Correction method of zero-crossing offset

Abstract

The present invention provides a correction method for zero-crossing offset, wherein a sampling interval is set according to the linear interval length of a differential confocal signal curve, and the differential confocal signal curve is calibrated, and the original data obtained by sampling is screened according to preset conditions, and then the interpolation method is used to obtain evenly spaced differential signal data, and then the calibration data and the most suitable standard data in the original data are obtained in combination with the constraint algorithm of cosine similarity, so as to obtain an accurate zero-crossing position. The correction method provided by the present invention solves the contradiction between the measurement rate and the sampling accuracy, and is very suitable for practical engineering applications. While increasing the sampling step length and the sampling rate, the robustness of the accuracy is guaranteed, and through the correction method provided, both the measurement speed can be increased and the uniform sampling interval can be obtained, and the measurement error caused by the zero-crossing offset is avoided, so that the differential confocal measurement system obtains a larger application space and more application occasions.

Description

Technical Field

[0001] The present invention relates to the field of optical detection technology, and in particular to a method for correcting zero-crossing offset. Background Art

[0002] In recent years, with the deepening of research on nano-scale technologies such as ultra-precision machining and their wider application, ultra-precision detection, as a key link, is of great importance. Traditional contact measurement technology is easy to damage the measuring components and has poor real-time performance, so it is no longer used in the field of ultra-precision detection. Sub-micron or even nano-level measurement accuracy has brought new challenges to the research of non-contact measurement methods.

[0003] Laser differential confocal microscopy (LDCM) is a modern optical microscopy technology with high resolution, which is widely used in biomedicine and optical precision measurement. LDCM uses two symmetrically placed defocus pinhole detection light paths. When the surface of the test piece is longitudinally scanned, the light intensity signals of the two detection light paths are collected, and the axial light intensity response curve is obtained by differential processing. After processing, the height information of the scanning position of the test piece can be obtained. LDCM detection technology has the characteristics of significantly improving the focus recognition ability, environmental anti-interference ability and linearity near the focus, and has the characteristics of fast measurement speed and high measurement accuracy.

[0004] However, the laser differential confocal microscope technology in the prior art has the following disadvantages: the laser differential confocal microscope technology obtains the differential confocal signal curve through the difference of the data collected by two detectors, and uses the objective lens driver to carry the objective lens to scan the points on the measured surface to obtain their spatial coordinates. In order to increase the detection rate, the sampling interval needs to be increased, but this will cause a large zero-crossing offset, resulting in measurement errors. At the same time, the accuracy of the scanning mechanism of the differential confocal microscope in the prior art cannot guarantee uniformly spaced sampling, and the uneven sampling interval will also bring about a considerable error, which will cause the discrete data points on the differential confocal signal curve to be unevenly distributed, resulting in a decrease in measurement accuracy. Summary of the invention

[0005] The purpose of the present invention is to overcome the defects of the prior art and to propose a correction method for zero-crossing offset.

[0006] To achieve the above object, the present invention adopts the following specific technical solutions:

[0007] A method for correcting zero-crossing offset, the method comprising the following steps:

[0008] S1, placing a plane reflector in a differential confocal measurement system, obtaining a differential confocal signal curve, and setting a sampling interval in a linear interval of the differential confocal signal curve so that the linear interval contains at least three sampling points;

[0009] S2. Calibrate the differential confocal signal curve of the differential confocal measurement system using a plane reflector: obtain a first differential signal sequence as calibration data, and filter the data in the first differential signal sequence according to a first preset condition;

[0010] S3, placing the object to be measured in a differential confocal measurement system, performing equal-interval sampling with the sampling interval as the scanning measurement step length of the differential confocal measurement system, obtaining position information and a second differential signal corresponding to the position information, obtaining a second differential signal sequence, filtering data in the second differential signal sequence according to a first preset condition, and performing interpolation processing on the filtered second differential signal sequence using interpolation;

[0011] S4. Based on cosine similarity, determine a group of differential signals with the highest similarity between the filtered first differential signal sequence and the interpolated second differential signal sequence, obtain first position information corresponding to the group of differential signals, filter the interpolated second differential signal sequence according to a second preset condition, and obtain the corresponding second position information, and obtain the offset corrected zero-crossing point position information in combination with the first position information and the second position information.

