JP2582272B2 - Linear encoder - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a linear encoder capable of eliminating the influence of a Fourier image caused by a change in the thickness of a grating scale or a distance between a diffraction grating and a reflecting system.

[Prior Art] As a conventional linear encoder, for example, Japanese Patent Application Laid-Open No. 60-19 / 1985
0812, which is configured as shown in FIG.

In FIG. 8, reference numeral 1 denotes a light source, 2 denotes a collimator lens, and 3 denotes a diffraction grating attached to a test object. 4
Numerals 4 and 4 'denote m-th order positive / negative diffracted light, 7 denotes a beam splitter, and 8 and 8' denote polarizing plates provided so that their deflecting directions are at 45 degrees to each other. Polarizing plates 6 and 6 'are provided for the optical paths of the light 4 and 4' reflected and diffracted at a predetermined angle by the diffraction grating 3 disposed orthogonal to the output optical path of the beam splitter 7 (the polarization directions are mutually different). Arranged orthogonally). Reference numerals 10 and 10 'denote corner cubes, which reflect the diffracted lights 4, 4' through the polarizers 6, 6 'so as to return to the same optical path. In the above configuration, the light emitted from the light source 1 becomes a parallel light beam by the collimator lens 2 and is reflected and diffracted by the diffraction grating 3. Reflected and diffracted light 4
And 4 'are reflected by the corner cubes 10 and 10' so as to pass through the polarizers 6 and 6 ', and irradiate the diffraction grating 3 again. Then, the light is again diffracted by the diffraction grating 3 and overlaps, and is split by the beam splitter 7. The split light passes through polarizers 8 and 8 ', respectively, and enters photodetectors 9 and 9'. The diffracted lights superposed on the polarizers 8 and 8 ′ interfere with each other, and change in brightness as the diffraction grating 3 moves. Also, the combination of the polarizers 6, 6 'and 8, 8' makes the output signals of the photodetectors 9 and 9 '90 deg.
A phase difference of ゜ is provided, and the moving direction of the diffraction grating 3 can be discriminated.

[Problems to be Solved by the Invention] However, in the linear encoder having the above configuration, the coherent light is obliquely incident on the diffraction grating as a parallel light beam, as shown in FIG. An image is generated at an interval R = P ² / λ (λ: light source wavelength, P: lattice constant). That is, Fourier image images having the same pitch as the grating pitch of the diffraction grating are generated at intervals R. Thus, when the distance from the incident point to the output point changes, the magnitude of the output amplitude changes according to the interval R as shown in FIG. In FIG. 3, when the distance from the diffraction grating 23 to the reflection system 24 changes, the change in the distance is such that the incidence part diffraction grating and the emission part diffraction grating move symmetrically in opposite directions because light is oblique. The same condition as the above occurs, and a bright signal and a dark signal are generated along the curve in FIG. 4, and the output signal fluctuates.

Conventionally, in order to prevent this fluctuation, as shown in FIG. 5, the diffraction grating 23 and the reflected light are positioned so as to be perpendicular to each other, and an optical system 36 such as a corner cube or a cat's eye is used instead of the reflector. Was. However, in such a method, the direction of the diffracted light can be selected only in the direction perpendicular to the lattice plane, and the apparatus becomes expensive due to the use of a cat's eye or the like.

The present invention has been made in view of the above-described prior art, and has been made to eliminate the influence of a Fourier image and to be able to be used even at an incident angle where the diffraction angle of irradiation light is not perpendicular to the diffraction grating. It is an object to provide a linear encoder.

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a diffraction grating having a grating formed at a predetermined interval on one surface of a glass substrate at right angles to the moving direction, and reflecting diffracted light on the other surface. And a grating scale provided with a reflecting plate that divides the coherent light into two directions whose deflection directions are orthogonal to each other. Each of the coherent lights is diffracted in the state of a focused light beam or a divergent light beam. A light irradiating means for transmitting and diffracting the light twice in a reciprocating manner and superimposing the diffraction light on the substantially same position on the surface of the diffraction grating; A light-collecting lens, and light detecting means for causing output light of the light-collecting lens to interfere with each other and photoelectrically converting the interference light. Non-perpendicular angle of incidence It is made to be incident.

