A scanning method and control system for improving the uniformity of lidar point cloud distribution

Abstract

This invention belongs to the field of lidar technology and provides a scanning method to improve the uniformity of lidar point cloud distribution. It transforms the uniform scanning method within one cycle of the prism into a multi-segment variable-speed scanning method, specifically including the following steps: S1, determining the number of speed changes in segmented scanning within one cycle; S2, determining the initial scanning speed of the prism; S3, determining the scanning speed of the prism after speed changes; S4, determining the speed change delay time; S5, determining the angular position of each speed change within one cycle. The technical solution of this invention can effectively improve the problem of uneven point cloud distribution in lidar imaging on airborne platforms, improve point cloud utilization, save point cloud resources, reduce laser repetition rate, and save costs.

Description

Technical Field

[0001] This invention belongs to the field of lidar technology, specifically relating to a scanning method and control system for improving the uniformity of lidar point cloud distribution. Background Technology

[0002] LiDAR (Light Detection and Ranging) is an active detection and imaging system that uses laser light as the detection medium. Its principle is to emit a laser beam towards the target and detect the echo signal to determine the target's distance information. Combined with a 3D scanning system, the target's spatial coordinates can be obtained. The scanning system determines not only the size of the LiDAR but also its angular resolution. Most existing mechanical and semi-solid-state LiDARs contain movable mechanical scanning components that rotate at a constant speed, resulting in a fixed imaging range and angular resolution. On certain platforms, uniform scanning imaging LiDARs cannot meet high-resolution requirements. For example, on vehicle-mounted platforms, uniform imaging of the road surface is required. This is typically achieved through localized, intensified scanning of the region of interest (ROI) to improve target imaging capabilities. On airborne platforms, prisms are typically used as scanning elements to achieve 3D imaging as the aircraft moves forward. The prism scanning trajectory is usually arranged in a forward overlapping pattern in the form of circles. Due to the geometric shape, there is usually more overlap on the sides and no overlap in the middle. This results in the point cloud density on the sides being much greater than that in the middle. In order to achieve the requirement of high resolution for ground targets, the point cloud density at the sparsest part in the middle is usually used as the feasibility criterion. This seriously wastes point cloud resources, limits imaging efficiency, increases laser power consumption, and raises development costs. Summary of the Invention

[0003] (a) Technical problems to be solved

[0004] The technical problem to be solved by this invention is the uneven distribution of point clouds caused by prism scanning in lidar.

[0005] (II) Technical Solution

[0006] To address the aforementioned technical problems, this invention provides a scanning method for improving the uniformity of point cloud distribution in lidar, transforming the uniform-speed scanning method within one prism cycle into a multi-segment variable-speed scanning method, specifically including the following steps:

[0007] S1. Determine the number of speed changes for segmented scanning within one cycle;

[0008] S2. Determine the initial scanning speed of the prism;

[0009] S3. Determine the scanning speed of the prism after speed change;

[0010] S4. Determine the speed change delay time;

[0011] S5. Determine the angular position of each speed change within one cycle;

[0012] S6. Determine the acceleration value based on the initial scan speed, the scan speed after speed change, and the speed change delay time;

[0013] S7. Determine the equipment's altitude, laser repetition rate, lateral flight speed, and beam deflection angle to determine the point cloud density distribution curve after the speed change.

[0014] S8. Using the variance of the point cloud density distribution curve as a criterion, adjust the magnitude of the acceleration value to optimize the uniformity of the point cloud density distribution.

[0015] S9. Calculate the optimal variable speed scanning conditions based on the actual achievable acceleration level and the initial state of the equipment.

[0016] Furthermore, the point cloud density distribution curve is obtained by counting the number of points in a unit area by dividing the scan trajectory map into squares of a certain size, and then counting the point cloud density in a certain direction.

[0017] The variance of the point cloud density distribution curve is obtained by solving for the variance of the data on the point cloud density distribution curve.

