Method for automatically detecting the angle of inclination of a light column on a laser sheet

Abstract

The application discloses a method for automatically detecting the light column inclination angle of light column laser paper, and relates to the technical field of testing material performance by measuring material physical properties, which comprises the following steps: fixing the light column laser paper on a rotating platform; irradiating the surface of the light column laser paper with a linear light source at a set angle γ, and collecting the reflection image of the surface at the same time; controlling the rotating platform to rotate the light column laser paper and collecting images; calculating the evaluation value of each collected image and the corresponding rotation angle; comparing each evaluation value, setting the collected image corresponding to the evaluation value with the largest value as the target collected image; and obtaining the light column inclination angle α1 according to the set angle γ and the rotation angle β1 corresponding to the target collected image. The method can detect the light column inclination angle of the light column laser paper with any initial deviation, solves the problem of the maximum allowable inclination angle limitation in the prior art, and greatly enhances the fault tolerance of the initial placement position of the light column laser paper.

Description

Technical Field

[0001] This invention relates to the technical field of testing material properties by measuring their physical properties, and specifically to a method for automatically detecting the tilt angle of a laser beam in a laser paper. Background Technology

[0002] Laser grating paper (also known as laser cylindrical grating paper) is a functional specialty packaging material that utilizes physical optics principles to achieve unique visual effects. Its basic structure involves forming a series of microscopic grooves or protrusions with nanometer to micrometer scale periodicity, equal spacing, and parallel arrangement on the surface of a paper or film substrate through a precision molding process. This microscopic structure is called "grating stripes" or "grating structure".

[0003] These grating fringes form a one-dimensional diffraction grating. When ambient light shines on its surface, the light is not diffusely reflected, but selectively reflected to a specific viewing angle according to the laws of diffraction and interference. Therefore, with minute changes in the observer's viewing angle or the sample angle, a bright and continuously moving "beam" or "blaze" will appear on the paper.

[0004] With its unique optical effects and difficult-to-replicate physical structure, holographic paper is widely used in high-end product packaging (such as tobacco, alcohol, and cosmetics), publication covers, anti-counterfeiting labels, and cultural and creative products. The consistency of the direction of its grating stripes (which can also be reflected in the tilt angle of the holographic columns) is a key quality indicator determining the visual quality of the final product and the precision of subsequent printing, die-cutting, and other processing. Therefore, it is necessary to measure the tilt angle of the holographic columns in holographic paper.

[0005] The existing method for measuring the tilt angle of a laser beam on a laser paper typically involves using a spectrophotometer to perform grid-like sampling on the sample surface and analyzing the periodic changes in color data to calculate the angle. While this method offers high accuracy, it has a significant drawback: it has a finite maximum allowable tilt angle (usually less than 10 degrees). When the initial sample placement deviation exceeds this range, extreme point mismatches can occur, leading to detection failure or incorrect results. Summary of the Invention

[0006] To address the aforementioned shortcomings of existing technologies, this invention provides a method for automatically detecting the tilt angle of a laser beam in holographic paper. This method can detect the tilt angle of a laser beam in holographic paper with arbitrary initial placement deviations, solving the problem of the maximum allowable tilt angle limitation in existing technologies and greatly enhancing the tolerance for errors in the initial placement position of the laser beam in holographic paper.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following solution:

[0008] A method for automatically detecting the tilt angle of a laser beam on holographic paper includes the following steps:

[0009] The laser paper to be tested is fixed on the rotating platform, and the reference edge of the laser paper is aligned with the positioning edge on the rotating platform.

[0010] A linear light source is used, which illuminates the surface of the laser paper at a set angle γ with the rotating platform. At the same time, an image acquisition device is set directly above the rotating platform to acquire the reflected image of the laser paper surface.

[0011] The rotating platform is controlled to rotate the laser paper within a range greater than 180° by a set angle each time. Each time the rotating platform rotates the laser paper by a set angle, the image acquisition device captures a reflected image from the surface of the laser paper. For all images acquired by the image acquisition device, an evaluation value reflecting the sharpness of the grating stripes and the corresponding rotation angle of the rotating platform are calculated.

[0012] The evaluation values ​​of the grating stripe sharpness of all acquired images are compared and measured. The acquired image with the largest evaluation value is set as the target acquired image, and the rotation angle β1 of the rotating platform corresponding to the target acquired image is recorded.

[0013] The tilt angle α1 of the laser beam paper is calculated based on the set angle γ and the rotation angle β1 of the rotating platform corresponding to the target acquired image.

