Abstract:
A brake system comprises a first electrohydraulic open-loop and closed-loop control unit. The first electrohydraulic control unit comprises a master brake cylinder actuatable by a brake pedal; a first electrically controllable pressure-providing device; and an electrically controllable pressure-modulating device sets wheel-specific brake pressures for the wheel brakes. The electrically controllable pressure-modulating device has at least one electrically actuatable inlet valve for each wheel brake. A first pressure-medium reservoir for supplying the first electrohydraulic control unit with pressure medium is arranged on the first electrohydraulic control unit. The brake system also comprises a second electrohydraulic open-loop and closed-loop control unit, which comprises a second electrically controllable pressure-providing device for actuating at least some of the wheel brakes and electrically actuatable valves. A second pressure-medium reservoir for supplying the second electrohydraulic control unit with pressure medium is provided, the second pressure-medium reservoir being arranged on the second electrohydraulic control unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of PCT International application No. PCT/EP2015/078947, filed Dec. 8, 2015, which claims the benefit of German patent application No. 10 2014 225 958.4, filed Dec. 16, 2014, each of which are hereby incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention is related to a hydraulic brake system for a motor vehicle. 
       BACKGROUND 
       [0003]    Hydraulic vehicle braking systems can be implemented as external vehicle braking systems and in addition to a master brake cylinder actuated by muscle power, to which the wheel brakes are hydraulically connected and which provides the pressure and volume for operating wheel brakes. An electrically controlled pressure and volume supply device actuates the wheel brakes in a “brake-by-wire” operating mode. In the event of failure of the electrically controlled pressure and volume supply device, operation of the wheel brakes is carried out solely by the muscle power of the driver of the vehicle. 
         [0004]    It is to be expected that in the future vehicle braking systems will also come into use that are suitable for automatically driving vehicles. Said braking systems could be in principle be externally actuated systems or “brake-by-wire” systems. This means that a braking demand is made by means of electronic or electrical control signals and can be implemented by the system without the aid of the driver. In this case, for safety reasons sufficiently high availability of the braking system or the externally actuated braking function is desired. 
         [0005]    In DE 10 2012 205 861 A1, a “brake-by-wire” braking system is described, which in addition to a brake pedal-operated master brake cylinder comprises a first and a second pressure supply device. The second pressure supply device is arranged to increase the pressure in each of the brake circuits for supplying the wheel brakes compared to the pressure supplied by the master brake cylinder. The driver of the vehicle can thus comfortably achieve an adequate service brake deceleration despite a failure of the first pressure supply device. 
         [0006]    The master brake cylinder, the first pressure supply device, the isolating valves, the supply valves and the simulator with the simulator release valve are disposed in the first module with the first electronic control and regulating unit, and the second pressure supply device and the inlet and outlet valves are disposed in the second module with the second electronic control and regulating unit. Just one pressure medium reservoir is provided, which is disposed on the first module. 
         [0007]    The suction sides of the pumps of the second pressure supply device of the second module are likewise hydraulically connected to said pressure medium reservoir. In the event of a small leak in the hydraulic connection between the second module and the pressure medium reservoir disposed on the first module, the leak could not be detected rapidly enough by the container warning device of the pressure medium reservoir, so that the pump of the second pressure supply device sucks air in via the small leak and thus contaminates the two brake circuits with air. 
         [0008]    Furthermore, the pressure ports of the second pressure supply device are connected to the input-side connections of the inlet valves of the wheel brakes, so that the inlet valves are disposed between the respective pressure port of the second pressure supply device and the wheel brake. Because of the throttling effect of the inlet valves, obstruction of the flow of pressure medium that is output by the second pressure supply to the wheel brake can occur. 
         [0009]    The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
       SUMMARY OF THE INVENTION 
       [0010]    It is the object of the present invention to provide a braking system for a motor vehicle that high reliability in relation to the availability of the braking system. In particular, the safety requirements of highly automated driving or of autonomous driving will be met by the braking system. 
         [0011]    The braking system has a first electrohydraulic control and regulating unit with a first electrically controlled pressure supply device and a first pressure medium reservoir disposed on the first electrohydraulic control and regulating unit for supplying the first electrohydraulic control and regulating unit, and a second electrohydraulic control and regulating unit with a second electrically controlled pressure supply device and a second pressure medium reservoir disposed on the second electrohydraulic control and regulating unit for supplying the second electrohydraulic control and regulating unit. 