[0012] Furthermore, the specific steps of the calibration in step S2 are:

[0013] S201, placing a plane reflector at a focal position of a differential confocal measurement system, performing equal-interval sampling with the sampling interval as a scanning measurement step length of the differential confocal measurement system, changing the position of the plane reflector with a preset offset and tilt angle, collecting position information of a measurement point of the plane reflector at different offsets and tilt angles and first differential signals corresponding to the different position information, and acquiring a first differential signal sequence;

[0014] S202: Filter data in the first differential signal sequence according to a first preset condition.

[0015] Furthermore, the first preset condition for screening is specifically:

[0016] The differential signal data of the t1th sampling point is obtained, and the differential signal data of the sampling points t1-N to t1+N are extracted; the differential signal data of the t1th sampling point satisfies: the differential signal strength of the t1th sampling point is greater than or equal to 0, and the differential signal strength of the t1+1th sampling point is less than 0; wherein N>1.

[0017] Furthermore, the second preset condition for screening is specifically:

[0018] The differential signal data of the t2th sampling point is obtained, wherein the differential signal data of the t2th sampling point: the differential signal strength of the t2th sampling point is greater than or equal to 0, and the differential signal strength of the t2+1th sampling point is less than 0.

[0019] Furthermore, the preset offset and tilt angle change the position of the plane reflector specifically as follows:

[0020] Place the plane mirror at the focal position of the differential confocal measurement system and The position of the plane reflector is changed by an offset and a tilt angle of [0, Δθ, ..., n×Δθ];

[0021] Wherein, m and n represent the equal division coefficients of the offset and the tilt angle respectively; Δd represents the sampling interval; and Δθ represents the unit tilt.

[0022] Furthermore, the specific steps of step S3 are:

[0023] S301, placing the object to be measured at the focal position of the differential confocal measurement system, performing equal-interval sampling with the sampling interval as the scanning measurement step length of the differential confocal measurement system, and acquiring position information and a second differential signal sequence corresponding to the position information;

[0024] S302, screening the data in the second differential signal sequence according to a first preset condition;

[0025] S303, determine the interpolation offset, and use the linear interpolation method to interpolate the filtered second differential signal sequence; the interpolation offset is determined by formula group (1), and the linear interpolation is calculated by formula (2) and formula group (3):

[0026]

[0027] Where ε represents the interpolation offset; Δd represents the sampling interval; d j represents the position information corresponding to the jth interpolation point in the second differential signal sequence after screening; d1 represents the position information corresponding to the first interpolation point in the second differential signal sequence after screening; d j -d1+ε represents the jth interpolation point in the second differential signal sequence after screening, which is the same as d j The distance between; rem represents the remainder operator;

[0028] d j ′=d1+ε+(j-1)×Δd (2)

[0029]

[0030] Among them, i j ' represents the differential signal strength corresponding to the jth interpolation point in the second differential signal sequence after interpolation, d j ' represents the position information corresponding to the jth interpolation point in the second differential signal sequence after interpolation; d j+1 represents the position information corresponding to the j+1th interpolation point in the second differential signal sequence after screening; d j-1 represents the position information corresponding to the j-1th interpolation point in the second differential signal sequence after screening; i j represents the differential signal strength corresponding to the j-th interpolation point in the second differential signal sequence after screening; i j-1 represents the differential signal strength corresponding to the j-1th interpolation point in the second differential signal sequence after screening; i j+1 represents the differential signal strength corresponding to the j+1th interpolation point in the second differential signal sequence after screening; j∈{1,…2N+1}.