[0014] Preferably, the formula for calculating the tilt angle α1 of the laser beam paper is: α1 = γ - β1.

[0015] Preferably, the set angle γ is obtained by the following steps:

[0016] A laser paper with a known beam tilt angle α0 is fixed on a rotating platform as a reference sample, and the reference edge of the reference sample is aligned with the positioning edge on the rotating platform.

[0017] The surface of the reference sample is illuminated by a linear light source, and an image acquisition device is placed directly above the rotating platform to acquire the reflected image of the surface of the reference sample.

[0018] The rotating platform is controlled to rotate the reference sample by a set angle within a range of greater than 180° each time. Each time the rotating platform rotates the reference sample by a set angle, the image acquisition device acquires the reflected image of the reference sample surface to obtain the reference sample acquisition image.

[0019] For all reference sample images acquired by the image acquisition device, calculate the evaluation value of the reference sample image to reflect the sharpness of the grating stripes, and the rotation angle of the rotating platform corresponding to the reference sample image;

[0020] The evaluation values ​​of the grating stripe sharpness of all reference sample images obtained by comparison measurement are used to set the reference sample image with the largest evaluation value as the target reference sample image, and the rotation angle β0 of the rotating platform corresponding to the target reference sample image is recorded.

[0021] The set angle γ is obtained based on the rotation angle β0 of the rotating platform corresponding to the image acquired from the target reference sample and the calculation of the reference sample.

[0022] Preferably, the formula for calculating the set angle γ is: γ = α0 + β0.

[0023] Preferably, the rotating platform is controlled to rotate the laser paper by a set angle each time, and each time the rotating platform rotates the laser paper by a set angle, the image acquisition device acquires a reflected image from the surface of the laser paper. For all acquired images, the evaluation value of the acquired image reflecting the sharpness of the grating stripes and the rotation angle of the rotating platform corresponding to the acquired image are calculated, including:

[0024] First stage: Control the rotating platform to drive the laser paper to rotate at a first set angle each time. Every time the rotating platform drives the laser paper to rotate by a first set angle, the image acquisition device acquires the reflected image on the surface of the laser paper to obtain a first acquired image. The evaluation value of the first acquired image is used to reflect the clarity of the grating stripes. Based on the calculated evaluation value of the first acquired image, the first angle range where the preset extreme value is located is located.

[0025] Second stage: Control the rotating platform to drive the laser paper to rotate at a second set angle each time within the first angle range. The second set angle is smaller than the first set angle. Each time the rotating platform drives the laser paper to rotate by the second set angle, the image acquisition device acquires the reflected image on the surface of the laser paper to obtain a second acquired image. Calculate the evaluation value of the second acquired image to reflect the clarity of the grating stripes, and the rotation angle of the rotating platform corresponding to the second acquired image.

[0026] Preferably, in the first stage, when the rotating platform drives the laser paper to rotate at a first set angle each time, the angle by which the rotating platform drives the laser paper to rotate is not less than 180°.

[0027] The width of the first angle range is greater than 10 times the second set angle, and the second set angle is not greater than half of the set angle precision.

[0028] Preferably, the evaluation value of the acquired image is obtained by calculating at least one of the following methods: the sum of gradient magnitudes of the acquired image, the energy of the characteristic peaks in the Fourier transform spectrum, or the intensity of the linear characteristic detected by the Hough transform.

[0029] Preferably, the method for calculating the sum of gradient magnitudes of the acquired images to obtain the evaluation value of the acquired images is as follows:

[0030] Calculate the spatial gradient information of the acquired image;

[0031] Based on the spatial gradient information, the cumulative gradient magnitude of all images or a portion of selected regions in the acquired image is calculated, and this cumulative gradient magnitude is used as the evaluation value of the acquired image.

[0032] Preferably, the method for calculating the energy of the characteristic peaks in the Fourier transform spectrum of the acquired image to obtain the evaluation value of the acquired image is as follows:

[0033] Perform a two-dimensional Fourier transform on the acquired image to obtain the frequency domain spectrum of the acquired image;

[0034] Analyze the energy distribution of the periodic features of the grating fringes on the laser paper corresponding to the frequency domain spectrum;

[0035] The evaluation value of the acquired image is calculated based on the concentration or characteristic peak intensity of the energy distribution.