         [0012]    Thus, the availability of externally actuated or electrically controlled braking is increased. In the case of a leak in the region of a suction line of one of the pressure supply devices, braking can continue to be carried out by means of the other pressure supply device, without the brake circuits being contaminated with air. 
         [0013]    According to one development, the first pressure medium reservoir and the second pressure medium reservoir each comprise a level detector or a level alarm. This enables the first electrohydraulic control and regulating unit and the second electrohydraulic control and regulating unit to be monitored separately from each other for a loss of pressure medium. If a loss of pressure medium in one of the electrohydraulic control and regulating units is detected, then only said electrohydraulic control and regulating unit can be deactivated. 
         [0014]    The second pressure medium reservoir may be designed to act as an air separator. Thus, even if there is a leak in the region of a suction line of the second electrohydraulic control and regulating unit, suction of air and contamination of the brake circuits is initially prevented. 
         [0015]    The first electrohydraulic control and regulating unit preferably has a first electronic control and regulating unit and a first hydraulic control and regulating unit. 
         [0016]    The second electrohydraulic control and regulating unit preferably has a second electronic control and regulating unit and a second hydraulic control and regulating unit. 
         [0017]    The braking system can preferably be actuated in a “brake-by-wire” operating mode both by the driver of the vehicle and also independently of the driver of the vehicle, in the context of an autonomous driving function. The braking system is preferably usually operated in the “brake-by-wire” operating mode and can be operated in at least one fallback mode. 
         [0018]    A first electrical power supply unit and a second electrical power supply unit, which is independent of the first power supply unit, are provided. The first electrohydraulic control and regulating unit is supplied with electrical energy by the first power supply unit and the second electrohydraulic control and regulating unit is supplied with electrical energy by the second power supply unit. In the event of a failure of one of the power supply units, electrically controlled braking can continue to be carried out. 
         [0019]    In one embodiment, the first pressure supply device is implemented for operating the wheel brakes of the braking system, whereas the second pressure supply device is only implemented for operating some of the wheel brakes. 
         [0020]    The first pressure medium reservoir and the second pressure medium reservoir can be structurally separate. 
         [0021]    The first electrohydraulic control and regulating unit and the second electrohydraulic control and regulating unit may be disposed spaced apart in the motor vehicle. 
         [0022]    According to another embodiment of the braking system, the second pressure supply device comprises at least two hydraulic pumps, both commonly driven by an electric motor and each with an intake port and a pressure port, wherein each of the pressure ports is connected to one of the wheel brakes without the interposition of a valve. This means that the pressure port is connected to the wheel brake directly, i.e. without the interposition of a valve or via a further hydraulic component. Throttling effects are avoided by this. Each of the pressure ports of the pumps may be connected to just one wheel brake. For example, the wheel brakes are the wheel brakes of the front wheels. 
         [0023]    The pressure ports of the pumps may be connected to a connecting line between the wheel brake and the output-side port of the associated inlet valve. 
         [0024]    The pressure ports of the pumps may be connected to the wheel brakes of different master brake cylinder brake circuits. 
         [0025]    The second pressure medium reservoir preferably comprises a hydraulic port, wherein the intake ports of the second pressure supply device are connected to the port. 
         [0026]    Alternatively, the second pressure medium reservoir comprises a first hydraulic port and a second hydraulic port, wherein the first port is connected to the intake port of one pump and the second port is connected to the intake port of the other pump. 
         [0027]    The intake ports of the second pressure supply device are preferably connected to the port via respective non-return valves opening towards the intake port. 
         [0028]    According to another development, a third pressure medium reservoir, at atmospheric pressure, has a filling opening, and is provided for supplying the first and the second pressure medium reservoirs with pressure medium and is connected to the first and the second pressure medium reservoirs. 
         [0029]    Alternatively, the first pressure medium reservoir comprises a filling opening and is connected to the second pressure medium reservoir for supplying pressure medium to the second pressure medium reservoir. 
