Backup Behavior

name: backup-behavior description: 'Configure Nav2 backup recovery behavior. Use when tuning backup_dist, backup_speed, or reverse motion for stuck recovery.'

BackUp Recovery Behavior

Overview

The BackUp behavior drives the robot backward by a specified distance at a controlled speed. It is used when the robot is stuck and spinning alone cannot free it — typically when the robot has driven into a tight space and needs to retreat. Plugin: nav2_behaviors::BackUp.

BT Node

<BackUp backup_dist="0.30" backup_speed="0.05" server_name="behavior_server" server_timeout="10"/>

• backup_dist: distance in meters (positive value, despite moving backwards). Default: 0.15.
• backup_speed: linear speed in m/s. Keep slow (0.05–0.15) for safety.

How It Works

1. On receiving the goal, the behavior records the robot's starting pose via TF.
2. Each cycle, it publishes a negative linear velocity (cmd_vel.linear.x = -backup_speed) with zero angular.
3. Before each command, it checks the costmap behind the robot by projecting the footprint along the reverse trajectory using simulate_ahead_time.
4. The behavior monitors accumulated distance by comparing current pose to start pose.
5. Returns SUCCESS when backup_dist is reached, FAILURE if rearward collision is detected.

Duration ≈ backup_dist / backup_speed. At 0.30 m / 0.05 m/s = 6 seconds.

YAML Configuration

behavior_server:
  ros__parameters:
    cycle_frequency: 10.0
    local_costmap_topic: local_costmap/costmap_raw
    global_costmap_topic: global_costmap/costmap_raw
    local_footprint_topic: local_costmap/published_footprint
    global_footprint_topic: global_costmap/published_footprint
    behavior_plugins: ["spin", "backup", "wait"]
    backup:
      plugin: "nav2_behaviors::BackUp"
    simulate_ahead_time: 1.0
    transform_tolerance: 0.1

backup_dist and backup_speed are set in the BT node, not the YAML. The behavior server YAML controls the collision checking and TF parameters.

Why BackUp Fails — Rear Sensor Self-Occlusion

The most common failure mode: the robot's own rear-facing sensors (LIDAR, depth cameras) mark the area immediately behind the robot as occupied in the local costmap. When the behavior checks for rearward clearance, it finds lethal cells and refuses to move.

Root cause: sensor min_range is set too low, or the sensor publishes returns from the robot's own body or mounting hardware.

Fixes:

1. Set appropriate min_range on obstacle layers for rear sensors:

   local_costmap:
     local_costmap:
       ros__parameters:
         obstacle_layer:
           observation_sources: scan rear_scan
           rear_scan:
             topic: /rear_scan
             min_obstacle_height: 0.05
             max_obstacle_height: 0.5
             clearing: true
             marking: true
             min_range: 0.20   # ignore returns closer than 20 cm
   

2. Enable footprint_clearing_enabled on the obstacle layer so the robot's own footprint cells are always cleared:

   obstacle_layer:
     footprint_clearing_enabled: true
   

3. Adjust the robot footprint to precisely match the physical chassis so that self-returns fall inside the cleared footprint zone.

transform_tolerance

The transform_tolerance parameter on behavior_server controls how stale a TF lookup can be before the behavior aborts. If your TF tree has jitter or occasional delays (common with hardware drivers), increase from the default 0.1 to 0.2–0.5.

Symptom of too-tight tolerance: BackUp returns FAILURE immediately with TF lookup errors in the log.

Common Issues

BT Recovery Pattern

A common escalating recovery sequence:

<RoundRobin name="RecoveryActions">
  <Sequence name="WaitThenRetry">
    <Wait wait_duration="3"/>
  </Sequence>
  <Sequence name="SpinThenRetry">
    <Spin spin_dist="1.57"/>
  </Sequence>
  <Sequence name="BackUpThenSpin">
    <BackUp backup_dist="0.20" backup_speed="0.08"/>
    <Spin spin_dist="1.57"/>
  </Sequence>
  <Sequence name="AggressiveRecovery">
    <ClearEntireCostmap name="ClearLocal" service_name="local_costmap/clear_entirely_local_costmap"/>
    <BackUp backup_dist="0.30" backup_speed="0.10"/>
  </Sequence>
</RoundRobin>

Debugging

ros2 topic echo /cmd_vel            # verify negative linear.x during backup
ros2 action send_goal /backup nav2_msgs/action/BackUp \
  "{target: {x: -0.20, y: 0.0, z: 0.0}, speed: 0.05}"

Check local costmap behind the robot in RViz — look for unexpected lethal cells at the rear of the footprint.