JP2023136314A - Straightness measurement device and method - Google Patents

Abstract

To provide a straightness measurement device and method which are less affected by air disturbance, thereby enabling straightness to be stably measured.SOLUTION: A straightness measurement device comprises: a laser beam generator 10 which includes a laser oscillator and generates a first laser beam having a first frequency f1 and a second laser beam having a second frequency f2 different from the first frequency f1; a diffraction grating 14 which is arranged in a moving body 16A of a linear motion mechanism 16 and in which a first laser beam having the same traveling direction as the moving direction of the moving body 16A is incident and which emits the diffracted light of the first laser beam; a beam splitter 22 and a condensing lens 26 which cause the diffracted light emitted from the diffraction grating 14 to interfere with the second laser beam and cause the interference beam to enter a photodetector 28; the photodetector 28 which detects an interference signal indicating the interference beam; and a frequency counter 30 which detects a modulation frequency δ from the interference signal. The straightness of the linear motion mechanism 16 is measured based on the modulation frequency δ.SELECTED DRAWING: Figure 1

Description

The present invention relates to a straightness measuring device and method, and more particularly to a technique for stably measuring straightness by suppressing the influence of air turbulence.

FIG. 4 is a diagram showing the principle of measurement by a conventional straightness measuring device.

The optical system of the conventional straightness measuring device shown in FIG. 4 includes a beam splitter 5, a quarter-wave plate 6, a polarizing prism 7, a reflecting prism 8, and a corner cube 9.

First, the locus of the laser beam will be explained.

The laser light emitted from the laser oscillator is single polarized light, and when this light enters the polarizing film 5A of the beam splitter 5, it is separated into transmitted light and reflected light. At this time, the optical axis of the polarizing film 5A has an inclination of 45° with respect to the polarization direction of the laser beam.

Here, the transmitted light will be referred to as "measuring light" and the reflected light will be referred to as "reference light." After passing through the quarter-wave plate 6, the light emitted from the polarizing film 5A passes through the optical path 1 and enters the polarizing prism 7, where it is given an inclination of approximately 1.3°. The light reflected from the reflecting prism 8 while maintaining this inclination passes through the same optical path 1 and passes through the quarter-wave plate 6 again. At this time, since the polarization direction of the light changes by 90 degrees, it is reflected by the polarizing film 5A, and the reflected light enters the corner cube 9.

The light emitted from the corner cube 9 is reflected again by the polarizing film 5A, passes through the optical path 2, and returns in the same manner as in the optical path 1.

By arranging such an optical path, it is possible to reduce the influence of pitching and yawing of the polarizing prism 7, which is one of the error components in straightness and squareness measurements.

In addition, the trajectory of the "reference light" reflected by the first polarizing film 5A first travels back and forth through optical path 3 using the same principle as explained in optical path 1 of the "measuring light", and then through optical path 4 in the same way as optical path 2. go back and forth.

Both the "measuring light" and the "reference light" finally enter the interference system on the same optical path as lights with orthogonal polarization directions. Then, the interference signal is photoelectrically converted and the optical path difference between the "measurement light" and the "reference light" is counted.

In this way, a measurement system is created in which both the "measuring light" and the "reference light" are two parallel beams and spread out at an angle of about 2.6 degrees.

In this measurement system, when the polarizing prism 7 or the reflecting prism 8 (generally the polarizing prism 7) moves in a direction spread by 2.6 degrees, an optical path difference occurs there and the straightness/squareness is measured.

In addition, the straightness measurement method described in Patent Document 1 matches the moving direction of the moving body of the linear movement mechanism with the normal to the processed surface of the diffraction grating installed on the moving body, and the coherent Light is incident parallel to the normal line, and after the +1st and -1st order diffracted lights from the diffraction grating are collimated by a collimating optical system, both diffracted lights are superimposed on a photodetector by an interference optical system and interfered. A signal is obtained and the straightness of the linear movement mechanism is measured by the phase change of the interference signal.

Japanese Patent Application Publication No. 11-325863

The conventional straightness measuring device shown in FIG. 4 spreads laser light in two directions and determines straightness based on the optical path difference or phase difference between the "measuring light" and the "reference light."

However, as the light is spread out, each optical path travels through a completely different space, making it more susceptible to air turbulence. That is, even if a change in the refractive index occurs in one optical path, it is detected as a change in the optical path difference, making stable measurement difficult.

