A linear displacement measurement system and measurement method based on image grating

Abstract

The present invention relates to a linear displacement measurement system and method based on an image grating, wherein the displacement measurement system comprises an LED backlight source, a glass linear scale, a displacement drive module, an image acquisition module, and a data processing and display module. During measurement, the glass linear scale is placed on the LED backlight source, and is allowed to perform translational motion along with the linear displacement slide in the displacement drive module. The image acquisition module can then acquire stripe images at different positions, and then the data processing and display module is used to perform phase correlation on the images before and after the translation, so as to obtain the number of pixels translated between the images, and finally, the proportional relationship between the object and the image can be used to obtain the actual displacement value of the displacement stage. The displacement measurement system and displacement measurement method proposed by the present invention solve the problem of insufficient installation flexibility of traditional grating sensors, and have higher measurement accuracy and a larger range than previous visual measurement methods.

Description

Technical Field

[0001] The invention relates to the field of optical measurement, and specifically designs a linear displacement measurement system and a measurement method based on an image grating. Background Art

[0002] Linear displacement measurement is widely used in industrial production, such as obtaining object position information, restoring three-dimensional shape, processing various parts, etc. However, with the development of advanced manufacturing, the demand for high-precision and large-range displacement measurement in many fields has become increasingly prominent, such as: processing and measuring of ultra-large wafers in the semiconductor field, measuring the size and shape of aviation blades in the aerospace field, positioning of high-end CNC machine tools in the field of free-form surface processing, and research and development of three-coordinate measuring machines in the field of metrology.

[0003] At present, most high-precision, large-range displacement measurement systems are realized through laser interferometers or traditional grating sensors. Laser interferometers have the highest accuracy and can reach resolutions of nanometers or even sub-nanometers. However, they not only require precise alignment and installation during use, but also have high requirements for the external environment to avoid external factors such as temperature and vibration that cause a decrease in detection accuracy, thereby limiting their application in industry. Traditional grating displacement sensors include gratings, magnetic gratings, capacitive gratings, and inductive synchronizers. The measurement accuracy of these sensors is slightly lower than that of laser interferometers. Their resolution depends on the scale lines and there are obvious cumulative errors. In addition, they all use a structure in which the scale and the reading head are close together. The narrow installation distance limits the flexibility of their installation, which brings great trouble to the installation of new displacement sensors on old equipment and the structural optimization of new equipment.

[0004] Visual measurement technology has developed very rapidly in recent years. With its advantages of simple installation, low cost, non-contact, long distance, and high resolution, it has been widely used in the fields of three-dimensional reconstruction, target detection, deformation analysis, etc. For large displacement measurement in vision, Liu Xueling and others from Tsinghua University proposed a linear displacement measurement method based on image grayscale information. This method solves the displacement of the translation stage by establishing a mapping function model between the grayscale value of each pixel point in a periodic image and the linear motion displacement. The experimental results show that the standard deviation of this method is 4μm within the measurement range of 10mm. However, this method will significantly increase the computing burden of the computer in high-frequency displacement information sampling.

[0005] Image correlation method is a common method for studying in-plane deformation or rigid body displacement in visual measurement. It has strong adaptability and real-time performance, and can measure sub-pixel displacement through interpolation. Therefore, applying image correlation method to linear displacement measurement has great research prospects. Zhao Jiwen and others from Anhui University used a camera that moved with the translation stage to capture non-periodic sinusoidal fringe images, and measured the displacement of the translation stage by calibrating the object-image size ratio. They proposed various sub-pixel interpolation algorithms based on phase correlation, and compared and analyzed the speed, accuracy and anti-interference problems of various related algorithms in sub-pixel interpolation, and obtained a better solution for calculating image translation.

[0006] These two types of vision-based linear displacement measurement methods have their own advantages, but neither of them solves the problem of how to achieve high-precision measurement in a large displacement range. The reasons can be divided into two aspects: first, the influence of optical distortion on displacement measurement accuracy is ignored; second, the limitation of a single field of view is not broken to achieve a wider range of measurement. Summary of the invention

[0007] The object of the present invention is to provide a linear displacement measurement system based on an image grating for realizing high-precision measurement in a large displacement range.

