rotary encoder

Abstract

A rotary scale device for an encoder device is provided which includes a planar disk provided with at least one track containing scale features, in which the planar disk has a hole through its center for receiving a cylindrical shaft, and the rotary scale member includes at least three cantilevered spring members disposed substantially planar with the planar disk and spaced around the edge of the hole for engaging the cylindrical shaft inserted into the hole and radially positioning the planar disk on the cylindrical shaft.

Description

[Technical Field]

[0001] The present invention relates to a rotary encoder, and more particularly to an encoder device comprising a rotary scale and a read head for reading the rotary scale. [Background technology]

[0002] Measuring scales are used in measuring the position of machine parts that can move relative to one another. They typically have a series of features that are read by a readhead, which can provide a measurement of their position along or around the scale. A measuring scale can be attached to one part of a machine and read by a suitable readhead attached to another part of the machine. Types of measuring scales include magnetic scales (where the scale features are provided by features with specific magnetic properties), capacitive scales (where the features are provided by features with specific capacitive properties), inductive scales (where the features are provided by features with specific inductive properties), and optical scales (where the features are provided by features with specific optical properties). Optical scales can be transmissive or reflective. Examples of optical scale configurations are also disclosed in U.S. Patent Nos. 5,629,999 and 5,729,999. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] European Patent No. 0207121 [Patent Document 2] U.S. Patent No. 4,974,962 [Patent Document 3] U.S. Patent No. 6,255,644 [Patent Document 4] U.S. Patent No. 6,293,021 Summary of the Invention [Problem to be solved by the invention]

[0004] To measure rotational displacement, such scales are provided on a rotating member that rotates with a shaft or other rotating part relative to a read head. In particular, the member having scale features and that rotates with a shaft can be a disk. In particular, glass or metal disks are commonly used in high performance encoders, and typically the scale features are formed directly in or on the glass or metal material. The inventors are interested in providing improvements to such glass or metal disk scales, and in particular to provide low-profile, high-performance (e.g., high-resolution / low-error) disk scales.

[0005] Accordingly, the present invention relates to an improved rotary encoder, and more particularly to an improved disk encoder. [Means for solving the problem]

[0006] According to a first aspect of the present invention, there is provided a rotary scale device for an encoder device comprising a track of at least one scale feature for reading by a read head to determine its relative position, the rotary scale device comprising a planar disk having a through hole for receiving a cylindrical shaft, a rotary scale member provided substantially planar with the planar disk and spaced around an edge of the through hole, the rotary scale member comprising at least three cantilevered spring members that engage the cylindrical shaft inserted through the through hole and that contact the cylindrical shaft to radially position (e.g., center) the planar disk.

[0007] Such an arrangement has been found to provide a particularly effective and compact self-locating rotary encoder device. For example, it has been found to be difficult to provide an effective (e.g., highly repeatable) in-plane self-localization solution for disk scales, particularly metal or glass disks. The use of cantilevered spring members has been found to be particularly advantageous over other arrangements that use non-cantilevered spring configurations. The increased self-localization performance of the improved rotary encoder device provided by the arrangement of the present invention (e.g., predictably, e.g., repeatably, or better, to locate itself relative to the shaft, e.g., to self-center the disk / scale markings relative to the shaft) can provide an improved setup of the encoder device, thereby improving the metrology performance of the encoder.

[0008] Although attempts have already been made to produce such so-called "self-locating" (e.g., "self-centering") rotary encoders (e.g., those described in U.S. Patent Nos. 5,999,298 and 5,999,298), the inventors have found that the existing solutions are insufficient, especially for thin / flat / compact rotary encoders, and that there are high demands on the predictability of the self-localization performance of the scale member.

[0009] Compared to a flexure supported at both ends (e.g., a flexure provided by a slot in the material), the spring force provided by a cantilevered spring member is less affected by its width. Therefore, due to manufacturing tolerances that can vary the width of the spring / flexure, it has been found that the use of a cantilevered spring member (instead of a flexure supported at both ends) can provide more predictable self-location capabilities for disk scales with in-plane spring members. Indeed, such differences can be significant with today's ever-increasing demand for higher precision position encoders. For example, inventors have faced the requirement of being able to self-center a scale on a shaft with an eccentricity of less than 10 μm (microns), and in some cases less than 5 μm (microns), and the claimed configurations have been instrumental in helping inventors meet such requirements.

