Rotary encoder

Abstract

A rotary scale device for an encoder apparatus includes a flat disk on which at least one track including scale features is provided, wherein the flat disk includes a hole through its center for receiving a cylindrical shaft, and wherein the rotary scale member includes at least three cantilever spring members arranged substantially in the same plane as the flat disk and spaced around the edge of the hole to engage with a cylindrical shaft inserted through the hole and to radially position the disk on the cylindrical shaft.

Description

Technical Field

[0001] This invention relates to a rotary encoder, and more particularly to a rotary scale, and an encoder device including a rotary scale and a reading head for reading the rotary scale. Background Technology

[0002] Measuring scales are used for measuring the position of machine parts that can move relative to each other. A measuring scale typically has a series of features that can be read by a reading head, allowing the reading head to provide a measurement of its position along or around the scale. A measuring scale can be mounted on one part of the machine and read by a suitable reading head attached to another part of the machine. Types of measuring scales include: magnetic scales (where scale features are provided by features having specific magnetic properties), capacitive scales (where features are provided by features having specific capacitive properties), inductive scales (where features are provided by features having specific inductive properties), and optical scales (where features are provided by features having specific optical properties). Optical scales can be transmissive or reflective. Examples of optical scale configurations are disclosed in EP-A-0 207 121 and US-A-4,974,962.

[0003] To measure rotational displacement, a scale can be provided on a component that rotates relative to the read head during use, together with the shaft or other rotating parts. In particular, the component with scale features and that rotates with the shaft during use can be a disc. Specifically, glass or metal discs are commonly used in high-performance encoders, and typically, the scale features are formed directly in / on the glass or metal material. The inventors are interested in providing improvements to such glass or metal disc scales, particularly providing disc scales with low profile, high performance (e.g., high resolution / low error).

[0004] Accordingly, the present invention relates to an improved rotary encoder, and more particularly to an improved disc encoder. Summary of the Invention

[0005] According to a first aspect of the invention, a rotating scale device for an encoder apparatus is provided, comprising at least one track for scale features to be read by a read head to determine their relative position, the rotating scale device comprising a flat disk including a through hole for receiving a cylindrical shaft, and wherein the rotating scale member comprises at least three cantilever spring members arranged substantially in the same plane as the flat disk and spaced around the edge of the hole to engage with a cylindrical shaft inserted through the hole and to radially position (e.g., center) the flat disk on the cylindrical shaft.

[0006] Such a configuration has been found to provide a particularly efficient and compact self-positioning rotary encoder device. For example, it has been found difficult to provide an efficient (e.g., highly repeatable) in-plane self-positioning solution for disc scales, especially metal or glass discs. It has been found that the use of cantilever spring members is particularly advantageous compared to other configurations using non-cantilever spring arrangements. The improved self-positioning performance of the rotary encoder device provided by the configuration of the present invention (e.g., better, predictably, e.g., repeatably positioning itself relative to the shaft, for example, so that the disc / scale mark is self-centered relative to the shaft) can provide an improved setup for the encoder device and thereby improve the metrological performance of the encoder.

[0007] Although attempts have been made to manufacture so-called “self-positioning” (e.g., “self-centering”) rotary encoders (e.g., those described in US6255644 and US 6293021), our inventors have found that existing solutions are not satisfactory, especially for thin / planar / compact rotary encoders and where there are high requirements for the predictability of the self-positioning performance of the scale component.

[0008] Compared to flexures supported at both ends (e.g., flexures provided by slots in the material), the spring force provided by a cantilevered spring member is less affected by its width. Accordingly, due to manufacturing tolerances (which can lead to variations in the width of the spring / flexure), it has been found that using a cantilevered spring member (instead of a flexure supported at both ends) can provide a more predictable self-positioning capability for disc scales with in-plane spring members. Indeed, such a difference can be very important given the growing demand for high-accuracy position encoders today. For example, the inventors faced the requirement that the scale be self-centered on the shaft, with an eccentricity of less than 10 μm (micrometers) and in some cases less than 5 μm (micrometers), and the claimed configuration helps the inventors meet such a requirement.

