Example URDF for diff_drive_controller, see https://answers.ros.org/question/300763/is-it-necessary-to-have-urdf-model-for-diff_drive_controller/

mir_simplified.urdf

<?xml version="1.0" ?>
<robot name="mir_100" xmlns:xacro="http://ros.org/wiki/xacro">

  <link name="base_footprint"/>
  
  <joint name="base_joint" type="fixed">
    <parent link="base_footprint"/>
    <child link="base_link"/>
    <origin rpy="0 0 0" xyz="0 0 0"/>
  </joint>
  
  <link name="base_link">
    <visual>
      <origin rpy="0 0 0" xyz="0.037646 0 0"/>
      <geometry>
        <mesh filename="package://mir_description/meshes/visual/mir_100_base.stl"/>
      </geometry>
      <material name="white">
        <color rgba="1 1 1 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0.037646 0 0"/>
      <geometry>
        <mesh filename="package://mir_description/meshes/collision/mir_100_base.stl"/>
      </geometry>
    </collision>
  </link>
  
  <joint name="left_wheel_joint" type="continuous">
    <origin rpy="0 0 0" xyz="0.0 0.222604 0.0625"/>
    <parent link="base_link"/>
    <child link="left_wheel_link"/>
    <axis xyz="0 1 0"/>
  </joint>
  
  <link name="left_wheel_link">
    <visual>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://mir_description/meshes/visual/wheel.stl"/>
      </geometry>
      <material name="dark_grey">
        <color rgba="0.3 0.3 0.3 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://mir_description/meshes/visual/wheel.stl"/>
      </geometry>
    </collision>
  </link>
  
  <joint name="right_wheel_joint" type="continuous">
    <origin rpy="0 0 0" xyz="0.0 -0.222604 0.0625"/>
    <parent link="base_link"/>
    <child link="right_wheel_link"/>
    <axis xyz="0 1 0"/>
  </joint>
  
  <link name="right_wheel_link">
    <visual>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://mir_description/meshes/visual/wheel.stl"/>
      </geometry>
      <material name="dark_grey">
        <color rgba="0.3 0.3 0.3 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://mir_description/meshes/visual/wheel.stl"/>
      </geometry>
    </collision>
  </link>
  
  <!-- caster hub -->
  <joint name="fl_caster_rotation_joint" type="continuous">
    <origin rpy="0 0 0" xyz="0.341346 0.203 0.1565"/>
    <parent link="base_link"/>
    <child link="fl_caster_rotation_link"/>
    <axis xyz="0 0 1"/>
    <dynamics damping="0.01" friction="0.0"/>
  </joint>
  
  <link name="fl_caster_rotation_link">
    <visual>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://mir_description/meshes/visual/caster_wheel_base.stl"/>
      </geometry>
      <material name="silver">
        <color rgba="0.79 0.82 0.93 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://mir_description/meshes/collision/caster_wheel_base.stl"/>
      </geometry>
    </collision>
  </link>
  
  <!-- caster wheel -->
  <joint name="fl_caster_wheel_joint" type="continuous">
    <origin rpy="0 0 0" xyz="-0.0382 0 -0.094"/>
    <parent link="fl_caster_rotation_link"/>
    <child link="fl_caster_wheel_link"/>
    <axis xyz="0 1 0"/>
  </joint>
  
  <link name="fl_caster_wheel_link">
    <visual>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://mir_description/meshes/visual/wheel.stl"/>
      </geometry>
      <material name="dark_grey">
        <color rgba="0.3 0.3 0.3 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://mir_description/meshes/visual/wheel.stl"/>
      </geometry>
    </collision>
  </link>
  
  <!-- caster hub -->
  <joint name="fr_caster_rotation_joint" type="continuous">
    <origin rpy="0 0 0" xyz="0.341346 -0.203 0.1565"/>
    <parent link="base_link"/>
    <child link="fr_caster_rotation_link"/>
    <axis xyz="0 0 1"/>
    <dynamics damping="0.01" friction="0.0"/>
  </joint>
  
  <link name="fr_caster_rotation_link">
    <visual>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://mir_description/meshes/visual/caster_wheel_base.stl"/>
      </geometry>
      <material name="silver">
        <color rgba="0.79 0.82 0.93 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://mir_description/meshes/collision/caster_wheel_base.stl"/>
      </geometry>
    </collision>
  </link>
  