[0010] Such a configuration also helps to increase the repeatability of the encoder disk's self-location performance. This can be important because the scale features on a glass or metal scale disk are typically formed by mounting the glass or metal disk on a shaft of a manufacturing device, then forming marks with the disk on the shaft, so that the glass or metal disk can be rotated by the shaft and the scale features formed around the surface of the disk. When the disk is removed from the shaft at the manufacturer site and then mounted on the shaft at the customer site, it is important to position the disk relative to the customer shaft in the same relative radial position as the shaft at the manufacturer site. Given that the shafts at the customer and manufacturer sites are unlikely to be exactly the same diameter, it is desirable to ensure that the bends that radially position the disk on the shaft operate repeatedly, even for shafts that are not the same size. It has been found that even small differences in diameter between the customer and manufacturer site shafts can have a significant adverse effect on where the disk is located on the customer site shaft compared to the manufacturer site shaft, affecting encoder performance.

[0011] As will be appreciated, the cantilevered spring members are elastically deformable, particularly in the radial dimension (relative to the disc scale), and thus preferably, in use (i.e. when attached to a shaft), each cantilevered spring member is capable of radial displacement within its elastic limits, and in return, they each provide a force that acts together to radially locate the disc on the shaft.

[0012] Preferably, each spring element provides substantially the same spring force for a given radial deflection. The cantilevered spring elements may be nominally / substantially identical in shape and size. This simplifies the design and manufacture of the cantilevered spring elements and provides nominally / substantially equal and balanced self-localization forces. This may be particularly preferred where repeatability of the radial self-location of the disk between shafts of different sizes is important.

[0013] As will be appreciated, each cantilevered spring element can be configured such that its length extends generally circumferentially around the edge of the hole. Preferably, at least one cantilevered spring element extends in an opposite direction to another cantilevered spring element. For example, preferably, at least one cantilevered spring element extends generally clockwise around the edge of the hole, and at least one other cantilevered spring element extends generally counterclockwise around the edge of the hole. It has been found that such a configuration helps avoid adverse forces between the disk and shaft during installation, which may affect the stability of the disk. For example, if the length of the cantilevered spring elements changes with temperature, each cantilevered spring element can exert a pivoting force on the shaft / disk when installed on the shaft. The effect of such a pivoting force can be reduced / neutralized by arranging at least one cantilevered spring element to extend in an opposite direction to the other elements. This may be particularly desirable for applications requiring maximum precision and stability.

[0014] The rotary scale member may include at least three pairs of cantilevered spring members disposed in a plane with the planar disk and spaced around the edge of the hole. The planar disk may have a combination of cantilevered springs and single spring pairs, although providing all cantilevered spring members in pairs may be advantageous to ensure balance of the self-localization spring forces.

[0015] In line with the previous two paragraphs, the rotary scale member can include at least three pairs of cantilevered spring members, with the cantilevered spring members of each pair extending in opposite directions. In other words, the rotary scale member can include at least three pairs of cantilevered spring members, with each spring member of a pair configured such that their free ends are proximal to each other and their fixed (or "root") ends are distal to each other. In such a case, within each pair of cantilevered spring members, any change in length of each cantilevered spring member (e.g., due to temperature changes) will have an opposite effect on the rotation / twisting of the disk relative to the shaft. In preferred embodiments in which the configuration of each cantilevered spring member is substantially identical, the pivoting forces exerted by changes in length of each cantilevered spring member should be substantially equal and opposite, and thus the net pivoting force exerted by a pair of cantilevered spring members can be substantially zero.

[0016] Preferably, the cantilever spring (or pairs of cantilever springs) are equiangularly arranged around the hole. In a particularly preferred embodiment, three pairs of cantilever springs are provided, with one pair of cantilever springs equiangularly arranged around the hole.

[0017] It may be preferable for the width of the cantilever spring member, measured between the side toward the center of the bore and the side away from the center, to gradually narrow toward its free end. Preferably, at least the side of the cantilever spring member toward the center of the bore is substantially straight. The inventors have discovered that a theoretically ideal cantilever spring member has a parabolic shape to ensure equal stress distribution along the length of the cantilever spring member. However, the inventors have also discovered that providing a cantilever spring member with a parabolic shape has disadvantages. For example, the rate of change of the force applied by the bend may change with the bend as the contact point between the shaft and the bend moves. Using a spring member with a straight edge facing / engaging the shaft during use reduces this effect, thereby providing a more stable and practically superior solution than the theoretically ideal parabolic shape.

