Laser auto-collimation differential alignment angle measuring method and device

Abstract

The invention relates to the technical field of precision optical detection, and discloses a laser auto-collimation differential alignment angle measuring method and device. The device comprises a point light source, a first spectroscope located in the emergent direction of the point light source, and a collimating mirror and a measured prism element which are sequentially arranged along the reflection direction of the first spectroscope, the rotating table bears the prism element to be measured and is connected with an angle measuring module for measuring the rotating angle of the prism element; the detection unit comprises a slit plate and a photoelectric detector which are sequentially arranged in the direction opposite to the reflection direction of the first spectroscope, the photoelectric detector is an array type detector or a quadrant type detector which are symmetrically distributed, and the slit plate is provided with a first slit and a second slit which are arranged in parallel; according to the invention, high-resolution alignment of the surface of the measured prism element is realized, the influence of the element tower difference on the measurement result is inhibited, and the measurement precision of the plane included angle of the measured prism element is improved.

Description

Technical Field

[0001] This invention relates to the field of precision optical inspection technology, and in particular to a laser autocollimation differential alignment angle measurement method and device for high-precision measurement of the plane angle of optical prisms, common mirrors and prism elements. Background Technology

[0002] Planar prism elements are widely used in aerospace, satellite exploration, and military weaponry. The error of the plane angle of the prism seriously affects important performance parameters such as imaging quality, reliability, and stability of the optical system. With the improvement of my country's military industry and the development of national defense needs, higher precision and quality requirements have been put forward for the processing of high-precision optical prisms and many optical and mechanical parts with strict requirements on angles in plane mirror prism optical systems. Therefore, high-precision measurement of the plane angle of prism elements is of great significance.

[0003] For measuring the included angle of prism components, there are mainly several methods, including the optical indexing head method, circular grating method, optical internal reflection method, interferometry, fiber optic method, ring laser method, and autocollimation method. The optical indexing head method offers high accuracy and sensitivity, but its reliance on visual reading of the measured information results in low efficiency and cumbersome operation. The circular grating method offers high accuracy and stability, enabling full-circumference measurements, but its dynamic characteristics are poor, and precise alignment between the grating and the turntable is crucial. The optical internal reflection method features small instrument size and simple structure, but its measurement accuracy is not high. The interferometry method offers high resolution and accuracy, but its operating conditions are demanding, its environmental adaptability is poor, and its cost is high. The fiber optic method offers high sensitivity and wide bandwidth, but its measurement resolution is low and its dynamic range is small. The ring laser method offers a wide dynamic range but is costly, and it is sensitive to environmental factors, resulting in significant measurement errors in conventional environments. Among existing methods, autocollimation is widely used as an important means of measuring angles due to its advantages of high measurement resolution and high measurement accuracy. However, when autocollimation uses a long-focal-length autocollimating tube for high-sensitivity alignment, the focused spot is large and easily affected by the surface area and reflectivity of the measured element, which limits the angle measurement accuracy of autocollimation.

[0004] To address the aforementioned issues, the inventors developed a laser self-collimation peak alignment angle measurement method. This method utilizes the self-collimation peak alignment principle to align the surface of the measured prism element. By combining a long-focal collimation optical path with lateral confocal detection, the surface alignment resolution of the measured prism element is significantly improved. Combined with a circular grating, the included angle of the measured prism element is measured. This method can achieve high-resolution and high-precision measurement of the included angle of the measured prism element. However, during use, it was found that the bell curve of the signal sensed by the detector has a relatively flat top, and the signal strength is not sensitive to small changes in position, resulting in limited angle positioning accuracy. At the same time, signal processing requires finding the peak point of the signal strength as a reference point, but the absolute signal strength is affected by factors such as light source fluctuations, sample reflectivity, and surface tilt, leading to zero-point drift.

[0005] Therefore, the inventors made further improvements and proposed a laser autocollimation differential alignment angle measurement method and device to overcome the above problems. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a laser autocollimation differential alignment angle measurement method and apparatus, which achieves high-resolution alignment of the surface of the measured prism element, suppresses the influence of element tower difference on the measurement results, and improves the measurement accuracy of the plane angle of the measured prism element.

