Abstract:
A control device for controlling a brake system of a vehicle comprises a first output for controlling a first solenoid valve; a second output for controlling a valve device; an input for receiving a requested brake pressure; and a port for receiving messages on a databus. The control device is capable of receiving an automatic brake request on the databus; determining a desired brake pressure in response to the automatic brake request; receiving a requested brake pressure in response to a driver actuating a service brake valve; comparing the desired brake pressure to the requested brake pressure; transmitting a control signal to close the first solenoid valve and transmitting a control signal to change the valve device to a first operating position in response to the requested brake pressure being less than the desired brake pressure in order to provide pressure to at least one brake actuator.

Description:
RELATED APPLICATION INFORMATION 
       [0001]    The present application is a continuation of U.S. patent application Ser. No. 13/402,654 “Traction-Slip Controlled Brake System of a Motor Vehicle Approaching Stops” filed on Feb. 22, 2012, which claims priority to and the benefit of German patent application no. 10 2011 012 270.2, which was filed in Germany on Feb. 24, 2011, the disclosure of which is incorporated herein by reference. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention is directed to a traction-slip controlled brake system of a vehicle having a service-brake valve which generates an actuation-dependent pressure at at least one outlet port in response to an actuation, as well as having a valve device, which, in accordance with a first operating position, establishes a direct or indirect connection from a pressure medium reserve and, in accordance with a second operating position, from at least one 2/2 solenoid valve to at least one brake actuator; in accordance with the definition of the description set forth herein, the at least one 2/2 solenoid valve being connected, on the one hand, to the outlet port of the service-brake valve and, on the other hand, to a first connection of the valve device. 
       BACKGROUND INFORMATION 
       [0003]    Such a traction-slip controlled brake system is known from the German Patent Application DE 10 2009 016 982 A1. The known brake system includes a halt brake of the type found in today&#39;s electropneumatic brake systems in commercial vehicles, such as buses or garbage trucks that frequently approach stops (stopping places or station stops). Such a halt brake, which supplements the service brake device with a halt brake function, is activated, respectively applied at the stops. A manually operable switch is provided for this purpose, upon whose actuation, a constant pressure of a predefined magnitude is introduced into the service brake cylinders of at least one axle in order to brake the motor vehicle at the stops. In contrast to a normal service braking triggered via a pedal-operated brake valve, the brake pressure is, therefore, not controllable by the driver in the case of the halt brake function. 
         [0004]    To realize the halt brake, a pressure limiting device is connected within a pressure line that is branched off from a supply pressure line. Also connected therein is a switch-operable solenoid valve which controls the supply pressure that is limited to the constant halt pressure, through a shuttle valve which, in turn, is connected to a pneumatic channel of the service-brake valve in order to further control the greater pressure in the particular case. In addition, an ASR valve for regulating traction slip is provided in a downstream pressure line. This design is relatively complex, however. 
         [0005]    It is, therefore, an object of the exemplary embodiments and/or exemplary methods of the present invention to further improve a traction-slip controlled brake system of the type mentioned at the outset so as to provide it with a greater range of function in the context of a simpler design. 
       SUMMARY OF THE INVENTION 
       [0006]    This objective is achieved in accordance with the exemplary embodiments and/or exemplary methods of the present invention by the features described herein. 
         [0007]    Underlying the exemplary embodiments and/or exemplary methods of the present invention is the idea that at least one 2/2 solenoid valve and the valve device are controllable by a control device in a way that allows the 2/2 solenoid valve to be switched to the open position and the valve device to be switched to the first operating position in order to implement traction-slip control; and a brake pressure to be generated at a predetermined level in the at least one brake actuator on the basis of the pressure medium from the pressure medium reserve; and the 2/2 solenoid valve to be switched to the closed position and the valve device to the first operating position. 
         [0008]    “To implement traction-slip control” signifies the case where the drive slip exceeds a predefined drive slip limit upon acceleration of the vehicle, and the (unacceptably high) actual drive slip must, therefore, be adjusted to a setpoint drive slip. 
         [0009]    The “brake pressure at a predetermined level” refers to the halt brake pressure, for example, for the case where the halt brake-actuating element is actuated into a position where the halt brake is applied. Moreover, the “brake pressure at a predetermined level” could also refer to the brake pressure last generated by the driver in the context of a starting assist (hill holder) function to brake the vehicle that is to be held in the brake actuators for a certain period of time, in particular until the vehicle starts off once more. 
