Laser tracking interferometer

A laser tracking interferometer has a carriage provided with a first displacement gauge outputting a displacement signal associated with a relative displacement from a reference sphere; a second retroreflector provided to the carriage; a laser interferometer provided to the carriage and outputting a displacement signal associated with a relative displacement between the first retroreflector and the second retroreflector; and a data processor calculating a displacement of the first retroreflector with reference to the reference sphere based on the displacement signal output from the first displacement gauge and the displacement signal output from the laser interferometer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 of Japanese Application No. 2013-109550 filed on May 24, 2013, the disclosure of which is expressly incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a laser tracking interferometer.

2. Description of Related Art

A fixed-datum type laser tracking interferometer is known (refer to U.S. Pat. No. 6,147,748, Japanese Patent No. 2603429, and Japanese Patent No. 4776454, for example), in which a first retroreflector as a measured body is attached at a front end of a Z axis or the like of a three-dimensional measuring apparatus and a laser beam is emitted toward the first retroreflector. Interference of the laser beam reflected by the first retroreflector in a return direction (direction opposite to emission) is used to measure a change in a distance from the center of a reference sphere, which is a reference point of measurement, to the first retroreflector. Tracking is also performed based on a change in a position of an optical axis of the laser beam.

Furthermore, a laser interferometer is known which measures a change in a distance between two points opposite to each other with the interferometer therebetween (refer to Japanese Patent Laid-Open Publication No. H7-190714, for example).

FIG. 1is a schematic configuration of a main portion of a laser tracking interferometer disclosed in Japanese Patent No. 4776454. In a device illustrated inFIG. 1, a first retroreflector105as a measured body is attached at a front end of a Z axis or the like of a three-dimensional measuring apparatus. The device tracks the first retroreflector105moving in space and measures in a highly accurate manner an amount of change ΔL in a distance L from a center point C of a reference sphere101to the first retroreflector105, the reference sphere101having excellent sphericity and being fixed in space.

The reference sphere101is produced such that a radius thereof is identical around an entire surface in a highly accurate manner. Thus, the amount of change ΔL in the distance L can be obtained from an amount of change ΔL2and an amount of change ΔL1as shown below, the amount of change ΔL2being measured by a displacement gauge103fixated onto a carriage102rotating around the point C, the amount of change ΔL1being measured by a laser interferometer104similarly fixated onto the carriage102.
ΔL=ΔL1+ΔL2

The amount of change ΔL2represents an amount of change in a distance L2from the surface of the reference sphere101to a reference point P2of displacement measurement of the displacement gauge103. The amount of change ΔL1represents an amount of change in a distance L1from a reference point P1of displacement measurement of the laser interferometer104to the first retroreflector105.

A situation is assumed herein where, for example, a general Michelson interferometer is used as the laser interferometer104, which is fixated to the carriage102at a connection point P on the carriage102.

In a state where the first retroreflector105stands still without moving in space, when a housing of the laser interferometer104undergoes thermal expansion due to a change in surrounding temperature or the like, an amount of change ΔL4is generated in a distance L4from the connection point P to the reference point P1, and thus the distance L4from the connection point P to the reference point P1increases by the amount of change ΔL4. As a result, even though the first retroreflector105stands still and the distance L1from the reference point P1to the first retroreflector105is not supposed to change, the distance L1is measured shorter by the amount of change ΔL4since a position of the reference point P1of the laser interferometer104is pushed out and changed. Thus, when the housing of the laser interferometer104undergoes thermal expansion due to a change in surrounding temperature or the like, a problem arises where an error occurs in a measurement value of the amount of change ΔL.

Similarly, when the carriage102undergoes thermal expansion due to a change in surrounding temperature or the like, an amount of change ΔL3is generated in a distance L3from the connection point P to the reference point P2of displacement measurement of the displacement gauge103, and thus the distance L3from the connection point P to the reference point P2increases by the amount of change ΔL3. As a result, even though the first retroreflector105stands still and the distance L2from the reference point P2to the reference sphere101is not supposed to change, the distance L2is measured shorter by the amount of change ΔL3since a position of the reference point P2of the displacement gauge103is pushed out and changed. Thus, when the carriage102undergoes thermal expansion due to a change in surrounding temperature or the like, a problem arises where an error occurs in a measurement value of the amount of change ΔL.

