The present invention relates to a rotary angle sensor for an electric power assisted steering system of a motor vehicle with the features of the preamble of claim 7. Rotary angle sensors have the ability to collectively measure steering wheel angle and speed with a number of steering wheel turns. In addition for differential torque measurement, a torque sensor is used in an electric power assisted steering system, which comprises a ring magnet which is fixed to the upper steering shaft and flux conductor parts which are fixed to the lower steering shaft. The torque sensor measures the relative shift angle between two rotating shafts of the steering column linked by a torsion bar. This angular data is converted into a voltage output and is fed to an electronic control unit of the electric power assisted steering system to determine how much power assistance is required . Furthermore, the electric motor of the electric power assisted steering system is provided with a rotor position sensor attached to the motor shaft.

Besides magnetic sensors, optical sensors are known. Current optical steering angle sensors are expensive as they use image sensors or multiple light sources. A disadvantage of commonly used rotor position sensors is, that they require counter to store the revolution number.

US 2014/0360804 Al discloses an optical torque sensor, wherein the torsion between the two shafts is read out in an analogue way based on measured polarization of transmissive light or reflected light.

US 7,327,056 B2 discloses a pattern readable for sensing a rotational speed of a motor shaft, wherein the marks in the pattern have a different reflectivity than the spaces and a photodetector receives the reflected light.

It is an object of the invention to provide an improved and simplified optical rotary angle sensor for an electric power assisted steering system of a motor vehicle with a high resolution which can provide signals after battery switch off.

This problem is solved by a rotary angle sensor with the features listed in claim 1 and an electric power steering apparatus with the features listed in claim 7.

Accordingly, a rotary angle sensor for an electric power assisted steering system of a motor vehicle, is provided, the rotary angle sensor comprises an optical sensor unit with a light source, optical components and a photodetector and a disc with an optical pattern, wherein the optical sensor unit and the disc are arranged in such a way, that they are able to rotate relative to each other around a rotary axis, and wherein the optical sensor unit is build in such a way, that light reflected from the optical pattern is measured by the

photodetector, wherein the optical pattern comprises steps and spaces separating the steps, designed such that light reflected by steps and spaces destructively interferes leading to an intensity modulation of the reflected light according to the optical pattern which encodes a binary type code for the rotary angle of the disc. This sensor is very simple and inexpensive compared to image sensors or sensors with multiple light sources. By reading out the optical pattern with the optical sensor unit, the position and the angle of the disc relative to the unit can be determined. Preferably the light source is a laser or a light-emitting diode (LED) or an integrated encoder IC.

Preferably, the steps and spaces are arranged in a circle or spiral concentric to the rotary axis. Advantageously, the light has a wavelength which is in a range between the visible light and the infrared light. Preferably the light beam has a wavelength between 230 nm and 800 nm. More preferably the light has a wavelengths between 350 nm and 550 nm.

In order to keep the optical sensor unit small and compact, it is preferred, that the optical components include a polarizing prism and a quarter wave plate.

The optical pattern can transmit a binary type code by adjusting the lengths of the steps and of the spaces. The sensitivity of the sensor can be amended by adjusting the radius of the optical pattern and the pattern layout and the density of the pattern. Preferably, the rotary angle sensor is separated into two parts, which are relative to each other moveable along the rotational axis with rotation around that axis in such a way, that the linear motion results in light attenuation, which can be detected by the photodetector and converted into the absolute rotary angle of the disc over multiple turns. This allows to provide right away at start-up of the sensor the absolute steering angle. Further an actuator can be implemented which provides low resolution at the start-up position of the battery. So a rotary position signal is provided without a storage unit. Further an electric power assisted steering system for assisting steering of a motor vehicle by conferring a support torque generated by an electric motor to a steering mechanism, the electric power steering system comprising a steering column with an upper steering shaft and a lower steering shaft linked by a torsion bar and a rotary angle sensor, as described above, is provided.

Preferably, to measure absolute steering angle, one part of the rotary angle sensor is the optical sensor unit which is arranged apart from a focusing lens, which is fixed to a housing and the other part is the focusing lens, which is moveable along the rotational axis of the steering shaft on a thread of the upper steering shaft. The linear motion and resulting light attenuation are translated into turns of the disc, allowing to measure the absolute steering angle or to detect the actuator position as a start-up signal. In one embodiment, the rotary angle sensor is a torque sensor, wherein the optical sensor unit is arranged to measure the torsion between the upper steering shaft and the lower steering shaft. In this configuration it is

advantageous, when the optical sensor unit is arranged torque-proof to the upper steering shaft and the disc is rotating with the lower steering shaft, wherein the steps and spaces are arranged in a circle or a spiral or radially concentric to the rotary axis.

