforce simulation means can be coupled to the force input element via a hydraulic system, the hydraulic system being realized with a throttle valve which is provided in the hydraulic connection to the pedal-counterforce simulation means and can be optionally switched into different throttle valve positions.  
         [0010]     The provision of a throttle valve, which can be switched into different throttle valve positions, in a hydraulic connection between the pedal-counterforce simulation means and the actuating-force generation means can substantially influence the pedal feel. Depending on the selected throttle valve position, the driver perceives, on the brake pedal, a greater or lesser resistance to a pedal actuation, this making it possible, for example, to simulate a sporty or comfortable behaviour of the braking system.  
         [0011]     According to a development of the invention, provision may be made whereby the throttle valve is provided in a hydraulic line between a hydraulic chamber of the actuating-force generation means and the pedal-counterforce simulation means, preferably between the pedal-counterforce simulation means and a pilot-valve arrangement. The pilot-valve arrangement may be provided, for example, to fully decouple the pedal-counterforce simulation means from the actuating-force generation means. This may be necessary, for example, if parts of the vehicle braking system have failed, so that, in an emergency operating mode, the pedal actuation force has to be used fully to generate an actuation force on the primary piston. According to an advantageous development of the invention, the throttle valve and the pilot-valve arrangement may also be realized as a common function unit.  
         [0012]     Provision may be made, in the case of an embodiment of the invention, whereby the throttle valve can be shifted into a certain, essentially unchanged, throttle position according to a predetermined throttle behaviour. It is thus possible, for example during fitting of the braking-force generator, to set a certain throttle valve position which is then lastingly maintained. As an alternative to this, provision may be made whereby the throttle valve can be activated electromagnetically, the throttle valve position being variable according to the electromagnetic activation. It is thereby possible for the throttle valve to be activated according to certain operating parameters of the vehicle. Thus, for example, the throttle valve position can be adjusted according to a switch position of a certain switch which, for example, gives the driver the choice between a sporty and a comfortable brake tuning. Certain vehicle dynamics parameters may also affect the throttle valve position.  
         [0013]     According to an exemplary embodiment of the invention, the throttle valve has a valve casing and a valve piston displaceably guided in the valve casing. The valve piston is preferably guided in a pressure-relieved state in the valve casing. This means that the valve piston does not have to work against applied hydraulic pressures in the case of an electromagnetic actuation. In this connection, provision may furthermore be made according to the invention whereby the valve casing accommodates a energizable coil, and the valve piston is realized integrally with an armature element or is coupled to same for the purpose of common movement.  
         [0014]     A development of the invention makes provision whereby the valve casing has a throttle aperture which is located in the fluidic connection between the actuation-force generation means and the pedal-counterforce simulation means, and the valve piston has a throttle portion which can be positioned in dependence on the throttle valve position in the throttle aperture. The throttle aperture is preferably conical in form. Furthermore, in the case of a development of this embodiment, provision may be made whereby the valve piston is biased by a spring element into the throttle aperture, preferably into a throttle valve position of maximum throttle effect. It is thereby possible to achieve the situation in which a sufficient release of the throttle aperture, permitting a perceptible activation of the pedal-counterforce simulation means, is only achieved following energizing of the coil.  
         [0015]     As an alternative to the previously described embodiment, provision may also be made, according to the invention, whereby the valve piston is cylindrical in form and is accommodated in a sealed manner in a valve bore in the valve casing or in the basic casing, the valve bore being located in the fluidic connection between the actuation-force generation means and the pedal-counterforce simulation means, and the valve piston is realized with a throttle groove which, in dependence on the throttle valve position, provides a throttled connection between the actuation-force generation means and the pedal-counterforce simulation means. Depending on the position of the valve piston within the valve bore, the effective diameter for the hydraulic connection between the actuation-force generation means and the pedal-counterforce simulation means can thus be adjusted and, consequently, the throttle effect of the throttle valve can also be adjusted. Provision may preferably be made, in this context, whereby the throttle groove is provided with a tapering profile.  
         [0016]     A development of this embodiment makes provision whereby the valve piston is biased by a spring element into a predetermined throttle valve position, preferably into a throttle valve position of minimum throttle effect.  
