Patent Publication Number: US-2012035820-A1

Title: Method and device for operating a vehicle, in particular a hybrid vehicle

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for operating a vehicle, in particular a hybrid vehicle, in which each of the two axles of the vehicle, which are not mechanically coupled, is driven by at least one drive unit, thus transmitting a torque to the wheels of the respective axle, and a device for carrying out the method. 
     BACKGROUND INFORMATION 
     A generic device is discussed in German patent document DE 35 42 059 C1. The vehicle has a main drive axle which is drivable by an internal combustion engine in a customary manner. When there is increased slip of the wheels of the main drive axle, the wheels of a connectable auxiliary drive axle may be automatically driven with the aid of a separate auxiliary drive unit, in particular an electric motor. This connection is always established in situations when the vehicle may be moving on a slippery surface. 
     When the vehicle starts to move, different coefficients of friction result at the two drive axles when the wheels of one drive axle are on a slippery surface, for example black ice, and the wheels of the other drive axle are on the paved roadway. If a possible friction is exceeded for the axle on the slippery surface, the wheel speeds of this axle greatly increase, accompanied by a high degree of slip. In many cases the vehicle slides laterally, so that the drive force is either not converted to propulsion at all, or is converted to propulsion with an unintended change in the direction of the vehicle. 
     The increase in the wheel speeds may be prevented using a traction control system (TCS) which limits the axle drive setpoint torque of the drive axle which on the slippery surface. 
     SUMMARY OF THE INVENTION 
     The method according to the present invention for operating a vehicle, in particular a hybrid vehicle, has the advantage that optimal use may be made of the different coefficients of friction at the two drive axles. As a result of the rotational speeds of the wheels of both drive axles being ascertained and averaged, a difference being formed from the averaged rotational speeds of both axles, and based on this difference the torque of at least one axle being influenced in such a way that differences in the averaged rotational speeds of the wheels are counteracted, optimal traction of the vehicle is achieved by making use of the different coefficients of friction at the drive axles. For vehicles whose drive torques are generated separately by associated drive units, a high degree of slip is thus avoided. 
     Based on the difference in the rotational speeds, an axle differential torque which, with an opposite algebraic sign, acts on the drive setpoint torques of the two axles is advantageously determined. The drive setpoint torque on the second axle, which is on the solid road surface, is thus increased in order to compensate for the lack of friction at the first drive axle which is on the slippery surface. The overall drive setpoint torque specified by the driver is thus maintained. The predistribution of the torques on the axles is corrected by the regulation. The use of limited slip differentials in the transmissions may thus be dispensed with, thereby saving on system costs. 
     In one refinement, the overall drive setpoint torque of the axles is divided between the drive setpoint torques of the two axles as a function of the instantaneous driving state of the vehicle. A predistribution of the overall drive torque on the axles, which may constitute either an equal distribution (50:50) or an unequal distribution (40:60, for example), is thus set. The predistribution may be influenced by an operating strategy of the vehicle and/or by a driving dynamics system. 
     The differences in the averaged rotational speeds of the individual axles are advantageously not compensated for in a steady state. This allows rotational speed differences which result from the vehicle geometry. The driving stability during cornering and the steering willingness are maintained. 
     In one embodiment, the steady-state lack of compensation is achieved by proportional or proportional-differential feedback of the rotational speed differences to the drive torques of the axles. The effect of this procedure corresponds to a mechanical central differential or an axle differential, which allows rotational speed compensation and which has an increasing blocking effect with increasing rotational speed difference, but which on account of the method according to the present invention may be dispensed with. 
     In one refinement, the feedback of the averaged rotational speed to the drive torques and/or the intensification of the feedback is/are influenced by the instantaneous driving state. The driving state is a function of numerous factors, such as the overall drive setpoint torque requested by the driver, the steering wheel angle, the brake pedal actuation, the longitudinal and/or transverse acceleration of the vehicle, the yaw rate, and the intervention by electronic stabilizing systems of the vehicle. The regulation is always influenced as a function of the instantaneous occurrence of friction forces at the wheels of the axles, for which reason an optimal driving state is set in every situation. 