         [0010]    Thus, a very simple valve design, including, namely, one single 2/2 solenoid valve and the valve device, which may be constituted of one single 3/2 solenoid valve, may be used to realize a plurality of auxiliary functions for a brake system, such as traction-slip control or hill holder, as are customary in today&#39;s commercial vehicles. The result is an advantageously simple and, therefore, inexpensive brake system design. In particular, a halt brake realization no longer requires a pressure limiting device since the valve device, which is already used for traction-slip control, supplies the constant halt brake pressure from the compressed-air reserve. Thus, this valve device (for example, a 3/2 solenoid valve) advantageously performs a dual function. 
         [0011]    Advantageous further refinements of the exemplary embodiments and/or exemplary methods of the present invention delineated herein and improvements thereto are rendered possible by the measures further set forth herein. 
         [0012]    One exemplary embodiment provides for at least one ABS pressure control valve, which is controlled by the control device, to be interposed between the valve device and the at least one brake actuator. Such ABS pressure control valves are reasonably well known and are used for maintaining, building up and reducing pressure in the course of an anti-lock braking control. To implement traction-slip control in response to unacceptably high drive slip when a vehicle drives off, the at least one ABS pressure control valve may then be cyclically controlled in order to adjust the drive slip to a predefined drive slip on the basis of a pressure from the pressure medium reserve introduced by the valve device into the at least one ABS pressure control valve. In this case, the valve device is then used to switch a supply pressure that is still to be modulated by the ABS pressure control valves, to the ABS pressure control valves. 
         [0013]    As already mentioned above, the brake system may include a halt brake having a halt brake-actuating element, upon whose actuation by the driver, a predetermined halt brake pressure is applied to the brake actuators of at least one axle. The halt brake includes an electrical signal-modulating, manually actuated switching element, for example, which is actuated by the driver to effect halt braking. The at least one 2/2 solenoid valve is then switched to the closed position in response to the electrical halt brake signal from the control device. In addition, the control device cyclically switches the valve device between the first operating position and the second operating position in order to generate the predetermined halt brake pressure in the brake actuators of the at least one axle on the basis of the pressure of the pressure medium from the pressure medium reserve. The predetermined halt brake pressure may be generated in this manner in all brake actuators of the vehicle. The cyclical operation, respectively switching of the valve device is performed as a function of the predefined halt brake pressure to be achieved. 
         [0014]    However, the “brake pressure at a predetermined level” could also be a brake pressure that is predefined by a headway distance control or adaptive cruise control (ACC) to be able to maintain a preset distance to a vehicle in front. 
         [0015]    Last but not least, the “brake pressure at a predetermined level” could also be the brake pressure which was used to brake a vehicle in the course of a starting assist function implemented in the control device and which, without further actuation of the brake, is to be held for a certain period of time. For this starting-traction control, the 2/2 solenoid valve is switched to the closed position in order to interrupt the pressure line between the service-brake valve and the brake actuators, respectively “lock in” the pressure in the brake actuators of the at least one axle, thereby allowing the brake pressure most recently generated by the driver to be maintained. 
         [0016]    However, since a pressure loss from pneumatic systems can never be completely ruled out, it may be additionally provided in the context of the starting assist (hill holder) function for the control device to cyclically switch the valve device configured downstream of the 2/2 solenoid valve between the first operating position and the second operating position in order to at least generate the most recently introduced brake pressure in the brake actuators of the at least one axle on the basis of the pressure of the pressure medium from the pressure medium reserve, respectively to maintain the same there also over a longer period of time. If a pressure loss is ascertained or estimated from other quantities, the valve device may then reproduce, respectively maintain the most recently generated, driver-dependent brake pressure by cyclical switching, i.e., by the one-time or repeated switching between the first operating position and the second operating position. In this context, a higher brake pressure may also be generated by the cyclical switching of the valve device than, for example, by the most recent driver actuation-dependent brake pressure that led to the braking of the vehicle. The value of the previously generated, driver-dependent brake pressure may be estimated, respectively measured using a brake-pressure sensor, for example. 