         [0017]     The invention furthermore relates to a braking system for a motor vehicle, comprising a braking-force generator of the previously described type.  
         [0018]     Other advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  shows a schematic general representation of the braking-force generator according to the invention and of the vehicle components coupled to same;  
         [0020]      FIG. 2  shows an enlarged schematic detailed sectional view of the area of the braking-force generator that includes the throttle valve;  
         [0021]      FIG. 3  shows a representation according to  FIG. 2  with a slight modification of the embodiment of the throttle valve according to  FIG. 2 ;  
         [0022]      FIG. 4  shows a further embodiment of a throttle valve in a representation corresponding to  FIG. 2 , and  
         [0023]      FIG. 5  shows the embodiment of the throttle valve according to  FIG. 4  in a position of maximum throttle effect. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     In  FIG. 1 , a braking system according to the invention is represented schematically and denoted generally by the reference  10 . This braking system comprises a braking-force generator  12 , and a master brake cylinder  14  which is coupled to the latter. The master brake cylinder  14  communicates, in conventional manner, with a braking system  16 , which controls via an electronic control unit  18 . In this case, the electronic control unit  18  receives signals from various feedback-control systems within the vehicle, such as, for example, an electronic stability program and an anti-lock system  20 , an automatic cruise control system  22  or the like. The signals flowing from these programs to the electronic control unit  18  are evaluated and used for activating the braking-force generator  12  according to the invention. In addition, the electronic control unit  18  receives signals from a rotational-angle sensor  24 , which detects the current position of a brake pedal  36  and thereby provides a signal corresponding to the current pedal actuation. According to the signal characterizing the current pedal actuation, the electronic control unit  18  activates the braking-force generator  12 , the structure and functioning of which are explained in the following.  
         [0025]     In respect of its basic structure, the braking-force generator  12  according to the invention consists of two modules, namely, on the one hand, of the master brake cylinder  14  and, on the other hand, of a braking-force generator casing  28  in which the master brake cylinder  14  is inserted and to which it is detachably connected. A force input element  30 , realized in the form of a bar, extends into the, in  FIG. 1 , right-hand portion of the braking force generator  12 , particularly of the casing  28 . A control valve  32  is provided in this region. The control valve  32  comprises a control-valve casing  34 , which is displaceable relative to the casing  28 . Provided within the control-valve casing  34  is a valve sleeve  36  which is displaceable relative to the latter.  
         [0026]     The braking-force generator  12  furthermore comprises a chamber arrangement which is disposed within the casing  28  and comprises a vacuum chamber  38  and a working chamber  40 , which are separated from one another in a tight manner by a movable wall  42 . The movable wall  42  is coupled to the control-valve casing  34  for the purpose of common movement.  
         [0027]     An electrically controllable coil  46  of an electromagnetic actuator  48  is disposed in the control-valve casing  34 . The actuator  48  additionally comprises a magnetic armature  50 , which is displaceable relative to the control-valve casing  34  and to the coil  46  in the direction of the longitudinal axis A of the braking-force generator  12  and is realized integrally with the valve sleeve  36 . Furthermore, the armature  50 , and the valve sleeve  36 , is provided with an axial through-bore, in which a transmission piston  52  extends in a movable manner. The armature  50  is biased, by means of a spring  54 , into the position shown in  FIG. 1 . The spring  54  bears, with its one end, on the movable wall  42  and, with its other end, on an inner flange  55  on the armature  50 . At its end which is on the right in  FIG. 1 , the transmission piston  52  has a receiving piston portion  57  which workingly accommodates the force input element  30 .  
         [0028]     Provided between the right end face of the flange  55  and the left end face of the receiving piston portion  57  is a safety clearance s, which must first be overcome before the receiving piston portion  57  comes into abutment with the flange  55 . Furthermore, provided between the left end face of the armature  50  and the portion of the movable wall  42  opposite said armature is a further clearance r, which must first be overcome before a mechanical coupling of armature  50  and movable wall  42 , and thus of armature  50  and primary piston  64 , exists.  
