Three-degree-of-freedom angular displacement measuring method and device

Abstract

The invention relates to a three-degree-of-freedom angular displacement measurement method and device, and the method comprises the steps: carrying out the measurement based on an inverted digital shear speckle interference method, and obtaining two phase difference distribution diagrams caused by the angular displacement of a measured object plane; based on the two phase difference distribution diagrams, three-degree-of-freedom angular displacement of the measured object plane is determined, and the three-degree-of-freedom angular displacement comprises a pitch angle, a deflection angle and a roll angle. According to the embodiment of the invention, the target-lens-free measurement of the three-degree-of-freedom angular displacement of the measured object surface can be effectively realized, and the measurement precision is high.

Description

Technical Field

[0001] This disclosure relates to the field of full-field optical measurement technology, and in particular to a method and apparatus for measuring three-degree-of-freedom angular displacement. Background Technology

[0002] In manufacturing, research on three-degree-of-freedom angular displacement measurement methods for components and structures has significant engineering implications, with important applications in attitude detection, precision machining, accuracy assessment, and deformation measurement. For example, in precision machine tools, the three-degree-of-freedom angular displacement error of moving parts needs to be measured periodically for error compensation to improve machining accuracy. In the aerospace field, thermal deformation of robotic arm joints can cause three-degree-of-freedom angular displacement at their end faces, leading to a decrease in control accuracy. Commonly used optical measurement methods for three-degree-of-freedom angular displacement include interferometry, collimation, and a combination of collimation and interferometry. These methods generally require the installation of a cooperative target mirror on the object being measured. If the object is very small, such as a microelectromechanical system (MEMS), it may be impossible to fix the target mirror on the object. Furthermore, the target mirror can introduce additional installation errors, or its own weight can affect the motion state of the measured component, leading to a decrease in the accuracy of the three-degree-of-freedom angular displacement measurement. In some cases, local deformation of the end face can also cause unexpected displacement of the target mirror, introducing significant measurement errors. Digital speckle interferometry (DFI) and transverse shear speckle interferometry (TSH) are two novel interferometric angular displacement measurement methods with the unique advantage of not requiring a target mirror, thus showing broad application prospects. However, the three-degree-of-freedom angular displacement measurement method based on DFI requires an independent reference light, is highly sensitive to environmental interference, is difficult to adapt to actual industrial measurement environments, and has a slow measurement speed. On the other hand, the angular displacement measurement method based on TSH cannot achieve simultaneous measurement of three-degree-of-freedom angular displacement, and has low measurement sensitivity, difficulty in phase unwrapping, and difficulty in obtaining absolute phase. Summary of the Invention

[0003] In view of this, this disclosure proposes a three-degree-of-freedom angular displacement measurement method and device to achieve high-precision non-contact three-degree-of-freedom angular displacement measurement.

[0004] According to one aspect of this disclosure, a three-degree-of-freedom angular displacement measurement method is provided, comprising: measuring two phase difference distribution maps caused by the three-degree-of-freedom angular displacement of the surface of the object being measured based on the inverted digital shear speckle interferometry.

[0005] Based on the two phase difference distribution maps, the three-degree-of-freedom angular displacement of the measured object surface is determined, wherein the three-degree-of-freedom angular displacement includes: pitch angle, yaw angle and roll angle.

[0006] In one possible implementation, two expanded laser beams are used to symmetrically illuminate the surface of the object under test at an angle θ relative to the optical axis, generating two diffuse speckle fields; the two expanded laser beams are used to illuminate the surface of the object under test in a beam-splitting multiplexing manner, and the beam-splitting multiplexing manner can be at least one of the following: time-division multiplexing, wavelength-division multiplexing, polarization-division multiplexing, time-frequency multiplexing, and spatial-frequency multiplexing.

[0007] The diffuse speckle field is split into a first beam and a second beam. A first transformation operation is applied to the wavefront distribution of the first beam, and a second transformation operation, which is the opposite of the first transformation operation, is applied to the wavefront distribution of the second beam, so that the reflected light from any point P(x, y) in the object space of the measured object and the corresponding point P′(-x, -y) form an inverted interference with the optical axis as the center.

[0008] The first and second transformation operations can be: applying a horizontal and vertical mirror symmetry transformation about the plane containing the optical axis to the wavefront distributions of the first and second beams, respectively; or applying a clockwise and counterclockwise relative rotation transformation about the optical axis of the wavefront distributions of the first and second beams, such that the wavefront distribution of the second beam is symmetrical about the origin of the optical axis relative to the wavefront distribution of the first beam; the transformed first and second beams are coaxially combined relative to the optical axis to generate a pair of spatially inverted and overlapping light fields in the image plane; the inverted shearing interference field is acquired and received by an image sensor, and the phase distribution maps of the state before and after angular displacement are extracted using the phase shift method.

