Publish to topic cmd_val

[MoonWalkerAZ]

Is there a good example how to control two motors for differential drive robot ? I would like to publish Twist message to cmd_val topic to control velocity of two motors.

[borongyuan]

Hi, there's a good example provied by @kallaspriit. Please refer to issue #1.

[kallaspriit]

Yeah my robot uses this library for physical and simulated differential drive robot, works nicely :)

https://github.com/kallaspriit/rosbot
https://www.youtube.com/watch?v=t-1GAfGHRLs

[MoonWalkerAZ]

@kallaspriit I have checked your library, but I don't understand which part do I need to create a node that creates cmd_vel topic and sends the correct data to odrive. I'm new at this. Can you help me ? I have seen that you use joystick to send a Twist message, but I dont know where is that twist message being read and then converted to values that odrive driver understands.

[kallaspriit]

For manual testing /cmd_vel is published by teleop_twist_joy package (which uses joy package to get joystick input).

See this for reference
https://github.com/kallaspriit/rosbot/blob/main/ros/src/teleop_twist_joy/launch/teleop.launch.py

To run in, you can run . launch_teleop.bash in ros or

. install/local_setup.bash
ros2 launch teleop_twist_joy teleop.launch.py

Which does the same thing.

[MoonWalkerAZ]

Ok, but who is subscribing to /cmd_vel topic ? And how is the Twist message being converted to input so that odrive can understand it ? Thats the part that I don't understand.

[borongyuan]

Before using this package, you need to be familiar with ros2_control framework.
http://control.ros.org/ros2_controllers/diff_drive_controller/doc/userdoc.html

[MoonWalkerAZ]

Ok. Some example of how to use an implement diff_drive_controller with odrive package would be nice. @kallaspriit could you help me with this ?

[kallaspriit]

Well the rosbot repo IS an example of how to implement diff drive controller with odrive, not sure what else you need. Familiarize yourself with ROS2, ros2_control, URDF's and read the repo. It's all configuration, no actual code is needed but it takes time to understand what does what, no way around it.

Main configuration is in robot URDF (Universal Robot Descriptor File), take a look at
https://github.com/kallaspriit/rosbot/blob/main/ros/src/rosbot_description/urdf/rosbot.urdf.xacro
https://github.com/kallaspriit/rosbot/blob/main/ros/src/rosbot_description/ros2_control/rosbot_system.ros2_control.xacro
https://github.com/kallaspriit/rosbot/blob/main/ros/src/rosbot_description/urdf/rosbot_links.urdf.xacro
https://github.com/kallaspriit/rosbot/blob/main/ros/src/rosbot_description/config/diff_drive_controller.yaml

[MoonWalkerAZ]

Ok, thank you @kallaspriit and @borongyuan four your help.