CN114594458B - A calibration method for multi-line laser radar - Google Patents

Abstract

The invention discloses a multi-line laser radar distance calibration method, including laser ranging module calibration and laser radar horizontal rotation angle calibration. In the laser ranging module calibration link, the measured value of the distance from the laser ranging module to the geometric center of the light spot is obtained respectively; the true value of the distance from the geometric center of the light spot to the laser transmitter is obtained, and the model parameters of the distance error model are determined according to the measured value and the true value; the pitch angle error of the laser ranging module is determined according to the coordinates of the geometric center of the light spot and the coordinates of the laser transmitter; in the laser radar horizontal angle calibration link, the rotation angles corresponding to multiple groups of targets in the laser radar coordinate system are measured by a total station as the true value, and the laser radar measures the rotation angles corresponding to the multiple groups of targets in the laser radar coordinate system as the measured value, and determines the relationship between the true value and the measured value. When the invention calculates the laser radar horizontal rotation angle, only the x-axis coordinates and the y-axis coordinates of the center coordinates of the cardboard are used, and the amount of calculation is reduced.

Description

A calibration method for multi-line laser radar

Technical Field

The invention belongs to the field of laser radar calibration, and in particular is a calibration method for a multi-line laser radar.

Background technique

LiDAR has outstanding features such as high resolution, strong anti-interference ability, and direct imaging. It can be used to measure distance, speed, angle, and tracking, and is widely used in target measurement, terrain mapping, navigation and guidance, etc. However, the widely used LiDARs are mainly designed and produced by North American and European countries. The design and calibration schemes of imported instruments are often confidential and expensive. In the existing reports, the existing LiDAR calibration methods all use mature LiDARs, which have been calibrated once before leaving the factory, and then calibrated again on this basis. In order to calibrate the independently developed multi-line LiDAR, a calibration scheme must be formulated according to its structural characteristics.

Summary of the invention

The object of the present invention is to provide a calibration method for a multi-line laser radar.

In order to achieve the above object, the present invention adopts the following technical solution: a calibration method of a multi-line laser radar, including laser ranging module calibration and laser radar horizontal rotation angle calibration:

(1) Calibration of laser ranging module: Let the laser beam emitted by the laser ranging module to be calibrated be horizontally irradiated on a standard reflectivity curtain, mark the geometric center of the light spot, and record the measured value of the distance from the laser ranging module to the geometric center of the light spot; use a total station to measure the coordinates of the geometric center of the light spot on the standard reflectivity curtain and the coordinates of the laser emitter, obtain the true value of the distance from the geometric center of the light spot to the laser emitter, and determine the model parameters of the distance error model based on the measured value and the true value; determine the pitch angle error of the laser ranging module based on the coordinates of the geometric center of the light spot and the coordinates of the laser emitter;

(2) LiDAR horizontal angle calibration: Use a total station to measure the rotation angles of multiple groups of targets in the LiDAR coordinate system as true values, and use the LiDAR to measure the rotation angles of the multiple groups of targets in the LiDAR coordinate system as measured values to determine the relationship between the true value and the measured value.

Preferably, the specific method of allowing the laser beam emitted by the laser distance measuring module to be calibrated to irradiate horizontally onto the standard reflectivity curtain is:

Fix the laser radar on the turntable and keep it from rotating, select a laser ranging module to be measured with a pitch angle of β ₀ and tilt the turntable by β ₀ to ensure that the laser beam emitted by the laser ranging module is parallel to the standard reflectivity curtain.

Preferably, the specific method of using a total station to determine the coordinates of the geometric center of the light spot on the standard reflectivity curtain and the coordinates of the laser emitter is:

Step 1: Place the total station between the standard reflectivity curtain and the laser radar, aim the laser beam of the total station at the marker on the standard reflectivity curtain, set the horizontal angle HR1 to 0 and record the slant distance SD1 and zenith distance VZ1 at this time;

Step 2, keep the total station fixed, rotate the total station so that the red laser beam is aimed at the laser transmitter of the laser ranging module, and record the slant distance SD2, horizontal angle HR2 and zenith distance VZ2 at this time;

Step 3: Convert the data obtained by the total station into rectangular coordinates to obtain the coordinates of the marker and the laser transmitter in the total station coordinate system as (x ₁ , y ₁ , z ₁ ) and (x ₂ , y ₂ , z ₂ ) respectively;

Calculate the true distance S between the laser transmitter and the marker and the elevation error Δβ of the laser beam:

Step 4: Change the distance between the laser ranging module and the standard reflectivity curtain to obtain multiple sets of real distance and distance measurement values, and determine the model parameters of the ranging error model and the average value of the pitch angle error.