[0031] The second differential signal sequence after screening is expressed as The second differential signal sequence after interpolation is expressed as

[0032] Furthermore, the specific steps of step S4 are:

[0033] S401, based on cosine similarity, determining a group of differential signals with the highest similarity between the first differential signal sequence after screening and the second differential signal sequence after interpolation; the cosine similarity formula is determined by formula group (4);

[0034]

[0035] in, represents a differential signal sequence after filtering the data in the first differential signal sequence according to a first preset condition;

[0036] S402, obtaining position information corresponding to the set of differential signals, screening the interpolated second differential signal sequence according to a second preset condition, and obtaining corresponding second position information;

[0037] S403: Acquire offset-corrected zero-crossing point position information by combining the first position information and the second position information.

[0038] Furthermore, the linear interval in step S1 is specifically:

[0039] The linear interval of the differential confocal signal curve satisfies: the slope of any point in the linear interval ≥ the zero-crossing slope / 2.

[0040] The present invention proposes a correction method for zero-crossing offset, which plans the sampling interval, uses the interpolation method to obtain the differential signal data with uniform intervals after screening the original data by preset conditions, and then obtains the most suitable standard data in combination with the constraint algorithm of cosine similarity, so as to obtain the accurate zero-crossing position. The correction method provided by the present invention solves the contradiction between the measurement rate and the sampling accuracy, and is very suitable for practical engineering applications. While increasing the sampling step length and the sampling rate, the robustness of the accuracy is guaranteed, and through the correction method provided, the requirements for the scanning mechanism of the differential confocal measurement system are reduced, and the requirements for the sampling uniformity of the scanning mechanism are relaxed, so that the differential confocal measurement system obtains a larger application space and more application occasions. It solves the problem that the accuracy of the scanning mechanism of the differential confocal microscope in the prior art cannot guarantee uniformly spaced sampling, resulting in uneven distribution of discrete data points on the differential confocal signal curve. The correction method provided by the present invention can both improve the measurement speed and obtain uniform sampling intervals, avoid measurement errors caused by zero-crossing offset, and improve measurement accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 is a schematic diagram of a differential confocal measurement system according to an embodiment of the present invention;

[0042] Figure 2 4 is a flow chart of a method for correcting zero-crossing offset according to an embodiment of the present invention.

[0043] Reference numerals include:

[0044] Laser light source 1, collimating lens 2, first beam splitter 3, focusing lens 4, object to be measured 5, second beam splitter 6, post-focus focusing lens 7, post-focus pinhole 8, post-focus photoelectric detector 9, pre-focus focusing lens 10, pre-focus pinhole 11, pre-focus photoelectric detector 12. DETAILED DESCRIPTION

[0045] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, the same modules are represented by the same reference numerals. In the case of the same reference numerals, their names and functions are also the same. Therefore, the detailed description thereof will not be repeated.

[0046] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and do not constitute a limitation of the present invention.

[0047] The specific working mode of the present invention is described in detail below with reference to the accompanying drawings:

