WO2006087627A1

WO2006087627A1 - Monitoring device

Info

Publication number: WO2006087627A1
Application number: PCT/IB2006/000322
Authority: WO
Inventors: Rudi De Winter
Original assignee: Melexis Nv
Priority date: 2005-02-17
Filing date: 2006-02-17
Publication date: 2006-08-24
Also published as: GB0503317D0

Abstract

A shaft 102 is constrained such that as it rotates it moves axially. A magnet 101 is mounted at the end of the shaft 102, the magnet having its magnetic axis substantially perpendicular to and intersecting with the rotational axis of said shaft 102. A monitoring device 107 is mounted adjacent to the end of the shaft. The monitoring device is provided for monitoring the position of the shaft 102 as it is rotated through angles greater than 360 degrees. The monitoring device comprises a pair of Hall elements operable to generate electrical signals in response to a local magnetic field component in the plane of the surface of the integrated circuit, said pair of Hall elements being aligned perpendicular to each other. As shaft 102 rotates the magnet 101 also rotates relative to the Hall elements in the monitoring device 107 and hence the angular position of the shaft 102 relative to the monitoring device may be determined. As axial motion occurs when shaft 102 rotates, the distance between the magnet 101 and the monitoring device 107 varies. As a result, by monitoring the overall magnitude of the local magnetic field at monitoring device 107 the axial position of shaft 102 can also be determined.

Description

MONITORING DEVICE

The present invention relates to a monitoring device for a shaft and in particular to a monitoring device for a shaft operable to determine the position of the shaft as it rotates and more particularly to such a monitoring device which is additionally operable to determine the number of rotations of the shaft.

Many applications for manual control of a mechanical apparatus require input from a shaft mounted manual control arrangement. A typical example is a steering wheel mounted on a steering column. The manual input range in such an application may cover an angular rotation range of more than one complete revolution. In such applications the desired position of the mechanical apparatus is defined not simply by the angular position of the angular input shaft but also by the number of complete 360 degree rotations that shaft has performed since the last known absolute or reference position of the mechanical apparatus.

A common type of monitoring device for such shaft rotation comprises a Hall effect sensing element (Hall element) operable to detect local magnetic fields and output a corresponding electrical signal. In this context, one or more Hall elements are provided adjacent to the shaft, the Hall elements being operable to detect fluctuations in the local magnetic field as the shaft rotates. The field fluctuations are caused by variations in the magnetic properties of the shaft around its circumference. These may be due to projections/cavities in the shaft surface, if the shaft material exhibits magnetic properties, or magnetic material attached to the shaft. As the shaft rotates, these fluctuations can be detected and the rotation of the shaft calculated. The accuracy of the determined rotational position depends upon the variation in magnetic field with rotation of the shaft and the measurement accuracy of the Hall element. To determine the rotational position typically requires two or more Hall elements mounted in proximity to the shaft and to one another such that any change in magnetic field due to rotation of the shaft is detected by two or more sensors, the phase difference between the output of said two or more Hall elements being sufficient to determine rotational position. One such system is disclosed in US patent 6,545,462 "Sensor for the detection of the direction of a magnetic field having flux concentrators and hall elements".

To determine the number of complete rotations of the shaft requires an integration of rotational information obtained since the shaft and its associated mechanical apparatus were in a known position. Any such dead reckoning system cannot account for any movement of the shaft and the mechanical apparatus that may take place whilst the sensing system is not powered. To accommodate such eventualities a further sensor or sensors are required. Typically such additional sensors are arranged to detect the position of the mechanical apparatus in a limit position or at a central or frequently occupied position. Such additional sensing can confirm the integrated dead reckoning position or can be used to correct said integrated dead reckoning position. Choice of position for such additional sensing can be critical to the application since under certain circumstances the system may be operated from start up for some time before the confirmation of position is obtained. Such additional sensing means adds to the cost and complexity of such systems. It is therefore an object of the present invention to provide a device for monitoring a shaft that alleviates or overcomes these problems.

According to a first aspect of the present invention there is provided a monitoring device for a rotatable shaft, the shaft being constrained such that rotation of the shaft results in axial movement of the shaft, the monitoring device comprising: a magnetic sensing means mounted adjacent to said shaft, said magnetic sensing means operable to sense variations in the magnitude and direction of a local magnetic field due to rotation and axial movement of the shaft; and a processing means operable to determine the rotational and axial position of the shaft from variations in the local magnetic field sensed by the magnetic sensing means.

