JPH01295116A - Correcting method for variation in reflected light quantity of linear encoder - Google Patents

Abstract

PURPOSE:To prevent an output signal from varying even if the thickness of a grating scale and the distance to a reflecting surface vary by irradiating the surface of a diffraction grating which crosses a moving direction at right angles with luminous flux generated by diverging coherent light split by a beam splitter. CONSTITUTION:The laser light from a laser light source 22 is diverged by a lens 21 and split into two by the beam splitter 20. Its reflected light and straight traveling light are reflected by reflecting mirrors 24a and 24b to get pieces 34 and 35 of diverged luminous flux, the diffraction grating 23 on a glass substrate 25 is illuminated by said luminous flux. The pieces 34 and 35 of luminous flux are diffracted, reflected by the reflecting surface 26 of the glass plate 25, and diffracted at the same position of the diffraction grating 23 to get parallel light through a convex lens 27 then circular polarized light through a wavelength plate 28. This circular polarized light goes to straight traveling light and reflected light through a beam splitter 29 and they pass through polarizing plates 30 and 32 and are converted 31 and 33 photoelectrically. The polarizing plates 30 and 32 make the mutual diffracted light beams interfere with each other and the moving direction of the diffraction grating 23 is discriminated to generate a phase difference required for signal processing. Consequently, even if the distance between the surface of the diffraction grating and reflecting surface varies, the output signal is prevented from varying.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a reflection method in a linear encoder that can eliminate the influence of Fourier images caused by changes in the thickness of the grating scale or the distance between the diffraction grating and the reflection system. This invention relates to a method of correcting light intensity fluctuations.

[Prior art] For example, as a conventional linear encoder,
90812, and is configured as shown in FIG.

In Fig. 8, 1 is a light source, 2 is a collimator lens,
3 is a diffraction grating attached to the object to be measured. 4 and 4' are m-order positive and negative diffracted lights, 7 is beam splitter 1
8 and 8' are polarizing plates provided so that the polarization directions are at 45 degrees to each other. Polarizing plates 6 and 6' are arranged for the optical paths of the lights 4 and 4' which are reflected and diffracted at a predetermined angle by the diffraction grating 3 arranged orthogonally to the output optical path of the beam splitter 7 (the polarization direction is (arranged perpendicularly to each other). Corner cubes 10 and 10' reflect the diffracted light 4.4' from the polarizing plate 6.6' so that it is returned to the same optical path. In the above configuration, the light emitted from the light source 1 becomes a parallel beam of light by the collimator lens 2, and is reflected and diffracted by the diffraction grating 3. The reflected and diffracted lights 4 and 4' pass through the polarizing plates 6 and 6', are reflected by the corner cubes 10 and 10', and reirradiate the diffraction grating 3. Then, the beams are diffracted again by the diffraction grating 3, overlap each other, and are split by the beam splitter 7. The divided lights pass through polarizing plates 8 and 8', respectively, and are sent to a photodetector 9.
and 9'. The overlapping diffracted lights by the polarizing plates 8 and 8' interfere with each other, causing changes in brightness and darkness as the diffraction grating 3 moves.

Also, by the combination of polarizing plates 6.6' and 8.8',
A phase difference of 90° is provided between the output signals of the photodetectors 9 and 9', and the direction of movement of the diffraction grating 3 can be discriminated.

[Problems to be Solved by the Invention] However, in the linear encoder having the above configuration, since the coherent light is parallel and obliquely enters the diffraction grating, as shown in FIG. Rie image is R-
occurs at a pitch R of P2/λ (λ: light source wavelength, P: grating constant) (i.e., an image identical to the grating exists at a pitch R
). On the other hand, when the distance from the input point to the output point changes, the magnitude of the output amplitude changes according to the pitch R as shown in FIG. In the tjS3 diagram, the reflection system 2 from the diffraction grating 23
When the distance to 4 changes, since the incident light is oblique, the same situation occurs as the entrance diffraction grating and the exit diffraction grating relatively move in opposite directions, as shown in Figure 4. Bright and dark signals are generated along the curve, causing variations in the amount of reflected light.

Conventionally, in order to prevent this fluctuation, as shown in FIG. 5, the diffraction grating 23 and the reflected light are positioned so that they are perpendicular to each other, and an optical system 36 such as a corner cube or cat's eye is used instead of a reflector. Ta.

However, in the darning method, the direction of the diffracted light can only be selected perpendicular to the lattice plane, and the apparatus is expensive because it uses a cat's eye or the like.

The present invention has been made in view of the above-mentioned state of the prior art, and it eliminates the influence of the 7-lier image and allows use even at an incident angle where the diffraction angle of the irradiated light is not perpendicular to the diffraction grating. The present invention aims to provide a method for correcting variations in the amount of reflected light in a linear encoder.

