JP7523866B2 - Optical Encoders - Google Patents

Description

The present invention relates to an optical encoder.

A conventional optical encoder includes a plate-shaped scale having a grid pattern formed at a predetermined period along a measurement direction on one surface thereof, and a detection head provided so as to be movable relative to the scale along the measurement direction. The detection head includes a light source that irradiates light onto the scale, and a light receiving means that receives light that has passed through the scale.
In such optical encoders, light emitted from a light source becomes multiple diffracted light beams via the grid pattern on the scale. The multiple diffracted light beams generate interference fringes with the same period as the grid pattern. The light receiving means receives the interference fringes, and the detection head detects a signal from the interference fringes received by the light receiving means. The optical encoder calculates the amount of relative movement between the scale and the detection head from the detection result (signal) by the detection head.

For example, the three-grating principle is applied to optical encoders. Specifically, the optical encoder includes a light source that irradiates light onto the scale, a scale having a lattice-like pattern corresponding to the first grating, a grating plate having a second grating, and a light receiving means having a plurality of light receiving elements (third grating). The scale is irradiated with light from the light source. The second grating of the grating plate is formed at a predetermined period. The grating plate is disposed between the scale and the plurality of light receiving elements, and irradiates light from the light source through the scale toward the plurality of light receiving elements. The plurality of light receiving elements, which are the third grating, are disposed at a predetermined period and filter the interference fringes generated by the light through the grating plate. The optical encoder calculates the relative movement amount between the scale and the detection head based on the interference fringes filtered by the plurality of light receiving elements.
Here, in the optical encoder using the three-grating principle as described above, the following problems may occur.

Figure 8 is a diagram showing a conventional optical encoder. Specifically, Figure 8(A) is a diagram showing an optical encoder 100A in which a first grating, a second grating, and a third grating are arranged at equal intervals. Also, Figure 8(B) is a diagram showing an optical encoder 100B in which a first grating, a second grating, and a third grating are arranged at different intervals.

As shown in FIG. 8, the optical encoders 100A and 100B include plate-shaped scales 200A and 200B having a grid pattern corresponding to a first grid, a light source 300 that irradiates light onto the scales 200A and 200B, a grid plate 400 having a second grid, and a light receiving means 500 having a plurality of detection elements corresponding to a third grid. The light source 300 is disposed between the scales 200A and 200B and the grid plate 400. The grid plate 400 is disposed between the scales 200A and 200B and the light receiving means 500. The light receiving means 500 is disposed on the opposite side of the grid plate 400 from the scales 200A and 200B.

In the optical encoders 100A and 100B, the scales 200A and 200B reflect the light from the light source 300 to the lattice plate 400. The lattice plate 400 passes the light reflected from the scales 200A and 200B toward the light receiving means 500. The light receiving means 500 receives the light that has passed through the lattice plate 400. For convenience of explanation, in FIG. 8, the optical path of the light irradiated from the light source 300 to be received by the light receiving means 500 is shown by a solid arrow. In addition, the measurement direction is the X direction, the direction perpendicular to the X direction on the plate surface of the scales 200A and 200B or the lattice plate 400 (the width direction of the scales 200A and 200B or the lattice plate 400) is the Y direction, and the direction perpendicular to the X direction and the Y direction is the Z direction.

As shown in FIG. 8(A), the optical encoder 100A arranges the scale 200A, the lattice plate 400, and the light receiving means 500 at an interval of distance D1 in the Z direction. That is, the optical encoder 100A arranges the scale 200A, the lattice plate 400, and the light receiving means 500 at equal intervals in the Z direction. On the other hand, as shown in FIG. 8(B), the optical encoder 100B arranges the scale 200B and the lattice plate 400 at intervals of distance D1 and distance D2 in the Z direction. Also, the optical encoder 100B arranges the lattice plate 400 and the light receiving means 500 at an interval of distance D1 in the Z direction. That is, the optical encoder 100B arranges the scale 200B and the lattice plate 400 at an interval of distance D2, which is different from the interval between the scale 200B and the lattice plate 400 and the light receiving means 500.

In FIG. 8A, the light emitted from the light source 300 is split into multiple beams by the scale 200A, reflected, passes through the lattice plate 400, and is combined by the light receiving means 500. In contrast, in FIG. 8B, the light emitted from the light source 300 is split into multiple beams by the scale 200B, reflected, passes through the lattice plate 400, and is irradiated by the light receiving means 500 at a distance D3 in the X direction. When the split beams are offset and irradiated to the light receiving means 500, the amount of overlap of the multiple beams is reduced compared to when they are not offset. That is, in the optical encoder 100B, the amount of overlap of the split beams is reduced compared to the optical encoder 100A. When the amount of overlap of the beams is reduced, interference fringes may not be generated or a signal may not be acquired.

Therefore, conventional optical encoders that use a grid-like pattern on the scale as the first grid have the problem that they may not be able to maintain the detection accuracy of signals such as interference fringes if the positions of the scale, grid plate, light receiving means, etc. change and a deviation occurs in the distance between them in the Z direction.

In contrast, the photoelectron scale reading device described in Patent Document 1 attempts to solve the problem by devising a light source design. Specifically, the size of the light source is limited to a predetermined size. The light source is arranged so as to define a small angle range with respect to the spectrometer grating (a grating plate having a grating). The size of the light source is designed based on an area that produces good and easily detectable interference fringes, the grating pitch of the spectrometer grating, the distance between the light source and the scale, the distance between the scale and the spectrometer grating, and the like.
The photoelectron scale reading device aims to maintain the signal detection accuracy by designing the light source based on the parameters of each component part in the photoelectron scale reading device.

Patent No. 4750998

However, the photoelectron scale reading device described in Patent Document 1 requires that the light source be designed according to the parameters of each component, etc. This creates a problem in that restrictions are placed on the light source.

