JP2018072273A - Encoder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an encoder that can suppress errors in a measurement using a coherent fringe.SOLUTION: An encoder 1A comprises: a light source 10; a scale 20 that has a diffraction grating 20a; a light reception unit 30; and a reflection unit that configures a first optical path and a second optical path having a mutually equal optical path length as an optical path forming diffraction of a plurality of times by the diffraction grating 20a from the light source 10 to the light reception unit 30 via the scale 20. In the encoder, light to be emitted from the light source is low coherent light. Further, when the optical path length of the first optical path and the second optical path is a normal optical path length, and an optical path length of an optical path other than the first optical path and the second optical path is a non-normal optical path length, the reflection unit is configured so that the normal optical path length is different from the non-normal optical path length.SELECTED DRAWING: Figure 1

Description

The present invention relates to an encoder, and more particularly to an encoder that outputs a signal that is 1/2 ⁿ (n is a positive integer) times a scale pitch.

  Patent Document 1 discloses an encoder that uses interference fringes. In this encoder, a coherent beam is incident on a diffraction grating attached to a moving object a plurality of times, the diffracted lights from the diffraction grating interfere with each other to form interference fringes, and the bright and dark fringes of the interference fringes are counted. Thus, the amount of movement of the moving object is measured. A coherent light source such as a laser beam is used as the light source. A plurality of corner cubes are provided on the optical path from the light source to the light receiving element via the scale. With such a configuration, a sine wave signal output of 1/8 times the scale pitch is obtained by multiple times of diffraction.

Japanese Patent Laid-Open No. 2-85715

  However, in the optical path from the light source to the light receiving element, there are a normal optical path assumed in the measurement (normal optical path) and an optical path caused by stray light that is not assumed (stray light optical path). The interference between the light passing through the regular optical path and the light passing through the stray light optical path, or the interference of light passing through the plurality of stray light optical paths causes an error in the sine wave signal necessary for the measurement.

  An object of this invention is to provide the encoder which can suppress an error in the measurement using an interference fringe.

  In order to solve the above problems, an encoder of the present invention forms a plurality of diffractions by a diffraction grating between a light source, a scale having a diffraction grating, a light receiving unit, and the light source to the light receiving unit through the scale. The optical path includes a first optical path and a second optical path that have the same optical path length. In this encoder, the light emitted from the light source is low coherent light. Further, when the optical path lengths of the first optical path and the second optical path are normal optical path lengths, and the optical path lengths of the optical paths other than the first optical path and the second optical path are non-normal optical path lengths, the reflecting unit has an abnormal optical path length of the normal optical path length. It is configured to be different from the length.

  According to such a configuration, the normal optical path length is different from the non-normal optical path length in the reflection portion that configures the optical path that forms the diffraction of a plurality of times by the diffraction grating of the scale. By emitting light from the light source, interference occurs between the light passing through the first optical path, which is the normal optical path, and the light passing through the second optical path. The light source emits low-coherence light (light with a coherent length shorter than the optical path difference between the normal optical path length and the non-normal optical path length), so there is interference between the light passing through the normal optical path and the light passing through the non-normal optical path. It is suppressed. Thereby, the error contained in the sine wave signal produced | generated by the interference of the light which passes along a regular optical path can be suppressed.

  In the encoder of the present invention, the reflecting portion includes a first reflecting member, a second reflecting member, an intermediate reflecting member provided between the first reflecting member and the second reflecting member in the first optical path and the second optical path, You may have. In the direction perpendicular to the surface on which the diffraction grating of the scale is provided, the distance between the first reflecting member and the scale is different from the distance between the second reflecting member and the scale. Thereby, an optical path difference can be provided between the non-regular optical path via the first reflecting member and the non-regular optical path via the second reflecting member.

  In the encoder of the present invention, the light source, the light receiving unit, the first reflecting member, the second reflecting member, and the intermediate reflecting member may be arranged on one side of the scale. Thereby, a reflection type optical path can be constituted with respect to the scale.

  In the encoder of the present invention, the light source, the light receiving unit, and the intermediate reflecting unit may be disposed on one side of the scale, and the first reflecting member and the second reflecting member may be disposed on the other side of the scale. Thereby, a transmissive optical path can be configured with respect to the scale.

