CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. national phase application of PCT International Application No. PCT/EP2007/051440, filed Feb. 14, 2007, which claims priority to German Patent Application No. DE102006008956.1, filed Feb. 23, 2006, and German Patent Application No. DE102006059949.7, filed Dec. 19, 2006, the contents of such applications being incorporated by reference herein in their entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a method of operating a motor vehicle brake system of the ‘brake-by-wire’ type, including
         a) a brake booster operable both by means of an input member that is movable into a force-transmitting connection with a brake pedal and by means of an electronic control unit, with a distance being provided between the brake pedal and the brake booster which allows decoupling a force-transmitting connection between the brake pedal and the brake booster in the ‘brake-by-wire’ operating mode,   b) a master brake cylinder connected downstream of the brake booster,   c) means to detect the actuating travel of the brake pedal,   d) a pedal travel simulator which cooperates with the brake pedal and allows simulating a resetting force that acts on the brake pedal in the ‘brake-by-wire’ operating mode independently of an actuation of the brake booster, and   e) a connecting and disconnecting device connecting the pedal travel simulator in the ‘brake-by-wire’ operating mode when the force-transmitting connection between the brake pedal and the brake booster is decoupled and disconnecting it outside the ‘brake-by-wire’ operating mode.       

     BACKGROUND OF THE INVENTION 
     A brake system is disclosed in the applicant&#39;s German patent application DE 10 2004 011 622 A1. The above-mentioned connecting and disconnecting device in a design of the prior art brake system is provided by a hydraulic cylinder-and-piston arrangement, the pressure chamber of which is connected to a pressure fluid volume take-up element by means of a closable connection and on the pistons of which a simulator housing is supported. The connection between the pressure chamber and the pressure fluid volume take-up element that is designed as a low-pressure accumulator is closed by means of an electromagnetically operable shut-off valve which is configured as a normally open switch valve. The mentioned piston is moved when the connecting and disconnecting device is tested, and the pressure rising in the hydraulic pressure chamber is measured and the corresponding pressure signal is evaluated. The testing operation can be performed at standstill in a first application of the brake pedal after the ignition has been turned on. This state can be referred to as ‘conventional mode’. Upon termination of the testing operation a change is made into the actual ‘by-wire mode’, and the change-over is executed as soon as a fully released brake pedal is detected. The brake pedal feeling imparted to the vehicle driver in the ‘conventional mode’ differs greatly from the feeling in the ‘by-wire mode’. 
     In view of the above, an object of the invention is to propose appropriate measures which allow changing over from the ‘conventional mode’ into the ‘by-wire mode’ and, thus, adapting the brake pedal feeling to the respective situation. 
     SUMMARY OF THE INVENTION 
     According to aspects of the invention, the foregoing object is achieved in that the travel covered upon application of the brake pedal is determined and subsequently reduced by the operator, and in that upon reduction of the actuating travel by a predetermined value or in the event of a detected vehicle movement or a positive result of a monitoring function of the connecting and disconnecting device running in the background, the connecting and disconnecting device is activated and the brake booster is actuated by the electronic control unit. 
     More specifically, the connecting and disconnecting device is formed of a hydraulic cylinder-and-piston arrangement, whose piston on which a housing of the pedal travel simulator is supported delimits a pressure chamber, which can be connected to a pressure fluid volume take-up element by way of a connection which is closable by means of a shut-off valve, and in that the connecting and disconnecting device is activated by change-over of the shut-off valve into its closed switch position. 
     In this context, it is especially favorable when the nominal value of the hydraulic pressure that is introduced into the master brake cylinder by actuation of the brake booster is taken from a characteristic curve, which associates a pressure value with an actuating travel of the brake pedal and which, compared to a nominal characteristic curve (along the axis on which the actuating travel values are plotted), is shifted by a value which corresponds to the shortest actuating travel since the activation of the connecting and disconnecting device and the actuation of the brake booster minus a correction value. 
     These and other aspects of the invention are illustrated in detail by way of the embodiments and are described with respect to the embodiments in the following, making reference to the Figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the invention will be explained in detail in the following description making reference to the accompanying drawings. In the drawings: 
         FIG. 1  is a representation of the brake system in which an exemplary method can be implemented, according to one aspect of the invention; 
         FIG. 2  is a partial cross-sectional view of a design of the brake actuation unit which is used in the brake system according to  FIG. 1 ; 
         FIGS. 3   a, b  show graphs of the time variations of the brake pedal actuating travel; and 
         FIG. 4  shows characteristic curves which associate values of the hydraulic pressure introduced into the master brake cylinder to defined brake pedal actuating travels. 