         [0030]    The braking system has for each pressure chamber of the master brake cylinder an electrically operated, normally open, isolating valve for the hydraulic connection or isolation of the master brake cylinder to or from the wheel brakes associated with the pressure chamber. The isolating valve is in each case disposed in a hydraulic connecting line between the pressure chamber of the master brake cylinder and a section of brake circuit supplying the associated inlet valves with pressure, and thus enables the selective closing or opening of the hydraulic connection between the pressure chamber and the section of brake circuit. 
         [0031]    For each pressure chamber of the master brake cylinder an electrically operated, normally closed, supply valve for the hydraulic connection or isolation of the first pressure supply device to or from the wheel brakes is provided. The supply valve is disposed in each case in a hydraulic connecting line between the first pressure supply device and the section of brake circuit and thus enables the selective opening or closing of the hydraulic connection between the first pressure supply device and the section of brake circuit. 
         [0032]    The braking system has a simulator, which provides the driver of the vehicle with a pleasant brake pedal feel in the “brake-by-wire” operating mode. The simulator can be hydraulically connected to at least one pressure chamber of the master brake cylinder by means of an electrically or mechanically operated simulator release valve. 
         [0033]    A first electronic control and regulating unit is implemented for actuating the first electrically controlled pressure supply device in the sensor of regulating or controlling the hydraulic pressure that is output by said pressure supply device. Furthermore, the isolating valves and supply valves, the simulator release valve and the pressure modulator of the braking system are regulated or controlled by the first electronic control and regulating unit. 
         [0034]    The pressure modulator has an electrically operated inlet valve and an electrically operated outlet valve for adjusting wheel-specific brake pressures for each wheel brake. 
         [0035]    The first electrically controlled pressure supply device is formed by a cylinder-piston arrangement, the piston of which can be operated by an electromechanical actuator. Such an electromechanical actuator operates dynamically and very quietly and tolerates the number of load changes necessary for braking systems without problems. 
         [0036]    Other objects, features and characteristics of the present invention, as well as the methods of operation and the functions of the related elements of the structure, the combination of parts and economics of manufacture will become more apparent upon consideration of the following detailed description and appended claims with reference to the accompanying drawings, all of which form a part of this specification. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the disclosure, are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]    Further embodiments of the invention are revealed by the subordinate claims and the following description using figures wherein: 
           [0038]      FIG. 1  shows schematically a first exemplary embodiment of a braking system according to the invention; and 
           [0039]      FIG. 2  shows schematically a second exemplary embodiment of a braking system according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    In  FIG. 1 , a first exemplary embodiment of a braking system is represented schematically. The braking system essentially comprises a first electrohydraulic control and regulating unit  60  and a second electrohydraulic control and regulating unit  160 . Said units are implemented as separate units or standalone assemblies. 
         [0041]    The first electrohydraulic control and regulating unit  60  comprises a first electronic control and regulating unit  60   a  (ECU1) and a first hydraulic control and regulating unit  60   b  (HCU1). Essentially, a master brake cylinder  2  that can be operated by means of a brake pedal  1 , a simulator  3  that works in conjunction with the master brake cylinder  2 , a first electrically controlled pressure supply device  5  and an electrically controlled pressure modulator  61  for setting wheel-specific brake pressures are disposed in the first hydraulic control and regulating unit  60   b.    
         [0042]    A first pressure medium reservoir  4  at atmospheric pressure for supplying the first electrohydraulic control and regulating unit  60  with pressure medium is disposed on the first electrohydraulic control and regulating unit  60 . 
         [0043]    The second electrohydraulic control and regulating unit  160  comprises a second electronic control and regulating unit  160   a  (ECU2) and a second hydraulic control and regulating unit  160   b  (HCU2). Essentially, a second electrically controlled pressure supply device  105  and electrically operated valves  145   a,    145   b,    146   a,    146   b,    147   a,    147   b  are disposed in the second hydraulic control and regulating unit  160   b.    
         [0044]    A second pressure medium reservoir  4  at atmospheric pressure for supplying the second electrohydraulic control and regulating unit  160  with pressure medium is disposed on the second electrohydraulic control and regulating unit  160 . 
         [0045]    For example, a third pressure medium reservoir  304  at atmospheric pressure is provided, which is connected to the first pressure medium reservoir  4  and the second pressure medium reservoir  104  and supplies said reservoirs with pressure medium as a type of primary container. The third pressure medium reservoir  304  comprises a filling opening for filling the braking system with pressure medium during maintenance processes. 
