Adjusting method for improving measurement performance of autocollimator

Abstract

The invention provides an adjusting method for improving the measurement performance of an autocollimator, is suitable for the field of precision manufacturing and machine tool calibration, and solves the problem of measurement errors of the autocollimator caused by various error sources. The method is based on two core characteristics: 1, the measurement error of the autocollimator has the characteristic of continuous change; and secondly, the positions, horizontally deviating from the optimal focal plane along the optical axis, of the CCD in the autocollimator are different, and the angle error variation trends of the CCD in the autocollimator are different when the CCD measures the same angle and different working distances. And in the full-scale working distance range of the autocollimator, the angle error difference between the nearest working distance and the farthest working distance during maximum angle measurement is enabled to be lower than a preset threshold value by searching a proper defocusing amount. And then fixing the CCD at a position corresponding to the defocusing amount, and recalibrating the autocollimator. According to the method, the overall measurement error of the autocollimator under the working conditions can be reduced, and the measurement performance is improved. The method has good universality and can be applied to all autocollimators.

Description

Technical Field

[0001] This invention belongs to the field of optical precision measurement technology, specifically relating to an installation and adjustment method to improve the measurement accuracy of a system. It is applicable to the installation and adjustment of all autocollimators. Background Technology

[0002] An autocollimator is an instrument that measures minute angular offsets using the principle of optical autocollimation. It is widely used in precision measurements such as angle detection of optical components, flatness detection of platforms, wobble detection of mechanical shafts, and straightness detection of precision guideways. Its basic principle is to use optical autocollimation imaging to convert the deflection angle of the target into the displacement of the image point on a photodetector (such as a CCD or PSD). The angle value is obtained by calculating the geometric relationship between the displacement and the system's focal length. With the increasing demand for high-precision and high-resolution angle measurements in engineering applications, many researchers have conducted various studies on error sources affecting the measurement performance of autocollimator systems, proposing various methods to improve their measurement performance from different perspectives.

[0003] Since the measurement performance of autocollimators can be affected by environmental conditions, system optical aberrations, and internal system noise, improvement schemes have been proposed in CN202410412838.5, CN202411137144.1, and CN202311607839.7 to enhance their measurement performance. These schemes involve modifying the autocollimator structure, optimizing some components in the system, and using corresponding algorithms to improve the measurement performance of the autocollimator.

[0004] Autocollimators acquire measurement angle information by measuring the offset of the measurement spot on a CCD (Computer-Controlled Disk Array). However, the sensitivity of the CCD to light signals introduces stray light reflected from the optical elements in the system. Furthermore, the CCD itself introduces systemic errors, such as non-uniformity in CCD manufacturing, inconsistencies in photoelectric response efficiency, electronic circuitry inconsistencies, and electronic noise. Existing technologies, such as CN202510291588.9, CN202410359194.8, and CN202111285021.9, address issues such as optical noise introduced by reflections from internal optical elements. They utilize specific algorithms to process the spot information received by the CCD, eliminating these error sources and improving the autocollimator's measurement performance. However, these studies have not fully explored the potential of CCD defocusing characteristics to enhance measurement performance. The present invention utilizes CCD defocusing to improve the measurement performance of the autocollimator. Summary of the Invention

[0005] To address the two major shortcomings of existing autocollimators: firstly, during long-distance, large-angle measurements, the energy of the beam reflected back to the autocollimator decreases and its symmetry is severely disrupted, leading to a significant increase in angle measurement error; secondly, component installation errors and optical element processing errors during installation and debugging further degrade measurement performance. This invention aims to achieve the following objectives:

[0006] Without altering the optical element structure of the autocollimator, the system error distribution is reconstructed by translating the CCD back and forth along the optical axis, re-determining the CCD position, and recalibrating the system. This achieves error balance within the full-scale working distance range and solves the problems of large errors in long-distance, large-angle measurements and the impact of installation and processing errors on performance.

[0007] It can significantly improve the overall performance of the instrument within the working distance range according to the working distance requirements, breaking the performance limitations in this scenario; the method is universal and can be directly applied to various types of autocollimators without the need for adaptation and adjustment for specific models, thus lowering the technical application threshold.