This provides a monitoring device whereby the rotational position of a shaft can be determined and whereby the axial position can be determined without tracking the rotation of the shaft or without any alternative dedicated reference signal.

The local magnetic field is may be generated by the shaft, if it has magnetic properties, or by a magnet, if the shaft does not have magnetic properties. If the field is generated by a magnet, the magnet is preferably a permanent magnet and may be mounted on an end of said shaft. Preferably, the magnet is a bar magnet. The magnet may be mounted with its magnetic axis substantially aligned with the rotational axis of said shaft or aligned substantially perpendicular to the rotational axis of said shaft or at any other known angle to said shaft. Preferably, the magnet is mounted with its magnetic axis substantially perpendicular to the rotational axis shaft and positioned such that the magnetic axis of the magnet intersects the rotational axis of the shaft. - A - The magnetic sensing means is preferably mounted adjacent to the portion of the shaft upon which the magnet is mounted, hi embodiments wherein the magnet is mounted on an end of the shaft, the magnetic sensing means is preferably positioned substantially in line with a continuation of the axis of the shaft, adjacent to the end of the shaft.

The magnetic sensing means and the processing means are preferably formed on a single integrated circuit. The magnetic sensing means may comprise one or more Hall effect elements or more preferably, two or more Hall effect elements, the or each said Hall element being operable to generate an electrical signal in response to the local magnetic field, the magnitude of said electrical signal being determined by the magnitude of the local magnetic field. The Hall elements are preferably operable to generate an electrical signal in response to local magnetic fields in the plane of the integrated circuit but may alternatively (or additionally) be operable to generate an electrical signal in response to local magnetic fields perpendicular to the plane of the integrated circuit.

In a preferred embodiment, the magnetic sensing means comprises two Hall elements provided with their respective axes at an angle to one another, each Hall element operable to generate an electrical signal in response to the magnitude of the component of the magnetic field aligned with its axis. Preferably said angle between the respective axes is 90 degrees. Preferably said pair of Hall elements each measure a magnetic field component in a plane substantially perpendicular to the rotational axis of said shaft. Preferably, the processing means is operable to determine the rotational position of the shaft from the local direction of the magnetic field and is operable to determine the axial position of the shaft from the magnitude of the local magnetic field. Preferably, the processing means is operable to determine the direction of the local magnetic field from the magnitude of the components of the local magnetic field aligned with each Hall element. The processing means may also be operable to track a number of complete revolutions of said shaft by inferring the linear axial movement of said shaft from the variation in the overall magnitude of the magnetic field.

The device may comprise an additional Hall element operable to determine the magnitude of the magnetic field in a direction parallel to the rotational axis of the shaft, hi such embodiments, the processing means may also be operable to track a number of complete revolutions of said shaft by inferring the linear axial movement of said shaft from the variation in the magnitude of the magnetic field in a direction parallel to the rotational axis of the shaft.

The shaft may be a shaft operable to cause operation of a mechanical apparatus in response to rotation of the shaft. The shaft may have at one or more positions on its external surface one or more shaped external formations. One or more bearings may be arranged to support said shaft whilst allowing rotation of said shaft. Said bearings may be provided with shaped internal formations adapted to couple with said shaped external formations of said shaft such that rotation of said shaft causes said shaft to move through said bearing in an axial direction. The amount of axial motion of the shaft per full rotation is determined by the configuration of said external and said internal extrusions. The formations may comprise corresponding threads provided on each of said shaft and said bearing.

In an alternative embodiment, the shaft being directly monitored may be a secondary shaft that is mechanically connected to a primary shaft and wherein information on movement of the primary shaft is desired. In such embodiments, the primary shaft need not move axially as well as rotationally as long as said secondary shaft is mechanically connected to the primary shaft in such a manner that rotation of the primary shaft generates corresponding axial movement of the secondary shaft.

