Optical component scratch removal method based on variable removal function

Abstract

The present invention relates to the field of optical processing technology, and in particular to a method for removing scratches from optical elements based on a variable removal function, wherein a defect detection unit is controlled to extract the position coordinates of scratches on a large-caliber optical element, and then the position coordinates are fitted to obtain a scratch curve, a normal vector is solved based on the scratch curve, and the fitted scratch curve data is inserted into an ideal processing trajectory file to form a new trajectory file, and while modifying the trajectory, the direction of a magnetorheological removal function at a trajectory point is changed to be the same as the calculated normal vector method, and finally a magnetorheological polishing device is used to quickly remove scratches. The present invention can improve the efficiency of removing scratches by magnetorheological polishing, and has obvious advantages, especially when removing scratches on the surface of large-caliber optical elements.

Description

Technical Field

[0001] The invention belongs to the technical field of optical processing, and in particular relates to a method for removing scratches from an optical element based on a variable removal function. Background Art

[0002] Surface scratches may occur during the manufacturing, transportation and use of optical components, which will reduce the laser loss threshold of high-intensity laser systems and increase the scattering loss of imaging optical systems. Therefore, scratches must be suppressed before the optical components are formally assembled into the system.

[0003] Traditional scratch removal methods include contact removal technologies such as magnetorheological polishing technology and airbag polishing technology, and non-contact removal technologies such as HF (hydrogen fluoride) etching and plasma etching. Non-contact removal technology achieves the purpose of passivation and scratch removal through chemical action or ion impact, and has high removal efficiency, but the caliber of the optical components that can be processed is small, and the products generated after the reaction will limit the improvement of the laser loss threshold of the optical components.

[0004] Contact removal technologies such as magnetorheological polishing technology and airbag polishing technology need to traverse the entire surface of the optical element to remove scratches. Among them, magnetorheological polishing technology is favored due to its flexible manufacturing characteristics. When using a magnetorheological polishing device to traverse the entire surface to remove scratches, a grating track is generally used, and the direction of the magnetorheological removal function is fixed. Due to the randomness of the distribution of scratches on the surface of the optical element, the ideal grating track point may not necessarily coincide with the point where the scratch is located (the scratch is regarded as a curve composed of discrete points); the ideal removal function direction may not be perpendicular to the scratch. The uncertainty of these two points causes the efficiency of magnetorheology in removing scratches to be uncertain. Therefore, the problem with magnetorheological polishing technology is that the direction of magnetorheology in removing scratches is random, and the maximum efficiency of scratch removal cannot be achieved, which is very unfriendly for the removal of scratches on large-diameter optical elements. Summary of the invention

[0005] In view of this, the invention aims to provide an optical element scratch removal method based on a variable removal function, extracting the position coordinates of scratches on a large-caliber optical element by a defect detection unit, then fitting the position coordinates to obtain a scratch curve, solving the normal vector based on the scratch curve, inserting the fitted scratch curve data into an ideal processing trajectory file to form a new trajectory file, modifying the trajectory while changing the direction of the magnetorheological removal function at the trajectory point to make it the same as the calculated normal vector method, and finally using a magnetorheological polishing device to quickly remove scratches. The present invention can improve the efficiency of magnetorheological polishing in removing scratches, especially when removing scratches on the surface of large-caliber optical elements, and has obvious advantages.

[0006] To achieve the above object, the technical solution created by the present invention is implemented as follows:

[0007] A method for removing scratches from an optical element based on a variable removal function comprises the following steps:

[0008] S1: Setting a defect detection unit at the tool end of a magnetorheological polishing driving unit, and setting an optical element for scratches to be detected within a detection range of the defect detection unit;

[0009] S2: Control the defect detection unit to extract the scratch position coordinates on the optical element, and then fit the scratch position coordinates to obtain a scratch curve; calculate the normalized normal vector of each point on the scratch curve;

[0010] S3: replace the defect detection unit with a magnetorheological polishing module and install it on the tool end of the magnetorheological polishing drive unit; generate a processing trajectory file according to the surface shape of the optical element; insert the scratch curve into the processing trajectory file point by point;

[0011] S4: changing the direction of the magnetorheological removal function of the insertion point to the corresponding normalized normal vector, and generating a new processing trajectory file after changing the polishing trajectory and the magnetorheological removal function direction;

[0012] S5: Control the magnetorheological polishing module to perform full-aperture processing on the optical element according to the new processing trajectory file.