[0048] In order to explain the embodiments of the present invention in detail, the differential confocal measurement system in the prior art is described in detail. The differential confocal measurement system used in the embodiments of the present invention is as follows: Figure 1 As shown, it includes a laser light source 1, a collimator 2, a first beam splitter 3, a focusing lens 4, a second beam splitter 6, a pre-focus focusing lens 10, a pre-focus pinhole, a pre-focus photodetector 12, a post-focus focusing lens 7, a post-focus pinhole 8, a post-focus photodetector 9, an interferometer, and a transmission assembly arranged in sequence along the optical path. The transmission assembly used in the embodiment of the present invention is an objective lens positioner. The transmission assembly is fixedly connected to the focusing lens 4, and the transmission assembly controls the focusing lens 4 to move back and forth at a uniform speed along the direction of the laser beam to control the imaging position of the surface to be measured, that is, the focal position. The interferometer is used to measure the displacement value of the focusing lens 4. The laser light source 1 emits a laser beam, which is collimated by the collimator 2 and then incident on the first beam splitter 3 for beam splitting. The transmitted light beam transmitted by the first beam splitter 3 is focused by the focusing lens 4 and then incident on the object to be measured 5, and then reflected by the object to be measured 5 along the original optical path, and then reflected by the first beam splitter 3 to the second beam splitter 6 for beam splitting. The light beam reflected by the second beam splitter 6 is used as the pre-focus light beam, which is incident on the pre-focus photodetector 12 via the pre-focus focusing lens 10 and the pre-focus pinhole. The light beam transmitted by the second beam splitter 6 is used as the post-focus light beam, which is incident on the post-focus photodetector 9 via the post-focus focusing lens 7 and the post-focus pinhole 8. The light intensity data of the pre-focus photodetector 12 and the post-focus photodetector 9 are collected and recorded as I(u, -u m ) and I(u, +u m The light intensity data of the pre-focus photodetector 12 is subtracted from the light intensity data of the post-focus photodetector 9 to obtain the differential confocal signal curve FES = I(u, -u m )-I(u,+u m ), and obtain the differential signal data of the differential confocal signal curve. When the light beam is focused on the surface of the object 5 to be measured, the light intensity received by the pre-focus photodetector 12 and the post-focus photodetector 9 is equal. The position of the focusing lens 4 is recorded as u by the interferometer, that is, the displacement value of the focusing lens 4 corresponding to the zero-crossing position measured by the interferometer is u, and the position information u of the surface of the object 5 to be measured is obtained. Among them, FES represents the differential confocal characteristic curve, u m represents the defocus amount, and u represents the position information of the point to be measured on the surface of the object to be measured 5.

[0049] The correction method provided by the present invention can be applied to different differential confocal measurement systems in the prior art. Therefore, the differential confocal measurement system may have other forms of changes, which are subject to actual usage. The present invention is not limited to this, and only gives examples to explain the embodiments of the present invention in detail.

[0050] The embodiment of the present invention provides a method for correcting zero-crossing offset, such as Figure 2 As shown, the correction method includes the following steps:

[0051] S1. Place a plane reflector in a differential confocal measurement system, collect and obtain the light intensity data of the pre-focus photodetector 12 and the post-focus photodetector 9, and then perform subtraction to obtain a differential confocal signal curve, and set a sampling interval within the linear interval of the differential confocal signal curve. The linear interval of the differential confocal signal curve satisfies: the slope of any point in the linear interval ≥ the zero-crossing slope / 2, so that the linear interval contains at least three sampling points.

[0052] The shape of the differential confocal signal curve is mainly affected by the tilt angle of the object to be measured 5. In the example of the present invention, the transmission component, that is, the minimum moving interval of the objective lens positioner is 3nm, and the measurement range of the differential confocal measurement system is a tilt angle range of 0-5°. Therefore, the shortest length l of the linear area of ​​the differential confocal signal curve generated at different tilt angles is determined. The embodiment of the present invention sets the sampling interval Δd to take a value of 1 / 3 to ensure that the differential signal has three sampling points in the linear area. Among them, Δd represents the sampling interval, and l represents the shortest length of the linear area. The linear interval contains at least three samples in order to retain the quadratic curvature characteristics of the subsequent sampling signal in the linear interval as much as possible, so as to make the zero-crossing position information obtained by the correction method provided by the present invention more accurate.

[0053] The sampling interval can also be obtained by simulation in the prior art, which is not limited in the present invention and can be selected according to actual conditions. In the embodiment of the present invention, step S1 uses the linear interval length of the differential confocal signal curve to determine the sampling interval. The method used in step S1 to obtain the linear interval length is more accurate than simulation.

[0054] S2. Calibrate the differential confocal signal curve of the differential confocal measurement system using a plane reflector: obtain a first differential signal sequence as calibration data, and filter the data in the first differential signal sequence according to a first preset condition.