[0036] Preferably, the method for calculating the evaluation value of the acquired image by determining the intensity of the linear features detected by the Hough transform is as follows:

[0037] Edge detection is performed on the acquired image to obtain a binary edge image of the acquired image;

[0038] Perform a Hough transform on the binary edge image to detect straight line features;

[0039] The evaluation value of the acquired image is calculated based on the intensity, length, or number of the linear features.

[0040] Compared with existing technologies, the present invention, when detecting the tilt angle of the light column, first fixes the light column laser paper to be tested on a rotating platform and aligns the reference edge of the light column laser paper with the positioning edge on the rotating platform; then, a linear light source illuminates the surface of the light column laser paper at a set angle γ, and an image acquisition device acquires the reflected image of the surface of the light column laser paper; then, by controlling the rotating platform to drive the light column laser paper to rotate at a set angle each time, the image acquisition device acquires the reflected image of the surface of the light column laser paper at each rotation angle to obtain the acquired image, and calculates the evaluation value reflecting the sharpness of the grating stripes and the corresponding rotation angle for all acquired images; by comparing the evaluation values ​​of the sharpness of the grating stripes of all acquired images, the acquired image corresponding to the largest evaluation value is set as the target acquired image, and the rotation angle β1 of the rotating platform corresponding to the target acquired image is recorded; finally, the tilt angle α1 of the light column laser paper is calculated based on the set angle γ and the rotation angle β1 of the rotating platform corresponding to the target acquired object. Therefore, this method is based on the evaluation value reflecting the clarity of the grating stripes in the acquired image. At the same time, the rotating platform can rotate the laser paper at any angle in the plane. Therefore, no matter what the initial placement deviation angle of the laser paper is, the rotating platform can rotate the laser paper to the target position. Therefore, this solution can theoretically effectively detect any tilt angle of the laser paper within the range of 0° to 180°, completely solving the problem of the maximum allowable angle limitation of the traditional point scanning method, and greatly enhancing the fault tolerance of the initial placement position of the laser paper. Attached Figure Description

[0041] Appendix Figure 1 This is a flowchart of the method for automatically detecting the tilt angle of the laser beam in the laser paper according to the present invention;

[0042] Appendix Figure 2 This is a flowchart illustrating the process of obtaining the angle γ in the method for automatically detecting the tilt angle of the laser paper light column of the present invention. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0044] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0045] Furthermore, in describing representative embodiments, the specification may have presented methods and / or processes as a specific sequence of steps. However, the method or process should not be limited to the specific order of steps described herein, to the extent that it does not depend on such a specific order. As will be understood by those skilled in the art, other sequences of steps are also possible. Therefore, the specific order of steps set forth in the specification should not be construed as a limitation on the embodiments. Moreover, the method and / or process should not be limited to the steps performed in the written order; those skilled in the art will readily understand that these orders can be varied and still remain within the spirit and scope of the embodiments of this application.

[0046] This specific embodiment provides a method for automatically detecting the tilt angle of the light column in laser paper, as shown in the attached figure. Figure 1 As shown, the specific steps include:

[0047] Step S1) Fix the laser paper to be tested onto the rotating platform and align the reference edge of the laser paper with the positioning edge on the rotating platform.

[0048] For example, the rotary platform has an angular resolution of no less than 0.01°, a repeatability of no less than ±0.005°, and is capable of precise point-to-point rotation and continuous rotation with adjustable speed under computer control, providing a power basis for the accurate measurement of the tilt angle of the light column. In actual use, the rotary platform can be a precision worm gear rotary table with closed-loop servo control or a direct-drive rotary table, and the rotary platform can be driven by a high-precision stepper motor.

[0049] Meanwhile, the laser paper to be tested needs a clear reference edge, such as the paper edge, and this paper edge should have a fixed relationship with the direction of the grating stripes, such as parallelism. To ensure the accuracy of the measurement results, each batch of incoming materials is pre-calibrated, for example, by randomly selecting several samples and using high-precision offline metrology equipment (such as a microscope with a rotating stage) to directly measure the actual angle between the "reference edge and the grating" of multiple samples, obtaining the compensation value δ1. Furthermore, each piece of laser paper to be tested must be placed in the same way, that is, its reference edge must be strictly parallel and aligned with the positioning edge on the rotating platform (such as a mechanical positioning block set on the rotating platform). This is also the key to achieving absolute measurement and ensuring the consistency of the measurement benchmark each time.

[0050] Step S2) A linear light source is used to illuminate the surface of the laser paper at a set angle γ with respect to the rotating platform. At the same time, an image acquisition device is set directly above the rotating platform to acquire the reflected image of the laser paper surface.

