David,

Thanks for uncovering the issues with that particular plugin, but I think I
forgot to mention a very important thing...  I think I'd approach this
problem differently.  A better way to apply torque on the wheels is to use
the pr2_controllers stack -- by adding a simple transmission and using the
gazebo_ros_controller_manager plugin.
I recommend taking a look at the package/example
pr2_examples_gazebo/single_link.launch, and replacing the position
controller with an effort controller.

John



On Mon, May 3, 2010 at 12:53 PM, David Ferguson <dhf@andrew.cmu.edu> wrote:

> Hi,
>
> I am trying to write create a gazebo simulation of a 4 wheeled vehicle
> (think golf cart). I am having an issue with trying to create forces in the
> simulation. I want to apply a torque the rear wheels (equivalent to pressing
> the gas pedal). I can't figure out however how to use the gazebo plugin
> which is supposed to apply the force. In the .h file for the plugin it says
> to use it as such:
>
>   This is a controller that collects data from a ROS topic and applies
> wrench to a body accordingly.
>
>   Example Usage:
>   \verbatim
>   <model:physical name="box_model">
>     <body:empty name="box_body">
>      ...
>     </body:empty>
>     <controller:gazebo_ros_force name="box_force_controller"
> plugin="libgazebo_ros_force.so">
>         <alwaysOn>true</alwaysOn>
>         <updateRate>15.0</updateRate>
>         <topicName>box_force</topicName>
>         <bodyName>box_body</bodyName>
>     </controller:gazebo_ros_force>
>   </model:phyiscal>
>   \endverbatim
>
> To test it I essentially copied the tutorial showing how to use the iface
> plugin (sets a models position). My urdf file is below:
>
> <robot name="object_model"
>        xmlns:controller="
> http://playerstage.sourceforge.net/gazebo/xmlschema/#controller"
>        xmlns:interface="
> http://playerstage.sourceforge.net/gazebo/xmlschema/#interface" >
>   <!-- urdf requires at least one joint and one link, so here's the joint
> -->
>   <joint name="my_mug_joint" type="floating" >
>     <!-- axis is in the parent link frame coordintates -->
>     <axis xyz="0 0 0" />
>     <parent link="world" />
>     <child link="object_link" />
>     <!-- initial pose of my_mug joint/link in the parent frame coordiantes
> -->
>     <origin xyz="0 0 0" rpy="0 0 0" />
>   </joint>
>   <!-- one floating link -->
>   <link name="object_link">
>     <inertial>
>       <mass value="1"/>
>       <origin xyz="0 0 0.0"/>
>       <inertia ixx="0.05" ixy="0" ixz="0" iyy="0.05" iyz="0" izz="0.05"/>
>     </inertial>
>     <visual>
>       <origin rpy="0 0 0" xyz="0 0 0.0"/>
>       <geometry>
>         <box size="1.0 1.0 1.0"/>
>       </geometry>
>     </visual>
>     <collision>
>       <origin rpy="0 0 0" xyz="0 0 0.0"/>
>       <geometry>
>         <box size="1.0 1.0 1.0"/>
>       </geometry>
>     </collision>
>
>   </link>
>   <gazebo reference="object_link">
>     <material>Gazebo/Black</material>
>     <mu1>10.0</mu1>
>     <mu2>10.0</mu2>
>     <kp>10</kp>
>     <kd>1.0</kd>
>     <turnGravityOff>false</turnGravityOff>
>     <dampingFactor>0.01</dampingFactor>
>   </gazebo>
>     <gazebo>
>         <controller:gazebo_ros_force name="box_force_controller"
> plugin="libgazebo_ros_force.so">
>          <alwaysOn>true</alwaysOn>
>          <updateRate>15.0</updateRate>
>          <topicName>box_force</topicName>
>          <bodyName>object_link</bodyName>
>         </controller:gazebo_ros_force>
>     </gazebo>
> </robot>
>
> I then have a python script publishing a wrench message to the box_force
> topic however no matter what settings I use it never seems to actually
> affect the box in the simulation. Am I going about this completely wrong? Is
> there a flag I'm not setting?
>
> Thanks in advance for any help,
>
> David Ferguson
>
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