JPH11160102A - Optical rotary encoder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform the detection with high sensitivity by making the light focused by a Fresnel zone plate where a sectional level of a pattern is changed stepwise, on a slit, and detecting the light by a photodetector. SOLUTION: A fixed plate 1 fixed in parallel with a X-Y plane comprises plural Fresnel zone plates 2 arranged at equal intervals in the X direction. A light source 3 projects the light toward the zone plates 2 from the back of the zone plates 2. A rotary slit plate 4 is mounted at a fixed distance (d) from the fixed plate 1 in parallel with the X-Y plane orthogonally to the optical axis at a specific position from the center of rotation, and comprises radial slits 4a formed at specific intervals on the whole periphery of the position. A photodetector 5 is mounted in parallel with the X-Y plane on the back of the rotary slit plate 4. By applying this construction, the intervals Ls of the slits 4a can be remarkably reduced while keeping the distance between the rotary plate 1 and the rotary slit plate 4 at an arbitrary manufacturable value, and the detecting sensitivity can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical rotary encoder.

[0002]

2. Description of the Related Art FIG. 7 is a perspective view showing the structure of a conventional general optical rotary encoder. In the figure, L
Two members having slits are interposed in the optical path between the light source 101 such as an ED (or laser) and the light receiving element 102. Among them, the member on the light source 101 side is a rotating slit member 103 that rotates together with the object to be measured, and slits 103a that act as diffraction gratings are formed radially on the disk at equal intervals in the circumferential direction. The other member is a fixed slit member 104 which is fixed, and has slits 104a formed at equal intervals with the rotating slit member 103.

Light from a light source 101, for example, LED light,
Light emitted from the front of the rotating slit member 103 and passing through the slits 103a and 104a is detected by the light receiving element 102 disposed behind the fixed slit member 104. Therefore, in the light receiving element 102, light is detected for each rotation amount corresponding to the slit interval of the movable slit member 103, and a pulse output is obtained. The amount of rotation can be detected by counting the pulse output. Therefore, the detection sensitivity (resolution) is determined by the number of output pulses per rotation amount, and it is necessary to reduce the interval between the slits 103a and 104a in order to improve the detection sensitivity. In the optical rotary encoder having such a configuration, the interval between the slits 103a and 104a is P,
Assuming that the wavelength of the LED light is λ, the rotating slit member 103
The following relationship exists between the distance between the fixed slit member 104 and the fixed slit member 104. δ = {(P / 2) ² } / λ. . . (1)

[0004]

However, in the conventional optical rotary encoder as described above, if the interval P between the slits 103a and 104a is reduced in order to improve the detection sensitivity, the interval P is calculated by the following equation (1). P
The distance δ between the rotating slit member 103 and the fixed slit member 104 must be reduced in proportion to the square of However, since there is a limit in manufacturing to reduce the distance δ, the reduction of the interval P is also limited, and thus the detection sensitivity is limited.

[0005] In view of the above-mentioned conventional problems, an object of the present invention is to provide an optical rotary encoder that enables more sensitive detection without increasing the manufacturing cost.

[0006]

SUMMARY OF THE INVENTION The present invention is an optical rotary encoder in which light from a light source is converged by a Fresnel zone plate and then incident on a slit to be detected by a light receiving element. It is a multi-level Fresnel zone plate in which the sectional level of the pattern changes stepwise (claim 1).

In the optical rotary encoder thus configured, the light converged by the Fresnel zone plate is made incident on the slit and detected by the light receiving element. The focal length of the Fresnel zone plate is determined from the radius of each zone, and the radius of each zone is calculated based on the radius of the innermost circle. Therefore, the focal length of the Fresnel zone plate is the radius of the innermost circle. It can be said that it depends on. By appropriately selecting the radius, the beam diameter can be reduced to a value close to the diffraction limit while securing the focal length, that is, the distance between the Fresnel zone plate and the slit, to a value that can be manufactured. Therefore, the interval between the slits can be made extremely small, and high detection sensitivity can be obtained for detecting the amount of rotation. Further, the Fresnel zone plate has a 2
Diffraction efficiency is higher than that of a stepped unevenness type. Therefore, high brightness is not required for the light source. Also, the light receiving element does not need to have particularly high sensitivity.

