JP3517506B2 - Optical displacement measuring device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical displacement measuring apparatus, and in particular, a coherent light beam such as a laser beam is made incident on a fine grating array such as a diffraction grating provided on a displacement object to obtain a specific order from the diffraction grating. This is suitable for measuring the displacement amount of the displacement object by causing the diffracted lights to interfere with each other to generate an interference signal and counting the brightness of the interference signal.

[0002]

2. Description of the Related Art Conventionally, there has been a rotary encoder as a displacement measuring device capable of highly accurately measuring rotation information such as a rotation amount and a rotation direction of a rotating object in an NC machine tool or the like, for example, in a submicron unit. It is used in the field.

In particular, as a high-precision and high-resolution rotary encoder, a coherent light beam such as a laser beam is made incident on a diffraction grating provided on a rotating object, and diffracted lights of a specific order generated from the diffraction grating interfere with each other. A well-known rotary encoder of the diffracted light interference system, which measures the moving state such as the moving amount and the moving direction of the moving object by counting the brightness of the interference fringes.

FIG. 4 is a schematic view of a main part of an optical displacement measuring device disclosed in Japanese Patent Publication No. 4-62002. This device is a diffracted light interference type rotary encoder.

In the figure, 31 is a light source such as a laser,
32 is a collimator lens, 33-35, 36-38 are cylindrical lenses, 39-42 are reflection mirrors, 43
Is a disk having a radiation diffraction grating 44, and 45 and 46 are 1 /
The four-wave plate is arranged so that its axis is ± 45 ° with respect to the linearly polarized light of the laser 31. Reference numeral 47 is a beam splitter, and 48 and 49 are polarizing plates arranged so that the polarization directions thereof are 45 ° to each other. Reference numerals 50 and 51 are light receiving elements.

The operation of this conventional example will be described. Laser 3
The light flux emitted from the beam No. 1 linearly irradiates the radiation diffraction grating 44 of the disk 43 via the collimator lens 32 and the cylindrical lens 33. When the pitch of the diffraction grating at the irradiation position M1 is P and the wavelength of the light flux is λ, ±
The diffraction angle θ _m of the reflected diffracted lights L1 and L2 of _{m-th order} is sin θ _m
= ± mλ / P The m-th order diffracted light beams are converted into substantially parallel light beams by the cylindrical lenses 34 and 35, respectively, and then are deflected by the reflection mirrors 39 and 40 toward the reflection mirrors 41 and 42, and are deflected by the mirrors toward the disk 43. Go to

Next, the two diffracted light beams are divided into a quarter wave plate 45,
After passing through 46, they become circularly polarized lights having mutually opposite rotations, and linearly irradiate a position M2, which is substantially symmetrical with respect to the disk rotation center of the irradiation position M1 on the radiation diffraction grating 44, through the cylindrical lenses 36 and 37. .

The incidence angle of each diffracted light on the irradiation position M2 is equal to the m-th order reflection diffraction angle θ _m thereof.
1, 42 and cylindrical lenses 36 and 37 are arranged. When the two diffracted lights are reflected and diffracted at the irradiation position M2, they are overlapped with each other and travel toward the cylindrical lens 38.
At this time, the two diffracted lights become linearly polarized light again, but their polarization directions change with the rotation of the radiation diffraction grating 44.

The light beam traveling to the cylindrical lens 38 passes through it and becomes a substantially parallel light beam again. Then, it is split into two light beams by the beam splitter 47, and the polarizing plate 4
The light passes through 8 and 49 and enters the light receiving elements 50 and 51.

At this time, the radiation diffraction grating 4 on the disk 43
Assuming that the number of four rotations of 4 is N and the rotation angle of the disc 43 is θ, the phase of the ± m-order diffracted light diffracted twice at the irradiation positions M1 and M2 is ± 2 mNθ with respect to the phase of the 0-order light. Is generated, and the light beams reflected and diffracted at the irradiation position M2 and overlapped with each other have a phase change of 2m − (− 2m) = 4mNθ. Therefore, it is possible to detect a change in brightness of 4 m times while the disk 43 rotates by one pitch (θ = 2π / N) of the radiation diffraction grating 44. Further, since the polarization plates 48 and 49 are arranged so that the polarization directions thereof are different from each other by 45 °, it is possible to give a phase difference of 90 ° between the output signals of the light receiving elements 50 and 51, thereby rotating the disc. The direction can be determined.

