Laser triangulation method and sensor calibration structure

Abstract

The invention relates to the technical field of machine vision measurement, in particular to a laser triangulation method and a sensor calibration structure. The optical axis of the imaging lens is located on the side, closest to the laser line, of the plate-level camera. According to the invention, the defect of lack of a universal high-precision laser measurement system in the prior art is overcome, the measurement performance of the sensor can be greatly developed under the same hardware condition, the nonlinear error of the sensor can be lower, and the measurement result of the sensor is more stable.

Description

Technical Field

[0001] The invention relates to the technical field of machine vision measurement, in particular to a laser triangulation measurement method and a sensor calibration structure. Background Art

[0002] At present, the known line laser measurement sensor is composed of a board-level camera, an imaging lens, and a laser, and its measurement structure follows the laser triangulation method. However, different tests and performance requirements will produce different structures. Reference [1] creatively demonstrates an image sensor that uses two symmetrically arranged around the laser beam axis. This structure improves the performance of optical triangulation by reducing the shadow effect while maintaining resolution. In addition, it is easy to obtain a very large field of view. However, the structure is large in size and uses two board-level cameras, which further increases the cost of the sensor. Reference [2] achieves fast scanning by adding a fast turning mirror to the outside of the existing sensor to get rid of the limitation of the measurement time by the moving speed of the traditional translation stage. However, the installation of the fast turning mirror invisibly increases the size of the sensor and reduces its flexibility of use. Reference [3] also shows the addition of a reflector inside the sensor to reduce the size of the sensor. The above structural designs are all suitable for their respective application scenarios, but none of them consider how the sensor structure should be arranged under the same hardware conditions to maximize the stability and sensitivity of the sensor.

[0003] Literature [1]: F. Blais, "Review of 20 years of range sensor development," Journal of Electronic Imaging, vol.13, no.1, pp.231-243, Jan, 2004.

[0004] Literature [2]: J.Schlarp, ​​E.Csencsics, and G.Schitter, "Optical Scanning of aLaser Triangulation Sensor for 3-D Imaging," Ieee Transactions on Instrumentation and Measurement, vol.69, no.6, pp.3606-3613, Jun, 2020.

[0005] Literature [3]: ZJNan, W.Tao, and H.Zhao, "Development of a small-size lasertriangulation displacement sensor and temperature drift compensation method," Measurement Science and Technology, vol.32, no.9, Sep, 2021. Summary of the invention

[0006] In order to overcome the defect of the above-mentioned prior art that there is a lack of a universal high-precision laser measurement system, the present invention proposes a laser triangulation measurement method, which can greatly develop the measurement performance of the sensor under the same hardware conditions, and can reduce the nonlinear error of the sensor, making its measurement results more stable.

[0007] The present invention proposes a laser triangulation measurement method, which first arranges a laser, an imaging lens, a board-level camera and an object motion track; the laser emitted light is reflected by the object to be measured on the object motion track, and then forms an image on the board-level camera through the imaging lens; the end of the board-level camera close to the laser is recorded as the inner end, and the end of the board-level camera far from the laser is recorded as the outer end; the straight line where the optical axis of the imaging lens is located passes through the inner end; the object to be measured is made to move from the starting point to the end point on the object motion track, and the laser and the board-level camera are started for tracking imaging, and the position of the object to be measured is calculated in combination with the position of the image point on the board-level camera, and the calculation formula is as follows:

[0008]

[0009] Among them, x' is the distance from the image point to the inner end of the board-level camera on the image point motion trajectory, x is the distance from the measured object to the starting point on the object motion trajectory; d0 is the distance from the intersection of the laser optical axis and the imaging lens optical axis on the imaging lens optical axis to the lens principal point, d1 is the distance from the lens principal point to the inner end on the imaging lens optical axis; β is the angle between the laser optical axis and the imaging lens optical axis, and μ is the angle between the imaging lens optical axis and the photosensitive surface of the board-level camera.

[0010] Preferably, the object motion trajectory is located on the optical axis of the laser.

[0011] Preferably, the starting point of the object's motion trajectory is the intersection of the optical axis of the imaging lens and the optical axis of the laser.