     In particular for braking or for ABS or driving dynamics interventions, decoupling of the axles by reducing or stopping the feedback may be advantageous, for example to allow independent brake slip control or control about the vertical axis of the vehicle. Ascertaining the vehicle speed which is required by the driving dynamics systems may also require decoupling of the axles for motor vehicles having all-wheel drive. Decoupling should occur during recognized maneuvering or parking operations, or when driving using a compact spare tire, while strong coupling occurs for active TCS or process measurement and control (PMC) interventions having more intense feedback of the rotational speed difference to the axle drive torques. 
     Alternatively, when there is little friction between the wheels and the roadway, the differences in the averaged rotational speeds of the individual axles are compensated for in a steady-state manner. A high level of traction is thus achieved; i.e., the drive force is optimally converted to propulsion. This may be achieved, for example, by proportional-integral feedback or proportional-integral differential feedback of the rotational speed difference. In addition, an effect is thus achieved which is similar to what would be obtained by using a mechanical limited slip differential. 
     The overall drive setpoint torque, which represents the sum of the drive torques on the axles, is advantageously influenced, in particular limited, by a driving dynamics system. As a result of the described regulation, the use of conventional driving dynamics systems, such as an electronic stability program, which always influence only a summed drive torque of all vehicle wheels, is also possible for vehicles having single-axle drives. Specialized development and manufacture of driving dynamics systems for the particular application in vehicles having single-axle drives may therefore be dispensed with. 
     In one embodiment, the influencing of the drive torques on the axles by virtue of the difference in the averaged rotational speeds affects an operating strategy of the vehicle. Thus, in a hybrid vehicle, not only the charging strategy for charging the energy store by the internal combustion engine, but also the operating point of the internal combustion engine may be adjusted in an improved manner, and/or the predistribution of the overall drive torque may be influenced. 
     Another refinement of the exemplary embodiments and/or exemplary methods of the present invention relates to a method for operating a vehicle, in particular a hybrid vehicle, having at least one axle at which the wheels are separately driven by at least one respective drive unit, thereby transmitting the torques thus generated to the wheel, directly or with the aid of a transmission. To be able to make optimal use of the coefficients of friction of the wheels, the rotational speeds of both wheels are ascertained and a difference in the rotational speeds is formed, this difference being used to influence the torque on at least one wheel in such a way that the difference in the rotational speed of the wheels of the axle is counteracted. Optimal traction of the vehicle is thus achieved by making use of the different coefficients of friction at the wheels. For vehicles whose drive torques are separately generated by associated drive units, a higher degree of slip is thus avoided. Based on the difference in the wheel speeds, a wheel differential torque which, with a different algebraic sign, acts on the wheel torques of the wheels in order to reduce the difference in the wheel speeds is advantageously determined. The predistribution of the torques on the wheels is thus corrected. A predefined overall drive setpoint torque on both wheels is maintained. The use of limited slip differentials may thus be dispensed with, thereby saving on component costs. 
     In one embodiment, a drive setpoint torque specified by the driver is limited to an overall drive setpoint torque on the wheels which is specified by a driving dynamics system. As a result of the described regulation, the use of conventional driving dynamics systems, such as an electronic stability program, which always influences only a summed drive torque on all vehicle wheels, is also possible for vehicles having single-axle drives. Specialized development and manufacture of driving dynamics systems for the particular application in vehicles having single-axle drives may therefore be dispensed with. 
     In one refinement, the differences in the rotational speeds of the individual wheels are not compensated for in a steady-state manner. Rotational speed differences which result from the vehicle geometry are thus allowed. The driving stability during cornering and the steering willingness are maintained. 
     The steady-state lack of compensation is advantageously achieved by proportional or proportional-differential feedback of the rotational speed differences to the drive torques of the wheels. The driving state is a function of numerous factors, such as the overall drive setpoint torque requested by the driver, the steering wheel angle, the brake pedal actuation, the longitudinal and/or transverse acceleration of the vehicle, the yaw rate, and the intervention of electronic stabilizing systems of the vehicle. The regulation is always influenced as a function of the instantaneous occurrence of friction forces at the wheels, for which reason an optimal driving state is set in every situation. 
