A transparent flat plate two-dimensional thickness measurement system and method based on line spectrum confocal technology

Abstract

The present invention discloses a transparent flat plate two-dimensional thickness measurement system and method based on line spectrum confocal technology. The light emitted by a wide spectrum linear array LED light source is transmitted to a slit through an optical fiber bundle, and then dispersed on the confocal plane through a symmetrical confocal linear dispersion mirror group. The linear light spot focused on the surface of the transparent flat plate will be reflected back and split by a line imaging spectrometer. The spectrum-distance mapping relationship is decoded by a microprocessor to obtain the thickness measurement result on the confocal line. With the help of a two-dimensional precision translation stage, the two-dimensional surface thickness measurement of a large area transparent flat plate can be achieved. This method breaks through the problems existing in the traditional spectral confocal sensor for thickness measurement and single-point thickness measurement, and has the advantages of simple measuring device, easy operation, easy integration in an automated production line, and high efficiency in two-dimensional surface thickness measurement.

Description

Technical Field

[0001] The invention relates to the technical field of precision optical measurement, and in particular to a transparent flat plate two-dimensional thickness measurement system and method based on line spectrum confocal technology. Background Art

[0002] With the rapid development of science and technology and precision manufacturing, higher requirements are placed on the precise measurement of the three-dimensional morphology of various transparent material products. Spectral confocal sensors, as an optical non-contact displacement sensor developed in recent years, are widely used in the field of industrial automation. Because of its ability to break through the optical diffraction limit and achieve super-resolution measurement, its measurement accuracy can often reach tens of nanometers. However, in addition to the need to characterize the unilateral morphology of various transparent flat parts including mobile phone screens, thickness is also an important parameter that determines their working performance. Traditional measurement methods often require differential comparison through cross-beam to obtain the thickness of transparent flat parts.

[0003] Chinese patent application CN111928788A discloses a bidirectional spectral confocal flat plate thickness detection system and its dual optical axis calibration method, which is achieved by setting spectral confocal probes on both sides of the measured flat plate and fine-tuning the five-axis displacement platform so that the two probes are located on the same axis, and finally used for thickness measurement of the measured flat plate after parallelism calibration. However, this method has many disadvantages such as complex measuring device, cumbersome assembly and operation process, high cost, and difficulty in realizing automated detection. Chinese patent application CN112229338A discloses a dual-spectral confocal thickness measurement method without standard plate zeroing. This method avoids the process of using a standard thickness plate for zeroing in the opposite-beam thickness measurement by setting reference surfaces for the two sensors. Although this method omits the process of zeroing the device for thickness measurement, it also introduces the additional operation process of requiring the sensors to set reference surfaces for each other. At the same time, this method also has a series of disadvantages existing in the above-mentioned opposite-beam thickness measurement method.

[0004] In addition to the above problems, the currently disclosed spectral confocal transparent flat plate thickness measurement methods are all single-point measurement methods, which require an additional two-dimensional precision scanning platform to measure the two-dimensional thickness distribution of the transparent flat plate parts, and the measurement efficiency is very low. Summary of the invention

[0005] In view of the deficiencies in the prior art, the present invention proposes a transparent flat plate two-dimensional thickness measurement system and method based on line spectrum confocal technology.

[0006] The technical solution of the present invention is as follows:

[0007] A transparent flat plate two-dimensional thickness measurement system based on line spectrum confocal technology includes a wide spectrum linear array LED light source, an optical fiber bundle, a slit, a symmetrical confocal linear dispersion mirror group, a line imaging spectrometer, a microprocessor, a probe fixing bracket and a two-dimensional precision translation stage.

[0008] The light emitted by the wide-spectrum linear array LED light source is transmitted through the optical fiber bundle, passes through the slit to become an ideal linear light source, and then passes through the symmetrical confocal linear dispersion mirror group to be dispersed on the confocal plane. The linear light spot focused on the surface of the transparent plate will be reflected back to the symmetrical confocal linear dispersion mirror group and split by the line imaging spectrometer. The spectrum-distance mapping relationship is decoded by the microprocessor to obtain the measurement value. The symmetrical confocal linear dispersion mirror group is vertically fixed on the probe fixing bracket, and the transparent plate to be measured is set on a two-dimensional precision translation stage.

[0009] Furthermore, the symmetrical confocal linear dispersion mirror group can image the ideal line light source after passing through the slit at different wavelengths at different depths on the confocal plane. The focal line length is the same as the slit length, which is 2 to 10 mm, and the imaging dispersion depth ranges from 1 to 6 mm.

