Six-degree-of-freedom displacement measurement apparatus and method based on two-dimensional grating heterodyne interference

Abstract

The application discloses a six-degree-of-freedom displacement measuring device and method based on two-dimensional grating heterodyne interference, which comprises a reflective two-dimensional orthogonal grating (1), an interference element group one (5), an interference element group two (6) and an incident light adjusting optical element; the interference element group one (5) is arranged on the left side of the incident light adjusting optical element, and the interference element group two (6) is arranged below the incident light adjusting optical element. The two-dimensional grating heterodyne interference is carried out on the diffraction light generated at two incident points by using the spatially separated double-frequency laser incident to the measuring grating, the six-degree-of-freedom displacement information of the measured grating is obtained by measuring and solving two groups of heterodyne interference signals. The whole transformation of the diffraction light is designed, the two groups of interference signals indirectly related to the displacement information are solved, the interference signals of each axis can be kept when the grating performs omnidirectional motion around the Z axis, and the continuous measurement of the in-plane rotary motion is realized.

Description

Technical Field

[0001] This invention belongs to the field of optical precision displacement measurement technology, specifically relating to a six-degree-of-freedom displacement measurement device and method based on two-dimensional grating heterodyne interference, which is suitable for nanometer-level displacement detection scenarios, and is especially suitable for large-scale omnidirectional continuous measurement needs with in-plane rotation angles exceeding ±5°. Background Technology

[0002] As a core piece of equipment in the field of ultra-precision measurement, the planar grating six-degree-of-freedom precision displacement measurement device based on a single measurement point has significant application value and development prospects in high-precision industrial scenarios such as nanoscale displacement measurement, optical precision machining, and semiconductor manufacturing. This technology achieves precise measurement of the spatial pose of the measured object through innovative optical path design and signal processing algorithms. In-plane rotational motion, as the core motion form in the main working direction, is the most critical measurement parameter among the three rotational degrees of freedom in the six-degree-of-freedom system, directly determining the overall measurement accuracy and application range of the system.

[0003] However, the traditional optical path design commonly used in existing technologies has significant technical limitations: the measurement of its three rotational degrees of freedom all rely on the detection of interference spot shift by quadrant photodiodes. When the grating rotates at a large angle, the traditional optical path design struggles to maintain a stable interference state of the interference spot, resulting in a significant decrease in the signal-to-noise ratio of the measurement signal. Furthermore, due to the physical limitations of the optical system, the movement range of all interference spots is restricted to the quadrant photodiode detection area of ​​only a few millimeters. This limitation severely restricts the system's measurement range, resulting in a measurement range of less than 1 degree for the three-axis rotation angle, which cannot meet the measurement requirements of typical industrial scenarios such as wafer alignment (requiring ±2°) and precision turntable calibration (requiring ±5°). Summary of the Invention

[0004] To achieve large-angle or even omnidirectional measurement of the rotation angle of an object within a plane, this invention provides a six-degree-of-freedom displacement measurement device and method based on two-dimensional grating heterodyne interference. Based on the principles of grating diffraction and laser heterodyne interference, and based on the designed interference optical path, the device acquires and calculates signals from the moving interference spot through a spatially separated dual-frequency laser incident design, a ring photodiode array layout, and an overall optical path transformation strategy. This yields the six-degree-of-freedom displacement information of the grating under test, and provides the capability for omnidirectional continuous measurement of the rotational motion of the grating within a plane.

[0005] The technical solution adopted in this invention is: a six-degree-of-freedom displacement measuring device based on two-dimensional grating heterodyne interference, comprising a reflective two-dimensional orthogonal grating, an interference element group one, an interference element group two, and an incident light adjustment optical element;

[0006] The first interference element group is arranged to the left of the incident light adjustment optical element, and the second interference element group is arranged below the incident light adjustment optical element.

