Abstract:
A “brake-by-wire” type brake system for a motor vehicle, having a brake pressure sensor which can be activated by a brake pedal of a pedal unit and can be connected to wheel brakes of the vehicle outside the “brake-by-wire” operating mode, having a pressure source which can be actuated by an electronic control unit and can be connected to the wheel brakes of the vehicle in the “brake-by-wire” operating mode, and having a pedal travel simulator which interacts with the brake pedal and is formed by at least one simulator element, and a restoring force which acts on the brake pedal independently of the actuation of the pressure source can be simulated in the “brake-by-wire” operating mode. In order to provide a simpler and more cost-effective “brake-by-wire” type brake system, a force/travel characteristic of the pedal travel simulator is provided in a controllable fashion.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is the U.S. National Phase Application of PCT International Application No. PCT/EP2010/050368, filed Jan. 14, 2010, which claims priority to German Patent Application No. 10 2009 000 235.9, filed Jan. 15, 2009, and German Patent Application No. 10 2009 002 885.4, filed May 7, 2009, the contents of such applications being incorporated by reference herein. 
     FIELD OF THE INVENTION 
     A “brake-by-wire” type brake system for a motor vehicle, having a brake pressure generator which can be activated by means of a brake pedal of a pedal unit and can be connected to wheel brakes of the vehicle outside the “brake-by-wire” operating mode, having a pressure source which can be actuated by means of an electronic control unit and can be connected to the wheel brakes of the vehicle in the “brake-by-wire” operating mode, having means for detecting a driver&#39;s deceleration request, and having a pedal travel simulator which interacts with the brake pedal and is formed by at least one simulator element, and by means of which a restoring force which acts on the brake pedal independently of the actuation of the pressure source can be simulated in the “brake-by-wire” operating mode. 
     BACKGROUND OF THE INVENTION 
     The “brake-by-wire” type brake system for a motor vehicle, illustrated in  FIG. 1 , is known from the prior art and is composed essentially of a brake activation unit  10 , a brake pedal  10 , a pedal travel simulator  2 , an electronic control unit  7 , which is illustrated only schematically, and wheel brakes (not shown) which are connected to the brake activation unit  10  with the optional intermediate connection of a hydraulic open-loop or closed-loop control unit. The brake activation unit  10  is formed by a brake booster, preferably an underpressure brake booster  3 , a master brake cylinder, for example a tandem master cylinder  4 , which is connected downstream of the brake booster  3  and to whose pressure spaces (not illustrated) the abovementioned wheel brakes of the motor vehicle are connected, as well as a pressure medium reservoir vessel  5  which is assigned to the master brake cylinder  4 . The brake pedal  1 , which serves for the activation of the brake booster  3  by the driver, interacts with the pedal travel simulator  2 , in particular in the “brake-by-wire” operating mode, which pedal travel simulator  2  gives the driver the desired brake pedal sensation. A sensor device  6 , which is preferably of redundant design, for detecting a driver&#39;s deceleration request generates, as a function of the activation of the brake pedal  1 , control signals which are fed to the electronic control unit  7 , by means of the output signals of which control unit  7  it is possible, inter alia, to actuate an electromagnet (not illustrated) which is assigned to the brake booster  3  and which permits a pneumatic control valve to be activated independently of the driver&#39;s will, which pneumatic control valve controls an air supply to the brake booster  3 . An axial gap which is provided between the end of a piston rod  8 , coupled to the brake pedal  1 , and a control piston of the abovementioned control valve ensures, in the “brake-by-wire” operating mode, decoupling of the force transmitting connection between the brake pedal  1  and the brake booster  3 . 