[0009] This configuration also helps ensure highly repeatable self-positioning performance of the encoder disc. This can be important because scale features for glass or metal scale discs are typically formed by mounting the glass or metal disc onto a shaft in a manufacturing device and then marking the disc on the shaft, where the glass or metal disc is rotated by the shaft so that the scale features can be formed around the disc surface. Importantly, when the disc is removed from the shaft at the manufacturer's site and subsequently mounted on a shaft at the customer's site, it is located in the same relative radial position relative to the customer's shaft as it is on the manufacturer's site shaft. Given that the diameters of the customer's shaft and the manufacturer's site shaft cannot be exactly the same, it is desirable to ensure repeatable performance of the flexure that radially positions the disc on the shaft, even when the shaft sizes are different. It has been found that even small differences in diameter between the customer's shaft and the manufacturer's site shaft compared to the manufacturer's site shaft can have a significant adverse effect on the disc's position on the customer's site shaft, which in turn can affect encoder performance.

[0010] It should be understood that the radial dimension, in particular (relative to the disc scale), of the cantilever spring members is elastically deformable. Accordingly, preferably, when in use (i.e., when mounted on a shaft), each of the cantilever spring members can be radially displaced within its elastic limit, and thus each provides a force that works together to radially position the disc on the shaft.

[0011] Preferably, for a given radial deflection, each spring member provides substantially equal spring force. The shape and size of the cantilever spring members can be nominally / substantially identical. This simplifies the design and manufacture of the cantilever spring members to provide nominally / substantially equal balanced self-positioning forces. Furthermore, this can be particularly preferred when the repeatability of radial self-positioning of the disc on shafts of different sizes is important.

[0012] It should be understood that each cantilever spring member can be configured such that its length extends around the edge of the hole in a generally circumferential direction. Preferably, at least one cantilever spring member extends in the opposite direction to the other cantilever spring member. For example, preferably, at least one cantilever spring member extends around the edge of the hole in a generally clockwise direction, and at least one other cantilever spring member extends around the edge of the hole in a generally counterclockwise direction. It has been found that such a configuration can help avoid unfavorable forces between the disc and the shaft during installation, which could affect the stability of the disc. For example, if the length of the cantilever spring member varies with temperature, each cantilever spring member can exert a rotational force on the shaft / disc when mounted on the shaft. The effect of such rotational forces can be reduced / eliminated by arranging at least one cantilever spring member (extending in the opposite direction to the other spring members). This may be particularly desirable for applications requiring maximum accuracy and stability.

[0013] The rotating scale component may include at least three pairs of cantilever spring components arranged in the same plane as the disc and spaced apart around the edge of the hole. The disc may have a combination of multiple pairs of cantilever springs and a single spring. However, it is advantageous to arrange all the cantilever spring components in pairs to help ensure that the self-positioning spring forces are balanced.

[0014] Based on the preceding two paragraphs, the rotating scale component may include at least three pairs of cantilever spring components, wherein these pairs of cantilever spring components extend in opposite directions relative to each other. In other words, the rotating scale component may include at least three pairs of cantilever spring components, wherein each spring component in a pair may be configured such that its free end is close to each other and its fixed (or "root") end is far from each other. In this case, within each pair of cantilever spring components, any variation in the length of each cantilever spring component (e.g., due to temperature changes) will have opposite effects on the rotation / torsion of the disc relative to the shaft. In a preferred embodiment, with each cantilever spring component configured substantially identically, the rotational force applied due to the variation in the length of each cantilever spring component should be substantially equal and opposite, and therefore the net rotational force applied by a pair of cantilever spring components can be substantially zero.

[0015] Preferably, the cantilever springs (or pairs of cantilever springs) are arranged at equal angles around the hole. In a particularly preferred embodiment, three pairs of cantilever springs are provided, wherein the pairs of cantilever springs are arranged at equal angles around the hole.

[0016] Preferably, the width of the cantilever spring member, measured between the center of the side facing the center of the hole and the side facing away from the center of the hole, gradually narrows towards its free end. Preferably, at least the side of the cantilever spring member facing the center of the hole is substantially straight. The inventors have discovered that, in order to ensure equal stress distribution along the length of the cantilever spring member, the theoretically ideal shape is parabolic. However, the inventors have also found that providing a parabolic cantilever spring member has disadvantages, such as the rate of change of the force applied by the flexure can change with deflection when the contact point between the shaft and the flexure moves. A spring member having a straight edge facing / engaging the shaft in use reduces this effect, thereby providing a more stable and practically better solution than the theoretically ideal parabolic shape.

[0017] Suitable materials for flat discs include metal or glass. In a preferred embodiment, the flat disc includes a steel disc, particularly a stainless steel disc, but it should be understood that other metals, such as aluminum or titanium, can also be used.

