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Hi, With the help I got, I was able to do the following:

I create the xacro file for the object:

<?xml version="1.0"?>

<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="box_obstacle">
   <xacro:arg name="row" default="0"/>
   <xacro:arg name="col" default="0"/>
   <xacro:arg name="width" default="1"/>
   <xacro:arg name="height" default="1"/>

  <link name="obstacle" type="fixed">
    <inertial>
      <origin xyz="$(arg row) $(arg col) 0.5" />
      <mass value="1.0" />
      <inertia  ixx="0.0" ixy="0.0"  ixz="0.0"  iyy="0.0"  iyz="0.0"  izz="0.0" />
    </inertial>
    <visual>
      <origin xyz="$(arg row) $(arg col) 0.5"/>
      <geometry>
        <box size="$(arg width) $(arg height) 1" />
      </geometry>
    </visual>
    <collision>
      <origin xyz="$(arg row) $(arg col) 0.5"/>
      <geometry>
        <box size="$(arg width) $(arg height) 1" />
      </geometry>
    </collision>
  </link>
  <gazebo reference="obstacle">
    <static>true</static>
    <material>Gazebo/Black</material>
  </gazebo>

</robot>

and my launch file looks like this:

 <launch>
     <arg name="world" default="$(find my_package)/worlds/empty.world"/>

      <!-- We resume the logic in empty_world.launch, changing only the name of the world to be launched -->
      <include file="$(find gazebo_ros)/launch/empty_world.launch">
          <arg name="world_name" value="$(arg world)"/>
      </include>

     <param command="$(find xacro)/xacro.py $(find my_package)/objects/obstacle.urdf.xacro row:=6.5 col:=4.0 width:=3 height:=6" name="obstacle_0" />
     <node args="-urdf -param obstacle_0 -model obstacle_0" name="spawn_obstacle_0" output="screen" pkg="gazebo_ros" respawn="false" type="spawn_model" />
     <param command="$(find xacro)/xacro.py $(find my_package)/objects/obstacle.urdf.xacro row:=4.5 col:=4.0 width:=1 height:=2" name="obstacle_1" />
     <node args="-urdf -param obstacle_1 -model obstacle_1" name="spawn_obstacle_1" output="screen" pkg="gazebo_ros" respawn="false" type="spawn_model" />
     <param command="$(find xacro)/xacro.py $(find my_package)/objects/obstacle.urdf.xacro row:=-4.5 col:=-9.0 width:=9 height:=2" name="obstacle_2" />
     <node args="-urdf -param obstacle_2 -model obstacle_2" name="spawn_obstacle_2" output="screen" pkg="gazebo_ros" respawn="false" type="spawn_model" />
</launch>

This works fine with few "obstacles", but when I try about 10 (instead of 3 here) the simulator doing weired things (I'm trying to attach images but >5 points are needed :()

 <launch>
     <arg name="world" default="$(find my_package)/worlds/empty.world"/>

      <!-- We resume the logic in empty_world.launch, changing only the name of the world to be launched -->
      <include file="$(find gazebo_ros)/launch/empty_world.launch">
          <arg name="world_name" value="$(arg world)"/>
      </include>

     <param command="$(find xacro)/xacro.py $(find my_package)/objects/obstacle.urdf.xacro row:=6.5 col:=4.0 width:=3 height:=6" name="obstacle_0" />
     <node args="-urdf -param obstacle_0 -model obstacle_0" name="spawn_obstacle_0" output="screen" pkg="gazebo_ros" respawn="false" type="spawn_model" />
     <param command="$(find xacro)/xacro.py $(find my_package)/objects/obstacle.urdf.xacro row:=4.5 col:=4.0 width:=1 height:=2" name="obstacle_1" />
     <node args="-urdf -param obstacle_1 -model obstacle_1" name="spawn_obstacle_1" output="screen" pkg="gazebo_ros" respawn="false" type="spawn_model" />
     <param command="$(find xacro)/xacro.py $(find my_package)/objects/obstacle.urdf.xacro row:=-4.5 col:=-9.0 width:=9 height:=2" name="obstacle_2" />
     <node args="-urdf -param obstacle_2 -model obstacle_2" name="spawn_obstacle_2" output="screen" pkg="gazebo_ros" respawn="false" type="spawn_model" />
    <param command="$(find xacro)/xacro.py $(find my_package)/objects/obstacle.urdf.xacro row:=-5.0 col:=-7.5 width:=2 height:=1" name="obstacle_3" />
    <node args="-urdf -param obstacle_3 -model obstacle_3" name="spawn_obstacle_3" output="screen" pkg="gazebo_ros" respawn="false" type="spawn_model" />
    <param command="$(find xacro)/xacro.py $(find my_package)/objects/obstacle.urdf.xacro row:=-2.0 col:=-1.5 width:=16 height:=1" name="obstacle_4" />
    <node args="-urdf -param obstacle_4 -model obstacle_4" name="spawn_obstacle_4" output="screen" pkg="gazebo_ros" respawn="false" type="spawn_model" />
    <param command="$(find xacro)/xacro.py $(find my_package)/objects/obstacle.urdf.xacro row:=-6.0 col:=-0.5 width:=4 height:=1" name="obstacle_5" />
    <node args="-urdf -param obstacle_5 -model obstacle_5" name="spawn_obstacle_5" output="screen" pkg="gazebo_ros" respawn="false" type="spawn_model" />
    <param command="$(find xacro)/xacro.py $(find my_package)/objects/obstacle.urdf.xacro row:=-0.5 col:=0.5 width:=19 height:=1" name="obstacle_6" />
    <node args="-urdf -param obstacle_6 -model obstacle_6" name="spawn_obstacle_6" output="screen" pkg="gazebo_ros" respawn="false" type="spawn_model" />
    <param command="$(find xacro)/xacro.py $(find my_package)/objects/obstacle.urdf.xacro row:=-2.5 col:=1.5 width:=1 height:=1" name="obstacle_7" />
    <node args="-urdf -param obstacle_7 -model obstacle_7" name="spawn_obstacle_7" output="screen" pkg="gazebo_ros" respawn="false" type="spawn_model" />
    <param command="$(find xacro)/xacro.py $(find my_package)/objects/obstacle.urdf.xacro row:=-0.5 col:=-5.0 width:=5 height:=4" name="obstacle_8" />
    <node args="-urdf -param obstacle_8 -model obstacle_8" name="spawn_obstacle_8" output="screen" pkg="gazebo_ros" respawn="false" type="spawn_model" />
    <param command="$(find xacro)/xacro.py $(find my_package)/objects/obstacle.urdf.xacro row:=0.0 col:=8.5 width:=20 height:=1" name="obstacle_9" />
    <node args="-urdf -param obstacle_9 -model obstacle_9" name="spawn_obstacle_9" output="screen" pkg="gazebo_ros" respawn="false" type="spawn_model" />
 </launch>

Is there a way to fix this? How can I tell the pyhsics engine to ignore these obstacles. They should be static and fixed to the ground.

Thanks a lot