[0055] S201, place the plane mirror at the focal position of the differential confocal measurement system, that is, Figure 1 The position of the object 5 under test is measured. The scanning measurement step length is 1 / 3. The position of the plane reflector is changed by the offset and the tilt angle [0, Δθ, ..., n×Δθ], and the differential confocal measurement system is controlled to perform equally spaced sampling. Wherein, m and n represent the equal division coefficients of the offset and the tilt angle respectively; Δd represents the sampling interval; Δθ represents the unit tilt. In the embodiment of the present invention, m is 100, n is 50, and Δθ is 0.1°. The position information of the measuring point of the plane reflector at different offsets and tilt angles and the first differential signal corresponding to the different position information are collected to obtain the first differential signal sequence as calibration data. Axial scanning is performed using the transmission assembly, and the position of the focusing lens 4 is recorded by the interferometer rangefinder during the scanning process to obtain 2m×n groups of first differential signal sequences. Wherein, i represents the first differential signal strength, d represents the position information corresponding to each first differential signal sampling point, and k represents the number of sampling points of each differential signal sequence. In the embodiment of the present invention, the value of k is 100.

[0056] The larger the m and n values ​​are, the higher the measurement resolution is. Even if the same differential confocal signal curve is sampled at the same sampling interval, the sampling results are different for different offsets, so it is necessary to use different offsets to obtain the differential signal sampling results under different offsets as much as possible. Under different tilt angles, the shape of the differential confocal signal curve is different, so different tilt angles are used to obtain the corresponding differential signal sampling results. The differential signals of different offsets and tilt angles are collected in order to find the group of calibration data with the highest similarity to the actual measurement data based on cosine similarity during the subsequent actual measurement of the object to be measured 5. The higher the similarity, the higher the accuracy of the zero-crossing position obtained by the correction method provided in the embodiment of the present invention.

[0057] S202: According to a first preset condition, the first differential signal sequence The data in is filtered, and the first differential signal sequence after filtering is expressed as Among them, u1...u 2N+1 Respectively represent the first differential signal strength after screening, d1...d 2N+1 Respectively represent the position information corresponding to each first differential signal sampling point after screening. The differential signal data of the t1th sampling point is extracted, the differential signal strength of the t1th sampling point is greater than or equal to 0, and the differential signal strength of the t1+1th sampling point is less than 0, and the differential signal data of the sampling points t1-N to t1+N are extracted, and the total data is 2N+1; wherein, N>1. The value of N in the embodiment of the present invention is 5. The larger the value of N, the higher the measurement resolution, but the amount of calculation will also increase, which will also cause the measurement speed to slow down. Therefore, the value of N can be set according to the actual situation to balance the resolution and the measurement rate, and the present invention is not limited to this.

[0058] S3, placing the object to be measured 5 in a differential confocal measurement system, performing equal-interval sampling with a sampling interval of 1 / 3 as the scanning measurement step of the differential confocal measurement system, obtaining position information and a second differential signal corresponding to the position information, obtaining a second differential signal sequence, filtering the data in the second differential signal sequence according to the first preset condition, and using interpolation to interpolate the filtered second differential signal sequence.

[0059] S301, placing the object to be measured 5 at the focal position of the differential confocal measurement system, taking a sampling interval of 1 / 3 as the scanning measurement step of the differential confocal measurement system to perform equal-interval sampling, and obtaining position information and a second differential signal sequence corresponding to the position information in, represents the second differential signal sequence.

[0060] S302: Screening the second differential signal sequence The second differential signal that meets the following conditions: the differential signal data of the t1th sampling point, the differential signal strength of the t1th sampling point is greater than or equal to 0, and the differential signal strength of the t1+1th sampling point is less than 0, and the differential signal data of the t1-Nth to t1+Nth sampling points are extracted, and the total data is 2N+1. The second differential signal sequence after screening is represented by Where N>1, i a1 ...i a2N+1 Respectively represent the second differential signal intensity after screening, d a1 ...d a2N+1 Respectively represent the position information corresponding to each second differential signal sampling point after screening.

[0061] S303, first use formula group (1) to determine the interpolation offset ε.

