Abstract:
A system and method for operating a brake system for motor vehicles, includes a master brake cylinder actuated by a brake pedal and separably connected to at least one hydraulically acutuatable wheel brake by at least one brake circuit, a hydraulically operable pedal travel simulator is connectable to the master brake cylinder, an electrically controllable pressure provision apparatus is separably connected to the brake circuit, and an electronic open-loop and closed-loop control unit. Upon brake pedal actuation, a pressure of the pressure provision apparatus is applied to the brake circuit in a first operating mode and the pressure of the master brake cylinder is applied to the brake circuit in a second operating mode. In the event of transition from the second operating mode to the first operating mode while the brake pedal is actuated, the actuation travel of the brake pedal is adjusted by electronic open-loop or closed-loop control.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. National Phase application of PCT/EP2013/051436, filed Jan. 25, 2013, which claims priority to German Patent Application No. 10 2012 201 515.9, filed Feb. 2, 2012, the contents of such applications being incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a method for operating a brake system and to a brake system for motor vehicles. 
       BACKGROUND OF THE INVENTION 
       [0003]    In motor vehicle engineering, “brake-by-wire” brake systems are becoming ever more widespread. Such brake systems often comprise not only a master brake cylinder which can be actuated by the vehicle driver but also an electrically controllable pressure supply device by means of which the wheel brakes or the master brake cylinder is actuated in the “brake-by-wire” operating mode. In order to give the vehicle driver a pleasant pedal sensation in the “brake-by-wire” operating mode, the brake systems usually comprise a brake pedal sensation simulation device. In these brake systems, the wheel brake can also be actuated on the basis of electronic signals, without active involvement of the vehicle driver. These electronic signals can, for example, be output by an electronic stability program or a distance control system. 
         [0004]    International patent application WO 2011/029812 A1, which is incorporated herein by reference, discloses a “brake-by-wire” brake system with a master brake cylinder which can be actuated by a brake pedal, a travel simulator and a pressure supply device. In a “brake-by-wire” operating mode pressure is applied to the wheel brakes by the pressure supply device. In a fallback operating mode, pressure is applied to the wheel brakes by means of the master brake cylinder which can be actuated by the brake pedal. In the application, no further details are given as to whether and, if appropriate, how a changeover can be carried out from the fallback operating mode into the “brake-by-wire” operating mode. 
       SUMMARY OF THE INVENTION 
       [0005]    An aspect of the present invention is a method for operating a brake system and a brake system which provides the vehicle driver with a continuous brake pedal sensation, in particular even at a changeover from one operating mode into another operating mode of the brake system. In particular, the driver is to be provided with the accustomed brake pedal sensation and the “brake-by-wire” operating mode with boosted braking force as quickly and comfortably as possible in the case of a start or restart of the brake system. 
         [0006]    An aspect of the invention is based on the concept that in the case of a changeover from the second operating mode into the first operating mode during actuation of the brake pedal adjustment of the actuation travel of the brake pedal is carried out under electronic open-loop or closed-loop control. 
         [0007]    In the case of the changeover from the second operating mode into the first operating mode during actuation of the brake pedal the actuation travel of the brake pedal is preferably adjusted by discharging pressure medium, under electronic open-loop or closed-loop control, from the master brake cylinder or by feeding pressure medium into the master brake cylinder under electronic open-loop or closed-loop control. 
         [0008]    According to one preferred embodiment of the method according to the invention, a first characteristic curve, which describes a relationship between a first and second variable in the first operating mode, and a second characteristic curve which describes a relationship between the first and second variables in the second operating mode, are predefined, and by comparing the current values of the first and second variables with the first characteristic curve and/or the second characteristic curve a decision is made as to whether pressure medium will be discharged or fed in. The actuation travel is particularly preferably adjusted to a value corresponding to the first characteristic curve. 
         [0009]    The actuation travel is preferably reduced by feeding in pressure medium if the value pair composed of the first and second variables is below the second characteristic curve after the pedal travel simulator is activated, in order to compensate the pressure medium volume which has flowed off into the pedal travel simulator. 
         [0010]    The actuation travel is preferably increased by discharging pressure medium if the value pair composed of the first and second variables is above the first characteristic curve after the pedal travel simulator is activated, in order to compensate the pressure medium excess volume originating from the wheel brake or brakes. 
