Gazebo Simulation

2. Validating Simulation Physics

When Gazebo is launched, the ros_gz_bridge connects Gazebo transport topics to ROS 2 topics. If sensor data or robot movement fails, introspect Gazebo directly.

# List all active Gazebo topics
gz topic -l

# Echo a Gazebo topic (1 message)
gz topic -e -t "/model/ackermann/odometry_with_covariance" -n 1

# Check IMU sensor data (z should be ≈ -9.81 or +9.81 depending on convention)
gz topic -e -t "/world/warehouse/model/ackermann/link/base_link/sensor/imu_sensor/imu" -n 1

# Check motor command arriving at Gazebo
gz topic -e -t /model/ackermann/command/motor_speed -n 3

# Check steering servo
gz topic -e -t /model/ackermann/servo_0 -n 3

# Check rover position (confirm movement)
gz topic -e -t /model/ackermann/odometry_with_covariance -n 1 | grep -A3 'position {'

# List Gazebo resources
gz resource list

3. ROS ↔ Gazebo Bridge Topics

The ros_gz_bridge exposes these Gazebo sensors as ROS 2 topics:

4. Common Issues & Debugging

Robot Not Moving:

• Check ros2 topic echo /ackermann/cmd_vel shows commands.
• Check ros2 node list | grep bridge — ros_gz_bridge must be running.
• Check Gazebo side: gz topic -e -t /model/ackermann/cmd_vel.

No Lidar/Camera Data:

• Same bridging issue. Check gz topic -e -t /lidar/points.

Gazebo Crashes:

• Check startup logs heavily. Look for plugin loading errors ([Err] [Plugin...]) or missing mesh files.
• Check for duplicate /clock publishers: ros2 topic info /clock.

RViz Resets / TF Time Jump:

• Symptom: Detected jump back in time. Resetting RViz.
• Cause: Multiple conflicting simulation stacks (multiple /clock publishers).
• Fix:

  pkill -f "ros2 launch robot_bringup" || true
  pkill -f "rviz2" || true
  pkill -f "gz sim" || true
  

  Then start a single clean bringup instance.

5. PX4 SITL with Gazebo

When enable_px4_sitl:=true, PX4 SITL co-simulates inside Gazebo — the gz_bridge module links PX4 directly to Gazebo without a ROS bridge.

Startup order is critical:

1. Start Gazebo: ros2 launch robot_bringup robot_bringup.launch.py enable_px4_sitl:=true
2. Start MicroXRCEAgent: ./scripts/start_microxrce_agent.sh
3. Start PX4 SITL: ./scripts/start_px4_sitl.sh (or --vio for VIO-only EKF2)
4. Start PX4 bridge: ros2 launch px4_bringup px4_bringup.launch.py mode_type:=speed_steering

Or from host with a single command:

./scripts/start_ros2_nodes.sh --px4
./scripts/start_ros2_nodes.sh --px4 --rtabmap --nav2

PX4 Airframe: 51000 (gz_rover_ackermann) World: warehouse.sdf (contains <spherical_coordinates> for GPS reference)

Key PX4 ↔ Gazebo sensor topic paths (for PX4's gz_bridge):

/world/warehouse/model/ackermann/link/base_link/sensor/imu_sensor/imu
/world/warehouse/model/ackermann/link/base_link/sensor/air_pressure_sensor/air_pressure
/world/warehouse/model/ackermann/link/base_link/sensor/magnetometer_sensor/magnetometer
/world/warehouse/model/ackermann/link/base_link/sensor/navsat_sensor/navsat

PX4 actuator topic paths (from gz_bridge):

• Motor: /model/ackermann/command/motor_speed
• Steering: /model/ackermann/servo_0

6. T265 Fisheye Simulation Bridges

In simulation, the T265 fisheye cameras and IMU are bridged via gazebo_bringup.launch.py when enable_t265:=true or use_cuvslam_odom:=true:

# Ros 2 topics produced by sim T265 bridges:
/t265/fisheye1/image_raw
/t265/fisheye2/image_raw
/t265/fisheye1/camera_info
/t265/fisheye2/camera_info
/t265/imu

7. Stopping All Simulation Processes

# From host — kills all ROS 2, Gazebo, PX4, and MicroXRCE processes inside the container:
docker compose -f docker/docker-compose.yml exec ackermann_slam bash -c \
  "pkill -9 -f 'ros2|rviz2|gz|ruby|px4|MicroXRCE'"

CRITICAL: Always terminate the simulation after completing any task. Leaving Gazebo or ROS nodes running causes /clock conflicts and stale process issues for future runs.