In addition, the straightness measurement method described in Patent Document 1 is also similar to the above because the diffraction grating separates the +1st-order and -1st-order diffracted lights, and the +1st-order diffracted light and the -1st-order diffracted light travel through different spaces. There's a problem.

Furthermore, conventionally, straightness has been measured using phase difference, but since phase difference has repeatability, there is a problem that straightness cannot be measured over one period of laser light.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a straightness measuring device and method that are less affected by air turbulence and can perform stable straightness measurements.

In order to achieve the above object, a straightness measuring device according to a first aspect of the present invention includes a laser oscillator and emits a first laser beam having a first frequency and a second laser beam having a second frequency different from the first frequency. a laser beam generator that generates a laser beam; and a laser beam generator that is disposed on a moving body of a linear movement mechanism, receives a first laser beam in the same traveling direction as the moving direction of the moving body, and emits diffracted light of the first laser beam. a diffraction grating, an interference optical system that causes interference between the first laser beam and the second laser beam emitted from the diffraction grating and emits interference light, a photodetector that detects an interference signal indicating the interference light, and an interference signal. a frequency detector that detects a modulation frequency that is modulated due to fluctuations in a direction that intersects the designed movement direction of the moving body; Measure straightness.

In the straightness measuring device according to the second aspect, in the first aspect, the straightness of the linear movement mechanism is measured based on the modulation frequency detected by the frequency detector while the moving body is moving. This is performed by detecting the velocity of the moving body in a direction that intersects with the designed moving direction, or by integrating the velocity and detecting the displacement of the moving body in the direction that intersects with the designed moving direction.

In the straightness measuring device according to the third aspect, in the first or second aspect, the laser oscillator has a first laser beam having a first frequency and a second frequency having a different frequency from the first laser beam. and a second laser beam having a polarization direction different from that of the first laser beam, and the laser beam generating device is a Zeeman laser that emits a first laser beam of a first frequency and a second laser beam of a second frequency oscillated from the Zeeman laser. and a polarizing plate that adjusts the polarization direction so that the separated first and second laser beams can interfere with each other.

In the straightness measuring device according to a fourth aspect, in the first or second aspect, the laser oscillator oscillates a laser beam of a single frequency, and the laser beam generator oscillates a laser beam of a single frequency in two directions. and a frequency shifter that frequency-modulates one of the laser beams in two directions separated by the beam splitter. One of the laser beams and the laser beam frequency-modulated by the frequency shifter is generated as a first laser beam with a first frequency, and the other laser beam is generated as a second laser beam with a second frequency. .

In the straightness measuring device according to a fifth aspect, in any one of the first to fourth aspects, the interference optical system focuses at least the first laser beam emitted from the diffraction grating on the light receiving surface of the photodetector. It has a condensing lens that emits light.

A straightness measuring method according to a sixth aspect of the present invention includes disposing a diffraction grating on a moving body of a linear movement mechanism, and emitting a first laser beam of a first frequency from a laser beam generator in the same traveling direction as the moving direction of the moving body. a step of making the light incident on the diffraction grating; a step of generating a second laser beam having a second frequency different from the first frequency from the laser beam generator; diffracted light of the first laser beam by the diffraction grating, and a second laser beam. A step of causing interference between the two using an interference optical system and emitting an interference light from the interference optical system, a step of detecting an interference signal indicating the interference light with a photodetector, and a step of detecting an interference signal indicating the interference light using a frequency detector. and detecting a modulation frequency that is modulated with variation in a direction intersecting the movement direction, and measuring the straightness of the linear movement mechanism based on the modulation frequency detected by the frequency detector.

According to the present invention, it is possible to perform stable straightness measurement with little influence of air turbulence, and it is also possible to measure straightness over one cycle of laser light.

FIG. 1 is a diagram showing a first embodiment of a straightness measuring device according to the present invention. FIG. 2 is a diagram showing a second embodiment of the straightness measuring device according to the present invention. FIG. 3 is a flowchart showing an embodiment of the straightness measuring method according to the present invention. FIG. 4 is a diagram showing the principle of measurement by a conventional straightness measuring device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of a straightness measuring device and method according to the present invention will be described below with reference to the accompanying drawings.

[First embodiment of straightness measuring device]
FIG. 1 is a diagram showing a first embodiment of a straightness measuring device according to the present invention.

In FIG. 1, a laser oscillator 10 has a first laser beam having a first frequency _f1 , a second frequency _f2 which is different in frequency from the first laser beam, and has a polarization direction with respect to the first laser beam. This is a Zeeman laser that oscillates a different second laser beam.

