Magnetic encoder

Abstract

The present invention discloses a magnetic encoder, comprising: an encoder element, the encoder element including a magnetized element fixed to a backing member in use, the magnetized element together defining an encoder region of at least two tracks, each region including a magnetic pole, the magnetic poles along each track being arranged in an alternating pattern of north and south poles; a sensor fixed in position to determine the position of the magnetized element of the track corresponding to the position; the magnetic encoder including at least one feature that provides a position for a mechanical fixing member that fixes the magnetized element relative to a moving part and contributes to a periodic change along the first track of the magnetic field emitted by the first track and detected by a sensing element associated with the first track, the periodic change at least partially canceling out a corresponding periodic change of the fields of other tracks, also detected by the sensing element associated with the first track.

Description

Technical Field

[0001] The present invention relates to improvements in magnetic encoders and, in particular, to small multi-track linear or rotary encoders. Background Art

[0002] Linear or rotary magnetic encoders can be used to measure position and are commonly used to measure the linear movement or rotational position of an object such as a motor rotor. An encoder typically includes three main components. The first is the encoder element, which has a plurality of encoding regions arranged along at least one elongated track. The track of a linear encoder should be linear and extend from one end of the encoder to the other, while the track of a rotary encoder should be curved to form an endless annular track around the axis of rotation. A set of sensors is located near the encoder, which detects the passage of the encoder regions as the encoder translates or rotates about the axis, and finally provides a signal processor to process the output signals from the sensors. The encoder can be used as an incremental encoder or an absolute position encoder, which differ in the way the sensors respond to the passing encoder regions and the way the signal processor processes the output signals.

[0003] In a typical magnetic encoder, the encoder element includes a set of magnetized regions arranged along the track, which includes an alternating pattern of north and south poles. As the encoder moves, the magnetic poles move relative to the sensors. The sensors detect the changing magnetic field, and this causes a corresponding change in the output of the sensors.

[0004] The magnetic poles can be formed by locally magnetizing regions of a material that can be permanently magnetized, and these magnetic poles can be formed in a disk shape or an annular shape or other shapes. In other arrangements, a plurality of individual magnets can be fixed to or embedded in a substrate at spaced positions to define the track.

[0005] In some applications, it is beneficial to provide two tracks arranged in parallel for a linear encoder and two tracks around a common axis of rotation for a rotary encoder. If one track has wider magnetic poles and the other track has narrower magnetic poles such that for a given length of track, there are more magnetic poles on one track than on the other track, then the sensors can be used to make a high-resolution incremental measurement by combining the position of the track with many magnetic poles with the low-resolution position of the track with fewer magnetic poles. For example, in the case of measuring the position of a motor rotor using a rotary encoder, when the motor starts, the first switch of the low-resolution track can determine the direction in which the rotor is pointing, and subsequently the track with many magnetic poles can provide a very high-resolution position for fine motor control.

[0006] The use of two parallel or concentric tracks can also provide opportunities for fault diagnosis. For example, if the signals from the track with fewer magnetic poles provided by the sensor do not match the signals from the track with more magnetic poles, then one of the sensors may be faulty.

[0007] In the case of having two tracks, each track can be associated with a set of sensors, where each set responds to changes in the magnetic field caused by the movement of the magnets of the corresponding track. By using a different number of encoder regions in each track and by offsetting at least some of the transitions between adjacent encoder regions of one track from the transitions of the other track - or by offsetting the sensors - and comparing the outputs from the two sensing elements, the rotational position and rotational direction can be determined. This is well known in the art.

[0008] The applicant has found that there are problems with magnetic encoders having two adjacent tracks, and this problem is amplified in the case of smaller encoders. In some applications, such as the measurement of the angular position and angular direction of a motor rotor, the packaging constraints can be very tight, which means that the encoder disk carrying these two concentric tracks with magnets must have a small diameter. This requires the use of smaller magnetic encoder regions and packing these regions closely together. The applicant has observed that magnets that are small in size and in close proximity may cause the magnetic field of the magnets in the inner track to affect the magnetic field of the magnets in the outer track and vice versa. This interference will vary around the tracks due to the different number of magnets used in each track and the offset of the magnets between the tracks. Also, due to manufacturing tolerances, the position of the field sensors may vary between encoders, and thus when the disk rotates past the sensor elements, the intensity and harmonic content of the fields sensed from each different track will be different, and thus the harmonic content of the sum of these fields will be different, and thus the harmonic distortion of the sensor output will vary between encoders.

