JPH1038517A - Optical displacement measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To always accurately measure the displacement of a scale by irradiating a lens system which is positioned so that a diffraction grating can substantially come to its focal point with a parallel luminous flux from a light projecting means and forming a plurality of diffracted light rays from the diffraction grating to parallel luminous fluxes which are parallel with each other. SOLUTION: A divergent luminous flux from a laser diode 1 is made a parallel luminous flux through a collimator lens 2 and converged onto a diffraction grating G1 as a spot light through a lens 3 which is positioned above the grating grating G1 so that the distance from the grating G1 can become equal to its focal distance and the grating G1 emits + primary diffracted light rays by reflecting and diffracting the luminous flux. The diffracted light rays are again passed through the lens 3 and become two parallel luminous fluxes having parallel optical axes even when the scale 4 rotationally moves along the grating forming surface. Then the parallel luminous fluxes are polarized through blazed diffraction gratings 5a and 5b of the same pitch and synthesized into one luminous flux by truing their wave fronts through a diffraction grating 6. The synthesized luminous flux is received by means of a four-divided sensor 7. Then, the information on the moving amount and direction of the scale 4 is obtained by processing sine wave signals obtained when the scale 4 moves.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical displacement measuring device. In particular, the present invention can be favorably applied to an encoder that utilizes the fact that an interference light beam is modulated by diffraction and interference generated when an object is irradiated with light.

[0002]

2. Description of the Related Art Conventionally, optical encoders, laser Doppler velocimeters, laser interferometers, and the like have been used as devices for irradiating an object with light to obtain a physical quantity such as movement or displacement of the object with high accuracy. The features of these devices using light are high accuracy and high resolution, and recently, further miniaturization is required for application to a wider field.

In such an apparatus, Japanese Patent Application Laid-Open No. 62-121
No. 314, a miniaturized encoder using three diffraction gratings is disclosed.
As disclosed in Japanese Patent Application Laid-Open No. 2 (1999) -210, there is known a device in which such a device is simultaneously miniaturized with high accuracy.

[0004]

Although the encoder as shown in the prior art has been reduced in size and increased in accuracy, it has the following problems.

In the devices disclosed in these publications, the light emitted from the light emitting source is split into two parts on the left and right, or the light path is further divided and made incident on the moving diffraction grating. Light is received by the photoelectric element. In such a configuration, if there is an inclination between the light beams synthesized by the multiplexing diffraction grating to cause interference, fringes are generated on the light receiving surface of the photoelectric element. In this case, in the temporal change of the interference light,
Due to the presence of the fringes, the respective phases are different for each interference region on the light receiving surface.

Therefore, it is necessary to strictly prevent the occurrence of a tilt between the multiplexed light beams, and to adjust the interference state on the light receiving surface to a uniform state. However, it is very difficult to adjust the interference state of the interference light beam synthesized by the diffraction grating to be constant in all interference regions. In particular, as the resolution of the main body is improved, the interference state is more likely to fluctuate due to the influence of a mounting error or the like.

[0007] Such fluctuations in the interference state cause the amplitude of the detected signal to decrease by averaging the temporal change in the intensity of the interference light on the light receiving surface when an interference fringe occurs, thereby making the signal amplitude unstable. I will. Further, for example, in an apparatus in which a multiplexed diffraction grating is configured by a plurality of diffraction gratings having different phases and an interference light emitted from each of the diffraction gratings is individually detected to obtain an AB phase signal, an inclination occurs between the multiplexed light fluxes. If so, the phase difference between signals obtained in different interference regions is not stable.
Also, the amplitude of the signal detected from each becomes unstable.

