JP2683098B2 - encoder - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an encoder, and more particularly, to a relative displacement of a light beam incident on a diffraction grating of a diffraction grating by causing some diffracted light beams emitted from the diffraction grating to interfere with each other. The present invention relates to an encoder that detects photoelectric interference of formed interference light.

(Prior art)

Conventionally, encoders have been used as sensors for detecting the position and angular displacement of an object to be inspected in NC machine tools and the like. In recent years, there has been an increasing demand for higher resolution and higher accuracy for this type of encoder.

Using a diffraction grating as an optical scale for displacement detection,
A high-resolution, high-precision encoder that obtains a periodic signal according to the displacement of the scale by making the recording density of the diffraction grating several microns / pitch and interfering some diffracted light emitted from the diffraction grating is already known. However, if the recording density of the diffraction grating is increased to about the wavelength order for higher precision and higher resolution, the diffraction angle of the diffracted light (exit angle from the diffraction grating) increases, and There is a problem that the arrangement becomes troublesome.

For example, the conventional encoder shown in FIG. 6 generates a signal according to the displacement of the scale by the following operation.

The light beam from the laser diode 1 is collimated by the collimator lens 2 and is made incident vertically on the point P1 on the diffraction grating 5, and the + 1st-order reflected diffracted light (R1 +) emitted from the point P1 is passed through the mirror 6 through the beam splitter. Point at the same time as returning to 4
The −1st order reflected diffracted light (R1−) emitted from P1 is returned to the beam splitter 4 via the mirror 62, and the ± 1 reflected diffracted light is overlapped and interfered via the beam splitter 4. While the diffraction grating 5 moves by one pitch of the grating, the phase of the wavefront of the + 1st-order diffracted light is "2π advanced" and the phase of the wavefront of the -1st-order diffracted light is "2π delayed".
With the interference light formed by causing the light beams to interfere with each other, a change in brightness of two cycles is observed according to the movement of one pitch of the grating. That is, a periodic signal having twice the number of gratings of the diffraction grating 5 can be taken out.

However, as described above, the finer the recording density (pitches) of the diffraction grating 5, the larger the diffraction angle of the diffracted light, so the exit angle of the diffracted light emitted from the diffraction grating 5 is 90 degrees.
It will be near. Therefore, the mirrors 61 and 62 must be installed in the vicinity of the diffraction grating 5 so as not to contact the diffraction grating 5, and such installation is extremely troublesome. Further, if the grating pitch of the diffraction grating 5 is set to be equal to or less than the wavelength of the light beam from the laser diode 1, the diffracted light cannot be extracted, and even the change of the diffraction grating 5 cannot be detected.

[Summary of the Invention]

An object of the present invention is to solve the above-mentioned conventional problems and to provide an encoder capable of easily achieving high resolution.

In order to achieve this object, the encoder of the present invention makes a light beam R ₁ and a light beam R ₂ incident on a diffraction grating, and the light beam R ₁
Interfere with the first diffracted light generated by being reflected and diffracted by the diffraction grating and the second diffracted light generated by the light flux R ₂ being reflected and diffracted by the diffraction grating to form interference light, and the interference light is photoelectrically converted. By doing so, in the encoder that detects the relative displacement of the light beams R ₁ and R ₂ and the diffraction grating, the first direction toward the optical path substantially the same as the optical path when the light beam R ₁ enters the diffraction grating.
Diffracted light is emitted, the light beam R ₂ is allowed to enter the diffraction grating the light beam R ₁ and R ₂ so that the second diffraction light is emitted toward the substantially same optical path as the optical path when incident on the diffraction grating It is characterized by having optical means.

Further features and specific forms of the present invention will be described in the embodiments described later.

