A laser interferometer that is insensitive to angular errors and simultaneously measures straightness and displacement.

Abstract

This invention discloses a laser interferometric device for simultaneous measurement of straightness and displacement that is insensitive to angular errors. It comprises an interferometric optical path and a measurement mirror assembly. The measurement mirror assembly consists of a Wollaston prism, a cornerstone prism, and a beam splitter, forming a moving measurement mirror. As this measurement mirror moves, it generates two measurement beams with different Doppler frequency differences. These beams are received by two photodetectors, outputting two measurement signals. These signals, along with a reference signal from a dual-frequency laser, are processed by a signal processing board and a computer to obtain the straightness error and displacement of the measured object. This invention achieves simultaneous measurement of straightness and displacement within a single laser interferometric device through the combination of the Wollaston prism, cornerstone prism, and beam splitter. By utilizing the insensitivity of the Wollaston prism and cornerstone prism to angular errors, the influence of the angular error of the measured object on the straightness error and displacement measurement results is reduced, thus improving measurement accuracy and stability.

Description

Technical Field

[0001] This invention relates to a metrology device characterized by employing optical methods, and more particularly to a laser interferometric straightness and displacement measurement device that is insensitive to angular errors. Background Technology

[0002] Straightness is an important geometric parameter characterizing the performance of various precision guideways or motion platforms. Laser interferometers for straightness measurement offer advantages such as high accuracy, large measurement range, and traceability. However, they typically only provide the magnitude of the measured straightness error, without specifying its exact location. This makes it difficult to pinpoint the exact location of the straightness error during actual performance testing of precision guideways or motion platforms, causing significant inconvenience for guideway manufacturing and maintenance, or motion platform calibration.

[0003] In its previous research, the applicant disclosed a Wollaston prism-based laser interference straightness and displacement simultaneous measurement device (patent number: ZL201811569945.X) and a dual-phase measurement laser heterodyne interference straightness and displacement simultaneous measurement device and method (patent number: ZL202010554665.2), using a combination of a semi-transparent and semi-reflective mirror and a Wollaston prism as the measuring mirror, which realizes the simultaneous measurement of the straightness error and position of the measured object.

[0004] However, the semi-transparent mirror in the above device is used as a displacement measuring mirror and is quite sensitive to angle changes. If the object to be measured has an angular error, the measuring mirror will rotate with the object to be measured, causing the measuring beam returning from the measuring mirror to deviate at an angle, resulting in a decrease in the interference quality between the measuring and reference beams, or even the inability to form interference. This limits the applicability and measurement accuracy of the measuring device. Summary of the Invention

[0005] To address the problems existing in the background technology, the present invention discloses a laser interference straightness and displacement measurement device that is insensitive to angle error. The device combines a Wollaston prism, a corner bevel prism, and a beam splitter prism as a measuring mirror. By moving the measuring mirror, the straightness error and displacement of the object being measured can be measured simultaneously.

[0006] This invention achieves simultaneous measurement of straightness error and displacement in a single laser interferometer by combining a Wollaston prism, a cornerstone prism, and a beam splitter. By utilizing the insensitivity of the Wollaston prism and the cornerstone prism to rotation, the influence of the rotation error of the measured object on the straightness error and displacement measurement results is reduced, thereby improving the accuracy and stability of the straightness error and displacement measurement.

[0007] The technical solution adopted by this invention to solve its technical problem is:

[0008] The overall device includes a dual-frequency laser with beat frequency signal output, a first beam splitter, a first polarizing beam splitter, a first polarizer, a first photodetector, a second polarizer, a second photodetector, a second beam splitter, a cornerstone prism, a Wollaston prism, and a double right-angle reflecting prism.

[0009] The device is divided into two polarizers, two photodetectors, an interference optical path section, and a measurement mirror section. The interference optical path section includes a dual-frequency laser, a first beam splitter, and a first polarizing beam splitter.