[0009] In a practical arrangement, the encoder elements will need to be supported by a backing part, which enables the encoder elements to be fixed to the moving part. Therefore, suitable means for fixing the backing part to the encoder elements are required. SUMMARY OF THE INVENTION

[0010] An object of the present invention is to provide an encoder assembly that can firmly fix the encoder to the backing part in a compact manner while reducing the undesired effects of harmonic noise caused by the interaction of the fields between the tracks.

[0011] According to a first aspect, the present invention provides a magnetic encoder, the magnetic encoder comprising: an encoder element including one or more magnetized elements fixed to a backing member in use, the one or more elements together defining an encoder region of at least two tracks, each region including a magnetic pole, the magnetic poles along each track being arranged in an alternating pattern of north and south poles;

[0012] and

[0013] at least one sensor fixed in position to determine the position of the magnetized elements of the track relative to the position;

[0014] characterized in that the magnetic encoder includes at least one feature that not only provides a position for a mechanical fixing for fixing the magnetized elements relative to a moving part, but also contributes to a periodic variation along the first track of a magnetic field emitted by the first track and detected by a sensing element associated with the first track, the periodic variation at least partially canceling out a corresponding periodic variation of the fields of other tracks, also detected by the sensing element associated with the first track.

[0015] The feature may include at least one of the following: a characteristic difference of one or more magnetic poles in the first track relative to other magnetic poles in the first track; and a characteristic difference of at least one non-magnetized track portion between a pair of adjacent magnetic poles in the first track or adjacent to at least one magnetic pole in the first track.

[0016] The feature may define a keying portion that provides a mechanical attachment point for the mechanical fixing.

[0017] The keying portion may be a hole within the encoder element, or an edge of a magnetic pole, or an inter-pole region of the encoder element, or a region of the encoder element offset from the first track and adjacent to one or more magnetic poles in the first track, which in each case may engage with a part of the mechanical fixing.

[0018] The feature may receive a part of the mechanical fixing, the part of the mechanical fixing engaging with the track, or with a space between magnetic poles in the track or adjacent to one or more magnetic poles of the track, the space interfering with the magnetic field of one or more adjacent magnetic poles.

[0019] The encoder may include a backing member supporting the encoder element and a mechanical fixing engaging both with the backing member.

[0020] Those skilled in the art understand that in the context of the rotational movement of a rotary encoder, the term "periodic variation" refers to the harmonic frequencies that can be observed in the output of the sensor assembly, which are harmonics of the rotational frequency of the encoder. For example, the 4th harmonic means there are 4 cycles per revolution. For a linear encoder where the magnetic poles move in a linear motion, the term "harmonic" refers to the frequency that can be observed in the output of the sensor assembly, which is a function of the linear motion speed, and as the speed increases, the frequency of the periodic variation increases.

[0021] The periodic variation of the field along the length of the first track and the engagement portion can be achieved simultaneously by forming the first track using at least two different designs of magnetic poles that differ in shape characteristics and optionally also differ in the characteristics of at least one of the following: area, magnetic strength, and orientation compared to another set of magnetic poles, or proximity to other magnetic materials.

[0022] These magnetic poles can be considered to form two sets of magnetic poles; one set has the first design and the other set has a different second design. In some arrangements, there can be more than two sets of magnetic poles in the track. One set can include one or more than one magnetic pole.

[0023] Mechanical fasteners that engage with one or more engagement portions of the encoder element can prevent the encoder from moving relative to the backing member in all directions.

[0024] The engagement portion can include an extended edge portion of the magnetic pole relative to the shape of the magnetic pole in the case where there is no engagement portion. Alternatively, the keying portion can include a radial shoulder defined by the common magnetic limit of a pair of magnetic poles, and the common magnetic pole is placed side by side with the magnetic pole that is cut away to expose the radial shoulder. In another alternative, a pair of magnetic poles can define a wider gap between the magnetic poles, and the gap includes a hole that serves as the engagement portion. The wider gap can be produced by reducing the width of one or both magnetic poles along the track, so that the field varies compared to other magnetic poles that define a smaller gap.

