Laser-Tracking-Interferometer

Abstract

Laser tracking interferometer for detecting a displacement of a first retroreflector by utilizing the interference of a laser beam emitted in the direction of the first retroreflector serving as a measuring body and reflected in the opposite direction by the first retroreflector, wherein the laser tracking interferometer performs tracking by utilizing a change in position along an optical axis of the laser beam, the laser tracking interferometer comprising: a fixed reference sphere; a sled with a first linear encoder configured to output a displacement signal associated with a relative displacement between the first linear encoder and the reference sphere; a second retroreflector provided on the sled; a laser interferometer configured to output a path signal associated with a relative displacement between the first retroreflector and the second retroreflector; and a data processor that is trained to calculate a displacement of the first retroreflector with respect to the sphere based on the displacement signal output by the first linear encoder and the displacement signal output by the laser interferometer.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority under 35 USC §119 to Japanese Patent Application No. 2013-109550, filed on May 24, 2013, the disclosure of which is hereby expressly incorporated by reference in its entirety. BACKGROUND OF THE INVENTION 1. Field of the invention

[0002] The present invention relates to a laser tracking interferometer. 2. Description of related technology

[0003] A laser tracking interferometer with a fixed reference point is known (referring to, for example, U.S. Patent No. 6,147,748, Japanese Patent No. 2603429, and Japanese Patent No. 4776454). A first retroreflector as a measuring body is attached to a front end of a Z-axis or the like of a three-dimensional measuring device, and a laser beam is emitted toward the first retroreflector. The interference of the laser beam reflected by the retroreflector in the reverse direction (the direction opposite to the emission direction) is used to measure a change in the distance from the center of a reference sphere, which serves as a measurement reference point, to the first retroreflector. Tracking is also performed based on a change in the position of an optical axis of the laser beam.

[0004] Furthermore, a laser interferometer is known which measures a change in distance between two opposite points, wherein the interferometer is positioned between the points (referring to, for example, Japanese Patent Laid-Open No. H7-190714).

[0005] Fig. 1 is a schematic structural diagram of a main part of a laser tracking interferometer disclosed in Japanese Patent No. 4776454. In a Fig. In the device shown in Figure 1, a first retroreflector 105 is attached as a measuring body to a front side of a Z-axis or the like of a three-dimensional measuring device. The device follows the first retroreflector 105 moving in space and measures with high precision an amount of change ΔL of a distance L from a center point C of a measuring sphere 101 to the first retroreflector 105, wherein the reference sphere 101 has excellent roundness and is spatially fixed.

[0006] The reference sphere 101 is manufactured such that its radius is identical around its entire surface with high accuracy. Thus, the change amount ΔL of the distance L can be obtained from a change amount ΔL2 and a change amount ΔL1, as shown below. The change amount ΔL2 is measured by a displacement gauge 103 fixed on a carriage 102 rotating around point C, and the change amount ΔL1 is measured by a laser interferometer 104 similarly fixed on the carriage 102. ΔL=ΔL1+ΔL2

[0007] The change amount ΔL2 represents a change amount of a distance L2 from the surface of the reference sphere 101 to a reference point P2 of the distance measurement of the distance measuring gauge 103. The change amount ΔL1 represents a change amount of a distance L1 from a reference point P1 of the distance measurement of the laser interferometer 104 to the first retroreflector 105.

[0008] Here, a situation is assumed in which, for example, a general Michelson interferometer is used as the laser interferometer 104, which is attached to a connection point P on the carriage 102.

[0009] In a state where the first retroreflector 105 is stationary without moving in space, and when a housing of the laser interferometer 104 is subjected to thermal expansion due to a change in ambient temperature or the like, a change amount ΔL4 of the distance L4 from the connection point P to the reference point P1 is generated, and thus the distance L4 from the connection point P to the reference point P1 increases by the change amount ΔL4. Therefore, although the first retroreflector 105 is stationary and the distance L1 from the reference point P1 to the first retroreflector 105 should not change, the distance L1 is measured shorter by the change amount ΔL4 because a position of the reference point P1 of the laser interferometer 104 is shifted outward and changed.Thus, when the housing of the laser interferometer 104 is subjected to thermal expansion due to a change in ambient temperature or the like, a problem arises in that an error occurs in the measured value of the change amount ΔL.

