Follow Dynamic Point

This behavior tree implements a navigation behavior from a starting point, attempting to follow a dynamic point over time. This “dynamic point” could be a person, another robot, a virtual carrot, anything. The only requirement is that the pose you’d like to follow is published to the topic outlined in the GoalUpdater BT node.

In this tree, we replan at 1 hz just as we did in Navigate To Pose using the ComputePathToPose node. However, this time when we replan, we update the goal based on the newest information in on the updated goal topic. After we plan a path to this dynamic point, we use the TruncatePath node to remove path points from the end of the path near the dynamic point. This behavior tree node is useful so that the robot always remains at least distance away from the obstacle, even if it stops. It also smooths out any off path behavior involved with trying to path plan towards a probably occupied space in the costmap.

After the new path to the dynamic point is computed and truncated, it is again passed to the controller via the FollowPath node. However, note that it is under a KeepRunningUntilFailure decorator node ensuring the controller continues to execute until a failure mode. This behavior tree will execute infinitely in time until the navigation request is preempted or cancelled.

<root main_tree_to_execute="MainTree">
  <BehaviorTree ID="MainTree">
    <PipelineSequence name="NavigateWithReplanning">
      <ControllerSelector selected_controller="{selected_controller}" default_controller="FollowPath" topic_name="controller_selector"/>
      <PlannerSelector selected_planner="{selected_planner}" default_planner="GridBased" topic_name="planner_selector"/>
      <RateController hz="1.0">
          <GoalUpdater input_goal="{goal}" output_goal="{updated_goal}">
            <ComputePathToPose goal="{updated_goal}" path="{path}" planner_id="{selected_planner}" error_code_id="{compute_path_error_code}"/>
        <TruncatePath distance="1.0" input_path="{path}" output_path="{truncated_path}"/>
        <FollowPath path="{truncated_path}" controller_id="{selected_controller}" error_code_id="{follow_path_error_code}"/>