     In particular for braking or for ABS or driving dynamics interventions, decoupling of the wheels by reducing or stopping the feedback may be advantageous, for example to allow independent brake slip control of the individual wheels. Ascertaining the vehicle speed, which is required by the driving dynamics systems, may also require decoupling of the wheels for motor vehicles having all-wheel drive. Decoupling should occur during recognized maneuvering or parking operations, or when driving using a compact spare tire, while strong coupling occurs for active TCS or PMC interventions having more intense feedback of the rotational speed difference to the wheel drive torques. 
     In one embodiment, the feedback of the difference in the rotational speed of the wheels to the drive torques of the wheels and/or the intensification of the feedback is/are influenced by the instantaneous driving state. In particular for braking, for ABS, or for driving dynamics interventions, decoupling of the wheels by reducing or stopping the feedback may be advantageous, for example to allow control about the vertical axis of the vehicle. 
     Alternatively, when there is little friction between the wheels and the roadway, the differences in the rotational speeds of the individual wheels are compensated for in a steady-state manner. A high level of traction is thus achieved. This may be achieved, for example, by proportional-integral feedback or proportional-integral differential feedback of the rotational speed difference. In addition, an effect is thus achieved which is similar to what would be obtained by using a mechanical limited slip differential. 
     Another refinement of the exemplary embodiments and/or exemplary methods of the present invention relates to a device for operating a vehicle, in particular a hybrid vehicle, in which each of the axles of the hybrid vehicle, which are not mechanically coupled, is driven by at least one drive unit, thus transmitting a torque to the wheels of the respective axle. In order to make better use of different coefficients of friction at the drive axles, a measuring arrangement measures the rotational speeds of the wheels of both drive axles and average same, then form a difference of the averaged rotational speeds of both axles, and use this difference to influence the torque on at least one axle in such a way that differences in the averaged rotational speeds of the wheels of an axle are counteracted. The device has the advantage that optimal traction of the vehicle is achieved by making optimal use of the different coefficients of friction at the drive axles. For vehicles whose drive torques are generated separately by associated drive units, a high degree of slip is thus avoided. 
     One rotational speed sensor advantageously measures the rotational speed of each wheel of an axle, the two rotational speed sensors for an axle each leading to an averaging unit, and the two averaging units being connected to a controller which determines an axle differential torque based on the difference in the rotational speeds, and which outputs this differential torque, with an opposite algebraic sign, to the drive setpoint torques of the two axles. As a result of this regulation, the distribution of the torques on the individual axles is corrected and adapted to the instantaneous roadway conditions. The use of a mechanical limited slip differential may be dispensed with. 
     In one embodiment, a drive force setpoint generator, a driver assistance system, and/or a driving dynamics system is/are connected to a limiter which outputs a drive setpoint torque and which leads to at least one drive unit of at least one axle. The overall drive setpoint torque is thus set, either by the driver or by the driving dynamics system, to a specified value which is distributed on the two drive axles in equal or unequal portions, depending on the driving state. 
     In one refinement, an operating strategy element is connected between the limiter and the drive unit. In such an operating strategy element, an axle drive setpoint torque is converted to the gear ratio of a transmission. 
     The limiter is advantageously connected to two drive units, each drive unit controlling a wheel, directly or with the aid of a transmission, and the two wheels are situated in an axle-free manner, two rotational speed sensors which detect the rotational speed of each wheel being connected to a summer which forms a difference, and which leads to a second controller which generates a wheel differential torque, and which outputs the wheel differential torque, with an opposite algebraic sign, to the torques of the two drive units of the wheels. 
     Optimal traction of the vehicle is thus achieved by making use of the different coefficients of friction at the wheels. The device has a second controller which makes optimal use of the different coefficients of friction of the two wheels. In interaction with the first controller which regulates the drive torque on an axle and the drive torque on the two wheels regulated by the second controller, a very flexible system is obtained for controlling drive axles or drive wheels which are not mechanically coupled. 
     The exemplary embodiments and/or exemplary methods of the present invention allows numerous design options. One of these design options is explained in greater detail with reference to the wheels illustrated in the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a device for driving drive axles, which are not mechanically coupled, according to the related art. 
         FIG. 2  shows a first exemplary embodiment of a device for regulating drive axles which are not mechanically coupled. 
         FIG. 3  shows a schematic flow chart for the device according to  FIG. 2 . 
