JP2629606B2 - encoder - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an encoder, and more particularly to an encoder, in which a coherent light beam is made incident on a diffraction grating attached to a moving object, and diffracted lights from the diffraction grating interfere with each other to form interference fringes. The present invention relates to an encoder such as a rotary encoder or a linear encoder that measures a movement amount of a diffraction grating, that is, a movement amount of a moving object by counting light and dark fringes.

[0002]

2. Description of the Related Art In recent years, in precision machines such as NC machine tools and semiconductor printing apparatuses, precise measuring instruments capable of measuring in units of 1 μm or less (submicron) have been required. Conventionally, as a measuring instrument capable of measuring in submicron units, a coherent light beam such as a laser is used to form interference fringes from diffracted light from a moving object, and a rotary encoder or linear encoder using the interference fringes is formed. Encoders are well known.

FIG. 6 is a configuration diagram of an example of a conventional linear encoder. In FIG. 1, reference numeral 1 denotes a laser, 2 denotes a collimator lens, and 3 denotes a diffraction grating having a grating pitch P attached to a moving object (not shown), and moves at a speed v in the direction of an arrow, for example. 5 ₁ and 5 ₂ are quarter-wave plates, 4 ₁ ,
4 ₂ roof prism for preventing axial displacement of the re-diffracted light caused by the inclination of the diffraction grating 3, or a corner cube reflector, 6 beamsplitter 7 _1, 7 ₂ are each polarization axis orthogonal to each other by the polarizing plate and which is further arranged at an angle of 5 _1, 5 ₂ of the polarization axis 45 ° quarter-wave plate. 8 ₁ and 8 ₂ are light receiving elements, respectively.

In FIG. 1, a light beam from a laser 1 is converted into a substantially parallel light beam by a collimator lens 2 and is incident on a diffraction grating 3. The positive and negative mth-order diffracted lights diffracted by the diffraction grating 3 are reflected by the corner cube reflecting mirrors 4 ₁ and 4 ₂ via the quarter-wave plates 5 ₁ and 5 _{2, and} are incident on the diffraction grating 3 again. Then, the diffracted lights become positive and negative m-th order diffracted lights again, are overlapped and split into two light beams by the beam splitter 6, and are polarized by the polarizing plates 7 ₁ and 7 _2.
And enters the light receiving elements 8 ₁ and 8 ₂ .

Here, the light beams incident on the light receiving elements 8 ₁ and 8 ₂ are given a phase difference of 90 degrees from each other by a combination of the ４ wavelength plates 5 ₁ and 5 ₂ and the polarizing plates 7 ₁ and 7 ₂ , and the diffraction grating is provided. 3 is used for discrimination in the moving direction. The amount of movement of the diffraction grating 3 is obtained by counting the bright and dark fringes of the interference fringes received by the light receiving elements 8 ₁ and 8 ₂ .

[0006] m order diffracted light from the light beam is incident on the diffraction grating 3, for example a diffraction grating 3 from the laser 1 in the linear encoder shown in Fig. 6 through the corner cube reflector 4 _2, 1/4-wave plate 5 ₂ The m-order diffracted light, which is re-incident on the diffraction grating 3 by the + m-th order, is incident on the light receiving elements 8 ₁ and 8 ₂ as signal light. Regular reflected 0-order diffracted quarter-wave plate 5 ₁ by the diffraction grating 3 of the light beam at this time and re-incident on the diffraction grating 3 is incident again on the diffraction grating 3 through the corner cube reflector 4 _1, and diffraction Specularly reflected by grating 3, corner cube reflecting mirrors 4 ₂ and 1 /
4 is incident on the diffraction grating 3 through the wave plate 5 _2, the light receiving element 8 diffracted light m order diffracted by the diffraction grating 3 as a noise light
_1, there is a case that comes incident on the 8 _2. As a result, there is a problem that the detection accuracy is reduced.

[0007]

SUMMARY OF THE INVENTION According to the present invention, by properly arranging the polarizing beam splitter and the quarter-wave plate in the axial direction, unnecessary diffracted light (noise light) is prevented from being incident on the light receiving element, and the detection accuracy is improved. It is an object of the present invention to provide an encoder that achieves an improvement, for example, a rotary encoder or a linear encoder.