[0007] Furthermore, the incident light adjustment optical element includes a quarter-wave plate, a first polarizing beam splitter, a first depolarizing beam splitter, a first half-wave plate, and a second half-wave plate. The quarter-wave plate is disposed directly below the reflective two-dimensional orthogonal grating. The first polarizing beam splitter and the first depolarizing beam splitter are disposed sequentially below the quarter-wave plate. The first half-wave plate is disposed on the upper left side of the first depolarizing beam splitter, and the second half-wave plate is disposed on the lower left side of the first depolarizing beam splitter.

[0008] Furthermore, the interference element group one includes a second polarizing beam splitter, a first reflecting mirror, a second reflecting mirror, a second depolarizing beam splitter, a first annular photodiode array, a second photodiode, and a first photodiode. The second polarizing beam splitter is located to the left of the first depolarizing beam splitter. The first reflecting mirror is disposed to the left of the second polarizing beam splitter. The second reflecting mirror is disposed below the second polarizing beam splitter. The second depolarizing beam splitter is disposed to the left of the second reflecting mirror. The first annular photodiode array is disposed below the second depolarizing beam splitter. The second photodiode and the first photodiode are disposed to the left of the second depolarizing beam splitter.

[0009] Furthermore, the first photodiode is a four-quadrant photodiode.

[0010] Furthermore, the second interference element group includes a third polarizing beam splitter, a first reflecting mirror, a second reflecting mirror, a first pentagonal reflecting prism, a second pentagonal reflecting prism, a third depolarizing beam splitter, and a second annular photodiode array. The third polarizing beam splitter is located below the first depolarizing beam splitter. The first reflecting mirror is located to the right of the third polarizing beam splitter. The second reflecting mirror is located below the second half-wave plate. The first pentagonal reflecting prism and the second pentagonal reflecting prism are located to the right and front of the second reflecting mirror, respectively. The third depolarizing beam splitter is located to the right of the second pentagonal reflecting prism, and the second annular photodiode array is located to the right of the third depolarizing beam splitter.

[0011] A displacement measurement method based on the aforementioned six-degree-of-freedom displacement measuring device using two-dimensional grating heterodyne interferometry includes the following steps:

[0012] Step 1: Adjustment of incident laser and guidance of diffracted light: The spatially separated dual-frequency laser source is incident on the first polarization beam splitter, reflected and passes through the quarter-wave plate, and reaches the reflective two-dimensional orthogonal grating. The diffracted light returns and passes through the quarter-wave plate and then through the first polarization beam splitter. It is split into beam splitter group 1 and beam splitter group 2 by the first depolarization beam splitter, and is incident on interference element group 1 and interference element group 2 respectively.

[0013] Step Two: Interference Implementation and Signal Acquisition in Interference Element Group One: Two lasers of different frequencies in the beam-splitting laser group one are incident on the second polarization beam splitter and separated according to their different polarization states. Laser A is reflected by the second polarization beam splitter and, after reflection by the second mirror, is incident on the second depolarization beam splitter. Laser B passes through the second polarization beam splitter, is reflected by the first mirror, and is incident on the second depolarization beam splitter, coinciding with laser A. The second depolarization beam splitter splits the coinciding beam, and the zero-order diffracted light is incident on the first photodiode. The interference beams of the first-order diffracted light are then incident on the second photodiode and the first ring photodiode array. The offset of the zero-order diffracted spot in its photosensitive quadrant measured by the first photodiode is denoted as S. x ,S y Four sets of interference signals, denoted as I, were obtained after signal processing from the first ring photodiode array. 1a ,I 1b ,I 1c ,I 1d , where I 1a ,I 1b For signals with the same phase change but opposite direction along the X-axis, I 1c ,I 1d These are signals with the same phase change but opposite directions along the Y-axis.

[0014] Step 3: Interference Implementation and Signal Acquisition in Interference Element Group Two: Two lasers of different frequencies in the second beam-splitting laser group are incident on the third polarization beam splitter and separated according to their different polarization states. Laser A passes through the third polarization beam splitter, is reflected by the second triangular mirror, and then incident on the second pentagonal mirror, where it is reflected twice before being incident on the third depolarization beam splitter. Laser B is reflected by the third polarization beam splitter, reflected by the first triangular mirror, and then incident on the first pentagonal mirror, where it is reflected twice before being incident on the third depolarization beam splitter, coinciding with laser A. The interference beam formed by the overlap of lasers A and B is acquired by the second ring photodiode array. After processing the acquired signals, four sets of interference signals I are obtained. 2a ,I 2b ,I 2c ,I 2d , where I 2a ,I 2bFor signals with the same phase change but opposite directions along the X-axis, I 2c ,I 2d These are signals with the same phase change but opposite directions along the Y-axis.