         [0029]     The valve sleeve  36 , the control-valve casing  34 , and a valve element  58  which is displaceable relative to said valve sleeve and control-valve casing constitute the actual pilot valve  22 . In the state shown in  FIG. 1 , the valve sleeve  36  bears, with its sleeve sealing seat  60  which faces the valve element  58 , on the valve element  58 . Furthermore, in this state a casing sealing seat  62  realized on the control-valve casing  34  is raised from the valve element  58 . In the sate shown in  FIG. 1 , the control valve  32  connects the vacuum chamber  38  to the working chamber  40 . The vacuum chamber  38  is in this case coupled to a vacuum source, namely to a separately realized vacuum pump  63  which, activated by means of the electronic control unit  18 , is driven by an electric motor  65 . The force input element is biased, by means of a return spring  56 , into the position shown in  FIG. 1 .  
         [0030]     The transmission piston  52  extends, with its end which is on the left in  FIG. 1 , into a primary piston  64 , which is realized with an axial through-bore. The primary piston  64  is guided, in a sealing manner, in a bore  66  which is open on one side and which is realized in the cylinder casing  14 . An actuating piston  68  is displaceably guided in the through-bore of the primary piston  64 . The actuating piston  68  likewise has a bore  70  which is open on one side, and which is closed by a separating piston  72  which is displaceable in said bore  70  and is integrally realized on the left end of the transmission piston  52 . The separating piston  72 , with the actuating piston  68 , encloses a hydraulic chamber  74 . Via a stop pin  75 , which is guided through an oblong slot  73  provided in the primary piston  64 , the actuating piston  68  bears on a diameter step within the cylinder casing  14 . It is thereby prevented from axial movement to the right in  FIG. 1 .  
         [0031]     The hydraulic chamber  74  is fluidically connected, via a connecting channel  76 , to a fluid channel  80  realized in the cylinder casing  18 . The fluid channel  80  leads, via a fluid line  78  having a pressure-measuring means  79  coupled to the electronic control unit  18 , to an electromagnetic pilot valve arrangement  82 , which is shown schematically. This pilot valve arrangement can be activated by the electronic control unit  18  and, in the state shown in  FIG. 1 , is in its passive position, which it assumes automatically owing to a biasing spring. Upon energizing of the pilot valve arrangement  82  through the electronic control unit  18 , the pilot valve arrangement  82  can be brought into its active position. The pilot valve arrangement  82  is coupled to two line branches. In the passive position, the fluid line  78  is fluidically connected to a pressure-limiting valve  84  which blocks a fluid flow out of the hydraulic chamber  74  until a pressure threshold has been reached at which the pressure-limiting valve  84  opens. In the active position, the pilot valve arrangement  82  permits a fluid flow out of the hydraulic chamber  74 , via the fluid line  78 , into a fluid line  86  adjoining the pilot valve arrangement  82 . Disposed in the fluid line  86  is a throttle means  88 , which can be activated electromagnetically and the structure and functioning of which are to be explained in the following. Furthermore, a line branch  90  branches off from the fluid line  86  to an unpressurized hydraulic fluid reservoir  92 . A throttle means  94  and a separating valve arrangement  96  are disposed before the hydraulic fluid reservoir  92 . The separating valve arrangement  96  is biased, by means of a biasing spring, into the passive position shown in  FIG. 1 , in which it fluidically connects the fluid line  86  to the hydraulic fluid reservoir  92 . The separating valve arrangement  96  can be switched over into its active position, in which it fluidically separates the fluid line  86  from the hydraulic fluid reservoir  92 , by being energized through the electronic control unit  18 .  
         [0032]     The fluid line  86  opens out, finally, into a pedal-counterforce simulation means  100 . The pedal-counterforce simulation means  100  is integrally realized in the cylinder casing of the master brake cylinder  14 . It comprises a simulation piston  102 , which is displaceable against the resistance of a simulation spring  104  and thereby opposes a movement of the transmission piston  52 , resulting from an actuation of the brake pedal  26 , with a resistance.  
         [0033]     It must also be added that non-return valves which, in certain operating situations, block an unwanted fluid flow to the hydraulic chamber  74 , are respectively disposed in the fluid line  86 , in parallel to the pressure-relief valve  84  and in parallel to the throttle means  88  and, in the line branch  90 , in parallel to the throttle means  94 .  
