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ball falling through ground instead of laying on ground plane

asked 2016-04-13 16:48:49 -0500

dcconner gravatar image

This should be easy, but I'm missing something obvious.

I'm trying to create a urdf file for a ball that that I can launch multiple times at different locations through roslaunch.

<node name="ball_1" pkg="gazebo_ros" type="spawn_model" args="-file $(find my_model/urdf/ball.urdf -urdf -x -1.2192 -y 2.56032 -z 0.0254 -s 0.0254 -c "green" -model ball_1" />

My attempt at urdf is :

image description

I first tried without the gazebo tag, but in either case the ball gets added to the gazebo simulation, but then it is not visible . I can select the model "ball_1" in Gazebo, but the pose continues to go negative as if it fell through the ground plane.

The robot model is working as expected.

I'm running ROS jade under Ubuntu 14.04 with a separate gazebo5 install

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Have you figured out what the problem was? If so, can you please submit it as an answer? I'm facing the same thing.

janindu gravatar imagejanindu ( 2017-05-07 01:00:15 -0500 )edit

Hey, even I am facing the same issue. Let me know the solution if anyone of you have figured out

Sam6 gravatar imageSam6 ( 2017-07-21 12:24:00 -0500 )edit

2 Answers

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answered 2018-10-03 03:02:48 -0500

kumpakri gravatar image

updated 2018-10-03 03:04:48 -0500

I believe you always have to define inertial of the model for the physics engine to be able to compute its interactions with other objects. So just add the

    <mass value="1" />
    <inertia  ixx="0.001" ixy="0.0"  ixz="0.0"  
              iyy="0.001"  iyz="0.0"  
              izz="0.001" /> 

section into the model's urdf file (and compute proper inertial matrix).

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answered 2017-10-28 11:16:44 -0500

dcconner gravatar image

updated 2017-10-28 11:23:48 -0500

I'm using the following:

<?xml version="1.0"?>
<robot xmlns:xacro="">

  <xacro:macro name="simple_ball" params="id:=red mass:=0.25 radius:=0.0254 gazebo_color:=Red ">

      <link name="${id}_ball">
            <mass value="${mass}" />
            <origin xyz="0 0 0" />
            <inertia  ixx="${radius*radius*mass*2./5.}" ixy="0.0"  ixz="0.0"  iyy="${radius*radius*mass*2./5.}"  iyz="0.0"  izz="${radius*radius*mass*2./5.}" />
          <origin xyz="0 0 0" rpy="0 0 0" />
            <sphere radius="${radius}"/>
          <origin xyz="0 0 0" rpy="0 0 0" />
            <sphere radius="${radius}"/>
      <gazebo reference="${id}_ball">


with setting a particular size color:

<?xml version="1.0"?>
<robot xmlns:xacro="">

  <xacro:include filename="$(find chris_world_models)/urdf/simple_ball.urdf.xacro"/>

  <simple_ball id="red" mass="0.25" radius="0.0254" gazebo_color="Red"/>


and launch:

  <arg name="ball_id" default="ball0" />
  <arg name="color"   default="red"/>
  <arg name="x_pos"   default="-1.0"/>
  <arg name="y_pos"   default="-1.0"/>
  <arg name="z_pos"   default= "0.05"/> <!-- slight drop for small ball -->

  <!-- make sure all colors are available -->
  <param name="red_ball"   command="$(find xacro)/xacro -v --inorder $(find chris_world_models)/urdf/red_ball.urdf.xacro" />
  <param name="green_ball" command="$(find xacro)/xacro -v --inorder $(find chris_world_models)/urdf/green_ball.urdf.xacro" />
  <param name="blue_ball"  command="$(find xacro)/xacro -v --inorder $(find chris_world_models)/urdf/blue_ball.urdf.xacro" />

  <!-- add ball of specified color and id to the gazebo simlation at the given coordinates -->
  <node name="$(arg ball_id)" pkg="gazebo_ros" type="spawn_model" args="-param $(arg color)_ball  -urdf -x $(arg x_pos) -y $(arg y_pos) -z $(arg z_pos) -model $(arg ball_id)" />


Full setup at:

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Asked: 2016-04-13 16:48:49 -0500

Seen: 3,878 times

Last updated: Oct 03 '18