Method for operating an electrical power steering system of a motor vehicle and an electric power steering system for a motor vehicle

1. A method for operating an electric power steering system of a motor vehicle, the method comprising:

receiving specification of a steering wheel angle by a steering handle as a measurement for a desired wheel steering angle for at least one steerable wheel of the motor vehicle;

providing steering assistance using an electric servo drive having an electric motor;

determining an additional restoring force based upon temperature to compensate for increased internal friction in the power steering system;

determining a desired steering direction at the steering handle and determining a directional reduction amount based on the desired steering direction; and

reducing the additional restoring force by the directional reduction amount.

2. The method of claim 1 , wherein the additional restoring force is reduced based upon a manual torque-dependent characteristic curve.

3. The method of claim 1 , wherein the additional restoring force is reduced based upon a steering direction such that, from an initial position, the additional restoring force is reduced by the directional reduction amount as a first amount responsive to determination that the desired steering direction is a first direction, and the additional restoring force is reduced by the directional reduction amount as a second amount, different from the first amount, responsive to determination that the desired steering direction is a second direction opposite the first direction.

4. The method of claim 3 , wherein the additional restoring force is reduced more sharply when steering away from a 0-position than when steering toward the 0-position.

5. An electric power steering system for a motor vehicle, the electric power steering system comprising:

a steering handle, manipulation of which, specifying a steering wheel angle as a measurement for a desired wheel steering angle for at least one steerable wheel of the motor vehicle;

an electric servo drive having an electric motor for providing steering assistance; and

an electronic control unit for the electric servo drive,

wherein the electronic control unit compensates for an increased internal friction of the power steering system causing the electrical servo drive to provide an additional restoring force based upon temperature, and

wherein the electronic control unit determines a desired steering direction at the steering handle and determines a directional reduction amount based on the desired steering direction, and reduces the additional restoring force by the directional reduction amount.

6. The electric power steering system of claim 5 , wherein the electronic control unit is configured to reduce the additional restoring force based upon the manual torque-dependent characteristic curve.

7. The electric power steering system of claim 5 , wherein the electronic control unit is configured to reduce the additional restoring force based upon a steering direction such that, from an initial position, the additional restoring force is reduced by the directional reduction amount as a first amount responsive to determination that the desired steering direction is a first direction, and the additional restoring force is reduced by the directional reduction amount as a second amount, different from the first amount, responsive to determination that the desired steering direction is a second direction opposite the first direction.

8. The electric power steering system of claim 7 , wherein the electronic control unit is configured to reduce the additional restoring force more sharply when steering away from the 0-position than when steering toward the 0-position.

9. A computer program having program code means to perform a method of operating an electric power steering system of a motor vehicle when the program is executed on a microprocessor of a computer, the method comprising:

receiving specification of a steering wheel angle by a steering handle as a measurement for a desired wheel steering angle for at least one steerable wheel of the motor vehicle;

providing steering assistance using an electric servo drive having an electric motor;

providing an additional restoring force based upon temperature to compensate for increased internal friction in the power steering system;

determining a desired steering direction at the steering handle and determining a directional reduction amount based on the desired steering direction; and

reducing the additional restoring force by the directional reduction amount.

10. The computer program of claim 9 , wherein the additional restoring force is reduced based upon a manual torque-dependent characteristic curve.

11. The computer program of claim 9 , wherein the additional restoring force is reduced based upon a steering direction such that, from an initial position, the additional restoring force is reduced by the directional reduction amount as a first amount responsive to determination that the desired steering direction is a first direction, and the additional restoring force is reduced by the directional reduction amount as a second amount, different from the first amount, responsive to determination that the desired steering direction is a second direction opposite the first direction.

12. The computer program of claim 9 , wherein the additional restoring force is reduced more sharply when steering away from a 0-position than when steering toward the 0-position.

13. A control unit of a power steering system, wherein the control unit:

controls an electric servo drive of an electric power steering system for a vehicle to provide steering assistance using an electric motor of the electric servo drive;

compensates for an increased internal friction of the electric power steering system causing the electrical servo drive to provide an additional restoring force based upon temperature;

determines a desired steering direction at a steering handle and a directional reduction amount to the additional restoring force based on the determined desired steering direction; and

reduces the additional restoring force by the determined directional reduction amount.

14. The electronic control unit of claim 13 , configured to reduce the additional restoring force based upon the manual torque-dependent characteristic curve.

15. The electronic control unit of claim 13 , configured to reduce the additional restoring force based upon a steering direction.

16. The electronic control unit of claim 13 , configured to reduce the additional restoring force more sharply when steering away from the 0-position than when steering toward the 0-position.

17. The electronic control unit of claim 13 , wherein the additional restoring force is reduced based upon a steering direction such that, from an initial position, the additional restoring force is reduced by the directional reduction amount as a first amount responsive to determination that the desired steering direction is a first direction, and the additional restoring force is reduced by the directional reduction amount as a second amount, different from the first amount, responsive to determination that the desired steering direction is a second direction opposite the first direction.