[0018] Furthermore, by considering the device's height h and the laser's repetition rate f... laser Horizontal flight speed of the aircraft v x The beam deflection angle α, the initial angular velocity w0, and the angle values ​​θ at different times, where t is time, are calculated using the following formula:

[0019]

[0020] Calculate the scanning trajectory of the prism within the time interval t;

[0021] By extracting the scanning trajectory range of the uniformly intersecting part, determining the number of point clouds per unit area, and calculating the point cloud density, a distribution map of the point cloud density can be obtained, and the variance of the data points on the curve can be calculated.

[0022] The optimal variable speed sweep condition is calculated as follows:

[0023] S901, Based on the range of angular velocity change [ω 1， ω2], the range of the speed change delay time Δt [t1, t2], the initial speed change angle position θ0, determine a set of boundary conditions, and calculate the corresponding point cloud density distribution variance;

[0024] S902. Compare the variance value obtained in S901 with the variance value of the point cloud distribution density obtained by uniform scanning. If it is greater than the variance value of the point cloud distribution density obtained by uniform scanning, then discard the boundary conditions and re-determine a new set of boundary conditions to return to S901 for recalculation. If the obtained variance value is less than the variance value of the point cloud density distribution obtained by uniform scanning, then proceed to S903.

[0025] S903. Save the variance that meets the conditions and the boundary conditions of this group of inputs, and determine the new boundary conditions again. Return to S901 to perform iterative calculations until all boundary conditions are calculated.

[0026] S904. Compare all the variance values ​​that meet the conditions, and select the input boundary condition corresponding to the smallest variance value as the optimal solution, i.e., the optimal variable speed scanning condition.

[0027] The present invention also provides a control system for improving the uniformity of lidar point cloud distribution, including a scanning prism module, a scanning prism driving module, and a scanning control device; the scanning prism module is used to deflect and scan the laser beam to form a continuous scanning trajectory; the scanning prism driving module is used to drive the prism to rotate in a predetermined manner; the scanning control device is used to download the optimal variable speed scanning conditions into the scanning control device in the form of a programming language to realize variable speed control of the scanning prism.

[0028] Furthermore, the scanning prism module can be a scanning rotating mirror or a scanning galvanometer.

[0029] Furthermore, the scanning prism drive module is a driving electronics device for other scanning components.

[0030] Furthermore, the scanning prism control module can be any electronic device with programming, storage, and communication functions.

[0031] Furthermore, both the scanning prism module and the scanning control device communicate with the host computer module, which is any computer reading and writing device with communication capabilities.

[0032] (III) Beneficial Effects

[0033] Compared with existing technologies, this invention has the following beneficial effects: it can effectively improve the problem of uneven point cloud distribution in airborne LiDAR imaging, increase point cloud utilization, save point cloud resources, reduce laser repetition rate, and save costs. In particular, those skilled in the art will clearly see that this method is not only applicable to prism scanning, but can also be used for point cloud homogenization adjustment in scanning imaging methods such as galvanometer and rotating mirror. Therefore, this method has good reference value for homogenization imaging of both mechanical and semi-solid-state LiDAR. Attached Figure Description

[0034] Figure 1This is a schematic diagram of the prism multi-segment variable speed scanning method;

[0035] Figure 2 This is a flowchart of the point cloud density distribution variance calculation process;

[0036] Figure 3 It is a schematic diagram showing the determination of the uniform scanning trajectory and point cloud density;

[0037] Figure 4 This is a flowchart of the calculation of optimal variable speed scanning conditions;

[0038] Figure 5 This is a schematic diagram showing the determination of the initial gear shift angle;

[0039] Figure 6 This is a schematic diagram of the variance statistics of point cloud density distribution under multi-scan boundary conditions;

[0040] Figure 7 It is a schematic diagram of the point cloud density distribution statistics obtained under a set of optimal variable speed scanning conditions;

[0041] Figure 8 This is a block diagram of the prism variable speed scanning control system. Detailed Implementation

[0042] To make the objectives, contents, and advantages of the present invention clearer, the specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples.

[0043] The first objective of this invention is to propose a prism variable speed scanning method. This method changes the prism's rotation from uniform speed to controllable multi-segment variable speed rotation. The optimal variable speed scanning mode can be obtained through calculation and simulation, thereby achieving uniform imaging in the entire area. This can effectively improve the imaging efficiency of lidar, save point cloud resources, and reduce costs.