         [0011]    In the case of the changeover from the second operating mode into the first operating mode, the sensors for measuring the first and second variables are firstly preferably initialized and then the pedal travel simulator is switched on by means of the simulator release valve. Changes to the values of the first and second variable during the activation of the pedal travel simulator can therefore be observed and evaluated. After the activation of the pedal travel simulator, the driver is connected hydraulically both to the pedal travel simulator and to the brake circuit/circuits, with the result that a pressure equilibrium can be established. The adjustment of the actuation travel of the brake pedal is particularly preferably carried out after the activation of the pedal travel simulator. 
         [0012]    The method is preferably carried out in a brake system with two or more brake circuits in which each brake circuit is connected to the master brake cylinder via a hydraulic connecting line with an isolating valve which is advantageously open in the currentless state, and to the pressure supply device via a further hydraulic connecting line with an activation valve which is advantageously closed in the currentless state. 
         [0013]    The brake system preferably also comprises a wheel brake pressure modulation unit which has, per wheel brake, an inlet valve and an outlet valve for setting a wheel-specific brake pressure which is derived from the pressure in the brake circuit. In the non-activated state, the inlet valves particularly preferably pass on the respective brake circuit pressures. The outlet valves are likewise blocked in the non-activated state. 
         [0014]    According to one development of the method according to the invention, the actuation travel of the brake pedal is reduced in that an activation valve which is arranged between the pressure supply device and the brake circuit is opened, and the pressure supply device is activated in such a way that pressure medium volume is forced into the master brake cylinder. The pressure supply device is therefore connected to the brake circuit and thus to the master brake cylinder, with the result that pressure medium can then be conducted into the master brake cylinder via the opened activation valve and the open connection between the brake circuit and the master brake cylinder by means of the pressure supply device, as a result of which the pedal position is corrected and/or the actuation travel of the brake pedal is adjusted. The actuation travel is particularly preferably adjusted if the value pair composed of the first and second variables is below the second characteristic curve after the activation of the pedal travel simulator. 
         [0015]    According to one development of the method according to the invention, the actuation travel of the brake pedal is increased in that an outlet valve which is arranged between a wheel brake and a pressure medium reservoir container is opened. As a result, the pressure medium excess volume originating from the wheel brake/brakes can flow off into the pressure medium reservoir container. The actuation travel is particularly preferably adjusted if the value pair composed of the first and second variable is above the first characteristic curve after the activation of the pedal travel simulator. 
         [0016]    The pressure supply device is preferably formed by a cylinder-piston arrangement whose piston can be actuated by means of an electromechanical actuator. 
         [0017]    According to one preferred embodiment of the method according to the invention, in addition, advantageously simultaneously, an activation valve which is arranged between the pressure supply device and the brake circuit is opened in order to open the outlet valve. It is therefore possible for pressure equalization to take place between the components comprising the pressure supply device, the brake circuit or circuits and the master brake cylinder. Particularly preferably the piston of the pressure supply device is additionally, advantageously simultaneously, set to a predetermined piston position. The piston is then located at the correct position for the first operating mode. The predetermined piston position can be determined easily according to a predefined piston position pressure characteristic curve, wherein the currently measured first variable is taken into account. 
         [0018]    After the adjustment of the actuation travel of the brake pedal an isolating valve which is arranged between the master brake cylinder and the brake circuit is preferably closed, and then a predetermined setpoint pressure is set by the pressure supply device. The brake system is then in the first operating mode, wherein the driver experiences the accustomed brake pedal sensation. The setpoint pressure is particularly preferably determined on the basis of the current values of the first and second variables. 
         [0019]    According to one preferred development of the method according to the invention, the actuation travel of the brake pedal is not adjusted if the current value of the second variable is lower than a predefined first threshold value. In these cases, the actuation of the brake pedal by the driver is so small that deviations from the accustomed brake pedal sensation are not disruptive. In addition to the current value of the second variable the current value of the first variable is particularly preferably used. The actuation travel of the brake pedal is then not adjusted if the current value of the second variable is lower than the predefined first threshold value and the current value of the first variable is lower than a predefined second threshold value. 
         [0020]    The brake system is preferably a brake system for motor vehicles which can be activated in a so-called “brake-by-wire” operating mode either by the vehicle driver or independently of the vehicle driver, is preferably operated in the “brake-by-wire” operating mode and can be operated in at least one fallback operating mode in which only the operation by the vehicle driver is possible. 