  In the encoder of the present invention, the light source may include a super luminescence diode (SLD). Thereby, it is possible to achieve both strong interference with light passing through the regular optical path and suppression of interference with light passing through the non-regular optical path.

(A) And (b) is a schematic diagram which illustrates the structure of the encoder which concerns on 1st Embodiment. (A) And (b) is a schematic diagram which illustrates a non-regular optical path. (A) And (b) is a schematic diagram which shows a reference example. (A) And (b) is a schematic diagram which shows a reference example. (A) And (b) is a schematic diagram which illustrates the structure of the encoder which concerns on 2nd Embodiment. (A) And (b) is a schematic diagram which illustrates a non-regular optical path.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following description, the same members are denoted by the same reference numerals, and the description of the members once described is omitted as appropriate.

[First Embodiment]
FIG. 1A to FIG. 2B are schematic views illustrating the configuration of an encoder according to the first embodiment.
The encoder 1 </ b> A according to the present embodiment detects a relative movement amount between the scale 20 and the optical system based on interference fringes generated by interference of diffracted light. The encoder 1 </ b> A includes a light source 10, a scale 20, a light receiving unit 30, and a reflecting unit 40.

  The light emitted from the light source 10 has a coherent length shorter than the optical path difference between a regular optical path length and an irregular optical path length, which will be described later. That is, the light emitted from the light source 10 is low coherent light. In the present embodiment, light that is less coherent than laser light (light that has low temporal coherence) is used. Examples of such a light source 10 that emits low-coherent light include a super luminescence diode (SLD). When an SLD is used as the light source 10, a light having a coherent length of several tens of μm to several hundreds of μm is applied.

  The scale 20 has a diffraction grating (not shown). The diffraction grating is constituted by irregularities, slits or the like having a predetermined pitch. The diffraction grating is provided on the surface 20 a of the scale 20. The linear scale 20 detects the amount of linear movement, and the circular scale 20 detects the amount of rotation (rotation angle). The light emitted from the light source 10 is irradiated in a direction orthogonal to the surface 20a on which the diffraction grating of the scale 20 is provided.

  The light receiving unit 30 includes, for example, a line sensor. The light receiving unit 30 converts the density of the interference fringes into an electrical signal and outputs it. A calculation unit (not shown) is provided at the subsequent stage of the light receiving unit 30 to perform processing on an electrical signal based on the received interference fringes.

  The reflection unit 40 is a reflection optical system for forming an optical path for forming a plurality of diffractions by the diffraction grating between the light source 10 and the light receiving unit 30 via the scale 20. As an optical path for forming a plurality of times of diffraction, the reflector 40 constitutes a first optical path and a second optical path that are normal optical paths.

  In order to configure the first optical path and the second optical path, the reflecting unit 40 includes a first reflecting member 41, a second reflecting member 42, and an intermediate reflecting member 43. These reflecting members can form diffracted light three times and output a signal that is 1/8 times the pitch of the diffraction grating provided on the scale 20.

  The 1st reflective member 41, the 2nd reflective member 42, and the intermediate | middle reflective member 43 are comprised by the corner cube, for example. The corner cube has two reflecting surfaces orthogonal to each other. The first reflecting member 41 is disposed on one side of the optical axis of the light source 10 on the surface 20 a side of the scale 20. The second reflecting member 42 is disposed on the other side of the optical axis of the light source 10 on the surface 20 a side of the scale 20. The intermediate reflecting member 43 is disposed between the first reflecting member 41 and the second reflecting member 42 on the surface 20a side of the scale 20. The light receiving unit 30 is disposed between the surface 20 a of the scale 20 and the intermediate reflecting member 43.

  In the present embodiment, the light source 10, the light receiving unit 30, the first reflecting member 41, the second reflecting member 42, and the intermediate reflecting member 43 are arranged on the surface 20 a side where the diffraction grating of the scale 20 is provided. Thereby, the reflective encoder 1A is configured.