[0044] A prism variable-speed scanning method transforms a uniform-speed scanning mode within one cycle of the prism into a multi-segment variable-speed scanning mode. This includes: determining the number of speed changes in the segmented scanning within one cycle; determining the initial scanning speed of the prism; determining the scanning speed after the speed change; determining the speed change delay time; determining the angular position of each speed change within one cycle; and determining the acceleration value from the initial scanning speed, the scanning speed after the speed change, and the speed change delay time. Simultaneously, by determining the device's altitude, laser repetition rate, lateral flight speed, and beam deflection angle, the point cloud density distribution curve after the speed change can be determined. The variance of the point cloud density distribution curve is used as a criterion to adjust the magnitude of the acceleration value to optimize the uniformity of the point cloud density distribution. Therefore, this method can calculate the optimal variable-speed scanning conditions based on the achievable acceleration level and the initial state of the device, thereby effectively improving the imaging uniformity of the lidar, increasing point cloud utilization, and saving point cloud resources.

[0045] The point cloud density distribution curve is obtained by counting the number of points in a unit area of ​​the scan trajectory map according to a certain size grid, and then counting the point cloud density in a certain direction.

[0046] The variance of the point cloud density distribution curve is obtained by solving for the variance of the data on the point cloud density distribution curve.

[0047] The second objective of this invention is to provide a prism variable speed scanning control system, which includes a scanning prism module, a scanning prism drive module, and a scanning control device. The scanning prism module is used to deflect and scan the laser beam, forming a continuous scanning trajectory; the scanning prism drive module is used to drive the prism to rotate in a predetermined manner; and the scanning control device is used for programming, downloading the optimal variable speed scanning conditions described in the prism variable speed scanning method into the scanning control device in the form of a programming language to achieve variable speed control of the scanning prism.

[0048] The optimal variable speed scanning conditions refer to the variable speed angle position, initial scanning speed, scanning speed after variable speed, and variable speed delay time when the variance of the point cloud density distribution curve is minimized, as calculated by the prism variable speed scanning method.

[0049] Optionally, during programming, the register addresses for four variables—variable shift angle position, initial scan speed, post-shift scan speed, and shift delay time—can be made available for easy reading, writing, and modification. These register addresses are the memory integrated into the scanning control device.

[0050] Example 1

[0051] The segmented variable speed scanning method in this embodiment divides a uniform speed scanning cycle into N segments of variable speed scanning. Optionally, the number of segments N of the variable speed scanning can be determined according to actual needs.

[0052] like Figure 1 As shown, in this embodiment, one scan cycle is divided into four variable-speed scan strokes. These four variable-speed scan strokes correspond to two sets of scan strokes with different angular velocities.

[0053] ω1 is the initial scanning speed, and ω2 is the scanning speed after speed change, with ω2 > ω1. If the positive X-axis direction is taken as 0°, then in this embodiment, the scanning speeds near 0° and 180° are less than those near 90° and 270° to allow for adjustment of the number of scanning points in different regions. Additionally, this embodiment includes four acceleration / deceleration regions, where Δt is the speed change delay time, i.e., the time required for the angular velocity to increase from ω1 to ω2, or decrease from ω2 to ω1. θ0 is the initial speed change angle position, i.e., the prism rotates from the initial speed ω1 to the θ0 position to begin speed change. For ease of calculation, the other three speed change positions in this embodiment are 180°-θ0, 180°+θ0, and 360°-θ0, respectively. Thus, variable scanning within one cycle can be achieved.

[0054] In this embodiment, in order to obtain the optimal variable speed scanning parameters, the initial scanning speed ω1, the scanning speed after the speed change ω1, the initial speed change angle position θ0, and the speed change delay time Δt can be adjusted, and the evaluation criterion is the magnitude of the variance value of the point cloud density distribution curve.