[0012] Preferably, when the imaging lens is a thin lens, the principal point of the lens is the coincidence point of the object-side principal point and the image-side principal point; when the imaging lens is a thick lens, d0 is the distance from the intersection of the laser optical axis and the imaging lens optical axis on the imaging lens optical axis to the object-side principal point of the lens, and d1 is the distance from the image-side principal point of the lens to the inner end on the imaging lens optical axis; β is the angle between the laser optical axis and the imaging lens optical axis, and μ is the angle between the imaging lens optical axis and the photosensitive surface of the board-level camera.

[0013] Preferably, the laser is a point laser or a line laser.

[0014] A laser triangulation calibration device proposed in the present invention comprises: a first linear displacement stage, a first rotating stage, a lens barrel, an optical fiber adapter, a second linear displacement stage and a second rotating stage; the first linear displacement stage is arranged on the first rotating stage, the first linear displacement stage is used for placing a board-level camera, and is used for driving the board-level camera to move linearly; the first rotating stage is used for adjusting the placement angle of the board-level camera; the second linear displacement stage is arranged on the second rotating stage, the lens barrel is arranged on the second linear displacement stage through a lens support frame, the imaging lens is located in the lens barrel, and the optical axis of the imaging lens coincides with the central axis of the lens barrel; the optical fiber adapter is arranged on a side of the lens barrel away from the first linear displacement stage, and the optical fiber adapter is used for providing an optical fiber socket.

[0015] Preferably, the optical fiber is first connected through a fiber optic adapter so that the output light of the optical fiber is emitted through the imaging lens; the relative angle of the imaging lens and the board-level camera is adjusted by the first rotating table and the second rotating table so that the output light of the imaging lens forms a light spot on the board-level camera; the relative position of the imaging lens and the board-level camera is adjusted by relative movement of the first linear displacement table and the second linear displacement table so that the light spot is at the inner end of the board-level camera and the light spot is in a minimum state.

[0016] The advantages of the present invention are:

[0017] (1) The present invention proposes a laser triangulation measurement method in which the optical axis of the imaging lens is located on the side of the board-level camera closest to the laser line. According to the object-image relationship curve and sensitivity curve derived from the laser triangulation method, when the measurement range, i.e., the object motion trajectory, is in the positive direction of the measurement, the nonlinearity of the measurement is lower and the overall sensitivity is greater, which fully guarantees and improves the stability and measurement accuracy of the sensor under the same hardware conditions.

[0018] (2) Under the same hardware conditions, the present invention can reduce the nonlinear error of the measurement system and improve the measurement accuracy by simply adjusting the position of the board-level camera on the optical axis, which fully explores the performance limit of the existing line laser triangulation measurement sensor under the same hardware conditions. It can not only greatly develop the measurement performance of the sensor under the same hardware conditions, but also make the nonlinear error of the sensor lower, making its measurement results more stable.

[0019] (3) The laser triangulation calibration device proposed in the present invention can accurately adjust the relative positions of the imaging lens and the board-level camera in advance, further improving the measurement accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 A laser triangulation system for an existing symmetrical structure;

[0021] Figure 2 The laser triangulation measurement system with an asymmetric structure proposed in this application;

[0022] Figure 3 is the sensitivity curve of the laser triangulation system;

[0023] Figure 4 Flow chart of the laser triangulation method proposed for this application;

[0024] Figure 5 is a schematic diagram of a measurement system of an embodiment;

[0025] Figure 6 Comparison of measurement errors between asymmetric and symmetric structures;

[0026] Figure 7 Schematic diagram of the laser triangulation calibration device.

