JPH0615977B2 - Grating interference displacement meter - Google Patents

Description

Detailed Description of the Invention [Industrial applications]

The present invention relates to a grating interference type displacement meter, and more particularly to a grating interference type displacement meter capable of maintaining stable interference even when the surface of the scale has poor surface quality.

[Prior art]

2. Description of the Related Art Photoelectric encoders that generate a periodic detection signal by using a scale having a fixed pitch optical scale are widely used. The resolution of this photoelectric encoder is determined by the width and pitch of the optical grating and the characteristics of the electronic circuit that divides the photoelectrically converted signal. In general, since the optical grating is manufactured by the etching method, the optical grating of about 4 μm is close to the limit of the accuracy of the final measurement, and if the electronic circuit is used within a range without a large cost up, the final resolution is It was around 1 μm, and it was difficult to further improve the precision. On the other hand, with the spread of photoelectric encoders, there is a demand for those that generate detection signals with higher resolution and higher accuracy. As one of the efforts to improve the resolution of the photoelectric encoder, a fine pitch (usually about 1 μm) scale is formed on the scale using holographic technology, and the scale is used as a diffraction grating to actively generate diffraction. A grating interferometric displacement sensor has been proposed that obtains a detection signal by means of the above method. FIG. 3 shows a grating interference type displacement gauge proposed in Japanese Patent Laid-Open No. 47-10034. This grating interference type displacement meter includes a scale on which a diffraction grating 10A of pitch d is formed and a laser beam 14 as a light beam on the diffraction grating 10A.
He-Ne laser light source 12 that irradiates (wavelength λ), mirrors 16 and 18 that respectively reflect the 0th-order and 1st-order diffracted light (light flux) generated by the diffraction grating 10A, and 1
The mixed wave of the 0th-order light of the 1st-order light reflected by the secondary-side mirror 18 and the 1st-order light of the 0th-order light reflected by the 0th-side mirror 16 is set to 3
A beam splitter (coarse diffraction grating) 20 that divides the beam splitting unit 20 and light receiving elements 22A, 22B, and 22C that photoelectrically convert the mixed waves divided into three by the beam splitter 20 are included. Here, each element except the scale is
It constitutes the detector. In FIG. 3, the polarization directions of the polarizers 24 and 26 inserted in the optical paths of the 0th-order light and the 1st-order light are set to be orthogonal to each other, and the central mixed wave divided into three parts is divided. The light receiving element 22A that receives the light does not generate interference fringes. Therefore, in the light receiving element 22A, not a fringe pattern but a mere sum signal is obtained, and thus the reference signal Vr is used. In addition, immediately before the light receiving element 22B, the analyzer 2 for generating the interference fringes is formed.
8B is arranged, and an As phase detection signal φA based on interference fringes is generated from this light receiving element 28B. Further, a quarter wave plate 30 and an analyzer 28C are arranged immediately before the light receiving element 22C, and from the light receiving element 22C, a B phase detection signal φ having a 90 ° phase difference from the A phase detection signal φA.
B is generated. Here, the incident angle θ of the laser beam 14 and the diffraction angle φ of the primary light are related by the following equation. d (sin θ + sin φ) = λ (1) According to such a grating interference type displacement meter, for example, by manufacturing the diffraction grating 10A by a hologram method, 1 μ
Since an optical grating of m or less can be formed, 0.0
It is also possible to achieve a resolution of 1 μm.

[Problems to be achieved by the invention]

However, if the flatness of the glass surface of the scale 10 on which the diffraction grating 10A is formed is poor, in the case of a transmission type grating interference displacement meter as shown in FIG. (In case of failure), as shown by an arrow A, a difference in refraction angle occurs and the light beam is bent.
The traveling directions of the two lights incident on B and 22C are inclined with respect to each other, and the light wavefronts orthogonal to the directions are combined into a stripe shape, and uniform light interference can be obtained over the entire cross section of the beam. I couldn't do it. Therefore, conventionally, the flatness of the scale must be maintained at 5 μm / 100 mm or less, and due to other factors, there is a problem that a signal cannot be detected when the optical axis direction is tilted. The present invention has been made to solve the above-mentioned conventional problems, and it is possible to obtain a stable interference signal by reducing the influence of poor flatness of the scale surface, inclination, pitching, and the like. An object is to provide a displacement meter.

