Added ground plane to URDF for simulators (#226)

* Added ground plane to urdf
* Added condition on simulation for ground plane

urdf/ur.urdf.xacro

@@ -53,10 +53,38 @@
 
    <!-- convert to property to use substitution in function -->
    <xacro:property name="initial_positions_file" default="$(arg initial_positions_file)"/>
+   <xacro:property name="is_sim_gazebo" value="$(arg sim_gazebo)"/>
+   <xacro:property name="is_sim_ignition" value="$(arg sim_ignition)"/>
 
    <!-- create link fixed to the "world" -->
    <link name="world" />
 
+   <xacro:if value="${is_sim_gazebo or is_sim_ignition}">
+     <link name="ground_plane">
+       <visual>
+         <origin rpy="0 0 0" xyz="0 0 0"/>
+         <geometry>
+           <box size="5 5 0"/>
+         </geometry>
+         <material name="ground_white">
+           <color rgba="1 1 1 0.5"/>
+         </material>
+       </visual>
+       <collision>
+         <origin rpy="0 0 0" xyz="0 0 0"/>
+         <geometry>
+           <box size="5 5 0"/>
+         </geometry>
+       </collision>
+     </link>
+
+     <joint name="ground_plane_joint" type="fixed">
+       <origin xyz="0 0 -0.01" rpy="0 0 0"/>
+       <parent link="world"/>
+       <child link="ground_plane"/>
+     </joint>
+   </xacro:if>
+
    <!-- arm -->
    <xacro:ur_robot
      name="$(arg name)"