  In the direction perpendicular to the surface 20a of the scale 20, the distance D1 between the first reflecting member 41 and the scale 20 is different from the distance D2 between the second reflecting member 42 and the scale 20. In the example shown in FIG. 1, the distance D1 is shorter than the distance D2.

  The line and arrow in FIG. 1A indicate the first optical path. The first optical path is constituted by light rays c1 and c11 to c17. The first optical path is an optical path that goes in the order of the light source 10, the scale 20, the second reflecting member 42, the scale 20, the intermediate reflecting member 43, the scale 20, the first reflecting member 41, the scale 20, and the light receiving unit 30.

  Specifically, first, the light ray c <b> 1 due to the light emitted from the light source 10 goes perpendicular to the surface 20 a of the scale 20. Diffracted light is generated by the diffraction grating provided on the surface 20 a of the scale 20. Of the diffracted light, a light ray c11 directed to the second reflecting member 42 is reflected by the second reflecting member 42 to become a light ray c12 and travels toward the scale 20. The light ray c12 is diffracted by the diffraction grating provided on the surface 20a of the scale 20 to become the light ray c13, and travels toward the intermediate reflecting member 43. The light ray c13 is reflected by the intermediate reflecting member 43 to become the light ray c14, and goes toward the scale 20 again. The light beam c14 is diffracted by the diffraction grating provided on the surface 20a of the scale 20 to become the light beam c15, and travels toward the first reflecting member 41. The light ray c15 is reflected by the first reflecting member 41 to become the light ray c16, and goes again to the scale 20. The light beam c <b> 16 is diffracted by the diffraction grating provided on the surface 20 a of the scale 20 to become the light beam c <b> 17 and travels toward the light receiving unit 30.

  The line and arrow in FIG. 1B indicate the second optical path. The second optical path is constituted by light rays c1 and c21 to c27. The second optical path is an optical path that goes in the order of the light source 10, the scale 20, the first reflecting member 41, the scale 20, the intermediate reflecting member 43, the scale 20, the second reflecting member 42, the scale 20, and the light receiving unit 30.

  Specifically, first, the light ray c <b> 1 due to the light emitted from the light source 10 goes perpendicular to the surface 20 a of the scale 20. Diffracted light is generated by the diffraction grating provided on the surface 20 a of the scale 20. Of the diffracted light, the light beam c21 that travels toward the first reflecting member 41 is reflected by the first reflecting member 41 to become the light beam c22 and travels toward the scale 20. The light ray c22 is diffracted by the diffraction grating provided on the surface 20a of the scale 20 to become the light ray c23, and travels toward the intermediate reflecting member 43. The light ray c23 is reflected by the intermediate reflecting member 43 to become the light ray c24, and goes again to the scale 20. The light ray c24 is diffracted by the diffraction grating provided on the surface 20a of the scale 20 to become the light ray c25, and travels toward the second reflecting member 42. The light ray c25 is reflected by the second reflecting member 42 to become the light ray c26, and goes toward the scale 20 again. The light ray c26 is diffracted by the diffraction grating provided on the surface 20a of the scale 20 to become the light ray c27, and travels toward the light receiving unit 30.

  The optical path length by the first optical path is equal to the optical path length by the second optical path. The light beam c17 by the first optical path and the light beam c27 by the second optical path interfere with each other to generate interference fringes, and the light receiving unit 30 can obtain a light and dark signal due to the interference fringes.

  FIGS. 2A and 2B are schematic views illustrating non-regular optical paths. FIG. 2A illustrates a third optical path that is one of the non-regular optical paths, and FIG. 2B illustrates a fourth optical path that is the other of the non-regular optical paths.

  The third optical path shown in FIG. 2A is constituted by light rays c1, c31 to c37. The third optical path is an optical path that goes in the order of the light source 10, the scale 20, the first reflecting member 41, the scale 20, the intermediate reflecting member 43, the scale 20, the first reflecting member 41, the scale 20, and the light receiving unit 30. That is, the third optical path that is an irregular optical path is an optical path that is not reflected by the second reflecting member 42 but is reflected twice by the first reflecting member 41.