         FIG. 4  shows a second exemplary embodiment of a device for regulating drive wheels which are not mechanically coupled. 
         FIG. 5  shows a schematic flow chart for the device according to  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     Identical features are denoted by the same reference numerals. 
       FIG. 1  shows a drive train of a hybrid vehicle. An internal combustion engine  1  is coupled to a first electric motor  2 , which leads to a first transmission  3 . Transmission  3  is connected to a first axle  4  on which two wheels  5 ,  6  are situated. Torque M 1  of internal combustion engine  1  and torque. M 2  of first electric motor  2  are added to form a drive torque which is converted by transmission  3 . An axle drive torque M 4  of first drive axle  4  results at the output of transmission  3 ; the first drive axle may be the front axle of the vehicle, for example. This drive torque M 4  is relayed to drive wheels  5 ,  6 . 
     A second drive axle  7  is driven by a second electric motor  8 , which generates drive torque M 8 . Drive torque M 8  is converted with the aid of a second transmission  9 , and is relayed to wheels  10 ,  11  as axle drive torque M 7  of second drive axle  7 . 
     Both transmissions  3  and  9  contain axle differentials, so that the sum of the two wheel torques corresponds to axle drive torque M 4  or M 7 , respectively. In most driving situations an axle drive torque is equally distributed over the two wheel torques. 
     The driver or a driver assistance system specifies an overall drive setpoint torque M driver  which is distributed, via a distributor  12  according to a distribution factor α, to axle drive setpoint torques M 4setpoint  and M 7setpoint  of the two drive axles  4 ,  7 , respectively. Distribution factor α is influenced by the operating strategy of the vehicle. A driving dynamics system may also influence distribution factor α. An element  13  for determining the operating strategy converts axle drive setpoint torque M 4setpoint , using the gear ratio of transmission  3 , and distributes it over electric motor  2  having torque M 2  and internal combustion engine  1  having torque M 1 . By use of this distribution, a charging strategy for an electrical energy store, not illustrated in greater detail, which implements boost and recuperation operations, among others, is achieved. In total, an axle drive torque M 4  results at drive wheels  5 ,  6  which approximately corresponds to axle drive setpoint torque M 4setpoint . 
     Second element  14  for an operating strategy uses the gear ratio of transmission  9  to torque M 8  of electric motor  8  to convert axle drive setpoint torque M 7setpoint  for second drive axle  4 . In total, an axle drive torque M 7  which approximately corresponds to axle drive setpoint torque M 7setpoint  results at drive wheels  10 ,  11 . 
       FIG. 2  illustrates a first exemplary embodiment of the present invention. Internal combustion engine  1 , first electric motor  2 , and transmission  3  are the same as described in  FIG. 1 , drive axle  4  being associated with wheels  5 ,  6 . The same applies for second electric motor  8 , which is associated with transmission  9  of second drive axle  7 , and therefore associated with wheels  10 ,  11 . 
     The wheel speeds of wheels  5 ,  6  and  10 ,  11  are detected by sensors. Sensor  15  is situated opposite wheel  5 , sensor  16  is opposite wheel  6 , sensor  17  is opposite wheel  10 , and sensor  18  is opposite wheel  11 . Sensors  15 ,  16  are connected to an averaging unit  19 , and sensors  17 ,  18  are connected to an averaging unit  20 . Both averaging units lead to a first controller  21 . 
     A limiter  22  which receives input signals from the driver, from a driver assistance system, and from a driving dynamics system  23  is situated upstream from distributor  12 . 
     The sequence of the method is explained with the aid of  FIG. 3 . In block  100  the vehicle starts to move, with different coefficients of friction at the two drive axles  4  and  7 . This means that wheels  5 ,  6  of one drive axle  4  are on a slippery surface, such as black ice, while wheels  10 ,  11  of second drive axle  7  are on pavement. In block  101  rotational speeds n of each wheel  5 ,  6  and  10 ,  11  are measured by sensors  15 ,  16 ,  17 ,  18 . In block  102  rotational speeds n of wheels  5 ,  6  of first drive axle  4  are averaged in averaging unit  19 , and a rotational speed average value n 4  is obtained. Rotational speeds n of wheels  10 ,  11  of second drive axle  7  are averaged in averaging unit  20  to form a rotational speed average value n 7 . For the described roadway conditions, rotational speed average value n 4  increases compared to rotational speed average value n 7  as a result of the black ice. 
