JP2011075581A - Optical encoder - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To overcome the problem in an optical encoder, wherein the division accuracy deteriorates in performing electrical division when harmonic distortion is superimposed on a displacement signal. <P>SOLUTION: The optimum value is used properly in desired gap setting for the optical effective numerical aperture of a first diffraction grating so as not to include harmonic components in the light intensity distribution itself of interference fringes because the harmonic distortion superimposed on the displacement signal is caused by the inclusion of higher-order spatial frequency components in the interference fringes formed by the diffraction grating. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an encoder, and causes a light beam from a light source to enter a diffraction grating formed on a main scale, and counts bright and dark fringes of interference fringes formed at a portion where a plurality of diffraction light beams from the diffraction grating overlap. The present invention relates to an optical encoder such as a rotary encoder or a linear encoder that measures the amount of movement of the diffraction grating.

  Conventional optical encoders generally use a main scale composed of a rectangular wave amplitude grating in which a light transmitting portion and a non-transmitting portion (or a light reflecting portion and a non-reflecting portion) are arranged at a predetermined pitch with a width of 1: 1. In the case of a transmissive optical encoder, an index composed of a rectangular wave amplitude grating in which light of a rectangular wave pattern transmitted by irradiating parallel light to such a scale is further arranged with a light transmitting portion and a non-transmitting portion at a predetermined pitch. Light is received through the scale. By receiving the light / dark pattern of the transmitted light of the index scale modulated by the relative movement of the main scale and the index scale, a displacement signal that varies periodically with the relative movement can be obtained, and by processing this displacement signal The amount of displacement can be measured. Generally, as an index scale, like a rectangular wave amplitude grating on a main scale, light transmitting portions and non-transmitting portions (or light reflecting portions and non-reflecting portions) are alternately arranged at a constant pitch with a width of 1: 1. It is supposed to have been done.

  Also, a pair of rectangular wave amplitude grating regions are prepared so as to obtain so-called A-phase and B-phase sine wave signals that are 90 ° out of phase with each other. Further, as a well-known method, in order to remove the DC offset voltage from the sine wave signal, a total is obtained so as to obtain an A phase reverse phase signal (180 ° phase difference) and a B phase reverse phase signal (270 ° phase difference). Four rectangular wave amplitude grating regions are prepared. Recently, a system that integrates an index scale and a light receiving element disposed behind the index scale and uses a light receiving element array including a similar function has become widespread.

  Further, in such measurement and measurement, it is required to further subdivide the measurement resolution, and means for increasing the number of interpolations in the attached counting circuit, that is, electric dividing means is taken. There are several methods for the electrical division of the displacement signal, but in general, an ideal sine wave signal is often required as a condition for the displacement signal.

  The displacement signal obtained from the optical encoder described above is a triangular wave or a trapezoidal wave corresponding to the change in the overlap of the scale grating in terms of geometric optics. This is particularly noticeable when the pitch dimension is relatively coarse. In addition, when a light source having excellent coherence, for example, a semiconductor laser is used as the light source, the tendency becomes strong. In either case, a higher-order diffracted light component has an influence. The displacement signal composed of such a pseudo sine wave signal has a large deviation from an ideal sine wave signal waveform, that is, a waveform distortion, and the distortion rate varies greatly due to a change in the interval between the main scale and the index scale. This variation in distortion factor is mainly due to odd-order (third, fifth,...) Harmonic components contained in the displacement signal, and position measurement is performed using a displacement signal with such distortion factor variation. If this is done, a large interpolation error occurs, which becomes a position measurement error. In particular, when the scale lattice pitch is as fine as several tens of μm, the effect of gap variation between the main scale and the index scale increases, and the distortion component varies greatly even with slight gap variations of several tens of μm. It is known to do. Therefore, the gap adjustment is extremely strict. On the other hand, when the scale pitch is relatively coarse of 100 μm or more, the distortion rate variation is insensitive to the gap variation as compared with the case of the fine pitch. Since the diffraction angle is small, higher-order diffracted light components are taken into the effective light beam region, and the displacement signal includes many higher-order harmonic components. As a result, it becomes a clear triangular wave or trapezoidal wave, which is far from being called a pseudo sine wave. It is known that the distortion rate is considerably large with respect to the fundamental wave amplitude. Further, even a light source with high coherence such as a semiconductor laser is reproduced without collapsing the phase of the higher-order diffracted light component, and similarly, a harmonic component is superimposed on the displacement signal.

In view of this, several methods for reducing the harmonic distortion of the optical encoder described above have been proposed. For example,
(1) A method using a so-called sine wave grating in which the light transmitting portion of the index scale has a sine wave shape (see, for example, Patent Document 1).
(2) A method of using a pair of rectangular wave gratings provided with a phase difference that cancels odd-order harmonic distortion on the index scale side (see, for example, Patent Document 2).
(3) A method of setting the width of the light transmitting portion and the non-transmitting portion on the index scale side to a ratio that can cancel the harmonic distortion instead of 1: 1 (for example, see Patent Document 3).

In this way, the distortion of the rectangular wave amplitude grating on the index scale, the aperture ratio, the arrangement, or the shape, aperture ratio, arrangement, etc. of the light receiving element array that performs the same function is sought to eliminate or reduce the distortion. Met.
The essential distortion reduction effect of these measures can be interpreted as follows. In other words, when the diffraction grating arranged on the main scale is irradiated with a monochromatic light beam, multi-beam interference fringes are formed in the overlapping portion of the diffracted light beam in the rear space of the main scale as a result of overlapping of the generated diffracted lights. The The light intensity distribution of the interference fringes includes harmonic components other than the fundamental wave, and the conventional measures described above are based on optical methods of various methods for the harmonic components formed on the index scale. This is equivalent to performing a special spatial frequency filtering.

As an improvement measure for parts other than the index scale, the light intensity distribution itself of the multi-wave interference fringe formed in the diffracted light beam overlapping part in the space behind the main scale does not contain or reduce harmonic components. Several methods have been proposed in which improvements are made to the square wave amplitude grating on the main scale. For example,
(4) The light transmitting portion and the non-transmitting portion without changing the pitch so that the width of the light transmitting portion and the non-transmitting portion of the rectangular wave amplitude grating on the main scale is substantially the same as that of the sine wave grating instead of 1: 1. A method of forming a non-uniform lattice width (see, for example, Patent Document 4).
(5) A method of setting the width of the light transmitting portion and the non-transmitting portion of the rectangular wave amplitude grating on the main scale to a ratio of 2: 1 instead of 1: 1 (for example, see Patent Document 5). Etc.
Further, when the diffraction grating arranged on the main scale is irradiated with the light beam from the light source, much research has been conducted on the intensity distribution of the light formed in the diffracted light beam overlapping portion in the space behind the grating (Fresnel region). (For example, see Non-Patent Documents 1 and 2.)