[0015] Step 4: Six-DOF Displacement Information Calculation: Based on the principles of grating diffraction and laser heterodyne interference, the phase changes of eight interference signals acquired by the first and second ring photodiode arrays are obtained through signal processing circuits. The phase changes of each interference signal are related to the displacement (x) of the reflective two-dimensional orthogonal grating relative to the two incident points A and B. A ,y A ),(x B ,y B The relationship is:

[0016]

[0017] Where p is the grating pitch of the reflective two-dimensional orthogonal grating, ΔΦ x1 ,ΔΦ y1 These are the interference signals I obtained by the signal processing circuit on the X-axis of the first interference element group. 1a ,I 1b The phase change of the signal, and the interference signal I obtained by the signal processing circuit on the Y-axis. 1c ,I 1d Phase change, ΔΦ x2 ,ΔΦ y2 These are the interference signals I obtained by the signal processing circuit on the X-axis of the second interference element group. 2a ,I 2b The phase change of the signal, and the interference signal I obtained by the signal processing circuit on the Y-axis. 2c ,I 2d Phase change;

[0018] Therefore, the displacement of the grating plane relative to the incident points A and B is calculated by the phase change of the acquired and processed interference signal:

[0019]

[0020] Motion in the grating plane (ΔX, ΔY, θ) z The information is obtained by measuring the displacement (x) of the grating relative to the incident points A and B. A ,y A ),(x B ,y B This was obtained through geometric calculations.

[0021] The rotational motion of the grating about the X and Y axes (θ) x θ yThe offset S of the zero-order diffraction spot in its photosensitive quadrant is measured by the first photodiode. x ,S y With calibrable fixed transformation coefficients C x C y calculate:

[0022]

[0023] The phase change ΔΦ of the interference signal is measured by fundamental interferometry through the reflection of light from the grating, where the Z-axis displacement ΔZ of the grating is transmitted. z The result is obtained from the laser wavelength λ used:

[0024]

[0025] This enables six degrees of freedom (ΔX, ΔY, ΔZ, θ) of the grating. x θ y θ z Displacement measurement.

[0026] The beneficial effects of this invention are as follows: This invention achieves full-angle measurement of the rotational motion of the object under test within a plane through a spatially separated dual-frequency laser incident design, a ring photodiode array layout, and an overall optical path transformation strategy; its dual-frequency laser spatial separation design significantly reduces the periodic error caused by frequency aliasing due to frequency leakage from optical components; when the grating undergoes large-angle rotational motion within the plane, the overall optical path transformation strategy ensures the continuous maintenance and stability of the laser interference state; the ring photodiode array layout ensures that interference spots at any position within the sensing plane can be collected, enabling the measurement of large-angle rotational motion within the plane; furthermore, the orthogonal installation orientation of each optical component facilitates assembly; and the compact design of the optical components facilitates integration and application.

[0027] In addition to the objectives, features, and advantages described above, the present invention has other objectives, features, and advantages. The invention will now be described in further detail with reference to the figures. Attached Figure Description

[0028] Figure 1 This is a schematic diagram illustrating the usage of a six-degree-of-freedom displacement measuring device based on two-dimensional grating heterodyne interference according to the present invention;

[0029] Figure 2 This is a detailed schematic diagram of the overall structure of the reading head of a six-degree-of-freedom displacement measuring device based on two-dimensional grating heterodyne interference according to the present invention;

[0030] Figure 3 This is a detailed schematic diagram of the optical element group used for incident light adjustment in the reading head of a six-degree-of-freedom displacement measuring device based on two-dimensional grating heterodyne interference according to the present invention.