[0010] Similarly, when the carriage 102 is subjected to thermal expansion due to a change in ambient temperature or the like, a change amount ΔL3 of the distance L3 from the connection point P to the reference point P2 of the displacement measurement of the displacement gauge 102 is generated, and thus the distance L3 from the connection point P to the reference point P2 increases by the change amount ΔL3. Therefore, even though the first retroreflector 105 is stationary and the distance L2 from the reference point P2 to the reference sphere 101 should not change, the distance L2 is measured to be shorter by the change amount ΔL3 because a position of the reference point P2 of the displacement gauge 103 is shifted outward and changed. Therefore, when the carriage 102 is subjected to thermal expansion due to a change in ambient temperature or the like, a problem arises in that an error occurs in the measured value of the change amount ΔL. SUMMARY OF THE INVENTION

[0011] In view of the above-mentioned common circumstances, a primary advantage of the present disclosure provides a laser tracking interferometer capable of highly accurately measuring a change amount of a distance even when a housing of a laser interferometer is subjected to thermal expansion in the laser tracking interferometer. A second advantage of the present disclosure provides a laser tracking interferometer capable of highly accurately measuring a change amount of a distance even when a carriage is subjected to thermal expansion in the laser tracking interferometer.

[0012] One aspect of the present disclosure provides a laser tracking interferometer that detects a displacement of a first retroreflector by utilizing the interference of a laser beam radiated toward the first retroreflector serving as a measuring body and reflected in the reverse direction by the first retroreflector, wherein the laser tracking interferometer performs tracking by utilizing a position change of an optical axis of the laser beam.The laser tracking interferometer includes a fixed reference sphere; a carriage equipped with a first displacement gauge that outputs a signal associated with a relative displacement of the reference sphere; a second retroreflector provided on the carriage; a laser interferometer that outputs a displacement signal associated with a relative displacement between the first retroreflector and the second retroreflector; and a data processor that calculates a displacement of the first retroreflector with respect to the sphere based on the displacement signal output by the first displacement gauge and the displacement signal output by the laser interferometer.

[0013] The laser tracking interferometer may further include a displacement sensor provided on the carriage and measuring a change in the distance between the first displacement gauge and the second displacement gauge. The data processor may calculate a displacement of the first retroreflector with respect to the reference sphere based on the displacement signal output by the first displacement gauge, the displacement signal output by the laser interferometer, and the change measured by the displacement sensor.

[0014] The odometer may comprise a target object and a second odometer positioned from the target object at a distance between the first odometer and the second retroreflector, the second odometer outputting a displacement signal associated with a relative displacement of the target object.

[0015] The odometer may further comprise a third odometer positioned between the first odometer and the second retroreflector, the third odometer outputting a displacement signal associated with a relative displacement of the second retroreflector.

[0016] The odometer may further comprise a holder positioned to cover a front surface of the second retroreflector.

[0017] The laser tracking interferometer may further comprise a fourth displacement gauge provided on the carriage at a position opposite the first displacement gauge with respect to the intermediate reference sphere, wherein the fourth displacement gauge outputs a displacement signal associated with a relative displacement from the reference sphere. The data processor may calculate a displacement of the first retroreflector with respect to the reference sphere based on the displacement signal output by the first displacement gauge, the displacement signal output by the laser interferometer, and the displacement signal output by the fourth displacement sensor.

[0018] The laser tracking interferometer may further include a position detector that outputs 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 its optical axis; and a controller that, based on the position signal from the position detector, controls rotation of the carriage such that the amount of misalignment is zero.

[0019] According to the present disclosure, the amount of change in the distance can be measured with high accuracy even when the housing of the laser interferometer is subjected to thermal expansion. Furthermore, the amount of change in the distance can be measured with high accuracy even when the carriage is subjected to thermal expansion. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The present invention will be further described in the following detailed description with reference to the several drawings by way of non-limiting examples of embodiments of the present invention, wherein like reference numerals represent like parts throughout the several views of the drawings, and wherein: Fig. 1 shows a schematic structural diagram of a main part of a conventional laser tracking interferometer; Fig. 2 is a schematic diagram showing a structure of a main part of a laser tracking interferometer according to a first embodiment of the present disclosure; Fig. 3 is a schematic diagram illustrating an internal structure of the laser tracking interferometer according to the first embodiment of the present disclosure; Fig. Figure 4 shows a perspective view representing the overall setup of the laser tracking interferometer according to the first embodiment of the present disclosure; Fig. Figure 5 shows a schematic diagram of the construction of a main part of a laser tracking interferometer according to a second embodiment of the present disclosure; Fig. Figure 6 shows a schematic diagram of the structure of a main part of a laser-tracking interferometer according to a third embodiment of the present disclosure; and Fig. Figure 7 shows a schematic diagram illustrating the construction of a main part of a laser tracking interferometer according to a fourth embodiment of the present disclosure. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0021] The details shown here are exemplary and serve only to illustrate the embodiments of the present invention. They are presented with the intention of providing the most helpful and easily understandable description of the principles and conceptual aspects of the present invention. In this respect, no attempt is made to describe constructive details of the present invention in more detail than is necessary for a basic understanding of the invention, and the description, together with the drawings, makes it clear to those skilled in the art how the forms of the present invention can be implemented in practice.