[0007] The first aspect of this invention provides a laser autocollimation differential alignment angle measuring device, comprising: a point light source, a first beam splitter located in the emission direction of the point light source, a collimating mirror and a prism element to be measured arranged sequentially along the reflection direction of the first beam splitter; a rotary stage carrying the prism element to be measured, and an angle measuring module for measuring its rotation angle connected to the rotary stage; a detection unit comprising a slit plate and a photodetector arranged sequentially in the opposite direction to the reflection direction of the first beam splitter, the photodetector being a symmetrically distributed array detector or a quadrant detector, and the slit plate having a first slit and a second slit arranged in parallel; a controller connected to the point light source, the rotary stage, the angle measuring module, and the laser autocollimation differential alignment angle measuring module. The photodetector is connected, and the controller controls the point light source to emit a diverging beam and controls the rotation of the rotary table. The diverging beam is collimated into parallel light after passing through the first beam splitter and collimating lens and then illuminates the prism element under test. The rotation of the rotary table causes the prism element under test to rotate. The focused light spot on the slit plate passes through the first slit and the second slit in sequence. The photodetector detects the two light intensity signals that pass through the first slit and the second slit respectively. The controller uses the difference between the two light intensity signals as the vertical axis and the rotation angle of the rotary table as the horizontal axis to draw a differential alignment curve. The differential alignment curve is used to obtain the angles of each face of the prism element under test based on the angle corresponding to its zero crossing point.

[0008] Optionally, the slot plate is made of an opaque material and the thickness of the slot plate is greater than 0.1mm.

[0009] Optionally, the width of the first slit and the second slit is 0.1 to 1 times the diameter of the light spot.

[0010] Optionally, a coarse aiming unit located between the first beam splitter and the collimating lens is also included. The coarse aiming unit detects the optical path information in front of the photodetector. The controller is connected to the coarse aiming unit and adjusts the tilt angle and position of the prism element under test through the optical path information obtained by the coarse aiming unit.

[0011] Optionally, the coarse aiming unit includes a second beam splitter, a field mirror located sequentially in the reflection direction of the second beam splitter, and an image detector, with the image detector connected to the controller.

[0012] Optionally, the rotary table includes: a motor and an adjustable and self-aligning table connected by a coupling and bearings, the motor and a motor driver are connected, the motor driver is connected to a controller, and the motor speed is 0.01 degrees / second to 10 degrees / second.

[0013] Optionally, the point light source includes a laser and a light source converging mirror arranged coaxially.

[0014] Optionally, the angle measurement module includes a circular grating, which is mounted on the rotating shaft of the rotary table.

[0015] Optionally, the photodetector can be one of a large-area two-quadrant light intensity detector, a large-area four-quadrant light intensity detector, or a large-area CCD.

[0016] A second aspect of the present invention provides a method for measuring laser autocollimation differential alignment angles, using the aforementioned laser autocollimation differential alignment angle measuring device, comprising the following steps: The divergent beam emitted by the point light source is reflected by the first beam splitter and then collimated into parallel light by the collimating lens. The parallel light illuminates the prism element under test. The light reflected by the prism element under test is converged by the collimating lens and then by the first beam splitter to form a focused spot on the slit plate. As the prism element being measured rotates with the rotary table, the focused spot moves laterally, passing through the first slit and the second slit in sequence. The photodetector detects the light intensity passing through the first slit and the second slit respectively, and obtains two light intensity signals. The angle measurement module measures the corresponding rotation angle of the rotary table. The difference between the two light intensity signals is used as the vertical axis and the rotation angle of the rotary table is used as the horizontal axis to draw a differential alignment curve. Each vertical face of the prism element under test can be measured to obtain a differential alignment curve. The angle corresponding to the zero point of each differential alignment curve is the angle of each face of the prism element under test.

[0017] The technical solution provided by the embodiments of the present invention has the following advantages compared with the prior art: The laser autocollimation differential alignment angle measurement method and device provided in this invention employs autocollimation differential alignment, which can overcome the diffraction limit. While leveraging the high sensitivity of the long-focal autocollimation optical path, it suppresses the influence of long-focal diffraction, improves alignment resolution, and achieves high-precision alignment of the surface of the measured prism element. The influence of the prism element's structural deviation is suppressed through a photodetector and a one-dimensional double-slit design. Alignment is performed using the zero-crossing point of the differential alignment signal, effectively suppressing the influence of differences in the reflective surface area and reflectivity of the measured prism element. This improves the adaptability and repeatability of alignment for each face of the measured prism element. Compared with previous laser autocollimation peak alignment angle measurement devices and methods, the single slit is replaced with a double slit, obtaining dual-path light intensity signals and calculating the difference to obtain a differential curve for differential detection. This achieves the zero-crossing point position (zero point) of the curve. The slope of the S-shaped curve is the largest, meaning that even a small axial displacement will cause a huge change in the differential signal. This makes the system extremely sensitive to the detection of defocusing, thus achieving nanometer or even sub-nanometer level resolution. It uses the zero point of the differential signal as the absolute reference for the focal position. Regardless of changes in light source intensity or sample reflectivity, as long as the photodetector's performance is symmetrical, it always bases its position on the zero-crossing point of the differential signal. This greatly suppresses common-mode interference and improves the long-term stability and reliability of measurements. Its S-curve has a region with excellent linearity near the zero point. This not only facilitates high-precision linear measurements but also allows the device to directly determine the direction and distance of sample rotation based on the magnitude and sign of the differential signal during alignment, facilitating fast and accurate closed-loop servo alignment. Due to the use of differential technology, any noise that simultaneously affects the photodetector (such as light source intensity fluctuations, electromagnetic interference, etc.) is significantly canceled out. This makes the differential confocal system more suitable for operation in environments with significant vibration and temperature fluctuations, such as industrial sites. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the structure of a laser autocollimation differential alignment angle measuring device provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of the movement of the converging light spot on the surface of the slit plate provided in an embodiment of the present invention; Figure 3 A schematic diagram of a large-area four-quadrant detector provided in an embodiment of the present invention; Figure 4 This is a structural diagram of a large-area array two-quadrant detector provided in an embodiment of the present invention; Figure 5 This is a schematic diagram of the rotary table structure provided in an embodiment of the present invention.