     The pedal travel simulator  2 , by means of which, as already mentioned, a restoring force which acts on the brake pedal is simulated in the “brake-by-wire” operating mode independently of activation of the brake booster  3 , is embodied in such a way that in the “brake-by-wire” operating mode said pedal travel simulator  2  can be activated when the force transmitting connection between the brake pedal  1  and the brake booster  3  is decoupled, and can be deactivated outside the “brake-by-wire” operating mode. The activation and deactivation of the pedal travel simulator  2  are carried out by an electro-hydraulic device  9 , which is formed essentially by a hydraulic cylinder-piston arrangement  11 ,  12  which can be shut off by means of a check valve  14  which can be activated electro-magnetically. The cylinder-piston arrangement  11 ,  12  has a hydraulic pressure space  12  which is bounded by a piston  11  and a hydraulic low pressure chamber  13  which is connected to the pressure space  12 , wherein the check valve  14  permits the shutting off and the opening of the connection. The check valve  14 , which can be activated or switched over by means of the actuation signals of the electronic control unit  7 , is embodied as a 2/2-way valve which can be activated electro-magnetically and is open in the currentless state (SO). A nonreturn valve, which is open toward the pressure space  12  and which is not illustrated in  FIG. 1 , serves to equalize the pressure between the pressure space  12  and the low pressure chamber  13 . 
     In the arrangement which is previously known from the prior art, the pedal travel simulator has a spring element  16  which is provided as a compression spring and which is clamped between the brake pedal  1  or a force transmitting part  17 , which is connected in a rotationally fixed fashion to the brake pedal  1 , and a two-armed lever  18 . The two-armed lever  18  is mounted opposite the brake pedal  1 , offset coaxially with respect thereto in such a way as to be rotatable to a limited degree, wherein the first arm  19  of said two-armed lever  18  forms a support face for the spring element  16 , while the second arm  20  thereof is supported on the hydraulic piston  11  of the piston-cylinder arrangement  9 . 
     The force transmitting part  17  interacts with an elastic simulator element  15 , preferably manufactured from rubber, for the brake pedal  1 . In order to generate a hysteresis which, during the rising stroke of the pedal travel simulator  2 , generates frictional forces in addition to the force of the spring element  16 , which frictional forces counteract the activation force acting on the brake pedal  1 , a friction element  21  bears against the force transmitting part  17  under the effect of the spring element  16 . The friction element  21  interacts with a friction face  22 . The friction element  21  bears against the force transmitting part  17  by means of obliquely arranged bearing faces  23 ,  24 , in such a way that during the activation of the pedal travel simulator  2  a force component is produced which presses the friction element  21  against the friction face  22 . The abovementioned elements  16 ,  17 ,  21 ,  22  are preferably arranged in a housing  25  which is preferably embodied in one piece with the first arm  19  of the two-armed lever  18 . 
     The method of functioning of the brake system described above is known to a person skilled in the art who is active in the technical field of “brake-by-wire” systems. In the position of rest of the brake system (illustrated in  FIG. 1 ) which also corresponds to the fallback level, the hydraulic pressure space  12  of the electro-hydraulic device  9  is connected to the low pressure chamber  13  via the check valve  14  which is open in the currentless state. During the activation of the brake pedal  1 , the movement thereof is detected by the sensor device  6  and signaled to the electronic control unit  7 , which at the same time generates control signals for actuating the abovementioned electromagnet and the check valve  14 , with the result that the connection between the pressure space  12  and the low pressure chamber  13  is interrupted, causing the pedal travel simulator  2  to be activated. The check valve  14  is therefore closed in the “brake-by-wire” operating mode, with the result that neither the arm  20  nor the housing  25  of the pedal simulator can move. When the brake pedal  1  is activated by the vehicle driver, the spring element  16  is therefore compressed. Outside the “brake-by-wire” operating mode the valve  14  is opened, and when the brake pedal  1  is activated the simulator housing  25  rotates along with the brake pedal  1 , as a result of which the force applied by the vehicle driver is not absorbed in the pedal travel simulator  2  but rather acts on the underpressure brake booster  3 . 
     In the event of failure of the vehicle electronics or failure of the on-board power system, the check valve  14  cannot be switched over, with the result that the braking occurs in the fallback level. When the brake pedal  1  is activated, the two-armed lever  18  rotates along with the brake pedal  1  under the effect of the spring element  16 , with the result that the piston  11  of the electro-hydraulic device  9  is pushed to the left in the drawing and the pressure medium is pushed out of the pressure space  12  into the low pressure chamber  13 . The simulator  2  is deactivated. 
     The basic design and the method of functioning of the “brake-by-wire” brake system are known as is described in the prior art, with the result that they are not included in the description of the following exemplary embodiments of a brake system according to the invention and details are only given on the differences according to aspects of the invention. 