The object of the present invention is to provide an optical encoder that can maintain signal detection accuracy without considering constraints in the design of the light source, even if the relative positions of the components change and deviations occur in the distance between them.

The optical encoder of the present invention includes a plate-shaped scale having a grid pattern formed at a predetermined period along the measurement direction on one surface thereof, and a detection head that is arranged to be movable relative to the scale along the measurement direction. The detection head includes a light source that irradiates light toward the scale, a plurality of grid plates that are arranged between the light source and the scale and parallel to the plate surface of the scale, and have grids formed at a predetermined period on one surface thereof, and a light receiving means that receives light reflected by the scale and passing through the plurality of grid plates. The plurality of grid plates include a first grid plate that is irradiated with light from the light source, a second grid plate that is arranged between the first grid plate and the scale and that irradiates light passing through the first grid plate toward the scale, a third grid plate that is arranged between the scale and the light receiving means and that is irradiated with light reflected by the scale, and a fourth grid plate that is arranged between the third grid plate and the light receiving means and that irradiates light passing through the third grid plate toward the light receiving means. Here, the predetermined period of the grid pattern is g, and a predetermined constant between 0 and 1 that determines the predetermined period of the grid is n. In this case, the grating of the first grating plate is formed with a period of g÷n. The grating of the second grating plate is formed with a period of g÷n÷2. The grating of the third grating plate is formed with a period of g÷|1-n|÷2. The grating of the fourth grating plate is formed with a period of g÷|1-n|.

According to the present invention, the optical encoder can suppress changes in the direction of light travel caused by changes in the relative positions of the components by designing the predetermined period of each of the gratings of the first grating plate, the second grating plate, the third grating plate, and the fourth grating based on the predetermined period of the grating pattern of the scale. The optical encoder can maintain signal accuracy by devising the design of the gratings of the multiple grating plates without designing a light source. Therefore, the optical encoder can maintain signal detection accuracy without considering constraints in the design of the light source, even if the relative positions of the components change and deviations occur in the separation distance.

The optical encoder of the present invention includes a plate-shaped scale having a lattice pattern formed at a predetermined period along the measurement direction on one surface thereof, and a detection head that is provided so as to be movable relative to the scale along the measurement direction. The detection head includes a light source that irradiates light toward the scale, a plurality of lattice plates that are arranged between the light source and the scale and parallel to the plate surface of the scale, and have lattices formed at a predetermined period on one surface thereof, and a light receiving means having a plurality of light receiving elements that receive light reflected by the scale and passing through the plurality of lattice plates. The plurality of lattice plates include a first lattice plate that is irradiated with light from the light source, a second lattice plate that is arranged between the first lattice plate and the scale, and that irradiates light passing through the first lattice plate toward the scale, and a third lattice plate that is arranged between the scale and the light receiving means, and that is irradiated with light reflected by the scale. The plurality of light receiving elements are formed at a predetermined period on one surface of the light receiving means that is parallel to the plate surface of the scale. Here, the predetermined period of the lattice pattern is g, and a predetermined constant between 0 and 1 that determines the predetermined period of the lattice and the predetermined period of the plurality of light receiving elements is n. In this case, the grating of the first grating plate is formed with a period of g÷n. The grating of the second grating plate is formed with a period of g÷n÷2. The grating of the third grating plate and the multiple light receiving elements are characterized by being formed with a period of g÷|1-n|÷2.

According to the present invention, the optical encoder can suppress changes in the direction of light travel caused by changes in the relative positions of the components by designing the predetermined period of the gratings of the first grating plate, the second grating plate, and the third grating plate and the predetermined period of the multiple light receiving elements based on the predetermined period of the grid pattern of the scale. The optical encoder can maintain signal accuracy by devising the design of the gratings and multiple light receiving elements of the multiple grating plates without designing a light source. Therefore, the optical encoder can maintain signal detection accuracy without considering constraints in the design of the light source, even if the relative positions of the components change and deviations occur in the separation distance.

The optical encoder of the present invention does not include the fourth grating plate that the optical encoder described above includes, and instead uses a plurality of light receiving elements of the light receiving means as a substitute for the grating in the fourth grating plate. Therefore, the optical encoder does not need to include the fourth grating plate described above, which allows for cost reduction.

In this case, it is preferable that the predetermined constant n is 0.5.

With this configuration, the predetermined constant n is 0.5, which makes it possible to stabilize the amount of overlap of the divided light in the light receiving means, compared to when a constant n other than 0.5 is used. In addition, the predetermined period can be calculated more easily, compared to when a constant n other than 0.5 is used, which makes it possible to improve efficiency in design.

In this case, it is preferable that the first lattice plate and the fourth lattice plate, or the first lattice plate and the light receiving means, and the second lattice plate and the third lattice plate are arranged on the same plane.

Here, if the distances between the first grating plate, the second grating plate, the scale, the third grating plate, and the fourth grating plate or the light receiving means are different, the amount of overlap of the light irradiated to the light receiving means may decrease. However, with this configuration, it is possible to make at least the distances between the first grating plate and the fourth grating plate, the second grating plate and the third grating plate, and the distances between the second grating plate and the third grating plate and the scale equal. Therefore, the optical encoder can suppress a decrease in the amount of overlap of the light in the light receiving means.

In this case, it is preferable that the distances between the first grating plate, the second grating plate, the scale, the third grating plate, and the fourth grating plate or the light receiving means are equal.

Here, in a conventional optical encoder, for example, when the grid pattern of the scale is the first grid, the grid of the third grid plate is the second grid, and the grid of the fourth grid plate or the light receiving element of the light receiving means is the third grid, and the separation distances of the respective components are different, the amount of overlap of the divided light in the light receiving means may decrease. However, in the optical encoder of the present invention, as described above, by designing the grids of the multiple grid plates and the predetermined period of the multiple light receiving elements, it is possible to suppress the reduction in the amount of overlap. In addition, according to this configuration, the optical encoder can further suppress the reduction in the amount of overlap of the divided light in the light receiving means by making the respective separation distances between the first grid plate, the second grid plate, the scale, the third grid plate, and the fourth grid plate or the light receiving means equal.