Further, the optical rotary encoder of the present invention comprises a light source, a fixed plate having a Fresnel zone plate for converging light projected from the light source, and the Fresnel zone plate in a projection direction of the light from the fixed plate. A rotating slit plate having a slit that is rotatably held at a position separated by a focal length and that allows converged light to be incident thereon, and a light receiving element that is arranged close to the rotating slit plate and detects light that has entered the slit Wherein the Fresnel zone plate is a multi-level Fresnel zone plate in which the cross-sectional level of a pattern changes stepwise to form a cross-sectional shape similar to a blazed Fresnel zone plate. (Claim 2).

In the optical rotary encoder thus configured, the light converged by the Fresnel zone plate is made incident on the slit and detected by the light receiving element. Since the focal length of the Fresnel zone plate is determined by the radius of the innermost circle of the Fresnel zone plate, the focal length, that is, the distance between the fixed plate and the rotary slit plate can be manufactured by appropriately selecting the radius. , While reducing the beam diameter to a value close to the diffraction limit. Therefore, the interval between the slits can be made extremely small, and high detection sensitivity can be obtained for detecting the amount of rotation. In addition, the cross-sectional level of the Fresnel zone plate changes stepwise to have a cross-sectional shape similar to that of the blazed Fresnel zone plate, so that the diffraction efficiency is extremely higher than that of the two-step concavo-convex type.
Therefore, high brightness is not required for the light source. Also, the light receiving element does not need to have particularly high sensitivity. In addition, it is easier to manufacture than a blazed Fresnel zone plate.

[0010]

FIG. 4 is a perspective view showing a main configuration of an optical rotary encoder according to an embodiment of the present invention in an XYZ rectangular coordinate system (however, the dimensions shown are not necessarily proportional to the actual dimensions). .). In the figure, X-
A plurality of (for example, 2 rows and 5 columns) Fresnel zone plates 2 are formed on a fixed plate 1 fixed and provided in parallel with the Y plane, and these are arranged at equal intervals in the X direction.
The light source 3 composed of an LED is arranged so as to form an optical axis in the Z direction perpendicular to the fixed plate 1 from the rear of the Fresnel zone plate 2 sufficiently, and the light is projected toward the Fresnel zone plate 2. The rotating slit plate 4 is a member that rotates together with the rotation amount detection target object.
They are arranged parallel to the XY plane at a distance d in the direction. The entire shape of the rotary slit plate 4 is a disk shape as shown in FIG. The slits 4a are formed at regular slit intervals Ls (FIG. 4) in the circumferential direction. The light receiving element 5 is located close to the rear side of the rotary slit
-It is arranged parallel to the Y plane. The above radius rs
Since the slit interval Ls is very small as compared with FIG. 4, the arrangement of the slits 4a of the number as shown in FIG. 4 can be regarded as being on a straight line. Therefore, the Fresnel zone plate 2 can be linearly arranged in the X direction.

In the above configuration, if the number of pulses detected by the light receiving element 5 per rotation of the rotary slit plate 4 is Ps, the width W of each slit 4a is W = π · rs / Ps. Since the slit interval Ls is set to twice the slit width W, Ls = 2π · rs / Ps. The distance d in the Z direction between the fixed plate 1 and the rotary slit plate 4 is also the focal length d of the Fresnel zone plate 2, and is given by d = (s1) ＾ 2 / λ. Here, s1 is the radius of the innermost circle of the Fresnel zone plate 2, and λ is the LED wavelength.

In order to make an optical system configuration similar to a rotary encoder utilizing Fraunhofer diffraction by a conventional diffraction grating, the Fresnel zone plates 2 are arranged at intervals of an integral multiple of the slit interval Ls. That is, the interval between the Fresnel zone plates 2 is n · Ls. here,
The integer n is

[0013]

(Equation 1)

[0014] The required number of Fresnel zone plates 2 is determined from the relationship between the amount of light collected on the same light receiving element through the slit 4a and the minimum sensitivity of the element. Specific numerical examples are as follows.

