JPH06174495A - Optical displacement detecting device - Google Patents

Abstract

PURPOSE:To improve the resolution of a geometrical/optical encoder and expand the interval between a slit pattern and a mask pattern faced to each other. CONSTITUTION:A displacement member 2 having a periodic slit pattern 1 and displaceably mounted along the first plane is used. A coherent light source 4 made of a laser diode is arranged behind the displacement member 2 to generate a primary bright/dark image 3 moved along the first plane. The primary bright/dark image 3 is projected at the preset magnifying power with a lens member 6, and the expanded secondary bright/dark image 5 moved along the preset second plane is formed. A light reception section 8 is fixed and arranged on the second plane, and it receives the light according to a periodic mask pattern in response to the moving secondary bright/dark image 5 and outputs the electric signal 7 indicating the displacement of the displacement member 2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical encoder or an optical displacement detecting device which irradiates an encoder plate which is rotationally displaced or linearly displaced with light and detects the displacement based on a change in the amount of light.

[0002]

2. Description of the Related Art Optical encoders using a wave optical system and a geometrical optical system are known. The encoder of the wave optical system utilizes interference and diffraction of coherent light, and is composed of a coherent light source such as a semiconductor laser, an encoder plate having a diffraction grating formed thereon, a light receiving element and the like. The diffraction grating formed on the encoder plate has a grating constant approximately equal to the wavelength of coherent light, and it is possible to obtain an optical encoder of high resolution and small size. However, when attempting to realize an absolute encoder using a wave optics system, a plurality of tracks consisting of slit rows containing different address information are arranged side by side on the surface of the encoder plate, thus achieving miniaturization. I can't do it. That is, if the track interval is made fine, light interference occurs and it is impossible to separate light for each track.

On the other hand, the geometrical optical encoder uses the straightness of light and is composed of a combination of an incoherent light source such as an LED, a movable slit and a fixed slit, a light receiving element and the like. Incident light is intermittently switched using a combination of a moving slit and a fixed slit, and displacement detection is performed based on a change in the light amount.

[0004]

By the way, in the encoder of the geometrical optical system, if the pitch of the moving slit or the fixed slit is reduced to the wavelength order of light in order to improve the resolution, it is formed by the light passing through the slit due to the diffraction phenomenon. There is a problem that the image becomes blurred. That is, the conventional geometrical optical system encoder has a limit of resolution due to the wave nature of light. In order to prevent this image blurring, it is necessary to arrange the moving slit and the fixed slit as close to each other as possible. However, the moving slit is formed in the encoder plate, and surface wobbling occurs with the movement. Further, the surface wobbling is promoted by externally applied vibration or impact. For this reason, the distance between the movable slit and the fixed slit must be set in consideration of a predetermined clearance, and it is actually difficult to suppress the blurring of the image described above. It was an obstacle to miniaturization.

In view of the problems of the geometrical optical encoder described above, it is an object of the present invention to provide an improved geometrical optical encoder capable of achieving high resolution without disposing a moving slit and a fixed slit in close proximity. It is another object of the present invention to provide a geometrical optical encoder in which blurring of an image is suppressed regardless of the limit of resolution due to the wave nature of light.

[0006]

Means taken for achieving the object of the present invention will be described with reference to FIG. The illustrated optical displacement detection device includes a displacement member 2 that has a periodic slit pattern 1 and is displaceably mounted along a predetermined first plane, and the slit pattern 1 is illuminated with coherent light to illuminate the first plane. A coherent light source 4 for generating a primary bright-dark image 3 that moves along a line, and a magnified beam 2 that projects the primary bright-dark image 3 at a predetermined magnification and moves along a predetermined second plane.
A lens member 6 for forming the next bright and dark image 5, and an electric signal 7 representing the displacement of the displacing member 2 having an effective light receiving area corresponding to the moving secondary bright and dark image fixedly arranged on the second plane.
And a light receiving portion 8 for outputting Assuming that the periodic array pitch of the slit pattern 1 is P, the wavelength of coherent light is λ, and the numerical aperture on the first plane side of the lens member 6 is NA, the optical system shown in the figure will satisfy the relation of P> λ / NA. Is set to. The displacement member 2 has a bit-coded slit pattern 1 representing the absolute displacement amount. Alternatively, the displacement member 2 may have a regular slit pattern 1 indicating the relative displacement amount.

