A six-degree-of-freedom error correction three-dimensional motion measurement system and a measurement method

Abstract

The application discloses a kind of six-degree-of-freedom error correction three-dimensional motion measurement system and measurement method, including optical air floatation platform, six-degree-of-freedom error correction three-dimensional motion measurement system, two-dimensional motion machine table, reference optical flat, color confocal measurement bridge component, camera measurement bridge component and thin film interference component;Camera measurement bridge frame module in the application acquires the image of concentric circle in the image grid sensor system, the concentric circle image is based on subpixel processing and concentric circle identification to obtain high-precision transverse displacement, based on linear interpolation calculation mode forms two virtual measurement lines in X, Y two directions, the intersection of measurement line is the placement position of color confocal head, the thin film interference component is placed directly below the head, the thin film interference component uses super-precision optical flat as measurement reference, so that the application satisfies three-dimensional zero Abbe error, three-axis zero coupling error, improve the measurement accuracy of three-dimensional motion measurement platform.

Description

Technical Field

[0001] This invention relates to the field of optical measurement, specifically to a three-dimensional motion measurement system and method with six-degree-of-freedom error correction. Background Technology

[0002] With the rapid development of modern technology, the precision of industrial products is gradually improving, and precision and ultra-precision machining technologies have become important directions for industrial development and scientific research. Along with ultra-precision machining and manufacturing, three-dimensional morphology inspection of industrial products is a key technology for quality control and evaluation in ultra-precision machining. For example, in the semiconductor processing field, sapphire substrates are crucial substrate materials due to their stable chemical properties. The surface quality of sapphire substrates has a certain impact on the growth of epitaxial films such as GaN; for instance, the processing curvature of sapphire substrates has a constant influence on the total warpage of epitaxial films. Therefore, in wafer processing processes such as wire cutting and grinding, comprehensive morphology inspection and tracking of wafers are required. Precision and ultra-precision machining have extremely high requirements for surface measurement; the measurement system needs to meet three-dimensional zero Abbe error and three-dimensional zero coupling error. Common three-dimensional morphology measurement systems often use rigid sensors on their measurement lines. As slave devices of the instrument, these sensors move with the instrument, and the Abbe arm of the instrument constantly changes with the system's angular error, making it difficult to ensure that the XYZ axis measurement lines all pass through the measurement point. Therefore, they do not meet the three-dimensional zero Abbe error principle. Common sensors used for measuring the lateral XOY plane displacement of a 3D measurement motion platform include optical grating displacement sensors, magnetic grating displacement sensors, and capacitive grating displacement sensors. These sensors offer advantages such as high precision and high integration. However, simply installing these sensors cannot eliminate the Abbe error caused by changes in the Abbe arm due to system yaw and roll; that is, it cannot eliminate the longitudinal Abbe error of the system. The Abbe principle is a crucial principle to consider during instrument design. By placing two grating sensor systems diagonally on the instrument, interpolation is used to create a virtual measurement line between the two pairs of grating systems. This virtual measurement line is independent of the platform and does not move with platform movement, thus always measuring the position and meeting the requirement of zero Abbe error in two dimensions in real time to compensate for horizontal Abbe error. Furthermore, the grating sensor system, based on high-precision sub-pixel processing and concentric circle processing, has high positioning accuracy, serving as a lateral measurement reference sufficient to compensate for coupling errors in the XY directions. The rational mechanical design based on diagonally positioned sensors provides more flexible installation space for the measuring instrument. Therefore, there is space in the vertical direction to install a thin-film interferometer system. The thin-film interferometer component detects the Abbe arm changes caused by system yaw and roll, further compensating for Z-axis Abbe error. The thin-film interferometer system and the color confocal probe together constitute a differential measurement structure. This differential measurement structure compensates for Z-axis coupling error based on an ultra-smooth optical reference. Ultimately, this overcomes the difficulty of achieving zero Abbe error and zero coupling error in three-dimensional topography measurement systems. Summary of the Invention

