Method and device for operating a motor vehicle, and motor vehicle

Abstract

A method for operating a motor vehicle which includes at least one wheel axle having two drive wheels, each drive wheel being drivable with the aid of a wheel-specific drive unit for the purpose of moving the motor vehicle on a roadway. It is provided that the drive units of the wheel axle are controlled as a function of a difference between the longitudinal forces applicable at the drive wheels of the wheel axle to the roadway.

Claims

1. A method for operating a motor vehicle, which includes at least one wheel axle having two drive wheels, the method comprising: controlling, via a control unit, each of the drive units of the wheel axle as a function of a difference between the longitudinal forces applicable to a roadway at the drive wheels of the wheel axle to the roadway, so as to ensure stable driving operation without static friction loss; and driving each of the drive wheels with a wheel-specific drive unit to move the motor vehicle on a roadway; wherein each of the drive wheels is assigned a corresponding one of the drive units, wherein the drive units each include an electric machine which is operable by a motor and/or by a generator, wherein each of the drive units are controlled by the control unit and receive electrical energy necessary for their drive from an electrical energy store, and wherein the control unit carries out a process which predefines setpoint wheel torques for the drive units in a wheel-specific manner starting from a requested axle drive torque for one of the wheel axles and starting from a differential torque determined as a function of the difference, so that wheel-specific brake interventions by a braking system are not needed.

2. The method of claim 1, wherein the difference is determined as a function of an instantaneous friction coefficient between the roadway and the particular drive wheel, a wheel speed of the particular drive wheel, a longitudinal acceleration of the motor vehicle and/or a contact force of the particular drive wheel.

3. The method of claim 1, wherein a torque difference is determined for the drive wheels as a function of the difference and is adjusted when controlling the drive units.

4. The method of claim 1, wherein the drive units are controlled to generate in each case setpoint wheel torque as a function of a requested axle drive torque for the wheel axle.

5. The method of claim 1, wherein the setpoint wheel torque is determined from the half of the axle drive torque from the half of the torque difference.

6. The method of claim 1, wherein the half of the torque difference is added to the half of the drive torque at one of the drive wheels and subtracted from the half of the drive torque at the other one of the drive wheels.

7. The method of claim 1, wherein a maximum torque difference is predefined as a function of an instantaneous driving situation.

8. The method of claim 1, wherein the driving situation is ascertained as a function of an adjusted steering angle, a steering torque applied by the driver of the motor vehicle, a transverse force, an acceleration force, a rotation rate and/or a driving speed of the motor vehicle.

9. A device for operating a motor vehicle, which includes at least one wheel axle having two drive wheels, comprising: a control unit configured to perform the following: controlling each of the drive units of the wheel axle as a function of a difference between the longitudinal forces applicable to a roadway at the drive wheels of the wheel axle to the roadway, so as to ensure stable driving operation without static friction loss; and driving each of the drive wheels with a wheel-specific drive unit to move the motor vehicle on a roadway; wherein each of the drive wheels is assigned a corresponding one of the drive units, wherein the drive units each include an electric machine which is operable by a motor and/or by a generator, wherein each of the drive units are controlled by the control unit and receive electrical energy necessary for their drive from an electrical energy store, and wherein the control unit carries out a process which predefines setpoint wheel torques for the drive units in a wheel-specific manner starting from a requested axle drive torque for one of the wheel axles and starting from a differential torque determined as a function of the difference, so that wheel-specific brake interventions by a braking system are not needed.

10. A motor vehicle, comprising: at least one wheel axle having two drive wheels, wherein each of the drive wheels is assigned an individually controllable drive unit; and a control unit configured to perform the following: controlling each of the drive units of the wheel axle as a function of a difference between the longitudinal forces applicable to a roadway at the drive wheels of the wheel axle to the roadway, so as to ensure stable driving operation without static friction loss; and driving each of the drive wheels with a wheel-specific drive unit to move the motor vehicle on a roadway; wherein each of the drive wheels is assigned a corresponding one of the drive units, wherein the drive units each include an electric machine which is operable by a motor and/or by a generator, wherein each of the drive units are controlled by the control unit and receive electrical energy necessary for their drive from an electrical energy store, and wherein the control unit carries out a process which predefines setpoint wheel torques for the drive units in a wheel-specific manner starting from a requested axle drive torque for one of the wheel axles and starting from a differential torque determined as a function of the difference, so that wheel-specific brake interventions by a braking system are not needed.

11. The motor vehicle of claim 10, wherein the drive units include wheel-proximal electric machines.

12. The motor vehicle of claim 9, wherein the drive units include wheel-proximal electric machines.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows in a simplified top view a motor vehicle according to a first exemplary embodiment.

(2) FIG. 2 shows in a simplified top view a motor vehicle according to a second exemplary embodiment.

DETAILED DESCRIPTION

(3) In a simplified top view, FIG. 1 shows a motor vehicle 1 which includes a front wheel axle 2 and a rear wheel axle 3. Both wheel axles 2, 3 each include two drive wheels 4, 5 and 6, 7, respectively. Each of drive wheels 4 through 7 is assigned a drive unit 8, 9, 10, and 11 in each case. Drive units 8 through 11 are coupled or mechanically operatively connected to particular drive wheels 4 through 7, respectively. According to the present exemplary embodiment, drive units 8 through 11 are each configured as electric machines which are operable, in particular, by a motor and, optionally, also by a generator. Drive machines 8 through 11 are controlled by a control unit 12 and receive the electrical energy necessary for their drive from an electrical energy store 13.