         [0021]    An aspect of the invention also relates to a brake system in whose open-loop and closed-loop control unit adjustment of the actuation travel of the brake pedal is carried out in the case of a changeover from the second operating mode into the first operating mode during actuation of the brake pedal. Furthermore, the invention relates to a brake system in whose electronic open-loop and closed-loop control unit a method according to the invention is carried out. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0022]    Further preferred embodiments of the invention can be found in the dependent claims and the following description with reference to figures. 
           [0023]    In the drawings, in each case in schematic form: 
           [0024]      FIG. 1  shows an exemplary brake system, 
           [0025]      FIG. 2  shows exemplary relationships between the pressure in the master brake cylinder and the actuation travel of the brake pedal for two different operating modes of the exemplary brake system from  FIG. 1 , 
           [0026]      FIG. 3  shows a schematic flowchart illustrating an exemplary method for operating the exemplary brake system from  FIG. 1 , 
           [0027]      FIG. 4  shows a diagram of the pedal travel and the associated pressure illustrating a first exemplary method, 
           [0028]      FIG. 5  shows a diagram of the pedal travel and the associated pressure illustrating a second exemplary method, and 
           [0029]      FIG. 6  shows an exemplary characteristic curve of a pressure supply device. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]      FIG. 1  is a schematic illustration of an exemplary brake system. The brake system comprises an actuation device  2  which can be actuated by a vehicle driver by means of an actuation pedal or brake pedal  1 , a pressure medium reservoir container  3  which is assigned to the actuation device  2 , an electrically controllable pressure supply device  4 , an electrically controllable pressure modulation device  5  to whose output connections wheel brakes  6  of a motor vehicle (not illustrated) are connected, and an electronic open-loop and closed-loop control unit  7  (ECU; electronic control unit) which serves to process sensor signals and to activate the electrically controllable components. 
         [0031]    The actuation device  2  comprises a dual-circuit master brake cylinder or tandem master cylinder  12  with two hydraulic pistons  8 ,  9  which are arranged in series in a (master brake cylinder) housing and which bound hydraulic pressure chambers  10 ,  11 . The pressure chambers  10 ,  11  are connected to the pressure medium reservoir container  3  via radial bores which are formed in the pistons  8 ,  9 , wherein these bores can be shut off by means of a relative movement of the pistons  8 ,  9  in the housing. Furthermore, each pressure chamber  10 ,  11  is connected to a brake circuit I, II, each with two wheel brake circuits with hydraulically actuable wheel brakes  6 , by means of a hydraulic line  13   a ,  13   b . An isolating valve  14   a ,  14   b , which is embodied as an electrically actuable 2/2 way valve, which is preferably open in the currentless state, is inserted into each of the hydraulic lines  13   a ,  13   b . A pressure sensor which is connected to the pressure chamber  11  and is preferably of a redundant design detects the pressure P THZ  which is built up in the pressure chamber  11  by moving the second piston  9  and which corresponds to the pedal force generated by the driver. Furthermore, the pressure chambers  10 ,  11  receive restoring springs (not denoted in more detail) which prestress the pistons  8 ,  9  counter to the actuation direction. A piston rod  16  which is coupled to the brake pedal  1  interacts with the first (master cylinder) piston  8 , wherein a variable S Pedal  which characterizes the actuation travel of the brake pedal  1 , for example the actuation travel or actuation angle of the brake pedal  1  itself or the actuation travel of the piston  8  which is coupled to the brake pedal, is detected by a travel sensor  17  which is preferably of redundant design. 
         [0032]    The actuation device  2  also comprises a pedal travel simulator (also referred to as pedal sensation simulator device)  19  which interacts with the master brake cylinder  12  and gives the vehicle driver a pleasant pedal sensation in a first operating mode (a so-called “brake-by-wire” operating mode). The pedal travel simulator  19  can be actuated hydraulically and is connected to at least one pressure chamber  10 ,  11  of the master brake cylinder  12 . The pedal travel simulator  19  can be activated and deactivated by means of an electrically actuable simulator release valve  20 . 
