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Unknown environment exploration

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Introduction​

Environment exploration task is to determine robot desired positions in such a sequence which gives as much information regarding environment as possible. One of the approaches for this task is to frontiers of occupancy grid. As a frontier we define line between free space ant territory marked as unknown. Moving towards frontiers, unknown area can be explored and marked as free or occupied and frontiers are moved into an unknown territory. Process is repeated until all frontiers are investigated, this means free area must be surrounded by occupied cells.

Environment exploration in ROS​

In ROS it is possible to explore environment with use of occupancy grid frontiers. One of the nodes, that perform this task is explore node from explore_lite package. This node uses occupancy grid e.g. created by slam_gmapping and publishes goal position to /move_base/goal topic subscribed by path planner e.g. move_base node.

Requirements regarding robot​

Before continuing with explore_lite node certain requirements must be met, robot should:

  • subscribe /move_base/goal topic with message type geometry_msgs/PoseStamped in which robot desired positions are included.

  • Publish map to /map topic with message type nav_msgs/OccupancyGrid

Above configuration is met by the robot created in previous manual.

Configuration of explore node​

Before starting experimenting with explore_lite you need to have working move_base for navigation. You should be able to navigate with move_base manually through rviz.

You should be also able to to navigate with move_base though unknown space in the map. If you set the goal to unknown place in the map, planning and navigating should work. With most planners this should work by default. Navigation through unknown space is required for explore_lite.

If you want to use costmap provided by move_base you need to enable unknown space tracking by setting track_unknown_space: true.

All required configuration should be set properly doing previous tutorials, so you can start experimenting with explore_lite. Provided explore.launch should work out-of-the box in most cases, but as always you might need to adjust topic names and frame names according to your setup.

Based on occupancy grid, explore node determines frontiers between free and unknown area and using them determines robot destinations.

Parameters​

  • robot_base_frame - The name of the base frame of the robot. This is used for determining robot position on map.

  • costmap_topic - Specifies topic of source nav_msgs/OccupancyGrid.

  • costmap_updates_topic - Specifies topic of source map_msgs/OccupancyGridUpdate. Not necessary if source of map is always publishing full updates, i.e. does not provide this topic.

  • visualize - Specifies whether or not publish visualized frontiers.

  • planner_frequency - Rate in Hz at which new frontiers will computed and goal reconsidered.

  • progress_timeout - Time in seconds. When robot do not make any progress for progress_timeout, current goal will be abandoned.

  • potential_scale - Used for weighting frontiers. This multiplicative parameter affects frontier potential component of the frontier weight (distance to frontier).

  • orientation_scale - Used for weighting frontiers. This multiplicative parameter affects frontier orientation component of the frontier weight.

  • gain_scale - Used for weighting frontiers. This multiplicative parameter affects frontier gain component of the frontier weight (frontier size).

  • transform_tolerance - Transform tolerance to use when transforming robot pose.

  • min_frontier_size - Minimum size of the frontier to consider the frontier as the exploration goal. Value is in meter.

Save configuration as exploration.yaml in tutorial_pkg/config directory.

Your file should look like below:

robot_base_frame: base_link
costmap_topic: map
costmap_updates_topic: map_updates
visualize: true
planner_frequency: 0.33
progress_timeout: 30.0
potential_scale: 3.0
orientation_scale: 0.0
gain_scale: 1.0
transform_tolerance: 0.3
min_frontier_size: 0.75

Launching exploration task​

To test above configuration you will need to run explore_lite node with nodes from path planning configuration.

To remind, you will need to run following nodes:

  • CORE2 bridge node - roslaunch rosbot_ekf all.launch - publishes tf, connect to CORE2 and run extended Kalman filter for odometry.

