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The URDF is just a xml file which describes how your robot looks, where your linkages are created, what joints join two linkages together, what sensors you have and where they are placed with respect to the robot. It does not magically take care of transforming coordinate frames (linkages) for you; you still need to setup your transforms using either the transform_broadcaster class, or like in your case the static_transform_publisher node (only for sensors rigidly mounted to your frame).

You can think about it like a message's header frame: msg.header.frame = "camera_link"

The message is being published with a description of how it looks, but you need to setup the transform between the coordinate frame "camera_link" (because obviously we just said the header frame of that msg is "camera_link") and whatever the parent coordinate frame would be. You can do that with the static tf publisher or tf broadcaster like i said