         [0033]    The pedal travel simulator  19  is composed for example essentially from two simulator chambers, a simulator spring chamber with a simulator spring  21  and a simulator piston (stepped piston) which separates these chambers from one another. In this context, the simulator chambers are each connected to a pressure chamber  10 ,  11  of the master brake cylinder  12 , while the simulator spring chamber can be connected to the pressure medium reservoir container  3  with intermediate connection of the simulator release valve  20 . A non-return valve which is connected in parallel with the simulator release valve  20  is connected to the simulator spring chamber and permits a largely unthrottled flow of the pressure medium into the simulator spring chamber independently of the switch state of the simulator release valve  20  and independently of a throttling effect of the hydraulic simulator outlet connections. The simulator release valve  20  is embodied as an electrically actuable 2/2 way valve which is preferably closed in the currentless state. 
         [0034]    The electrohydraulic pressure supply device  4  is embodied as a hydraulic cylinder-piston arrangement whose piston  22  can be actuated by a schematically indicated electric motor  23  with the intermediate connection of a rotational-translational gear mechanism (not illustrated). The electric motor  23  and the rotational-translational gear mechanism form a linear actuator, wherein, in order to detect a variable which is characteristic of the position of the piston  22  of the pressure supply device  4 , a sensor  24  is present which is embodied for example as a rotor position sensor  24  which serves to detect the rotor position of the electric motor  23 . Further sensors such as for example a temperature sensor supply the electronic open-loop and closed-loop control unit  7  with state information on the electric motor  23  or the linear actuator. The piston  22  bounds a pressure chamber  25  which can be connected to the brake circuits I, II via hydraulic lines  26   a ,  26   b , each with an electrically actuable activation valve  27   a ,  27   b . In this context, in each case a non-return valve, which closes toward the pressure chamber  25 , is connected in parallel with the activation valves  27   a ,  27   b . Furthermore, the pressure chamber  25  is connected to the pressure medium reservoir container  3  via a non-return valve  34  which closes toward said pressure medium reservoir container  3 . The activation valves  27   a ,  27   b  are embodied as electrically actuable 2/2 way valves which are preferably closed in the currentless state. Arranged in the pressure chamber  25  is a spring  33  which loads the piston  22  counter to the pressure build-up direction. 
         [0035]    In order to modulate the pressure at the wheel brakes  6 , the hydraulic pressure modulation device  5  comprises, per wheel brake  6 , for example an inlet valve  28   a - 28   d  and an outlet valve  29   a - 29   d . The input connections of the inlet valves  28   a - 28   d  or the brake circuits I, II can be supplied with the pressure of the master brake cylinder  12  (via the lines  13   a ,  13   b  with the isolating valves  14   a ,  14   b ) or with the pressure of the pressure supply device  4  (via the lines  26   a ,  26   b  with the activation valves  27   a ,  27   b ). The output connections of the outlet valves  29   a - 29   d  are connected via return lines  32   a ,  32   b  to the pressureless pressure medium reservoir container  3  (so-called open system). In addition it is also to be noted that all the circular symbols represent hydraulic lines leading to the pressure medium reservoir container  3 . The inlet valves  28   a - 28   d  are advantageously embodied as electrically actuable pressure regulating valves, which are open in the currentless state, and the outlet valves  29   a - 29   d  are embodied as electrically actuable 2/2 way valves, which are closed in the currentless state. 
         [0036]    For example, a pressure sensor  30 ,  31  for detecting the pressure present at the input connections of the associated inlet valves  28   a ,  28   b ;  28   c ,  28   d  is arranged in each brake circuit I, II. Alternatively it is conceivable that a pressure sensor is arranged in just one of the two brake circuits I, II or that a pressure sensor is arranged in the line  26   a ,  26   b  between the pressure chamber  25  and the activation valves  27   a ,  27   b . By means of this pressure sensor or these pressure sensors it is possible to determine the pressure P act  of the pressure supply device  4  (when the isolating valves  14   a ,  14   b  are closed). 
         [0037]    In a first operating mode Z A  (a so-called “brake-by-wire” operating mode with boosting of the braking force) the pressure of the pressure supply device  4  is applied to the brake circuits I, II. For this purpose, for example the activation valves  27   a ,  27   b  are opened, with the result that the pressure supply device  4  is connected hydraulically to the brake circuits I, II. The master brake cylinder  12  is disconnected from the brake circuits I, II or wheel brakes  6  by means of the closed isolating valves  14   a ,  14   b . The simulator release valve  20  is opened, with the result that the pedal travel simulator  19  is activated or switched on. In the case of actuation S Pedal  of the brake pedal  1 , pressure medium volume is forced out of the pressure chambers  10 ,  11  of the master brake cylinder  12  into the simulator chambers of the pedal travel simulator  19 . The pressure supply device  4  is activated by means of the electronic open-loop and closed-loop control unit  7  in order to supply a setpoint pressure P Setp  to be applied to the brake circuits I, II. The value for the setpoint pressure P Setp  of the pressure supply device  4  is calculated, for example in the electronic open-loop and closed-loop control unit  7 , on the basis of a predefined braking force boosting function and the driver&#39;s braking request (S Pedal , P THZ ) which is determined, for example, by means of the sensors  15  and  17 . 