         [0046]    Alternatively, the first pressure medium reservoir that is disposed on the first electrohydraulic control and regulating unit  60  can also be implemented as a primary container with a filling opening that supplies the second pressure medium reservoir  104  with pressure medium. 
         [0047]    A respective level detector  9 ,  109  or level alarm is provided for the first pressure medium reservoir  4  and for the second pressure medium reservoir  104 , by means of which in each case the level of pressure medium is detected or the level of pressure medium falling below a predetermined level of the respective pressure medium reservoir  4 ,  104  is detected. This enables the loss of pressure medium at the second electrohydraulic control and regulating unit  160  to be sensed separately from a loss of pressure medium at the first electrohydraulic control and regulating unit  60 . Conversely, the loss of pressure medium at the first control and regulating unit  60  can also be detected by the level detector  9  of the pressure medium reservoir  4 . Accordingly, the respective control and regulating units  60 ,  160  can thus sense such leakage defects separately from each other. In the case of a leak, the affected control and regulating unit can then be deactivated, whereas the other control and regulating unit continues to be ready to carry out braking. 
         [0048]    If only one pressure medium reservoir were to be provided for both control and regulating units  60 ,  160 , on detecting a leak at the pressure medium reservoir both control and regulating units  60 ,  160  would have to be deactivated. An externally actuated or electrically controlled build-up of brake pressure would then not be possible at all. 
         [0049]    The pressure medium reservoir  104  of the second control and regulating units  160  is implemented such that the pressure medium reservoir  104  acts as an air separator, for example owing to the geometry thereof. That is, that air present in the intake line can settle in the pressure medium reservoir  104  when the pumps  142  are sucking. However, the air does not pass further to the pumps  142 , at least not before the level detector  109  or level alarm indicates a low level. 
         [0050]    The pressure modulator  61  comprises at least one electrically operated inlet valve  6   a - 6   d  for each wheel brake. For example, for each wheel brake  8   a - 8   d  of a motor vehicle that is not shown the pressure modulator  61  comprises an inlet valve  6   a - 6   d  and an outlet valve  7   a - 7   d,  which are connected together hydraulically in pairs via center ports and are connected to the wheel brakes  8   a - 8   d.  The input ports of the inlet valves  6   a - 6   d  are supplied via brake circuit sections  13   a,    13   b  with pressures that are derived from a system pressure in a “brake-by-wire” operating mode, said system pressure being present in a system pressure line  38  that is connected to a pressure chamber  37  of the first electrically controlled pressure supply device  5 . 
         [0051]    A respective non-return valve that opens towards the brake circuit sections  13   a,    13   b  is connected in parallel with each of the inlet valves  6   a - 6   d  and is not specified in detail. In a fallback mode, the brake circuit sections  13   a,    13   b  can be supplied with a pressure of the master brake cylinder  2  via hydraulic lines  22   a,    22   b.  The output ports of the outlet valves  7   a - 7   d  are connected to the first pressure medium reservoir  4  via a return line  14   a.  For detecting the pressure prevailing in the system pressure line  38 , a redundantly implemented pressure sensor  19  is provided. For example, the wheel brakes  8   a  or  8   b  are associated with the left front wheel FL or the right rear wheel RR and the wheel brakes  8   c  or  8   d  are associated with the right front wheel FR or the left rear wheel RL (so-called diagonal split). 
         [0052]    The master brake cylinder  2  has two pistons  15 ,  16  that are disposed one after the other in a housing  21  and that bound pressure chambers  17 ,  18 . On the one hand, the pressure chambers  17 ,  18  are connected to the first pressure medium reservoir  4  via radial bores formed in the pistons  15 ,  16  and suitable pressure equalization lines  41   a,    41   b  (the pressure equalization line  41   a  is shown partly combined with the return line  14   a,  but separate lines are also possible), wherein the connections can be shut off by a relative movement of the pistons  15 ,  16  in the housing  21 . 