[0022] Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. However, the present invention is not limited to the following preferred embodiments. Furthermore, the following descriptions and drawings may be simplified for clarity. First embodiment

[0023] A first embodiment of the present disclosure will be described with reference to the Fig. 2 to 4 described. Fig. 2 is a schematic diagram illustrating a structure of a main part of a laser tracking interferometer according to the present embodiment. Fig. 3 is a schematic diagram illustrating an internal structure of the laser tracking interferometer of the present embodiment. Fig. Figure 4 is a perspective view showing the overall setup of the laser tracking interferometer according to the present embodiment.

[0024] In the Fig. In the embodiment shown in Figures 2 to 4, a first retroreflector 206 as a measuring body is attached to an end face of a Z-axis or the like of a three-dimensional measuring device. A laser tracking interferometer 203 is used to track the moving first retroreflector 206 and simultaneously measure a change amount ΔL of the distance L from a center point C of a reference sphere 201 to the first retroreflector 206, the reference sphere 201 having excellent roundness and being fixedly fixed (i.e., spatially fixed without moving). Conventional components of the structure are described in detail in, for example, U.S. Patent No. 6,147,748 and Japanese Patent Laid-Open No. H7-190714, both of which are expressly incorporated herein by reference in their entirety, and therefore, their descriptions are omitted below. Functions of the above-mentioned components and deviations from the conventional structure will be described.

[0025] With reference to the Fig. Figures 2 to 4 of the laser tracking interferometer of the present embodiment include the reference sphere 201. The reference sphere 201 is attached to a base plate 409 via a holder 410. A support frame 406 is provided on the base plate 409. A slide 202 is rotatably attached to the support frame 406. The support frame 406 rotates in an azimuthal direction around the center point C of the reference sphere 201 (direction of rotation about a Y-axis of Fig. 4) by an azimuth angle rotation motor 408 provided on the base plate 409. Furthermore, an elevation angle rotation motor 407 is provided on the support frame 406. The rotation drive of the elevation angle rotation motor 407 rotates the carriage in an elevation direction (rotation direction about an X-axis in Figs. 1 and 4). The elevation angle rotation motor 407 and the azimuth angle rotation motor 408 are connected to a data processor 411, which controls the rotation of the carriage 202. The carriage 202 has a laser interferometer 203 that emits a measuring light (laser beam). The laser beam is emitted along the Z-axis as an optical axis direction orthogonal to the X and Y axes. Furthermore, a first displacement gauge 204 for the carriage 202 is provided between the reference sphere 201 and the laser interferometer 203 along the same axis as the laser beam.A second retroreflector 205 is provided on the carriage 202 between the first linear encoder 204 and the laser interferometer 203 along the same axis as the laser beam. The first linear encoder 204 and the laser interferometer 203 are connected to the data processor 411.

[0026] The optical axis Z, directed from the laser interferometer 203 to the first retroreflector 206, rotates along with the rotation of the carriage 202, while following the first retroreflector 206. The center point C of the reference sphere 201, the first linear encoder 204, the second retroreflector 205, and the laser interferometer 203 are all positioned along the optical axis Z.

[0027] The first linear encoder 204 outputs a displacement signal corresponding to a relative displacement between the reference sphere 201 and the first linear encoder 204. In particular, the first linear encoder 204 is used to measure a change ΔL2 of a distance L2 from a surface of the reference sphere 201 to the first linear encoder 204 in conjunction with the rotation of the slide 202.

[0028] The laser interferometer 203 has a displacement detector (described later) and outputs a displacement signal corresponding to a relative displacement between the second retroreflector 205 and the first retroreflector 206, simultaneously with the measurement of the change ΔL2 of the distance L2 by the first displacement gauge 204. In particular, the laser interferometer 203 is used to measure a change ΔL1 of the distance L1 from the second retroreflector 205 to the first retroreflector 206.

[0029] The data processor 411 calculates a displacement of the first retroreflector 206 with respect to the reference sphere 201 based on the displacement signal output from the first displacement gauge 204 and the displacement signal output from the laser interferometer 203.

