07. AMCL Launch File: AMCL Node

AMCL Node

The next node to be set up is the amcl node. As it was introduced in the previous concepts, it takes odometry and laser scan data to perform the AMCL localization.

Add amcl Node

First, add the amcl node in your amcl.launch file: 

<launch>

  <!-- Map Server -->
  ...

  <!-- AMCL Node -->
  <node name="amcl" pkg="amcl" type="amcl" output="screen">
  </node>

</launch>

Ok, we created the amcl node. However it needs more information to localize the robot! For example, what are the sensor readings from the LiDAR?

Remap scan topic

By default, amcl package will look for the scan topic for LiDAR data. In the simulation, the Hokuyo LiDAR sensor actually publishes on the <YOUR PACKAGE NAME>/laser/scan topic. We will use the remap tag to remap the topic name scan to the actual topic name so that the amcl package could use it!

Add this remap line to your amcl node in the amcl.launch file: 

...
<!-- AMCL Node -->
<node name="amcl" ...>
  <remap from="scan" to="<YOUR PACKAGE NAME>/laser/scan"/>
</node>

For more information on remap , check out the ROS Wiki here: http://wiki.ros.org/roslaunch/XML/remap

Add AMCL Parameters

The AMCL node also requires a set of parameters in order to connect the world ( map frame) with the robot ( odom frame).

Add the following parameter tags to the amcl node in the amcl.launch file: 

...
<!-- AMCL Node -->
<node name="amcl" ...>
  <remap from="scan" to="<YOUR PACKAGE NAME>/laser/scan"/>
  <param name="odom_frame_id" value="odom"/>
  <param name="odom_model_type" value="diff-corrected"/>
  <param name="base_frame_id" value="robot_footprint"/>
  <param name="global_frame_id" value="map"/>
</node>

From the ROS Wiki ( http://wiki.ros.org/amcl) , we could find the purpose of the parameters added above:

  • odom_frame_id (string, default: "odom"): Which frame to use for odometry
  • odom_model_type (string, default: "diff"): Which model to use, either "diff", "omni", "diff-corrected" or "omni-corrected"
  • base_frame_id (string, default: "base_link"): Which frame to use for the robot base
  • global_frame_id (string, default: "map"): The name of the coordinate frame published by the localization system

Remember, AMCL package 'links' the robot ( odom frame) with the world ( map frame). These parameters are required for amcl package to localize the robot in the world.

Optional: Set Initial Position

You could use the RViz 2D Pose Estimate function to give AMCL a pose estimate as position, but you could also have it defined in the launch file.

Add the following parameters to the AMCL node, the values should correspond to your world.launch file: 

<param name="initial_pose_x" value="<YOUR X VALUE>"/>
<param name="initial_pose_y" value="<YOUR Y VALUE>"/>

The Launch File

Now, your amcl.launch file should look like this: 

<launch>

  <!-- Map Server -->
    <arg name="map_file" ... />
    <node name="map_server" ... />

  <!-- AMCL Node -->
  <node name="amcl" pkg="amcl" type="amcl" output="screen">
    <remap from="scan" to="<YOUR PACKAGE NAME>/laser/scan"/>
    <param name="odom_frame_id" value="odom"/>
    <param name="odom_model_type" value="diff-corrected"/>
    <param name="base_frame_id" value="robot_footprint"/>
    <param name="global_frame_id" value="map"/>

    <!-- If you choose to define initial pose here -->
    <param name="initial_pose_x" value="0"/>
    <param name="initial_pose_y" value="0"/>
  </node>

</launch>

Now, the robot has a map and the amcl package to localize itself. But how could it navigate to other positions to collect more information about its surroundings? ROS Navigation Stack is the answer.

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