[0031] Figure 4 This is a schematic diagram of the structure and principle of the first interference optical element group in the reading head of a six-degree-of-freedom displacement measuring device based on two-dimensional grating heterodyne interference according to the present invention.

[0032] Figure 5 This is a schematic diagram illustrating the structure and principle of the second interferometric optical element group in the reading head of a six-degree-of-freedom displacement measuring device based on two-dimensional grating heterodyne interference according to the present invention.

[0033] The figure includes the following reference numerals: 1. Reflective two-dimensional orthogonal grating; 2. Reading head; 3. Spatially separated dual-frequency laser (reading head incident on the grating); 4. Incident laser source; 5. Interference element group one; 6. Interference element group two; 7. Quarter-wave plate; 8. First polarizing beam splitter prism; 9. First half-wave plate; 10. First depolarizing beam splitter prism; 11. Beam-splitting laser group one; 12. Second half-wave plate; 13. Beam-splitting laser group two; 14. Second polarizing beam splitter prism; 15. First reflecting mirror; 16. Second reflecting mirror; 17. Second depolarizing beam splitter prism; 18. First ring photodiode array; 19. First photodiode; 20. Second photodiode; 21. Third polarizing beam splitter prism; 22. First triangular reflecting mirror; 23. First pentagonal reflecting prism; 24. Second triangular reflecting mirror; 25. Second pentagonal reflecting prism; 26. Third depolarizing beam splitter prism; 27. Second ring photodiode array. Detailed Implementation

[0034] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0035] The present invention discloses a six-degree-of-freedom displacement measuring device based on two-dimensional grating heterodyne interferometry, and its usage is as follows: Figure 1 As shown, a reflective two-dimensional orthogonal grating 1 is fixed to the object under test, while the reading head 2 remains stationary. Spatially separated dual-frequency lasers are incident from the reading head 2 onto the reflective two-dimensional orthogonal grating 1. The two frequencies of laser light are represented by solid and dashed lines, denoted as laser A and laser B, respectively, and their incident points are denoted as incident point A and incident point B, respectively. The diffracted light generated by the reflective two-dimensional orthogonal grating 1 returns to the reading head 2 for interference and measurement. Solid and dashed arrows represent lasers of different frequencies, respectively. When the object under test moves the reflective two-dimensional orthogonal grating 1, the reading head 2 can read the six degrees of freedom displacement information of the object under test based on the interference signal. The X-axis and Y-axis are defined as the directions along the two orthogonal grids of the reflective two-dimensional orthogonal grating, and the Z-axis is the direction perpendicular to the plane of the reflective two-dimensional orthogonal grating.

[0036] The six-degree-of-freedom displacement measuring device based on two-dimensional grating heterodyne interferometry of the present invention is the aforementioned reading head 2. Its overall structure includes an interferometer group 5, an interferometer group 6, and an incident light adjustment optical element. The interferometer group 5 is arranged to the left of the incident light adjustment optical element, and the interferometer group 6 is arranged below the incident light adjustment optical element, as shown below. Figure 2 As shown.

[0037] The incident light adjustment optical element includes a quarter-wave plate 7, a first polarizing beam splitter 8, a first depolarizing beam splitter 10, a first half-wave plate 9, and a second half-wave plate 12. The quarter-wave plate 7 is positioned directly below the reflective two-dimensional orthogonal grating 1. Below it are, in order, the first polarizing beam splitter 8 and the first depolarizing beam splitter 10. The first half-wave plate 9 is positioned to the upper left of the first depolarizing beam splitter 10, and the second half-wave plate 12 is positioned to the lower left of the first depolarizing beam splitter 10. Figure 3 As shown.

