URDF robot root joint

urdf

asked 2013-01-24 18:36:55 -0500

Tobias Kunz
3 ●1 ●1 ●3

updated 2013-01-24 19:10:15 -0500

How do I specify in the URDF file format how the root link of a robot is connected to the world? The root link of the robot could either be free floating in the world or it could be fixed. E.g. for a PR2 the root link should be free floating as the robot should be able to move around. In contrast, a robot arm is usually bolted into something and thus the root link is fixed.

EDIT:

I'll try to make more clear what I am trying to achieve. I am not trying to implement anything in ROS. Thus, a publisher or node is not going to help me. I am trying to load a URDF model into our own simulator and I am trying to understand the URDF file format. A dynamics simulator needs to know whether a robot root link is attached rigidly to the world or free floating. I am not sure exactly how gazebo does this.

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answered 2013-01-25 05:37:48 -0500

David Lu

10932 ●123 ●219 ●250 http://MetroRobots.com/

There is no way to specify what a URDF model's relationship to the world is. You can

A) Include part of the world as part of the URDF, thus specifying the relationship with a joint, as you would any other part.

B) Add some custom field to the URDF to specify such, although such a feature would most likely not be natively supported by the URDF standard.

C) Specify the relationship some other way.

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answered 2013-01-24 18:44:48 -0500

Thomas

4478 ●39 ●58 ●84 http://thomas.moulard.net/

It is not directly specified by the model (the robot motion w.r.t the world may vary depending on your application). However, an external node usually provides the "base_link" to "world" frame (or other global frames, see REP 105). If you have a fixed manipulator then a static_transform_publisher is enough but if you have a mobile robot then you need a localization node that will estimate the robot motion.

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Hi Thomas, could you please better explain what you mean with "localization node"? I have the same problem as Tobias but to me is not clear how to create a localizatione node nor how to implement it. Thank you

Andromeda ( 2014-08-28 03:31:25 -0500 )edit

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