hi one preferred secondary shaft arrangement, the secondary shaft is provided coaxially with said primary shaft and rotational motion of the primary shaft causes the secondary shaft to extend axially from the end of the primary shaft by a greater or lesser distance. Preferably, in such embodiments, the secondary shaft is magnetised and has a magnetisation axis substantially parallel to its rotational axis, hi such embodiments, the magnetic sensing means are preferably provided adjacent to the end of the primary shaft wherefrom the secondary shaft extends and may comprise at least one Hall element arranged to detect a magnetic field component in a direction substantially parallel to the rotational axis of the shafts. Preferably, a magnet may be provided on the end of said primary shaft with a magnetic axis substantially perpendicular to the axis of rotation of said shafts and said magnetic sensing means additionally comprises Hall elements arranged to detect magnetic field components in two substantially mutually perpendicular directions in a plane substantially perpendicular to the axis of rotation of the shafts, m such embodiments, the pair of

Hall elements arranged to detect magnetic field components in two substantially mutually perpendicular directions in a plane substantially perpendicular to the axis of rotation of the shafts may be used to determine the rotational position of the shafts as described above and the Hall element arranged to detect a magnetic field component in a direction substantially parallel to the rotational axis of the shafts may be used to determine the axial position of the secondary shaft as described above. In this manner, a particularly accurate indication of the axial position of the secondary shaft can be obtained and thus it is relatively easy to distinguish between shaft positions.

According to a second aspect of the present invention there is provided a monitoring device for a rotatable primary shaft, the monitoring device comprising: a secondary shaft mechanically connected to said primary shaft such that rotation of the primary shaft drives corresponding rotation of the secondary shaft and the secondary shaft being constrained such that rotation of the secondary shaft results in axial motion of the secondary shaft; a magnetic sensing means mounted adjacent to said secondary shaft, said magnetic sensing means operable to sense variations in the magnitude and direction of a local magnetic field due to rotation and axial movement of the secondary shaft; and a processing means operable to determine the rotational and axial position of the secondary shaft from variations in the local magnetic field sensed by the magnetic sensing means and operable to determine the rotational position of the primary shaft from the determined rotational and axial position of the secondary shaft.

The device according to the second aspect of the present invention may comprise any or all suitable features described in relation to the first aspect of the present invention as desired or as appropriate. According to a third aspect of the present invention there is provided a method of monitoring a rotatable shaft, the shaft being constrained such that rotation of the shaft results in axial motion of the shaft, the method comprising the steps of: providing a magnetic sensing means adjacent to said shaft, said magnetic sensing means operable to sense variations in the magnitude and direction of the local magnetic field due to rotational and axial motion of said shaft; and determining the rotational position of the shaft from the direction of the local magnetic field and determining the axial position of the shaft from the magnitude of the local magnetic field.

The method of the third aspect of the present invention may incorporate any or all features described in respect of the first aspect of the present invention as desired or as appropriate.

According to a fourth aspect of the present invention there is provided a method of monitoring a rotatable primary shaft, said primary shaft being mechanically connected to a secondary shaft such that rotation of the primary shaft drives corresponding rotation of the secondary shaft and the secondary shaft being constrained such that rotation of the secondary shaft results in axial motion of the secondary shaft comprising the steps of: providing a magnetic sensing means adjacent to said secondary shaft, said magnetic sensing means operable to sense variations in the magnitude and direction of the local magnetic field due to rotational and axial motion of said secondary shaft; determining the rotational position of the secondary shaft from the direction of the local magnetic field and determining the axial position of the secondary shaft from the magnitude of the local magnetic field; and hence determining the rotational position of the primary shaft from the determined rotational position and axial position of the secondary shaft.

The method of the fourth aspect of the present invention may incorporate any or all features described in respect of the second aspect of the present invention as desired or as appropriate.

According to a fifth aspect of the present invention there is provided an apparatus for monitoring the position of a steering column comprising: a shaft, said shaft being an extension of said steering column and mechanically connected to said steering column such that rotational movement of the steering column is transferred to said shaft but axial motion of said shaft is not transferred back to the steering column; a mechanical arrangement whereby said shaft is constrained to move axially in response to rotation; a magnet mounted on said shaft; a magnetic sensing means mounted adjacent to said shaft, said magnetic sensing means operable to sense variations in the magnitude and direction of a local magnetic field due to rotation and axial movement of the shaft; and a processing means operable to determine the rotational and axial position of the shaft from variations in the local magnetic field sensed by the magnetic sensing means and to thereby determine the rotational position of the steering column. The apparatus of the fifth aspect of the present invention may incorporate any or all features described in respect of any one of the first to fourth aspects of the present invention as desired or as appropriate.

It is of course possible that the invention may be adapted to monitor the position of other rotatable manual input devices.