         [0017]    For functions such as halt brake, starting-traction control or headway distance control, the control device drives the at least one above mentioned ABS pressure control valve to switch it to the open position. In other words, the ABS pressure control valve may have no influence on the brake pressure buildup in the mentioned cases and, for these purposes, may also be eliminated if the vehicle is not provided with any anti-lock braking control during braking. 
         [0018]    All of the examples described above relate to cases where a specific pressure level is to be adjusted, respectively built up. However, within the scope of functions such as a headway distance control, cases are also conceivable where brake pressure must be reduced. For example, in the context of a headway distance control, the situation may arise where a comparatively high brake pressure, as described above, initially adjusted to set the predefined minimum distance to a vehicle suddenly cutting into the lane, is subsequently no longer needed to keep a constant distance to the vehicle ahead on a downhill stretch, for example. The previously adjusted high brake pressure must then be reduced. 
         [0019]    To reduce pressure in the at least one brake actuator, the at least one 2/2 solenoid valve may then be cyclically controlled by the control device, and the valve device is switched to the second operating position. This enables too high brake pressure to be reduced to the desired lower level via the then through-connected valve device and the through-connected 2/2 solenoid valve by way of the service-brake valve, which typically has a pressure reduction or bleed feature. 
         [0020]    Via the at least one 2/2 solenoid valve and the valve device having merely two operating positions, brake pressures may then be built up, maintained, as well as reduced for additional brake-system functions, such as traction slip control, halt brake, starting assist (hill holder) or adaptive cruise control (ACC), while entailing a low degree of complexity. This enumeration is not conclusive. Rather, the exemplary embodiments and/or exemplary methods of the present invention may also be conceivably used for other purposes where building up, maintaining and/or reducing pressure are important. Between the outlet port of service-brake valve and the at least one brake actuator, one further embodiment provides for a bypass connection to bridge the 2/2 solenoid valve and the valve device and within which at least one nonreturn valve is connected in a way that allows it to close toward the outlet port of the service-brake valve and to open toward the at least one brake actuator. This inventive refinement makes it possible for the brake pressure level, that had been preset by the at least one 2/2 solenoid valve and the valve device in the context of the particular function, such as the starting assist, to be increased via the parallel branch of the bypass connection. Thus, when the driver actuates the pedal of the service-brake valve, the pressure prevailing in the at least one brake actuator may be increased via this parallel bypass connection and the thereby opening nonreturn valve. However, pressure may not be reduced via the bypass connection since the nonreturn valve closes toward the service-brake valve. 
         [0021]    To control greater volumes of air, a relay valve, which is connected to the valve device on the control side, may be interposed between the valve device and the at least one brake actuator. 
         [0022]    The valve device may be constituted of a 3/2 solenoid valve, the first connection being connected to the 2/2 solenoid valve, a second connection to the pressure medium reserve, and a third connection indirectly or directly to the at least one brake actuator. In its open position, the 2/2 solenoid valve establishes a connection between the first connection of 3/2 solenoid valve and the outlet port of the service-brake valve; in its closed position, this connection being blocked. 
         [0023]    In accordance with one further specific embodiment, the 2/2 solenoid valve and the valve device, respectively the 3/2 solenoid valve may be accommodated in a shared housing and be constituted of one single valve which then encompasses all switching functionalities of the 2/2 solenoid valve and of the valve device, respectively of the 3/2 solenoid valve. 
         [0024]    A more detailed description is provided in the following with reference to an exemplary embodiment. 
         [0025]    Exemplary embodiments of the present invention are illustrated in the drawing and are described in greater detail herein below. Various embodiments of an apparatus 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  shows a schematic circuit diagram of a pressure medium-actuated brake system having a valve unit in accordance with one exemplary embodiment of the present invention. 
           [0027]      FIG. 2  shows a schematic circuit diagram of the valve unit of  FIG. 1  in accordance with one exemplary embodiment, including a 2/2 solenoid valve that cooperates with a 3/2 solenoid valve. 
           [0028]      FIG. 3  shows a schematic circuit diagram of the valve unit of  FIG. 1  in accordance with one further specific embodiment, including a 2/2 solenoid valve that cooperates with a 3/2 solenoid valve, and also including a bypass having a nonreturn valve. 
           [0029]      FIG. 4  shows a schematic circuit diagram of the valve unit of  FIG. 1  in accordance with one further specific embodiment, including a 2/2 solenoid valve that cooperates with a 3/2 solenoid valve, and also including a relay valve. 
