JP2007303957A - Encoder - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an encoder which detects the position information of a scale with high precision. <P>SOLUTION: A concave cylindrical lens 25 is provided between a movable scale 24 and a light receiving element 26. This allows light passed through the movable scale 24 to be a cylindrical plane wave and prevents the occurrence of interference fringes in the Y-axis direction on a light receiving surface of the light receiving element 26. A change in the light intensity of interference light by the interference fringes is included as noise in a photoelectric conversion signal detected by the light receiving element 26. By preventing the occurrence of the interference fringes, the position information of the movable scale 24 can be detected with high precision. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an encoder, and more particularly, to an encoder that optically detects position information of a moving body.

  Conventionally, as a general optical encoder, a scale that moves with a moving body and has a pattern periodically arranged in the moving direction, and an irradiation optical system that irradiates the scale with two coherent light beams, A light-receiving element that detects a change in the intensity of the interference light of the positive and negative diffracted light diffracted at the scale, and is used to change the intensity of the interference light due to the phase difference between the two coherent light beams generated as the moving body moves. Based on this, a so-called diffraction interference type encoder that detects relative position information between a scale and an irradiation optical system is known (see, for example, Patent Document 1).

  In such an encoder, when the scale is rotated with respect to the moving direction of the moving body, interference fringes may be generated on the light receiving surface of the light receiving element in the direction intersecting the moving direction of the moving body. is there. Since the intensity of light passing through the scale changes due to the interference fringes, the interference fringes hinder high-precision detection of scale position information.

Japanese Patent Laying-Open No. 2005-3438

  The present invention includes a scale on which a pattern extending in a predetermined direction is formed and irradiated with light having a planar wavefront; a direction extending in the predetermined direction after being irradiated on the scale and intersecting the predetermined direction A light-receiving element that receives the light converted into a cylindrical wavefront extending in a wide area.

  According to this, light having a planar wavefront is received by the light receiving element in a state where it is converted into light having a wavefront on a cylinder extending in a predetermined direction and extending in a direction intersecting the predetermined direction. As a result, the phase difference of the light collected at each point on the light receiving surface of the light receiving element is canceled out, and the light intensity becomes substantially constant with respect to the predetermined direction.

  Hereinafter, an embodiment of the present invention will be described with reference to FIGS.

  FIG. 1 shows a schematic configuration of a main part of an encoder 10 according to an embodiment of the present invention. As shown in FIG. 1, the encoder 10 is a so-called diffraction interference type encoder, and is a linear encoder that detects position information of a moving body (not shown) that moves in the X-axis direction.

  As shown in FIG. 1, the encoder 10 includes a light source 12, a vibrating mirror 14, a collimator lens 18, index scales 20 and 22, a moving scale 24, a cylindrical lens 25, and a light receiving element 26. ing.

  The light source 12 is a light source that emits, for example, coherent light, for example, laser light having a wavelength λ (= 850 nm) in the + X direction in FIG.

  The vibration mirror 14 reflects the laser light from the light source 12 toward the index scale 20. The oscillating mirror 14 is periodically oscillated in the rotational direction around the Y axis by a driving device 16 having an actuator. Due to this rotational vibration, the reflection direction of the light incident on the oscillating mirror 14 varies depending on the direction of the reflecting surface, and the angle of the illumination light incident on the collimator lens 18 is periodically modulated.

  The collimator lens 18 converts the laser light reflected by the vibrating mirror 14 into parallel light.

  The index scale 20 is a transmission type phase grating formed of a plate on which a diffraction grating having the X-axis direction as the periodic direction of the score line is formed, and parallel light transmitted through the collimator lens 18 enters the index scale 20. In the index scale 20, a part of the incident parallel light is diffracted to generate a plurality of diffracted lights. In FIG. 1, among these diffracted lights, ± first-order diffracted light generated at the index scale 20 (diffracted light emitted to the + X side in FIG. The light is assumed to be -1st order diffracted light).