[0003] To achieve the above objectives, the present invention provides the following technical solution: a six-degree-of-freedom error-corrected three-dimensional motion measurement system, comprising an optical air-bearing platform, an image grating sensor system, a two-dimensional motion stage, a reference optical flat, a color confocal measurement bridge assembly, a camera measurement bridge assembly, a calibration plate, and a thin-film interferometer assembly; the optical air-bearing platform serves as a base, the two-dimensional motion stage is mounted on the base and includes a guide rail drive mechanism, a reference optical flat, and a central platform disposed on the base; the central platform is disposed on the guide rail drive mechanism, the reference optical flat is disposed on the central platform to support the workpiece to be measured, the color confocal measurement bridge assembly is disposed on the base and spans both sides of the two-dimensional motion stage to collect surface information of the workpiece to be measured, the image grating sensor system is fixed on the two-dimensional motion stage and can move with the two-dimensional motion stage, and the calibration plate is composed of multiple sets of concentric circles, each concentric circle having the same interval. The camera measurement bridge assembly is positioned above the grating sensor system and acquires concentric circle patterns from the calibration plate. The camera measurement bridge assembly acquires concentric circle images of the two-dimensional motion stage at different positions. Subpixel image processing eliminates XY-direction coupling errors and guide rail positioning errors. The grating sensor system measures the position to eliminate lateral Abbe errors of the two-dimensional motion stage. The thin-film interference assembly is located on one side of the two-dimensional motion stage and can eliminate longitudinal Abbe errors and Z-axis coupling errors by measuring the displacement of the thin-film interference fringes generated with the lower surface of the reference optical flat. Two sets of camera measurement bridge assemblies and the grating sensor system are diagonally arranged on the two-dimensional motion stage. Based on the displacement data acquired by the grating sensor system and the camera measurement bridge assembly, two virtual measurement lines are linearly interpolated in the X and Y directions, respectively, and the intersection of the virtual measurement lines is the position of the color confocal probe.

[0004] Preferably, the six-degree-of-freedom error-corrected three-dimensional motion measurement system includes a white light source, and the grating sensor system includes a grating displacement sensor. The system calculates the pixel offset of the center point of the circle by acquiring a concentric circle pattern on the calibration plate through the camera measurement bridge assembly and performing sub-pixel image processing. Based on the pixel-to-true distance ratio, it is converted into a true displacement, and a virtual measurement line is calculated using linear interpolation of similar triangles to compensate for Abbe error. The linear interpolation of similar triangles is as follows: the displacements acquired by the two sets of grating displacement sensors from left to right are respectively... and Linear interpolation based on similar triangles can obtain the displacement value at the measurement point. The true displacement of the measuring point along the X-axis when the motion machine moves in the X-direction. ,in Let X be the similarity ratio, satisfying ,in and Let be the distances between the measurement optical axes of the two cameras in the Y direction and the measurement points, respectively. ,in Let the similarity ratio in the Y direction satisfy... ,in and These represent the distances between the measurement optical axes of the two cameras in the X direction and the measurement points.

[0005] Preferably, the subpixel image processing is as follows: the first concentric circle acquired is an image acquired from the stationary state before the two-dimensional motion machine starts running, and its center point is... Assume the second concentric circle acquired is an image captured after the two-dimensional motion machine has moved, and its center point is... The pixel offset between the second concentric circle and the first concentric circle ,in Pixel offset simplified to There is a linear relationship between the pixel offset and the actual displacement; that is, the linearity ratio can be obtained through the relationship between the pixel and the actual displacement. The actual displacement is .

[0006] Preferably, the center position of the concentric circles is determined as follows: Gaussian blur algorithm is used to denoise the image; a Sobel convolution kernel is applied at a certain pixel to obtain a two-dimensional gradient value; when the gradient value at that point is greater than a set threshold, the point is considered an edge point; then, sub-pixel edge points of the image are extracted based on the edge points and gradient values. The sub-pixel edge point extraction method is to first determine the edge points... Four horizontal lines are drawn at adjacent pixels, and the gradient direction of the horizontal lines relative to the edge points is... They were handed over to , , , The four points mentioned above do not fall on integer pixels; the pixels are linearly distributed within a small area, and linear interpolation is used for calculation. , , , Its edge points exhibit a Gaussian distribution, which can be obtained by fitting a Gaussian function. The optimal sub-pixel edge point is obtained, and finally, based on the sub-pixel edge point and gradient direction and Hough circle, the center of the inner and outer circles is obtained respectively.

[0007] Preferably, the color confocal measurement bridge assembly includes an arched bridge and a color confocal probe. The arched bridge is disposed on the base, and the color confocal probe is fixed at the center of the arched bridge and used to collect surface information of the workpiece to be measured.

[0008] Preferably, the thin-film interference assembly includes a laser, a right-angle prism, an imaging screen, and a camera. A wedge-shaped air gap is formed between the right-angle prism and the optical flat. The incident light is dispersed into two beams after passing through the right-angle prism and the optical flat, and there is an optical path difference between the two beams. ,in Represents the height of the wedge-shaped air gap. Represents phase shift, and the longitudinal error of the system is obtained based on the phase shift of the interference fringe image.

[0009] Preferably, the guide rail drive mechanism includes an active drive component, a driven component, and a built-in grating, and the central platform is rigidly connected to the active drive component and the driven component.