SUMMARY OF THE INVENTION

In view of the above conventional circumstances, a primary advantage of the present disclosure provides a laser tracking interferometer capable of measuring an amount of change in a distance in a highly accurate manner even when a housing of a laser interferometer undergoes thermal expansion in the laser tracking interferometer. A secondary advantage of the present disclosure provides a laser tracking interferometer capable of measuring an amount of change in a distance in a highly accurate manner even when a carriage undergoes thermal expansion in the laser tracking interferometer.

An aspect of the present disclosure provides a laser tracking interferometer detecting a displacement of a first retroreflector by utilizing interference of a laser beam emitted toward the first retroreflector as a measured body and reflected by the first retroreflector in a return direction, the laser tracking interferometer performing tracking by using a positional change of an optical axis of the laser beam. The laser tracking interferometer includes a reference sphere provided fixatedly; a carriage provided with a first displacement gauge outputting a displacement signal associated with a relative displacement from the reference sphere; a second retroreflector provided to the carriage; a laser interferometer outputting a displacement signal associated with a relative displacement between the first retroreflector and the second retroreflector; and a data processor calculating a displacement of the first retroreflector with reference to the reference sphere based on the displacement signal output from the first displacement gauge and the displacement signal output from the laser interferometer.

The laser tracking interferometer may further include a displacement measurer provided to the carriage and measuring an amount of change in a distance between the first displacement gauge and the second retroreflector. The data processor may calculate a displacement of the first retroreflector with reference to the reference sphere based on the displacement signal output from the first displacement gauge, the displacement signal output from the laser interferometer, and the amount of change measured by the displacement measurer.

The displacement measurer may include a target and a second displacement gauge positioned from the target with a distance between the first displacement gauge and the second retroreflector, the second displacement gauge outputting a displacement signal associated with a relative displacement from the target.

The displacement measurer may further include a third displacement gauge positioned between the first displacement gauge and the second retroreflector, the third displacement gauge outputting a displacement signal associated with a relative displacement from the second retroreflector.

The displacement measurer may further include a holder positioned so as to cover a front surface of the second retroreflector.

The laser tracking interferometer may further include a fourth displacement gauge provided to the carriage at a position opposite to the first displacement gauge with the reference sphere therebetween, the fourth displacement gauge outputting a displacement signal associated with a relative displacement from the reference sphere. The data processor may calculate a displacement of the first retroreflector with reference to the reference sphere based on the displacement signal output from the first displacement gauge, the displacement signal output from the laser interferometer, and the displacement signal output from the fourth displacement gauge.

The laser tracking interferometer may further include a position detector outputting a position signal associated with an amount of misalignment of the laser beam reflected by the first retroreflector and returning to the laser interferometer when the laser beam is misaligned in a direction orthogonal to the optical axis thereof; and a controller controlling, based on the position signal from the position detector, rotation of the carriage such that the amount of misalignment is zero.

According to the present disclosure, the amount of change in the distance can be measured in a highly accurate manner even when the housing of the laser interferometer undergoes thermal expansion. Furthermore, the amount of change in the distance in a highly accurate manner even when the carriage undergoes thermal expansion.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention are described below with reference to the attached drawings. However, the present invention is not limited to the preferred embodiments below. Furthermore, the descriptions and drawings below are simplified as appropriate for clarification purposes.

First Embodiment

A first embodiment of the present disclosure is described with reference toFIGS. 2 to 4.FIG. 2is a schematic diagram illustrating a configuration of a main portion of a laser tracking interferometer according to the present embodiment.FIG. 3is a schematic diagram illustrating an internal configuration of the laser tracking interferometer according to the present embodiment.FIG. 4is a perspective view illustrating an overall configuration of the laser tracking interferometer according to the present embodiment.