[0010] Furthermore, the transparent plate to be tested is a transparent flat plate to be tested, a multi-layer transparent flat plate, a transparent composite material plate or a transparent curved plate.

[0011] A two-dimensional thickness measurement method for a transparent flat plate based on line spectrum confocal technology comprises the following steps:

[0012] S1: A symmetrical confocal linear dispersion mirror set is vertically fixed on a probe fixing bracket, and the transparent plate to be measured is set on a two-dimensional precision translation stage. A plurality of LED light sources with uniform spectral distribution and high brightness in the working band are arranged into a wide spectrum line array LED light source, which is transmitted through an optical fiber bundle and passes through a slit to become an ideal wide spectrum line light source with the same length as the slit;

[0013] S2: After the light beam passes through the exit mirror group of the symmetrical confocal linear dispersion mirror group, it is uniformly dispersed on the confocal plane perpendicular to the measured surface, that is, different wavelengths of the above-mentioned broad spectrum light source are imaged at different focusing depths. The light beams that are just focused on the upper and lower surfaces of the transparent plate will be reflected back to the incident mirror group of the symmetrical confocal linear dispersion mirror group, while the remaining defocused light beams can only form diffuse spots, and the reflected light intensity is extremely weak;

[0014] S3: After the incident mirror group of the symmetrical confocal linear dispersion mirror group receives the reflected light, it images the reflected light into the line imaging spectrometer for spectroscopic decoding, and establishes a mapping relationship between the peak wavelength of the reflection spectrum and the reflection depth;

[0015] S4: using a microprocessor to perform data analysis and processing, locate the peak wavelength of the reflection spectrum of the upper and lower surfaces of the transparent plate, and calculate the thickness of the measured sample according to the line spectrum thickness measurement model;

[0016] S5: By controlling the two-dimensional precision translation stage to perform reciprocating scanning motion, the two-dimensional full-area thickness measurement of the transparent flat plate to be measured can be achieved.

[0017] Furthermore, the expression of the line spectrum thickness measurement model is: Where t(θ) represents the ratio of the reflectivity of the light beam at an incident angle of θ to the total reflectivity of all analyzed light beams, n(λ) is the refractive index of the transparent plate at wavelength λ, θ is the incident angle corresponding to the incident light beam, and the incident angle range of the light beam is θ min and θ max Between, Δθ is θ max and θ min The difference between x ′ is the difference in distance measurement results of the peak wavelengths corresponding to the upper and lower surfaces of the transparent plate in the reflection spectrum.

[0018] Furthermore, when controlling the two-dimensional precision translation stage to perform reciprocating scanning motion, it is necessary to ensure that there is a certain repeated scanning area between each scanning stroke, and based on the three-dimensional point cloud morphology data of the upper surface of the transparent plate in the repeated scanning area, an iterative closest point algorithm is used for splicing, and the measured two-dimensional thickness data is mapped through the upper surface point cloud morphology data to realize the two-dimensional thickness measurement of the transparent plate.

[0019] Furthermore, the line spectrum thickness measurement model can also be extended to the thickness measurement of multi-layer transparent plates and transparent composite materials.

[0020] Furthermore, the measurement types also include the two-dimensional thickness of transparent surfaces. By increasing the swing freedom of the probe fixing bracket and controlling the symmetrical confocal linear dispersion mirror group to approximately maintain the normal plane position on the upper surface of the transparent surface during the scanning movement, the two-dimensional thickness measurement of the transparent surface can be achieved.

[0021] The beneficial effects of the present invention are:

[0022] 1) A transparent flat plate line thickness measurement method suitable for a symmetrical confocal linear dispersion mirror group is proposed. The thickness values ​​of all measurement points on the confocal line can be obtained simultaneously through a single single-side detection. The device is simple and easy to operate.

[0023] 2) The transparent flat plate to be measured is placed on a two-dimensional precision translation stage, which can also realize efficient measurement of the two-dimensional surface shape and thickness of a large-area transparent flat plate, making it easy to integrate into an automated production line.

[0024] 3) By increasing the swinging degree of freedom for the probe fixing bracket, the proposed method can also be applied to the thickness measurement of curved transparent plates. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1It is a schematic diagram of a transparent flat plate two-dimensional thickness measurement system based on the line spectrum confocal technology of the present invention.