[0030] Furthermore, the laser interferometer 203 includes a position detector (described later) and outputs a position signal associated with the amount of misalignment of the laser beam reflected by the first retroreflector 206 and returning to the laser interferometer when the laser beam is misaligned in a direction orthogonal to the optical axis. The data processor 411 includes a controller that controls the rotation of the carriage 202 based on a position signal from a two-dimensional PSD 308 such that the amount of misalignment is zero.

[0031] The components of the Fig. The two setups shown are described in detail below. The reference sphere 201, for example, can have a quality of G3 and a diameter of 5 to 25.4 mm. The reference sphere 201 can be made of conductive quartz, cordierite ceramic, or BK7, each coated with a high-carbon chromium bearing steel, Super-Invar, precious metal, or the like. The reference sphere 201 can be fixed in place by attaching it to the base plate 409 via the holder 410. The base plate 409 is made of Super-Invar, cordierite ceramic, or an aluminum alloy. The holder 410 is made of a material with a low coefficient of linear expansion, such as Super-Invar or cordierite ceramic, and is rod-shaped or conical.

[0032] The carriage 202 is preferably formed from a material with a low coefficient of linear expansion, such as super-Invar or cordierite ceramic, to reduce the amount of change ΔL3 due to thermal expansion of the distance L3 between the second retroreflector 205 and the first displacement gauge 204. However, the material of the carriage is not limited to the above materials. The carriage 202 may be formed from a material such as an aluminum alloy or carbon steel. The second retroreflector 205 and the first displacement gauge 204 may then be placed in the same holder with a low coefficient of linear expansion, and one end of the holder may be fixed to the carriage 202. Although the coefficient of linear expansion is higher than that of a super-Invar or the like, the material cost can be lower in this case.

[0033] The laser interferometer 203 can have a housing made of a material with a low coefficient of linear expansion, such as super-invar or cordierite ceramic. In the present embodiment, the laser interferometer 203 can measure the change ΔL without being affected by thermal expansion of the housing, as will be described later. The material of the laser interferometer 203 is therefore not limited to the materials mentioned above. Although the coefficient of linear expansion is greater than that of super-invar or the like, a more cost-effective material, such as carbon steel or an aluminum alloy, can be used for the housing of the laser interferometer 203.

[0034] The first displacement gauge 204 may be a capacitive displacement gauge or an eddy current displacement gauge.

[0035] The second retroreflector 205 can, for example, be a prismatic quartz angle reflector, a hollow retroreflector, or a spherical retroreflector formed from a spherical glass having a refractive index of 2.0 and partially coated with a metal that reflects a laser beam. The second retroreflector 205 can be configured to be attached to the carriage 202 via a holder made of a material with a low coefficient of linear expansion, such as super-invar. The second retroreflector 205 can be made of a less expensive material than the first retroreflector 206.

[0036] The first retroreflector 206 can be, for example, a hollow retroreflector, a combined hemispherical retroreflector, or a spherical retroreflector formed from a spherical glass having a refractive index of 2.0 and partially coated with a metal that reflects a laser beam. The first retroreflector 206 can be attached to the end face of the Z-axis or the like of the three-dimensional measuring device (measuring body) by means of a holder made of a material with a low coefficient of linear expansion, such as super-invar or cordierite ceramic.

[0037] With reference to Fig. 3, the laser interferometer 203 includes an optical fiber 301 that emits a laser beam from a light source; a collimator lens 302; a polarization beam splitter (PBS) 303; a non-polarization beam splitter (NPBS) 304; a λ / 4 plate 305; a λ / 4 plate 306; a phase meter 307 that serves as a displacement detector; and a two-dimensional position detector (2D PSD) 308 that serves as a position detector.

[0038] With a Fig. In the setup of the laser interferometer 203 shown in Figure 3, the change ΔL1 of the distance L1 from the second retroreflector 205 to the first retroreflector 206 can be measured. Furthermore, a displacement amount and direction of the first retroreflector 206 required for tracking can be detected in a direction orthogonal to the optical axis Z.

[0039] Regarding the respective components of the structure in Fig. 3, the optical fiber 301 can be, for example, a polarization-maintaining fiber or a single-mode fiber. A PBS and a λ / 2 plate can be provided between the collimator lens 302 and the PBS 303. The PBS increases an extinction ratio of the laser output from the optical fiber 301. The λ / 2 plate adjusts a ratio of polarization intensity P to polarization intensity S. For the 2D PSD 308, a quadrant photodiode can be used instead of a two-dimensional position detector. A lens for reducing a beam diameter can be provided between the 2D PSD 308 and the non-polarization beam splitter 304. The phase meter 307 can be, for example, a quadrature phase meter capable of detecting four phases of Sinθ, -Sinθ, Cosθ, and -Cosθ. Instead of a four-phase meter, a two-phase meter can be used, which is capable of detecting two phases of sine and cosine.An optical system can be provided to reduce the beam diameter between the PBS 303 and the phase meter 307.

