Method for operating a steering system for a motor vehicle and steering system

1. A method for operating a steering system for a motor vehicle, having at least one steerable vehicle wheel, having a change speed gear mechanism in the transmission path between a steering wheel and the vehicle wheel and having a servomotor (M), wherein a set point torque predefined value is fed to the servomotor (M) as a function of the manual steering torque (M Hand ) generated by the driver, in order to generate a motor actuating torque (M Servo ), wherein a hysteresis torque (M Hys ) which at least approximately compensates for the friction in the steering system is taken into account in the torque predefined value for the servomotor (M) in accordance with the function:

M soll, komp =M soll +M Hys

where

M soll, komp is the compensated set point torque which is to be fed to the servomotor,

M soll is the noncompensated set point torque which is to be determined from the manual steering torque (M Hand ) and

M Hys is the hysteresis torque,

and wherein:

the hysteresis torque (M Hys ) is acquired from a hysteresis characteristic curve as a function of the noncompensated set point torque (M soll ).

2. The method as claimed in claim 1 , wherein the noncompensated set point torque (M soll ) is included, as an input variable, in the hysteresis characteristic curve, and an actual torque (M ist ) is determined as an output variable, wherein the hysteresis torque (M ist ) from the noncompensated set point torque (M soll ) according to the relationship:

M Hys =M soll +M ist .

3. The method as claimed in one of claim 1 or 2 , wherein the noncompensated set point torque (M soll ) is acquired as a function of a noncompensated motor set point torque (M Mot, soll ) and of a manual steering torque (M Hand, Ü ), generated by the driver, according to the relationship:

M soll =M Mot, soll +M Hand, Ü .

4. The method as claimed in claim 3 , wherein the manual steering torque (M Hand, Ü ) is determined from the manual steering torque (M Hand ) which is transmitted to the steering wheel by the driver and modified in the change speed gear mechanism.

5. A steering system for a motor vehicle, in particular a steering system for carrying out the method as claimed in one of claim 1 or 2 , having at least one steerable vehicle wheel, comprising a change speed gear mechanism in the transmission path between a steering wheel and the vehicle wheel, and a servomotor (M), wherein a set point torque predefined value can be fed to the servomotor (M) as a function of the manual steering torque generated by the driver, in order to generate a motor actuating torque (M servo ) which is to be transmitted to the vehicle wheel, in which the set point torque predefined value for the servomotor (M) in order a hysteresis torque (M Hys ) which at least approximately compensates for the friction in the steering system can be acquired according to a function, wherein, in a control unit, a hysteresis function which characterizes the friction in the steering system is stored, and wherein the at least approximately compensating hysteresis torque (M Hys ) can be acquired according to the function:

M soll, komp =M soll +M Hys

where:

M soll, komp is the compensated set point torque which is to be fed to the servomotor,

M soll is the noncompensated set point torque, and

M Hys is the hysteresis torque,

and the hysteresis torque (M Hys ) can be acquired from a hysteresis characteristic curve as a function of the noncompensated set point torque (M soll ).