Method for operating a steering system

1. A method for operating a steering system for

a motor vehicle with at least one steerable wheel, a servo drive and a superimposed gear mechanism, comprising:

initiating a steering movement by the driver of the vehicle to create a steering wheel angle (δ s );

creating a control signal (δ Md ) based upon useful functions (VSR, LAFN), each of said useful functions corresponding to a steering component by which the steering wheel angle (δ S ) is adjusted at the superimposed gear mechanism to achieve a desired wheel angle (δ Fm ) of the at least one steerable wheel;

triggering the servo drive for initiating movement of the servo drive creating a motor angle (δ M ) by an amount indicated by the control signal (δ Md ), said control signal being generated based upon said useful functions (VSR, LAFN);

superimposing said motor angle (δ M ) on the steering wheel angle (δ S ) by operation of the superimposed gear mechanism to create a pinion angle (δ G ) for producing the desired wheel angle (δ Fm ) of the at least one steerable wheel; and

turning back the servo drive in a controlled manner by at least one said steering component to eliminate an inclined position of the at least one steerable wheel when at least one of said useful functions of the steering system, corresponding to at least one said steering component for turning back the servo drive, is disabled.

2. The method of claim 1 , wherein steering portions formed by the useful functions (VSR, LAFN) are superimposed for producing the control signal (δ Md ), the steering portions (δ Md , δ Md ) being limited appropriately for the turning back of the servo drive in the controlled manner.

3. The method of claim 1 or 2 , wherein a control device receives the steering wheel angle (δ S ), the pinion angle (δ G ) and further vehicle-specific parameters as input signals, a substitute mode of said at least one of said useful functions being switched on or off because of defective input signals (v X , δ G , δ S ) of the control device.

4. The method of claim 3 , wherein said further vehicle-specific parameters include a vehicle speed (v X ).

5. The method of claim 1 or 2 , wherein a variable steering gear ratio (VSR), a lead steering or a vehicle dynamics stabilization is used as one of the useful functions.

6. The method of claim 1 or 2 , wherein the turning back of the servo drive is performed slowly in the controlled manner by means of a low angular velocity.

7. The method of claim 1 or 2 , wherein a course of the control signal (δ Md ) for the motor angle (δ M ) of the servo drive is steady for the turning back in the controlled manner.

8. A method for operating a steering system for a motor vehicle including a steering wheel, at least one steerable wheel, a servo drive and a superimposed gear mechanism, comprising:

orienting the steering wheel at a steering wheel angle (δ S );

creating a control signal (δ Md ) based upon at least one useful function of vehicle-specific data corresponding to a steering component by which the steering wheel angle (δ S ) is adjusted at the superimposed gear mechanism to achieve a desired wheel angle (δ Fm ) of the at least one steerable wheel, said at least one useful function including a variable steering gear ratio (VSR);

turning the servo drive to a motor angle (δ M ) in accordance with the control signal (δ Md );

superimposing said motor angle (δ M ) on the steering wheel angle (δ S ) by the superimposed gear mechanism to create a pinion angle (δ G ) for producing a steering angle of the at least one steerable wheel (δ Fm ); and

eliminating said motor angle (δ M ) in a controlled manner by appropriate movement of the servo drive when said at least one usable function is disabled in order to prevent an inclined position of the at least one steerable wheel (δ Fm ).

9. The method of claim 8 , wherein the variable steering gear ratio (VSR) is disabled in an absence of an input signal of the steering wheel angle (δ S ).