[0011]

In the conventional optical displacement measuring apparatus, if the eccentricity due to the wavelength variation of the light flux from the light source due to the variation of the ambient temperature and the mounting error between the disc and the other elements exists, the irradiation position is increased. The diffraction angle θ _{m at} M1 and M2 changes, and the error of this diffraction angle θ _m causes a reduction in the contrast of the interference signal formed by the overlap of the two diffracted lights.

Further, the conventional optical displacement measuring device produces a light-dark change of 4 m times with respect to the displacement of one pitch of the radiation diffraction grating, but it is hoped that an optical displacement measuring device with higher resolution will appear. It is rare.

The present invention separates a "detection unit" and a "disk" for irradiating a diffraction grating provided on a displacement object with a coherent light beam and detecting the displacement amount of the displacement object from the interference signal obtained thereby. In built-in type optical displacement measuring device, good interference condition and measurement even if wavelength of light flux from light source fluctuates due to fluctuation of environmental temperature or error in relative positional relationship between detection unit and disk. It is an object of the present invention to provide an optical displacement measuring device which is highly accurate and can measure the amount of displacement of a displaced object with high resolution.

[0014]

An optical system according to the invention of claim 1
The displacement measuring device separates the light beam from the light source means into the first and second light fluxes by the first light beam splitting means and makes them incident on a point D1 on the rotating diffraction grating on the rotating disk, and the point D1
Two diffracted light beams of a predetermined order based on the first and second light beams from the light condensing optical system and a reflecting surface in the vicinity of their focal planes are incident respectively.
First, which has the effect of reflecting the incident light beam in the same direction as the first direction
The light is made incident again on the point D1 via the reflecting means, and two diffracted lights of a predetermined order based on the first and second light fluxes from the point D1 are superposed by the first light flux splitting means, and then through the relay optical system. Is made incident on the second light beam splitting means to be separated into two diffracted lights based on the first and second light beams, and the point D1 on the radiation diffraction grating.
And at a position approximately point-symmetric with respect to the center of rotation of the disk,
The two diffracted lights of a predetermined order based on the first and second light beams which are incident on a point D2 having a conjugate relationship with the relay optical system and diffracted at the point D2 are respectively in the vicinity of the focusing optical system and its focal plane.
Reflects incident light in the same direction as it has a reflecting surface
After being made incident again on the point D2 via the second reflecting means having the action of , the two diffracted lights of a predetermined order based on the first and second luminous fluxes from the point D2 are superposed by the second luminous flux splitting means. The interfering means causes the first light flux and the second light flux to interfere with each other to form an interfering light flux, and the light receiving means receives the interfering light flux to detect rotation information of the disk.

The invention of claim 2 is the same as the invention of claim 1.
The light source means, the first and second light beam splitting means, the first and second reflecting means, the relay optical system, the interfering means, and the light receiving means are housed in one housing to form a unit. It is characterized by being.

The invention of claim 3 is the invention of claim 1 or 2.
In the first luminous flux splitting means, the polarized light beam splitting means
A polarizing film splitting film
On the other hand, the first light flux is transmitted and the second light flux is reflected.
It has a feature.

The invention of claim 4 is the invention of claim 1 or 2.
In the second beam splitting means, the polarized beam splitter
A polarizing film splitting film
On the other hand, the first light flux is transmitted and the second light flux is reflected.
It has a feature.

[0018]

1 and 2 are schematic views of the essential parts of an embodiment of an optical displacement measuring apparatus of the present invention. The present embodiment is a diffracted light interference type rotary encoder. FIG. 1 is a plan view of the optical system, and FIG. 2 is a side view thereof.

In the figure, 1 is a light source, which is composed of a semiconductor laser that emits a coherent light beam. A collimator lens 2 converts the light flux from the semiconductor laser 1 into substantially parallel light. The light source 1, the collimator lens 2 and the like constitute one element of the light source means.

