POSITION SENSOR AND METHOD FOR DETERMINING A POSITION OF A TIMEPIECE SETTING STEM

Abstract

A position sensor and method for determining an axial and/or an angular position of a setting stem of a timepiece. A magnet is provided on the setting stem, and at least one magnetic field sensor is configured to detect changes in magnetic field strength along at least a first axis and a second axis as the rotatable element rotates; the second axis is not parallel to the first axis. The changing magnetic field sensed by the magnetic sensor is converted into a characteristic signature path which may then be mapped onto a circular signature path in two dimensions to derive the angular position of the setting stem.

Claims

1-15. (canceled)

16. A position sensor for determining an axial and/or an angular position of a rotatable element of a timepiece, the position sensor comprising: a permanent magnet configured to rotate with the rotatable element about a rotation axis of the rotatable element; a magnetic field sensor, static with respect to the timepiece and configured to measure a magnitude and an orientation of an incident magnetic field along at least a first axis and a second axis, and detect changes in the magnetic field magnitude and orientation, due to rotation of the magnetic field of the permanent magnet, along the first axis and the second axis as the rotatable element rotates, the second axis being not parallel to the first axis; means for deducing angular orientation from the measured magnitude and orientation of the magnetic field.

17. The position sensor according to claim 16, wherein the permanent magnet is arranged in a distal axial region of the rotatable element.

18. The position sensor according to claim 16, wherein the permanent magnet is arranged in a circumferential region of the rotatable element,

19. The position sensor according to claim 16, wherein the magnetic field sensor comprises at least three mutually non-parallel measurement axes.

20. The position sensor according to claim 16, further comprising a transform unit to perform a mapping transform between a first frame of reference to a second frame of reference.

21. The position sensor according to claim 20, wherein the second frame of reference comprises a circular signature path in two dimensions.

22. The position sensor according to claim 20, wherein the first frame of reference comprises an elliptical signature path in three dimensions.

23. The position sensor according to claim 16, wherein the permanent magnet is configured to move axially with the rotatable element during an axial movement of the rotatable element.

24. The position sensor according to claim 23, wherein the rotatable element has at least a first and a second predetermined axial position, wherein a rotation of the rotatable element in the first predetermined axial position is associated with a first timepiece function, and a rotation of the rotatable element in the second predetermined axial position is associated with a second timepiece function or no timepiece function.

25. The position sensor according to claim 23, wherein the magnetic sensor comprises a first sensing mode, in which the magnetic sensor is configured to detect the axial movement of the rotatable element, and a second sensing mode, in which the magnetic sensor is configured to detect the rotation of the rotatable element.

26. The position sensor according to claim 25, wherein the magnetic sensor is configured to perform sensing at a first resolution and/or at a first sampling rate when the position sensor is in the first sensing mode, and to perform sensing at a second resolution and/or at a second sampling rate when the position sensor is in the second sensing mode, wherein the first resolution is lower than the second resolution and/or the first sampling rate is lower than the second sampling rate.

27. The position sensor according to claim 16, further comprising a shielding arranged around the position sensor to shield components of the timepiece from the magnetic field of the permanent magnet.

28. The position sensor or method according to claim 16, wherein the rotatable element is assembled to, or unitary with, a setting stem of the timepiece.

29. A method of determining a position of a rotatable element of a timepiece, the method comprising: arranging a permanent magnet to rotate with the rotatable element about a rotation axis of the rotatable element; using a magnetic field sensor to measure a magnitude and an orientation of an incident magnetic field along at least a first axis and a second axis and detect changes in the magnetic field magnitude and orientation, due to rotation of the magnetic field of the permanent magnet, along the first axis and second axis as the rotatable element rotates, the second axis being not parallel to the first axis; deducing the angular orientation from the measured magnitude and orientation of the magnetic field.

30. The method according to claim 29, further comprising performing a mapping transform between a first frame of reference to a second frame of reference.

31. The method according to claim 29, wherein the second frame of reference comprises a circular signature path in two dimensions.

