Steer-by-wire system, motor vehicle, and method for operating a steer-by-wire system

1. A steer-by-wire system for a motor vehicle, comprising:

at least two wheels which are steerable independently of one another in a normal operating mode of the steer-by-wire system,

at least two steering actuators, each of which is assigned to one of the steerable wheels and is configured for adjusting a steering angle of a particular steerable wheel, and

at least one steering electronics system which is signally connected to the steering actuators and which is configured for controlling the steering actuators individually on the basis of steering commands,

wherein, upon detection of a failure of a steering actuator of the at least two steering actuators, thereby indicating an emergency operating mode of the steer-by-wire system, the at least one steering electronics system selects a specific steering ratio from one or multiple steering ratios for each failure scenario and uses said selected steering ratio for controlling at least one steering actuator of the at least two steering actuators that has not failed, said selected steering ratio having a directness that deviates from a directness of a steering ratio in the normal operating mode.

2. The steer-by-wire system as claimed in claim 1 , wherein upon detection of the failure of the steering actuator, said at least one steering electronics system controls the at least one steering actuator that has not failed with a more direct steering ratio than in the normal operating mode.

3. The steer-by-wire system as claimed in claim 1 , wherein the steering actuator that has failed blocks a steering movement of a steerable wheel assigned thereto.

4. The steer-by-wire system as claimed in claim 1 , wherein the steering actuator that has failed releases a steering movement of a steerable wheel assigned thereto.

5. The steer-by-wire system as claimed in claim 1 , wherein the steering actuator that has failed dampens a steering movement of a steerable wheel assigned thereto.

6. The steer-by-wire system as claimed in claim 1 , further comprising at least one coupling device which is signally connected to the at least one steering electronics system and which is configured for mechanically coupling the steerable wheels of a wheel axle to each other in a coupling state of the coupling device and for mechanically decoupling said wheels in a decoupling state of the coupling device,

wherein the at least one steering electronics system is configured for controlling the coupling device so that the coupling device is in the decoupling state in the normal operating mode of the steer-by-wire system and is in the coupling state in the emergency operating mode of the steer-by-wire system.

7. The steer-by-wire system as claimed in claim 1 , wherein a specific steering ratio for each failure scenario is stored in the at least one steering electronics system or in a memory unit contained therein.

8. The steer-by-wire system as claimed in claim 1 , wherein said system has no torque sensor.

9. A motor vehicle comprising a steer-by-wire system as claimed in claim 1 .

10. The motor vehicle as claimed in claim 9 , further comprising at least one brake module which is signally connected to the at least one steering electronics system and which is configured for controlling at least the wheel brakes assigned to the steerable wheels,

wherein the at least one steering electronics system is configured for controlling the brake module so that, in the emergency operating mode of the steer-by-wire system, the brake module limits a maximum braking force, which is generated by the wheel brake assigned to a steerable wheel having a failed steering actuator, to a predefined limiting value.

11. A method for operating a steer-by-wire system of a motor vehicle, wherein the steer-by-wire system comprises at least two wheels which are steerable independently of each other in a normal operating mode of the steer-by-wire system, and at least two steering actuators, each of which is assigned to one of the steerable wheels and is configured for adjusting a steering angle of a particular steerable wheel, said method comprising:

detecting a failure of a steering actuator of the at least two steering actuators, thereby indicating an emergency operating mode of the steer-by-wire system; and

at least one steering electronics system selecting a specific steering ratio from one or multiple steering ratios for each failure scenario and using said selected steering ratio for controlling at least one steering actuator of the at least two steering actuators that has not failed, wherein said selected steering ratio has a directness that deviates from a directness of a steering ratio in the normal operating mode.

12. The method as claimed in claim 11 , wherein upon detection of the failure of the steering actuator, the at least one steering actuator that has not failed is controlled with a more direct steering ratio than in the normal operating mode.

13. The method as claimed in claim 11 , wherein at least one steering actuator of the at least two steering actuators, in the event of the failure thereof, blocks a steering movement of a steerable wheel assigned thereto.

14. The method as claimed in claim 11 , wherein at least one steering actuator of the at least two steering actuators, in the event of the failure thereof, releases a steering movement of a steerable wheel assigned thereto.

15. The method as claimed in claim 11 , wherein at least one steering actuator of the at least two steering actuators, in the event of the failure thereof, dampens a steering movement of a steerable wheel assigned thereto.

16. The method as claimed in claim 11 , wherein a specific steering ratio for each failure scenario is stored in the at least one steering electronics system or in a memory unit contained therein, and is selected depending on the particular actuator which has failed.