     Rotational speed average value n 7  of second axle  7  which is ascertained in this way, having an opposite algebraic sign, is combined with rotational speed average value n 4  of first axle  4  (block  103 ). This results in an axle rotational speed difference n Adiff , which is supplied to controller  21  in block  104 . When controller  21  is supplied with a positive axle rotational speed difference n Adiff , as is the case in the described start-up situation, controller  21  generates a positive axle differential torque M Adiff . In block  105  this axle differential torque M Adiff  is supplied to axle drive setpoint torque M 4setpoint  with a negative algebraic sign, while it is supplied to axle drive setpoint torque M 7setpoint  with a positive algebraic sign. When axle differential torque M Adiff  is positive, in block  106  this results in a decrease in axle drive setpoint torque M 4setpoint  for first axle  4 , and an increase in axle drive setpoint torque M 7setpoint  on second drive axle  7 , which counteracts rotational speed difference n Adiff . Overall drive setpoint torque M driver  specified by the driver is thus maintained. 
     If it is determined in block  107  that the two drive axles  4 ,  7  have excessively high slip, driving dynamics system  23  or a TCS system, not illustrated in greater detail, is activated in block  108 . Overall drive setpoint torque M driver  is reduced by limiter  22 , resulting in an overall machine drive setpoint torque M Asetpoint  which is less than overall drive setpoint torque M driver  which has been requested by the driver. This means that overall machine drive setpoint torque M Asetpoint  is not reduced compared to the overall drive setpoint torque specified by the driver until optimal use has already been made of the different coefficients of friction at the two drive axles  4 ,  7 , which ensures good traction. Also during the intervention by the TCS system or driving dynamics system  23 , the regulation and therefore the optimal distribution of the two drive setpoint torques M 4setpoint  and M 7setpoint  remains active. 
     By recognition of an appropriate driving situation or by specification by the driver, an integral portion of controller  21  which compensates for axle rotational speed difference n Adiff  in a steady-state manner or regulates same to zero is enabled in block  109 . The traction of the vehicle is optimized in this way. 
     Not illustrated in greater detail is a taking into account of axle differential torque M Adiff  in operating strategy element  13  which, corresponding to increased axle drive setpoint torque M 7setpoint  and the resulting increase in the power requirements of second electric motor  8 , shifts the operating points of first electric motor  2 , having torque M 2 , and of internal combustion engine  1 , having torque M 1 , in order to generate more electrical power. Likewise not illustrated is an influencing of distribution factor α by axle differential torque M Adiff . If it is not possible to maintain a distribution factor α specified by the operating strategy on account of the instantaneous roadway friction conditions or the instantaneous driving state, this results in a longer intervention by controller  21  with the aid of axle differential torque M Adiff . Such an intervention is used to correct distribution factor α, and therefore the predistribution, for a fairly long period of time, and thus to terminate the intervention. 
     As an alternative to averaging the wheel speeds on the basis of the axles, the rotational speeds of electric motors  2 ,  8  or of internal combustion engine  1  may be used, taking the transmission gear ratios into account. Slip at transmission elements, for example at a starting clutch or a torque converter, must likewise be taken into account. 
     Dedicated control units which communicate with one another via bus connections are usually used for internal combustion engine  1  and electric motors  2  and  8 . In that case it is meaningful to ascertain axle differential torque M Adiff  or wheel differential torque M Rdiff  at the same time in multiple control units in order to allow the feedback of a rotational speed to a setpoint torque, without, or with the smallest possible, time delays as a result of the bus systems. 
     In  FIG. 2 , rotational speed n 7  of second drive axle  7  may be computed from the rotational speed of second electric motor  8  which is present in the control unit of second electric motor  8 . Axle differential torque M Adiff , the feedback to axle drive setpoint torque M 7setpoint  and operating strategy element  14  are then likewise computed in the control unit of second electric motor  8 . The total signal flow from the rotational speed of second electric motor  8  to setpoint torque M 8  of second electric motor  8  is then present in the control unit of second electric motor  8 . The signal flow does not occur via a bus connection, and therefore does not have time delays, which improves the control quality. A corresponding procedure may be selected for the control unit of internal combustion engine  1  and the control unit of first electric motor  2 . Algorithms of operating strategy element  13  must likewise be computed simultaneously in these two control units. 
