ROS Discussion Group, Tonight! Tuesday, Sep 23 @ 7pm PT

[camp .]

    It's Tuesday again! Whether you're a beginner or an expert, please join us if you're interested in ROS.  :-] 

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Camp

[agmiller]

I'm hoping to get some guidance from "the brain trust" on accessing URDF limb/joint position/orientation/size properties, from within ROS2 modules, for use in movement/pose calculations.

- A -  

[Mark Rose]

You should really use the TF2 API to do all the calculations of frame locations and orientations. You can add additional frames if there are positions you want to calculate that are not already in your model. Or do you mean things like wheel diameter and spacing? (I've typically configured that separately from the URDF, for, say, differential drive calculations, since you sometimes have to tweak those a little bit based on empirical tests, especially if there's any sort of tread on your wheels. But I've always used TF2 to calculate arm or camera frame transformations.)

Mark

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Mark Rose

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[Michael Wimble]

What Mark said. 

The URDF is published as a topic by robot_state_publisher and you can subscribe to the topic and parse the XML file and then pull out what you need. You can pick up constants that you put in the URDF like wheel_separation but since there aren’t standards for what is in a URDF you need to specialize your code for your robot. What a lot of programs do is read the URDF and then pick up all of the joints. That’s how robot_state_publisher knows how to listen to the joint_states topic to pull out joint positions for all the movable bits. 

Then you create tf2 listeners for the frames associated with your interesting links, wait for the transform to first appear, the read your link messages (sensors or odometry), transform them to a common frame, like base_link, after checking the timestamps to see that everything is reasonably current, and Bob’s your uncle. There are samples of parts of this in the documentation and I could probably point you to code in my repositories that does this. 

On Sep 23, 2025, at 2:02 PM, Mark Rose <mark....@gmail.com> wrote:



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[camp .]

Tonight! - cp

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