JP2013134211A - Optical encoder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an excellent light intensity distribution contrast of a Fourier image of a grating pattern formed on a slit which forms a slit pair and is on a side close to a light source on a grating pattern of the other slit with respect to an optical encoder, and to generate an excellent pseudo-sine wave signal without depending upon the numerical aperture of an illumination optical system.SOLUTION: The two grating patterns formed on the two slits forming the slit pair are arranged at such an interval that a contrast based upon the interval between the two grating patterns as a variable has an extremal value when it is considered that the two grating patterns are a plane light source.

Description

  The present invention relates to an optical encoder that detects a position, a displacement amount, a rotation amount, a speed, a rotation speed, and the like, and is particularly suitable for application to an optical linear encoder, an optical rotary encoder, an optical linear gauge sensor, and the like. Is.

  Conventionally, optical encoders have been employed as means for detecting position, displacement, rotation amount, speed, rotation speed, and the like (see Patent Documents 1 and 2).

  In this optical encoder, in recent years, high resolution has been remarkable. Since miniaturization of a scale serving as a reference for measuring a displacement amount, a rotation amount, and the like is difficult, a multiplication process is used as a means for achieving high resolution. In order to perform multiplication processing, two pseudo sine waves that are excellent in S / N ratio and well approximated to a sine wave signal that are shifted by a quarter of a period according to the amount of displacement or rotation of the object to be measured A signal needs to be generated.

  1 and 2 are a perspective view and a side view, respectively, showing the basic configuration of the optical system linear gauge.

  As means for generating two good pseudo sine wave signals, for example, an optical linear gauge conventionally has the basic configuration shown in FIGS. Here, the optical axis direction is the Z-axis, the movable direction of the movable slit 3 is the X-axis, and the direction perpendicular to both these axes is the Y-axis.

  The light source unit 1 often uses a light source with low spatial coherence such as an LED (Light Emitting Diode) because of demands for durability and cost. Let λ be the wavelength of the luminous flux emitted by the LED of the light source unit 1. However, in general, the emission spectrum of an LED is not monochromatic but has a continuous spread. Λ shown here includes λ when the maximum value of the spectrum is shown, λ when the average value is shown, and the like, and is a representative wavelength emitted by the LED of the light source unit 1.

  A light beam having a wavelength λ emitted from the light source unit 1 enters the illumination optical system 2, is converted into a substantially parallel light beam, and enters a movable slit 3 disposed substantially perpendicular to the optical axis. The movable slit 3 is connected to a shaft 6 for measuring the displacement of the object to be measured.

  FIG. 3 is a schematic view (A) of the movable slit and a partially enlarged view (B) thereof.

  On the back surface of the movable slit 3, as shown in FIG. 3B, a rectangular gray lattice pattern in which the lattice vector faces in the direction parallel to the X axis, the constant is d, and the gray length ratio is 1: 1. Is carved.

  The light beam having the wavelength λ that has passed through the rectangular shading grating pattern of the movable slit 3 is diffracted by the grating pattern and is incident on the fixed slit 4 disposed substantially parallel to the movable slit 3 (see FIGS. 1 and 2). ).

  FIG. 4 is a schematic diagram (A) of the fixed slit and a partially enlarged view thereof.

  As shown in FIG. 4, the fixed slit 4 is a pattern similar to the light and shade grating pattern carved in the movable slit 3, the grating vector direction is parallel to the X axis, and the phase difference is 1/4 period. Two types of shaded grid patterns that are separated are carved on the surface of the fixed slit 4. Hereinafter, these two types of gray grid patterns will be represented as A phase and B phase, respectively, according to the custom.

  Furthermore, in order for the Fourier image of the rectangular gray lattice pattern on the back surface of the movable slit 3 to be satisfactorily formed on the surface of the fixed slit 4, the gap length z between these surfaces is

Is set in the vicinity. This condition is the most desirable gap length when the light source unit 1 is a point light source.