32. The method according to claim 29, wherein the first frame of reference comprises an elliptical signature path in three dimensions

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Other features and advantages of the invention will become apparent from the following description of a non-limiting exemplary embodiment, which is described with reference to the appended drawings, in which:

[0020] FIG. 1 shows a schematic isometric illustration of an example setting stem and sensor according to the present invention.

[0021] FIG. 2 shows a simplified example of a three-dimensional magnetic rotation signature path resulting from a rotation of the setting stem of FIG. 1.

[0022] FIG. 3 shows a transform into two dimensions of the magnetic rotation signature path of FIG. 2.

[0023] FIG. 4 shows a shielding arrangement for the sensor of FIG. 1.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

[0024] An embodiment of the present invention will now be described in detail, with reference to the attached figures. Identical or corresponding functional and structural elements which appear in the different drawings are assigned the same reference numerals. The drawings are not to scale, and are not intended to convey any relative or absolute dimensions.

[0025] FIG. 1 shows an example of a rotatable element 3, preferably mounted on a setting stem which may be found in a timepiece. The rotatable element 3 has a rotation axis 4. In the preferred embodiment described hereunder, a setting stem of a wristwatch will be taken as an illustrative example of an application of the position sensor of the invention, wherein the rotatable element just consists of a part of the setting stem. A setting stem may also be known as a crown stem. Thus, the rotatable element 3 of this example would be a small piece, with a diameter of 0.5 mm or 1 mm, for example. FIG. 1 only shows the distal end portion of such a setting stem. The rotatable element 3 is provided with a magnet 2 attached at its distal end (i.e. in the case of a setting stem terminated by a crown, at the end of the setting stem opposite to the crown). The magnet is shown as a broad cylinder of the same diameter as the rotatable element 3, because this configuration occupies minimum extra space. However, in practice the magnet may have any practical shape. The polarity of the magnet 2 is indicated in FIG. 1 by dashed lines representing the magnetic field lines 70 of the magnet 2. The magnet 2 preferably comprises a permanent magnet, for example, comprising a ferromagnetic or ferrimagnetic or other magnetic material, but it may also alternatively comprise a piece of magnetisable material such as a ferrous metal which has, or acquires a magnetisation.

[0026] The magnetic field lines 70 impinge on a magnetic field sensor 5, which may for example be a two-axis or three-axis sensor measuring the incident magnetic field 7 along two or three measurement axes. The magnetic sensor 5 may for example be a miniature surface-mount device (SMD) fitted to a printed circuit board (PCB) 1 and connected to the PCB via SMD connections 6. The magnitude and orientation of the magnetic field lines 7 detected by the magnetic sensor 5 as the rotatable element 3 rotates through a full rotation form what will be referred to as a signature path, which effectively comprises a succession of magnitude/orientation data points. Each of the data points on the signature path (i.e. each instantaneous measured magnitude/orientation datum) corresponds to a unique angular orientation of the rotatable element 3. As will be discussed below, the sensor may thus be provided with a means for deducing the angular orientation of the rotatable element 3 from the measured magnitude and orientation of the magnetic field, for example by referring to a lookup table or by performing a calculation.

[0027] The setting stem of a timepiece such as a wristwatch may typically be movable between multiple axial positions. These may include for example a home position, in which the setting function of the stem is disabled, a time setting position, in which the rotation of the stem is used to set the time displayed by the timepiece, and a date setting position, in which the rotation of the stem is used to set the date displayed by the timepiece. These axial positions are indicated symbolically by reference 8 in FIG. 1. If the rotatable element 3 is also axially moveable between discrete axial positions in such a manner, then the signature path referred to above may be used to deduce not only the angular orientation of the rotatable element 3, but also its axial position 8, especially thanks to the change of magnitude of the magnetic field 7 when passing from one discrete axial position to another. The more space between each axial position is provided, the better the leap between magnitude values can be detected.