  • rplidarNode - driver for rpLidar laser scanner

Or instead ot these, Gazebo:

  • roslaunch rosbot_gazebo maze_world.launch

And:

  • static_transform_publisher - tf publisher for transformation of laser scanner relative to robot

  • slam_gmapping - map building node

  • move_base - trajectory planner

  • explore_lite - exploration task

  • rviz - visualization tool

For the explore_lite node you will need to specify path for .yaml configuration file:

    <node pkg="explore_lite" type="explore" respawn="false" name="explore" output="screen">
        <rosparam file="$(find tutorial_pkg)/config/exploration.yaml" command="load" />
    </node>

You can use below launch file, save this as tutorial_8.launch :

<launch>

  <arg name="rosbot_pro" default="false" />
  <arg name="use_gazebo" default="false" />

  <!-- Gazebo -->
  <group if="$(arg use_gazebo)">
    <include file="$(find rosbot_gazebo)/launch/maze_world.launch" />
    <include file="$(find rosbot_description)/launch/rosbot_gazebo.launch"/>
    <param name="use_sim_time" value="true" />
  </group>

  <!-- ROSbot 2.0 -->
  <group unless="$(arg use_gazebo)">
    <include file="$(find rosbot_ekf)/launch/all.launch">
      <arg name="rosbot_pro" value="$(arg rosbot_pro)" />
    </include>

    <include if="$(arg rosbot_pro)" file="$(find rplidar_ros)/launch/rplidar_a3.launch" />
    <include unless="$(arg rosbot_pro)" file="$(find rplidar_ros)/launch/rplidar.launch" />
  </group>

  <node unless="$(arg use_gazebo)" pkg="tf" type="static_transform_publisher" name="laser_broadcaster" args="0 0 0 3.14 0 0 base_link laser 100" />

  <node pkg="teleop_twist_keyboard" type="teleop_twist_keyboard.py" name="teleop_twist_keyboard" output="screen"/>

  <node pkg="rviz" type="rviz" name="rviz"/>

  <node pkg="gmapping" type="slam_gmapping" name="gmapping_node" output="log">
    <param name="base_frame" value="base_link" />
    <param name="odom_frame" value="odom" />
    <param name="delta" value="0.01" />
    <param name="xmin" value="-5" />
    <param name="ymin" value="-5" />
    <param name="xmax" value="5" />
    <param name="ymax" value="5" />
    <param name="maxUrange" value="5" />
    <param name="map_update_interval" value="1" />
    <param name="linearUpdate" value="0.05" />
    <param name="angularUpdate" value="0.05" />
    <param name="temporalUpdate" value="0.1" />
    <param name="particles" value="100" />
  </node>

  <node pkg="move_base" type="move_base" name="move_base" output="screen">
    <param name="controller_frequency" value="10.0"/>
    <rosparam file="$(find tutorial_pkg)/config/costmap_common_params.yaml" command="load" ns="global_costmap" />
    <rosparam file="$(find tutorial_pkg)/config/costmap_common_params.yaml" command="load" ns="local_costmap" />
    <rosparam file="$(find tutorial_pkg)/config/local_costmap_params.yaml" command="load" />
    <rosparam file="$(find tutorial_pkg)/config/global_costmap_params.yaml" command="load" />
    <rosparam file="$(find tutorial_pkg)/config/trajectory_planner.yaml" command="load" />
  </node>

  <node pkg="explore_lite" type="explore" respawn="true" name="explore" output="screen">
    <rosparam file="$(find tutorial_pkg)/config/exploration.yaml" command="load" />
  </node>

</launch>

Launch this with commands:

ROSbot 2.0:

roslaunch tutorial_pkg tutorial_8.launch

ROSbot 2.0 PRO:

roslaunch tutorial_pkg tutorial_8.launch rosbot_pro:=true

Gazebo:

roslaunch tutorial_pkg tutorial_8.launch use_gazebo:=true

Explore​

If everything was set correctly exploration will start immediately after node initialization. Exploration will finish when whole area is discovered.

If you are using gazebo you should see a maze with rosbot:

image

Inspect how it works using rviz:

image

Summary​

After completing this tutorial you should be able to configure explore_lite node to find frontiers on occupancy grid map and set goals for trajectory planner to explore all unknown area.

by Adam Krawczyk, Husarion

Do you need any support with completing this tutorial or have any difficulties with software or hardware? Feel free to describe your thoughts on our community forum: https://community.husarion.com/ or to contact with our support: support@husarion.com