Abstract:
A brake system includes a main brake cylinder and a pedal decoupling unit provided with a holding piston, the first ring surface thereof together with a first primary brake cylinder piston defining a first hydraulic chamber that is hydraulically pressurized with an electrically controllable pressure supply device. In order to improve the pedal characteristics of the brake system, in particular in a brake-by-wire brake system, the holding piston is embodied as a differential piston, the second ring surface thereof defining a second, blockable hydraulic chamber, and a piston effect in the second chamber corresponds to a force that acts counter to the direction of actuation on the holding piston.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. national phase application of PCT International Application No. PCT/EP2009/059124, filed Jul. 16, 2009, which claims priority to German Patent Application No. DE 10 2008 033 785.4, filed Jul. 18, 2008, and German Patent Application No. DE 10 2009 033 499.8, filed Jul. 15, 2009, the contents of such applications being incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a brake system for motor vehicles. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a brake system for motor vehicles which can be actuated in a brake-by-wire operating mode both by the vehicle driver and independently of the vehicle driver, and is preferably operated in the brake-by-wire operating mode and can be operated in a fallback operating mode without brake boosting, in which fallback operating mode only operation by the vehicle driver is possible. Such a brake system is known from international patent application WO 2004/080772 A1, which is incorporated by reference. The elastic element of the simulator device is embodied as a compression spring which is arranged in terms of its effect between the input force element and the restraining piston. Consequently, the restraining piston has to be held in the (normal) brake-by-wire operating mode against a stop which is fixed to the housing in order to make available a stable abutment to the aforementioned compression spring. When rapid activation of the pedal occurs in the previously known brake system, there is the problem that the electrically controllable pressure supply device, which is embodied as a motor pump assembly, cannot build up the necessary restraining pressure with the same dynamic so that a temporary movement of the restraining piston away from its stop can take place. This movement results in a brake pedal sensation which is unpleasant for the vehicle driver. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is therefore to ensure improvement of the pedal sensation in a brake system of the generic type mentioned at the beginning, in particular in brake-by-wire brake systems. In this context, the temporary dipping of the restraining piston when dynamic activation occurs is to be largely avoided. 
     This object is achieved in a brake system according to aspects of the present invention in that the restraining piston is embodied as a stepped piston whose annular face bounds a hydraulic space which can be shut off, wherein a pressure effect in the space corresponds to a force which acts on the restraining piston counter to the activation direction. 
     The hydraulic space can preferably be connected to the pressure medium reservoir vessel via a check valve. 
     The electrically controllable pressure supply device can preferably be connected directly to one of the master brake cylinder pressure spaces. A 2/2 way valve which is closed when no current is flowing (CC) and which can be activated electromagnetically is particularly preferably inserted into the connection between the pressure supply device and the master brake cylinder pressure space. 
     The electrically controllable pressure supply device of the brake system according to aspects of the invention is preferably embodied as a hydraulic cylinder-piston arrangement whose piston can be driven by means of an electric motor. 
     According to one preferred embodiment of the brake system according to aspects of the invention, a high pressure accumulator is provided which can be charged by the electrically controllable pressure supply device. A check valve (also referred to as an accumulator charging valve) is advantageously inserted into the hydraulic connection between the high pressure accumulator and the electrically controllable pressure supply device. 
     A hydraulic brake pressure modulation unit, which permits brake-circuit-specific or wheel-selective braking processes, is preferably inserted between the master brake cylinder and the wheel brakes. 
     The brake pressure modulation unit advantageously has pressure regulating valves for carrying out a braking process according to what is referred to as the multiplex principle. 
     According to one development of the brake system according to aspects of the invention, all the 2/2 way valves which can be activated electromagnetically, the pressure sensors, the pressure supply device and a rotor position sensor which senses the rotor position of an electric motor which forms a component of the pressure supply device are integrated in a hydraulic valve block. 
     The valve block preferably contains the pedal decoupling unit with its housing and the simulator device and can be attached to a splash board of the motor vehicle. 
     It is likewise preferred that the sensor device for sensing the driver deceleration request be integrated into the valve block and connected to the electronic open-loop and closed-loop control unit without external cables. 
