A method for calibrating a lidar

Abstract

The application discloses a laser radar calibration method, by rotating the reflecting plate forward and backward, the reflecting plate is switched between two positions of laser vertical and inclined incidence on the reflecting surface, and then the reflected signal strength of the laser reflected on the reflecting surface is continuously changed, the fitting calculation of the reflected signal strength received by the photoelectric detector is carried out, and the condition formula X=peak+scalea_peak and Y=real distance-TOF distance measurement value are met, the respective fitting degrees are calculated by searching the scale coefficient in the whole range, the maximum fitting degree is taken as the optimal solution, the correction coefficient of the global distance fitting of the TOF radar is calculated, the TOF distance measurement value is corrected according to the determined correction coefficient and fitting coefficient and the peak and a_peak combined with each distance measurement, and finally the TOF distance correction value is obtained, so that the calibration method of the whole laser radar can be simplified under the premise of the same reflecting plate, and the test cost is reduced.

Description

Technical Field

[0001] This invention relates to the field of lidar technology, and in particular to a lidar calibration method. Background Technology

[0002] Smart robotic vacuum cleaners on the market often use LiDAR as their eyes, and mainly use a single-line 360° scanning method. When the LiDAR is working, the laser emitter of its ranging module emits a laser. After the laser reaches the obstacle, it is reflected. After a period of time, the reflected light is received by the photodetector of the ranging module. By calculating the time difference between the photodetector receiving the photon and the photon being emitted, the distance to the obstacle that needs to be measured can be obtained.

[0003] In practical applications of distance measurement, the time of flight (TOF) calculated by the ranging module is also related to the surface material characteristics of the obstacle, including specular, mirror-like, rough, textured, highly absorbent, and highly reflective surfaces. For example, highly reflective surfaces result in extremely strong reflected signals received by the photodetector; while black textured surfaces produce weaker reflected signals. Even with specular or mirror-like surfaces (smooth planes), when incident at large angles, most light is reflected, and only a small portion is diffusely reflected back to the photodetector, resulting in extremely weak measured reflected signals. Different reflected signal intensities significantly affect the TOF calculation error. Therefore, calibrating the TOF error for different reflective surfaces is a pressing issue that needs to be addressed. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a lidar calibration method.

[0005] To achieve the above objectives, the technical solution provided by the present invention is as follows: This invention provides a lidar calibration method, comprising the following steps: S1, setting a rotatable reflector in the detection optical path of the lidar, ensuring that the laser spot emitted by the lidar falls on the rotation axis of the reflector; S2, rotating the reflector clockwise and counterclockwise, switching the reflector between a first incident position where the laser is perpendicularly incident on the reflector surface and a second incident position where the laser is obliquely incident on the reflector surface, thereby continuously varying the intensity of the reflected signal returned by the laser on the reflector surface; S3, fitting and calculating the intensity of the reflected signal received by the photodetector of the lidar, satisfying the following condition: X = peak + scale a_peak, Y = true distance - TOF distance measurement, where the TOF distance measurement is the distance converted from the time of flight measured by the conventional channel of the photoelectric detector, peak is the reflected signal intensity value measured by the conventional channel of the photoelectric detector, a_peak is the intensity value measured by the attenuation channel of the photoelectric detector, and scale is the fitting coefficient of the intensity value measured by the attenuation channel of the photoelectric detector. The fitting degree of each scale coefficient is calculated by searching the entire range, and the maximum fitting degree is taken as the optimal solution to calculate the correction coefficient of the global range fitting of the TOF radar. Then, based on the determined correction coefficient and fitting coefficient, and by combining peak and a_peak in each ranging measurement, the TOF distance measurement value is corrected to finally obtain the corrected TOF distance value.

[0006] Furthermore, the correction coefficients a, b, c, and d for the global range fitting of the TOF radar are calculated by fitting the cubic polynomial equation Y=aX³+bX²+cX+d.

[0007] Furthermore, the rotation angle range of the reflector is -60 degrees to +60 degrees, so as to ensure that the distance between the lidar and the center line of symmetry of the reflective surface changes within 3 mm during the continuous rotation of the reflector.