In the present embodiment shown inFIGS. 2 to 4, a first retroreflector206as a measured body is attached at a front end portion of a Z axis or the like of a three-dimensional measuring apparatus. A laser tacking interferometer203is used to track the moving first retroreflector206, and concurrently measure an amount of change ΔL in a distance L from a center point C of a reference sphere201to the first retroreflector206, the reference sphere201having excellent sphericity and being fixed in place (i.e., fixed in space and without moving). Conventional configuration components are described in detail in U.S. Pat. No. 6,147,748 and Japanese Patent Laid-Open Publication No. H7-190714, for example, both disclosures of which are expressly incorporated by reference herein in their entireties, and thus descriptions thereof are omitted below. Functions of the components above and changes from the conventional configuration components are described.

With reference toFIGS. 2 to 4, the laser tracking interferometer of the present embodiment has the reference sphere201. The reference sphere201is attached to a base plate409via a holder410. A support frame406is provided on the base plate409. A carriage202is rotatably attached to the support frame406. The support frame406is rotated in an azimuthal direction around the center C of the reference sphere201(rotational direction around a Y axis ofFIG. 4) by an azimuth angle rotation motor408provided on the base plate409. Furthermore, an elevation angle rotation motor407is provided to the support frame406. Rotary drive of the elevation angle rotation motor407rotates the carriage202in an elevation direction (rotational direction around an X axis ofFIGS. 2 and 4). The elevation angle rotation motor407and the azimuth angle rotation motor408are connected to a data processer411, which controls rotation of the carriage202. The carriage202has a laser interferometer203that emits a measurement light (laser beam). The laser beam is emitted along the Z axis as an optical axis direction orthogonal to X and Y axes. Furthermore, a first displacement gauge204is provided to the carriage202between the reference sphere201and the laser interferometer203along the same axis as the laser beam. A second retroreflector205is provided on the carriage202between the first displacement gauge204and the laser interferometer203along the same axis as the laser beam. The first displacement gauge204and the laser interferometer203are connected to the data processer411.

The optical axis Z directed from the laser interferometer203to the first retroreflector206rotates in conjunction with the rotation of the carriage202while tracking the first retroreflector206. The center point C of the reference sphere201, the first displacement gauge204, the second retroreflector205, and the laser interferometer203are all positioned along the optical axis Z.

The first displacement gauge204outputs a displacement signal associated with a relative displacement between the reference sphere201and the first displacement gauge204. Specifically, the first displacement gauge204is used to measure an amount of change ΔL2in a distance L2from a surface of the reference sphere201to the first displacement gauge204in association with the rotation of the carriage202.

The laser interferometer203has a displacement detector (described later) and outputs a displacement signal associated with a relative displacement between the second retroreflector205and the first retroreflector206, concurrently with the measurement of the amount of change ΔL2in the distance L2by the first displacement gauge204. Specifically, the laser interferometer203is used to measure an amount of change ΔL1in a distance L1from the second retroreflector205to the first retroreflector206.

The data processor411calculates a displacement of the first retroreflector206with reference to the reference sphere201based on the displacement signal output from the first displacement gauge204and the displacement signal output from the laser interferometer203.

Furthermore, the laser interferometer203has a position detector (described later) and outputs a position signal associated with an amount of misalignment of the laser beam reflected by the first retroreflector206and returning to the laser interferometer203when the laser beam is misaligned in a direction orthogonal to the optical axis. The data processor411has a controller that controls the rotation of the carriage202based on a position signal from a two-dimensional PSD308such that the amount of misalignment is zero.

Each configuration component shown inFIG. 2is described in more detail below. The reference sphere201can have, for example, a grade of G3 and a diameter of 5 to 25.4 mm. For the reference sphere201, a sphere can include, as a material, a conductive quart, cordierite ceramics, or BK7, each of which is coated with a high carbon-chromium bearing steel, super invar, noble metal, or the like. The reference sphere201can be fixed in space by fixating it to the base plate409via the holder410. The base plate409is formed of a super invar, cordierite ceramics, or aluminum alloy. The holder410is formed of a material having a small linear expansion coefficient, such as a super invar or cordierite ceramics, and has a rod or conical shape.