[Means for achieving the object]

The present invention provides a detector including a scale having a diffraction grating, a light source for irradiating the diffraction grating with a light beam, and a light receiving element for photoelectrically converting a mixed wave of a plurality of light beams generated by the diffraction grating. And a grating interferometric displacement meter that generates a detection signal that changes periodically according to the relative displacement of the scale and the detector, an optical element that splits the light beam from the light source and enters the diffraction grating. And a plurality of reflecting means for returning each 0th-order light transmitted through the diffraction grating in the same direction and re-incident on the diffraction grating, and by the 0th-order reflected light re-incident from the plurality of reflecting means, The above-mentioned object is achieved by mixing the first-order diffracted lights generated by the diffraction gratings.

[Action and effect]

Conventionally, when a difference in a refraction angle (in the case of a transmission type) or a diffraction angle (in the case of a reflection type) occurs due to the flatness of the scale surface or the inclination of the scale, the light is incident on the light receiving elements 22B and 22C.
The light traveling directions of the books are inclined to each other, and the light wavefronts orthogonal to the light traveling directions are combined into a stripe shape, and uniform light interference cannot be obtained over the entire cross section of the beam. Therefore, the flatness of the scale must be maintained with high accuracy, and due to other factors, when the relative traveling directions of both beams are tilted, signals may not be detected, resulting in measurement error. There was a problem. By the way, as shown in FIG. 4 above, the 0th-order light transmitted through the scale 10 is also affected by the flatness in the same manner as the 1st-order light diffracted by the diffraction grating 10A, and in terms of diffraction efficiency, The amount of 0th-order light (about 80%) is much larger than the amount of 1st-order light (about 20%). The present invention has been made by paying attention to this point, and as shown in FIG. 2, the 0th-order light transmitted through the diffraction grating (10) is converted into, for example, a right-angle prism 40, a triangular cube such as a corner cube, and a triangular prism. By returning the light in the same direction with and re-entering the diffraction grating, the multiple light fluxes generated by the diffraction grating are made to travel in parallel to the respective designed traveling directions. Degree 15
A stable interference signal can be obtained even when the thickness is 100 μm / 100 mm or more. Further, due to other factors, a stable signal can be obtained even if the optical axis is tilted. In this way, pitching (tilt) due to poor flatness of the scale surface or alignment fluctuation during scale installation
Regardless of this, the diffracted light beam can always be made to travel in the same direction, and stable interference can be maintained. In this way, if the optical system is made more stable, the effects of poor flatness of the scale surface, pitching, etc. are reduced,
A more stable signal can be obtained. Therefore, the flatness of the scale used and the tolerance of the alignment at the time of attachment are improved, an inexpensive scale can be used, and the alignment can be simplified. Further, since the light flux emitted from the light source does not return to the light source, the oscillation is stable and the noise is reduced even when using a light source such as a laser diode which is affected by the returning light. In the conventional example shown in FIG. 3, the mirrors 16 and 18 may be changed to, for example, right-angle prisms. In that case, the light reflected by the mirror 18 is first-order diffracted light, Since it is already about 20% of the light quantity of the light source, the light quantity of the 1st-order diffracted light when it is further diffracted becomes about 4% of the light quantity of the light source. It is necessary to use or increase the capacity of the light source. On the other hand, according to the present invention, since the 0th-order light is reflected, the light quantity of the 0-1st-order diffracted light can be secured at about 16% of the light quantity of the light source, and the response speed of the detector can be improved and the light source can be improved. Can be miniaturized.

