Wait Behavior

name: wait-behavior description: 'Configure Nav2 wait recovery behavior. Use when adding timed waits for dynamic obstacle clearing or sensor stabilization.'

Wait Recovery Behavior

Overview

The Wait behavior pauses the robot for a specified duration, publishing zero velocity. It is the simplest recovery behavior and the appropriate first response when the obstruction is likely a dynamic obstacle (person, pet, door) that will clear on its own. Plugin: nav2_behaviors::Wait.

BT Node

<Wait wait_duration="5" server_name="behavior_server" server_timeout="10"/>

wait_duration is in seconds (integer). The behavior always returns SUCCESS after the duration elapses — it cannot fail.

How It Works

1. The behavior server receives the wait duration from the BT action node.
2. Each cycle, it publishes geometry_msgs/msg/Twist with all fields zero, ensuring the robot is stationary.
3. It monitors elapsed time since the behavior started.
4. After wait_duration seconds, it returns SUCCESS.

There are no collision checks, costmap interactions, or TF requirements beyond basic operation. This is intentionally minimal.

YAML Configuration

Wait requires no behavior-specific parameters. It only needs to be registered in the behavior plugin list:

behavior_server:
  ros__parameters:
    cycle_frequency: 10.0
    behavior_plugins: ["spin", "backup", "wait"]
    spin:
      plugin: "nav2_behaviors::Spin"
    backup:
      plugin: "nav2_behaviors::BackUp"
    wait:
      plugin: "nav2_behaviors::Wait"

The wait_duration is set per-invocation in the BT node, not in YAML.

Where Wait Fits in Recovery BTs

Wait is most effective as the first recovery action in a RoundRobin sequence. The logic: try the cheapest, least-disruptive recovery first. If the obstacle clears during the wait, navigation resumes without any robot motion.

Recommended Recovery Ordering

<RecoveryNode number_of_retries="6" name="NavigateRecovery">
  <PipelineSequence name="NavigateWithReplanning">
    <RateController hz="1.0">
      <ComputePathToPose goal="{goal}" path="{path}"/>
    </RateController>
    <FollowPath path="{path}"/>
  </PipelineSequence>
  <RoundRobin name="RecoveryActions">
    <!-- 1st attempt: just wait, obstacle may clear -->
    <Wait wait_duration="5"/>
    <!-- 2nd attempt: small spin to clear costmap artifacts -->
    <Spin spin_dist="1.0"/>
    <!-- 3rd attempt: wait longer -->
    <Wait wait_duration="8"/>
    <!-- 4th attempt: back up and spin -->
    <Sequence>
      <BackUp backup_dist="0.15" backup_speed="0.05"/>
      <Spin spin_dist="1.57"/>
    </Sequence>
    <!-- 5th attempt: clear costmaps entirely -->
    <Sequence>
      <ClearEntireCostmap service_name="local_costmap/clear_entirely_local_costmap"/>
      <ClearEntireCostmap service_name="global_costmap/clear_entirely_global_costmap"/>
    </Sequence>
    <!-- 6th attempt: aggressive backup -->
    <Sequence>
      <BackUp backup_dist="0.30" backup_speed="0.10"/>
      <Spin spin_dist="3.14"/>
    </Sequence>
  </RoundRobin>
</RecoveryNode>

The RoundRobin node cycles through children on each failure. So the first navigation failure triggers Wait(5s), the second triggers Spin, the third triggers Wait(8s), and so on. After all children are exhausted, number_of_retries resets the cycle.

When to Use Wait vs. Other Recoveries

Combining Wait with Decorators

You can use BT decorator nodes to add conditions to Wait:

<!-- Wait up to 10 seconds, but recheck the path every 2 seconds -->
<RetryUntilSuccessful num_attempts="5">
  <Sequence>
    <Wait wait_duration="2"/>
    <ComputePathToPose goal="{goal}" path="{path}"/>
    <FollowPath path="{path}"/>
  </Sequence>
</RetryUntilSuccessful>

This pattern retries navigation after short waits rather than committing to a single long pause.

House Patrol Considerations

For a home patrol robot, Wait is particularly important because:

• Most obstructions are household members or pets that will move within seconds.
• Aggressive recoveries (spinning, backing up) in a home setting risk bumping furniture or alarming occupants.
• A patient robot that waits quietly is preferable to one that thrashes around.

Consider using longer wait_duration values (8–15 seconds) for the first recovery attempt in domestic environments.

Debugging

Wait is the simplest behavior to debug. If it is not executing:

ros2 action list           # verify /wait action exists
ros2 topic echo /cmd_vel   # should show all-zero Twist during wait

If the BT skips Wait, the issue is in BT structure, not the behavior itself.