When the apparatus described in Japanese Patent Application Laid-Open No. 3-279812 is taken as an example, there is a problem in particular that the interference state varies due to an error in mounting the head and the scale provided with the light projecting means and the light receiving means. . If a mounting error occurs such that rotation about an axis perpendicular to the surface on which the diffraction grating is formed occurs, interference fringes parallel to the arrangement direction of the diffraction grating occur.
When a mounting error occurs such that rotation occurs around an axis perpendicular to the arrangement direction of the diffraction gratings in the plane on which the diffraction grating is formed, interference fringes parallel to the rotation axis direction are generated. In the following, the rotation angle of the rotation mounting error about the axis perpendicular to the surface on which the diffraction grating is formed is referred to as “azimuth angle (η)”, and the rotation angle in the direction in which the diffraction grating is arranged within the surface on which the diffraction grating is formed. The rotation angle of the rotation mounting error about the vertical axis is referred to as “rotation angle (φ)”.

When parallel light is used for two beams to be interfered,
Even if there is a slight mounting error in the rotation angle direction, if the difference is small, there will not be much difference in the angle between the two beams of interference, but if there is an extreme deviation in the rotation angle direction, the interference state will be unstable. In addition, a slight deviation in the mounting error in the azimuth angle direction causes an angle difference between the two light beams of the interference, which makes the interference state sensitively unstable.

The present invention has been made in view of the above-mentioned prior art, and has as its object to provide an apparatus which in principle can prevent the occurrence of interference fringes due to mounting errors and can always accurately measure displacement regardless of the mounting state of a diffraction grating or a detection side. With the goal.

[0011]

According to a first aspect of the present invention, there is provided a light-emitting device for spot-projecting a light beam onto a diffraction grating provided on an object whose relative displacement is to be detected; Diffracted light beams of different orders from the spot projection position are compared with each other in a form in which the diffracted lights of different orders are both parallel light beams and parallel to each other, or in a form in which the wavefronts of the diffracted lights of different orders have the same center of curvature. An optical displacement measuring device comprising: a light detecting unit that receives an interference light beam obtained by multiplexing.

According to a second aspect of the present invention, there is provided a light projecting means for spot-projecting a light beam onto a diffraction grating provided on an object whose relative displacement is to be detected, and an order from the spot projection position of the diffraction grating. And a light detecting means for receiving an interference light beam obtained by multiplexing the diffracted lights of different orders in such a manner that the wavefronts of the diffracted lights of different orders are aligned with each other.

The third invention further provides an optical system for guiding diffracted light of different orders from the spot projection position of the diffraction grating as divergent light or parallel light to guide the combined light to the light detecting means. It is characterized by having.

According to a fourth aspect of the present invention, in the optical system, substantially two light beam multiplexing diffraction gratings are arranged for each of the plurality of light beams of the diffracted light, and the substantially two light beam multiplexing diffraction gratings are arranged. A grating portion having a different grating arrangement phase is provided in one of the light beam transmitting regions of the light beam combining diffraction grating.

According to a fifth and a sixth aspect of the present invention, the light projecting means further irradiates a parallel light beam to a lens system arranged such that the diffraction grating is substantially at a focal position, and a plurality of diffracted lights from the diffraction grating. Are collimated by the lens system and are made parallel to each other.

[0016]

FIG. 1 is a schematic explanatory view showing an optical configuration of an optical displacement measuring apparatus according to a first embodiment of the present invention.

In the figure, 1 is a laser diode, 2 is a collimator lens, and 3 is an objective lens. Reference numeral 4 denotes a scale provided on the moving object side, on which a diffraction grating G1 is provided. 5a and 5b are blazed diffraction gratings, 6 is a diffraction grating for multiplexing light beams having four grating regions whose grating phases are shifted by λ / 4, and 7 is a diffraction grating 6 corresponding to four grating regions.
This is a four-divided sensor provided with two light receiving areas.

