JPH0642981A - Optical encoder - Google Patents

Abstract

PURPOSE:To obtain an optical encoder which has a simple constitution, high light utilization efficiency and high resolution for a grating pitch and enable obtaining output signals with excellent accuracy. CONSTITUTION:The coherent light emitted from a light source 1 is made parallel light by the use of a colimate lens 2 and is then made incident on a fixed diffraction plate 3A perpendicularly. The +1st order diffraction light 11 having passed the fixed diffraction plate 3A passes a moving diffraction plate 4A as a -1st order diffraction light 21 and is converged on a photodetector 6 with a condenser lens 5 and the -1st order diffraction light 12 having passed the moving diffraction plate 3A passes a moving diffraction plate 4A as a +1st order diffraction light 22 and is converged on a photodetector 6 with a focus lens 5. If the + or -1st order diffraction light 21 and 22 interface each other, sine wave output having two times frequency of a sine wave obtained by the interference between the 0th order diffraction light having passed the fixed diffraction plate 3A and the moving diffraction plate 4A and + or -1st order diffraction light 21 and 22.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical encoder, and more particularly to a coherent light beam incident on a diffraction grating attached to a moving object such as a rotating object, and diffracted light beams passing through the diffraction grating interfere with each other to cause interference. The present invention relates to an optical encoder for observing a moving state, for example, a rotating state of a diffraction grating by measuring the intensity of the light.

[0002]

2. Description of the Related Art Optical encoders such as photoelectric encoders are widely used for positioning in mechanical devices. In this photoelectric encoder, slits are provided on a rotating disk and a fixed mask provided at a predetermined distance from the rotating disk, and light passing through both slits is converted into an electric signal by a photodetector and output. It measures linear length and rotation angle.
In this photoelectric encoder, the detection accuracy can be improved by making the pitch of the slits fine.

However, according to this photoelectric encoder, if the pitches of the slits provided on the rotating disk and the fixed mask are made too small, the signal-to-noise ratio S of the output signal from the photodetector is affected by the diffracted light. There is a problem that the / N ratio is lowered and the detection accuracy is lowered.

Further, if the slit spacing is enlarged to such an extent that the output signal from the photodetector is not affected by the diffracted light, the diameter of the rotating disk must be increased,
As a result, the size of the entire apparatus becomes large, and there is a problem that the load on the driving body that rotationally drives the rotating disk becomes large.

On the other hand, as an optical encoder, an interference fringe detection type encoder using diffracted light that has passed through a diffraction grating is also known. This interference fringe detection type encoder converts an interference fringe generated by diffraction and interference of light passing through a fixed diffraction plate and a moving diffraction plate, which are arranged substantially perpendicular to the optical axis, into an electric signal by a photodetector. It is something to take out.

However, in this interference fringe detection type encoder, since the diffracted light of a plurality of orders is emitted from the moving diffraction plate and the fixed diffraction plate, the intensity of the diffracted light of a specific order required for measurement is lowered and the detection sensitivity is lowered. There was a problem that it decreased.

There is also a problem that the S / N ratio at the time of detecting the interference fringes decreases because diffracted light of an unnecessary order for measurement becomes flare or causes generation of ghost light.

Furthermore, when the interference fringes due to the diffracted light that has passed through the moving diffracting plate and the fixed diffracting plate are read by the photodetector, a large number of different diffracted lights including the 0 th diffracted light interfere with each other, so that the moving diffraction There is also a problem that the light intensity fluctuates due to fluctuations in the gap between the plate and the fixed diffraction plate. The range in which this gap variation is allowed is at most 2p ²
/ Λ (where p is the pitch of the grating and λ is the wavelength of the light being measured) (Optics and Laser Tec
hnology, (1985) p89-95).