           [0030]      FIG. 5  shows a schematic circuit diagram of the valve unit of  FIG. 1  in accordance with one further specific embodiment, including a 2/2 solenoid valve that cooperates with a 3/2 solenoid valve, a bypass having a nonreturn valve, and also including a relay valve. 
           [0031]      FIG. 6  shows a schematic diagram of a control device of the brake system from  FIG. 1 . 
           [0032]      FIG. 7  shows a flow chart which represents a service braking. 
           [0033]      FIG. 8  shows a flow chart which represents a service braking including ABS activity. 
           [0034]      FIG. 9  shows a flow chart which represents an automatically triggered braking. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    The schematic circuit diagram shown in  FIG. 1  of a brake system  1  of a bus is limited to essential assemblies and elements of the present invention, respectively to components that cooperate therewith. It is assumed here that merely rear axle  2   a  of the bus is powered, front axle  2   b  being a nonpowered, but steerable and brakable front axle. 
         [0036]    With the exception of auxiliary functions, such as a halt brake and traction-slip control (ASR), brake system  1  of the exemplary embodiment is controlled and actuated purely pneumatically and also has ABS functionalities. Alternatively, brake system  1  may also be an electropneumatic braking device or also an electronically controlled brake system (EBS). 
         [0037]    Accordingly, brake system  1  has a two-channel service-brake valve  4 , for example, that is actuatable by a pedal  3  in response to the action of the driver&#39;s foot; a front-axle channel  6  of service-brake valve  4  being assigned to a front-axle service-brake circuit, and a rear-axle channel  8  being assigned to a rear-axle service-brake circuit. For this, an inlet of front-axle channel  6  of service-brake valve  4  is connected to a front-axle compressed-air reserve  10 , and an inlet of rear-axle channel  8  is connected to a rear-axle compressed-air reserve  12  via a supply line  14 ,  16 , respectively. 
         [0038]    On the other hand, an outlet of front-axle channel  6  of service-brake valve  4  is connected via a pressure line  18  and ABS pressure control valves  20  to brake actuators  22  of front axle  2   b , and an outlet of rear-axle channel  8  of service-brake valve  4  is connected via a pressure line  24 , a valve unit  26 , as well as downstream ABS pressure control valves  28  of each wheel, to brake actuators  30  of the rear axle. Valve unit  26  is connected via an additional supply line  32  to rear-axle compressed-air reserve  12 , for example. Valve unit  26  is connected within pressure line  24  of rear-axle service-brake circuit. 
         [0039]    Brake actuators  22 ,  30  of front axle  2   b  and of the rear axle are pneumatic, active service brake cylinders; at rear axle  2   a , they are combination cylinders which include passive spring-type cylinders. A traction slip control (ASR) is provided for powered rear axle  2   a  of the bus and is used to adjust an actual drive slip in a manner known per se to a setpoint drive slip. In addition, an ABS, which is controlled for each wheel, is provided for front axle  2   b , as well as for rear axle  2   a.    
         [0040]    ABS pressure control valves  20 ,  28 , which are designed as solenoid valves and are reasonably well known, and electromagnetic valve unit  26  may be controlled by an electronic control device  34 . In addition, wheel-speed sensors  36  are provided for each wheel in order to transmit wheel speed information to control device  34  at least in the context of ABS and ASR. The halt brake includes an electrical switch  38 , for example, which may transmit a halt brake signal to electronic control device  34  in order to apply or release the halt brake. 
         [0041]    Therefore, rear-axle service-brake valve encompasses rear-axle compressed-air reserve  12 , rear-axle channel  8  of service brake valve  4 , valve unit  26 , ABS pressure control valves  28  of the wheels of rear axle  2   a  and brake actuators  30 . Together with the components of front-axle service-brake circuit, such as front-axle compressed-air reserve  10 , front-axle channel  6  of service brake valve  4 , ABS pressure control valves  20  of the wheels of front axle  2   b , the components of rear-axle service-brake circuit, together with control device  34 , make up a service brake device of a service brake of the bus that is used for braking the vehicle that is underway. 
         [0042]    The halt brake circuit, respectively the halt brake which here, for example, cooperates with the rear-axle service-brake circuit, respectively overlaps with the same, includes switch  38  as halt brake-actuating element, and control device  34 , as well as valve unit  26 . 
