[DiffBot] Inertia fix (ros-controls#373)

Author: luis-camero

ros2_control_demo_description/diffbot/urdf/diffbot_description.urdf.xacro

@@ -72,9 +72,9 @@
         <origin xyz="0 0 0" rpy="${PI/2} 0 0"/>
         <mass value="${wheel_mass}"/>
         <inertia
-          ixx="${wheel_mass / 12.0 * (3*wheel_radius + wheel_len*wheel_len)}" ixy="0.0" ixz="0.0"
-          iyy="${wheel_mass / 12.0 * wheel_radius*wheel_radius}" iyz="0.0"
-          izz="${wheel_mass / 12.0 * wheel_radius*wheel_radius}"/>
+          ixx="${wheel_mass / 12.0 * (3*wheel_radius*wheel_radius + wheel_len*wheel_len)}" ixy="0.0" ixz="0.0"
+          iyy="${wheel_mass / 12.0 * (3*wheel_radius*wheel_radius + wheel_len*wheel_len)}" iyz="0.0"
+          izz="${wheel_mass / 2.0 * wheel_radius*wheel_radius}"/>
         </inertial>
       </link>
 
@@ -107,9 +107,9 @@
         <origin xyz="0 0 0" rpy="${PI/2} 0 0"/>
         <mass value="${wheel_mass}"/>
         <inertia
-          ixx="${wheel_mass / 12.0 * (3*wheel_radius + wheel_len*wheel_len)}" ixy="0.0" ixz="0.0"
-          iyy="${wheel_mass / 12.0 * wheel_radius*wheel_radius}" iyz="0.0"
-          izz="${wheel_mass / 12.0 * wheel_radius*wheel_radius}"/>
+          ixx="${wheel_mass / 12.0 * (3*wheel_radius*wheel_radius + wheel_len*wheel_len)}" ixy="0.0" ixz="0.0"
+          iyy="${wheel_mass / 12.0 * (3*wheel_radius*wheel_radius + wheel_len*wheel_len)}" iyz="0.0"
+          izz="${wheel_mass / 2.0 * wheel_radius*wheel_radius}"/>
         </inertial>
       </link>