Therefore, in order to detect a stable signal with respect to the wavelength fluctuation of the light source and the fluctuation of the gap between the moving diffraction plate and the fixed diffraction plate, Japanese Patent Laid-Open Publication No. 3-279812.
And an optical encoder as shown in FIG. 10 of the present application is proposed. That is, the coherent light source 30
A collimator lens 32 for collimating the light emitted from the coherent light source 30, and a collimator lens 32.
A fixed diffractive plate 34 and a reflective moving diffractive plate 36, which are arranged substantially perpendicular to the optical axis of the light that has passed through and at appropriate intervals and have diffraction gratings of equal periodic pitch. The 0th and ± 1st order diffracted lights, which are emitted from 30 and passed through the collimating lens 32 and then passed through the first diffraction grating 34a of the fixed diffraction plate 34, are converted into the second diffraction gratings 36a and 36a of the moving diffraction plate 36. , 36a,
After being reflected, the third diffraction gratings 34b and 34c of the fixed diffraction plate 34 are passed again, and the third diffraction gratings 34b and 34c are passed.
Of the diffracted light components that intersect at 34c, the light that is diffracted and interferes in the direction parallel to the light that has passed through the collimator lens 32 is detected by the photodetectors 38A and 38B.

[0010]

However, in the above case, the number of diffractions by the diffraction grating is 3 times in total, and there is a problem that the utilization efficiency of light is reduced.

Further, since the 0th-order and ± 1st-order diffracted lights are all used, it is necessary to make the diffraction efficiency equal for the diffracted lights of the respective orders. Therefore, for example, assuming that the diffraction efficiency is about 20%, the total light utilization efficiency is (0.2) ³ = 0.
008, that is, about 0.01, and only a light amount of less than 1% can be used. Therefore, there is a problem that the output of the coherent light source 30 needs to be increased in order to improve the S / N of the output signal.

Further, as described above, the inconvenience that diffracted light of an unnecessary order for measurement causes flare or ghost light cannot be avoided.

In view of the above, the present invention provides an optical encoder having a simple structure, high light utilization efficiency, high resolution performance with respect to a grating pitch, and capable of obtaining an accurate signal. With the goal.

[0014]

To achieve the above object, the invention of claim 1 spatially filters only the ± 1st order diffracted light of the diffracted light passing through the fixed grating plate and the moving grating plate. The sine wave having a frequency twice that of the sine wave formed by the interference of the 0th-order diffracted light and the ± 1st-order diffracted light is output.

[0015] Specifically, the means for solving the problems according to the invention of claim 1 is to provide an optical encoder, a light source which emits coherent light, and a light source which is substantially perpendicular to the optical axis of the light emitted from the light source. A fixed diffractive plate and a moving diffractive plate which are provided in parallel and have a phase grating that mainly passes only the ± 1st-order diffracted light; and a photodetector that receives light passing through the fixed diffractive plate and the movable diffractive plate, A configuration is provided including a condenser lens that condenses the ± 1st-order diffracted light that has passed through the fixed diffraction plate and the moving diffraction plate on the photodetection unit of the photodetector.

According to a second aspect of the present invention, the phase grating is formed in a rectangular wave shape, and the rectangular wave is shaped so that the light quantity ratio of the 0th order diffracted light passing through the fixed grating plate and the moving grating plate can be ignored. Specifically, in the structure of claim 1, both the fixed diffraction plate and the moving diffraction plate have a rectangular wave-shaped phase grating, and the peaks and valleys of the rectangular wave of the phase grating are symmetrical. And a step d between the peak and the valley is d = (1/2) × λ ×
(1 + 2m) × (1 / | n−n ₀ |) (where m = 0,
± 1, ..., λ = wavelength of light emitted from the light source, n = refractive index of materials forming the fixed diffraction plate and the moving diffraction plate, n ₀ =
The configuration in which the refractive index of the medium between the fixed diffraction plate and the moving diffraction plate is set) is added.

According to the third aspect of the present invention, the phase grating is formed in a sine wave shape, and the shape of the sine wave is such that the light quantity ratio of the 0th-order diffracted light passing through the fixed grating plate and the moving grating plate can be ignored. Therefore, specifically, in the structure of claim 1, the fixed diffractive plate and the moving diffractive plate both have a sinusoidal phase grating, and a step d between a peak and a valley of the sinusoidal wave of the phase grating is provided. Is d
= (1/2) × λ × (1 / | n−n ₀ |) ÷ (1-2 /
π) (where m = 0, ± 1, ..., λ = wavelength of light emitted from the light source, n = refractive index of the material forming the fixed diffraction plate and the moving diffraction plate, n ₀ = fixed diffraction plate and moving diffraction The refractive index of the medium between the plate and the medium is added.

