• http://wiki.ros.org/xacro
• http://wiki.ros.org/urdf/Tutorials/Using%20Xacro%20to%20Clean%20Up%20a%20URDF%20File
• Frames of Reference, Links Configuration and Appearance

• Properties act like constants, they are global in your xacro file (they're also (most likely, haven't tested) seen by the xacro files that you include in this xacro file
• They can be defined before or after the point they're used.
• Properties are local if defined inside of a xacro:macro

<xacro:property name="width" value="0.2" />

<xacro:property name="width" value="0.2" />
<xacro:property name="bodylen" value="0.6" />
<link name="base_link">
    <visual>
        <geometry>
            <cylinder radius="${width}" length="${bodylen}"/>
        </geometry>
        <material name="blue"/>
    </visual>
    <collision>
        <geometry>
            <cylinder radius="${width}" length="${bodylen}"/>
        </geometry>
    </collision>
</link>

We first define a macro, then call it

<xacro:macro name="default_origin">
    <origin xyz="0 0 0" rpy="0 0 0"/>
</xacro:macro>
<xacro:default_origin />

This will generate: <origin rpy="0 0 0" xyz="0 0 0"/>

If the xacro with a specified name is not found, it will not be expanded and will NOT generate an error.

wtf?

<xacro:macro name="default_inertial" params="mass">
    <inertial>
            <mass value="${mass}" />
            <inertia ixx="1.0" ixy="0.0" ixz="0.0"
                    iyy="1.0" iyz="0.0"
                    izz="1.0" />
    </inertial>
</xacro:macro>

Then call it like this:

<xacro:default_inertial mass="10"/>

For block parameters of a xacro macro, prefix with an asterisk, *.

<xacro:macro name="blue_shape" params="name *shape">
    <link name="${name}">
        <visual>
            <geometry>
                <xacro:insert_block name="shape" />
            </geometry>
            <material name="blue"/>
        </visual>
        <collision>
            <geometry>
                <xacro:insert_block name="shape" />
            </geometry>
        </collision>
    </link>
</xacro:macro>

Then call it like:

<xacro:blue_shape name="base_link">
    <cylinder radius=".42" length=".01" />
</xacro:blue_shape>

<xacro:macro name="pr2_caster" params="suffix *origin **content **anothercontent">
  <joint name="caster_${suffix}_joint">
    <axis xyz="0 0 1" />
  </joint>
  <link name="caster_${suffix}">
    <xacro:insert_block name="origin" />
    <xacro:insert_block name="content" />
    <xacro:insert_block name="anothercontent" />
  </link>
</xacro:macro>

<xacro:pr2_caster suffix="front_left">
  <pose xyz="0 1 0" rpy="0 0 0" />
  <container>
    <color name="yellow"/>
    <mass>0.1</mass>
  </container>
  <another>
    <inertial>
      <origin xyz="0 0 0.5" rpy="0 0 0"/>
      <mass value="1"/>
      <inertia ixx="100"  ixy="0"  ixz="0" iyy="100" iyz="0" izz="100" />
    </inertial>
  </another>
</xacro:pr2_caster>

• Multiple blocks will be processed in the specified order

<xacro:macro name="pr2_arm" params="suffix parent reflect">
  <pr2_upperarm suffix="${suffix}" reflect="${reflect}" parent="${parent}" />
  <pr2_forearm suffix="${suffix}" reflect="${reflect}" parent="elbow_flex_${suffix}" />
</xacro:macro>

<xacro:pr2_arm suffix="left" reflect="1" parent="torso" />
<xacro:pr2_arm suffix="right" reflect="-1" parent="torso" />

• Failed to find root link: Two root links found
  • Each URDF / Xacro file must have a single root link
  • A root link is denoted as <link name="base_footprint"/>