[0038] Interference element group 5 includes a second polarizing beam splitter 14, a first reflecting mirror 15, a second reflecting mirror 16, a second depolarizing beam splitter 17, a first annular photodiode array 18, a second photodiode 20, and a first photodiode 19, wherein the first photodiode 19 is a four-quadrant photodiode. The second polarizing beam splitter 14 is located to the left of the first depolarizing beam splitter 10. The first reflecting mirror 15 is disposed to the left of the second polarizing beam splitter 14. The second reflecting mirror 16 is disposed below the second polarizing beam splitter 14. The second depolarizing beam splitter 17 is disposed to the left of the second reflecting mirror 16. The first annular photodiode array 18 is disposed below the second depolarizing beam splitter 17. The second photodiode 20 and the first photodiode 19 are disposed to the left of the second depolarizing beam splitter 17.

[0039] Interference element group 26 includes a third polarizing beam splitter 21, a first reflecting mirror 22, a second reflecting mirror 24, a first pentagonal reflecting prism 23, a second pentagonal reflecting prism 25, a third depolarizing beam splitter 26, and a second annular photodiode array 27. The third polarizing beam splitter 21 is located below the first depolarizing beam splitter 10. The first reflecting mirror 22 is located to the right of the third polarizing beam splitter 21. The reflecting mirror 24 is located below the polarizing beam splitter 12. The first pentagonal reflecting prism 23 and the second pentagonal reflecting prism 25 are located to the right and front of the second reflecting mirror 24, respectively. The third depolarizing beam splitter 26 is located to the right of the second pentagonal reflecting prism 25, and the second annular photodiode array 27 is located to the right of the third depolarizing beam splitter 26.

[0040] The overall working principle of the six-degree-of-freedom displacement measuring device based on two-dimensional grating heterodyne interference of the present invention is as follows: the incident laser source 4 is a spatially separated dual-frequency laser, which is incident on the incident light adjustment optical element, and after adjustment, it is incident on the reflective two-dimensional grating 1, generating several levels of diffraction light, which then returns to the reading head 2, and enters the interference element group 5 and the interference element group 6 respectively to interfere and be collected.

[0041] The specific implementation principle of the three-degree-of-freedom displacement measuring device based on two-dimensional grating heterodyne interference of the present invention is as follows.

[0042] Step 1: Adjusting the incident laser and guiding the diffracted light. For example... Figure 2 As shown, the spatially separated dual-frequency laser source 4 is incident on the first polarizing beam splitter 8, reflected through the quarter-wave plate 7, and reaches the reflective two-dimensional orthogonal grating 1. Figure 3 As shown, the returned diffracted light passes through the quarter-wave plate 7 and then through the first polarizing beam splitter 8. It is then split into two beams, laser group 11 and laser group 2 13, by the first depolarizing beam splitter 10. These beams are then incident on interference element group 1 5 and interference element group 2 6, respectively. The first half-wave plate 9 and the second half-wave plate 12 are used to adjust the laser polarization state.

[0043] Step Two: Interference Implementation and Signal Acquisition in Interference Element Group 5. For example... Figure 4 As shown, two lasers of different frequencies in the laser beam splitting group 11 are incident on the second polarizing beam splitter 14 and separated according to their different polarization states. Laser A is reflected by the second polarizing beam splitter 14, and after being reflected by the second reflecting mirror 16, it is incident on the second depolarizing beam splitter 17. Laser B passes through the second polarizing beam splitter 14, is reflected by the first reflecting mirror 15, and is incident on the second depolarizing beam splitter 17, coinciding with laser A. The second depolarizing beam splitter 17 splits the coinciding beam, and the zero-order diffracted light after splitting is incident on the first photodiode 19. The interference beams of the first-order diffracted light after splitting are incident on the second photodiode 20 and the first ring photodiode array 18. The offset of the zero-order diffracted spot in its photosensitive quadrant measured by the first photodiode 19 is denoted as S. x ,S y Four sets of interference signals, denoted as I, were obtained after signal processing by the first ring photodiode 18. 1a ,I 1b ,I 1c ,I 1d , where I 1a ,I 1b For signals with the same phase change but opposite directions along the X-axis, I 1c ,I 1dThe first photodiode 20 is a signal with the same phase change and opposite direction along the Y axis; the second photodiode 20 is used to determine the rotation state of the light spot and assist the first ring photodiode 18 in judging and calculating the acquired signal.

