RioD URDF extensions

URDF describes a robot as a tree of links and joints. A closed kinematic loop - a four-bar linkage, a delta robot, a Stewart platform - does not fit in a tree. These extensions, used by ORIGITECH's RioD, add the two things that are missing: joints without a motor, and the constraints that close the loops.

A file using them is still a URDF file. The new elements live in an XML namespace of their own, so a parser that does not know them - urdfdom, for one - reads the tree and skips the rest.

Namespace

Declare it on <robot>, with the prefix riod:

<robot name="four_bar" xmlns:riod="https://github.com/Origitech/urdf-extensions">

The URI is an identifier, compared character by character and case-sensitively. RioD currently recognises the elements by their prefix, so use exactly riod.

Modelling a closed loop

1. Cut every loop once, so that what remains is a tree, and write that tree as ordinary URDF.
2. Mark the joints no motor drives with riod:actuated value="false".
3. Close every cut with a riod:loop_closure.

Write the tree so that the loops are closed when all joints are at zero: it tells an importer which way the mechanism is assembled.

riod:actuated

Inside a moving <joint> - revolute, continuous or prismatic.

Attribute	Value
value	true or false	Required. A joint without the element is actuated.

A passive joint (false) is not driven: its position follows from the actuated joints through the loop closures, so at least one closure has to constrain it.

<joint name="rocker" type="revolute">
  <origin xyz="0.25 0 0" rpy="0 0 1.5708"/>
  <parent link="base_link"/>
  <child link="rocker_link"/>
  <axis xyz="0 0 1"/>
  <limit lower="-3.14" upper="3.14" effort="0" velocity="1"/>
  <riod:actuated value="false"/>
</joint>

riod:loop_closure

Inside <robot>. Ties the frames of two links together.

Attribute	Value
frameA, frameB	link names	Required. The two frames to bring together.
type	point (default) or rigid	point: the origins coincide and the orientations stay free - a ball joint, 3 constraints. rigid: the frames coincide entirely - 6 constraints.
name	text	Optional name of the closure.

A link's frame is the frame of the joint that leads to it. To close a loop elsewhere on a link, put an empty link there on a fixed joint:

<link name="coupler_end"/>
<joint name="coupler_end_frame" type="fixed">
  <origin xyz="0.25 0 0"/>
  <parent link="coupler_link"/>
  <child link="coupler_end"/>
</joint>

<riod:loop_closure name="closure" type="point" frameA="coupler_end" frameB="rocker_end"/>

Units and conventions are URDF's own: metres, radians, rpy about the fixed axes.

References

• URDF XML specification: robot, link, joint
• URDF tutorials (ROS 2)
• urdfdom (The reference parser)
• Namespaces in XML (W3C)