The Zeeman laser of this example is a transverse Zeeman laser, and the first laser beam and the second laser beam are linearly polarized laser beams orthogonal to each other. Further, the frequency difference (f ₁ −f ₂ ) between the first laser beam and the second laser beam emitted from the Zeeman laser is about 10 MHz.

The first laser beam and the second laser beam oscillated from the laser oscillator 10 are incident on the polarizing beam splitter 12, where the first laser beam with the first frequency _f1 and the second laser beam with the second frequency _f2 are input. It is separated into light and light.

One of the first laser beams having the first frequency f ₁ that has passed through the polarizing beam splitter 12 is incident on the diffraction grating 14 disposed on the moving body 16A of the linear movement mechanism 16.

The linear movement mechanism 16 is provided in a measuring device, a machining device, a manufacturing device, etc., and moves the moving body 16A linearly. The moving body 16A can move in the left-right direction in FIG. 1, and can move from the position shown by the solid line to the position shown by the broken line.

The first laser beam having the first frequency f ₁ travels in the same traveling direction as the designed moving direction of the moving body 16A, and is incident on the diffraction grating 14. The diffraction grating 14 is arranged on the movable body 16A so that the incident surface on which the first laser beam is incident intersects (perpendicularly intersects) the moving direction of the movable body 16A.

This diffraction grating 14 is a transmission type diffraction grating, and the direction in which the incident light is transmitted through the diffraction grating 14 and diffracted is determined by the wavelength of the incident light and the grating interval of the diffraction grating 14.

Now, when the grating interval of the diffraction grating 14 is d, the wavelength of the incident light is λ, and the diffraction angle is θ, the following equation holds true.

[Number 1]
λ=(sinθ)×d/m
m=order Note that the first laser beam enters the incident surface of the diffraction grating 14 perpendicularly, and the incident angle of the incident light is 0°, so the above formula [Equation 1] holds true.

In this example, +1st order diffracted light transmitted through the diffraction grating 14 is used. A mirror 18 and a mirror 20 are arranged in accordance with the diffraction angle of the +1st-order diffracted light, and the diffracted light (first laser light) reflected by the mirror 18 and mirror 20 is transmitted to a beam splitter 22 constituting an interference optical system. guided by. The diffracted light of an order different from the +1st order has a diffraction angle different from that of the +1st order diffracted light, so it is not guided to the beam splitter 22.

When measuring the straightness of the linear movement mechanism 16, the moving body 16A is moved. When the moving body 16A moves and changes in a direction intersecting (perpendicular to) the designed moving direction of the linear movement mechanism 16 (that is, when a straightness change component is added to the moving body 16A), this With the fluctuation, the first laser light (diffraction light) undergoes frequency modulation due to the Doppler effect and is diffracted.

When the modulation frequency due to the straightness change component is δ, the first laser beam that undergoes frequency modulation and is diffracted has a frequency (f ₁ +δ) that is modulated from the original first frequency f ₁ by the modulation frequency δ.

As shown in the formula [Equation 1], the diffraction angle θ of the diffracted light is determined by the grating interval d of the diffraction grating 14 and the wavelength λ, so the diffraction direction of the diffracted light does not change due to a change in the straightness.

On the other hand, the other second laser beam having the second frequency f2 reflected by the polarizing beam splitter ₁₂ is reflected by the mirror 24, and then enters the 1/2 wavelength plate 25, which is a polarizing plate. Since the second laser beam with the second frequency _f2 in this example is linearly polarized light, the 1/2 wavelength plate 25 can change the polarization direction of the second laser beam by π/2. It becomes possible to interfere with the first laser beam of frequency _f1 .

The second laser beam with the second frequency f2 whose polarization direction has been adjusted by the half-wave plate ₂₅ is guided to the beam splitter 22, where it is diffracted by the diffraction grating 14 and reflected by the mirrors 18 and 20. It is combined with the first laser beam having (f ₁ +δ).

The combined first laser beam and second laser beam are condensed onto the light receiving surface of the photodetector 28 by the condensing lens 26.

The first laser beam having a frequency (f ₁ +δ) that is diffracted by the diffraction grating 14 is incident on different positions of the mirror 18 depending on the movement position of the moving body 16A. As a result, the first laser beam reflected by the mirror 18 and the mirror 20 is incident on different positions of the beam splitter 22 depending on the movement position of the moving body 16A.