US Pat. No. 4,782,229 (FIG. 6) Japanese Patent No. 2539269 (FIG. 2) Japanese Patent No. 3045452 (FIGS. 2 and 3) Japanese Patent No. 2695623 JP 09-196705 A

E. Keren, O .; Kafri, 1985, "Diffraction effects in moire defectometry," J. Opt. Soc. Am. A / Vol. 2, no. 2 / February 111-120. Optics Vol.16 No.2 (February 1987) Proximity Image Calculation Hiroshi Oki

  However, all of the above-described conventional techniques (1) to (3) improve the rectangular wave amplitude grating on the index scale on the light receiving side to reduce harmonic distortion, and the main scale itself is the same as the conventional one.

  In principle, these methods require that the luminous flux from the light source is uniformly applied to the index scale and the main scale. If not, the desired effect can be obtained. I can't get it. When the illuminance distribution from the light source on the main scale or the index scale is not uniform, none of the proposals of the conventional techniques (1) to (3) can provide a sufficient effect.

  In the case of a relatively rough pitch, the effect of removing and reducing the pitch is further reduced. Furthermore, from the viewpoint of manufacturing technology, in the method (1), it is technically difficult to form the lattice pattern itself in a sine wave shape particularly in the case of a fine pitch.

  Originally, the harmonic component of the displacement signal is a high-order spatial frequency in the multi-beam interference fringes formed on the diffracted beam overlap part as a result of the overlapping of the diffracted beams generated by the diffraction gratings arranged on the main scale. This is because the components are superposed. This is well known to those skilled in the art.

  Therefore, as a more effective distortion removal and reduction method, it is important to prevent the harmonic component from being included in the light intensity distribution itself of the multi-wave beam interference fringes projected on the index scale. That is, it is effective to improve the diffraction image on the main scale and eliminate the cause of distortion.

  In this respect, the methods of the prior arts (4) and (5) are basically rectangular on the main scale so as not to include harmonic components in the light intensity distribution of interference fringes projected and superimposed on the index scale. This is an example of a measure for improving the amplitude grating.

  However, in the prior art (4), the illuminance distribution on the scale is required to be uniform as in the prior arts (1) to (3). Although a structure in which lattices having a non-uniform aperture ratio are appropriately distributed on the main scale surface is adopted, this method is affected by the illuminance distribution unevenness previously pointed out. In addition, it is also in terms of manufacturing technology to maintain a high pitch accuracy with respect to the main scale, which is the main component that determines the detection accuracy as a position detection device, and to accurately form a non-uniform aperture ratio. difficult.

  According to the method of the prior art (5), harmonic distortion can be reduced only by changing the opening width of the rectangular wave amplitude grating on the main scale. Since the aperture width of the grating is changed uniformly over the entire effective area of the grating on the main scale, the distortion reduction effect is not lost even if there is uneven illumination, and the desired pitch accuracy on the main scale is maintained while maintaining a high accuracy. Setting the aperture ratio is easy in terms of manufacturing technology and is considered to have a great economic effect, and can be evaluated as a practically effective means.

  However, the method of the prior art (5), that is, the method of setting the width of the light transmitting portion and the non-transmitting portion at a ratio of 2: 1 instead of 1: 1 can be realized only at a specific gap position. Therefore, detailed simulations and experimental studies have revealed that it is impossible to reduce distortion at any gap.

  As for the problems in the prior art (5), the text of the reference document 5 and figures are appropriately cited, and the prior art (5) will be described below with reference to FIGS. 17A and 17B.

  FIG. 17A is a diagram showing the configuration of the entire main part. In the figure, a main scale 1 is a transmissive main scale in which light transmitting portions 11 and non-transmitting portions 12 are arranged at a pitch P, and the width of the light transmitting portion 11 is set to 2P / 3. Is an index scale 3 in which light transmitting portions and non-transmitting portions are arranged at a pitch P with a width of 1: 1.

In this configuration, in the prior art (5), the operation and effect of the invention are described as follows.
In the literature reference, FIG. 4 is replaced with FIG. 17B in this specification.
"... In this embodiment, as described above, the light transmission part 11 of the main scale 1 is set to a width of 2P / 3, whereby the third harmonic of the displacement output signal which is a pseudo sine wave. The principle will be described with reference to Fig. 4 (⇒ Fig. 17B), where the pitch of the main scale 1 is P, the width of the light transmission part 11 is generally L, and parallel light is irradiated. When this is done, the transmitted light pattern of the main scale 1 is a rectangular wave pattern as shown in Fig. 4 (⇒ Fig. 17B) considering only the straight light component. The pattern I (x) is expressed by the following formula 1 by Fourier expansion.

In Equation 1, C and D are constants. Such a transmitted light quantity pattern of the main scale 1 is further modulated by the index scale 3 on the light receiving side, and a pseudo sine wave output is obtained. As is clear from Equation 1, when attention is paid to the third harmonic, which is the largest odd-order harmonic component, that is, n = 3, when L = P / 3 or L = 2P / 3, the right side The coefficient is 0 for both the first and second terms. Therefore, according to this embodiment in which L = 2P / 3 is set, a displacement signal closer to a sine wave from which the third harmonic component is removed can be obtained. ...... "
Citation from reference end.

To summarize the problems included in the above reference description, first, the intensity pattern I (x) expressed by Equation 1 represents the intensity transmittance distribution of the grating, and this is certainly the light intensity distribution immediately after the main scale. There is no problem in assuming such a rectangular pattern, but in the actual encoder configuration, as shown in FIG. 17A, the main scale and the index scale are provided with an interval (gap = z). The light intensity distribution of the interference fringes superimposed on the index scale in that state is not shown.

  That is, it is necessary to express at least I (x, z) as the light intensity distribution.

The expression of the intensity distribution I (x) in Equation 1 is approximately established only at the following specific gap position = z _n .

This intensity distribution proportional to the intensity transmittance distribution of the grating appearing at this specific gap position z _n is called a Fourier image.