[0051] Specifically, the set angle γ here refers to the angle between the linear light source and the mounting plane of the rotating platform. This set angle γ is generally less than 10°. When the light from the linear light source passes over the surface of the laser paper at an extremely low angle, the micro-grooves or ridges of the grating stripes on the laser paper will cast shadows on one side while the other side is illuminated, thus greatly exaggerating the micro-undulations of the surface. The resulting reflected image has extremely high contrast between bright and dark stripes, but the illumination is uneven, with a very obvious characteristic of one side being bright and the other dark, and the edge information of the grating stripes will be extremely enhanced. That is, when the direction of the linear light source is parallel to the direction of the grating stripes, the illumination conditions of each grating groove are consistent, forming the most periodically stable and edge-sharp alternating bright and dark stripe projection in the direction perpendicular to the grating. This projected image has the largest gradient energy in the spatial domain and the most concentrated characteristic spectral energy in the frequency domain, thus maximizing the image sharpness evaluation value.

[0052] Among them, as attached Figure 2 As shown, this set angle is obtained by measuring through the following steps:

[0053] Step S21) Fix a laser-like paper with a known beam tilt angle α0 as a reference sample on a rotating platform, and align the reference edge of the reference sample with the positioning edge on the rotating platform. The beam tilt angle of the laser-like paper used in this step has been measured beforehand by other methods, such as using a high-magnification optical microscope with a precision rotating stage to directly observe the grating fringes of the laser-like paper and measure its direction through image analysis or laser diffraction. This method, through direct imaging, does not have matching problems based on the periodicity of color changes, and therefore has no angular range limitations, allowing for accurate identification of the beam tilt angle α0 of the reference sample laser-like paper, as well as the angle δ0 between its reference edge and the direction of the grating fringes.

[0054] Meanwhile, the reference edge of the reference sample is aligned with the positioning edge on the rotating platform, so that when testing the laser beam paper to be tested, the reference sample and the laser beam paper to be tested have the same measurement reference, ensuring the accuracy of the test results.

[0055] Step S22) Illuminate the surface of the reference sample with a linear light source, and simultaneously place an image acquisition device directly above the rotating platform to acquire the reflected image of the reference sample surface. At this time, the illumination angle of the linear light source is unknown, but in order to obtain better image quality, the illumination angle of the linear light source is generally set within 10°, and the specific illumination angle data will be calculated according to the following steps.

[0056] Step S23) Control the rotating platform to rotate the reference sample within a 180° range, rotating it by a set angle each time. Each time the rotating platform rotates the reference sample by a set angle, the image acquisition device captures a reflection image of the reference sample surface. As the rotating platform rotates the reference sample, the angular relationship between the grating fringes on the reference sample and the illumination direction of the linear light source changes accordingly. This causes a change in the reflection image captured by the image acquisition device. This change in the reflection image allows the determination of the parallel position between the grating fringes and the illumination direction of the linear light source in subsequent steps. In this step, the rotation angle of the rotating platform should be greater than 180°, ensuring that even with a large angular deviation in the initial placement position, the correct direction of the grating fringes can be found through the rotation of the platform.

[0057] Step S24) For all reference sample images acquired by the image acquisition device, calculate the evaluation value of the reference sample image used to reflect the sharpness of the grating stripes, and the rotation angle of the rotating platform corresponding to the reference sample image.

[0058] Step S25) Compare the evaluation values ​​of the grating stripe sharpness of all the reference sample acquisition images obtained in the measurement, set the reference sample acquisition image corresponding to the largest evaluation value as the target reference sample acquisition image, and record the rotation angle β0 of the rotating platform corresponding to the target reference sample acquisition image; at this time, the position of the reference sample corresponding to the target reference sample acquisition image is the position where the grating stripe direction of the reference sample is parallel to the illumination direction of the linear light source.

[0059] Step S26) Calculate the set angle γ based on the rotation angle β0 of the rotating platform corresponding to the image acquired by the target reference sample and the tilt angle α0 of the light column of the reference sample.

[0060] The formula for calculating the angle γ is: γ = α0 + β0 - δ0. When δ0 = 0, then γ = α0 + β0.

[0061] For example, an image acquisition device can be a camera.

[0062] Step S3) Control the rotating platform to drive the laser paper to rotate at a set angle within a range of more than 180° each time. Every time the rotating platform drives the laser paper to rotate by a set angle, the image acquisition device will acquire the reflected image on the surface of the laser paper to obtain the acquired image. For all the acquired images acquired by the image acquisition device, calculate the evaluation value of the acquired image to reflect the clarity of the grating stripes, as well as the rotation angle of the rotating platform corresponding to the acquired image.