[0009] The phase distribution maps obtained before and after the angular displacement are used to obtain a wrapped phase map through differential operation; the phase difference value at the origin of the optical axis is known, there exists an absolute phase difference value, and this absolute phase difference value is 0. The phase is unwrapped using the origin of the optical axis as a reference to obtain a continuous phase difference distribution map. Two phase difference distribution maps are obtained by two beam expanding lasers respectively, and this phase difference value is the absolute phase difference value; the phase unwrapping method includes, but is not limited to: spatial phase unwrapping method, temporal phase unwrapping method, and spatiotemporal phase unwrapping method.

[0010] In one possible implementation, based on the two phase difference distribution maps, at least two feature points are selected in each phase difference distribution map, for a total of at least four feature points; in the phase difference distribution map, the feature points should be any points other than the optical axis origin and singular points, and the coordinate positions of the feature points selected in the two phase difference distribution maps are the same;

[0011] Based on the phase difference values ​​at the at least four feature points, the three-degree-of-freedom angular displacement of the measured object surface is determined according to the following mathematical model:

[0012]

[0013] Among them, R x R y and R z Δφ1 and Δφ2 represent the pitch angle, yaw angle, and roll angle of the measured object surface, respectively. Δφ1 and Δφ2 represent the phase difference values ​​of the two expanded laser beams at the feature point, respectively. λ1 and λ2 represent the wavelengths of the two expanded laser beams, respectively. θ is the angle between the illumination beam and the optical axis. (x,y), (x1,y1), and (x2,y2) represent the coordinates of any point on the phase difference distribution map except for the origin of the optical axis and the singular point, respectively. The singular point is defined as x2y1 = x1y2.

[0014] According to another aspect of this disclosure, a three-degree-of-freedom angular displacement measuring device is provided, comprising: a phase measurement module, used to acquire two interference light phase distribution maps of the measured object surface before and after angular displacement based on the inverted digital shear speckle interferometry, and to perform differential operations on the phase distribution maps obtained before and after the angular displacement to obtain two phase difference distribution maps; and a three-degree-of-freedom angular displacement measuring module, used to determine the three-degree-of-freedom angular displacement of the measured object surface based on the two phase difference distribution maps, wherein the three-degree-of-freedom angular displacement includes: pitch angle, yaw angle and roll angle.

[0015] In one possible implementation, the phase measurement module includes: an illumination submodule configured to use two expanded laser beams incident symmetrically about the optical axis at an angle θ using a beam-splitting multiplexing method, wherein the wavelengths of the two expanded laser beams can be the same or different; the beam-splitting multiplexing method includes, but is not limited to: time-division multiplexing, wavelength-division multiplexing, polarization-division multiplexing, time-frequency multiplexing, and spatial-frequency multiplexing; an inverted shearing submodule, used to apply inverted shearing to the laser speckle field reflected from the surface of the object under test, forming an inverted shearing interference between the reflected light from any point P(x, y) in the object space and the corresponding point P′(-x, -y) with the optical axis as the center; a phase shifting submodule, configured to use the phase shifting method to obtain the phase distribution map of the interference field before and after the angular displacement; an image acquisition submodule, used to record the image of the interference field; and a phase difference calculation submodule, used to perform differential operations on the phase distribution maps obtained before and after the angular displacement to obtain a wrapped phase map, and based on the wrapped phase map, a de-wrapping algorithm to obtain a continuously distributed phase difference distribution map with the origin of the optical axis as the reference.

[0016] In one possible implementation, the three-degree-of-freedom angular displacement measurement module includes: an angular displacement calculation submodule, used to select at least two feature points in each phase difference distribution map based on the two phase difference distribution maps, for a total of at least four feature points; in the phase difference distribution map, the feature points should be any points other than the optical axis origin and singular points, and the coordinate positions of the feature points selected in the two phase difference distribution maps are the same, and the three-degree-of-freedom angular displacement of the measured object surface is determined based on the at least four feature points.

[0017] Based on the inverted digital shear speckle interferometry, by obtaining two phase difference maps caused by angular displacement, only the phase information of two phase difference feature points needs to be extracted to simultaneously calculate the three-degree-of-freedom angular displacement. This method is significantly superior to the three-degree-of-freedom angular displacement measurement method based on digital speckle interferometry in terms of anti-interference and measurement speed. It overcomes the limitations of transverse digital shear speckle interferometry, such as the inability to obtain three-degree-of-freedom angular displacement simultaneously and the small measurement range. It has the advantages of high resolution, targetless measurement, and strong anti-environmental interference capability.

[0018] Other features and aspects of this disclosure will become clear from the following detailed description of exemplary embodiments with reference to the accompanying drawings. Attached Figure Description

[0019] The accompanying drawings, which are included in and form part of this specification, illustrate exemplary embodiments, features, and aspects of this disclosure together with the specification and serve to explain the principles of this disclosure.

[0020] Figure 1 A flowchart illustrating a three-degree-of-freedom angular displacement measurement method according to an embodiment of this disclosure is shown.