[0025] The engagement portion can include a hole located within the boundary of one or both of the magnetic poles in a pair of magnetic poles, and a part of the mechanical fastener can pass through the hole.

[0026] Most preferably, the engagement portion can include a cut-away edge portion of the magnetic pole relative to the shape of the magnetic pole in the case where there is no engagement portion.

[0027] In each case, for a given magnetic pole area, the magnetic field strength of the magnetic pole can be constant, so a smaller magnetic pole has a lower total field strength compared to a larger magnetic pole.

[0028] Placing the magnetic poles with cut - out portions for receiving mechanical fasteners in regions where the influence of the inter - track field is compensated ensures that the cut - outs for the mechanical fasteners have a beneficial effect on performance without increasing the overall size of the assembly, since some parts are cut away to receive the fasteners.

[0029] The terms "extended" or "cut - out" mean that when observed in a plan view and a registration view, the shape of the magnetic pole with an engaging portion is substantially similar to the shape of the magnetic pole without an engaging portion and has the same size, such that they completely overlap except for an additional or missing portion having a second shape (which can be considered as cut away). Some parts of the magnetic pole may have been physically cut away. It will be understood that this does not necessarily mean that it has been physically cut away, only that it is missing from the shape compared to other magnetic poles, and this can be achieved by magnetizing regions of material of different shapes, or by removing material portions, or by shaping magnetized components.

[0030] The applicant has realized that changing the design of a set of magnetic poles (perhaps by removing material from some of the magnetic poles compared to others on the first track) can reduce the magnetic pole area so that the field in some magnetic poles is moderated, thereby reducing the superposition effect of the fields of adjacent tracks. Thus, although there is an additional unwanted magnetic field of the second track in some regions, it can be compensated by an equal and opposite change in the actual magnetic field of the first track in these regions, so that the sensor generally does not observe an unwanted magnetic field.

[0031] The encoder element can comprise a single item, such as a disk or strip of magnetizable material. Alternatively, each track can be provided on a separate item, and the two are fixed so that they do not move relative to each other.

[0032] By removing portions of magnetized material from the encoder element during manufacture, different - shaped magnetic poles can be provided, for example, providing holes through the material, straight edges, slots, corners, or other engaging features.

[0033] The encoder element can be arranged such that the magnetic poles in the first track (e.g., by reducing the area) have a reduced field strength relative to other magnetic poles in that track. The smaller - field magnetic poles can be positioned at locations where the field of other tracks has the greatest enhancement on the field of the first track. Larger magnetic poles can be provided at locations where the field of other tracks has the greatest cancellation on the field of the first track. Thus, the smaller magnetic poles may only be located at half of the region with the greatest interference.

[0034] The magnetic poles of the second track may all have the same shape and may be evenly spaced along the track on the encoder element. Thus, it is possible that only the shape of the magnetic poles of one track has changed. The magnetic poles of the second track may or may not all have the same shape, and the magnetic poles of the second track may or may not be evenly spaced along the track on the encoder element. It is possible that only the shape of the magnetic poles of one track has changed, or it is possible that not only the shape of the magnetic poles of one track has changed.

[0035] The encoder may be a linear encoder, in which case each track will include a linear array of magnetic poles arranged parallel to the other tracks.

[0036] Alternatively, the encoder may be a rotary encoder having two tracks arranged around a common axis. These tracks may be arranged concentrically on a disc-shaped element, or may have the same radius but be axially displaced from each other and thus be parallel. In the latter case, these tracks may be formed on a tubular element.

[0037] In the case where these tracks are concentric, the first track may be the outer track or the inner track of the rotary encoder.

[0038] Compared with the wider magnetic poles of the second track, the first track may include a plurality of narrower magnetic poles such that the first track has more magnetic poles along a given length compared to the second track. Wide or narrow refers to the width of the magnetic poles measured along the direction of the track, and in the case of a rotary encoder, the pole width will be around the circumference.

[0039] The narrower magnetic poles in the first track may be cut at the inner radius if the inner radius of the track is closer to the second track, or at the outer radius if the outer radius of the first track is closer to the second track, in order to effectively cancel the interference of the field of the second track on the field of the first track.