[0063] This step uses a two-step method. Of course, in actual use, a three-step or more-step method can also be used, depending on the specific requirements for detection accuracy and efficiency.

[0064] Step S31) In the first stage of coarse search, the rotating platform is controlled to rotate the laser paper by a first set angle each time. When the rotating platform rotates the laser paper by a first set angle, the image acquisition device acquires the reflected image on the surface of the laser paper to obtain the first acquired image. The evaluation value of the first acquired image is used to reflect the clarity of the grating stripes. Based on the evaluation value of the calculated first acquired image, the first angle interval where the preset extreme value is located is located.

[0065] In the first stage, when the rotating platform drives the laser paper to rotate at a first set angle each time, the angle of rotation of the laser paper driven by the rotating platform is not less than 180°.

[0066] In the first stage of coarse search, if the initial position of the laser paper on the rotating platform is far from the optimal position, it would be extremely time-consuming to start scanning from scratch with small step rotations. Therefore, in the first stage, a larger step (such as 3°) can be used to quickly rotate 180°. This way, only a few dozen sampling points are needed to lock the first angle range where the optimal angle is located, skipping the fine-grained search of invalid areas. This is the main source of efficiency improvement.

[0067] Secondly, due to the periodicity of the grating fringes, a similar peak appears in the sharpness evaluation value every 180°. The first-stage coarse search, through global scanning, can plot the overall trend of the evaluation value as a function of angle, thus clearly identifying the main peak and the secondary peaks caused by periodicity. This provides the correct range for the subsequent fine search, preventing the algorithm from converging to the wrong secondary peak due to starting the search from a local point.

[0068] Step S32) In the second stage of fine search, the rotating platform is controlled to drive the laser paper to rotate at a second set angle each time within the first angle range. The second set angle is less than the first set angle. Every time the rotating platform drives the laser paper to rotate by a second set angle, the image acquisition device acquires the reflected image on the surface of the laser paper to obtain a second acquisition image. The evaluation value of the second acquisition image used to reflect the clarity of the grating stripes and the rotation angle of the rotating platform corresponding to the second acquisition image are calculated.

[0069] The width of the first angle interval is greater than 10 times the second set angle, and the second set angle is not greater than half of the set angle accuracy. The set angle accuracy is the accuracy of the tilt angle of the laser paper light column to be obtained. For example, if the set angle accuracy is 0.1°, then the second set angle should not be greater than 0.05°, so as to ensure that the sampling requirements of the set angle accuracy are met.

[0070] For example, within the first angle range (e.g. ±5°) determined by the first stage coarse search, a rotating platform is used to perform dense sampling with small steps (e.g. 0.03°), thereby achieving dense sampling within a small angle range, taking into account both sampling accuracy and sampling effect.

[0071] Furthermore, in practical applications, interpolation or curve fitting algorithms can be used to process the sampled evaluation values ​​and angle data to calculate the precise location of the peak between two sampling points. This allows the final angle accuracy (e.g., ±0.005°) to be far higher than the platform's minimum physical step size, achieving super-resolution measurement. This is because the sampling points in the second-stage fine search are discrete, while the actual sharpness curve is continuous. Its theoretical peak will not, in most cases, fall exactly on a single sampling point. If the sampling point with the highest evaluation value is simply selected as the optimal angle, the accuracy will only reach a maximum of one sampling step size (e.g., a step size of 0.03° results in an accuracy of ±0.03°). However, through interpolation or curve fitting, the position of the continuous curve peak can be estimated, thereby improving the accuracy to a level far exceeding the step size.

[0072] In addition, in the second stage of fine search, a rotating platform can be used to perform a back-and-forth rotation scan at the predicted peak position. For example, first take a few points to the left of the predicted peak position, and then take a few points to the right of the predicted peak position. This can obtain denser data near the peak, thereby obtaining more accurate fitting data and thus more accurate measurement data.

