JPH02176420A - Lattice interference type displacement gage and its scale - Google Patents

Abstract

PURPOSE:To obtain a reflecting type scale readily and to make it possible to manufacture a reflection type lattice-interference displacement gage which is advantageous in assembling readily by providing a transmitting type scale on the rear surface of which a reflecting film is formed, and providing first and second reflecting means. CONSTITUTION:A reflecting film 50B is formed on the rear surface of a transmitting type scale having a diffraction grating 50A wherein density is changed periodically. Thus a reflecting type scale 50 is provided. A zero-order side rectangular prism 58 and a first-order side rectangular prism 62 are provided. A laser beam 54 which is emitted from a laser diode 52 and inputted into the scale 50 is partially diffracted and becomes a first-order diffracted light 60. The remaining part is reflected and become zero-order reflected light 56. The zero-order light and the first-order light are reflected with the prisms 58 and 62 and sent back to the same direction. Thus the inclination of the reflecting surface of the scale 50 is corrected, and a stable interference signal is obtained. The positions of the prisms 58 and 62 are set so that the light obtained by reflecting the diffracted light 60 with the scale 50 is overlapped with the light obtained by diffracting the reflected light 56 with the lattice 50A. Then, the interference which is resistant to the fluctuation in wavelength of a light source and is stable for the fluctuation of the scale surface is obtained.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a grating interference type displacement meter and its scale,
In particular, the present invention relates to a grating interference displacement meter and its scale that can configure a reflective grating interference displacement meter using a transmission scale on which a diffraction grating whose density periodically changes is formed. [Prior Art 1] Photoelectric encoders that generate periodic detection signals using a scale on which optical graduations are formed at a constant pitch are in widespread use. 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 signal after photoelectric conversion. Generally, optical gratings are manufactured by the etching method, so an optical grating of around 4μ is close to the upper limit of final measurement accuracy, and if electronic circuits are to be used without a significant increase in cost, the final The resolution is around 1 μm, and it has been difficult to further improve the accuracy. On the other hand, as photoelectric encoders become more widespread, there is a demand for encoders that can generate detection signals with higher resolution and higher accuracy. One of the ways to improve the resolution of photoelectric encoders is to use holography technology on the scale to form graduations with a fine pitch (usually about 1μ), and use the graduations as a diffraction grating to actively cause diffraction. A grating interferometric displacement meter has been proposed that obtains a detection signal by FIG. 3 shows a grating interference type displacement meter proposed in Japanese Patent Application Laid-Open No. 47-10034. This grating interference type displacement meter includes a transmission type scale in which a diffraction grating 10A with a pitch d is formed, a He-Ne laser light source 12 that irradiates the diffraction grating 10A with a laser beam 14 (wavelength λ) as a light beam, A mirror 16.1 that reflects the 0th-order transmitted light and the 1st-order diffracted light (luminous flux) generated by the diffraction grating 10A.
8, a beam splitter (coarse diffraction grid) 20
and light receiving elements 22A, 22B, and 22C that photoelectrically convert the mixed waves divided into three by the beam splitter 20, respectively. Here, each element except the scale constitutes a detector. In addition, in FIG. 3, the polarization directions of the polarizers 24 and 26 inserted into the optical paths of the 0th-order light and the 1st-order light are set to be perpendicular to each other, and the central mixed wave divided into three The light receiving element 22A that receives the light is designed to prevent interference fringes from occurring. Therefore, in this light-receiving element 22A, a simple sum signal is obtained instead of interference fringes, so this is referred to as a reference signal Vr. In addition, just before the light receiving element 22B, an analyzer 2 for generating interference fringes is installed.
8B is arranged, and an A-phase detection signal φA based on interference fringes is generated from this light receiving element 22B. Further, a 174-wavelength plate 30 and an analyzer 28C are arranged immediately before the light receiving element 22C, and from this light receiving element 22C, a B phase detection signal φ having a phase different by 90 degrees from the A phase detection signal φA is outputted from the light receiving element 22C.
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(sinθ+sinφ)−λ −−−−・−−−
(1) According to such a grating interference type displacement meter, for example, by manufacturing the diffraction grating 10A using a hologram method, an optical grating of 1 μm or less can be formed.
It is also possible to achieve a resolution of 0.01 μm. In particular, when using a transmission type scale 10 whose density changes sinusoidally as shown in FIG.
Since the scale can be easily produced by exposure by using the original plate, it is characterized by low cost. On the other hand, a reflection type grating interference displacement meter, in which both the light source and detection system are placed on one side of the reflection scale, can be used with a built-in scale such as a separate type, since it is only necessary to place the light source and detection system on one side of the scale. suitable for (Problem to be achieved by the invention 1) However, conventionally, since it is necessary to simultaneously and symmetrically generate +1st-order and -1st-order diffracted light from a single vertically incident light source, the scale 40 is As shown in Figure 5, it is formed by changing its height in a sinusoidal manner.Therefore, compared to the transmission type scale 10, it is difficult to manufacture, requires large-scale equipment, and is expensive. The present invention has been made to solve the above-mentioned conventional problems, and its first object is to provide a reflection-type grating interference type displacement meter that uses a transmission-type scale that is easy to produce. A second object of the present invention is to provide a reflective scale using a transmission scale.

