Incorrect time_increment in LaserScan Topic Published by Webots Lidar Causes Cartographer Mapping Drift

[LiuXianBang]

Title: Incorrect time_increment in LaserScan Topic Published by Webots Lidar Causes Cartographer Mapping Drift

Description:

When using Webots to publish Lidar data, the LaserScan topic is published correctly and initially works well for mapping in Cartographer. Below is my Lidar configuration:

<device reference="lidar" type="Lidar">
    <ros>
        <enabled>true</enabled>
        <updateRate>5</updateRate>
        <topicName>/scan</topicName>
        <alwaysOn>true</alwaysOn>
        <frameName>laser</frameName>
    </ros>
</device>

However, after running for some time, Cartographer logs the following error:

[cartographer logger]: W1006 07:09:36.000000 84738 sensor_bridge.cpp:211] Ignored subdivision of a LaserScan message from sensor scan because previous subdivision time 638953025761062191 is not before current subdivision time 638953025761062191

Once this error occurs, the mapping process fails due to significant drift.

Problem Analysis:

After reviewing related resources, such as the following GitHub Issues:

• Ignored subdivision · Issue #1326 · cartographer-project/cartographer_ros
• Sensor Bridge Warning Ignored Subdivision of a Laserscan · Issue #907 · cartographer-project/cartographer_ros

It appears that the issue is likely caused by an incorrect setting of the time_increment parameter in the sensor_msgs/LaserScan message. According to the ROS documentation (sensor_msgs/LaserScan Documentation), time_increment is a critical parameter that represents the time interval between laser scan points. However, there seems to be no direct way to configure or fix the time_increment parameter in Webots' Lidar settings.

I hope the Webots development team can provide guidance on correctly setting the time_increment parameter in Webots, or confirm if this is a bug in Webots

[LiuXianBang]

I confirmed that the issue was indeed caused by the incorrect setting of the time_increment parameter. I successfully resolved the problem by writing a simple callback function to forward and modify the LaserScan message. Here is the specific solution:

def scan_callback(self, msg: LaserScan):
    # If time_increment is 0 or incorrect, calculate and set
    if msg.time_increment == 0.0 and len(msg.ranges) > 0 and msg.scan_time > 0.0:
       msg.time_increment = msg.scan_time / len(msg.ranges)

    # Republish the modified message
    self.publisher.publish(msg)

[Mega-kitty]

Hey, buddy. I'm also experiencing map drift when building maps. When the car is driving straight, the map is built fine. However, when the car turns, the radar point cloud follows the turn and tries to build a new map instead of matching the original one. This causes the maps to overlap. Has this happened to you?

[LiuXianBang]

I confirmed that the issue was indeed caused by the incorrect setting of the time_increment parameter. I successfully resolved the problem by writing a simple callback function to forward and modify the LaserScan message. Here is the specific solution:

def scan_callback(self, msg: LaserScan):
# If time_increment is 0 or incorrect, calculate and set
if msg.time_increment == 0.0 and len(msg.ranges) > 0 and msg.scan_time > 0.0:
msg.time_increment = msg.scan_time / len(msg.ranges)

# Republish the modified message
self.publisher.publish(msg)

My fix seems to be ineffective again, which is a very troublesome problem

[LiuXianBang]

@omichel @DavidMansolino I'm sorry to bother, but I couldn't find a solution. I hope to receive your help