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Below is the launch file, which I call hybrid_localization because it allows you to have pose in two different map referenced frames fused into a single ekf. I know that is bad practice to have to absolute position's fused together because it causes jitter.

This however is predicated on the fact that where you have GPS you will not be able to use AMCL and where you have AMCL you will not have GPS. In my experience on a university campus this holds true, and I have found this setup very useful.

I would like to pretend that the launch and config files "explain themselves", but I understand they probably do not, and therefore will be happy to field questions, or provide more information to anyone who is interested.

hybrid_localization.launch

<launch>
   <arg name="amcl_map_frame" default="map_amcl"/>
       <arg name="map_file" default="/home/administrator/philbart_config/maps/pma_23_09_2020_8_15.yaml"/>
   <arg name="ekf_localization_map_file" default="/home/administrator/philbart_config/ekf_localization_map.yaml"/>
   <arg name="navsat_transform_file" default="/home/administrator/philbart_config/navsat_transform.yaml"/>

   <!---Static Transform world-to-map_amcl -->
       <node pkg="tf2_ros" type="static_transform_publisher" name="world_to_map_amcl_broadcaster" args="-13.62 19.02 0 -0.121 0 0 world $(arg amcl_map_frame)" />

      <!---Static Transform world-to-map -->
      <node pkg="tf2_ros" type="static_transform_publisher" name="world_to_map_broadcaster" args="0 0 0 0 0 0 world map" />

     <!---AMCL Map -->
     <node name="map_server_amcl" pkg="map_server" type="map_server" args="$(arg map_file)">
           <param name="frame_id" value="$(arg amcl_map_frame)" />
           <remap from="map" to="$(arg amcl_map_frame)"/>
          <remap from="map_metadata" to="$(arg amcl_map_frame)_metadata"/>
     </node>

     <!---AMCL Node -->
    <include file="$(find husky_navigation)/launch/amcl.launch">
          <arg name="global_frame_id" default="$(arg amcl_map_frame)"/>
          <arg name="map_topic" default="$(arg amcl_map_frame)"/>
          <arg name="tf_broadcast" default="false"/>
    </include>

    <!--- Global EKF -->
   <node pkg="robot_localization" type="ekf_localization_node" name="ekf_localization_map" output="screen">
        <rosparam command="load" file="$(arg ekf_localization_map_file)" />
       <remap from="/odometry/filtered" to="/odometry/filtered_map"/>
    </node>

    <!--- Navsat Transform Node-->
    <node pkg="robot_localization" type="navsat_transform_node" name="navsat_transform" output="screen">
          <rosparam command="load" file="$(arg navsat_transform_file)" />
          <remap from="odometry/filtered" to="odometry/filtered_map"/>
          <remap from="gps/fix" to="/piksi_reference/piksi/navsatfix_best_fix"/>
    </node>

</launch>

Where ekf_localization_map.yaml is:

 map_frame: map
 odom_frame: odom
 base_link_frame: base_link
 world_frame: map

 imu0: /gx5/imu/data
 imu0_config: [false, false, false,
          true,  true,  false,
          false, false, false,
          true,  true,  true,
          true,  true,  true]
imu0_nodelay: true
imu0_differential: false
imu0_relative: false
imu0_queue_size: 10
imu0_remove_gravitational_acceleration: true

odom0: /husky_velocity_controller/odom
odom0_config: [false, false, false,
           false, false, false,
           true,  true,  true,
           false, false, true,
           false, false, false]
odom0_queue_size: 10
odom0_nodelay: true
odom0_differential: false
odom0_relative: false

odom1: odometry/gps
odom1_config: [true,  true,  false,
           false, false, false,
           false, false, false,
           false, false, false,
           false, false, false]
odom1_queue_size: 10
odom1_nodelay: true
odom1_differential: false
odom1_relative: false
odom1_rejection_threshold: 2.0

pose0: /amcl_pose
pose0_config: [true, true, false,
          false, false, true,
          false, false, false,
          false, false, false,
          false, false, false]
pose0_queue_size: 10
pose0_nodelay: true
pose0_differential: false
pose0_relative: false
pose0_rejection_threshold: 2.0

use_control: false