LED wavelength λ = 0.74 μm Number of pulses Ps = 5000 pulses / rotation Focal length d = 200 μm Radius rs = 14 mm

[0016]

(Equation 2)

## EQU1 ## In the above-described configuration, when light is projected from the light source 3 toward the Fresnel zone plate 2, the converged minute spot light is formed by the lens action of the plurality of Fresnel zone plates 2 arranged at equal intervals (X direction). Obtained at equal intervals. Since the interval between the minute spot lights is an integral multiple (n · Ls) of the interval Ls between the slits 4a, the process of passing through the slits 4a at once according to the movement of the rotary slit plate 4 and then simultaneously not passing is repeated. It is. As a result, an on / off signal corresponding to the amount of rotation of the rotating slit plate 4 is output from the light receiving element 5.

When the focal length of the Fresnel zone plate 2, that is, the distance d between the fixed plate 1 and the rotary slit plate 4, is d = d
Since (s1) ＾ 2 / λ, the distance d can be arbitrarily set by changing the radius s1 of the innermost circumference circle of the Fresnel zone plate 2. It is also known that a beam converged by the Fresnel zone plate 2 can be converged to a beam diameter close to the diffraction limit.
Therefore, the drawing accuracy of the Fresnel zone plate 2 is increased by making full use of a semiconductor drawing technique such as lithography, and the minimum spot diameter is set to a predetermined value, whereby the distance d between the fixed plate 1 and the rotary slit plate 4 is increased. Is maintained at an arbitrarily manufacturable value, the interval Ls between the slits 4a is made extremely small, and the detection sensitivity can be improved. Further, in order to narrow the LED light into a spot by the Fresnel zone plate 2, the S / N at the time of signal ON / OFF due to the LED light passing / non-passing through the slit 4a.
The ratio can be increased. Further, since the amount of received light can be adjusted by the number of Fresnel zone plates 2, the radius of the center circle, and the number of zone zones, the degree of freedom in design is large.

The center of the Fresnel zone plate is L
A so-called A / B two-phase output can be easily obtained by drawing by shifting by s / 4.

FIG. 2 is a view showing an example of the shape of the Fresnel zone plate 2. FIG. 3 is a graph showing a general sectional shape of such an annular zone of the Fresnel zone plate 2.
It is. The horizontal axis represents the distance in the radial direction, and the vertical axis represents the length based on the wavelength of light. It has been found that the imaging efficiency of the Fresnel zone plate 2 when such a two-level uneven structure is formed in a black and white pattern is an extremely low value of 20% or less. Therefore, the light source 3 needs high brightness. Also, instead of a black-and-white pattern, even in an uneven level structure using lithography,
The theoretical efficiency is as low as 41%. On the other hand, FIG.
The Fresnel zone plate 2 having a cross-sectional shape shown in FIG.
% Is also possible. Such a blazed Fresnel zone plate is manufactured by irradiating ultraviolet light, an electron beam, or the like. However, it is generally difficult to manufacture the blazed Fresnel zone plate because the irradiation time is difficult to control.

Therefore, in the present embodiment, a semiconductor lithography technique is used to create a stepped level shape at another stage so as to approximate a blazed Fresnel zone plate, and to give various types of phase changes to increase the diffraction efficiency. Improve. A Fresnel zone plate constituted by such a stepwise level configuration of another stage is called a multi-level Fresnel zone plate. According to theoretical studies and experiments, for example, the diffraction efficiency in the case of the four-stage multi-level Fresnel zone plate shown in (b) is 81%,
The diffraction efficiency in the case of the eight-stage multi-level Fresnel zone plate shown in (c) is 94.8%. As described above, by using the multi-level Fresnel zone plate, the diffraction efficiency is greatly improved as compared with the two-step concavo-convex Fresnel zone plate (FIG. 3), and a diffraction efficiency close to that of the blazed Fresnel zone plate is obtained. Can be. As a result, it is not necessary to increase the brightness of the light source 3 so much. In addition, it is not necessary to use a light receiving element 5 having particularly high sensitivity. Therefore, the light source 3 and the light receiving element 5 need only be inexpensive. Further, unlike a blazed Fresnel zone plate, it does not require delicate control of the irradiation time at the time of manufacture, and can be manufactured by an electroforming mold / plastic press method. Therefore, productivity is improved.

Although the rotary slit plate 4 in the above embodiment is of the incremental type, the rotary slit plate 4 can also be configured in the same manner by using a rotary encoder using an absolute type rotary slit plate.