According to a fourth aspect of the present invention, when the phase grating is formed in a triangular wave shape, the triangular wave is shaped so that the light quantity ratio of the 0th-order diffracted light passing through the fixed grating plate and the moving grating plate can be ignored. Specifically, in the structure of claim 1, both the fixed diffraction plate and the moving diffraction plate have a triangular wave-shaped phase grating, and the peaks and valleys of the triangular wave of the phase grating are It has a symmetrical shape and the step d between the peak and the valley
Is d = λ × (1 / | n−n ₀ |) (where m = 0, ±
1, ..., λ = wavelength of light emitted from the light source, n = refractive index of the material forming the fixed diffraction plate and the moving diffraction plate, n ₀ = refractive index of the medium between the fixed diffraction plate and the moving diffraction plate ) Is added to the configuration set in.

[0019]

According to the structure of claim 1, a fixed diffractive plate and a moving diffractive plate having a phase grating that mainly passes only ± 1st order diffracted light of the coherent light emitted from the light source,
Since a ± 1st-order diffracted light beam that has passed through the fixed diffractive plate and the moving diffractive plate is provided on the photodetector of the photodetector, a 0-th order diffracted light beam and a ± 1st-order diffracted light beam are provided. A double frequency sine wave output waveform having a frequency twice that of the sine wave formed when light interferes can be obtained.

According to the second to fourth aspects, when the phase grating has a rectangular wave shape, a sinusoidal wave shape, or a triangular wave shape, the step size between the peak and the valley is specified. In addition, the light quantity ratio of the 0th-order diffracted light passing through the fixed grating plate and the moving grating plate can be suppressed to a negligible level.

[0021]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a schematic structure of an optical encoder according to a first embodiment of the present invention, and FIG. 2 shows a structure of a main part of the optical encoder according to the first embodiment. In FIG. 1 is a semiconductor laser or L having a relatively high coherence
A light source made up of an ED (light emitting diode), 2 is a collimating lens for collimating the light emitted from the light source 1, and 3A is a fixed diffraction plate which has a phase grating with a rectangular wave cross section and is arranged perpendicular to the optical axis. Reference numeral 4A denotes a moving diffraction plate which has a phase grating with a rectangular wave section and is arranged perpendicular to the optical axis and is movable in the vertical direction. The phase grating of the fixed diffraction plate 3A and the phase grating of the moving diffraction plate 4A are denoted by 4A. And have the same cycle as each other. Further, in the figure, 5 is a condenser lens for condensing the light that has passed through the moving diffraction plate 3A, and 6 is an electric signal for converting the diffraction image condensed by the condenser lens 5 and formed in the photodetector 7 into an electric signal. The photodetector for outputting 8 is a frequency discriminating filter.

In the optical encoder according to the first embodiment, the light emitted from the light source 1 is generated by the collimator lens 2
After being made into parallel light by, the light is incident on the fixed diffraction plate 3A from a direction substantially perpendicular to the fixed diffraction plate 3A. The light incident on the fixed diffractive plate 3A is diffracted by the fixed diffractive plate 3A and is emitted as 0th-order diffracted light 10, + 1st-order diffracted light 11, −1st-order diffracted light 12, .... These diffracted lights 10, 11 and 12 are incident on the moving diffractive plate 4A and then further emitted as diffracted lights. If the diffracted light emitted from the moving diffraction plate 4A is expressed as (n, m) (where n is the diffraction order of the fixed diffraction plate 3A and m is the diffraction order of the moving diffraction plate 3B, respectively), the moving diffraction is obtained. As the diffracted light passing through the plate 3B, as shown in FIG.
0) diffracted light 20, (+1, -1) diffracted light 21, (-)
1, + 1) diffracted light 22, (-2, + 2) diffracted light,
There is (+2, -2) diffracted light, .... However, in FIG. 2, for the convenience of illustration, the diffracted light of (-2, +2),
The (+2, -2) diffracted light and the diffracted light of a higher order than the diffracted light of the second order are omitted.