         [0038]    For example, the setpoint pressure P Setp  is determined from the measured variables P THZ  and S Pedal  according to the following formula: 
         [0000]        P   Setp =λ S   *f   S ( S   Pedal )+λ P   *f   P ( P   THZ )
 
         [0000]    where:
 
f S : is the brake pressure boosting function in dependence on the pedal travel S Pedal  
 
f P : is the brake pressure boosting function in dependence on the pressure P THZ ,
 
λ S : is the weighting factor or weighting function for the pressure component f S , and
 
λ P : is the weighting factor or weighting function for the pressure component f P .
 
         [0039]    The setpoint pressure P Setp  is therefore determined by weighted superimposition of a pressure component f S , based on the pedal/piston travel variable S Pedal  and of a pressure component f P  based on the pressure variable P THZ . The weighting factors can each assume for example values between zero and one. Specific adaptation of the setpoint pressure calculation to the present brake system and the requests relating to the configuration of the brake system is possible by virtue of the corresponding predefinition or definition and parameter selection with respect to the predefined brake pressure boosting functions f S , f P  and the predefined weighting factors λ S , λ P . 
         [0040]    In a second operating mode Z P  (a so-called fallback operating mode), the pressure of the master brake cylinder  12  is applied to the brake circuits I, II. For this purpose, the isolating valves  14   a ,  14   b  are opened, with the result that the master brake cylinder  12  is hydraulically connected to the brake circuits I, II. The pressure supply device  4  is disconnected from the brake circuits I, II or wheel brakes  6  by means of the closed activation valves  27   a ,  27   b . The simulator release valve  20  is closed, with the result that the pedal travel simulator  19  is switched off. In the case of actuation S Pedal  of the brake pedal  1 , pressure medium volume is forced out of the pressure chambers  10 ,  11  of the master brake cylinder  12  into the brake circuits I, II or wheel brakes  6 . 
         [0041]      FIG. 2  is a schematic illustration of exemplary relationships between the pressure P THZ  in the master brake cylinder  12  and the associated actuation travel S Pedal  for the two operating modes Z A , Z P . The pressure P THZ  is plotted against the actuation travel S Pedal  wherein the characteristic curve  40  represents the first operating mode Z A  and the characteristic curve  41  represents the second operating mode Z P . Characteristic curves  40  and  41  represent the nominal relationship between the pressure P THZ  and the actuation travel S Pedal  in the respective operating mode Z A  and Z P , respectively. For example, corresponding characteristic curves  40 ,  41  are stored for the two operating modes Z A , Z P  in the electronic open-loop and closed-loop control unit  7 . 
         [0042]    The nominal pedal characteristic curve of the brake system in the first operating mode Z A , i.e. the relationship between the pedal force and the pedal travel S Pedal  is determined essentially by the simulator spring  21  (preferably a progressive compression spring, other spring elements are conceivable, for example an elastomer spring) of the pedal travel simulator  19 . The displacement volume of the pedal travel simulator  19  depends linearly on the pedal travel S Pedal . Likewise, the pedal force and the pressure in the pedal travel simulator  19  are in a linear relationship. Correspondingly, a characteristic curve  40  is produced which describes the pressure in the pedal travel simulator  19 , which corresponds to the pressure P THZ  which can be measured by means of the pressure sensor  15 , as a function of the pedal travel S Pedal . This nominal characteristic curve  40 , which represents the normal braking function (“brake-by-wire” operating mode, first operating mode Z A ), is stored in the electronic open-loop and closed-loop control unit  7 . 