In the above embodiment, an example is shown in which a plurality of Fresnel zone plates 2 are arranged, but basically one Fresnel zone plate 2 may be provided. FIG. 6 is a perspective view showing an optical rotary encoder using one Fresnel zone plate. It is the same as the above-described embodiment except that only one Fresnel zone plate 2 is provided on the fixed plate 1 and that the light receiving element 5 is reduced in the X direction in accordance with the Fresnel zone plate 2.

[0024]

The present invention configured as described above has the following effects. According to the optical rotary encoder of the first aspect, since the focal length of the Fresnel zone plate is determined by the radius of the innermost circle of the zone plate, the focal length, that is, the Fresnel zone plate and the slit can be selected by appropriately selecting the radius. The beam diameter can be reduced to a value close to the diffraction limit while securing the distance to the extent that can be manufactured. Therefore, the interval between the slits can be made extremely small, and high sensitivity can be obtained for detecting the amount of rotation. In the Fresnel zone plate, the cross-sectional level of the pattern changes stepwise,
Diffraction efficiency is higher than that of the two-step concavo-convex type. Therefore, high brightness is not required for the light source. Also, the light receiving element does not need to have particularly high sensitivity. As a result, an inexpensive optical rotary encoder with high resolution can be provided.

According to the rotary encoder of the second aspect,
Since the focal length of the Fresnel zone plate is determined by the radius of the innermost circle of the zone plate, by appropriately selecting the radius, the focal length, that is, the distance between the fixed plate and the rotary slit plate can be set to a value that can be manufactured. While securing, the beam diameter can be reduced to a value close to the diffraction limit. Therefore, the interval between the slits can be made extremely small, and high sensitivity can be obtained for detecting the amount of rotation. In addition, the cross-sectional level of the Fresnel zone plate changes stepwise to have a cross-sectional shape similar to that of the blazed Fresnel zone plate, so that the diffraction efficiency is extremely higher than that of the two-step concavo-convex type. Therefore, high brightness is not required for the light source. Also, the light receiving element does not need to have particularly high sensitivity. In addition, it is easier to manufacture than a blazed Fresnel zone plate. As a result, with high resolution,
An optical rotary encoder that is inexpensive and has excellent productivity can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram showing a cross-sectional shape of a Fresnel zone plate in an optical rotary encoder according to an embodiment of the present invention.

FIG. 2 is a diagram showing a shape of a Fresnel zone plate.

FIG. 3 is a diagram showing a cross-sectional shape of a two-step concavo-convex type of a Fresnel zone plate.

FIG. 4 is a perspective view showing a configuration of an optical rotary encoder according to one embodiment of the present invention.

FIG. 5 is a front view showing the entire shape of the rotary slit plate in FIG. 4;

FIG. 6 is a perspective view showing a configuration of an optical rotary encoder in a case where there is one Fresnel zone plate.

FIG. 7 is a perspective view showing a configuration of a conventional optical rotary encoder.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Fixed plate 2 Fresnel zone plate 3 Light source 4 Rotating slit plate 4a Slit 5 Light receiving element

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Tatsuo Matsushima 3-5-8 Minamisenba, Chuo-ku, Osaka City Inside Koyo Seiko Co., Ltd. (72) Inventor Manabu Tsukamoto 3-58 Minamisenba, Chuo-ku, Osaka City Koyo Seiko Co., Ltd. (72) Inventor Yasushi Fujimoto 3-5-8 Minamisenba, Chuo-ku, Osaka-shi Koyo Seiko Co., Ltd.

Claims (2)

[Claims] 1. An optical rotary encoder, wherein light from a light source is converged by a Fresnel zone plate and then made incident on a slit to be detected by a light receiving element. An optical rotary encoder characterized by a variable multi-level Fresnel zone plate. 2. A light source, a fixed plate having a Fresnel zone plate for converging light projected from the light source, and rotating at a position separated from the fixed plate by a focal length of the Fresnel zone plate in a projection direction of the light. A rotary slit plate that is freely held and has a slit that allows converged light to enter, and an optical rotary encoder that includes a light receiving element that is arranged close to the rotary slit plate and detects light that has entered the slit. The optical rotary encoder according to claim 1, wherein the Fresnel zone plate is a multi-level Fresnel zone plate in which a sectional level of a pattern changes stepwise to form a sectional shape similar to a blazed Fresnel zone plate.