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URDF = Unified Robot Description Format

It is an XML format that describes:

  • Links (rigid bodies)
  • Joints (connections)
  • Visual shape
  • Collision shape
  • Inertial properties

Robot = Links + Joints

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<?xml version="1.0"?>
<robot name="box_robot">

  <link name="base_link">

    <visual>
      <geometry>
        <box size="0.5 0.3 0.2"/>
      </geometry>
      <origin xyz="0 0 0"/>
    </visual>

    <collision>
      <geometry>
        <box size="0.5 0.3 0.2"/>
      </geometry>
    </collision>

    <inertial>
      <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>

</robot>
Tag Purpose
visual What you see
collision What physics uses
inertial Mass + inertia tensor

quick preview

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URDF rules

  • Rule1: urdf is a tree (not a graph) no loop allowed
  • Rule2: Every link need inertia if it move
  • Rule3: Visual and collision should usually math, but collision can be simplified for performance

Joints

A joint defines:

  • Where the child frame is placed relative to the parent
  • Around which axis it moves
  • Motion limits

Info

The joint defines the child link frame.

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joint types:

  • fixed
  • revolute
  • continuous
  • prismatic
  • floating
  • planar

Joint does TWO things

  • Places child frame relative to parent
  • Allows motion of child frame relative to parent

Limits

effort

maximum torque allowed (Nm)

Physics engines simulate motors. If no effort limit exists:

  • The motor could apply infinite torque
  • That is non-physical
  • Simulation becomes unstable
Example

Arm: - mass = 0.5 kg - COM = 0.2 m - gravity = 9.81

\[\tau = 0.5 \cdot 9.81 \cdot 0.2 ≈ 0.98 \text{ Nm}\]

so effort 20 Nm is plenty but if we set the effort="0.5" the motor might not lift the arm

velocity

Maximum allowed joint speed.

In real motors: - Motors have max RPM. - Gearboxes limit speed.

Physics engine enforces: - If you command faster motion, - It will clamp at velocity limit.

Demo: Revolute Joint

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base_link → revolute joint → arm_link
Arm 
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<?xml version="1.0"?>
<robot name="simple_arm">

  <!-- ===================== -->
  <!-- Base Link             -->
  <!-- ===================== -->
  <link name="base_link">

    <!-- Visual -->
    <visual>
      <origin xyz="0 0 0.05"/>
      <geometry>
        <box size="0.2 0.2 0.1"/>
      </geometry>
      <material name="gray">
        <color rgba="0.6 0.6 0.6 1"/>
      </material>
    </visual>

    <!-- Collision -->
    <collision>
      <origin xyz="0 0 0.05"/>
      <geometry>
        <box size="0.2 0.2 0.1"/>
      </geometry>
    </collision>

    <!-- Inertia (solid box approx) -->
    <inertial>
      <origin xyz="0 0 0.05"/>
      <mass value="2.0"/>
      <inertia
        ixx="0.0067"
        ixy="0.0"
        ixz="0.0"
        iyy="0.0067"
        iyz="0.0"
        izz="0.0067"/>
    </inertial>

  </link>

  <!-- ===================== -->
  <!-- Arm Link              -->
  <!-- ===================== -->
  <link name="arm_link">

    <!-- Visual -->
    <visual>
      <origin xyz="0.2 0 0.025"/>
      <geometry>
        <box size="0.4 0.05 0.05"/>
      </geometry>
      <material name="blue">
        <color rgba="0.1 0.1 0.8 1"/>
      </material>
    </visual>

    <!-- Collision -->
    <collision>
      <origin xyz="0.2 0 0.025"/>
      <geometry>
        <box size="0.4 0.05 0.05"/>
      </geometry>
    </collision>

    <!-- Inertia -->
    <inertial>
      <origin xyz="0.2 0 0.025"/>
      <mass value="0.5"/>
      <inertia
        ixx="0.002"
        ixy="0.0"
        ixz="0.0"
        iyy="0.02"
        iyz="0.0"
        izz="0.02"/>
    </inertial>

  </link>

  <!-- ===================== -->
  <!-- Revolute Joint        -->
  <!-- ===================== -->
  <joint name="shoulder_joint" type="revolute">

    <parent link="base_link"/>
    <child link="arm_link"/>

    <!-- Joint position relative to base -->
    <origin xyz="0 0 0.1" rpy="0 0 0"/>

    <!-- Rotate around Z axis -->
    <axis xyz="0 0 1"/>

    <!-- Joint limits -->
    <limit
      lower="-1.57"
      upper="1.57"
      effort="20"
      velocity="2.0"/>

    <!-- Optional damping (important for stability) -->
    <dynamics damping="0.1" friction="0.01"/>

  </joint>

</robot>

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<joint name="shoulder_joint" type="revolute">
    <parent link="base_link"/>
    <child link="arm_link"/>
    <origin xyz="0 0 0.1" rpy="0 0 0"/>

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base_link frame
   |
   | 0.1m up
   ↓
arm_link frame (this is defined by joint origin)

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Center Of Mass (COM)

The center of mass is the point where the mass of the object is balanced

Physics engine assume:

  • Gravity acts at COM
  • Force apply at COM
  • Inertia is compute around COM

origin

Where is the element locate relative to is parent frame

Tag Meaning
visual/origin Where the shape is drawn
collision/origin Where collision shape is
inertial/origin Where COM is