Reference numerals 3 and 3'indicate first and second light beam splitting means, respectively, which are composed of Kester prisms, and are provided with polarization beam splitting films 5 and 5'on their inner split surfaces. 7 and 7'are mirrors, 9 and 9'and 10 and 10 'are 1/4
Wave plates, 15 and 16 are first reflecting means, 15 'and 1 respectively.
Reference numerals 6'denotes second reflecting means, respectively. The first and second reflecting means have a condensing optical system and a reflecting surface in the vicinity of the focal plane thereof, and have a function of reflecting the light beam in the same direction as the incident direction.

Among the above elements, the Kester prism 3, the mirror 7, the quarter wave plates 9 and 10, the first reflecting means 15 and 16
Etc. are elements of the first irradiation means, and the Kester prism 3 ', the mirror 7', the quarter wave plates 9 ', 10', the second reflection means 15 ', 16', etc. are the elements of the second irradiation means. It is an element. The first irradiation means and the second irradiation means have the same configuration.

Reference numeral 17 is a radiation diffraction grating provided on the surface of the disk 18, and its pitch is P. 19, 1
9'is a mirror, 20 is a half-wave plate, and 21 and 21 'are lenses. The mirrors 19 and 19 ', the half-wave plate 2
0, the lenses 21 and 21 ', etc. form one element of the relay optical system.

Reference numeral 24 is a quarter wavelength plate (interference means), and 25 is a non-polarizing beam splitter. Reference numerals 26 and 27 denote polarizing plates, respectively, but their polarization axes are inclined by 45 ° with respect to each other. 28 and 29 are sensors. Quarter-wave plate 24,
The non-polarizing beam splitter 25, the polarizing plates 26 and 27, the sensors 28 and 29, etc., each constitute one element of the optical system 23. In addition, the non-polarizing beam splitter 25 and the polarizing plate 2
6, 27, sensors 28, 29, etc. constitute one element of the light receiving means. This light receiving means generates two-phase interference signals that are 90 ° out of phase with each other.

Further, in this embodiment, the disk 18 is attached to a rotating object (not shown), and the other components are arranged in one housing to form a detection unit.

The operation of this embodiment will be described. A linearly polarized light beam having a wavelength λ emitted from a light source (semiconductor laser) 1 is converted into substantially parallel light by a collimator lens 2 and is incident on a Kester prism (first light beam splitting means) 3. This light beam is split by a polarized beam splitting film 5 into two light beams set at 45 ° so that the intensity ratio of P and S polarized light is 1: 1, and the P polarized light beam (first light beam) is transmitted, and the S polarized light beam is transmitted. The light flux (second light flux) is reflected. P and S split into these two beams
Of the polarized light flux, the P-polarized light flux (first light flux) becomes circularly polarized light after passing through the optical path 11 and passing through the quarter-wave plate 9 and becomes circularly polarized light at the point D ₁ of the radiation diffraction grating 17 at the incident angle θ _i and is reflected. Diffract.
Of the plurality of diffracted lights generated here, the diffracted light of order −1 (this is the first light flux) is a predetermined angle θ _m
And proceeds to the optical path 13.

The relationship between the incident angle θ _i and the diffraction angle θ _m is positive when the rotation direction of the radiation diffraction grating 17 is the direction a and when θ _i is counterclockwise with respect to the normal direction n, When θ _m is clockwise, sin θ _m −sin θ _i = λ / P.

The first light flux that has traveled along the optical path 13 is reflected by the first reflecting means 15 and travels backward through the optical path 13, and again the radiation diffraction grating 1
It is incident on the point D ₁ on 7 and the second reflection diffraction is performed. Then, of the plurality of diffracted lights generated here, the diffracted light of order −1 (this is the first light flux) travels to the optical path 11,
After passing through the wave plate 9, it becomes S-polarized light, which is reflected by the polarized beam splitting film 5 of the Kester prism 3 and is reflected in a predetermined optical path 1.
Proceed to C.

The other S-polarized light beam (second light beam) has an optical path 1
After passing through 2 and passing through the quarter-wave plate 10, it becomes circularly polarized light and enters the point D ₁ of the radiation diffraction grating 17 at an incident angle of −θ _i , and is reflected and diffracted. Of the plurality of diffracted lights generated here, the diffracted light of the order +1 (this is the second light flux) advances in the direction of a predetermined angle −θ _m and advances to the optical path 14.

