JP2009069033A - Photoelectric incremental type encoder - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve high speed, low cost and compactness by enabling detection with a light receiving element array with a long pitch similarly as in the past even for a narrower scale lattice pitch. <P>SOLUTION: In a three-lattice type photoelectric incremental type encoder having a first lattice for modulating light from a light source 10, a second lattice 22 formed on a (main) scale 20, and a third lattice onto which interference fringes formed by the first and second lattices are projected, first and third common lattices 32, 33 which work for both the first and third lattices are provided to an index (scale) 30, and moire fringes 34 produced on the first and third common lattices 32, 33 are detected by a lens optical system 40. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a photoelectric incremental encoder that generates and detects moiré fringes on a third grid surface of a three-grid type. In particular, even if the grid pitch of a scale is narrow, it is as long as the conventional one. The present invention relates to a photoelectric incremental encoder that can be detected by a light-receiving element having a pitch and can realize high speed, low price, and downsizing.

  For example, as shown in FIG. 5 of Patent Document 1, a first grating for modulating light from a light source, a second grating formed on a main scale (simply referred to as a scale), the first, first A light receiving element array in which a third grating is integrated, on which interference fringes formed by two gratings are projected, and an optical grating (second grating) arranged on a scale is formed by the three grating principle and the light receiving element array. There is a way to detect.

  In the case of a reflective encoder using the three-grid principle, as shown in FIG. 6 of Patent Document 1, the pitch of the light receiving element array is determined in accordance with the grating pitch of the detected scale.

  Here, if the lattice pitch of the scale to be detected is narrowed, it cannot be detected unless the pitch of the light receiving element array is also narrowed. For example, if the grating pitch is 1 μm, the pitch of the light receiving element array must be 0.25 μm (when four phases are detected).

Japanese Patent Laying-Open No. 2005-164515 (FIGS. 5 and 6)

  However, narrowing the pitch of the light receiving element array (photodiode structure) that can be detected as a semiconductor device is technically limited, leading to an increase in cost. Even if it can be realized, the peripheral length of the pattern of each photodiode is increased, and the junction capacitance formed at the pattern boundary is increased, which makes it difficult to increase the detection speed. Further, when light is incident obliquely, there is a problem that the size of the detection head is increased.

  The present invention has been made to solve the above-mentioned conventional problems, and even if the grating pitch of the scale is narrowed, it can be detected by a light receiving element (array) having the same long pitch as the conventional one, and the speed and cost are reduced. -The issue is to provide technology that can achieve miniaturization.

  The present invention includes a first grating for modulating light from a light source, a second grating formed on a scale, and a third grating on which interference fringes formed by the first and second gratings are projected. In the three-grid photoelectric incremental encoder having the first and third combined gratings having the functions of the first and third gratings on an index scale (simply referred to as an index), the first and third gratings are provided. The above-mentioned problem is solved by detecting the moire fringes generated in the dual-purpose grating with a lens optical system.

  Here, the light from the light source is branched by a beam splitter, irradiated perpendicularly to the scale surface via the first and third combined gratings, and reflected on the first and third combined gratings by the reflected light from the scale surface. The generated moire fringes can be detected by the lens optical system.

  The pitch of the light receiving element array can be made larger than the pitch of the first and third combined gratings and the second grating.

  The magnification of the lens optical system can be set so that the period of the moire fringes matches the pitch of the light receiving element array.

  Further, the light from the light source can be incident from the lateral direction with respect to the scale moving direction.

  The numerical aperture of the lens optical system can be set so that only the moire fringes can be detected without detecting the patterns of the first and third combined gratings.

  The lens optical system may be a telecentric optical system in which an aperture is disposed at a projection lens and a focal position thereof.

  Further, the aperture diameter of the telecentric optical system can be set to a cutoff frequency that does not resolve the pattern of the first and third combined gratings.

  Further, the first and third combined gratings can be formed on the scale side surface of the index.

  The origin pattern can be formed in parallel with the first and third combined gratings.

  According to the present invention, moire fringes are generated on the first and third combined grating surfaces of the index, and this is detected by the lens optical system, so that it can be detected by the light receiving element (array) having a long pitch. . Therefore, it is possible to observe a fine cycle without reducing the pitch of the light receiving elements (array). Therefore, a light receiving element having a wide pitch similar to the conventional one can be used, and it is possible to reduce the price and increase the speed by reducing the junction capacitance.