〔Example〕

FIG. 1 is a schematic view showing an embodiment of the present invention, in which 1 is a light source composed of a laser diode, 2 is a collimator lens, 4 is a polarization beam splitter, 5 is a diffraction grating having a pitch P, Reference numerals 61 and 62 are mirrors, 7 is a quarter-wave plate, 9 is a non-polarizing beam splitter, 10 and 11 are polarizing elements (such as a polarizing plate and a polarizing beam splitter), and S1 and S2 are light receiving elements. A collimator lens 2 collimates a laser beam having a wavelength λ emitted from a light source 1 and makes this collimated beam incident on a polarization beam splitter 4 to split it into two beams R1 and R2 whose polarization directions are orthogonal to each other. Here, the light beam R1 is S-polarized light reflected by the polarized beam splitter 4, and the light beam R2 is P-polarized light transmitted through the polarized beam splitter 4. The light flux R1 travels on the optical path L1 formed via the mirror 61, and the light flux R2 travels on the optical path L2 formed via the mirror 62. Then, the luminous fluxes R1 and R2 are respectively
After passing through the quarter-wave plate 7, it enters the point P1 on the diffraction grating 5 at an incident angle θ ₀ [= sin ^-1 (λ / 2p)], and the light beams R ₁ and R ₂ are reflected by the diffraction grating 5. The + 1st-order diffracted light (R1 +) of the light beam R1 and the -1st-order diffracted light (R2-) of the light beam R2, which are obtained by being diffracted, respectively travel to the original optical paths L1 and L2 via the 1/4 wavelength plate 7. Light path
The + _1st- order diffracted light that goes backwards in L ₁ and the -1st-order diffracted light that goes backwards in the optical path L ₂ are reflected by mirrors 61 and 62, respectively, and are directed to the polarization beam splitter 4, and are again superposed by the polarization beam splitter 4. Since the + 1st-order diffracted light is made into P-polarized light by the action of the quarter-wave plate 7, and the -1st-order diffracted light is made into S-polarized light by the action of the 1 / 4-wave plate 7, these luminous fluxes are The polarized beam splitter 4 overlaps and emits without any loss. Since the two overlapping light beams pass through the quarter-wave plate 7 and become circularly polarized light whose polarization planes rotate in opposite directions, the polarization states of the light beams in which the mutually opposite circularly polarized light beams are combined are It becomes linearly polarized light. The polarization direction of this light flux is determined by the phase difference of the wavefronts of the light flux (R1 +) and the light flux (R2-) that change according to the displacement of the diffraction grating 5, and the phase difference is 0,
While changing to 1 / 4π, 2 / 4π, 3 / 4π, 4 / 4π, 5 / 4π,…, 8 / 4π, the polarization direction of this linearly polarized light beam is 45 °, 6
7.5 °, 90 °, 112.5 °, 135 °, 157.5 °,…, 225 ° (45
゜) and rotate. Therefore, after splitting this light flux into two light fluxes of equal light quantity by the non-polarizing beam splitter 9, only a specific polarization component is split from one light flux using the polarizing element 10 and is made incident on the light receiving element S1. , A specific polarization component is separated from the other light beam by using the polarization element 11 and is extracted, and is made incident on the light reception element S2.
Periodic signals are output from 1 and S2 respectively. If the azimuths of the polarization components extracted by the polarization elements 10 and 11 are shifted from each other by 45 °, the timing of the change in brightness of the interference light incident on the light-receiving elements S1 and S2 is 1/4 cycle (1 in the output signal phase).
/ 2π) only shift. Therefore, the amount of change and the direction of displacement of the diffraction grating 5 can be detected by electrically amplifying and binarizing these two-phase periodic signals that are 90 ° out of phase with each other. Since this detection method is a well-known technique, a detailed description thereof will be omitted.

According to this encoder, even if the pitch of the diffraction grating 5 becomes equal to the wavelength λ of the light beam from the light source 1,
Incident angle θ ₀ = 30 ° of light fluxes R ₁ and R ₂ , ± 1st order diffracted light (R 1
The exit angle of +, R2−) is 30 °, and ± 1st order diffracted light for forming interference light can be easily extracted. Moreover, the configuration of the optical system is extremely simple.

FIG. 2 is a perspective view showing a second embodiment in which the present invention is applied to a rotary encoder. In FIG. 2, 1 is a light source composed of a laser diode, 2 is a collimator lens, 31 and 32 are prisms, 41 and 42. Is a polarized beam splitter surface in prisms 31, 32, 5 is a rotating disk plate (diffraction grating), 63, 64, 65, 66 are mirrors, 71, 72, 73 are quarter-wave plates, 8
Is a half-wave plate, 9 is a non-polarizing beam splitter, 10 and 11 are polarizing elements (for example, polarizing plates and polarizing prisms), and S1 and S2 are light receiving elements.