  Like the index scale 20, the index scale 22 is a transmission type phase grating composed of a plate on which a diffraction grating having a periodic direction in the X-axis direction is formed, and is arranged between the index scale 20 and the moving scale 24. Yes. The index scale 22 diffracts the −1st order diffracted light generated by the index scale 20 to generate + 1st order diffracted light. The + 1st order diffracted light travels toward the moving scale 24. The index scale 22 diffracts the + 1st order diffracted light generated by the index scale 20 to generate a −1st order diffracted light. This −1st order diffracted light travels to the moving scale 24.

  Similar to the index scales 20 and 22, the moving scale 24 is a transmissive phase grating formed of a plate on which a diffraction grating having a periodic direction in the X-axis direction is formed, and is fixed to a moving body (not shown). The moving scale 24 diffracts the + 1st order diffracted light generated by the index scale 22 to generate a −1st order diffracted light, and diffracts the −1st order diffracted light to generate a + 1st order diffracted light. These diffracted lights overlap and proceed in the −Z direction while interfering with each other.

  In this embodiment, it is assumed that the lattice pitch of the index scale 20 and the moving scale 24 is the same, and the lattice pitch of the index scale 22 is ½ of the lattice pitch of the index scale 20 and the moving scale 24.

  The ± first-order diffracted lights emitted from the moving scale 24 enter the cylindrical lens 25 in a state of interfering with each other. The cylindrical lens 25 is a concave cylindrical lens with respect to the Y-axis direction. The interference light that has entered the cylindrical lens 25 has a planar wavefront, but the cylindrical lens 25 has a cylindrical wavefront that extends in the Y-axis direction and extends in the Z-axis direction. Convert to

  The light emitted from the cylindrical lens 25 enters the light receiving element 26. As a result, the light receiving element 26 comes to output a photoelectric conversion signal indicating the interference intensity of the interference light. The interference intensity of the interference light changes periodically as the moving scale 24 moves in the X-axis direction. The encoder 10 detects the fluctuation of the interference intensity of the interference light from the photoelectric conversion signal, and detects the position information of the moving scale 24 based on the fluctuation.

  FIG. 2A shows the moving scale 24 viewed from the + Z side. Originally, the moving scale 24 is installed so that the periodic direction of the engraving line is parallel to the X axis, but actually, as shown in FIG. While moving in the direction, it also rotates slightly around the Z axis. In FIG. 2A, the rotation amount of the moving scale 24 around the Z axis is represented by θ. This amount of rotation θ is also called a yawing amount.

  By the way, when the moving scale 24 rotates around the Z axis (tilt with respect to the X axis), as shown in FIG. 2B, the light is also diffracted in the Y axis direction, and a plurality of diffracted lights traveling in different directions. Comes to occur. In this case, if the cylindrical lens 25 is not provided as in the encoder 10 according to the present embodiment, interference fringes in the Y-axis direction are generated on the light receiving surface of the light receiving element 26. Since the signal level of the photoelectric conversion signal detected by the light receiving element 26 varies due to the interference fringe, the interference fringe causes a detection error in the position information of the moving scale 24.

  Therefore, in the encoder 10 according to the present embodiment, as shown in FIG. 3, a concave cylindrical lens 25 is disposed between the moving scale 24 and the light receiving element 26. The focal position of the cylindrical lens 25 matches the pattern forming surface of the moving scale 24. By arranging this cylindrical lens 25, the light emitted from the moving scale 24 is converted into light having a cylindrical wavefront extending in the Y-axis direction and expanding in the −Z direction. To reach. In this way, changes in the optical path length difference are reduced in the light reaching each point on the light receiving surface of the light receiving element 24, so that the light intensity on the light receiving surface is substantially constant in the Y-axis direction. , No interference fringes in the Y-axis direction occur. As a result, noise included in the photoelectric conversion signal output from the light receiving element 26 is reduced, and position information of the moving scale 24 can be detected with high accuracy based on the photoelectric conversion signal.

  As described above in detail, in the encoder 10 according to the present embodiment, the planar wavefront light incident on the moving scale 24 is received in a state of being converted into cylindrical wavefront light by the action of the cylindrical lens 25. Since the light is received by the element 26, the light intensity on the light receiving surface of the light receiving element 26 can be made substantially constant in the Y-axis direction. As a result, it is possible to detect the position information of the moving scale 24 (and consequently the moving body) with high accuracy.