[0027] Illustration: 1. Laser; 2. Imaging lens; 3. Board-level camera; 4. First linear translation stage; 5. Lens barrel; 6. Fiber optic adapter; 7. Second linear translation stage; 71. Lens support frame; 8. Second rotating stage; 10. Dual-frequency laser interferometer; 9. Second rotating stage; 100. Object to be measured. DETAILED DESCRIPTION

[0028] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] like Figure 1As shown, in the existing laser triangulation measurement system, the straight line where the optical axis of the imaging lens 2 is located passes through the center point of the photosensitive surface of the board-level camera 3, that is, the midpoint of the motion trajectory of the image point on the photosensitive surface. In this way, the object motion trajectory BC is set on the optical axis of the laser 1, and the object motion trajectory is the system measurement range. The laser emitted by the laser 1 is projected onto the surface of the object to be measured, and the generated diffuse reflection light is partially imaged on the photosensitive surface of the board-level camera 3 through the imaging lens 2. When the object to be measured moves within the measurement range, the position of the light spot on its surface relative to the laser 1 changes, and the position of the corresponding image point on the photosensitive surface of the board-level camera 3 also changes. The displacement of the object to be measured can be obtained by accurately measuring the displacement of the image point on the board-level camera 3. Figure 1 In the figure, the straight line where the optical axis of the imaging lens 2 is located passes through the center point of the photosensitive surface of the board-level camera 3, that is, the lengths of the board-level camera 3 on both sides of the straight line where the optical axis of the imaging lens 2 is located are equal. This structure is referred to as a symmetrical structure.

[0030] In this embodiment, the laser is a line laser.

[0031] Figure 2 The laser triangulation measurement system proposed in the present application is shown in FIG. 2 , in which the straight line where the optical axis of the imaging lens 2 is located passes through the end of the board-level camera 3 close to the laser 1. In this way, the photosensitive surface of the board-level camera 3 is completely distributed on the same side of the straight line where the optical axis of the imaging lens 2 is located, and this structure is recorded as an asymmetric structure.

[0032] Reference Figure 1 , Figure 2 , let the intersection of the optical axis of the imaging lens 2 and the optical axis of the laser 1 be point A, let there be point C and point B on the optical axis of the laser 1, point C is located on the side of point A close to the laser 1, and point B is located on the side of point A away from the laser 1. When the object to be measured is located at point C, the image point of the object to be measured on the board-level camera 3 is located at the end of the photosensitive surface away from the imaging lens 2; when the object to be measured is located at point B, the image point of the object to be measured on the board-level camera 3 is located at the end of the photosensitive surface close to the laser; obviously, point B only exists in symmetrical structures; point B does not exist in asymmetrical structures, or point B coincides with point A.

[0033] Let the image point of the object under test at point A on the board-level camera 3 be recorded as A', the image point of the object under test at point B on the board-level camera 3 be recorded as B', and the image point of the object under test at point C on the board-level camera 3 be recorded as C'; stipulate The direction is positive, The direction is negative, and the corresponding The direction is positive, The direction is negative.

[0034] The sensor based on the laser triangulation measurement principle uses the proportional relationship of similar triangles such as Figure 3 As shown, according to Figure 3The dotted area in the figure can be derived as follows:

[0035]

[0036] Among them, x' is the distance from the image point to the inner end of the board-level camera 3 on the image point motion trajectory, x is the distance from the measured object to the starting point on the object motion trajectory; d0 is the distance from the intersection of the optical axis of the laser 1 and the optical axis of the imaging lens 2 on the optical axis of the imaging lens 2 to the lens principal point, d1 is the distance from the lens principal point to the inner end on the optical axis of the imaging lens 2; β is the angle between the optical axis of the laser 1 and the optical axis of the imaging lens 2, and μ is the angle between the optical axis of the imaging lens 2 and the photosensitive surface of the board-level camera 3.

[0037] In a specific embodiment, the displacement of the object 100 to be measured is measured by using an asymmetric structure and a symmetric structure. When measuring the symmetric structure, the motion trajectory of the object 100 to be measured is BAC; Figure 2 When measuring the asymmetric structure shown, the motion trajectory of the object 100 is AC. The motion trajectory of the object 100 corresponds to the measurement range. Obviously, Figure 2 The measurement range of the asymmetric structure shown is less than Figure 1 Measuring range for the symmetrical structure shown.