[0040] With reference to the Fig. 3 and Fig. 4 A measurement principle regarding the change ΔL of the distance L from the center point C of the reference sphere 201 to the first retroreflector 206 according to the present embodiment is described below.

[0041] A laser beam from the light source (not shown in the drawings) is directed through the optical fiber 301 onto the laser interferometer 203. The laser light source (not shown in the drawings) can, for example, be a laser light source with a single wavelength of 633 nm. The directed laser beam is converted into parallel light by the collimator lens 302. The laser beam converted into parallel light exhibits p-polarized and s-polarized light and is split into measurement light and reference light by the PBS 303.

[0042] The p-polarized light enters the phase meter 307 as the reference light. The s-polarized light, however, is reflected by the PBS 303 and emitted as measuring light in the direction of the first retroreflector 206 through the NPBS 304 and the λ / 4 plate 305.

[0043] The measuring light, which is reflected back by the first retroreflector 206, passes again through the A / 4 plate 305. The measuring light is then partially reflected by the NPBS 304 and detected by the 2D PSD 308.

[0044] The position of the measuring light detected by the 2D PSD 308 changes depending on the magnitude and direction of the displacement of the first retroreflector 206 when it is moved in a direction orthogonal to the optical axis Z. The data processor 411 thus receives the position of the measuring light detected by the 2D PSD 308, and the controller of the data processor 411 drives the elevation rotary motor 407 and the azimuth rotary motor 408 to rotate the carriage 202 such that the position of the measuring light detected by the 2D PSD 308 remains constant. This allows the optical axis Z of the measuring light emitted from the laser interferometer 203 to continuously follow the first retroreflector 206.

[0045] Meanwhile, after passing through the λ / 4 plate 305 again, the measurement light reflected back by the first retroreflector 206 partially passes through without being reflected by the NPBS 304, passes through the PBS 303 and the λ / 4 plate 306, and is then emitted toward the second retroreflector 205. The measurement light reflected back by the second retroreflector 205 passes through the λ / 4 plate 306 again and is then reflected by the PBS 303 toward the phase meter 307. The measurement light reflected back by the second retroreflector 205 overlaps the reference light passing through the PBS 303. A phase difference between the measurement light and the reference light is detected by the phase meter 307. The data processor 411 obtains a value of the phase difference and uses the value to calculate the change amount λL1.

[0046] The data processor 411 adds the change amount ΔL2 measured by the first displacement gauge 204 and the change amount ΔL1 measured by the laser interferometer 203 to obtain the change amount of the distance L as shown below. ΔL=ΔL1+ΔL2

[0047] According to the setup of the present embodiment, the second retroreflector 205, which is positioned on the same axis as the laser beam, is additionally provided on the carriage 202 to measure a relative displacement between the second retroreflector 205 and the first retroreflector 206. Even if the housing of the laser interferometer 203 is subjected to thermal expansion, the distance L1 from the second retroreflector 205 to the first retroreflector 206 does not change without being affected. Consequently, an error due to thermal expansion of the housing of the laser interferometer, which is very likely to occur in the conventional method, does not occur. Even if the housing of the laser interferometer 203 is subjected to thermal expansion, the change ΔL of the distance can be measured with high accuracy.

[0048] Furthermore, in the conventional method, a material with a low coefficient of linear expansion, such as super-invar or cordierite ceramic, can be considered as the material for the housing of the laser interferometer to reduce errors due to thermal expansion of the housing of the laser interferometer 203. In contrast, in the design of the present embodiment, a material such as an aluminum alloy or carbon steel, which has a higher coefficient of linear expansion than super-invar or the like, can be used, thus enabling manufacturing with a more cost-effective material. Therefore, a laser tracking interferometer can be provided at a lower price. Second embodiment

[0049] Fig. Figure 5 is a schematic diagram illustrating the structure of a main part of a laser-tracking interferometer according to the present embodiment. In the following description of the present embodiment, components already described are identified with the same reference symbols, and their descriptions are omitted or simplified. With reference to Fig. 5 The laser tracking interferometer of the present embodiment comprises, as an exemplary displacement measuring device, a second displacement measuring gauge 501 and a second displacement measuring gauge target object 502.