[0062]

[0063] Where ε represents the interpolation offset; Δd represents the sampling interval; d aj represents the position information corresponding to the jth interpolation point in the second differential signal sequence after screening; d a1 Indicates the position information corresponding to the first interpolation point in the second differential signal sequence after screening; d aj -d a1 +ε indicates the difference between the jth interpolation point in the second differential signal sequence after screening and d aj The distance between them; rem represents the remainder operator, that is, d aj -d a1 +ε remainder with respect to Δd.

[0064] The interpolation points using the interpolation algorithm should be infinitely close to the original data points.aj -d a1 +ε is the remainder with respect to Δd, which is the jth interpolation point, and is related to d aj The distance between them. Formula group (1) can minimize the sum of the distances between all interpolation points and the original data by summing and then performing argmin operation. The correction method provided by the present invention can avoid measurement errors caused by zero-crossing offset by combining uniform sampling intervals with interpolation data processed by an algorithm, thereby improving measurement accuracy.

[0065] Then use the linear interpolation method to filter the second differential signal sequence Interpolation processing is performed. The second differential signal sequence after interpolation processing is expressed as Among them, i1...i 2N+1 'represent the differential signal strength after interpolation, d1'...d 2N+1 'respectively represent the position information corresponding to each differential signal sampling point after interpolation. The position information after linear interpolation is calculated by formula (2), and the differential signal strength after interpolation is calculated by formula group (3).

[0066] d j '=d a1 +ε+(j-1)×Δd(2)

[0067]

[0068] Among them, i j ' represents the differential signal strength corresponding to the jth interpolation point in the second differential signal sequence after interpolation, d j ' represents the position information corresponding to the jth interpolation point in the second differential signal sequence after interpolation; d aj+1 represents the position information corresponding to the j+1th interpolation point in the second differential signal sequence after screening; d aj-1 represents the position information corresponding to the j-1th interpolation point in the second differential signal sequence after screening; i aj represents the differential signal strength corresponding to the j-th interpolation point in the second differential signal sequence after screening; i aj-1 represents the differential signal strength corresponding to the j-1th interpolation point in the second differential signal sequence after screening; i aj+1 Represents the differential signal strength corresponding to the j+1th interpolation point in the second differential signal sequence after screening; j∈{1,…2N+1}.

[0069] S4. Determine the first differential signal sequence after screening based on cosine similarity And the second differential signal sequence after interpolation A group of differential signals with the highest similarity in the differential signal sequence is obtained, and the first position information corresponding to the group of differential signal sequences is obtained. The interpolated second differential signal sequence is screened according to the second preset condition, and the corresponding second position information is obtained. The offset corrected zero-crossing point position is obtained by combining the first position information and the second position information.

[0070] S401: Determine the first differential signal sequence after screening based on cosine similarity And the second differential signal sequence after interpolation The cosine similarity formula is determined by formula group (4);

[0071]

[0072] in, represents a differential signal sequence after filtering the data in the first differential signal sequence according to the first preset condition;

[0073] S402, obtaining the position information F corresponding to a group of differential signal sequences with the highest similarity p,q , F p,q As the first position information. The second differential signal sequence U after screening and interpolation interp The following conditions are met: Get the differential signal data of the t2th sampling point, the differential signal data of the t2th sampling point: the differential signal strength of the t2th sampling point is greater than or equal to 0, and the t 2+1 The differential signal strength of the sampling point is less than 0. Get the position information d corresponding to the t2th sampling point t2 ', that is, the position information corresponding to the differential signal d t2 ',d t2 ' as the second position information.

[0074] S403, combining the first position information and the second position information, obtaining the offset corrected zero-crossing position information: F p,q +d t2 '.