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a diffraction grating formed by forming gratings at predetermined intervals on the surface of a moving object perpendicular to the moving direction, and a coherent light beam. light irradiation means that divides the light into two directions and causes each of the coherent lights to overlap and interfere at the same position on the surface of the diffraction grating; and interference light that the light irradiated by the means is diffracted by the diffraction grating and emitted. In the linear encoder, the linear encoder includes a condensing lens that converts the light into parallel light, and a light detection means that photoelectrically converts interference light based on the output light of the lens, when the light irradiated by the light irradiation means is not a parallel light flux. It is designed to irradiate the surface of the diffraction grating.

[Operation] By giving the incident light to the diffraction grating not as a parallel beam but in the form of a patterned beam or a diverging beam, the pitch of the 7-lie image of the diffraction grating and the beam angle of the diffraction grating can be changed. There is no possibility of causing a mismatch with the output signal and causing fluctuations in the output signal.

[Embodiment] The present invention focuses on the fact that fluctuations in the output signal occur when incident light is applied to a diffraction grating in the form of a parallel beam, and the present invention is designed so that the incident light becomes a shaped beam or a diverging beam. be. DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings in the case where the incident light is a divergent luminous flux. As shown in FIG. 1, for a polarizing beam splitter 20 that splits incident light into transmitted light and reflected light according to the polarization direction of the light, a lens system 21 and a lens system 2
A laser diode 22 is provided as a light source for irradiating laser light to the laser diode 1 . Reflection mirrors 24a, 241 to reflect each of the laser beams output from the polarizing beam splitter 20 to the same position on the surface of the diffraction grating 23]
are arranged symmetrically with respect to the polarizing beam splitter 20. The diffraction grating 23 is formed on one side of a glass substrate 25, and a reflection plate 26 is provided on the other side of the glass substrate 25 to reflect the 10 - diffracted light from the diffraction grating 23 side to the diffraction grating 23 side. The light from the reflection plate 26 is further diffracted by the diffraction grating 23 and then emitted in the vertical direction, but a convex lens 27 is disposed in its forward path, and a wavelength plate 28 and a beam splitter 29 are further disposed in this order. There is. A polarizing plate 30 and a photodetector 31 are sequentially arranged on the straight optical path of the beam splitter 29, and the beam splitter/ter 29
Similarly, a polarizing plate 32 and a photodetector 33 are placed on the reflected optical path of 9.
are arranged in sequence. Next, the operation in the above configuration will be explained.

The laser beam generated by the laser diode 22 is reflected by a reflecting mirror 26 by a lens system 21, and is adjusted to be sufficiently divergent on a diffraction grating 23, and then enters a polarizing beam splitter 20.

The light separated by the beam splinter 20 into a P component and an S component is reflected by reflection mirrors 24a and 24. It is emitted to each of b. These reflecting mirrors 24a and 24. The reflected lights 34 and 35 from b are set to be incident on the diffraction grating 23 at a predetermined angle. Well,
The angle of incidence α with respect to the diffraction grating 23 is determined by equation (1) or equation (2).

Sin β = Sin α −nλ /P ・・・・
... (1) Sinγ=Sinα-2n^/P...
...(2) (In addition, γ is the output angle, n is the diffraction order, P is the lattice constant, input is the laser beam wavelength, and β is the diffraction angle.) As shown in FIG. 2, the light enters the diffraction grating 23. The reflected light 34 is
The light is diffracted twice: when it enters the glass substrate 251, 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 beam is made into parallel light by a convex lens 271, and further made into circularly polarized light by a wavelength plate 28 (for example, a λ/4 wavelength plate). At this time, the convex lens 27 is arranged so that its focal point coincides with the focal point of the lens system 21.

This circularly polarized diffracted light is split by a beam splitter 29 into passing 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's diffracted lights to create a phase difference (for example, 9
0 degrees).

The lens system 21 shown in FIG. 1 causes the light incident on the diffraction grating 23 to become a diverging beam of light, so that the 7-lier image is magnified on the surface of the diffraction grating. As a result, the pitches of the diffraction grating 23 and the 7-lier image of the diffraction grating 23 are completely different and do not interact with each other. Therefore, even if the distance between the grating surface and the reflection plate 26 changes, the output signal does not change.