[0008] Furthermore, the reflective surface is a metallic-looking glossy surface to ensure that the intensity of the reflected signal changes continuously from high signal intensity to weak signal intensity when the laser is incident from a vertical angle to a large angle.

[0009] Furthermore, the metallic-looking glossy surface is a polyvinyl chloride reflective glossy surface.

[0010] Furthermore, the lidar can move linearly relative to the reflector; or the lidar can be fixed in place.

[0011] Furthermore, the photodetector is a single-photon avalanche diode assembly.

[0012] Furthermore, the axis of rotation of the reflector coincides with the center line of symmetry of the reflector; and / or, the reflector is configured to rotate at a uniform speed.

[0013] The technical solution provided by this invention has the following beneficial effects: 1. Compared with the prior art, the present invention can solve the problem of inaccurate measurement of reflective surfaces of glossy materials, making the lidar more accurate and adaptable in complex real-world application environments.

[0014] 2. Compared with the prior art, the present invention does not require reflectors of different materials and can perform repeated sampling at different test distances, thereby simplifying the calibration method of the entire lidar and reducing the test cost. Attached Figure Description

[0015] Figure 1 The flowchart shown is a process for calibrating a lidar in an embodiment. Figure 2 The diagram shown is a schematic of a lidar calibration device containing lidar in the embodiment. Figure 3 The figure shown is a schematic diagram of the state of the lidar calibration device in the embodiment at 0 degrees. Figure 4 The figure shown is a schematic diagram of the state of the lidar calibration device in the embodiment at +60 degrees. Figure 5 The figure shown is a schematic diagram of the state of the lidar calibration device in the embodiment at -60 degrees. Figure 6 The figure shown is a graph of peak versus time in the embodiment. Figure 7 The figure shown is a curve of the reflected signal data fitting in the embodiment. Detailed Implementation

[0016] To further illustrate the various embodiments, the present invention provides accompanying drawings. These drawings are part of the disclosure of the present invention and are mainly used to illustrate the embodiments, and can be used in conjunction with the relevant descriptions in the specification to explain the operating principles of the embodiments. With reference to these drawings, those skilled in the art should be able to understand other possible implementations and the advantages of the present invention. Components in the drawings are not drawn to scale, and similar component symbols are generally used to represent similar components.

[0017] The present invention will now be further described in conjunction with the accompanying drawings and specific embodiments.

[0018] Reference Figures 1 to 7 This embodiment provides a lidar calibration method and uses a lidar calibration device (hereinafter referred to as calibration device 100) for testing to ensure that the TOF error of lidar 200 under different reflective surfaces (such as mirror, mirror-like, rough, velvety, high-absorption, and high-reflectivity surfaces) in actual application scenarios is within the specified requirements, thereby ensuring stable, accurate, and reliable ranging.

[0019] like Figure 2As shown, the calibration device 100 of this embodiment includes a base 5, a reflector 1, a rotary motor motion module 2, a linear screw motion module 3 with a slide rail 31, a mounting bracket 4 for fixing the lidar 200, a camera 6 for monitoring the position of the laser spot on the reflective surface 11 of the reflector 1, and a computer with calibration software. The reflector 1 has a polyvinyl chloride reflective surface and serves as a metallic-looking reflective surface 11 to ensure that the intensity of the reflected signal changes continuously from high to weak signal intensity when the laser is incident from perpendicular to a large angle on the reflective surface 11. The linear screw motion module 3 and the rotary motor motion module 2 are mounted on the base 5. The reflector 1 is mounted on the rotating end of the rotary motor motion module 2 to drive the reflector 1 to rotate uniformly at a fixed point for only 360 degrees. The center line 111 of the reflective surface 11 of the reflector 1 is aligned with its rotation axis in the vertical direction (the center line 111 coincides with the rotation axis of the reflector 1). This ensures that the laser beam from the lidar 200 hits the center line 111 of the reflective surface 11 and the measurement distance remains constant during the rotation of the reflector 1. The mounting bracket 4 is mounted on the linear motion end of the linear screw motion module 3 to drive the lidar 200 on the mounting bracket 4 to move closer to or further away from the reflective surface 11 of the reflector 1. A computer is electrically connected to the communication port of the lidar 200 via a data cable to transmit measurement data in real time and to perform data processing and display calibration results. Of course, in other embodiments, the rotation axis of the reflector 1 may not coincide with the center line 111 of the reflector 1.