A collimator lens 2 collimates a laser beam having a wavelength λ emitted from a light source 1, makes the collimated beam incident on a prism 31, and causes a symmetry by a mirror surface or a polarization beam splitter surface 41 provided at a predetermined position of the prism 31. Divided into two light beams R1 and R2 that travel along the optical paths L1 and L2,
R1 and R2 are reflected by the mirror 63 and passed through the quarter-wave plate 71, and are then simultaneously made incident on the first point (P1) of the radial diffraction grating of the grating pitch P provided on the rotating disk plate 5. Here, among the plurality of diffracted lights diffracted by the diffraction grating and emitted from the point P1, the + 1st-order reflected diffracted light of the light beam R1 and the −1st-order reflected diffracted light of the light beam R2 respectively travel backward in the original optical paths L1, L2. The incident angle θ ₀ of the light fluxes R1 and R2 is previously set to θ ₀ = sin ⁻¹ so as to be emitted in the direction.
Set to (λ / 2P). Further, the light fluxes R1 and R2 are linearly polarized lights whose polarization planes are orthogonal to each other at the time of being split by the polarization beam splitter surface 41, but by passing back and forth through the 1/4 wavelength plate 71, the light fluxes R1 and R2 The polarization planes are swapped. The operation of the quarter-wave plate 71 is similar to that of the two quarter-wave plates 7 of the embodiment of FIG. 1, and the light beam R1 is linearly polarized light (P-polarized light) transmitted through the polarization beam splitter surface 41. Because there is a luminous flux
The + 1st-order diffracted light (R1 +) of R1 becomes S-polarized light through the quarter-wave plate 71, is reflected by the polarization beam splitter surface 41, and is emitted from the prism 31. Further, since the light flux R2 is linearly polarized light (S-polarized light) reflected by the polarization beam splitter surface, the −1st-order diffracted light (R2-) of the light flux R2 becomes P-polarized light through the 1/4 wavelength plate 71, and the polarization beam splitter. Prism 31 through surface 41
Exits with the light flux (R1 +) overlapping. + Of light flux R1
First-order diffracted light (R1 +) and -1st-order diffracted light of light flux R2 (R2-)
Are transmitted by the mirrors 64 and 65 of the prism 3 while being overlapped with each other, pass through the half-wave plate 8 and enter the prism 32. Then, the light flux (R1 +) is caused to travel along the optical path L3 and the light flux (R2−) is caused to travel along the optical path L4 by the mirror surface or the polarization beam splitter surface 42 provided at a predetermined position of the prism 32, and each of them is reflected by the mirror 66. After being reflected and passed through the quarter-wave plate 72, it is incident on the second point (P2) of the radial diffraction grating provided on the rotating disk plate 5 at an angle θ ₀ , where:
The 1/2 wavelength plate 8 converts the polarization plane of the + 1st order diffracted light (R1 +) from S polarization to P polarization, and converts the polarization plane of the -1st order diffracted light (R2-) from P polarization to S polarization. Further, the points P1 and P2 are set in a symmetrical positional relationship with respect to the rotation axis 0 of the rotating disk plate 5. Among the plurality of reflected diffracted lights reflected and diffracted by the diffraction grating and emitted from the point P, the + 1st-order re-diffracted light (R1 ++) of the light flux (R1 +) travels backward in the original optical path L3 and the It passes again and becomes S-polarized light, which is reflected by the polarized beam splitter surface 42 in the prism 32 and exits the prism 32. on the other hand,
The -1st order re-diffracted light (R2−−) of the luminous flux (R2−) is the original optical path L4.
In the reverse direction to pass through the quarter-wave plate 72 again to become P-polarized light and pass through the polarization beam splitter surface 42 in the prism 32,
The second re-diffracted light (R1 ++) overlaps and exits the prism 32. The two overlapping light fluxes become circularly polarized light whose polarization planes rotate in opposite directions by passing through the quarter-wave plate 73. Therefore, the polarization states of the light fluxes obtained by combining the mutually opposite circularly polarized light rays are linear. It becomes polarized light. The polarization azimuth of this linearly polarized light flux changes according to the rotation of the rotating disk plate +1.
It is determined by the phase difference between the wavefronts of the second-order diffracted light (R1 ++) and the -1st-order diffracted light (R2--), and the phase difference is 0, π / 4,2π / 4,3π.
While changing to /4,4π/4,5π/4,...,8π/4, the polarization direction of the linearly polarized light beam is 45 °, 67.5 °, 90 °, 112.5 °, 135
It rotates at ゜, 157.5 ゜,…, 225 ゜ (45 ゜). Therefore, after splitting this light flux into two light fluxes of equal light quantity by the non-polarization beam splitter 9, one of the light fluxes is separated by using the polarization element 10 and only a specific polarization component is taken out to obtain the light receiving element S1.
To the light-receiving element S2 by separating only a specific polarization component from the light-receiving element S2 by using the polarizing element 11 and separating the other into the light-receiving element S2. A periodic signal is output. Here, if the polarization components extracted by the polarization elements 10 and 11 are shifted from each other by 45 °, the timing of the light / dark change of the interference light entering the light receiving elements S1 and S2 becomes 1/4 cycle (π / 2 in the phase of the output signal). ). Therefore, as in the embodiment of FIG.
If the two-phase periodic signals that are 90 ° out of phase are electrically amplified or binarized, the rotation angle and the rotation direction of the rotary disk plate 5 can be detected.