The carriage202is preferably formed of a material having a small linear expansion coefficient, such as a super invar or cordierite ceramics, in order to reduce an amount of change ΔL3due to thermal expansion in a distance L3between the second retroreflector205and the first displacement gauge204. However, the material of the carriage202is not limited to the above. The carriage202may be Ruined of a material, such as an aluminum alloy or carbon steel. Then, the second retroreflector205and the first displacement gauge204may be placed in the same holder including a material having a small linear expansion coefficient, and one end of the holder may be fixated to the carriage202. In this case, although the linear expansion coefficient is larger than that of a super invar or the like, the material cost can be lower.

The laser interferometer203may have a housing formed of a material having a small linear expansion coefficient, such as a super invar or cordierite ceramics. In the present embodiment, the laser interferometer203can measure the amount of change ΔL without being affected by thermal expansion of the housing, as described later. Thus, the material of the laser interferometer203is not limited to the above. Although the linear expansion coefficient is larger than that of a super invar or the like, a lower-cost material, such as a carbon steel or aluminum alloy, may be used for the housing of the laser interferometer203.

The second retroreflector205can be, for example, a quart corner cube prism, a hollow retroreflector, or a spherical retroreflector formed of a spherical glass having a refraction index of 2.0 and having a surface partially coated with a metal that reflects a laser. The second retroreflector205may be designed so as to be fixated to the carriage202via a holder formed of a material having a small linear expansion coefficient, such as a super invar. The second retroreflector205can be formed from a lower-cost material than the first retroreflector206.

The first retroreflector206can be, for example, a hollow retroreflector, a hemisphere-combined retroreflector, or a spherical retroreflector formed of a spherical glass having a refraction index of 2.0 and having a surface partially coated with a metal that reflects a laser. The first retroreflector206may be attached to the front end portion of the Z axis or the like of the three-dimensional measuring apparatus (measured body) via a holder formed of a material having a small linear expansion coefficient, such as a super invar or cordierite ceramics.

With reference toFIG. 3, the laser interferometer203has an optical fiber301transmitting a laser beam from a light source; a collimate lens302; a polarization beam splitter (PBS)303; a non-polarization beam splitter (NPBS)304; a λ/4 plate305; a λ/4 plate306; a phase meter307serving as a displacement detector; and a two-dimensional position detector (2D PSD)308serving as a position detector.

With a configuration of the laser interferometer203shown inFIG. 3, the amount of change ΔL1in the distance L1from the second retroreflector205to the first retroreflector206can be measured. Furthermore, a displacement amount and a displacement direction of the first retroreflector206in the direction orthogonal to the optical axis Z required for tracking can be detected.

With respect to each configuration component shown inFIG. 3, the optical fiber301can be, for example, a polarization-preserving fiber or single-mode fiber. A PBS and a λ/2 plate may be provided between the collimate lens302and the PBS303. The PBS increases an extinction ratio of the laser output from the optical fiber301. The λ/2 plate sets a ratio of P polarization intensity and S polarization intensity. For the 2D PSD308, a quadrant photodiode may be used instead of a two-dimensional position detector. A lens for reducing a beam diameter may be provided between the 2D PSD308and the non-polarization beam splitter304. The phase meter307can be, for example, a four-phase meter capable of detecting four phases of Sin θ, −Sin θ, Cos θ, and −Cos θ. Instead of a four-phase meter, a two-phase meter capable of detecting two phases of Sin θ and Cos θ may be used. An optical system for reducing a beam diameter may be provided between the PBS303and the phase meter307.

With reference toFIGS. 3 and 4, a measurement principle is described below with respect to the amount of change ΔL in the distance L from the center C of the reference sphere201to the first retroreflector206according to the present embodiment.

A laser beam from a laser light source (not shown in the drawings) is directed to the laser interferometer203through the optical fiber301. The laser light source (not shown in the drawings) can be a single wavelength laser light source of 633 nm, for example. The directed laser beam is converted to parallel light by the collimate lens302. The laser beam converted to the parallel light has p-polarized light and s-polarized light and is divided by the PBS303into measurement light and reference light.

The p-polarized light enters the phase meter307as is as the reference light. Meanwhile, the s-polarized light is reflected by the PBS303and is emitted as the measurement light in the direction of the first retroreflector206through the NPBS304and the λ/4 plate305.