[Means for Achieving the Object] The present invention provides a grating interference type displacement meter in which a diffraction grating whose density periodically changes is formed, a scale whose back surface reflects light, and a scale which reflects light on the back surface, and a light source that obliquely impinges a luminous flux onto the grating at an incident angle different from the diffraction angle; a first reflecting means for re-entering the light reflected by the scale onto the diffraction grating; A detector including a second reflecting means for input, and a light receiving element that charges and converts a mixed wave of the diffracted light of the zero-order reflected light and the reflected light of the first-order diffracted light generated by the diffraction grating and the scale. , the first problem is achieved by generating a detection signal that changes periodically according to the relative displacement between the scale and the detector. Furthermore, the present invention achieves the second object by forming a reflective film on the back surface of a transmission scale on which a diffraction grating whose density changes periodically is formed. [Operations and Effects] The present invention focuses on the fact that if a reflective scale can be obtained by utilizing a transmission scale that is easy to produce, a reflective grating interference displacement meter that is advantageous for integration can be easily produced. It was made by That is, if the back surface of a current transmission type scale on which a diffraction grating whose density periodically changes is formed into a reflective surface 50B by, for example, mirror vapor deposition, an apparently reflective scale 50 will be formed as shown in FIG. can be obtained. However, in this case, the density is constant in the height direction of the scale 50, so like the conventional reflective grating interference displacement meter,
Diffraction cannot occur if the incident light beam is perpendicularly incident. Therefore, as shown in FIG. 2, it is necessary to make the incident light beam obliquely incident, but in this case, only one first-order diffracted light is generated, and interference cannot be caused as it is. Therefore, in the present invention, as shown in FIG.
A second reflecting means (62) for making the (first-order) diffracted light by 0A enter the scale 50 again is provided. Thereby, it is possible to obtain two light beams, the diffracted light of the 0th-order reflected light and the reflected light of the 1st-order diffracted light, generated by the diffraction grating 50A and the scale 50, and by photoelectrically converting the mixed wave, a detection signal is generated. can be obtained. At this time, if the incident angle θ and the diffraction angle φ match, it becomes difficult to separate the incident light and the first-order diffracted light, so θ and φ are set to different angles. In the case of a reflective type that uses diffraction of reflected light, for example, when a small and inexpensive laser diode is used as a light source, there are problems not only with wavelength fluctuations, but also with respect to scale flatness and running accuracy. The required accuracy is stricter than that of the conventional one, and installation and adjustment must be carried out strictly. Therefore, it is desirable that these also be undetectable. This is because triangular prisms such as a right angle prism, a corner cube, or a cat's eye are used as the first and second reflecting means to return the light flux in the same direction, thereby correcting angular fluctuations in reflection and diffraction. By making the optical system more stable in this way, the influence of the flatness and running accuracy of the scale surface is reduced, and stable interference can be maintained to obtain more stable signals. Therefore, the tolerance value for the flatness of the used scale is improved. In addition, it is resistant to angular fluctuations in the mounting position and is stable with respect to scale running accuracy, so the alignment tolerance when mounting the detection system is improved and alignment can be simplified. There is no need to strictly design the shape and parallelism of the beam. Therefore, it is possible to realize a reflective grating interference displacement meter using a small light source such as a laser diode. Further, the reflective scale can be obtained by forming a reflective film on the surface of a transmission scale on which a diffraction grating whose density periodically changes is formed. Therefore, a reflection type scale can be obtained by using a transmission type scale on hand, and a reflection type grating interference type displacement meter which is advantageous for integration can be easily produced.