[0021] Figure 2 This diagram shows the optical path structure of an inverted digital shear speckle interferometry system according to an embodiment of the present disclosure.

[0022] Figure 3 This diagram shows the phase distribution before and after angular displacement in an embodiment of the present disclosure.

[0023] Figure 4 This diagram illustrates the optical path principle of a shearing device for an inverted digital shearing speckle interferometry according to an embodiment of the present disclosure.

[0024] Figure 5 This diagram illustrates the optical path principle of a shearing device for an inverted digital shearing speckle interferometry according to an embodiment of the present disclosure.

[0025] Figure 6 This diagram illustrates an image space inversion shearing of an embodiment of the present disclosure.

[0026] Figure 7 Two package phase diagrams are shown according to embodiments of this disclosure;

[0027] Figure 8 Two phase difference distribution diagrams of embodiments of this disclosure are shown;

[0028] Figure 9 This diagram illustrates the geometric relationship between in-plane and out-of-plane displacement and angular displacement according to an embodiment of the present disclosure.

[0029] Figure 10 This diagram illustrates an example of feature point selection according to an embodiment of the present disclosure.

[0030] Figure 11 This diagram illustrates an example of feature point selection according to an embodiment of the present disclosure.

[0031] Figure 12 A block diagram of a three-degree-of-freedom angular displacement measuring device according to an embodiment of the present disclosure is shown;

[0032] Figure 13 A block diagram of a phase measurement module according to an embodiment of the present disclosure is shown. Detailed Implementation

[0033] Various exemplary embodiments, features, and aspects of this disclosure will now be described in detail with reference to the accompanying drawings. The same reference numerals in the drawings denote elements that have the same or similar functions. Although various aspects of the embodiments are shown in the drawings, they are not necessarily drawn to scale unless specifically indicated otherwise.

[0034] The term “exemplary” as used herein means “serving as an example, embodiment, or illustration.” Any embodiment illustrated herein as “exemplary” is not necessarily to be construed as superior to or better than other embodiments.

[0035] Furthermore, to better illustrate this disclosure, numerous specific details are set forth in the following detailed description. Those skilled in the art will understand that this disclosure can be practiced without certain specific details. In some instances, methods, means, components, and circuits well known to those skilled in the art have not been described in detail in order to highlight the main points of this disclosure.

[0036] Digital speckle interferometry (DFI) and transverse shear speckle interferometry (TSH) are two novel interferometric angular displacement measurement methods with the unique advantage of not requiring a target mirror, thus showing broad application prospects. The DFI three-degree-of-freedom (DOF) angular displacement measurement method boasts high measurement accuracy, but requires an independent reference light, is highly sensitive to environmental interference, is difficult to adapt to practical industrial measurement environments, and has a slow measurement speed. TSH, on the other hand, offers strong robustness, but its resolution for small angular displacements is limited by the shearing amount, and it is difficult to simultaneously measure three-degree-of-freedom angular displacements, nor can it directly measure the absolute phase. Therefore, we propose a method for measuring the three-degree-of-freedom angular displacement of an object surface based on inverted digital shear speckle interferometry. Unlike related three-degree-of-freedom angular displacement measurement methods, the method provided in this disclosure enables simultaneous, target-free, and accurate measurement of the three-degree-of-freedom angular displacement of the measured object surface, has a wide range of applications, and exhibits strong measurement robustness.

[0037] The three-degree-of-freedom angular displacement measurement method provided in the embodiments of this disclosure will be described in detail below.

[0038] Figure 1 A flowchart illustrating a three-degree-of-freedom angular displacement measurement method according to an embodiment of this disclosure is shown; as follows: Figure 1 As shown, the method may include:

[0039] In step S11, based on the inverted digital shear speckle interferometry, interference is formed using an inverted shearing device, and the phase distribution is extracted by phase shifting to obtain two phase difference distribution maps.

[0040] In step S12, the three-degree-of-freedom angular displacement is calculated using the phase difference values ​​at selected feature points in the two phase difference distribution maps.

[0041] The three-degree-of-freedom angular displacement measurement method of this disclosure, based on the inverted digital shear speckle interferometry, can effectively achieve target-free measurement of the surface of the object being measured, and the measured three-degree-of-freedom angular displacement has high accuracy.

[0042] Figure 2The diagram illustrates the structure of a complete optical system according to an embodiment of this disclosure. In one possible implementation, the test system hardware includes devices such as a laser, a beam expander, a shearing device, a phase shifting module, and an image sensor. Two lasers are used in conjunction with two beam expanders to generate two expanded laser beams, which symmetrically illuminate the surface of the test object at an angle θ relative to the optical axis, thereby generating two diffuse speckle fields. In one possible implementation, two right-angle prisms and a beam splitter are used to form a shearing device, which inverts and shears the diffuse speckle fields, and the shearing is received by the image sensor. In one possible implementation, the phase shifting module consists of a piezoelectric ceramic and its controller. The piezoelectric ceramic is mounted on one of the right-angle prisms to drive the right-angle prism to move by a specific step size, thereby performing phase extraction based on time phase shift, and extracting the phase distribution maps of the state before and after the angular shift, respectively. Figure 3 The disclosed embodiment shows a possible implementation in which a beam of expanded laser is obtained by time phase shifting to obtain phase distribution maps of the state before and after angular displacement; the phase distribution maps obtained before and after angular displacement are subjected to differential operation to obtain a wrapped phase map, and the two continuously distributed phase difference distribution maps are obtained by unwrapping algorithm.