The first laser beam and the second laser beam that are incident on the beam splitter 22 are usually combined (interfered) at the beam splitter 22, but the first laser beam is Since the light is incident on different positions of the beam splitter 22, it may not be combined with the second laser light on the reflective surface of the beam splitter 22, or may not be combined sufficiently.

The condensing lens 26 focuses the first laser beam on a specific position on the light-receiving surface of the photodetector 28 regardless of the incident position of the first laser beam that enters the beam splitter 22. The two laser beams can be combined (interfered) well.

Note that the condenser lens 26 may be arranged between the mirror 20 and the beam splitter 22. In this case, the condenser lens 26 condenses the first laser beam onto the light receiving surface of the photodetector 28 and combines it with the second laser beam, regardless of the moving position of the moving body 16A.

The combined light (interference light that has undergone heterodyne interference) has a frequency difference (|f ₁ - f ₂ | + δ) between the frequency of the first laser beam (f ₁ + δ) and the frequency f ₂ of the second laser beam due to interference. Detected as a beat-down beat frequency wave. The photodetector 28 converts the incident interference light into a current signal corresponding to the intensity of the interference light, and converts the current signal into a voltage signal to generate a voltage signal (interference signal) corresponding to the intensity of the interference light. Detect.

The frequency counter 30, which functions as a frequency detector, measures the beat frequency (|f ₁ −f ₂ |+δ) from the interference signal. Here, since the frequency difference Δf (=|f ₁ −f ₂ |) is known, it is possible to obtain the modulation frequency δ by signal processing.

This modulation frequency δ corresponds to a straightness change component in a direction perpendicular to the moving direction when the moving body 16A (diffraction grating 14) moves.

Therefore, the straightness measuring device of the first embodiment can measure the 16 straightnesses of the linear movement mechanism based on the modulation frequency δ. Here, the straightness of the linear movement mechanism 16 is measured based on the modulation frequency δ detected by the frequency counter 30, while the moving body 16A is moving in a direction perpendicular to the designed moving direction of the moving body 16A. This can be done by detecting the velocity of the moving body 16A, or by integrating the detected velocity and detecting the displacement in the direction orthogonal to the designed moving direction of the moving body 16A.

As described above, according to the straightness measuring device of the first embodiment, the straightness of one first laser beam can be measured more easily than the conventional technology which measures the straightness from the optical path difference between two laser beams passing through two optical paths. Since the straightness is measured by detecting the modulation frequency δ corresponding to the degree change component, the influence of air turbulence is small and the straightness can be measured stably. Furthermore, compared to conventional techniques that measure straightness based on phase difference (optical path difference), since straightness is measured from modulation frequency δ, it is also possible to measure straightness over one period of laser light.

In the first embodiment, the 1/2 wavelength plate 25 is arranged in the optical path of the second laser beam with the second frequency _f2 , but the 1/2 wavelength plate 25 is placed in the optical path of the first laser beam with the first frequency _f1 . 25 may be arranged.

Further, in the first embodiment, a transverse Zeeman laser is used as the laser oscillator 10, but a vertical Zeeman laser may also be used. In the case of a vertical Zeeman laser, the first laser beam and the second laser beam, which have a frequency difference, have rotational polarization with different rotation directions, so the optical path is set appropriately so that the first laser beam and the second laser beam interfere with each other. It is necessary to place a polarizer to adjust the polarization direction.

[Second embodiment of straightness measuring device]
FIG. 2 is a diagram showing a second embodiment of the straightness measuring device according to the present invention.

In FIG. 2, parts common to those of the straightness measuring device of the first embodiment shown in FIG. 1 are denoted by the same reference numerals, and detailed explanation thereof will be omitted.

The straightness measuring device of the second embodiment shown in FIG. 2 is different from the straightness measuring device of the first embodiment shown in FIG. 1 in the laser light generating device.

That is, the straightness measuring device according to the second embodiment shown in FIG. The difference is that a laser oscillator 11, a beam splitter 13, and an acousto-optic modulator (AOM) 32 are used.

The laser oscillator 11 oscillates a laser beam with a single frequency f ₁ and makes it incident on the beam splitter 13 .

The beam splitter 13 separates the incident laser beam with a single frequency _f1 into two directions, transmits a part of the laser beam, makes it enter the diffraction grating 14 as a first laser beam with a frequency _f1 , and The laser beam is reflected and made to enter the AOM 32 via the mirror 24.