The second light flux that has traveled along the optical path 14 is reflected by the first reflecting means 16 and travels backward through the optical path 14, and again the radiation diffraction grating 1
It is incident on the point D ₁ on 7 and the second reflection diffraction is performed. Then, of the plurality of diffracted lights generated here, the diffracted light of the order +1 (this is the second light flux) travels to the optical path 12,
After passing through the quarter-wave plate 10, it becomes P-polarized light, passes through the polarized beam splitting film 5 of the Kester prism 3, and advances to a predetermined optical path 1C. In this way, the first light flux is an S-polarized light flux that has been diffracted twice with the order −1, and the second light flux is a P-polarized light flux that has been diffracted twice with the order + 1st order along the predetermined optical path 1C. Overlap and proceed. This first, second
The luminous flux is converted into P-polarized light and S-polarized light by the half-wave plate 20, and then enters the second irradiation means.

In the second irradiating means, these light fluxes are converted into two light fluxes of P and S polarization by the polarized beam splitting film 5'of the Kester prism (second light flux splitting means) 3'as in the case of the first irradiating means. The P-polarized light flux (first light flux) is transmitted and the S-polarized light flux (second light flux) is reflected.

[0031] Among the divided two light beams, P polarized (first light flux) point D ₁ of the radiation diffraction grating 17 becomes a passage after yen polarize 'quarter-wave plate 9 by taking the' optical path 11 disk The light is incident on a point D ₂ that is substantially symmetrical with respect to the rotation center at an incident angle θ _i , and the third diffraction is performed. Diffracted light of order -1 among a plurality of diffracted lights generated here (this is referred to as a first light flux)
Travels in the direction of a predetermined angle θ _m and travels to the optical path 13 ′. The relationship between the incident angle θ _i and the diffraction angle θ _m at this time is the same as in the above case.

The first light beam traveling through the optical path 13 'is reflected by the second reflecting means 15', travels backward through the optical path 13 ', enters the point D ₂ on the radiation diffraction grating 17 again, and is reflected and diffracted for the fourth time. do. Then, of the plurality of diffracted lights generated here, the diffracted light of the order −1 (this is the first light flux) is the optical path 11 ′.
After passing through the quarter-wave plate 9 ', it becomes S-polarized light and is reflected by the polarization beam splitting film 5'of the Kester prism 3'and travels to a predetermined optical path 1M.

The other S-polarized light beam (second light beam) has an optical path 1
After taking 2 ′ and passing through the 1/4 wavelength plate 10 ′, it becomes circularly polarized light and is incident on a point D ₂ on the radiation diffraction grating 17 at an incident angle −θ _i ,
Do the third reflection diffraction. Of the plurality of diffracted lights generated here, the diffracted light of order +1 (this is the second light flux) advances in the direction of a predetermined angle −θ _m and advances to the optical path 14 ′.

The second light flux traveling along the optical path 14 'is reflected by the second reflecting means 16', travels backward through the optical path 14 ', enters the point D ₂ on the radiation diffraction grating 17 again, and is reflected and diffracted for the fourth time. do. Then, of the plurality of diffracted light generated here, the diffracted light of order +1 (this is the second light flux) is the optical path 12 '.
The polarized beam splitting film 5'of the Kester prism 3'becomes P-polarized after passing through the 1/4 wavelength plate 10 '.
And travels to a predetermined optical path 1M.

In this way, the first luminous flux is 4 of the order -1.
As the S-polarized light beam that has undergone the diffraction, the second light beam has an order +
Predetermined optical path 1 as a P-polarized light beam that has been diffracted four times in the first order
The light beams travel along the line M, and enter the optical system 23.

Then, these diffracted light beams are converted into a quarter wave plate 24.
The circularly polarized light beams that rotate in opposite directions by passing through the two circularly polarized light beams are overlapped with each other to form an interference light beam, which becomes linearly polarized light having a certain polarization direction. This linearly polarized light beam is split into two light beams by the non-polarizing beam splitter 25, and each light beam passes through polarizing plates 26 and 27 which are arranged with their polarization axes inclined by 45 °, and then are respectively detected by sensors 28 and 29. It receives light and generates two-phase signal outputs that are 90 ° out of phase with each other.