     A brake-by-wire system is known from DE 10 2004 011 622 A1, which is incorporated by reference, and is described, in particular, in conjunction with  FIG. 15  in said document. The pedal travel simulator is accommodated in the previously known brake system by a housing in which a force transmitting element which is connected in a rotationally fixed fashion to the brake pedal projects. An electro-hydraulic device which can be actuated by means of the electronic control unit and which is formed by a piston-cylinder arrangement permits activation of the pedal travel simulator in the “brake-by-wire” operating mode and at least partial deactivation of the pedal travel simulator outside the “brake-by-wire” operating mode by virtue of the fact that the housing of the pedal travel simulator is supported on the electro-hydraulic device by means of a lever arm which is securely connected to the housing. It is considered disadvantageous in the previously known brake system that the function of the electro-hydraulic device has to be monitored. In addition, the known system is costly and has a high number of components. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of at least one embodiment of the present invention to provide a “brake-by-wire” type brake system which is more cost-effective, and has a simpler and more space-saving design. 
     There is provision here that a force/travel characteristic of the pedal travel simulator is provided in a controllable fashion. Controlling the pedal characteristic permits an additional travel with a limited force level to be enabled if the “brake-by-wire” system fails or a higher braking performance is necessary, with the result that the pedal travel simulator no longer has to be deactivated in the fallback level. It is therefore advantageous that there is no longer any need for a device for deactivating or activating the pedal travel simulator, and as a result of which the monitoring thereof is eliminated. 
     The control of the force/travel characteristic of the pedal travel simulator can preferably be provided in a force-dependent fashion, with the result that, starting from a predetermined foot force applied to the brake pedal, prolongation of the pedal travel is made possible and the reaction force which occurs in the process does not exceed a defined limiting value. Alternatively, the control of the force/travel characteristic of the pedal travel simulator, and therefore also of the pedal characteristic, could also be provided in a travel-dependent fashion. 
     An advantageous embodiment of the invention provides that the pedal travel simulator is coupled to a component, which is fixed to the vehicle, and to the brake pedal. In this context, the space which is present between the component, which is fixed to the vehicle, and the brake pedal can be used at least partially as an installation space for the pedal travel simulator. 
     Another advantageous embodiment of the invention provides, on the other hand, that the pedal travel simulator is arranged between a component, which is fixed to the vehicle, and a component which can rotate coaxially with respect to the brake pedal, as a result of which the dimensioning of the pedal travel simulator does not need to be adapted to the very limited installation space between the component, which is fixed to the vehicle, and the brake pedal. 
     The component which is fixed to the vehicle is preferably a part of the pedal unit, with the result that the pedal travel simulator can be provided as a component of the pre-mountable pedal unit. 
     Simple control of the pedal characteristic can advantageously be achieved in that, in order to prolong the pedal travel, a spring element is provided which, starting from a predetermined foot force applied to the brake pedal, can be compressed and therefore permits prolongation of the pedal travel. 
     According to one advantageous embodiment of the invention, the spring element is provided as one or more prestressed springs. 
     The prestressed spring element is preferably provided as a spring packet composed of disk springs or as at least one helical compression spring. 
     An alternative advantageous embodiment of the invention provides that the spring element is provided as an elastomer. In addition, it is conceivable within the scope of the invention to provide a combination of various spring types and a combination of springs and other sprung elements as a spring element. 
     An advantageously pre-mountable structural unit is made possible by virtue of the fact that the prestressed spring element is arranged secured between a first activation element, which supports the simulator element, and a second activation element. 
     According to one advantageous embodiment, a third activation element is provided onto which the simulator element is pressed and which is attached indirectly or directly to the brake pedal or to the component which can move coaxially with respect to the brake pedal. 
     A particularly simple design of the pedal travel simulator is achieved in accordance with one advantageous embodiment by virtue of the fact that the spring element is arranged between the component, which can rotate coaxially with respect to the brake pedal, and an activation element, and the simulator element is attached by means of a receptacle to the component which is fixed to the vehicle. 
     In order to be able to easily change the characteristic curve of the brake system in addition to the predefined properties of the other components of the pedal travel simulator, one advantageous embodiment of the invention provides that a distance is provided in an adjustable fashion between the simulator element and the third activation element. 
