The present invention relates to a motor assembly for coupling with a gearbox of an electric power steering system, the motor assembly including a position sensor. The invention also relates to an electric power-assisted steering system.

It is known to provide a motor assembly including an electric motor for providing torque inputs to the steering column of a vehicle . In order to provide the required control for the electric motor, it is required to sense the position of the motor during operation. Generally, the position is sensed by a position sensor including a part held stationary relative to the rotor shaft and a part held stationary relative to a housing of the motor assembly. However, the space available within the motor assembly is limited and therefore it is desirable to save space, where possible. According to a first aspect of the invention, there is provided a motor assembly for coupling with a gearbox, the motor assembly comprising: a motor including a stator and a rotor having a rotor shaft; a flexible coupling means attached to the rotor shaft for transferring torque from the rotor shaft to the gearbox; a motor position sensing means for sensing a position of the rotor shaft, the motor position sensing means including a target element and detecting means for detecting the position of the target element; wherein the target element is supported by the flexible coupling means.

By providing a motor assembly as described above, it is possible to achieve a compact motor assembly without sacrificing the accuracy of the position sensor and whilst retaining the advantages of the flexible coupling means. Without such an arrangement, a compact arrangement would not be possible and space-saving would have to be accomplished elsewhere, for example by the use of a smaller motor or a disadvantageous coupling means.

The flexible coupling means may comprise a first coupler rotationally fixed to the rotor shaft, a second coupler for transferring torque to a shaft of the gearbox, and a connecting element that interengages the first coupler and the second coupler for transferring torque therebetween. The use of a flexible coupling provides advantages to the motor assembly by preventing or limiting backlash during reversals of the motion of the electric motor and by helping to prevent vibration of the shaft of the gearbox. The connecting element may comprise a spider element including a plurality of legs and the first coupler and the second coupler each include a plurality of drive teeth, the drive teeth engaging with the legs of the spider element, in use. The connecting element may be resilient. Preferably, the target element may be supported by the first coupler. More preferably, the target element may be adhered to the first coupler.

The first coupler may be integrally formed with the rotor shaft. Providing these components as one may allow the assembly to be even more compact.

Preferably, the target element may be supported on a side of the flexible coupling means facing the rotor and the detecting means may be interposed between the target element and the rotor. The target element may include an annular portion. Thus, the target element may be positioned about the rotational axis of the flexible coupling means.

The motor assembly may include two target elements adjacent to one another. Inclusion of a plurality of target elements may allow greater resolution or accuracy of the motor position sensing means.

The target element may comprise a target magnet.

Where the target element comprises a target magnet, the target element may be magnetised with a magnetisation pattern such that, as the rotor shaft rotates, the detecting means senses changes in the magnetic field detected from the target element.

Preferably, the motor assembly may further comprise keying means for orientating the target element. The keying means may include at least one detent that engages with at least one corresponding recess. The use of keying means may allow enhanced accuracy and repeatability during assembly of the motor assembly.

According to a second aspect of the invention, there is provided an electric power- assisted steering system for a vehicle, the electric power-assisted steering system comprising a housing, an electric motor fixed relative to the housing, the electric motor including a stator and a rotor having a rotor shaft, a worm shaft operatively connected to the rotor shaft, an output shaft operatively connected to a steering column, and a torque sensor adapted to produce an output signal indicative of the torque in the output shaft, the motor being adapted to apply a torque, dependent upon the output signal from the torque sensor, to the output shaft through a worm gear provided on the worm shaft, which is adapted to mesh with a wheel gear operatively connected to the output shaft,

the electric power-assisted steering system further comprising a flexible coupling means attached to the rotor shaft for transmitting rotation of the rotor shaft to the worm shaft, a motor driving circuit for providing drive signals to the electric motor, and a motor position sensing means for sensing a position of the rotor shaft, the motor position sensing means being in communication with the motor driving circuit, the motor position sensing means including a target element and detecting means for detecting the position of the target element, wherein the target element is supported by the flexible coupling means.

The electric power-assisted steering system may include any additional features as described with reference to the first aspect of the invention.

Preferably, the second coupler may be integrally formed with the worm shaft.

The invention will now be described with reference to the accompanying drawings, of which:

Figure 1 is an exploded perspective view of an electric power-assisted steering system in accordance with the second aspect of the invention;

Figure 2 is a cross-sectional view of the engagement of the rotor shaft and worm shaft of the electric power-assisted steering system of Figure 1 ; Figure 3 is an exploded perspective view of the electric motor and first coupler of the electric power-assisted steering system of Figure 1 ; Figure 4 is an exploded perspective views of the flexible coupling means, worm shaft, and target elements of Figure 1 ; and

Figure 5 is an end view of the target elements on the flexible coupling means of Figure 1.