[0026] Figure 2 This is a flow chart of the two-dimensional thickness measurement method of a transparent flat plate of the present invention.

[0027] Figure 3 This is a schematic diagram of the line spectrum confocal two-dimensional thickness measurement principle of the present invention.

[0028] Figure 4 It is a schematic diagram of the line spectrum confocal two-dimensional thickness measurement reflection spectrum signal of the present invention. DETAILED DESCRIPTION

[0029] To make the purpose, technical solution and advantages of the present invention clearer, the following is further described in detail with reference to the accompanying drawings and embodiments. However, it should be understood that the embodiments specifically described herein are only used to explain the present invention and are not intended to limit the scope of protection of the present invention in any way.

[0030] The present invention discloses a transparent flat plate two-dimensional thickness measurement system based on line spectrum confocal technology. Figure 1 As shown, it includes a wide spectrum linear array LED light source 1, an optical fiber bundle 2, a slit 3, a symmetrical confocal linear dispersion mirror group 4, a line imaging spectrometer 5, a microprocessor 6, a probe fixing bracket 7 and a two-dimensional precision translation stage 8.

[0031] The light emitted by the wide-spectrum linear array LED light source 1 is transmitted through the optical fiber bundle 2, passes through the slit 3 to become an ideal linear light source, and then passes through the symmetrical confocal linear dispersion mirror group 4 to be dispersed on the confocal plane. The linear light spot focused on the surface of the transparent plate will be reflected back to the symmetrical confocal linear dispersion mirror group 4, and split by the line imaging spectrometer 5. The spectrum-distance mapping relationship is decoded by the microprocessor 6 to obtain the measurement value. The symmetrical confocal linear dispersion mirror group is vertically fixed on the probe fixing bracket 7, and the transparent plate to be measured is set on the two-dimensional precision translation stage 8.

[0032] The symmetrical confocal linear dispersion mirror set 4 can image the ideal line light source after passing through the slit 2 at different wavelengths at different depths on the confocal plane. The focal line length is the same as the slit length, which is 2 to 10 mm, and the imaging dispersion depth ranges from 1 to 6 mm.

[0033] A two-dimensional thickness measurement method for transparent flat plates based on line spectrum confocal technology, such as Figure 2 As shown, the following steps are included:

[0034] S1: A plurality of LED light sources with uniform spectral distribution and high brightness in the working band are arranged into a wide spectrum line array LED light source 1, which is transmitted through the optical fiber bundle 2 and passes through the slit 3 to become an ideal wide spectrum line light source with the same length as the slit;

[0035] S2: After the light beam passes through the exit mirror group of the symmetrical confocal linear dispersion mirror group 4, uniform dispersion occurs on the confocal plane perpendicular to the measured surface, that is, different wavelengths of the above-mentioned broad spectrum light source are imaged at different focusing depths, and the light beams just focused on the upper and lower surfaces of the transparent plate will be reflected back to the incident mirror group of the symmetrical confocal linear dispersion mirror group 4, while the remaining defocused light beams can only form diffuse spots, and the reflected light intensity is extremely weak;

[0036] S3: After the incident mirror group of the symmetrical confocal linear dispersion mirror group 4 receives the reflected light, it images the reflected light into the line imaging spectrometer 5 for spectroscopic decoding, and establishes a mapping relationship between the peak wavelength of the reflection spectrum and the reflection depth;

[0037] S4: using the microprocessor 6 to perform data analysis and processing, locate the peak wavelength of the reflection spectrum of the upper and lower surfaces of the transparent plate, and calculate the thickness of the measured sample according to the line spectrum thickness measurement model;

[0038] S5: Fix the sensor on the probe fixing bracket 7, and set the transparent plate to be measured on the two-dimensional precision translation stage 8. By controlling the two-dimensional precision translation stage to perform reciprocating scanning motion, the two-dimensional full-area thickness of the transparent plate to be measured can be measured.

[0039] The expression of the line spectrum thickness measurement model is: In the formula, t(θ) represents the ratio of the reflectivity of the light beam at the incident angle θ to the total reflectivity of all analyzed light beams, n(λ) is the refractive index of the transparent plate at wavelength λ, θ is the incident angle corresponding to the incident light beam, and the incident angle range of the light beam is θ min and θ max Between, Δθ is θ max and θ min The difference between x ′ is the difference in distance measurement results of the peak wavelengths corresponding to the upper and lower surfaces of the transparent plate in the reflection spectrum.