         [0043]    In the case that brake device  1  is an electronically controlled brake system (EBS) having an electronic EBS control unit, electronic control device  34  may also be integrated in the EBS control unit. Electronic control device  34  is connected via control, respectively signal lines (drawn as dashed lines in  FIG. 1 ) to wheel speed sensors  36 , ABS pressure control valves  20 ,  28 , as well as to valve unit  26 . 
         [0044]    The exemplary embodiment of valve unit  26  shown in  FIG. 2  includes a 3/2 solenoid valve  40 , for example, whose first connection  42  is connected to a connection  44  of a 2/2 solenoid valve  46 ; whose second connection  48  is connected via supply line  32  to rear-axle pressure medium reserve  12 , and whose third connection  50  is connected via ABS pressure control valves  28  of rear axle  2   a  to brake actuators  30  of rear axle  2   a . Further connection  52  of 2/2 solenoid valve  46  is directly connected to pressure line  24  of rear-axle channel  8  of service brake valve  4 , for example. 
         [0045]    In its de-energized and spring-loaded open position shown exemplarily in  FIG. 2 , 2/2 solenoid valve  46  forms a connection between first connection  42  of 3/2 solenoid valve  40  and rear-axle channel  8  of service-brake valve  4 ; in its exemplarily energized closed position, this connection being blocked. In addition, in accordance with a first operating position, 3/2 solenoid valve  40  connects rear-axle pressure medium reserve  12  and, in accordance with a second operating position shown in  FIG. 2 , connects connection  44  of 2/2 solenoid valve  46  to ABS pressure control valves  28  of rear axle  2   a , and thus directly to brake actuators  30  of the rear axle. 
         [0046]    Valve  26  may form one unit having a shared housing for 2/2 solenoid valve  46 , 3/2 solenoid valve  40 , and for further components referred to in the specific embodiments described in the following, and is provided with corresponding connections. 
         [0047]    In this context, the operating principle of brake system  1  is explained in the following: Service-brake valve  4  is actuated by the driver for service braking operations; appropriate brake pressures being generated in front-axle channel  6 , respectively in rear-axle channel  8 , and being introduced into ABS pressure control valves  20 ,  28 . These allow the brake pressures to flow through if no unacceptable brake slips are occurring at the wheels. Otherwise, i.e., in the event that unacceptable brake slips are occurring, the brake pressure is reduced, maintained or built up by ABS pressure control valves  20 ,  28 . 
         [0048]    To implement traction-slip control when accelerating, i.e., when the drive slip exceeds a setpoint drive slip, 2/2 solenoid valve  46  is switched to the open position shown in FIGS. 2, and 3/2 solenoid valve  40  is switched to the first operating position in which rear-axle compressed-air reserve  12  is connected to ABS pressure control valves  28  of rear axle  2   a . On the basis of the introduced supply pressure, ABS pressure control valves  28  then regulate the brake pressure in brake actuators  30  of rear axle  2   a  for each wheel as a function of the system deviation. However, to regulate brake pressure by axle in the context of ASR, 2/2 solenoid valve  46  could also be driven by control device  34  to compensate for the system deviation, for example, by switching cyclically, back and forth between the open and closed position. 
         [0049]    When, in the context of the halt brake, a predetermined halt brake pressure is to be applied to brake actuators  30  of rear axle  2   a  by actuation of switch  38 , control device  34  then responds to the corresponding electric halt brake signal and induces 2/2 solenoid valve  46  to switch to the closed position. In addition, 3/2 solenoid valve  40  is cyclically switched by control device  34  between the first operating position and the second operating position in order to generate the predetermined halt brake pressure for brake actuators  30  of rear axle  2   a  on the basis of the pressure of the pressure medium from rear-axle pressure medium reserve  12 . The cyclical operation, respectively switching of 3/2 solenoid valve  40  is carried out as a function of the predefined halt brake pressure to be achieved. 
         [0050]    However, a valve unit  26  of the type described could be additionally provided at front axle  2   b . Such a valve unit  34  would then be supplied with compressed air from front-axle compressed-air reserve  10 , for example, and would be connected within pressure line  18 . The brake pressure at a predetermined level adjusted by valve units  34  at front axle  2   b  and at rear axle  2   a  could then be a brake pressure that is predefined by a headway distance control or adaptive cruise control (ACC) to be able to maintain a preset distance to a vehicle in front. 