[0044] Step 3: Interference implementation and signal acquisition in Interference Signal Group 2.6. (For example...) Figure 5 As shown, two laser frequencies in the second laser beam splitter 13 are incident on the third polarization beam splitter 21 and separated according to their different polarization states. Laser A passes through the third polarization beam splitter 21, is reflected by mirror 24, and then incident on the second pentagonal reflecting prism 25, where it is reflected twice before incident on the third depolarization beam splitter 26. Laser B is reflected by the third polarization beam splitter 21, reflected by mirror 22, and then incident on the first pentagonal reflecting prism 23, where it is reflected twice before incident on the third depolarization beam splitter 26, where it coincides with laser A. The interference beam formed by the overlap of laser A and laser B is collected by the second ring photodiode array 27. After processing the collected signal, four sets of interference signals I are obtained. 2a ,I 2b ,I 2c ,I 2d , where I 2a ,I 2b For signals with the same phase change but opposite directions along the X-axis, I 2c ,I 2d These are signals with the same phase change but opposite directions along the Y-axis.

[0045] Step 4: Six-DOF displacement information calculation. Based on the principles of optical diffraction and laser heterodyne interference, the phase changes of eight interference signals acquired by the first ring photodiode array 18 and the second ring photodiode array 27 can be obtained through the signal processing circuit. The phase change of each interference signal is related to the displacement (x) of the reflective two-dimensional grating 1 relative to the two incident points A and B. A ,y A ),(x B ,y B The relationship is:

[0046]

[0047] Where p is the grating pitch of the reflective two-dimensional orthogonal grating 1, and ΔΦ x1 ,ΔΦ y1 The interference signals I on the X-axis of interference element group 5 are obtained by the signal processing circuit. 1a ,I 1b The phase change of the signal, and the interference signal I obtained by the signal processing circuit on the Y-axis. 1c ,I 1d Phase change, ΔΦ x2 ,ΔΦy2 The interference signals I on the X-axis of interference element group 26 are obtained by the signal processing circuit. 2a ,I 2b The phase change of the signal, and the interference signal I obtained by the signal processing circuit on the Y-axis. 2c ,I 2d Phase change.

[0048] Therefore, the displacement of the grating plane relative to the incident points A and B is calculated by the phase change of the acquired and processed interference signal:

[0049]

[0050] At this point, the motion (ΔX, ΔY, θ) within the grating plane is complete. z The information is obtained by measuring the displacement (x) of the grating relative to the incident points A and B. A ,y A ),(x B ,y B This was obtained through geometric calculations.

[0051] The rotational motion of the grating about the X and Y axes (θ) x θ y The offset S of the zero-order diffraction spot in its photosensitive quadrant is measured by the first photodiode 19. x ,S y With calibrable fixed transformation coefficients C x C y calculate:

[0052]

[0053] The phase change ΔΦ of the interference signal is measured by fundamental interferometry through the reflection of light from the grating, where the Z-axis displacement ΔZ of the grating is transmitted. z The result is obtained from the laser wavelength λ used:

[0054]

[0055] Thus, six degrees of freedom (ΔX, ΔY, ΔZ, θ) of the grating are achieved. x θ y θ z Displacement measurement.

[0056] This invention is based on the principles of grating diffraction and laser heterodyne interference. It utilizes spatially separated dual-frequency lasers incident on a measurement grating. The diffracted light generated at the two incident points undergoes two different sets of heterodyne interference. By measuring and calculating the two sets of heterodyne interference signals, the six-degree-of-freedom displacement information of the grating under test is obtained. The photoelectric sensor devices include a four-quadrant photodiode and a specially designed photodiode array. The four-quadrant photodiode is used to measure out-of-plane motion of the grating (rotational motion around the X and Y axes), and the photodiode array is used to measure in-plane motion of the grating (translational motion along the X and Y axes, and rotation around the Z axis). Translation along the Z axis is measured by interference of the reflected light from the grating. By calculating the interference signals of multiple sensing elements, the six-degree-of-freedom displacement information of the grating is obtained. This invention, through the design of a comprehensive transformation of the diffracted light and by calculating the two sets of interference signals indirectly related to the displacement information, can maintain the interference signals of each axis when the grating undergoes omnidirectional motion around the Z axis, achieving continuous measurement of in-plane rotational motion.