When the moving diffractive plate 4A is moved at a constant speed in the vertical direction (vertical direction in FIGS. 1 and 2) with respect to the optical axis, the phase of diffracted light higher than 0th order is 0 due to the movement. Since it changes with respect to the phase of the next diffracted light, for example (+
1, -1) diffracted light 21 and (-1, + 1) diffracted light 22
The light intensity of the interference wave obtained by interference between and changes in a sine wave shape. Similarly, (+1, -1) diffracted light 21 and (0, 0)
Light intensity of the interference wave with the diffracted light 20 of, or (-1, +
The light intensity of the interference wave due to the interference between the diffracted light 22 of 1) and the diffracted light 20 of (0,0) also periodically changes with the movement of the moving diffraction plate 4A.

By the way, the step d between the peak and the valley of the moving diffraction plate 4A having a rectangular wave-shaped cross section is | n−n ₀ | × d = (λ / 2) × (with respect to the wavelength λ of the light source 1. 1 + 2m) (1) (where m = 0, ± 1, ± 2, ..., n is the refractive index of the material forming the fixed diffraction plate 3A and the moving diffraction plate 4A, N
₀ is the refractive index of the medium between the fixed diffraction plate 3A and the moving diffraction plate 4A. ) Is formed.

In this case, it is well known that the components of the even-order diffracted light such as the 0-th order become 0, and most of the light energy (each about 40%) is concentrated in the ± 1st-order diffracted light. is there. In reality, however, some even-order diffracted light is generated due to manufacturing errors of the fixed diffraction plate 3A and the moving diffraction plate 4A.

In the above formula (1), n is approximately 1. when the fixed diffraction plate 3A and the moving diffraction plate 4A are SiO ₂ substrates.
46, λ becomes 633 nm when the light source 1 is a He-Ne gas laser, and n ₀ becomes 1 when the medium between the fixed diffraction plate 3A and the moving diffraction plate 4A is air. Further, m is set to 0, the grating period p of the fixed diffraction plate 3A and the moving diffraction plate 4A is set to about 10 μm, and the step d produced by the method of lithography and dry etching is about 0.68.
According to the results of experiments using the fixed diffracting plate 3A and the moving diffracting plate 4A having a rectangular wave shape of 8 μm, the diffraction efficiencies of the ± 1st order diffracted light are about 37 to 40%, respectively. The components were 2% or less, and the components of the third-order diffracted light were about 4 to 5%, respectively. In this case, the ratio (duty ratio) of peaks and valleys of the rectangular wave of the phase grating of the fixed diffraction plate 3A and the moving diffraction plate 4A was 6: 4.

Now, the (0, 0) diffracted light 20 is (+
When the crests and troughs of the rectangular waves of the fixed diffractive plate 3A and the moving diffractive plate 4A coincide with each other, the light intensity interferes with the diffracted light 21 of (1, -1) or the diffracted light 22 of (-1, + 1). Is the maximum, and the output of the fundamental wave (see FIG. 3 (b)) is a sine waveform such that the light intensity becomes minimum when the peaks and valleys of the rectangular wave deviate from each other by a half period p / 2. can get. On the other hand, the (+1, −1) diffracted light 21 and the (−1, +1) diffracted light 22 occupying the main light amount interfere with each other and are formed by a sine wave having a frequency twice that of the fundamental wave. A double frequency output (see FIG. 3B) is obtained. Incidentally, FIG.
, 23 is the (0, + 1) diffracted light, 24 is the (0, -1) diffracted light, 25 is the (+1,0) diffracted light, and 26 is the (+ 1, + 1) diffracted light. Shown respectively.