[0040] As a preferred implementation, when controlling the two-dimensional precision translation stage 8 to perform a reciprocating scanning motion, it is necessary to ensure that there is a certain repeated scanning area between each scanning stroke, and based on the three-dimensional point cloud morphology data of the upper surface of the transparent plate in the repeated scanning area, an iterative closest point algorithm is used for splicing, and the measured two-dimensional thickness data is mapped through the upper surface point cloud morphology data to realize the two-dimensional thickness measurement of the transparent plate.

[0041] As a preferred implementation, the line spectrum thickness measurement model can also be extended to be applied to the thickness measurement of multi-layer transparent flat plates and transparent composite materials.

[0042] As a preferred embodiment, the measurement type also includes the two-dimensional thickness of the transparent curved surface. By increasing the swing freedom of the probe fixing bracket and controlling the symmetrical confocal linear dispersion mirror group 4 to approximately maintain the normal plane position on the upper surface of the transparent curved surface during the scanning movement, the two-dimensional thickness measurement of the transparent curved surface can be achieved. It should be further explained that the thickness of the transparent curved surface at a certain point is defined as the Euler distance between the upper surface measurement point of the curved surface and its lower surface.

[0043] The line spectrum thickness measurement model is further explained below.

[0044] like Figure 3 As shown in the figure, at a distance x from the edge of the transparent plate, a cross section coplanar with the symmetrical confocal linear dispersion mirror group is taken for study, where the x direction is the confocal line direction of the symmetrical confocal linear dispersion mirror group, and the y direction is the scanning direction of the two-dimensional thickness measurement. i (i = 1 ~ m) is the incident angle of the light focused on the lower surface. The refractive index n (λ) of the transparent plate is determined by the inherent properties of the material. So, by tracing a bunch of light focused on the lower surface of the transparent plate, applying the Snell principle, we can get

[0045] n0 sinθ i =nsinθ t ,

[0046] Where n0 is the refractive index of air, θ t is the incident angle θ i The corresponding refraction angle is known from the geometric relationship

[0047] θ t =arcsin(sinθ i / n).

[0048] So for a single analytical ray, the thickness of the transparent plate can be expressed as

[0049]

[0050] However, since the outgoing light of the symmetrical confocal linear dispersion lens group is obliquely dispersed to the confocal plane, the light of the same wavelength focused at the same depth (i.e., the lower surface of the transparent plate) has different incident angles. Figure 3 The θ1 and θ shown m Corresponding to the minimum and maximum incident angles θ at the confocal wavelength min and θ max Taking into account the influence of light at different incident angles, the thickness of the transparent plate at position x can be expressed as:

[0051]

[0052] It can be further written in the form of accumulation

[0053]

[0054] Where, the adjacent incident angle θ i With θ i-1 The spacing is taken as unit length, and t(θ) represents the ratio of the reflectivity of the light at an incident angle of θ to the sum of the reflectivities of all analyzed light rays. This value is given by the Fresnel formula and is related to the polarization state of the incident light. For most naturally polarized light, the reflectivity can be expressed as

[0055]

[0056] Where, t all is the sum of all analysis light reflectances. Figure 4 The figure shows a schematic diagram of the reflection spectrum when the symmetrical confocal linear dispersion mirror set 4 is used to measure the transparent flat plate. The x-coordinate direction in the figure coincides with the confocal plane. The thickness d at the point can be calculated by the difference of the spectrum peaks located at different x-coordinates with the help of the above thickness measurement model. x The thickness of the confocal line is determined by a series of d x The measured value is obtained.