[0010] In the aforementioned interference optical path section, the dual-frequency laser outputs two orthogonally linearly polarized beams of different frequencies f1 and f2. Both beams are reflected and transmitted when they are split by the first beam splitter. The reflected light reflected by the first beam splitter is used as the reference beam, and the transmitted light transmitted by the first beam splitter is used as the measurement beam.

[0011] After the measurement beam passes through the measuring lens section, a portion of it is reflected back. The reflected portion forms a displacement measurement beam, which returns to the first polarizing beam splitter next to the first beam splitter. The displacement measurement beam and the reference beam reflected by the first beam splitter are both reflected and transmitted when they are incident on the first polarizing beam splitter, thus splitting the beam. The s-polarization component (frequency f2) of the displacement measurement beam reflected by the first polarizing beam splitter and the p-polarization component (frequency f1) of the reference beam transmitted by the first polarizing beam splitter are combined and then incident on the first polarizer, becoming two linearly polarized beams with the same polarization direction. They interfere and are received by the first photodetector, which outputs a displacement measurement signal.

[0012] The polarization components of the displacement measurement beam transmitted by the first polarizing beam splitter and the polarization components of the reference beam reflected by the first polarizing beam splitter both exit from the first polarizing beam splitter and do not require detection or acquisition.

[0013] Simultaneously, the measuring beam, after passing through the measuring lens section, forms two straightness measuring beams. These two straightness measuring beams are incident on the second polarizer and become two linearly polarized beams with the same polarization direction. They then interfere and are incident on the second photodetector, where they are received. The second photodetector outputs a straightness error measurement signal.

[0014] The frequency of the s-polarization component of the displacement measurement beam reflected by the first polarization beam splitter is f2, and the frequency of the p-polarization component of the reference beam transmitted by the first polarization beam splitter is f1. The two frequencies are different and correspond to two different frequencies of orthogonally linearly polarized beams.

[0015] The measuring lens assembly includes a second beam splitter, a corner bevel prism, a Wollaston prism, and a double right-angle reflecting prism. The second beam splitter is arranged close to one side of the Wollaston prism, and the second beam splitter and the Wollaston prism are arranged sequentially along the optical axis. The corner bevel prism is arranged close to the side of the second beam splitter, and the line connecting the corner bevel prism and the second beam splitter is perpendicular to the optical axis. The other side of the Wollaston prism is used to output a straightness measurement beam.

[0016] In the measurement mirror assembly, when the measuring mirror, composed of the second beam splitter, a corner bevel prism, and a Wollaston prism, moves, the measurement beam transmitted by the first beam splitter is incident on the second beam splitter and splits due to reflection and transmission. The reflected light, which is reflected by the second beam splitter, serves as the displacement measurement beam, containing a Doppler frequency difference Δf. After being reflected internally by the corner bevel prism and then reflected again by the second beam splitter, it returns as part of the inverse return path. The transmitted light, transmitted by the second beam splitter, is incident on the Wollaston prism and splits into two divergent first measurement beams (f1±Δf) with different Doppler frequency differences. p (frequency p-polarized beam) and the second measurement beam (f2±Δf) s Two beams (frequency s-polarized beams) are incident on a double right-angle reflecting prism and reflected back to the Wollaston prism to rejoin. The combined first and second measurement beams are then transmitted from the Wollaston prism as two straightness measurement beams.

[0017] In the measuring mirror assembly, the second beam splitter, the corner bevel prism, and the Wollaston prism together form the measuring mirror. The measuring mirror is mounted on the object being measured, and as the object moves, the position of the double right-angle reflecting prism is fixed in a suitable position.

[0018] In the aforementioned measuring mirror assembly, when the measuring mirror, composed of the second beam splitter prism, corner bevel prism, and Wollaston prism, moves with the object being measured, the displacement measuring beam contains the Doppler frequency difference Δf of the object being measured. The first measuring beam and the second measuring beam contain different Doppler frequency differences, wherein the first measuring beam is f1±Δf. p The frequency of the p-polarized beam and the second measurement beam are f2±Δf s An s-polarized beam of frequency.

[0019] The dual-frequency laser outputs orthogonally linearly polarized beams with two different frequencies, f1 and f2, to the first beam splitter.