     A further advantageous embodiment of the invention provides that the second activation element is attached indirectly to the brake pedal or to the component which can move coaxially with respect to the brake pedal, and, in the “brake-by-wire” operating mode, the simulator element is pressed onto a side wall of a housing of the pedal travel simulator, wherein the prestressed spring element is arranged in series with a restoring spring of the brake pedal. 
     For the purpose of easily monitoring the pedal travel simulator stroke, according to one advantageous embodiment, it is possible to provide that the pedal travel simulator has a travel sensor. 
     The travel sensor preferably comprises a permanent magnet as a signal generator and a sensor element. However, other sensor variants can also be used within the scope of the invention. 
     The response force and restoring force of the brake pedal can easily be adapted to the different requirements by virtue of the fact that a restoring spring of the brake pedal is provided on the pedal travel simulator and/or as a separate spring element. 
     According to one advantageous embodiment of the invention, for the purpose of improved mounting of the pedal unit with the brake booster, a securing element, which has a funnel-shaped receptacle which is oriented in the direction of a piston rod of the brake booster, is attached to the brake pedal. At the same time, the arrangement serves for guiding the piston rod after mounting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is best understood from the following detailed description when read in connection with the accompanying drawings. Included in the drawings are the following figures: 
         FIG. 1  shows a sectional illustration of the brake system which is known from the prior art and is of the generic type specified at the beginning; 
         FIG. 2  shows a simplified schematic side view of the arrangement of the pedal travel simulator of the brake system according to the invention according to a first exemplary embodiment; 
         FIG. 3  shows an enlarged sectional illustration of the pedal travel simulator according to  FIG. 2 ; 
         FIG. 4  shows a simplified schematic front view of the arrangement of the pedal travel simulator of the brake system according to aspects of the invention according to the first exemplary embodiment in accordance with  FIG. 2 ; 
         FIG. 5  shows a sectional illustration of a pedal travel simulator according to a second exemplary embodiment; 
         FIG. 6  shows a sectional illustration of a pedal travel simulator according to a third exemplary embodiment; 
         FIG. 7  shows a force/travel diagram of the brake system according to aspects of the invention; 
         FIG. 8  shows a pedal unit according to a fourth exemplary embodiment; 
         FIG. 9  shows the pedal unit according to  FIG. 8  in another view; 
         FIG. 10  shows an enlarged view of the pedal unit according to  FIG. 8 ; 
         FIG. 11  shows part of the pedal travel simulator according to the fourth exemplary embodiment in accordance with  FIGS. 8 to 10 ; 
         FIG. 12  shows a further view of the pedal unit according to the fourth exemplary embodiment; 
         FIG. 13  shows an enlarged illustration of the adjustment screw of the pedal unit according to  FIG. 8 ; 
         FIG. 14  shows a pedal unit according to a fifth exemplary embodiment; 
         FIG. 15  shows an enlarged view of the pedal unit according to  FIG. 14 ; and 
         FIG. 16  shows a longitudinal section through a pedal unit according to a sixth exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In order to present a simpler and more cost-effective “brake-by-wire” type brake system, according to the exemplary embodiments described below there is provision that a force/travel characteristic of the pedal travel simulator  2 , which is formed by at least one simulator element  15 , is provided in a controllable fashion. Controlling the pedal characteristic allows additional travel with a limited force level to be enabled if the “brake-by-wire” system fails or a higher braking performance is necessary, with the result that separate deactivation of the pedal travel simulator in the fallback level no longer has to take place outside the “brake-by-wire” operating mode according to the described prior art. It is therefore particularly advantageous that there is no longer any need for a device for deactivating or activating the pedal travel simulator, as a result of which monitoring thereof can also be dispensed with. 
     The force/travel characteristic, i.e. the pedal travel, is controlled in a force-dependent fashion in the exemplary embodiments described below, with the result that, starting from a predetermined foot force applied to the brake pedal  1 , prolongation of the pedal travel is made possible and the reaction force which occurs in the process does not exceed a defined limiting value. 
     The pedal travel simulator  2  is arranged between a component, which is fixed to the vehicle, of the pedal unit, referred to as the pedal mount  27 , and the brake pedal  1  or a component  26  which can rotate coaxially with respect to the brake pedal  1  and is in the form of a lever. Basically, under corresponding spatial conditions it is possible to couple the pedal travel simulator  2  directly to the pedal mount  27  and to the brake pedal  1 . Under certain circumstances it is also possible to use a component which is not part of the pedal unit as the component which is fixed to the vehicle. 