In this case, the light source is preferably an LED.

As described above, conventional optical encoders that use a grid-like pattern on the scale as the first grid have a problem that, when the relative positions of the components change and deviations occur in the distance between them, they may not be able to maintain detection of signals such as interference fringes. On the other hand, this problem can be solved by using a light source with a wide coherence region, such as a He-Ne laser. Specifically, for example, when the light source is a He-Ne laser, its coherence length (coherence region) is several meters. For this reason, even if the difference in the optical path length between the two beams split by the first grid plate and the light receiving means is large, interference fringes can be generated. In other words, when a light source with a very short coherence length of several centimeters is used as the light source, if the difference in the optical path length from the split point of the two beams to the light receiving means is several centimeters or more, interference fringes will not be generated. However, light sources with a wide coherence region, such as a He-Ne laser, are expensive, and therefore costly. In addition, when such a light source is used, the optical encoder may become large.

In contrast, in the optical encoder of the present invention, the optical paths of the two split beams are approximately linearly symmetrical about an axis in the orthogonal direction perpendicular to the plate surfaces of the multiple grating plates or scales. Therefore, the optical path lengths of the two beams are approximately the same. Therefore, the optical encoder can keep the difference in the optical path lengths of the split beams within a few centimeters, which is the coherence length of the light source, and reliably generate interference fringes within the limits of the coherence of the light source. Therefore, the optical encoder can avoid the limitations due to the coherence of the light source, and can use, for example, an LED, which is less expensive and has greater coherence limitations than a He-Ne laser, as a light source instead of an expensive He-Ne laser, thereby reducing costs.

FIG. 1 is a perspective view showing an optical encoder according to a first embodiment; A diagram showing the optical path of light in the optical encoder. FIG. 2 is a schematic diagram showing the arrangement of components in the optical encoder; FIG. 2 is a schematic diagram showing a case where the scale of the optical encoder has an inclination; FIG. 13 is a perspective view showing an optical encoder according to a second embodiment; FIG. 2 is a schematic diagram showing the arrangement of components in the optical encoder; FIG. 13 is a perspective view showing an optical encoder according to a third embodiment; Diagram showing a conventional optical encoder

First Embodiment
A first embodiment of the present invention will now be described with reference to FIGS.
FIG. 1 is a perspective view showing an optical encoder 1 according to the first embodiment.
1, the optical encoder 1 is a linear encoder including a plate-shaped scale 2 and a detection head 3 provided so as to be movable relative to the scale 2 along the X direction which is the measurement direction. The detection head 3 includes a light source 4 which irradiates light towards the scale 2, a plurality of grating plates 5 which are arranged between the light source 4 and the scale 2, and a light receiving means 6 which receives the light. The detection head 3 provided with these is provided so as to be able to advance and retreat as a unit in the X direction which is the measurement direction relative to the scale 2. The linear encoder moves the detection head 3 along the scale 2 to obtain position information from the amount of relative movement between the scale 2 and the detection head 3.
In the following description and in each drawing, the measurement direction, that is, the longitudinal direction of the scale 2 is referred to as the X direction, the lateral direction of the scale 2 is referred to as the Y direction, and the height direction perpendicular to the X and Y directions is referred to as the Z direction.

The scale 2 is made of glass or the like. A grid pattern 20 is formed on one surface of the scale 2 at a predetermined period g along the X direction, which is the measurement direction. The grid pattern 20 reflects light irradiated from the light source 4 via a plurality of grid plates 5 toward the light receiving means 6. For this reason, the grid pattern 20 is made of a material that reflects light. The light reflected by the grid pattern 20 is irradiated to the light receiving means 6 via the plurality of grid plates 5.
The light source 4 irradiates parallel light toward one surface of the scale 2. An LED (Light Emitting Diode) is used as the light source 4. In the following description and in each drawing, the optical path of the light irradiated from the light source 4 is indicated by an arrow. The reason why an LED is used as the light source 4 will be described later.

The light receiving means 6 is arranged parallel to the XY plane, which is the plate surface of the scale 2. The light receiving means 6 has a plurality of light receiving elements 60 that receive light reflected by the scale 2 and passing through the plurality of grating plates 5. Specifically, the plurality of light receiving elements 60 are formed at a predetermined period P on one surface of the light receiving means 6. The plurality of light receiving elements 60 are also installed facing the grating pattern 20 of the scale 2 with the plurality of grating plates 5 in between. The plurality of light receiving elements 60 receive light passing through the scale 2 and the plurality of grating plates 5, and detect a signal from the interference fringes generated by the light. The plurality of light receiving elements 60 are made of a PDA (Photo Diode Array). A PDA is a detector that has the property of being able to measure a plurality of interference fringes at once. The plurality of light receiving elements 60 are not limited to a PDA, and any detector such as a PSD (Position Sensitive Detector) or a CCD (Charge-Coupled Device) may be used.

The multiple grating plates 5 include a first grating plate 51, a second grating plate 52, and a third grating plate 53. The multiple grating plates 5 are arranged parallel to the XY plane which is the plate surface of the scale 2.
Specifically, the first grating plate 51 is disposed between the light source 4 and the scale 2, and is irradiated with light from the light source 4. The second grating plate 52 is disposed between the first grating plate 51 and the scale 2, and is irradiated with light that has passed through the first grating plate 51 toward the scale 2. The third grating plate 53 is disposed between the scale 2 and the light receiving means 6, and is irradiated with light reflected by the scale 2. The third grating plate 53 is irradiated with light reflected by the scale 2 toward the light receiving means 6.