In the illustrated example, the displacement member 2 is a transparent substrate 21.
In addition to having a slit pattern 1 formed of a periodic arrangement of a transmissive portion 22 and a non-transmissive portion 23 formed on the substrate 1, the coherent light source 4 illuminates the transparent substrate 21 with coherent light from the back surface to generate a primary bright-dark image 3. It is located in.
Further, the light receiving portion 8 includes a fixed mask plate 83 having an array of transmissive portions 81 and non-transmissive portions 82 formed according to a periodic mask pattern, and a light receiving element 85 having a uniform light receiving region 84 arranged behind it. It is a two-layer structure consisting of. This two-layer structure defines an effective light receiving area according to the secondary bright and dark image. However, the light receiving unit 8 is not limited to such a two-layer structure. For example, a single layer structure composed of a light receiving element having a periodic effective light receiving area formed according to a periodic mask pattern may be used as the light receiving portion.
Further, the lens member 6 is composed of an aspherical lens and is adapted to form a secondary bright-dark image 5 in which aberration is substantially removed.

[0008]

When the slit pattern 1 is illuminated by the coherent light source 4, a primary bright-dark image 3 moving along the first plane is obtained. The first plane is set at the position of the distance L in the optical axis direction with reference to the lens member 6. The primary bright-dark image 3 has a peak pitch corresponding to the cycle of the slit pattern 1. This primary bright-dark image 3 is enlarged and projected by the lens member 6 and is formed as a secondary bright-dark image 5 on the second plane. The second plane is set at a position having a distance N in the optical axis direction with reference to the lens member 6. These distances L and N are the lens formula 1
Calculated according to / N + 1 / L = 1 / F. However, F is the focal length of the lens member 6. As is apparent from this lens formula, the magnification M of the secondary bright-dark image 5 with respect to the primary bright-dark image 3 is given by N / L.

When the primary bright-dark image 3 moves as indicated by the arrow, the secondary bright-dark image 5 also moves accordingly. However, the moving directions are opposite. The peak of the moving secondary bright-dark image 5 is intermittently received by the light receiving element 85 via the fixed mask plate 83, and the AC electric signal 7 corresponding to the change in the amount of received light is output.
When the slit pattern 1 is a simple regular pattern representing the relative displacement amount, the frequency of the AC electric signal 7 represents the displacement velocity of the displacement member 2 and the number of waves represents the displacement amount. With this configuration, it is possible to obtain a high-resolution encoder that uses the straightness of light without causing the displacement member 2 and the light-receiving unit 8 to approach each other. For high resolution, it is not necessary to miniaturize the mask pattern on the second plane side even if the slit pattern on the first plane side is miniaturized. The above-mentioned enlargement factor is applied between the cycle of the slit pattern and the cycle of the mask pattern.

In the present invention, when the periodic array pitch of the slit pattern 1 is P, the wavelength of coherent light is λ, and the numerical aperture on the first plane side of the lens member 6 is NA,
The geometrical optical system is set so as to satisfy the relation of P> λ / NA. By using the coherent light source 4 and satisfying the above relationship, the periodic array pitch P of the slit patterns is
Can be miniaturized to the order of the wavelength λ of coherent light, and a secondary bright-dark image 5 with extremely clear contrast can be obtained. Hereinafter, a detailed description will be added to this point.

Generally, the wave distribution U of a point image formed by an aplanatic lens has a circular aperture of Fraunforfer (Fr).
aunhofer diffraction (eg M. Bo)
rn, E.I. Wolf Translated by Kusakawa and Yokota "Principles of Optics II"
pp600, Tokai University Press, 1975). The result is expressed by the following formula 1.

[Equation 1]

Further, when the light intensity distribution I is obtained based on the wave distribution U obtained by the equation 1, the following equation 2 is obtained.

[Equation 2] The above Equation 2 is shown in a graph as shown in FIG. That is,
As is well known, the light intensity distribution of a point image formed by an aplanatic lens has a predetermined spread based on the wave nature of light given by a so-called Airy disc. That is, even an aplanatic lens has a limit in terms of wave optics, an ideal point image cannot be obtained, and a disc image having a certain degree of spread is always obtained.

