LiDAR-based SLAM (gmapping, cartographer) in ROS

Introduction

LiDAR-based SLAM helps a robot build a map of its surroundings while figuring out where it is. It uses laser scans to see the environment clearly.

When a robot needs to explore an unknown indoor space and create a map.

When you want a robot to navigate safely without GPS, like inside buildings.

When you want to track a robot's position precisely using laser sensors.

When you need a real-time map update as the robot moves around.

When you want to combine sensor data to improve robot localization.

Syntax

ROS

roslaunch gmapping slam_gmapping.launch
roslaunch cartographer_ros demo_backpack_2d.launch

gmapping is a ROS package that uses laser scans to create 2D maps.

cartographer is a more advanced SLAM system that can do 2D and 3D mapping with better accuracy.

Examples

This command starts gmapping to build a 2D map using LiDAR data.

ROS

# Run gmapping for 2D SLAM
roslaunch gmapping slam_gmapping.launch

This command runs a cartographer demo that uses a backpack LiDAR sensor for 2D mapping.

ROS

# Run cartographer demo for 2D SLAM
roslaunch cartographer_ros demo_backpack_2d.launch

This command runs cartographer for 3D mapping using a 3D LiDAR sensor.

ROS

# Run cartographer for 3D SLAM
roslaunch cartographer_ros demo_backpack_3d.launch

This saves the map created by gmapping to a file named 'my_map'.

ROS

# Save the map after gmapping
rosrun map_server map_saver -f my_map

Sample Program

This launch file starts gmapping with basic parameters, along with robot state and laser drivers. It lets the robot build a 2D map using LiDAR data.

ROS

# This is a ROS launch file example for gmapping

<launch>
  <!-- Start gmapping node -->
  <node pkg="gmapping" type="slam_gmapping" name="slam_gmapping" output="screen">
    <param name="base_frame" value="base_link" />
    <param name="odom_frame" value="odom" />
    <param name="map_update_interval" value="5.0" />
  </node>

  <!-- Start robot state publisher -->
  <node pkg="robot_state_publisher" type="robot_state_publisher" name="robot_state_publisher" />

  <!-- Start laser scan publisher (example) -->
  <node pkg="laser_drivers" type="laser_node" name="laser_node" />
</launch>

Important Notes

Gmapping works well for 2D indoor environments but can struggle with large or complex spaces.

Cartographer can handle 3D data and provides better accuracy but needs more setup.

Both require good LiDAR data and odometry for best results.

Summary

LiDAR-based SLAM helps robots map and localize using laser scans.

Gmapping is simple and good for 2D maps; Cartographer is more advanced and supports 3D.

Use ROS launch files to start these SLAM systems easily.

Practice

1. What is the main purpose of LiDAR-based SLAM in ROS?

easy

A. To process camera images for object detection

B. To control robot motors directly

C. To help robots create maps and know their location using laser scans

D. To send commands to robot arms

Solution

1. Step 1: Understand SLAM basics

   SLAM stands for Simultaneous Localization and Mapping, which means building a map and knowing the robot's position at the same time.

2. Step 2: Identify LiDAR role

   LiDAR uses laser scans to sense the environment, which helps SLAM create accurate maps and localize the robot.

3. Final Answer:

   To help robots create maps and know their location using laser scans -> Option C

4. Quick Check:

   LiDAR-based SLAM = mapping + localization with lasers [OK]

Hint: SLAM = map + location using sensors like LiDAR [OK]

Common Mistakes:

• Confusing SLAM with motor control
• Thinking SLAM processes camera images only
• Assuming SLAM sends commands to robot arms

2. Which ROS launch command correctly starts the gmapping SLAM node?

easy

A. rosstart gmapping slam.launch

B. roslaunch gmapping slam_gmapping.launch

C. rosrun gmapping slam_gmapping.py

D. roslaunch cartographer slam.launch

Solution

1. Step 1: Recall ROS launch syntax

   ROS uses roslaunch to start nodes with launch files, specifying package and launch file name.

