minimal example

acrobot_missing_tip_body.urdf

<?xml version="1.0"?>

<robot name="DoublePendulum">
  <link name="base_link">
    <visual>
      <geometry>
         <box size=".2 .2 .2" />
      </geometry>
      <material name="green">
	       <color rgba="0 1 0 1" />
      </material>
    </visual>
  </link>

  <link name="upper_link">
    <inertial>
      <origin xyz="0 0 -.5" rpy="0 0 0" />
      <mass value="1" />
      <inertia ixx="1" ixy="0" ixz="0" iyy="0.083" iyz="0" izz="1" />
    </inertial>
    <visual>
      <origin xyz="0 0 -.5" rpy="0 0 0" />
      <geometry>
         <cylinder length="1.1" radius=".05" />
      </geometry>
      <material name="red">
	       <color rgba="1 0 0 1" />
      </material>
    </visual>
  </link>

  <link name="lower_link">
    <inertial>
      <origin xyz="0 0 -1" rpy="0 0 0" />
      <mass value="1" />
      <inertia ixx="1" ixy="0" ixz="0" iyy="0.33" iyz="0" izz="1" />
    </inertial>
    <visual>
      <origin xyz="0 0 -1" rpy="0 0 0" />
      <geometry>
         <cylinder length="2.1" radius=".05" />
      </geometry>
      <material name="blue">
	       <color rgba="0 0 1 1" />
      </material>
    </visual>
  </link>

  <link name="even_lower_link"></link>

  <joint name="optical_frame_joint" type="fixed">
    <origin rpy="-1.570795 0 -1.570795" xyz="0 0 0"/>
    <parent link="lower_link"/>
    <child link="even_lower_link"/>
  </joint>
	
  <joint name="shoulder" type="continuous">
    <parent link="base_link" />
    <child link="upper_link" />
    <origin xyz="0 .15 0" />
    <axis xyz="0 1 0" />
    <dynamics damping="0.1" />
  </joint>

  <joint name="elbow" type="continuous">
    <parent link="upper_link" />
    <child link="lower_link" />
    <origin xyz="0 .1 -1" />
    <axis xyz="0 1 0" />
    <dynamics damping="0.1" />
  </joint>
</robot>