Assuming that the primary bright-dark image obtained by the slit pattern of the displacement member is represented by a rectangular wave, when the light source is incoherent light, the point image intensity distribution of Equation 2 is expanded to the rectangular wave by the lens expansion. The intensity distribution of the secondary bright and dark image can be obtained by integrating in the range multiplied by the magnification. On the other hand, when the slit pattern is illuminated by using coherent light, it is necessary to take into account the phase of the wave, and therefore Equation 1 must be integrated over the entire angle of view of the rectangular wave. Therefore, the lens aperture is divided into minute components, and Fresnel-K
The simulation was performed using specific parameters based on the Irchhoff diffraction formula. The parameter used is 0.2 numerical aperture NA of the lens viewed from the first plane side.
5, the slit pitch was 5.5 μm, the lens focal length was 3.2 mm, the lens magnification was 15.2 times, and the light / dark duty of the slit pattern and the mask pattern was 1: 1. Also, the lens was treated as aberration-free. Further, for incoherent light, an LED was used as a light source and the wavelength λ was set to 680 nm, and in the case of coherent light, a laser diode was used as a light source and the wavelength λ was set to 780 nm. The simulation result of incoherent light is shown in FIG. The electric output is shown in FIG. On the other hand, the secondary bright and dark image light intensity distribution of the coherent light is shown in FIG. 5, and the electrical output is shown in FIG.

From the above-mentioned relationship of Z = k · a · r / R,
a = Z · R · λ / 2πr is obtained. Focusing on the light intensity I at a certain constant value Z, a is proportional to λ. Therefore, when λ becomes large, the Airy disk also becomes large, and the contrast of the secondary bright-dark image should deteriorate. However, in actuality, as is clear by comparing FIGS. 3 and 5 and FIGS. 4 and 6, the coherent light with the wavelength λ = 780 nm has a wavelength λ
= 2 than when using incoherent light of 680 nm
The contrast of the next bright and dark images is greatly improved.

FIG. 7 is a graph showing the output characteristics of the encoder. As shown in the upper part of FIG. 7, the light receiving element moves 2
An AC electric signal is output according to the change in the amount of received light of the next bright and dark image. The electrical signal includes an AC signal component V _P -V _B and the DC offset component V _B. In the graph of FIG. 7, the horizontal axis represents the surface deflection of the encoder plate, and the vertical axis represents the magnitude of the AC signal component. In this graph, the curve connecting the black dots shows the case of using a coherent light source (λ = 780 nm), and the curve connecting the white dots shows the case of using an incoherent light source (λ = 680 nm). As is clear from this graph, when the coherent light source is used, a large AC signal component can be extracted as compared with the incoherent light source. Even if there is a considerable amount of surface wobbling, a stable AC signal component can be obtained by using a coherent light source.

Next, conditions under which the above-mentioned phenomenon occurs will be described. It is known that the optical transfer function OTF of a coherent light source is equal to the pupil function G. Therefore, if the lens has no aberration and the aperture is a circle with radius A, the OT on the second plane side
F is as shown in FIG. 8, and the cutoff frequency is A / λR
Given in. Note that R is the back focus as described above. On the other hand, the optical transfer function in the case of the incoherent light source is the autocorrelation function H of the pupil function, which is given by the following Expression 3.

[Equation 3] Similarly, if the lens aperture is a circle with radius A, then Equation 3
The right side of is equal to the shaded area shown in FIG. 9, and when this value is calculated, the OTF of the incoherent light source shown in FIG. 8 is obtained. The cutoff frequency is 2A / λR.

Paying attention to FIG. 8, the spatial frequency is A / λR.
It can be seen that the OTF of the coherent light source is higher than the OTF of the incoherent light source in the following range. When the amplitude of the primary bright-dark image changes sinusoidally, the contrast of the secondary bright-dark image is given by this OTF. However, when the secondary light-dark image is a general rectangular wave,
As shown in, the contrast of the second-order bright and dark images is represented by the series sum of OTFs (see Kubota, Ukita, and Aida “Optical Technology Handbook” p153, Asakura Shoten, 1968).

[Equation 4] For both coherent light and incoherent light, the OTF value becomes 0 at spatial frequencies 3A / λR, 5A / λR, 7A / λR, ... No significant change occurs in the contrast T. Therefore, the condition for producing the effect of the present invention is limited to the case where the spatial frequency on the secondary bright-dark image is A / λR or less.

The limit spatial frequency value A / λR is converted into a slit pitch. First, the pitch of the secondary bright and dark image is given by the following mathematical expression 5.

[Equation 5] When the sine condition is satisfied, the following Expression 6 is satisfied.

[Equation 6] Therefore, based on the above Equations 5 and 6, the limit value of the slit pitch P is given by the following Equation 7.