         [0051]    Last but not least, the “brake pressure at a predetermined level” could also be the brake pressure that was used to brake a vehicle in the course of a starting assist function implemented in control device  34  and that is to be maintained for a certain period of time, without further actuation of the brake. To implement starting-traction control, 2/2 solenoid valve  46  is switched to the closed position in order to interrupt pressure line  24 , respectively  18  between service-brake valve  4  and brake actuators  22 ,  30 , respectively “lock in” the pressure in brake actuators  22 ,  30  of at least one axle  2   a ,  2   b , thereby allowing the brake pressure most recently generated by the driver to be maintained. 
         [0052]    To compensate for potential pressure losses, it may be additionally provided in the context of the starting assist (hill holder) function for control device  34  to cyclically switch 3/2 solenoid valve  40  configured downstream of 2/2 solenoid valve  46  between the first operating position and the second operating position in order to at least generate the most recently introduced brake pressure in brake actuators  22 ,  30  on the basis of the pressure of the pressure medium from rear-axle pressure medium reserve  12 , respectively front-axle pressure medium reserve  10 , respectively to maintain the same there also over a longer period of time. The value of the previously generated, driver-dependent brake pressure may be estimated, respectively measured using a brake-pressure sensor, for example. 
         [0053]    For functions such as halt brake, starting-traction control or headway distance control, ABS pressure control valves  20 ,  28  are driven by control device  34  to be switched to the open position, so that ABS pressure control valves  20 ,  28  may have no influence on the brake pressure buildup in the mentioned cases. However, it is also conceivable to enable ABS pressure control valves  20 ,  28  to participate in the build-up of predefined brake pressures by the cyclical operation thereof. 
         [0054]    For example, in the context of a headway distance control, the situation may arise where it is necessary to reduce a comparatively high, previously adjusted brake pressure. To reduce pressure in brake actuators  22 ,  30 , 2/2 solenoid valve  46  may then be cyclically controlled by control device  34 , and 3/2 solenoid valve  40  is switched to the second operating position. 
         [0055]    In this manner, too high brake pressure may then be reduced to the desired lower level via then through-connected 3/2 solenoid valve  46  and temporarily through-connected 2/2 solenoid valve  46  by way of service-brake valve  4  since service-brake valve  4  has a pressure reduction or bleed feature. 
         [0056]    In the specific embodiments of  FIG. 3 through 5 , identical, respectively identically functioning components and assemblies are denoted by the same reference numerals as in  FIG. 2 . 
         [0057]    A bypass connection  54 , which bridges 2/2 solenoid valve  46  and 3/2 solenoid valve  40  in pressure line  24  and which may be likewise integrated in valve unit  26 , respectively the unit, is provided in the specific embodiment of  FIG. 3  between rear-axle channel  8  of service-brake valve  4  and ABS pressure control valves  28 , respectively brake actuators  30  of the rear axle. Therefore, bypass connection  54  branches off from pressure line  24  upstream of 2/2 solenoid valve  46  and leads again into pressure line  24  downstream of 3/2 solenoid valve  40 , viewed from service-brake valve  4 . 
         [0058]    At least one nonreturn valve  56  is connected within this bypass connection  54  in a way that allows it to close toward rear-axle channel  8  of service-brake valve  4  and open toward ABS pressure control valves  28 , respectively brake actuators  30 . This makes it possible to increase the predefined, respectively desired brake pressure level preset by 2/2 solenoid valve  46  and 3/2 solenoid valve  40 , for example in the context of the starting assist, via the parallel branch of bypass connection  54 . Thus, when the driver actuates the pedal of service-brake valve  4 , the pressure prevailing in brake actuators  30  may be increased via this parallel bypass connection  54  and the thereby opening nonreturn valve  56 . On the other hand, pressure may not be reduced via bypass connection  54  since nonreturn valve  56  closes toward service-brake valve  4 . Apart from that, the operating principle of valve unit  26  is comparable to that described in the context of  FIG. 2 . 