[0050] The second linear encoder 501 and the second linear encoder target object 502 are attached to the carriage 202 at a predetermined distance L3 between them. With reference to the Fig. 5 corresponds to the distance L3 equal to a distance between a reference point of the path measurement of the first measuring gauge 204 and the second retroreflector 205.

[0051] The second displacement gauge 501 outputs a displacement signal associated with a relative displacement of the second displacement gauge target 502. The second displacement gauge 501 is used to measure an amount of change in the distance between the displacement measurement reference point of the first displacement gauge 204 and the second retroreflector 205.

[0052] The second displacement gauge 501 can be a laser interferometric displacement gauge or a capacitive displacement gauge. If the laser interferometric displacement gauge is used as, for example, the second displacement gauge 501, the second displacement gauge target 501 can be, for example, a flat mirror or retroreflector. If the capacitive displacement gauge is used as, for example, the second displacement gauge 501, the second displacement gauge target 502 can be a block of a material with a low coefficient of linear expansion, such as Super-Invar.

[0053] With reference to Fig. Section 5 below describes a measurement principle for the change ΔL of the distance L from the center of the reference sphere 201 to the first retroreflector 206 according to the present embodiment. The laser interferometer 203 measures the change ΔL1; the first displacement gauge 204 measures the change ΔL2; and simultaneously, the second displacement gauge 501 measures the change ΔL3 of the distance L3. The data processor 411 adds the change ΔL2 measured by the first displacement gauge 204 and the change ΔL1 measured by the laser interferometer 203, and also adds the change ΔL3 measured by the second displacement gauge 501. The data processor 411 then corrects the amount of change ΔL3 associated with the thermal expansion of the carriage 202 and obtains the amount of change ΔL of the distance L shown below. ΔL=ΔL1+ΔL2+ΔL3

[0054] Similar to the first embodiment described above, the change in distance ΔL1 does not change according to the setup of the present embodiment, even when the housing of the laser interferometer 203 is subjected to thermal expansion. Thus, an error due to thermal expansion of the laser interferometer housing, as occurs in the conventional method, does not occur. Consequently, the change in distance ΔL can be measured with high accuracy even when the laser interferometer housing is subjected to thermal expansion.

[0055] Furthermore, according to the design of the present embodiment, the second linear encoder 501 is additionally provided on the slide 202 to measure the change in the distance between the reference point of the displacement measurement of the first linear encoder 204 and the second retroreflector 205. This corrects the error ΔL3 caused by thermal expansion of the slide 202, which is likely to occur in the conventional method. Thus, the change in distance ΔL can be measured with high accuracy even when the slide 202 is subjected to thermal expansion. Third embodiment

[0056] Fig. Figure 6 is a schematic diagram illustrating the structure of a main part of a laser-tracking interferometer according to the present embodiment. In the following description of the present embodiment, components already described are identified with the same reference symbols, and their descriptions are omitted or simplified. With reference to the Fig. 6 and in comparison to the first embodiment described above, the laser tracking interferometer of the present embodiment also includes, as an alternative exemplary displacement measuring device, a third displacement measuring gauge 601 and a holder 602.

[0057] The third linear encoder 601 is provided on the slide 202 between the first linear encoder 204 and the second retroreflector 205 and is attached to a rear surface of the first linear encoder 204. The holder 602 is attached to the slide 202 to cover a front surface of the second retroreflector 205, which faces the third linear encoder 601. The holder 602 has a flat rear surface. The second retroreflector 205 is attached to the slide 202 via the holder 602. In a case where the second retroreflector 205 has a spherical structure, the present embodiment can be implemented without the holder 602.

[0058] The third linear encoder 601 and the holder 602 are at a predetermined distance L 3A attached between them to the sled 202. With reference to the Fig. 6 is the distance L 3Aa distance between a reference point of the path measurement of the third measuring gauge 601 and a front surface of the holder 602.

[0059] The third linear encoder 601 outputs a displacement signal corresponding to a relative displacement of the second retroreflector 205 (holder 602). The third linear encoder 601 is used to measure a change ΔL. 3A to measure, which is part of the change in the distance between the reference point of the displacement measurement of the first displacement gauge 204 and the second retroreflector 205.

[0060] The third linear encoder 601 can be a laser interferometric linear encoder or a capacitive linear encoder. The holder 602 can be made from a super-invar with a low coefficient of linear expansion.