The multiple lattice plates 5 each have a lattice formed at a predetermined period on one surface. Specifically, the first lattice plate 51 has a lattice 510 formed at a predetermined period P1. The second lattice plate 52 has a lattice 520 formed at a predetermined period P2. The third lattice plate 53 has a lattice 530 formed at a predetermined period P3. The lattices 510, 520, and 530 of the multiple lattice plates 5 will be described later.

In the optical encoder 1, the first grating plate 51 and the light receiving means 6 are arranged on the same plane. In addition, in the optical encoder 1, the second grating plate 52 and the third grating plate 53 are arranged on the same plane. Specifically, the first grating plate 51 and the light receiving means 6 are arranged on the same XY plane S1. The second grating plate 52 and the third grating plate 53 are arranged on the XY plane S2 located a distance d away from the XY plane S1 in the -Z direction (downward on the paper). In addition, the first grating plate 51, the second grating plate, the scale 2, the third grating plate, and the light receiving means 6 are arranged with an equal separation distance d between them.

FIG. 2 is a principle diagram showing the optical path of light in the optical encoder 1. As shown in FIG.
2, the optical path of the light irradiated from the light source 4 and a method for designing the gratings 510, 520, 530 of the multiple grating plates 5 and the multiple light receiving elements 60 will be described. Note that, for convenience of explaining the optical path of the light, in FIG. 2, the light irradiated to the scale 2 is shown in a state where it has passed through the scale 2, unlike in FIG. 1.

Here, the light passing through the scale 2 and the multiple grating plates 5 is split and diffracted into multiple diffracted light beams. The multiple diffracted light beams include diffracted light beams that travel in the same direction as the optical axis of the light emitted from the light source, diffracted light beams that travel on both sides of the optical axis at a predetermined diffraction angle, and diffracted light beams that travel on both sides of the optical axis at a diffraction angle larger than the predetermined diffraction angle.
If the diffracted light traveling in the same direction as the optical axis is defined as the 0th order diffracted light, the multiple diffracted lights can be ordered as ±1st order diffracted lights, ±2nd order diffracted lights, etc. in the direction in which the diffraction angle increases with respect to the 0th order diffracted light.

The light receiving means 6 detects a signal from the interference fringes generated mainly by the ±1st order diffracted light. Therefore, the ±1st order diffracted light becomes the signal diffracted light, and the diffracted light of orders higher than the ±1st order diffracted light becomes the noise diffracted light. In the following description and each drawing, the signal diffracted light generating the interference fringes is indicated by a solid arrow, and in Figure 2, the ± direction of the ±1st order diffracted light is indicated in parentheses.
A method of designing the gratings 510, 520, and 530 of the plurality of grating plates 5 and the optical path of the signal diffracted light from the light source 4 to the light receiving means 6 will be described below.

First, the predetermined period of the grid pattern 20 of the scale 2 is g. Next, a predetermined constant, which is a numerical value between 0 and 1, that determines the predetermined periods P1, P2, and P3 of the grids 510, 520, and 530 of the multiple grid plates 5 and the predetermined period P of the multiple light receiving elements 60 is n. In this case, the period P1 of the grid 510 of the first grid plate 51 is formed by "g÷n". Furthermore, the period P2 of the grid 520 of the second grid plate 52 is formed by "g÷n÷2". Furthermore, the period P3 of the grid 530 of the third grid plate 53 and the period P of the multiple light receiving elements 60 are formed by "g÷|1-n|÷2".

In this embodiment, the predetermined constant n is 0.5. If the predetermined constant n is 0.5 and the predetermined period g of the lattice pattern 20 is "4 μm", for example, the period P1 of the lattice 510 of the first lattice plate 51 is "8 μm". The period P2 of the lattice 520 of the second lattice plate 52 is "4 μm". The period P3 of the lattice 530 of the third lattice plate 53 and the period P of the multiple light receiving elements 60 are "4 μm". Although a simple period is used for the purpose of explanation, by determining the predetermined periods P1, P2, and P3 of the lattices 510, 520, and 530 of the multiple lattice plates 5 and the period P of the multiple light receiving elements 60 based on the predetermined period g of the lattice pattern 20 of the scale 2, these can be easily designed and the detection accuracy of the signal can be maintained. The predetermined period g of the lattice pattern 20 may be any period.

Next, the optical path of the signal diffracted light from the light source 4 to the light receiving means 6 will be described.
First, light from the light source 4 is diffracted into a plurality of diffracted beams by the grating 510 of the first grating plate. The ±1st order diffracted beams, which become the signal diffracted beams, are irradiated onto the second grating plate 52 at an angle _θ0 . In this case, when the wavelength of the light from the light source 4 is λ, the angle _θ0 can be expressed as "sin _θ0 = n × λ ÷ g".
Next, the ±1st order diffracted light irradiated to the second grating plate 52 is diffracted by the grating 520 and irradiated to the scale 2. The ±1st order diffracted light diffracted by the second grating plate 52 and serving as the signal diffracted light is irradiated to the scale 2 with the ± inverted from when it was diffracted by the first grating plate 5. At this time, the ±1st order diffracted light is incident on the scale 2 at an angle _θ0 and intersects with each other on the scale 2.

The ±1st order diffracted light is reflected by the scale 2 and is irradiated onto the second grating plate 53 at an angle θ _1. At this time, the angle θ ₁ can be expressed as "sin θ ₁ = λ÷g - sin θ ₀ = λ÷g × (1 - n)".
The ±1st order diffracted light irradiated to the third grating plate 53 is diffracted by the grating 530 and irradiated to the light-receiving means 6. The ±1st order diffracted light diffracted by the third grating plate 53 and serving as signal diffracted light is irradiated to the light-receiving means 6 with the ± inverted from when it was reflected by the scale 2. At this time, the ±1st order diffracted light is incident on the light-receiving means 6 at an angle _θ1 and generates interference fringes on the light-receiving means 6. A plurality of light-receiving elements 60 of the light-receiving means 6 detects a signal relating to the amount of movement of the detection head 3 relative to the scale 2 from the interference fringes.

