Unable to see /odom topic in ros2

Course Support ROS2 In 5 Days Python
1

Hi,

I’ve run in Shell 1

source ~/.bashrc_bridge
ros2 run ros1_bridge dynamic_bridge

In Shell 2, I get below error.

user:~/ros2_ws$ ros2 topic echo /odom nav_msgs/msg/Odometry
Traceback (most recent call last):
  File "/opt/ros/foxy/bin/ros2", line 11, in <module>
    load_entry_point('ros2cli==0.9.7', 'console_scripts', 'ros2')()
  File "/opt/ros/foxy/lib/python3.8/site-packages/ros2cli/cli.py", line 67, in main
    rc = extension.main(parser=parser, args=args)
  File "/opt/ros/foxy/lib/python3.8/site-packages/ros2topic/command/topic.py", line 41, in main
    return extension.main(args=args)
  File "/opt/ros/foxy/lib/python3.8/site-packages/ros2topic/verb/echo.py", line 81, in main
    return main(args)
  File "/opt/ros/foxy/lib/python3.8/site-packages/ros2topic/verb/echo.py", line 97, in main
    message_type = get_message(args.message_type)
  File "/opt/ros/foxy/lib/python3.8/site-packages/rosidl_runtime_py/utilities.py", line 30, in get_message
    raise ValueError("Expected the full name of a message, got '{}'".format(identifier))
ValueError: Expected the full name of a message, got 'nav_msgs/msg/Odometry'

Any idea why ?

2

Hello, firstly, I’m also a ROS newbie.

Secondly, is the topic listed under ros2 topic list?

I had (have) a similar question asking why /cmd_vel is not listed under topic list - but of course it makes sense that if it’s not listed you cannot ‘echo’ it…

3

Hi, do you have the simulation running? Can you see the output of ros2 topic list?

I noticed that you reserved the real robot lab, however we had a cable disconnect over the weekend so the robot wasn’t powered, maybe that is the issue you encountered. I will attend to the robot and let you know when it is ready to reserve again.

Thank you,

4

OK, thanks for the info. I just wanted to solve the exercise, We should be able to receive /odom data without real robots right ?

user:~/ros2_ws$ ros2 topic list
/parameter_events
/rosout
5

Basically I am unable to list /odom topic, despite running ros1 bridge. Do I have to run some node that publishes it ? As per the notes in Exercise 5.2, there is no such step required. Please confirm. Thanks.

6

Hello @msp ,

By default, the dynamic_bridge doesn’t bridge all the topics available, only the ones that have a publisher-subscriber connection. That’s why you won’t be able to see the topic (with a ros2 topic list) until you create a subscriber for it. I’ve been doing some tests here and it should work OK.

Shell 1:

source .bashrc_bridge
ros2 run ros1_bridge dynamic_bridge

Shell 2:

source .bashrc_ros2
ros2 topic echo /odom nav_msgs/msg/Odometry

When you run the echo command in Shell 2, this is the same as creating a subscriber for the topic, so the bridge is now created for the /odom topic. You should see a message like this now in the bridge Shell output:

created 1to2 bridge for topic '/odom' with ROS 1 type 'nav_msgs/Odometry' and ROS 2 type 'nav_msgs/msg/Odometry'

An alternative is to run the dynamic_bridge with the following arguments:

ros2 run ros1_bridge dynamic_bridge --bridge-all-topics

With this bridge-all-topics argument you will bridge all the topics automatically, so you will be able to see the odom topic (with a ros2 topic list) directly.

Does all this make sense?

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7

Thank you so much @albertoezquerro . That was very useful info. IMO, it may be worth to add this in the course for beginners.

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