[0061] With reference to Fig. 6, a measurement principle regarding a change amount ΔL of the distance L from the center C of the reference sphere 201 to the first retroreflector 206 according to the present embodiment is described below. The laser interferometer 203 measures the change amount ΔL1; the first displacement gauge 204 measures the change amount ΔL2; and simultaneously, the third displacement gauge 601 measures the change amount ΔL 3A , which is part of the change distance ΔL3 of the distance L3. The data processor 411 adds the change amount ΔL2 measured by the first displacement gauge 204 and the change amount ΔL1 measured by the laser interferometer 203, and also adds the change amount ΔL measured by the third displacement gauge 601. 3A The data processor 411 then corrects the change amount ΔL 3A , which is part of the change ΔL3 of the distance L3 associated with the thermal expansion of the slide 202, and receives the change ΔL of the distance L shown below. ΔL=ΔL1+ΔL2+ΔL3A

[0062] Similar to the first embodiment described above, the change in distance ΔL1 does not change according to the setup of the present embodiment, even when the housing of the laser interferometer 203 is subjected to thermal expansion. Thus, an error due to thermal expansion of the laser interferometer housing, as occurs in the conventional method, does not occur. Consequently, the change in distance ΔL can be measured with high accuracy even when the laser interferometer housing is subjected to thermal expansion.

[0063] Furthermore, according to the design of the present embodiment, the third linear encoder 601 is additionally provided on the slide 202 in order to measure part of the change in the distance between the reference point of the displacement measurement of the first linear encoder 204 and the second retroreflector 205. This corrects the change ΔL 3A , which is part of the error ΔL3 caused by thermal expansion of the carriage 202 and which is likely to occur in the conventional method. Thus, the change ΔL of the distance can be measured with high accuracy even when the carriage 202 is subjected to thermal expansion.

[0064] Furthermore, by using the measured values ​​of the change amounts ΔL1, ΔL2 and ΔL3 as well as a value of the distance L3 and a value of the distance L 3A , which are measured in advance, the amount of change ΔL can also be obtained as below to correct the thermal expansion of the slide 202. [Expression 1] ΔL=ΔL1+ΔL2+L3L3AΔL3A Fourth embodiment

[0065] Fig. Fig. 7 is a schematic diagram illustrating a structure of a main part of a laser tracking interferometer according to the present embodiment. In the following description of the present embodiment, previously described components are denoted by the same reference numerals, and their descriptions are omitted or simplified. Fig. 7 and compared to the first embodiment described above, the laser tracking interferometer of the present embodiment further comprises a fourth displacement gauge 701.

[0066] The fourth measuring gauge 701 is attached to the slide 202 to be on the same axis as the laser beam and opposite the first measuring gauge 204 with reference to the intermediate reference sphere 201.

[0067] The fourth linear encoder 701 outputs a displacement signal corresponding to a relative displacement between the reference sphere 201 and the fourth linear encoder 701. In particular, the fourth linear encoder 701 is used to measure a change ΔL3 in the distance L3 from the surface of the reference sphere 201 to the fourth linear encoder 701, which is associated with the rotation of the slide 202. Measuring the change in distance ΔL5 allows for a correction of the thermal expansion of the reference sphere 201. The fourth linear encoder 701 can be a capacitive or an eddy current encoder.

[0068] With reference to Fig.7, a measurement principle regarding a change amount ΔL of the distance L from the center of the reference sphere 201 to the first retroreflector 206 according to the present embodiment will be described below. The laser interferometer 203 measures the change amount ΔL1; the first displacement gauge 204 measures the change amount ΔL2; and simultaneously, the fourth displacement gauge 701 measures the change amount ΔL5 of the distance L5. The data processor 411 adds the change amount ΔL2 measured by the first displacement gauge 204 and the change amount ΔL1 measured by the laser interferometer 203, and further adds the change amount ΔL5 measured by the fourth displacement gauge 701. The data processor 411 then corrects the change amount ΔL5 associated with the thermal expansion of the reference sphere 201, and obtains the change amount ΔL of the distance L shown below. [Expression 2] ΔL=ΔL1+ΔL2+ΔL52

[0069] Similar to the first embodiment described above, the change in distance ΔL1 does not change according to the setup of the present embodiment, even when the housing of the laser interferometer 203 is subjected to thermal expansion. Thus, an error due to thermal expansion of the laser interferometer housing, as occurs in the conventional method, does not occur. Consequently, the change in distance ΔL can be measured with high accuracy even when the laser interferometer housing is subjected to thermal expansion.

