JP6838954B2 - Photoelectric encoder - Google Patents

Description

This case relates to a photoelectric encoder.

An encoder that generates an interference signal by converging the optical paths of the + s-th order diffracted light and the −s-th order diffracted light from the scale by an index grid inside the detector is disclosed. In such an encoder, if unnecessary diffracted light other than the ± s-order diffracted light is spatially mixed with the ± s-order diffracted light, the profile of the interference fringes is disturbed and a measurement error occurs. Therefore, a technique for physically shielding unnecessary diffracted light in the detector is disclosed (for example, Patent Documents 1 and 2).

Japanese Unexamined Patent Publication No. 4-184218 Japanese Unexamined Patent Publication No. 2004-069702

However, in this technique, in order to shield the unnecessary diffracted light, it is necessary to widen the distance between the scale and the index grid to spatially separate the signal diffracted light and the unnecessary diffracted light. In this case, the detector may become larger by the amount that the distance between the scale and the index grid is widened.

On one aspect, it is an object of the present invention to provide a photoelectric encoder capable of suppressing the influence of unnecessary diffracted light from the interference fringes of ± s-th order diffracted light while suppressing the increase in size.

In one aspect, in the photoelectric encoder according to the present invention, a light source that emits light and light from the light source are incident, and a plurality of lattices are formed along an axis that intersects the optical axis of the light from the light source. The main scale and the diffracted light from the main scale are incident on the central axis, and the central axis has a central axis in the length direction of the lattice of the main scale and is perpendicular to the optical axis of the diffracted light. A first region in which a plurality of first lattices formed at predetermined intervals are symmetrically inclined at a first angle with respect to the central axis, and a first region symmetrical with respect to the central axis. An index lattice having a second region formed with a plurality of second lattices formed at predetermined intervals and inclined at two angles, and a light receiving element for receiving diffracted light transmitted through the index lattice are provided . The lattice formed in the first region is only the first lattice, and the lattice formed in the second region is only the second lattice .

In the photoelectric encoder, the arrangement direction of the plurality of lattices of the main scale is orthogonal to the optical axis of the light incident on the main scale from the light source, and the central axis is the light incident on the main scale from the light source. It may pass through the optical axis of.

In the photoelectric encoder, the light receiving element is arranged at a position where the ± primary diffracted light transmitted through the index lattice is collected, and the photodiode of the light receiving element collects ± secondary diffracted light transmitted through the index lattice. It may be shorter than the distance between the shining part and the part where the 0th-order diffracted light is focused.

In the photoelectric encoder, a plurality of photodiodes are arranged in parallel to the arrangement direction of the plurality of lattices of the main scale in the light receiving element, and ± primary diffracted light transmitted through the index lattice is focused. When the pitch of the interference fringes is Λ, the pitches of the plurality of photodiodes may be Λ, Λ / 4 or 3Λ / 4.

It is possible to provide a photoelectric encoder capable of suppressing the influence of unnecessary diffracted light from the interference fringes of ± s-order diffracted light while suppressing the increase in size.

It is a perspective view of the photoelectric encoder which concerns on embodiment. FIG. 2A is a perspective view of a photoelectric encoder according to a comparative embodiment, and FIG. 2B is a diagram illustrating the light intensity of interference fringes when the light intensity of unnecessary light is 10% of ± primary diffracted light. Is. It is a figure which illustrates the detail of the index grid. (A) is a perspective view of a photoelectric encoder according to an embodiment, and (b) is a diagram illustrating interference fringes at a location where ± primary diffracted light is focused. It is a figure for demonstrating the preferable inclination angle θ. It is a figure which illustrates the light receiving element. It is a figure which illustrates the light receiving element. It is a figure which illustrates the light receiving element.

Hereinafter, embodiments will be described with reference to the drawings.