         [0034]     Returning to the structure of the braking-force generator  12  according to the invention, as it is represented in  FIG. 1 , it can be seen from this representation that, in addition to the primary piston  64 , a secondary piston  106  is also movably accommodated in the cylinder casing  14 . The primary piston  64 , together with the boundary wall of the bore  66  and the secondary piston  106 , and the end of the actuating piston  68  that is on the left in  FIG. 1 , delimits a primary pressure chamber  108 . The secondary piston  106 , together with the boundary wall of the bore  66 , delimits a secondary pressure chamber  110 . The primary piston and secondary piston are biased, by means of return springs  112  and  114 , into the position shown in  FIG. 1 .  
         [0035]     Finally, a position sensor  116  is also shown in  FIG. 1 . The position sensor  116  has a tappet  118 , which is spring-biased to the right in  FIG. 1  and which, with its free end, bears continuously on the movable wall  42  and detects its current position.  
         [0036]     The functioning of the braking-force generator  12  according to the invention is to be explained in the following with reference to  FIG. 1 .  
         [0037]     Following an actuation of the brake pedal  26 , the force input element  30  is subjected to the force F and displaced along the longitudinal axis A of the braking-force generator, relative to the initial position shown in  FIG. 1 . If all components are functioning fully—i.e., in a normal operating situation—the brake-pedal actuation is detected directly by the rotational-angle sensor  24  shown in  FIG. 1 , and forwarded to the electronic control unit  18 . The latter activates the coil  46  and energizes it according to predefined characteristics and, possibly, taking account of further parameters, for example from the stability program and the anti-lock system  20  or the cruise control means  22 . The energizing of the coil  46  causes a magnetic field to be built up in the latter, said magnetic field drawing the armature  50  into the coil, to the left in  FIG. 1 . In this case, the valve sleeve  36  is drawn along by the armature  50 . The valve element  58  moves with the valve sleeve  36  until it comes into abutment on the casing sealing seat  62 . The sleeve sealing seat  60  is then raised from the valve element  58 . As a consequence, the vacuum chamber  38  is isolated from the working chamber  40 , and the working chamber  40  is connected to the ambient atmosphere. An above-atmospheric pressure, which results in a displacement of the control-valve casing  34  against a force of a return spring  44  and also, consequently, in a displacement of the primary piston  64  and of the secondary piston  106 , builds up in the working chamber  40 . As a result, there is respectively built up, in the primary pressure chamber  108  and in the secondary chamber  110 , a brake pressure which is used, in a vehicle braking system connected to the braking-force generator  12 , to brake the vehicle. The movable wall  42  moves with the control-valve casing  34  until both sealing seats, namely the sleeve sealing seat  60  and the casing sealing seat  62 , are again in abutment on the valve element  58 . In this state, the system is in equilibrium, and no further change occurs without external action.  
         [0038]     As previously explained, the actuation of the control valve  32  is effected through a displacement of the armature  50 , which is moved, through the magnetic force generated in the coil  46 , along the longitudinal axis A. In the actuated state shown in  FIG. 1 , however, the movement of the force input element  30  and the force F which initiates this movement are not transmitted to the armature  50 . Rather, this movement of the force input element  14  is transmitted to the transmission piston  52 . The transmission piston  52  is consequently displaced within the primary cylinder  64 , in particular within the bore  70 , open on one side, of the actuating piston  68 , and in this case moves the separating piston  72  to the left in  FIG. 1 , the actuating piston  68  remaining in its position relative to the casing  28  owing to the hydraulic pressure prevailing in the primary pressure chamber  108 .  
         [0039]     The movement of the separating piston  72  causes hydraulic fluid to be delivered out of the hydraulic chamber  74 , via the connecting channel  76  and the fluid channel  80 , to the electromagnetic pilot valve arrangement  82 . As a result of the detected pedal actuation, the electromagnetic pilot valve arrangement  82  is switched by the electronic control unit  18  into its active position, in which it allows a fluid flow out of the hydraulic chamber  74 . Furthermore, owing to the detected pedal actuation, the separating valve arrangement  96  is switched by the electronic control unit  18  into its active position, in which it blocks a fluid flow out of the hydraulic chamber  74  into the fluid reservoir  92 . Consequently, the hydraulic fluid forced out of the hydraulic chamber  74  cannot flow into the hydraulic fluid reservoir  102 , but is delivered, against the resistance of the pedal-counterforce simulation device  100 , into the latter. The simulation piston  102  is then displaced, with the simulation spring  104  being compressed. The behaviour of the pedal-counterforce simulation means  100  is affected by the position of the controllable throttle valve  88 .  