       FIG. 4  illustrates a device for regulating drive wheels which are not mechanically coupled. Wheel  10  is driven by an electric motor  24 , and wheel  11  is driven by an electric motor  25 . A rotational speed sensor  26  is situated opposite wheel  10 , and a rotational speed sensor  27  is opposite wheel  11 . Both rotational speed sensors  26  and  27  are connected to a second controller  29  via a summer  28 . In this design as well, signals from the driver and/or from a driving dynamics system  23  are directed to a limiter  22 , whose output signal is supplied to each of distributors  30 ,  31 . Distributor  30  is connected via summation point  32  to electric motor  24  which drives first wheel  10 , while second distributor  31  is led via summation point  33  to electric motor  25  which drives wheel  11 . 
     The mode of operation of this device is illustrated in  FIG. 5 . The driver outputs an overall drive setpoint torque M driver  in block  201 . A drive setpoint torque M Asetpoint  for wheels  10 ,  11  results from limiting overall drive setpoint torque M driver  specified by the driver, to a torque limit in limiter  22  which is specified by driving dynamics system  23 . In block  202  this drive setpoint torque M Asetpoint  is directed to the two distributors  30 ,  31 , which divide drive setpoint torque M Asetpoint  in half, electric motor  24  being supplied with setpoint torque M 24setpoint  by distributor  30 , and electric motor  25  being supplied with setpoint torque M 25setpoint  by distributor  31 . Setpoint torque M 24setpoint  and M 25setpoint  approximately correspond to the wheel torques of wheels  10 ,  11  which are driven by the respective electric motor. 
     Actual wheel speeds n of wheels  10 ,  11 , which are based on the actual conditions of the state of the vehicle and the surface beneath the vehicle, are measured in block  203 . In block  204 , summer  28  uses measured wheel speeds n to form a wheel speed difference n Rdiff , which is supplied to controller  29 . In block  205 , controller  29  forms a wheel differential torque M Rdiff  based on wheel speed difference n Rdiff . This wheel differential torque M Rdiff  is included in block  206  with a negative algebraic sign, resulting in setpoint torque M 24setpoint . Setpoint torque M 25setpoint  results from addition of wheel differential torque M Rdiff  with a positive algebraic sign. A rotational speed difference n Rdiff  is counteracted in this way. For multiple driven axles, drive axle A from  FIG. 4  may replace drive axle  7  from  FIG. 2 , for example. Drive setpoint torque M Asetpoint  in  FIG. 4  then corresponds to axle drive setpoint torque M 7setpoint  in  FIG. 2 . Both controllers  21  and  29  are used, controller  21  counteracting the differences in the averaged wheel speeds of individual drive axles  4 ,  7 , and controller  29  counteracting the differences in the wheel speeds of axle A. 
     As an alternative to the wheel speeds, the rotational speeds of the electric motors may be used, possibly taking transmission gear ratios into account. 
     In both exemplary embodiments, possible operating ranges of the units, such as electric motors, internal combustion engines, an electrical energy store, among others, must be maintained. For example, based on a positive overall drive setpoint torque M driver  which is specified by the driver, no increase is allowed in the total generated drive torque as a result of the unit limitations. 
     It is also possible for axle drive setpoint torques M 4setpoint  and M 7setpoint  from  FIG. 2 , which already include differential torque M Adiff , to be influenced or limited separately with the aid of the driving dynamics system, thus allowing, for example, the self-guidance behavior or control about the vertical axis of the vehicle to be optimized in a targeted manner. 
     Likewise, setpoint torques M 24setpoint  and M 25setpoint  from  FIG. 4 , which already include wheel differential torque M Rdiff , may be separately influenced with the aid of the driving dynamics system. 