[0019] Explanation of reference numerals in the attached figures: 1. Point light source; 2. First beam splitter; 3. Collimating lens; 4. Measured prism element; 5. Rotary stage; 6. Circular grating; 7. Detection unit; 8. Differential alignment curve; 9. Coarse aiming unit; 10. Laser; 11. Light source converging lens; 12. Slit plate; 13. Detector; 14. Second beam splitter; 15. Field lens; 16. Image detector; 17. First slit; 18. Second slit; 19. Focused spot; 20. Quadrant 4 detector; 21. Quadrant 2 detector; 22. Quadrant 3 detector; 23. Quadrant 1 detector; 24. Quadrant 2 detector; 25. Quadrant 1 detector; 26. Controller; 27. Bearing; 28. Coupling; 29. ​​Motor; 30. Motor driver; 31. Tilt and centering stage. Detailed Implementation

[0020] The following detailed description of a specific embodiment of the present invention is provided in conjunction with the accompanying drawings. However, it should be understood that the scope of protection of the present invention is not limited to the specific embodiment.

[0021] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the technical solution of this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0022] The present invention will be described below through several specific embodiments. To keep the following description of the embodiments clear and concise, detailed descriptions of known functions and components may be omitted. When any component of an embodiment of the present invention appears in more than one drawing, the component may be represented by the same reference numerals in each drawing.

[0023] like Figure 1 and Figure 2As shown, the first embodiment of the present invention provides a laser self-collimating differential alignment angle measuring device, including: a point light source 1, a first beam splitter 2 located in the emission direction of the point light source 1, a collimating mirror 3 arranged sequentially along the reflection direction of the first beam splitter 2, and a prism element 4 to be measured. Under the command of the controller 26, the point light source 1 emits a diverging beam. After being reflected by the first beam splitter 2, the diverging beam enters the collimating mirror 3, which converts the diverging beam into high-quality parallel light. A rotary table 5 carries the prism element 4 to be measured. The rotary table 5 is connected to an angle measuring module that measures its rotation angle. The parallel light is perpendicularly irradiated on the working surface of the prism element 4. According to the principle of optical self-collimation, if the working surface is completely perpendicular to the optical axis, the light will return along the original path. If there is a slight tilt angle (i.e., angular deviation) on the working surface, the returned light will deviate from the original optical path by an angle, which is twice the tilt angle of the working surface. Under the control of the controller 26, the rotary table 5 drives the measured prism element 4 to perform a precise rotary motion. In this way, each working surface of the measured prism element 4 will pass through the measurement optical path in sequence, and the angular error information of each working surface is encoded in the deflection angle of the returned beam. The detection unit 7 includes a slit plate 12 and a photodetector 13 arranged in the opposite direction of the reflection direction of the first beam splitter 2. The photodetector 13 is a symmetrically distributed array detector or quadrant detector. The slit plate 12 has a first slit 17 and a second slit 18 arranged in parallel. The first slit 17 and the second slit 18 are arranged symmetrically with respect to the optical axis centerline. The returned beam carrying the angular information passes through the collimating lens 3 again. At this time, the collimating lens 3 acts as a converging lens, converging the returned light incident at different angles to different positions on its back focal plane (i.e., the plane where the slit plate 12 is located). For a working surface without angular error, the returned light will converge at the center of the optical axis. For a working surface with angular error, the returned light will converge into a focused spot 19 off-center. The angular deviation is converted into the position offset of the spot. When the rotary table 5 rotates, the focused spot 19 will sweep across the slit plate 12. It will pass through the first slit 17, the area between the two slits, and the second slit 18 in sequence. The photodetector 13 (array type or quadrant type) is located behind the slit plate 12 and is used to detect the light intensity transmitted through the slits. During the process of the spot sweeping, when the center of the spot coincides with the center of a certain slit, the light intensity transmitted through that slit is the maximum.Since the two slits are separate, the light intensity signal passing through the first slit 17 and the light intensity signal passing through the second slit 18 will form a waveform with a phase difference. The controller 26 is connected to the point light source 1, the rotary table 5, the angle measurement module and the photodetector 13 respectively. The controller 26 controls the point light source 1 to emit a diverging beam and controls the rotary table 5 to rotate. After passing through the first beam splitter 2 and the collimating lens 3, the diverging beam is collimated into parallel light and illuminates the prism element 4 under test. The rotation of the rotary table 5 drives the prism element 4 under test to rotate. The focused spot 19 on the slit plate 12 passes through the first slit 17 and the second slit 18 in sequence. The photodetector 13 detects the two light intensity signals passing through the first slit 17 and the second slit 18 respectively. The controller 26 uses the difference between the two light intensity signals as the vertical axis and the rotation angle of the rotary table 5 as the horizontal axis to draw the differential alignment curve 8. The differential alignment curve 8 is used to obtain the angles of each face of the prism element 4 under test according to the angle corresponding to its zero point. The controller 26 acquires the rotation angle value (horizontal axis) of the rotary table 5 provided by the angle measurement module in real time, and simultaneously calculates the real-time difference (vertical axis) between the two light intensity signals. When the light spot is located exactly in the middle of the two slits, the light intensity transmitted through the two slits is equal, and the difference is zero. This "zero point" is the optical alignment point. The plotted differential alignment curve 8 is a typical "S"-shaped curve. The zero-crossing point of the curve precisely corresponds to the instant when the working surface of the measured prism reaches the optical alignment state. By accurately capturing this zero-crossing point and corresponding it with the reading of the angle measurement module at this time, the controller can achieve ultra-high precision calibration and measurement of the corner position of each working surface of the prism.