         [0059]    In the specific embodiment of  FIG. 4 , a relay valve  58  is interposed between 3/2 solenoid valve  40  and ABS pressure control valves  28 , respectively brake actuators  30  in order to control greater volumes of air. In this context, a control connection  60  of relay valve  58  is connected to third connection  50  of 3/2 solenoid valve  40 , a supply connection  62  to rear-axle compressed-air reserve  12 , for example, and a working connection  64  to ABS pressure control valves  28 , respectively directly to brake actuators  30 . Last but not least, relay valve  58  also has a bleed feature. As a function of the pressure prevailing at control connection  60  thereof, relay valve  58  modulates an appropriate brake pressure from the pressure of rear-axle compressed-air reserve  12 . Apart from that, the operating principle of valve unit  26  is comparable to that described in the context of  FIG. 2 . 
         [0060]    In the specific embodiment of  FIG. 5 , both a bypass connection  54  having a nonreturn valve  56 , as well as a relay valve  58  are provided, so that, in terms of the operating principle, reference is made to the above explanations for  FIGS. 3 and 4 . It is evident here that the functions described therein are provided in combinations thereof 
         [0061]    In accordance with one further specific embodiment, the two valves, namely 2/2 solenoid valve  46  and 3/2 solenoid valve  40  of  FIG. 2 , which are installed in valve unit  26  and configured in a shared housing, may be designed as one single valve which encompasses all switching functionalities of 2/2 solenoid valve  46  and of 3/2 solenoid valve  40 . Such a single valve then has a connection to compressed-air reserve  12 , a connection for rear-axle channel  8  of service-brake valve  4 , as well as a connection for an ABS pressure control valve  28 . 
         [0062]    Control device  34  is shown in detail in  FIG. 6 . It features the following electrical connections: 
         [0063]    connections  36   a  for wheel speed sensors  36 ; 
         [0064]    a connection  38   a  for the “halt brake” switch; 
         [0065]    a connection  46   a  for 2/2 solenoid valve  46  of valve unit  26  assigned to rear axle  2   a;    
         [0066]    a connection  40   a  for 3/2 solenoid valve  40  of valve unit  26  assigned to rear axle  2   a;    
         [0067]    connections  20   a ,  28   a  for ABS pressure control valves  20 ,  28  of rear axle  2   a  and of front axle  2   b ; and 
         [0068]    in the case, as already indicated above, a valve unit  26  (as in  FIG. 2 ) is likewise assigned to front axle  2   b , a connection  46   b  for 2/2 solenoid valve  46  of valve unit  26  assigned to rear axle  2   b , as well as a connection  40   b  for 3/2 solenoid valve  40  of valve unit  26  assigned to front axle  2   b;    
         [0069]    as well as a connection  66  for external signals, for example, via a data-bus connection of the vehicle. 
         [0070]    Against this background, control device  34  executes the following functions: 
         [0071]    Via connections  36   a , it receives the wheel-speed values from wheel-speed sensors  36  and determines whether the brake slip or drive slip exceeds or falls below permissible values. 
         [0072]    Via connection  38   a , it receives electric signals from the “halt brake” sensor. 
         [0073]    Via connection  66 , it receives a delay signal for an automatic brake application for the vehicle, for example, from an adaptive cruise control (ACC), an ESP system or a starting assist function (hill holder). 
         [0074]    It features a memory for predefined limit values of brake pressures for each brake function or brake type. In particular, different limit values for brake pressures may be stored for rear axle  2   a  and front axle  2   b  in the case that an additional ASR valve is provided for front axle  2   b  (in the case of ABS, different limit values for brake pressures at each individual wheel may be stored. Last but not least, a brake pressure value for the halt brake pressure in the case of “halt brake, a brake pressure value for the starting-assist brake pressure (hill holder brake pressure), as well as a brake pressure value for an automatically triggered braking procedure may be stored). 
         [0075]    It controls 2/2 solenoid valve  44  via connection  44   a.    
         [0076]    It controls 3/2 solenoid valve  40  via connection  40   a.    
         [0077]    It controls ABS pressure control valves  20 ,  28  as a function of the particular brake slip via connections  20   a ,  28   a.    