       FIG. 2  shows a simplified schematic side view of the arrangement of the pedal travel simulator  2  according to a first exemplary embodiment, which is illustrated in an enlarged form in  FIG. 3 . 
     As is apparent from  FIG. 3 , the pedal travel simulator  2  according to the first exemplary embodiment has an essentially sleeve-shaped housing  28  in which a first disk-shaped activation element  29  which supports the simulator element  15  is arranged in a displaceable fashion. In this context, a stop  31 , which is provided on an inner wall  30  of the housing  28 , limits the movement of the activation element  29  which is secured against the stop  31  by a prestressed spring element  32  which is provided as a spring packet. The prestressed spring element can be replaced, within the scope of the invention, by any other spring element which can be compressed starting from a predetermined foot force applied to the brake pedal  1 . An elastomer or a combination of springs is possible, for example, as a spring element. 
     The spring packet  32 , which has a plurality of disk springs connected in series in this exemplary embodiment, is supported at its end on a second activation element  33  which has a hollow-cylindrical projection  35  for guiding a centrally arranged cylindrical protrusion  34  of the first activation element  29 . For the purpose of attachment to the pedal mount  27 , a threaded protrusion  36  is integrally formed onto the second activation element  33 , which threaded protrusion  36  can be screwed into an attachment element  37 . 
     A restoring spring  16 , which serves to position the brake pedal  1  and to make available a response force, is supported in this exemplary embodiment on a collar  38  of the housing  28  and on a stop  39 . 
     As is apparent from  FIG. 3 , the pedal travel simulator  2  has a further, third activation element  40  onto which the simulator element  15  is pressed. A cylindrical protrusion  41  of the third activation element  40  is connected by means of a connecting element  42  to a threaded rod  43  which can be screwed into an attachment element  44  in order to attach the pedal travel simulator  2  to the lever  26 . The stop  39  of the restoring spring  16  is provided attached to the connecting element  42 . 
     When the brake pedal  1  is activated by the vehicle driver in the activation direction, the restoring spring  16  is compressed by the simultaneous activation of the lever  26 , and the third activation element  40  is displaced in the direction of the simulator element  15 . 
     As a result of the bending of the spring packet  32 , prolongation of the pedal travel is possible and an additional travel can be enabled if the “brake-by-wire” system fails or a higher braking performance is necessary. As a result, there is no longer any need for an additional device for deactivating or activating the pedal travel simulator  2 , as a result of which monitoring thereof is dispensed with. In contrast to the known brake system, the pedal travel simulator  2  is therefore also active in the fallback level and moves whenever the brake pedal is activated. 
     The characteristic curve of the brake system which is shown in  FIG. 7  and which is explained in more detail below is virtually the same in the “brake-by-wire” operating mode and in the fallback level. In the fallback level all that is necessary is to add a force component of the brake booster  3 , which component advantageously decouples the feedback to the driver in certain operating modes. Such brake boosters are known, for example, as a mechanical braking assistant. The characteristic curve in the fallback level is illustrated by dashes in  FIG. 7 . 
       FIG. 4  shows a simplified schematic front view of the arrangement of the pedal travel simulator  2  of the first exemplary embodiment. It is apparent that the pedal travel simulator  2  is arranged outside the brake pedal  1  and the pedal mount  27 . The pedal travel simulator  2  can also be advantageously provided within the pedal mount  2 , with the result that the installation space of the entire pedal unit can be reduced further. In addition it is also conceivable to couple the pedal travel simulator  2  directly to the brake pedal  1 . 
     In addition to the restoring spring  16 , a further restoring spring  45  can be provided which is not directly arranged on the pedal travel simulator  2 . It is basically also possible to allow the restoring spring  16  to be dispensed with, with the result that the restoring spring  45  entirely performs the function with respect to the response force and the restoring force. 
       FIGS. 5 and 6  each show a sectional illustration of a pedal travel simulator  2  according to a second and a third exemplary embodiment, the function of which is the same as that of the first exemplary embodiment. Details will be given below only on the structural differences. Identical components are provided with identical reference symbols. 
     According to  FIG. 5  it is apparent that the restoring spring  16  is provided in a housing  46  of the pedal travel simulator  2 . 