(Embodiment)
FIG. 1 is a perspective view of the photoelectric encoder 100 according to the embodiment. As illustrated in FIG. 1, the photoelectric encoder 100 includes a collimating light source 10, a main scale 20, an index grid 30, and a light receiving element 40. In the following description, the arrangement direction of each grid 21 formed on the main scale 20 is defined as the X-axis. The optical axis direction of the collimated light orthogonal to the X-axis and incident on the main scale 20 from the collimating light source 10 is defined as the Z-axis. The direction in which each grid 21 of the main scale 20 extends orthogonal to the X-axis and the Z-axis is defined as the Y-axis.

The collimating light source 10 is not particularly limited as long as it is a light source that emits collimated light. For example, the collimating light source 10 includes a light emitting element such as a light emitting diode and a collimating light generating means such as a collimating lens and a reflector. The reflector is, for example, a reflector having a reflecting surface of a partial paraboloid.

The main scale 20 includes a grid 21 having a predetermined scale period along the X-axis direction. That is, the main scale 20 has a grid arrangement direction in the X-axis direction. Therefore, the measurement axis of the main scale 20 is the X axis. Each grid 21 extends in the Y-axis direction. That is, each grid 21 has a length direction in the Y-axis direction. The main scale 20 is movable relative to the collimating light source 10, the index grid 30, and the light receiving element 40 in the X-axis direction.

The index grid 30 includes a grid having a predetermined period. The light receiving element 40 is provided with a light receiving region 41. In the light receiving region 41, a plurality of photodiodes 42 are arranged side by side in the X-axis direction at a predetermined cycle. For example, the light receiving element 40 is a photodiode array. The index grid 30 forms an image of diffracted light transmitted through the main scale 20 in the light receiving region 41 of the light receiving element 40. The light receiving region 41 uses the outputs of the plurality of photodiodes 42 to detect periodic light and darkness according to the grid 21 of the main scale 20. Thereby, the relative position fluctuation of the main scale 20 can be detected. Specifically, the amount of position variation can be obtained based on the light receiving intensity detected by the plurality of photodiodes 42.

Here, the comparative form will be described. FIG. 2A is a perspective view of the photoelectric encoder 200 according to the comparative mode. In the photoelectric encoder 200, an index grid 30a is arranged instead of the index grid 30. As illustrated in FIG. 2A, the index grid 30a includes a grid having a predetermined period along the X-axis direction. That is, the index grid 30a has a grid arrangement direction in the X-axis direction. Each grid of the index grid 30a extends in the Y-axis direction. Therefore, the grid of the index grid 30a is formed parallel to the grid of the main scale 20. The grid pitch of the index grid 30a is, for example, 1/2 of the grid pitch of the main scale 20.

In this configuration, not only the signal diffracted light (± 1st-order diffracted light) but also unnecessary diffracted light (0th-order diffracted light, ± 2nd-order diffracted light, ± 3rd-order diffracted light, ...) Is focused on one point by the light receiving element 40. In this case, the strength of the interference fringes will appear. That is, distortion appears in the interference fringes. For example, FIG. 2B illustrates the light intensity of the interference fringes when the light intensity of unnecessary light (0th-order diffracted light, ± 2nd-order diffracted light, ± 3rd-order diffracted light, ...) Is 10% of the ± 1st-order diffracted light. Has been done. In FIG. 2B, the horizontal axis represents the X-axis direction in the light receiving region of the light receiving element 40, and the vertical axis represents the light intensity incident on the light receiving region of the light receiving element 40. As described above, in the photoelectric encoder 200 according to the comparative form, high measurement accuracy may not be obtained.

Therefore, in the photoelectric encoder 100 according to the embodiment, the ± s-order diffracted light is spatially separated by providing the index grid 30 with a grid that is inclined symmetrically with respect to the central axis. The details will be described below.

FIG. 3 is a diagram illustrating the details of the index grid 30. As illustrated in FIG. 3, the index grid 30 includes a plurality of grids that are symmetrically inclined with the central axis CL as the central axis in a predetermined plane. Each grid 33 on one side (hereinafter, first region 31) has the same inclination angle (θ) with respect to the central axis CL, and each grid 34 on the other side (hereinafter, second region 32) has the same inclination angle (hereinafter, second region 32). It has −θ). The lattice pitch p is the same in both the first region 31 and the second region 32.