【Example】

Embodiments of the present invention will be described in detail below with reference to the drawings. This embodiment, as shown in FIG.
A transmissive scale 10 on which a diffraction grating is formed, a laser diode (LD) 42 as a light source, and a PIN, for example.
Photodetectors 22A ₁ and 22 composed of photodiodes
A ₂ , 22B, 22C, polarizing plates 24, 26, analyzer 28
B, 28C, a detector including a quarter wave plate 30,
In a transmissive grating interference type displacement meter that generates a detection signal that periodically changes according to the relative displacement of the scale 10 and the detector, the laser beam 14 from the laser diode 42 is divided into two, and the scale 10 The beam splitter 44 incident on the diffraction grating above and the scale 10 are symmetrically arranged on the scale 10 at the same incident angle θ (when θ is set so that θφ is set, φ is more stable). Right-angle prisms 40A and 40B for returning the 0th-order lights that have been incident and passed through the diffraction grating on the scale 10 in the same direction and re-enter the diffraction grating, and are re-incident from the right-angle prisms 40A and 40B. The beam splitters 46A and 4A that separate the first-order diffracted lights generated by the diffraction grating by the 0th-order reflected light while reflecting and separating the first-order diffracted lights, respectively.
6B and the reference signal Vr = (Vra + Vrb) obtained by photoelectrically converting the diffracted light separated by the beam splitters 46A and 46B.
And the light receiving element _22A 1, 22A ₂ to obtain ÷ 2,
And a half mirror 48 for remixing the diffracted light reflected by the beam splitters 46A and 46B. As described above, the right-angle prisms 40A and 40B are provided, and the 0th-order light transmitted through the diffraction grating is returned in the same direction and re-enters the diffraction grating, thereby refracting according to the flatness of the scale 10. Since the light is made incident again at the same angle, the influence of refraction is canceled out, and the diffraction angle φ
Are always equal. Therefore, the first-order diffracted light in a fixed direction can always be obtained regardless of the flatness of the scale surface or the pitching of the scale due to the poor alignment, and a stable interference signal can be obtained. In this embodiment, the diffracted light is reflected by the beam splitter 46.
After reflecting once using A and 46B, the half mirror 48
Because of the so-called multi-stage structure of bilateral symmetry, the first-order light is common to the diffracted light φ even if the wavelength λ of the laser diode 42 changes.
The incident directions on 2B and 22C are almost constant. Therefore, large but symmetrical fluctuations such as wavelength fluctuations of the light source and rolling and gear fluctuations on the scale 10 are also absorbed and are not affected by such fluctuations. Further, the reflected light on the surface of the scale 10 does not directly enter the light receiving element. In this embodiment, the right-angled prism is used as the means for reflecting the 0th-order light that has passed through the diffraction grating, but it is also possible to use a triangular prism such as a corner cube or a cassette eye. In this case, the diffraction points can be laterally displaced in the direction perpendicular to the paper surface of FIG. Although the laser diode 42 is used as the light source in the above embodiment, the type of the light source is not limited to this.

[Brief description of drawings]

FIG. 1 is a front view showing a configuration of an embodiment of a grating interference type displacement meter according to the present invention, FIG. 2 is an optical path diagram for explaining the principle of the present invention, and FIG. 3 is a conventional grating interference. FIG. 4 is a front view showing the configuration of an example of a die displacement meter, and FIG. 4 is a cross-sectional view showing a state in which a light beam is bent due to poor flatness in a conventional example. 10 ... Scale, 10A ... Diffraction grating, 14 ... Laser beam (light flux), 40, 40A, 40B ... Right angle prism, 42 ... Laser diode (LD), 44 ... Beam splitter, 48 ... Half mirror.

Claims (1)

[Claims] 1. A detection including a scale having a diffraction grating, a light source for irradiating the diffraction grating with a light beam, and a light receiving element for photoelectrically converting a mixed wave of a plurality of light beams generated by the diffraction grating. And a grating interferometric displacement meter that generates a detection signal that periodically changes according to relative displacement between the scale and the detector, in which the light beam from the light source is split and incident on the diffraction grating. An element and a plurality of reflecting means for returning the 0th-order light transmitted through the diffraction grating to the same direction and re-incident on the diffraction grating, and the 0th-order reflected light re-incident from the plurality of reflecting means. A grating interference type displacement meter characterized by mixing the first-order diffracted lights respectively generated by the diffraction grating.