  <!-- caster wheel -->
  <joint name="fr_caster_wheel_joint" type="continuous">
    <origin rpy="0 0 0" xyz="-0.0382 0 -0.094"/>
    <parent link="fr_caster_rotation_link"/>
    <child link="fr_caster_wheel_link"/>
    <axis xyz="0 1 0"/>
  </joint>
  
  <link name="fr_caster_wheel_link">
    <visual>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://mir_description/meshes/visual/wheel.stl"/>
      </geometry>
      <material name="dark_grey">
        <color rgba="0.3 0.3 0.3 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://mir_description/meshes/visual/wheel.stl"/>
      </geometry>
    </collision>
  </link>
  
  <!-- caster hub -->
  <joint name="bl_caster_rotation_joint" type="continuous">
    <origin rpy="0 0 0" xyz="-0.270154 0.203 0.1565"/>
    <parent link="base_link"/>
    <child link="bl_caster_rotation_link"/>
    <axis xyz="0 0 1"/>
    <dynamics damping="0.01" friction="0.0"/>
  </joint>
  
  <link name="bl_caster_rotation_link">
    <visual>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://mir_description/meshes/visual/caster_wheel_base.stl"/>
      </geometry>
      <material name="silver">
        <color rgba="0.79 0.82 0.93 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://mir_description/meshes/collision/caster_wheel_base.stl"/>
      </geometry>
    </collision>
  </link>
  
  <!-- caster wheel -->
  <joint name="bl_caster_wheel_joint" type="continuous">
    <origin rpy="0 0 0" xyz="-0.0382 0 -0.094"/>
    <parent link="bl_caster_rotation_link"/>
    <child link="bl_caster_wheel_link"/>
    <axis xyz="0 1 0"/>
  </joint>
  
  <link name="bl_caster_wheel_link">
    <visual>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://mir_description/meshes/visual/wheel.stl"/>
      </geometry>
      <material name="dark_grey">
        <color rgba="0.3 0.3 0.3 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://mir_description/meshes/visual/wheel.stl"/>
      </geometry>
    </collision>
  </link>
  
  <!-- caster hub -->
  <joint name="br_caster_rotation_joint" type="continuous">
    <origin rpy="0 0 0" xyz="-0.270154 -0.203 0.1565"/>
    <parent link="base_link"/>
    <child link="br_caster_rotation_link"/>
    <axis xyz="0 0 1"/>
    <dynamics damping="0.01" friction="0.0"/>
  </joint>
  
  <link name="br_caster_rotation_link">
    <visual>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://mir_description/meshes/visual/caster_wheel_base.stl"/>
      </geometry>
      <material name="silver">
        <color rgba="0.79 0.82 0.93 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://mir_description/meshes/collision/caster_wheel_base.stl"/>
      </geometry>
    </collision>
  </link>
  
  <!-- caster wheel -->
  <joint name="br_caster_wheel_joint" type="continuous">
    <origin rpy="0 0 0" xyz="-0.0382 0 -0.094"/>
    <parent link="br_caster_rotation_link"/>
    <child link="br_caster_wheel_link"/>
    <axis xyz="0 1 0"/>
  </joint>
  
  <link name="br_caster_wheel_link">
    <visual>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://mir_description/meshes/visual/wheel.stl"/>
      </geometry>
      <material name="dark_grey">
        <color rgba="0.3 0.3 0.3 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://mir_description/meshes/visual/wheel.stl"/>
      </geometry>
    </collision>
  </link>
  
  <transmission name="left_wheel_trans">
    <type>transmission_interface/SimpleTransmission</type>
    <joint name="left_wheel_joint">
      <hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface>
    </joint>
    <actuator name="left_wheel_motor">
      <hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface>
      <mechanicalReduction>1</mechanicalReduction>
    </actuator>
  </transmission>
  <transmission name="right_wheel_trans">
    <type>transmission_interface/SimpleTransmission</type>
    <joint name="right_wheel_joint">
      <hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface>
    </joint>
    <actuator name="right_wheel_motor">
      <hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface>
      <mechanicalReduction>1</mechanicalReduction>
    </actuator>
  </transmission>
</robot>