  When the light source unit 1 is a point light source such as a laser, the Fourier image of the rectangular gray lattice pattern on the back surface of the movable slit 3 formed on the surface of the fixed slit 4 is distributed in the same rectangular manner as the lattice pattern. However, when the light source unit 1 is a surface light source such as an LED, the substantially parallel light beam provided by the illumination optical system 2 has a numerical aperture. If the focal length of the illumination optical system 2 is optimized, the rectangular image is affected by the numerical aperture of the illumination optical system 2, and the image appears to be blurred, as if it were a sine wave. This image is generally used as a pseudo sine wave.

  The light flux having the wavelength λ having the light intensity distribution like the sine wave passes through the light and shade grating pattern carved in the fixed slit 4 so that the light intensity varies according to the displacement of the movable slit 3. It will be. For example, when the movable slit 3 is moving at a constant speed in the moving direction, the light intensity of the light beam passing through the fixed slit 4 will oscillate in a pseudo sine wave as time passes. At this time, the quasi-sinusoidal light intensity due to the light flux having the wavelength λ that has passed through the two grayscale patterns carved in the fixed slit 4, the A phase and the B phase has a phase difference of ¼ period.

  The light flux having the wavelength λ that has passed through the two rectangular gray-scale grating patterns of the fixed slit 4, the A phase and the B phase, is incident on the separate photosensors 5 a and 5 b constituting the photodetector 5. Converted. In many cases, these photocurrents are converted into a voltage, and an electric signal converted into a finer resolution than the density grid pattern carved in the movable slit 3 and the fixed slit 4 is output by a multiplication processing circuit.

  Conventionally, such a configuration corresponds to high multiplication.

JP 05-346330 A JP 2006-58116 A

  The above-described gap length setting value between the light and shade grid patterns is an optimum value when the light source can be regarded as a point light source such as a laser beam. At this time, as is well known, the Fourier image of the rectangular gray lattice pattern on the back surface of the movable slit exhibits the maximum contrast and provides a signal with a good S / N ratio.

  However, when a surface light source typified by an LED is used as the light source as in the background art described above, the rectangular gray lattice on the back surface of the movable slit is set at the gap length setting value between the gray lattice patterns represented by Equation (1). Depending on the pattern, the light intensity distribution of the rectangular gray lattice pattern Fourier image on the back of the movable slit formed on the gray lattice pattern carved in the fixed slit does not exhibit good contrast.

  Usually, the linear motion in which the rotation of the movable slit to which the shaft or the like is attached is constrained by swinging such as pitching and yawing. At this time, the light intensity distribution contrast of the rectangular gray lattice pattern Fourier image on the back of the movable slit formed on the gray lattice pattern carved in the fixed slit shows an extreme value with respect to the gap length between the rectangular gray lattice patterns. Otherwise, the amplitude of the obtained sine wave will fluctuate, making it difficult to perform a good multiplication function. Also, if the light intensity distribution does not show the maximum contrast, a signal with a good S / N ratio cannot be obtained. Here, if the light intensity distribution of the rectangular shading grid pattern Fourier image on the back of the movable slit formed on the shading grid pattern carved in the fixed slit shows the maximum contrast, the rectangular shading grid pattern Fourier image It is obvious that the contrast of the light intensity distribution is the same as showing the extreme value with respect to the gap length between the rectangular grayscale grating patterns.

  In addition, when expressing two pseudo sine wave signals for multiplication processing by blurring of the Fourier image of the movable slit rectangular grayscale pattern depending on the numerical aperture of the illumination optical system, the rectangular grayscale grating of the fixed slit The light intensity distribution of the rectangular shading grid pattern Fourier image on the back of the movable slit formed on the pattern is that the sine and non-stationary conditions of the illumination optical system are almost satisfied. This is a convolution of the Fourier image and blur due to the numerical aperture of the illumination optical system, but in order to obtain a good quasi-sine wave signal, in principle, the blur width must be increased. Since the focal length is reduced and, as a result, the signal amplitude is reduced, the rate of signal amplification is increased in the electric processing system, and the S / N ratio of the pseudo sine wave signal is increased. Detrimental to the improvement.

  In the above-mentioned Patent Document 1, an attempt is made to obtain a signal with good contrast on the premise that an LED having a relatively large light emitting area is used. However, in Patent Document 1, an equation assuming a point light source is adopted. And only incorrect results have been obtained.