Figure 3 is a schematic diagram showing the arrangement of components in the optical encoder 1. Specifically, Figure 3(A) is a schematic diagram in which the components on the XY plane S1, the components on the XY plane S2, and the scale 2 are arranged at equal intervals with a separation distance d between them. Figure 3(B) is a schematic diagram in which the components on the XY plane S1 and the components on the XY plane S2 are arranged with a separation distance d between them, and the components on the XY plane S2 and the scale 2 are arranged with a separation distance d1 in addition to the separation distance d between them.

As shown in FIG. 3A, in the optical encoder 1, the first grating plate 51, the second grating plate 52, the scale 2, the third grating plate 53, and the light receiving means 6 are arranged so that they are equally spaced apart at a distance d.

Here, in conventional optical encoders, if the separation distances between the respective components are different, the amount of overlap of the divided light in the light receiving means 6 may decrease. However, in the optical encoder 1, as described above, the reduction in the amount of overlap can be suppressed by designing the gratings 510, 520, 530 of the multiple grating plates 5 and the predetermined periods P1, P2, P3, P of the multiple light receiving elements 60. In addition, since the scale 2 is a reflective type that reflects light, the light can be folded back by the scale 2, so that the separation distance d between the components on the XY plane S1 and the components on the XY plane S2 and the scale 2 can be easily designed to be equal.

In addition, by arranging the components of the optical encoder 1 at equal intervals, the optical paths of the two beams of light irradiated from the light source 4 and split by the first grating plate 51 are approximately linearly symmetrical about an axis in a direction perpendicular to the multiple grating plates 5 and the plate surfaces of the scale 2. As a result, the optical path lengths of the two beams are approximately the same. This allows the optical encoder 1 to use an LED with a short coherence length of several centimeters as the light source 4. The optical encoder 1 can keep the difference in the optical path lengths of the split beams within the coherence length of the light source 4, which is several centimeters, and can reliably generate interference fringes within the limits of the coherence of the light source 4.

As shown in FIG. 3B, unlike FIG. 3A, even if the components on the XY plane S2 and the scale 2 are arranged with a separation distance d1 in addition to the separation distance d, the accuracy may be reduced compared to the optical encoder 1 shown in FIG. 3A, but the same effect can be obtained. Specifically, the optical encoder 1 can suppress the reduction in the overlap amount by designing the gratings 510, 520, 530 of the multiple grating plates 5 and the predetermined periods P1, P2, P3, P of the multiple light receiving elements 60. Also, the optical paths of the two lights irradiated from the light source 4 and divided by the first grating plate 51 are approximately line symmetrical with respect to the axis of the orthogonal direction perpendicular to the plate surfaces of the multiple grating plates 5 and the scale 2. Therefore, the optical encoder 1 can suppress the difference in the optical path length of each divided light within several centimeters, which is the coherence length of the light source 4, and can reliably generate interference fringes within the limit of the coherence of the light source 4.

FIG. 4 is a schematic diagram of the optical encoder 1 when the scale 2 has an inclination.
As shown in FIG. 4, the optical encoder 1 can suppress the occurrence of measurement errors and maintain the signal detection accuracy even if the scale 2 is inclined.
Specifically, when the scale 2 rotates about the Y direction and is tilted at an angle α, the light irradiated from the second grating plate 52 to the scale 2 is reflected by the scale 2 and is irradiated to the third grating plate 53 at a position shifted in the -X direction by a distance T1 from the irradiation position on the second grating plate 52. The light irradiated to the third grating plate 53 is irradiated to the light receiving means 6 at a position shifted in the -X direction by a distance T2 from the irradiation position on the first grating plate 51. At this time, an optical lever acts on the light that passes from the scale 2 through the third grating plate 53 and is irradiated to the light receiving means 6. As a result, even if the scale 2 is tilted at an angle α, the effect of the tilt is offset, so the optical encoder 1 can suppress the occurrence of measurement errors and maintain the detection accuracy of the signal.

According to the first embodiment, the following actions and effects can be achieved.
(1) The optical encoder 1 is designed based on the predetermined period P1, P2, P3 of the gratings 510, 520, 530 of the first grating plate 51, the second grating plate 52, and the third grating plate 53, respectively, and the predetermined period P of the multiple light receiving elements 60, based on the predetermined period g of the grating pattern 20 of the scale 2, thereby suppressing changes in the direction of light propagation caused by fluctuations in the relative positioning of the components.
(2) The optical encoder 1 can maintain signal accuracy by devising the design of the gratings 510, 520, 530 of the multiple grating plates 5 and the multiple light receiving elements 60 without designing the light source 4. Therefore, the optical encoder 1 can maintain signal detection accuracy without considering constraints in the design of the light source 4 even if the relative arrangement positions of the components change and deviations occur in the separation distances.

(3) The first grating plate 51 and the light receiving means 6 are arranged on the same plane S1, and the second grating plate 52 and the third grating plate 53 are arranged on the same plane S2, so that the respective separation distances d can be made equal. Therefore, the optical encoder 1 can suppress a decrease in the amount of overlap of light in the light receiving means 6.
(4) By making the distances between the first grating plate 51, the second grating plate 52, the scale 3, the third grating plate 53, and the light receiving means 6 equal, the optical encoder 1 can further suppress the reduction in the amount of overlap of the divided light in the light receiving means.

(5) The optical encoder 1 can keep the difference in the optical path length of each divided light within a few centimeters, which is the coherence length of the light source 4, and can reliably generate interference fringes within the coherence limitations of the light source 4. Therefore, the optical encoder 1 can avoid limitations due to the coherence of the light source 4 and can use, for example, an LED, which is less expensive than a He-Ne laser and has larger coherence limitations, as a light source instead of an expensive He-Ne laser, thereby reducing costs.