[0057] The above embodiments are preferred implementations of the present invention, and are only used to illustrate the technical concept and features of the present invention. The purpose is to enable people familiar with the technology to understand the content of the present invention and implement it accordingly, and it cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A two-dimensional thickness measurement method for a transparent flat plate based on line spectrum confocal technology, characterized in that: The thickness measurement system used includes a wide spectrum linear array LED light source, a fiber bundle, a slit, a symmetrical confocal linear dispersion mirror set, a line imaging spectrometer, a microprocessor, a probe fixing bracket and a two-dimensional precision translation stage; The light emitted by the wide-spectrum linear array LED light source is transmitted through the optical fiber bundle, passes through the slit to become an ideal linear light source, and then passes through the symmetrical confocal linear dispersion mirror group to be dispersed on the confocal plane. The linear light spot focused on the surface of the transparent plate to be measured is reflected back to the symmetrical confocal linear dispersion mirror group and split by the line imaging spectrometer. The spectrum-distance mapping relationship is decoded by the microprocessor to obtain the measurement value. The symmetrical confocal linear dispersion mirror group is vertically fixed on the probe fixing bracket, and the transparent plate to be measured is set on a two-dimensional precision translation stage. The following steps are involved: S1: A symmetrical confocal linear dispersion mirror set is vertically fixed on a probe fixing bracket, and the transparent plate to be measured is set on a two-dimensional precision translation stage. A plurality of LED light sources with uniform spectral distribution and high brightness in the working band are arranged into a wide spectrum line array LED light source, which is transmitted through an optical fiber bundle and passes through a slit to become an ideal wide spectrum line light source with the same length as the slit; S2: After the light beam passes through the exit mirror group of the symmetrical confocal linear dispersion mirror group, it is uniformly dispersed on the confocal plane perpendicular to the measured surface, that is, different wavelengths of the above-mentioned broad spectrum light source are imaged at different focusing depths. The light beams that are just focused on the upper and lower surfaces of the transparent plate will be reflected back to the incident mirror group of the symmetrical confocal linear dispersion mirror group, while the remaining defocused light beams can only form diffuse spots, and the reflected light intensity is extremely weak; S3: After the incident mirror group of the symmetrical confocal linear dispersion mirror group receives the reflected light, it images the reflected light into the line imaging spectrometer for spectroscopic decoding, and establishes a mapping relationship between the peak wavelength of the reflection spectrum and the reflection depth; S4: Using a microprocessor to perform data analysis and processing, locate the peak wavelength of the reflection spectrum of the upper and lower surfaces of the transparent plate, and calculate the thickness of the measured sample according to the line spectrum thickness measurement model; the expression of the line spectrum thickness measurement model is: Where t(θ) represents the ratio of the reflectivity of the light beam at an incident angle of θ to the total reflectivity of all analyzed light beams, n(λ) is the refractive index of the transparent plate at wavelength λ, θ is the incident angle corresponding to the incident light beam, and the incident angle range of the light beam is θ min and θ max Between, Δθ is θ max and θ min The difference between x ′ is the difference in distance measurement results of the peak wavelengths corresponding to the upper and lower surfaces of the transparent plate in the reflection spectrum; S5: By controlling the two-dimensional precision translation stage to perform reciprocating scanning motion, the two-dimensional full-area thickness measurement of the transparent flat plate to be measured is achieved.

2. According to claim 1, a transparent flat plate two-dimensional thickness measurement method based on line spectrum confocal technology is characterized in that: When controlling the two-dimensional precision translation stage to perform reciprocating scanning motion, it is necessary to ensure that there is a repeated scanning area between each scanning stroke, and based on the three-dimensional point cloud topography data of the upper surface of the transparent plate in the repeated scanning area, an iterative closest point algorithm is used for splicing, and the measured two-dimensional thickness data is mapped through the upper surface point cloud topography data to realize the two-dimensional thickness measurement of the transparent plate.

3. The method for two-dimensional thickness measurement of a transparent flat plate based on line spectrum confocal technology according to claim 1, characterized in that: Applied to the thickness measurement of multi-layer transparent plates and transparent composite materials.

4. The method for two-dimensional thickness measurement of a transparent flat plate based on line spectrum confocal technology according to claim 1, characterized in that: It is used to measure the two-dimensional thickness of transparent curved plates. By increasing the swing freedom of the probe fixing bracket, the symmetrical confocal linear dispersion mirror group is controlled to approximately maintain the normal plane position on the upper surface of the transparent curved surface during the scanning movement, thereby realizing the two-dimensional thickness measurement of the transparent curved surface.

5. The method for two-dimensional thickness measurement of a transparent flat plate based on line spectrum confocal technology according to claim 1, characterized in that: The symmetrical confocal linear dispersion mirror group can image the ideal line light source after passing through the slit at different wavelengths at different depths on the confocal plane. The focal line length is the same as the slit length, which is 2 to 10 mm, and the imaging dispersion depth ranges from 1 to 6 mm.

6. The method for two-dimensional thickness measurement of a transparent flat plate based on line spectrum confocal technology according to claim 1, characterized in that: The transparent plate to be tested is a transparent flat plate to be tested, a multi-layer transparent flat plate, a transparent composite material plate or a transparent curved plate.

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