  The 4th optical path shown in Drawing 2 (b) is constituted by light rays c1 and c41-c47. The fourth optical path is an optical path that goes in the order of the light source 10, the scale 20, the second reflecting member 42, the scale 20, the intermediate reflecting member 43, the scale 20, the second reflecting member 42, the scale 20, and the light receiving unit 30. That is, the fourth optical path that is an irregular optical path is an optical path that is not reflected by the first reflecting member 41 but is reflected twice by the second reflecting member 42.

  In the present embodiment, since the distance D1 and the distance D2 are different from each other, the optical path length by the third optical path is different from the optical path length by the fourth optical path. Due to this difference in optical path length, interference between the light beam c37 by the third optical path and the light beam c47 by the fourth optical path is suppressed.

  Further, in the present embodiment, the optical path lengths of the first optical path and the second optical path that are normal optical paths are different from the optical path length of the third optical path that is an irregular optical path. The optical path lengths of the first optical path and the second optical path are also different from the optical path length of the fourth optical path. Since the light source 10 emits low coherent light (light having a coherent length shorter than the optical path difference between the normal optical path length and the non-normal optical path length), the light in the first optical path and the light in the second optical path, which are normal optical paths. However, strong interference does not occur between the light in the first optical path and the second optical path, which are normal optical paths, and the light in the third optical path and the fourth optical path, which are non-regular optical paths. That is, by making the optical path difference between the regular optical path and the non-regular optical path longer than the coherent length of the light source 10, it is possible to clearly generate interference fringes due to light in the regular optical path without generating stray light interference fringes. it can. Therefore, interference fringes due to interference in the regular optical path appear clearly, and errors due to light in the irregular optical path can be suppressed.

[Reference example]
FIG. 3A to FIG. 4B are schematic diagrams illustrating reference examples.
FIGS. 3A to 4B show an example in which the distance D1 of the first reflecting member 41 and the distance D2 of the second reflecting member 42 are the same.
3 (a) and 3 (b) show light rays along the normal optical path (first optical path and second optical path in the reference example), and FIGS. 4 (a) and 4 (b) show non-normal optical paths (first reference path in the reference example). The light rays by (3 optical paths and 4 optical paths) are shown.

  The first optical path of the reference example shown in FIG. 3A includes the light source 10, the scale 20, the second reflecting member 42, the scale 20, the intermediate reflecting member 43, the scale 20, the first reflecting member 41, the scale 20, and the light receiving unit 30. It is an optical path heading in the order of. The second optical path of the reference example shown in FIG. 3B includes the light source 10, the scale 20, the first reflecting member 41, the scale 20, the intermediate reflecting member 43, the scale 20, the second reflecting member 42, the scale 20, and the light receiving unit 30. It is an optical path that goes to

  The third optical path of the reference example shown in FIG. 4A includes the light source 10, the scale 20, the first reflecting member 41, the scale 20, the intermediate reflecting member 43, the scale 20, the first reflecting member 41, the scale 20, and the light receiving unit 30. It is an optical path heading in the order of. The fourth optical path of the reference example shown in FIG. 4B includes the light source 10, the scale 20, the second reflecting member 42, the scale 20, the intermediate reflecting member 43, the scale 20, the second reflecting member 42, the scale 20, and the light receiving unit 30. It is an optical path heading in the order of.

  In such a reference example, the optical path lengths of the first optical path, the second optical path, the third optical path, and the fourth optical path are equal to each other. Therefore, interference occurs due to the light in each of the first optical path, the second optical path, the third optical path, and the fourth optical path toward the light receiving unit 30, and interference in the non-regular optical path affects the interference in the regular optical path. Become.

  In the encoder 1A of the present embodiment shown in FIG. 1A to FIG. 2B, it is possible to generate clear light and dark by interference in the normal optical path, and to suppress the influence of interference in the non-normal optical path. Therefore, highly accurate detection can be performed by interference in the regular optical path.

  In particular, when a measurement accuracy narrower than the pitch of the diffraction grating is obtained by generating the diffracted light a plurality of times, the influence of slight interference due to the non-regular optical path tends to cause an error as the number of reflections increases. In the present embodiment, since the influence of the non-regular optical path can be suppressed, it is effective for improving the measurement accuracy of the encoder 1A using a plurality of generations of diffracted light.