         FIG. 5  illustrates the dependency of the correction value s corr  on value s 0 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The motor vehicle brake system shown in  FIG. 1  of the drawings which preferably can be operated in the ‘brake-by-wire’ operating mode, essentially consists of an actuating unit  1 , a hydraulic control and regulation unit (HCU)  17 , vehicle wheel brakes  13 ,  14 ,  15 ,  16  connected to the hydraulic control and regulation unit (HCU)  17 , a first electronic control and regulation unit  7  associated with the actuating unit  1  as well as a second electronic control and regulation unit  12  associated with the hydraulic control and regulation unit (HCU)  17 . The actuating unit  1 , in turn, consists of a brake booster, preferably a vacuum brake booster  2 , a master brake cylinder connected downstream of the brake booster  2 , preferably a tandem master cylinder  3 , to the pressure chambers (not shown) of which the above-mentioned wheel brakes  13 ,  14 ,  15 ,  16  are connected by the intermediary of the hydraulic control and regulation unit  17 , and a pressure fluid supply tank  4  associated with the master brake cylinder  3 . A brake pedal  5  is used for actuation of the brake booster  2  by the driver, and a pedal travel simulator  6  is provided which cooperates with the brake pedal  5 , in particular in the ‘brake-by-wire’ operating mode, and imparts the customary brake pedal feeling to the driver. A driver&#39;s deceleration request or the actuating travel of the brake pedal  5  is detected by means of at least one sensor device  21 , the signals of which are sent to the above-mentioned first electronic control unit  7 . The output signals of the first electronic control unit  7  enable, among others, actuation of an electromagnet  8  associated with the brake booster  2  which renders it possible to actuate a pneumatic control valve  9  independently of the driver&#39;s wish, the said control valve controlling the supply of air to the brake booster  2 . As will be explained in detail in the following description, the first electronic control and regulation unit  7  comprises a control circuit for controlling a characteristic quantity of the brake booster  2 , preferably the travel covered by the output member  20  of the brake booster  2 , and/or a quantity for controlling the hydraulic pressure that prevails in the system. 
     An axial slot or distance ‘a’ provided between the end of an input member (piston rod)  10  coupled to the brake pedal  5  and a control piston  11  of the above-mentioned control valve  9  ensures decoupling the force-transmitting connection between the brake pedal  5  and the brake booster  2  in the ‘brake-by-wire’ operating mode. A travel sensor  18  is used to detect the travel of a movable wall  19  that generates the boosting power of the brake booster  2 , or the travel of the above-mentioned output member  20  of the brake booster  2 , which transmits its output force onto a non-illustrated first piston of the master brake cylinder  3 . In addition, a pressure sensor  34  is integrated in the hydraulic control unit  17  and senses the hydraulic inlet pressure that prevails in the system. 
     The pedal travel simulator  6  by which, as has been mentioned above, a resetting force acting on the brake pedal  5  in the ‘brake-by-wire’ operating mode can be simulated irrespective of an actuation of the brake booster  2 , is designed in such a fashion that it can be enabled in the ‘brake-by-wire’ operating mode by means of a connecting and disconnecting device  60  illustrated in an axial cross-section in  FIG. 2  when the force-transmitting connection between the brake pedal  5  and the brake booster  2  is decoupled, and can be disabled outside the ‘brake-by-wire’ operating mode. 
     Further, it can be taken from the drawing that the hydraulic control and regulation unit (HCU)  17  includes all hydraulic and electrohydraulic components required to perform brake pressure control operations such as ABS, TCS, ESP, etc. Among these are per brake circuit: each one separating valve  22   a, b , one electric change-over valve  23   a, b, a  hydraulic pump  24   a, b , in each case two electrically actuatable pressure control valves or inlet and outlet valves  25   a, b ,  26   a, b ,  27   a, b , and  28   a, b  for the selective adjustment of the brake pressure at the wheel brakes  13  to  16 , each one low-pressure accumulator  29   a, b  as well as pressure sensors  30 ,  33  associated with the wheel brakes  13  to  16 . 