2. Step 2: Match gmapping launch file

   The gmapping package provides slam_gmapping.launch to start the SLAM node.

3. Final Answer:

   roslaunch gmapping slam_gmapping.launch -> Option B

4. Quick Check:

   Use roslaunch + package + launch file [OK]

Hint: Use roslaunch with package and launch file name [OK]

Common Mistakes:

• Using rosrun instead of roslaunch for launch files
• Typing rosstart which is invalid
• Confusing cartographer launch with gmapping

3. Given this ROS launch snippet for Cartographer:

<launch>
  <include file="$(find cartographer_ros)/launch/demo_backpack_2d.launch" />
</launch>

What will happen when you run this launch file?

medium

A. It launches a robot simulation without SLAM

B. It starts gmapping SLAM with a 3D map

C. It causes a syntax error due to missing parameters

D. It starts Cartographer SLAM with a 2D LiDAR backpack demo configuration

Solution

1. Step 1: Understand the include tag

   The <include> tag loads another launch file from the cartographer_ros package.

2. Step 2: Identify the included launch file

   demo_backpack_2d.launch is a Cartographer demo for 2D SLAM using a backpack LiDAR sensor.

3. Final Answer:

   It starts Cartographer SLAM with a 2D LiDAR backpack demo configuration -> Option D

4. Quick Check:

   Include demo_backpack_2d.launch = Cartographer 2D SLAM [OK]

Hint: Include demo_backpack_2d.launch means Cartographer 2D SLAM [OK]

Common Mistakes:

• Confusing Cartographer with gmapping
• Assuming missing parameters cause errors here
• Thinking it launches simulation without SLAM

4. You try to run gmapping with this command:

roslaunch gmapping slam_gmapping.launch

But get an error: "Could not find node of type slam_gmapping in package gmapping". What is the likely cause?

medium

A. The gmapping package is not installed or sourced properly

B. You used rosrun instead of roslaunch

C. The launch file name is misspelled

D. Your LiDAR sensor is not connected

Solution

1. Step 1: Analyze the error message

   The error says it cannot find the node type in the gmapping package, meaning the package or node is missing.

2. Step 2: Check common causes

   This usually happens if the gmapping package is not installed or the ROS environment is not sourced to find it.

3. Final Answer:

   The gmapping package is not installed or sourced properly -> Option A

4. Quick Check:

   Missing package or source = node not found error [OK]

Hint: Check package installed and sourced if node not found error [OK]

Common Mistakes:

• Assuming rosrun vs roslaunch causes this error
• Thinking a typo in launch file name causes node type error
• Blaming LiDAR connection for node not found

5. You want to create a 3D map using Cartographer with a LiDAR sensor on a robot. Which approach is best?

hard

A. Use Cartographer's 3D SLAM launch files and configure sensor topics properly

B. Use gmapping with default 2D settings and add a 3D camera

C. Run gmapping and Cartographer simultaneously for better accuracy

D. Use Cartographer's 2D demo launch files and convert output to 3D later

Solution

1. Step 1: Understand SLAM capabilities

   Gmapping supports only 2D SLAM, so it cannot create 3D maps directly.

2. Step 2: Choose Cartographer for 3D

   Cartographer supports 3D SLAM and has launch files designed for 3D mapping with LiDAR sensors.

3. Step 3: Configure sensor topics

   Properly setting sensor topics in Cartographer launch files ensures correct data flow for 3D mapping.

4. Final Answer:

   Use Cartographer's 3D SLAM launch files and configure sensor topics properly -> Option A

5. Quick Check:

   3D map needs Cartographer 3D launch + sensor config [OK]

Hint: 3D mapping needs Cartographer 3D launch and sensor setup [OK]

Common Mistakes:

• Trying to use gmapping for 3D maps
• Running both SLAM systems at once
• Using 2D demos and expecting 3D output