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 determination of an multi-turn steering wheel angle in a steer-by-wire steering system of a motor vehicle according to the preamble of claim 8. In an electromechanical power steering mechanism a steering shaft is connected to a steering wheel for operation by the driver. The steering shaft is coupled to a steering rack via a gear pinion. Steering rack rods are connected to the steering rack and to steered wheels of the motor vehicle. A rotation of the steering shaft causes an axial displacement of the steering rack by means of the gear pinion which is connected to the steering shaft in a torque-proof manner. Assist force is applied to a steering mechanism by driving an electric motor. In electromechanical power steering mechanism the multi-turn steering wheel angle can be measured via multiple sensors with gears or with a single turn sensor with sleep-mode functionality. The sleep-mode functionality turns on the sensor periodically to detect and count the rotation of the steering wheel . With this functionality the system can provide absolute (>360°) steering wheel angle information. This information is lost when the power supply is disconnected. But it is possible to recover the position information from other vehicle signals such as the individual rotation speed of the road wheels.

In steer-by-wire systems there is no mechanical connection between the steering wheel and the steering rack and the steerable wheels and steering movement is achieved by an electrically controlled motor. It is therefore not possible to recover the multi-turn steering wheel angle by observing physical values measured by the motor vehicle. It is known to add single turn or multi turn angle sensors to recover the absolute steering wheel angle information.

It is an object of the present invention to provide a steer-by-wire steering system of a motor vehicle with a cost-effective and easy multi-turn steering wheel angle determination.

This object is achieved by a steer-by-wire steering system of a motor vehicle having the features of claim 1 and a method having the features of claim 8.

Accordingly, a steer-by-wire steering system of a motor vehicle is provided comprising a steering wheel, which is mounted non-rotatably to a steering shaft, a feedback actuator for providing road feedback to a driver, wherein the feedback actuator has an electric motor with a rotor shaft and wherein the rotor shaft is connected to the steering shaft via a transmission, an index sensor arranged on the steering shaft and a rotor angle sensor, wherein the ratio of the transmission is a non-integer value. This transmission ratio creates a phase shift between the signals of the two sensors, which allows absolute angle signal recovery with rotation of the steering wheel by less than 360°. With an integer valued transmission ratio the rotor angle signal and the index sensor signal would have no phase shift over multiple steering wheel rotations, which would prevent recovering the absolute steering wheel angle over a small movement of the steering wheel .

Preferably, the index sensor is a single turn sensor, which provides signal pulses, if the steering wheel angle is within pre-defined ranges. These predefined ranges can be spaced equidistant or in a pseudorandom encoding circumferentially around the steering shaft. It is advantageous, if the

pseudorandom encoding consists of bits. Preferably, the rotor angle sensor is a single turn sensor. The transmission can be any kind of transmission as long as it has appropriate mechanical accuracy. It is preferred that the transmission is a belt drive.

Further a method for determination of an multi-turn steering wheel angle in steer-by-wire steering system of a motor vehicle is provided, the steer-by-wire steering system comprising a steering wheel, which is mounted non-rotatably to a steering shaft, a feedback actuator for providing road feedback to a driver, wherein the feedback actuator has an electric motor with a rotor shaft and wherein the rotor shaft is connected to the steering shaft via a

transmission, an index sensor arranged on the steering shaft and a rotor angle sensor, wherein said method comprises the following steps:

- Providing signal pulses by the index sensor, if the steering wheel angle is within pre-defined ranges over a single turn;

- Providing a single turn rotor angle signal by the rotor angle sensor, wherein the two signals have a phase shift as a result of a non-integer transmission ratio;

- Determination of an multi-turn steering wheel angle using the two signals.

This method presents an easy way to determine the multi-turn steering wheel angle without the need for sensors with sleep-mode functionality and multi- turn sensors.

Preferably, the pre-defined ranges are spaced equidistant or in a

pseudorandom encoding circumferentially around the steering shaft. It is advantageous, if the pseudorandom encoding consists of bits.

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 with an multi-turn steering wheel angle sensor according to the present invention; and

Figure 2 : is a graph of the signals measured by the sensors. Figure 1 shows a simplified overview of a steer-by-wire steering system 1 of a motor vehicle. A steering wheel 2 is connected to a steering shaft 3 on which an index sensor 4 is arranged . A feedback actuator 5 acts on the shaft 3 to generate a steering feel. DThe feedback actuator 5 has an electric motor 6 connected to a transmission 7 via a motor shaft 6' in form of a belt drive. An electronic control unit (ECU) 8 controls the electric motor 6 based on the rotor angle, the steering wheel angle and other signals not shown in this figure. The rotor angle is measured by a rotor angle sensor 9 arranged between the electric motor 6 and the ECU 8. At the end of the steering shaft 3 an optional steering wheel lock 10 is arranged . The optional steering wheel lock 10 is used to limit the movement of the steering wheel 2, allowing to mechanically lock the steering system 1. The transmission ratio between the motor shaft 6' and the steering shaft 3 is a non-integer value. This means that the transmission ratio is not an integer ratio (1 : N, where N is not an integer). The index sensor 4 provides signal pulses, when the steering wheel angle is within pre-defined ranges, which are determined during design and can be placed equidistant or in a pseudorandom fashion over a single turn (single turn sensor). The resulting signal periodicity of the index sensor 4 is therefore not larger than 360° on the steering wheel. The rotor angle sensor is a single turn sensor, as well.

Figure 2 shows the sensors' signals and the resulting multi-turn steering wheel angle. At the top the signal output of the index sensor 4 is shown. This signal is periodic over 360° of steering wheel rotation and in this example it consists of four pulses per steering wheel rotation. Below the signal output of the rotor angle sensor is shown . This signal is also periodic over 360° of rotor rotation. Because the transmission between the steering wheel and the rotor has a transmission ration, which is a non-integer value, the periodicity of the rotor rotation is not equivalent to the periodicity of steering wheel rotation . The signal output of the index sensor shifts in phase compared to the rotor angle signal output. This phase shift allows to recover the multi-turn (absolute) steering wheel angle without rotating the steering wheel from lock to lock when the steering wheel is at an angle, where the index signal output is high. To recover the steering wheel angle, the steering wheel has to be rotated to the next index pulse location.

In the shown example the transmission ratio from the electric motor to the steering wheel is 97 : 277 (1 : 2.85567), so one steering wheel rotation equals 2.85567 rotations of the electric motor. The periodicity of the index signal can carry a pseudorandom encoding. As an example a 7 bit pseudorandom generator can create 128 bits of which can be placed on the circumference of the sensor plate of the index sensor. After reading 7 bits from the sensor plate the absolute position of the steering wheel can be determined with a resolution of 2.8°. To get the single turn steering wheel angle value the steering wheel has to be moved by ~20°. With 5 bits of resolution, 32 bits can be placed on the circumference of the sensor plate. One bit equals 11.25°, so the steering wheel has to be moved by ~57° to read the necessary 5 bits to recover the single turn steering wheel angle value. Even though the resolution is relatively low, it is significantly higher than the resolution of a simple index sensor, and the locations of the edges of the signal can have much higher precision, than the bit width of the encoding .

The present invention provides an absolute angle sensor with an index sensor having a signal with a periodicity less than or equal to 360° with either equidistant spacing or pseudorandom encoding and a transmission which creates a phase shift between the signal of the index sensor and the rotor angle sensor allowing absolute steering wheel angle recovery without full lock- to-lock rotation of the steering wheel.