[0008]

SUMMARY OF THE INVENTION An encoder according to the present invention is characterized in that a coherent light beam is made incident on a diffraction grating, interference light is formed from the diffraction light from the diffraction grating, and the interference light is photoelectrically converted. In an encoder for measuring a moving state of a grating, the coherent light beam is split into two light beams by a polarizing beam splitter, and each of the two light beams is arranged with a fast axis for incident light in the same direction as each other. And diffracted light of each of the two light beams from the diffraction grating is returned to the polarizing beam splitter through the quarter-wave plate, and the polarized light is split by the polarizing beam splitter. It is characterized in that interference light is formed by superimposing diffracted light beams, and the interference light is photoelectrically converted.

[0009]

FIG. 1 is a schematic view of an optical system according to an embodiment of the present invention. 6, the same elements as those shown in FIG. 6 are denoted by the same reference numerals. 9 polarization beam splitter, 5 _1, 5 ₂ are each quarter-wave plate in FIG. 1/4
As described later, the wave plates 5 ₁ and 5 ₂ have their fast axes oriented in the same direction as each other, and are arranged in a predetermined direction with respect to the polarizing beam splitter 9.

Reference numerals 10 ₁ and 10 ₂ denote reflecting mirrors, 11 denotes an end-surface imaging type refractive index distribution type optical member, and a reflection film 12 is provided on one end.
Is given. The optical member 11 and the reflection film 12 constitute a light converging system 20.

In the present embodiment, the coherent light beam from the laser 1 is converted into a substantially parallel light beam by the collimator lens 2 and is made incident on the polarization beam splitter 9. It is split into luminous flux. At this time, the mounting position of the laser 1 is adjusted such that the linear polarization direction of the light beam emitted from the laser 1 is 45 degrees with respect to the polarization beam splitter 9. Thereby, the intensity ratio between the transmitted light beam and the reflected light beam from the polarizing beam splitter 9 is set to be approximately 1: 1.

The reflected light beam and the transmitted light beam from the polarization beam splitter 9 are converted into circularly polarized light via quarter-wave plates 5 ₁ and 5 ₂ , reflected by the reflecting mirrors 10 ₁ and 10 ₂ , and made incident on the diffraction grating 3. At this time, the m-order diffracted light from the target diffraction grating 3 is incident so as to be reflected substantially perpendicularly from the diffraction grating 3.

That is, when the grating pitch of the diffraction grating 3 is P, the wavelength of the coherent light beam is λ, and m is an integer, and the incident angle of the coherent light beam on the diffraction grating 3 is θ _m , θ _m ≒ sin ^{− 1} (mλ / P) ‥‥‥ (1).

The m-th order diffracted light emitted substantially perpendicularly from the diffraction grating 3 is made incident on the optical member 11. Since the reflecting film 12 is provided near the focal plane of the optical member 11, the incident light flux is reflected by the reflecting film 12 as shown in FIG. The light enters the diffraction grating 3.

The m-order reflected diffracted light diffracted again by the diffraction grating 3 returns to the original optical path, is reflected by the reflecting mirrors 10 ₁ and 10 ₂ , passes through the quarter-wave plates 5 ₁ and 5 ₂ , and is polarized. Re-enter the beam splitter 9.

At this time, the re-diffracted light is divided into quarter wave plates 5 ₁ , 5
₂ reciprocates, the light flux first reflected by the polarizing beam splitter 9 will be transmitted when re-entering, because the polarization direction differs by 90 degrees with respect to the polarizing beam splitter 9. Conversely, the light beam first transmitted by the polarizing beam splitter 9 is reflected when re-entering.

[0017] After thus through the quarter-wave plate 5 ₃ not overlapping the two diffracted light by the polarization beam splitter 9,
Circularly polarized light is split into two light beams by the beam splitter 6, and after passing through the polarizing plates 7 ₁ and 7 ₂ , the light is converted into linearly polarized light and incident on the light receiving elements 8 ₁ and 8 ₂ , respectively.

Incidentally, the angle θ _{m in the} expression (1) indicates that the angle should be within a range in which the diffracted light can enter the condensing system 20 and be incident again on the diffraction grating 3.

In this embodiment, the phase of the m-th order diffracted light changes by 2mπ when the diffraction grating moves by one pitch. Therefore, since the light receiving elements 8 ₁ and 8 ₂ receive the interference of the luminous flux having received the positive and negative m-order diffraction twice each, when the diffraction grating moves by one pitch of the grating, 4m sine wave signals are obtained. Is obtained.

For example, if the pitch of the diffraction grating 3 is 3.2 μm and the first order (m = 1) is used as the diffracted light, the diffraction grating 3
Is moved by 3.2 μm, four sine wave signals are obtained from the light receiving elements 8 ₁ and 8 ₂ . That is, the resolution per sine wave is 1/4 of the pitch of the diffraction grating 3, that is, 3.2 / 4 = 0.8 .mu.m.
m is obtained.