[0073] Furthermore, before entering the second stage, it can be pre-judged whether the evaluation value changes within the first angle interval located in the first stage meet a monotonic or unimodal trend. If so, the second stage can proceed; otherwise, the first stage's coarse search is re-executed or a system alarm is triggered. This is because, under correct theoretical models and ideal experimental conditions, when the grating stripe direction of the laser paper under test gradually approaches and becomes parallel to the direction of the linear light source, the sharpness evaluation value of the acquired image should show a smooth, continuous upward trend, reaching a single peak at the parallel point, and then smoothly declining. If this trend is not met, it indicates that the first stage's coarse search has located the wrong interval, or that there is a serious anomaly in the laser paper under test or the environment (such as sudden instability of the linear light source, camera defocusing, severe mechanical vibration, etc.). This allows for the immediate interception of problematic laser paper or the detection of system malfunctions, preventing them from affecting subsequent processes and ensuring that the data submitted to the second stage's fine search is reliable and accurate, thereby guaranteeing the accuracy of the final result and the stability of the system.

[0074] Step S4) Compare the evaluation values ​​of the grating stripe sharpness of all acquired images obtained in the measurement, set the acquired image corresponding to the evaluation value with the largest value as the target acquired image, and record the rotation angle β1 of the rotating platform corresponding to the target acquired image.

[0075] Specifically, the evaluation value of the acquired image can be obtained by calculating at least one of the following methods: the sum of the gradient magnitudes of the acquired image, the energy of the characteristic peaks in the Fourier transform spectrum, or the intensity of the linear characteristic detected by the Hough transform.

[0076] The method for calculating the sum of gradient magnitudes of the acquired image to obtain the evaluation value of the acquired image is to directly analyze the rate of change (i.e., gradient) of the pixel grayscale values ​​of the acquired image. The clearer the stripes, the greater the gradient at the edge.

[0077] Specifically, the method for calculating the sum of gradient magnitudes of the acquired images to obtain the evaluation value of the acquired images is as follows:

[0078] Calculate the spatial gradient information of the acquired image; the spatial gradient information can be calculated using the Sobel operator, the Scharr operator, or the Prewitt operator;

[0079] Based on spatial gradient information, the cumulative gradient magnitude of all images or a portion of selected regions in the acquired image is calculated, and this cumulative gradient magnitude is used as the evaluation value of the acquired image.

[0080] The method of calculating the energy of the characteristic peaks in the Fourier transform spectrum of the acquired image to obtain the evaluation value of the acquired image is based on the fact that after a clear periodic stripe image undergoes a two-dimensional Fourier transform, a pair (or a group) of symmetrical bright spots that are far from the center (corresponding to high frequencies) and have highly concentrated energy will appear on the spectrum. The more blurred the image, the more these bright spots will spread and get closer to the center.

[0081] Specifically, the method for calculating the energy of the characteristic peaks in the Fourier transform spectrum of the acquired image to obtain the evaluation value of the acquired image is as follows:

[0082] Perform a two-dimensional Fourier transform on the acquired image to obtain the frequency domain spectrum of the acquired image;

[0083] Analyze the energy distribution of the periodic characteristics of the grating fringes on the laser paper corresponding to the light column in the frequency domain spectrum;

[0084] The evaluation value of the acquired image is calculated based on the concentration of energy distribution or the intensity of characteristic peaks.

[0085] The method of calculating the evaluation value of the acquired image by measuring the intensity of the straight line features detected by the Hough transform is based on the fact that clear stripes can be extracted into a series of continuous edge points by edge detection algorithms (such as Canny), and the Hough transform can detect these points as straight lines. The clearer the image, the more stable the number of detected straight lines, the longer their length, and the higher their cumulative intensity.

[0086] Specifically, the method for calculating the evaluation value of the acquired image by measuring the intensity of the linear features detected by the Hough transform is as follows:

[0087] Edge detection is performed on the acquired image to obtain a binary edge image of the acquired image;

[0088] Perform Hough transform on the binary edge image to detect straight line features;

[0089] The evaluation value of the acquired image is calculated based on the intensity, length, or number of linear features.

[0090] Step S5) Calculate the tilt angle α1 of the laser beam paper based on the set angle γ and the rotation angle β1 of the rotating platform corresponding to the target acquisition object.

[0091] Specifically, the formula for calculating the tilt angle α1 of the laser beam on the laser beam paper is: α1 = γ - β1 - δ1. Since γ = α0 + β0 - δ0, when δ1 = 0 and δ0 = 0, then α1 = γ - β1 or α1 = α0 + β0 - β1. This gives the tilt angle α1 of the laser beam on the laser beam paper to be tested.