  Therefore, in view of such a point, the present invention provides a Fourier image of a lattice pattern formed in a slit on the side close to the light source that constitutes a slit pair even when a surface light source typified by an LED is used as the light source. It is an object of the present invention to provide a good contrast of light intensity distribution on the other slit lattice pattern, and to generate a good pseudo sine wave signal regardless of the numerical aperture of the illumination optical system.

The optical encoder of the present invention is
A light source that emits a light beam with low coherence,
An illumination optical system that receives the incident light beam emitted from the light source and collects the light beam so as to be substantially parallel light;
A movable slit formed in the first one-dimensional lattice pattern and moving in the direction of the lattice vector of the first one-dimensional lattice pattern; a lattice pattern parallel to the lattice vector of the first one-dimensional lattice pattern; and the same lattice constant A second one-dimensional lattice pattern having a movable slit between the illumination optical system and a fixed slit placed at a position sandwiched between the illumination optical system and the movable slit. A slit pair that receives an incident light beam emitted from the system and acts on the light beam; and a photodetector that converts the light beam affected by the slit pair into a photocurrent.
In the slit pair, the gap between the first one-dimensional lattice pattern and the second one-dimensional lattice pattern is an expression

  However,

Is the nth order Bessel function,

Is the numerical aperture of the illumination optical system,

Is the wave number of the luminous flux,

Is a lattice constant common to both the first one-dimensional lattice pattern and the second one-dimensional lattice pattern.
It is characterized by having a movable slit and a fixed slit at each position that satisfies | z | or | z |

  As will be apparent from the equation derivation process described later, the movable slit and the first one-dimensional lattice pattern and the second one-dimensional lattice pattern are positioned at positions where the above-mentioned | z | or | z | By providing the fixed slit, a good pseudo sine wave signal can be generated using a surface light source.

  Here, one of the movable slit and the fixed slit has two one-dimensional grating patterns whose phases are shifted from each other by 90 ° in the grating vector direction, and the photodetector includes the two one-dimensional gratings. It is preferable to have two photosensors for converting each light flux subjected to the action of each pattern into each photocurrent.

  With this configuration, two good pseudo sine wave signals having different phases can be obtained, and highly accurate multiplication processing can be performed.

In the optical encoder of the present invention,
(1) The illumination optical system is a substantially telecentric optical system. (2) The slit pair includes a movable slit and a fixed slit, the normal line of the first one-dimensional grating pattern plane and the second one-dimensional grating pattern. (3) The illumination optical system is an optical system that satisfies a substantially sinusoidal condition. (4) The illumination optical system is (5) The slit pair includes a movable slit and a fixed slit, the numerical aperture

(6) In this case, the numerical aperture is determined to be provided at each position where the gap | z | or the gap in the vicinity of | z |

(7) The slit pair includes a movable slit and a fixed slit.

As described above, it is preferable that the light source unit is provided at each position serving as a gap in the vicinity of | z | or | z | calculated by adopting a wavelength representative of the wavelength distribution of the light beam emitted from the light source unit.

  By satisfying these, | z | is calculated with higher accuracy.

  The illumination optical system may be a lens system (spherical lens, aspheric lens, etc.), a diffractive optical system (Fresnel lens, etc.), or an optical system composed of a combination thereof. Also good.

  According to the present invention described above, a good light intensity distribution contrast of the Fourier image of the lattice pattern formed in the slit closer to the light source that constitutes the slit pair is provided on the lattice pattern of the other slit, A good pseudo sine wave signal is generated regardless of the numerical aperture of the illumination optical system.

It is the perspective view which showed the basic composition of the optical system linear gauge. It is the side view which showed the basic composition of the optical system linear gauge. It is the schematic diagram (A) of a movable slit, and its partial enlarged view (B). It is the schematic diagram (A) of a fixed slit, and its partial enlarged view (B). In the basic configuration shown in FIGS. 1 and 2, the distance z and the contrast when the distance between the lattice pattern on the movable slit and the lattice pattern on the fixed slit is changed based on the above-described equation (14). It is the figure which showed the correspondence.

  Embodiments of the present invention will be described below.