【Example】

Embodiments of the present invention will be described in detail below with reference to the drawings. As shown in FIG. 1, this embodiment uses a reflective scale 50 according to the present invention in which a diffraction grating whose density changes in a sinusoidal manner and a reflection 1150A formed on the back surface (lower surface in the figure); A laser diode (LD) 5 as a light source that makes a laser beam 54 obliquely enter the diffraction grating 50A.
2. A first beam for re-injecting the zero-order reflected light 56 of the laser beam 54 by the scale 50 into the diffraction grating 50A.
a zero-order right-angle prism 58 which is a reflection means, a first-order right-angle prism 62 which is a second reflection means for making the first-order diffracted light 60 by the diffraction grating 50A enter the scale 50 again, and polarizing plates 64 and 66. , a beam splitter similar to the conventional example for photoelectrically converting a mixed wave of the first-order diffracted light of the zero-order reflected light 56 and the zero-order reflected light of the first-order diffracted light 60, which are generated by the diffraction grating 50A and the scale 50. 2
0A. 20B (which was shared in the conventional example, but is divided into two in this embodiment), light receiving elements 22A and 22B, each of which is, for example, a PIN photodiode. It is equipped with 22C1 analyzers 28B, 28G, and a detector including a quarter-wave plate 30, and is configured to generate a detection signal that changes periodically according to the relative displacement between the scale 50 and the detector. . The reflective scale 50 includes, for example! It can be easily formed by mirror-depositing an anti-rI4 film on the back surface of an existing transmission scale on which a diffraction grating whose power changes in a sinusoidal manner is formed. The effects of the embodiment will be explained below. The laser beam 54 emitted from the laser diode 52 and incident on the scale 50 is partially diffracted and becomes first-order diffracted light 6.
0, and the remainder is reflected to become zero-order reflected light 56. By reflecting this 0th-order and 1st-order light by the right-angle prisms 58 and 62 and returning them in the same direction, the inclination of the reflecting surface of the scale 50 is corrected, and it is also possible to prevent poor flatness and running accuracy of the scale surface. Regardless, it is possible to obtain a stable interference signal. Also, since the first-order diffracted light 60 reflected by the scale 50 and the zero-order reflected light 56 diffracted by the diffraction grating 50A are always at the same angle, the right-angle prism 58 is used so that the two beams overlap. If the position is set at .62, stable interference can be obtained that is resistant to fluctuations in the wavelength of the light source and even to surface fluctuations of the scale. In this embodiment, right angle prisms 58 and 62 were used as the first and second reflecting means, but the structure of the reflecting means is not limited to this, and for example, a triangular prism such as a corner cube or a cat's eye may be used. It is also possible to use prisms. When using a triangular prism, the luminous flux is
The diffraction points 68A and 68B, which are separated in the longitudinal direction of the scale in the embodiment of FIG. 1, can be made to appear to be integrated in the longitudinal direction of the scale. Normally, the manufacturing error of the diffraction grating in the scale width direction is very small, so in this case, a stable interference signal can be obtained. In this example, even if the wavelength λ of the laser diode 42 changes, the direction of incidence on the light receiving element remains almost constant. Therefore, large but symmetrical fluctuations such as wavelength fluctuations of the light source and rolling and gap fluctuations on the scale are also absorbed and are not affected by such fluctuations. Further, the reflected light on the surface of the scale 50 does not directly enter the light receiving element. Further, in the embodiment described above, the laser diode 52 was used as a light source, but the type of light source is not limited to this. Furthermore, the configuration of the detection system also includes two beam splitters 2OA, 20B and three light receiving elements 22A, 228, 2.
It is not limited to those including 20.

[Brief explanation of the drawing]

FIG. 1 is an optical path diagram showing the 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 present invention in detail, and FIG. 3 is a conventional grating interference type displacement meter. An optical path diagram showing the configuration of an example of a mold displacement meter, FIG. 4 is a cross-sectional view showing the shape of a transmission type scale used in the conventional example, and FIG. 5 is a cross-sectional view showing the shape of a conventional reflective scale. It is a diagram. 50... Reflective scale, 50A... Diffraction grating, 50B... Reflective film, 52... Laser diode (LD) 54... Laser beam, θ... Incident angle, φ... times 56...0th order reflected light, 58...0th order right angle prism, 60...1st order diffracted light, 62...1st order right angle prism.

Claims (3)

[Claims] (1) A scale in which a diffraction grating whose density periodically changes is formed and the back surface reflects light, and a light source that obliquely impinges a luminous flux on the diffraction grating at an incident angle different from the diffraction angle. , a first reflecting means for making the light reflected by the scale enter the diffraction grating again; a second reflecting means for making the light diffracted by the diffraction grating enter the scale again; a detector including a light-receiving element that photoelectrically converts a mixed wave of the diffracted light of the zero-order reflected light and the reflected light of the first-order diffracted light generated by the A grating interferometric displacement meter that generates a detection signal that changes. (2) The grating interference type displacement meter according to claim 1, wherein the first and second reflecting means are rectangular prisms or triangular prisms. (3) A scale for a grating interference type displacement meter, characterized in that a reflective film is formed on the back surface of a transmission type scale on which a diffraction grating whose density changes periodically is formed.