[0040] Alternatively, compared with the first track, the second track may include a plurality of narrower magnetic poles such that the second track has more magnetic poles along a given length compared to the first track. In this case, the narrower magnetic poles of the second track may be cut at the inner radius if the inner radius of the track is closer to the first track, or at the outer radius if the outer radius of the second track is closer to the first track, in order to effectively cancel the interference of the field of the first track on the field of the second track.

[0041] In one embodiment, the encoder may be a rotary encoder. The first track may be positioned concentrically around the second track, and the magnetic poles of the first track may extend to the edge of the disc, and the fixing mechanism engages with the engaging portion provided at the outer periphery of the disc.

[0042] The first track may include arcuate magnetic poles having a curved outermost peripheral edge that defines the curved outer edge of the encoder disk along a path of radius r, and other magnetic poles including features defining an engagement portion may have an outermost edge that extends along a straight line of a chord formed for the path r. The total area of these magnetic poles of the first shape will thus be less due to bits being missing or cut away than the area of the magnetic poles of the first shape. The cut-away portion will include a circular segment that would otherwise be enclosed between the chord and an arc of a circle having the radius r of the outer edge of the encoder disk, the ends of the arc being joined to the ends of the chord.

[0043] The fixing mechanism may engage the flat outermost edge. Because they extend along the chord, this engagement will prevent the encoder disk from rotating.

[0044] As an alternative to the straight outer edge of the magnetic poles of the second shape, another shape may be used for the engagement portion as long as the shape does not exactly follow the path of radius r.

[0045] The encoder disk may have an outer edge that tapers inwardly away from the backing portion such that the disk forms a frustum, and the fixing means may extend upwardly along the outer edge and taper inwardly towards the center of the encoder disk to form an undercut for receiving the outer edge. In this way, the fixing mechanism surrounds the outer edge, thereby helping to prevent axial movement of the disk relative to the backing element.

[0046] The side of the encoder disk facing away from the sensor (forming the lower side of the encoder disk) may be conical such that the disk has a larger radius near the center and the radius decreases as it moves away from the center. The surface of the encoder disk facing the sensor may be flat.

[0047] The backing portion may be captured between the fixing element and the backing portion. The fixing element may include a body portion located on the side of the backing portion opposite the encoder element and side walls that surround the edges of both the backing portion and the encoder element to hold them together.

[0048] The fixing mechanism may include a can-shaped portion having a base portion that is substantially complementary to the outer surface of the backing portion facing away from the encoder disk and an upright peripheral wall that surrounds the outermost edge of the backing portion and also surrounds the outermost edge of the encoder disk.

[0049] The peripheral wall may extend continuously around the circumference of the base portion or may be discontinuous to define a set of engagement fingers, each engagement finger engaging a corresponding engagement portion.

[0050] The mechanical fixing mechanism may include metal components.

[0051] The backing part may include a disc-shaped body having an upper surface that is substantially complementary to the lower side of the encoder element. For example, if the lower side of the encoder disc is convex, the upper surface may be concave.

[0052] The rod may extend away from the side of the backing part that faces away from the encoder disc.

[0053] The rod may be hollow to allow the backing part to be screwed onto a shaft, such as the rotor shaft of a motor.

[0054] The encoder device may further include an adhesive pad that adheres the backing part to the lower side of the encoder disc. This will help prevent axial and radial movement when supporting the fixing mechanism.

[0055] Each magnetic pole may have a curved inner edge and two linearly extending outer edges that extend radially.

[0056] The curved inner edge (if provided) may be located on a circular path that extends around the axis of rotation of the encoder.

[0057] One or more of these magnetic poles may be provided with at least one hole within the perimeter of the magnetized pole. The hole may be unfilled or may be filled with a magnetic material having a different field strength than the area surrounding the magnetic pole to alter the magnetic field of that magnetic pole compared to the other magnetic poles in the track.

[0058] The reader will understand that the two tracks may include a first track that is concentrically arranged outside the second track. Alternatively, the first track may be concentrically arranged outside the second track.

[0059] The outer track may include more magnetic poles than the inner track.

[0060] Each magnetic pole of the first track may have a different shape. These magnetic poles are fully customized for the application, or there may be multiple magnetic poles of a first shape and multiple magnetic poles of a second shape in the first track. There may be a third shape, or even more shapes of magnetic poles.