[0024] The laser autocollimation differential alignment angle measurement method and device provided in this invention employs autocollimation differential alignment, which can overcome the diffraction limit. While leveraging the high sensitivity of the long-focal autocollimation optical path, it suppresses the influence of long-focal diffraction, improves alignment resolution, and achieves high-precision alignment of the surface of the measured prism element. The influence of the prism element's structural deviation is suppressed through a photodetector and a one-dimensional double-slit design. Alignment is performed using the zero-crossing point of the differential alignment signal, effectively suppressing the influence of differences in the reflective surface area and reflectivity of the measured prism element. This improves the adaptability and repeatability of alignment for each face of the measured prism element. Compared with previous laser autocollimation peak alignment angle measurement devices and methods, the single slit is replaced with a double slit, obtaining dual-path light intensity signals and calculating the difference to obtain a differential curve for differential detection. This achieves the zero-crossing point position (zero point) of the curve. The slope of the S-shaped curve is the largest, meaning that even a small axial displacement will cause a huge change in the differential signal. This makes the system extremely sensitive to the detection of defocusing, thus achieving nanometer or even sub-nanometer level resolution. It uses the zero point of the differential signal as the absolute reference for the focal position. Regardless of changes in light source intensity or sample reflectivity, as long as the photodetector's performance is symmetrical, it always bases its position on the zero-crossing point of the differential signal. This greatly suppresses common-mode interference and improves the long-term stability and reliability of measurements. Its S-curve has a region with excellent linearity near the zero point. This not only facilitates high-precision linear measurements but also allows the device to directly determine the direction and distance of sample rotation based on the magnitude and sign of the differential signal during alignment, facilitating fast and accurate closed-loop servo alignment. Due to the use of differential technology, any noise that simultaneously affects the photodetector (such as light source intensity fluctuations, electromagnetic interference, etc.) is significantly canceled out. This makes the differential confocal system more suitable for operation in environments with significant vibration and temperature fluctuations, such as industrial sites.

[0025] Specifically, the slit plate 12 is made of an opaque material with a thickness greater than 0.1 mm. For example, the material of the slit plate 12 can be metal, plastic, glass coated with a light-absorbing material, crystal, etc. Opaqueness means that the material has extremely low optical transmittance and extremely high extinction ratio for the laser wavelength used in the device (usually visible or near-infrared light). This ensures that only light that strictly passes through the slit can be received by the photodetector, while light illuminating the slit region is completely absorbed or reflected. Any stray light leaking from the slit region will create background noise, drowning out the weak effective signal, or causing baseline drift of the differential curve and blurred zero-crossing point judgment, thus severely degrading measurement accuracy. If the slit plate is too thin (e.g., a foil only tens of micrometers thick), the edge of the laser spot may bypass the physical edge of the slit through diffraction, or the optical boundary of the slit may be blurred due to the penumbra effect of the material. This will make the rising and falling edges of the light intensity signal smoother when the spot sweeps across, reducing the slope of the differential curve. A thickness greater than 0.1 mm ensures the slit possesses a high aspect ratio tunnel structure. This provides the beam with a clear, sharp geometric aperture. As the focused spot sweeps across, the light intensity signal undergoes an extremely steep change, resulting in a differential curve with extremely sharp edges and a very high slope. This allows the controller to determine the zero-crossing point more accurately and repeatedly. According to optical principles, when light passes through an aperture, diffraction occurs at the edges. The thinner the aperture, the more significant the diffraction effect, resulting in a complex diffraction pattern behind the slit, rather than a clear bright band. The slit plate with a certain thickness acts as an aperture stop, effectively blocking and absorbing light diffracted at large angles, making the shape of the transmitted light spot closer to the geometric projection of the slit, improving the linearity and determinism between the spot position and the light intensity signal. The substrate with a thickness greater than 0.1 mm provides the necessary mechanical rigidity and stability. It is not easily deformed by internal stress, assembly forces, or slight touches, and can maintain the accuracy of the slit spacing and parallelism over a long period of time. At the same time, the thickness also allows the slit to be manufactured using processes such as precision electrical discharge machining or laser cutting. These processes can achieve extremely high dimensional accuracy and edge quality, and are not easily damaged, ensuring the durability and consistency of the product.

