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Day02 - How Robotic Systems Communicate And How You Can Listen

Intro

  • Robot as system of systems: control, perception, navigation, task orchestration, each complex system with subcomponents and all must share info in real time
  • ROS as Discord for Robots: a well structured, heavily moderated chat where nodes (single purpose, unitary programs) exchange information in predefined formats, by publishing and/or subscribing to channels dedicated to specific topics
  • brushed up on basic ros2 CLI commands to explore an unfamiliar robot system: list nodes and topics, listen to a topic, graphing the system with rqt_graph . Beyond that:

Linux command line crash course

To open a terminal inside the docker

  • In VsCode with Dev Containers : View -> Terminal or CTRL + ñ or clicking on the +icon

  • either through the GUI with click on Terminator in the VNC GUI

  • A short intro to linux commands, arguments and (short hand and long hand) flags, the command prompt <user>@<computer>:<directory>$

  • bash is a programming language!

  • why both empty?

$ echo $ROS_DISTRO

$ echo $IGN_GAZEBO_RESOURCE_PATH

Troubleshooting

The refresher on ros2 CLI commands was helpful to troubleshoot because Krytn teleop task runs but the robot does not move, even though /cmd_vel topic has traffic.

Task throws error "Failed to activate controller":

[...]
[gazebo-1] [ERROR] [1738886097.960364961] [controller_manager]: Switch controller timed out after 5.000000 seconds!
[spawner-7] [ERROR] [1738886097.962109904] [spawner_joint_state_broadcaster]: Failed to activate controller
[ERROR] [spawner-7]: process has died [pid 4396, exit code 1, cmd '/opt/ros/jazzy/lib/controller_manager/spawner joint_state_broadcaster diff_drive_base_controller --ros-args'].
[...]
$ ros2 doctor
[...]
/opt/ros/jazzy/lib/python3.12/site-packages/ros2doctor/api/topic.py: 42: UserWarning: Publisher without subscriber detected on /diff_drive_base_controller/cmd_vel.
/opt/ros/jazzy/lib/python3.12/site-packages/ros2doctor/api/topic.py: 42: UserWarning: Publisher without subscriber detected on /dynamic_joint_states.
/opt/ros/jazzy/lib/python3.12/site-packages/ros2doctor/api/topic.py: 42: UserWarning: Publisher without subscriber detected on /joint_state_broadcaster/transition_event.
[...]
/opt/ros/jazzy/lib/python3.12/site-packages/ros2doctor/api/topic.py: 42: UserWarning: Publisher without subscriber detected on /tf.
/opt/ros/jazzy/lib/python3.12/site-packages/ros2doctor/api/topic.py: 42: UserWarning: Publisher without subscriber detected on /tf_static.

All 5 checks passed

$ ros2 node list
/controller_manager
/gz_ros_control
/joint_state_broadcaster
/robot_state_publisher
/ros_gz_bridge
/rqt_gui_py_node_4393
/static_transform_publisher_6yXulYBNtRWkegAo
/twist_stamper
$ #/diff_drive_base_controller and one additional /static_transform_publisher_XXX are missing!

$ros2 topic list
[...]
$ #/odom missing!
$ ros2 topic echo /cmd_vel
linear:
  x: 0.0
  y: 0.0
  z: 0.0
angular:
  x: 0.0
  y: 0.0
  z: -1.21
---
linear:
  x: 0.0
  y: 0.0
  z: 0.0
angular:
  x: 0.0
  y: 0.0
  z: -1.21
---
[...]
$ ros2 topic info /cmd_vel
Type: geometry_msgs/msg/Twist
Publisher count: 1
Subscription count: 1
$

My conclusion after a couple of days struggling:

  • Recovered the error by running purge + build tasks iaw John in the Discord purge task deletes installand build folders and then build forces to rebuild and repopulate the folder structure inside the docker which I probably messed up calling colcon from outside the docker in the first place.

  • Apparently the repo is symlinked so it can be accessed both from inside or outside the container, but the path is different in eahc case. Worth trying purge + build to see if this fixes ubuntu

  • may want to add a 3s TimerAction to the joint_state_broadcaster spawner in gazebo.launch.py as proposed in the Discord by Danial Othman.

  • More and more intrigued with docker and devcontainers

Important note

/twist_stamper subscribes to Twist messages in /cmd_vel and translates to TwistStamped messages which it publishes on /diff_drive_base_controller/TwistStamped

$ros2 interface show geometry_msgs/msg/TwistStamped

Bonus

A couple of tools beyond the old rqt_graph to interrogate an unfamiliar ROS2 system

Seen here: https://www.linkedin.com/posts/ahcorde_ros-ros2-opensource-activity-7291100454443216897-L__N

Insight - a modern, user-friendly GUI for ROS 2

Current GUI Features

The application already supports several core functionalities for interacting with ROS2:

- Viewing Nodes: Display detailed information about running nodes (similar to ros2 node list and ros2 node info), including their topics, services, and more.

- Viewing Topics: List all topics (similar to ros2 topic list), inspect details of specific topics (similar to ros2 topic info), and view message definitions.

- Viewing Services: List all services (similar to ros2 service list), inspect specific services (similar to ros2 service info), and view message definitions.

- Viewing Actions: List all actions (similar to ros2 action list), inspect specific actions, and view message definitions.

- Viewing Message Definitions: Access detailed message structures, including raw definitions. This was the first feature developed and the original motivation for creating the GUI.

- Viewing Transformations (TFs): Visualize all coordinate frames and calculate transformations between frames (similar to ros2 run tf2_ros tf2_echo source_link target_link).

Key GUI Features

- Filtering: Each page includes robust filtering options for quickly finding relevant content.

- Navigation: User-friendly navigation, no menu bar.

- Nested Pages: Navigate through nested message definitions.

- Copy Button: Simply press a button to copy relevant content (like the command to execute a node, eg. ros2 run turtlesim turtlesim_node)

- View Online: See a message definition in the browser.

- Open folder: Open the folder of a package.

Seen here: https://www.linkedin.com/posts/ahcorde_ros-ros2-network-activity-7288201345226919936-eufA

ros_network_viz This is a utility to visualize the state of an entire ROS 2 network in a graphical way. This utility will show all of the nodes in a graph, all of the topics, services, and actions that connect them, as well as some additional metadata about them.

ROSboard is a node that turns your robot into a web server to visualize ROS topics

Danial Othman mentioned in the Discord he saw ROSboard in use in the nanosaur project (https://nanosaur.ai/) by Rafaello Bonghi and decided to install it into the BAR container and run it as part of ros2 package.