[0061] The encoder may be arranged such that when the north pole on the track with a high number of magnetic poles approaches the north pole on another track with a low number of magnetic poles, the area of the north pole on the track with a large number of magnetic poles can be reduced to achieve a larger measurement gap.

[0062] Alternatively, the area of the special magnetic poles may be increased on the track with a larger number of magnetic poles to achieve a smaller measurement gap. The measurement gap refers to the physical spacing between the track of the encoder element and the associated sensor of the entire encoder.

[0063] In a most preferred arrangement, the encoder element includes a magnetic encoder region of two concentric annular tracks, each region including a north or south pole, the outer track defining a first track which includes a first set of ordinary encoder regions and a second set of special regions, whereby the disc has a generally circular perimeter which defines the outer perimeter of the ordinary encoder regions, and wherein the inner edges of one or more magnetic poles of the outer track are different from the inner edges of the other magnetic poles of the outer track to achieve a desired compensation for inter-track field distortion and optionally provide a region for placement of mechanical fixings.

[0064] A rotary magnetic encoder may include a signal processor which receives the outputs of sensors from two sensor assemblies and processes the plurality of outputs to provide a signal indicative of the angular position of the encoder element. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Various embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

[0066] Figure 1 is a cross-sectional view showing key components of a rotary encoder assembly according to the present invention;

[0067] Figure 2 is Figure 1 a perspective view of the rotary encoder assembly;

[0068] Figure 3 shows in plan view an encoder element having four flat portions which define engaging portions arranged around an otherwise circular outer perimeter;

[0069] Figure 4 shows an alternative arrangement of the encoder disc, in which the central magnetic pole group is enlarged compared to two end magnetic poles;

[0070] Figure 5 shows in more detail the actual arrangement of the encoder disc, in which steel inserts are provided on the outer edges of the end magnetic poles to align with the outer edges of the enlarged central magnetic poles, thereby providing magnetic poles of different strengths, such that the lighter shaded magnetic poles have a reduced field strength compared to the darker shaded magnetic poles;

[0071] Figure 6 is a schematic view of an encoder element of a linear encoder used in a linear embodiment of the present invention.

[0072] Figure 7 is an exploded view of a rotary component of an embodiment of a rotary magnetic encoder according to the present invention, in which the sensor assemblies are omitted for clarity.

[0073] Figure 8 is Figure 7Partial cross-sectional view of the encoder, showing how the encoder disk is fixed to the backing member;

[0074] Figures 9(a) and 9(b) are views of the backing member;

[0075] Figures 10(a) and 10(b) are views of the mechanical fasteners for fixing the encoder disk to the backing member; and

[0076] Figures 11(a) and 11(b) are a pair of alternative views of the mechanical fasteners before they are installed and deformed to conform to the outer edge of the encoder disk. Detailed Description

[0077] As Figure 1 and Figure 2 shown, an example of a rotary encoder 1 according to the present invention includes an encoder element 2 of magnetizable material. The disk has a central hole to allow the disk to be screwed onto the rotor shaft of a motor or other rotating object. The disk has a plurality of magnetized regions 8, 9 separated by regions of non-magnetized material. Each magnetized region forms a north or south magnetic pole. The magnetic poles are arranged to form two concentric magnetic tracks 3, 4, each concentric track being centered on the axis of the metal disk. In this example, the outer magnetic track includes 32 magnetic poles, which are arranged as alternating north and south poles extending all the way to the outer edge of the metal disk. The inner magnetic track includes 8 magnetic poles, which are arranged as alternating north and south poles extending all the way to the inner edge of the disk. In other examples, the number of magnetic poles in the inner and outer magnetic tracks may be different. Moving the magnetic poles to the edge maximizes the size of the magnetic poles for a given disk size, which is important when the disk size is limited due to the geometry of the location where the encoder is to be placed during use. Figure 2 Shows how the disk forms a frustum due to the coning of the outer edge of the disk.