(6) The optical encoder 1 does not include a fourth grating plate 54 (see FIG. 7) that is included in the optical encoder 1B of the third embodiment described later, and instead employs a plurality of light receiving elements 60 of the light receiving means 6 as a substitute for the grating 540 in the fourth grating plate 54. Therefore, the optical encoder 1 does not need to include the fourth grating plate 54 in the third embodiment, and therefore costs can be reduced.
(7) The predetermined constant n is 0.5, which makes it possible to stabilize the amount of overlap of the divided light in the light receiving unit 6, compared to when a constant n is other than 0.5. In addition, the predetermined period can be calculated more easily, compared to when a constant n is other than 0.5, which makes it possible to improve efficiency in design.

Second Embodiment
A second embodiment of the present invention will now be described with reference to Figures 5 and 6. In the following description, the same reference numerals will be used to designate parts that have already been described, and description thereof will be omitted.

FIG. 5 is a perspective view showing an optical encoder 1A according to the second embodiment.
In the first embodiment, the predetermined constant n that determines the predetermined periods P1, P2, P3 of the gratings 510, 520, 530 of the plurality of grating plates 5 and the predetermined period P of the plurality of light receiving elements 60 is 0.5.
In the second embodiment, the predetermined constant n that determines the predetermined periods P1, P2, and P3 of the gratings 510A, 520A, and 530A of the multiple grating plates 5A and the predetermined period P of the multiple light receiving elements 60A is different from the first embodiment in that it is a numerical value between 0 and 1 and is not 0.5. Specifically, in the second embodiment, the predetermined constant n is 0.25.

As shown in FIG. 5, the multiple grating plates 5A in the detection head 3A of the optical encoder 1A include a first grating plate 51A having a grating 510A formed at a predetermined period P1. The multiple grating plates 5A also include a second grating plate 52A having a grating 520A formed at a period P2 smaller than the predetermined period P1. The multiple grating plates 5A also include a third grating plate 53A having a grating 530A formed at a period P3 smaller than the predetermined period P2. The light receiving means 6 has multiple light receiving elements 60A formed at the same period P as the predetermined period P3.

As in the first embodiment, the period P1 of the grating 510A of the first grating plate 51A is formed by "g÷n". The period P2 of the grating 520A of the second grating plate 52A is formed by "g÷n÷2". The period P3 of the grating 530A of the third grating plate 53A and the period P of the multiple light receiving elements 60A are formed by "g÷|1-n|÷2".

The predetermined periods P1, P2, P3, and P are calculated as follows, assuming that the predetermined constant n is 0.25 and the predetermined period g of the lattice pattern 20 is "4 μm". First, the period P1 of the lattice 510A of the first lattice plate 51A is "16 μm". Next, the period P2 of the lattice 520A of the second lattice plate 52A is "8 μm". Next, the period P3 of the lattice 530A of the third lattice plate 53A and the period P of the multiple light receiving elements 60A are "2.66666667 μm". Although a simple period was used for the purpose of explanation, by calculating the predetermined periods P1, P2, and P3 of the lattice 510A, 520A, and 530A of the multiple lattice plates 5A and the period P of the multiple light receiving elements 60A based on the predetermined period g of the lattice pattern 20 of the scale 2, these can be easily designed and the detection accuracy of the signal can be maintained. The predetermined period g of the grid pattern 20 may be any period.

Figure 6 is a schematic diagram showing the positional relationship of components in optical encoder 1A. Specifically, Figure 6(A) is a schematic diagram in which the components on XY plane S1, the components on XY plane S2, and the scale 2 are arranged at equal intervals with a separation distance d. Figure 6(B) is a schematic diagram in which the components on XY plane S1 and the components on XY plane S2 are arranged with a separation distance d, and the components on XY plane S2 and the scale 2 are arranged with a separation distance d1 in addition to the separation distance d.

As shown in FIG. 6(A), in the optical encoder 1A, the first grating plate 51A and the second grating plate 52A, the second grating plate 52A and the scale 2, the scale 2 and the third grating plate 53A, and the third grating plate 53A and the light receiving means 6A are arranged so that the respective separation distances are equal at a distance d. As described above, the optical encoder 1A can suppress a decrease in the amount of overlap by designing the gratings 510A, 520A, 530A of the multiple grating plates 5A and the predetermined periods P1, P2, P3, P of the multiple light receiving elements 60A.

In addition, by arranging the components of the optical encoder 1A at equal intervals, the optical paths of the two beams split by the first lattice plate 51A are approximately linearly symmetrical about an axis in a direction perpendicular to the plate surfaces of the multiple lattice plates 5A. As a result, the optical path lengths of the two beams are approximately the same, so the optical encoder 1A can keep the difference in the optical path lengths of the split beams within a few centimeters, which is the coherence length of the light source 4, and can reliably generate interference fringes within the limits of the coherence of the light source 4.

As shown in FIG. 6(B), unlike FIG. 6(A), even if the components on the XY plane S2 and the scale 2 are arranged with a separation distance d1 in addition to the separation distance d, the accuracy may be reduced compared to the optical encoder 1A of FIG. 6(A), but the same effect can be obtained. Specifically, the optical encoder 1A can suppress a decrease in the overlap amount by designing the gratings 510A, 520A, 530A and the predetermined periods P1, P2, P3, and P of the multiple light receiving elements 60A. Also, as with the optical encoder 1A of FIG. 6(A), the optical path lengths of the two lights split by the first grating plate 51A are approximately the same, so an LED can be used for the light source 4.

In the second embodiment, in addition to the same actions and effects as those (1) to (6) in the first embodiment, the following actions and effects can be achieved.
(8) The predetermined constant n that determines the predetermined periods P1, P2, and P3 of the gratings 510A, 520A, and 530A of the multiple grating plates 5A and the predetermined period P of the multiple light receiving elements 60A can be a value other than 0.5 in the first embodiment. Therefore, the optical encoder 1A can have an improved degree of freedom in design.