[0020] like Figure 1 , Figures 3 to 5 As shown, based on the calibration device 100 described above, the lidar calibration method used in this embodiment specifically includes the following steps: Step S1: The lidar 200 is mounted on the slide rail 31 in a linearly movable manner using the mounting bracket 4, and the rotatable reflector 1 is placed on the detection optical path of the lidar 200. It is ensured that the laser spot emitted by the lidar 200 falls on the symmetrical center line 111 of the reflective surface 11 of the reflector 1, and the distance between the lidar 200 and the reflective surface 11 of the reflector 1 remains unchanged during the calibration test.

[0021] Step S2 involves rotating the reflector 1 at a constant speed between -60 degrees and +60 degrees, alternating between forward and reverse rotations. This switches the reflector 1 between a first incident position (vertical incident position) where the laser is perpendicularly incident on the reflecting surface 11 and a second incident position (large angle incident position) where the laser is obliquely incident on the reflecting surface 11, thereby continuously varying the intensity of the reflected signal from the reflecting surface 11. Simultaneously, during the continuous rotation of the reflector 1, the distance between the lidar 200 and the symmetrical center line 111 of the reflecting surface 11 varies within 3 mm. In this embodiment, the two second incident positions correspond to the incident positions during the forward and reverse rotation processes of the reflector 1, respectively. Of course, in other embodiments, the reflector 1 may not rotate at a constant speed.

[0022] Step S3: The reflected signal intensity and TOF distance measurement data received by the photodetector (such as a single-photon avalanche diode component) of the lidar 200 are transmitted to the computer via a data cable. Combined with the actual distance used in calibration, a fitting calculation is performed on the computer, satisfying the following condition: X = peak + scale a_peak, Y = true distance - TOF distance measurement value, calculate the fitting degree of each by searching the scale coefficient in the whole range, and then take the maximum fitting degree as the optimal solution to calculate the correction coefficient of the global range fitting of the TOF radar. Then, based on the determined correction coefficient and fitting coefficient, and combining peak and a_peak for each range measurement, the TOF distance measurement value is corrected, and finally the TOF distance correction value is obtained, thus completing the calibration test.

[0023] Wherein, the TOF distance measurement value is the distance converted from the time of flight measured by the conventional channel of the photodetector, peak is the reflected signal intensity value measured by the conventional channel of the photodetector, a_peak is the intensity value measured by the attenuation channel of the photodetector, and scale is the fitting coefficient of the intensity value measured by the attenuation channel of the photodetector (the scale coefficient is iterated from 0 to 1 during the calculation process).

[0024] In this specific embodiment, the result is obtained by fitting the cubic polynomial equation Y = aX³ + bX² + cX + d to obtain Y = 0.000000513. X 3 -0.000396889 X 2 +0.156133443 X-610.184936523, where 0.000000513, -0.000396889, 0.156133443, and -610.184936523 are correction coefficients for the global range fitting of the TOF radar, specifically as follows: Figure 6 and Figure 7 As shown.

[0025] The distance calculated by fitting using the above calibration method can simulate the TOF error encountered in real-world application scenarios under different material surfaces and different usage distances, and the point cloud straightness performance is excellent.

[0026] Furthermore, this embodiment can adaptively adjust the scale fitting coefficient to achieve optimal fitting, making the LiDAR 200 perform better in measuring different material surfaces and different usage distances encountered in actual application scenarios.