[0075] The zero crossing point obtained by the differential confocal measurement system scan does not move with the inclination of the measured surface. However, the slope at the zero crossing point changes with the inclination angle of the object to be measured 5. The greater the inclination of the object to be measured 5, the smaller the slope, that is, the lower the sensitivity and resolution. Therefore, under the premise that the noise remains unchanged, if the object to be measured 5 is tilted, it will cause the value obtained by the zero crossing point to change, and the inaccurate zero crossing point position information will affect the measurement accuracy. The correction method for the zero crossing point offset provided by the present invention, by planning the sampling interval, the original data is screened by preset conditions, and then the interpolation method is used to obtain the differential signal data with uniform intervals, and then the most suitable standard data is obtained in combination with the constraint algorithm of cosine similarity, so as to obtain the accurate zero crossing point position. The correction method provided by the present invention solves the contradiction between the measurement rate and the sampling accuracy, and is very suitable for practical engineering applications. While increasing the sampling step length and the sampling rate, the robustness of the accuracy is guaranteed. The correction method provided reduces the requirements on the scanning mechanism of the differential confocal measurement system and relaxes the requirements on the sampling uniformity of the scanning mechanism, so that the differential confocal measurement system has a larger application space and more application occasions.

[0076] The present invention verifies the effectiveness of the correction method of the technical solution of the present invention through experiments. The system parameters of the selected differential confocal measurement system are as follows: the wavelength of the laser light source 1 is 642nm, and after being collimated by the collimator 2, the collimation is a Gaussian collimated beam with a radius of 3.2mm. The focusing lens 4 uses a microscope with a focal length of 9mm, the front focusing lens 10 and the rear focusing lens 7 use a plano-concave lens with a focal length of 10mm, the front focus pinhole and the rear focus pinhole 8 have an offset of 650μm, and the pinhole size is 20μm.

[0077] Through experimental comparison, the measured surface of the object to be measured 5 is randomly placed in different positions of the differential confocal measurement system within the range of inclination angle [0°, 5°], and measured using a dual-frequency laser interferometer. A total of 1000 groups of measurement experiments were conducted. The correction method provided by the embodiment of the present invention is compared with the traditional method of linear fitting the linear interval to extract the zero-crossing point for ranging. The comparison results are as follows:

[0078] Comparison Table

[0079] Method of the present invention Traditional methods Average distance measurement error (nm) 48.15nm 117.42nm

[0080] By comparison, the measurement accuracy of the technical solution of the present invention and the traditional method is improved by 59%. It can be seen that the correction method provided by the present invention can not only improve the measurement speed but also obtain a uniform sampling interval, avoid the measurement error caused by the zero-crossing offset, and improve the measurement accuracy.

[0081] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0082] Although the embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and cannot be understood as limiting the present invention. Those skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention.

[0083] The above specific implementations of the present invention do not constitute a limitation on the protection scope of the present invention. Any other corresponding changes and modifications made based on the technical concept of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A method for correcting zero-crossing offset, characterized in that: The correction method comprises the following steps: S1, placing a plane reflector in a differential confocal measurement system, obtaining a differential confocal signal curve, and setting a sampling interval in a linear interval of the differential confocal signal curve so that the linear interval contains at least three sampling points; S2. Calibrate the differential confocal signal curve of the differential confocal measurement system using a plane reflector: obtain a first differential signal sequence as calibration data, and filter the data in the first differential signal sequence according to a first preset condition; S3, placing the object to be measured in a differential confocal measurement system, performing equal-interval sampling with the sampling interval as the scanning measurement step length of the differential confocal measurement system, obtaining position information and a second differential signal corresponding to the position information, obtaining a second differential signal sequence, filtering data in the second differential signal sequence according to a first preset condition, and performing interpolation processing on the filtered second differential signal sequence using interpolation; S4. Based on the cosine similarity, determine a group of differential signals with the highest similarity between the first differential signal sequence after screening and the second differential signal sequence after interpolation, and obtain position information corresponding to the group of differential signals with the highest similarity as the first position information; The interpolated second differential signal sequence is screened according to a second preset condition, and the corresponding second position information is obtained, and the offset corrected zero-crossing point position information is obtained by combining the first position information and the second position information; The first preset condition for screening is specifically: Acquire the differential signal data of the t1th sampling point, and extract the differential signal data of the t1-Nth to t1+Nth sampling points; the differential signal data of the t1th sampling point satisfies: the differential signal strength of the t1th sampling point is greater than or equal to 0, and the differential signal strength of the t1+1th sampling point is less than 0; wherein N>1; The second preset condition for screening is specifically: The differential signal data of the t2th sampling point is obtained, and the differential signal data of the t2th sampling point satisfies: the differential signal strength of the t2th sampling point is greater than or equal to 0, and the differential signal strength of the t2+1th sampling point is less than 0.

