initial IMU output

[asherikov]

Apparently IMU (may be other sensors as well) outputs data before simulation is properly initialized, e.g., I see this in linear accelerations:

   z: -1.7763568394002505e-15
   z: -1.7763568394002505e-15
   z: -1.7763568394002505e-15
   z: -1.7763568394002505e-15
   z: 808.1787561478695
   z: 9.7989163952117
   z: 9.798916361883988
   z: 9.79891636183127
   z: 9.798916361782853

I've played with always_on flag, but it doesnt seem to have any effect.
Such behaviour may have negative impact on some applications, e.g., state estimators, so it would be nice to delay publication until simulation is fully initialized.

[ShahazadAbdulla]

Hi @asherikov this looks like a classic initialization artifact where the sensor publishes before the physics engine has settled (hence the near-zero/garbage values in the first few ticks).

I'm setting up a test case to reproduce this. If confirmed, I think we can solve it by adding a valid_start_time check or verifying the initialization flag in the Update() loop to gate the first few frames. I'll update here once I've reproduced it.

[ShahazadAbdulla]

Hi @asherikov and @azeey , I attempted to reproduce this on Rolling using a static box with update_rate: 1000.

Even capturing the very first simulation step (nsec: 1000000) using real_time_factor: 0.01, I see valid gravity immediately:

header { stamp { nsec: 1000000 } }
linear_acceleration { z: 9.8 }

It seems the initialization race condition might be resolved in the current Rolling binaries or is specific to certain hardware/physics configurations.

[azeey]

Thanks for testing this @ShahazadAbdulla. @asherikov can you let us know what version of Gazebo you are using and provide a small reproducible example?

[asherikov]

This happens in a PX4 SITL setup, drones are spawned dynamically

gz topic -e --topic <drone_path>/link/base_link/sensor/imu_sensor/imu | grep 'linear_acceleration ' -A 3 | grep 'z:'

> gz sim --version
Gazebo Sim, version 8.10.0

I'll try to find time to reproduce it in a clean minimal setup.

[asherikov]

Ubuntu 24 / ROS Jazzy

# install gazebo
sudo apt install ros-jazzy-ros-gz
source /opt/ros/jazzy/setup.bash
# clone PX4 models
git clone --depth 1 https://github.com/PX4/PX4-gazebo-models.git
# launch sim
env GZ_SIM_SERVER_CONFIG_PATH=`pwd`/PX4-gazebo-models/server.config GZ_SIM_RESOURCE_PATH=`pwd`/PX4-gazebo-models/models/ gz sim --verbose 4 -r -s PX4-gazebo-models/worlds/default.sdf
# echo topic in a separate terminal
gz topic -e --topic /world/default/model/drone_1/link/base_link/sensor/imu_sensor/imu | grep 'linear_acceleration ' -A 3 | grep 'z:'
# spawn a drone
gz service -s "/world/default/create" --reqtype gz.msgs.EntityFactory --reptype gz.msgs.Boolean --timeout 5000 --req "name: 'drone_1', allow_renaming: false, sdf: '<sdf version=\"1.6\"><include><uri>file://`pwd`/PX4-gazebo-models/models/x500/model.sdf</uri><pose>0 0 0 0 0 0</pose></include></sdf>'"

output

  z: -0.00898948596555798
  z: -0.0053523418576785558
  z: -0.00062249387030061294
  z: 0.0078281462731318325
  z: 0.0026329317151760779
  z: 0.0027502878888070949
  z: 0.00050589167986171159
  z: 0.0035673641234468321
  z: 0.000838648442012188
  z: -0.01056422553692251
  z: -0.0047373643924107246
  z: -0.010973802875281683
  z: 0.0019638907650514252
  z: 137.453290869822
  z: 9.7967108500409417
  z: 9.7933425967007643
  z: 9.8132079902376432
  z: 9.8064683548656753
  z: 9.802628903048662
  z: 9.8049654007300671
  z: 9.79504569036311
...