  In addition, since the optical axis is inclined in the case of the oblique incidence configuration, it is necessary to dispose the optical components obliquely. However, by making the vertical incidence, the difficulty of designing and aligning the lens optical system is reduced. be able to. Further, the detection head can be reduced in size by reducing the size of the detection head.

  Here, by defining the numerical aperture NA of the lens optical system, it is possible to detect only the moire fringes without distortion without detecting the patterns of the first and third combined gratings.

  Also, by setting the aperture diameter of the telecentric optical system to a cutoff frequency that does not resolve the pattern of the first and third combined gratings, it is possible to improve contrast and detect only moire fringes without distortion. It becomes possible.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  The first embodiment of the present invention is formed on a scale 20 and a first grating for modulating light from the light source 10 as shown in FIG. 1 (perspective view) and FIG. 2 (sectional view showing an optical path). In the three-grating photoelectric incremental encoder having the second grating 22 and the third grating on which the interference fringes formed by the first and second gratings are projected, the functions of the first grating and the third grating are combined. The first and third combined gratings 32 are provided on the index 30, and the light from the light source 10 is branched by the beam splitter 36 disposed on the index 30, and is perpendicular to the scale surface via the first and third combined gratings 32. And a moire fringe 34 (see FIG. 4) generated on the first and third combined gratings 32 by the reflected light from the scale surface, using a lens optical system 40 composed of a projection lens 42, a light receiving element array. To 50 It is obtained so as to shadow.

  The light source 10, the index 30, the beam splitter 36, the lens optical system 40, and the light receiving element array 50 are disposed in a detector (not shown) and are movable relative to the scale 20 in the measurement direction.

  Here, the first and third combined gratings 32 formed on the index 30 have a film structure that does not reflect light from the upper surface, for example, a Cr film 32a as shown in FIG. 3, and an antireflection film 32b made of, for example, CrO. The two-layer film. The configuration of the first / third combined grating 32 is not limited to this.

Assuming that the period of the moire fringes 34 generated is P _m and the period of the second grating 22 is P ₂ , the period P ₁₃ of the first and third combined gratings 32 on the index 30 is expressed by the following equation.

_{P 13 = {P 2 + (} P 2 P m) / (P m -P 2)} / 2 ... (1)

Therefore, as shown in FIG. 4, for example, if the period P _{13 of} the first and third combined gratings 32 is 1.005 μm and the period P ₂ of the second grating 22 is 1 μm, the period P _m of the moire fringes 34 is 101 μm. The moire fringes P _m having a period 101 times the period P ₂ of the second grating 22 can be observed with a relatively simple optical system.

Here, the magnification of the lens optical system 40 is set so that the pitch of the light receiving element array 50 matches the period P _{m of the} moire fringes 34. Specifically, the pitch of the light receiving element array 50 matches the cycle of the projection image 46 (see FIG. 4) that passes through the projection lens 42 and is generated on the light receiving element array 50.

  Here, as in the second embodiment shown in FIG. 4, when a telecentric optical system including the projection lens 42 and the aperture 44 disposed at the focal position is used as the lens optical system 40, the light receiving element array 50 is obtained. The degree of freedom of the air gap of the projection lens 42 can be set high. Furthermore, the contrast can be improved by setting the diameter of the aperture 44 of the telecentric optical system to a cutoff frequency that does not resolve the pattern of the first and third combined gratings 32.

More specifically, in order to cut short pitch fringes than half the moiré fringe period P _m, to set the numerical aperture NA of an optical system as in the following equation.

    NA = f × λ (2)

  Here, f is the spatial frequency of the moire fringes 34, and λ is the wavelength of light.

  Assuming that the focal length of the projection lens 42 is L and the aperture diameter of the aperture 44 is D, the following relationship is established.

    D = 2 × L × NA (3)

  As a result, the lens optical system 40 does not resolve the pattern of the first and third combined gratings 32, so that a waveform without distortion can be detected.

  Next, a third embodiment of the present invention is shown in FIG.

  In the present embodiment, the first and third combined grids 33 are formed on the side surfaces of the scale 20 of the index 30 in the same configuration as the first embodiment.

  The first and third combined gratings 33 have a film structure that does not reflect light from the upper and lower surfaces, for example, a three-layer film of a Cr film 33a and an antireflection film 33b made of, for example, CrO as shown in FIG. Yes. As a result, stray light (such as a light beam indicated by a dotted arrow in the figure) that directly enters the light receiving element array 50 from the index 30 can be prevented. The configuration of the first and third combined gratings 33 is not limited to this.

