Robot Modeling

The urdf XML specification contains nine sub-specifications. Many of the specifications are not used or only used with specific packages. The important specifications are described below.

• model - This specification provides a general guide to the .urdf format.
• robot - The <robot> element is the top-level element in all urdf files. It contains links and joints.
• link - Describes properties of a rigid-body in your robot
  • Has a name attribute to name the link.
  • <inertial> Optionally specify mass and rotational inertia properties of the link.
  • <visual> Optionally describes how to draw the link. There are basic primitives or you can use a 3D model from CAD.
    • <geometry> specifies the shape of the object. Can be <box>, <cylinder>, <sphere> or <mesh>.
    • <material> specifies the color of the object
  • <collision> Optionally describe geometry used for collision detection. Used by motion planning and simulation tools.
• joint - Connects links together with various types of joint freedoms.
  • Has a name attribute to name the joint.
  • Has a type attribute which determines what freedoms and constraints are imposed on the joint.
    • revolute: a 1-degree of freedom (DOF) rotational joint with upper and lower joint limits (specified by a <limit> element)
    • continuous: a 1-DOF rotational joint without limits
    • prismatic: a 1-DOF translational joint with limits (specified by a <limit> element)
    • fixed: A transformation that does not change between two links
    • floating: A joint where all 6 DOF can move
    • planar: A 2-DOF joint allowing translation in the plane
  • Joints have a <parent> link and a <child> link and determine where the <child> link is relative to the parent.
  • <origin> specifies the location of the child in the parent link frame
    • The child frame is translated by xyz
    • The child frame is rotated by rpy, roll ,pitch, yaw (which are rotations about the fixed x, y, and z axes).
  • Other joints can be created by locating multiple joints at the same location. For example: a universal joint is two co-located continuous joints. Creating such joints may require using "dummy" links that lack inertial, visualization, or collision properties.
  • There are many optional tags. The <safety_controller> tag is only used by the pr2_controller_manager and the ros_control packages.
• gazebo Gazebo extensions to URDF files, which it uses when converting to its SDF file format. For more information see Gazebo Tutorial for Using URDF.
• Go through the ROS URDF Tutorials to learn more.
• Even though the documentations linked here are for ROS 1, the specification is the same for ROS 2

There are other formats used in robotics (not just ROS) that relate to URDF files.

• Kinematics and Dynamics Library (KDL)
  • The kdl_parser converts urdf to a format used by KDL
• Gazebo (the primary ROS simulation tool) uses SDF files, but can convert URDF files to SDF automatically
  • Many tags in a URDF file are mainly used when Gazebo converts to SDF.
• Collada is a generic 3D file format.
• SolidWorks is a 3D computer aided design (CAD) program.
  • If using solidworks (or other CAD program) it often makes sense to export models as .vrml files, edit them in Blender to reduce the polygon count for visualization purposes

1. Remember to install the urdf file in setup.py
2. Use a Node Action, along with the Command, PathJointSubsitution, ExecutableInPackage, and FindPackageShare substitutions

Node(
    package="robot_state_publisher",
    executable="robot_state_publisher",
    parameters=[
        {"robot_description" :
        Command([ExecutableInPackage("xacro", "xacro"), " ",
                 PathJoinSubstitution(
                [FindPackageShare("mypackage"), "my.urdf.xacro"])])}
            ]
            ),

• This Action

  1. Starts a node using the robot_state_publisher executable in the robot_state_publisher package
  2. Loads the robot_state_publisher robot_description parameter with the contents of the urdf created by running xacro on my.urdf.xacro
  3. The Command substitution executes whatever string is created from concatenating elements in its list argument and substitutes it with the output of the command.
     • So we are essentially building a command to run whose output will be filling the robot_description
     • ExecutableInPackage() finds the xacro executable (2nd argument) within the xacro package (1st argument)
     • We add a space (" ") to separate the xacro command from its arguments
     • FindPackageShare() finds the share directory of mypackage
     • PathJoinSubstitution() concatenates the mypackage share directory with the location of the xacro file
  1. See Launch Substitutions for details.

An example package that creates a URDF file and launches rviz to display it can be found at https://github.com/m-elwin/me495_urdf

• Use the <xacro:property name="name" value="value"> to define a constant in xacro
  • Basic constants (like pi) are already defined.
  • You can use any python expression anywhere using the ${} syntax

  • It often makes sense to load non-changeable robot parameters directly from a yaml file

    <!-- Get the path to the yaml file relative to the package -->
    <!-- config.yaml should have been installed in setup.py -->
    <xacro:property name="yaml_file" value="$(find package)/config.yaml" />
    <!-- load values from yaml file which are stored in a dictionary called props-->
    <xacro:property name="props" value="${xacro.load_yaml(yaml_file)}" />
    <!-- access an individual variable -->
    <xacro:property name="myprop"  value="${props['myprop']}" />
    

• In ROS2, yaml files often start with the node name (or a pattern) and the ros__parameters keyword, as these parameters are associated with individual nodes

  node_name_or_wildcards:
     ros__parameters:
  

  • When loading the yaml file from a xacro file, you can access the elements by indexing: for example: <xacro:property name="props" value="${xacro.load_yaml(yaml_file)['/**']['ros__parameters']}" /> will retrieve a dictionary of the parameters that you actually wish to use.

• Macro's are the most useful tools when creating urdf files for eventual use with gazebo

• A common case: you want the visual, inertial, and collision elements of a link to correspond to each other:

   <!-- Create a link with
        visual, inertial, and collision for a z-axis aligned cylinder.
        Also enables specifying a material block.
        Assumes the default cylinder orientation
   -->
  <xacro:macro name="cylinder_link" params="name length radius mass *material">
    <link name="${name}">
      <visual>
        <geometry>
          <cylinder length="${length}" radius="${radius}" />
        </geometry>
        <xacro:insert_block name="material" />
      </visual>
      <collision>
        <geometry>
          <cylinder length="${length}" radius="${radius}" />
        </geometry>
      </collision>
      <inertial>
        <mass value="${mass}"/>
        <inertia ixx="${(1.0/12.0)*mass*(3*radius**2 + length**2)}"
                 ixy="0"
                 ixz="0"
                 iyy="${(1.0/12.0)*mass*(3*radius**2 + length**2)}"
                 iyz="0"
                 izz="${0.5*mass*radius**2}" />
      </inertial>
    </link>
  </xacro:macro>
  

• You can include xacro files in other xacro files using <xacro:include filename = "$(find package)/urdf/myfile.xacro">
• Build your own library of useful xacro macros.
• Keep the gazebo tags for your urdf in a separate file and include it. Thus you can maintain a clean separation between the gazebo and non-gazebo parts of your file.

• Conditionally include a block using <xacro:if value = "expression">

• Pass arguments to xacro using xacro model variable:=value syntax..
• Access arguments within xacro using $(arg name)
• Declare that an argument is expected and provide a default value with <xacro:arg name="argname" default="default"/>