[0013] Further, step S2 includes the following steps:

[0014] S21: Calculate the distance between adjacent points on each scratch according to the scratch position coordinates, and then obtain the normalized cumulative arc length of each point on each scratch;

[0015] S22: setting the B-spline order of the fitting, and calculating the B-spline control points according to the normalized cumulative arc length of each point on each scratch;

[0016] S23: setting a B-spline node vector, and performing B-spline fitting in combination with the B-spline order and the B-spline control points to obtain a scratch curve of each scratch;

[0017] S24: Calculate the normalized normal vector of each point on the scratch curve.

[0018] Furthermore, in step S21, the distance between adjacent points on each scratch is calculated by the following formula, namely:

[0019] ;

[0020] in, represents the distance between the i-th point and the i-1-th point in the k-th scratch, represents the position coordinates of the i-th point of the k-th scratch, represents the position coordinates of the i-1th point of the kth scratch, , n represents the total number of points on each scratch;

[0021] According to distance Get the cumulative arc length of each point on each scratch :

[0022] ;

[0023] Where j represents the number of points on each scratch;

[0024] The cumulative arc length of each point on each scratch is calculated by the following formula Perform normalization to obtain the normalized cumulative arc length of each point on each scratch :

[0025] .

[0026] Further, in step S22, the normalized cumulative arc length of each point on each scratch is calculated by the following formula: The corresponding fitting position coordinates :

[0027] ;

[0028] in, , m represents the number of B-spline control points, M represents the maximum number of B-spline control points, and p represents the B-spline order. represents the mth B-spline control point when fitting the kth scratch, represents the B-spline basis function of the m-th B-spline control point under the B-spline order p when fitting the k-th scratch;

[0029] The position coordinates of the i-th point of the k-th scratch are calculated by the following formula: The corresponding fitting position coordinates The gap between E:

[0030] ;

[0031] Minimize the above formula to get the B-spline control point when fitting the k-th scratch .

[0032] Further, in step S23, the B-spline node vector u is:

[0033] ;

[0034] At this time, each scratch curve is obtained :

[0035] .

[0036] Further, in step S24,

[0037] Find each scratch curve The derivative of , and each scratch curve is obtained The tangent vector ,Right now:

[0038] ;

[0039] Find the tangent vector of each scratch The derivative of , and each scratch curve is obtained The normal vector ,Right now:

[0040] ;

[0041] Normal vector Normalize to get the normalized normal vector ,Right now:

[0042] .

[0043] Further, in step S3, the surface error E0 of the optical element is measured by an interferometer, and a test piece made of the same material as the optical element is processed by a magnetorheological polishing module to obtain a magnetorheological removal function RF; the residence time T is obtained by the following formula:

[0044] ;

[0045] in, Represents convolution calculation; converts the dwell time T into a processing trajectory file.

[0046] Compared with the prior art, the invention can achieve the following beneficial effects:

[0047] In the optical element scratch removal method based on the variable removal function created by the present invention, the scratch point cloud data of the full-aperture optical surface is extracted through the defect detection unit to obtain the position coordinates of the scratches on the large-aperture optical element; then the position coordinates are fitted to obtain the scratch curve, the normal vector is solved based on the scratch curve, and the fitted scratch curve data is inserted into the ideal processing trajectory file to form a new trajectory file, and the direction of the magnetorheological removal function at the trajectory point is changed while modifying the trajectory to make it the same as the calculated normal vector method, and finally the magnetorheological polishing device is used to quickly remove the scratches. Compared with the traditional method, the method proposed by the present invention can improve the efficiency of magnetorheological polishing to remove scratches, especially when removing scratches on the surface of large-aperture optical elements, it has obvious advantages. The method proposed by the present invention has significant advantages in improving the post-processing efficiency of high-power laser optical elements and reducing manufacturing costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The drawings constituting part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments and descriptions of the present invention are used to explain the present invention and do not constitute an improper limitation on the present invention. In the drawings:

[0049] Figure 1 A schematic flow chart of a method for removing scratches from an optical element based on a variable removal function according to an embodiment of the present invention;

[0050] Figure 2 A schematic diagram of the detection and use of the optical element scratch removal method based on a variable removal function according to an embodiment of the present invention;

[0051] Figure 3 To create a motion trajectory of the defect detection unit described in the embodiment of the present invention;

[0052] Figure 4 It is a schematic diagram of vertical removal and parallel removal;

[0053] Figure 5 A schematic diagram of the processing and use of the optical element scratch removal method based on a variable removal function according to an embodiment of the present invention;

[0054] Figure 6 Schematic diagram of the trajectory and magnetorheological removal function described in the embodiment of the present invention.

[0055] Description of reference numerals:

[0056] 1. Magnetorheological polishing drive unit; 2. Defect detection unit; 3. Optical element; 4. Support platform; 5. Magnetorheological polishing module. DETAILED DESCRIPTION

[0057] In order to make the purpose, technical solution and advantages of the invention more clear, the 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 here are only used to explain the invention and do not constitute a limitation of the invention.

[0058] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0059] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. In addition, the terms "first", "second", etc. are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, features defined as "first", "second", etc. may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, "multiple" means two or more.

[0060] In the description of the invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installation", "connection" and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the invention can be understood according to specific circumstances.

[0061] The present invention will be described in detail below with reference to the accompanying drawings and in combination with embodiments.

[0062] like Figure 1 As shown, the optical element scratch removal method based on the variable removal function described in the embodiment of the present invention comprises the following steps:

[0063] S1: Figure 2As shown, the defect detection unit 2 is arranged at the tool end of the magnetorheological polishing driving unit 1, and the optical element 3 to be detected for scratches is placed on the supporting platform 4, and the optical element 3 is within the detection range of the defect detection unit 2, ensuring that the defect detection unit 2 can completely scan the optical element 3. The coordinates of the optical element 3 and the coordinates of the magnetorheological polishing driving unit 1 are unified in a coordinate system, thereby facilitating the extraction and fitting of the position coordinates of the scratches.

[0064] Among them, the magnetorheological polishing drive unit 1 can be a magnetorheological polishing machine tool (such as a conventional CNC machine tool) or a new magnetorheological polishing processing equipment such as an industrial robot. In an embodiment of the present invention, the magnetorheological polishing drive unit 1 preferably adopts a robotic arm, and the defect detection unit 2 is arranged at the tool end of the robotic arm.

[0065] The defect detection unit 2 in the embodiment of the present invention preferably adopts a high-resolution industrial camera of model MV-CE200-11UM / C, a resolution of 5472×3648, and a pixel size of 2.4μm×2.4μm, which is a device for defect detection on the surface of the optical element 3 and can distinguish scratches with a width of tens of microns. The defect detection unit 2 uses a scattering imaging method to detect defects on the surface of the optical element 3. When the incident light is irradiated on the surface defect, the defect area will produce chaotic scattering. Although the roughness of the reflector surface will also cause a certain degree of scattering, the scattering directionality and intensity of the surface defects are significantly higher in comparison. In the dark field imaging mode, the defect detection unit 2 uses six ring lights for grazing incidence illumination, which can effectively highlight the scattering of the surface defect area. When there are defects on the surface, these areas will show bright scratch morphology in the image captured by the camera. This grazing incidence illumination has the characteristics of high efficiency and is suitable as a rough scanning method to find surface defects. The fine scanning mode of the defect detection unit 2 uses high-power white light for vertical illumination. At surface defects, scattering also occurs, but scratches appear as dark shadows. This type of illumination has a high signal-to-noise ratio and is therefore often used to accurately calculate the size and shape of surface defects.