[0078] Two sensors 5, 6 are provided, each sensor including a plurality of magnetic sensor elements responsive to a magnetic field. One sensor 5 is placed such that the position of its sensor elements and its detection area are adjacent to the first magnetic track 3, while the other sensor 6 is placed such that the sensing area of its sensor elements is adjacent to the second magnetic track 4. The outputs of the two sensors 5, 6 are fed into a signal processing unit 7. The signal processing unit processes the signals in a conventional manner to produce a measurement of the angular position of the metal disk relative to the sensors.

[0079] In an example of the rotary encoder, the total diameter of the encoder element is relatively small, with a nominal inner diameter of about 12 mm and a nominal outer diameter of about 19 mm. The axial height range between the surface of the encoder element and each of the two sensors is between 0.8 mm and 1.5 mm.

[0080] The Applicant has realized that for encoders with a smaller diameter, there is a greater possibility of magnetic interference between tracks. This can manifest as: compared to the ideal signal that would exist without interference, the signals output from each sensor have harmonic distortion. For an encoder as Figure 2 shown (which has two tracks, each track having the same magnetic poles spaced around the track), the Applicant has observed that at the minimum height, the field of the inner track is strong enough so that the field of the outer track has a very small fourth-order effect on this inner track and does not cause interference. However, the farther the sensor is, the higher the crosstalk between the inner track and the outer track becomes, mainly because the field amplitude of the outer track is lower compared to the inner track. On the other hand, stronger fourth-order harmonics have been observed where the magnetic field of the inner track interferes with the outer track. For smaller-diameter sensor elements, this problem is also more severe because the field strength of relatively larger magnetic poles decays more slowly with distance compared to smaller magnetic poles, so the interference is greater at larger gaps between the magnetic poles and the sensor elements.

[0081] The Applicant has proposed an alternative arrangement of encoder elements, which can improve the magnetic field interference between tracks and also allow the backing part to firmly support the encoder elements. Figure 3 An example of a suitable encoder disk is shown in, which includes features that achieve the desired effect. The encoder disk has a plurality of magnetic poles in the outer track, and these magnetic poles have cut-out portions at their outermost edges to form four flat engaging portions, thereby reducing the magnetic pole area and thus having a smaller field compared to other magnetic poles. These magnetic limits define four equally spaced key areas, each key area being spaced 90 degrees around the disk.

[0082] The magnetic poles with flat portions define features that simultaneously provide firm fixation and a beneficial modification of the field characteristics along the track.

[0083] In the example, the arrangement of the cut-out portions reduces the interference between the inner track and the outer track in the magnetic track, thereby reducing the fourth-order interference.

[0084] It can be seen that reducing the magnetic poles in the outer track that have the same polarity as the adjacent magnetic poles in the inner track most effectively reduces the fourth-order cross-coupling. On the other hand, if the magnetic poles in the outer track with opposite polarity to the adjacent inner track magnetic poles are reduced, this will increase the fourth-order interference.

[0085] Figure 4 and Figure 5 show other arrangements, where Figure 4 a step change in the radius of the disk is used as an engaging feature, and Figure 5 the strength of certain magnetic poles is changed by adding steel inserts at the edges. Weaker magnetic poles are shown in light shading, while stronger magnetic poles are shown in darker shading.

[0086] Figures 7 to 1 1 shows how a backing member supports an encoder disk. In Figure 7 and best seen in FIGS. 9, the backing member includes a disk-shaped body having an upper surface complementary to the lower side of the encoder disk. The encoder disk 2 is fixed to this surface using Figure 7 the adhesive pads 11 shown. A tubular rod 14 extends away from the side of the backing member facing away from the encoder disk 2. Its size can be set to slide fit onto a shaft (not shown) that is rotating and on which the sensor will provide measurements.

[0087] To further secure the encoder disk 2 to the backing member 12, a mechanical fastener 13 is provided. This can be best seen in FIGS. 10(a) and 10(b).

[0088] As shown in FIGS. 10(a) and 10(b), the mechanical fastener includes a can-shaped portion having a disk-shaped base portion that is substantially complementary to the outer surface of the backing portion facing away from the encoder disk. An upright peripheral wall projects from the outer edge of the disk and surrounds the outermost edge of the backing portion and also surrounds the outermost edge of the encoder disk. This engages with the engaging portion, and this engagement prevents any rotation of the encoder disk relative to the backing portion. In Figure 8 the assembled configuration shown, a wall 15 surrounds the conical edge of the encoder disk 2 to form an undercut, thereby preventing axial displacement of the disk relative to the backing member in the event of adhesive pad failure.