[Equation 7] Wavelength λ = 780 nm, numerical aperture NA = 0.25
, The limit slit pitch value P is 3.12μ.
m. In the above-mentioned simulation, the slit pitch was set to 5.5 μm. Therefore, this value is within the limit, and compared with the case of using incoherent light,
It is theoretically supported that the contrast of the secondary bright-dark image can be improved by using the coherent light.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 10 is a schematic exploded perspective view showing an embodiment in which the optical displacement detection device according to the present invention is applied to an incremental rotary encoder. In addition,
It goes without saying that the present invention is applicable not only to rotary encoders but also to linear encoders. The rotary encoder shown in the figure uses a coherent light source 4 composed of a laser diode as an illumination light source. A rotary encoder plate 2 is rotatably arranged in front of it. The encoder plate 2 is made of a transparent substrate such as a glass plate, and the slit pattern 1 is radially formed on the lower surface side thereof. The slit pattern 1 has a rotary encoder plate 2 at a predetermined cycle.
Are arranged along the circumferential direction. A reference slit 1Z is also formed on the inner side of the regular slit pattern 1 in the radial direction. The reference slit represents the reference position of the encoder plate 2. The slit pattern 1 and the reference slit 1Z can be formed by using photolithography and etching, and fine processing is possible.

An enlarged image forming lens member is arranged at a predetermined position in front of the encoder plate 2. This member is composed of an aspherical lens 6 so as to eliminate aberration as much as possible from the enlarged image of the slit pattern 1. A fixed mask plate 83 is arranged on the front imaging plane of the aspherical lens 6. Fixed mask plate 8
A first slit row 83A, a second slit row 83B, and an opening 83Z are formed in the No. 3. The array period of the slit row corresponds to the stripe interval of the enlarged secondary bright-dark image. First
The phase of the slit row 83A and the second slit row 83B is 90.
The angle is deviated so that the rotation direction of the encoder plate 2 can be detected. The opening 83Z is formed at a position where only the magnified image of the reference slit 1Z can be selectively transmitted. A light receiving element 85 is arranged on the back surface of the fixed slit plate 83. The light receiving element 85 is composed of a photodiode having a uniform light receiving surface.

FIG. 11 is a graph showing an output voltage waveform obtained from the light receiving element 85 shown in FIG. The output voltage waveform is actually measured by assembling an incremental rotary encoder under the same conditions as the above-mentioned simulation and using a laser diode as a light source. On the other hand, in FIG. 12, the light source is simply L under the same conditions.
The figure shows an actually measured output voltage waveform when an incoherent light source including an ED is used. Compared with the simulation results (Figs. 4 and 6) due to the effects of lens aberrations, magnification errors, scattered light, etc., the amplitude values are lower in both cases, but it is clearly higher when a laser diode is used as the light source. Amplitude values can be achieved.

FIG. 13 is a schematic exploded perspective view showing another embodiment of the present invention. The illustrated rotary encoder is an absolute type, which is different from the incremental type shown in FIG. The incremental encoder optically counts the number of radial slits based on a predetermined reference position. On the other hand, the absolute encoder optically detects the coded pattern formed on the surface of the rotary disk to directly read the absolute position. As shown, the absolute encoder includes a plurality of tracks 101 to 108 arranged concentrically with each other.
The rotary disc 100 having the structure is used. These tracks 101 to 108 define bit-coded slit patterns and represent the absolute angular position of the disc. These tracks 101 to 1
08 corresponds to the high-order bit to the low-order bit in order radially outward. A coherent light source 109 composed of a laser diode or the like is arranged on the lower surface side of the disc 100 and illuminates the tracks 101 to 108. A magnifying lens member 110 is arranged on the upper surface side of the disc 100 and faces the coherent light source 109 to project the illuminated slit pattern of the tracks 101 to 108 at a predetermined magnifying power. The light-receiving element array 111 is arranged via the fixed mask plate 112, receives the enlarged and projected pattern, and outputs a detection signal for each track. That is, the mask plate 112 has a plurality of windows 121 to 128 aligned in the radial direction at a predetermined pitch, and separates the projected light for each track. By processing the output detection signal, the slit pattern is decoded to determine the absolute position or address of the rotary disc 100.