It is also possible to use two rotary angle sensors to read out one disc arranged on the upper steering shaft and one disc arranged on the lower steering shaft, such that the two disks are rotating together with steering wheel motion, while having an angular offset relative to their torsion. Thereby it is preferred, that the optical patterns of the two discs are arranged concentrically without overlap.

In another embodiment, the rotary angle sensor is a rotor position sensor of the electric motor, wherein the disc is arranged at one end of the motor shaft, and wherein the rotation axis of the motor shaft is equal to the axis of symmetry of the disc and its optical pattern.

Preferred embodiments of the present invention will be described with reference to the drawings. In all figures the same reference signs denote the same components or functionally similar components. Figure 1 shows a schematic illustration of an electric power steering system;

Figure 2 shows a schematic illustration of an altenative embodiment of an electric power steering system;

Figure 3 shows schematically the functionalty of the rotary angle sensor;

Figure 4 shows the use of the rotary angle sensor as a rotor position sensor; Figure 5 shows the arrangement of the sensor on a steering shaft;

Figure 6 shows the arragement of the sensor as a torque sensor;

Figure 7 shows the arrangement of Figure 6, whereas the sensor is moveable on a thread of the steering shaft; Figure 8 shows the arrangement of a sensor, which detects the torsion of two steering shafts;

Figure 9 shows the arrangement of two sensors to detect rotation of the upper and lower steering shaft, respectively;

Figure 10 shows the arrangement of a sensor with an actuator to detect

rotation of the steering shaft;

Figure 11 shows the arrangement of two sensors with two actuators to detect the rotation of the upper and lower steering shaft, respectively; and

Figure 12 shows an exemplary optical pattern read out by the sensor.

Figure 1 and figure 2 are schematic drawings of an electric power steering system 1. A steering wheel 2 is fixed to a upper steering shaft 3, the steering movement of the driver is transmitted via a torsion bar 19 to a lower steering shaft 4. The lower steering shaft 4 is coupled to a rack 6 via a rack-and-pinion mechanism 5. Rotation of the upper and lower steering shaft 3,4

accompanying a steering operation is converted into a reciprocating linear motion of the toothed rack 6 by the rack-and-pinion mechanism 5. The linear motion of the rack 6 changes the steering angle of the steered road wheels 7. To provide steering assistance, the electric motor 8 can be mounted to the side of the steering shaft 3, shown in figure 1 or to the side of the rack 6, shown in figure 2. Transferring the assist torque from the motor 8 to the lower steering shaft 4 or the rack 6, respectively, provides the steering assistance.

The electric power steering systems according to figure 1 and 2 are equipped with a rotary angle sensor. The operation of the rotary angle sensor is explained in figure 3. An optical sensor unit 9 reads out information stored on a disk 10. The optical sensor unit 9 comprises a light source 11, an optical collimator lens 12, a polarizing prism 13, a first and second focusing lens 14, 15, a quarter wave plate 16 and a photodetector 17. Light from the light source 11 is transmitted to the lens 12, functioning as an optical collimator aligning the light. The light source 11 can be a laser or light-emitting diode (LED) or an integrated encoder IC. After that the light impinges on the polarizing prism 13 making the beam plane polarized. Next the light passes through the quarter wave plate 16, that introduces a 90° phase difference between the two components of the electric field vector. The beam is now circularly polarized. After passing through the quarter wave plate 16, the light is focused by the first focusing lens 14. The focused light impinges on the patterned disc 10 and is reflected . Passing through the quarter wave plate 16 a second time introduces a further 90° phase difference between the components, which makes the beam polarized in a plane 90° rotated from that of the beam emerging from the polarizing prism 13. The plan of polarization is now such that the polarizing prism 13 efficiently reflects the returning beam towards the photodetector 17 rather than transmitting the beam back towards the light source 11. The light is focused by the second focusing lens 15 and detected by the photodetector 17 which transforms the light into electrical current. The optical pattern 10' on the disc 10 is formed by steps 18 and spaces 18' between the steps 18. The path difference between a beam reflected from a step 18 and one reflected from a space 18' is half a wave length. This path difference is achieved in the outward and return journey of the beam by a step height of a quarter wavelength. Light from the steps 18 will destructively interfere with light reflected from the spaces 18' and thus appear dark to the photodetector 17. If light is reflected solely from steps or spaces destructive interference does not occur and the photodetector detects light. Thus interrogation of the sensor is based on reflectometry. The lengths of the steps 18 and of the spaces 18' between them transmit a binary type code, which is processed to reconstitute the absolute angular position. The steps 18 are arranged in a circle or in a spiral. Preferably, the light source beam has a wavelength which is in the range of infrared and visible light.