The present invention enables highly accurate position detection by utilizing the above-mentioned double frequency component. The amount of light incident on the photodetector 6 is (+/− 100) ² = 4 × 10 ⁻⁴ while the (0,0) diffracted light 20 is at most (+/−).
1, -1) diffracted light 21 and (-1, + 1) diffracted light 2
2 are (40/100) ² = 0.16, respectively. That is, the amplitude ratio of light waves contributing to interference is √ (4 × 10 ⁻⁴ ):
√ (0.16) = 2 × 10 ^-2 : 0.4 = 1: 20. Therefore, the signal amplitude U _{0 of} the fundamental wave shown in FIG.
Is at most (1/20) compared to the double-frequency signal amplitude U _1.
X2 = only 1/10. On the other hand, (+3, -3) diffracted light that gives the maximum amplitude as an inconvenient component and (-3, +)
When the diffracted light of 3) interferes with each other, an output waveform of 6 times frequency is generated, but the ratio of the amplitude U ₃ of 6 times frequency and the amplitude U ₁ of 2 times frequency is about 1: 8 to 1:10. Also, (+3, -3)
The component of the light in which the diffracted light of (1) and the diffracted light of (+1, -1) and the component of the light in which the diffracted light of (-3, +3) and the (-1, +1) diffracted light interfere are doubled. Frequency, but the amplitude of these doubled frequencies, like the amplitudes of other doubled frequencies, is (+
1, -1) diffracted light 21 and (-1, + 1) diffracted light 22
Is smaller than the amplitude U _{1 of the} double frequency generated due to the interference between and.

The photoelectric conversion signal output of the photodetector 6 of the first embodiment is obtained as a distorted waveform as shown in FIG. 3A when exaggeratedly shown. This distorted waveform is shown in Figure 3.
As shown in (b), it can be decomposed into a fundamental wave component, a double frequency component, a sixth frequency component (a sixth frequency component is not shown), etc. The components other than the frequency component are weak and change little due to the interference of the diffracted waves that are conjugate with each other, so that the double frequency component can be sufficiently detected.

If necessary, the cutoff frequency band of the frequency discriminating filter circuit 10 shown in FIG. 1 can be narrowed so that the frequency discriminating filter circuit 8 does not output the 6-fold frequency component.

Further, particularly when it is desired to obtain only the double frequency component with higher accuracy, the purpose can be easily achieved by adding a bandpass filter circuit to the frequency discrimination filter circuit 8.

Further, although weak, as interference light components other than the above-mentioned interference light, for example, interference light of (+2, -1) diffracted light and (-2, +1) diffracted light is shown in FIG. It is obtained at the spot 9a on the image plane 9 and similarly (-2, +
The interference light of the diffracted light of 1) and the diffracted light of (+1, -2) is obtained at the spot 9b on the image plane 9, but these interference lights are not reflected on the portion other than the photodetector 7 of the photodetector 6. It is easy to understand that it does not matter at all because it collects light.

FIG. 4 shows the structure of the main part of an optical encoder according to the second embodiment of the present invention. In the second embodiment, the fixed grid plate 3B and the moving grid plate 4B are the first one.
A phase grating having a rectangular wave-shaped cross section is provided as in the embodiment, but the ratio of the width of the peaks and the valleys of the rectangular wave is about 5: 5.
Is formed. That is, the peak and the valley of the rectangular wave are symmetric, and the step d between the peak and the valley of the rectangular wave is d.
= (1/2) × λ × (1 + 2 m) × (1 / | n−n
₀ |) is set.

In the second embodiment, the same members and diffracted light as those in the first embodiment are designated by the same reference numerals as those in the first embodiment, and the description thereof will be omitted.

Similar to the first embodiment, the light emitted from the light source 1 made of a semiconductor laser or an LED having a relatively strong coherence passes through the collimating lens 2 and then is substantially perpendicular to the fixed grating plate 3B. It is incident parallel from the direction. After passing through the fixed grating plate 3B, about 80% of the light incident on the fixed grating plate 3B becomes a ± 1st order diffracted light, which is about 10%.
% Light amount becomes ± 3rd order diffracted light, and the remaining light amount is distributed to ± 5th order or higher and odd order diffracted light at a ratio of several% or less.

In the second embodiment, since the ratio of the width of the peak to the width of the valley of the rectangular wave is set to about 5: 5, the light quantity ratio of the 0th order diffracted light can be suppressed to a negligible level. Only the output of the double frequency component can be taken out from the photodetector 6 without requiring a special filter circuit. Further, in the second embodiment, a sufficiently small opening 9c is provided on the front side of the photodetector 8 of the photodetector 6 so that unnecessary light enters the photodetector 7 of the photodetector 6. Is preventing.