         [0053]    On the other hand, the pressure chambers  17 ,  18  are connected to the aforementioned brake circuit sections  13   a ,  13   b  by means of the hydraulic lines  22   a,    22   b.  A normally open (SO—) diagnostic valve  28  is disposed in the pressure equalization line  41   a.  The pressure chambers  17 ,  18  accommodate restoring springs that are not specified in detail and that position the pistons  15 ,  16  in an initial position when the master brake cylinder  2  is not being actuated. A piston rod  24  couples the pivoting motion of the brake pedal  1  resulting from pedal actuation to the translational motion of the first (master cylinder) piston  15 , the operating displacement of which is detected by a preferably redundantly implemented displacement sensor  25 . As a result, the corresponding piston displacement signal represents a measure of the brake pedal actuation angle. It represents a braking demand of a driver of the vehicle. 
         [0054]    An isolating valve  23   a,    23   b,  by means of which the pressure chambers  17 ,  18  can be hydraulically isolated from the brake circuit sections  13   a,    13   b,  is disposed in each of the hydraulic connections  22   a,    22   b.    
         [0055]    The hydraulic connection between the master brake cylinder  2  and the wheel brakes  8   a - 8   d  can also be shut off by the isolating valves  23   a,    23   b.  The isolating valves  23   a,    23   b  are implemented as electrically operated, normally open (SO—) 2/2-way valves. A pressure sensor  20  connected to the line section  22   b  detects the pressure built up in the pressure chamber  18  by a displacement of the second piston  16 . 
         [0056]    The simulator  3  is hydraulically coupled to the master brake cylinder  2  and consists essentially of a simulator chamber  29 , a simulator spring chamber  30  and a simulator piston  31  isolating the two chambers  29 ,  30  from each other. The simulator piston  31  is supported on the housing  21  by an elastic element (for example a spring) that is disposed in the simulator spring chamber  30  and that is pretensioned. The simulator chamber  29  can be connected to the first pressure chamber  17  of the master brake cylinder  2 , for example by means of an electrically operated simulator release valve  32 . With a determined pedal force and the simulator release valve  32  activated, pressure medium flows from the master brake cylinder pressure chamber  17  into the simulator chamber  29 . A non-return valve  34  that is disposed hydraulically antiparallel to the simulator release valve  32  enables a substantially unhindered return flow of the pressure medium from the simulator chamber  29  to the master brake cylinder pressure chamber  17  independently of the switch state of the simulator release valve  32 . 
         [0057]    The simulator  3  imparts a pleasant brake pedal feel to the driver of the vehicle in the “brake-by-wire” operating mode. 
         [0058]    The first electrically controlled pressure supply device  5  is implemented as a hydraulic cylinder-piston arrangement or a single-circuit electrohydraulic actuator (linear actuator), the pistons  36  of which can be actuated by a schematically indicated electric motor  35  with the interposition of a likewise schematically represented rotations-translation gearbox. A rotor position sensor that is used for the detection of the rotor position of the electric motor  35  and that is only schematically indicated is denoted by the reference character  44 . 
         [0059]    In addition, a temperature sensor can also be used for detecting the temperature of the motor winding. The piston  36  bounds the pressure chamber  37 . The actuator pressure produced by the force action of the piston  36  on the pressure medium enclosed within the pressure chamber  37  is fed into the system pressure line  38  and detected by the system pressure sensor  19 . In the “brake-by-wire” operating mode, the system pressure line  38  is connected to the brake circuit sections  13   a,    13   b  via the supply valves  26   a,    26   b.  In this way, a build-up and reduction of brake pressure are carried out for all the wheel brakes  8   a - 8   d  during normal braking. 
         [0060]    During the build-up of brake pressure, the pressure medium previously displaced from the pressure chamber  37  of the actuator  5  into the wheel brakes  8   a - 8   d  flows back into the pressure chamber  37  of the actuator  5  via the same paths. By contrast, during braking with different wheel-specific wheel brake pressures that are regulated using the pressure modulation valves  6   a - 6   d,    7   a - 7   d,  the component of the pressure medium discharged via the outlet valves  7   a - 7   d  flows into the first pressure medium reservoir  4 . It is possible to suck pressure medium back into the pressure chamber  37  by returning the piston  36  with the supply valves  26   a,    26   b  closed, wherein pressure medium flows from the first reservoir  4  into the actuator pressure chamber  37  via a connecting line  46  with a suction valve  52  that is implemented as a non-return valve that opens in the flow direction to the actuator. 
