The Nav2 loopback simulator is a stand-alone simulator to create a "loopback" for non-physical simulation to replace robot hardware, physics simulators (Gazebo, Bullet, Isaac Sim, etc). It computes the robot's odometry based on the command velocity's output request to create a perfect 'frictionless plane'-style simulation for unit testing, system testing, R&D on higher level systems, and testing behaviors without concerning yourself with localization accuracy or system dynamics.

This was created by Steve Macenski of Open Navigation LLC and donated to Nav2 by the support of our project sponsors. If you rely on Nav2, please consider supporting the project!

**⚠️ If you need professional services related to Nav2, please contact Open Navigation at info@.nosp@m.open.nosp@m.nav.o.nosp@m.rg.**

It is drop-in replaceable with AMR simulators and global localization by providing:

Map -> Odom transform
Odom -> Base Link transform, nav_msgs/Odometry odometry
Accepts the standard /initialpose topic for transporting the robot to another location

Note: This does not provide sensor data, so it is required that the global (and probably local) costmap contain the StaticLayer to avoid obstacles.

It is convenient to be able to test systems by being able to:

Arbitrarily transport the robot to any location and accurately navigate without waiting for a particle filter to converge for testing behaviors and reproducing higher-level issues
Write unit or system tests on areas that are not dependent on low-level controller or localization performance without needing to spin up a compute-heavy process like Gazebo or Isaac Sim to provide odometry and sensor data, such as global planning, autonomy behavior trees, etc
Perform R&D on various sensitive systems easily without concerning yourself with the errors accumulated with localization performance or imperfect dynamic models to get a proof of concept started
Simulate N robots simultaneously with a lower compute footprint
When otherwise highly compute constrained and need to simulate a robotic system

How to Use

ros2 run nav2_loopback_sim loopback_simulator  # As a node, if in simulation
ros2 launch nav2_loopback_sim loopback_simulation.launch.py  # As a launch file
ros2 launch nav2_bringup tb3_loopback_simulation.launch.py  # Nav2 integrated navigation demo using it
ros2 launch nav2_bringup tb4_loopback_simulation.launch.py  # Nav2 integrated navigation demo using it

API

Parameters

update_duration: The duration between updates (default 0.01 – 100hz)
base_frame_id: The base frame to use (default base_link)
odom_frame_id: The odom frame to use (default odom)
map_frame_id: The map frame to use (default map)
scan_frame_id: The can frame to use to publish a scan to keep the collision monitor fed and happy (default base_scan for TB3, rplidar_link for TB4)
enable_stamped_cmd_vel: Whether cmd_vel is stamped or unstamped (i.e. Twist or TwistStamped). Default false for Twist.
scan_publish_dur: : The duration between publishing scan (default 0.1s – 10hz)
publish_map_odom_tf: Whether or not to publish tf from map_frame_id to odom_frame_id (default true)
publish_clock: Whether or not to publish simulated clock to /clock (default true)
scan_range_min: Minimum measurable distance from the scan in meters. Values below this are considered invalid (default: 0.05)
scan_range_max: Maximum measurable distance from the scan in meters. Values beyond this are out of range (default: 30.0)
scan_angle_min: Starting angle of the scan in radians (leftmost angle) (default: -π / -3.1415)
scan_angle_max: Ending angle of the scan in radians (rightmost angle) (default: π / 3.1415)
scan_angle_increment: Angular resolution of the scan in radians (angle between consecutive measurements) (default: 0.02617)
scan_use_inf: Whether to use inf for out-of-range values. If false, uses scan_range_max - 0.1 instead (default: True)

Topics

This node subscribes to:

initialpose: To set the initial robot pose or relocalization request analog to other localization systems
cmd_vel: Nav2's output twist to get the commanded velocity

This node publishes:

clock: To publish a simulation clock for all other nodes with use_sim_time=True
odom: To publish odometry from twist
tf: To publish map->odom and odom->base_link transforms
scan: To publish a range laser scan sensor based on the static map