With this configuration, as shown in FIG. 5, the amplitude of the signal of the fundamental wave component (see FIG. 5A) is U ₀ substantially 0, and the signal of the double frequency component is Only the output was obtained (see FIG. 5 (b)). In this case, the sixth frequency component produces a photoelectric conversion output that is substantially the same as that of the first embodiment, but this output can be blocked by limiting the band of the frequency discrimination filter circuit 8.

FIG. 6 shows the structure of the main part of an optical encoder according to a third embodiment of the present invention. In the third embodiment, the fixed grid plate 3C and the moving grid plate 4C are each sinusoidal in cross section. It has a phase grating of.

In the third embodiment, the same members and diffracted light as those in the first embodiment are designated by the same reference numerals as those in the first embodiment, and the description thereof will be omitted.

The light incident on the fixed grating plate 3C is divided into diffracted light of several orders by the fixed grating plate 3C and diffracted.
The condition for setting the intensity of the 0th-order diffracted light to 0 is determined by the shape of the grating, the refractive index of the material forming the grating, and the depth of the grating. For convenience, it should be calculated by the "center of gravity" of the phase of the diffracted light. You can For example, in the fixed diffraction plate 3B and the moving diffraction plate 3C of the second embodiment, as shown in FIG. 8A, the center of gravity G and G'of the phase are on the surface of the rectangular wave, and the center of gravity G of the grating for advancing the phase. When the phase difference between the center of gravity of the grating and the center of gravity G ′ of the grating which delays the phase becomes 1/2 of the wavelength λ of the light source, it is considered that 0th-order light is not emitted. The condition of the step d of this lattice is as shown in equation (1).

Next, in the sine wave grating shown in FIG. 8 (b), the center of gravity of the phases is obtained by averaging the phases of the sine waves to find the heights of G and G '.

[Equation 1]

It becomes When the heights of G and G'are equal to λ / 4, d = (1/2) × λ ×
(1 / | n−n ₀ |) / (1-2 / π).

When the step d between the peak and the valley of the sine wave is set as described above, the 0th-order diffracted light component that has passed through the fixed diffraction plate 3C and the moving diffraction plate 3C exits from the valley and the peak of the sine wave. The lights are out of phase with each other by 0.5λ and cancel each other out to be 0.
On the other hand, since the phase of the 1st-order diffracted light is different by 1λ, it is considered that the lights emitted from the troughs and the peaks of the sine wave strengthen each other and the intensity of the 1st-order diffracted light increases.

FIG. 9 shows the structure of the main part of an optical encoder according to a fourth embodiment of the present invention. In the fourth embodiment, the fixed grating plate 3D and the moving grating plate 4D have triangular wave-shaped cross sections, respectively. It has a phase grating of.

Also in the fourth embodiment, the same members and diffracted lights as those in the first embodiment are designated by the same reference numerals as those in the first embodiment, and the description thereof will be omitted.

In the fourth embodiment, as in the second embodiment, the light incident on the fixed grating plate 3D is the fixed grating plate 3D.
Is diffracted into several orders of diffracted light. Figure 8
From the drawing for obtaining the center of gravity G of the triangular wave phase shown in (c), the height of G is λ / 4, so the step d between the peak and the valley of the triangular wave is d = λ × (1 / | n−n ₀ |).

When the step d of the triangular wave is formed as described above, the 0th-order diffracted light component that has passed through the fixed diffractive plate 3D and the moving diffractive plate 3D is the light emitted from the troughs and peaks of the triangular wave-shaped grating. The phases are offset by 0.5λ and cancel each other out to 0. On the other hand, since the 1st-order diffracted light has a phase difference of 1λ, it is considered that the light emitted from the troughs and the peaks of the triangular wave grating strengthens each other and the intensity of the 1st-order diffracted light increases.
In particular, if the triangular wave pitch and the refractive index n of the fixed grating plate 3D and the moving grating plate 4D are set appropriately so that the triangular wave-shaped inclination matches the diffraction angle of the first-order diffracted light, the intensity of the first-order diffracted light is maximized. Can be converted.