[0043] Figure 4 and Figure 5 A schematic diagram of the optical path of an inverted shearing device according to an embodiment of the present disclosure is shown. In one possible implementation, such as Figure 4 and Figure 5 As shown, the device includes a beam-splitting prism and two orthogonally distributed right-angle prisms, as well as an image sensor for capturing speckle patterns; Figure 5 As shown, right-angle prism B is placed horizontally on the xoz plane. Right-angle prism A is rotated 90° around the z-axis and then placed vertically, maintaining orthogonality with right-angle prism B. The diffuse speckle field is split into a first beam and a second beam by a beam splitter. These beams are reflected by two orthogonally placed right-angle prisms, achieving horizontal and vertical mirror symmetry transformations about the plane of the optical axis. Finally, the beams are coaxially combined relative to the optical axis by the beam splitter and sent to the image sensor. Figure 6 A schematic diagram of image space inverted shearing according to an embodiment of the present disclosure is shown; this structure enables the reflected light from any point P(x, y) in object space and the corresponding point P′(-x, -y) to form inverted interference centered on the optical axis, achieving maximum shearing without mechanical adjustment and omnidirectional shearing. It should be noted that, except... Figure 4 and Figure 5 In addition to the inverted shearing device shown, other optical devices can also be used to achieve the inverted shearing function. This disclosure does not limit the optical path implementation of the inverted shearing device.

[0044] For two-beam illumination and beam-expanding lasers, a beam multiplexing method is employed. In one possible implementation, a wavelength-division multiplexing method is used, where the system provides laser beam-expanding illumination from two lasers of different wavelengths and a beam expander. Because the laser wavelengths are different, the two lasers are simultaneously measured using the same shearing device and a color image sensor. In another possible implementation, a time-division multiplexing method is used, where the system provides laser beam-expanding illumination from two lasers of the same wavelength and a beam expander. The two lasers are alternately opened using a shutter, and the corresponding phase difference distribution maps of the two beams are measured sequentially using the same shearing device and an image sensor.

[0045] Figure 7 The diagram illustrates two encapsulated phase maps measured according to an embodiment of this disclosure. In one possible implementation, phase distribution maps are obtained using the two beams of light before and after the angular displacement of the object surface. Differential operations are then performed on these phase distribution maps to obtain the two encapsulated phase maps. The phase difference at the origin of the optical axis is known, and an absolute phase difference exists, which is 0. According to an embodiment of this disclosure, in one possible implementation, after filtering the encapsulated phase maps, a spatial unwrapping algorithm is used to unwrap the phases relative to the origin of the optical axis, resulting in two consecutive phase difference distribution maps as shown below. Figure 8 As shown; according to the feature point selection method, feature points are selected in the phase difference distribution map, that is, specific phase difference values ​​are selected, and the angular displacement angle is calculated according to the mathematical model.

[0046] Based on the fundamental principle of optical interference, the phase of the interfering light is shown in the following formula (1):

[0047]

[0048] Where φ represents the phase of the interference light, λ represents the wavelength of the interference light, n represents the refractive index of the medium, and L represents the optical path. When the object undergoes angular displacement, the resulting surface displacement will cause a change in the optical path of the reflected light, which in turn leads to a change in the phase of the interference light. The derivative of the above equation is shown in the following formula (2):

[0049]

[0050] Where Δφ represents the change in phase, and δλ, δn, and δL represent the changes in wavelength of the interference light, refractive index of the medium, and optical path, respectively. Generally, the wavelength of the interference light remains constant, and the medium through which the light passes is air (n=1). Therefore, the above equation can be simplified as shown in the following formula (3):

[0051]

[0052] Where δu, δv, and δw are the components of the displacement vector in the x, y, and z directions, and A, B, and C correspond to displacement sensitivity factors determined by geometric relationships. In one possible implementation, [the following is a partial translation of the original text, which is incomplete and requires further context]. Figure 2 As shown, a geometric model is established on the surface of the object to be measured. The pitch axis is the x-axis, the y-axis is the y-axis, the roll axis is the z-axis, and the origin of the optical axis is the coordinate origin. Based on this coordinate system, the displacement sensitivity factor can be calculated and simplified. The phase difference between the two laser beams before and after the angular displacement is defined as Δφ1 and Δφ2, respectively. Then, for any point on the surface of the object to be measured in the object space, when the size of the object to be measured is much smaller than the working distance, the phase difference distribution obtained by the two laser beams and the relationship between the in-plane deformation and the out-of-plane deformation are shown in the following formula (4):

[0053]

[0054] Where λ1 and λ2 represent the wavelengths of the two laser beams, which can be the same or different; Δu represents the difference in in-plane displacement along the positive x-axis between the two corresponding points where interference occurs; Δw represents the difference in out-of-plane displacement along the positive z-axis between the two corresponding points where interference occurs; and θ is the angle between the illumination direction and the positive z-axis direction.