[0018] Suitable disc materials for the planar disc include metal or glass, in a preferred embodiment the planar disc comprises a steel, particularly a stainless steel disc, although it will be appreciated that other metals such as aluminium or titanium may also be used.

[0019] The cantilevered spring member and planar disk can be formed (e.g., cut) from a single sheet of material. The use of sheet material and the formation (e.g., cutting) of features from the sheet material helps ensure that the root of the cantilevered spring member is planar with the disk and also ensures that forces exerted on the disk remain planar, thereby reducing / avoiding moments on the disk that could distort the disk. Such a configuration also helps facilitate a very compact rotary encoder.

[0020] Preferably, the track of scale features comprises a complete circular track of scale features (e.g., not just a partial arc). Preferably, at least one track of scale features is provided on a plane of the planar disc. The scale features within a track may be incremental or absolute features. The disc scale may comprise multiple tracks.

[0021] The disc scale may include at least one additional hole disposed between the hole for the shaft and the outer edge of the planar disc. Such an additional hole may be provided to help facilitate fastening the disc scale to the shaft, for example, to facilitate fastening the disc scale to a body on which the shaft is provided. For example, a bolt may be passed through the hole to fasten the disc to the body, and / or an adhesive may be disposed in the hole to fasten the disc to the body.

[0022] Preferably, the thickness of the planar disc is 5 mm or less, for example 2.5 mm or less, for example 1.5 mm or less. As will be appreciated, the disc needs to be thick enough to be self-supporting (i.e., not collapse under its own weight) and / or to ensure an effective spring member. Such a suitable minimum thickness will depend on various factors, including the diameter of the planar disc and the material of the planar disc. Typically, the inventors have found it preferable to have a disc that is 0.5 mm or more, for example 0.6 mm or more, especially 0.7 mm or more. Preferably, the planar disc has a substantially uniform thickness.

[0023] According to another aspect of the present invention, there is provided an apparatus comprising first and second relatively rotatable parts, a rotary scale device mounted to one of the first and second relatively rotatable parts for rotation therewith, and at least one readhead mounted to the other of the first and second relatively rotatable parts for reading scale features of the rotary scale device.

[0024] Embodiments of the present invention will now be described, by way of example only, with reference to the following drawings, in which: [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1 is an isometric view of a disk scale member according to the present invention mounted on a shaft with a read head positioned to read the scale. [Figure 2] FIG. 2 is a plan view of the arrangement of FIG. 1. [Figure 3] FIG. 2 is a side view of the arrangement of FIG. 1. [Figure 4] FIG. 2 is a plan view of the disk scale member of FIG. 1 shown in isolation. [Figure 5] FIG. 2 is a detailed view of a spring member of the disk scale member of FIG. 1. [Figure 6] FIG. 10 is a plan view of a disk scale member according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0026] 1-3, there is shown an encoder device 2 including a disk scale member 4 according to one example embodiment of the present invention. The disk scale member 4 is shown alone in FIG.

[0027] In Figures 1-3, the disc scale member 4 is shown attached to a cylindrical shaft 6 of the machine. The disc scale member 4 is planar in configuration. In particular, the disc scale member 4 is formed from a thin sheet of material, in this embodiment, stainless steel, approximately 1 mm thick. For context, the diameter of the disc scale member 4 in this embodiment is approximately 55 mm. It will be appreciated that the invention is not limited to discs of such size, and such dimensions are given merely as an example of a disc. The discs could also be made from other metallic materials, such as aluminum.

[0028] The disc scale member 4 has a scale track 8 on one of its planes, extending completely annularly around the disc scale member. The scale track 8 comprises a series of features that can be read by a readhead 10 (mounted on a component 12 that is fixed relative to the cylindrical shaft 6) to determine the relative position / movement of the scale disc 4 and readhead 10. In the described embodiment, the encoder device is an optical encoder device, but this does not have to be the case. For example, the encoder device could be a magnetic encoder device, an inductive encoder device, or a capacitive encoder device. Also, in the described embodiment, the encoder device is a reflective optical encoder device (light from the readhead is reflected by the scale back to the readhead, and the readhead illumination and scale detection are on the same side of the scale). However, this does not have to be the case and the encoder device could be a transmissive optical encoder.