In order that the invention is more clearly understood, it will now be described further herein with reference to the accompanying drawings, in which:

Figure 1 shows a sectional view of an embodiment of a monitoring device for a shaft according to the present invention; and

Figure 2 shows a sectional view of an alternative embodiment of a monitoring device for a shaft according to the present invention.

Referring now to figure 1, shaft 102 has a screw thread 103 provided on its outer surface. Shaft 102 passes through a bearing 104, which has on its inner surface a screw thread (not visible) corresponding to the screw thread 103 on shaft 102. When shaft 102 is rotated 106, the coupling of the corresponding screw threads causes said shaft 102 to move in an axial direction 105. The distance moved in the axial direction 105 is proportional to the angle of rotation 106. A magnet 101 is also provided on the end of shaft 102, the magnet aligned such that its magnetic axis is perpendicular to and intersects the axis of shaft 102. A monitoring device 107 is mounted adjacent to the end of the shaft. The monitoring device is provided for monitoring the position of the shaft 102 as it is rotated through angles greater than 360 degrees. The monitoring device comprises an integrated circuit 107, the integrated circuit incorporating magnetic sensing means and processing means.

The magnetic sensing means comprises a pair of Hall elements operable to generate electrical signals in response to a local magnetic field component in the plane of the surface of the integrated circuit, said pair of Hall elements being aligned perpendicular to each other. As shaft 102 rotates the magnet 101 also rotates relative to the Hall elements in the monitoring device 107. For each Hall element, the magnetic field component in the same direction as its axis will vary sinusoidally as the shaft rotates. As the two Hall elements are aligned perpendicular to one another, there will be a 90 degree phase difference between the outputs of each Hall element. Processing of the resultant sinusoidal electrical signals generated by the pair of Hall elements can determine the rotational position of the shaft 102. Such a system is described in US patent 6,545,462 "Sensor for the detection of the direction of a magnetic field having flux concentrators and hall elements".

As axial motion occurs when shaft 102 rotates, the distance between the magnet 101 and the monitoring device 107 varies. As a result, the overall magnitude of the local magnetic field at monitoring device 107 varies correspondingly. If the rotation is continuous, this can be seen as variation in the amplitude of the sinusoidal output signals from the pair of Hall elements. Determination of the overall magnitude of the signal provides an indication of the separation between the monitoring device 107 and the end of shaft 102 and can thus provide an indication of the axial position o the shaft. By considering the overall magnitude of the field and the field direction, the rotational and axial position of the shaft can be determined, as due to the thread 103 on the shaft a given axial position of the shaft can only be achieved at a given rotational position.

hi an alternative embodiment, one or more additional Hall elements may be provided on the integrated circuit to measure the strength of the magnetic field and supply additional data to the processing means to improve the accuracy of the calculations.

hi a further alternative embodiment, the above arrangement may be adapted to monitor the position of a steering column in a vehicle. This can be achieved by providing a suitable mechanical arrangement whereby shaft 102 is an extension of the steering column and is connected to the steering column such that whilst rotational movement of the steering column is transferred to shaft 102, the axial movement of shaft 102 is not transferred back to the steering column, hi this way monitoring the axial position and rotational position of the shaft 102 can provide information on the absolute position of the steering column.

Referring now to figure 2, an alterative shaft monitoring arrangement is shown, hi this embodiment, shaft 202 rotates within a bearing 204 but does not move axially. The shaft 202 is provided with a magnet 201 on its end. The magnet 201 has a magnetic axis substantially perpendicular to the shaft rotation axis. A monitoring device 207 is provided adjacent to the end of shaft 202. The monitoring device 207 is similar to the monitoring device 107 described above and is provided with a pair of

Hall elements operable to detect magnetic field components in a plane substantially perpendicular to the shaft rotation axis. The monitoring device is operable as described previously to thereby determine the rotational or angular position of shaft

202.

A further arrangement is provided to determine an overall position of the shaft i.e. how many turns in one direction it has made. The alternative arrangement comprises a threaded secondary shaft 203 positioned coaxially within shaft 202. The secondary shaft 203 is moveable axially, 205, in response to rotation of shaft 202. The secondary shaft is provided with a magnet 209 having a magnetic axis substantially parallel to the shaft rotation axis. As a result, the magnitude of the local magnetic field component parallel to the axis of rotation varies as the shaft 202 rotates. The monitoring device 207 is thus provided with an additional Hall element or Hall elements operable to detect magnetic fields components parallel to the rotational axis. Variation in this component can thus be used to determine the axial position of the secondary shaft 203 and thus the overall number of turns in a particular direction that have been made by shaft 202. It is of course clear that this arrangement may also be adapted to monitor the position of a vehicle steering column or any other shaft.