[0018] The cantilever spring member and the flat disk can be formed from a single sheet of material (specifically, cut from it). The use of a sheet of material and the characteristic of being formed from a sheet of material (e.g., cut from it) helps ensure that the root of the cantilever spring member is in the same plane as the disk, and also helps ensure that the force applied to the disk remains in the plane, thereby reducing / avoiding moments on the disk that could cause it to warp. Such a configuration also helps to achieve a very compact rotary encoder.

[0019] Preferably, the track of the scale feature includes a complete circular track of the scale feature (e.g., as opposed to including only a partial arc). Preferably, at least one track of the scale feature is provided on the plane of the disc. The scale feature in the track can be an incremental feature or an absolute feature. A disc scale can include more than one track.

[0020] The disc scale may include at least one additional hole located between the hole for the shaft and the outer edge of the disc. Providing such an additional hole can facilitate securing the disc scale to the shaft, for example, to the body on which the shaft is situated. For instance, bolts can be passed through the hole to secure the disc to the body, and / or adhesive can be applied into the hole to secure the disc to the body.

[0021] Preferably, the thickness of the flat disk is no greater than 5 mm, for example, no greater than 2.5 mm, or even no greater than 1.5 mm. It should be understood that the disk must have sufficient thickness in order to be self-supporting (i.e., not collapsing under its own weight) and / or to ensure an effective spring mechanism. This appropriate minimum thickness depends on several different factors, including the diameter of the flat disk and the material of the flat disk. Typically, the inventors have found that a disk having a thickness of not less than 0.5 mm, for example, not less than 0.6 mm, and especially not less than 0.7 mm, is preferred. Preferably, the flat disk has a substantially uniform thickness.

[0022] According to another aspect of the invention, an apparatus is provided comprising first and second rotatable portions, wherein a rotating scale device as described above is mounted on one of the first and second rotatable portions to rotate therewith, and at least one reading head is mounted on the other of the first and second rotatable portions to read scale features of the rotating scale device. Attached Figure Description

[0023] Embodiments of the invention will now be described by way of example only with reference to the following accompanying drawings, in which:

[0024] Figure 1 This is an isometric view of a disc-type scale member mounted on a shaft according to the present invention, wherein a reading head is arranged for reading the scale;

[0025] Figure 2 yes Figure 1 A plan view of the layout;

[0026] Figure 3 yes Figure 1 A side view of the arrangement;

[0027] Figure 4 yes Figure 1 A plan view of the disc-type scale component shown separately;

[0028] Figure 5 yes Figure 1 Detailed views of the spring component of the disc scale component; and

[0029] Figure 6 This is a plan view of a disc-type scale component according to another embodiment of the present invention. Detailed Implementation

[0030] See Figures 1 to 3 The image shows an encoder device 2 comprising a disc scale member 4 according to an exemplary embodiment of the present invention. Figure 4 The disc-type scale component 4 is shown separately.

[0031] exist Figures 1 to 3 In the diagram, the disc-shaped scale member 4 is shown mounted on the cylindrical shaft 6 of the machine. The disc-shaped scale member 4 is planar in shape. Specifically, the disc-shaped scale member 4 is formed from a thin sheet of material, in this embodiment stainless steel, and is approximately 1 mm thick. For the context purposes of this embodiment, the diameter of the disc-shaped scale member 4 is approximately 55 mm. It should be understood that the invention is not limited to discs of this size, and such dimensions are given only as an example of a disc. Furthermore, the disc can be made of other metallic materials, such as aluminum.

[0032] The disc scale member 4 has a scale track 8 on one of its planes, which extends in a complete loop around the disc scale member. The scale track 8 includes a series of features that a read head 10 (mounted on a component 12, which is fixed relative to the cylindrical shaft 6) can read to determine the relative position / movement of the scale disc 4 and the read head 10. In the described embodiment, the encoder device is an optical encoder device, but this is not necessarily the case. For example, the encoder device could be a magnetic, inductive, or capacitive encoder device. Furthermore, in the described embodiment, the encoder device is a reflective optical encoder device (where light from the read head is reflected backwards from the scale towards the read head, and the illumination component of the read head and the scale detection component are on the same side of the scale). However, this is not necessarily the case, and the encoder device could be a transmissive optical encoder.