[0008] To achieve the above objectives, the present invention employs the following technical means:

[0009] This invention mainly relies on two core features:

[0010] First, the measurement error of an autocollimator exhibits a continuously changing characteristic. During angle measurement, the error of the autocollimator changes continuously with the dynamic changes in the working distance and the measurement angle. In other words, within the working distance and angle measurement range of the autocollimator, theoretically, the measurement error of the autocollimator is a continuously changing function of the angle and the working distance.

[0011] Secondly, when the CCD deviates horizontally from the optimal focal plane at different positions along the optical axis, the angular error variation trend of the autocollimator varies significantly under the condition of "same measurement angle, different working distances." Specifically, during angle measurement, even at the same measurement angle, different working distances lead to differences in the symmetry of the beam energy returning to the autocollimator; this symmetry difference results in significantly different angular errors at the same angle due to different working distances. Further detail: within the full-scale working distance range, the returned beam energy is the least at the farthest working distance, and the beam is only distributed on one side of the collimating lens. This situation causes the angular error to have opposite signs on both sides of the focal plane when the CCD defocuses.

[0012] Based on the above two core features, an adjustment method for improving the measurement performance of an autocollimator is proposed, which includes the following steps:

[0013] 1. Determining basic parameters:

[0014] First, for the autocollimator to be adjusted, we need to define three adjustment parameters: the instrument's maximum range, full-scale working distance, and preset error difference threshold, so as to provide a quantitative basis for subsequent adjustments.

[0015] Determination of full-scale range and working distance: Based on the actual application requirements of the autocollimator, and combined with the autocollimator's own aperture and maximum measurement angle, the "maximum range" of the autocollimator is determined, denoted as ±θ. max "Full-scale working distance" is denoted as [L] min ,L max ].

[0016] Preset error difference threshold determination: Based on the accuracy level requirements of the autocollimator, a "preset error difference threshold" is set, denoted as ΔE. The ΔE is preferably set to a value close to 0, such as the single-point stability error limit of the autocollimator. Its function is to determine whether the near-distance and far-distance errors need to be optimized. When the difference between the two errors is less than the threshold, it means that the current accuracy meets the requirements and no further adjustment is needed. When the difference is greater than or equal to the threshold, the subsequent defocus optimization step is started.

[0017] 2. Error Data Extraction

[0018] After determining the basic parameters, the maximum measurement angle θ of the autocollimator is then determined. max Collect L min With L max The initial measurement angle errors at the two locations are calculated, and their absolute values ​​are compared with ΔE. If the absolute value error is less than ΔE, it indicates that the error distribution of the autocollimator within the full-scale working distance meets the accuracy requirements, and there is no need to adjust the CCD defocus position; the adjustment process can be terminated directly. If the absolute value error is greater than ΔE, the following CCD defocus position optimization steps need to be initiated to achieve error balancing.

[0019] 3. Find the appropriate defocus amount

[0020] This step involves acquiring and linearly fitting multiple sets of data on defocus amount and maximum angle measurement errors to accurately position L. min With L max The CCD defocus position where the difference between the two maximum angular errors is less than ΔE is determined by the following steps:

[0021] 31) In L minAt the same location, keeping the maximum measurement angle constant, the CCD defocusing amount is changed along the optical axis of the autocollimator (the defocusing amount is defined as "the distance between the actual position of the CCD and the initial optimal focal plane", where "the direction closer to the collimating lens" is recorded as positive defocusing and "the direction farther from the collimating lens" is recorded as negative defocusing). For each defocusing amount, the error value corresponding to the maximum measurement angle is measured. The angle error under each defocusing amount is measured repeatedly and the average value is calculated (the purpose of repeated measurement is to reduce the influence of random errors in the system), forming a dataset of "defocusing amount - maximum measurement angle error" at the closest working distance.

[0022] 32) In L max Repeat the above measurement process to form a dataset of "defocus amount - maximum measurement angle error" at the farthest working distance.

[0023] 33) Using the defocus amount as the independent variable and the error value at the corresponding working distance as the dependent variable (vertical axis), the least squares method is used to perform linear fitting on the two sets of datasets mentioned above, resulting in two linear fitting equations.