[0008] In order to achieve the above-mentioned purpose, the present invention adopts the following technical scheme: a linear displacement measurement system based on an image grating, a linear displacement measurement system based on an image grating, including an LED backlight source, a glass linear scale, a displacement drive module, an image acquisition module, and a data processing and display module; the glass linear scale is placed on the LED backlight source, and is allowed to perform translational motion with the linear displacement slide in the displacement drive module, and stripe images at different positions are acquired by the image acquisition module, and then the images before and after the translation are phase correlated by the data processing and display module to obtain the number of pixels translated between the images, and the actual displacement value of the displacement stage is acquired based on the obtained number of pixels and the proportional relationship between the object and the image.

[0009] Preferably, the glass linear ruler includes two types of stripe linear rulers: one is a full-striped linear ruler used for focusing, adjusting the verticality of the camera optical axis, and calibrating pixel equivalents; the other is a partial-striped linear ruler for realizing displacement measurement, with specially designed stripes engraved on its surface.

[0010] Preferably, the image acquisition module includes an ultra-low distortion lens group, a high-resolution linear array camera and a computer.

[0011] Preferably, the ultra-low distortion lens group is mounted on the high-resolution line array camera, and the output end of the high-resolution line array camera is connected to the computer.

[0012] Preferably, the data processing and display module processes and calculates the translation image obtained by the image acquisition module through a computer, and displays the calculation result.

[0013] Preferably, the LED backlight source is placed on the electric linear displacement slide of the displacement driving module, and the glass linear ruler is placed on the LED backlight source. The light emitted by the LED backlight source can pass through the glass linear ruler to form a stripe image which is acquired by the image acquisition module. The image acquisition module then transmits the stripe image to the data processing and display module for calculation and display of the displacement value.

[0014] A method for measuring linear displacement using the linear displacement measurement system based on an image grating as described above comprises the following steps:

[0015] (1) Focus: Place the full-striped linear ruler on the LED backlight, and adjust the position of the linear array camera and the lens group up and down to make a clear stripe image appear in the camera;

[0016] (2) Adjust the verticality of the camera optical axis: According to the distribution of the number of pixels occupied by the fringe images of a specified number at each position in the full field of view, adjust the camera optical axis so that it is perpendicular to the plane of the line scale;

[0017] (3) Pixel equivalent calibration: Divide the actual length of a specified number of stripes by the number of pixels they occupy on the image plane to obtain the pixel equivalent value used to calculate the displacement of the line scale;

[0018] (4) Obtaining a translational image: Replace the full-stripe line ruler with a partial-stripe line ruler for measurement, drive the electric linear displacement slide, drive the LED backlight source and the partial-stripe line ruler to move, and the line array camera can capture the translational stripe image.

[0019] (5) Sub-pixel registration: Use the DFT (discrete Fourier transform) local upsampling phase correlation method to obtain the number of pixels translated between images with sub-pixel accuracy;

[0020] (6) Obtaining the actual displacement value: multiplying the pixel equivalent calibrated in step (3) by the number of translation pixels obtained in step (5) to obtain the actual displacement of the glass linear scale.

[0021] Preferably, in step (1), the light luminance of the LED backlight source and the exposure time of the line array camera are adjusted to enable a clear image of the linear ruler at the middle position of the depth of field.

[0022] Preferably, in step (2), according to the perspective projection principle, when the camera optical axis is completely perpendicular to the object plane, the image lengths of any objects of equal length in the object plane are equal in the image plane. Therefore, the verticality of the camera optical axis is adjusted by determining whether the pixel number distribution of stripes of the same width (specified number) at different positions in the field of view is equal on the image plane.

[0023] Preferably, in step (4), each group of stripes of the partial stripe line ruler is arranged separately on the line ruler, and the width of a line pair can be set between 90 μm and 150 μm, with a total width of 3600 μm.