As illustrated in FIG. 4A, the index grid 30 is arranged so that the central axis CL is parallel to the Y axis. Further, the index grid 30 is arranged so that the grids 33 and 34 form a plane perpendicular to the optical axis of the diffracted light from the main scale 20. The angle of the grid that goes to the plus side of the X-axis is positive as it goes to the plus side of the Y-axis, and the angle of the grid that goes to the minus side of the X-axis is minus as it goes to the plus side of the Y-axis.

Since each lattice 33 of the first region 31 has an inclination angle of θ, the traveling direction of the diffracted light transmitted through the first region 31 is separated on the positive side in the Y-axis direction. Since each lattice 34 of the second region 32 has an inclination angle of −θ, the traveling direction of the diffracted light transmitted through the second region 32 is also separated on the Y-axis plus side. Since each grid 33 in the first region 31 and each grid 34 in the second region 32 are inclined symmetrically with respect to the central axis CL, the light is collected at the same location. Each ± s-th order diffracted light is focused on the same axis, but if the value of s is different, they are separated on the same axis. In the present embodiment, the light receiving element 40 is arranged at a position where the ± primary diffracted light is focused. If each photodiode 42 of the light receiving element 40 is shorter than the distance between the portion where the ± second-order diffracted light on the same axis is focused and the 0th-order diffracted light, it is possible to avoid the incident of unnecessary diffracted light on each photodiode. ..

FIG. 4B exemplifies the interference fringes at the points where the ± primary diffracted light is focused. Since the incident of unnecessary diffracted light is avoided, the strength of the interference fringes is suppressed. That is, the distortion of the interference fringes is suppressed. The interference fringes in FIG. 4B are interference fringes when the grid pitch of the main scale 20 is 4 μm, the grid pitch of the index grid 30 is 2 μm, and θ is 10 degrees. In this way, an ideal sinusoidal interference fringe can be obtained.

According to the present embodiment, it is not necessary to spatially separate the signal diffracted light and the unnecessary diffracted light by widening the distance between the main scale 20 and the index grid 30, so that the increase in size of the photoelectric encoder 100 can be suppressed. Can be done. Moreover, the influence of unnecessary diffracted light can be suppressed. That is, it is possible to suppress the influence of unnecessary diffracted light from the interference fringes of ± s-th order diffracted light while suppressing the increase in size.

Subsequently, a preferable inclination angle θ will be described. First, as illustrated in FIG. 5, the ± primary diffraction angle θ ₁ from the main scale 20 is represented by the following equation (1). In addition, λ is the wavelength of collimated light, and g is the lattice pitch of the main scale 20.

_{Let θ 2} be the traveling direction angle of the 0th-order diffracted light when the ± 1st-order diffracted light from the index lattice 30 is projected onto the plane formed by the diffracted light of the main scale 20. In this case, the following equation (2) holds. Note that p is the grid pitch of the index grid 30, and θ is the tilt angle of each grid of the index grid 30 from the central axis CL.

The period Λ of the interference fringes generated on the light receiving element 40 when the light receiving element 40 is irradiated with the ± primary diffracted light at _{an angle of ± θ 2 can be expressed by the following equation (3).} ..

For example, if the grid pitch g of the main scale 20 is 4 μm, the grid pitch p of the index grid 30 is 2 μm, and the inclination angle θ is 10 degrees, the period Λ is 1.94 μm. In this case, when the fluctuation of the period Λ is suppressed within ± 0.1% from the design value, the inclination angle θ is preferably within 10 ± 0.16 degrees.

Subsequently, the pitch of the photodiode 42 in the light receiving element 40 will be described. The pitch p of the photodiode 42 is preferably a period Λ. This is because a sine wave signal can be obtained. The pitch p of the photodiodes 42 is the distance between the centers of the photodiodes 42 in the arrangement direction of the photodiodes 42.

