The invention relates to a method of controlling a servo motor in an electric vehicular steering system.
Generally speaking, the steering mechanism of a motor vehicle converts rotary motions of a steering wheel into steering motions of steerable vehicle wheels. To assist the driver in turning the vehicle wheels, an electric motor can be provided to apply a moment which is coupled into the steering mechanism. Usually, a sensor for determining the steering moment applied by the driver is provided in the steering mechanism, the sensor data being used by the steering system to establish an assistance moment to be applied by the motor.
As a rule, the assistance moment is established based on the driver steering moment by a non-linear amplification function, as is described in EP 0 947 413 A2, for example. Incidentally, the assistance moment not only depends on the manually applied driver steering moment in most cases, but also on the vehicle speed, for instance.
An electronic control unit transmits the desired assistance moment into control signals for the electric motor so that the latter generates a corresponding motor moment; as the case may be, a mechanical gear transmission of the steering gearbox and further relevant factors such as the temperature of the power transistors in the electronic control unit may be taken into consideration.
Further, from EP 0 640 903 A1 it is known that the electronic control unit may comprise further control functions requesting additional moments from the electric motor. These control functions are a damping control, an active reset and feedback control or a lane assist system, for instance, and usually result in a so-called superposition of moments, i.e. an additional moment which is added to the assistance moment or subtracted from it.
WO 2008/071926 A1 points out the disadvantages and problems of conventional concepts for the superposition of moments and suggests a concept to overcome them, in which a requested superposition moment is considered twice during the determination of a motor moment. On the one hand, the manually applied driver steering moment is combined with the requested superposition moment to determine the input value for the non-linear amplification function and, on the other hand, the obtained assistance moment is again combined with the requested superposition moment in order to determine the motor moment.
As already mentioned in WO 2008/071926 A1, an amount of the requested superposition moment is often limited to a predeterminable maximum value for reasons of safety, allowing the driver to override this superposition moment without difficulty by a manually applied driver steering moment. Thus, the driver always keeps control of the electric steering system.
However, it has been found that the maximum values of the superposition moment usually are in a magnitude with which an effective steering motion of the vehicle wheels hardly occurs due to the friction in the vehicular steering system. This is particularly unsatisfactory in case an automatic lane assist system is used.