         [0078]      FIG. 7 through 9  illustrate flow charts for various brake modes that are implemented by the control routines of the control device. Thus,  FIG. 7  shows a flow chart for a normal service braking for which valve unit  26 , i.e., 2/2 solenoid valve  46 , as well as 3/2 solenoid valve  40  are switched to the un-energized state upon program start, and are in the operating positions shown in  FIG. 2  in which they allow the brake pressure that had been fed from rear-axle channel  8  of service brake valve  4 , through to ABS pressure control valve  28  which likewise allows this pressure, unchanged, through to the corresponding wheel brake. 
         [0079]      FIG. 8  shows a flow chart which illustrates a service braking including ABS activity. Upon program start, valve unit  26 , i.e., 2/2 solenoid valve  46 , as well as 3/2 solenoid valve  40  are switched again to the un-energized state, so that they are in the operating positions shown in  FIG. 2 . However, control device  34  cyclically controls ABS pressure control valve  28 , for example, in order to modulate the brake pressure generated by the driver in rear-axle channel  8  in accordance with an optimal brake slip and to then introduce this modulated brake pressure into the corresponding wheel brake(s). 
         [0080]    Finally,  FIG. 9  shows a flow chart where the brake pressure is not generated by the driver, but rather automatically, for example, by a starting assist function (hill holder), an ESP function, an ACC function or a halt brake. 
         [0081]    Once the program starts, control device  34  calculates desired brake pressure P desired  from a request for the particular automatic brake function in accordance with step  122 . If the driver actuates the service brake in parallel to the particular automatic brake function, a query is made in step  124  to determine whether brake pressure P desired  is greater than brake pressure P requested  requested by the driver. If this is the case (“yes”), then the wheel brake pressure(s) is/are determined in accordance with the brake pressure P requested  requested by the driver in accordance with step  126 . However, if this is not the case (“no”), then, in step  128 , control device  34  energizes 2/2 solenoid valve  46  of valve unit  26  switching it to its closed position. At the same time, 3/2 solenoid valve  40  of valve unit  26  is energized to enable it to switch the supply pressure in rear-axle compressed-air reserve  12  to ABS pressure control valve  28  in accordance with step  130 . In response thereto, compressed air then flows under supply pressure to ABS pressure control valve  28  in accordance with step  132 . A current actual service brake pressure P service  service is derived herefrom. 
         [0082]    In step  134 , it is queried whether current actual service brake pressure P service  is equal to desired brake pressure P desired . If the response is “yes,” then, in accordance with step  140 , ABS pressure control valve  28  remains in its position allowing throughflow, so that the current actual service brake pressure P service  conveyed from rear-axle compressed-air reserve  12  is allowed through to the wheel brakes. If the response is “no,” then 2/2 solenoid valve  46  of valve unit  26  is cyclically switched by control device  34  in such a way that a service brake pressure P service  that is too high, for example, is reduced to the desired lower level via then through-connected 3/2 solenoid valve  40  and temporarily through-connected 2/2 solenoid valve  46  via service brake valve  4  in accordance with step  136 , since service-brake valve  4  has a pressure reduction, respectively bleed feature. Otherwise, thus when a service brake pressure P service  that is too low, for example, is to be increased, 3/2 solenoid valve  40  of valve unit  26  is then cyclically switched by control device  34  to allow compressed air under supply pressure to be supplied temporarily from rear-axle compressed-air reserve  12  in accordance with step  138 . 
         [0083]    The list of Reference Numerals is as follows:
         1  brake system     2   a  rear axle     2   b  front axle     4  service-brake valve     6  front-axle channel     8  rear-axle channel     10  front-axle compressed-air reserve     12  rear-axle compressed-air reserve     14  supply line     16  supply line     18  pressure line     20  ABS pressure control valves     20   a  connection     22  brake actuator     24  pressure line     26  valve unit     28  ABS pressure control valves     28   a  connection     30  brake actuator     32  supply line     34  control device     36  wheel-speed sensors     36   a  connection     38  switch     38   a  connection     40  3/2 solenoid valve     40   a  connection     40   b  connection     42  first connection     44  connection     46  2/2 solenoid valve     46   a  connection     48  second connection     50  third connection     52  connection     54  bypass connection     56  nonreturn valve     58  relay valve     60  control connection     62  supply connection     64  working connection     66  connection       
 
         [0126]    While the present invention has been illustrated by the description of embodiments thereof, and while the embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention, in its broader aspects, is not limited to the specific details, the representative apparatus, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the applicant&#39;s general inventive concept.