Secondly, according to the description of the prior art (5), the optical effective aperture ratio of the main scale (= light transmission portion) at the gap position: z _n where the above Fourier image in which Equation 1 is approximately established is generated. Let us consider a case in which the ratio of the width of the non-transmissive portion is set to a ratio of 2: 1 instead of 1: 1. The rectangular wave amplitude grating contributes to the generation of the third-order harmonic component included in the displacement waveform. Among the generated diffracted light beams, the zero-order, ± 1st-order, ± 2nd-order, and so on, which cannot be ignored as the intensity, A combination of ± 3rd order diffracted light components is formed by (0, +3), (0, −3), (+1, −2), and (−1, +2). In the prior art (5), when the aperture ratio is 2: 1 (it may be 1: 2), the third-order diffracted light becomes a missing order (0, +3), (0, -3). ) Is removed. However, the component in the combination of (+1, −2) and (−1, +2) remains, and (+1, −2) and (−1, +2) are canceled with a phase relationship of 180 °. Therefore, the distortion of the third order component is not removed.

  From the above, I (x) in the prior art (5) is merely a Fourier expansion of the transmitted light pattern on the main scale surface, and does not show the light intensity distribution in the diffraction space behind the main scale. Therefore, in order to remove higher-order harmonic components by changing the ratio of the optical effective aperture of the main scale, the optical effective aperture ratio of the diffraction grating is set to “AR” and the gap dimension is set to “z”. Light intensity distribution formed behind the diffraction grating:

It is necessary to clarify.

  It is necessary to deal with the light intensity distribution based on the so-called diffraction theory, but research on “diffracted images in the Fresnel region of the grating” in that field is in general publications (academic papers or books). It has been studied in detail. (For example, refer nonpatent literatures 1 and 2.). However, there are few studies on the diffraction grating aperture width and harmonic distortion.

This invention was made in consideration of the above circumstances,
Deriving the relational expression I (x, y, z, AR, λ) by a simple method for the interference image by the diffraction grating,
Aperture ratio of rectangular wave amplitude grating on main scale: AR and gap: gap: harmonic distortion contained in x-direction light intensity distribution I (x) of interference image on index scale arranged at z To understand and clarify their relationship,
Based on the result, at the appropriate gap position (z), the harmonic distortion of the displacement signal can be effectively reduced and removed by controlling the aperture ratio: AR of the rectangular wave amplitude grating on the main scale. An object of the present invention is to provide an optical encoder with improved accuracy.

In order to solve the above-described problem, in the present invention, a light source, a scale that moves relative to the light source, and has a first diffraction grating for diffracting a light beam emitted from the light source, A light receiving element array that receives interference fringes formed by superimposing the diffracted light beams diffracted by the first diffraction grating and obtains an output signal; and from the output signal, the light source and the scale In an optical encoder having a calculation unit for calculating a relative movement amount,
The optical effective aperture ratio AR of the first diffraction grating is 40 ± 5% or 72 ± 5%.

Further, the present invention includes a light source, a scale that moves relative to the light source, and has a first diffraction grating for diffracting a light beam emitted from the light source, and a plurality of light receiving elements. A light receiving element array that receives an interference fringe formed by superimposing diffracted light beams diffracted by the first diffraction grating and obtains an output signal, and calculates a relative movement amount of the light source and the scale from the output signal. In an optical encoder having a calculation unit,
When the optical effective aperture ratio of the first diffraction grating is AR, and the gap between the first diffraction grating and the light receiving element array is z,
In the range of z <2 mm, the optical effective aperture ratio AR is 44 ± 5% or 78 ± 5%.
It is characterized by being in the range of

  Conventionally, there has been no optical encoder that can accurately remove harmonic distortion components with the optical effective aperture ratio of the diffraction grating for the purpose of removing and reducing harmonic distortion in the output signal waveform.

  In the present invention, the light intensity distribution I (x, y, z, AR) in the space behind the grating is obtained based on the scalar diffraction theory, and the gap position in which the harmonic component is minimized is related to the aperture ratio AR. By determining and setting the gap with that value, the targeted component is reduced or removed accurately.

  In particular, the effect is tremendous in removing third-order harmonic components that have the greatest influence on waveform distortion.

  Moreover, since the effect can be obtained only by controlling the optical effective aperture ratio of the grating on the main scale, it is economical. Since a uniform aperture ratio is created over the entire main scale, it is not necessary to create non-uniform apertures as in the prior art, and it is easier to form and manufacture pitches and aperture widths uniformly at equal intervals. .

  Furthermore, it is easy to use together with other waveform distortion removing means, and the entire distortion removing effect is further enhanced. For example, it is possible to share the removal component and take countermeasures by simultaneously performing optical filtering using the index scale found in the prior art. It is also possible to remove the third-order harmonic component with the main scale according to the present invention and remove the fifth-order component with the index scale.

  The present invention can be applied to both transmission and reflection types as scales to be applied, and is not limited to the linear type shown in the embodiment, and can also be applied to a rotary type radial grating. In addition, the present invention can be similarly applied to a cylindrical scale arranged on a cylindrical curved surface in consideration of the influence of the curved surface to some extent.

  Further, the luminous flux irradiating the main scale is not limited to the parallel luminous flux illumination, but can be applied to a type illuminating the main scale with a divergent luminous flux, and the same effect can be obtained. This can be done by applying various ideas.

FIG. 1 is a diagram showing the overall configuration of an embodiment of the present invention and an optical system Diagram for explaining the outline of the electrical signal unit The figure for demonstrating the optical scale of this invention Diagram for explaining the calculation formula of diffraction theory Diagram showing diffraction efficiency of square wave amplitude grating Diagram showing the relationship between the scale aperture ratio AR and the actual diffracted light The light intensity distribution is shown by the relationship between the displacement direction x calculated based on the scalar theory and the gap z. The graph for demonstrating the application example 1 of Example 1 of this invention The graph for demonstrating the application example 2 of Example 1 of this invention The graph for demonstrating the application example 3 of Example 1 of this invention The graph for demonstrating the application example 3 of Example 1 of this invention The graph for demonstrating the specific application example of Example 1 of this invention The figure which shows the structure of the reflective encoder of Example 2 of this invention. Optical equivalent model diagram of divergent light beam illumination system of Embodiment 2 of the present invention The graph for demonstrating the specific application example 4 of Example 2 of this invention The figure for demonstrating the structure of the type attached to the cylindrical body of Example 3 of this invention. Prior art diagram

The present invention changes the grating shape on the main scale of the optical encoder, and the aperture ratio of the diffraction grating that minimizes the optical intensity distribution harmonic distortion in the scale displacement direction x in the rear space after the transmission through the scale. The AR and the gap position z are calculated, and the index scale is arranged at the aperture ratio AR of the main scale and the gap position z.
A specific method is shown below.

