[ros-users] gazebo controller plugins and spawning multiple robots

I'm trying to spawn several robots of the same type in gazebo through ros and am 
hitting what seems to be a controller namespacing issue.  What I would like is 
for each robot to live in its own namespace like:

robot1/odom
robot1/cmd_vel
...
robot2/odom
robot2/cmd_vel
...

The urdf's I'm currently using (erratic_description/urdf/erratic_base.xacro from 
ua-ros-pkg) take in a namespace directly inside the URDF file (example below):

            <controller:diffdrive_plugin name="differential_drive_controller" 
plugin="libdiffdrive_plugin.so">
                <alwaysOn>true</alwaysOn>
                <update>100</update>
                <updateRate>100.0</updateRate>
                <leftJoint>base_link_right_wheel_joint</leftJoint>
                <rightJoint>base_link_left_wheel_joint</rightJoint>
                <wheelSeparation>${caster_wheel_offset_y*2}</wheelSeparation>
                <wheelDiameter>${wheel_radius*2}</wheelDiameter>
                <torque>5</torque>
                <interface:position name="position_iface_0"/>
                <robotNamespace>robot1</robotNamespace>
                <topicName>cmd_vel</topicName>
            </controller:diffdrive_plugin>

So basically the problem is this: Since the controller converts gazebo objects 
into ros topics and the controller is inside the definition of a robot, how do I 
uniquely specify namespaces for multiple robots based on the same URDF?

I could just make several copies of the erratic_base.xacro and just change the 
<robotNamespace> tag in each, but that seems kludgy.  Is there some way to pass 
in variables to a URDF from launch file or something?

PS is this even the general method for working with multiple robots with ros + 
gazebo (ie spawning several robots under different namespaces?) or am I heading 
in the wrong direction...

Jeff




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