[0010] A measurement method for a three-dimensional motion measurement system applying the six-degree-of-freedom error correction described above includes the following steps:

[0011] (1) The concentric circle pattern in the calibration plate is acquired by the camera measurement bridge assembly, and the pixel offset of the center point of the circle is calculated by sub-pixel image processing. Based on the pixel-to-real distance ratio, it is converted into real displacement, and the virtual measurement line is calculated by linear interpolation of similar triangles to compensate for Abbe error.

[0012] (2) The first concentric circle acquired is an image of the two-dimensional motion machine in its static state before operation, and its center point is... Assume the second concentric circle acquired is an image captured after the two-dimensional motion machine has moved, and its center point is... The pixel offset between the second concentric circle and the first concentric circle ,in Pixel offset simplified to There is a linear relationship between the pixel offset and the actual displacement; that is, the linearity ratio can be obtained through the relationship between the pixel and the actual displacement. The actual displacement is The linear interpolation method for the similar triangles is as follows: the displacements acquired by the two sets of grating displacement sensors from left to right are respectively... and Linear interpolation based on similar triangles can obtain the displacement value at the measurement point. The true displacement of the measuring point along the X-axis when the motion machine moves in the X-direction. ,in Let X be the similarity ratio, satisfying ,in and Let be the distances between the measurement optical axes of the two cameras in the Y direction and the measurement points, respectively. ,in Let the similarity ratio in the Y direction satisfy... ,in and These represent the distances between the measurement optical axes of the two cameras in the X direction and the measurement points, respectively.

[0013] (3) Obtain the interference pattern through the thin film interference component, and obtain the longitudinal error of the system through the phase shift of the interference pattern.

[0014] Preferably, the center position of the concentric circles is determined as follows: Gaussian blur algorithm is used to denoise the image; a Sobel convolution kernel is applied at a certain pixel to obtain a two-dimensional gradient value; when the gradient value at that point is greater than a set threshold, the point is considered an edge point; then, sub-pixel edge points of the image are extracted based on the edge points and gradient values. The sub-pixel edge point extraction method is to first determine the edge points... Four horizontal lines are drawn at adjacent pixels, and the gradient direction of the horizontal lines relative to the edge points is... They were handed over to , , , The four points mentioned above do not fall on integer pixels; the pixels are linearly distributed within a small area, and linear interpolation is used for calculation. , , , Its edge points exhibit a Gaussian distribution, which can be obtained by fitting a Gaussian function. The optimal sub-pixel edge point is obtained, and finally, based on the sub-pixel edge point and gradient direction and Hough circle, the center of the inner and outer circles is obtained respectively.

[0015] This invention offers the following advantages: It provides a three-dimensional motion measurement platform with zero Abbe error and zero coupling error, featuring a diagonally mounted planar grating. This solves the problem of the system not adhering to the Abbe principle due to the inherent precision issues of the guide rail drive mechanism and the cross-stacking arrangement of the X and Y guide rails. The camera measurement bridge module in this invention acquires images of concentric circles in the grating sensor system. These images are processed using sub-pixel technology and concentric circle recognition to obtain high-precision lateral displacement. Two virtual measurement lines are formed in the X and Y directions using linear interpolation. The intersection of these lines marks the placement position of the color confocal probe. The thin-film interference component is placed directly below the probe, using an ultra-precision optical flat as the measurement reference. This ensures that the invention achieves zero Abbe error in three dimensions and zero coupling error in three axes, improving the measurement accuracy of the three-dimensional motion measurement platform. Furthermore, the invention uses the color confocal measurement bridge component to acquire surface information of the workpiece, thus addressing the problems of existing precision and ultra-precision machining technologies where three-dimensional topography measurement systems struggle to eliminate Abbe and coupling errors, have stringent assembly and adjustment requirements, and are costly. Attached Figure Description

[0016] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained from these drawings without creative effort.

[0017] Figure 1 This is a top view of the system of the present invention;

[0018] Figure 2 This is a three-dimensional schematic diagram of the system of the present invention;

[0019] Figure 3 for Figure 2 A schematic diagram of a thin-film interference assembly;

[0020] Figure 4 for Figure 3 A schematic diagram of thin-film interference measured in the image;

[0021] Figure 5 This is a schematic diagram of linear interpolation of similar triangles according to the present invention;

[0022] Figure 6 This is a schematic diagram of the concentric circle sub-pixel edge point extraction of the present invention;

[0023] Figure 7 This is a flowchart of the present invention for extracting edge points from a concentric circle image;

[0024] Figure 8 For the present invention to Figure 7 A flowchart for extracting sub-pixel edge points from edge points in a graph;

[0025] Figure 9 This invention relates to Figure 8 A flowchart illustrating the process of processing mid-sub-pixel edge points to obtain the new position of concentric circles.