[0066] Before using the defect detection unit 2 to perform defect detection on the optical element 3, it is necessary to calibrate the defect detection unit 2 using a standard part with known defect information, that is:

[0067] The standard part is placed on the support platform 4, and the magnetorheological polishing driving unit 1 is driven, so that the magnetorheological polishing driving unit 1 drives the defect detection unit 2 to move along the surface of the standard part. At the same time, the defect detection unit 2 performs defect detection on the surface of the standard part to obtain the detected defect position coordinates. . Get the coordinates of the defect location The actual defect position coordinates of the standard part The difference ,Right now:

[0068]

[0069] If the difference If the error range is within the error range, it is considered that the defect detection unit 2 has been calibrated. Otherwise, the defect detection unit 2 needs to be recalibrated. In the embodiment of the present invention, the error range is less than 0.01 mm, that is, , and All are less than 0.01mm.

[0070] S2: Control the defect detection unit 2 to move along the surface of the optical element 3 as follows: Figure 3 The scratch position coordinates on the optical element 3 are extracted and then the scratch position coordinates are fitted to obtain the scratch curve. Figure 3 The direction of the blue arrow in the middle is the step direction of the defect detection unit 2, and the direction of the red arrow is the track gauge direction of the motion trajectory of the defect detection unit 2. Then, the normalized normal vector of each point on the scratch curve is calculated.

[0071] The fitting method for the scratch curve in step S2 includes but is not limited to the least square method, the polynomial fitting method and the B-spline fitting method. In the embodiment of the present invention, the B-spline fitting method is preferably used. Step S2 includes the following steps:

[0072] S21: Calculate the distance between adjacent points on each scratch according to the scratch position coordinates, and then obtain the normalized cumulative arc length of each point on each scratch.

[0073] The distance between adjacent points on each scratch is calculated by the following formula:

[0074] ;

[0075] in, represents the distance between the i-th point and the i-1-th point in the k-th scratch, represents the position coordinates of the i-th point of the k-th scratch, represents the position coordinates of the i-1th point of the kth scratch, , n represents the total number of points on each scratch.

[0076] According to distance Get the cumulative arc length of each point on each scratch :

[0077] ;

[0078] Where j represents the number of points on each scratch;

[0079] The cumulative arc length of each point on each scratch is calculated by the following formula Perform normalization to obtain the normalized cumulative arc length of each point on each scratch :

[0080] .

[0081] S22: setting the B-spline order of the fitting, and calculating the B-spline control points according to the normalized cumulative arc length of each point on each scratch.

[0082] The normalized cumulative arc length of each point on each scratch is calculated by the following formula: The corresponding fitting position coordinates :

[0083] ;

[0084] in, , m represents the number of B-spline control points, M represents the maximum number of B-spline control points, that is, there are M+1 B-spline control points, p represents the B-spline order, represents the mth B-spline control point when fitting the kth scratch, represents the B-spline basis function of the m-th B-spline control point at B-spline order p when fitting the k-th scratch.

[0085] The higher the order of B-spline fitting, the smoother the fitting curve will be, but the corresponding calculation will be more complicated. The third-order B-spline is sufficient to fit the scratches on the surface of the optical element 3. Therefore, in the embodiment of the present invention, the third-order B-spline fitting is preferably used, that is, p=3.

[0086] The position coordinates of the i-th point of the k-th scratch are calculated by the following formula: The corresponding fitting position coordinates The gap between E:

[0087] ;

[0088] Use the least squares method to minimize the above formula and get the B-spline control points when fitting the k-th scratch. .

[0089] S23: setting a B-spline node vector, and performing B-spline fitting in combination with the B-spline order and the B-spline control points to obtain a scratch curve of each scratch.