[0055] In the above steps, the variance of the point cloud density distribution curve is mainly obtained according to... Figure 2 The process shown requires first determining the known conditions in S201, including the device's height h and the laser's repetition rate f. laser Horizontal flight speed of the aircraft v x The beam deflection angle α and initial angular velocity w0, with angle values ​​θ at different times and t representing time, are calculated using the following formula:

[0056]

[0057] The scanning trajectory within the time interval t can be calculated from the above formula, such as Figure 3 The image shows the scanning trajectory of the prism during uniform scanning. (Press...) Figure 2 In step S203, the scanning trajectory range of the uniformly intersecting portion is selected, and the number of point clouds per unit area is calculated, i.e., using... Figure 3 The black grid lines shown divide the scan trajectory, count the number of point clouds within each grid, and calculate the point cloud density. From this, the point cloud density can be obtained along... Figure 3 The distribution map of point cloud density in the Y direction is obtained, and the variance of the data points on the curve is calculated.

[0058] In the above steps, the optimal variable speed sweep parameters can be obtained by following the steps outlined above. Figure 4 The process is as shown. First, the known parameters in step S401 need to be determined, and then the boundary conditions in S402 need to be input, mainly including: the range of angular velocity variation [ω]. 1，ω2], the range of the speed change delay time Δt [t1, t2], the initial speed change angle position θ0; in S403, a set of boundary conditions is determined, and the corresponding point cloud density distribution variance is calculated by steps S404 to S406 respectively. The obtained variance value is compared with the point cloud distribution density variance value obtained by uniform speed scanning. If it is greater than the point cloud distribution density variance obtained by uniform speed scanning, the set of calculation conditions is discarded, and a new set of boundary conditions is determined and calculated cyclically. If the obtained variance value is less than the point cloud density distribution variance obtained by uniform speed scanning, step S407 is entered, the obtained variance and the set of input boundary conditions are saved, and new boundary conditions are determined again and calculated cyclically until all boundary conditions are calculated. All variance values ​​that meet the conditions are compared, and the input boundary condition corresponding to the smallest obtained variance value is selected as the optimal solution, that is, the optimal speed change scanning parameters.

[0059] In the above steps, the initial gear shift angle position can be determined according to calculation rules. Figure 5 The figure shows the point cloud density distribution calculated under the same set of boundary conditions at different initial acceleration angle positions θ0. As can be seen, the point cloud density at the center position is independent of the initial acceleration angle position θ0. At the edge positions, the distribution of the lowest point density gradually shifts to both sides as θ0 increases, and the value of the lowest point density also gradually increases. In this embodiment, θ0 = 60° was selected as the initial acceleration angle position. Because under this condition, the lowest point at the edge is relatively close to the lowest point at the center, thus satisfying the goal of uniform point cloud distribution.

[0060] Figure 6 The diagram shows the variance distribution calculated under multiple boundary conditions. As can be seen from the diagram, there are convergent boundary conditions in a certain region across the entire range, that is, there is a set of optimal scanning parameters.

[0061] The above steps can be used to calculate a set of relatively ideal variable speed scanning parameters. Figure 7 The figure shows a set of point cloud density distribution maps obtained through calculation. As can be seen, using a segmented variable-speed scanning method can reduce the point cloud density at the edges and increase the point cloud density in the middle. This improves both the uniformity of the point cloud distribution and the overall point cloud density, thereby enhancing the imaging resolution. Those skilled in the art will clearly recognize that the optimal scanning parameters obtained under ideal conditions may not perfectly match the actual scanning device; further constraints on the range of parameter variables are needed to obtain compromise scanning parameters.

[0062] To achieve the scanning method shown in this embodiment, a prism variable speed scanning system is also proposed.

[0063] Figure 8The diagram shows the structural composition of the prism variable speed scanning system in this embodiment. S801 is the power supply module, used to power all modules of the system. S802 is the scanning prism control module, used for programming and implementing the variable speed scanning control mode. It contains a storage module and can communicate with the S805 host computer module via an open register address to modify scanning parameters. S804 is the scanning prism drive module, used to drive the scanning prism to rotate. S803 is the scanning prism module, used to deflect the emitted light rays to achieve circular trajectory scanning. This module has a built-in encoder and can output angle values.

[0064] Optionally, the scanning prism module can be other mechanical scanning components, such as scanning rotating mirrors, scanning galvanometers, etc.

[0065] Optionally, the scanning prism drive module can be a drive electronics for other scanning components.