Example 1)
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a schematic configuration of the optical encoder of the present embodiment. The main scale 110 on which lattices with equal intervals are formed is fixed to a moving measurement object and moves in the lattice arrangement direction (x-axis direction) indicated by an arrow in the figure. The main scale 110 receives light from the light source 112 and forms interference fringes behind it. In this embodiment, the grating formed on the main scale 110 functions as a rectangular wave amplitude grating (first grating).

  The light receiving element array 114 is fixedly arranged at a position where an interference fringe is formed by diffraction of the mains scale 110. On the light receiving element array 114, light receiving elements are arranged in four areas in a lattice pattern. In the figure, the four light receiving element areas will be described.

  In the figure, an upper right light receiving element (hereinafter a phase light receiving element) 116, an upper left light receiving element (b phase light receiving element) 117, a lower right light receiving element (c phase light receiving element) 118, and a lower left light receiving element (d The phase light-receiving elements 119 are each formed with a lattice having a pitch equal to the pitch of the interference fringes, and are arranged with a quarter pitch, that is, with a phase of 90 °. When the a phase is used as a reference, the b phase is 90 degrees, the c phase is 180 degrees, and the d phase is 270 degrees. When each of the a-phase, b-phase, c-phase, and d-phase light receiving elements 1116, 117, 118, and 119 receives light, it generates a voltage corresponding to the amount of light. Therefore, as the scale 110 moves, the a-phase to d-phase light receiving elements 116 to 119 output varying voltage signals. In this manner, the A-phase to D-phase light receiving elements 116 to 119 formed in a lattice shape are replaced with the index scale 120 (second lattice) having the a-phase to d-phase detection optical grating, and each detection is performed behind the index scale 120. The light receiving elements a to d corresponding to the optical gratings may be arranged. Output signals of the a-phase to d-phase light receiving elements 116 to 119 are sent to the electric circuit unit 121.

FIG. 2 shows a schematic circuit configuration of the electric circuit unit 121.
The electric circuit unit 121 includes a light emitting circuit of the light source 112, an analog signal processing unit 123, and a position calculation unit 122 that calculates the amount of movement of the scale 110 to obtain the position of the measurement object.

  In the analog signal processing unit 123, the amplification circuit of each light receiving element, the light receiving element having the opposite phase, that is, the A phase as a differential output between the a phase and the c phase, and the B phase output as a difference between the b phase and the d phase. Getting a signal. The output signals VA and VB from the analog signal processing unit 23 input to the position calculation unit 122 can be divided into AC components Va and Vb and a DC component Vref2.

The position calculator 122 first obtains the number of interference fringes due to diffraction by counting the signal peaks from the output signal of the A phase (VA = Va + Vref2) or the B phase (VB = Vb + Vref2). If the pitch of the interference fringes is multiplied by the counted number, the amount of movement of the scale 110 can be roughly understood. The amount of movement below the interference fringe pitch can be calculated by calculating the phase angle based on the AC components of the A-phase and B-phase output signals.
In this embodiment, the phase angle is calculated by calculating an arctangent value of the A and B phase sine wave signals to obtain an arctangent value (ArcTan value).

FIG. 3 shows a cross-sectional view of a main scale 110 that is a component to which the present invention is applied.
The diffraction grating disposed on the main scale 110 is a rectangular wave amplitude grating having a pitch of P, and having a pitch dimension of P, and has a width d (= optical effective aperture width) of the light transmitting portion. The ratio of the optical effective aperture width to the pitch dimension P = the optical effective aperture ratio (Aperture Ratio) is AR, and is defined as follows.

A light beam diverging and radiating from the light source 112 to the main scale is irradiated to the main scale 110 as a substantially parallel light beam by the collimator lens 113. At this time, taking the displacement direction of the main scale as the x-axis and the gap direction as the z-axis, the intensity transmittance distribution I (x, z) of the main scale 110 becomes a rectangular pattern as shown in the lower part of FIG. Is expressed as follows.

  In the present invention, the aperture ratio AR of the main scale is handled as a main design parameter, and the value of AR is optimized so that the target harmonic distortion component is minimized. The present embodiment aims to reduce the amplitude of unnecessary third-order harmonic components included in the A and B phase signals, which are displacement signals, and a method for reducing the amplitude will be described below.

For this purpose, the diffraction calculation method according to the present invention will be briefly described first.
In the present invention, when the diffraction grating disposed on the main scale is irradiated with the light beam from the light source, as a result of natural superposition of the diffracted light generated by the grating, the diffraction grating is formed in the diffracted light beam overlapping portion in the space behind the grating. Diffraction calculation is performed on the intensity distribution of the light.

The diffraction theory includes a scalar diffraction theory that ignores the influence of the polarization state of the light and the electromagnetic properties of the aperture, and a vector diffraction theory that takes those influences into account. In the present invention, the above diffraction calculation is performed based on the scalar diffraction theory. In applying the scalar theory, the gap between the main scale and the index scale is premised on at least one wavelength, and if the light source wavelength is λ and the value is assumed to be 0.65 μm, it is about 10 times. It has already been shown that a profile that is not different from the exact solution can be obtained by the scalar theory in the range of pitch, that is, P = 6.5 μm or more (see Non-Patent Document 2).
In scalar theory, it is generally obtained in the form of Fresnel integrals.

  The method using Fresnel integration is shown in FIG.

And is obtained by performing diffraction integration over the entire opening.
It is given by the following equation, with the Inclination factor being constant.

By the way, what is generated on the diffractive surface is only Fraunhofer diffraction, and even with the scalar theory, the image amplitude distribution must be expressed in the form of the sum of plane waves.
The intensity distribution I is

Given in. Here, Tn is the amplitude of the nth-order diffracted light.

Equations (6) and (7) give slightly different results in the actual calculation, but the calculation results are almost the same in the previous application range, and the calculation result in the Fresnel integral expression has a fine ripple in the intensity profile. When the integration range is changed, the shape of the ripple also changes. In the point where the validity of the approximation of equation (5) is questioned, ripples do not appear in equation (7) expressed by superposition of traveling plane waves, and the calculation is simple. Furthermore, when considering the spatial frequency components involved in the waveform distortion to be handled here, the target harmonic components are at most in the range from the 3rd order to the 9th order at most, so the number of diffracted waves involved is 5 It is considered sufficient to consider even the folding light.