[0033] In this embodiment, encoder device 2 is an incremental encoder device. Accordingly, in this embodiment, scale disk 4 is an incremental scale disk, and scale track 8 includes a series of periodically arranged features that read head 10 can read to provide a count of the relative position / movement of scale disk 4 and read head 10. As is common in the field of incremental encoder devices, scale disk may include one or more reference marks that can be read by the read head as it passes through, allowing the read head to identify reference positions on the scale disk component. Of course, the encoder device may be an absolute encoder device rather than an incremental encoder device. Accordingly, scale disk 4 may be an absolute scale disk, wherein scale track(s) thereon may include features defining a series of unique absolute positions, such that the absolute positions of scale disk and read head can be determined at startup without requiring relative movement between scale disk component and read head.

[0034] As in Figure 4 As best viewed, the scale disc 4 includes a through-hole 14 through which the cylindrical shaft 6 can extend when the scale disc 4 is mounted. According to the invention, the disc-type scale member 4 includes a plurality of cantilevered spring members 16 arranged in the same plane as the disc and spaced around the edge of the through-hole 14 to engage the cylindrical shaft 6 when it is inserted through the scale disc 4 and to radially position the scale disc on the cylindrical shaft.

[0035] like Figure 4 As shown, the disc scale member 4 includes three pairs of cantilever spring members 16a, 16b, and 16c. Each pair of cantilever spring members 16a, 16b, and 16c is arranged in the same plane as the flat scale disc 4 and spaced apart around the edge of the hole 14. Furthermore, each pair of cantilever spring members 16a, 16b, and 16c is configured such that the free ends 18 of these cantilever spring members 16 are close to each other, and their fixed ends 20 are far apart (see, for example, [reference needed]). Figure 5 In other words, the cantilever spring members 16 in each pair of 16a, 16b, 16c point towards each other, rather than away from each other.

[0036] like Figure 4 and Figure 5 As shown, each cantilever spring member 16 is tapered, narrowing towards its free end 18. Accordingly, the width w1 of the cantilever spring member at its fixed end 20 is greater than its width w2 at its free end 18. It should be understood that the exact desired dimensions of the cantilever spring members depend on several factors, including the material, the size of the disc, and the desired spring force. Our inventors have found that a good desired spring force of approximately 20 Newtons for each flexure in each pair provides a good balance between providing sufficient self-positioning capability and not over-clamping the shaft.

[0037] In the described embodiment, the shaft engagement side / edge of the cantilever spring member 16 is straight. From a manufacturing and performance point of view, this configuration has been found to be particularly advantageous. Specifically, our inventors have recognized that while the ideal shape of the cantilever spring member might be parabolic for uniform force distribution, manufacturing a flexure of this shape can be difficult, and the rate of force change can vary significantly due to variations in the contact point between the cantilever spring member and the shaft. Accordingly, our inventors have found that providing a contact surface with a straight edge for the cantilever spring member represents the optimal trade-off between achieving good force distribution near the free end contact of the cantilever spring member and ease of manufacture, thus being cost-effective. Furthermore, in the described embodiment, the width of the cantilever spring member 16 decreases linearly.

[0038] In the described embodiment, each cantilever spring member 16 is formed by creating a slot 22 (located behind the cantilever spring member 16) and a gap 24 (located between the free ends 18 of the cantilever spring member 16) in the same material / sheet as the scale disk 4. The slot 22 and the gap 24 allow the cantilever spring member 16 to flex along its length into the slot 22. Such slots 22 and gaps 24 can be formed, for example, by etching and / or machining the scale disk 4. Alternatively, the scale disk 4 and its cantilever spring members 16 are formed by molding, casting, and / or additive manufacturing processes.

[0039] When the scale disc 4 is pushed onto the shaft 6 (which is slightly larger than the space between the paired cantilever spring members 16a, 16b, 16c), the shaft 6 engages the side of each cantilever spring member 16 facing the center of the hole 14, causing them to bend slightly into the slot 22. The elasticity of the material of the cantilever spring members 16 generates a reaction force on the shaft 6. Preferably, the reaction force provided by each cantilever spring member 16 is nominally the same, such that the scale disc 4 is self-centered on the shaft 6. This nominally identical reaction force can be achieved by configuring the cantilever spring members 16 such that their shape and size are nominally identical, as in the described embodiment.