[0029] In this embodiment, the encoder device 2 is an incremental encoder device. Accordingly, in this embodiment, the scale disc 4 is an incremental scale disc, and the scale track 8 comprises a series of periodically arranged features that can be read by the readhead 10 to provide a count of the relative position / movement of the scale disc 4 and the readhead 10. As is common in the field of incremental encoder devices, the scale disc may include one or more reference marks that can be read by the readhead passing the scale disc, thereby enabling the readhead to identify a reference position on the disc scale member. Of course, instead of an incremental encoder device, the encoder device may be an absolute encoder device. Accordingly, the scale disc 4 may be an absolute scale disc, and the scale track(s) on the scale disc 4 may comprise features that define a series of unique absolute positions, such that the absolute position of the scale disc and readhead may be determined at start-up without requiring relative movement of the disc scale member and the readhead.

[0030] As best shown in Figure 4, the scale disc 4 has a hole 14 through its center, through which the cylindrical shaft 6 can extend when the scale disc 4 is attached to the cylindrical shaft 6. In accordance with the present invention, the disc scale member 4 is provided with a number of cantilevered spring members 16 that are flush with the planar disc and spaced around the edge of the hole 14 and that engage the cylindrical shaft 6 when the cylindrical shaft 6 is inserted through the hole, thereby radially positioning the scale disc 4 on the cylindrical shaft 6.

[0031] As shown in Figure 4, the disk scale member 4 includes three pairs of cantilevered spring members 16a, 16b, and 16c. Each pair of cantilevered spring members 16a, 16b, and 16c is disposed in a plane with the planar scale disk 4 and spaced apart around the edge of the hole 14. Each pair of cantilevered spring members 16a, 16b, and 16c is configured such that the free ends 18 of the cantilevered spring members 16 are proximal to one another and their fixed ends 20 are distal to one another (see, for example, Figure 5). In other words, the cantilevered spring members 16 of each pair 16a, 16b, and 16c face each other rather than away from one another.

[0032] As shown in Figures 4 and 5, each cantilevered spring member 16 tapers narrowly toward its free end 18. Thus, the width w1 of the cantilevered spring member at its fixed end 20 is greater than the width w2 at its free end 18. As will be appreciated, the exact desired dimensions of the cantilevered spring members will depend on several factors, including the material, the size of the disk, and the desired spring force. The inventors have found that a good desired spring force for each bend in the pair is approximately 20 Newtons, which provides a good balance between providing sufficient self-localization capability and not overly gripping the shaft.

[0033] In the described embodiment, the shaft-engaging side / edge of the cantilevered spring member 16 is straight. Such a configuration has been found to be particularly advantageous from a manufacturing and performance standpoint. In particular, the inventors have determined that, while the ideal shape for the cantilevered spring member would be parabolic for force distribution reasons, manufacturing a bend of such a shape can be difficult and the rate of change of force can vary significantly as the contact point between the cantilevered spring member and the shaft changes. Therefore, the inventors have found that providing the cantilevered spring member with a straight-edge contact surface is an optimal compromise between good force distribution, allowing contact close to the free end of the cantilevered spring member, and being simple and therefore cost-effective to manufacture. Also, in the described embodiment, the width of the cantilevered spring member 16 decreases linearly.

[0034] In the described embodiment, each cantilevered spring element 16 is formed by creating a slot 22 (at the rear of the cantilevered spring element 16) and a gap 24 (between the free ends 18 of the cantilevered spring element 16) in the same piece / sheet of material as the scale disk 4. The slots 22 and gaps 24 allow the cantilevered spring element 16 to flex along its length into the slot 22. Such slots 22 and gaps 24 may be formed, for example, by etching and / or machining the scale disk 4. Optionally, the scale disk 4, along with its cantilevered spring elements 16, is formed by molding, casting, and / or additive processes.

[0035] When the scale disc 4 is press-fit onto the shaft 6, which is slightly larger than the space between the pair of cantilevered spring members 16a, 16b, 16c, the shaft 6 engages the side of each of the cantilevered spring members 16 that faces the center of the hole 14, causing each of them to bend slightly into the slot 22. The resilience of the material of the cantilevered spring members 16 induces a reaction force on the shaft 6. Preferably, the reaction force provided by each cantilevered spring member 16 is such that the scale disc 4 is pressed onto the shaft 6. Becoming self-centeredSuch nominally identical reaction forces can be achieved by configuring the cantilever spring members 16 to be nominally identical in shape and size, as they are in the described embodiment.