[0020] The optical axes of the first polarizer and the second polarizer are adjusted to form an angle of 45° relative to the optical axis.

[0021] The two photodetectors and the dual-frequency laser in the interference optical path are all electrically connected to the signal processing board, which is then connected to the computer.

[0022] In the interference optical path section, the two measurement signals output by the first and second photodetectors and the reference signal output by the dual-frequency laser are processed together by the signal processing board and the computer to obtain the straightness error and displacement of the measured object.

[0023] This invention comprises an interference optical path section and a measurement mirror section. In the measurement mirror section, a Wollaston prism, a cornerstone prism, and a beam splitter prism form a moving measurement mirror. When the measurement mirror moves, it generates two measurement beams with different Doppler frequency differences. These beams are received by two photodetectors and output as two measurement signals. Together with the reference signal output by the dual-frequency laser, these signals are processed by a signal processing board and a computer to obtain the straightness error and displacement of the measured object.

[0024] The beneficial effects of this invention are:

[0025] 1) The measuring device of the present invention utilizes a combination of Wollaston prism, cornerstone prism and beam splitter prism to achieve simultaneous measurement of straightness error and displacement in a set of laser interferometers;

[0026] 2) The measuring device of the present invention uses a Wollaston prism, a cornerstone prism and a beam splitter as moving measuring mirrors. By taking advantage of the fact that the Wollaston prism and the cornerstone prism are insensitive to rotation angles, the influence of the rotation angle error of the measured object on the straightness error and displacement measurement results is reduced, thereby improving the accuracy and stability of straightness error and displacement measurement.

[0027] 3) The measuring device of the present invention adopts laser interferometry technology, and has the advantages of meter traceability, high measurement resolution and large measurement range in straightness error and displacement measurement.

[0028] This invention is applicable to the simultaneous measurement of straightness error and displacement of precision guideways and precision motion tables in fields such as ultra-precision machining, microelectronics manufacturing, and microelectromechanical systems. Attached Figure Description

[0029] Figure 1 This is the optical path diagram of the device of the present invention.

[0030] Figure 2 This is a schematic diagram showing the relationship between straightness error and optical path change in displacement measurement.

[0031] Figure 3 This is a schematic diagram of straightness error measurement in an embodiment.

[0032] In the figure: 1. Dual-frequency laser with beat frequency signal output; 2. First beam splitter; 3. First polarizing beam splitter; 4. First polarizer; 5. First photodetector; 6. Second polarizer; 7. Second photodetector; 8. Second beam splitter; 9. Pyramidal prism; 10. Wollaston prism; 11. Double right-angle reflecting prism. Detailed Implementation

[0033] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0034] like Figure 1 As shown, taking the laser heterodyne interferometry method as an example, this invention selects a dual-frequency laser 1 capable of outputting beat frequency signals, and outputs orthogonally linearly polarized beams with two different frequencies f1 and f2, wherein the polarization direction of the linearly polarized light at frequency f1 is perpendicular to the [missing information - likely a specific frequency]. Figure 1 On the paper, the polarization direction of the linearly polarized light at frequency f2 is parallel to... Figure 1 The paper in the middle.

[0035] In the interference optical path section, the dual-frequency laser 1 outputs two orthogonally linearly polarized beams with different frequencies f1 and f2. After being split by the first beam splitter 2, the beams are reflected and transmitted. The reflected beam serves as the reference beam, and the transmitted beam serves as the measurement beam. After passing through the measurement lens section, part of the measurement beam forms a displacement measurement beam, which returns to the first polarizing beam splitter 3. The displacement measurement beam and the reference beam reflected by the first beam splitter 2 are polarized and combined at the first polarizing beam splitter 3. The s-polarization component (frequency f2) of the reflected displacement measurement beam and the p-polarization component (frequency f1) of the transmitted reference beam are combined. After passing through the first polarizer 4, they become two linearly polarized beams with the same polarization direction and interfere with each other. This is received by the first photodetector 5, which outputs a displacement measurement signal. At the same time, after passing through the measurement lens section, another part of the measurement beam forms two straightness measurement beams. After passing through the second polarizer 6, the two straightness measurement beams become two linearly polarized beams with the same polarization direction and interfere with each other. This is received by the second photodetector 7, which outputs a straightness error measurement signal.