[0089] Other arrangements of features are possible, such as providing holes within one or more magnetic poles (where parts of the fixing mechanism are located), or holes between or beside the magnetic poles within the tracks. In each case, features that allow positioning (such as holes) should also contribute to reducing or eliminating any inter-track distortion of the magnetic field.

[0090] Figure 6 Shows how the present invention can be applied to a linear encoder element, where a section of the encoder element is shown, which can be repeated as needed depending on the length of the encoder. As shown, for every 12 elements in one track, there are three elements in the other track, and the track with narrower magnetic poles has two different magnetic pole shapes. In both designs, the inner edges of the magnetic poles facing the track with wider magnetic poles are different, and the magnetization intensity, shape, and material are the same in other respects.

Claims

1. A magnetic encoder, comprising: A disc-shaped rotatable encoder element (2), the encoder element comprising one or more magnetized elements fixed in use to a backing member (12), the one or more magnetized elements together defining an encoder region of a first track (3) and a second track (4), the first track (3) being positioned concentrically around the second track (4), each encoder region including magnetic poles, the magnetic poles along each of the first track (3) and the second track (4) being arranged in an alternating pattern of north and south poles, the magnetic poles of the first track (3) extending to the edge of the disc of the encoder element to form a perimeter that extends at least partially around a circular path of radius r. And At least one sensor (5, 6), the at least one sensor being fixed in a position relative to which the positions of the magnetized elements of the first track (3) and the second track (4) are to be determined. Characterized in that the magnetic encoder includes at least one feature that not only provides a position for a mechanical fixing member (13) that fixes the magnetized elements relative to a moving part, but also contributes to a periodic variation along the first track (3) of a magnetic field emitted by the first track (3) and detected by a sensing element associated with the first track (3), the periodic variation at least partially cancelling out a corresponding periodic variation of the magnetic field of the second track, also detected by the sensing element associated with the first track (3), the at least one feature including the outermost edge of the magnetic poles of the first track (3), the outermost edge extending along a straight line that forms a chord for the circular path of radius r.

2. The magnetic encoder according to claim 1, wherein, The at least one feature includes one of the following: a characteristic difference of one or more magnetic poles in the first track (3) relative to other magnetic poles in the first track (3); a characteristic difference of at least one non-magnetized track portion between a pair of adjacent magnetic poles in the first track (3); or a characteristic difference of at least one non-magnetized track portion adjacent to at least one magnetic pole in the first track (3).

3. The magnetic encoder according to any one of the preceding claims, wherein, The at least one feature is capable of defining a clamping portion that provides a mechanical attachment point for the mechanical fixing member (13).

4. The magnetic encoder according to claim 3, wherein, The clamping portion includes a hole within the encoder element (2), or an edge of a magnetic pole, or an inter-pole region of the encoder element (2), or a region of the encoder element (2) that is offset from the first track (3) and adjacent to one or more magnetic poles in the first track (3), the clamping portion in each case being capable of engaging a part of the mechanical fixing member (13).

5. The magnetic encoder according to claim 1 or 2, wherein, The at least one feature receives a part of the mechanical fixing member (13), the part of the mechanical fixing member being engaged with the first track or the second track, or with a space between the magnetic poles in the first track or the second track, or with a space adjacent to one or more magnetic poles in the first track or the second track, the space interfering with the magnetic field of one or more adjacent magnetic poles.

6. The magnetic encoder according to claim 1, wherein, The backing member (12) supports the encoder element (2), and the mechanical fastener (13) engages both the backing member (12) and the at least one feature.

7. The magnetic encoder according to claim 6, wherein, The encoder element (2) has an outer edge that tapers inwardly away from the backing member such that the disc forms a frustum, and the mechanical fastener (13) extends upwardly along the outer edge and tapers inwardly towards the center of the disc to form an undercut for receiving the outer edge.

8. The magnetic encoder according to claim 6 or claim 7, wherein, The mechanical fastener (13) includes a pot-shaped portion having a base portion that is substantially complementary to the outer surface of the backing member facing away from the disc, and an upright peripheral wall that surrounds the outermost edge of the backing member and also surrounds the outermost edge of the disc.

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