Fake links gets transformed in actual links with non-zero mass and inertia

[traversaro]

In the context of URDF files, "fake" links (i.e. links attached to their parent with a fixed joint and without any inertial tag) are commonly used to represent frames, as no "frame" concept is present in URDF (see https://discourse.ros.org/t/urdf-ng-link-and-frame-concepts/56).

This is possible as the URDF specification (http://wiki.ros.org/urdf/XML/link) clearly specify that for the inertial element the following hold:

(optional: defaults to a zero mass and zero inertia if not specified)

urdf-modifiers however does not seem to respect this, and expands "fake" links to links with unit mass and unit inertia.

Minimal Reproducible Example

In this example, we have a simple model with a real link and a fake link. If this is loaded and saved back to file when using urdf-modifiers, the mass of the model is duplicated.

How to run:

git clone https://gist.github.com/traversaro/b251019c117c14faee9d9808ac46600b
cd b251019c117c14faee9d9808ac46600b
python test.py

test.py:

from urdfModifiers.core.linkModifier import LinkModifier
from urdfModifiers.core.jointModifier import JointModifier
from urdfModifiers.core.modification import Modification
from urdfModifiers.utils import *

urdf_path ="./model_with_fake_links.urdf"
output_file = "./model_with_fake_links_converted.urdf"
dummy_file = 'no_gazebo_plugins.urdf'

robot, gazebo_plugin_text = utils.load_robot_and_gazebo_plugins(urdf_path,dummy_file)
utils.write_urdf_to_file(robot, output_file, gazebo_plugin_text)

model_with_fake_links.py :

<?xml version='1.0' encoding='UTF-8'?>
<robot name="modelWithFakeLinks">
    <link name="link1">
        <inertial>
            <mass value="1" />
            <inertia ixx="0.01" ixy="0.0" ixz="0.0" iyy="0.01" iyz="0.0" izz="0.01" />
        </inertial>
    </link>
    <joint name="fakeLinkFixedJoint" type="fixed">
        <origin xyz="0 0 0" rpy="0 0 0" />
        <axis xyz="0 0 0" />
        <parent link="link1" />
        <child link="fakeLink" />
    </joint>    
    <link name="fakeLink"/>
</robot>

After running the script, the model (model_with_fake_links_converted.urdf) becames:

<?xml version='1.0' encoding='UTF-8'?>
<robot name="modelWithFakeLinks">
  <link name="link1">
    <inertial>
      <origin xyz="0.0 0.0 0.0" rpy="0.0 -0.0 0.0"/>
      <mass value="1.0"/>
      <inertia ixx="0.01" ixy="0.0" ixz="0.0" iyy="0.01" iyz="0.0" izz="0.01"/>
    </inertial>
  </link>
  <link name="fakeLink">
    <inertial>
      <origin xyz="0.0 0.0 0.0" rpy="0.0 -0.0 0.0"/>
      <mass value="1.0"/>
      <inertia ixx="1.0" ixy="0.0" ixz="0.0" iyy="1.0" iyz="0.0" izz="1.0"/>
    </inertial>
  </link>
  <joint name="fakeLinkFixedJoint" type="fixed">
    <parent link="link1"/>
    <child link="fakeLink"/>
    <axis xyz="nan nan nan"/>
    <origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0"/>
  </joint>
</robot>

[traversaro]

I am afraid that this bug is actually in urdpy . If this is the case, the problem is that urdfpy does not seems to be really responsive to upstream PRs, so once we have a fix in urdfpy we may need to understand how to handle this.

[traversaro]

fyi @GrmanRodriguez @AlexAntn @Nicogene @CarlottaSartore @fabiodinatale

[CarlottaSartore]

C.C. @lrapetti @mebbaid