    
    
     
       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 first embodiment of a brake system according to aspects of the present invention, 
         FIG. 2  shows a second embodiment of a brake system according to aspects of the present invention, 
         FIG. 3  shows a simplified illustration of a brake system according to aspects of the invention which is modified compared to  FIG. 1  and is in the unactivated state, and 
         FIGS. 4 to 11  show various operating modes of the brake systems according to aspects of the invention which are illustrated in  FIGS. 1 and 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The brake system according to aspects of the invention which is illustrated in  FIG. 1  is composed essentially of an activation device  1 , a pedal decoupling unit  2 , an electrically controllable pressure supply device  3 , wherein the activation device  1  and the pressure supply device  3  form a brake booster, as well a master brake cylinder or tandem master cylinder  4  which is connected downstream of the brake booster in terms of effect and whose pressure spaces  14 ,  15  can be connected by means of known, so-called expansion holes (not illustrated), to which chambers, under atmospheric pressure, of a pressure medium reservoir vessel  11  can be connected. Furthermore, the aforementioned pressure spaces  14 ,  15  are connected to the pressure medium reservoir vessel  11  via 2/2 way valves  41 ,  42  which are preferably open when no current is flowing (OC) and can be activated electromagnetically, and their function will be explained later. On the other hand, wheel brake circuits I, II, which supply the wheel brakes  5 - 8  of a motor vehicle with hydraulic pressure medium with intermediate connection of a hydraulic assembly or of a controllable wheel brake pressure modulation module  9 , are connected to the pressure spaces  14 ,  15 . Furthermore, the wheel brake circuits I, II can be connected by means of a hydraulic connection in which a 2/2 way valve  43  which is preferably closed when no current is flowing (CC) and can be activated electromagnetically is inserted, the function of which 2/2 way valve  43  will also be explained later. 
     The wheel brake pressure modulation module  9  has pressure modulation valves (not denoted in more detail) which are connected upstream of the wheel brakes  5 - 8  and which permit, for example, ABS braking processes and/or ESP braking processes. In this context, eight pressure modulation valves are necessary for carrying out the ABS braking operations and twelve pressure modulation valves are necessary for carrying out the ESP braking operations. Of course, as an alternative to the wheel brake pressure modulation module illustrated, it is also possible to use a significantly simpler wheel brake pressure modulation module with four pressure modulation valves, which permits wheel brake pressure modulation according to what is referred to as a multiplex principle, which is known to a person skilled in the art (see in particular  FIG. 3 ). Furthermore, the brake system which is shown has an electronic open-loop and closed-loop control unit  10  which is merely indicated schematically. The activation device  1 , which is arranged in a housing  20  in which the tandem master cylinder  4  is also integrated, can be actuated by means of a brake pedal  12  which is connected in effective terms to an input force element  16  via an activation rod  13 . The activation travel of the brake pedal  12  is sensed indirectly using the rotational angle of the brake pedal  12  by means of a sensor device  17 . The previously mentioned pedal decoupling unit  2  has essentially a restraining piston  18  in the interior of which the input force element  16  is guided in a displaceable fashion and which is supported on a compression spring  19  which represents a simulation device which gives the driver of the vehicle a pleasant pedal sensation in the brake-by-wire operating mode. An end face, facing the master brake cylinder  4 , of the restraining piston  18 , which is embodied as a stepped piston in the example shown, is embodied as a first annular face  26 , serves as a guide of an axial protrusion  21  of a primary piston  22  and bounds therewith a first hydraulic chamber  23  which is connected to the electrically controllable pressure supply device  3 , on the one hand, via a 2/2 way valve  24  which is preferably open when no current is flowing (OC) and can be activated electromagnetically, and, on the other hand, via a second 2/2 way valve  25  which is also open when no current is flowing (OC) and can also be activated electromagnetically. The axial protrusion  21  is in the state of rest shown in  FIG. 1 , at an axial distance from the input force element  16  which is denoted by d. 
       FIG. 1  also shows that a second annular face  27  of the restraining piston  18  bounds, in the housing  20 , a hydraulic second chamber  28  which can be shut off and which is connected to the pressure medium reservoir vessel  11  via a 2/2 way valve  29  which is open when no current is flowing (OC) and can be activated electromagnetically. The hydraulic pressure which is present in the second chamber  28  is sensed by means of a first pressure sensor which is provided with the reference sign  34 . Finally, a third annular face  30  of the restraining piston  18  in the housing  20  bounds a third hydraulic chamber  31  to which pressure supplied by the electrically controllable pressure supply device  3  can be applied. In this context, the ratio of the third annular face  30  to the first annular face  26  of the restraining piston  18  constitutes a pressure transmission ratio which makes it possible to generate a higher pressure in the master brake cylinder  4  at a given value of the pressure supplied by the electrically controllable pressure supply device  3 . Finally, a line  32  which can be shut off by means of a fourth 2/2 way valve  33  which can be activated electromagnetically connects the outlet of the electrically controllable pressure supply device  3  to the first or primary pressure space  14  of the master brake cylinder  4 . In this context, the pressure which is supplied by the pressure supply device  3  is sensed by means of a second pressure sensor  35 , while the value of the hydraulic pressure which is applied in the primary pressure space  14  can be determined by a third pressure sensor  36 . 
