The present invention relates to a steer-by-wire steering system of a motor vehicle according to the preamble of claim 1 and a method for detecting movement of a steering wheel in a steer-by-wire steering system for a motor vehicle according to the preamble of claim 6.

In a steer-by-wire steering system, the vehicle's steering wheel is disengaged from the steering mechanism. In such a steering system, there is no mechanical coupling between the steering wheel and the steering gear. A steering actuator with an electric motor achieves steering movement. The steering actuator operates in response to detected values of various steering parameters, such as steering wheel angle and vehicle speed etc. The detected values are communicated electronically to the steering actuator from sensors, whereby the electric motor drives the rack and orients the steerable wheels in the desired direction. Even though the mechanical linkage between the steering wheel and the road wheels has been eliminated, a steer-by-wire steering system is expected to produce the same functions and steering feel as a conventional mechanically linked steering system. The forces generated in moving the road wheels have to be fed back to the steering wheel to provide information for directional control to the driver. The feedback also contributes to a feeling of steering referred to as steering feel . In steer-by- wire steering systems the feedback and steering feel respectively is generated with a feedback actuator connected to the steering wheel . The feedback actuator has a steering and angle sensor (SAS), which measures the absolute angle of the steering wheel from end lock position to end lock position. This way the steering system is controlled. In case the SAS is faulty or not present, the feedback actuator does not know the position of the steering wheel and steering is not possible. Thus, the SAS cannot be dispensed.

It is an object of the present invention to provide a steer-by-wire steering system of a motor vehicle with cost-effective measurement of the position of the steering wheel and a method therefor.

This object is achieved by a steer-by-wire steering system of a motor vehicle having the features of claim 1 and a method for detecting movement of a steering wheel in a steer-by-wire steering system for a motor vehicle having the features of claim 6. Accordingly, a steer-by-wire steering system for a motor vehicle having a steering wheel and a feedback actuator connected to the steering wheel for providing road feedback to a driver, is provided, wherein the feedback actuator comprises an electric motor and an ECU, and wherein the feedback actuator has two redundant rotor position sensors, each part has at least one rotor position sensor, and wherein the ECU is equipped with a sleep-mode functionality, which in case the ignition is off wakes-up the rotor position sensors periodically to detect and measure the rotation of the electric motor's rotor. Waking up function can also be triggered by movement e.g . turn of the steering wheel. This way the position is not lost, if the steering wheel is moved externally e.g . in the workshop. A SAS can be dispensed, because the rotor angle can be measured redundantly with the rotor position sensors.

Preferably, the feedback actuator has two battery connectors without common ground . In that regard, it is advantageous if the feedback actuator has a first part and a second part connected via the two battery connectors to a first battery and a second battery, respectively. In case one of the batteries is disconnected from the system's electronic control unit, the other part of the feedback actuator connected to the other battery can take over control and the rotor position is not lost.

Preferably, the first part of the feedback actuator is connected to a first rotor position sensor of the two redundant rotor position sensors and the second part of the feedback actuator is connected to the second rotor position sensor likewise forming to independent working systems.

It is advantageous, if the functions of the first part and the second part of the feedback actuator are identical.

Further, a method for detecting movement of a steering wheel in a steer-by- wire steering system for a motor vehicle is provided, the steer-by-wire steering system having a steering wheel and a feedback actuator connected to the steering wheel for providing road feedback to a driver, wherein the feedback actuator comprises an electric motor and an ECU, and wherein the feedback actuator is calibrated during assembly of the steer-by-wire steering system by measurement of the two end lock positions of the steering wheel, and wherein the feedback actuator has two redundant rotor position sensors, and the method further comprises the following steps:

• If the ignition is off, o waking-up the rotor position sensors periodically to detect and

measure the rotation of the electric motor's rotor by a sleep-mode functionality of the ECU or; o waking-up the rotor position sensors by the movement of the rotor to detect and measure the rotation of the electric motor's rotor by a sleep-mode functionality of the ECU;

• Storing the measured position of the electric motor's rotor in a storage means;

• With ignition on, reading out the stored position of the electric motor's rotor and measurement of the current position of the electric motor's rotor;

• Crosschecking the current position of the electric motor's rotor by reading out both rotor position sensors.

With this method the feedback actuator always knows the position of the steering wheel even if the steering wheel is turned externally while the ignition is off. An SAS can be dispensed . Preferably, the feedback actuator has two battery connectors without common ground . In that regard, it is advantageous if the feedback actuator has a first part and a second part connected via the two battery connectors to a first battery and a second battery, respectively. In case one of the batteries is disconnected from the system's electronic control unit, the other part of the feedback actuator connected to the other battery can take over control and the rotor position is not lost.

Preferably, the first part of the feedback actuator is connected to a first rotor position sensor of the two redundant rotor position sensors and the second part of the feedback actuator is connected to the second rotor position sensor likewise forming to independent working systems.

It is advantageous, if the first part and the second part of the feedback actuator are identical.

Further it is preferred, that the method includes the following step : In case one of the batteries is disconnected from the system's ECU, taking over control by the other part of the feedback actuator connected to the other battery.

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

Figure 1 shows a schematic illustration of a feedback actuator 1 with two separate battery connectors 2,3 without common ground . A first part of the feedback actuator 10 is connected to a first battery 20 and a second part of the feedback actuator 11 is connected to a second battery 30 likewise. The functions of the two parts of the feedback actuator 10,11 are identical. The feedback actuator 1 is connected to a steering wheel in order to generate a steering feel (not shown). A steering shaft connected to the steering wheel is driven by the feedback actuator having an electric motor, which is connected to the steering shaft via a reduction gear. An electronic control unit (ECU) controlling the electric motor forms part of the feedback actuator 1. The feedback actuator 1 further has two independently working rotor position sensors 4,5. A first rotor position sensor 4 of those redundant sensors 4,5 is connected to the first part of the feedback actuator 10 and the second rotor position sensor 5 is connected to the second part of the feedback actuator 11, respectively. During the assembly of the steering system the feedback actuator 1 is calibrated by measurement of the two end lock positions of the steering wheel. After this scenario, the feedback actuator knows the current position of steering wheel any time. The feedback actuator is equipped with a dual sleep-mode; The ECU checks the rotor position sensors' values

periodically to detect and measure the rotation of the electric motor's rotor (e.g 10 ms). This dual sleep-mode allows measuring the rotation of the steering wheel after the ignition is off. This can be necessary e.g . in workshop. If one of the batteries is disconnected from the system's electronic control unit (ECU), the rotor position is not lost, because the other part of the feedback actuator connected to the second battery takes over. The dual sleep-mode allows to crosscheck the rotor position sensors after the ignition on cycle. A steering and angle sensor is not needed to measure the absolute angle, which reduces the prize and the size of the feedback actuator, because an extra connection is not needed. In the case a SAS is present but faulty, the above described setup can be used to determine the steering wheel angle.