Table 1 shows the relationship between the combination of diffracted waves and the order of the generated harmonic components. In this embodiment, the removal of the third harmonic component is considered.
In deriving the calculation formula, the intensity level of the fifth-order diffracted light was ignored as the diffraction intensity, and up to ± 3rd-order diffracted light was used as the calculation target. Therefore, as a combination of the diffracted light of the third harmonic component, the generation of the third harmonic component included in the displacement waveform is mainly influenced by the 0th order, ± 1st order, A combination of ± 2nd order and ± 3rd order diffracted light components is formed only at (0, +3), (0, −3), (+1, −2), and (−1, +2). In this embodiment, the aperture width of each light receiving element of the index scale is set to 1/2 of the index scale pitch P, and the 180 ° inverted signal is differentially amplified by the electric signal processing circuit configuration. Therefore, the harmonic component of the even-numbered order is canceled and does not need to be considered.

  FIG. 6 shows the generation of the 0th and ± 1st to ± 3rd order diffracted waves involved in this example. In FIG. 5, the diffraction intensities of the respective orders are shown. A and c in the figure correspond to aperture ratios AR = 33.33% and 66.67%. In this part, the third-order diffracted light lacks the order. When AR = 50%, the order of the second-order diffracted light is lacking. 6B and 6C correspond to each.

  As described above, in the present invention, when obtaining the profile of the diffraction image by the rectangular wave amplitude grating on the main scale, diffraction calculation is performed based on plane wave expansion even if the observation point is not the far field. Further, in order to simplify the calculation and increase the speed, the diffraction beam amplitude corresponding to the coefficient Tn in the equation (7) is expressed by the following equation. Expressions (9) and (10) representing the diffraction intensity of each order of the rectangular wave amplitude grating were used. m represents the diffraction order.

I ₀ is omitted, and the amplitudes are as follows.

FIG. 5 shows the relationship between the optical effective aperture ratio AR of the diffraction grating and the diffraction amplitude value.
Next, using these amplitude values, each diffracted wave is expressed as a plane wave in a complex number display.
However, in the following mathematical notation, the opening width of the main scale: d
Main scale pitch: P
Light source wavelength: λ
Gap direction: z
Scale displacement direction: x.
Here, the y-axis direction is a lattice extending infinitely, and its influence is ignored.
The plane waves of the diffracted light of a total of 7 waves from the 0th order and the ± 1st order to the 3rd order were U0, U1p, U1m, U2p, U2m, U3p, and U3m, respectively.
Each diffracted light is expressed by the following equations (13) to (19).

0th order diffracted light U0

+ 1st order diffracted light amplitude U1p

−1st order diffracted light amplitude U1m

+ 2nd order diffracted light amplitude U2p

-2nd order diffracted light amplitude U2m

+ 3rd order diffracted light amplitude U3p

-3rd order diffracted light amplitude U3m

It becomes.

Next, the combined amplitude of the diffracted light of 7 waves from the 0th order and the ± 1st order to the 3rd order, that is, the sum thereof, is multiplied by the complex conjugate number to obtain the combined intensity,
The respective composite conjugates are UU0, UU1p, UU1m, UU2p, UU2m, UU3p, UU3m, and the synthesized waves f1 and f2 are as follows.

  The interference image from the diffraction grating on the main scale is obtained by the product of the complex amplitude sum f1 and its complex conjugate f2. That is, the combined intensity I (x, z, AR, λ) is replaced with AR = d / p,

The above expression (23) expresses the intended light intensity distribution.
However, since all the diffracted light of 7 waves from 0th order and ± 1st order to 3rd order is synthesized, it contains 2nd order harmonics, 4th order harmonics, 5th order harmonics and 6th order harmonics. The above-described intensity distribution is derived by simplifying the equation by removing even-order harmonics that are canceled out in this case and fifth-order harmonics that are less affected here.

If only the fundamental wave and the third harmonic component are handled by excluding the even frequency component and the fifth harmonic component from the combined intensity equation (23), the following f3 becomes the intensity combined equation. That is, the fundamental wave and the third harmonic component wave (0, +3), (0, -3), (+1, -2), (-1, +2) of (0, +1), (0, -1) Therefore, it is simplified to the following formula f3.

It is simplified as follows.
7A is a light intensity distribution represented by the equation (23). FIG. 7B is a light intensity distribution represented by the equation (25). This data includes the main scale pitch P = 128 μm, the light source wavelength λ = 0. It is calculated at 65 μm.
The light intensity distribution is represented by a gap z and a displacement direction x. The bright area indicates the strong area. In either case, the horizontal axis of the graph indicates the gap position z, and the vertical axis indicates the x-axis direction.
In (A), since all distortion components are taken into the equation, even-order spatial frequencies are included, and it can be seen that the secondary components are particularly strong. On the other hand, (B) shows only the fundamental wave and the third-order component, and the portion where the amplitude of the third-order harmonic component is large and the portion where the amplitude is small are well understood.

As described above, the light intensity distribution in the displacement direction x is particularly derived with respect to the two parameters of the aperture ratio of the diffraction grating (AR = d / p: Aperture Ratio) and the gap dimension (z: GAP) between the main scale and the index scale. .
Next, the effects of high-order harmonic distortion removal and reduction by this method will be described specifically with some designs and configuration examples.

Now, the relationship between the aperture ratio AR and the gap z is specifically evaluated for the third-order harmonic distortion using Equation (25) below.
From the relational expression (25) of the light intensity distribution I (x, z, AR, λ), a method of evaluating the distortion rate in the distribution and deriving the condition that minimizes the distortion was shown.

Application example 1)
In FIG. 8, when the grid pitch of the main scale 110 is 128 μm, the optimum aperture ratio of the main scale in the case where the design center is set to 1500 μm and the center position is set will be obtained. The light source wavelength λ is 0.65 μm.