[0036] When the measuring mirror, composed of the second beam splitter 8, the corner bevel prism 9, and the Wollaston prism 10, moves, the measuring beam incident from the first beam splitter 2 is split by the second beam splitter 8. The reflected light, as the displacement measuring beam (containing a Doppler frequency difference Δf), returns to the interference path after being reflected by the corner bevel prism 9 and the beam splitter 8. The transmitted light from the second beam splitter 8 is split by the Wollaston prism 10 into two diverging first measuring beams (f1±Δf) containing different Doppler frequency differences. p (frequency p-polarized beam) and the second measurement beam (f2±Δf) sThe two measurement beams (frequency s-polarized beam) are reflected by the double right-angle reflecting prism 11 and return to the Wollaston prism 10 to rejoin and combine. The two straightness measurement beams after being combined return to the interference optical path.

[0037] The measuring mirror, consisting of the second beam-splitting prism 8, the cornerstone prism 9, and the Wollaston prism 10, is mounted on the object being measured. As the object moves, the double right-angle reflecting prism 11 is fixed in a suitable position. The dual-frequency laser 1 outputs orthogonally linearly polarized beams with two different frequencies, f1 and f2, to the first beam-splitting prism 2.

[0038] In the interference optical path section, the optical axes of the first polarizer 4 and the second polarizer 6 are adjusted to be 45° relative to the optical path. The two measurement signals output by the first photodetector 5 and the second photodetector 7, together with the reference signal output by the dual-frequency laser 1, are processed by the signal processing board and the computer to obtain the straightness error and displacement of the measured object.

[0039] The specific dual-frequency laser used is the Agilent Technologies 5517B dual longitudinal mode stabilized He-Ne laser, and the signal processing system hardware uses the AX7010 Black Gold development board from ChipEasy Electronics Technology (Shanghai) Co., Ltd. (the development board is equipped with two 2-channel high-speed AD modules AN9238). The computer used is a Dell Inspiron 3690 desktop computer.

[0040] exist Figure 1 In the optical path, the solid black dot represents linearly polarized light at frequency f1, the black triangular arrow represents linearly polarized light at frequency f2, the horizontal black triangular arrow with double diagonal lines represents linearly polarized light at frequency f2±Δf containing Doppler frequency differences, and the solid black dot with double diagonal lines and the vertical black triangular arrow with double diagonal lines represent linearly polarized light at frequencies f1±Δf containing different Doppler frequency differences, respectively. p Frequency p-polarized beam and f2±Δf s A frequency-polarized beam.

[0041] The device of this invention achieves simultaneous measurement of straightness and displacement in a single laser interferometer by combining a Wollaston prism, a cornerstone prism, and a beam splitter. By utilizing the characteristic that the Wollaston prism and the cornerstone prism are insensitive to angular errors as measuring mirrors, the influence of the angular error of the measured object on the straightness error and displacement measurement results is reduced, thereby improving the measurement accuracy and stability.

[0042] Combination Figure 2 and Figure 3 The specific implementation of the laser interferometry straightness and displacement measurement device that is insensitive to angle errors is as follows:

[0043] (1) Before measurement, the measuring mirror consisting of beam splitter prism 8, corner bevel prism 9 and Wollaston prism 10 is installed on the object to be measured, and the double right-angle reflecting prism 11 is fixedly placed in a suitable position on the other side. Among them, beam splitter prism 8 and corner bevel prism 9 are used as measuring mirrors for displacement measurement, and Wollaston prism 10 is used as measuring mirror for straightness error measurement.