It is of course to be understood that the invention is not to be limited to the details of the above embodiments which are described by way of example only.

Claims

1. A monitoring device for a rotatable shaft, the shaft being constrained such that rotation of the shaft results in axial movement of the shaft, the monitoring device comprising: a magnetic sensing means mounted adjacent to said shaft, said magnetic sensing means operable to sense variations in the magnitude and direction of a local magnetic field due to rotation and axial movement of the shaft; and a processing means operable to determine the rotational and axial position of the shaft from variations in the local magnetic field sensed by the magnetic sensing means.

2. A monitoring device as claimed in any preceding claim wherein the processing means is operable to determine the rotational position of the shaft from the local direction of the magnetic field and is operable to determine the axial position of the shaft from the magnitude of the local magnetic field.

3. A monitoring device as claimed in claim 1 or claim 2 wherein the shaft has magnetic properties and the local magnetic field is generated by the shaft.

4. A monitoring device as claimed in claim 1 or claim 2 wherein the local magnetic field is generated by a magnet.

5. A monitoring device as claimed in claim 4 wherein the magnet is a permanent magnet.

6. A monitoring device as claimed in claim 4 or claim 5 wherein the magnet is mounted on an end of said shaft.

7. A monitoring device as claimed in any one of claims 4 to 6 wherein the magnet is a bar magnet.

8. A monitoring device as claimed in any one of claims 4 to 7 wherein the magnet is mounted with its magnetic axis substantially aligned with the rotational axis of said shaft.

9. A monitoring device as claimed in any one of claims 4 to 8 wherein the magnet is mounted with its magnetic axis aligned substantially perpendicular to the rotational axis of said shaft.

10. A monitoring device as claimed in of claim 9 wherein the magnetic axis of the magnet intersects the rotational axis of the shaft.

11. A monitoring device as claimed in any preceding claim wherein the magnetic sensing means is mounted adjacent to the portion of the shaft upon which the magnet is mounted.

12. A monitoring device as claimed in any one of claims 4 to 11 wherein the magnet is mounted on an end of the shaft and the magnetic sensing means is positioned substantially in line with a continuation of the axis of the shaft, adj acent to the end of the shaft.

13. A monitoring device as claimed in any preceding claim wherein the magnetic sensing means and the processing means are formed on a single integrated circuit.

14. A monitoring device as claimed in any preceding claim wherein the magnetic sensing means comprises two Hall elements provided with their respective axes at an angle to one another, each Hall element operable to generate an electrical signal in response to the magnitude of the component of the magnetic field aligned with its axis.

15. A monitoring device as claimed in claim 14 wherein said angle between the respective axes of the Hall elements is 90 degrees.

16. A monitoring device as claimed in claim 14 or claim 15 wherein said Hall elements each measure a magnetic field component in a plane substantially perpendicular to the rotational axis of said shaft.

17. A monitoring device as claimed in any preceding claim wherein the processing means is operable to determine the direction of the local magnetic field from the magnitude of the components of the local magnetic field aligned with each Hall element.

18. A monitoring device as claimed in any preceding claim wherein the processing means is operable to track a number of complete revolutions of said shaft by inferring the linear axial movement of said shaft from the variation in the overall magnitude of the magnetic field.

19. A monitoring device as claimed in claim 18 wherein an additional Hall element is provided, said additional Hall element operable to determine the magnitude of the magnetic field in a direction parallel to the rotational axis of the shaft.

20. A monitoring device as claimed in claim 19 wherein the processing means is operable to track a number of complete revolutions of said shaft by inferring the linear axial movement of said shaft from the variation in the magnitude of the magnetic field in a direction parallel to the rotational axis of the shaft.

21. A monitoring device as claimed in any preceding claim wherein the shaft being directly monitored is a secondary shaft that is mechanically connected to a primary shaft and wherein said secondary shaft is mechanically connected to the primary shaft in such a manner that rotation of the primary shaft generates corresponding axial movement of the secondary shaft.

