[URDF] What is a dummy link needed for?

edit

It's possible/likely the "dummy links" you've seen were included to avoid the well known

The root link base_link has an inertia specified in the URDF, but KDL does not support a root link with an inertia. As a workaround, you can add an extra dummy link to your URDF.

See #q192817 for an example (but there are many more).

This is printed by treeFromUrdfModel(..) in kdl_parser_py/kdl_parser_py/urdf.py:

def treeFromUrdfModel(robot_model, quiet=False):
    """
    Construct a PyKDL.Tree from an URDF model from urdf_parser_python.

    :param robot_model: URDF xml string, ``str``
    :param quiet: If true suppress messages to stdout, ``bool``
    """

    root = robot_model.link_map[robot_model.get_root()]

    if root.inertial and not quiet:
        print("The root link %s has an inertia specified in the URDF, but KDL does not support a root link with an inertia.  As a workaround, you can add an extra dummy link to your URDF." % root.name);
    ...

As you can see, the warning suggests to add a "dummy link" (with an identity transform between it and the actual root of your urdf), which you would use to specify dynamics properties (such as the inertia).

But we can't really know for sure, as such a "dummy link" is not standard practice necessarily. Anyone is free to use the name for whatever they want/need, so it could be the dummy links you've seen are used for something else entirely.

And note:

I've seen that often URDF models have a dummy link (almost always called base_link) [..]

base_link would not be the dummy link here. base_link would be the root of the URDF's kinematic chain/tree. The dummy would be either before or after base_link in the tree.

Edit:

OK, so in general:

• the dummy link is to avoid the warning (or for other purposes)
• the base_link is the first link of the robot chain/tree (hence the name "base")

Yes, that seems to be correct.

Although again: that's just one possible use of a "dummy link".

I'm building a mobile robot (simple car-like stuff with a caster wheel), and of course if I don't include the first "empty" link I get the warning you've mentioned, since the chassis (first link) has inertia. So then I've introduced a base_link and a fixed joint between that and the chassis to get rid of the warning.

Doesn't this make the two purposes mix? I've kinda introduced a dummy base, in a sense, right?

Whenever I wanted to get rid of the warning (wanted, as depending on what you use a KDL URDF parser for, it doesn't really always affect anything important), I've used the following structure:

base_link
└ some_dummy_link
  └ <rest of my tree/chain>

With an identity transform, fixed joint between base_link and some_dummy_link and the dynamics properties specified on some_dummy_link.

This keeps base_link the root of the tree, avoids the KDL-related warning, and keeps the tree relatively 'clean'.

I've kinda introduced a dummy base, in a sense ...