Then, according to the rotation of the disk 18, it becomes 1/4.
The polarization azimuth of the linearly polarized light formed by passing through the wave plate 24 changes, so that the sensors 28 and 29 detect the brightness and the rotation angle of the disk 18 can be detected. Further, since the output from the sensor 28 and the output from the sensor 29 are out of phase with each other by 90 °, the rotation direction of the disk 18 can be detected from the two-phase signal output.

In FIG. 3, the first light flux and the second light flux are the first light flux.
FIG. 7 is an explanatory diagram of an action of blocking unnecessary diffracted light that occurs when passing through the irradiation means of FIG. In FIG. 3 (A), the P-polarized light beam (first light beam) takes the optical path 11 and passes through the point D on the radiation diffraction grating 17.
_When incident on ₁ and diffracted, unnecessary diffracted light of order 0 is generated and travels to the optical path 12. This unnecessary diffracted light is generated by the 1/4 wavelength plate 1.
Since it is converted into S-polarized light by 0 and reflected by the polarized beam splitting film 5 of the Kester prism 3, it is not mixed in the predetermined optical path 1C.

Further, the P-polarized light beam passes from the optical path 11 to a point D ₁
When the diffracted light of the order −1 generated upon incidence on the optical disk reciprocates in the optical path 13 and is incident on the point D ₁ again and is diffracted, unnecessary diffracted light of the order 0 is generated. Since the direction of the unnecessary diffracted light overlaps the optical path 14, the unnecessary diffracted light reciprocates in the optical path 14 and is incident on the point D ₁ again to generate a plurality of reflected diffracted lights. Since the direction of the unnecessary diffracted light of order +1 overlaps with the optical path 12, the unnecessary diffracted light travels along the optical path 12, but this is 1 /
4 Since it is converted into S-polarized light by the wavelength plate 10 and reflected by the polarized beam splitting film 5 of the Kester prism 3,
It does not enter the predetermined optical path 1C.

However, as shown in FIG. 3B, when the 1/4 wavelength plate 30 is on the optical path in a predetermined direction, the above-mentioned unnecessary diffracted light is not converted into S-polarized light and the predetermined optical path 1C is obtained. Therefore, it is impossible to block unnecessary diffracted light with this configuration.

As described above, in this embodiment, unnecessary diffracted light is effectively blocked, and the displacement of the radiation diffraction grating is detected by the first and second light beams that have been diffracted four times, respectively. Since the light and dark changes are generated eight times with respect to the displacement of one pitch, the displacement information of the displaced object can be detected with extremely high accuracy.

In this embodiment, the first irradiation means 2 is used.
The diffracted light beam diffracted once is guided along the optical path 1C to the second irradiating means by the relay optical system. At this time, the relay optical system irradiates the point D ₁ irradiated by the first irradiating means to the second irradiation means. Since an image is formed on the point D ₂ irradiated by the irradiation means (the points D ₁ and D ₂ are in a conjugate relationship with the relay optical system), the outer peripheral portion and the inner peripheral portion of the radiation diffraction grating 17 are thereby set. The wavefronts of the light beams closer to each other coincide with each other, and the above-mentioned superposed state of the two light beams is improved.

Further, the first reflecting means 15 and 16 and the second reflecting means 15 ', which are provided with reflecting surfaces near the focal plane of the condensing optical system,
Since 16 'is used, even if the wavelength change due to the environmental temperature change or the change in the diffraction direction due to the eccentricity of the mounting of the disk occurs, the two light fluxes can be made to substantially match in the optical paths 1C and 1M and proceed. As a result, it is possible to prevent the contrast of the interference signal from decreasing and obtain a good interference signal.

Further, since the above-mentioned first reflecting means and second reflecting means are provided independently for each of the first light flux and the second light flux, the difference in optical path length between the two light fluxes is corrected and the interference signal It is possible to prevent a decrease in contrast and obtain a good interference signal.

Although the above embodiment is the embodiment of the rotary encoder, the linear encoder is also provided with the linear diffraction grating on the moving object and is basically the same as the first embodiment in one housing. An element is installed to irradiate a point D ₁ and a point D ₂ which are separated by a predetermined distance on the diffraction grating by the first irradiating means and the second irradiating means. The displacement information of the moving object can be detected with high accuracy by obtaining two light fluxes and causing the two light fluxes to interfere with each other by the interference means and received by the light receiving means.