22. A monitoring device for a rotatable primary shaft, the monitoring device comprising: a secondary shaft mechanically connected to said primary shaft such that rotation of the primary shaft drives corresponding rotation of the secondary shaft and the secondary shaft being constrained such that rotation of the secondary shaft results in axial motion of the secondary shaft; a magnetic sensing means mounted adjacent to said secondary shaft, said magnetic sensing means operable to sense variations in the magnitude and direction of a local magnetic field due to rotation and axial movement of the secondary shaft; and a processing means operable to determine the rotational and axial position of the secondary shaft from variations in the local magnetic field sensed by the magnetic sensing means and operable to determine the rotational position of the primary shaft from the determined rotational and axial position of the secondary shaft.

23. A monitoring device as claimed in claim 22 wherein the secondary shaft is provided coaxially with said primary shaft and rotational motion of the primary shaft causes the secondary shaft to extend axially from the end of the primary shaft by a greater or lesser distance.

24. A monitoring device as claimed in claim 22 or claim 23 wherein the secondary shaft is magnetised and has a magnetisation axis substantially parallel to its rotational axis.

25. A monitoring device as claimed in any one of claims 22 to 24 wherein the magnetic sensing means are provided adjacent to the end of the primary shaft wherefrom the secondary shaft extends and comprise at least one Hall element arranged to detect a magnetic field component in a direction substantially parallel to the rotational axis of the shafts.

26. A monitoring device as claimed in any preceding claim wherein the Hall element arranged to detect a magnetic field component in a direction substantially parallel to the rotational axis of the shafts is used to determine the axial position of the secondary shaft.

27. A monitoring device as claimed in claim 26 wherein a magnet is provided on the end of said primary shaft with a magnetic axis substantially perpendicular to the axis of rotation of said shafts and said magnetic sensing means additionally comprises Hall elements arranged to detect magnetic field components in two substantially mutually perpendicular directions in a plane substantially perpendicular to the axis of rotation of the shafts.

28. A monitoring device as claimed in claim 27 wherein the pair of Hall elements arranged to detect magnetic field components in two substantially mutually perpendicular directions in a plane substantially perpendicular to the axis of rotation of the shafts are used to determine the rotational position of the shafts.

29. A method of monitoring a rotatable shaft, the shaft being constrained such that rotation of the shaft results in axial motion of the shaft, the method comprising the steps of: providing a magnetic sensing means adjacent to said shaft, said magnetic sensing means operable to sense variations in the magnitude and direction of the local magnetic field due to rotational and axial motion of said shaft; and determining the rotational position of the shaft from the direction of the local magnetic field and determining the axial position of the shaft from the magnitude of the local magnetic field.

30. A method as claimed in claim 29 wherein the method is carried out by a monitoring device according to any one of claims 1 to 21.

31. A method of monitoring a rotatable primary shaft, said primary shaft being mechanically connected to a secondary shaft such that rotation of the primary shaft drives corresponding rotation of the secondary shaft and the secondary shaft being constrained such that rotation of the secondary shaft results in axial motion of the secondary shaft comprising the steps of: providing a magnetic sensing means adjacent to said secondary shaft, said magnetic sensing means operable to sense variations in the magnitude and direction of the local magnetic field due to rotational and axial motion of said secondary shaft; determining the rotational position of the secondary shaft from the direction of the local magnetic field and determining the axial position of the secondary shaft from the magnitude of the local magnetic field; and hence determining the rotational position of the primary shaft from the determined rotational position and axial position of the secondary shaft.

32. A method as claimed in claim 31 wherein the method is carried out by a monitoring device according to any one of claims 22 to 28.

33. An apparatus for monitoring the position of a steering column comprising: a shaft, said shaft being an extension of said steering column and mechanically

connected to said steering column such that rotational movement of the steering column is transferred to said shaft but axial motion of said shaft is not transferred back to the steering column; a mechanical arrangement whereby said shaft is constrained to move axially in response to rotation; a magnet mounted on said shaft; a magnetic sensing means mounted adjacent to said shaft, said magnetic sensing means operable to sense variations in the magnitude and direction of a local magnetic field due to rotation and axial movement of the shaft; and a processing means operable to determine the rotational and axial position of the shaft from variations in the local magnetic field sensed by the magnetic sensing means and to thereby determine the rotational position of the steering column.