[0024] Preferably, in step (4), the stripe design of the partial stripe line ruler also needs to meet the following conditions: (i) the features of at least three adjacent stripes are different; (ii) the interval between each two stripes is sufficient to fill the camera field of view. In addition, according to the relative position of the stripes in the field of view, the sampling interval of the stripe image is adjusted to ensure that there is a complete set of stripes in the field of view to match the same stripe image at the previous position, so as to ensure the accuracy and continuity of the matching.

[0025] Preferably, in step (5), the DFT local upsampling phase correlation method is a sub-pixel registration method based on the frequency domain proposed by scholars. It has high robustness, accuracy and real-time performance, and is an excellent method for calculating image translation under high-frequency acquisition. It can be divided into two steps: (i) integer pixel level positioning. The translation of the image in the spatial domain is converted into phase shift through Fourier transform, and then the cross-power spectrum is taken to obtain the pure phase change representing the image translation. Finally, the δ function can be obtained through inverse transformation. At this time, the position coordinates at the peak of the δ function represent the number of pixels translated by the reference image; (ii) sub-pixel level positioning. The core of this step is to refine the local area around the peak position in the spatial domain while keeping the power spectrum unchanged, and use the sinc function to realize interpolation during the inverse transformation, and finally obtain the number of translated pixels with sub-pixel accuracy.

[0026] The present invention has the following beneficial effects: 1. The camera is used to shoot the line scale to obtain the actual displacement of the target object, which changes the design structure of the traditional grating displacement sensor (grating, magnetic grating, etc.) in which the scale and the reading head are closely attached to each other, has stronger installation flexibility, and provides a low-cost, convenient and flexible displacement measurement solution for old equipment or new products; 2. A strong backlight LED is used as a light source and placed under the line scale, and the DFT local upsampling phase correlation method is used to achieve sub-pixel registration of the image before and after the displacement, so that the measurement has higher accuracy, adaptability and real-time performance; 3. The displacement measurement depends on the matching between the images before and after the displacement. 4. The image grating displacement measurement system can achieve high resolution with high magnification lens, which can avoid the deformation of the fringe image before and after the displacement caused by lens distortion, and keep a high consistency of the image before and after the displacement, which not only reduces the change of pixel equivalent, but also improves the accuracy of sub-pixel registration; 5. Based on the principle of phase correlation method, multiple groups of stripes with different characteristics are designed on the linear scale, thus breaking the limitation of camera field of view and realizing a large range of measurement; 6. The image grating displacement measurement system can achieve high resolution with high magnification lens, which also provides a new solution for small size and high precision displacement measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without paying creative work.

[0028] Figure 1 It is a schematic diagram of the composition of the measurement system of the present invention;

[0029] Figure 2 This is a schematic diagram of a full-striped linear ruler of the present invention;

[0030] Figure 3 A schematic diagram of a partial stripe line ruler of the present invention;

[0031] FIG4 is a comparison diagram of the number of pixels occupied by 20 line pairs of stripes at each position in the field of view before and after the verticality adjustment of the optical axis of the camera of the present invention, wherein FIG4(a) is a schematic diagram of the change in the number of pixels when the optical axis of the camera is tilted, and FIG4(b) is a schematic diagram of the change in the number of pixels when the optical axis of the camera is vertical;

[0032] Figure 5 It is a flowchart of the DFT local up-sampling phase correlation method of the present invention.

[0033] Legend: 1-image acquisition module; 2-glass line ruler; 3-LED backlight; 4-mobile platform; 5-computer. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work 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 drawings is not intended to limit the scope of the invention claimed for protection, but merely represents the selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0035] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present invention.

[0036] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0037] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0038] In the present invention, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may include that the first and second features are in direct contact, or may include that the first and second features are not in direct contact but are in contact through another feature between them. Moreover, a first feature being "above", "above" and "above" a second feature includes that the first feature is directly above and obliquely above the second feature, or simply indicates that the first feature is higher in level than the second feature. A first feature being "below", "below" and "below" a second feature includes that the first feature is directly below and obliquely below the second feature, or simply indicates that the first feature is lower in level than the second feature.