Preferably, the specific method for calibrating the laser radar horizontal angle is:

Set up several cardboards around the laser radar, measure the center coordinates of the ith cardboard in the laser radar coordinate system, and solve the measured value of the horizontal rotation angle corresponding to the center coordinate; use the total station to measure the center coordinates of the ith cardboard in the total station coordinate system, and use the total station to measure the translation matrix and rotation matrix between the laser radar coordinate system and the total station coordinate system; convert the center coordinates of the ith cardboard in the total station coordinate system to the laser radar coordinate system, and solve the corresponding true value of the horizontal rotation angle, and determine the model parameters of the horizontal rotation angle error model.

Preferably, the angle between the line connecting the center line of the paperboard and the center line of the laser radar and the x-axis in the laser radar coordinate system is taken as the measured value of the horizontal rotation angle corresponding to the central coordinate.

Preferably, the specific calculation method of the measured value of the horizontal rotation angle corresponding to the center coordinate is:

In the formula, (xi _,j ,yi _,j ,zi _,j ) is the coordinate of the jth point cloud data on the i-th cardboard.

Preferably, the specific method of using the total station to measure the translation matrix T and the rotation matrix R between the laser radar coordinate system and the total station coordinate system is:

Step 1: Use the total station to obtain the coordinates of the laser radar base in the total station coordinate system, and obtain the translation matrix T between the laser radar coordinate system and the total station coordinate system;

Step 2: Use the midpoint coordinates of one of the cardboards in the laser radar coordinate system The rotation matrix R between the two coordinate systems is calculated using the midpoint coordinates (x _i ′, y _i ′, z _i ′) in the total station coordinate system.

Preferably, the method for calculating the true value of the horizontal rotation angle is:

Where: The coordinates of the i-th cardboard in the lidar coordinate system are ( _xi ”, _yi ”).

Preferably, the formula for determining the coordinates of the i-th cardboard in the laser radar coordinate system is:

Where ( _xi ', _yi ') is the center coordinate of the ith cardboard in the total station coordinate system, R is the rotation matrix, and T is the rotation matrix.

Preferably, several groups of measured values _α0 and true values α of the horizontal rotation angle are substituted into the error model of the horizontal rotation angle of the laser radar, and the model parameters are solved according to the least squares indirect adjustment principle. The error model of the horizontal rotation angle of the laser radar is:

α＝α ₀ +k ₀ +k ₁ sin(α ₀ +k ₂ )

Where k ₀ , k ₁ and k ₂ are the parameter models to be determined.

Compared with the prior art, the present invention has the following significant advantages: (1) the present invention uses a high-precision turntable to compensate for the pitch angle of the laser ranging module, so that the laser beam is irradiated on a standard reflectivity curtain, and then the center position of the light spot can be easily found in conjunction with an infrared CCD camera; (2) the present invention uses a total station to measure the coordinate information of the target object and the laser emitter, and can simultaneously obtain the true distance from the laser emitter to the marker and the true pitch angle of the laser beam, and calibrate the distance error and the pitch angle error at the same time; (3) the present invention uses the angle between the connecting line of the center axis of the cardboard and the center line of the laser radar and the x-axis in the laser radar coordinate system to represent the horizontal rotation angle of the laser radar; (4) the present invention uses a total station to measure the center position of the laser radar, and can directly obtain the translation matrix between the laser radar coordinate system and the total station coordinate system; (5) when calculating the horizontal rotation angle of the laser radar, the present invention only uses the x-axis coordinate and the y-axis coordinate of the center coordinate of the cardboard, thereby reducing the amount of calculation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the structure of a laser radar of the present invention.

FIG. 2 is a flow chart of a laser ranging module ranging calibration method according to the present invention.

FIG3 is a schematic diagram of a ranging calibration experiment scenario of a laser ranging module of the present invention.

FIG4 is a flow chart of the laser radar horizontal angle calibration method of the present invention.

FIG. 5 is a point cloud diagram of an experimental scene for horizontal angle calibration of a laser radar according to the present invention.

FIG. 6 is a schematic diagram of an experimental scene for laser radar horizontal angle calibration of the present invention.