[0046]

According to the present invention having the above-mentioned structure, a "detection unit" and a "detection unit" for irradiating a diffraction grating provided on a displacement object with a coherent light beam and detecting the displacement amount of the displacement object from an interference signal obtained thereby. In the built-in type optical displacement measuring device that is configured by separating the `` disc '', even if there is a wavelength variation of the light flux from the light source due to a variation of the environmental temperature, or there is an error in the relative positional relationship between the detection unit and the disc,
(EN) An optical displacement measuring device capable of obtaining a good interference state and measuring accuracy and measuring a displacement amount of a displaced object with high resolution.

[Brief description of drawings]

FIG. 1 is a schematic view (plan view) of a main part of an embodiment of an optical displacement measuring device of the present invention.

FIG. 2 is a schematic view of a main part (side view) of an embodiment of an optical displacement measuring device of the present invention.

FIG. 3 is an explanatory diagram of an action of blocking unnecessary diffracted light in the embodiment.

FIG. 4 is a schematic view of a main part of a conventional optical displacement measuring device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Light source (semiconductor laser) 1C, 1M Predetermined optical path 2 Collimator lens 3, 3 ′ First and second light beam splitting means (Kester prism) 5, 5 ′ Polarized beam splitting film 7, 7 ′ Mirror 9, 9 ′ 1 / 4 wave plates 10, 10 '1/4 wave plates 11, 12, 13, 14, 11', 12 ', 13', 1
4'optical path 15, 16 first reflecting means 15 ', 16' second reflecting means 17 radiation diffraction grating 18 disks 19, 19 'mirror 20 1/2 wave plate 21, 21' lens 23 optical system 24 1/4 wave plate (Interference means) 25 Non-polarizing beam splitter 26, 27 Polarizing plate 28, 29 Sensor 30 1/4 wavelength plate

Front page continued (58) Fields surveyed (Int.Cl. ⁷ , DB name) G01B 11/00-11/30 G01D 5/26-5/38

Claims (4)

(57) [Claims] 1. A light flux from a light source means is split into a first light flux and a second light flux by a first light flux splitting means and is made incident on a point D1 on a radiation diffraction grating on a rotating disk, Two diffracted lights of a predetermined order based on the first and second light beams from D1 are respectively provided with a converging optical system and a reflecting surface in the vicinity of its focal plane.
Has the function of reflecting the incident light beam in the same direction as the incident direction.
The first is incident again point D1 through the reflection means having, after superposing the first from the point D1, the two diffracted light of a predetermined order based on the second light flux by the first beam splitter, a relay optical It is incident on the second beam splitting means through the system to be separated into two diffracted beams based on the first and second beams, with respect to the point D1 on the radiation diffraction grating and the rotation center of the disc.
Conjugate relation with respect to the relay optical system at a position of substantially point symmetry
At a point D2, and two diffracted lights of a predetermined order based on the first and second light beams diffracted at the point D2 are reflected to the focusing optical system and the vicinity of its focal plane, respectively.
It has a surface and reflects the incident light beam in the same direction as the incident direction.
Incident again on the point D2 via the second reflecting means having
After the two diffracted lights of a predetermined order based on the first and second light fluxes from the point D2 are superposed by the second light flux splitting means, the interference light flux is caused to interfere with the first light flux and the second light flux. And the rotation information of the disk is detected by the light receiving means receiving the interference light beam. 2. The light source means, the first and second light beam splitting means, the first and second reflecting means, the relay optical system,
The optical displacement measuring device according to claim 1 , wherein the interfering means and the light receiving means are housed in one housing to form a unit. 3. The first beam splitting means is a polarized beam sp
A polarizing beam splitting device having a litting film.
The first light flux is transmitted and the second light flux is reflected by the coating film.
The optical displacement measuring device according to claim 1 or 2, characterized in that
Place 4. The second beam splitting means is a polarization beam splitter.
A polarizing beam splitting device having a litting film.
The first light beam is transmitted through the second film and the second light beam Is reflective
The optical displacement measuring device according to claim 1 or 2, characterized in that
Place