The divergent light beam emitted from the laser diode 1 is converted into a parallel light beam by the collimator lens 2, and is condensed by the lens 3 adjusted to the focal position f by adjusting the distance from the diffraction grating G1 on the scale 4 to the light on the diffraction grating G1. Point irradiation. The diffraction grating G1 is a phase type diffraction grating having a grating height of substantially a quarter wavelength so as to particularly strongly reflect and diffract ± 1st-order diffracted light. At this time, if the laser wavelength is λ and the diameter of the laser parallel light beam is a,
The beam waist ω1 on the diffraction grating G1 is as follows: ω1 = 1.273 × λ × | f | / a. Beam waist ω1 on this diffraction grating G1
Selects the diameter a and the focal position f of the laser parallel beam so as to be several or more with respect to the grating pitch p of the diffraction grating G1.

The light beam irradiated on the diffraction grating G1 is reflected and diffracted, and emits ± first-order diffracted light.

At this time, the diffraction angle θ of the ± 1st-order diffracted optical axis is
It is obtained from the following equation.

P sin θ = λ The ± 1st-order diffracted light passes through the lens whose distance from the diffraction grating G1 is adjusted to the focal position f again, and becomes two parallel light beams whose optical axes are parallel to each other. This optical system realizes an optical system in which the ± 1st-order diffracted lights obtained from the diffraction grating G1 are both parallel light beams and the optical axes are parallel even if the scale is rotationally moved along the grating forming surface.

The two parallel light beams are deflected by blazed diffraction gratings 5a and 5b of equal pitch, which are disposed so as to be deflected in the direction of the optical axis of the entire optical system, and are blazed at the intersection of the deflected light beams. Diffraction gratings 5a, 5b
The wavefronts are aligned and multiplexed by a diffraction grating 6 having four grating regions having the same pitch as that and having a grating phase shifted by λ / 4 in the two light beam transmitting regions.

Then, the light is received by a four-divided sensor 7 in which each light receiving region is arranged at a position where the light beam combined in each of the four grating regions is received. From the four-divided sensor 7, four sine wave signals having a quarter cycle phase difference from each other are obtained from each light receiving area as the scale 4 moves. The four signals are processed by a signal processing system (not shown) to obtain information on the movement amount and movement direction of the scale 4. Such signal processing is already known, and therefore, the details are omitted.

As described above, as an optical system for combining two parallel light beams with their wavefronts aligned, two diffraction gratings are arranged for one light beam, and the two diffraction gratings are disposed in a light transmission area of one of the two diffraction gratings. By providing portions having different grating arrangement phases, it is possible to generate a phase shift signal without taking a complicated optical configuration such as a combination of a quarter-wave plate and a deflecting prism. Enables accuracy detection.

Here, consider the case where the scale 4 rotates in the azimuth direction in this embodiment.

FIG. 2 is an optical path diagram when the scale 4 is displaced in the azimuth direction.

As can be seen from the figure, if the scale 4 has a misalignment of the azimuth angle η, the ± 1st-order diffracted light is shifted from the diffraction grating in the azimuth angle direction as compared with a state where there is no misalignment (dotted line). It emits while diverging.

The two divergent light beams are transmitted and deflected by the lens 3. However, since the diffraction grating G1 is set at the focal position of the lens 3, the optical axes are always parallel to each other regardless of the azimuth angle η. It becomes two parallel light beams. As a result, even when the blazed diffraction gratings 5a and 5b and the diffraction grating 6 combine the light beams, the optical axes of the two light beams remain parallel even if a slight displacement occurs between the two light beams. That is, regardless of the value of the azimuth angle η of the mounting deviation, no angle deviation occurs between the combined two light beams, and no spatial interference fringes occur on the light receiving surface of the four-divided sensor 7.

FIG. 3 is an optical path diagram when the scale 4 is displaced in the rotational direction.

As can be seen from the figure, if the scale 4 has a displacement of the rotation angle φ, the lens 3 of the optical axis of ± 1st-order diffracted light
The angles (apparent diffraction angles) formed with respect to the optical axis differ from each other in ± 1 order. Each apparent diffraction angle θ ± 2φ
Is obtained by the following equation.