[0026] Optionally, the widths of the first slit 17 and the second slit 18 are 0.1 to 1 times the spot diameter. The quality of the system's final output signal—the difference in intensity between the two beams—depends entirely on the shape of the curve (optical transfer function) showing the transmission intensity changing with position as the spot scans the slits. The ratio of the slit width to the spot diameter directly determines the shape of this curve. When the slit width is approximately equal to the spot diameter (e.g., a ratio of 0.8 to 1.0), the slit is wide enough to almost contain the entire spot. When the spot center is aligned with the slit center, most of the light energy passes through, resulting in the maximum signal strength. The intensity-position curve is very close to a straight line within a certain range near the center point. This means that the difference signal is well proportional to the spot displacement within this linear region. This facilitates linear correction and accurate zero-crossing interpolation calculations by the controller. Due to the high transmitted light intensity, the effective signal is much higher than the detector's noise floor. Although the linear region is wide, the slope at the top of the curve (i.e., the sensitivity) is relatively gentle. This may not be optimal in applications requiring extreme sensitivity. When the slit width is approximately 0.5 times the spot diameter (optimal sensitivity region), this is the most common compromise and optimization choice. When the spot center coincides with the slit center, the spot energy is clipped by the slit, and only about half of the light energy passes through. At this point, the intensity-position curve reaches its steepest slope near the zero-crossing point. This means that an extremely small displacement of the spot will cause a very large change in the transmitted light intensity. The slope of the differential curve is the steepest at the zero-crossing point, and the system is most sensitive to small angular deviations (corresponding to spot displacement). Although the total light flux is smaller than with a wide slit, the signal rate of change is extremely large, and the overall signal-to-noise ratio is still excellent. The steep slope minimizes the uncertainty range of the zero-crossing point, thus achieving the highest alignment accuracy. When the slit width is less than or equal to the spot diameter (e.g., a ratio of 0.1 to 0.3), the slit is very narrow, allowing only a very small portion of the energy at the spot center to pass through. The signal it detects is approximately the derivative of the spot's energy distribution function. Its signal curve itself is a very sharp peak. It is extremely sensitive to minute changes in the spot position, and can be used to detect the fine structure or edges of the spot. However, the total luminous flux is very low, which can lead to a significant decrease in the signal-to-noise ratio, and it is susceptible to electronic noise and shot noise. The linear region of the differential curve becomes very short, resulting in a very small operating range (capture range) for the system. If the spot energy distribution is non-uniform (non-ideal Gaussian distribution), it can cause signal distortion.

[0027] The widths of the first slit 17 and the second slit 18 are 0.1 to 1 times the diameter of the light spot. This ratio is determined by the optical designer through a trade-off and optimization among several interdependent factors, such as sensitivity, linearity, signal-to-noise ratio, and measurement range. It ensures that the entire differential detection system can stably, reliably, and accurately convert minute light spot displacements into high-quality differential electrical signals, laying a solid foundation for the final nanometer-scale angle measurement.

[0028] While slit-based differential detection systems offer extremely high precision (down to the nanometer-radian level), their linear operating range is very limited (typically only a few tens of microradians). If the light spot deviates too far from the slit region, the photodetector will fail to generate an effective differential signal, causing the system to "lose lock." At the start of the measurement, the installation of the measured prism element 4 will inevitably have significant initial angular and positional deviations, far exceeding the acquisition range of a precision aiming system. Manually finding and approaching the alignment point is an extremely time-consuming process dependent on operator experience, making automated measurement impossible.

[0029] Based on the above issues, refer again Figure 1 The first embodiment of the present invention provides a laser autocollimation differential alignment angle measuring device, which also includes a coarse aiming unit 9 located between the first beam splitter 2 and the collimating lens 3. The coarse aiming unit 9 detects the optical path information in front of the photodetector 13. The controller 26 is connected to the coarse aiming unit 9 and adjusts the tilt angle and position of the measured prism element 4 through the optical path information obtained by the coarse aiming unit 9.