Detailed ways

The present invention is further described below in conjunction with the accompanying drawings and specific embodiments. The present invention includes but is not limited to the following embodiments.

As an embodiment, the schematic diagram of the multi-line laser radar structure used in the present invention is shown in Figure 1, which is composed of several laser ranging modules and a brushless DC motor. The laser ranging modules are distributed in a ring shape around the motor. The pitch angle of each module is different, which is determined by its mechanical structure.

In some embodiments, 21 laser ranging modules are distributed in a ring around the motor, and the pitch angle of each module is different, which is determined by its mechanical structure.

As an embodiment, a multi-line laser radar calibration method includes two steps: calibration of a laser ranging module and calibration of a laser radar horizontal angle:

Laser ranging module calibration: Using a high-precision turntable and total station, the true distance from the laser ranging module to the target and the true elevation angle of the laser ranging module can be obtained, and the ranging error and elevation angle error of the laser ranging module can be calibrated at the same time.

LiDAR horizontal rotation angle calibration: Use a high-precision total station to measure the horizontal rotation angle of the target object in the LiDAR coordinate system as the true value to calibrate the LiDAR's horizontal rotation angle error.

(1) Laser ranging module calibration

The flow chart of the laser ranging module calibration method is shown in Figure 2. The laser ranging module calibration includes ranging error calibration and pitch angle calibration. The schematic diagram of the experimental scene of the calibration method is shown in Figure 3. First, the laser radar is fixed on a high-precision turntable and the laser radar is kept fixed. The pitch angle β ₀ of the currently calibrated laser ranging module is recorded and the turntable is used to tilt the laser radar by β ₀ so that the laser beam emitted by the laser ranging module can be almost parallel to the standard reflectivity curtain.

Then, with the help of an infrared CCD camera and a display, the position of the light spot is found on the standard reflectivity curtain and the measured distance value at this time is saved in the host computer as S ₀ , which is the distance measurement value. At the same time, the position of the geometric center of the light spot is determined, and an obvious marker is attached. Finally, the coordinates of the marker on the standard reflectivity curtain and the coordinates of the laser transmitter are determined using a total station.

Furthermore, the specific method for determining the coordinates of the marker and the laser emitter is:

When the total station measures the coordinates of the target object, it obtains the slant distance SD, horizontal angle HR and zenith distance VZ of the target object from the total station. Therefore, the rectangular coordinate conversion formula in the total station coordinate system is formula (1):

First, place the total station between the standard reflectivity curtain and the laser radar, level it and turn on the laser pointing function of the total station; then aim the red laser beam at the marker on the standard reflectivity curtain, set the horizontal angle HR1 to 0, and record the slant distance SD1 and zenith distance VZ1 at this time; then keep the total station in place, only rotate the total station, aim the red laser beam at the laser emitter of the laser ranging module, and record the slant distance SD2, horizontal angle HR2 and zenith distance VZ2 at this time; finally, calculate the coordinates ( _x1 , _y1 , z1) of the marker on the standard reflectivity curtain and the coordinates ( _x2 , _y2 , _z2 ) of the laser emitter according to formula ( ₁ ). Therefore, the true value S of the distance between the marker and the laser emitter can be calculated by formula (2):

The pitch angle error of the laser ranging module is calculated by formula (3):

For this laser ranging module, within the distance range of 10m-40m from the standard reflectivity curtain, an experiment is performed every 2m, so that 16 sets of distance measurement values _S0 and true values S can be obtained to calculate the model parameters of the distance error model in the form of formula (4); at the same time, 16 sets of pitch angle error values Δβ can be obtained and the average value is obtained Reduce accidental errors.

S＝a*S+b (4)

Wherein, a and b are model parameters of the distance calibration error model, and the parameter values can be calculated according to the least squares principle.

The actual pitch angle β of the laser ranging module is:

Finally, the same experiment is performed on each laser ranging module to obtain the model parameters and pitch angle error of the distance error model of each laser ranging module.

(2) LiDAR horizontal rotation angle calibration

The flow chart of the laser radar horizontal rotation angle calibration method is shown in Figure 4. In the laser radar horizontal rotation angle calibration link, the laser radar is first placed in an open environment, and 17 cardboards are erected around the laser radar at a distance of about 10m from the laser radar; then the laser radar is started to scan the environment, a point cloud image is obtained, and the top view of the point cloud image of the 17 cardboards is distinguished in the point cloud image, as shown in Figure 5.