Next, as in the second embodiment, the fixed grid plate 3D is used.
Guide the ± 1st order diffracted light passing through to the moving diffraction plate 4D,
By passing through the moving diffraction plate 4D, the (+1, −1) diffracted light in the direction perpendicular to the moving diffraction plate 4D and the (−)
1, + 1) diffracted light can be obtained. Then, when the moving diffraction plate 4D moves in the vertical direction in FIG. 9, a double-frequency sine wave signal having a frequency twice that of the fundamental wave is obtained as shown in FIG. 7C with the movement. be able to.

In each of the above embodiments, the fixed diffraction plates 3A, 3B, 3C and 3D are arranged on the light source 1 side and the movable diffraction plate 4 is formed.
Although A, 4B, 4C, and 4D were arranged on the photodetector 6 side, respectively, instead of this, the moving diffraction plates 4A, 4B, and 4 were used.
C, 4D on the light source 1 side, fixed diffraction plates 3A, 3B, 3C,
3D may be arranged on the photodetector 6 side.

[0051]

According to the optical encoder of the first aspect of the present invention, a fixed diffractive plate and a moving diffractive plate having a phase grating that mainly passes only the ± 1st order diffracted light of the coherent light emitted from the light source are provided. Since it has a condenser lens for condensing the ± 1st-order diffracted light that has passed through the fixed diffracting plate and the moving diffractive plate on the photodetector of the photodetector, the 0th-order diffracted light and the ± 1st-order diffracted light Since it is possible to obtain an output waveform having a frequency twice that of the sine wave formed when the diffracted light interferes with each other, it is possible to obtain a highly accurate sine wave output signal.

Further, since ± 1st-order diffracted light (conjugate wave) that passes through the fixed diffractive plate and the movable diffractive plate and is symmetric with respect to the optical axis and conjugate with each other is mainly used, the wavelength variation of the light source and the spread of the wavelength, Since it is possible to cancel the gap variation between the fixed diffraction plate and the moving diffraction plate substrate, flare, ghost light, etc., it is possible to obtain an output signal with a high SN ratio.

Further, since the number of constituent parts is small, the optical encoder can be manufactured at a low cost, and the optical system is bilaterally symmetrical even if the fixed diffractive plate or the movable diffractive plate expands due to humidity or temperature rise. Therefore, the output is not affected and the industrial value is high.

Further, when the fixed diffractive plate and the movable diffractive plate are formed of resin, they can be easily manufactured at low cost because they are easily duplicated.

According to the optical encoder of the second aspect of the present invention, the fixed diffractive plate and the moving diffractive plate have a shape in which peaks and valleys are symmetrical, and a step d between peaks and valleys is d = (1 / 2) ×
Since it has a rectangular wave phase grating of λ × (1 + 2m) × (1 / | n−n ₀ |), the light quantity of the 0th-order diffracted light can be made substantially zero, so that the distortion is A more accurate sine wave output signal can be obtained.

According to the optical encoder of the third aspect of the present invention, in the fixed diffractive plate and the movable diffractive plate, the step d between peaks and valleys is d = (1/2) × λ × (1 / | n−n ₀ |) ÷ (1
Since it has a sine wave phase grating that is −2 / π),
Since the light quantity of the 0th-order diffracted light can be made substantially zero, it is possible to obtain a more accurate sine wave output signal with no distortion, as in the second aspect of the invention.

According to the optical encoder of the fourth aspect of the present invention, the fixed diffractive plate and the moving diffractive plate have a shape in which peaks and valleys are symmetrical and the step d between peaks and valleys is d = λ × ( 1 / |
(n−n ₀ |) has a triangular wave phase grating, so that the light quantity of the 0th-order diffracted light can be made approximately 0. Therefore, as in the invention of claim 2, there is no distortion, and A highly accurate sine wave output signal can be obtained.

Further, according to the optical encoders of the second to fourth aspects of the invention, since the phase gratings of the fixed diffractive plate and the moving diffractive plate have a simple shape, they are highly accurate, but
It can be manufactured at low cost.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an optical encoder according to a first embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of a main part of the optical encoder according to the first embodiment.