Even in this rotary encoder, even if the pitch of the radial diffraction grating of the rotating disk plate 5 becomes equal to the wavelength λ of the light beam from the light source 1, the points P1 and R2 of the light beams R1 and R2
The incident angle with respect to P2 can be about 30 °, and
The exit angles of the ± 1st-order diffracted light and the ± 1st-order diffracted light can be set to about 30 °. Therefore, it is possible to create a high-resolution encoder without being restricted by the arrangement of the optical system.

Further, since the rotary encoder diffracts the light beams R1 and R2 on the rotating disk plate 5 at points P1 and P2 which are symmetrical to each other, the rotation center 0 of the rotating disk plate 5 and the center (radiation center) of the radial diffraction grating are Achieves highly accurate detection of the rotational state with reduced influence of eccentricity.

Moreover, since the optical paths of the ± 1st-order diffracted lights transmitted from the prism 31 to the prism 32 are almost common, the optical path length difference does not largely occur due to the ambient temperature fluctuation, and extremely stable detection can be performed.

In the above description of the embodiment, as the orders of diffracted light, + 1st order and −
Although the first order is used, in the present application, the ± sign indicates that the moving direction of the diffraction grating 5 and the direction in which the traveling direction of the light beam are displaced are the same as + as shown in FIGS. 5 (A) and 5 (B). ,
5 (A) schematically shows the states of incident light and diffracted light in the conventional example of FIG. 6, and FIG. 5 (B) of FIG. The state of the incident light (R2) and the diffracted light in the first embodiment is shown. As is clear from a comparison between the two figures, in the conventional example, ± 1st order diffracted light emitted toward an optical path that is completely deviated from the optical path of the incident light is used, whereas in the present invention, it is almost the same as the optical path of the incident light ( The −1st order diffracted light emitted toward the optical path of the same or a near optical path is used. With such a shape, the above-mentioned effects are produced. It goes without saying that the order 1 of the diffracted light used in the encoder of each of the above embodiments is merely an example and may be second order or higher.
In addition, in each of the above-described embodiments, if the diffracted light of the second or higher order is used, it is possible to detect the displacement with a higher resolution. This also applies to other embodiments described later.