Referring firstly to Figure 1 , there is shown an electric power-assisted steering system 100 in accordance with the second aspect of the invention. The electric power-assisted steering system 100 includes a motor assembly 102 in accordance with the first aspect of the invention and a gearbox 104.

The motor assembly 102 includes a stator 106 and a rotor 108, each visible in Figure 2, the rotor 108 being attached to a rotor shaft 1 10 and the stator 106 being held stationary relative to a housing 1 12. The rotation of the rotor 108 relative to the stator 106 therefore results in respective rotation of the rotor shaft 1 10.

Further included in the motor assembly 102 are the control electronics required for operation of the motor assembly 102. In the depicted embodiment, these take the form of a motor driving circuit included on a printed circuit board (PCB) 1 14. The motor assembly 102 also includes a motor position sensing means 1 16 that senses a position of the rotor shaft 1 10. The motor position sensing means 1 16 communicates with the motor driving circuit in order that the motor driving circuit can provide the correct signals to the motor 102 to cause effective rotation of the rotor 108. In order to do so, accurate position information relating to the rotor shaft 1 10 is required to be supplied to the motor driving circuit.

The motor position sensing means 1 16 of the present embodiment includes a detecting means 1 18, having a plurality of magnetic sensors 120, and a plurality of target element, which comprise, in the present embodiment, target magnets 122. The magnetic sensors 120 are mounted on the PCB 1 14 whilst the target magnets 122 are configured to rotate with the rotor shaft 1 10 and pass by the magnetic sensors 120, the position of the target magnets 122 being detectable by a varying magnetic field that is sensed by the magnetic sensors 120. Although a plurality of magnetic sensors 120 and target magnets 122 are described in the present embodiment, it will be apparent to the skilled person that the motor position sensing means 1 16 may be provided with one or more magnetic sensor 120 and one or more target magnets 122.

Although described as comprising target magnets 122, the target elements could include any other form of detectable element, the choice of which will be apparent to the skilled person.

The rotor shaft 1 10 is coupled to a worm shaft 124 of the gearbox 104 by a flexible coupling means 126. The worm shaft 124 then acts upon a wheel gear (not shown) that is attached to an output shaft 128 of a steering column. Thus, the electric motor 102 can provide torque to the steering column. The flexible coupling means 126 of the present embodiment is configured to prevent or reduce backlash between the rotor shaft 1 10 and the worm shaft 124, which may be caused by reversal of the direction of the rotor 108, for instance.

The flexible coupling means 126 comprises a first coupler 130, a second coupler 132, and a connecting element 134, as shown in Figure 4. The first coupler 130 is rotationally fixed to the rotor shaft 1 10 and the second coupler 132 is rotationally fixed to the worm shaft 124. In fact, the second coupler 132 is, in the present embodiment, integrally formed with the worm shaft 124 and the flexile element 134 is interposed to transmit torque between the rotor shaft 1 10 and the worm shaft 124. In the present embodiment, the connecting element 134 transmits torque through its engagement with both the first coupler 130 and the second coupler 132. Engagement is provided by a number of drive teeth 136 provided on each of the first coupler 130 and the second coupler 132. These drive teeth 136 engage with a number of legs 138 of the connecting element 134.

In the present embodiment, the connecting element 134 comprises eight legs 138, whilst the first coupler 130 and second coupler 132 each have four drive teeth 136. The drive teeth 136 and legs 138 are equally spaced relative to one another. When assembled, the drive teeth 136 of the first coupler 130 and the second coupler 132 will be flanked on either side by legs 138 of the connecting element 134, drive teeth 136 of the first coupler 130 and the second coupler 132 being alternately arranged about the connecting element 134. In this way, torque may be transmitted evenly through the flexible coupling means 126. This is but one possible arrangement and other such flexible coupling means 126 and their implementations will be known to the person skilled in the art.

Although the flexible coupling means 126 is beneficial in terms of reducing backlash between the rotor shaft 1 10 and the worm shaft 124, it may take up additional space within the electric power-assisted steering system 100, this space being at a premium due to the position at which it is required within a vehicle . Due to the small amount of available space in such a system, space saving may be required elsewhere in order to include the flexible coupling means 126.