[0044] (2) During measurement, the measuring mirror consisting of the beam splitter prism 8, the corner bevel prism 9, and the Wollaston prism 10 is moved from the initial position to the current position. Let the time be t, the distance moved be Δz, and the current position have a straightness error of Δx. According to Figure 2 The diagram shown illustrates displacement measurement. The frequency change of the displacement measurement beam caused by the Doppler effect can be expressed as:

[0045]

[0046] In the formula: Δf represents the frequency change of the measurement beam used for displacement measurement; λ is the center wavelength of the laser in air; v z The velocity v is the velocity along the measurement reference axis (z-axis direction). This velocity v is measured when the measuring mirror and the laser move in opposite directions. z A positive value indicates a positive value, and vice versa.

[0047] according to Figure 3 The diagram shown illustrates the measurement of straightness error. The frequency change of the two straightness measurement beams caused by the Doppler effect can be expressed as:

[0048]

[0049]

[0050] In the formula: Δf p , Δf s This represents the frequency variation of the two measurement beams used for straightness measurement, where θ is half the beam-splitting angle of the Wollaston prism, and v x The velocity is perpendicular to the measurement reference axis (x-axis direction). When the measuring mirror moves upwards relative to the measurement reference line, the velocity v is... x A positive value indicates a positive value, and vice versa.

[0051] (3) The displacement measurement beam interferes with the reference beam and is received by the first photodetector 5. The two straightness error measurement beams interfere with each other and are received by the second photodetector 7. The frequencies of the two measurement signals output by the two photodetectors are f1-f2±Δf and f1-f2±(Δf), respectively. p -Δf s The frequency differences Δf and Δf between the two measured signals can be obtained by calculating the frequency difference between them and the reference signal (frequency f1-f2) output by the dual-frequency laser 1. p -Δfs .according to Figure 2 and Figure 3 The geometric relationships shown indicate that the straightness error and displacement of the measured object can be expressed as follows:

[0052] Straightness error:

[0053] Displacement:

[0054] When the measuring mirror moves upward relative to the measuring baseline, that is, from Figure 3 If the dashed line position is moved to the solid line position, Δx is positive; otherwise, it is negative.

[0055] The specific signal processing is performed by a high-speed signal processing board based on an FPGA. The process is divided into analog processing and digital processing. The analog processing section preprocesses the measured and reference signals, including signal reception from the detector to the signal processing board, phase-locked loop tracking, mixing, filtering, and shaping. The digital processing section performs phase counting on the measured and reference signals, including integer-cycle counting and non-integer-cycle counting, which is handled by the digital logic gates within the FPGA. After the FPGA completes the data processing, it transmits the data to a computer for software processing and display.

[0056] The above steps enable simultaneous measurement of straightness error and displacement. By utilizing the characteristics of Wollaston prism and cornerstone prism as measuring mirrors that are insensitive to rotation, the influence of the rotation error of the measured object on the straightness error measurement results is reduced, thereby improving the accuracy and stability of straightness error and displacement measurement.

[0057] The above specific embodiments are used to explain and illustrate the present invention, but not to limit the present invention. Any modifications and changes made to the present invention within the spirit and scope of the claims shall fall within the protection scope of the present invention.