[Second Embodiment]
FIG. 5A to FIG. 6B are schematic views illustrating the configuration of an encoder according to the second embodiment.
The encoder 1B according to this embodiment includes a light source 10, a scale 20, a light receiving unit 30, and a reflecting unit 40. The reflection unit 40 includes a first reflection member 41, a second reflection member 42, and an intermediate reflection member 43. Among these, the light source 10, the light receiving unit 30, and the intermediate reflecting member 43 are provided on one side of the scale 20, and the first reflecting member 41 and the second reflecting member 42 are provided on the other side of the scale 20. Thereby, the transmissive encoder 1B is configured.

  In the direction perpendicular to the surface 20a of the scale 20, the distance D1 between the first reflecting member 41 and the scale 20 is different from the distance D2 between the second reflecting member 42 and the scale 20. In the example shown in FIG. 5, the distance D1 is shorter than the distance D2.

  The line and arrow in FIG. 5A indicate the first optical path. The first optical path is constituted by light rays c1 and c51 to c57. The first optical path is an optical path that goes in the order of the light source 10, the scale 20, the second reflecting member 42, the scale 20, the intermediate reflecting member 43, the scale 20, the first reflecting member 41, the scale 20, and the light receiving unit 30.

  Specifically, first, the light ray c <b> 1 due to the light emitted from the light source 10 goes perpendicular to the surface 20 a of the scale 20. Diffracted light is generated when the light beam c1 passes through the diffraction grating provided on the surface 20a of the scale 20. Of the diffracted light, the light beam c 51 that travels toward the second reflecting member 42 is reflected by the second reflecting member 42 to become a light beam c 52 that travels toward the scale 20. The light beam c52 passes through the diffraction grating provided on the surface 20a of the scale 20 and is diffracted into the light beam c53, which travels toward the intermediate reflecting member 43. The light ray c53 is reflected by the intermediate reflecting member 43 to become the light ray c54, and goes again to the scale 20. The light ray c54 passes through the diffraction grating provided on the surface 20a of the scale 20 and is diffracted to become the light ray c55, which travels toward the first reflecting member 41. The light ray c55 is reflected by the first reflecting member 41 to become the light ray c56, and goes toward the scale 20 again. The light beam c56 passes through the diffraction grating provided on the surface 20a of the scale 20 and is diffracted to become the light beam c57, which travels toward the light receiving unit 30.

  The line and arrow in FIG. 5B indicate the second optical path. The second optical path is constituted by light rays c1 and c61 to c67. The second optical path is an optical path that goes in the order of the light source 10, the scale 20, the first reflecting member 41, the scale 20, the intermediate reflecting member 43, the scale 20, the second reflecting member 42, the scale 20, and the light receiving unit 30.

  Specifically, first, the light ray c <b> 1 due to the light emitted from the light source 10 goes perpendicular to the surface 20 a of the scale 20. Diffracted light is generated when the light beam c1 passes through the diffraction grating provided on the surface 20a of the scale 20. Of the diffracted light, the light beam c 61 that travels toward the first reflecting member 41 is reflected by the first reflecting member 41 to become a light beam c 62 and travels toward the scale 20. The light ray c62 passes through the diffraction grating provided on the surface 20a of the scale 20 and is diffracted to become the light ray c63, which travels toward the intermediate reflecting member 43. The light ray c63 is reflected by the intermediate reflecting member 43 to become the light ray c64, and again travels toward the scale 20. The light beam c64 passes through the diffraction grating provided on the surface 20a of the scale 20 and is diffracted to become the light beam c65, which travels toward the second reflecting member 42. The light ray c65 is reflected by the second reflecting member 42 to become a light ray c66, and goes toward the scale 20 again. The light beam c66 passes through the diffraction grating provided on the surface 20a of the scale 20 and is diffracted into the light beam c67, which travels toward the light receiving unit 30.

  The optical path length by the first optical path is equal to the optical path length by the second optical path. The light beam c57 by the first optical path and the light beam c67 by the second optical path interfere with each other to generate interference fringes, and the light receiving unit 30 can obtain a light / dark signal due to the interference fringes.

