Magnetic encoder

Abstract

A magnetic encoder comprising an encoder element having at least two tracks of encoder regions, each region comprising a magnetic pole, the poles along each track being arranged as an alternating pattern of North and South poles, and one or more sensors, each sensor comprising one or more sensing elements associated with a respective track and generating an output that is indicative of the magnetic field associated with that track in the vicinity of the sensor, in which at least one track has a differing number of poles to at least one of the other tracks, and in which the properties of the poles of a first one of the tracks differ along the track such that there is a periodic variation along the first track of the magnetic field emitted by the first track that is detected by the sensing elements associated with the first track which at least partially cancels out a corresponding periodic variation in field from the other tracks that is also detected by the sensing elements associated with the first track.

Claims

1. A magnetic encoder comprising: a disk shaped, rotary encoder element (2) including one or more magnetized elements that in use is fixed to a backing member (12), the one or more elements together defining first and second tracks (3, 4) of encoder regions in which the first track (3) is located concentrically around the second track (4), each region comprising a magnetic pole, the poles along each track (3, 4) being arranged as an alternating pattern of North and South poles, the poles of the first track (3) extending to the edge of the encoder element (2) to give a perimeter that at least partially extends around a circular path of radius r, and at least one sensor (5, 6) fixed in a location relative to which the location of the magnetized elements of the tracks (3, 4) are to be determined; wherein the magnetic encoder includes at least one feature for simultaneously providing a location for a mechanical fixing (13) that secures the magnetized elements relative to the moving part and that contributes to a periodic variation along the first track (3) of the magnetic field emitted by the first track (3) that is detected by the sensing elements associated with the first track (3) which at least partially cancels out a corresponding periodic variation in field from the other tracks that is also detected by the sensing elements associated with the first track (3), the at least one feature comprising an outermost edge of the poles of the first track (3) that each extends in a straight line forming a chord to the path r.

2. A magnetic encoder according to claim 1 in which the feature comprises at least one of: a difference in the properties of one or more of the poles of the first track (3) relative to other poles in the first track (3), the properties of at least one non-magnetized portion in between a pair of adjacent poles of the first track (3), and the properties of the at least one non-magnetized portion adjacent to at least one pole of the first track (3).

3. A magnetic encoder according to claim 1 in which the feature receives a portion of the mechanical fixing (13) that engages the first track or a space between poles in the first track or adjacent one or more poles in the first track that interferes with the magnetic field of one or more of the poles it is adjacent to.

4. A magnetic encoder according to claim 1 in which the backing member (12) supports the encoder element (2) and the mechanical fixing (13) engages both the backing member (12) and the at least one feature.

5. A magnetic encoder according to claim 4 in which the encoder element (2) has an outer edge that is tapered inwards away from the backing portion, so that the disk forms a truncated cone, and the mechanical fixing (13) extends up along the outer edge and tapers inwards towards the centre of the encoder disk to form an undercut which receives the outer edge.

6. A magnetic encoder according to claim 4 in which the mechanical fixing (13) comprises a can having a base part that is substantially complimentary to an external face of the backing portion that faces away from the encoder disk, and an upstanding perimeter wall that embraces an outermost edge of the backing portion and also an outermost edge of the encoder disk.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) There will now be described, by way of example only, various embodiments of the present invention with reference to the accompanying drawings of which:

(2) FIG. 1 is a cross sectional view showing the key components of a rotary encoder assembly in accordance with the present invention;

(3) FIG. 2 is a perspective view of the rotary encoder assembly of FIG. 1;

(4) FIG. 3 shows an encoder element in plan view having four flats defining keying portions provided around an otherwise circular outer perimeter;

(5) FIG. 4 shows an alternative arrangement of encoder disk having a central set of poles are enlarged compared with the two end poles;

(6) FIG. 5 shows in more detail a practical arrangement of an encoder disk where the steel inserts are provided on the outer edge of the end poles to align with the outer edge of the enlarged central poles giving magnetic poles of different strengths, the lighter shaded poles having a reduced field strength compared with the darker shaded poles as a consequence;

(7) FIG. 6 is a schematic of an encoder element of a linear encoder use in a linear implementation of the invention.

(8) FIG. 7 is an exploded view of the rotating components of an embodiment of a rotary magnetic encoder in accordance with the present invention with the sensors assembly omitted for clarity.

(9) FIG. 8 is a partial cross sectional view of the encoder of FIG. 7 showing how the encoder disk is fixed to the backing member;

(10) FIGS. 9(a) and (b) are views of the backing member;

(11) FIGS. 10 (a) and (b) are views of the mechanical fixing that secures the encoder disk to the backing member; and

(12) FIGS. 11 (a) and (b) is an alternative pair of views of the mechanical fixing prior to being installed and deformed to conform to the outer edge of the encoder disk.

DESCRIPTION

(13) As shown in FIGS. 1 and 2, an example of a rotary encoder 1 in accordance with the present invention comprises an encoder element 2 of magnetisable material. The disk has a central hole allowing it to be threaded onto a rotor shaft of a motor or other rotating object. The disk has a number of regions 8, 9 that are magnetised separated by regions of unmagnetised material. Each magnetised region forms a North or South magnetic pole. The poles are arranged to form two concentric tracks 3, 4 each centred on the axis of the metal disk. The outer track in this example comprises 32 poles arranged as alternating North and South poles that extend right to the outer edge of the metal disk. The inner track comprise 8 magnetic poles arranged as alternating North and South poles that extend right to the inner edge of the disk. In other examples there may be different numbers of poles in the inner and outer tracks. Taking the poles to the edges allows the size of the poles to be maximised for a given size of disk, which is important where the size of the disk is limited due to the geometry of the location in which the encoder is to be placed in use. FIG. 2 shows how the disk forms a truncated cone due to a taper of the outer edge of the disk.