[0026] In the diagram: 1-Optical air-bearing platform; 2-Camera measurement bridge assembly; 3-Raster sensor system; 4-Color confocal measurement bridge assembly; 5-Guide rail drive mechanism; 6-Central platform; 7-Reference optical flat; 8-White light source; 9-Camera; 10-Lens; 11-Calibration plate; 12-Color confocal probe; 13-Arch bridge; 14-Camera support frame; 15-Mechanism; 16-Right-angle prism; 17-Imaging screen; 18-Camera; 19-Thin-film interference assembly. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of the present invention. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to represent selected embodiments of the invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of the present invention.

[0028] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0029] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0030] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0031] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature being directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature being directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0032] Example

[0033] The following are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the following embodiments. All technical solutions that fall within the scope of the present invention are within the scope of protection of the present invention.

[0034] Reference manual attached Figure 1-2A six-degree-of-freedom error-corrected three-dimensional motion measurement system includes an optical air-bearing platform 1, an image grating sensor system 3, a two-dimensional motion stage, a reference optical flat 7, a color confocal measurement bridge assembly 4, a camera measurement bridge assembly 2, and a thin-film interferometer assembly 19. The optical air-bearing platform 1 serves as a base. The two-dimensional motion stage is mounted on this base and includes a guide rail drive mechanism, the reference optical flat 7, and a central platform 6 disposed on the base. The central platform 6 is mounted on the guide rail drive mechanism. The reference optical flat 7 is mounted on the central platform 6 to support the workpiece to be measured. The color confocal measurement bridge assembly 4 is mounted on the base and spans both sides of the two-dimensional motion stage to acquire surface information of the workpiece to be measured. The grating sensor system 3 is placed on and rigidly connected to the two-dimensional motion platform. The camera measurement bridge assembly 2 is positioned above the grating sensor system 3 and acquires images from it. The concentric circle images of the two-dimensional motion platform at different positions acquired by the camera measurement bridge assembly 2 are used to eliminate XY direction coupling errors through subpixel image processing. The lateral Abbe error of the two-dimensional motion platform is eliminated by measuring the position of the grating sensor system 3. The thin-film interference assembly 19 is located on one side of the two-dimensional motion platform and can eliminate the longitudinal Abbe error and Z-axis coupling error by measuring the displacement of the fringes generated during thin-film interference with the lower surface of the reference optical flat 7. The longitudinal error is sensed by the thin-film interference assembly. Assuming the measurement value of the color confocal sensor is H1 (at which point the value of the color confocal sensor has coupling error and longitudinal motion error), and the measurement value of the thin-film interference system is H2, the coupling error and motion error can be eliminated by subtracting H1 from H2. The lateral Abbe error is sensed by a three-dimensional motion measurement system with six degrees of freedom error correction. In fact, the displacement sensed by the diagonal plane grating sensor is the most accurate displacement Xt, while the displacement identified by the motor is a displacement value Xe with Abbe error. Therefore, the value of Abbe error is obtained by Xt-Xe. In this invention, only Xt is used as an accurate displacement value, which compensates for the Abbe error in the system.

[0035] The six-degree-of-freedom error-corrected three-dimensional motion measurement system includes a white light source 8 and a calibration plate 11. The calibration plate 11 consists of multiple sets of concentric circles, each with the same spacing. The camera measurement bridge assembly 2 acquires the concentric circle pattern on the calibration plate and calculates the pixel offset of the center point of the circle using sub-pixel image processing. Based on the pixel-to-real distance ratio, it is converted into real displacement, and a virtual measurement line is calculated using linear interpolation of similar triangles to compensate for Abbe error. The main function of the white light source 8 is to ensure the uniformity of light in the grating sensor system 3, enabling the camera measurement bridge assembly 2 to better capture images of the calibration plate 11. The pattern on the calibration plate 11 consists of 11 sets of concentric circles, each spaced 10mm apart. The camera 9 and lens 10 image the concentric circle pattern onto the image at approximately a 1:1 scale. Therefore, the camera measurement bridge assembly 2 acquires the concentric circle pattern on the calibration plate 11 and calculates the pixel offset of the center point of the circle using sub-pixel image processing. Based on pixel-to-actual distance ratio Converted into real displacement The virtual measurement line is calculated using linear interpolation of similar triangles, thus compensating for Abbe error in the system.