  In an embodiment described below, in order to solve the above-described problem, an optical encoder that detects a position, a displacement amount, a rotation amount, a speed, a rotation speed, and the like, for example, an optical linear gauge sensor, includes an LED. A surface light source with low coherence, which is representative, an illumination optical system that converts the light beam from the light source into substantially parallel, and a light source that is arranged substantially perpendicular to the optical axis of the light source, and is movable to detect displacement, rotation, etc. And a rectangular slit pattern having a rectangular vector having a lattice vector set parallel to the movable direction on the lower surface thereof, and being arranged substantially parallel to the movable slit and having a rectangular gray pattern on the upper surface thereof. Are fixed slits having patterns such as A phase and B phase set in parallel to the rectangular gray lattice pattern engraved in the movable slit, and rectangular shades such as the A phase and B phase. It consists of a group of photodetectors that separately detect the light flux of wavelength λ that has passed through the pattern, and the amount of change in the amplitude of the pseudo sine wave signal is small and good pseudo sine even when the movable slit swings. In order to generate a wave signal, it is converted into approximately parallel by the illumination optical system, and the numerical aperture

The gap length between the light and shade grating patterns so that the effect of interference between the 0th order light and the ± 1st order light is maximized among the diffracted light fluxes of the rectangular light and shade grating pattern of the movable slit that is received by the light flux of wavelength λ. An optical encoder characterized in that is set.

  In other words, the contrast of the pseudo sine wave signal provided on the rectangular gray grid pattern of the fixed slit takes an extreme value with respect to the gap length between the gray grid patterns, and at the same time from the rectangular gray grid pattern of the movable slit. Of the diffracted light that is emitted, to quantitatively express the gap length between the light and shade grating patterns that maximizes the effect of interference between the 0th order light and the ± 1st order light. The numerical aperture of the illumination optical system, the light flux is the wavelength λ, and the rectangular grayscale grating pattern constant of the movable slit

The problem is to be solved by expressing it as a function.

  Also in the present embodiment, the basic configuration is as described with reference to FIGS. 1 to 4. In the following, FIGS. 1 to 4 are treated as diagrams representing the basic configuration of the present embodiment, and the present embodiment is described. explain.

  In the following, the optimum value of the gap length between the light and shade grid patterns is obtained.

  The coordinate system employed here is the above-described system, that is, the optical axis direction of the light source unit 1 is the Z axis, the movable direction of the movable slit 3 is the X axis, and the directions orthogonal to both of these axes are the Y axis. The Cartesian coordinate system.

  First, the electric field between the shaded grid patterns is determined.

  Strictly speaking, the electric field should be treated as a vector quantity, but the lattice constant of the rectangular gray lattice pattern

Is a scalar wave that is an effective approximation under these conditions

Express the electric field with Scalar wave of luminous flux of wavelength λ diffracted by rectangular gray grating pattern of movable slit

Is the steady state wave equation,

And z = 0 on the rectangular gray grid pattern of the movable slit as a boundary condition, it is expected to have an amplitude distribution that matches the gray grid pattern. In addition

Is a wave vector representing the luminous flux. Also later scalar wave

The light intensity is calculated from

Ignore any phase term of

Is a scalar wave

Is

Can be expressed as Substituting Equation (3) and Equation (4) into Equation (2) and approximating to the Fresnel diffraction region,

Is

Can be determined. Here, the wave vector representing the luminous flux

Each ingredient of

And

It was.

  Next, a scalar wave

To the light intensity of the light flux between the light and shade grating patterns

To decide. Or later,

And the assumption of steady state, the light intensity

Is a function of x and z.

  Numerical aperture of illumination optics

Is conical, and the illumination optical system substantially satisfies the sine condition and the non-stationary condition, the normalized light intensity of the light flux between the grayscale grating patterns

Is

Can be expressed as Therefore, the normalized light intensity of the light flux between the light and shade grating patterns

The specific expression of is a condition that is valid for many optical encoders

Under

It becomes. However,

Is an nth-order first-type Bessel function. Here, in equation (9a)

Represents a phenomenon known as the Talbot effect, where the parallel light beam supplied from the illumination optical system is diffracted by the gray grating pattern,

Is the numerical aperture of the light beam supplied from the illumination optical system

Represents the correction term. In this way, the light intensity of the light flux between the light and shade grating patterns

Was decided.