[0066] Optionally, the scanning prism control module can be any electronic device with programming, storage, and communication functions.

[0067] Optionally, the host computer module can be any computer reading and writing device with communication capabilities.

[0068] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A scanning method for improving the uniformity of a laser radar point cloud, characterized in that, The uniform speed scanning mode of the prism in a period is changed into a multi-section variable speed scanning mode, and the method comprises the following steps: S1, determining the number of variable speeds in a period of segmented scanning; S2, determining the initial scanning speed of the prism; S3, determining the scanning speed of the prism after the speed change; S4, determining the variable speed delay time; S5, determining the angular position of each variable speed in a period; S6, determining the acceleration value from the initial scanning speed, the scanning speed after the speed change, and the variable speed delay time; S7, determining the point cloud density distribution curve after the speed change according to the height of the device, the laser frequency, the transverse flight speed, and the light beam deflection angle; the point cloud density distribution curve is obtained by counting the number of points in a unit area in the scanning track diagram, and then counting the point cloud density in a certain direction; S8, taking the variance value of the point cloud density distribution curve as a criterion, adjusting the size of the acceleration value to optimize the uniformity of the point cloud density distribution; the variance value of the point cloud density distribution curve is obtained by solving the variance of the data on the point cloud density distribution curve, and the height h of the device, the laser frequency f laser , the lateral flight speed of the aircraft v x , the beam deflection angle α and the initial angular velocity w 0 , the angle value at different times θ , t is time, and the following calculation formula is used: The scanning track of the prism in the t time period is calculated; The scanning track range of the uniform intersection part is intercepted, the number of points in a unit area is calculated, and the point cloud density is calculated, so that the point cloud density distribution diagram is obtained, and the variance of the data points on the curve can be calculated; S9, according to the actual reachable acceleration level and the initial state of the device, the optimal variable speed scanning condition is calculated, and the calculation method of the optimal variable speed scanning condition is as follows: S901、according to the angular velocity variation range ω 1， ω 2 ] the range of the variable speed delay time Δt t 1 t 2 ] the initial variable speed angle position θ 0 determine a set of boundary conditions, and calculate the corresponding point cloud density distribution variance;​​ S902, the variance value obtained in S901 is compared with the point cloud distribution density variance obtained by uniform speed scanning, if the variance value is greater than the point cloud distribution density variance obtained by uniform speed scanning, the group of boundary conditions is discarded, a new group of boundary conditions is determined and returned to S901 for recalculation, if the variance value is less than the point cloud density distribution variance obtained by uniform speed scanning, S903 is entered; S903, the variance and the input boundary condition of the group meeting the condition are saved, and new boundary conditions are determined again and returned to S901 for cyclic calculation until all boundary conditions are calculated; S904, all the variance values meeting the condition are compared, and the input boundary condition corresponding to the minimum variance value is selected as the optimal solution, that is, the optimal variable speed scanning condition.

2. A control system for improving the uniformity of distribution of a lidar point cloud, the system being used to implement the scanning method for improving the uniformity of distribution of a lidar point cloud according to claim 1, characterized in that, The scanning prism module, the scanning prism driving module, and the scanning control device are included; the scanning prism module is used for realizing the deflection scanning of the laser beam to form a continuous scanning track; the scanning prism driving module is used for driving the prism to rotate in a predetermined manner; and the scanning control device is used for downloading the optimal variable speed scanning condition in the form of a programming language to the scanning control device to realize the variable speed control of the scanning prism.

3. The control system for improving uniformity of distribution of a ladar point cloud of claim 2, wherein, The scanning prism module can be a scanning rotating mirror or a scanning oscillating mirror.

4. The control system for improving uniformity of distribution of a ladar point cloud of claim 2, wherein, The scanning prism driving module is a driving electronic device of other scanning components.

5. The control system for improving uniformity of distribution of a ladar point cloud of claim 2, wherein, The scanning prism control module is any electronic device with programming, storage, and communication functions.

6. The control system for improving uniformity of distribution of a ladar point cloud of claim 2, wherein, The scanning prism module and the scanning control device are in communication with the upper computer module, and the upper computer module is any computer read-write device with communication function.

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