[0024] 34) Since the angle transformation trends are different at the closest and furthest working distances, the two fitted lines must intersect. The x-coordinate value of the intersection point is the CCD defocus amount. Translate the CCD along the optical axis to the position corresponding to this defocus amount and fix it to complete the determination of the CCD defocus position.

[0025] 35) Detecting L min With L max Check if the absolute value of the difference between the measurement angle errors at the two locations is less than the preset error difference threshold ΔE. If it is not less than ΔE, it may be because the defocus amount set in steps 31) and 32) is too small or the average number of times when taking the angle error is too few, resulting in a large deviation between the defocus amount obtained from the intersection of the fitted straight lines and the true value. The defocus amount and the average number of times should be increased and steps 31) and 32) should be repeated until the condition is met.

[0026] 4. Recalibrate the autocollimator

[0027] After fixing the CCD target's out-of-focus position, in L min The autocollimator was recalibrated, and the calibrated autocollimator exhibited good measurement performance at the full-scale working distance.

[0028] Compared with the prior art, the present invention has the following significant advantages:

[0029] 1. Redefine the optimal image plane determination scheme for autocollimators;

[0030] 2. Significantly improved measurement accuracy: By optimizing the CCD defocusing amount, the maximum error of the closest and farthest working distances within the full scale is balanced, and the overall measurement error is reduced;

[0031] 3. High versatility: This method does not require changes to the autocollimator structure. As long as the absolute value of the difference between the selected measurement angle error at the closest and farthest working distances within the selected autocollimator's working distance range is higher than its measurement accuracy, this invention can be used to improve measurement performance. It is applicable to any type of autocollimator.

[0032] 4. High-performance angle measurement can be achieved using low-cost lenses, reducing the system's optical design and lens manufacturing costs. Attached Figure Description

[0033] Figure 1 A diagram illustrating beam loss at the same angle using existing technology at both near and far working distances; Figure 1 (a) is a schematic diagram of the beam during close-range measurement; Figure 1 (b) is a schematic diagram of the beam during long-distance measurement;

[0034] Figure 2 A schematic diagram of the autocollimator structure in a specific embodiment;

[0035] Figure 3 A specific embodiment of the single-lens autocollimator CCD at the focal plane shows the relationship between the deflection angle and the angle measurement error at different working distances.

[0036] Figure 4 A specific embodiment of the linear fitting relationship between angle measurement error and defocusing amount of a single-lens autocollimator under the same measurement angle of 1000″ and working distances of 100mm and 2600mm;

[0037] Figure 5 A specific embodiment shows the relationship between the deflection angle and the angle measurement error at different working distances, determined by translating the CCD to the intersection of the fitted straight line with the optimal image plane. Detailed Implementation

[0038] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0039] Please see Figure 2 The autocollimator model in the specific embodiment includes a light source 1, a beam splitter 2, a collimating lens 3, a plane mirror 4, and a CCD 5.

[0040] This embodiment provides an adjustment method for improving the measurement performance of an autocollimator, including the following steps:

[0041] (1) Determination of basic parameters

[0042] Figure 2The autocollimator model in the image uses a single lens with a 50mm aperture and a 500mm focal length. The full-scale range of the autocollimator model is set to ±1000″, and the full-scale working distance is [100, 2600]mm. The preset error difference threshold is set to 0.05″.

[0043] (2) Error data extraction

[0044] After determining the basic parameters, control the rotation and movement of the reflector, using a working distance step of 500mm and a measurement angle step of 100″, to obtain the following results: Figure 3 The graph shows the relationship between the actual angle and the angle measurement error at different working distances. For the autocollimator's maximum measurement angle of 1000″, initial measurement errors were collected at 100mm and 2600mm. Figure 3 It can be seen that the absolute value of the error between the two measured angles reaches 0.46″, which is greater than the preset error difference threshold. Proceed to the next step.

[0045] (3) Find the appropriate defocus amount

[0046] 31) At 100mm, keep the maximum measurement angle of 1000″ constant, change the CCD defocus amount along the optical axis of the autocollimator, set the defocus amount range to [-0.2, 0.2]mm (here negative defocus is the direction away from the lens, positive defocus is the direction closer to the lens), and the step size is 0.02mm. For each defocus amount, measure the error value corresponding to the maximum measurement angle to form a dataset of "defocus amount - maximum measurement angle error" at a working distance of 100mm.

