How to Localize in a known map of the environment#

🏁 Goal of this tutorial

By the end of this tutorial, you will know how to localize the robot in a 2D map of the environment (AKA OccupancyGrid).

Pre-requisites#

You should be able to remotely connect to your robot Set up ROS 2 communication with the robot.

You should be familiar with ROS’s RViz

You should be familiar with The startup process of the robot and application management

You should first have completed the How to create a 2D map of the environment

Start the Localization#

By default, PAL Robots start in localization mode. However, if you are in mapping mode, you first need to stop the slam module with the command:

Note

To check the current operating mode of the robot (localization or slam), you can use the command:

pal module list

The output will show the status of the localization and slam modules so that you can check which one is running.

pal module stop slam

And then start the localization module with the command:

pal module start localization

Attention

Both the localization and the slam modules will publish a transform between the frames map and base_footprint. For this reason, you should not start both modules at the same time. When starting the slam module make sure you first shut down the localization module and vice-versa.

Otherwise, if you are using the simulated robot, you can refer to Simulation to learn how to start the Localization.

Load the Map#

Once you created and saved a map of the environment using the steps described in the How to create a 2D map of the environment, you can now use this map to localize the robot.

To do so, the localization uses the map hosted by the map_server node, defined in its configuration file.

If you want to use a different map, you can dynamically change it using the /map_server/load_map with the following command:

ros2 service call /map_server/load_map nav2_msgs/srv/LoadMap "map_url: '/home/pal/my_map.yaml'"

Otherwise, you can change the default map that the map_server loads by creating a new configuration file. To do so, you first need to create a new directory in the .pal folder of your robot.

mkdir -p ~/.pal/config

And then, within this folder, you can create a new map_server configuration file.

touch ~/.pal/config/99_my_map_server_config.yaml

Attention

The name of the file should start with a number to ensure that it is loaded last and overrides the default PAL configurations.

Within the newly created 99_my_map_server_config.yaml file, you can insert your custom map_server parameters. For the list of available parameters, their meaning and how they affect the map_server, you can refer to the map_server configuration guide. When creating a new Navigation configuration file, you need to specify the node name it refers to, in this case, map_server.

/map_server:
   ros__parameters:
     # Your map_server parameters

     yaml_filename: "/home/pal/my_map.yaml"

Once created, to start using your custom map_server configuration, you need to restart the localization module with the command:

pal module restart localization

Note

This change is is persistent and will be loaded every time you start the localization module. If you want to revert to the default PAL configuration, you can simply delete the custom configuration file you created in the ~/.pal/config folder.

Localize the Robot#

To visualize the map the robot is using and its current position, you need to establish the communication with the robot as explained in Set up ROS 2 communication with the robot.

The MCL algorithm requires an initial pose, of the current position of the robot. This doesn’t need to be an accurate estimate of the current pose of the robot, but can be an initial guess of an area on the map where the robot is located.

This initial pose can be set either through the /initialpose topic with the command:

ros2 topic pub /initialpose geometry_msgs/msg/PoseWithCovarianceStamped \
"{header: {stamp: {sec: 0, nanosec: 0}, frame_id: 'map'}, \
pose: {pose: {position: {x: 0.0, y: 0.0, z: 0.0}, orientation: {x: 0.0, y: 0.0, z: 0.0, w: 1.0}}, \
covariance: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, \
0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]}}" --once

Note

Change the values of the position and orientation fields to the desired initial pose of the robot.

You can use the same /initialpose through the 2D Pose Estimate RViz Tool.

Alternatively, you can set an initial pose for the amcl using the /set_initial_pose service with the following command:

ros2 service call /set_initial_pose nav2_msgs/srv/SetInitialPose \
"pose: {header: {stamp: {sec: 0, nanosec: 0}, frame_id: 'map'}, \
pose: {pose: {position: {x: 0.0, y: 0.0, z: 0.0}, orientation: {x: 0.0, y: 0.0, z: 0.0, w: 1.0}}, \
covariance: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, \
0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]}}"

Next steps#

Once the robot is properly localized in a map, all is ready to send goals to the robot and making it autonomously navigate in the environment using the 🥅 Goal Navigation features. To learn how to send an Autonomous Navigation goal to the robot, you can refer to the How to Autonomously Navigate while avoiding obstacles.