JP2557967B2 - Grating interference displacement meter - Google Patents

Description

Detailed Description of the Invention

BACKGROUND OF THE INVENTION 1. Field of the Invention 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 if the scale surface has a poor surface degree.

[Prior art]

2. Description of the Related Art Photoelectric encoders that generate periodic detection signals using a scale having an optical scale with a constant pitch 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. One of the efforts to improve the resolution of the photoelectric encoder is to use a holographic technique to form a fine pitch (usually about 1 μm) scale on the scale, and use the scale 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. 7 shows a grating interference type displacement gauge proposed in Japanese Patent Laid-Open No. 47-10034. This grating interferometer type displacement meter includes a scale on which a diffraction grating 10A of pitch d is formed, a He-Ne laser light source 12 for irradiating the diffraction grating 10A with a laser beam 14 (wavelength λ) as a light beam, and the diffraction grating. Mirrors 16 and 18 that respectively reflect the 0th-order and 1st-order diffracted light (light flux) generated by 10A, and 1 that is reflected by the primary-side mirror 18
A beam splitter (coarse diffraction grating) 20 that divides a mixed wave of the 0th-order light of the next-order light and the 1st-order light of the 0th-order light reflected by the 0th-order side mirror 3 into three, and the beam splitter 20 It is configured to include light receiving elements 22A, 22B, 22C that photoelectrically convert the divided mixed waves. Here, each element other than the scale constitutes a detector. In FIG. 7, the polarization directions of the polarizers 24 and 26 respectively inserted in the optical paths of the 0th-order light and the 1st-order light are set so as to be orthogonal to each other, and the mixed wave in the center is divided into three. The light receiving element 22A that receives the light is designed not to 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. Immediately before the light receiving element 22B, an analyzer 28B for generating an interference fringe is formed.
Are arranged, and an A-phase detection signal φA based on interference fringes is generated from the light receiving element 22B. Immediately before the light receiving element 22C, a quarter wave plate 30 and an analyzer 28C
Is arranged, and a B-phase detection signal φB having a 90 ° phase difference from the A-phase detection signal φA is generated from the light receiving element 22C. Here, the incident angle θ of the laser beam 14 and the first-order diffraction angle φ 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, 0.01μ
It is also possible to achieve a resolution of m.

[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. As shown by the arrow A in the case of (defective), the light flux is bent due to the difference in the diffraction angle, and the light receiving elements 22B, 22C
The directions of travel of the two light beams incident on each other are inclined with respect to each other, and light and dark stripes due to interference occur on the light wavefront orthogonal to that direction, so that 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 kept below 5 μm / 100 mm.
There is a problem that a signal cannot be detected when the optical axis is tilted due to other factors. On the other hand, in a reflection type grating interference displacement meter in which both the light source and the detection system are arranged on one side of the reflective case, the light source and the detection system may be arranged on one side of the scale, so a built-in scale such as a separate type is used. Suitable for However, since the diffraction of the reflected light is used, the deviation of the optical path due to the inclination of the scale and the poor flatness is more severe than in the case of the transmissive type as shown in FIG. 7, and it is necessary to perform mounting and adjustment strictly. Yes, these tasks were difficult. The present invention has been made to solve the above-mentioned conventional problems, and is caused by poor flatness or inclination of the scale surface,
To provide a grating interference type displacement meter capable of obtaining a stable signal by reducing the influence of a small but large change and thus simplifying the alignment when the detection system is attached. Is an issue.

[Means for achieving the object]

The present invention provides a scale on which a diffraction grating is formed, and a light source for irradiating the diffraction grating with a collimated light beam.
And an optical system including a light receiving element that photoelectrically converts the diffracted light of the same number order that forms a bilaterally symmetric optical path by the diffraction grating and photoelectrically converts the light. Accordingly, in a grating interference type displacement meter that generates a detection signal that changes periodically, the diffracted light of the same order that forms a symmetrical optical path generated by the diffraction grating is focused on the surface of the diffraction grating. The above object is achieved by providing a convex lens or a concave mirror having the above-mentioned means with each of which forms a parallel light beam before interference.