  Finally, among the diffracted light beams of the rectangular slit grating pattern of the movable slit received by the light beam having the wavelength λ, the effect of interference between the 0th order light and the ± 1st order light is the maximum, and the optical pseudo sine wave signal exhibits the maximum contrast. Determine the conditions.

  In general, the definition of contrast V is

However, since the analytical handling is difficult first, and the information of the optical pseudo-sinusoidal wave cannot be obtained, the definition by the equation (10) is not adopted here. Here, the subscripts max and min represent the maximum value and the minimum value, respectively. Instead, the desired optical pseudo-sinusoidal signal determined from the boundary conditions when solving the wave equation (2)

Is the signal amplitude

, Signal offset

As

Therefore, the amplitude is calculated by the least square method.

Determine the contrast definition here,

And To that end, the evaluation function

From the above formula

Ask for. This is easy to calculate

It becomes.

  Since the extreme value of the contrast of the light pseudo-sinusoidal wave with respect to the gap length between the light and shade lattice patterns is differentiated from the equation (14) and the derivative is zero,

It is possible to obtain the optimal gap length between the light and dark grid patterns that can be expressed as Expression (14) means that only 0th-order light and ± 1st-order light among the diffracted light beams by the rectangular shade grating pattern contribute to a desired signal. Therefore, among the diffracted light beams produced by the rectangular shading grating pattern of the movable slit received by the light beam having the wavelength λ, the effect of the interference between the 0th order light and the ± 1st order light is the maximum, and the optical pseudo sine wave signal exhibits the maximum contrast. Was decided.

  The cosine function on the right side of Equation (14) represents the normal Talbot effect, and the other term on the right side of Equation (14) represents the numerical aperture of the illumination optical system.

Represents the correction by. From Expression (14), the numerical aperture of the illumination optical system

Due to the influence of the correction, the gap length between the rectangular gray-scale grating patterns in which the optical pseudo sine wave signal exhibits the maximum contrast is shorter when the light source is a surface light source than when it is a point light source.

  Hereinafter, returning to FIGS. 1 to 4, additional description of the present embodiment will be given.

  The present invention can be applied to an optical rotary encoder, an optical linear encoder, an optical linear gauge, and the like. In the present embodiment, the present invention is applied to an optical linear cage.

  The light source unit 1 is formed of an LED, and a light beam emitted from the light source unit 1 enters an illumination optical system 2 including a diffractive aspheric lens for illuminating the movable slit 3. The diffractive aspherical lens constituting the illumination optical system 2 is substantially an fsin θ lens, and converts the light beam from the light source unit 1 into a substantially parallel light beam.

  Further, the diffractive aspherical lens constituting the illumination optical system 2 substantially satisfies the sine condition and the non-free condition, and has a substantially uniform numerical aperture on the movable slit.

Illuminated with.

  The movable slit 3 is of an optically transparent glass shape, and has a rectangular lattice constant as shown in FIG.

The diffraction grating is patterned. A shaft 6 for measuring the displacement of the object to be measured is also attached to the movable slit 3.

  The light beam incident on the movable slit 3 is diffracted by the diffraction grating patterned on the movable slit 3 and enters the fixed slit 4. The fixed slit 4 is also an optically transparent glass, and has a lattice pattern similar to that of the movable slit on the surface thereof.

  The gap length z between the pattern surface of the diffraction grating patterned on the movable slit 3 and the pattern surface of the diffraction grating patterned on the fixed slit 4 substantially satisfies the following conditions.

  However,

Is the nth order Bessel function,

Is the numerical aperture of the illumination optical system,

Is the wave number of the luminous flux,

Is a lattice constant common to both the gray lattice pattern carved in the movable slit and the gray lattice pattern carved into the fixed slit.

  The light beam incident on the fixed slit 4 is filtered by the lattice pattern of the fixed slit 4 and enters the optical sensors 5a and 5b. The optical sensors 5a and 5b generate a photocurrent from the incident optical sine wave signal, convert it to an electrical sine wave signal, and send the signal to the signal processing mechanism.