         [0043]    In the hydraulic fallback level (second operating mode Z P ), the valves, in particular the valves  14   a ,  14   b ,  20 ,  27   a ,  27   b , are currentless, and the driver is directly connected hydraulically to the wheel brakes  6  when the brake pedal  1  is actuated. In accordance with the hydraulic configuration of the brake system and of the vehicle-specific volumetric capacity of the wheel brakes  6 , therefore a different pedal characteristic curve is produced and the relationship between the pressure P THZ , which can be measured by means of the pressure sensor  15 , and the pedal travel S Pedal  is described by the characteristic curve  41 . This nominal characteristic curve  41 , which represents the second operating mode Z P , is also stored in the electronic open-loop and closed-loop control unit  7 . As is apparent from  FIG. 2 , the characteristic curves of the two operating modes Z P  and Z A  are significantly different. 
         [0044]      FIG. 3  illustrates a schematic flowchart showing an exemplary method for operating a brake system. 
         [0045]    If the vehicle is firstly shut down or the brake system is not supplied with electrical energy, the brake system is in a passive or shut-down state  50  (assuming no actuation of the brake pedal by the driver). The brake system is usually started, for example, by the driver by switching on the engine ignition or in advance by opening the driver&#39;s door (optionally by means of a corresponding surroundings sensor system, for example keyless entry). Firstly, in block  51 , the sensors for the brake system, for example the pressure sensors  15 ,  30 ,  31  and the position sensors  17 ,  24  are initialized, with the result that they provide measured values after the initialization. In block  52 , the corresponding valves, for example the isolating valves  14   a ,  14   b , the simulator release valve  20  and the activation valves  27   a ,  27   b , as well as the brake boosting function, are activated, with the result that in the case of actuation of the brake pedal by the driver a “brake-by-wire” braking operating can be carried out. The brake system is then in an operationally ready active state (first operating mode Z A , block  65 ), in which, when the brake pedal  1  is actuated, the isolating valves  14   a ,  14   b  Of the actuation device  2  are closed, and the simulator release valve  20  and the activation valves  27   a ,  27   b  of the pressure supply device  4  are opened. The driver is then not directly connected hydraulically to the brake circuits I, II or wheel brakes  6  but rather actuates the pedal travel simulator  19  while the pressure P Setp  is applied (in accordance with a predefined brake force boosting function) to the brake circuits I, II by the pressure supply device  4 . The initialization path without actuation by the driver (blocks  50 ,  51 ,  52 ,  65 ) is generally the usual case, i.e. the valves  14   a ,  14   b ,  20 ,  27   a ,  27   b  are energized directly and the pressure supply device  4  is activated. 
         [0046]    If the vehicle is shut down (or if the brake system is not supplied with electrical energy) and the driver actuates the brake pedal  1 , the brake system is in the second operating mode Z P  (fallback operating mode). If the engine ignition is then switched on (or if the brake system is supplied with electrical energy again), the brake system is changed over into the first operating mode Z A  (block  65 ), in accordance with the exemplary method described below, wherein an advantage of the method is that the changeover is carried out as comfortably as possible and without distracting the driver. 
         [0047]    In the text which follows, for example the two following cases are differentiated: 
         [0048]    The first case relates to the situation in which the driver actuates the brake pedal during a passive state or switched-off state and then the brake system is started or initialized while the driver actuates the brake pedal further. This corresponds, for example, to the case in which the driver is seated in the shut-down vehicle, actuates the brake pedal and then switches on the ignition. In order to illustrate the first case, an exemplary diagram of the pedal travel S Pedal  and the associated pressure P THZ  is illustrated in  FIG. 4 . 
         [0049]    If the driver therefore actuates the brake pedal during a passive or switched-off state (i.e. without previous initialization of the brake system), the brake system is in the second operating mode Z P  (fallback operating mode) and the characteristic curve  41  of  FIG. 4  is active. The pedal travel S Pedal  and the associated system pressure in the brake circuits I, II, which corresponds to the pressure P THZ  of the master brake cylinder  12  owing to the opened isolating valves  14   a ,  14   b , set themselves to the point  80  (S 1 , P 1 ) in accordance with the volumetric capacity of the wheel brakes  6  and the pedal force applied by the driver. If the brake system is then started, for example by the driver by means of the engine ignition, the sensors and the ECU  7  of the brake system are firstly initialized in block  53  of  FIG. 3 , with the result that measured values of the pressure sensor  15  for the pressure P THZ  and measured values of the travel/position sensor  17  for the pedal travel S Pedal  are subsequently available. Then, in block  53 , the simulator release valve  20  is opened, with the result that the pedal travel simulator  19  is actuated. The valves  14   a ,  14   b ,  27   a ,  27   b  firstly remain currentless. As a result of the activation of the pedal travel simulator  19 , the pedal travel S Pedal  becomes longer while the pedal force applied by the driver remains the same (i.e. constant pressure P 1  in  FIG. 4 ) by the capacity of the pedal travel simulator  19 , i.e. the pedal travel is lengthened from S 1  to S 2  in  FIG. 4 . 