[0036] Reference manual attached Figure 5 The linear interpolation method for the similar triangles is as follows: the displacements acquired by the two sets of grating displacement sensors from left to right are respectively... and Linear interpolation based on similar triangles can obtain the displacement value at the measurement point. The true displacement of the measuring point along the X-axis when the motion machine moves in the X-direction. (Only when the motion platform is completely horizontal will the displacement of the two cameras and the displacement value measured at the measurement point be the same. The error caused by the motion platform is that their actual displacement is in a similar triangular relationship to their longitudinal distance.) Let X be the similarity ratio, satisfying ,in and Let be the distances between the measurement optical axes of the two cameras in the Y direction and the measurement points, respectively. ,in Let the similarity ratio in the Y direction satisfy... ,in and These represent the distances between the measurement optical axes of the two cameras in the X direction and the measurement points. This design of the present invention can correct displacement measurement errors along the X and Y axes, and the Abbe error of the platform can be corrected by adjusting the yaw angle of the quantization platform in the XOY plane.

[0037] The subpixel image processing involves capturing the first concentric circle of an image taken from a stationary state before the two-dimensional motion machine begins operation; the center point of this circle is... Assume the second concentric circle acquired is an image captured after the two-dimensional motion machine has moved, and its center point is... The pixel offset between the second concentric circle and the first concentric circle ,in Pixel offset simplified to There is a linear relationship between the pixel offset and the actual displacement; that is, the linearity ratio can be obtained through the relationship between the pixel and the actual displacement. The actual displacement is .

[0038] Reference manual attached Figure 6-9 The method for determining the center position of the concentric circles is as follows: Gaussian blur algorithm is used to denoise the image. A Sobel convolution kernel is applied at a certain pixel to obtain a two-dimensional gradient value. When the gradient value at that point is greater than a set threshold, the point is considered an edge point. Then, sub-pixel edge points of the image are extracted based on the edge points and gradient values. The sub-pixel edge point extraction method is to first... Four horizontal lines are drawn at adjacent pixels, and the gradient direction of the horizontal lines relative to the edge points is... They were handed over to , , , The four points mentioned above do not fall on integer pixels; the pixels are linearly distributed within a small area, and linear interpolation is used for calculation. , , , Its edge points exhibit a Gaussian distribution, which can be obtained by fitting a Gaussian function. The optimal sub-pixel edge point is obtained, and finally, based on the sub-pixel edge point and gradient direction and Hough circle, the center of the inner and outer circles is obtained respectively.

[0039] Due to inherent accuracy issues with the guide rail drive mechanism 5 and the problem of the system not adhering to the Abbe principle caused by the cross-stack arrangement of the X and Y guide rail drive mechanisms, coupling errors and Abbe errors exist in the transverse XOY plane when measuring the workpiece. The camera measurement bridge assembly 2 and the image grating sensor system 3 are described in the attached manual. Figure 5 Install diagonally as shown in the instructions. Figure 5In this system, using the color confocal probe 12 as the common intersection point of similar triangles, and based on the displacement data collected by the two sets of image grating sensor systems and the camera measurement bridge assembly, two virtual measurement lines are linearly interpolated in the X and Y directions, respectively, according to the similarity ratio of the similar triangles. The intersection of the virtual measurement lines is the position of the color confocal probe 12. The elimination of lateral Abbe error is achieved based on the virtual measurement lines. The color confocal probe 12 belongs to the color confocal measurement bridge assembly 4, and its function is to collect surface information of the workpiece under test. Overall, the image grating sensor system 3, based on its reasonable diagonal mounting design and high-precision sub-pixel concentric circle processing algorithm, ultimately achieves the elimination of lateral Abbe error and coupling error.

[0040] The color confocal measurement bridge assembly 4 includes an arched bridge 13 and a color confocal probe 12. The arched bridge 13 is disposed on the base 1, and the color confocal probe 12 is fixed at the center of the arched bridge. The reference optical flat 7 is disposed on the central platform 6. The upper surface of the reference optical flat 7 is used to support the workpiece to be measured, wherein the workpiece to be measured is a low-steepness workpiece such as a wafer, a substrate, or a mobile phone screen.

[0041] The thin-film interference assembly 19 includes a laser 15, a right-angle prism 16, an imaging screen 17, and a camera 18. A wedge-shaped air gap is formed between the right-angle prism 16 and the optical flat 7. The incident light is dispersed into two beams by the right-angle prism 16 and the optical flat 7, and there is an optical path difference between the two beams. ,in and Refractive indices of air and glass, respectively, and angles. All quantities related to the incident angle are constants, therefore the optical path difference can be simplified to... Simplifying the above equation, we get: ,in Represents the height of the wedge-shaped air gap. Representing phase shift, the longitudinal error of the system is obtained based on the phase shift of the interference fringe image. By adding a thin-film interferometer component, users can calculate the height change based on the fringe translation, and determine the platform's pitch and roll angles based on the fringe rotation and density variations.