     Furthermore, a first, essentially disk-shaped activation element  47  which supports the simulator element  15  is provided with a second activation element  48  and the spring packet  32  as a unit composed of securely joined parts, said unit advantageously constituting a premountable assembly. For securely joining the parts, the first activation element  47  has a cylindrical protrusion  49  which projects through an opening in the second activation element  48  and is secured therein by means of an attachment ring  50 . 
     When activation occurs, the second activation element  48 , and therefore also the spring packet  32  and the first activation element  47 , are pushed by means of a further activation element  51  which is connected to a threaded rod  43  by the connecting element  42  which is described with respect to the first exemplary embodiment. 
     The restoring spring  16  is supported between a collar  53  which is integrally formed onto the first activation element  47  and a side wall  52  of the housing  46 . 
     Furthermore, it is apparent from  FIG. 5  that the activation element  51  is attached to the second activation element  48  by shaped clips  54  of the second activation element  48 . The unit composed of secured-together parts and the restoring spring  16  are secured in a captive fashion in the housing  46  by means of a stop  55 . 
     For the purpose of attachment to the pedal mount  27 , an attachment element  56  is attached to an outer side of the side wall  52 , for example by welding on. 
     It becomes clear that the second exemplary embodiment differs from the first in that, when activation occurs, the completely securely joined structural unit comprising the first and the second activation elements  47 ,  48  and the spring packet  32  moves in the direction of the side wall  52  counter to the spring force of the restoring spring  16 . The restoring spring  16  and the spring packet  32  are connected directly in series in this embodiment. As a result, a third activation element can be dispensed with and the design of the pedal travel simulator  2  can be simplified. 
     In contrast to  FIG. 5 ,  FIG. 6  shows a pedal travel simulator  2  which additionally has a travel sensor  57 . The latter comprises a permanent magnet  58  as a signal generator and a sensor element  59  which is arranged on the housing  56 . As is apparent from  FIG. 6 , the permanent magnet  58  is arranged secured between an element  60  and a stop disk  61  attached thereto. Other types of sensors can also be used for this function within the scope of the invention. Other functions, such as sensing brake pressure, can monitored using any brake pressure sensor  600  known to a person having ordinary skill in the art. 
     The element  60  and a second activation element  62  are connected to one another in a positively locking fashion as parts of the securely joined spring packet unit. 
     Since all the inner components of the pedal travel simulator  2  according to the described embodiments are moved when activation occurs, what are referred to as “creeping” errors can be avoided and additional monitoring of the function is not necessary. 
       FIGS. 8 to 13  show a further, fourth exemplary embodiment.  FIGS. 8 to 10 ,  12  and  13  therefore merely show a pedal unit of the brake system with a pedal travel simulator  2  in various spatial illustrations, which pedal travel simulator  2  is partially illustrated in  FIG. 11 . 
     In contrast to the exemplary embodiments described above, this exemplary embodiment does not have a housing and is distinguished by a particularly simple design. The essential design of the pedal travel simulator  2  is apparent, in particular, from  FIG. 11 , said pedal travel simulator  2  having a first stepped and sleeve-shaped activation element  63  which supports the simulator element  15 . A first step  64  serves, on the one hand, for accommodating the simulator element  15  and, on the other hand, for the abutment of a prestressed spring element  65  which fulfills the same function as the spring element  32  described above. As is apparent from  FIG. 11 , the spring element  65  comprises in this exemplary embodiment two helical compression springs  66 ,  67  which abut against a second stepped, disk-shaped activation element  68 . The two activation elements  63  and  68  are provided securely joined by means of a shank  69 , wherein the prestress of the spring element  65  secures the activation element  63 ,  68  against stops  70 ,  71  of the shank  69 . In this context, the stop  70  is integrally formed onto the shank  69  and, for the purpose of positioning, engages in a very small step  72  on the second activation element  68 . Steps  73 ,  74  formed on the opposite side serve to position the two compression spring ends. 
     A second and a third step  75 ,  76  of the first activation element  63  are embodied in such a way that the first activation element  63  is supported on the stop  71  by the prestress of the spring element  65 . The stop  71  is secured on the shank  69  by means of a circlip  77  secured in a groove  78  in the shank  69 . 