In the present invention, reflected diffracted light is used as the diffracted light that forms the interference light. Therefore, a diffraction grating having a high reflectance is useful. The diffraction grating used in each of the above examples is a so-called amplitude type diffraction grating in which light reflecting portions made of chrome or the like are periodically arranged on a glass substrate, but a periodic groove is formed in glass or plastic. It is also possible to use a so-called phase type diffraction grating in which a reflection film of Al or the like is applied to the uneven portion. This phase type diffraction grating is suitable for the encoder of the present invention because it is excellent in mass productivity and can increase the diffraction efficiency of ± first-order diffracted light.

FIGS. 3 and 4 are obtained by partially changing the optical paths of the optical systems of the embodiments of FIGS. 1 and 2, respectively.

FIG. 3 shows that the incident angles of the light beams R1 and R2 incident on the diffraction grating 5 are slightly changed from the case of FIG.
2) and the return optical path (L5, L6) are slightly shifted,
The basic structure is the same as that of the optical system shown in FIG. Points in Figure 1
The + 1st-order diffracted light of the light beam R1 and the specularly-reflected light (zero-order diffracted light) of the light beam R2 emitted from P1 travels in the same optical path L1 and the + 1st-order diffracted light of the light beam R1 is generated by the polarization beam splitter 4 due to the difference in polarization planes. Although it should be reflected only, the specularly reflected light of the luminous flux R2 is slightly reflected by the polarization beam splitter 4 and guided to the light receiving elements S1 and S2 depending on the imperfections of the optical components, and the ghost light (noise) )Become. Also,
Even if the specularly reflected light (zero-order diffracted light) of the light flux R1 passes through the polarized beam splitter 4 through the optical path L2 of the −1st-order reflected diffracted light, it becomes a ghost light as well. So the third
If you set the optical path as shown by L1, L2, L5, L6 in the figure,
Since the optical path of the ghost light and the optical path of the desired diffracted light forming the interference light are deviated from each other, the ghost light receiving element S
It is possible to avoid the incidence on 1, S2.

In FIG. 4, the incident angles of the light beams R1 and R2 incident on the rotating disk plate 5 are slightly changed from those in the case of FIG.
1, L2, L3, L4) and the optical path of the return path (L5, L6, L7, L8) are slightly shifted, and basically the same optical parts as the encoder of Fig. 2 can be used. In FIG. 2, the + 1st-order diffracted light (R1 +) of the light beam R1 and the specularly reflected light (zero-order diffracted light) of the light beam R2 emitted from the point P1 travels on the same optical path L1, and the polarization planes of the polarization beam splitter surface 41 are mutually polarized. Luminous flux by the difference of
Only the + 1st order diffracted light (R1 +) of R1 should be reflected, but part of the specularly reflected light of the light flux R2 is also reflected and transmitted by the polarized beam splitter surface 41 depending on the imperfections of the optical components. As described above, ghost light (noise) may occur. Specular reflection light (zero-order diffracted light) of light flux R1, point
The same applies to the regular reflection light (zero-order re-diffracted light) of the light flux (R1 +) emitted from P2 and the regular reflection light (zero-order re-diffraction light) of the light flux (R2-). However, in Figure 4, L1, L2, L5, L6, L3, L4, L
If the optical path is set as shown at 7, L8, the optical path of the ghost light deviates from the optical path of the diffracted light that forms the interference light, so that the ghost light can be prevented from entering the light receiving elements S1 and S2. .

〔The invention's effect〕

As described above, by appropriately setting the incident angles of the two light beams incident on the diffraction grating and substantially returning the desired diffracted light to the original optical path (optical path at the time of incidence), the grating of the diffraction grating Even if the pitch is on the order of the wavelength of light, the diffracted light for forming the interference light can be easily extracted. Therefore, a highly accurate and high resolution encoder is realized. Furthermore, since the optical component that guides the light beam to the diffraction grating and the optical component that extracts the desired diffracted light generated from the diffraction grating can be shared,
The reading optical system of the diffraction grating is very simple and compact.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing one embodiment of the present invention. FIG. 2 is a perspective view showing a second embodiment adapted to the rotary encoder of the present invention. FIGS. 3 and 4 are views showing modified examples of the embodiment of FIGS. 1 and 2, respectively. FIGS. 5A and 5B are explanatory views for explaining the signs of the diffraction orders of diffracted light. FIG. 6 is an explanatory view showing a conventional encoder utilizing interference of diffracted light. 1 ...... Light source 2 ...... Collimator lens 3, 31, 32 ...... Prism 4 ...... Polarization beam splitter 41, 42 …… Polarization beam splitter surface 5 …… Diffraction grating or rotating disk plate 61, 62, 63, 64,65,66 …… Mirror 7,71,72,73 …… 1/4 wave plate 8 …… 1/2 wave plate 9 …… Non-polarized beam splitter 10,11 …… Polarizing element S1, S2 …… Light receiving element