(14) Two sensors 5, 6 are provided, each comprising multiple magnetic sensor elements that are responsive to magnetic field. One sensor 5 is placed with its sensor elements position with their detecting regions adjacent the first track 3 and the other sensor 6 is placed such that the sensing region of its sensor elements is adjacent the second track 4. The output of the two sensors 5, 6 is fed into a signal processing unit 7. This signal processing unit processes the signals to produce a measurement of the angular position of the metal disk relative to the sensors in a conventional manner.

(15) In the examples of a rotary encoder, the encoder element has a small overall diameter with a nominal inner diameter of around 12 mm and a nominal outer diameter of around 19 mm. The range of the axial height between the surface of the encoder element and each of the two sensors is between 0.8-1.5 mm.

(16) The applicant has appreciated that for small diameter encoders there is a high likelihood of inter-track magnetic interference. This may manifest as harmonic distortion of the signal output form each sensor compared with the ideal signal that would be present when there was no interference. For an encoder as shown in FIG. 2 with two tracks each having identical poles spaced around the track, the applicant observed that at the minimum height, the inner track field is strong enough that the outer track field 4th order effect on that inner track is very low and does not interfere. However, the further the sensor, the cross-talk between the inner and outer track becomes higher which is mainly due to lower field amplitude of the outer track compared to the inner track. On the other hand a strong fourth order harmonic was observed where the magnetic field of the inner track interferes with the outer track. The problem is also more exaggerated for small diameter sensor elements because the field strength of the relatively large poles decays less quickly with distance than from the small poles, hence more interference at larger gaps between the poles and sensor elements.

(17) The applicant has proposed an alternative arrangement of encoder element which ameliorates the inter-track magnetic field interference and also allows for the encoder element to be securely supported by a backing portion. An example of a suitable encoder disk that includes features that achieve the desired effect is shown in FIG. 3. This has a number of poles of the outer track that have cutaway portions on their outermost edge to form four flat keying portions 10, resulting in a reduction in the area of the pole and hence a smaller field compared with the other poles. These define four equal-spaced key regions 10, each 90 degrees apart around the disk.

(18) The poles with the flats define features that simultaneously provide for a secure fixing and a beneficial modification of the field pattern along the track.

(19) The arrangement of the cut-aways in the example reduces the interference between the inner one of the tracks on the outer track to reduce 4.sup.th order interference.

(20) It can be shown that reducing poles in the outer track of like polarity to the neighbouring poles in the inner has is most effective at reducing the 4th order cross-coupling. On the other hand, if outer track poles of opposite polarity to the neighbouring inner track poles are reduced, this will increase the 4th order interference.

(21) Other arrangements are shown in FIGS. 4 and 5, with FIG. 4 having a step change in radius of the disk as the keying feature, and FIG. 5 varying the strength of certain poles by adding steel inserts to the edges. Weaker poles are shown with light shading and stronger poles by dark shading.

(22) FIGS. 7 to 11 show how the encoder disk is supported by a backing member. The backing member, seen best in FIGS. 7 and 9, comprises a disk shaped body that has an upper surface that is complimentary to the underside of the encoder disk. The encoder disk 2 is secured to this surface using an adhesive pad 11 shown in FIG. 7. A tubular stem 14 extends away from the side of the backing member that faces away from the encoder disk 2. This may be sized to be a sliding fit onto a shaft (not shown) which is rotating and on which the sensor is to provide measurements.

(23) To further secure the encoder disk 2 to the backing member 12 a mechanical fixing 13 is provided. This is best seen in FIGS. 10(a) and 10(b).

(24) As shown in FIGS. 10(a) and 10(b), the mechanical fixing comprises a can having a disk shaped base part that is substantially complimentary to an external face of the backing portion that faces away from the encoder disk. An upstanding perimeter wall projects from the outer edge of that disk and embraces an outermost edge of the backing portion and also an outermost edge of the encoder disk. This engages the keying portions and this engagement prevents any rotation of the encoder disk relative to the backing portion. In the assembled configuration shown in FIG. 8 the walls 15 wrap around the conical edge of the encoder disk 2 to form an undercut, preventing the disk being axially displaced relative to the backing member if the adhesive pad was to fail.

(25) Other arrangements of features are possible, such as providing holes within one or more poles into which portions of the fixing mechanism are located, or holes between poles within the tracks or alongside a track. In each case the feature that allows location, such as a hole, should also simultaneously contribute to the reduction or removal of any inter track distortion of the magnetic fields.

(26) FIG. 6 shows how the invention can be applied to a linear encoder element, with a section of an encoder element 40 being shown, the section being repeated as required depending on the length of the encoder. As shown one track 42 has three elements for every 12 of the other track 44 and the track 44 with the narrower poles has two different pole shapes. The inner edge of the poles facing the track 42 with the wider poles is varied between the two designs, the magnetisation and shape and material otherwise being the same.