  In the present embodiment, the conversion from the planar wavefront to the cylindrical wavefront is realized by the concave cylindrical lens 25, but any optical element that converts from a planar wavefront to a cylindrical wavefront can be used. Can be used. For example, it is also possible to apply a Fresnel type cylindrical lens which is a kind of the same cylindrical lens. Since the Fresnel type cylindrical lens is thinner than a normal cylindrical lens (cylindrical lens 25) having the same optical effect, the distance between the moving scale 24 and the light receiving element 26 can be shortened. Thereby, the apparatus can be miniaturized.

  Further, it is not always necessary to dispose an optical element between the moving scale 24 and the light receiving element 26. For example, the moving scale 24 ′ shown in FIG. 4A may be adopted instead of the moving scale 24 without disposing the concave cylindrical lens 25. As shown in FIG. 4A, the moving scale 24 'is defined to have a shorter engraving length than the moving scale 24 shown in FIG.

Let this marking length be L. FIG. 4B is a diagram for explaining the engraving length L. In FIG. 4B, two engraving lines are shown. Assuming that the yawing allowable rotation amount of the moving scale 24 ′ is θ, the score line length L is defined by the following equation.
L ≦ p / tan θ (1)
Here, p is the score pitch of the moving scale 24. For example, when θ = 30 minutes and p = 5 μm, the marking length L can be allowed up to 573 μm. That is, the length L of the score line in the Y-axis direction is defined so that the ratio of the pitch p of the score line to the length is equal to or greater than the tangent value in the allowable value of the rotation angle of the moving scale 24.

  When the engraving length L is defined by the above equation (1), as shown in FIG. 4C, the light that has passed through the moving scale 24 becomes light having a cylindrical wavefront, and the encoder 10 according to the present embodiment. As in the case where the cylindrical lens 25 is arranged between the moving scale 24 and the light receiving element 26, the generation of interference fringes in the Y-axis direction of the light on the light receiving surface of the light receiving element 26 is prevented. The light intensity on the light receiving surface is substantially constant with respect to the Y-axis direction.

  Note that the configuration of the encoder 10 according to the above embodiment can be appropriately changed in design. For example, the diffraction grating on each scale may not be formed by engraving and may be a chrome pattern. Further, instead of the index scale 20, an optical element such as a beam splitter that splits the laser light into two may be arranged. Further, instead of the index scale 22, two reflecting mirrors may be arranged.

  In the above embodiment, the case where the moving scale 24 is a transmission type has been described. However, the present invention is not limited to this, and the moving scale 24 may be a reflective scale. In this case, the light reflected by the moving scale 24 may be reflected by using a beam splitter or the like, and the concave cylindrical lens 25 and the light receiving element 26 may be provided on the optical path of the reflected light. Also in this case, it is possible to perform the same measurement as in the present embodiment.

  In the above embodiment, the oscillating mirror 14 is used. However, instead of this, a crystal, a tuning fork type crystal, or the like may be used. In short, a mechanism that periodically changes the incident angle of the illumination light to the index scale 20 may be provided. For example, a plurality of point light sources arranged in a line may be used as the light source, and each point light source may be turned on periodically.

  Further, in the above embodiment, the traveling direction of the laser light is changed and the incident angle to the index scale 20 is periodically oscillated, but the traveling direction of the laser light is fixed and the posture of the index scale 20 is periodically changed. You may make it rotate and vibrate.

  Further, the mechanism for detecting the drift of the moving scale 24 in the Z-axis direction can be applied to encoders other than the encoder 10 of the above embodiment. For example, the present invention is also applicable to an encoder that removes the drive device 16 that drives the oscillating mirror 14 and causes the light source 12 to oscillate periodically along the Z-axis instead of simply oscillating the mirror. It is possible. Further, the index scale 20 may be vibrated in the X-axis direction.