[0055] Figure 9 This diagram illustrates the principle of angular displacement-in-plane / out-of-plane displacement geometric relationship mapping in an embodiment of this disclosure. A coordinate system is established with the optical axis origin as the coordinate origin, the pitch axis as the x-axis, the yaw axis as the y-axis, and the roll axis as the z-axis. R z R y R x These represent the roll angle, yaw angle, and pitch angle, respectively. Point P is any point on the surface of the object being measured, and point P′ is the inverted shear point corresponding to point P. After angular motion, point P moves to position P1, and correspondingly, point P′ moves to position P′1. u represents the magnitude of the in-plane displacement component generated by point P's motion in the positive x-direction, and w represents the magnitude of the out-of-plane displacement component generated by point P's motion in the positive z-direction. Correspondingly, u′ represents the magnitude of the in-plane displacement component generated by point P′'s motion in the positive x-direction, and w′ represents the magnitude of the out-of-plane displacement component generated by point P′'s motion in the positive z-direction. Figure 9 Taking the trajectory of point P as an example, when a point in the object space undergoes angular displacement and moves from point P to P1, analyze the displacement effects of pitch, yaw, and roll motions on each axis. For example... Figure 9 As shown in (a), the rolling and yaw motions cause point P to move to point P1 and generate an equivalent in-plane displacement of magnitude u in the positive x direction; as Figure 9As shown in (b), the pitch and yaw motions cause point P to move to point P1 and generate an equivalent out-of-plane displacement of magnitude w in the positive z direction; that is, the difference in in-plane displacement Δu in the x direction is mainly caused by the yaw and rolling motions, while the difference in out-of-plane displacement Δw in the z direction is only caused by the pitch and yaw motions; by establishing a geometric mapping relationship between in-plane and out-of-plane displacements and angular displacements, the mapping relationship from phase difference to angular displacement is established. The simplified mapping relationship between in-plane and out-of-plane displacements and angular displacements is shown in the following formula (5):

[0056]

[0057] Combining formulas (4) and (5), the mapping relationship between phase difference and angular displacement is obtained, as shown in formula (6) below:

[0058]

[0059] From the above equation, it can be seen that the corresponding phase difference of each laser beam is related to the three angular displacements. Therefore, when the angular displacement exists alone or simultaneously, it can cause a change in the corresponding phase difference, revealing the principle that the inverted digital shearing interferometry is sensitive to the three-degree-of-freedom angular displacement. In addition, from the above equation, when x = y = 0, that is, when the origin of the optical axis is selected, the phase difference Δφ1(x, y) = Δφ2(x, y) = 0, which means that there is an absolute phase difference value at the origin, and this absolute phase difference value is 0. Using this absolute phase difference value as the reference point, the phase unwrapping can be performed to directly calculate the angular displacement. Separating the angular displacement variables in the above equation, the model for the measurement of three-degree-of-freedom angular displacement is obtained as shown in the following formula (7):

[0060] Where (x0, y0), (x1, y1), and (x2, y2) represent the coordinates of any point on the phase difference distribution map other than the origin of the optical axis and the singular point, and the singular point is defined as x2y1 = x1y2. Figure 10 This embodiment of the present disclosure illustrates an example of feature point selection under this mathematical model. From the mathematical model shown in formula (7), the basic principle for feature point selection can be obtained: based on the two phase difference distribution maps, at least two feature points are selected in each phase difference distribution map, for a total of at least four feature points; in the phase difference distribution maps, the feature points should be any points other than the origin of the optical axis and the singular point, where the singular point is defined as x2y1 = x1y2; the feature points selected in the two phase difference distribution maps have the same coordinate position. The three-degree-of-freedom angular displacement of the measured object surface is determined based on the phase difference values ​​at the at least four feature points.

[0061] In one possible implementation, the mathematical model shown in equation (7) can be further simplified. Figure 2As shown, based on the geometric model established on the surface of the object being measured, a coordinate system is established with the pitch axis as the x-axis, the yaw axis as the y-axis, the roll axis as the z-axis, and the origin of the optical axis as the coordinate origin. In R... x During the solution process, select any point (0, y) on the y-axis other than the origin of the optical axis; in R... y During the solution process, select any point (x, 0) on the x-axis other than the origin of the optical axis; in R... z During the calculation, any point (0, y) on the y-axis other than the origin of the optical axis is selected; variables can be further eliminated to simplify the formula. The simplified mathematical model is shown in the following formula (8):

[0062]