[0070] Furthermore, according to the structure of the present embodiment, a material such as an aluminum alloy or carbon steel, which has a higher coefficient of linear expansion than Super Invar or the like, can be used as the material for the housing of the interferometer 203, thereby enabling manufacturing with a lower-cost material. Thus, a laser tracking interferometer can be provided at a lower price.

[0071] Furthermore, according to the structure of the present embodiment, the fourth displacement gauge 701 is additionally provided on the slider 202 to measure the relative displacement between the reference ball 201 and the fourth displacement gauge 701. When the reference ball 201 is subjected to uniform thermal expansion from the center point C, the thermal expansion of the reference ball 201 can be compensated. Thus, the amount of change in the distance can be measured with high accuracy.

[0072] The present invention is not limited to the embodiments described above and can be modified within a scope not deviating from the concept.

[0073] It is noted that the preceding examples were given for illustrative purposes only and are in no way to be construed as limiting the present invention. Although the present invention has been described with reference to exemplary embodiments, it is understood that the wording used herein is a description and explanation rather than a limitation. Amendments may be made within the scope of the pending claims, as indicated herein and in the present version, without altering the scope of protection and the inventive concept of the present invention in any aspect.Although the present invention has been described herein with reference to particular structures, materials, and embodiments, the present invention is not intended to be limited to the details disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods, and uses as fall within the scope of the appended claims.

[0074] The present invention is not limited to the embodiments described above, and various variations and modifications are possible without deviating from the scope of protection of the present invention.

Claims

[1] Laser tracking interferometer for detecting a displacement of a first retroreflector by utilizing the interference of a laser beam emitted in the direction of the first retroreflector serving as a measuring body and reflected in the opposite direction by the first retroreflector, wherein the laser tracking interferometer carries out tracking by utilizing a change in position of an optical axis of the laser beam, the laser tracking interferometer comprising: a fixed reference sphere; a carriage having a first displacement gauge configured to output a displacement signal associated with a relative displacement between the first displacement gauge and the reference ball; a second retroreflector provided on the carriage; a laser interferometer configured to output a displacement signal associated with a relative displacement between the first retroreflector and the second retroreflector; and a data processor configured to calculate a displacement of the first retroreflector with respect to the sphere based on the displacement signal output by the first displacement gauge and the displacement signal output by the laser interferometer. [2] The laser tracking interferometer according to claim 1, further comprising an odometer provided on the carriage and configured to measure an amount of change in the distance between the first odometer and the second retroreflector, wherein the data processor is configured to calculate a displacement of the first retroreflector with respect to the reference sphere based on the displacement signal output from the first odometer, the displacement signal output from the laser interferometer, and the amount of change measured by the odometer. [3] Laser tracking interferometer according to claim 2, wherein the displacement sensor comprises: a target object; and a second displacement gauge positioned from the target object at a distance between the first displacement gauge and the second retroreflector, the second displacement gauge being configured to output a displacement signal associated with a relative displacement between the second displacement gauge and the target object. [4] The laser tracking interferometer of claim 2, wherein the displacement sensor further comprises a third displacement gauge positioned between the first displacement gauge and the second retroreflector, the third displacement gauge configured to output a displacement signal associated with a relative displacement between the third displacement gauge and the second retroreflector. [5] The laser tracking interferometer of claim 4, wherein the odometer further comprises a holder covering a front surface of the second retroreflector. [6] Laser tracking interferometer according to one of claims 1 to 3, further comprising an additional displacement gauge provided on the carriage at a position opposite the first displacement gauge with respect to the intermediate reference sphere, wherein the additional displacement gauge is configured to output a displacement signal associated with a relative displacement between the additional displacement gauge and the reference sphere, wherein the data processor is further configured to calculate a displacement of the first retroreflector with respect to the reference sphere based on the displacement signal output by the first displacement gauge, the displacement signal output by the laser interferometer and the displacement signal output by the additional displacement gauge. [7] The laser tracking interferometer according to claim 4 or claim 5, further comprising a fourth displacement gauge provided on the carriage at a position opposite the first displacement gauge with respect to the intermediate reference sphere, the fourth displacement gauge being configured to output a displacement signal associated with a relative displacement between the fourth displacement gauge and the reference sphere, the data processor being further configured to calculate a displacement of the first retroreflector with respect to the reference sphere based on the displacement signal output by the first displacement gauge, the displacement signal output by the laser interferometer, and the displacement signal output by the fourth displacement gauge. [8] Laser tracking interferometer according to one of the preceding claims, further comprising: a position detector configured to output 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 its optical axis; and a controller configured to control the rotation of the carriage based on the position signal from the position detector such that the misalignment amount is zero.

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