[0030] Specifically, the coarse aiming unit 9 includes a second beam splitter 14, a field lens 15 located sequentially in the reflection direction of the second beam splitter 14, and an image detector 16. The image detector 16 is connected to the controller 26. The main function of the second beam splitter 14 is to couple a small portion of the beam energy from the main optical path with a low reflectivity (e.g., 10%~20%) and reflect it to the coarse aiming detection optical path. The vast majority (80%~90%) of the laser energy passes through the second beam splitter 14 and continues to the collimating lens 3 and the measured prism element 4, ensuring that the light signal returning to the photodetector 13 has sufficient intensity to maintain its extremely high signal-to-noise ratio and measurement accuracy. The reflected 10%~20% of the light energy is sufficient for the image detector 16 to obtain a clear and bright beacon spot, completing the coarse aiming task. The field lens 15 is usually a convex lens, placed in the reflection optical path of the second beam splitter 14. Its key function is to bring the spot information from infinity closer and image it onto the target surface of the image detector 16. It converges the returning parallel light from infinity, carrying angular information, into a clear spot image. The position of this spot on the detector directly corresponds to the tilt angle of the working surface of the measured prism. The aperture of the beam from the collimating lens may be large, while the target surface size of the image detector 16 is limited. The field lens 15 can converge the large-aperture parallel light, making it effectively received by a small-sized detector. The focal length of the field lens 15 determines the field of view of the coarse aiming system. The shorter the focal length, the larger the field of view, and the larger the range of initial deviations that can be captured, which is very advantageous for rapid initial acquisition. The image detector 16 is an area array CCD or CMOS camera. It converts the optical image of the spot arriving at its target surface into a digital signal containing pixel coordinates and grayscale information.

[0031] The controller 26 drives the image detector 16 to perform continuous or triggered acquisition, obtaining the raw image containing the light spot. The controller 26 runs a built-in image processing algorithm. First, threshold segmentation is performed to separate the light spot from the background. Then, the centroid algorithm is used to calculate the precise position of the light spot in the pixel coordinate system. The controller 26 compares the calculated centroid position of the light spot with a preset reference position to obtain the pixel-level deviation. The controller 26 converts the pixel deviation into an angular deviation based on the focal length of the field lens 15 and the pixel size of the detector. Subsequently, it generates a corresponding control signal to drive the two-dimensional tilt adjustment mechanism on the rotary table 5 to adjust the posture of the measured prism element 4. The above process forms a closed-loop control. The controller 26 continuously monitors the position of the light spot and continuously adjusts it until the centroid of the light spot stabilizes within the tolerance range allowed by the reference position, indicating that the coarse aiming is complete.

[0032] refer to Figure 5The rotary table 5 includes a motor 29 and a tilting and centering table 31 connected via a coupling 28 and bearings 27. The motor 29 is connected to a motor driver 30, which in turn is connected to a controller 26. The motor 29 rotates at speeds of 0.01 degrees / second to 10 degrees / second, with the high-speed setting (10 degrees / second) used for rapid addressing and initial positioning. When multiple working faces of a prism need to be measured, the high-speed mode can quickly rotate the next working face to its approximate working position, greatly improving measurement efficiency and avoiding long waiting times. The low-speed setting (0.01 degrees / second = 36 arcseconds / second) is the core guarantee for achieving nanometer-level angle measurement accuracy. During the precision aiming stage, when the light spot approaches the slit, the system needs to enter an ultra-high-resolution data acquisition mode. The extremely slow rotation speed allows the controller to acquire enough data points in the short time the light spot sweeps across the slit, thus accurately depicting the complete shape of the "differential alignment curve," especially the steep linear region near the zero point. High-speed rotation introduces dynamic errors such as centrifugal force, vibration, and wind resistance. Extremely low-speed operation ensures the entire system operates in a quasi-static measurement environment, effectively avoiding these interferences and allowing the angle measurement module (such as a circular grating) readings to accurately reflect the static angular position of the prism. Matching the response time with the photodetector and data acquisition card ensures distortion-free signal transmission. The rotary table 5 is driven by stepper, continuous, and servo speed control methods, including forward, reverse, and reciprocating rotation. The tilt and centering table 31 is a secondary micro-motion platform integrated into the rotary table 5. Its core function is to fine-tune the pitch and yaw angles (tilt adjustment) and lateral translation (centering) of the measured prism element 4, based on the instructions of the controller 26, during the coarse aiming stage. Its drive mechanism is typically a piezoelectric ceramic actuator or a differential head / precision lead screw. Piezoelectric ceramics can achieve nanometer-level resolution micro-displacement with fast response speed, making them an ideal choice for automated closed-loop coarse aiming. "Tilt adjustment" directly addresses the angular deviation of the working face of the prism and is the main task of rough aiming. "Alignment" ensures that the rotation center of the prism coincides with the rotation axis of the worktable, avoiding additional periodic errors during rotation due to eccentricity.