[0057] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A six-degree-of-freedom displacement measuring device based on two-dimensional grating heterodyne interferometry, characterized in that, It includes a reflective two-dimensional orthogonal grating (1), an interference element group one (5), an interference element group two (6), and an incident light adjustment optical element; The first interference element group (5) is arranged on the left side of the incident light adjustment optical element, and the second interference element group (6) is arranged below the incident light adjustment optical element.

2. The six-degree-of-freedom displacement measuring device based on two-dimensional grating heterodyne interferometry according to claim 1, characterized in that, The incident light adjustment optical element includes a quarter-wave plate (7), a first polarizing beam splitter (8), a first depolarizing beam splitter (10), a first half-wave plate (9), and a second half-wave plate (12). The quarter-wave plate (7) is located directly below the reflective two-dimensional orthogonal grating (1). The first polarizing beam splitter (8) and the first depolarizing beam splitter (10) are arranged sequentially below the quarter-wave plate (7). The first half-wave plate (9) is located on the upper left side of the first depolarizing beam splitter (10), and the second half-wave plate (12) is located on the lower left side of the first depolarizing beam splitter (10).

3. The six-degree-of-freedom displacement measuring device based on two-dimensional grating heterodyne interferometry according to claim 2, characterized in that, The interference element group 1 (5) includes a second polarizing beam splitter (14), a first reflecting mirror (15), a second reflecting mirror (16), a second depolarizing beam splitter (17), a first annular photodiode array (18), a second photodiode (20), and a first photodiode (19). The second polarizing beam splitter (14) is located to the left of the first depolarizing beam splitter (10). The first reflecting mirror (15) is located to the left of the second polarizing beam splitter (14). The second reflecting mirror (16) is located below the second polarizing beam splitter (14). The second depolarizing beam splitter (17) is located to the left of the second reflecting mirror (16). The first annular photodiode array (18) is located below the second depolarizing beam splitter (17). The second photodiode (20) and the first photodiode (19) are located to the left of the second depolarizing beam splitter (17).

4. The six-degree-of-freedom displacement measuring device based on two-dimensional grating heterodyne interferometry according to claim 3, characterized in that, The first photodiode (19) is a four-quadrant photodiode.

5. The six-degree-of-freedom displacement measuring device based on two-dimensional grating heterodyne interferometry according to claim 3 or 4, characterized in that, The interference element group two (6) includes a third polarizing beam splitter (21), a first triangular mirror (22), a second triangular mirror (24), a first pentagonal mirror (23), a second pentagonal mirror (25), a third depolarizing beam splitter (26), and a second annular photodiode array (27). The third polarizing beam splitter (21) is located below the first depolarizing beam splitter (10). The first triangular mirror (22) is located to the right of the third polarizing beam splitter (21). The second triangular mirror (24) is located below the second half-wave plate (12). The first pentagonal mirror (23) and the second pentagonal mirror (25) are located to the right and front of the second triangular mirror (24), respectively. The third depolarizing beam splitter (26) is located to the right of the second pentagonal mirror (25). The second annular photodiode array (27) is located to the right of the third depolarizing beam splitter (26).