[0040] If necessary, the scale disc 4 can be further secured to the shaft. For example, in the described embodiment, the scale disc 4 includes three holes 26 through which mechanical fasteners 28, such as bolts 28, can be passed to secure the scale disc 4 to the shaft 6. Optionally, an adhesive can be applied to the area of ​​the holes 26 to secure the scale disc 4 to the shaft 6 (with or without the mechanical fasteners 28). In the illustrated embodiment, the holes 26 for the fasteners are connected to the slots 22. This means that the manufacturing process is much simpler and more efficient, as the holes 26 and the slots 22 can be formed in a continuous process. However, it should be understood that this is not necessarily the case, and therefore the holes 26 and the slots 22 can be disconnected / separated. Furthermore, in the illustrated embodiment, an alignment feature (in this case, a hole) 30 is provided. Such an alignment hole 30 can be used to ensure that the disc 4 is accurately oriented when mounted on the shaft 6. For example, the shaft 6 may include a protruding pin (not shown) such that the disc 4 can only be mounted against the shaft 6 when the protruding pin and the alignment hole 30 are aligned.

[0041] It should be understood that such holes 26 for fasteners are optional, and fewer than three such holes may exist (e.g., as shown in the image). Figure 6 (as shown), or alternatively, the disc scale component may not have any such holes for fasteners.

[0042] In the described embodiment, the shape of the scale plate hole 14 is irregular. Specifically, the side of each cantilever spring facing the center of the hole is straight, while the shape of the hole between the pairs of cantilever springs is curved. Although it has been found advantageous to provide cantilever springs with straight edges for engagement with the shaft, the shape of the hole between the pairs of cantilever springs is less important and its shape and size only need to be confirmed to be sufficient so that it does not engage with the cylindrical shaft extending through it (so as not to affect the self-positioning effect of the impact cantilever spring).

[0043] In the described embodiment, three pairs of cantilever springs 16a, 16b, and 16c are provided. However, while this has been found to be the optimal solution, other configurations are possible. For example, four pairs of cantilever springs may be provided. Alternatively, multiple (e.g., three or four) independent cantilever springs may be provided around the side of the hole, instead of providing cantilever springs in pairs.

[0044] As shown in the figure, in the described embodiment, preferably, the cantilever springs (or pairs of cantilever springs) are arranged at equal angles around the hole. This is not necessarily the case, but such an arrangement can be the simplest when attempting to balance the forces provided by the cantilever springs to achieve a self-centering effect.

Claims

1. A rotary scale device for an encoder apparatus, comprising at least one track for scale features read by a read head, the rotary scale device comprising a flat disk including a through hole for receiving a cylindrical shaft, wherein, The flat disk includes at least three pairs of integral cantilever spring members, the cantilever spring members being arranged substantially in the same plane as the flat disk and spaced apart around the edge of the hole to engage with a cylindrical shaft inserted through the hole and radially position the flat disk on the cylindrical shaft, wherein each cantilever spring member is configured such that its longer direction extends around the edge of the hole in a generally circumferential direction, wherein a pair of cantilever spring members extend in opposite directions relative to each other, and each spring member in a pair is configured such that its free ends are close to each other and its fixed ends are far apart.

2. The rotating scale device as described in claim 1, wherein, One of the pair of cantilever spring members extends in the opposite direction to the other cantilever spring member in the pair, such that one of the cantilever spring members in the pair extends generally clockwise around the edge of the hole, while the other cantilever spring member in the pair extends generally counterclockwise around the edge of the hole.

3. The rotating scale device as described in claim 1, wherein, The width of the cantilever spring member, measured between the side facing the center of the hole and the side away from the center of the hole, gradually narrows toward its free end.

4. The rotating scale device as described in claim 1, wherein, At least one face of the cantilever spring member facing the center of the hole is substantially straight.

5. The rotating scale device as described in any of the preceding claims, wherein, The flat disk includes a metal disk.

6. The rotating scale device as described in claim 1, wherein, The cantilever spring component and the flat plate are formed from a single sheet of material.

7. The rotating scale device as described in claim 1, wherein, The at least one track, including the scale feature, is disposed on the plane of the flat disk.

8. The rotating scale device of claim 1, comprising at least one additional hole located between the hole for the shaft and the outer edge of the flat disk.

9. The rotating scale device as described in claim 1, wherein, The thickness of the flat disk is not less than 0.5 mm and not more than 5 mm.

10. An encoder device comprising first and second rotatable relative portions, wherein, The rotating scale device as claimed in any one of claims 1 to 9 is mounted on one of the first and second rotatable portions to rotate therewith, and at least one reading head is mounted on the other of the first and second rotatable portions to read scale features on the rotating scale device.

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