[0042] Using the grating sensor system 3, linear interpolation forms two virtual measurement lines, X and Y. The intersection of these two virtual measurement lines is the measurement point of the workpiece and also the placement point for the thin-film interferometer component 19 and the color confocal component 4, eliminating Abbe error present in the system. The grating sensor system 3, based on sub-pixel image processing, achieves high-precision center positioning, eliminates coupling errors in the XY direction, ensures the system's positioning accuracy in the transverse XOY plane, and is not limited by mechanical structures, offering strong installation flexibility.

[0043] The guide rail drive mechanism 5 includes two pairs of active motors and six pairs of rolling driven guide rails. The active and driven guide rails are stacked in a cross-shaped arrangement. When the device starts, the active motors drive the driven guide rails and the central platform 6 to perform two-dimensional linear motion, indirectly driving the reference optical flat 7 and the grating sensor system 3 to perform two-dimensional linear motion in the X and Y directions. This system is built with modular components. If further expansion of the stroke is required, it can be achieved by replacing the active motors and driven guide rails. It is worth noting that when the stroke of the motors and guide rails is increased, straightness accuracy and repeatability will affect the topographic measurement accuracy of the two-dimensional motion platform. Thin-film interferometers can effectively eliminate longitudinal Abbe errors and longitudinal coupling errors present in the measurement system.

[0044] The white light source 8 has a light-emitting area of ​​130 x 130 mm; the calibration plate 11 consists of 11 x 11 concentric circles, with each concentric circle spaced 10 mm apart, the inner circle having a diameter of 2 mm and the outer circle a diameter of 5 mm. Other types of calibration plates can also be used, which are not specifically limited here, but all of these solutions are within the protection scope of this invention; the camera 9 can be a Basler a2A2448-23gmBAS area array camera, equipped with a CMOS image sensor, with a target surface size of 1 / 1.8”. The system displays images at 23 frames per second with a 4-megapixel resolution (2448 x 2048). The lens 10 can be a MORITEX ML-MC35HR with a focal length of 35mm, a minimum magnification of 0.25, a maximum magnification of 0.7, and TV distortion of less than 0.05%. The magnification can be increased to 0.9 by adding washers. The camera measurement bridge assembly 2 includes a camera bridge mechanical structure, which can use a "bulb-type" mechanical design and side-drilled holes to fix the camera 9 and lens 10. Parallel holes facilitate adjustment of the distance between the camera 9 and the optical calibration plate 11. The imaging screen 17 can be frosted glass. The camera 18 can be a Basler acA2000-165um area array camera. The laser 15 can be a 50mW monochromatic red light source with an exit pupil diameter of 15mm, a divergence angle of 0.1 mrad, and a wavelength of 650nm. The right-angle prism 16 can be 30mm x 30mm x 30mm. A right-angle prism with a diameter of mm and a surface profile of λ / 10@632.8 nm.

[0045] The color confocal measurement bridge assembly includes a measurement bridge 13 and a color confocal probe 12. The color confocal measurement bridge 13 is mounted on the base 1, and the color confocal probe 12 is fixed at the center of the color confocal measurement bridge 13. The measuring end of the color confocal probe 12 is perpendicularly illuminated on the upper surface of the external reference optical flat 7. The virtual measurement lines in the X and Y directions formed by the two sets of image grating sensors 3 intersect at the color confocal probe 12. The external reference optical flat is a double-sided optical flat with a flatness ≤0.05μm and a parallelism ≤1". With the addition of an additional set of color confocal probes 12, the two sets of color confocal probes 12 focus precisely on the same area of ​​the reference optical flat, realizing a zero Abbe error three-dimensional measurement system with differential error separation. The measurement bridge 13 can be designed as an arched, centrally hollowed-out bridge, such as... Figure 2 As shown, the color confocal probe 12 is used to fix the probe. The color confocal probe 12 can use an IFS2406 color confocal sensor, with a measurement range of 2.5 mm, a measurement resolution of 24 nm, a nonlinearity error of 0.75 μm, and a minimum measurable thickness of 0.12 mm. A high-precision probe directly affects the system's measurement accuracy, but higher accuracy requires higher costs. Therefore, a probe should be selected rationally while meeting specific measurement needs.

[0046] After the camera measurement bridge assembly 2 saves the running image of the grating sensor system 3, the system performs preprocessing such as filtering on the image. Through the sub-pixel edge extraction algorithm, it extracts the edge information of concentric circles. Based on the self-developed concentric circle center and radius extraction algorithm, it obtains the center of each image. The movement of the center pixel coordinates between each image corresponds to the real displacement. Based on this, the system position in the XOY lateral direction is accurately obtained.