         [0040]     If the brake pedal is released again by the driver, the system moves back into the position shown in  FIG. 1 . Owing to the action of the pedal-counterforce simulation means  100  and further return springs, the force input element  30  is then moved back into its initial position. This return movement is effected, with hysteresis, in dependence on the position of the throttle valve  88 .  
         [0041]     The phases, described above, of the braking-force generation are always effected with maintenance of the safety clearance s, apart from small fluctuations due to lag. The safety clearance r, however, is changed owing to the displacement of the armature caused by the actuator, and possibly even used up in the case of very forceful braking.  
         [0042]     During the activation of the actuator  48 , the current position of the movable wall  42  is permanently detected by the electronic control unit  18 , via the position sensor  116 . The actual position of the control-valve casing  34  can thereby be detected and compared with a setpoint position predetermined through the pedal actuation. In the case of a discrepancy of the actual and setpoint positions, for example owing to an alteration of the pedal position by the driver or owing to other external influences, the electronic control unit  18  effects corrective actuation of the actuator  48 . In the case of an emergency braking, in which the brake pedal  26  is depressed rapidly and with great actuating force by the driver, the electronic control unit  18  can also effect disproportionately strong energizing of the actuator  48 , in order rapidly to build up a high pressure difference in the chamber arrangement and consequently to generate, with the braking-force generator  12 , a braking force that is sufficiently large for emergency braking.  
         [0043]     The preceding description shows that, in normal operation, the actuating force F exerted on the force input element effects only a displacement of the transmission piston  52  and, as a result of a hydraulic transmission, a movement of the simulation piston  102 , but has no direct effect whatsoever on the components of the control valve  32 . Rather, the actuating force which displaces the primary piston  64  is initiated through activation of the actuator  48  and displacement of the armature  50 , as a result of which the control valve  32  is actuated, in order to achieve a pressure difference in the chamber arrangement. Owing to this pressure difference, the control-valve casing  34  and, with the latter, the primary piston  64  and the secondary piston  106 , are displaced.  
         [0044]     The following describes an emergency operating situation in which the braking-force generator  12  according to the invention continues to function despite a defect on one or more components:  
         [0045]     An emergency operating situation occurs, for example, if the coil  46  is no longer properly activated. This may be due to the fact, for example, that the rotational-angle sensor  22  is defective, or that a defect occurs in the on-board power supply of the vehicle. This defect results in the electronic control unit  18  failing to bring the pilot valve arrangement  82  into its active position. In the case of such a defective operating state, the control valve  32  can no longer be actuated via the actuator  48 . Nevertheless, a sufficiently good braking effect can still be achieved with the braking-force generator  12  according to the invention. Upon actuation of the brake pedal, the force input element  30  is displaced to the left in  FIG. 1 . As a result, the transmission piston  62  is displaced to the right in  FIG. 1 , along the longitudinal axis A. Since, however, the pilot valve arrangement  82  is not activated by the electronic control unit  18  and thus remains in the passive position shown in  FIG. 1 , the hydraulic fluid contained in the hydraulic chamber  74  cannot escape. Owing to the non-compressibility of the hydraulic fluid, the head of liquid contained in the hydraulic chamber  74  first produces a direct hydromechanical force coupling between the transmission piston  52  and the actuating piston  68 , which, via the connecting pin  75 , finally displaces the primary piston  64  in the cylinder casing  14 . The brake pedal actuation is thus first transmitted directly, and without overcoming of the idle clearance s, to the primary piston  64 , this resulting in a reliable and rapid response of the braking system  10  in the case of emergency operation.  