  The upper diagram in FIG. 8 shows the relationship between the aperture ratio AR and the distortion factor of the third-order component when the gap is 1500 μm using the optical intensity distribution equation (25) derived in the present invention. In the figure, the horizontal axis represents the optical effective aperture ratio AR, and the vertical axis represents the fundamental wave amplitude on the left side and (distortion amplitude / fundamental wave amplitude) on the right side. This result does not represent the distortion superimposed on the actual displacement waveform of the encoder because the distortion is calculated by evaluating the optical intensity distribution. However, when converted to an actual encoder waveform, a certain degree of filtering action is performed on the index scale, so the value decreases to approximately 1/3 to 1/5. There is no problem in determining the optical effective aperture ratio that minimizes the distortion. From the upper graph, the optical effective aperture ratio AR that minimizes the distortion was determined to be 52%. The graph in the middle of FIG. 8 is a gap characteristic diagram when the aperture ratio determined above is 52%. In the upper graph of FIG. 8, only the horizontal axis is changed to the gap position (z). It is shown that the third-order component attenuates and takes a minimum value at a gap of 1500 μm. 8 is an intensity distribution density diagram in which the horizontal axis is the gap vertical axis and the displacement direction is x. The spatial frequency of the third-order component cannot be found at the gap position where the white portion is where the light intensity is strong and the distortion is minimal.

Application example 2)
Next, the optimum aperture ratio of the main scale in the case where the design center is set to 1000 μm and the setting center position is provided will be obtained. The lattice pitch of the main scale 110 was the same as the previous example. The light source wavelength λ is 0.65 μm.
The upper diagram in FIG. 9 shows the relationship between the aperture ratio AR and the distortion factor of the third-order component when the gap is 1000 μm, as in the previous application example.

In this application example, there are local minimum values as optical effective aperture ratios at two locations of AR = 44% and AR = 78%. Comparing the two points, when the magnitude of the fundamental wave amplitude is considered as the second selection criterion, the fundamental wave amplitude is larger at the gap point with the aperture ratio of 78%. In this case, it is preferable to set with an aperture ratio of 78%.
From this upper graph, the optical effective aperture ratio AR that minimizes the distortion was determined to be 78%. The middle graph in FIG. 9 is the same as the middle graph in FIG. 8 and shows the gap characteristics at this 78% aperture ratio.

  It is shown that the third-order component attenuates and takes a minimum value at a gap of 1000 μm. The lower diagram of FIG. 9 is an intensity distribution density diagram in which the horizontal axis is the gap vertical axis and the displacement direction is x. The spatial frequency of the third-order component cannot be found at the position of the gap of 1000 μm, which is the gap position where the distortion is minimal where the light intensity is high. In addition, it is easily estimated from this figure that a trapezoidal wave is clearly obtained from this density diagram when a closer gap is used, and a triangular wave is formed in the direction in which the gap is separated.

Application example 3)
Next, a case where the pitch P of the main scale 110 is 40 μm will be described.
In many cases, such a fine pitch is set to a gap position where interference fringes have a high contrast, that is, a gap point where a so-called Fourier image is formed.

  In the optical system in this embodiment, the scale is illuminated with a parallel light beam, so the position where the Fourier image is formed in the parallel light beam illumination system can be obtained from equation (26).

z _n indicates the formation position of the Fourier image and is also the period of change in contrast of the fundamental wave that changes periodically. n is a natural number and corresponds to the number of the Fourier image formed discretely with respect to the gap.

Here, the case where a gap is set at the position of the first Fourier image is handled.
(Hereinafter, this first Fourier image characteristic is referred to as a 1st.Peak characteristic.)
When P = 40 μm, λ = 0.65 μm, and n = 1, the gap position z ₁ ≈2461 μm.

  FIG. 10 is a diagram for explaining a case where the waveform distortion countermeasure action in the present invention is not performed. In general, the aperture ratio in a rectangular wave amplitude grating is set to 50%. FIG. 10 shows the result of evaluating the degree of occurrence of the third harmonic distortion at this time by the evaluation formula (25) of the present invention.

The upper diagram of FIG. 10 is a gap characteristic diagram in which the vertical axis represents the fundamental wave amplitude, the distortion rate, and the horizontal axis represents the gap.
In this characteristic diagram, at a gap position z ₁ ≈2461 μm at which a Fourier image is formed, a gap with poor conditions is observed with respect to waveform distortion overlapping with a partial peak position (position where distortion becomes large) of a distortion rate characteristic that varies periodically. Position.

  Further, considering the case where the aperture ratio is selected giving priority to the maximum condition of the amplitude value of the fundamental wave, 64.5% is selected as the aperture ratio from the lower graph of FIG. The value of the second harmonic distortion is large.

  In such a case, by setting the aperture ratio of the grating on the main scale according to the present invention, the minimum point of the harmonic distortion can be set to a desired gap position (= the gap position where the Fourier image is formed here). Become.

  This is specifically shown in FIG. According to the evaluation formula (25) according to the present invention, there are local minimum values as optical effective aperture ratios at two locations of AR = 40.55% and AR = 72.2%. Considering the magnitude of the fundamental wave amplitude as the second selection criterion as in the previous example when comparing the two points, the fundamental wave amplitude is larger at the aperture ratio of the aperture ratio of 72.2%. It is preferable to set the aperture ratio.

  In this example, the case where a main scale having a pitch dimension of 40 μm is used has been described. However, the same result as described above can be obtained at a gap point where a Fourier image is formed regardless of the pitch value.

FIG. 12 is a gap characteristic diagram in which the relationship of the equation (26) is applied to the equation (25), the gap size is normalized by the fundamental period z ₁ of Fourier image generation, and the relationship is shown.

12A, 12B, and 12C show the gap characteristics when the scale pitch is 20 μm, 200 μm, and 2000 μm, respectively. As described above, the distortion is minimized at the formation position in the Fourier image, that is, the value of 1 on the horizontal axis, and when the gap is normalized by the Fourier image generation cycle z ₁ , the same characteristics are shown. Yes.

Therefore, the following things were understood by the following present invention.
When the grating pitch of the main scale is P and the light source wavelength is λ, when the collimator lens is used to illuminate the parallel light flux with this light source, the gap in which the Fourier image is formed is indicated by (26) and is used for setting the gap. In this case, the third-order harmonic distortion is minimized by setting the aperture ratios to 40.55% and 72.2%.
As described above, three specific application examples of the present invention have been shown. However, the important point is that in all the application examples, the above relationship is the relational expression between the aperture ratio and diffraction efficiency of each diffracted light included in the expression (25) This holds true when the equations (11) and (12) are applied. That is, an ideal rectangular wave amplitude grating is assumed.

  However, if the diffraction efficiency of the main scale that is actually used is different, the above aperture ratio will deviate from this value. Specific examples of the scale used with the rectangular wave amplitude grating include various types such as a photoengraving film scale, a metal etching scale, and a metal vapor deposition film on a glass surface and a grating provided by etching. Further, the relationship between the aperture ratio and the diffraction efficiency does not necessarily match the expressions (11) and (12) due to the manufacturing accuracy of the grating, the linearity of the grating edge, and the like.