The horizontal rotation angle of the laser radar can be expressed by the angle between the center line of the cardboard and the center line of the laser radar and the x-axis in the laser radar coordinate system, as shown in Figure 6. In order to facilitate the calculation of the horizontal rotation angle of the laser radar, the angle of the laser ranging module with a pitch angle of 0° can be considered as the horizontal rotation angle. On each piece of cardboard, the point cloud of the laser ranging module with a pitch angle of 0° is extracted separately, and its point cloud image should be a curve. Assuming that there are n point cloud data on the i-th cardboard, and the coordinates of the j-th point cloud data are (xi _,j , _yi,j , _zi,j ), then the coordinates of the center of the symmetry axis of the cardboard at this height are The calculation formulas are formula (6), formula (7) and formula (8):

Because the point cloud data of the laser ranging module with a pitch angle of 0° is used, z _i,j is a constant and can be ignored. Therefore, the horizontal rotation angle measurement value α ₀ of the laser radar of the central axis of each cardboard can be calculated using formula (9):

Next, place the total station in the middle of a circle of cardboard, level the total station, and fix the zenith distance to 90°. This setting method can make the points pointed by the red laser beam of the total station all on the same horizontal plane, that is, there is no need to consider the z-axis coordinate, reducing the calculation workload. Turn on the laser pointing function, let the red laser beam align with the central axis of the first cardboard, set the horizontal angle HR to 0, and record the slant distance SD at this time. Then rotate the total station counterclockwise to align the red laser beam with the central axis of the second cardboard, and record the slant distance SD and horizontal angle HR at this time. Then record the slant distance SD and horizontal angle HR corresponding to the central axis of each cardboard in the same way, and use formula (1) to convert them into rectangular coordinates to obtain the coordinates corresponding to the central axis of the i-th cardboard in the total station coordinate system.

Then use the total station to measure the coordinates of the laser radar base. Because the laser radar base is a symmetrical cuboid, you only need to measure the coordinates of the two ends of the base diagonal and then use the midpoint coordinate formula to get the coordinates of the laser radar in the total station coordinate system, thereby obtaining the translation matrix T between the total station coordinate system and the laser radar coordinate system; on this basis, use the coordinates of the first cardboard centerline in the laser radar coordinate system and the coordinates (x ₁ ',y ₁ ') in the total station coordinate system to solve the rotation matrix R between the two coordinate systems;

Finally, the center coordinates ( _xi ’, _yi ’) of the i-th cardboard in the total station coordinate system are converted to the lidar coordinate system using formula (10) to obtain ( _xi ’, _yi ’), and then the true value α of the horizontal rotation angle corresponding to ( _xi ’, _yi ’) in the lidar coordinate system is calculated using formula (11).

The error model of the laser radar horizontal rotation angle is shown in formula (12). 17 sets of horizontal rotation angle measurement values _α0 and true values α are obtained from 17 pieces of cardboard. The model parameters _k0 , _k1 and _k2 are solved according to the least squares indirect adjustment principle.

α＝α ₀ +k ₀ +k ₁ sin(α ₀ +k ₂ ) (12)

Claims (8)