The measurement light, which is retroreflected by the first retroreflector206, passes through the λ/4 plate305again. Then, the measurement light is partially reflected by the NPBS304and is detected by the 2D PSD308.

A position of the measurement light detected on the 2D PSD308thereby changes depending on the amount and direction of displacement of the first retroreflector206when displaced in the direction orthogonal to the optical axis Z. Thus, the data processer411obtains the position of the measurement light detected by the 2D PSD308and the controller of the data processor411drives the elevation angle rotation motor407and the azimuth angle rotation motor408to rotate the carriage202such that the position of the measurement light detected by the 2D PSD308is constantly the same. Thereby, the optical axis Z of the measurement light emitted from the laser interferometer203can constantly track the first retroreflector206again.

Meanwhile, after passing through the λ/4 plate305again, the measurement light retroreflected by the first retroreflector206partially passes through without being reflected by the NPBS304, passes through the PBS303and the λ/4 plate306, and is then emitted toward the second retroreflector205. The measurement light retroreflected by the second retroreflector205passes through the λ/4 plate306again and is then reflected by the PBS303in the direction of the phase meter307. The measurement light retroreflected by the second retroreflector205overlaps the reference light passing through the PBS303. A phase difference between the measurement light and the reference light is detected by the phase meter307. The data processor411obtains a value of the phase difference and uses the value to calculate the amount of change ΔL1.

The data processor411adds the amount of change ΔL2measured by the first displacement gauge204and the amount of change ΔL1measured by the laser interferometer203, and thus obtains the amount of change ΔL in the distance L as shown below.
ΔL=ΔL1+ΔL2

According to the configuration of the present embodiment, the second retroreflector205positioned on the same axis as the laser beam is additionally provided on the carriage202to measure a relative displacement between the second retroreflector205and the first retroreflector206. Thus, even when the housing of the laser interferometer203undergoes thermal expansion, the distance L1from the second retroreflector205to the first retroreflector206does not change without being affected. Accordingly, an error due to thermal expansion of the housing of the laser interferometer, which is likely to occur in the conventional method, does not occur. Even when the housing of the laser interferometer203undergoes thermal expansion, the amount of change ΔL in the distance can be measured highly accurately.

Furthermore, in the conventional method, a material having a small linear expansion coefficient, such as a super invar or cordierite ceramics, can be considered as a material for the housing of the laser interferometer203to reduce an error due to thermal expansion of the housing of the laser interferometer. In contrast, according to the configuration of the present embodiment, a material such as an aluminum alloy or carbon steel, which has a larger linear expansion coefficient than a super invar or the like, can be used as a material for the housing of the laser interferometer203, thus enabling production using a more inexpensive material. This provides a laser tracking interferometer at a lower price.

Second Embodiment

FIG. 5is a schematic diagram illustrating a configuration of a main portion of a laser tracking interferometer according to the present embodiment. In the description below of the present embodiment, components already described are denoted with the same reference numerals and descriptions thereof are omitted or simplified. With reference toFIG. 5, the laser tracking interferometer according to the present embodiment further includes a second displacement gauge501and a second displacement gauge target502, as an exemplary displacement measurer.

The second displacement gauge501and the second displacement gauge target502are fixated to the carriage202with a predetermined distance L3therebetween. With reference toFIG. 5, the distance L3is equal to a distance between a reference point of displacement measurement of the first displacement gauge204and the second retroreflector205.

The second displacement gauge501outputs a displacement signal associated with a relative displacement from the second displacement gauge target502. The second displacement gauge501is used to measure an amount of change in the distance between the reference point of displacement measurement of the first displacement gauge204and the second retroreflector205.

The second displacement gauge501can be a laser interferometric displacement gauge or a capacitance displacement gauge. In the case of using the laser interferometric displacement gauge, for example, as the second displacement gauge501, the second displacement gauge target502can be a plane mirror or a retroreflector, for example. In the case of using the capacitance displacement gauge, for example, as the second displacement gauge501, the second displacement gauge target502can be a block composed of a material having a small linear expansion coefficient, such as a super invar.