[0063] Figure 11 This embodiment illustrates an example of feature point selection under this simplified mathematical model. The embodiment shown in Equation (8) further simplifies the mathematical model by restricting the selection of feature points, thus reducing the computational load. As can be seen from the mathematical models of Equations (7) and (8), the measurement resolution and measurement speed are mainly affected by two factors: phase resolution and feature point selection strategy. In the mathematical model, the phase difference value of the feature point needs to be selected in the phase difference distribution map and substituted into the model to calculate the angular displacement. Theoretically, the larger the feature point coordinate value, the higher the measurement resolution. However, in actual measurement, the farther the selected feature point is from the origin of the optical axis, the larger the angle of the incident wavefront relative to the normal of the measured object surface. The feature point is close to the illumination area of ​​the boundary, the larger the measurement area, the larger the error, and the larger the error of its model. It is also more susceptible to environmental noise interference at the edge. Experiments show that as the feature point coordinate value increases, the average relative error of the pitch angle, yaw angle, and roll angle measurement generally shows a decreasing trend. In summary, within the effective illumination range of the expanded beam laser, the feature point is close to the edge of the measured object surface, and its measurement error is the smallest. When selecting feature points, within the effective illumination area of ​​the expanded laser beam, points close to the edge of the object being measured or with larger coordinate values ​​should be selected for calculation. This helps to smooth phase noise and reduce the impact of errors. In one possible implementation, to optimize measurement speed and resolution, the feature point selection methods include: selecting points (0, y) and (x, 0) on the x and y axes where the origin of the optical axis is located; ensuring that the selected feature points for pitch and roll angle calculations satisfy y≠0, and that the selected feature points for yaw angle calculations satisfy x≠0; and prioritizing feature points close to the edge of the illumination area with larger coordinate values ​​|x| or |y| within the effective illumination area of ​​the expanded laser beam to improve calculation accuracy.

[0064] Experimental and theoretical calculations show that the method and apparatus provided in this disclosure can achieve high-precision synchronous measurement of three-degree-of-freedom angular displacement. For example, under the configuration conditions of laser wavelength λ1=λ2=532nm, system phase measurement resolution better than π / 20rad, illumination angle θ=20°, and feature point distance from the optical axis origin approximately 100mm, the measurement resolution of pitch and yaw angles can reach 0.03μrad, and the measurement resolution of roll angle can reach 0.2μrad, fully demonstrating the high-precision advantage of the method of this invention.

[0065] Figure 12 A block diagram of a three-degree-of-freedom angular displacement measuring device according to an embodiment of this disclosure is shown. Figure 12 As shown, the three-degree-of-freedom angular displacement measuring device 120 includes:

[0066] Phase measurement module 121 is used to apply inverted shearing to two laser speckle fields reflected from the surface of the object under test based on the inverted digital shearing speckle interferometry method, and extract the phase distribution maps of the interference light before and after the angular displacement of the surface of the object under test, respectively. The phase distribution maps of the interference light obtained before and after the angular displacement are subjected to differential operation to obtain two phase difference distribution maps.

[0067] The three-degree-of-freedom angular displacement measurement module 122 is used to select at least two feature points in each phase difference distribution map based on the two phase difference distribution maps, for a total of at least four feature points; in the phase difference distribution map, the feature points should be any points other than the origin of the optical axis and the singular point, and the coordinate positions of the feature points selected in the two phase difference distribution maps are the same, and the three-degree-of-freedom angular displacement of the measured object surface is determined based on the at least four feature points, wherein the three-degree-of-freedom angular displacement includes: pitch angle, yaw angle and roll angle.

[0068] Figure 13 A block diagram of one embodiment of the phase measurement module of this disclosure is shown. Figure 13 As shown, the phase measurement module 121 includes:

[0069] The illumination submodule 1211 is configured to provide λ1 and λ2 wavelength lasers incident symmetrically with respect to the optical axis at an angle θ, employing a beam multiplexing method. In one possible implementation, a wavelength division multiplexing method is used, where the system provides laser beam expansion illumination from two lasers of different wavelengths and a beam expander. Because the laser wavelengths are different, the two lasers are simultaneously measured using the same shearing device and a color image sensor. In another possible implementation, a time division multiplexing method is used, where the system provides laser beam expansion illumination from two lasers of the same wavelength and a beam expander. The two lasers are opened alternately using a shutter, and the corresponding phase difference distribution maps of the two beams are measured sequentially using the same shearing device and an image sensor.

[0070] Inverted shear submodule 1212, such as Figure 4 as well as Figure 5 As shown, it consists of a beam splitter prism and an orthogonally combined right-angle prism, used to achieve inverted shearing interference of the reflected light from any point P(x, y) in the object space and the corresponding point P′(-x, -y) with the optical axis as the center; the right-angle prism B is placed horizontally on the xoz plane, and the right-angle prism A is rotated 90° around the z axis and then placed vertically, and kept orthogonal to the right-angle prism B;

[0071] In one possible implementation, the phase modulation submodule 1213 consists of a piezoelectric ceramic and its controller. The piezoelectric ceramic is mounted on one of the right-angle prisms to drive the right-angle prism to move by a specific step size, thereby performing phase extraction based on time phase shift, and extracting the phase distribution maps of the state before and after the angular displacement, respectively.