     Finally, from  FIG. 1  it is apparent that the electrically controllable pressure supply device  3  is embodied as an electrohydraulic actuator which is composed essentially of a hydraulic cylinder-piston arrangement  37  and an electric motor  39  which drives the piston  38  of the aforementioned arrangement  37 , preferably with intermediate connection of a so-called Rot-Trans transmission (not illustrated). A travel sensor or position sensor  42  which is indicated only schematically senses the activation travel of the piston  38  and/or the angular position of the rotor (not illustrated) of the electric motor  39 . 
     The design of the second variant (illustrated in  FIG. 2 ) of the brake system according to aspects of the invention corresponds largely to the first embodiment shown in  FIG. 1 , wherein the same reference signs are used for the same components. In order to increase the dynamics of the brake system according to aspects of the invention shown in  FIG. 1 , in particular during pressure buildup processes, a high pressure accumulator  44  is provided which can preferably be charged by the electrically controllable pressure supply device  3 , described in conjunction with  FIG. 1 , with intermediate connection of a check valve or accumulator charging valve  45 . 
     The brake systems shown in  FIGS. 1 and 2  are each illustrated in the unactivated state. The illustrated state of the individual components of the brake systems according to aspects of the invention, except in the positions of the brake pedal  12 , input force elements  16 , the length of the simulator spring  19  and the switched state of the valves  24  and  29 , corresponds simultaneously to a brake operating mode in a hybrid vehicle in which the braking which is necessary in accordance with the brake pedal position is carried out by means of a generator operating mode of the drive motor (recuperative braking) which is present in the vehicle, and there is no need for a hydraulic buildup of brake pressure. 
     In the embodiment which is shown in  FIG. 3  and is simplified compared to the brake system illustrated in  FIG. 1 , a wheel brake pressure modulation module  9   a  which is indicated only schematically is integrated into the previously mentioned housing  20  and is composed essentially of four pressure modulation valves  46 ,  47 ,  48 ,  49 , which are individually assigned to the wheel brakes  5 ,  6 ,  7 ,  8  and are suitable for carrying out pressure regulating processes according to the so-called multiplex principle. A multiplex operating mode in brake systems is known to a person skilled in the art and does not need to be explained in more detail in the present context. 
     In the normal braking mode illustrated in  FIG. 4 , which corresponds to the so-called brake-by-wire operating mode, the 2/2 way valves  24 ,  29  which are open when no current is flowing and have been mentioned in conjunction with  FIGS. 1 and 2  are energized when there is a signal from the brake pedal travel sensor  17 , with the result that the hydraulic connections of the first chamber  23  and those of the second chamber  28  to the pressure medium reservoir vessel  11  are shut off. Accordingly, when there is subsequent activation of the electrically controllable pressure supply device  3  or  37 , no movement of the restraining piston  18  is possible. Pressure medium volume which is expelled by the pressure supply device  3  or  37  flows via the (OC) 2/2 way valve  25  (see  FIG. 1 ) which is not shown in  FIG. 4  into the first hydraulic chamber  23  and brings about a movement of the master brake cylinder piston  22  to the left, and therefore an increase in pressure in the master brake cylinder pressure spaces  14 ,  15  and in the motor vehicle wheel brakes  5 - 8  (not illustrated in  FIG. 4 ). In this context, the values of the pressure which is applied in the first chamber  23 , the second chamber  28  and in the primary pressure space  14  of the master brake cylinder  4  are sensed by the pressure sensors  35 ,  34  and  36  and signaled to the electronic open-loop and closed-loop control unit  10 . During the activation of the brake pedal, the simulator spring  19  is compressed, as a result of which a pleasant pedal sensation is conveyed to the vehicle driver. An operating mode (not illustrated) is also conceivable in which the first hydraulic chamber  23  is shut off and the connection of the second chamber  28  to the pressure medium reservoir vessel  11  is opened and the pressure supplied by the pressure supply device  3  is applied to the third chamber  31  so that a pressure increase occurs in the master brake cylinder  4 . In this operating mode, an additional pressure medium volume for the primary pressure space  14  can also be made available from the region of the pressure supply device  3  (without a boosting factor). 
     A so-called hybrid operating mode in which the motor vehicle is braked exclusively by the (electric) drive motor and in which no hydraulic buildup of brake pressure is necessary is illustrated in  FIG. 5 . If a signal appears at the output of the travel sensor or rotational angle sensor  17  which senses the driver&#39;s deceleration request, the 2/2 way valves  24 ,  29  (mentioned above) which are open when no current is flowing (OC) and are electromagnetically switchable are switched over into their closed position, as a result of which the hydraulic connections of the two hydraulic chambers  23 ,  28  to the pressure medium reservoir vessel  11  are shut off. In this context, the restraining piston  18  which is in its blocked position constitutes a fixed support of the simulator spring  19  which is correspondingly compressed by the braking force applied at the brake pedal  12  (the distance d becomes smaller). 
     A highly dynamic braking process is illustrated in  FIG. 6 . 