With reference toFIG. 5, a measurement principle is described below with respect to the amount of change ΔL in the distance L from the center C of the reference sphere201to the first retroreflector206according to the present embodiment. The laser interferometer203measures the amount of change ΔL1; the first displacement gauge204measures the amount of change ΔL2; and concurrently, the second displacement gauge501measures the amount of change ΔL3in the distance L3. The data processor411adds the amount of change ΔL2measured by the first displacement gauge204and the amount of change ΔL1measured by the laser interferometer203, and further adds the amount of change ΔL3measured by the second displacement gauge501. Then, the data processor411corrects the amount of change ΔL3associated with thermal expansion of the carriage202and obtains the amount of change ΔL in the distance L as shown below.
ΔL=ΔL1+ΔL2+ΔL3

According to the configuration of the present embodiment, similar to the first embodiment described above, the amount of change ΔL1in the distance does not change even when the housing of the laser interferometer203undergoes thermal expansion. Thus, an error due to thermal expansion of the housing of the laser interferometer, which occurs in the conventional method, does not occur. Accordingly, even when the housing of the laser interferometer203undergoes thermal expansion, the amount of change ΔL in the distance can be measured highly accurately.

Furthermore, according to the configuration of the present embodiment, the second displacement gauge501is additionally provided on the carriage202to measure the amount of change in the distance between the reference point of displacement measurement of the first displacement gauge204and the second retroreflector205. This corrects the error ΔL3due to thermal expansion of the carriage202, which is likely to occur in the conventional method. Thus, even when the carriage202undergoes thermal expansion, the amount of change ΔL in the distance can be measured highly accurately.

Third Embodiment

FIG. 6is a schematic diagram illustrating a configuration of a main portion of a laser tracking interferometer according to the present embodiment. In the description below of the present embodiment, components already described are denoted with the same reference numerals and descriptions thereof are omitted or simplified. With reference toFIG. 6, compared to the first embodiment described above, the laser tracking interferometer according to the present embodiment further includes a third displacement gauge601and a holder602, as an alternative exemplary displacement measurer.

The third displacement gauge601is provided on the carriage202between the first displacement gauge204and the second retroreflector205and is fixated to a rear surface of the first displacement gauge204. The holder602is fixated to the carriage202so as to cover a front surface of the second retroreflector205which is opposite to the third displacement gauge601. The holder602has a planar rear surface. The second retroreflector205is fixated to the carriage202through the holder602. In a case where the second retroreflector205has a spherical structure, the present embodiment can be achieved without the holder602.

The third displacement gauge601and the holder602are fixated to the carriage202with a predetermined distance L3Atherebetween. With reference toFIG. 6, the distance L3Ais a distance between a reference point of displacement measurement of the third displacement gauge601and a front surface of the holder602.

The third displacement gauge601outputs a displacement signal associated with a relative displacement from the second retroreflector205(holder602). The third displacement gauge601is used to measure an amount of change ΔL3A, which is a portion of the amount of change in the distance between the reference point of displacement measurement of the first displacement gauge204and the second retroreflector205.

The third displacement gauge601can be a laser interferometric displacement gauge or a capacitance displacement gauge. The holder602can be formed of a super invar having a small linear expansion coefficient.

With reference toFIG. 6, a measurement principle is described below with respect to the amount of change ΔL in the distance L from the center C of the reference sphere201to the first retroreflector206according to the present embodiment. The laser interferometer203measures the amount of change ΔL1; the first displacement gauge204measures the amount of change ΔL2; and concurrently, the third displacement gauge601measures the amount of change ΔL3A, which is a portion of the amount of change ΔL3in the distance L3. The data processor411adds the amount of change ΔL2measured by the first displacement gauge204and the amount of change ΔL1measured by the laser interferometer203, and further adds the amount of change ΔL3Ameasured by the third displacement gauge601. Then, the data processor411corrects the amount of change ΔL3A, which is a portion of the amount of change ΔL3in the distance L3associated with thermal expansion of the carriage202, and obtains the amount of change ΔL in the distance L as shown below.
ΔL=ΔL1+ΔL2+ΔL3A

According to the configuration of the present embodiment, similar to the first embodiment described above, the amount of change ΔL1in the distance does not change even when the housing of the laser interferometer203undergoes thermal expansion. Thus, an error due to thermal expansion of the housing of the laser interferometer, which occurs in the conventional method, does not occur. Accordingly, even when the housing of the laser interferometer203undergoes thermal expansion, the amount of change ΔL in the distance can be measured highly accurately.