(4) Control unit 12 controls drive units 8, 9, and 10, 11 of particular wheel axle 2, 3 as a function of a difference between the longitudinal forces which are applicable to the roadway at drive wheels 4, 5 and 6, 7 of particular wheel axle 2, 3 in order to ensure a stable driving operation without static friction loss. Here, control unit 12 carries out a method which predefines setpoint wheel torques for drive units 8, 9, and 10, 11 in a wheel-specific manner starting from a requested axle drive torque for one of wheel axles 2, 3 and starting from a differential torque determined as a function of the difference, so that wheel-specific brake interventions are dispensed with with the aid of a braking system (not illustrated in FIG. 1) for stabilizing motor vehicle 1.

(5) In this way, a comfortable driving operation of the motor vehicle is also achieved for inhomogeneous friction coefficients of a roadway in the case of which drive wheels 4, 6 are provided with a lower friction coefficient than drive wheels 5, 7, for example. Even in the case of changing or alternating friction coefficient conditions, a stable guidance of the motor vehicle is ensured due to sufficiently high cornering forces. The driver achieves a reproducible driving behavior at the same accelerator pedal position, regardless of whether motor vehicle 1 moves on a roadway having a homogeneous friction coefficient or on a roadway having an inhomogeneous friction coefficient, the braking system being spared and its service life thus increased.

(6) A total drive torque is determined for motor vehicle 1 as a function of the accelerator pedal actuation. It is then distributed among wheel axles 2, 3 to ensure an optimal driving operation. Drive torques M.sub.A are thus determined from the total drive torque for wheel axles 2, 3 which may be predefined to be identical or different.

(7) A control method usually provides the drive torque as a sum torque for the applicable axle drive torque of the particular wheel axle, the drive torque being implemented, in a classic configuration, by one single engine which is connected via a transmission and a differential to two drive wheels of a wheel axle. The known control method also provides a setpoint differential torque which is implemented as a hydraulic braking torque in the classic case. This setpoint differential torque or the torque difference is determined as a function of the friction coefficients of drive wheels 4, 5 of wheel axle 2 and of drive wheels 6, 7 of wheel axle 3 in the present case. Here, the friction coefficients may be ascertained in a manner known per se during the driving operation of motor vehicle 1, for example. The axle drive torque for wheel axle 3, for example, and differential torque M.sub.D are now used to determine setpoint wheel torque M.sub.10, M.sub.11 for particular drive unit 10 and 11 of wheel axle 3:
M.sub.10=0.5*(M.sub.A−M.sub.D)/i.sub.G
M.sub.11=0.5*(M.sub.A+M.sub.D)/i.sub.G

(8) Here, gear ratio i.sub.G between particular drive machine 10, 11 and associated drive wheel 6, 7 is also taken into account. Drive machines 8, 9 of wheel axle 2 may be controlled analogously thereto.

(9) This means that this method is used to compute the particular setpoint wheel torque for drive machines 8 through 11 as a function of torque difference M.sub.D. In this case, the steady portion is included in the half of axle drive torque M.sub.A, the wheel-specific portion is included in the half of torque difference M.sub.D in each case.

(10) As soon as a torque difference is needed, because motor vehicle 1 is moving on a roadway having an inhomogeneous friction coefficient, for example, setpoint wheel torque M.sub.10 is decreased by half of the value of torque difference M.sub.D on the side having the lower friction coefficient. On the side having the higher friction coefficient, setpoint wheel torque M.sub.11 is simultaneously increased by half of the value of torque difference M.sub.D.

(11) In contrast to conventional methods, in which a brake intervention is used to stabilize the driving operation, a torque increase is thus possible, thus allowing for an improved driving stability. The setpoint torques are delimited by the particular drive potential of drive machines 8 through 11. Drive torque M.sub.A is delimited by the vehicle control, for example based on the driver input or an otherwise determined value for the total drive torque of motor vehicle 1.

(12) Torque difference M.sub.D is delimited to a maximum value as a function of the driving situation. Here, reference is made in particular to the steering effort of the driver, i.e. for the applied steering torque, the vehicle speed, the used friction coefficient potential, as well as other known variables for characterizing the driving situation of a motor vehicle.

(13) FIG. 2 shows a second exemplary embodiment, the elements already known from FIG. 1 being provided with the same reference numerals. Essentially, the differences are to be further discussed in the following.

(14) In contrast to the preceding exemplary embodiment, it is provided in this case that front wheel axle 2 is only assigned one drive machine 8 which is operatively connected or coupled via a differential 14 to the two drive wheels 5, 4 of front axle 2. Drive machines 10, 11 of rear wheel axle 3 may be controlled as described above. In the case of front wheel axle 2, is implemented as a function of torque difference M.sub.D at front wheel axle 2 through active brake interventions at particular wheel 4, 5, as is known from the related art. In this case, the following applies for the overall drive:
M.sub.8=M.sub.A/i.sub.G14
M.sub.10=0.5*(M.sub.A−M.sub.D)/i.sub.G
M.sub.11=0.5*(M.sub.A+M.sub.D)/i.sub.G

(15) When determining setpoint wheel torque M.sub.8 for drive machine 8 or drive unit 8, only gear ratio i.sub.G14 as well as axle drive torque M.sub.A is taken into account for wheel axle 2. The torque difference is implemented, as already mentioned above, through brake interventions and differential 14.

(16) Naturally, the above-described method may also be carried out in a motor vehicle 1 which includes only one drive wheel at front wheel axle 2, for example, or more than two wheel axles, each having two drive wheels and two drive machines.