Further, the diffracted light can be taken in any direction with respect to the grating plane, and output fluctuations can be corrected in any direction. Furthermore, since corner cubes, cat's eyes, etc. are not required, the number of parts can be reduced, reliability can be improved, and maintenance and adjustment can be simplified. Furthermore, by filtering the light beam with the lens system 21 shown in FIG. 1, the pinch of the diffraction grating 23 and the 7-lier image become completely different as shown in FIG. 6, and it is possible to correct output fluctuations. I can do it. However, in this case, the focal point of the convex lens 27 needs to coincide with the condensing point of the reflected light beam. In addition, when using a shaped light beam, the convex lens 27 shown in FIG.
Instead, a concave lens 37 may 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 35 and 35' are focused on the focal position of the concave lens 37. Therefore, in this case as well, as in FIG. Fluctuations are prevented.

[Effects of the Invention] As explained above, according to the present invention, since the incident light on the diffraction grating is prevented from becoming a parallel beam, the 7 - Prevent fluctuations in the output signal due to ray images, and the diffraction angle of the illuminating light is not perpendicular to the diffraction grating. Correction of output fluctuations can be performed at the angle of incidence.

Furthermore, since a cat's eye or the like is not required, the number of parts can be reduced and maintenance can be simplified.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of a linear encoder to which the present invention is applied, FIG. 2 is a sectional view showing details of the configuration of the surrounding area of the diffraction grating in the linear encoder of FIG. 1, and FIG. 3 is a diagram showing incident light on the diffraction grating Fig. 4 is an output signal amplitude characteristic diagram corresponding to the thickness change of the glass substrate 25 when the incident light is a parallel light flux, and Fig. 5 is a conventional diagram. FIGS. 6 and 7 are cross-sectional views showing the structure of other diffraction grating peripheral parts used in the linear encoder of FIG. 1. FIGS. 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...Reflector plate 27...Convex lens 28...Wave plate 30.32...Polarizing plate
31.33... Photodetector 37... Concave lens Patent applicant Tokyo Seimitsu Co., Ltd. Figure 6 Procedural Amendment (Voluntary) 1. Indication of the case 1985 Patent Application No. 327817 2 Title of the invention Method for correcting reflected light amount variation in a linear encoder 3 Person making the correction Relationship to the case Patent applicant address 7-1 Shimotatsujaku 9-chome, Mitaka City, Tokyo No. ``Spontaneous'' 5, Number of inventions increased by amendment 6, ``Detailed explanation of the invention'' column 7 of the specification subject to the amendment, Contents of the amendment) ``Upstream'' on page 2, line 18 of the specification (2) ``to pass'' and (2) ``to do'' on page 3, line 3 of the specification.
and correct it. (3) Delete "for incidence" and "1- in the opening of FIG. 3" from lines 17 to 18 on page 3 of the specification. (4) In the 7th line of page 6 of the specification, add ``to prevent the incident light from becoming a parallel beam of light'' between ``to make it happen'' and ``tachi no deru.''. (5) “Lens system 2” on page 7, lines 13 to 16 of the specification
By 1・・・・・・・・・・・・・・・・・・
It is incident on... ” is corrected as follows. After being reflected by 26 reflecting plates by lens system 21, 2
The beam is adjusted to sufficiently diverge on the diffraction grating No. 3, and then enters the deflection beam splinter 20. ” (6) No. 800, lines 15 to 16 of the specification “At this time,
・・・・・・・・・・・・・・・・・・・・・・・・
It is arranged like... ” to be deleted. (7) "1 enlargement" on page 9, line 7 of the specification is corrected to "divergence." (8) "Left and right incident lights 35.35' are focused" in lines 7 and 8 of page 10 of the specification is corrected as follows. [i; 6/7'I X 1#honor 2 l q ζ
1 (9) Delete "." between "not perpendicular" and "at the angle of incidence" from page 10, line 2O to page 11, line ttiJ1 of the specification. Written amendment to the above procedure (method) 1. Indication of the case Patent Application No. 327817 of 1988 2
, Title of the invention: Method for correcting reflected light intensity fluctuations in linear encoders 3, Relationship with the person making the correction: Patent applicant address: 9-7-1 Shimorenjaku, Mitaka City, Tokyo, March 1988
March 29th (shipping mortar) 5. Drawing subject to correction 6. Contents of correction The characters in Fig. 4 are enlarged (no changes to the contents). Appropriate drawings are attached separately.

Claims (1)

[Claims] A diffraction grating is constructed by forming gratings at predetermined intervals on the surface of a moving object perpendicular to the direction of movement; A light irradiation means for overlapping the position and making it interfere; a condensing lens that converts the interference light emitted by the light emitted by the means into parallel light after being diffracted by the diffraction grating; and a photoconductor for converting the interference light based on the output light of the lens into a parallel light. A method for correcting a variation in reflected light amount of a linear encoder comprising a light detecting means for converting the light, in which the diffraction angle of the light irradiated by the light irradiation means can be incident even at an incident angle that is not perpendicular to the diffraction grating.