6. A displacement measurement method for a six-degree-of-freedom displacement measuring device based on two-dimensional grating heterodyne interferometry according to claim 5, characterized in that, Includes the following steps: Step 1: Adjustment of incident laser and guidance of diffracted light: The spatially separated dual-frequency laser source (4) is incident on the first polarization beam splitter (8), reflected and passes through the quarter-wave plate (7), and reaches the reflective two-dimensional orthogonal grating (1). The diffracted light that returns passes through the quarter-wave plate (7) and then passes through the first polarization beam splitter (8). It is split into beam split laser group one (11) and beam split laser group two (13) by the first depolarization beam splitter (10), and is incident on interference element group one (5) and interference element group two (6) respectively. Step 2: Interference realization and signal acquisition in the first interferometer group (5): Two lasers of different frequencies in the first laser beam splitter group (11) are incident on the second polarization beam splitter (14) and separated according to different polarization states. Laser A is reflected by the second polarization beam splitter (14), and after being reflected by the second mirror (16), it is incident on the second depolarization beam splitter (17). Laser B passes through the second polarization beam splitter (14), and after being reflected by the first mirror (15), it is incident on the second depolarization beam splitter (17) and coincides with laser A. The second depolarization beam splitter (17) splits the coincident beam. The zero-order diffracted light after beam splitting is incident on the first photodiode (19). The interference beams of the first-order diffracted light after beam splitting are incident on the second photodiode (20) and the first ring photodiode array (18). The offset of the zero-order diffracted spot in its photosensitive quadrant measured by the first photodiode (19) is recorded as S. x S y Four sets of interference signals were obtained after signal processing by the first ring photodiode array (18), denoted as I. 1a I 1b I 1c I 1d , where I 1a I 1b For signals with the same phase change but opposite directions along the × axis, I 1c I 1d These are signals with the same phase change but opposite directions along the Y-axis. Step 3: Interference realization and signal acquisition in the second interferometer element group (6): Two lasers of different frequencies in the second beam-splitting laser group (13) are incident on the third polarization beam splitter (21) and separated according to different polarization states. Laser A passes through the third polarization beam splitter (21), is reflected by the second triangular mirror (24), and is incident on the second pentagonal mirror (25) after being reflected twice by it. Then it is incident on the third depolarization beam splitter (26). Laser B is reflected by the third polarization beam splitter (21), is reflected by the first triangular mirror (22), and is incident on the first pentagonal mirror (23) after being reflected twice by it. Then it is incident on the third depolarization beam splitter (26) and coincides with laser A. The interference beam formed by the coincidence of laser A and laser B is collected by the second ring photodiode array (27). After processing the collected signal, a total of four sets of interference signals I are obtained. 2a I 2b I 2c I 2d , where I 2a I 2b For signals with the same phase change but opposite directions along the X-axis, I 2c I 2d These are signals with the same phase change but opposite directions along the Y-axis. Step 4: Six-DOF displacement information calculation: Based on the grating diffraction principle and the laser heterodyne interference principle, the phase changes of a total of 8 interference signals collected by the first ring photodiode array (18) and the second ring photodiode array (27) are obtained through the signal processing circuit. The phase changes of each interference signal are related to the displacement (x) of the reflective two-dimensional orthogonal grating (1) relative to the two incident points A and B. A y A ), (x B y B The relationship is: Where p is the grating pitch of the reflective two-dimensional orthogonal grating (1), and ΔΦ x1 , ΔΦ y1 The interference signals I on the × axis of the interference element group (5) are obtained by the signal processing circuit. 1a I 1b The phase change and the interference signal I obtained by the signal processing circuit on the Y-axis 1c I 1d Phase change, ΔΦ x2 , ΔΦ y2 The interference signals I obtained by the signal processing circuit on the × axis in interference element group two (6) are respectively. 2a I 2b The phase change and the interference signal I obtained by the signal processing circuit on the Y-axis 2c I 2d Phase change; Therefore, the displacement of the grating plane relative to the incident points A and B is calculated by the phase change of the acquired and processed interference signal: Motion in the grating plane (ΔX, ΔY, θ) z The information is obtained by measuring the displacement (x) of the grating relative to the incident points A and B. A y A ), (x B y B This was obtained through geometric calculations. The rotational motion of the grating about the X and Y axes (θ) x θ y The offset S of the zero-order diffraction spot in its photosensitive quadrant is measured by the first photodiode (19). x S y With calibrable fixed transformation coefficients C x C y calculate: The phase change ΔΦ of the interference signal is measured by fundamental interferometry through the reflection of light from the grating, where the Z-axis displacement ΔZ of the grating is transmitted. z The result is obtained from the laser wavelength λ used: This enables six degrees of freedom (ΔX, ΔY, ΔZ, θ) of the grating. x θ y θ z Displacement measurement.

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