FIG. 3 is a waveform of diffracted light that has passed through a fixed diffraction plate and a moving diffraction plate of the optical encoder of the first embodiment,
(A) shows the waveform of a wave in which the fundamental wave and the double frequency interfere with each other, and (b) shows the waveform of the fundamental wave and the waveform of the double frequency.

FIG. 4 is a schematic configuration diagram of an optical encoder according to a second embodiment of the present invention.

FIG. 5 is a waveform of diffracted light that has passed through the fixed diffractive plate and the moving diffractive plate of the optical encoder of the second embodiment.
(A) shows the waveform of the fundamental wave whose amplitude is 0, (b) shows the double frequency waveform.

FIG. 6 is a schematic configuration diagram of an optical encoder according to a third embodiment of the present invention.

FIG. 7 is a waveform of diffracted light that has passed through a fixed diffraction plate and a moving diffraction plate of the optical encoder of the third embodiment,
(A) shows the waveform of the fundamental wave whose amplitude is 0, (b) shows the double frequency waveform.

FIG. 8 is an explanatory diagram illustrating a center of gravity serving as a calculation reference for determining the shape of the phase grating in the second to fourth embodiments of the present invention.

FIG. 9 is a schematic configuration diagram of an optical encoder according to a fourth embodiment of the present invention.

FIG. 10 is a schematic configuration diagram of a conventional optical encoder.

[Explanation of symbols]

 1 Light Source 2 Collimating Lens 3A, 3B, 3C, 3D Fixed Lattice Plate 4A, 4b, 4c, 4d Moving Lattice Plate 5 Collective Lens 6 Photodetector 7 Photodetector 8 Frequency Discrimination Filter

Claims (4)

[Claims] 1. A light source that emits coherent light and a phase that is provided substantially perpendicular to and parallel to the optical axis of the light emitted from the light source and that mainly passes only ± 1st-order diffracted light. A fixed diffractive plate and a moving diffractive plate having a grating, a photodetector for receiving the light passing through the fixed diffractive plate and the moving diffractive plate, and ± 1st order diffracted light passing through the fixed diffractive plate and the moving diffractive plate. An optical encoder, comprising: a condenser lens for condensing the light on the photodetection section of the photodetector. 2. The fixed diffractive plate and the movable diffractive plate both have a rectangular wave-shaped phase grating, and the peaks and valleys of the rectangular wave of the phase grating have a symmetrical shape and the peaks and valleys. The step d is d = (1/2) × λ × (1 + 2m) × (1 / | n−n ₀
│) (where m = 0, ± 1, ..., λ = wavelength of light emitted from the light source, n = refractive index of the material forming the fixed diffraction plate and the moving diffraction plate, n ₀ = fixed diffraction plate and moving diffraction plate The optical encoder according to claim 1, wherein the optical encoder is set to have a refractive index of a medium between the plate and the plate. 3. The fixed diffractive plate and the moving diffractive plate both have a sinusoidal phase grating, and the step d between the peak and the valley of the sinusoidal wave of the phase grating is d = (1/2) × λ × (1 / | n−n ₀ |) ÷ (1-2
/ Π) (where m = 0, ± 1, ..., λ = wavelength of light emitted from the light source, n = refractive index of the material forming the fixed diffraction plate and the moving diffraction plate, n ₀ = moving with the fixed diffraction plate The optical encoder according to claim 1, wherein the optical encoder is set to have a refractive index of a medium between the diffractive plate and the medium. 4. The fixed diffractive plate and the moving diffractive plate both have a triangular wave phase grating, and the peaks and valleys of the triangular wave of the phase grating have a symmetrical shape and the step between the peaks and valleys. d
Is d = λ × (1 / | n−n ₀ |) (where m = 0, ± 1, ..., λ = wavelength of light emitted from the light source, n = constitutes a fixed diffraction plate and a moving diffraction plate The optical encoder according to claim 1, wherein the refractive index of the material is set to n ₀ = the refractive index of the medium between the fixed diffraction plate and the moving diffraction plate).