1. A multi-line laser radar calibration method, characterized in that it includes laser ranging module calibration and laser radar horizontal rotation angle calibration: (1) Calibration of laser ranging module: Let the laser beam emitted by the laser ranging module to be calibrated be horizontally irradiated on a standard reflectivity curtain, mark the geometric center of the light spot, and record the measured value of the distance from the laser ranging module to the geometric center of the light spot; use a total station to measure the coordinates of the geometric center of the light spot on the standard reflectivity curtain and the coordinates of the laser emitter, obtain the true value of the distance from the geometric center of the light spot to the laser emitter, and determine the model parameters of the distance error model based on the measured value and the true value; determine the pitch angle error of the laser ranging module based on the coordinates of the geometric center of the light spot and the coordinates of the laser emitter; wherein, the specific method of using a total station to measure the coordinates of the geometric center of the light spot on the standard reflectivity curtain and the coordinates of the laser emitter is as follows: Step 1: Place the total station between the standard reflectivity curtain and the laser radar, aim the laser beam of the total station at the marker on the standard reflectivity curtain, set the horizontal angle HR1 to 0 and record the slant distance SD1 and zenith distance VZ1 at this time; Step 2, keep the total station fixed, rotate the total station so that the red laser beam is aimed at the laser transmitter of the laser ranging module, and record the slant distance SD2, horizontal angle HR2 and zenith distance VZ2 at this time; Step 3: Convert the data obtained by the total station into rectangular coordinates to obtain the coordinates of the marker and the laser transmitter in the total station coordinate system as (x ₁ , y ₁ , z ₁ ) and (x ₂ , y ₂ , z ₂ ) respectively; Calculate the true distance S between the laser transmitter and the marker and the elevation error Δβ of the laser beam: Step 4: Change the distance between the laser ranging module and the standard reflectivity curtain to obtain multiple sets of real distances and distance measurement values, and determine the model parameters of the ranging error model and the average value of the pitch angle error; (2) Laser radar horizontal angle calibration: The rotation angles corresponding to multiple groups of targets in the laser radar coordinate system are measured by a total station as true values, and the laser radar measures the rotation angles corresponding to the multiple groups of targets in the laser radar coordinate system as measured values, and determines the relationship between the true value and the measured value. The specific method of laser radar horizontal angle calibration is as follows: Set up several cardboards around the laser radar, measure the center coordinates of the ith cardboard in the laser radar coordinate system, and solve the measured value of the horizontal rotation angle corresponding to the center coordinate; use the total station to measure the center coordinates of the ith cardboard in the total station coordinate system, and use the total station to measure the translation matrix and rotation matrix between the laser radar coordinate system and the total station coordinate system; convert the center coordinates of the ith cardboard in the total station coordinate system to the laser radar coordinate system, and solve the corresponding true value of the horizontal rotation angle to determine the model parameters of the horizontal rotation angle error model. 2. The calibration method of the multi-line laser radar according to claim 1 is characterized in that the specific method of allowing the laser beam emitted by the laser ranging module to be calibrated to irradiate horizontally on the standard reflectivity curtain is: Fix the laser radar on the turntable and keep it from rotating, select a laser ranging module to be measured with a pitch angle of β ₀ and tilt the turntable by β ₀ to ensure that the laser beam emitted by the laser ranging module is parallel to the standard reflectivity curtain. 3. The calibration method of the multi-line laser radar according to claim 1 is characterized in that the angle between the center axis of the cardboard and the center line of the laser radar and the x-axis in the laser radar coordinate system is taken as the measured value of the horizontal rotation angle corresponding to the center coordinate. 4. The calibration method of the multi-line laser radar according to claim 1 or 3, characterized in that the specific calculation method of the measured value of the horizontal rotation angle corresponding to the center coordinate is: In the formula, (xi _,j ,yi _,j ,zi _,j ) is the coordinate of the jth point cloud data on the i-th cardboard. 5. The calibration method of the multi-line laser radar according to claim 1 is characterized in that the specific method of using the total station to measure the translation matrix T and the rotation matrix R between the laser radar coordinate system and the total station coordinate system is: Step 1: Use the total station to obtain the coordinates of the laser radar base in the total station coordinate system, and obtain the translation matrix T between the laser radar coordinate system and the total station coordinate system; Step 2: Use the midpoint coordinates of one of the cardboards in the laser radar coordinate system The rotation matrix R between the two coordinate systems is calculated using the midpoint coordinates (x _i ′, y _i ′, z _i ′) in the total station coordinate system. 6. The calibration method of the multi-line laser radar according to claim 1, characterized in that the method for calculating the true value of the horizontal rotation angle is: Where: The coordinates of the i-th cardboard in the lidar coordinate system are ( _xi ”, _yi ”). 7. The calibration method of the multi-line laser radar according to claim 6, characterized in that the formula for determining the coordinates of the i-th paperboard in the laser radar coordinate system is: Where ( _xi ', _yi ') is the center coordinate of the ith cardboard in the total station coordinate system, R is the rotation matrix, and T is the rotation matrix. 8. the calibration method of multi-line laser radar according to claim 1, is characterized in that, the measured value α ₀ and true value α of several groups of horizontal rotation angles are substituted into the error model of laser radar horizontal rotation angle, and the model parameters are solved according to the least squares indirect adjustment principle. The error model of laser radar horizontal rotation angle is: α＝α ₀ +k ₀ +k ₁ sin(α ₀ +k ₂ ) Where k ₀ , k ₁ and k ₂ are the parameter models to be determined.