[0072] Image acquisition submodule 1214 uses an image sensor to record shearing interferograms;

[0073] The phase difference calculation submodule 1215 is used to perform differential operations on the phase distribution map obtained before and after the angular displacement to obtain a wrapped phase map. In one possible implementation, based on the absolute phase difference point of the wrapped phase map, i.e. the optical axis origin, a spatial unwrapping algorithm is used to obtain the continuously distributed phase difference distribution map.

[0074] In one possible implementation, the three-degree-of-freedom angular displacement measurement module 122 includes:

[0075] The angular displacement calculation submodule calculates the angular displacement based on the mathematical model. It selects characteristic points that meet the requirements according to the point selection method and calculates the three-degree-of-freedom angular displacement using the corresponding phase difference values ​​at the characteristic points. In one possible implementation, the characteristic points should be any points other than the optical axis origin and singular points. The coordinate positions of the selected characteristic points in the two phase difference distribution maps are the same. The three-degree-of-freedom angular displacement of the measured object surface is determined based on the at least four characteristic points.

[0076] It should be noted that, although... Figures 1-13 The above example illustrates a three-degree-of-freedom angular displacement measurement method of this disclosure; however, those skilled in the art will understand that this disclosure is not limited thereto. In fact, users can flexibly configure the measurement optical path and parameters according to their personal preferences and / or actual application scenarios.

[0077] Thus, based on the inverted digital shear speckle interferometry, two phase difference distribution maps caused by the angular displacement of the measured object surface are obtained. Based on the two phase difference distribution maps, the three degrees of freedom angular displacement of the measured object surface is determined, wherein the three degrees of freedom angular displacement includes pitch, yaw, and roll. Therefore, based on the inverted digital shear speckle interferometry, targetless measurement of the measured object surface can be effectively achieved, and the measured three degrees of freedom angular displacement has high accuracy.

[0078] In some embodiments, the functions or modules of the apparatus provided in this disclosure can be used to perform the methods described in the above method embodiments. The specific implementation can be referred to the description of the above method embodiments, and for the sake of brevity, it will not be repeated here.

[0079] This disclosure also proposes a computer-readable storage medium storing computer program instructions that, when executed by a processor, implement the above-described method. The computer-readable storage medium can be volatile or non-volatile.

[0080] This disclosure also proposes an electronic device, including: a processor; and a memory for storing processor-executable instructions; wherein the processor is configured to implement the above method when executing the instructions stored in the memory.

[0081] This disclosure also provides a computer program product, including computer-readable code, or a non-volatile computer-readable storage medium carrying computer-readable code, wherein when the computer-readable code is run in a processor of an electronic device, the processor in the electronic device performs the above-described method.

[0082] The various embodiments of this disclosure have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or technical improvements to the embodiments in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A method for measuring three-degree-of-freedom angular displacement, characterized in that, include: Based on the inverted digital shear speckle interferometry, the distribution maps of two phase differences caused by the three degrees of freedom angular displacement of the measured object surface are obtained. Based on the two phase difference distribution maps, the three-degree-of-freedom angular displacement of the measured object surface is determined, wherein the three-degree-of-freedom angular displacement includes: pitch angle, yaw angle and roll angle.

2. The method according to claim 1, characterized in that, The inverted digital shear speckle interferometry includes: Two expanded laser beams are used to symmetrically illuminate the surface of the object under test at an angle θ relative to the optical axis, generating two diffuse speckle fields. The two speckle fields are inverted and sheared, resulting in two pairs of spatially inverted and overlapping light fields on the image plane, forming an inverted shearing interference field. The inverted shear interference field is acquired and received by an image sensor, and the phase distribution maps of the state before and after the angular displacement are extracted using the phase shift method. The phase distribution maps obtained before and after the angular displacement are used to obtain a wrapped phase map through differential operation, and the two continuously distributed phase difference distribution maps are obtained through the unwrapping algorithm.

3. The method according to claim 2, characterized in that, The inverted shearing generates a total of two pairs of spatially inverted and overlapping light fields in the image plane, including: The diffuse speckle field is split into a first beam and a second beam. A first transformation operation is applied to the wavefront distribution of the first beam, and a second transformation operation, which is the opposite of the first transformation operation, is applied to the wavefront distribution of the second beam, so that the reflected light from any point P(x, y) in the object space of the measured object and the corresponding point P′(-x, -y) form an inverted interference with the optical axis as the center. The first transformation operation and the second transformation operation can be respectively applying a horizontal mirror symmetry transformation and a vertical mirror symmetry transformation about the plane where the optical axis is located to the wavefront distributions of the first beam and the second beam; or applying a clockwise and counterclockwise relative rotation transformation about 90° around the optical axis to the wavefront distributions of the first beam and the second beam, so that the wavefront distribution of the second beam is symmetrical about the origin of the optical axis relative to the wavefront distribution of the first beam. The transformed first beam and second beam are coaxially combined relative to the optical axis to generate a pair of spatially inverted and overlapping light fields on the image plane.

