initialbot.xml

<?xml version='1.0'?>
<sdf version='1.4'>
  <model name="my_robot">
    <static>false</static>
      <link name='chassis'>
        <pose>0 0 .1 0 0 0</pose>

        <collision name='collision'>
          <geometry>
            <box>
              <size>.4 .2 .1</size>
            </box>
          </geometry>
        </collision>

        <visual name='visual'>
          <geometry>
            <box>
              <size>.4 .2 .1</size>
            </box>
          </geometry>
        </visual>
        <collision name='caster_collision'>
          <pose>-0.15 0 -0.05 0 0 0</pose>
          <geometry>
              <sphere>
                <radius>.05</radius>
            </sphere>
          </geometry>

          <surface>
            <friction>
              <ode>
                <mu>0</mu>
                <mu2>0</mu2>
                <slip1>1.0</slip1>
                <slip2>1.0</slip2>
              </ode>
            </friction>
          </surface>
        </collision>

        <visual name='caster_visual'>
          <pose>-0.15 0 -0.05 0 0 0</pose>
          <geometry>
            <sphere>
              <radius>.05</radius>
            </sphere>
          </geometry>
        </visual>
      </link>

      <link name="left_wheel">
        <pose>0.1 0.13 0.1 0 1.5707 1.5707</pose>
        <collision name="collision">
          <geometry>
            <cylinder>
              <radius>.1</radius>
              <length>.05</length>
            </cylinder>
          </geometry>
        </collision>
        <visual name="visual">
          <geometry>
            <cylinder>
              <radius>.1</radius>
              <length>.05</length>
            </cylinder>
          </geometry>
        </visual>
      </link> 
 
      <link name="right_wheel">
        <pose>0.1 -0.13 0.1 0 1.5707 1.5707</pose>
        <collision name="collision">
          <geometry>
            <cylinder>
              <radius>.1</radius>
              <length>.05</length>
            </cylinder>
          </geometry>
        </collision>
        <visual name="visual">
          <geometry>
            <cylinder>
              <radius>.1</radius>
              <length>.05</length>
            </cylinder>
          </geometry>
        </visual>
      </link>  

      <joint type="revolute" name="left_wheel_hinge">
        <pose>0 0 -0.03 0 0 0</pose>
        <child>left_wheel</child>
        <parent>chassis</parent>
        <axis>
          <xyz>0 1 0</xyz>
        </axis>
      </joint>

      <joint type="revolute" name="right_wheel_hinge">
        <pose>0 0 0.03 0 0 0</pose>
        <child>right_wheel</child>
        <parent>chassis</parent>
        <axis>
          <xyz>0 1 0</xyz>
        </axis>
      </joint>

  </model>
</sdf>