Furthermore, according to the configuration of the present embodiment, the third displacement gauge601is additionally provided on the carriage202to measure a portion of the amount of change in the distance between the reference point of displacement measurement of the first displacement gauge204and the second retroreflector205. This corrects the amount of change ΔL3A, which is a portion of the error ΔL3due to thermal expansion of the carriage202, the error being likely to occur in the conventional method. Thus, even when the carriage202undergoes thermal expansion, the amount of change ΔL in the distance can be measured highly accurately.

Furthermore, using measurement values of the amounts of change ΔL1, ΔL2, and ΔL3, and a value of the distance L3and a value of the distance L3A, which are measured in advance, the amount of change ΔL can also be obtained as below to correct thermal expansion of the carriage202.

FIG. 7is a schematic diagram illustrating a configuration of a main portion of a laser tracking interferometer according to the present embodiment. In the description below of the present embodiment, components already described are denoted with the same reference numerals and descriptions thereof are omitted or simplified. With reference toFIG. 7, compared to the first embodiment described above, the laser tracking interferometer according to the present embodiment further includes a fourth displacement gauge701.

The fourth displacement gauge701is fixated on the carriage202so as to be on the same axis as the laser beam and opposite to the first displacement gauge204with the reference sphere201therebetween.

The fourth displacement gauge701outputs a displacement signal associated with a relative displacement between the reference sphere201and the fourth displacement gauge701. Specifically, the fourth displacement gauge701is used to measure an amount of change ΔL5in a distance L5from the surface of the reference sphere201to the fourth displacement gauge701associated with rotation of the carriage202. Measuring the amount of change ΔL5allows correction of thermal expansion of the reference sphere201. The fourth displacement gauge701can be a capacitance displacement gauge or an eddy current displacement gauge.

With reference toFIG. 7, a measurement principle is described below with respect to the amount of change ΔL in the distance L from the center C of the reference sphere201to the first retroreflector206according to the present embodiment. The laser interferometer203measures the amount of change ΔL1; the first displacement gauge204measures the amount of change ΔL2; and concurrently, the fourth displacement gauge701measures the amount of change ΔL5in the distance L5. The data processor411adds the amount of change ΔL2measured by the first displacement gauge204and the amount of change ΔL1measured by the laser interferometer203, and further adds the amount of change ΔL5measured by the fourth displacement gauge701. Then, the data processor411corrects the amount of change ΔL5associated with thermal expansion of the reference sphere201and obtains the amount of change ΔL in the distance L as shown below.

According to the configuration of the present embodiment, similar to the first embodiment described above, the amount of change ΔL1in the distance does not change even when the housing of the laser interferometer203undergoes thermal expansion. Thus, an error due to thermal expansion of the housing of the laser interferometer, which occurs in the conventional method, does not occur. Accordingly, even when the housing of the laser interferometer203undergoes thermal expansion, the amount of change ΔL in the distance can be measured highly accurately.

Furthermore, according to the configuration of the present embodiment, a material such as an aluminum alloy or carbon steel, which has a larger linear expansion coefficient than a super invar or the like, can be used as a material for the housing of the laser interferometer203, thus enabling production using a more inexpensive material. This provides a laser tracking interferometer at a lower price.

Furthermore, according to the configuration of the present embodiment, the fourth displacement gauge701is additionally provided on the carriage202to measure the relative displacement between the reference sphere201and the fourth displacement gauge701. When the reference sphere201undergoes thermal expansion evenly from the center C, the thermal expansion of the reference sphere201can be compensated. Thus, the amount of change in the distance can be measured highly accurately.

The present invention is not limited to the embodiments described above and may be modified appropriately within a range not deviating from the concept.

The present invention is not limited to the above-described embodiments, and various variations and modifications may be possible without departing from the scope of the present invention.