However, in the photoelectric encoder 100, it is preferable to use a plurality of sine wave signals having a phase difference in order to determine the moving direction of the main scale 30. Therefore, for example, as illustrated in FIG. 6, it is preferable to provide a plurality of photodiode arrays whose phases are shifted from each other in the displacement direction to obtain signals having different phases by 90 degrees. In the example of FIG. 6, signals having a phase difference of 90 degrees can be obtained in four photodiode arrays that are separated from each other.

Alternatively, as illustrated in FIG. 7, at least one set of photodiodes 42 may be arranged with the pitch p of the photodiode 42 being Λ/4 and four photodiodes 42 as one set. In this case, signals having different phases of 90 degrees are obtained from each of the four photodiodes 42. In such a configuration, since four photodiodes are integrated in one cycle of the interference fringes, the photoelectric encoder 100 can be made compact.

Alternatively, as illustrated in FIG. 8, the pitch p of the photodiode 42 may be 3Λ/4, the four photodiodes 42 may be one set, and at least one set of photodiodes 42 may be arranged. In this case as well, signals having different phases of 90 degrees can be obtained from each of the four photodiodes 42. In such a configuration, the pitch of the photodiode 42 is larger than that in FIG. 7, but the effect that the light receiving element 40 can be easily manufactured can be obtained.

In the embodiment, the arrangement direction of the grids of the main scale 20 is orthogonal to the optical axis of the collimated light, but it may intersect. When the arrangement direction of the grid of the main scale 20 is orthogonal to the optical axis of the collimated light, the central axis CL of the index grid 30 is the optical axis of the collimated light incident on the main scale 20 from the collimating light source 10. It is preferable to pass through. This is because the symmetry of the diffracted light from the index grid 30 is improved.

Although the examples of the present invention have been described in detail above, the present invention is not limited to the specific examples, and various modifications and modifications are made within the scope of the gist of the present invention described in the claims. It can be changed.

10 Collimated light source 20 Main scale 30 Index grid 31 1st region 32 2nd region 33,34 grid 40 Light receiving element 41 Light receiving area 42 Photodiode 100 Photoelectric encoder

Claims (4)

A light source that emits light and
A main scale in which light from the light source is incident and a plurality of grids are formed along an axis intersecting the optical axis of the light from the light source.
The diffracted light from the main scale is incident, has a central axis in the length direction of the lattice of the main scale, and is symmetrical with the central axis in a plane perpendicular to the optical axis of the diffracted light. A first region in which a plurality of first lattices formed at predetermined intervals are inclined at a first angle with respect to the central axis, and a second angle symmetrical with the first angle with respect to the central axis. An index lattice having a second region formed with a plurality of second lattices formed at predetermined intervals, and an index lattice having a plurality of second lattices formed at predetermined intervals.
A light receiving element that receives diffracted light transmitted through the index grid is provided .
A photoelectric encoder characterized in that the lattice formed in the first region is only the first lattice, and the lattice formed in the second region is only the second lattice. The arrangement direction of the plurality of grids of the main scale is orthogonal to the optical axis of the light incident on the main scale from the light source.
The photoelectric encoder according to claim 1, wherein the central axis passes through an optical axis of light incident on the main scale from the light source. The light receiving element is arranged at a position where the ± primary diffracted light transmitted through the index grid is focused.
The photodiode of the light receiving element according to claim 1 or 2, wherein the photodiode of the light receiving element is shorter than the distance between the portion where the ± 2nd-order refracted light passing through the index lattice is focused and the portion where the 0th-order diffracted light is focused. Photodiode encoder. In the light receiving element, a plurality of photodiodes are arranged in parallel with the arrangement direction of the plurality of lattices of the main scale.
When the pitch of the interference fringes at the location where the ± 1st-order diffracted light transmitted through the index grid is condensed is Λ, the pitches of the plurality of photodiodes are Λ, Λ / 4 or 3Λ / 4. The photoelectric encoder according to any one of claims 1 to 3.

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