         [0061]    The output ports of the inlet valves  6   a - 6   d  of the first electrohydraulic control and regulating unit  60  are each connected to just one of the wheel brakes  8   a - 8   d  via a respective hydraulic connection  51   a - 51   d.  In this case, the second electrohydraulic control and regulating unit  160  is for example hydraulically disposed between the first electrohydraulic control and regulating unit  60  and the wheel brakes  8   a  and  8   c.  The inlet valves  6   b,    6   d  are connected directly to the wheel brakes  8   b  and  8   d  via the hydraulic connections  51   b,    51   d,  which for example run outside the unit  160 . The second electrohydraulic control and regulating unit  160  is implemented for applying pressure or setting pressure at the wheel brakes  8   a  and  8   c.  In the case of a failure of the first pressure supply device  5 , pressure medium volumes can be made available for the wheel brakes  8   a  and  8   c  by activating the second pressure supply device  105 . 
         [0062]    It is also possible that the second electrohydraulic control and regulating unit  160  is implemented for applying pressure or setting pressure at all wheel brakes  8   a,    8   b,    8   c  and  8   d.    
         [0063]    For each wheel brake respective normally open, analogue-activated control valves  145   a,    145   b  with a parallel connected non-return valve that opens towards the wheel brake are disposed in the connections  51   a  and  51   c,  each of which connects the wheel brake  8   a  or  8   c  to the output-side port of the inlet valve  6   a  or  6   c  associated therewith or to the input-side port of the outlet valve  7   a  or  7   c  associated therewith. 
         [0064]    For building up wheel brake pressure, furthermore an electrically operated, normally closed, discharge valve  147   a,    147   b  is provided for each wheel brake  8   a  and  8   c,  by means of which the wheel brake can be connected to the second pressure medium reservoir  104 . 
         [0065]    The second electrohydraulic control and regulating unit  160  comprises for example two pumps  142  as the second electrically controlled pressure supply device  105 , which are commonly driven by an electric motor that is not shown. The second pressure medium reservoir  104  comprises a hydraulic port  114  to which the intake ports of the pumps  142  are connected, wherein a respective non-return valve  148   a,    148   b  that opens towards the intake port is disposed in a line section  150   a,    150   a  between the intake port and the port  114 . The intake ports of the pumps  142  can furthermore be connected to the associated connection section of the connection  51   a  or  51   c  between the inlet valve  6   a  or  6   c  and the control valve  145   a  or  145   b  via respective electrically operated, advantageously normally closed, pressure medium supply valves  146   a,    146   b.  The pressure port of one pump  142  is connected to the wheel brake  8   a  and the pressure port of the other pump  142  is connected to the wheel brake  8   c,  in each case without the interposition of a valve. 
         [0066]    The pressure supply device  105  thus sucks pressure medium from the second pressure medium reservoir  104  and supplies the pressure medium that it outputs into the wheel brakes  8   a  and  8   c  directly in each case (without interposed valves). 
         [0067]    The pressure supply function of the second electrohydraulic control and regulating unit  160  operates by means of the valves  145   a  and  145   b,  each of which regulates the pressure of the pressure medium volume supplied by the pumps  142  by suitable electronic actuation. Excess pressure medium volume that overflows to the valves  145   a,    145   b  can be directly made available to the corresponding pump  142  again by means of the valves  146   a,    146   b.    
         [0068]    According to an exemplary embodiment that is not shown of the braking system according to the invention, the valves  146   a,    146   b  and the connecting line thereof are not present. In said exemplary embodiment, the excess pressure medium volume flows to the pressure medium reservoir via the first control and regulating unit  60 . 
         [0069]    The second control and regulating unit  160  comprises for example a pressure sensor  120  for detection of the input pressure in the connection  51   a  and a respective pressure sensor  119  for detection of the wheel brake pressure in each wheel brake  8   a  and  8   c.    
         [0070]    The first pressure medium reservoir  4  is thus used for supplying the first electrohydraulic control and regulating unit  60 , in particular of the master brake cylinder  2 , and the first pressure supply device  5 . 
         [0071]    The second pressure medium reservoir  104  is used for supplying the second electrohydraulic control and regulating unit  160 , in particular the second pressure supply device  105 . 