[0047] 32) Keep the defocus range, step size and measurement conditions unchanged, repeat the above measurement process at 2600mm to form the "defocus amount - maximum measurement angle error" dataset at a working distance of 2600mm;

[0048] 33) Using the defocus amount as the independent variable and the error value at the corresponding working distance as the dependent variable, the least squares method is used to perform linear fitting on the two datasets mentioned above, and the fitted lines are given as follows: Figure 4 As shown;

[0049] 34) The two linear fitting equations obtained from the solution will have an intersection point because the angle transformation trends are different at the closest and farthest working distances. The x-coordinate of the intersection point, 0.0987mm, is the CCD defocusing amount. The CCD is then translated along the optical axis to the position corresponding to this defocusing amount and fixed to complete the determination of the CCD defocusing position.

[0050] 35) If the absolute value of the difference between the measurement angle errors at 100mm and 2600mm is 0.05″ and does not exceed the preset error difference threshold of 0.05″, then proceed to step (4) to recalibrate the autocollimator;

[0051] (4) Recalibrate the autocollimator

[0052] After fixing the CCD target defocus position, the autocollimator is recalibrated at a working distance of 100mm within a measurement angle range of ±1000″. Following the mirror angle deflection and working distance settings in step (2), the relationship between the actual angle and the angle measurement error is re-measured at a full-scale range of ±1000″ and a full-scale working distance range of [100, 2600]mm, as shown in the graph. Figure 5 As shown.

[0053] Depend on Figure 3 and Figure 5 It can be seen that the measurement accuracy of the autocollimator with the CCD at its optimal image plane at full scale is 0.92″ (PV value) within ±1000″ of full scale and within the working distance [100, 2600] mm. Following the above steps, the autocollimator with the CCD at its optimal image plane has an error of 0.46″ (PV value) within ±1000″ of full scale and within the working distance [100, 2600] mm, thus doubling the measurement accuracy.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for adjusting the measurement performance of an autocollimator, characterized in that, Includes the following steps: (1) For the autocollimator to be adjusted, determine its full-scale working distance and preset error difference threshold. (2) Select the nearest and farthest working distances corresponding to the maximum measurement angle of the autocollimator, and extract the measurement angle error at the two working distances respectively; (3) Determine whether the difference in measurement angle error under the above two working distances is less than the preset error difference threshold; (4) If the error difference is greater than the preset error difference threshold, the following steps are performed: the CCD is adjusted to the target defocus position by an appropriate method. The position must satisfy the following: within the full-scale working distance range, the maximum angle error at the nearest and farthest working distances is equal, or the difference between the two is less than the preset error difference threshold. The CCD is adjusted to the target defocus position by an appropriate method. This position must meet the following requirements: within the full-scale working distance range, the maximum angular error at the nearest and farthest working distances is equal, or the difference between the two is less than the preset error difference threshold. (5) Recalibrate the autocollimator at the closest working distance of the full-scale working distance.

2. The autocollimator adjustment method according to claim 1, characterized in that, The full-scale working distance in step (1) is determined based on the actual application requirements of the instrument and the adjustment range allowed by the autocollimator itself.

3. The autocollimator adjustment method according to claim 1, characterized in that, The preset error difference threshold mentioned in step (1) can be set to a value close to 0, such as the single-point error limit of the instrument.

4. The autocollimator adjustment method according to claim 1, characterized in that, The appropriate method described in step (4) for adjusting the CCD to the out-of-focus position of the target can be performed as follows: 41) At the closest point under the full-scale working distance, measure the error value of the maximum angle under different defocus amounts; 42) At the furthest point of the full-scale working distance, measure the error value of the maximum angle at different defocus amounts; 43) Using the defocus amount as the independent variable and the maximum angle measurement error value at the corresponding working distance as the dependent variable, perform linear fitting on the two sets of data obtained in steps 41) and 42) respectively to obtain the fitting lines of "defocus amount - maximum angle measurement error value" corresponding to the closest working distance and the farthest working distance. 44) By selecting the defocus amount corresponding to the intersection of the two fitted straight lines, the defocus position of the target CCD can be determined.

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