P (sin θ ± sin φ) = λ The ± 1st-order diffracted lights having different apparent diffraction angles are diverged from the diffraction grating in the direction of the rotation angle as compared with the state where there is no misalignment (dotted line). While emitting.

The two divergent light beams are transmitted and deflected by the lens 3, but since the diffraction grating G1 is adjusted to the focal position of the lens 3, the optical axes are always parallel to each other regardless of the rotation angle φ. Two parallel light beams. As a result, even when the blazed diffraction gratings 5a and 5b and the diffraction grating 6 combine the light beams, the optical axes of the two light beams remain parallel even if a slight displacement occurs between the two light beams. That is, regardless of the value of the rotation angle φ of the mounting deviation, no angular deviation occurs between the combined two light beams, and no spatial interference fringes occur on the light receiving surface of the four-divided sensor 7.

By adopting the above configuration, it is possible to provide an optical displacement measurement that hardly cares about the scale mounting accuracy.

As described with reference to FIGS. 2 and 3, when the scale is displaced, the optical paths of the combined two light beams slightly shift in parallel. An optical displacement measuring device that can be used under all conditions can be selected by selecting the diameter, grating pitch, and lens focal length of the incident light beam according to the scale mounting deviation assumed at the design stage so that the light beam arrives. Can be realized.

When an azimuth angle is generated, a measurement error corresponding to sinη is added to the displacement information finally obtained, but this is so small that the error due to the assumed displacement of the scale does not influence. What is necessary is just to select a scale range, a scale pitch, and the like.

In the first embodiment, an example is shown in which a parallel light beam is radiated to the objective lens 3 as a lens system adjusted to the focal position as a light projecting optical system which enters the diffraction grating, and a point is radiated onto the diffraction grating G1. However, it is also possible to adopt a configuration in which the objective lens 3 is configured to be separated on the projection side and the light receiving side, and a point irradiation is directly performed on the diffraction grating G1 without passing through such a diffraction grating at a focal position. Effects can be obtained. In this case, since the beam waist on the diffraction grating G1 is determined by the light projecting optical system, the number of irradiations with respect to the grating pitch p of the diffraction grating G1 can be set with a high degree of freedom.

In the first embodiment, a deflecting optical system for combining two parallel light beams with their wavefronts aligned is used for two light beams per light beam.
Although an example in which a plurality of diffraction gratings are arranged has been described, a deflecting optical system in which a prism system is arranged and wave surfaces are aligned and multiplexed may be employed.

Further, the diffraction grating 6 in the first embodiment is constituted by one grating, and the four-division sensor 7 is replaced by a sensor having one light receiving surface. Then, the laser diode 1 and the collimator lens 2 in the first embodiment are replaced. Even if the arrangement of the set 7 and the sensor 7 is reversed, an optical displacement measuring device which hardly cares about the accuracy of mounting the scale is also realized.

To explain this, two parallel light beams split by the diffraction grating 6 form a beam waist on the diffraction grating G1 via the blazed diffraction gratings 5a and 5b and the lens 3, and are multiplexed from the beam waist. The ± 1st-order diffracted light generated in the normal direction of the diffraction grating forming surface is converted into a parallel light beam by the lens 3 and enters the sensor. This ± 1st order diffracted light is the first
As in the case of the embodiment, after being converted into a parallel light beam by the lens 3, even if the mounting displacement occurs in the azimuth angle direction or the rotation angle direction, only the parallel displacement occurs, and no angle displacement occurs.

FIG. 4 is a schematic explanatory view showing an optical configuration of an optical displacement measuring device according to another embodiment of the present invention.

The same members as those described above are denoted by the same reference numerals. The description of the same parts as in the first embodiment is omitted.

The light beam condensed by the lens 2 and spot-irradiated on the diffraction grating G1 is reflected and diffracted, and emits ± 1st-order diffracted light.