         [0046]     In such an emergency operating situation, the brake pedal actuation also results in a large increase in pressure within the hydraulic chamber  74 . If the pressure prevailing within the hydraulic chamber  74  exceeds the pressure threshold value set by the pressure-limiting valve  84 , hydraulic fluid can escape from the hydraulic chamber  74 , via the pilot valve arrangement  82 , the pressure-limiting valve  84  and the separating valve  96 , into the reservoir, due to the action of the force F on the force input element  30 . This results in a displacement of the transmission piston  52  relative to the actuating piston  68 , the pressure threshold value set by the pressure-limiting valve  84  continuing to prevail as a pressure in the hydraulic chamber  74 . Upon the force input element  30  being further subjected to strong force, causing the pressure threshold value to be exceeded, the transmission piston  52  moves further relative to the actuating piston  68 , and also further relative to the valve sleeve  36  that is not actuated owing to the failure of the actuator. In this case, the safety clearance s is overcome until, finally, the receiving piston portion  57  comes into abutment on the flange  55 . The transmission piston  52 , and therefore also the force input element  30 , are then workingly connected to the valve sleeve  36 . Consequently, a further displacement of the force input element  30  to the left in  FIG. 1  also results in a displacement of the valve sleeve  36 , causing the sleeve sealing seat  60  to be raised from the valve element  58 . There consequently arises the previously described situation, in which a pressure difference can develop between the vacuum chamber  38  and the working chamber  40 . If the vacuum source coupled to the vacuum chamber is still functioning correctly, this mechanical displacement of the valve sleeve  36  causes a pressure difference to be built up between the working chamber  40  and the vacuum chamber  38 , said pressure difference causing a displacement of the movable wall  42  and, consequently, a displacement of the primary piston  68 . In this state, in which the actuator  48  has failed, the control valve  32  is therefore actuated mechanically, after overcoming of the safety clearance s.  
         [0047]     In the emergency operating situation described above, in which only the actuator, but not the vacuum source, has failed, the hydromechanical coupling of the actuating piston  68  and the transmission piston  52  first enables a direct minimum braking action to be achieved, which is determined by the level of the pressure threshold value. Subsequently, following overcoming of the safety clearance s, a pneumatic braking-force generation can be achieved in the conventional manner. This also applies to the case in which the vacuum source has also failed, but there is still a sufficient vacuum in the vacuum chamber  38  to achieve brake boosting. It is thus possible, for example, still to perform three to four braking operations, even if the vacuum source has failed, until a sufficient pressure difference can no longer be set between the vacuum chamber  38  and the working chamber  40 .  
         [0048]     Even in emergency operating situations, in which the vacuum source has also failed and the available vacuum has been “used up”, the braking force generator  12  according to the invention also enables purely mechanical braking to be performed. Again, in such cases, the safety clearance s is first used up following exceeding of the pressure threshold value in the hydraulic chamber  74 , causing the flange  55  and the receiving piston portion  57  to come into mutual abutment, and thus resulting in a mechanical coupling of the valve sleeve  36  and the force input element  30 . Subsequently—as already described above—upon further displacement of the force input element  30  to the left in  FIG. 1 , the valve sleeve  36  is displaced to the left in  FIG. 1 , with the result that the clearance r is also used up. Finally, through the receiving piston portion  57  and the valve sleeve  36 , the valve sleeve  36  comes, with its end face that is on the left in  FIG. 1 , into mutual abutment with the movable wall  42 , resulting in a direct mechanical coupling between the force input element  30  and the primary piston  64  that is coupled to the movable wall. A further displacement of the force input element  30  to the left in  FIG. 1  in the case of such a mechanical coupling therefore results in a direct displacement of the primary piston  64  and, consequently, in a direct transmission of the pedal actuation force to the primary piston  64 .  
         [0049]      FIG. 2  shows a sectional detailed representation of the throttle valve  88 . The throttle valve  88  is accommodated in the cylinder casing  14 , in a stepped bore. It comprises a valve casing  124 , in which a coil  126  is accommodated. The coil  126  can be energized via contacts  128  of a plug-in connector  130 . An armature  132  is movably guided in the coil  126 . The armature  132  is coupled to a valve piston  134  for the purpose of common movement. The armature  132  can thus be displaced together with the coil  126  within the valve casing  124 . The assembly consisting of the armature  132  and the valve piston  134  is biased by a return spring  136  into the position shown in  FIG. 2 . In this case, one end of the return spring  136  is applied to a support flange  138  of the valve casing  124 , and its opposite end is applied to an abutment flange  140  which is realized on the valve piston  134 . The valve piston  134  additionally has a pin-type throttle portion  142  which, in  FIG. 2 , projects into a conical throttle aperture  144 . The conical throttle aperture  144  is realized in a throttle washer  146 , which is accommodated in a firm, sealed manner in the valve casing  124 .  