[0033] The rotary table 5 includes forward, reverse, and reciprocating rotation, which allows multiple measurement curves to be obtained during measurement. The average value is then taken as the final measurement curve, further improving measurement accuracy.

[0034] Optionally, the point light source 1 includes a laser 10 and a light source converging mirror 11 arranged coaxially.

[0035] The function of the light source converging mirror 11 is to further converge the tiny diverging beam emitted by the laser 10, forming a very bright, extremely small virtual point source within a very short distance. This virtual point source is located on the object-side focal plane of the collimating mirror 3. Although the emitting surface of the laser 10 is small, it still has physical dimensions. By efficiently collecting and focusing the emitted light, the light source converging mirror 11 effectively creates a secondary light source that is smaller and more ideal than the physical emitting surface. According to the principles of geometric optics, the emitted light can only be perfectly parallel when the light source is located on the focal plane of the collimating mirror. The light source converging mirror 11 ensures that the image point of the light emitted by the laser falls precisely on the focal point of the collimating mirror 3 after passing through it. It more effectively couples the beam of light emitted by the laser 10, which has a certain divergence angle, into the subsequent collimating mirror 3, reducing light waste and improving the overall luminous flux and signal-to-noise ratio of the system.

[0036] Specifically, the angle measurement module includes a circular grating 6, which is mounted and fixed on the rotating shaft of the rotary table 5. The circular grating 6 consists of a main grating ruler and an indicator grating. The main grating ruler is a disk with tens of thousands to tens of millions of precise lines etched on a glass or metal substrate. These lines are radially distributed. When the main grating rotates with the shaft, while the indicator grating remains fixed, the interaction of the lines produces alternating bright and dark moiré fringes. Each time the shaft rotates through a tiny angle, the moiré fringes shift by one cycle. By reading the changes in the moiré fringes through a photodetector and processing them through an electronic subdivision circuit, one fringe cycle can be subdivided into thousands of parts, thus achieving high-resolution measurement of minute angle changes. The circular grating is one of the most accurate devices in direct angle measurement, with an accuracy of ±0.1 arcseconds or even higher. Through electronic subdivision, its resolution can easily reach the order of 0.01 arcseconds, sufficient to match or even surpass the sensitivity of optical autocollimation systems.

[0037] like Figure 3 and Figure 4As shown, the photodetector 13 is one of a large-area array two-quadrant light intensity detector, a large-area array four-quadrant light intensity detector, or a large-area CCD array. When the photodetector 13 is a large-area array two-quadrant light intensity detector, it includes a second quadrant 24 and a first quadrant 25. The first slit 17 can be directly aligned with the first quadrant 25, and the second slit 18 can be aligned with the second quadrant 24. When the light spot sweeps across the slit plate 12, the light passing through the first slit 17 is received by the first quadrant 25, generating signal S1; the light passing through the second slit 18 is received by the second quadrant 24, generating signal S2. The controller 26 directly calculates the difference = S1 - S2. When the photodetector 13 is a large-area four-quadrant light intensity detector, it includes quadrant four 20, quadrant two 21, quadrant three 22, and quadrant one 23. It divides the photosensitive surface into four sector regions in two directions (X and Y): quadrant one 23, quadrant two 21, quadrant three 22, and quadrant four 20. In this invention, differential detection is mainly achieved by utilizing two quadrant pairs in the X direction. For example, the sum of quadrant one 23 and quadrant four 20 can correspond to the first slit 17, and the sum of quadrant two 21 and quadrant three 22 can correspond to the second slit 18.

[0038] A second aspect of the present invention provides a method for measuring laser autocollimation differential alignment angles, using the aforementioned laser autocollimation differential alignment angle measuring device, comprising the following steps: The divergent beam emitted by the point light source 1 is reflected by the first beam splitter 2 and collimated into parallel light by the collimating lens 3. The parallel light illuminates the prism element 4 under test. The light path reflected by the prism element 4 under test is converged by the collimating lens 3 and then converged by the first beam splitter 2 onto the slit plate 12 to form a focused spot 19. As the rotary table 5 rotates, the focused spot 19 moves laterally, passing through the first slit 17 and the second slit 18 in sequence. The photodetector 13 detects the light intensity passing through the first slit 17 and the second slit 18 respectively, and obtains two light intensity signals. The angle measurement module measures the corresponding rotation angle of the rotary table 5. The difference between the two light intensity signals is used as the vertical axis, and the rotation angle of the rotary table 5 is used as the horizontal axis to draw the differential alignment curve 8. Each vertical surface of the prism element 4 under test can be measured to obtain a differential alignment curve 8. The angle corresponding to the zero point of each differential alignment curve 8 is the angle of each face of the prism element 4 under test.