  In the case of the basic configuration shown here, the side close to the illumination optical system 2 is a movable slit and the side away from the fixed slit is a fixed slit. A slit may be arranged.

  In addition, here, an example is shown in which two lattice patterns of A phase and B phase shown in FIG. 4 are formed on the fixed slit side, but these two lattice patterns of A phase and B phase are formed on the movable slit side. May be.

  FIG. 5 shows the basic configuration shown in FIGS. 1 and 2 when the distance between the lattice pattern on the movable slit 3 and the lattice pattern on the fixed slit 4 is changed based on the aforementioned equation (14). It is the figure which showed the correspondence of distance z and contrast.

  According to the background art described above, the lattice pattern on the movable slit and the lattice pattern on the fixed slit are

In this embodiment, the movable slit and the fixed slit are arranged, and according to the present embodiment, the movable slit and the fixed slit are arranged so as to have a distance z _a that is a distance slightly shorter than the distance. This distance z _a is the distance that the contrast is an extreme value, hence a change in contrast even if the distance z is less variation is slight, correspondingly, pitching with the movement of the movable slit, the swing, such as yawing Multiplication processing that is allowed and more accurate than the conventional one is possible.

DESCRIPTION OF SYMBOLS 1 Light source part 2 Illumination optical system 3 Movable slit 4 Fixed slit 5 Optical detector 5a, 5b Optical sensor 6 Shaft

Claims (8)

A light source that emits a light beam with low coherence,
An illumination optical system that receives the incident light beam emitted from the light source and collects the light beam so as to be substantially parallel light;
A movable slit formed in the first one-dimensional lattice pattern and moving in the direction of the lattice vector of the first one-dimensional lattice pattern; a lattice pattern parallel to the lattice vector of the first one-dimensional lattice pattern; and the same lattice A second one-dimensional lattice pattern having a constant is formed, and is fixed at a position where the movable slit is sandwiched between the illumination optical system or a position sandwiched between the illumination optical system and the movable slit. Comprising a slit, a slit pair that receives the light beam emitted from the illumination optical system and acts on the light beam, and a photodetector that converts the light beam subjected to the action of the slit pair into a photocurrent,
In the slit pair, a gap between the first one-dimensional lattice pattern and the second one-dimensional lattice pattern is an expression.

However,

Is the nth order Bessel function,

Is the numerical aperture of the illumination optical system,

Is the wave number of the luminous flux,

Is a lattice constant common to both the first one-dimensional lattice pattern and the second one-dimensional lattice pattern.
An optical encoder comprising the movable slit and the fixed slit at each position satisfying | z | or in the vicinity of the | z |. One of the movable slit and the fixed slit has two one-dimensional lattice patterns whose phases are shifted from each other by 90 ° in the lattice vector direction,
2. The optical encoder according to claim 1, wherein the optical detector includes two optical sensors that respectively convert the light beams subjected to the actions of the two one-dimensional grating patterns into respective photocurrents.   The optical encoder according to claim 1, wherein the illumination optical system is a substantially telecentric optical system.   The slit pair includes the movable slit and the fixed slit, and the normal line of the first one-dimensional lattice pattern plane and the normal line of the second one-dimensional lattice pattern are parallel to the optical axis of the illumination optical system. The optical encoder according to any one of claims 1 to 3, wherein the optical encoder is arranged in a direction such that.   The optical encoder according to claim 1, wherein the illumination optical system is an optical system that satisfies a substantially sine condition.   6. The optical encoder according to claim 5, wherein the illumination optical system is an optical system that further satisfies a substantially non-free condition. The slit pair includes the movable slit and the fixed slit, the numerical aperture

7 to 6 which are provided at each of the positions where | z | calculated by adopting a representative numerical aperture of the illumination optical system as or a gap in the vicinity of the | z | The optical encoder of any one of these. The slit pair includes the movable slit and the fixed slit, the wave number

As a feature, it is provided at each position that becomes | z | calculated by adopting a wavelength representative of the wavelength distribution of the light beam emitted from the light source or a gap in the vicinity of the | z |. The optical encoder according to any one of 1 to 7.

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