[0047] The output of the active drive component is electrically connected to the input of the terminal. The output of the white light source is electrically connected to the input of the terminal. The output of the camera measurement bridge component is electrically connected to the input of the terminal. The output of the color confocal measurement bridge component is electrically connected to the input of the terminal. The terminal is connected to the active drive guide rail and each sensing device. The sensing devices transmit signals to the host computer using different transmission methods for data storage and processing, ultimately acquiring the surface morphology of the workpiece under test.

[0048] In summary, in the six-degree-of-freedom error model of the measurement system, the XY axis translation error... and Furthermore, the yaw angle error will cause a lateral displacement feedback error, which consists of Abbe error and motor positioning error. This invention compensates for the lateral displacement feedback error by using a alignment plane grating sensor. The Z-axis translation error... Pitch angle error and roll angle error cause longitudinal motion error in the measurement system. This invention eliminates these errors by using a thin-film interferometer sensor. Therefore, this invention can achieve six-degree-of-freedom error correction.

[0049] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A three-dimensional motion measurement system with six degrees of freedom error correction, characterized in that, It includes an optical air-bearing platform, a grating sensor system, a two-dimensional motion stage, a reference optical flat, a color confocal measurement bridge assembly, a camera measurement bridge assembly, a calibration plate, and a thin-film interference assembly; The optical air-bearing platform serves as a base, upon which the two-dimensional motion stage is mounted. The two-dimensional motion stage includes a guide rail drive mechanism, a reference optical flat, and a central platform. The central platform is mounted on the guide rail drive mechanism, and the reference optical flat is mounted on the central platform to support the workpiece to be measured. The color confocal measurement bridge assembly is mounted on the base and spans both sides of the two-dimensional motion stage to acquire surface information of the workpiece. The grating sensor system is fixed to the two-dimensional motion stage and can move with it. The calibration plate consists of multiple sets of concentric circles with equal spacing between each circle. The camera measurement bridge assembly is positioned above the grating sensor system and acquires the concentric circle pattern on the calibration plate through the camera measurement bridge assembly. The measurement bridge assembly acquires concentric circle images of the two-dimensional motion stage at different positions. Subpixel image processing is used to eliminate XY direction coupling errors and guide rail positioning errors. The lateral Abbe error of the two-dimensional motion stage is eliminated by measuring the position through the grating sensor system. The thin-film interference assembly is located on one side of the two-dimensional motion stage and can eliminate the longitudinal Abbe error and Z-axis coupling error of the system by measuring the displacement of the thin-film interference fringes generated with the lower surface of the reference optical flat. Two sets of camera measurement bridge assemblies and grating sensor systems are diagonally arranged on the two-dimensional motion stage. Based on the displacement data collected by the grating sensor system and camera measurement bridge assemblies, two virtual measurement lines are linearly interpolated in the X and Y directions respectively, and the intersection of the virtual measurement lines is the position of the color confocal probe.

2. The six-degree-of-freedom error-corrected three-dimensional motion measurement system according to claim 1, characterized in that, The six-degree-of-freedom error-corrected three-dimensional motion measurement system includes a white light source, and the grating sensor system includes a grating displacement sensor. The system calculates the pixel offset of the center point of the circles by acquiring concentric circle patterns on the calibration plate through the camera measurement bridge assembly and performing sub-pixel image processing. Based on the pixel-to-true distance ratio, it is converted into true displacement, and a virtual measurement line is calculated using linear interpolation of similar triangles to compensate for Abbe error. The linear interpolation of similar triangles is as follows: the displacements acquired by the two sets of grating displacement sensors from left to right are... and Linear interpolation based on similar triangles can obtain the displacement value at the measurement point. The true displacement of the measuring point along the X-axis when the motion machine moves in the X-direction. ,in Let X be the similarity ratio, satisfying ,in and Let be the distances between the measurement optical axes of the two cameras in the Y direction and the measurement points, respectively. ,in Let the similarity ratio in the Y direction satisfy... ,in and These represent the distances between the measurement optical axes of the two cameras in the X direction and the measurement points.

3. The six-degree-of-freedom error-corrected three-dimensional motion measurement system according to claim 2, characterized in that, The subpixel image processing is as follows: the first concentric circle acquired is an image acquired from the stationary state of the two-dimensional motion machine before its operation, and its center point is... Assume the second concentric circle acquired is an image captured after the two-dimensional motion machine has moved, and its center point is... The pixel offset between the second concentric circle and the first concentric circle ,in Pixel offset simplified to There is a linear relationship between the pixel offset and the actual displacement; that is, the linearity ratio can be obtained through the relationship between the pixel and the actual displacement. The actual displacement is .