Claims

1. An angle error insensitive laser interferometer displacement and straightness measurement device, characterized in that: the device is divided into two polarizers, two photodetectors, an interference light path part and a measurement group mirror part, the interference light path part comprises a dual-frequency laser (1), a first light splitting prism (2) and a first polarization splitting prism (3); in the interference light path part, the dual-frequency laser (1) outputs two orthogonally linearly polarized beams with different frequencies f1 and f2 to the first light splitting prism (2) to be split by reflection and transmission; the reflected light reflected by the first light splitting prism (2) is used as a reference beam, and the transmitted light transmitted by the first light splitting prism (2) is used as a measurement beam; the measurement beam returns inversely after passing through the measurement group mirror part, and a part of the measurement beam returns to the first polarization splitting prism (3) beside the first light splitting prism (2); the displacement measurement beam and the reference beam are incident on the first polarization splitting prism (3) to be split by reflection and transmission; the s-polarization component of the displacement measurement beam reflected by the first polarization splitting prism (3) and the p-polarization component of the reference beam transmitted by the first polarization splitting prism (3) are incident on the first polarizer (4) to become two linearly polarized beams with the same polarization direction and interfere with each other to be received by the first photodetector (5); meanwhile, another part of the measurement beam after passing through the measurement group mirror part forms two straightness measurement beams, which are incident on the second polarizer (6) to become two linearly polarized beams with the same polarization direction and interfere with each other to be received by the second photodetector (7). The frequency of the s-polarization component of the displacement measurement beam reflected by the first polarization splitting prism (3) is f2, and the frequency of the p-polarization component of the reference beam transmitted by the first polarization splitting prism (3) is f1, the frequencies of the two are different, and correspond to the two different frequencies of the orthogonally linearly polarized beams respectively. The measurement group mirror part comprises a second light splitting prism (8), a corner cube prism (9), a Wollaston prism (10) and a double right-angle reflecting prism (11); the second light splitting prism (8) is arranged closely on one side of the Wollaston prism (10), and the corner cube prism (9) is arranged closely on the side of the second light splitting prism (8); the measurement beam transmitted by the first light splitting prism (2) is incident on the second light splitting prism (8) to be split by reflection and transmission; the reflected light reflected by the second light splitting prism (8) is used as a displacement measurement beam, which is reflected by the corner cube prism (9) and then reflected by the second light splitting prism (8) to return to the interference light path part as a part of the inverse return; the transmitted light transmitted by the second light splitting prism (8) is incident on the Wollaston prism (10) to be split into two beams of divergent first measurement beams and second measurement beams; the first measurement beams and the second measurement beams are reflected by the double right-angle reflecting prism (11) to return to the Wollaston prism (10) to be recombined, and the first measurement beams and the second measurement beams after recombination are transmitted from the Wollaston prism (10) as two straightness measurement beams. ​ ​ 2. The angle error insensitive laser interferometric straightness and displacement simultaneous measurement device of claim 1, wherein: ​ 3. The angle error insensitive laser interferometric straightness and displacement simultaneous measurement device of claim 1, wherein: ​ ​ 4. The angle error insensitive laser interferometric straightness and displacement simultaneous measurement device of claim 3, wherein: The second light splitting prism (8), the corner cube prism (9) and the Wollaston prism (10) together form a measuring mirror, which is installed on the measured object and moves with the measured object, and the double right-angle reflecting prism (11) is fixed.

5. The angle error insensitive laser interferometric straightness and displacement simultaneous measurement device of claim 4, wherein: The measuring group mirror part, the second light splitting prism (8), the corner cube prism (9) and the Wollaston prism (10) constitute the measuring mirror, when the measured object moves, the displacement measuring light beam contains the Doppler frequency difference Δf of the measured object, the first measuring light beam and the second measuring light beam contain different Doppler frequency difference, the first measuring light beam is f1±Δf p The second measuring light beam is the p-polarized light beam with the frequency f2±Δf s The second measuring light beam is the s-polarized light beam with the frequency f2±Δf 6. The angle error insensitive laser interferometric straightness and displacement simultaneous measurement device of claim 1, wherein: The dual-frequency laser (1) outputs two orthogonal linearly polarized beams with different frequencies f1 and f2 to the first light splitting prism (2).

7. The angle error insensitive laser interferometric straightness and displacement simultaneous measurement device of claim 1, wherein: The optical axes of the first polarizer (4) and the second polarizer (6) are adjusted to form an angle of 45° with the optical axis.

8. The angle error insensitive laser interferometric straightness and displacement simultaneous measurement device of claim 1, wherein: The dual-frequency laser (1) and the two photodetectors in the interference optical path part are electrically connected to a signal processing board, which is further connected to a computer.

9. The angle error insensitive laser interferometric straightness and displacement simultaneous measurement device of claim 8, wherein: The two measuring signals output by the first photodetector (5) and the second photodetector (7) in the interference optical path part and the reference signal output by the dual-frequency laser (1) are processed by the signal processing board and the computer to obtain the straightness error and displacement of the measured object.

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