  FIGS. 6A and 6B are schematic views illustrating non-regular optical paths. FIG. 6A illustrates a third optical path that is one of the non-regular optical paths, and FIG. 6B illustrates a fourth optical path that is the other of the non-normal optical paths.

  The third optical path shown in FIG. 6A is composed of light rays c1 and c71 to c77. The third optical path is an optical path that goes in the order of the light source 10, the scale 20, the first reflecting member 41, the scale 20, the intermediate reflecting member 43, the scale 20, the first reflecting member 41, the scale 20, and the light receiving unit 30. That is, the third optical path that is an irregular optical path is an optical path that is not reflected by the second reflecting member 42 but is reflected twice by the first reflecting member 41.

  The 4th optical path shown in Drawing 6 (b) is constituted by light rays c1 and c81-c87. The fourth optical path is an optical path that goes in the order of the light source 10, the scale 20, the second reflecting member 42, the scale 20, the intermediate reflecting member 43, the scale 20, the second reflecting member 42, the scale 20, and the light receiving unit 30. That is, the fourth optical path that is an irregular optical path is an optical path that is not reflected by the first reflecting member 41 but is reflected twice by the second reflecting member 42.

  In the present embodiment, since the distance D1 and the distance D2 are different from each other as in the first embodiment, the optical path length by the third optical path is different from the optical path length by the fourth optical path. Due to this difference in optical path length, interference between the light beam c77 by the third optical path and the light beam c87 by the fourth optical path is suppressed.

  Further, the optical path lengths of the first optical path and the second optical path that are normal optical paths are different from the optical path length of the third optical path that is an irregular optical path. The optical path lengths of the first optical path and the second optical path are also different from the optical path length of the fourth optical path. Since low coherent light is emitted from the light source 10, interference fringes due to interference in the regular optical path clearly appear, and errors due to light in the irregular optical path can be suppressed.

  As described above, according to the embodiment, encoders 1A and 1B that can suppress errors in measurement using interference fringes can be provided.

  Although the present embodiment has been described above, the present invention is not limited to these examples. For example, the configuration of the reflection unit 40 is not limited to the first reflection member 41, the second reflection member 42, and the intermediate reflection member 43, and is a configuration that realizes reflection of diffracted light four times or more by more reflection members. There may be. In addition, those in which those skilled in the art appropriately added, deleted, and changed the design of the above-described embodiments, and combinations of the features of each embodiment as appropriate also include the gist of the present invention. As long as it is within the scope of the present invention.

1A, 1B ... Encoder 10 ... Light source 20 ... Scale 20a ... Surface 30 ... Light receiving portion 40 ... Reflecting portion 41 ... First reflecting member 42 ... Second reflecting member 43 ... Intermediate reflecting member D1 ... Distance D2 ... Distance

Claims (5)

A light source;
A scale having a diffraction grating;
A light receiver;
Reflecting portions constituting a first optical path and a second optical path having the same optical path length as an optical path for forming a plurality of diffractions by the diffraction grating from the light source to the light receiving section through the scale,
With
When the optical path lengths of the first optical path and the second optical path are normal optical path lengths, and the optical path lengths of optical paths other than the first optical path and the second optical path are non-normal optical path lengths, the reflecting unit is configured to have the normal optical path length. Is configured to be different from the non-regular optical path length,
The encoder, wherein the light source emits light having a coherent length shorter than an optical path difference between the normal optical path length and the non-normal optical path length. The reflective portion is
A first reflecting member;
A second reflecting member;
An intermediate reflection member provided between the first reflection member and the second reflection member in the first optical path and the second optical path;
The distance between the first reflecting member and the scale is different from the distance between the second reflecting member and the scale in a direction perpendicular to the surface of the scale on which the diffraction grating is provided. The encoder described.   The encoder according to claim 2, wherein the light source, the light receiving unit, the first reflecting member, the second reflecting member, and the intermediate reflecting member are disposed on one side of the scale. The light source, the light receiving unit and the intermediate reflecting member are arranged on one side of the scale,
The encoder according to claim 2, wherein the first reflecting member and the second reflecting member are disposed on the other side of the scale.   The encoder according to claim 1, wherein the light source includes a super luminescence diode (SLD).

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