         [0072]    The first electronic control and regulating unit  60   a  (ECU1) is used for actuating the pressure supply device  5 , the valves  23   a,    23   b,    26   a,    26   b,    32  for changing between the “brake-by-wire” operating mode and the fallback mode and the valves  6   a - 6   d,    7   a - 7   d  of the pressure modulator  61 . The signals of the displacement sensors  25 , of the sensor  44  and the pressure sensors  19  and  20  are fed to the control and regulating unit  60   a.    
         [0073]    The second electronic control and regulating unit  160   a  (ECU2) is used for actuating the pressure supply device  105  and the valves  142   a,    142   b,    146   a,    146   b,    147   a,    147   b.  The signals of the pressure sensors  119  and  120  are fed to the control and regulating unit  160   a.    
         [0074]    For performing the regulation task, signals are also exchanged via one or more buses, for example. Depending on the implementation, a direct connection between the first and the second electronic control and regulating units can also be provided. To safeguard the functions, the level display of the second level detector  9  is read in directly into the second electronic control and regulating unit  160   a.  Furthermore, depending on the implementation, wheel sensor signals are read in by the second electronic control and regulating unit  160   a.    
         [0075]    The signals of the level detector  9  are advantageously fed to the first electronic control and regulating unit  60   a  (ECU1) and the signals of the level detector  109  are fed to the second electronic control and regulating unit  160   a  (ECU2) and are processed there. 
         [0076]    A first electrical power supply unit  70  and a second electrical power supply unit  170 , which is independent of the first power supply unit, are provided. The first electrohydraulic control and regulating unit  60  is supplied with electrical energy by the first power supply unit  70 , whereas the second electrohydraulic control and regulating unit  160  is supplied with electrical energy by the second power supply unit  170 . 
         [0077]    In a normal operating mode of the braking system, the wheel brakes  8   a - 8   d  are supplied with brake pressure by means of the first pressure supply device  5  of the first electrohydraulic control and regulating unit  60 . In the event of a failure of the first pressure supply device  5  or the first electrohydraulic control and regulating unit  60 , the wheel brakes  8   a,    8   c  can be supplied with brake pressure by means of the second pressure supply device  105  of the second electrohydraulic control and regulating unit  160 . Thus, redundant, externally actuated or electrically controlled brake pressure generation is highly reliably provided, for example for an autonomous driving function. 
         [0078]    In  FIG. 2  a second exemplary embodiment of a braking system is schematically represented. The braking system essentially corresponds to the braking system of the first exemplary embodiment and essentially comprises a first electrohydraulic control and regulating unit  60 , on which a first pressure medium reservoir  4  is disposed, and a second electrohydraulic control and regulating unit  160 , on which a second pressure medium reservoir  204  is disposed. The control and regulating units  60 ,  160  are implemented as separate units or standalone assemblies. Whereas the second pressure medium reservoir  104  of the first exemplary embodiment of  FIG. 1  only comprises one hydraulic port  114 , to which both pump intake sides are connected, the pressure medium reservoir  104  thus being implemented with one circuit in this sense, the second pressure medium reservoir  204  of the second exemplary embodiment of  FIG. 2  is implemented with two circuits. The second pressure medium reservoir  204  comprises a first hydraulic port  214   a  and a second hydraulic port  214   b,  wherein each of the ports  214   a,    214   b  is connected to just one of the intake ports of the pumps  142 . The remaining components of the second exemplary embodiment correspond to those of the first exemplary embodiment. 
         [0079]    According to the second exemplary embodiment a respective level detector  9 ,  109  or level alarm is also provided for the first pressure medium reservoir  4  and for the second pressure medium reservoir  104 , by means of which in each case the level of pressure medium or a decrease of the level of pressure medium below a predetermined level of the respective pressure medium reservoir  4 ,  204  is detected. 
         [0080]    The pressure medium reservoir  204  of the second control and regulating unit  160  is implemented such that the pressure medium reservoir  204  acts as an air separator. 
         [0081]    The foregoing preferred embodiments have been shown and described for the purposes of illustrating the structural and functional principles of the present invention, as well as illustrating the methods of employing the preferred embodiments and are subject to change without departing from such principles. Therefore, this invention includes all modifications encompassed within the scope of the following claims.