At this time, the diffraction angle θ of the ± 1st-order diffracted optical axis is
Assuming that the grating pitch of the diffraction grating G1 is p1, it can be obtained from the following equation as in the first embodiment.

P1 sin θ = λ The ± 1st-order diffracted lights are deflected by the provided diffraction gratings 5a and 5b having a grating pitch: p2 arranged so as to be deflected in the optical axis direction of the entire optical system, respectively.
The wavefronts are aligned and combined by the diffraction grating 6 having four grating regions at p3 and having a grating phase shifted by λ / 4 within the two light beam transmitting regions.

Then, similarly to the first embodiment, the four-division sensor 7 corresponding to each of the four grid regions is used.
With the movement of the scale, a sine wave signal having a 1/4 period phase difference can be obtained.

At this time, the grating pitch p of the three diffraction gratings
1, p2 and p3 satisfy the following relationship.

1 / p2 = 1 / p1 1 / p3 By setting the diffraction grating interval so that the optical axes of the two diffracted lights are aligned by the diffraction grating 6, the wavefront of the two diffracted lights emitted from the diffraction grating 6 is obtained. Can be matched. As a result, a deflecting optical system that finally combines the two diffracted lights with their wavefronts aligned is realized.

FIG. 5 is an optical path diagram when the scale 4 is displaced in the azimuth direction.

As can be seen from the figure, if the azimuth angle η is misaligned on the scale 4, the ± 1st-order diffracted light is shifted from the diffraction grating in the azimuth angle direction from the state where there is no misalignment (dotted line). It emits while diverging.

Since the two divergent light beams are adjusted to the light beam condensing position of the lens 2 by the diffraction grating G1, the wavefront whose irradiation point on the diffraction grating is always the center of curvature is always used regardless of the azimuth angle η. Will remain. Thus, even when the blazed diffraction gratings 5a and 5b and the diffraction grating 6 are multiplexed and the traveling directions are aligned, even if a slight displacement occurs between the two light beams, the centers of curvature of the wavefronts of the two light beams remain coincident with each other. It is. That is, regardless of the value of the azimuth angle η of the mounting deviation, the center of curvature of the wavefront does not deviate between the combined two light beams, and no spatial interference fringes occur on the light receiving surface of the four-divided sensor 7. .

FIG. 6 is an optical path diagram when the scale 4 is displaced in the rotational direction.

As can be seen from the drawing, if the scale 4 has a misalignment of the rotation angle φ, the lens 2 of the optical axis of the ± 1st-order diffracted light can be used.
The angles (apparent diffraction angles) formed with respect to the optical axis differ from each other in ± 1 order. Each apparent diffraction angle θ ± 2φ
Is obtained by the following equation.

P1 (sin θ ± sin φ) = λ The ± 1st-order diffracted lights having different apparent diffraction angles are diverged from the diffraction grating in the direction of the rotation angle as compared with the state where there is no misalignment (dotted line). While emitting.

Since the two divergent light beams are aligned with the light beam condensing position of the lens 2 by the diffraction grating G1, the wavefront having the irradiation point on the diffraction grating as the center of curvature is always required regardless of the rotation angle φ. Will remain. Thus, even when the blazed diffraction gratings 5a and 5b and the diffraction grating 6 are multiplexed and the traveling directions are aligned, even if a slight displacement occurs between the two light beams, the centers of curvature of the wavefronts of the two light beams remain coincident with each other. It is. That is, the center of curvature of the wavefront does not deviate between the combined two light beams regardless of the value of the rotation angle φ of the mounting deviation, and no spatial interference fringes occur on the light receiving surface of the four-divided sensor 7. .

By adopting the above configuration, it is possible to provide an optical displacement measurement that hardly cares about the scale mounting accuracy as in the first embodiment.