The target magnets 122 are attached to the flexible coupling means 126, causing the target magnets 122 to move relative to the magnetic sensors 120 during rotation of the rotor shaft 1 10. In the present embodiment, the target magnets 122 are mounted on the first coupler 130. This mounting is achieved by way of an adhesive but could be by way of physical interlocking, or fixtures, for example. Other mounting methods will be apparent to the skilled person. The target magnets 122 may therefore be mounted directly to the flexible coupling means 126.

By mounting the target magnets 122 to the flexible coupling means 126, space can be saved within the housing of the electric power-assisted steering system 100. Thus, the system can be made more compact or the size of the electric motor can be increased where it would otherwise not be possible .

The target magnets 122 of the present embodiment are mounted on a side of the first coupler 130 that faces the rotor 108. As such, the manufacture of the assembly is simplified and the target magnets 122 may be in close proximity to the detecting means 1 18, which are normally provided on the PCB 1 14 or otherwise adjacent to the rotor 108 and stator 106, and which in the present embodiment are interposed between the rotor 108 and the flexible coupling means 126. This arrangement allows a smaller form factor, which is beneficial when producing an electric power-assisted steering system 100. The target magnets 122 may alternatively or additionally be mounted to another face of the flexible coupling means 126, such as a circumferential face of the first coupler 130.

Preferably, the target magnets 122 will be mounted on a face of the flexible coupling means 126 that is closest to the magnetic sensors 120. In the present embodiment, where the magnetic sensors 120 are mounted to the PCB 1 14 which is itself positioned on an end of the motor, it is therefore desirable to mount the target magnets 122 on the end face of the first coupler 130. Were the magnetic sensors 120 to be mounted about the circumference of the flexible coupling means 126, for example, the mounting the target magnets on a circumferential face of the flexible coupling means 126 would be likely to be a more desirable position.

As is clear, the target magnets 122 are supported by the flexible coupling means 126. In order to provide the compact arrangement, the target magnets are not directly supported by the rotor shaft.

The flexible coupling means 126 described above is formed of three parts, two of which are discrete, with the third formed integrally with the worm shaft 124. However, it is possible for the flexible coupling means 126 to be provided with a different structure. For example, the first coupler 130 may be integrally formed with the rotor shaft 1 10 and/or the second coupler 132 may be discretely formed and coupled to the worm shaft 124.

The target magnets 122 of the present embodiment are provided with a magnetisation pattern which is detectable by the magnetic sensors 120. The magnetic sensors 120 may comprise, as in the depicted embodiment, Hall effect sensors that detect the changes in the magnetic field pattern as the target magnets 122 rotate. Alternatively, an integrated circuit containing an encoder that comprises an array of magnetically sensitive devices can be used. Such an encoder may be capable of detecting the passage of a magnetic pole on the target magnet 122 and thus reading the magnetisation pattern that relates to the position of the rotor shaft 1 10. Another option would be to use a dipole sensor that measures the angle of the magnetic field of the target magnets 122. The two target magnets 122 can provide enhanced accuracy and/or redundancy of the motor position sensing means 1 16. For example, if one of the target magnets 122 were to become dislodged, in use, the other target magnet 122 could continue to provide the motor position sensing means 1 16 with position information, although, depending on the method of operation, the motor position sensing means 1 16 may be forced to operate with lower accuracy.

In order to ensure that the target magnets 122 are positioned correctly during assembly, it may be advantageous to provide a keying means, as shown in Figure 5. The keying means may be particularly advantageous where, as in the present embodiment, the target magnets 122 are directly adhered to the flexible coupling means 126, as they provide specific alignment of the target magnets 122 relative to the first coupler 130, which allows swift and accurate assembly. The keying means comprises detents 140 on the first coupler 130 that engage with corresponding recesses 142 in the target magnets 122. Although it is possible to provide the keying means with a single detent 140 for each target magnet 122, three detents 140 and three recesses 142 are provided in relation to each target magnet 122, in the present embodiment. Due to the number and arrangement of these detents 140 and recesses 142, the target magnets 122 may only be attached to the first coupler 130 in a single orientation.

In the present embodiment, the detents 140 are upstanding from the first coupler 130 and engage with recesses 142 in the sides of the target magnets 122. However, where, for example, the target magnets 122 are received within depressions in the first coupler 130, the detents 140 need not be upstanding. Alternatively, the detents 140 and recesses 142 could be formed on the target magnets 122 and first coupler 130, respectively, or a combination of detents 140 and recesses 142 could be used on each component. Although detents 140 and recesses 142 are used in the depicted embodiment, other keying means may also be used, as will be apparent to the skilled person.