[0039] Example

[0040] The following are only preferred implementations of the present invention. The protection scope of the present invention is not limited to the following embodiments. All technical solutions under the concept of the present invention belong to the protection scope of the present invention.

[0041] Figure 1 FIG. 1 is a schematic diagram of the composition of the measurement system of the present invention. Figure 1 As shown in the figure, in the box on the left, the electric linear displacement slide is located at the bottom, which is connected to the computer and controller on the right to form a displacement drive module. At the same time, it drives the LED backlight and glass linear scale located above to do translational movement. The LED backlight is placed below the glass linear scale, and white light is projected from the bottom onto the glass linear scale. After the etched shadow area, a high-contrast stripe image of light and dark will be formed in the camera, which greatly improves the image's anti-noise ability and enables the calibration of pixel equivalents and image registration to achieve higher accuracy.

[0042] Figure 1The upper part of the left box is the linear array camera and ultra-low distortion lens group in the image acquisition module. The manufacturer of the linear array camera is Basler, the model is ral8192-12gm-racer, the horizontal / vertical resolution is: 8192px x 1px, the pixel size is 3.5μm, the small pixel can bring higher resolution to the image, so that the sub-pixel interpolation accuracy is higher during image registration, and the larger target surface provides more convenient conditions for increasing the field of view of the measurement. In the measurement, the sub-pixel registration between the images before and after the displacement is used to obtain the sub-pixel translation with higher accuracy. A very critical link is to ensure the consistency of the images before and after the displacement. However, for ordinary imaging lenses, the magnification will always show nonlinear changes with the field of view due to the deviations in the design, production and assembly of the lens. Therefore, when the stripes move in the field of view, it is difficult to ensure that the images in the front and rear positions remain highly consistent, which not only causes a large change in the pixel equivalent, but also causes a large error in the sub-pixel registration of the image. Therefore, the present invention adopts an ultra-low distortion lens of model VHK120 / 5.8 produced by OPT Machine Vision, with a magnification of 1 and a distortion of 0.01%, which greatly reduces the displacement measurement error caused by lens distortion.

[0043] Figure 1 The computer on the right is an important component of the displacement drive module, image acquisition module, data processing and display module. This system achieves rapid measurement of displacement by utilizing the computer's intelligent control and fast processing capabilities. Figure 2 This is a schematic diagram of a full-striped line ruler. Figure 2 As shown in the figure, the stripes are fully covered with a linear ruler, and the width of one line pair is 100μm. The full-striped linear ruler can be used for focusing, adjusting the verticality of the camera optical axis, and calibrating the pixel equivalent of the measurement system. Before displacement measurement, the full-striped linear ruler is placed on a uniformly luminous LED backlight source. After passing through the image acquisition module, a black and white striped image can be observed at each position in the field of view of the linear array camera.

[0044] Figure 3 The figure is a schematic diagram of a partial stripe line ruler. A partial stripe line ruler is also called a measuring line ruler. Figure 3 As shown, the present invention arranges stripes of different line widths on a glass line scale, with a certain interval between each group of stripes. The width of a line pair in each group of stripes can be set between 90μm and 150μm, and the total width is 3600μm. These stripes will form stripe images with different characteristics in the camera during the measurement process, providing a prerequisite for large-scale displacement measurement. At the same time, in order to avoid confusion in registration and ensure the uniqueness of stripe image matching during stripe design, the following two requirements must be followed: (1) The characteristics of at least three adjacent groups of stripes must be inconsistent. (2) The interval between each two groups of stripes must be large enough to fill the camera field of view as much as possible.

[0045] In this example, a measurement method based on a linear displacement measurement system of an image grating is performed according to the following steps:

[0046] 1. Focus. Figure 2 The full-stripe linear ruler shown is placed on an LED backlight source, and the positions of the linear array camera and the lens group, the brightness of the LED backlight source, and the exposure time of the linear array camera are adjusted to enable the linear ruler to be clearly imaged in the middle of the depth of field and to enable the stripe image to have a high contrast.

