lidar_align

A simple method for finding the extrinsic calibration between a 3D lidar and a 6-dof pose sensor

Note: Accurate results require highly non-planar motions, this makes the technique poorly suited for calibrating sensors mounted to cars.

The method makes use of the property that pointclouds from lidars appear more 'crisp' when the calibration is correct. It does this as follows:

1. A transformation between the lidar and pose sensor is set.
2. The poses are used in combination with the above transformation to fuse all the lidar points into a single pointcloud.
3. The sum of the distance between each point and its nearest neighbor is found. This process is repeated in an optimization that attempts to find the transformation that minimizes this distance.

Installation

To install lidar_align, please install ROS Indigo, ROS Kinetic or ROS Melodic.

The following additional system dependencies are also required:

sudo apt-get install libnlopt-dev

Input Transformations

The final calibrations quality is strongly correlated with the quality of the transformation source and the range of motion observed. To ensure an accurate calibration the dataset should encompass a large range of rotations and translations. Motion that is approximately planner (for example a car driving down a street) does not provide any information about the system in the direction perpendicular to the plane, which will cause the optimizer to give incorrect estimates in this direction.

Estimation proceedure

For most systems the node can be run without tuning the parameters. By default two optimizations are performed, a rough angle only global optimization followed by a local 6-dof refinement.

The node will load all messages of type sensor_msgs/PointCloud2 from the given ROS bag for use as the lidar scans to process. The poses can either be given in the same bag file as geometry_msgs/TransformStamped messages or in a separate CSV file that follows the format of Maplab.

Visualization and Results

The node will output it's current estimated transform while running. To view this your launchfile must set output="screen" in the <node/> section. See the given launchfile for an example.

Once the optimization finishes the transformation parameters will be printed to the console. An example output is as follows:

Active Transformation Vector (x,y,z,rx,ry,rz) from the Pose Sensor Frame to  the Lidar Frame:
[-0.0608575, -0.0758112, 0.27089, 0.00371254, 0.00872398, 1.60227]

Active Transformation Matrix from the Pose Sensor Frame to  the Lidar Frame:
-0.0314953  -0.999473  0.0078319 -0.0608575
  0.999499 -0.0314702 0.00330021 -0.0758112
 -0.003052 0.00793192   0.999964    0.27089
         0          0          0          1

Active Translation Vector (x,y,z) from the Pose Sensor Frame to  the Lidar Frame:
[-0.0608575, -0.0758112, 0.27089]

Active Hamiltonen Quaternion (w,x,y,z) the Pose Sensor Frame to  the Lidar Frame:
[0.69588, 0.00166397, 0.00391012, 0.718145]

Time offset that must be added to lidar timestamps in seconds:
0.00594481

ROS Static TF Publisher: <node pkg="tf" type="static_transform_publisher" name="pose_lidar_broadcaster" args="-0.0608575 -0.0758112 0.27089 0.00166397 0.00391012 0.718145 0.69588 POSE_FRAME LIDAR_FRAME 100" />

If the path has been set the results will also be saved to a text file.

As a method of evaluating the quality of the alignment, if the needed path is set all points used for alignment will be projected into a single pointcloud and saved as a ply. An example of such a pointcloud can be seen below.

CSV format

Column	Description
1	timestamp ns
2	vertex index (not used)
3	position x
4	position y
5	position z
6	orientation quaternion w
7	orientation quaternion x
8	orientation quaternion y
9	orientation quaternion z

Note that Maplab has two CSV exporters. This file-format is the same as produced by exportPosesVelocitiesAndBiasesToCsv but differs from the output of exportVerticesAndTracksToCsv

Parameters

---

Scan Parameters

Parameter	Description	Default
min_point_distance	Minimum range a point can be from the lidar and still be included in the optimization.	0.0
max_point_distance	Maximum range a point can be from the lidar and still be included in the optimization.	100.0
keep_points_ratio	Ratio of points to use in the optimization (runtimes increase drastically as this is increased).	0.01
min_return_intensity	The minimum return intensity a point requires to be considered valid.	-1.0
motion_compensation	If the movement of the lidar during a scan should be compensated for.	true
estimate_point_times	Uses the angle of the points in combination with lidar_rpm and clockwise_lidar to estimate the time a point was taken at.	false
lidar_rpm	Spin rate of the lidar in rpm, only used with estimate_point_times.	600
clockwise_lidar	True if the lidar spins clockwise, false for anti-clockwise, only used with estimate_point_times.	false

IO Parameters

Parameter	Description	Default
use_n_scans	Optimization will only be run on the first n scans of the dataset.	2147483647
input_bag_path	Path of rosbag containing sensor_msgs::PointCloud2 messages from the lidar.	N/A
transforms_from_csv	True to load scans from a csv file, false to load from the rosbag.	false
input_csv_path	Path of csv generated by Maplab, giving poses of the system to calibrate to.	N/A
output_pointcloud_path	If set, a fused pointcloud will be saved to this path as a ply when the calibration finishes.	""
output_calibration_path	If set, a text document giving the final transform will be saved to this path when the calibration finishes.	""

Alinger Parameters

Parameter	Description	Default
local	If False a global optimization will be performed and the result of this will be used in place of the inital_guess parameter.	false
inital_guess	Initial guess to the calibration (x, y, z, rotation vector, time offset), only used if running in local mode.	[0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
max_time_offset	Maximum time offset between sensor clocks in seconds.	0.1
angular_range	Search range in radians around the inital_guess during the local optimization stage.	0.5
translational_range	Search range around the inital_guess during the local optimization stage.	1.0
max_evals	Maximum number of function evaluations to run	200
xtol	Tolerance of final solution	0.0001
knn_batch_size	Number of points to send to each thread when finding nearest points	1000
knn_k	Number of neighbors to consider in error function	1
global_knn_max_dist	Error between points is limited to this value during global optimization.	1.0
local_knn_max_dist	Error between points is limited to this value during local optimization.	0.1
time_cal	True to perform time offset calibration	true