     The two activation elements  63  and  68  are provided with the spring element  65  and the shank  69  as a pre-mountable structural unit. For the purpose of attaching this structural unit to the pedal mount  27 , the shank  69  is used which projects, with its end  79  provided as a thread, through a drilled hole in the pedal mount  27  and is attached to the pedal mount  27  by means of a nut  80 , as is clearly apparent from  FIG. 12 . 
     Furthermore, the pedal travel simulator  2  has a third activation element  81  against which the simulator element  15  is pressed when activation occurs. A cylindrical protrusion  82 , provided with a thread, on the third activation element  81  projects for the purpose of attaching a shoulder  84  of the lever  26  and is attached to a nut  83 . 
     As is apparent, in particular, from  FIGS. 10 and 13 , the response force is provided here by means of a separate restoring spring  85  which is embodied as a leg spring. 
     A distance a is provided in an adjustable fashion between the simulator element  15  and the third activation element  81 , with the result that the pedal characteristic curve can be adjusted in addition to the predefined properties of the spring elements of the pedal travel simulator. The adjustment is carried out by means of the cylindrical protrusion  82 , provided with a thread, on the third activation element  81  and the nut  83 . 
     The final position of the brake pedal  1  is defined by a pedal end stop  86 . 
       FIGS. 14 and 15  show a pedal unit according to a fifth exemplary embodiment. In the text which follows, details are given only on the differences from the fourth exemplary embodiment since the rest of the design of the pedal unit is provided in the same way. 
     As is apparent, in particular, from  FIG. 15 , a securing element  87 , which has a funnel-shaped receptacle  88 , is attached to the brake pedal  1 , preferably by welding, said funnel-shaped receptacle  88  being oriented in the direction of a piston rod  90  of the brake booster  3 . 
     This receptacle  88  serves, on the one hand, to guide a piston rod end  89  of the piston rod  90  during the mounting of the pedal unit with the brake booster  3 . On the other hand, the piston rod  90  is guided through the receptacle  88  and/or through the securing element  87  after connection to the brake pedal  1 . 
     A damping element  91  in the form of a damping ring reduces noises when the piston rod end  89  is in contact with the securing element  87 . 
     A further advantage here is that the brake pedal  1  is not weakened in the region of the piston rod connection. 
     The guidance of the piston rod  90  by means of the receptacle  88  or the securing element  87  safeguards the gap in the “brake-by-wire” operating mode, i.e. the decoupling of the force transmitting connection between the brake pedal  1  and the brake booster  3 . 
     The parts which are important for the functioning of the pedal travel simulator  2  are moved during activation also in the embodiments described last, with the result that “creeping” faults are avoided and additional monitoring of the function is not necessary. 
       FIG. 16  shows a longitudinal section through a pedal unit according to a sixth exemplary embodiment. The latter differs from the fourth exemplary embodiment in that the simulator element  15  is attached to the pedal mount  27  by means of a receptacle  92 . 
     An activation element  93  is attached to the shoulder  84  by means of a shank  94 , as a result of which the spring element  65  is positioned prestressed between the activation element  93  and the shoulder  84 . As is apparent, the pedal travel simulator  2  according to this exemplary embodiment has a particularly simple design since just one activation element is provided. 
     Furthermore, the distance a between the simulator element  15  and the activation element  93  can be adjusted by means of the shank  94  and nuts  95  which are provided for the attachment thereof. 
     In contrast to the embodiments described above, according to one embodiment of the invention which is not shown, the control of the pedal travel can be provided as a function of the travel, with the result that, starting from a predetermined activation travel of the brake pedal  1 , prolongation of the pedal travel simulator and therefore of the pedal travel is enabled. 
       FIG. 7  shows a force/travel diagram of all the embodiments of the brake system according to aspects of the invention. It is apparent that the characteristic curve rises linearly up to the point A when the restoring spring/restoring springs  16 ,  45 ,  85  is/are compressed. As soon as the activation element  40 ,  81  or the side wall  52  is pressed against the simulator element  15 , the characteristic curve rises progressively (points A to B). If the foot force applied to the brake pedal  1  exceeds a force F K  predefined by the spring force of the spring element  32 ,  65 , the characteristic curve bends and extends between the points B and C with a significantly flatter gradient than at the section A to B.