     When a signal appears at the outlet of the travel sensor or rotational angle sensor  17  which senses the driver&#39;s deceleration request, which signal represents rapid activation of the brake pedal  12 , the 2/2 way valves  24 ,  29  (OC) are switched over into their closed position for the purpose of shutting off the two hydraulic chambers  23 ,  28 . At the same time as the activation of the electrically controllable pressure supply device  3 , the accumulator charging valve or check valve  45  which is mentioned in conjunction with  FIG. 2  is switched over into its open switched position so that, in addition to a quantity of pressure medium which is made available by the pressure supply device  3 , the pressure medium which is under high pressure in the high pressure accumulator  44  is applied to the first hydraulic chamber  23 . The described process results in a rapid buildup of pressure in the first chamber  23  and therefore in the master brake cylinder  4 . The pressure value which is applied to the first chamber  23  is sensed by the pressure sensor  35 , while the pressure prevailing in the first pressure space  14  is determined by the second pressure sensor  36 . 
     An autonomous buildup of pressure for assistance functions in which all the hydraulic components assume the switched positions described in conjunction with  FIG. 4  is illustrated in  FIG. 7 . The electrically controllable pressure supply device  3  is actuated by actuating signals of the electronic open-loop and closed-loop control unit  10  in accordance with the requests by an assistance system present in the motor vehicle, for example an electronic adaptive cruise control system, without activation of the brake pedal  12  by the driver. 
       FIGS. 8 and 9  each show a state of the brake system according to aspects of the invention which is due to a lack of a power supply and which is referred to as a fallback level. In the state shown in  FIG. 8 , the activation force which is applied to the brake pedal  12  by the driver is transmitted to the first piston  22  or primary piston  22  of the master brake cylinder  4  via the input force element  16 , the compressed simulator spring  19  and the restraining piston  18 . The activation travel carried out by the brake pedal  12  corresponds to the sum of a first travel component, which is due to the pressure medium volume takeup by the brake system connected downstream of the master brake cylinder  4 , and of a travel component which corresponds to the compression of a simulator spring  19 . When a relatively large brake pedal force is acting and results in relatively strong activation of the activation device  1  (see  FIG. 9 ), the further brake pedal travel corresponds to the state (shown in  FIG. 8 ) of the pressure medium volume takeup of the brake system which is connected downstream of the master brake cylinder  4 , since the activation of the master brake cylinder  4  takes place by means of direct mechanical transmission of the input force from the input force element  16  to the axial protrusion  21  of the master cylinder piston  22 . 
     The type of actuation of the brake system according to aspects of the invention which is shown in  FIG. 10  is appropriate in particular when self-resetting brake calipers are used which have no or little residual braking torque. In this context, the 2/2 way valves  25 ,  41 ,  42  which are open when no current is flowing (OC) and are mentioned in conjunction with  FIG. 1  are switched over into their closed switched position and the 2/2 way valve  43  which is closed when no current is flowing (CC) and is switched between the master brake cylinder pressure spaces  14 ,  15  as well as the 2/2 way valve  33  which is closed when no current is flowing (CC) and is inserted between the electrically controllable pressure supply device  3  and the primary pressure space  14  of the master brake cylinder  4  are switched over into their open switched position. Given subsequent actuation of the pressure supply device  3  in the direction opposite to the activation direction, a defined quantity of pressure medium volume is sucked out of the wheel brakes  5 - 8  (not illustrated in  FIG. 10 ), so that the resulting underpressure permits the brake linings to be pulled back and therefore allows the brake disks to freewheel. After this, all the abovementioned 2/2 way valves are switched back into their unactivated position. 
     Finally,  FIG. 11  shows an operating mode in which the high pressure accumulator  44  is charged by the pressure supply device  3 . In this context, both brake circuits I, II are shut off by isolating valves contained in the wheel pressure modulation module  9  which is only indicated schematically and the first and the second hydraulic chambers  23 ,  28  are shut off and the accumulator charging valve  45  is opened, so that the pressure medium volume which is expelled by the pressure supply device  3  charges the high pressure accumulator  44 . The pressure made available by the pressure supply device  3  is monitored by the pressure sensor  35 . 
     Within the scope of the present invention, further modifications are of course conceivable. It is therefore possible, for example in order to check the function of the fallback level, to move the restraining piston by actuating the pressure supply device  3 . Another conceivable measure relates to the configuration of the distance “d” which is virtually zero in this variant, and in the case of regenerative braking by means of the electric motor which operates in the generator mode and drives a hybrid vehicle the 2/2 way valve  33  which is closed when no current is flowing (CC) is opened in order to suppress a hydraulic buildup of pressure, wherein the pressure medium volume, displaced by the driver by means of the axial protrusion  21 , of the primary pressure space  14  is expelled into the first hydraulic chamber  23 .