[0027] Furthermore, compared to existing technologies such as the lidar distance testing system in Chinese Utility Model Patent CN215180893U, which uses multiple reflective plates with different reflectivities for calibration testing, this embodiment does not require reflective plates 1 of different materials. It only requires the same reflective plate 1 with a metallic glossy surface for calibration testing. Moreover, it can perform repeated sampling multiple times at various test distances, thereby simplifying the calibration method of the entire lidar 200 and reducing the testing cost. At the same time, it solves the problem of inaccurate measurement of reflective surfaces with glossy materials, making the lidar 200 more accurate and adaptable in complex real-world application environments.

[0028] Of course, in other embodiments, the reflector 1 can be first installed on the rotating end of the rotary motor motion module 2, and the rotary motor motion module 2 can be mounted on the linear motion end of the linear lead screw motion module 3. The linear lead screw motion module 3 is mounted on the base 5, and the lidar 200 is fixed to the other side of the base 5 by the mounting bracket 4. In this way, not only can the reflector 1 rotate, but it can also move closer to or further away from the lidar 200 in a straight line. Alternatively, the lidar 200 can be fixed on the base 5 at a certain installation distance, and the reflector 1 can be rotatably set.

[0029] Although the invention has been specifically shown and described in conjunction with preferred embodiments, those skilled in the art will understand that various changes in form and detail may be made to the invention without departing from the spirit and scope of the invention as defined in the appended claims, all of which shall be within the scope of protection of the invention.

Claims

1. A lidar calibration method, characterized in that, Includes the following steps: S1, a rotatable reflector is placed in the detection light path of the lidar, and the laser spot emitted by the lidar falls on the rotation axis of the reflector. S2, rotate the reflector in both directions to switch between a first incident position where the laser is perpendicularly incident on the reflector surface and a second incident position where the laser is obliquely incident on the reflector surface, thereby causing the intensity of the reflected signal reflected back by the laser on the reflector surface to change continuously. S3, by fitting the intensity of the reflected signal received by the photodetector of the lidar, and satisfying the following condition: X = peak + scale a_peak, Y = true distance - TOF distance measurement, where the TOF distance measurement is the distance converted from the time of flight measured by the conventional channel of the photoelectric detector, peak is the reflected signal intensity value measured by the conventional channel of the photoelectric detector, a_peak is the intensity value measured by the attenuation channel of the photoelectric detector, and scale is the fitting coefficient of the intensity value measured by the attenuation channel of the photoelectric detector. The fitting degree of each scale coefficient is calculated by searching the entire range, and the maximum fitting degree is taken as the optimal solution to calculate the correction coefficient of the global range fitting of the TOF radar. Then, based on the determined correction coefficient and fitting coefficient, and by combining peak and a_peak in each ranging measurement, the TOF distance measurement value is corrected to finally obtain the corrected TOF distance value.

2. The lidar calibration method according to claim 1, characterized in that: The correction coefficients a, b, c, and d for the global range fitting of the TOF radar are calculated by fitting the cubic polynomial equation Y=aX³+bX²+cX+d.

3. The lidar calibration method according to claim 1 or 2, characterized in that: The rotation angle range of the reflector is -60 degrees to +60 degrees, so as to ensure that the distance between the lidar and the rotation axis of the reflector changes within 3mm during continuous rotation.

4. The lidar calibration method according to claim 3, characterized in that: The reflective surface is a metallic-looking glossy surface to ensure that the intensity of the reflected signal changes continuously from high signal intensity to weak signal intensity when the laser is incident from a vertical angle to a large angle.

5. The lidar calibration method according to claim 4, characterized in that: The metallic-looking glossy surface is a polyvinyl chloride reflective glossy surface.

6. The lidar calibration method according to claim 1 or 2, characterized in that: The lidar can move linearly relative to the reflector; or the lidar can be fixed in place.

7. The lidar calibration method according to claim 1 or 2, characterized in that: The photodetector is a single-photon avalanche diode assembly.

8. The lidar calibration method according to claim 1 or 2, characterized in that: The axis of rotation of the reflector coincides with the center line of symmetry of the reflector; and / or, the reflector is configured to rotate at a uniform speed.

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