  The present invention can also be applied to an encoder that periodically vibrates the collimator lens 18 along the X axis without causing the light source 12 to vibrate. The present invention is also applicable to an encoder that periodically vibrates the traveling direction or passing position of laser light by arranging an acousto-optic element (AOM) or an electro-optic element (EOM) between the light source 12 and the collimator lens 18. The invention can be applied.

  Furthermore, the present invention can also be applied to an encoder that detects a signal corresponding to the interference light formed by the moving scale 24 without modulating the photoelectric conversion signal.

  Moreover, although the said embodiment demonstrated the case where the movement scale 24 moved, it is not restricted to this, This invention can be employ | adopted also when parts other than the movement scale 24 move. The point is that the moving scale 24 and the other optical member may move relative to each other.

  In the above-described embodiment, the case where the index scales 20 and 22, the moving scale 24, and the reference scale 25 have phase gratings has been described. However, the present invention is not limited to this, and an amplitude type diffraction grating (bright and dark type diffraction grating) is employed. May be. Further, an amplitude type diffraction grating and a phase grating may be mixed.

  Thus, the optical system in the encoder can be variously modified. For the sake of space, in the embodiment and the like, the various optical elements of the optical system in the encoder are all arranged in the XZ plane. However, the invention is not limited to this, and diffracted light generated by the index scale 20 is placed on the moving scale 24. Of course, various optical elements to be guided may be arranged three-dimensionally.

  In the above embodiment, ± 1st-order diffracted light is used as measurement light, but the present invention is not limited to this. Further, interference light of higher-order diffracted light may be used as measurement light, or interference between diffracted lights of different orders such as 0th order and nth order (or -nth order), + nth order and + (m + n) th order. Light may be used as measurement light. However, considering the intensity of the diffracted light, it can be said that it is desirable to use ± first-order diffracted light as the measurement light.

  The wavelength of the laser light and the value of the grating pitch of each diffraction grating in the above embodiment are merely examples, and are appropriately determined according to the resolution required for the encoder. Generally, the smaller the grating pitch of the diffraction grating, the better the resolution of the encoder.

  As described above, the encoder of the present invention is suitable for detecting the position information of the moving body.

It is a figure which shows schematic structure of the encoder which concerns on one Embodiment of this invention. FIG. 2A is a diagram of the moving scale 24 viewed from the + Z side, and FIG. 2B is a diagram of a part of the encoder viewed from the + X side. It is a figure for demonstrating the principle of the encoder of FIG. FIG. 4A is a diagram showing another example of the moving scale of the encoder, and FIG. 4B is a diagram for explaining the ruled line length of the moving scale. FIG. These are the figures for demonstrating the principle of this encoder.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Encoder, 12 ... Light source, 14 ... Vibrating mirror, 16 ... Drive apparatus, 18 ... Collimator lens, 20, 22 ... Index scale, 24, 24 '... Moving scale, 25 ... Cylindrical lens, 26 ... Light receiving element.

Claims (6)

A scale on which a pattern extending in a predetermined direction is formed and irradiated with light having a planar wavefront;
A light-receiving element that receives the light that has been converted into a cylindrical wavefront that extends in the predetermined direction and extends in a direction intersecting the predetermined direction after being irradiated on the scale.   An optical element for converting the light having the planar wavefront via the scale into light having the cylindrical wavefront extending in the predetermined direction and extending in a direction intersecting the predetermined direction; The encoder according to claim 1. The optical element is
3. The encoder according to claim 2, wherein a focal position is on the pattern forming surface of the scale and is a concave cylindrical lens with respect to the predetermined direction.   The encoder according to claim 3, wherein the cylindrical lens is a Fresnel type lens. On the scale, the pattern is periodically arranged in a direction orthogonal to a predetermined direction,
The length of the pattern with respect to the predetermined direction is
2. The method according to claim 1, wherein the scale is defined based on a rotation angle of the scale in a two-dimensional plane including a length direction of the pattern and a periodic direction of the pattern, and a period of the pattern. Encoder.   The length of the pattern in the predetermined direction is defined such that the ratio of the period of the pattern to the length of the pattern in the predetermined direction is equal to or greater than a tangent value in the allowable value of the rotation angle of the scale. The encoder according to claim 5.

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