In this example, eight tracks 101 to 108 are formed on the surface of the disk, which results in 2 ⁸
The angular position of the disk can be recorded with 8-bit parallel data with a resolution of. As can be easily understood, the resolution of absolute angular position detection can be increased as the number of tracks is increased. Typically, the disc has 12 concentric tracks formed therein. In such a case, the radial pitch of the track arrangement is set to be extremely small so that the size of the disk is not increased. Further, the outermost track corresponding to the least significant bit has a slit pattern having an extremely fine circumferential pitch. because,
This is because the pitch in the circumferential direction in this embodiment is set to 1/2 ^{12 of} the innermost track corresponding to the most significant bit. Of course, the pattern formation is not necessarily limited to this numerical value. The present invention is most effective in such a small size and high resolution absolute encoder. That is, since the extremely fine slit pattern is magnified by the inserted projection lens, the light receiving element array can detect the magnified secondary bright-dark image with a good S / N ratio, and the light receiving element array itself is practical. It has a typical light receiving area size. In other words, according to the present invention, an extremely fine slit pattern can be formed on the disc, the resolution can be improved, and the disc can be downsized. On the other hand, the light-receiving element array can have a practically sufficient light-receiving area to secure the S / N ratio and ensure the light separation between the adjacent tracks.

[0024]

As described above, according to the present invention, the encoder plate on which the slit pattern is formed is illuminated with coherent light to generate a primary bright-dark image, and this is magnified and projected by a lens to perform secondary projection. You have a bright and dark image. The displacement of the encoder plate is detected by receiving the enlarged secondary bright-dark image according to the mask pattern. Therefore, there is an effect that the slit pattern can be made finer than the conventional one, and a high-resolution encoder can be obtained. Moreover, since it is not necessary to dispose the slit pattern and the mask pattern close to each other as in the conventional case, the allowable range of the surface runout with respect to the encoder plate is widened, and an effect that a stable encoder that is strong against external shock and vibration can be obtained is obtained. is there. In addition, by using the coherent light source, there is an effect that a sufficient contrast of the secondary bright and dark image can be secured even when the slit pitch is miniaturized to the wavelength order. Further, since the slits can be miniaturized, there is an effect that it can be applied to an absolute encoder at a practical level.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a basic configuration of an optical displacement detection device according to the present invention.

FIG. 2 is a graph for explaining the operation of the present invention.

FIG. 3 is a graph showing the secondary bright and dark image light intensity when an incoherent light source is used.

FIG. 4 is a graph showing an electric output waveform when an incoherent light source is used.

FIG. 5 is a graph showing the intensity of secondary bright and dark image light when a coherent light source is used.

FIG. 6 is a graph showing an electric output waveform when a coherent light source is used.

FIG. 7 is a graph showing a relationship between AC amplitude of electric output and surface wobbling.

FIG. 8 is a graph showing a relationship between an optical transfer function and a spatial frequency of a coherent light source and an incoherent light source.

FIG. 9 is a graph showing a calculation process of a pupil function.

FIG. 10 is a schematic exploded perspective view showing an embodiment in which the present invention is applied to an incremental rotary encoder.

11 is a graph showing an output voltage waveform obtained by the embodiment of FIG.

FIG. 12 is a graph showing an output voltage waveform of a comparative example.

FIG. 13 is a schematic exploded perspective view showing another embodiment in which the present invention is applied to an absolute type rotary encoder.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Slit pattern 2 Displacement member 3 Primary bright-dark image 4 Coherent light source 5 Secondary bright-dark image 6 Lens member 7 Electric signal 8 Light receiving part

Claims (4)

[Claims] 1. A first predetermined pattern having a slit pattern formed of a periodic array of transparent portions and non-transparent portions formed on a transparent substrate.
A displacement member mounted so as to be displaceable along a plane, a coherent light source for illuminating the slit pattern with coherent light to generate a primary bright-dark image moving along the first plane, and the primary bright-dark image A lens member for projecting a magnified secondary bright-dark image projected at a predetermined magnification and moving along a predetermined second plane, and a moving secondary bright-dark image fixedly arranged on the second plane. An optical displacement detecting device having a corresponding effective light receiving area and a light receiving portion for outputting an electric signal representing the displacement of the displacement member. 2. The periodic array pitch of slit patterns is P
, The coherent light wavelength is λ, and the first lens member
The optical displacement detection device according to claim 1, wherein a relation of P> λ / NA is satisfied, where NA is a plane-side numerical aperture. 3. The optical displacement detector according to claim 1, wherein the displacement member has a bit-coded slit pattern representing an absolute displacement amount. 4. The optical displacement detection device according to claim 1, wherein the displacement member has a regular slit pattern indicating a relative displacement amount.