[0092] Compared with existing technologies, the present invention, when detecting the tilt angle of the light column, first fixes the light column laser paper to be tested on a rotating platform and aligns the reference edge of the light column laser paper with the positioning edge on the rotating platform; then, a linear light source illuminates the surface of the light column laser paper at a set angle γ, and an image acquisition device acquires the reflected image of the surface of the light column laser paper; then, by controlling the rotating platform, the light column laser paper rotates at a set angle each time, and the image acquisition device acquires the reflected image of the surface of the light column laser paper at each rotation angle to obtain the acquired image, and calculates the evaluation value reflecting the sharpness of the grating stripes and the corresponding rotation angle for all acquired images; compares the evaluation values ​​of the sharpness of the grating stripes of all acquired images obtained by measurement, sets the acquired image corresponding to the evaluation value with the largest value as the target acquired image, and records the rotation angle β1 of the rotating platform corresponding to the target acquired image; finally, the tilt angle α1 of the light column laser paper is calculated based on the set angle γ and the rotation angle β1 of the rotating platform corresponding to the target acquired object. Therefore, this method is based on the evaluation value reflecting the clarity of the grating stripes in the acquired image. At the same time, the rotating platform can rotate the laser paper at any angle in the plane. Therefore, no matter what the initial placement deviation angle of the laser paper is, the rotating platform can rotate the laser paper to the target position. Therefore, this solution can theoretically effectively detect any tilt angle of the laser paper within the range of 0° to 180°, completely solving the problem of the maximum allowable angle limitation of the traditional point scanning method, and greatly enhancing the fault tolerance of the initial placement position of the laser paper.

[0093] Secondly, by employing a strong shadow effect generated by a low-angle linear light source, this invention can produce high-contrast, clearly defined striped images even on completely opaque laser paper, thus broadening its applicability.

[0094] Furthermore, the final beam tilt angle measurement value of this invention comes directly from the angle feedback of a high-precision rotating platform. Its accuracy depends on the mechanical precision of the rotating platform itself (which can be designed to reach the arcsecond level), avoiding the secondary error caused by indirectly calculating the angle through image pixels in the prior art. The data is more direct and reliable.

[0095] Finally, the combination of machine vision and mechanical control used in this invention has a higher tolerance for environmental vibration, a more stable system, and is more suitable for long-term reliable operation in industrial environments.

[0096] This invention is described with reference to schematic and / or block diagrams illustrating the implementation of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the schematic and / or block diagrams can be implemented by computer program instructions, and combinations of blocks in the schematic and / or block diagrams can be implemented. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the schematic and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 Devices that specify the functions in one or more boxes.

[0097] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including an instruction device, which is implemented in the implementation flow diagram. Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0098] These computer program instructions can also be loaded onto a computer or other programmable data processing equipment, causing a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0099] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit the technical solutions. Those skilled in the art should understand that any modifications or equivalent substitutions to the technical solutions of the present invention without departing from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A method for automatically detecting the tilt angle of a laser beam on holographic paper, characterized in that, Includes the following steps: The laser paper to be tested is fixed on the rotating platform, and the reference edge of the laser paper is aligned with the positioning edge on the rotating platform. A linear light source is used, which illuminates the surface of the laser paper at a set angle γ with the rotating platform. At the same time, an image acquisition device is set directly above the rotating platform to acquire the reflected image of the laser paper surface. The rotating platform is controlled to rotate the laser paper within a range greater than 180° by a set angle each time. Each time the rotating platform rotates the laser paper by a set angle, the image acquisition device captures a reflected image from the surface of the laser paper. For all images acquired by the image acquisition device, an evaluation value reflecting the sharpness of the grating stripes and the corresponding rotation angle of the rotating platform are calculated. The evaluation values ​​of the grating stripe sharpness of all acquired images are compared and measured. The acquired image with the largest evaluation value is set as the target acquired image, and the rotation angle β1 of the rotating platform corresponding to the target acquired image is recorded. The tilt angle α1 of the laser beam paper is calculated based on the set angle γ and the rotation angle β1 of the rotating platform corresponding to the target acquired image.

2. The method for automatically detecting the tilt angle of the laser beam on laser paper according to claim 1, characterized in that, The formula for calculating the tilt angle α1 of the laser beam in holographic paper is: α1 = γ - β1.