4. The method according to claim 2, characterized in that, The two expanded laser beams include: The beam splitting multiplexing method is adopted, and the beam splitting multiplexing method includes, but is not limited to: time division multiplexing, wavelength division multiplexing, polarization state division multiplexing, time frequency division multiplexing, and spatial frequency division multiplexing.

5. The method according to claim 2, characterized in that, The phase difference distribution map obtained by the unwrapping algorithm, which is continuously distributed, includes: The phase difference value at the origin of the optical axis is known, and there exists an absolute phase difference value, which is 0. The unwrapping algorithm performs phase unwrapping based on the origin of the optical axis to obtain a phase difference distribution map. This phase difference value is the absolute phase difference value. The phase unwrapping method includes, but is not limited to: spatial phase unwrapping method, temporal phase unwrapping method, and spatiotemporal phase unwrapping method.

6. The method according to claim 1, characterized in that, The determination of the three-degree-of-freedom angular displacement of the measured object surface based on the two phase difference distribution maps includes: Based on the two phase difference distribution maps, at least two feature points are selected in each phase difference distribution map, for a total of at least four feature points; In the phase difference distribution map, the feature point should be any point other than the origin of the optical axis and the singular point, and the coordinate positions of the feature points selected in the two phase difference distribution maps are the same; The three-degree-of-freedom angular displacement of the measured object surface is determined based on the phase difference values ​​at the at least four feature points.

7. The method according to claim 6, characterized in that, The determination of the three-degree-of-freedom angular displacement of the measured object surface based on the phase difference values ​​at the at least four feature points includes: Based on the phase difference values ​​at the at least four feature points, the three-degree-of-freedom angular displacement of the measured object surface is obtained according to the following mathematical model: Among them, R x R y and R z Δφ1 and Δφ2 represent the pitch angle, yaw angle, and roll angle of the measured object surface, respectively. Δφ1 and Δφ2 represent the phase difference values ​​of the two expanded laser beams at the feature point, respectively. λ1 and λ2 represent the wavelengths of the two expanded laser beams, respectively. θ is the angle between the illumination beam and the optical axis. (x0, y0), (x1, y1), and (x2, y2) represent the coordinates of any point on the phase difference distribution map other than the origin of the optical axis and the singular point, respectively. The singular point is defined as x2y1 = x1y2.

8. A three-degree-of-freedom angular displacement measuring device, characterized in that, include: The phase measurement module is used to obtain two phase distribution maps of interference light before and after the angular displacement of the surface of the object under test based on the inverted digital shear speckle interferometry, and to perform differential operation on the phase distribution maps obtained before and after the angular displacement to obtain two phase difference distribution maps. The three-degree-of-freedom angular displacement measurement module determines the three-degree-of-freedom angular displacement of the measured object surface based on the two phase difference distribution maps, wherein the three-degree-of-freedom angular displacement includes: pitch angle, yaw angle and roll angle.

9. The apparatus according to claim 8, characterized in that, The phase measurement module includes: The illumination submodule is configured to use two beam-expanding lasers incident symmetrically with respect to the optical axis at an angle θ using a beam-splitting multiplexing method. The wavelengths of the two beam-expanding lasers can be the same or different. The beam-splitting multiplexing method includes, but is not limited to: time-division multiplexing, wavelength-division multiplexing, polarization-division multiplexing, time-frequency multiplexing, and spatial-frequency multiplexing. The inverted shearing submodule is used to apply inverted shearing to the laser speckle field reflected from the surface of the object under test, forming inverted shearing interference between the reflected light from any point P(x,y) in the object space and the corresponding point P′(-x,-y) with the optical axis as the center. The phase-shifting submodule is configured to use the phase-shifting method to obtain the phase distribution diagrams of the interference field before and after the angular displacement. The image acquisition submodule is used to record the image of the interference field; The phase difference calculation submodule is used to perform differential operations on the phase distribution map obtained before and after the angular displacement to obtain a wrapped phase map. Based on the wrapped phase map, the unwrapping algorithm obtains the continuously distributed phase difference distribution map with the optical axis origin as the reference.

10. The apparatus according to claim 8, characterized in that, The three-degree-of-freedom angular displacement measurement module includes: The angular displacement calculation submodule is used to select at least two feature points in each phase difference distribution map based on the two phase difference distribution maps, for a total of at least four feature points; In the phase difference distribution diagram, the feature point should be any point other than the origin of the optical axis and the singular point. The coordinate positions of the feature points selected in the two phase difference distribution diagrams are the same. The three-degree-of-freedom angular displacement of the measured object surface is determined based on the phase difference value on the at least four feature points.

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