         [0050]     In the position shown in  FIG. 2 , the throttle portion  146  largely closes the throttle aperture  144  completely, with the result that there is no fluidic connection between the hydraulic chamber  74  and the pedal-counterforce simulation means  100 . If, however, the coil  126  is energized, the armature  132  is displaced to the left, according to the arrow P in  FIG. 2 . The valve piston  134  is then carried along with it, with the result that the throttle portion  146  also moves out of the throttle aperture  144 , according to the displacement of the armature  132 , against the action of the return spring  136 . A throttled fluidic connection between the fluid chamber  74  and the pedal-counterforce simulation means  100  thereby becomes possible. The degree of throttle effect is determined by the position of the throttle portion  146  within the throttle aperture  144 .  
         [0051]      FIG. 3  shows an embodiment of the throttle valve  88  according to the invention which is slightly modified compared with  FIG. 2 . To facilitate description and avoid repetitions, the same references as used in the description of  FIG. 2 , but with a lower-case “a” suffix, are used for components which are of the same type or have the same function.  
         [0052]     The embodiment according to  FIG. 3  differs from the embodiment according to  FIG. 2  in that the valve piston  134   a  is guided in the cylinder casing  14   a  in a pressure-balanced state. This is achieved in that the hydraulic fluid from the hydraulic line  86   a  is also routed, via a bypass line  148   a  and the aperture  150   a,  into the area of the armature  132   a,  with the result that, ultimately, the valve piston  134   a  can be displaced in a pressure-balanced state along the arrow P. The ability of the valve piston  134   a  to be adjusted via the coil  126   a  is thereby improved, because the hydraulic pressure applied via the hydraulic line  86   a  no longer loads the valve piston  134   a  on the front face.  
         [0053]     Finally,  FIGS. 4 and 5  show a further exemplary embodiment for the. throttle valve according to the invention. Again, the same references as used in  FIG. 1 , but with a lower-case “b” suffix, are used for components which are of the same type or have the same function.  
         [0054]     In the exemplary embodiment according to  FIG. 4 , the valve piston  134   b  is guided in a sealed manner in a valve bore  152   b  within the cylinder casing  14   b,  ring seals  154   b  being let into the outer circumference of the valve piston  134   b.  A full-perimeter throttle groove  156   b,  which tapers radially inwards in the longitudinal section, according to  FIG. 4 , of the valve piston  134   b,  is provided between the two ring seals  154   b.  The valve piston  134   b  is realized integrally with the armature  132   b.  The component consisting of the valve piston  134   b  and the armature  132   b  is biased by the return spring  136   b  into the position shown in  FIG. 4 , in which the throttle groove  156   b  provides a connection, that is largely free from throttle action, between the two hydraulic lines  86   b  and  90   b.  Energizing of the coil  126   b,  however, causes the component consisting of the armature  132   b  and the valve piston  134   b  to be displaced to the left, according to the arrow P in  FIG. 4 , against the action of the return spring  136   b.  This can be seen in  FIG. 5 . In  FIG. 5 , the position of the valve piston  134   b  is shown at maximum throttle effect. The effective cross-section through which hydraulic fluid can flow to and fro between the lines  86   b  and  90   b,  via the throttle groove  156   b,  has been substantially reduced compared with the position from  FIG. 4 .  
         [0055]     The pedal feel perceived by a driver in actuations of a brake pedal of a vehicle braking system equipped with the braking-force generator according to the invention can be substantially influenced by means of the different embodiments of throttle valves according to the invention, shown in FIGS.  2  to  5 . Depending on the position of the respective throttle valve, the pedal-counterforce simulation means is hydraulically activated with greater or lesser throttle, which allows the driver to perceive a greater or lesser resistance when depressing the brake pedal, and results in a greater or lesser hysteresis when the brake pedal is released. The throttle valve may be set in advance or, alternatively, also activated differently in dependence on certain operating situations of the vehicle or as selected by the driver.  
         [0056]     In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiments. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.