[0047] 2. Adjust the verticality of the camera optical axis. During the installation of the camera, the camera optical axis will always be slightly tilted to the plane of the linear scale, which will introduce perspective error and make the linear scale unable to guarantee a constant optical magnification in the same field of view. Therefore, in the pinhole imaging model, in order to ensure high-precision measurement, the camera optical axis must be perpendicular to the object plane. According to the principle of perspective projection, when the camera optical axis is completely perpendicular to the object plane, the image lengths of any equal-length objects in the object plane are equal in the image plane. Therefore, this paper completes the adjustment of the verticality of the camera optical axis by judging whether the pixel distribution of stripes of the same width at different positions in the field of view is equal on the image plane. The changes in the number of pixels occupied by 20 line pairs of stripes at each position in the field of view when the camera optical axis is tilted and vertical are shown in Figure 4(a) and Figure 4(b), respectively. In order to accurately count the number of pixels occupied by the stripe image on the camera's photosensitive surface, this paper uses 100-fold cubic spline interpolation to interpolate the grayscale curve formed by the stripe image, and takes a straight line with a grayscale value of 150 to intersect it (this measure is used to count the number of pixels between intersections), and the number of pixels occupied by the stripes with an accuracy of 0.01 pixel can be obtained.

[0048] 3. Calibrate pixel equivalents. After the camera optical axis is adjusted to be perpendicular to the plane of the linear scale, the pixel equivalents in the measurement system are calibrated using the full-striped linear scale. 20 line pairs are grouped as a group, and the pixel equivalents are quantified from the left to the right of the field of view. Finally, the pixel equivalent changes in the entire field of view can be obtained. In order to reduce the influence of random errors, the present invention uses the average of the pixel equivalents at different positions in the entire field to calculate the displacement.

[0049] 4. Get the translation image. Replace the full-striped line ruler with a partial-striped line ruler for measurement, adjust the sampling interval of the fringe image according to the relative position of the stripes in the field of view, drive the electric linear displacement slide to drive the LED backlight and the partial-striped line ruler to move, and the linear array camera can capture the translational fringe image.

[0050]

[0051] Table 1 Fringe images at different positions

[0052] Table 1 shows a series of images captured by the camera when the stripes move to different positions during the measurement process. It can be seen that in any field of view, there is a complete set of stripes that match the previously selected reference position stripes, so that the accuracy of the matching between the stripe images at different positions can be guaranteed. Moreover, before the first set of stripes disappears from the field of view, the second set of different stripes will appear, thus ensuring the continuity of the matching, so that a large range of measurements can be achieved in a single field of view.

[0053] 5. Sub-pixel registration. One of the key steps of the image grating displacement measurement system is to obtain the number of pixels of the displacement between the fringe images before and after the displacement. In order to obtain the sub-pixel displacement of the characteristic target in the image faster and more accurately, this paper adopts the DFT (discrete Fourier transform) local upsampling phase correlation method, which is briefly described as follows: Figure 5 As shown in the flowchart, the whole process can be divided into two steps as follows:

[0054] The first step is to locate the integer pixel level and obtain the integer pixel displacement T. First, the image acquisition module is used to obtain two stripe images before and after the displacement, and then the Fourier transform is used to convert the translation in the spatial domain into the phase shift in the frequency domain. After that, the normalized cross-power spectrum of the two frequency domain expressions is taken to obtain the pure phase change representing the image translation, which is also called the cross-correlation function R(u,v) in the frequency domain, that is:

[0055] Finally, by inversely transforming the cross-correlation function, we can get the function δ that represents the number of pixels translated between images, that is:

[0056] R(x,y)=IFT[R(u,v)]=δ(x-x0,y-y0).

[0057] When the measurement accuracy needs to exceed the limit of integer pixels, that is, when the sampling multiple is greater than 1, the second step is to perform sub-pixel positioning to obtain the sub-pixel displacement ΔT. The core of this step is to refine the two-dimensional coordinates in the local area around the peak position in the spatial domain while keeping the power spectrum unchanged, and then use the sinc function to implement interpolation during the inverse transformation.

