Frontier Exploration Guide

Overview

Frontier exploration is a key capability for mobile robots navigating unknown environments. This guide explains the concept, scenarios, implementation strategies, and practical usage in ROS Noetic.

Author

Name: ZHENXU Chen
Date: Dec 10, 2024
ROS Version: Noetic

Concept

Key Strategies

Frontier exploration uses map updates to identify unexplored areas and direct robots safely. Integration with the ROS navigation stack ensures obstacle avoidance and adaptability to environmental changes.

Frontier Detection Algorithm

The detection process involves:

Robot Pose Retrieval: Using getRobotPose to determine the current position.
Frontier Search: searchFrom performs Breadth-First Search (BFS) to locate boundaries between known and unknown areas, sorted by cost.
Empty Frontier Handling: If no frontiers are found, exploration stops.
Visualization Markers: Publish visual markers for debugging or monitoring.
Target Selection: Selects a valid frontier centroid as the next goal.

Explore Lite

Explore Lite is a ROS package for lightweight autonomous exploration. It provides tools for efficient navigation and mapping in unknown environments.

Usage

Path Planning Example

<launch>
  <arg name="model" default="$(env TURTLEBOT3_MODEL)" doc="Model type [burger, waffle, waffle_pi]"/>
  <arg name="cmd_vel_topic" default="/cmd_vel" />
  <arg name="odom_topic" default="odom" />
  <arg name="move_forward_only" default="false"/>

  <node pkg="move_base" type="move_base" name="move_base" output="screen">
    <param name="base_local_planner" value="dwa_local_planner/DWAPlannerROS" />
    <param name="base_global_planner" value="global_planner/GlobalPlanner"/>
    <rosparam file="$(find turtlebot3_navigation)/param/costmap_common_params_$(arg model).yaml" command="load" ns="global_costmap" />
    <rosparam file="$(find turtlebot3_navigation)/param/costmap_common_params_$(arg model).yaml" command="load" ns="local_costmap" />
    <rosparam file="$(find turtlebot3_navigation)/param/local_costmap_params.yaml" command="load" />
    <rosparam file="$(find turtlebot3_navigation)/param/global_costmap_params.yaml" command="load" />
    <rosparam file="$(find turtlebot3_navigation)/param/move_base_params.yaml" command="load" />
    <rosparam file="$(find turtlebot3_navigation)/param/dwa_local_planner_params_$(arg model).yaml" command="load" />
    <remap from="cmd_vel" to="$(arg cmd_vel_topic)"/>
    <remap from="odom" to="$(arg odom_topic)"/>
    <param name="DWAPlannerROS/min_vel_x" value="0.0" if="$(arg move_forward_only)" />
  </node>
</launch>

SLAM Example

<launch>
  <arg name="model" default="$(env TURTLEBOT3_MODEL)" doc="Model type [burger, waffle, waffle_pi]"/>
  <arg name="slam_methods" default="gmapping" doc="SLAM type [gmapping, cartographer, hector, karto, frontier_exploration]"/>
  <arg name="configuration_basename" default="turtlebot3_lds_2d.lua"/>
  <arg name="open_rviz" default="true"/>

  <include file="$(find turtlebot3_bringup)/launch/turtlebot3_remote.launch">
    <arg name="model" value="$(arg model)" />
  </include>

  <include file="$(find turtlebot3_slam)/launch/turtlebot3_$(arg slam_methods).launch">
    <arg name="model" value="$(arg model)"/>
    <arg name="configuration_basename" value="$(arg configuration_basename)"/>
  </include>

  <group if="$(arg open_rviz)">
    <node pkg="rviz" type="rviz" name="rviz" required="true"
          args="-d $(find turtlebot3_slam)/rviz/turtlebot3_$(arg slam_methods).rviz"/>
  </group>
</launch>

Exploration Configuration Example

<launch>
  <node pkg="explore_lite" type="explore" respawn="false" name="explore" output="screen">
    <param name="robot_base_frame" value="base_footprint" />

    <param name="costmap_topic" value="move_base/global_costmap/costmap" />
    <param name="costmap_updates_topic" value="move_base/global_costmap/costmap_updates" />
    <param name="visualize" value="true" />
    <param name="planner_frequency" value="0.20" />

    <param name="progress_timeout" value="30.0" />
    <param name="potential_scale" value="3.0" />
    <param name="orientation_scale" value="0.0" />
    <param name="gain_scale" value="1.0" />

    <param name="transform_tolerance" value="0.3" />
    <param name="min_frontier_size" value="0.1" />
  </node>
</launch>

Communication Between Nodes

Robot Image

Troubleshooting

Path Planning Failures

Use recovery behaviors and parameters to handle path planning failures during robot navigation. The recovery behaviors include clearing costmaps at varying levels of conservativeness and performing rotation-based costmap clearing. Key parameters such as planner_patience, controller_patience, conservative_reset_dist, planner_frequency, and oscillation_timeout ensure robust and reliable robot navigation by preventing getting stuck.

recovery_behaviors:
  - name: 'super_conservative_reset1'
    type: 'clear_costmap_recovery/ClearCostmapRecovery'
  - name: 'conservative_reset1'
    type: 'clear_costmap_recovery/ClearCostmapRecovery'
  - name: 'aggressive_reset1'
    type: 'clear_costmap_recovery/ClearCostmapRecovery'
  - name: 'clearing_rotation1'
    type: 'rotate_recovery/RotateRecovery'

planner_patience: 2.0 
controller_patience: 5.0
conservative_reset_dist: 3.0
planner_frequency: 5.0 
oscillation_timeout: 5.0

Parameter Tuning

This configuration focuses on parameter tuning for trajectory scoring and simulation settings to optimize robot navigation. Trajectory scoring parameters, such as path_distance_bias, goal_distance_bias, and occdist_scale, influence the path planning preferences. Simulation settings include sim_time, ensuring accurate local navigation planning.

Trajectory Scoring

path_distance_bias: 6000.0 
goal_distance_bias: 0.1 
occdist_scale: 0.0
forward_point_distance: 0.325
stop_time_buffer: 0.3
scaling_speed: 0.25
max_scaling_factor: 0.2

Simulation Settings

sim_time: 4
vx_samples: 20
vy_samples: 0
vth_samples: 40
controller_frequency: 10.0