Hello,
Just finishing up the URDF for ROS 2 course. I have everything working except for my lidar when I teleop command my robot the lidar is seems to move back and forth slightly on the chassis. It rotates, but also moving back and forward whilst he chassis is moving.
I understand that we use two approaches for joint_states here, lidar is ros2_control stack through gz_ros2_control and wheels/caster Gazebo Sim systems: DiffDrive and JointStatePublisher.
I’m wondering why I get this judder on the lidar. I can’t seem to figure it out - i’m also confused why we use two joint state stacks, why not just ros2_control?
If anyone could guide me a bit as to what I’m going wrong that would be great. Everything online seems to tell me just to use ros2_control to avoid the suggested issue of j/oint_states topicsgetting stale data.
Below is my sim URDF plugins:
<!-- Differential Drive Plugin -->
<gazebo>
<plugin
filename="gz-sim-diff-drive-system"
name="gz::sim::systems::DiffDrive">
<left_joint>wheel_left_joint</left_joint>
<right_joint>wheel_right_joint</right_joint>
<wheel_separation>0.093</wheel_separation>
<wheel_radius>0.0325</wheel_radius>
<topic>cmd_vel</topic>
<publish_odom>true</publish_odom>
<publish_odom_tf>true</publish_odom_tf>
<publish_wheel_tf>true</publish_wheel_tf>
<odom_topic>odom</odom_topic>
<tf_topic>tf</tf_topic>
<frame_id>odom</frame_id>
<child_frame_id>base_link</child_frame_id>
<update_rate>50</update_rate>
</plugin>
</gazebo>
<!-- ros2_control config -->
<ros2_control name="GazeboSimSystem" type="system">
<hardware>
<!-- Gazebo Sim integration plugin -->
<plugin>gz_ros2_control/GazeboSimSystem</plugin>
</hardware>
<joint name="lidar_joint">
<command_interface name="velocity">
<param name="min">0.0</param>
<param name="max">2.0</param>
</command_interface>
<state_interface name="position"/>
<state_interface name="velocity"/>
</joint>
</ros2_control>
<!-- Gazebo Sim system plugin that boots the controller manager and parses the ros2_control tags -->
<gazebo>
<plugin filename="gz_ros2_control-system"
name="gz_ros2_control::GazeboSimROS2ControlPlugin">
<!-- Controller configuration YAML file -->
<parameters>$(find ros2_urdf_project)/config/controllers.yaml</parameters>
</plugin>
</gazebo>
My spawn launch:
#!/usr/bin/env python3
from ament_index_python.packages import get_package_share_directory
from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument
from launch.substitutions import LaunchConfiguration
from launch_ros.actions import Node, SetParameter
from launch.conditions import IfCondition
def generate_launch_description():
declare_model_name = DeclareLaunchArgument(
'model_name', default_value='my_robot',
description='Name to give the spawned robot model')
declare_x = DeclareLaunchArgument(
'x', default_value='0.0',
description='Spawn X position')
declare_y = DeclareLaunchArgument(
'y', default_value='0.0',
description='Spawn Y position')
declare_z = DeclareLaunchArgument(
'z', default_value='0.2',
description='Spawn Z position')
declare_yaw = DeclareLaunchArgument(
'yaw', default_value='0.0',
description='Spawn Yaw orientation')
gz_spawn_entity = Node(
package='ros_gz_sim',
executable='create',
name='spawn_entity',
output='screen',
arguments=[
'-name', LaunchConfiguration('model_name'),
'-allow_renaming', 'true',
'-topic', 'robot_description', # reads from /robot_description
'-x', LaunchConfiguration('x'),
'-y', LaunchConfiguration('y'),
'-z', LaunchConfiguration('z'),
'-Y', LaunchConfiguration('yaw'),
],
)
# Spawner for the Joint State Broadcaster (required for TF)
joint_state_broadcaster_spawner = Node(
package="controller_manager",
executable="spawner",
arguments=["joint_state_broadcaster"],
parameters=[{'use_sim_time': True}]
)
# Spawner for your specific LIDAR controller
# Note: The argument must match the name used in your controllers.yaml
lidar_controller_spawner = Node(
package="controller_manager",
executable="spawner",
arguments=["velocity_controller"],
parameters=[{'use_sim_time': True}],
)
return LaunchDescription([
SetParameter(name='use_sim_time', value=True),
declare_model_name,
declare_x,
declare_y,
declare_z,
declare_yaw,
gz_spawn_entity,
joint_state_broadcaster_spawner,
lidar_controller_spawner
])
My controller.yaml:
controller_manager:
ros__parameters:
update_rate: 50 # Hz
velocity_controller:
type: velocity_controllers/JointGroupVelocityController
joint_state_broadcaster:
type: joint_state_broadcaster/JointStateBroadcaster
velocity_controller:
ros__parameters:
joints:
- lidar_joint
interface_name: velocity
command_interfaces:
- velocity
state_interfaces:
- position
- velocity
joint_state_broadcaster:
ros__parameters:
joints:
- lidar_joint
I’m wondering if a bit of ghosting/juddering is just expected? But mine here feels a bit more than just a slow simulation load.
Thanks 