         [0050]    The position of the point  81  (S 2 , P 1 ), which is set, relative to the two nominal characteristic curves  40 ,  41  is checked in block  56  by plausibility checking and a comparison on the basis of the current measured values of the sensors  15  and  17  (P THZ  and S Pedal ) as well as the characteristic curve  41 , stored in block  54 , for the second operating mode Z P  and the characteristic curves  40 , stored in block  55 , for the second operating mode Z A . If it is detected in block  56  that the point  81  (S 2 , P 1 ) which is set is below the two nominal characteristic curves  40 ,  41 , an active pedal correction, for example by means of the pressure supply device  4 , is carried out (branch  57  to block  58 ). 
         [0051]    For this purpose, in block  58  the activation valves  27   a ,  27   b  are preferably firstly opened, with the result that the pressure supply device  4  is connected hydraulically to the brake circuits I, II and therefore also to the master brake cylinder  12 . In block  58 , the pressure supply device  4  is further activated in such a way that pressure medium volume is forced from the pressure supply device  4  into the pressure chambers  10 ,  11  of the master brake cylinder  12  via the opened activation valves  27   a ,  27   b  and isolating valves  14   a ,  14   b , with the result that the brake pedal  1  is reset counter to the acting pedal force of the driver, i.e. constant pressure P 1 . The resetting of the brake pedal  1  (the pedal correction travel) is illustrated in  FIG. 4  by the arrow  83 . The brake pedal  1  is set to the nominal point  82  of the characteristic curve  40  of the first operating mode Z A  (“brake-by-wire” operating mode), with the result that the pedal position which is set corresponds to the pedal travel S 3 , obtained from the characteristic curve  40 , for the (system) pressure P 1 . The pedal position is checked by means of the sensor  17 . 
         [0052]    After the ending of the pedal correction (block  58 ), in block  59  the master brake cylinder  12  is disconnected from the brake circuits I, II by closing the isolating valves  14   a ,  14   b . In block  60 , the boosting of the braking force is then slowly increased until the brake system is then in the first operating mode Z A  (“brake-by-wire” operating mode with boosting of the brake force) in block  65 . 
         [0053]    As a result of the procedure described above with brake pedal correction, the brake system is changed over from the second operating mode into the first operating mode quickly, comfortably and without distracting the driver during actuation of the brake pedal. In particular, the pedal correction changes over the brake system almost imperceptibly for the driver from the state at the point  81  into the “brake-by-wire” operating state which corresponds to the point  82  on the characteristic curve  40  of the first operating mode, to which the driver is accustomed. 
         [0054]    The second case relates to the situation in which the driver actuates the brake pedal in the “brake-by-wire” operating mode, and the brake system goes into the fallback operating mode (for example owing to a brief power failure of the brake system) and then a restart or initialization of the brake system is carried out while the driver continues to apply pressure to the brake pedal. In order to illustrate the second case,  FIG. 5  illustrates an exemplary diagram for the pedal travel S Pedal  and the associated pressure P THZ . 
         [0055]    If the brake system is restarted during a “brake-by-wire” braking operation initiated by actuation of the brake pedal, with boosting of the braking force (for example caused by a brief power failure), all the valves, in particular the valves  14   a ,  14   b ,  20 ,  27   a ,  27   b  as well as the pressure supply device  4  of the brake system are without an electrical power supply, i.e. currentless, for a limited time. The brake system is therefore reset from the first operating mode with actuation by the driver into the hydraulic fallback level, i.e. into the second operating mode Z P  in the case of actuation by the driver. 