[0090] The selection of the B-spline node vector will affect the shape of the B-spline. Common node vectors are uniformly distributed, but non-uniform node vectors can be used for complex data. In the embodiment of the present invention, uniform node vectors are preferably used to fit scratches. Therefore, for a p-order B-spline and M+1 control points, the B-spline node vector u is:

[0091] ;

[0092] In the above formula, for a B-spline order of p and M+1 control points, the length of the B-spline knot vector u is M+1+p+1 and is uniformly distributed in [0,1]. Taking the third-order B-spline with M=16 as an example, the length of the B-spline knot vector u is 21, and the corresponding B-spline knot vector u is:

[0093] .

[0094] According to the B-spline control points Perform B-spline fitting with the B-spline node vector u to obtain each scratch curve :

[0095] .

[0096] S24: Since the shape of the magnetorheological removal function is non-circularly symmetrical, it has a major axis and a minor axis. It is approximately axisymmetric in the major axis direction and asymmetric in the minor axis direction. If the major axis direction of the magnetorheological removal function is consistent with the normal vector direction of the scratch curve, then the magnetorheological removal function is considered to be perpendicular to the scratch removal, that is, the major axis direction of the magnetorheological removal function is the direction of the magnetorheological removal function. If the major axis direction of the magnetorheological removal function is perpendicular to the normal vector direction of the scratch curve, then the magnetorheological removal function is considered to be parallel to the scratch removal. Vertical removal and parallel removal are as follows: Figure 4 Therefore, in order to determine that the major axis direction of the magnetorheological removal function is the direction of the magnetorheological removal function, that is, the magnetorheological removal function is perpendicular to the scratch removal, it is necessary to solve the normal vector of the scratch parameterization curve.

[0097] First, find each scratch curve The derivative of , and each scratch curve is obtained The tangent vector ,Right now:

[0098] ;

[0099] Find the tangent vector of each scratch The derivative of , and each scratch curve is obtained The normal vector ,Right now:

[0100] ;

[0101] Normal vector Normalize to get the normalized normal vector ,Right now:

[0102] ;

[0103] S3: The magnetorheological polishing module 5 replaces the defect detection unit 2 and is installed on the tool end of the magnetorheological polishing drive unit 1, such as Figure 5 As shown. A processing trajectory file is generated according to the surface shape of the optical element 3; the scratch curve is inserted into the processing trajectory file point by point, and the position inserted into the processing trajectory file should be the same as the closest position of the scratch in the processing trajectory file.

[0104] In step S3, the magnetorheological polishing structure in the invention patent application with Chinese patent publication number CN113352152A, publication date September 7, 2021, and patent name "A magnetorheological polishing processing system based on a robotic arm" is preferably integrated into the magnetorheological polishing module 5 in the embodiment of the present invention.

[0105] In the embodiment of the present invention, the surface error E0 of the optical element 3 is measured by an interferometer, and the magnetorheological polishing module 5 is used to process the test piece of the same material as the optical element 3 to obtain the magnetorheological removal function RF; the residence time T is obtained by the following formula:

[0106]

[0107] in, Represents convolution calculation.

[0108] In an embodiment of the present invention, a python transcoding program is used to convert the residence time distribution into a processing code that can be recognized by the magnetorheological polishing drive unit 1 to obtain a processing trajectory file, which contains the ideal polishing trajectory, polishing speed and direction of the magnetorheological removal function RF of the magnetorheological polishing module 5 for the optical element 3.

[0109] S4: Change the direction of the magnetorheological removal function at the insertion point to the corresponding normalized normal vector, so that the direction of the magnetorheological removal function RF is parallel to the normal direction of the polishing trajectory curve at the insertion point (equivalent to the direction of the magnetorheological removal function RF being perpendicular to the scratch curve), and generate a new processing trajectory file after changing the polishing trajectory and the magnetorheological removal function direction. The polishing trajectory and magnetorheological removal function in the new processing trajectory file are as follows: Figure 6 As shown, Figure 6 The red line in the middle is the modified polishing trajectory in the new processing trajectory file.