[0058] After obtaining the integer pixel position (x0, y0) in the first step, a 1.5X1.5 pixel area is taken with (x0, y0) as the center, and then the DFT matrix multiplication is performed k times upsampling in the area. After the selected area is refined, the power spectrum function R(U, V) is subjected to the normalized inverse Fourier transform of the matrix multiplication to obtain the interpolated two-dimensional delta function r(X', Y').

[0059]

[0060] in,

[0061]

[0062] Finally, the number of pixels T+ΔT of translation with sub-pixel accuracy is obtained.

[0063] 6. Obtain the actual displacement value. Multiply the pixel equivalent calibrated in step 3 by the number of translation pixels obtained in step 5 to obtain the actual displacement of the linear scale.

[0064] In summary, the image grating-based linear displacement measurement system and measurement method proposed in the present invention solve the problem of insufficient installation flexibility of traditional grating sensors. Compared with previous visual measurement methods, the measurement accuracy is higher and the range is larger.

Claims

1. A method for measuring linear displacement using a linear displacement measurement system based on an image grid, characterized in that: The linear displacement measurement system based on the image grid includes an LED backlight source, a glass linear scale, a displacement drive module, an image acquisition module, and a data processing and display module; the glass linear scale is placed on the LED backlight source and is allowed to perform translational motion with the linear displacement slide in the displacement drive module, stripe images at different positions are acquired by the image acquisition module, and then the images before and after the translation are phase correlated by the data processing and display module to obtain the number of pixels translated between the images, and the actual displacement value of the displacement stage is obtained according to the obtained number of pixels and the proportional relationship between the object and the image, the glass linear scale includes two types of stripe linear scales: one is a full-striped linear scale for focusing, adjusting the verticality of the camera optical axis, and calibrating pixel equivalents; the other is a partial-striped linear scale for realizing displacement measurement, and specially designed stripes are engraved on its surface, the image acquisition module includes an ultra-low distortion lens group, a high-resolution linear array camera, and a computer; the linear displacement measurement method includes the following steps: (1) Focus: Place the full-striped linear ruler on the LED backlight source, and adjust the position of the linear array camera and the lens group up and down to make a clear stripe image appear in the camera; (2) Adjust the verticality of the camera optical axis: According to the distribution of the number of pixels occupied by the fringe images of a specified number at each position in the full field of view, adjust the camera optical axis so that it is perpendicular to the plane of the line scale; (3) Pixel equivalent calibration: Divide the actual length of a specified number of stripes by the number of pixels they occupy on the image plane to obtain the pixel equivalent value used to calculate the displacement of the line scale; (4) Obtaining a translational image: Replace the full-striped linear scale with a partial-striped linear scale for measurement, drive the electric linear displacement slide, drive the LED backlight and the partial-striped linear scale to move, and the linear array camera can capture the translational fringe image; (5) Sub-pixel registration: Use the discrete Fourier transform local upsampling phase correlation method to obtain the number of pixels translated between images with sub-pixel accuracy; (6) Obtaining the actual displacement value: multiplying the pixel equivalent calibrated in step (3) by the number of translation pixels obtained in step (5) to obtain the actual displacement of the glass linear scale.

2. The linear displacement measurement method according to claim 1, characterized in that: In step (1), the brightness of the LED backlight source and the exposure time of the line array camera are adjusted to allow the linear scale to be clearly imaged at the middle position of the depth of field.

3. The linear displacement measurement method according to claim 1, characterized in that: In step (4), each group of stripes of the partial stripe line ruler is arranged separately on the line ruler, and the width of a line pair in each group of stripes can be set between 90 μm and 150 μm, with a total width of 3600 μm.

4. The linear displacement measurement method according to claim 3, characterized in that: In step (4), the stripe design of the partial stripe line ruler also needs to meet the following conditions: (i) the characteristics of at least three adjacent groups of stripes are different; and (ii) the interval between every two groups of stripes is sufficient to fill the camera field of view.

Images