     Document DE 10 2004 011 622 A1 described previously discloses the layout of the above-mentioned brake actuating unit  1 . Therefore, a partial cross-sectional view of  FIG. 2  depicts only the control group of the vacuum brake booster  2  in detail. The pedal travel simulator  6 , which cooperates with the brake pedal  5  in particular in the ‘brake-by-wire’ operating mode imparting the usual brake pedal feeling to the driver, and which is outside the flux of forces between the brake pedal  5  and the brake booster  2  in the embodiment shown, cooperates with an electrohydraulic connecting and disconnecting device  60  that disconnects the pedal travel simulator  6  outside the ‘brake-by-wire’ operating mode. The connecting and disconnecting device  60  basically includes a piston-and-cylinder arrangement  51  and a hydraulic pressure fluid take-up element  56 . By means of an actuating rod  58 , the piston  52  of the piston-and-cylinder arrangement  51  is in a force-transmitting connection with a housing  61  of the pedal travel simulator  6  and delimits a pressure chamber  53 , which is connected to the pressure fluid take-up element  56  by means of a hydraulic connection  54  shown in dotted lines, the said take-up element being designed as a low-pressure accumulator in the illustrated example. An electromagnetically operable shut-off valve  55  is inserted into the hydraulic connection  54  and allows shutting off the mentioned connection  54 . The hydraulic pressure in the pressure chamber  53  of the piston-and-cylinder arrangement  51  can be determined by means of a pressure sensor  57 . The measured pressure value must be almost zero in the initially opened shut-off valve  55 , while an abrupt pressure rise must take place upon change-over of the shut-off valve  55  into its closing position. It is thus proven that the piston  52  has moved before the shut-off valve  55  is closed, that the shut-off valve  55  is sufficiently seal-tight and that the pressure sensor  57  is functioning. 
     In particular when performing a pre-drive or post-drive test, the use of a pressure sensor  57  is advantageous because the pressure sensor signal can be employed additionally as a plausibilisation of the signal that is produced by a pedal travel sensor associated with the brake pedal. The pressure sensor then makes it possible to detect certain fail conditions of the system, such as faulty detection of the brake pedal travel, thereby activating fallback modes. 
     As has been mentioned hereinabove, the representations according to  FIGS. 3   a  and  3   b  show temporal variations of the brake pedal actuating travel s and the quantity s o , which corresponds to the shortest actuating travel since the activation of the connecting and disconnecting device  60  and the actuation of the brake booster  2 . Point A corresponds to the maximum actuating travel reached upon depression of the brake pedal  5 , while the curve portion A to A 0  corresponds to a withdrawal of the actuating force which acts on the brake pedal  5  and results in a reduction of the actuating travel S A  by a predetermined value Δs to the value s 0 . The actuation is performed in the initially mentioned ‘conventional’ mode in period  0  to T 0 . At time T 0  the connecting and disconnecting device  60  of the pedal travel simulator  6  is activated and the brake booster  2  is driven by the electronic control unit  7  so that the actuating unit is subsequently operated in a mixed form of ‘conventional’ mode and the ‘by-wire mode’ in the interval T 0  to T 2 . At time T 1  lying between the points T 0  and T 2 , there is a new actuation or a continued depression of the brake pedal  5  by the operator, and a value s 01  is reached before the continued depression which is taken into consideration as the shortest actuating travel since the activation of the connecting and disconnecting device  60  and the actuation of the brake booster  2 . At time T 2 , the value s 02  corresponds to distance ‘a’ which serves for the decoupling of the force transmission between the brake pedal  5  and the brake booster  2 . The above-mentioned mixed form is terminated, and the brake system has fully adopted the ‘by-wire’ mode. At time T 3 , the brake pedal  5  is completely released and the actuation of the brake system is terminated. 
       FIG. 4  eventually shows the effect the measures explained above have on the characteristic curves, which represent the dependency of the nominal value p nominal  of the hydraulic pressure introduced into the master brake cylinder  3  on the brake pedal actuating travel s. While the characteristic curve p nominal [T 2 ] represents the nominal characteristic curve, the characteristic curves p nominal [T 0 ] and p nomina [T 1 ] correspond to the previously explained performance of the system at times T 0  and T 1 . In this context, it can be taken from  FIG. 4  that the characteristic curve which corresponds to the ‘first’ lowest actuating travel value s 0  is shifted along the abscissa by a predetermined value s shift . Characteristic curves are obtained by fixing further actuating travel values s 0  (see characteristic curve p nominal [T 1 ]) which move in the direction of the nominal characteristic curve. The predetermined value S shift  is calculated according to the equation s shift =s 0 −k*s corr  in which s corr  implies a correction value that depends on value s 0 .  FIG. 5  illustrates the dependency of the correction value s corr  on value s 0 . The factor k, which can adopt the values 0 or 1 in the simplest case, results from an assessment of the driving situation. A change-over from 0 to 1 is, for example, practicable when a rapid forward pedal movement above a threshold value is detected. 
     While preferred embodiments of the invention have been described herein, it will be understood that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the spirit of the invention. It is intended that the appended claims cover all such variations as fall within the spirit and scope of the invention.
In order to impart a pleasant brake pedal feeling to the operator in the transition from a 'conventional mode' to a 'brake-by-wire' mode, it is disclosed that the travel (s) covered upon application of the brake pedal is determined and subsequently reduced by the operator, and in that upon reduction of the actuating travel (s) by a predetermined value (Ds) or in the event of a detected vehicle movement or a positive result of a monitoring function of the connecting and disconnecting device running in the background, the connecting and disconnecting device is activated and the brake booster is actuated by the electronic control unit.