[0039] The surface of the prism element 4 under test includes a prism, a corner cabinet, a common reflector group, and a multifaceted prism. The reflecting surface of the prism element 4 under test includes an inner reflecting surface and an outer reflecting surface.

[0040] The above inventions are merely a few specific embodiments of the present invention. However, the embodiments of the present invention are not limited thereto, and any variations that can be conceived by those skilled in the art should fall within the protection scope of the present invention.

Claims

1. A laser autocollimation differential alignment angle measuring device, characterized in that, include: A point light source, a first beam splitter located in the emission direction of the point light source, a collimating mirror and a prism element to be measured arranged sequentially along the reflection direction of the first beam splitter; A rotary table carries the prism element to be measured, and the rotary table is connected to an angle measuring module that measures its rotation angle. The detection unit includes a slit plate and a photodetector arranged sequentially in the opposite direction to the reflection direction of the first beam splitter. The photodetector is a symmetrically distributed array detector or a quadrant detector. The slit plate has a first slit and a second slit arranged in parallel. The controller is connected to the point light source, the rotary table, the angle measurement module, and the photodetector. The controller controls the point light source to emit a diverging beam and controls the rotation of the rotary table. The diverging beam is collimated into parallel light after passing through the first beam splitter and collimating lens and then illuminates the prism element under test. The rotation of the rotary table causes the prism element under test to rotate. The focused light spot on the slit plate passes through the first slit and the second slit in sequence. The photodetector detects the two light intensity signals that pass through the first slit and the second slit respectively. The controller uses the difference between the two light intensity signals as the vertical axis and the rotation angle of the rotary table as the horizontal axis to draw a differential alignment curve. The differential alignment curve is used to obtain the angles of each face of the prism element under test based on the angles corresponding to its zero-crossing points.

2. The laser autocollimation differential alignment angle measuring device as described in claim 1, characterized in that, The slit plate is made of an opaque material and its thickness is greater than 0.1 mm.

3. The laser autocollimation differential alignment angle measuring device as described in claim 1, characterized in that, The width of the first slit and the second slit is 0.1 to 1 times the diameter of the light spot.

4. The laser autocollimation differential alignment angle measuring device as described in claim 1, characterized in that, It also includes a coarse aiming unit located between the first beam splitter and the collimating lens. The coarse aiming unit detects the optical path information in front of the photodetector. The controller is connected to the coarse aiming unit and adjusts the tilt angle and position of the prism element under test through the optical path information obtained by the coarse aiming unit.

5. The laser autocollimation differential alignment angle measuring device as described in claim 4, characterized in that, The coarse aiming unit includes a second beam splitter, a field mirror located sequentially in the reflection direction of the second beam splitter, and an image detector, wherein the image detector is connected to the controller.

6. The laser autocollimation differential alignment angle measuring device as described in claim 1, characterized in that, The rotary table includes a motor and an adjustable and self-aligning table connected by a coupling and bearings. The motor is connected to a motor driver, and the motor driver is connected to a controller. The speed of the motor is 0.01 degrees / second to 10 degrees / second.

7. The laser autocollimation differential alignment angle measuring device as described in claim 1, characterized in that, The point light source includes a laser and a light source converging lens arranged coaxially.

8. The laser autocollimation differential alignment angle measuring device as described in claim 1, characterized in that, The angle measurement module includes a circular grating, which is mounted and fixed on the rotating shaft of the rotary table.

9. The laser autocollimation differential alignment angle measuring device as described in claim 1, characterized in that, The photodetector is one of a large-area array two-quadrant light intensity detector, a large-area array four-quadrant light intensity detector, or a large-area array CCD.

10. A method for measuring the laser autocollimation differential alignment angle, characterized in that, Using the laser autocollimation differential alignment angle measuring device according to any one of claims 1 to 9 includes the following steps: The divergent beam emitted by the point light source is reflected by the first beam splitter and then collimated into parallel light by the collimating lens. The parallel light illuminates the prism element under test. The light reflected by the prism element under test is converged by the collimating lens and then by the first beam splitter to form a focused spot on the slit plate. As the prism element being measured rotates with the rotary table, the focused spot moves laterally, passing through the first slit and the second slit in sequence. The photodetector detects the light intensity passing through the first slit and the second slit respectively, and obtains two light intensity signals. The angle measurement module measures the corresponding rotation angle of the rotary table. The difference between the two light intensity signals is used as the vertical axis and the rotation angle of the rotary table is used as the horizontal axis to draw a differential alignment curve. Each vertical face of the prism element under test can be measured to obtain a differential alignment curve. The angle corresponding to the zero point of each differential alignment curve is the angle of each face of the prism element under test.

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