         [0056]    In the first operating mode, i.e. before the switching over into the hydraulic fallback level, the master brake cylinder  12  is connected to the pedal travel simulator  19  and the characteristic curve  40  in  FIG. 5  is effective. In accordance with the pedal force applied by the driver, the pressure P 2  is therefore present at the master brake cylinder  12  or pedal travel simulator  19 , and the pedal travel S 4  is present (point  84 ). Owing to the braking force boosting function of the pressure supply device  4  there is a relatively high (system) pressure level in the brake circuits I, II. At the changeover into the second operating mode Z P , the closing of the valves  20 ,  27   a ,  27   b  and the opening of the isolating valves  14   a ,  14   b  cause the brake pedal  1  to be forced back out of its pedal position S 4  with a constant pedal force, i.e. the same pressure P 2 , by the excess volume in the wheel brakes  6  until a corresponding pressure equalization has taken place. This is indicated in  FIG. 5  by the arrow  86 . The pedal travel S 5  is therefore set. At the same time, the piston  22  is returned to its hydraulic zero position by the spring  33 . The pressure space  25  is filled here via the non-return valve/suction valve  34 . 
         [0057]    If the electrical power supply is then restored, in block  53  in  FIG. 3  the sensors and the ECU  7  of the brake system are firstly initialized, with the result that measured values of the pressure sensor  15  and measured values of the travel/position sensor  17  are then available again. In block the simulator release valve  20  is also opened again. The valves  14   a ,  14   b ,  27   a ,  27   b  initially remain currentless. 
         [0058]    In block  56 , it can be detected, through plausibility checking or a comparison on the basis of the current measured values of the sensors  15  and  17  (P THZ  and S Pedal ) and the characteristic curves  41  and  40  stored in the blocks  54  and  55 , respectively, that the point  85  (S 5 , P 2 ) which is set is above the nominal characteristic curves  40 . If this is the case, an active pedal correction, for example by means of the outlet valves  29   a - d , is carried out (branch  61  to block  62 ). 
         [0059]    The pedal correction in block  62  occurs for example by the opening of the outlet valves  29   a - d  which are assigned to the wheel brakes  6 . The excess volume which is previously fed in via the wheel brakes  6  is discharged via the outlet valves  29   a - d  into the pressure medium reservoir container  3  until the pedal travel of the brake pedal  1  corresponds again to the corresponding value S 4  of the nominal characteristic curve  40 . The activation valves  27   a ,  27   b  are also opened in block  62  so that the pressure supply device  4  is connected to the brake circuits I, II. 
         [0060]    At the same time, the piston  22  of the pressure supply device is set to a piston position S Piston  which corresponds to a pressure of the pressure supply device  4  which is equal to the pressure P THZ  in the master brake cylinder  12  (P THZ =P 2 ). For this purpose, a pressure/travel characteristic curve  42  of the pressure supply device  4  is stored in the open-loop and closed-loop control unit  7 . An exemplary characteristic curve describing the relationship between the piston position S Piston  and the pressure P DBE  of the pressure supply device  4  is illustrated schematically in  FIG. 6 . A piston position S K1  corresponds to the pressure P DBE =P 2 . The piston position S Piston  can be checked by means of the sensor  24 . 
         [0061]    Once the pedal correction and setting of the piston  22  are concluded, in block  59  the isolating valves  14   a ,  14   b  are closed and then in block  60  the boosting of the braking force is slowly increased in accordance with the driver&#39;s request until the brake system is again in the first operating mode Z A  (“brake-by-wire” operating mode with boosting of the braking force). 
         [0062]    In addition to the two cases described above (first and second cases), in cases in which the actuation of the brake pedal  1  by the driver is low, no pedal correction is carried out. For this purpose, in block  56  it is checked whether the current pedal travel S Pedal  or the current operating point (S Pedal , P THZ ) of the brake system is below a predefined threshold value S g  or inside a predefined characteristic diagram range  44  which is defined, for example, by two threshold valves S g  and P g  (see  FIGS. 4 and 5 ). If the pedal travel S Pedal  is below the threshold value S g  or the operating point (S Pedal  P THZ ) is inside the characteristic diagram range  44 , the actual pedal travel differs only very slightly or to an acceptable degree from the pedal travel according to the nominal characteristic curve  40 . In block  64  the isolating valves  14   a ,  14   b  are then closed and the activation valves  27   a ,  27   b  opened directly (branch  63  in  FIG. 3 ), without pedal correction. Subsequently, as has already been stated for the first and second cases above, in block  60  the boosting of the braking force is slowly increased by means of the pressure supply device  4 . The brake system is then in the first operating mode Z A  (“brake-by-wire” operating mode with boosting of the braking force, normal braking function). After a subsequent release of the brake pedal  1 , the brake system then sets itself again to the nominal characteristic curve  40  as a result of the sucking in of the missing (small) volume (via the connection of the pressure chambers  10 ,  11  to the pressure medium reservoir container  3 ).