[0038] In this embodiment, the object to be measured moves in the direction of the optical axis of the laser 1. Figure 5 As shown, the actual moving distance of the measured object is measured by a dual-frequency laser interferometer 10 to calculate the measurement errors of the asymmetric structure and the symmetric structure. The error statistics of the two measurement methods are as follows: Figure 6 As shown. Figure 6 It can be seen that when measuring the symmetrical structure, the maximum error is 12.05 microns; when measuring the asymmetrical structure, the maximum error is 8.2 microns, and the maximum error is reduced by 31.95%. It can be seen that the displacement distance l that can be tested for the asymmetrical structure is smaller than the displacement distance l that can be tested for the symmetrical structure, but the test accuracy of the asymmetrical structure is higher.

[0039] When arranging an asymmetric structure, it is necessary to debug the relative positions of the imaging lens 2 and the board-level camera 3. For this purpose, the present application also proposes Figure 7The laser triangulation calibration device shown includes: a first linear displacement stage 4, a first rotating stage 9, a lens barrel 5, a fiber adapter 6, a second linear displacement stage 7 and a second rotating stage 8. The second linear displacement stage 7 is arranged on the second rotating stage 8, the lens barrel 5 is mounted on the second linear displacement stage 7, the imaging lens 2 is mounted on the side of the lens barrel 5 facing the board-level camera 3, and a lens support frame 71 is arranged on the second linear displacement stage 7 to support the portion of the imaging lens 2 extending out of the lens barrel 5. The optical axis of the imaging lens 2 coincides with the central axis of the lens barrel 5. The first linear displacement stage 4 is arranged on the first rotating stage 9; the first linear displacement stage 4 is used to place the board-level camera 3 and drive the board-level camera 3 to move linearly to adjust the three-dimensional spatial position of the board-level camera 3. The first rotating stage 9 is used to drive the first linear displacement stage 4 to rotate to adjust the angle of the board-level camera 3. The second linear displacement stage 7 is used to drive the lens barrel 5 to move linearly to adjust the three-dimensional spatial position of the imaging lens 2. The second rotating stage 8 is used to drive the lens barrel 5 to rotate to adjust the angle of the imaging lens 2. The optical fiber adapter 6 is disposed on a side of the lens barrel 5 away from the first linear translation stage 4 , and the optical fiber adapter 6 is used to provide an optical fiber socket.

[0040] During debugging, firstly, the optical fiber is connected through the optical fiber adapter 6, so that the optical fiber output light passes through the imaging lens 2 and is emitted to the first linear translation stage 4;

[0041] Then, the first rotating platform 9 and the second rotating platform 8 are adjusted so that the imaging lens 2 and the board-level camera 3 are positioned opposite to each other, the board-level camera 3 is located on the optical axis of the imaging lens 2, and the light emitted from the imaging lens 2 forms a light spot on the board-level camera 3;

[0042] Then, the first linear displacement stage 4 and the second linear displacement stage 7 are debugged to adjust the position and state of the light spot of the imaging lens 2 on the board-level camera 3 so that the light spot is at the inner end of the board-level camera 3 and is the smallest.

[0043] In this way, the relative position debugging of the imaging lens 2 and the board-level camera 3 is completed; during testing, it is only necessary to arrange the board-level camera 3 and the imaging lens according to the debugging results, and then set the laser 1 so that the optical axis of the laser 1 intersects with the optical axis of the imaging lens 2. When testing the object to be measured, the object to be measured is moved on the optical axis of the laser 1 from the intersection of the optical axis of the laser 1 and the optical axis of the imaging lens 2 toward the laser 1, and the moving distance of the image point of the feature point on the object to be measured on the board-level camera 3 is detected, thereby calculating the moving distance of the object to be measured.

[0044] Of course, it is obvious to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, but also includes the same or similar structures that can be implemented in other specific forms without departing from the spirit or essential features of the present invention. Therefore, no matter from which point of view, the embodiments should be regarded as exemplary and non-limiting, and the scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims are included in the present invention. Any reference numerals in the claims should not be regarded as limiting the claims involved.

[0045] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

[0046] The techniques, shapes, and structural parts not described in detail in the present invention are all well-known techniques.