Third Embodiment
A third embodiment of the present invention will now be described with reference to Fig. 7. In the following description, the same reference numerals will be used to designate parts that have already been described, and description thereof will be omitted.

FIG. 7 is a perspective view showing an optical encoder 1B according to the third embodiment.
In the third embodiment, the optical encoder 1B differs from the first embodiment in that it further includes a fourth grating plate 54 and includes a light receiving means 6B having a plurality of light receiving portions 61, 62, and 63. The optical encoder 1B has a configuration substantially similar to that of the optical encoder 1 in the first embodiment, except for the fourth grating plate 54 and the light receiving means 6B.

As shown in FIG. 7, a detection head 3B in an optical encoder 1B includes a plurality of grating plates 5B and a light receiving means 6B.
The plurality of lattice plates 5B include a first lattice plate 51, a second lattice plate 52, and a third lattice plate 53 similar to those in the first embodiment. The plurality of lattice plates 5B further include a fourth lattice plate 54 that is disposed between the third lattice plate 53 and the light receiving means 6B and irradiates the light that has passed through the third lattice plate 53 toward the light receiving means 6B.

The fourth grating plate 54 has a grating 540 formed at a predetermined period P4. The predetermined period P4 of the grating 540 of the fourth grating plate 54 is formed at "g÷|1-n|". In other words, although the fourth grating plate 54 is formed at a period P4 that is different from the period P "g÷|1-n|÷2" of the multiple light receiving elements 60 in the first embodiment, it functions in the optical encoder 1B in the same way as the multiple light receiving elements 60.

In this embodiment, the predetermined constant n is 0.5. If the predetermined constant n is 0.5 and the predetermined period g of the lattice pattern 20 is "4 μm", for example, the period P1 of the lattice 510 of the first lattice plate 51 is "8 μm". The period P2 of the lattice 520 of the second lattice plate 52 is "4 μm". The period P3 of the lattice 530 of the third lattice plate 53 is "4 μm". The period P4 of the lattice 540 of the fourth lattice plate 54 is "8 μm". Although a simple period is used for the purpose of explanation, by determining the predetermined periods P1, P2, P3, and P4 of the lattices 510, 520, 530, and 540 of the multiple lattice plates 5 based on the predetermined period g of the lattice pattern 20 of the scale 2, these can be easily designed and the detection accuracy of the signal can be maintained. The predetermined period g of the lattice pattern 20 may be any period.

The light receiving means 6B does not include the multiple light receiving elements 60 arranged at the predetermined period P in the first embodiment. The light receiving means 6B receives light reflected by the scale 2 and passing through the multiple grating plates 5B. Specifically, the light receiving means 6B receives light that has passed through the fourth grating plate 54.
The light receiving means 6B has a first light receiving portion 61 , a second light receiving portion 62 and a third light receiving portion 63 .
The light passing through the fourth grating plate 54 is mainly divided into 0th order diffracted light, ±1st order diffracted light, and ±2nd order diffracted light. The first light receiving unit 61, the second light receiving unit 62, and the third light receiving unit 63 each receive the diffracted light of the order that generates interference fringes as signal diffracted light. The second light receiving unit 62 mainly receives the 0th order diffracted light that has passed through the fourth grating plate 54 as signal diffracted light. The first light receiving unit 61, the second light receiving unit 62, and the third light receiving unit 63 generate interference fringes that are shifted in phase by 120°. The light receiving unit 6B detects the displacement amount of the detection head 3B relative to the scale 2 based on the interference fringes generated by the first light receiving unit 61, the second light receiving unit 62, and the third light receiving unit 63. Any detector such as a PDA, a PSD, or a CCD can be used as the light receiving unit 6B.

In the optical encoder 1B, the first grating plate 51 and the fourth grating plate 54 are arranged on the same plane. In addition, in the optical encoder 1B, the second grating plate 52 and the third grating plate 53 are arranged on the same plane. Specifically, the first grating plate 51 and the fourth grating plate 54 are arranged on the same XY plane S1. The second grating plate 52 and the third grating plate 53 are arranged on the XY plane S2 located at a distance d away from the XY plane S1 in the -Z direction (downward on the paper). In addition, the first grating plate 51, the second grating plate, the scale 2, the third grating plate, and the fourth grating plate 54 are arranged with an equal separation distance d between them.
In this way, in the optical encoder 1B, even if it is not possible to adopt the multiple light receiving elements 60 arranged at the predetermined period P in the first embodiment, by providing a fourth grating plate 54 having a grating 540 arranged at a predetermined period P4 determined based on the period g of the grating pattern 20, it is possible to suppress the occurrence of measurement errors and maintain the detection accuracy of the signal.

In the third embodiment, in addition to the same actions and effects as (1) to (5) and (7) in the first embodiment and (8) in the second embodiment, the following actions and effects can be achieved.
(9) In the optical encoder 1B, the distance d between the first grating plate 51 and the fourth grating plate 54, the distance between the second grating plate 52 and the third grating plate 53, and the distance between the third grating plate 53 and the scale 2 can be made equal. Therefore, in the optical encoder 1B, it is possible to suppress a decrease in the amount of overlap of light in the light receiving means 6B.
(10) In the optical encoder 1B, by designing the predetermined periods P1, P2, P3, and P4 of the gratings 510, 520, 530, and 540 of the multiple grating plates 5B, a reduction in the amount of overlap can be suppressed.

[Modifications of the embodiment]
It should be noted that the present invention is not limited to the above-described embodiments, and modifications and improvements within the scope of the present invention that can achieve the object of the present invention are included in the present invention.
For example, in each of the above embodiments, the present invention is described as being applied to the optical encoders 1, 1A to 1B, which are linear encoders. However, as long as the encoder is an optical encoder, the type of detector, the detection method, etc. are not particularly limited.
In each of the above-described embodiments, the light receiving means 6, 6A to 6B detects signals from interference fringes generated mainly by ±1st order diffracted light, but the light receiving means may receive any type of light as a signal as long as it is able to receive light and detect a signal.