  Thus, since the diffraction efficiency of the rectangular wave amplitude grating actually obtained is considered to depend on the material of the scale, the manufacturing method, the manufacturing accuracy, etc., in actually applying the present invention industrially, By actually measuring the diffraction efficiency of the scale to be used and correcting the value with respect to the equations (11) and (12), the harmonic distortion component can be accurately removed and the aperture ratio that can be reduced can be obtained accurately. The practical value of the method shown increases.

  When the pitch dimension of the main scale is relatively coarse, waveform distortion is significant because high-order diffracted waves are included in the effective optical path. For example, consider the case of 100 μm or more.

  The generation position of the Fourier image is 15 mm or more when the light source wavelength λ is set to about 0.65 μm, and is not actually set with such a gap size. It is often set at 1 mm or less. In this case, the waveform distortion superimposed on the displacement signal becomes a problem when electrical signal division is performed using such a coarse pitch main scale.

  Already, the distortion avoidance when the pitch is set to 128 μm in the application examples 1 and 2 has been described. However, as shown in this example, by appropriately setting the aperture ratio that minimizes the distortion with a desired setting gap, The harmonic distortion of can be avoided.

Example 2)
Next, a case where the main scale is irradiated with a divergent light beam will be described with an example.
In Example 1), equation (25) was derived for the diffraction image when the main scale was irradiated with a parallel light beam. In this example, consider the case where the main scale is irradiated with a divergent light beam.

  FIG. 13 is a configuration diagram of a reflective encoder. The main scale 110 is irradiated with the divergent light beam from the light source 112 as it is, and interference fringes are formed on the index scale 120 by the reflected diffracted light from the main scale 110. The index scale 120 and the main scale 110 are arranged with a gap Z. In this configuration, when the main scale pitch is P, interference fringes having a period twice the main scale pitch are formed on the index scale surface. Hereinafter, since it is the same as that of the first embodiment, the description thereof is omitted.

  The present invention can also be applied to an encoder having such a configuration. First, an equivalent optical system having the configuration shown in FIG. 13 is shown in FIG. In FIG. 14, the movement direction axis of the central main scale is X0, the index scale is X2, and the light source is arranged on the X1 axis, and is arranged at the coordinate L0. The gap corresponding to the main scale and index scale gap Z shown in FIG. 13 corresponds to the distance between X0-X2 and X0-X1 in FIG. A light beam diagram is drawn in which a divergent light beam is irradiated from the position of the light source coordinate L0 onto the main scale arranged on the X0 axis, and the light beam is diffracted on the diffraction grating surface. The light reaching the origin (0, 0) on the main scale from L0 is diffracted at the diffraction angle θ1 and reaches the index scale. If the direction of the diffracted light beam is extended in the direction opposite to the traveling direction of the light beam, and the point where the origin intersects with a circle with a radius Z having (0.0) is L + 1, the diffracted light beam has a light source arranged at L + 1. A wavefront similar to the light wave formed is formed. Therefore, the diffracted light beams of the respective orders are arranged on the circumference on the radius Z according to the diffraction orders as in such virtual light source points L + 1, L-1,. An interference image can be calculated as a superposition. By treating that the light emission intensity at each virtual point emits light with the intensity according to the diffraction efficiency shown in the first embodiment 1), the interference fringe light by superimposing the diffracted light beams on the main scale as in the embodiment. Intensity distribution can be calculated. However, in Example 1, the superposition of plane waves is used. In this case, the superposition of divergent light beams is performed, but the number of wave fronts to be handled can be considered only the number of diffracted light beams to be handled as in the previous calculation. As in the first embodiment, the interference fringe intensity distribution on the index scale surface can be obtained very easily.

In the following, the derivation process in the middle is omitted and the results are shown.
The expression representing the light wave of each diffraction order is as follows. The intensity of each diffracted wave is expressed by the above (11) and (12), and each diffracted wave is expressed as a spherical wave in a complex number display.
However, in the following mathematical expression, a term that is inversely proportional (1 / z) to the distance by which the light wave attenuates before reaching the observation point is omitted. Hereinafter, as in Example 1, the opening width of the main scale: d
Main scale pitch: P
Light source wavelength: λ
Gap direction: z
Scale displacement direction: x.
Here, the y-axis direction is a lattice extending infinitely, and its influence is ignored.
The spherical waves of the diffracted light of a total of 7 waves from the 0th order and the ± 1st order to the 3rd order were U0, U1p, U1m, U2p, U2m, U3p, and U3m, respectively.
Each diffracted light is expressed by the following equations (27) to (33).

0th order diffracted light U0

+ 1st order diffracted light amplitude U1p

−1st order diffracted light amplitude U1m

+ 2nd order diffracted light amplitude U2p

-2nd order diffracted light amplitude U2m

+ 3rd order diffracted light amplitude U3p

-3rd order diffracted light amplitude U3m

Next, the combined amplitude of the diffracted light of 7 waves from the 0th order and the ± 1st order to the 3rd order, that is, the sum thereof, is multiplied by the complex conjugate number to obtain the combined intensity,
The respective composite conjugates are UU0, UU1p, UU1m, UU2p, UU2m, UU3p, UU3m, and the synthesized waves f1 and f2 are as follows.

  The interference image from the diffraction grating on the main scale is obtained by the product of the complex amplitude sum f1 and its complex conjugate f2. That is, the combined intensity I (x, z, AR, λ) is replaced with AR = d / p,

  The above result is the following equation (37).

As in the first embodiment, only the combination of diffracted light waves that contribute to distortion of the third harmonic component is calculated.

And
Finally, the intensity distribution of the diffraction image on the main scale illuminated with the divergent light beam (the intensity distribution of only the component that affects the third-order harmonic component) is expressed by the following equation (38).

The effect of the third-order harmonic distortion removal in the reflective encoder will be specifically described with an application example using the equation (38).

Application example 4)
The case where the pitch P of the main scale 110 illuminated with the divergent light beam in the reflective encoder having the optical arrangement of the second embodiment is set to 40 μm will be described. In the same way as in the previous embodiment, when this fine pitch is obtained, in many cases, a gap position having a high contrast of interference fringes, that is, a gap point where a so-called Fourier image is formed is set.
In the optical system according to the second embodiment, unlike the first embodiment, the main scale is illuminated with a divergent light beam. Therefore, the position where the Fourier image is formed in the divergent light beam illumination system is obtained from the following equation (38). .

z _n indicates the formation position of the Fourier image and is also the period of change in contrast of the fundamental wave that changes periodically. n is a natural number and corresponds to the number of the Fourier image formed discretely with respect to the gap.