[0021] FIG. 7 is an explanatory view showing the positional relationship between the polarizing direction and a quarter-wave plate 5 ₁ of the polarizing beam splitter 9, 5 ₂ of the fast axis (fast axis) in this embodiment.
As shown in the figure, the fast axes (fast axes) of the quarter-wave plates 5 ₁ and 5 ₂ are set at 45 ° with respect to the polarization direction of the linearly polarized light reflected and transmitted through the polarizing beam splitter 9. by which, and quarter-wave plate 5 _1, 5 ₂ of the fast axis is set in the same direction. That is, the reflected light beam becomes right-handed circularly polarized light passes through the quarter-wave plate 5 ₁ enters the diffraction grating 3 by the polarization beam splitter 9, the focusing optical system 2
The m-order diffracted light reflected at 0 and returned is a quarter-wave plate 5 ₁
Is transmitted again, becomes linearly polarized light vibrating in a direction orthogonal to the outward path, passes through the polarization beam splitter 9, and is guided to the light receiving unit.

On the other hand, incident on the diffraction grating 3 is counterclockwise circularly polarized light light beam which has first passed through the polarizing beam splitter 9 passes through the quarter-wave plate 5 _2, reflected back by the condenser system 20 When come -m order diffracted light is transmitted through the quarter-wave plate 5 ₂ again, is reflected by the polarization beam splitter 9 is the forward becomes orthogonal direction of the linearly polarized light is guided to the light receiving portion. When two light beams enter the diffraction grating 3 at an angle θ _m represented by the expression (1), the specularly reflected light (0-order reflected diffraction light) by the diffraction grating 3 overlaps the optical path of ± m-order reciprocating diffraction light. The light enters the polarization beam splitter 9. Here, 1/4 fast axis of the wave plate 5 ₁ and 5 ₂ is because it is set in the same direction, clockwise transmitted through the first polarization beam splitter quarter-wave plate 5 ₁ is reflected by the 9 Diffraction grating 3 with circular polarization
In the positive reflected light beam is transmitted through the quarter-wave plate 5 _2, 1
/ 4 linearly polarized light since the direction of the linearly polarized light perpendicular to the having entered into the wavelength plate 5 ₁ polarization beam splitter 9
Through. Therefore, this light beam does not enter the light receiving section.
Similarly, the light beam transmitted through the polarization beam splitter 9 and regularly reflected by the diffraction grating does not enter the light receiving unit.

Further, of the diffracted light reflected by the light condensing system 20 and re-entering the diffraction grating 3, the -m-order diffracted light having the opposite sign to the m-order diffracted light returning on the outward path overlaps with the optical path on the opposite side and is polarized. enters the beam splitter 9 but, by setting 1/4-wave plate 5 ₁ and 5 ₂ axes as described above, are prevented from entering the light receiving portion as in the specular reflected light. In other words, 1/4-wave plate 5 _1, the fast axis of 5 ₂ by setting the same direction, only the ± m order reciprocating diffracted light becomes the signal light enters the light receiving portion, specular reflection light and the m-th order diffraction None of the noise light, such as the light that has been diffracted in the −m order and the light that has been diffracted in the + m order after the −m order, does not enter the light receiving portion. As described above, in the present embodiment, the S / N ratio of the output signals of the light receiving elements 8 ₁ and 8 ₂ is prevented from lowering, and highly accurate detection is possible.

Since the light condensing system 20 in this embodiment has a reflecting surface near the focal plane, for example, the diffraction angle changes with the change in the oscillation wavelength of the laser light, and the incident angle to the light condensing lens becomes small. Even if it changes slightly, it can be returned in substantially the same optical path. As a result, the two positive and negative diffracted lights are overlapped to prevent a decrease in the S / N ratio of the output signals of the light receiving elements 8 ₁ and 8 ₂ .

Further, in this embodiment, the angle of incidence of the coherent light beam on the diffraction grating 3 is set as described above, and the overall size of the apparatus is reduced by using a light condensing system.

For example, the grating pitch of the diffraction grating 3 is 3.2 μm,
Assuming that the wavelength of the laser 1 is 0.78 μ, the diffraction angle of ± 1st-order diffracted light is 14.2 degrees. Therefore, in the case where a refractive index distribution type lens having a diameter of about 2 mm is used as the optical member 11 and only the ± 1st-order diffracted light is reflected,
The distance to 1 is 2 / tan14.2 ° = 7.9 mm, and it is sufficient to leave the distance of about 8 mm, so that the entire apparatus can be made extremely small.