[0028] In order to reduce power consumption, the position sensor may be arranged so that, when the rotatable element 3 is in its home axial position (i.e. the setting function is disabled), the output of the magnetic sensor 5 is sampled infrequently and/or at a coarse resolution, since the only requirement when in this state is to detect when the rotatable element 3 is moved to an axial position 8 other than the home axial position. The infrequent sampling may be carried out once a second, for example. Then, when coarse and/or infrequent sampling detects that the rotatable element 3 is in the other axial position 8, or one of the other axial positions 8, the sampling of the magnetic sensor output may then be carried out at a higher rate and/or at a higher resolution while the setting function is enabled. The position sensor may thus be operated in an axial-detection mode, in which case the sampling rate and/or sensor resolution is relatively low, or a rotation-detection mode, in which the sampling rate and/or sensor resolution is higher.

[0029] The magnet 2 and/or the magnetic sensor 5 may be fully or partially enclosed within a magnetic shielding 11, as shown for example in FIG. 4. Such a shielding 11 may help to improve the accuracy of the position sensor by protecting the magnetic sensor 5 against stray magnetic fields from external magnetic sources, or even from other elements of the timepiece. Such shielding 11 may also help to protect other parts of the timepiece against becoming magnetised as a result of being near to the magnet 2.

[0030] The position sensor may comprise a transform unit (not shown in FIG. 1) for performing a mapping or transform operation between the magnetic field parameters detected by the magnetic sensor 2 and the rotational and/or axial position of the rotatable element 3. Alternatively, the mapping/transform function may be carried out by a separate device, such as a central processing unit (CPU) of the timepiece. FIG. 2 shows a simplified graphic representation of a three-dimensional signature path (for example a set of data points from a three-axis magnetic sensor 2) corresponding to a rotation of the magnet 2 and the rotatable element 3. FIG. 3 shows how the 3D signature path of FIG. 2 may appear when mapped on to a two-dimensional signature path. In this case, the mapping/transformation has been carried out such that the resulting 2D signature path is circular. This has the advantage that the angular position of the rotatable element may be deduced directly from the 2D data, without further calculation. For example, when a point 10 on the 3D signature path 9 maps onto a point 10′ on the 2D signature path 9′, the angular position may be simply derived from the angle θ subtended by the point 10′ relative to the x-axis.

[0031] The transform unit may perform a mathematical transform operation (such as a matrix transform, for example) in order to map the magnetic sensor output data (e.g. 3D signature path 10) on to the (preferably circular) 2D signature path. According to the preferred embodiment illustrated on FIGS. 2 and 3, this mapping corresponds to a projection of the 3D coordinates of FIG. 2 onto a plane orthogonal to the rotation axis 4. Alternatively, the mapping function may be defined in a correspondence table such as a look-up table. The former solution may provide a more accurate transformation, while the latter may be faster and require less power. These are simply examples; other mapping or transformation methods are possible.

[0032] While the invention has been described with reference to a particular example embodiment, it should be understood that many other configurations of the constituent elements are possible. For example, the magnet 2 shown in FIG. 1 as being located at the end of rotatable element 3 may alternatively be located in or on a peripheral surface of the rotatable element 3. In this case, the magnetic field lines would provide a different type of signature path, and would require a different type of transform operation in order to map the source signature path 9 on to a transformed (preferably circular) signature path 9′. Similarly, the magnetic sensor 5 may be mounted in one of many different positions relative to the rotatable element 2. It is also possible to use more than one magnetic sensor 5, and/or more than one magnet 2. Moreover, despite the fact that the permanent magnet 2 is, according to the preferred embodiment illustrated by the figures, preferably associated with a rotatable element 3 and the magnetic field sensor 5 accordingly preferably arranged to be static with respect to the timepiece, it can be appreciated that it is only essential that a relative rotational and/or axial movement between the permanent magnet 2 and the magnetic sensor 5 is provided to detect magnetic field 7 strength changes, so that according to an alternative embodiment, the magnet 2 and the sensor 5 could be swapped to be respectively arranged on a fixed element of the timepiece and on a rotatable element 3.

[0033] It will also be understood that since the invention provides an absolute angular position sensor and position detection method, incremental detection of position changes are also provided, e.g. by simply computing the difference between consecutive detected angular and/or axial positions.