Claims (5)

(57) [Claims] 1. A light beam R ₁ and a light beam R ₂ are made incident on a diffraction grating,
The first diffracted light produced by the light beam R ₁ being reflected and diffracted by the diffraction grating and the second diffracted light produced by the light beam R ₂ being reflected and diffracted by the diffraction grating.
In the encoder that detects the relative displacement of the light beams R ₁ and R ₂ and the diffraction grating by photoelectrically converting the interference light by forming interference light by interfering with the diffracted light, the light beam R ₁ is The first diffracted light is emitted toward an optical path that is substantially the same as the optical path when entering the diffraction grating, and the second diffracted light is directed toward an optical path that is substantially the same as the optical path when the light beam R ₂ enters the diffraction grating. An encoder having optical means for causing the light beams R ₁ and R ₂ to enter the diffraction grating so that the light beam is emitted. _{2. When} the wavelengths of the light beams R ₁ and R ₂ are λ, and the grating pitch of the diffraction grating is P, the optical means outputs the light beams R ₁ and R ₂ by θ ₀ = sin ⁻¹ (λ The encoder according to claim (1), wherein the encoder is incident on the diffraction grating at an incident angle θ ₀ of / 2P). 3. When the wavelengths of the light beams R ₁ and R ₂ are λ, and the grating pitch of the diffraction grating is P, the optical means outputs the light beams R ₁ and R ₂ by θ ₀ = sin ⁻¹ (λ The encoder according to claim (1), wherein the encoder is made to enter the diffraction grating at an incident angle different from an incident angle θ _{0 of} / 2P). 4. The optical means reflects a laser and a beam splitter for splitting a laser beam from the laser into the light beam R ₁ and the light beam R ₂ and the light beam R ₁ from the beam splitter. It has a first reflecting mirror for directing to the diffraction grating and a second reflecting mirror for reflecting the luminous flux R ₂ from the beam splitter and directing it toward the diffraction grating, and the first diffracted light is reflected by the first reflecting mirror. Is directed to the beam splitter, the second diffracted light is directed to the beam splitter via the second reflecting mirror, and the beam splitter causes the optical paths of the first and second diffracted lights to overlap with each other. The encoder according to claim (1), characterized in that: 5. A light beam R ₁ and a light beam R ₂ are made incident on a diffraction grating,
The first diffracted light produced by the light beam R ₁ being reflected and diffracted by the diffraction grating and the second diffracted light produced by the light beam R ₂ being reflected and diffracted by the diffraction grating.
In the encoder for measuring the relative detection of the light fluxes R ₁ and R ₂ and the diffraction grating by photoelectrically converting the interference light by interfering with the diffracted light, the diffraction grating is rotated. When the first diffracted light is emitted toward the optical path substantially the same as the optical path when the light beam R ₁ enters the diffraction grating and the light beam R ₂ enters the diffraction grating. The first diffracted light and the first optical means for making the light beams R ₁ and R ₂ incident on the point P ₁ on the diffraction grating so that the second diffracted light is emitted toward the almost same optical path as The first re-diffracted light is emitted toward almost the optical path when entering the diffraction grating, and the second re-diffracted light is directed toward the almost same optical path as when the second diffracted light enters the diffraction grating. in the point P ₁ and substantially symmetrical positions with respect but the rotation center of the rotating object a first and second diffracted light to exit A point on the diffraction grating is incident on P ₂ and a second optical means allowed to reflection diffraction, by interfering the first and second re-diffracted light interference light formed by, for photoelectrically converting the interference light that An encoder for detecting the rotation state of the rotating object by means of.