[Operation] The incident light to the diffraction grating is not converted into a parallel light beam,
By giving the light beam in the state of a focused light beam or a divergent light beam, the grating pitch of the Fourier image image of the diffraction grating and the grating pitch of the diffraction grating are made inconsistent, and the output signal does not fluctuate.

[Embodiment] The present invention focuses on the point that the fluctuation of the output signal occurs when the incident light is given to the diffraction grating as a parallel light beam, and makes the incident light a narrowed light beam or a divergent light beam. Does not become a parallel light beam. Hereinafter, an embodiment of the present invention will be described with reference to the drawings, in the case where incident light is a divergent light beam. As shown in FIG. 1, a polarizing beam splitter 20, which divides incident light into transmitted light and reflected light according to the polarization direction of light, applies a laser beam to a lens system 21 and the lens system 21 in accordance with the optical path of the transmitted light. A laser diode 22 as a light source for irradiating light is provided. The reflecting mirrors 24a and 24b are used to reflect each of the laser beams output from the polarizing beam splitter 20 to substantially the same position on the surface of the diffraction grating 23.
It is arranged symmetrically to 20. The diffraction grating 23 is formed on one surface of the glass substrate 25, and a reflection plate 26 for reflecting the diffracted light from the diffraction grating 23 to the diffraction grating 23 is provided on the other surface of the glass substrate 25. The light from the reflection plate 26 is further diffracted by the diffraction grating 23 and then emitted in the vertical direction. A convex lens 27 is provided in the optical path, and a wave plate 28 and a beam splitter 29 are sequentially provided. On a straight optical path of the beam splitter 29, a polarizing plate 30 and a photodetector 31 are sequentially arranged, and on a reflected optical path of the beam splitter 29, a polarizing plate 32 and a photodetector 33 are similarly sequentially arranged. I have. Next, the operation in the above configuration will be described.

The laser light generated by the laser diode 22 is adjusted by the lens system 21 so as to be reflected by the reflecting plate 26 and then sufficiently diverged on the diffraction grating 23 to be incident on the polarization beam splitter 20.

The light split into the P component and the S component by the beam splitter 20 is emitted to each of the reflection mirrors 24a and 24b. Lights 34 and 35 reflected by these reflecting mirrors 24a and 24b
Are set to enter the diffraction grating 23 at a predetermined angle. That is, the incident angle α with respect to the diffraction grating 23 is obtained by the equation (1) or (2).

Sinβ = Sinα−nλ / P (1) Sinγ = Sinα−2nλ / P (2) (where, γ is an emission angle, n is a diffraction order, P is a lattice constant, γ
Is the laser light wavelength, and β is the diffraction angle. As shown in FIG. 2, the reflected light 34 entering the diffraction grating 23 is
The light undergoes diffraction twice when it enters the glass substrate 25 and when it is reflected by the reflection plate 26, passes through the glass substrate 25, and passes through the diffraction grating 23 again, and is emitted while diverging. The emitted laser light is converted into parallel light by the convex lens 27, and further converted into circularly polarized light by the wave plate 28 (for example, a λ / 4 wave plate).
This circularly polarized diffracted light is split by the beam splitter 29 into transmitted light and reflected light. The transmitted light reaches the photodetector 31 via the polarizing plate 30, and the reflected light reaches the photodetector 33 via the polarizing plate 32, and is photoelectrically converted. The polarizing plates 30 and 32 interfere with each other to make the phase difference (for example, 90 degrees) necessary for discriminating the moving direction of the diffraction grating 23 and signal processing by interfering with each other.