[Action and effect]

When the refraction angle (transmissive type) and diffraction angle (reflective type) differ due to the flatness of the scale surface or the inclination of the scale, the two light beams entering the light receiving elements 22B and 22C travel The directions were inclined with respect to each other, and the light wavefronts orthogonal to the directions were combined into stripes, and uniform light interference could not 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, a signal may not be detected when the relative traveling directions of both beams are tilted, which causes a measurement error. I got it. According to the present invention, as shown in FIG. 2, for example, by arranging the convex lens 40 so that the surface (diffraction surface) of the diffraction grating is focused, a plurality of luminous fluxes generated by the diffraction grating are generated by the half mirror 50. Since they were made to proceed in parallel to the respective designed traveling directions before mixing, stable interference signals could be obtained even when the flatness of the scale was 15 μm / 100 mm or more. Further, due to other factors, a stable signal can be obtained even if the optical axis is tilted. Further, a concave mirror may be used to realize such a state. In this way, the light flux after passing through the convex lens or the concave mirror can always be advanced in the direction parallel to the designed optical axis regardless of the tolerance of the scale flatness and the inclination of the scale, that is, the passage of the half mirror 50. , Or, after reflection, they travel in parallel optical paths, so an inexpensive scale can be used, and the tolerance of detection alignment is improved,
The alignment can be simple. Furthermore,
It is not necessary to strictly design the shape and parallelism of the light source beam. In particular, in the case of a reflection type grating interference type displacement meter, where the deviation of the optical path due to the tilt of the scale etc. is more severe than that of the transmission type, and rigorous mounting and adjustment are required, the mounting and adjustment become easier, and the effect However, it is possible to realize a reflection type grating interference displacement meter using a small light source such as a laser diode.

【Example】

Embodiments of the present invention will be described in detail below with reference to the drawings. The first embodiment of the present invention is, as shown in FIG. 1, a transmission type scale 10 in which a diffraction grating is formed, similar to the conventional one.
And a laser diode (LD) 42 that forms a collimated parallel light flux as a light source, and a light receiving element 22A ₁ , 22A ₂ , 22B, 22C, which is a PIN photodiode, and an analyzer.
28B, 28C, comprising a detector including a 1/4 wavelength plate 30, in a transmission type grating interference displacement meter that generates a detection signal that periodically changes according to the relative displacement of the scale 10 and the detector, A P / S splitter 44 that divides the laser beam 14 from the laser diode 42 into two according to the deflection direction,
While reflecting only the first-order diffracted light, a pair of mirrors 46A and 46B for making the divided light flux symmetrically enter the same diffraction point C of the diffraction grating on the scale 10 at the same incident angle θ, respectively. Reference signals Vr = (Vr _a + Vr _b ) / 2 obtained by photoelectrically converting the beam splitters 48A and 48B provided to be decomposed and the diffracted light decomposed by the beam splitters 48A and 48B.
The light receiving elements 22A ₁ and 22A ₂ for obtaining the same, and a half mirror 50 for mixing and interfering the diffracted light reflected by the beam splitters 48A and 48B again, the half mirror 50 and the beam splitter. A convex lens 52 having a diffraction surface of the scale 10 as a focus, which is arranged between 48A and 48B.
It is equipped with A and 52B. Thus, before the two light beams diffracted by the diffraction grating are mixed by the half mirror 50 and interfered with each other, the convex lens 52
By passing through A and 52B, the convex lens 52A and 52B correct the curved optical axis to affect the flatness of the scale surface, the pitching of the scale, and small but asymmetrical fluctuations. It is possible to make it hard to receive. In this embodiment, the diffracted light is reflected by the beam splitter 48.
Since the light is reflected once by using A and 48B, and then mixed by the half mirror 50, a so-called left-right symmetrical so-called multi-stage structure is adopted. Therefore, even if the wavelength λ of the laser diode 42 changes, the diffraction angle φ of the primary light is changed. Since they are common, the direction of incidence on each of the light receiving elements 22B and 22C is substantially constant. Therefore, large symmetrical fluctuations such as wavelength fluctuations of the light source and rolling and gear fluctuations on the scale 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 the present embodiment, the convex lenses 52A, 52B were arranged between the beam splitters 48A, 48B and the half mirror 50, but the arrangement positions of the convex lenses 52A, 52B are not limited to this, for example, the scale 10 It is also possible to dispose between the beam splitter 48A and the beam splitter 48B. The point is that before the light fluxes generated by the diffraction grating are mixed and interfered with each other, and if there is a focus on the diffraction surface of the scale, it may be anywhere. Next, a second embodiment of the present invention will be described in detail with reference to FIG. The second embodiment is a multi-stage transmission grating interferometric displacement gauge similar to the first embodiment, as shown in FIG.
Instead of the convex lenses 52A and 52B, concave mirrors 54A and 54B are arranged at the positions of the beam splitters 48A and 48B. In the second embodiment, the reference signal Vr needs to be separately obtained by using a beam splitter (not shown) or the like arranged somewhere else. In addition, in the said 1st and 2nd Example, this invention was applied to the transmission-type grating | lattice interference type displacement meter containing the transmission-type scale 10, but the application range of this invention is not limited to this. For example, the present invention can be similarly applied to a reflection type grating interference type displacement meter including a reflection type scale 60 as shown in FIG. The third and fourth embodiments of the present invention, which are applied to the reflection type grating interference type displacement meter, will be described below. The third embodiment of the present invention is, as shown in FIG. 4, a beam splitter 48A, 48B in a reflection type grating interference type displacement meter.
The convex lenses 52A and 52B are arranged between the polarizing plates 24 and 26 so that the diffraction point C of the scale 60 is focused. Further, the luminous flux 14 from the laser diode 45 which is the light source is
As shown in FIG. 5, the scale 60 is obliquely incident, and the reflected light from the scale 60 is prevented from returning to the laser diode 42, and the return light automatically outputs the laser diode 42. The control (APC control) is prevented from going wrong. Also in this embodiment, since the optical system is symmetrical,
It is highly resistant to symmetric optical path variations such as wavelength variations, and can absorb the effects of wavelength variations of the laser diode 42. Since the other points are the same as those of the first embodiment, the description thereof will be omitted. Next, a fourth embodiment of the present invention will be described in detail with reference to FIG. The fourth embodiment is similar to the third embodiment in the reflection type grating interferometric displacement gauge, but instead of the convex lenses 52A and 52B, concave mirrors 54A and 54B are provided at the positions of the beam splitters 48A and 48B.
Is arranged. Since the other points are the same as those of the third or second embodiment, the description thereof will be omitted. Although the laser diode 42 is used as the light source in each of the embodiments, the type of light source is not limited to this.