The optical rotary angle sensor can also be realized as rotor position sensor of the electric motor 8, as shown in figure 4. The disc 10 is arranged at the end of the motor shaft 8' of the electric motor 8, wherein the rotation axis of the motor shaft 8' is equal to the axis of symmetry of the disc 10 and the optical pattern 10', respectively.

Figures 5 to 9 show different placements of the rotary angle sensor.

In figure 5 the disc 10 is arranged on the steering shaft 3, 4 or rather fixed to the steering shaft 3, 4 in a torque-proof manner. The optical pattern 10' is arranged concentric to the steering shaft 3, 4. The optical sensor unit 9 is fixed to a housing.

Figure 6 shows the rotary angle sensor arranged so that the sensor detects the torsion between the upper steering shaft 3 and lower steering shaft 4 connected via a torsion bar 19. The optical sensor unit 9 is arranged torque- proof to the upper steering shaft 3 and the disc 10 is rotating with the lower steering shaft 4. The torque sensor is not limited to this arrangement; the optical sensor unit 9 can be likewise fixed to the upper steering shaft and the disk to the lower steering shaft, accordingly. As shown in figure 7, in a preferred embodiment of the invention the optical sensor unit 9' apart from the focusing lens 14 is fixed to the housing. The lens 14 is moveable along the rotational axis of the steering shaft on a thread of the upper steering shaft 3. This arrangement translates the steering wheel rotation into linear motion of the focusing lens 14 between the optical sensor unit and the disk 10, which is, as described above, torque-proof fixed to the lower steering shaft 4. The linear motion of the lens 14 results in light attenuation, which can be detected by the photodetector 17 and converted into the position of the focusing lens 14 and a respective absolute angle of the steering wheel 2. Even after a restart of the system, this allows to read out the absolute angle over multiple turns accurately without the need of a revolution counter. In other preferred embodiments the disk 10 or the optical sensor unit 9' are movable with respect to the remaining components of the rotary angle sensor, resulting in the same measurement technique. Figure 8 shows that a single optical sensor unit 9 can detect the torsion of the upper and the lower steering shaft 3, 4 with respect to the sensor unit 9 by reading out the information of two discs 10 placed on each steering shaft 3, 4 respectively. Figure 9 shows that two sensor units 9 can be arranged each reading out one disc arranged on the upper and the lower steering shaft 3, 4. It is also possible to mount the disks 10 directly to the two ends of the torsion bar 19. The two disks 10 are rotating together with steering wheel motion, while having an angular offset relative to their torsion. When arranging the optical pattern 10' of both disks concentrically, the patterns can be read out from the same direction.

In figure 10 the optical sensor unit 9 is arranged on an actuator 20. The actuator 20 can move the optical sensor unit 9 in an axially direction dx with respect to the steering shaft 3,4. The torsion of the steering shaft 3,4 is detected by the optical sensor unit 9 by reading out the patterns 10' of the disk 10. The information of the optical sensor unit 9 is transferred back to the actuator 20, so in a failure or when the battery is switched-off, the lower resolution start-up steering angle can be provided by reading back the resolution of the actuator 20.

In figure 11 two disks 10 are arranged next to each other and connected to the steering shaft 3,4. The optical patterns 10' of each disk 10 are read out by one optical sensor 9 which is moved in an axially or radially direction dx2 to the steering shaft 3,4 by a first actuator 20. The spiral optical patterns 10' of the other disk are read out by a second optical sensor 9 which is moved in an axially or radially direction dxl by a second actuator 20'. This arrangement provides a multi-turn angle sensing and a start-up steering angle with lower resolution.

On the surface of the disc an optical pattern is arranged, which can be seen in figure 12. The pattern consists of steps 18 and spaces 18' with different lengths. When the steering shaft 3, 4 rotates, the disc 10 rotates respectively. The optical sensor detects the reflected light and converts it into a binary signal as for example disclosed in DE 33 09 779 Al . This way the absolute angle or the torsion between two shafts can be determined . The resolution of the sensor is highly scalable with adaptation of disk radius and pattern layout.

The disk is preferably made of polymer and coated with material of high reflectivity. The steps 18 and spaces 18' which are embossed on the surface of the plastic substrate are preferably formed by injecting moulding. The optical sensor unit is preferably based on low cost injection moulded lenses and optics. Redundancy can be introduced by additional optical pattern (tracks) and interrogating optics.

Start-up steering angle can be provided with lower resolution depending on the movement of the optical parts with respect to each other and the sensitivity of the photodetector. This can be further provided by using an actuator.

In order to read out information over more than one full turn, the optical sensor unit is moved in radial direction. The steps 18 are arranged in a spiral . It is further possible to have jumping focal point and/or two layers of optical patterns.