  When compared with the first embodiment of the parallel beam illumination system at the main scale pitch P in which the generation cycle of the Fourier image is the same, the fluctuation of the contrast occurs in the cycle twice. In this type of reflection type, the gap sensitivity is insensitive compared to the first embodiment, and the gap alignment tolerance is wide.

Here again, the case where a gap is set at the position of the first Fourier image is handled as in the case of the parallel beam illumination.
(Hereinafter, this first Fourier image characteristic is referred to as a 1st.Peak characteristic.)
When P = 40 μm, λ = 0.65 μm, and n = 1, the gap position z ₁ ≈4923 μm.

FIG. 15 shows an aperture ratio characteristic diagram and a gap characteristic in this application example.
According to the distortion evaluation formula (38) for the divergent light beam according to the present invention, there are minimum values as optical effective aperture ratios at two locations of AR = 40.55% and AR = 72.2%. This value is the same value as in the case of the parallel beam illumination described above. It is considered best to select this value with an aperture ratio of 72.2% as the amplitude of the fundamental wave is large.

  In the gap characteristic diagram of FIG. 15, the third-order harmonic distortion is well removed and reduced at the set gap position 4923 μm.

  Also in the case of this divergent light illumination, the same result as above can be obtained regardless of the pitch value at the gap point where the Fourier image is formed as in the first embodiment.

Example 3)
In the embodiments so far, the main scale is of the transmissive type or of the reflective type, but both are different from those in which a grid is formed on a plane. The scope of application of the present invention is broad and can be applied to a type having a special configuration from an encoder having a general configuration. In the third embodiment, a grid is arranged on a curved surface.

  FIG. 16 shows a specific product mounting example. This is a part of the configuration of an optical encoder mounted on an interchangeable lens of a single-lens reflex camera. FIG. 9A shows a portion 301 on the inner side surface of the cam ring 300 for the main scale 110 to drive the autofocus lens. FIG. 5 is a configuration diagram of a rotary encoder that is attached with a double-sided tape and measures the rotation angle of a cam ring 300 that is an angle measurement target member with a detection head 200; As shown in the figure, a configuration example in which the main scale is a curved lattice is shown in FIG. FIG. 3C shows the configuration of the detection head 200, which includes a light source 112 and a light receiving element array section 120 that performs the same optical action as the index scale. Z indicates a substantial gap dimension in this configuration.

  When such a curved main scale is used, it is not easy to accurately obtain the diffraction intensity by the diffraction grating because the divergent light beam from the light source is wavefront transformed on the main scale surface. It is also possible to apply to such a configuration by superimposing diffracted waves in consideration of the wavefront curvature change.

  Specifically, in the reflection type optical calculation model shown in FIG. 14, the change in curvature of the wavefront received by the curved grating is obtained by curved surface diffraction at the coordinate points of the virtual light source points L + 1, L-1,. By changing to match the wavefront curvature received by the grating, the intensity distribution of the diffraction image can be obtained in the same way as in the previous examples, and the third harmonic is obtained from the intensity distribution equation of the obtained interference image. The aperture ratio of the main scale 110 that minimizes the components can be determined.

  As described above in Examples 1 to 3, the present invention can be applied to both transmission and reflection types as the scale to be applied, and is not limited to the linear type shown in the embodiment. It is also applicable to a rotary type radial grating. In addition, the present invention can be similarly applied to a cylindrical scale arranged on a cylindrical curved surface in consideration of the influence of the curved surface to some extent.

  Further, the luminous flux irradiating the main scale is not limited to the parallel luminous flux illumination, but can be applied to a type illuminating the main scale with a divergent luminous flux, and the same effect can be obtained. This can be done by applying various ideas.

  In the embodiment, reduction and removal of the third-order harmonic component is mainly taken up. However, in this method, not only the third-order component but also other components can be applied, and a plurality of higher-order components can be handled. It is.

DESCRIPTION OF SYMBOLS 110 Main scale 112 Light source 113 Collimating lens 116 Photodiode array 117 Photodiode array 118 Photodiode array 119 Photodiode array 120 Index scale 121 Electric circuit unit 122 Position calculation part 123 Signal processing part 200 Detection head 300 Cylindrical body (301 scale to be measured 301 scale) Fixed part 1 Main scale 10 Transparent substrate 11 Light transmission part (signal transfer part)
12 Non-transparent part (non-transfer part)
2 Light irradiation means 20 LED
21 Collimating lens 22 Parallel light 3 Photodiode array 30 Silicon substrate 31 Photodiode

Claims (5)

A light source; a scale that moves relative to the light source and has a first diffraction grating for diffracting a light beam emitted from the light source; and a plurality of light receiving elements, the first diffraction grating A light receiving element array that receives the interference fringes formed by superimposing the diffracted light beams diffracted in step (b) and obtains an output signal; and an arithmetic unit that calculates a relative movement amount of the light source and the scale from the output signal. In optical encoder,
The optical encoder having an optical effective aperture ratio AR of 40 ± 5% or 72 ± 5% of the first diffraction grating. The wavelength of the light source is λ, the pitch of the first diffraction grating is P, the arrangement direction of the first diffraction grating is x, the gap between the first diffraction grating and the light receiving element array is z, and the light receiving element array When the period of the fundamental wave of the interference fringes superimposed on PO is PO,

If
The optical encoder according to claim 1 satisfying a range of z = Zn ± 0.1 × Z1. A light source; a scale that moves relative to the light source and has a first diffraction grating for diffracting a light beam emitted from the light source; and a plurality of light receiving elements, the first diffraction grating A light receiving element array that receives the interference fringes formed by superimposing the diffracted light beams diffracted in step (b) and obtains an output signal; and an arithmetic unit that calculates a relative movement amount of the light source and the scale from the output signal. In optical encoder,
When the optical effective aperture ratio of the first diffraction grating is AR, and the gap between the first diffraction grating and the light receiving element array is z,
In the range of z <2 mm, the optical effective aperture ratio AR is 44 ± 5% or 78 ± 5%.
An optical encoder characterized by being in the range. The optical encoder according to any one of claims 1 to 3, wherein the scale having the first diffraction grating has a curved surface shape. The lens apparatus carrying the optical encoder as described in any one of Claims 1 thru | or 4.

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