Although a gradient index lens is used as the optical member 11 in this embodiment, it may be constituted by a combination of the condenser lens 13 and the reflecting mirror 14 as shown in FIG.

In this embodiment, the reflected diffracted light is used. However, as shown in FIG. 4, the condensing system 20 may be arranged to use the transmitted diffracted light.

FIG. 5 shows another embodiment of the present invention. In the figure, the same elements as those in FIG. 1 are denoted by the same reference numerals.
In FIG. 5, a polarizing prism 15 has the same function as the polarizing beam splitter 9 in the embodiment of FIG. A folding prism 16 has the same function as the combination of the reflecting mirrors 10 ₁ and 10 ₂ in the embodiment of FIG.

In this embodiment, the light beam from the laser 1 is made substantially collimated by the collimator lens 2 and
5 is incident. The light beam incident on the polarizing prism 15 is reflected by the reflection surfaces 15a and 15b, and then split by the polarization beam splitting surface 15c into two light beams, a reflected light beam L1 and a transmitted light beam L2. The reflected light beam L1 and the transmitted light beam L2 are reflected by the reflecting surfaces 15a and 15d, respectively, are emitted from the surface 15b, and the quarter-wave plates 5 whose fast axes are oriented in the same direction.
_1, 5 ₂ passes through the, by a predetermined angle refracted by folding prism 16, thereby respectively incident so as to satisfy the above conditions the diffraction grating 3.

Then, the diffracted light of a predetermined order diffracted by the diffraction grating 3 is made incident on the diffraction grating 3 again through the light condensing system 20, and
A prism 1 that returns two re-diffracted lights from the diffraction grating 3
After through superimposed 6 and the polarizing prism 15, is transmitted through the embodiment similarly to the quarter-wave plate 5 ₃ of Figure 1, each polarizer is divided by the beam splitter 6 into two light beams 7 _1, 7
After through _2, are respectively received by the light receiving element _81, 82.

In each of the embodiments described above, the linear encoder is described. However, the present invention can be applied to a rotary encoder in the same manner.

[0033]

According to the present invention, a coherent light beam is split into two linearly polarized light beams by a polarizing beam splitter, and the two light beams pass through a quarter-wave plate having a fast axis directed in a predetermined direction. After that, by setting the incident angles of the two light beams to the diffraction grating as described above, it is possible to prevent unnecessary diffracted light from entering the light receiving unit, and to achieve a highly accurate encoder.

[Brief description of the drawings]

FIG. 1 is a schematic view of an optical system according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of an optical function of a part of FIG. 1;

FIG. 3 is an explanatory view of an optical function of a part of FIG. 1;

FIG. 4 is a partial explanatory view when a part is changed in the embodiment of FIG. 1;

FIG. 5 is a schematic view of an optical system according to another embodiment of the present invention.

FIG. 6 is an explanatory diagram of a conventional linear encoder.

FIG. 7 is an explanatory diagram showing the relationship between the polarization direction of the polarization beam splitter and the axial direction of the quarter-wave plate in the encoder of the present invention.

[Explanation of symbols]

1 laser 2 a collimator lens 3 diffraction grating 4 _1, 4 ₂ corner cube reflector 5 _1, 5 _2, 5 ₃ 1/4 wave plate 6 a beam splitter 7 _1, 7 ₂ polarizing plates _81, 82 light-receiving element 9 polarization Beam splitter 10 ₁ , 10 ₂ Mirror 20 Focusing system

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kimi Ishizuka 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Yoichi Kubota 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Co., Ltd. (56) References JP-A-59-163517 (JP, A)

Claims (1)

(57) [Claims] 1. An encoder for making a coherent light beam incident on a diffraction grating, forming interference light from the diffraction light from the diffraction grating, and measuring the moving state of the diffraction grating by photoelectrically converting the interference light. The coherent light beam is split into two light beams by a polarizing beam splitter, and each of the two light beams is directed in different directions to the diffraction grating via a quarter-wave plate in which the fast axes to the incident light are arranged in the same direction. And the diffracted light of each of the two light beams from the diffraction grating travels back to the polarizing beam splitter via the quarter-wave plate, and is superposed on the diffracted light of the two light beams by the polarizing beam splitter. An encoder that forms light and photoelectrically converts the interference light.