In the first and second embodiments, the first lattice plate 51, 51A and the second lattice plate 52, 52A, the second lattice plate 52, 52A and the scale 2, the scale 2 and the third lattice plate 53, 53A, and the third lattice plate 53, 53A and the light receiving means 6, 6A were arranged so that the respective separation distances d were equal. Also, in the third embodiment, the first lattice plate 51 and the second lattice plate 52, the second lattice plate 52 and the scale 2, the scale 2 and the third lattice plate 53, and the second lattice plate 53 and the fourth lattice plate 54 were arranged so that the respective separation distances d were equal. However, the separation distances of the respective components do not have to be equal.

In the first embodiment, the predetermined constant n was 0.5, and in the second embodiment, the predetermined constant n was 0.25, but the predetermined constant n may be any number between 0 and 1.
In the first and second embodiments, the first lattice plate 51, 51A and the light receiving means 6, 6A are disposed on the XY plane S1. The second lattice plate 52, 52A and the third lattice plate 53, 53A are disposed on the same plane by being disposed on the XY plane S2. In the third embodiment, the first lattice plate 51 and the fourth lattice plate 54 are disposed on the XY plane S1. The second lattice plate 52 and the third lattice plate 53 are disposed on the XY plane S2 by being disposed on the same plane. However, each component does not have to be disposed on the same plane.

In the third embodiment, the light receiving means 6B has a first light receiving section 61, a second light receiving section 62, and a third light receiving section 63, and interference fringes with a phase shift of 120° are generated in each section, but the light receiving means does not have to have three light receiving sections, and may have one light receiving section or four light receiving sections. If the light receiving means has four light receiving sections, for example, interference fringes with a phase shift of 90° may be generated in each section.

As described above, the present invention can be suitably used in optical encoders.

1, 1A to 1B Optical encoder 2 Scale 3, 3A to 3B Detection head 4 Light source 5, 5A to 5B Multiple grating plates 51, 51A First grating plate 510, 510A Grating of first grating plate 52, 52A Second grating plate 520, 520A Grating of second grating plate 53, 53A Third grating plate 530, 530A Grating of third grating plate 54 Fourth grating plate 540 Grating of fourth grating plate 6, 6A to 6B Light receiving means 60, 60A Light receiving element

Claims (6)

An optical encoder comprising: a plate-shaped scale having a lattice pattern formed at a predetermined period along a measurement direction on one surface thereof; and a detection head provided so as to be movable relative to the scale along the measurement direction,
The detection head includes:
A light source that irradiates light toward the scale;
a plurality of grating plates disposed between the light source and the scale and parallel to a plate surface of the scale, the grating plates having a grating formed at a predetermined period on one surface;
a light receiving means for receiving light reflected by the scale and passing through the plurality of grating plates,
The plurality of lattice plates include
A first lattice plate onto which light from the light source is irradiated;
a second grating plate disposed between the first grating plate and the scale and configured to irradiate light passing through the first grating plate toward the scale;
a third grating plate disposed between the scale and the light receiving means and onto which the light reflected by the scale is irradiated;
a fourth lattice plate disposed between the third lattice plate and the light receiving means and configured to irradiate the light passing through the third lattice plate toward the light receiving means;
Let g be the predetermined period of the grid pattern, and n be a predetermined constant between 0 and 1 that defines the predetermined period of the grid,
The grating of the first grating plate is formed with a period of g÷n,
The grating of the second grating plate is formed with a period of g÷n÷2,
The grating of the third grating plate is formed with a period of g÷|1-n|÷2,
An optical encoder, wherein the grating of the fourth grating plate is formed with a period of g÷|1-n|. An optical encoder comprising: a plate-shaped scale having a lattice pattern formed at a predetermined period along a measurement direction on one surface thereof; and a detection head provided so as to be movable relative to the scale along the measurement direction,
The detection head includes:
A light source that irradiates light toward the scale;
a plurality of grating plates disposed between the light source and the scale and parallel to a plate surface of the scale, the grating plates having a grating formed at a predetermined period on one surface;
a light receiving means having a plurality of light receiving elements that receive light reflected by the scale and passing through the plurality of grating plates;
The plurality of lattice plates include
A first lattice plate onto which light from the light source is irradiated;
a second grating plate disposed between the first grating plate and the scale and configured to irradiate light passing through the first grating plate toward the scale;
a third grating plate disposed between the scale and the light receiving means and onto which the light reflected by the scale is irradiated;
The plurality of light receiving elements include
are formed at a predetermined interval on one surface of the light receiving means parallel to the plate surface of the scale,
Let g be the predetermined period of the grid pattern, and n be a predetermined constant between 0 and 1 that defines the predetermined period of the grid and the predetermined period of the plurality of light receiving elements.
The grating of the first grating plate is formed with a period of g÷n,
The grating of the second grating plate is formed with a period of g÷n÷2,
The optical encoder according to claim 1, wherein the grating of the third grating plate and the plurality of light receiving elements are formed at a period of g÷|1-n|÷2. 3. The optical encoder according to claim 1,
2. An optical encoder according to claim 1, wherein the predetermined constant n is 0.5. 2. The optical encoder according to claim 1 ,
An optical encoder characterized in that the first grating plate and the fourth grating plate, or the first grating plate and the light receiving means, and the second grating plate and the third grating plate are each arranged on the same plane. 2. The optical encoder according to claim 1 ,
An optical encoder characterized in that the first grating plate, the second grating plate, the scale, the third grating plate, and the fourth grating plate or the light receiving means are spaced apart at equal intervals. In the optical encoder according to any one of claims 1 to 5,
The optical encoder according to claim 1, wherein the light source is an LED.

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