The present invention relates to a method to control a steering system of a motor vehicle according to the preamble of claim 1 and a steering system designed to carry out the method.

In motor vehicles it is known to implement fail-functional architectures of steering systems. These include at least two powerpacks which include separate and independent controllers and motors. In case of unilateral failure of the redundant system or extreme loads or short-term performance requirements, in electric power assisted steering system (EPAS) with autonomous driving mode or in steer-by-wire steering systems, the power available may not be sufficient for steering functionality in all driving situations. Therefore, it can be desirable to increase performance of the steering system at least temporarily.

It is an object of the present invention to provide a method to control a steering system of a motor vehicle which enables the driver to steer the motor vehicle with at all times full steering functionality as provided under normal conditions, even after failure of an actuator. This object is achieved by a method to control a steering system of a motor vehicle having the features of claim 1 and a steering system designed to carry out the method.

Accordingly, a method to control a steering system of a road vehicle is provided, wherein the steering system comprises an actuator with at least two redundant power packs, and the method includes the following steps: a) Detecting a failure and/or degradation in or of a power pack of the actuator and/or switching off the malfunctioning power pack or

Detecting extreme loads or short-term performance requirements; b) Increasing the supply voltage for the at least one functioning power pack of the actuator to a value higher than the supply voltage under normal conditions to provide a higher power output of the actuator than the power under normal conditions.

The method allows on hone hand to partially compensate the failure or degradation of one power pack and on the other hand in case of extreme loads or short-term performance requirements an increase in power compared to normal conditions to provide full steering functionality. Preferably, the actuator is an actuator of a steer-by-wire steering system or an actuator of an electric power assisted steering system, which provides an assistance torque. In steer- by-wire systems the actuator can be the road wheel actuator. In that case it is preferred that the supply voltage increases based on the gap between a target rack position and an actual rack position of the road wheel actuator. The supply voltage is increased at least temporarily. Preferably, until the gap between a target rack position and an actual rack position of the road wheel actuator becomes small, namely is under a defined threshold. However, the actuator can also be the feedback actuator. In that case it is preferred that the supply voltage increases based on the gap between a target feedback torque and an actual feedback torque of the feedback actuator. The supply voltage is increased at least temporarily. Preferably, until the gap between a target feedback torque and an actual feedback torque of the feedback actuator becomes small, namely is under a defined threshold. If the road vehicle is an electric vehicle, it is preferred that in step b) a voltage converter is used to increase the supply voltage. If the road vehicle has an internal combustion engine, it is preferred that in step b) the supply voltage is increased generator controlled.

Further, a steering system for a road vehicle designed to carry out the above described method is provided. The steering system can be a steer-by-wire system or an electric power assisted steering system.

A preferred embodiment of the present invention will be described with reference to the drawings.

Figure 1 : is a schematic illustration of a steer-by-wire steering system of a motor vehicle; and

Figure 2: shows a block diagram of a method to handle failure or degradation in a road wheel actuator of the steer-by-wire steering system.

Figure 1 is a schematic drawing of a steer-by-wire steering system 1 with a steering shaft 2 connected to a steering wheel 3. There is no mechanical connection between the steering wheel 3 and the road wheels 4. A road wheel actuator 5 operates a gear rack 6 via a rack-and-pinion gear 7.

When a driver operates the steering wheel 3, steering shaft 2 is rotated, which is detected by a shaft sensor, which is not shown in the drawings. A control unit calculates an operation signal for the road wheel actuator 5 from the signal detected by the shaft sensor. By operating gear rack 6 with the operation signal, the road wheels 4 are turned. At the same time, forces introduced in the gear rack 6 from the road wheels 4 are recognized by another sensor not shown in the drawings, and a feedback signal is calculated, which is applied to the steering shaft 2 by a steering wheel actuator 8, also called feedback actuator, so that the operator can recognize the feedback in the steering wheel 3. The road wheel actuator 5 has a redundant power pack system with at least two redundant power packs, preferably two redundant power packs (not shown).

Figure 2 shows schematically a control method of the steer- by- wire steering system 1.

If a failure of one of the actuators 5,8 is detected 9, which can be a failure on one side of the redundant system or any degradation, the respective power pack is switched off and the performance of the road wheel actuator is reduced to the remaining functional powerpack.

The power of one powerpack under normal conditions is however not sufficient for steering functionality in all driving situations. Therefore, after detection of failure, the supply voltage of the at least one remaining powerpack is increased to increase the power to allow improved steering functionality.

Another situation can be that degradation in one of the actuators 5,8 takes place, which can be a degradation on one or more sides of the redundant system, which may reduce the remaining power. In case the degradation leads to a reduction in power, the remaining power might not be sufficient for steering functionality in all driving situations. Therefore, after detection of degradation, the supply voltage of the at least one degraded and/or the at least one fully functional powerpack is increased to increase the power to allow improved steering functionality.

In a third situation, the actuator 5,8 may be subjected to extreme loads or short-term performance requirements. To meet the requirements, the supply voltage of at least one powerpack is increased to increase the power to allow improved steering functionality.

In general, the actuator can be the road wheel actuator. In that case it is preferred that the supply voltage increases based on the gap between a target rack position and an actual rack position of the road wheel actuator. The supply voltage is increased at least temporarily. Preferably, until the gap between a target rack position and an actual rack position of the road wheel actuator becomes small, namely is under a defined threshold. However, the actuator can also be the feedback actuator. In that case it is preferred that the supply voltage increases based on the gap between a target feedback torque and an actual feedback torque of the feedback actuator. The supply voltage is increased at least temporarily. Preferably, until the gap between a target feedback torque and an actual feedback torque of the feedback actuator becomes small, namely is under a defined threshold.

Generally, the increase is given by a power supply provider 10. In case of internal combustion engines, the voltage increase is generator controlled with a generator in voltage control mode. This requires to measure a voltage, compare the measurement to a reference and increase/decrease the reactive power flow out of the generating equipment. In case of electric vehicles, a voltage converter is used to increase the supply voltage. The power supply provider 10 provides the increased supply voltage to respective actuator 5, 8, which accordingly has an increased output power 11.

In another embodiment of the invention the steering system is an electric power assisted steering system with an actuator. The actuator is used to assist a driver in applying torque to the steering mechanism, by applying an assistance torque of the same sense, to make it easier to turn the steering wheel. Thus, operation of the actuator may assist in rotating the steering column shaft, or moving a portion of the steering rack mechanism. Of course, the actuator may be connected to any part of any typical steering mechanism as long as it can provide an assistance torque to aid the driver in turning the steering wheel. In analogy to above, the actuator is designed to be redundant with at least two power packs. In case of failure or degradation of one side of the redundant system, or in case of extreme loads or short-term performance requirements, the method described above also applies.