     When there is zero crossing of an axle drive setpoint torque M 4setpoint , M 7setpoint  or of setpoint torque M 24setpoint , M 25setpoint  a transition occurs between coasting mode and traction mode of the axle or the wheel. Mechanical slack occurs in the transmission or in the articulated joints of the drive shafts. The zero crossing of the reaction torque also causes the engine to tilt in its bearings, which may result in load impacts. For comfort reasons, a zero crossing should occur smoothly, which is achieved by limiting the dynamics of the axle drive setpoint torque or of the setpoint torque during its zero crossing, for example by gradient limitation. In one refinement, the dynamics of axle drive setpoint torques M 4setpoint , M 7setpoint  and/or of setpoint torques M 24setpoint , M 25setpoint  are limited in the range around 0 Nm, for example in a range of −100 Nm to +100 Nm. 
     Setpoint rotational speed differences have not been described in the exemplary embodiments illustrated above; i.e., it has been assumed that setpoint rotational speed differences n Adiffsetpoint  and n Rdiffsetpoint  are equal to 0 rpm. 
     In one refinement, in  FIG. 2 , instead of axle rotational speed difference n Adiff , a deviation n Adelta  of axle rotational speed difference n Adiff  from a setpoint rotational speed difference n Adiffsetpoint  is supplied to controller  21   
     
       
      
       n 
       Adelta 
       =n 
       Adiff 
       −n 
       Adiffsetpoint  
      
     
     In  FIG. 4 , instead of wheel speed difference n Rdiff , a deviation n Rdelta  of wheel speed difference n Rdiff  from a setpoint rotational speed difference n Rdiffsetpoint  may be supplied to controller  29   
     
       
      
       n 
       Rdelta 
       =n 
       Rdiff 
       −n 
       Rdiffsetpoint  
      
     
     The setpoint rotational speed differences n Adiffsetpoint  and n Rdiffsetpoint  are ascertained based on an instantaneous driving state and/or a desired setpoint driving state of the vehicle, for example based on the requested overall drive torque, the steering wheel angle, the brake pedal activation, the longitudinal and/or transverse acceleration of the vehicle, the yaw rate, and/or the vehicle speed. Conditions of the surroundings, such as roadway friction conditions, may also be taken into account. 
     Setpoint rotational speed differences n Adiffsetpoint  and n Rdiffsetpoint  are computed by a driving dynamics system or an electronic stabilizing system of the vehicle, for example, and thus specify that the instantaneous driving state of the vehicle approximates the setpoint driving state. This results in comfortable influencing of axle drive setpoint torques M 4setpoint , M 7setpoint  or of setpoint torques M 24setpoint , M 25setpoint  in which sum M 4setpoint +M 7setpoint  of the axle drive torques or sum M 24setpoint +M 25setpoint  of the setpoint torques is not changed. A high level of driving dynamics may also be provided in this way. At the same time, the instantaneous driving state is corrected, for example to stabilize skidding motions. Deviations n Adelta  of the axle rotational speed difference and n Rdelta  of the wheel speed difference may be compensated for in a steady-state manner, or not compensated for in a steady-state manner, depending on the instantaneous driving state and the desired setpoint driving state. 
     By use of the setpoint rotational speed differences, for example, different axle and wheel speeds during cornering may be taken into account, based on the vehicle geometry or different wheel diameters. Controllers  21 ,  29  adapt the instantaneous axle rotational speed difference and/or the instantaneous wheel speed difference to the setpoint rotational speed differences, which has a stabilizing effect on the driving dynamics. 
     The instantaneous driving state influences the behavior of controller  21  or of controller  29 . Controller parameters, the behavior in the large and small signal ranges, and/or a controller dead band are adapted to the instantaneous driving state. 
     The regulation illustrated in  FIG. 2  is advantageously active when only one axle is driven, for example when the vehicle in electric driving mode is driven only by second electric motor  8 , and internal combustion engine  1  together with first electric motor  2  is decoupled by engaging a neutral gear in transmission  3 . Large slip differences between driven axle  7  and nondriven first axle  4  are then avoided. 
     The regulation may also be used for actively damping drive train vibrations in which one wheel/axle vibrates against another axle/wheel, for example for vibration excitation resulting from an uneven roadway, from interventions of a driving dynamics or braking system, from the starting or switching off of internal combustion engine  1 , from sudden changes in the roadway friction conditions, or from shifting of the transmission.