  Since the other points are the same as those in the first embodiment, description thereof is omitted.

  According to this embodiment, the interstitial gap G between the scale 20 and the first and third combined gratings 33 can be reduced.

  A fourth embodiment using this point is shown in FIG.

  In the present embodiment, origin patterns 24 and 38 are provided on the scale 20 and the index 30 in parallel with the second grid 22 and the first and third combined grids 33, respectively, in the same configuration as the third embodiment. A light receiving element 54 for detecting the origin is provided on the light receiving substrate 52 in parallel with the light receiving element array 50. A light receiving device (IC) in which the light receiving element array and the light source for detecting the origin are integrated may be used.

  FIG. 8 shows the arrangement of the first and third combined grids 33 and the origin pattern 38 on the index 30. Here, the origin pattern 38 is a reverse pattern to the scale pattern. For example, when the scale is reflective, it is transmitted.

  According to the present embodiment, the gap between the lattices of the origin pattern 38 on the scale 20 and the index 30 can be reduced, stray light due to the diffracted light of the origin pattern 24 on the scale 20 is prevented, and the origin is detected. Efficiency is improved.

  In the third and fourth embodiments as well, the allowable range of the air gap can be widened by using the lens detection system 40 as a telecentric optical system as in the second embodiment.

  In each of the embodiments, the light receiving element array is used. However, the type of the light receiving element is not limited to an array.

The perspective view which shows the whole structure of 1st Embodiment of this invention. Sectional view showing optical path Sectional drawing which similarly shows the structure of the 1st and 3rd combined use lattice Sectional drawing which shows the structure of 2nd Embodiment of this invention. Sectional drawing which similarly shows the structure of 3rd Embodiment Sectional drawing which similarly shows the structure of the 1st and 3rd combined use lattice Sectional drawing which shows the structure of 4th Embodiment of this invention. (A) Plan view and (B) Side view showing the pattern on the index.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Light source 20 ... (Main) Scale 22 ... 2nd grating | lattice 24, 38 ... Origin pattern 30 ... Index (scale)
32, 33 ... 1st and 3rd combined grating 34 ... Moire fringe 36 ... Beam splitter 40 ... Lens optical system 42 ... Projection lens 44 ... Aperture 46 ... Projected image 50 ... Light receiving element array 52 ... Light receiving substrate 54 ... Light reception for origin detection element

Claims (10)

Three gratings having a first grating for modulating light from the light source, a second grating formed on the scale, and a third grating onto which the interference fringes formed by the first and second gratings are projected Type photoelectric incremental encoder,
The first and third combined gratings having the functions of the first and third gratings are provided in the index,
A photoelectric incremental encoder characterized in that a moire fringe generated in the first and third combined gratings is detected by a lens optical system.   The light from the light source is split by a beam splitter and irradiated perpendicularly to the scale surface via the first and third combined gratings, and the moire generated on the first and third combined gratings by the reflected light from the scale surface. The photoelectric incremental encoder according to claim 1, wherein fringes are detected by a lens optical system.   3. The photoelectric incremental encoder according to claim 1, wherein the pitch of the light receiving element array is larger than the pitch of the first and third combined gratings and the second grating. 4.   4. The photoelectric incremental encoder according to claim 1, wherein a magnification of the lens optical system is set so that a period of the moire fringes matches a pitch of the light receiving element array. 5.   5. The photoelectric incremental encoder according to claim 1, wherein light from the light source is incident from a lateral direction with respect to a scale moving direction. 6.   The numerical aperture of the lens optical system is set so that only moire fringes can be detected without detecting the pattern of the first and third combined gratings. The photoelectric incremental encoder as described.   6. The photoelectric incremental encoder according to claim 1, wherein the lens optical system is a telecentric optical system in which an aperture is disposed at a focal position of the projection lens.   8. The photoelectric incremental encoder according to claim 7, wherein the aperture diameter of the telecentric optical system is set to a cutoff frequency at which the pattern of the first and third combined gratings is not resolved.   9. The photoelectric incremental encoder according to claim 1, wherein the first and third combined gratings are formed on a scale side surface of the index.   10. The photoelectric incremental encoder according to claim 1, wherein an origin pattern is formed in parallel with the first and third combined gratings.

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