[0036] If desired, the scale disc 4 can be further secured to the shaft. For example, in the embodiment shown, the scale disc 4 includes three holes 26 through which mechanical fasteners 28, such as bolts 28, can pass to secure the scale disc 4 to the shaft 6. Optionally, adhesive can be applied to the hole 26 area to secure the scale disc 4 to the shaft 6 (with or without the mechanical fasteners 28). In the embodiment shown, the fastener holes 26 are connected to the slots 22. Doing so means that the holes 26 and slots 22 can be formed in one continuous process, making the manufacturing process much simpler and more efficient. However, as will be appreciated, this is not necessarily the case, and thus the holes 26 and slots 22 may be cut out / separate. Also, in the embodiment shown, alignment features (in this case holes) 30 are provided. Such alignment holes 30 can be used to ensure that the disc 4 is correctly oriented when attached to the shaft 6. For example, the shaft 6 may be provided with a protruding pin (not shown) so that the disc 4 can only sit flat against the shaft 6 when the protruding pin and alignment hole 30 are aligned.

[0037] As will be appreciated, such holes 26 for fasteners are optional, and there may be fewer than three such holes (e.g., as shown in FIG. 6), or optionally, the disk scale member may be provided without any such holes for fasteners.

[0038] In the described embodiment, the holes 14 in the scale disk are irregularly shaped. In particular, the side of each cantilever spring facing the center of the hole is straight, while the shape of the hole between a pair of cantilever springs is curved. While it has been found advantageous to provide cantilever springs with straight edges for engaging a shaft, the shape of the hole between a pair of cantilever springs is not critical, and it just needs to be of sufficient shape and size so as not to engage a cylindrical shaft extending therethrough (so as not to affect the self-localization effectiveness of the cantilever springs).

[0039] In the described embodiment, three pairs of cantilever springs 16a, 16b, 16c are provided. However, while this has been found to be the optimum solution, other configurations are possible. For example, four pairs of cantilever springs can be provided. Optionally, a number of individual cantilever springs (e.g., three or four) cantilever springs can be provided around the sides of the hole instead of providing the cantilever springs in pairs.

[0040] As shown, in the described embodiment, the cantilever spring (or pair of cantilever springs) are preferably equiangularly positioned around the hole. Although this is not necessarily the case, it is possible to balance the forces provided by the cantilever springs. The egocentric effect Such an arrangement may be the simplest if you are trying to have

Claims

1. 1. A rotary scale device for an encoder device having at least one track of scale features for reading by a read head, the rotary scale device comprising a planar disk having a through hole for receiving a cylindrical shaft, the planar disk comprising at least three integral cantilever spring members mounted substantially planarly by the planar disk and spaced around an edge of the through hole to engage a cylindrical shaft inserted into the through hole and radially position the planar disk on the cylindrical shaft, each cantilever spring member configured such that its longitudinal direction is generally circumferential of the planar disk around the edge of the through hole.

2. The rotary scale device according to claim 1 , wherein at least one of the cantilevered spring members extends in an opposite direction from another of the cantilevered spring members.

3. 3. The rotary scale device according to claim 1, wherein the rotary scale member is provided on a plane by the planar disk and includes at least three pairs of cantilever spring members spaced apart around an edge of the through hole, wherein one pair of the cantilever spring members extends in opposite directions to each other, and each pair of the cantilever spring members is configured such that their free ends are proximal to each other and their fixed ends are distal to each other.

4. The rotary scale device according to any one of claims 1 to 3, wherein a width of the cantilever spring is measured between a side toward a center of the through hole and a side away from the center of the through hole, and gradually narrows toward a free end of the cantilever spring.

5. The rotary scale device according to any one of claims 1 to 4, wherein at least a side surface of the cantilever spring member facing the center of the through hole is substantially straight.

6. The rotary scale device of any one of claims 1 to 5, wherein the planar disc comprises a metal disc.

7. The rotary scale device according to any one of claims 1 to 6, wherein the cantilevered spring member and the planar disk are formed from a single sheet of material.

8. The rotary scale device according to any one of claims 1 to 7, wherein the at least one track containing scale features is provided on a surface of the planar disk.

9. The rotary scale device according to any one of claims 1 to 8, further comprising at least one additional hole located between the through hole of the shaft and the outer edge of the planar disk.

10. 10. The rotary scale device according to claim 1, wherein the thickness of the planar disk is 0.5 mm or more and 5 mm or less.

11. 11. An apparatus comprising first and second relatively rotatable parts, wherein the rotary scale device according to any preceding claim is mounted on one of the first and second relatively rotatable parts for rotation therewith, and at least one readhead mounted on the other of the first and second relatively rotatable parts for reading the scale features on the rotary scale member.

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