4. The six-degree-of-freedom error-corrected three-dimensional motion measurement system according to claim 3, characterized in that, The center position of the concentric circles is determined as follows: Gaussian blur algorithm is used to denoise the image. A Sobel convolution kernel is applied at a certain pixel to obtain a two-dimensional gradient value. When the gradient value at that point is greater than a set threshold, the point is considered an edge point. Then, sub-pixel edge points of the image are extracted based on the edge points and gradient values. The sub-pixel edge point extraction method is to first determine the edge points... Four horizontal lines are drawn at adjacent pixels, and the gradient direction of the horizontal lines relative to the edge points is... They were handed over to , , , The four points mentioned above do not fall on integer pixels; the pixels are linearly distributed within a small area, and linear interpolation is used for calculation. , , , Its edge points exhibit a Gaussian distribution, which can be obtained by fitting a Gaussian function. The optimal sub-pixel edge point is obtained, and finally, based on the sub-pixel edge point and gradient direction and Hough circle, the center of the inner and outer circles is obtained respectively.

5. The six-degree-of-freedom error-corrected three-dimensional motion measurement system according to claim 2, characterized in that, The color confocal measurement bridge assembly includes an arched bridge and a color confocal probe. The arched bridge is mounted on the base, and the color confocal probe is fixed at the center of the arched bridge and used to collect surface information of the workpiece to be measured.

6. The six-degree-of-freedom error-corrected three-dimensional motion measurement system according to claim 2, characterized in that, The thin-film interference assembly includes a laser, a right-angle prism, an imaging screen, and a camera. A wedge-shaped air gap is formed between the right-angle prism and the optical flat. Incident light is dispersed into two beams by the right-angle prism and the optical flat, and there is an optical path difference between the two beams. ,in Represents the height of the wedge-shaped air gap. Represents phase shift, and the longitudinal error of the system is obtained based on the phase shift of the interference fringe image.

7. The three-dimensional motion measurement system with six-degree-of-freedom error correction according to claim 2, characterized in that, The guide rail drive mechanism includes an active drive component, a driven component, and a built-in grating, and the central platform is rigidly connected to the active drive component and the driven component.

8. A measurement method for a three-dimensional motion measurement system employing six-degree-of-freedom error correction as described in any one of claims 2-6, characterized in that, Includes the following steps: (1) The concentric circle pattern in the calibration plate is acquired by the camera measurement bridge assembly, and the pixel offset of the center point of the circle is calculated by sub-pixel image processing. Based on the pixel-to-real distance ratio, it is converted into real displacement, and the virtual measurement line is calculated by linear interpolation of similar triangles to compensate for Abbe error. (2) The first concentric circle acquired is an image of the two-dimensional motion machine in its static state before operation, and its center point is... Assume the second concentric circle acquired is an image captured after the two-dimensional motion machine has moved, and its center point is... The pixel offset between the second concentric circle and the first concentric circle ,in Pixel offset simplified to There is a linear relationship between the pixel offset and the actual displacement; that is, the linearity ratio can be obtained through the relationship between the pixel and the actual displacement. The actual displacement is The linear interpolation method for the similar triangles is as follows: the displacements acquired by the two sets of grating displacement sensors from left to right are respectively... and Linear interpolation based on similar triangles can obtain the displacement value at the measurement point. The true displacement of the measuring point along the X-axis when the motion machine moves in the X-direction. ,in Let X be the similarity ratio, satisfying ,in and Let be the distances between the measurement optical axes of the two cameras in the Y direction and the measurement points, respectively. ,in Let the similarity ratio in the Y direction satisfy... ,in and These represent the distances between the measurement optical axes of the two cameras in the X direction and the measurement points, respectively. (3) Obtain the interference pattern through the thin film interference component, and obtain the longitudinal error of the system through the phase shift of the interference pattern.

9. The measurement method according to claim 8, characterized in that, The center position of the concentric circles is determined as follows: Gaussian blur algorithm is used to denoise the image. A Sobel convolution kernel is applied at a certain pixel to obtain a two-dimensional gradient value. When the gradient value at that point is greater than a set threshold, the point is considered an edge point. Then, sub-pixel edge points of the image are extracted based on the edge points and gradient values. The sub-pixel edge point extraction method is to first determine the edge points... Four horizontal lines are drawn at adjacent pixels, and the gradient direction of the horizontal lines relative to the edge points is... They were handed over to , , , The four points mentioned above do not fall on integer pixels; the pixels are linearly distributed within a small area, and linear interpolation is used for calculation. , , , Its edge points exhibit a Gaussian distribution, which can be obtained by fitting a Gaussian function. The optimal sub-pixel edge point is obtained, and finally, based on the sub-pixel edge point and gradient direction and Hough circle, the center of the inner and outer circles is obtained respectively.

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