The AOM 32, which functions as a frequency shifter, modulates the incident laser beam of frequency f ₁ with a modulation frequency Δf, and sends the modulated laser beam of frequency f ₂ (=f ₁ +Δf) to the beam splitter 22 as a second laser beam. Make it incident. Note that the AOM 32 can provide modulation at a modulation frequency Δf of about 100 MHz, for example.

In the straightness measuring device of _the second embodiment shown in _FIG . Since this is common to the straightness measuring device of the embodiment, detailed description thereof will be omitted.

Further, like the straightness measuring device of the first embodiment, the straightness measuring device of the second embodiment is less affected by air turbulence, can perform stable straightness measurement, and also has the ability to measure straightness stably. Straightness measurements over one cycle are possible.

Note that in the straightness measuring device of the second embodiment, the AOM 32 is placed between the mirror 24 and the beam splitter 22, but the AOM 32 is not limited to this, and may be placed between the beam splitter 13 and the diffraction grating 14. .

[Embodiment of straightness measurement method]
FIG. 3 is a flowchart showing an embodiment of the straightness measuring method according to the present invention.

Note that the straightness measuring method shown in FIG. 3 is a measuring method performed by the straightness measuring device of the first embodiment shown in FIG.

In FIG. 3, first, the diffraction grating 14 is placed on the moving body 16A of the linear movement mechanism 16 (step S10).

Next, the laser oscillator 10 oscillates two modes of laser light with a first frequency _f1 and a second frequency _f2 , and the polarization beam splitter 12 separates the first laser light with the first frequency _f1 and the second frequency _f2 . and the second laser beam (step S12). This laser oscillator 10 is a Zeeman laser, and oscillates laser light in two modes, a first frequency _f1 and a second frequency _f2 , which have different polarization directions.

The first laser beam of the first frequency _f1 separated by the polarizing beam splitter 12 is made incident on the diffraction grating 14 disposed on the moving body 16A (step S14). The traveling direction of the first laser beam of the first frequency _f1 that is incident on the diffraction grating 14 is the same direction as the designed moving direction of the moving body 16A.

Diffracted light (for example, +1st-order diffracted light) that passes through the diffraction grating 14 is reflected by mirrors 18 and 20 and guided to a beam splitter 22 that constitutes an interference optical system. When measuring the straightness of the linear movement mechanism 16, the moving body 16A is moved, but if the moving body 16A moves in a direction perpendicular to the designed movement direction of the linear movement mechanism 16 (that is, the straightness When a change component is added), the first laser light (diffraction light) is subjected to frequency modulation due to the Doppler effect due to this variation.

If the modulation frequency that is modulated with the straightness change component is δ, the first laser beam that undergoes frequency modulation and is diffracted will change from the original first frequency f ₁ to a frequency (f ₁ + δ) modulated by the modulation frequency δ. Become.
A second laser beam with a second frequency f 2 is separated from the first laser beam diffracted by the diffraction grating 14 by the polarizing beam splitter 12 and guided to the beam splitter ₂₂ via the mirror 24 and the half-wave plate 25. The light is combined (interfered) with the light through the beam splitter 22 and the condensing lens 26 (step S16).

The photodetector 28 detects an interference signal indicating the combined light (interference light) (step S18). The interference signal detected by the photodetector 28 is beat down to the frequency difference (|f ₁ - f ₂ |+δ) between the frequency of the first laser beam (f ₁ +δ) and the frequency f ₂ of the second laser beam. Detected as a beat frequency wave.

Subsequently, the frequency counter 30 detects the beat frequency (|f ₁ −f ₂ |+δ) of the interference signal, and calculates the modulation frequency δ corresponding to the straightness component of the moving body 16A (step S20). Note that since the frequency difference Δf (=|f ₁ -f ₂ |) is known, the modulation frequency δ can be calculated from the beat frequency (|f ₁ -f ₂ |+δ) by signal processing.

The straightness measuring method of this embodiment measures the straightness of the linear movement mechanism 16 based on the modulation frequency δ calculated as described above (step S22). Here, the straightness of the linear movement mechanism 16 is measured by detecting the speed of the moving body 16A in a direction perpendicular to the designed moving direction of the moving body 16A while the moving body 16A is moving. Alternatively, the detection can be performed by integrating the detected velocity and detecting the displacement in the direction perpendicular to the designed moving direction of the moving body 16A.