Claims

1. A laser triangulation method, characterized in that: Firstly, a laser (1), an imaging lens (2), a board-level camera (3) and an object motion track are arranged; after the emitted light of the laser (1) is reflected by the object to be measured on the object motion track, it is imaged on the board-level camera (3) through the imaging lens (2); the end of the board-level camera (3) close to the laser (1) is recorded as the inner end, and the end of the board-level camera (3) far from the laser (1) is recorded as the outer end; the straight line where the optical axis of the imaging lens (2) is located passes through the inner end; The object to be measured is made to move from the starting point to the end point on the object motion trajectory, and the laser (1) and the board-level camera (3) are started to track and image. The position of the object to be measured is calculated based on the position of the image point on the board-level camera (3). The calculation formula is as follows: Wherein, x' is the distance from the image point on the image point motion trajectory to the inner end of the board-level camera (3), x is the distance from the measured object to the starting point on the object motion trajectory; d0 is the distance from the intersection of the optical axis of the laser (1) and the optical axis of the imaging lens (2) on the optical axis of the imaging lens (2) to the lens principal point, d1 is the distance from the lens principal point to the inner end on the optical axis of the imaging lens (2); β is the angle between the optical axis of the laser (1) and the optical axis of the imaging lens (2), and μ is the angle between the optical axis of the imaging lens (2) and the photosensitive surface of the board-level camera (3).

2. The laser triangulation method according to claim 1, characterized in that: The object's motion trajectory is located on the optical axis of the laser (1).

3. The laser triangulation method according to claim 2, characterized in that: The starting point of the object's motion trajectory is the intersection of the optical axis of the imaging lens (2) and the optical axis of the laser (1).

4. The laser triangulation method according to claim 1, characterized in that: When the imaging lens (2) is a thin lens, the principal point of the lens is the coincidence point of the object side principal point and the image side principal point; when the imaging lens (2) is a thick lens, d0 is the distance from the intersection of the optical axis of the laser (1) and the optical axis of the imaging lens (2) on the optical axis of the imaging lens (2) to the object side principal point of the lens, and d1 is the distance from the image side principal point of the lens on the optical axis of the imaging lens (2) to the inner end; β is the angle between the optical axis of the laser (1) and the optical axis of the imaging lens (2), and μ is the angle between the optical axis of the imaging lens (2) and the photosensitive surface of the board-level camera (3).

5. The laser triangulation method according to claim 1, characterized in that: The laser uses a point laser or a line laser.

6. A laser triangulation calibration device for the laser triangulation method according to any one of claims 1 to 5, characterized in that: include: A first linear displacement stage (4), a first rotating stage (9), a lens barrel (5), an optical fiber adapter (6), a second linear displacement stage (7) and a second rotating stage (8); the first linear displacement stage (4) is arranged on the first rotating stage (9), the first linear displacement stage (4) is used to place a board-level camera (3) and to drive the board-level camera (3) to move linearly; the first rotating stage (9) is used to adjust the placement angle of the board-level camera (3); the second linear displacement stage (7) is arranged on the second rotating stage (8), the lens barrel (5) is arranged on the second linear displacement stage (7) through a lens support frame (71), the imaging lens (2) is located in the lens barrel (5), and the optical axis of the imaging lens (2) coincides with the central axis of the lens barrel (5); the optical fiber adapter (6) is arranged on a side of the lens barrel (5) away from the first linear displacement stage (4), and the optical fiber adapter (6) is used to provide an optical fiber socket.

7. The laser triangulation calibration device according to claim 6, characterized in that: First, an optical fiber is connected through an optical fiber adapter (6) so that the optical fiber output light is emitted through an imaging lens (2); the relative angle between the imaging lens (2) and the board-level camera (3) is adjusted through a first rotating table (9) and a second rotating table (8) so that the output light of the imaging lens (2) forms a light spot on the board-level camera (3); and the relative position of the imaging lens (2) and the board-level camera (3) is adjusted through relative movement of a first linear displacement table (4) and a second linear displacement table (7) so that the light spot is at the inner end of the board-level camera (3) and is in a minimum state.

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