3. The method for automatically detecting the tilt angle of the laser beam on laser paper according to claim 2, characterized in that, The set angle γ is obtained through the following steps: A laser paper with a known beam tilt angle α0 is fixed on a rotating platform as a reference sample, and the reference edge of the reference sample is aligned with the positioning edge on the rotating platform. The surface of the reference sample is illuminated by a linear light source, and an image acquisition device is placed directly above the rotating platform to acquire the reflected image of the surface of the reference sample. The rotating platform is controlled to rotate the reference sample by a set angle within a range of greater than 180° each time. Each time the rotating platform rotates the reference sample by a set angle, the image acquisition device acquires the reflected image of the reference sample surface to obtain the reference sample acquisition image. For all reference sample images acquired by the image acquisition device, calculate the evaluation value of the reference sample image to reflect the sharpness of the grating stripes, and the rotation angle of the rotating platform corresponding to the reference sample image; The evaluation values ​​of the grating stripe sharpness of all reference sample images obtained by comparison measurement are used to set the reference sample image with the largest evaluation value as the target reference sample image, and the rotation angle β0 of the rotating platform corresponding to the target reference sample image is recorded. The set angle γ is calculated based on the rotation angle β0 of the rotating platform corresponding to the image acquired from the target reference sample and the tilt angle α0 of the light column of the reference sample.

4. The method for automatically detecting the tilt angle of the laser beam on laser paper according to claim 3, characterized in that, The formula for calculating the set angle γ is: γ = α0 + β0.

5. The method for automatically detecting the tilt angle of the laser beam on laser paper according to claim 4, characterized in that, The rotating platform is controlled to rotate the laser paper by a set angle each time. Each time the rotating platform rotates the laser paper by a set angle, the image acquisition device captures the reflected image from the surface of the laser paper. For all captured images, the evaluation value reflecting the sharpness of the grating stripes and the corresponding rotation angle of the rotating platform are calculated, including: First stage: Control the rotating platform to drive the laser paper to rotate at a first set angle each time. Every time the rotating platform drives the laser paper to rotate by a first set angle, the image acquisition device acquires the reflected image on the surface of the laser paper to obtain a first acquired image. The evaluation value of the first acquired image is used to reflect the clarity of the grating stripes. Based on the calculated evaluation value of the first acquired image, the first angle range where the preset extreme value is located is located. Second stage: Control the rotating platform to drive the laser paper to rotate at a second set angle each time within the first angle range. The second set angle is smaller than the first set angle. Each time the rotating platform drives the laser paper to rotate by the second set angle, the image acquisition device acquires the reflected image on the surface of the laser paper to obtain a second acquired image. Calculate the evaluation value of the second acquired image to reflect the clarity of the grating stripes, and the rotation angle of the rotating platform corresponding to the second acquired image.

6. The method for automatically detecting the tilt angle of the laser beam on laser paper according to claim 5, characterized in that, In the first stage, when the rotating platform drives the laser paper to rotate at a first set angle each time, the angle at which the rotating platform drives the laser paper to rotate is not less than 180°. The width of the first angle range is greater than 10 times the second set angle, and the second set angle is not greater than half of the set angle precision.

7. The method for automatically detecting the tilt angle of the laser beam on laser paper according to claim 6, characterized in that, The evaluation value of the acquired image is obtained by calculating at least one of the following methods: the sum of gradient magnitudes of the acquired image, the energy of the characteristic peaks in the Fourier transform spectrum, or the intensity of the linear characteristic detected by the Hough transform.

8. The method for automatically detecting the tilt angle of the laser beam on laser paper according to claim 7, characterized in that, The method for calculating the sum of gradient magnitudes of the acquired images to obtain the evaluation value of the acquired images is as follows: Calculate the spatial gradient information of the acquired image; Based on the spatial gradient information, the cumulative gradient magnitude of all images or a portion of selected regions in the acquired image is calculated, and this cumulative gradient magnitude is used as the evaluation value of the acquired image.

9. The method for automatically detecting the tilt angle of the laser beam on laser paper according to claim 7, characterized in that, The method for calculating the energy of the characteristic peaks in the Fourier transform spectrum of the acquired image to obtain the evaluation value of the acquired image is as follows: Perform a two-dimensional Fourier transform on the acquired image to obtain the frequency domain spectrum of the acquired image; Analyze the energy distribution of the periodic features of the grating fringes on the laser paper corresponding to the frequency domain spectrum; The evaluation value of the acquired image is calculated based on the concentration or characteristic peak intensity of the energy distribution.

10. The method for automatically detecting the tilt angle of the laser beam on laser paper according to claim 7, characterized in that, The method for calculating the evaluation value of the acquired image by measuring the intensity of the linear features detected by the Hough transform is as follows: Edge detection is performed on the acquired image to obtain a binary edge image of the acquired image; Perform a Hough transform on the binary edge image to detect straight line features; The evaluation value of the acquired image is calculated based on the intensity, length, or number of the linear features.

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