[Brief description of drawings]

FIG. 1 is a front view showing the configuration of a first 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. 4 is a front view showing the configuration of a second embodiment of the present invention, FIG. 4 is a front view showing the configuration of a third embodiment of the present invention, FIG. 5 is a side view of the same, and FIG. 6 is a fourth view of the present invention. FIG. 7 is a front view showing the configuration of the embodiment, FIG. 7 is a front view showing the configuration of an example of a conventional grating interference type displacement meter, and FIG. 8 is a view showing a state in which a light beam is bent due to poor flatness in the conventional example. It is sectional drawing shown. 10, 60 ... Scale, 10A ... Diffraction grating, 14 ... Laser beam (light flux), 40, 52A, 52B ... Convex lens, 42 ... Laser diode (LD), 48A, 48B ... Beam splitter, 50 ...... Half mirror, 54A, 54B ...... Concave mirror.

 ─────────────────────────────────────────────────── --- Continued from the front page (56) References JP-A-63-309817 (JP, A) JP-A-59-163517 (JP, A) JP-A-58-191906 (JP, A) JP-A-60- 190812 (JP, A)

Claims (1)

(57) [Claims] 1. A scale having a diffraction grating formed thereon, a light source for irradiating the diffraction grating with a collimated parallel light flux, and a scale generated by the diffraction grating.
An optical system including a light-receiving element that photoelectrically converts the diffracted light of the same order that forms a symmetrical optical path by interfering with each other, and generates a detection signal that periodically changes according to the relative displacement of the scale and the detected product. In the grating interferometric displacement meter, the diffracted light of the same order that forms a symmetrical optical path generated by the diffraction grating is collimated by a convex lens or a concave mirror having a focal point on the surface of the diffraction grating before the interference. A grating interferometer type displacement meter, characterized in that it is provided with means for making a light beam.