2. The correction method according to claim 1, characterized in that: The specific steps of the calibration in step S2 are: S201, placing a plane reflector at a focal position of a differential confocal measurement system, performing equal-interval sampling with the sampling interval as a scanning measurement step length of the differential confocal measurement system, changing the position of the plane reflector with a preset offset and tilt angle, collecting position information of a measurement point of the plane reflector at different offsets and tilt angles and first differential signals corresponding to the different position information, and acquiring a first differential signal sequence; S202: Filter data in the first differential signal sequence according to a first preset condition.

3. The correction method according to claim 2, characterized in that: The preset offset and tilt angle change the position of the plane reflector specifically as follows: Place the plane mirror at the focal position of the differential confocal measurement system and The position of the plane reflector is changed by an offset and a tilt angle of [0, △θ, ..., n×△θ]; Wherein, m and n represent the equal division coefficients of the offset and the tilt angle respectively; △d represents the sampling interval; △θ represents the unit tilt.

4. The correction method according to claim 3, characterized in that: The specific steps of step S3 are: S301, placing the object to be measured at the focal position of the differential confocal measurement system, performing equal-interval sampling with the sampling interval as the scanning measurement step length of the differential confocal measurement system, and acquiring position information and a second differential signal sequence corresponding to the position information; S302, screening the data in the second differential signal sequence according to a first preset condition; S303, determine the interpolation offset, and use the linear interpolation method to interpolate the filtered second differential signal sequence; the interpolation offset is determined by formula group (1), and the linear interpolation is calculated by formula (2) and formula group (3): Among them, ε represents the interpolation offset; △d represents the sampling interval; d j represents the position information corresponding to the jth interpolation point in the second differential signal sequence after screening; d1 represents the position information corresponding to the first interpolation point in the second differential signal sequence after screening; d j -d1+ε indicates the difference between the jth interpolation point in the second differential signal sequence after screening and d j The distance between; rem represents the remainder operator; d j ′=d1+ε+(j-1)×△d (2) Among them, i j ' represents the differential signal strength corresponding to the jth interpolation point in the second differential signal sequence after interpolation, d j ' represents the position information corresponding to the jth interpolation point in the second differential signal sequence after interpolation; d j+1 represents the position information corresponding to the j+1th interpolation point in the second differential signal sequence after screening; d j-1 represents the position information corresponding to the j-1th interpolation point in the second differential signal sequence after screening; i j represents the differential signal strength corresponding to the j-th interpolation point in the second differential signal sequence after screening; i j-1 represents the differential signal strength corresponding to the j-1th interpolation point in the second differential signal sequence after screening; i j+1 represents the differential signal strength corresponding to the j+1th interpolation point in the second differential signal sequence after screening; j∈{1,…2N+1}; The second differential signal sequence after screening is expressed as The second differential signal sequence after interpolation is expressed as 5. The correction method according to claim 4, characterized in that: The specific steps of step S4 are: S401, based on cosine similarity, determining a group of differential signals with the highest similarity between the first differential signal sequence after screening and the second differential signal sequence after interpolation; The cosine similarity formula is determined by formula group (4); in, represents a differential signal sequence after filtering the data in the first differential signal sequence according to a first preset condition; S402, obtaining position information corresponding to the set of differential signals, screening the interpolated second differential signal sequence according to a second preset condition, and obtaining corresponding second position information; S403: Acquire offset-corrected zero-crossing point position information by combining the first position information and the second position information.

6. The correction method according to claim 1, characterized in that: The linear interval in step S1 is specifically: The linear interval of the differential confocal signal curve satisfies: the slope of any point in the linear interval ≥ the zero-crossing slope / 2.

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