Further, similarly to the first embodiment, the diffraction grating 6 in the second embodiment is constituted by one grating, and the four-divided sensor 7 is replaced with a sensor having one light receiving surface. Even if the arrangement of the sensor 7 and the set of the laser diode 1 and the collimator lens 2 is reversed, an optical displacement measuring device which hardly cares about the mounting accuracy of the scale can be realized.

In addition, in FIG. 4, instead of the blazed diffraction gratings 5a and 5b, it is also possible to arrange parallel mirrors whose reflection surfaces face each other. In this case, the grating pitch between the diffraction grating G1 and the diffraction grating 6 is made to match. With this configuration, an optical system similar to that of the above-described embodiment can be realized.

[0058]

As described above, according to the first aspect, even if azimuth or rotational misalignment occurs relative to the scale, the interference state is stable, and a stable signal is always obtained.

According to the second aspect of the present invention, even if azimuth or rotational misalignment occurs relative to the scale, the interference state is stable, and a stable signal is always obtained.

According to the third aspect of the present invention, a device which is more resistant to azimuth and rotation can be realized.

According to the fourth aspect of the present invention, output signals having different phases can be detected without taking a complicated optical configuration, thereby enabling high-precision detection.

According to the fifth and sixth aspects of the present invention, spot projection can be performed with a simple optical system, and a plurality of diffracted lights from the diffraction grating can be made parallel light beams and parallel to each other.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory diagram illustrating a first embodiment of the present invention.

FIG. 2 is an optical path diagram when the scale is misaligned.

FIG. 3 Optical path diagram when scale azimuth shift

FIG. 4 is a schematic explanatory view illustrating another embodiment of the present invention.

FIG. 5 is an optical path diagram when the scale is misaligned.

FIG. 6 is an optical path diagram when a scale azimuth shift is performed.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Laser diode 2 Collimator lens 3 Lens 4 Scale G1 Phase type diffraction grating 5a, 5b Blazed type diffraction grating 6 Diffraction grating 7 Quadrant sensor

Claims (6)

[Claims] 1. A light projecting means for spot-projecting a light beam onto a diffraction grating provided on an object whose relative displacement is to be detected, and diffracted light beams of different orders from the spot projection position of the diffraction grating. A form in which the diffracted lights are both parallel beams and parallel to each other, or a form in which the wavefronts of the diffracted lights having the different orders have the same center of curvature, and a light detecting means for receiving an interference light beam obtained by combining the diffracted lights with each other. An optical displacement measuring device comprising: 2. A light projecting means for spot-projecting a light beam onto a diffraction grating provided on an object whose relative displacement is to be detected, and diffracted light beams of different orders from the spot projection position of the diffraction grating, the light beams having different orders. An optical displacement measuring device, comprising: light detecting means for receiving an interference light beam obtained by multiplexing the diffracted light beams in such a manner that their wavefronts are aligned. 3. An optical system for diffracting light of different orders from the spot projection position of the diffraction grating, as divergent light or parallel light conversion, to guide the combined light to the light detecting means. The optical displacement measuring device according to claim 2, wherein: 4. In the optical system, substantially two light beam multiplexing diffraction gratings are arranged for each of the light beams of the plurality of diffracted lights, and the two substantially light beam multiplexing diffraction gratings are arranged. 4. The optical displacement measuring apparatus according to claim 3, wherein grating portions having different grating arrangement phases are provided in one of the light beam transmitting regions of the diffraction grating. 5. The light projecting means irradiates a parallel light beam to a lens system arranged such that the diffraction grating is substantially at a focal position, and the optical system applies a plurality of diffracted lights from the diffraction grating to the lens system. The optical displacement measuring apparatus according to claim 2, wherein the system is configured to make the light beams parallel and mutually parallel by a system. 6. The light projecting means irradiates a parallel light beam to a lens system arranged so that the diffraction grating is substantially at the focal position, and emits a plurality of diffracted lights from the diffraction grating by the lens system. The optical displacement measuring device according to claim 1, wherein the optical displacement measuring devices are parallel to each other.