As described above, according to the straightness measuring method of the present embodiment, the straightness of one first laser beam can be measured in comparison with the conventional technique in which straightness is measured from the optical path difference between two laser beams passing through two optical paths. Since straightness is measured by detecting the modulation frequency δ corresponding to the changing component, the influence of air turbulence is small and stable straightness measurement can be performed. Furthermore, compared to conventional techniques that measure straightness based on phase difference (optical path difference), since straightness is measured from modulation frequency δ, it is also possible to measure straightness over one period of laser light.

[others]
A laser light generating device that includes a laser oscillator and generates a first laser light having a first frequency and a second laser light having a second frequency different from the first frequency has the first embodiment shown in FIGS. 1 and 2. The first laser beam of the first frequency and the second laser beam of the second frequency, which have an easy-to-handle frequency difference and cause heterodyne interference in an interference optical system, are generated without being limited to the form and the second embodiment. It is fine as long as it is done.

Further, although the diffraction grating 14 in this embodiment is a transmission type diffraction grating, a reflection type diffraction grating may also be used. In this case, it is necessary to arrange the mirror at a different position from mirrors 18 and 20.

Furthermore, it goes without saying that the present invention is not limited to the embodiments described above, and that various modifications can be made without departing from the spirit of the present invention.

10, 11... Laser oscillator, 12... Polarizing beam splitter, 13, 22... Beam splitter, 14... Diffraction grating, 16... Linear movement mechanism, 16A... Moving body, 18, 20, 24... Mirror, 25... 1/2 wavelength Plate, 26... Condensing lens, 28... Photodetector, 30... Frequency counter

Claims (6)

a laser beam generator that includes a laser oscillator and generates a first laser beam of a first frequency and a second laser beam of a second frequency different from the first frequency;
a diffraction grating that is disposed on a moving body of a linear movement mechanism, receives the first laser beam in the same traveling direction as the moving direction of the moving body, and emits diffracted light of the first laser beam;
an interference optical system that causes the first laser beam and the second laser beam emitted from the diffraction grating to interfere with each other and emits interference light;
a photodetector that detects an interference signal indicating the interference light;
a frequency detector that detects, from the interference signal, a modulation frequency that is modulated according to fluctuations in a direction that intersects the designed movement direction of the mobile body;
measuring the straightness of the linear movement mechanism based on the modulation frequency detected by the frequency detector;
Straightness measuring device. The straightness of the linear movement mechanism is measured based on the modulation frequency detected by the frequency detector. or by integrating the speed and detecting the displacement in a direction intersecting the designed moving direction of the moving body.
The straightness measuring device according to claim 1. The laser oscillator includes a first laser beam having a first frequency, and a second laser beam having a second frequency different from the first laser beam and having a polarization direction different from the first laser beam. It is a Zeeman laser that oscillates
The laser beam generator includes a polarizing beam splitter that separates the first laser beam of the first frequency and the second laser beam of the second frequency oscillated from the Zeeman laser, and the separated first laser beam. and a polarizing plate that adjusts the polarization direction so that the second laser beam and the second laser beam can interfere with each other.
The straightness measuring device according to claim 1 or 2. The laser oscillator oscillates a single frequency laser beam,
The laser beam generator includes a beam splitter that separates the single frequency laser beam into two directions;
a frequency shifter that frequency-modulates one of the laser beams in two directions separated by the beam splitter;
The other of the laser beams in the two directions separated by the beam splitter and one of the laser beams frequency-modulated by the frequency shifter are generated as a first laser beam of the first frequency. and generating the other laser beam as a second laser beam of the second frequency,
The straightness measuring device according to claim 1 or 2. The interference optical system includes a condenser lens that condenses at least the first laser beam emitted from the diffraction grating onto a light receiving surface of the photodetector.
The straightness measuring device according to any one of claims 1 to 4. arranging a diffraction grating on a moving body of the linear movement mechanism, and causing a first laser beam of a first frequency to be incident on the diffraction grating from a laser beam generator in the same traveling direction as the moving direction of the moving body;
generating a second laser beam having a second frequency different from the first frequency from the laser beam generator;
causing the diffracted light of the first laser beam by the diffraction grating to interfere with the second laser beam by an interference optical system, and emitting interference light from the interference optical system;
detecting an interference signal indicative of the interference light with a photodetector;
Detecting a modulation frequency that is modulated in accordance with fluctuations in a direction intersecting a designed moving direction of the moving object from the interference signal using a frequency detector,
measuring the straightness of the linear movement mechanism based on the modulation frequency detected by the frequency detector;
Straightness measurement method.