Method of controlling a servo motor in an electric vehicular steering system

A method of controlling a servo motor in an electric vehicular steering system in consideration of superposition moments and by compensating for influences of the system-inherent friction, the method comprising the following steps:

• a manually applied driver steering moment is detected;
• a requested superposition moment requested by an external driver assistance system is detected;
• the driver steering moment and the requested superposition moment are added to form an intermediate moment from which an assistance moment is generated by means of a predefined, non-linear amplification function;
• starting from the detected values of the driver steering moment and of the requested superposition moment, a modified superposition moment compensating for the friction in the electric vehicular steering system is determined such that the amount of the modified superposition moment is larger, by a factor greater than one, than the amount of the requested superposition moment at least for some pairs of variates comprised of driver steering moment and requested superposition moment;
• the assistance moment and the modified superposition moment are added to form a motor moment from which a corresponding input signal for the servo motor is established;
• and
• the servo motor is controlled with the established input signal in order to provide the motor moment.