Transforms in ROS

• In robotics, the positions and orientations of objects (e.g., robots, sensors, and obstacles) are often expressed as transformations of one coordinate frame into another
• As time advances, these transformations change (e.g., if the robot moves).
• Although the choice of frames (and therefore the coordinates used to represent the state of the system) is arbitrary, the right choice of frames can greatly simplify the math required
• ROS provides an API for tracking these frames as they evolve in time
• tf2 Tutorials
• Information about Transforms in ROS 2

• The transform (tf) tree is available on the /tf topic
• Any node can publish transforms on /tf or retrieve information from /tf
• ROS has a special API for manipulating the Transform Tree.
  • The tf2_ros package implements this API

ROS follows a set of conventions for frames.

• From REP 0103 Standard Units of Measure and Coordinate Conventions
  • All distances are in meters
  • All angles are in radians
  • All coordinate systems are right-handed
  • Transforms frame_id into child_frame_id: thus the transform converts points in child_frame_id to into the frame frame_id
  • There are four descriptions for orientation, in preferred order
    1. Quaternions
    2. Rotation matrices in \(SO(3)\)
    3. Fixed-axis rotations using roll-pitch-yaw (a.k.a. fixed XYZ Euler angles)
    4. Relative-axis ZYX Euler axis
• From REP 105 Coordinate Frames for Mobile Platforms
  • On a mobile robot, \(x\) is forward, \(y\) is left, and \(z\) is up
  • For cameras \(z\) is forward, \(x\) is right, \(y\) is down, and frame_id ends with _optical

There are several tools that can be used to debug the transform tree.

1. ros2 run tf2_tools view_frames: generates frames.pdf file image of the current frames and how they are connected.
2. ros2 run tf2_ros tf2_echo [from_frame] [to_frame] - print transformations between frames
3. ros2 run tf2_ros static_transform_publisher - Creates a static tf2 frame
4. ros2 doctor May scan for failures in the tf
5. ros2 run rqt_tf_tree rqt_tf_tree lets you see examine the tf tree interactively
   • You may need to separately install the rqt_tf_tree package
6. rviz can be used to visualize frames.
   • You must choose the correct fixed frame in rviz to see any of the frames.
   • All frames are displayed relative to the fixed frame
   • Having an invalid fixed frame will prevent your data from showing up in rviz.
   • You can add the Tf view to see all the frames on tf
   • You can also add individual axes objects

1. The timing of the transforms you look up is crucial.
   • If two frames came in too far apart tf will throw an exception rather than interpolating
   • The ideal way to solve this problem is to
     1. Understand the frequency at which the frames are coming in
     2. Only query the listener when recent-enough data is available
   • The practical way to solve this problem is to
     1. Use try-except around transform calls
     2. Ignore the exceptions and try again on your next iteration.
2. To ask for the most recent transforms, use rclpy.Time() as the time:
   • rclpy.Time.now() gets the current time, but a transform may not be available at the current time.
3. Make sure your transforms form a connected tree
   • There should be a path from the root of the tree to every node (i.e., no disconnected components)
   • Every frame should have only one parent.
   • Each transform should be provided from a single location
4. There are no default frames, only frames that you add.

• The activity that goes with this work can be found TF Activity
• tf2_ros API Docs
  • Jazzy documentation is very incomplete as of 10/2024
  • Rolling documentation is somewhat incomplete as of 10/24

    • Self generate the complete documentation:

      git clone https://github.com/ros2/geometry2
      rosdoc2 build --package-path geometry2/tf2_ros
      # Documentation is under docs_output/tf2_ros
      

• 3Blue1Brown Quaternion Video
• Interactive quaternions