In the lens system 21 shown in FIG. 1, the light incident on the diffraction grating 23 is converted into a divergent light beam, and the Fourier image is diverged on the diffraction grating surface. Thus, the diffraction grating 23 and the grating pitch of the Fourier image image of the diffraction grating 23 are completely different from each other, and do not interact with each other. Therefore, even if the distance between the grating surface and the reflector 26 changes, the output signal does not fluctuate.

Further, the diffracted light can be taken in any direction with respect to the lattice plane, and the output fluctuation can be corrected in any direction. Furthermore, since a corner cube, a cat's eye, and the like are not required, the number of parts can be reduced, and reliability can be improved and maintenance adjustment can be simplified. It should be noted that, even if the light beam is narrowed down by the lens system 21 shown in FIG. 1, the diffraction grating 23 and the grating pitch of the Fourier image image are completely different as shown in FIG. it can. However, in this case, the focal point of the convex lens 27 needs to coincide with the focused point of the focused light beam. When a focused light beam is used, the convex lens 27 shown in FIG.
Instead, a concave lens 37 can be used as shown in FIG. In this case, the positional relationship of the lens system 21 is adjusted so that the left and right incident lights 34 and 35 are collected at the focal position of the concave lens 37. Therefore, also in this case, the grating pitch of the diffraction grating 23 and the grating pitch of the Fourier image image can be greatly different from each other as in FIG. Fluctuations are prevented.

[Effects of the Invention] As described above, according to the present invention, the incident light of the diffraction grating is prevented from becoming a parallel light beam. Therefore, the Fourier irrespective of changes in the thickness of the grating scale and the distance from the diffraction grating surface to the reflector. The output signal fluctuation caused by the image can be prevented, and the fluctuation of the output signal can be corrected at the incident angle where the diffraction angle of the irradiation light is not perpendicular to the diffraction grating. Further, since a cat's eye or the like is not required, the number of parts can be reduced and maintenance can be simplified.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a linear encoder to which the present invention is applied, FIG. 2 is a cross-sectional view showing details of a configuration around a diffraction grating in the linear encoder of FIG. 1, and FIG. 3 is light incident on the diffraction grating. FIG. 4 is an explanatory diagram of generation of a Fourier image when is a parallel light beam, FIG. 4 is an output signal amplitude characteristic diagram corresponding to a change in thickness of the glass substrate 25 when incident light is a parallel light beam, and FIG. FIGS. 6 and 7 are cross-sectional views showing the configuration of the periphery of another diffraction grating used in the linear encoder of FIG. FIG. 8 is a diagram showing a conventional linear encoder. 20, 29 ... Beam splitter, 21 ... Lens system, 22 ...
Laser diode 23 ... Diffraction grating, 24a, 24b ... Reflection mirror, 25 ... Glass substrate 26 ... Reflection plate, 27 ... Convex lens, 28 ... Wave plate, 30, 32
...... Polarizing plates 31, 33 ... Photodetector, 37 ... Concave lens

 ────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-59-163517 (JP, A) JP-A-60-190812 (JP, A) JP-A-57-191513 (JP, A)

Claims (1)

(57) [Claims] 1. A grating scale having a grating formed on one surface of a glass substrate at right angles to a moving direction at predetermined intervals, a grating scale having a reflecting plate for reflecting diffracted light on the other surface, and a deflector for deflecting coherent light. Each direction is divided into two directions orthogonal to each other, and each of the coherent light beams is transmitted and diffracted twice in a reciprocating manner by the diffraction grating in a state of a narrowed light beam or a divergent light beam. A light irradiating unit that is superimposed on the same position, a condensing lens that irradiates the light irradiated by the light irradiating unit through transmission diffraction by the diffraction grating to convert the emitted light into parallel light, and an output light of the condensing lens. Let me interfere
A linear encoder, comprising: light detection means for photoelectrically converting the interference light, wherein incident light from the light irradiation means is incident on the diffraction grating at an incident angle that is not perpendicular to the diffraction grating.