[0110] S5: Control the magnetorheological polishing module 5 to perform full-aperture processing on the optical element 3 according to the new processing trajectory file.

[0111] It should be understood that the various forms of processes shown above can be used to reorder, add or delete steps. For example, the steps described in the disclosure of the present invention can be performed in parallel, sequentially or in different orders, as long as the desired results of the technical solution disclosed in the present invention can be achieved, and this document does not limit this.

[0112] The above specific implementations do not constitute a limitation on the protection scope of the present invention. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modification, equivalent substitution and improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for removing scratches from an optical element based on a variable removal function, characterized in that: The following steps are involved: S1: Setting a defect detection unit at a tool end of a magnetorheological polishing driving unit, and setting an optical element for scratches to be detected within a detection range of the defect detection unit; S2: Control the defect detection unit to extract the scratch position coordinates on the optical element, and then fit the scratch position coordinates to obtain a scratch curve; calculate the normalized normal vector of each point on the scratch curve; S3: replacing the defect detection unit with a magnetorheological polishing module and installing it on the tool end of the magnetorheological polishing drive unit; generating a processing trajectory file according to the surface shape of the optical element; and inserting the scratch curve into the processing trajectory file point by point; S4: changing the direction of the magnetorheological removal function of the insertion point to the corresponding normalized normal vector, and generating a new processing trajectory file after changing the polishing trajectory and the magnetorheological removal function direction; S5: controlling the magnetorheological polishing module to perform full-aperture processing on the optical element according to the new processing trajectory file; Step S2 includes the following steps: S21: Calculate the distance between adjacent points on each scratch according to the scratch position coordinates, and then obtain the normalized cumulative arc length of each point on each scratch; S22: setting the B-spline order of the fitting, and calculating the B-spline control points according to the normalized cumulative arc length of each point on each scratch; S23: setting a B-spline node vector, and performing B-spline fitting in combination with the B-spline order and the B-spline control points to obtain a scratch curve of each scratch; S24: Calculate the normalized normal vector of each point on the scratch curve; In step S21, the distance between adjacent points on each scratch is calculated by the following formula: ； in, represents the distance between the i-th point and the i-1-th point in the k-th scratch, represents the position coordinates of the i-th point of the k-th scratch, represents the position coordinates of the i-1th point of the kth scratch, , n represents the total number of points on each scratch; According to the distance Get the cumulative arc length of each point on each scratch : ； Where j represents the number of points on each scratch; The cumulative arc length of each point on each scratch is calculated by the following formula Perform normalization to obtain the normalized cumulative arc length of each point on each scratch : ； In step S22, the normalized cumulative arc length of each point on each scratch is calculated by the following formula: The corresponding fitting position coordinates : ； in, , m represents the number of the B-spline control points, M represents the maximum number of the B-spline control points, p represents the B-spline order, represents the mth B-spline control point when fitting the kth scratch, represents the B-spline basis function of the m-th B-spline control point at the B-spline order p when fitting the k-th scratch; The position coordinates of the i-th point of the k-th scratch are calculated by the following formula: The corresponding fitting position coordinates The gap between E: ； Minimize the above formula to get the B-spline control point when fitting the k-th scratch ; In step S23, the B-spline node vector u is: ； At this time, each scratch curve is obtained : ； In step S24, Find each scratch curve The derivative of , and each scratch curve is obtained The tangent vector ,Right now: ； Find the tangent vector of each scratch The derivative of , and each scratch curve is obtained The normal vector ,Right now: ； Normal vector Normalize to get the normalized normal vector ,Right now: ； In step S3, the surface error E0 of the optical element is measured by an interferometer, and the magnetorheological polishing module is used to process a test piece made of the same material as the optical element to obtain the magnetorheological removal function RF; the residence time T is obtained by the following formula: ； in, Represents convolution calculation; converts the residence time T into the processing trajectory file.

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