Thursday, May 21, 2015

Controlling a differential drive robot in Gazebo from ROS

Gazebo is a simulation suite targeted at robotics projects, used by DARPA to implement the virtual part of its Robotics Challenge. It's a great companion to the Robot Operating System, supporting several ways to interface with it.

One recurrent question regarding Gazebo is how to control robots described in Simulation Description Format (SDF) models from ROS. The more involved case of porting a URDF model to Gazebo and manipulating it through ROS Control interfaces is well documented, but what about a robot that already has a full SDF model available? Isn't it possible to just paste it into a Gazebo workspace and get some ROS topics to manipulate it?

The tutorial below covers this simpler case. It is based on the SDF model for the Pioneer 2DX differential drive robot, which is available from Gazebo's default library. It also assumes ROS and Gazebo are already installed, as well as the necessary integration packages and plugins.

First, save the SDF world model below to your local filesystem as pioneer2dx_ros.world:
<?xml version="1.0" ?>
<sdf version='1.4'>
  <world name='default'>
    <light name='sun' type='directional'>
      <cast_shadows>1</cast_shadows>
      <pose>0 0 10 0 -0 0</pose>
      <diffuse>0.8 0.8 0.8 1</diffuse>
      <specular>0.2 0.2 0.2 1</specular>
      <attenuation>
        <range>1000</range>
        <constant>0.9</constant>
        <linear>0.01</linear>
        <quadratic>0.001</quadratic>
      </attenuation>
      <direction>-0.5 0.1 -0.9</direction>
    </light>
    <model name='ground_plane'>
      <static>1</static>
      <link name='link'>
        <collision name='collision'>
          <geometry>
            <plane>
              <normal>0 0 1</normal>
              <size>100 100</size>
            </plane>
          </geometry>
          <surface>
            <friction>
              <ode>
                <mu>100</mu>
                <mu2>50</mu2>
              </ode>
            </friction>
            <bounce/>
            <contact>
              <ode/>
            </contact>
          </surface>
          <max_contacts>10</max_contacts>
        </collision>
        <visual name='visual'>
          <cast_shadows>0</cast_shadows>
          <geometry>
            <plane>
              <normal>0 0 1</normal>
              <size>100 100</size>
            </plane>
          </geometry>
          <material>
            <script>
              <uri>file://media/materials/scripts/gazebo.material</uri>
              <name>Gazebo/Grey</name>
            </script>
          </material>
        </visual>
        <velocity_decay>
          <linear>0</linear>
          <angular>0</angular>
        </velocity_decay>
        <self_collide>0</self_collide>
        <kinematic>0</kinematic>
        <gravity>1</gravity>
      </link>
    </model>
    <physics type='ode'>
      <max_step_size>0.001</max_step_size>
      <real_time_factor>1</real_time_factor>
      <real_time_update_rate>1000</real_time_update_rate>
      <gravity>0 0 -9.8</gravity>
    </physics>
    <scene>
      <ambient>0.4 0.4 0.4 1</ambient>
      <background>0.7 0.7 0.7 1</background>
      <shadows>1</shadows>
    </scene>
    <spherical_coordinates>
      <surface_model>EARTH_WGS84</surface_model>
      <latitude_deg>0</latitude_deg>
      <longitude_deg>0</longitude_deg>
      <elevation>0</elevation>
      <heading_deg>0</heading_deg>
    </spherical_coordinates>
    <model name='pioneer2dx'>
      <link name='chassis'>
        <pose>0 0 0.16 0 -0 0</pose>
        <inertial>
          <mass>5.67</mass>
          <inertia>
            <ixx>0.07</ixx>
            <iyy>0.08</iyy>
            <izz>0.1</izz>
            <ixy>0</ixy>
            <ixz>0</ixz>
            <iyz>0</iyz>
          </inertia>
        </inertial>
        <collision name='collision'>
          <geometry>
            <box>
              <size>0.445 0.277 0.17</size>
            </box>
          </geometry>
          <max_contacts>10</max_contacts>
          <surface>
            <bounce/>
            <friction>
              <ode/>
            </friction>
            <contact>
              <ode/>
            </contact>
          </surface>
        </collision>
        <collision name='castor_collision'>
          <pose>-0.2 0 -0.12 0 -0 0</pose>
          <geometry>
            <sphere>
              <radius>0.04</radius>
            </sphere>
          </geometry>
          <surface>
            <friction>
              <ode>
                <mu>0</mu>
                <mu2>0</mu2>
                <slip1>1</slip1>
                <slip2>1</slip2>
              </ode>
            </friction>
            <bounce/>
            <contact>
              <ode/>
            </contact>
          </surface>
          <max_contacts>10</max_contacts>
        </collision>
        <visual name='visual'>
          <pose>0 0 0.04 0 -0 0</pose>
          <geometry>
            <mesh>
              <uri>model://pioneer2dx/meshes/chassis.dae</uri>
            </mesh>
          </geometry>
        </visual>
        <visual name='castor_visual'>
          <pose>-0.2 0 -0.12 0 -0 0</pose>
          <geometry>
            <sphere>
              <radius>0.04</radius>
            </sphere>
          </geometry>
          <material>
            <script>
              <uri>file://media/materials/scripts/gazebo.material</uri>
              <name>Gazebo/FlatBlack</name>
            </script>
          </material>
        </visual>
        <velocity_decay>
          <linear>0</linear>
          <angular>0</angular>
        </velocity_decay>
        <self_collide>0</self_collide>
        <kinematic>0</kinematic>
        <gravity>1</gravity>
      </link>
      <link name='right_wheel'>
        <pose>0.1 -0.17 0.11 0 1.5707 1.5707</pose>
        <inertial>
          <mass>1.5</mass>
          <inertia>
            <ixx>0.0051</ixx>
            <iyy>0.0051</iyy>
            <izz>0.009</izz>
            <ixy>0</ixy>
            <ixz>0</ixz>
            <iyz>0</iyz>
          </inertia>
        </inertial>
        <collision name='collision'>
          <geometry>
            <cylinder>
              <radius>0.11</radius>
              <length>0.05</length>
            </cylinder>
          </geometry>
          <surface>
            <friction>
              <ode>
                <mu>100000</mu>
                <mu2>100000</mu2>
                <slip1>0</slip1>
                <slip2>0</slip2>
              </ode>
            </friction>
            <bounce/>
            <contact>
              <ode/>
            </contact>
          </surface>
          <max_contacts>10</max_contacts>
        </collision>
        <visual name='visual'>
          <geometry>
            <cylinder>
              <radius>0.11</radius>
              <length>0.05</length>
            </cylinder>
          </geometry>
          <material>
            <script>
              <uri>file://media/materials/scripts/gazebo.material</uri>
              <name>Gazebo/FlatBlack</name>
            </script>
          </material>
        </visual>
        <velocity_decay>
          <linear>0</linear>
          <angular>0</angular>
        </velocity_decay>
        <self_collide>0</self_collide>
        <kinematic>0</kinematic>
        <gravity>1</gravity>
      </link>
      <link name='left_wheel'>
        <pose>0.1 0.17 0.11 0 1.5707 1.5707</pose>
        <inertial>
          <mass>1.5</mass>
          <inertia>
            <ixx>0.0051</ixx>
            <iyy>0.0051</iyy>
            <izz>0.009</izz>
            <ixy>0</ixy>
            <ixz>0</ixz>
            <iyz>0</iyz>
          </inertia>
        </inertial>
        <collision name='collision'>
          <geometry>
            <cylinder>
              <radius>0.11</radius>
              <length>0.05</length>
            </cylinder>
          </geometry>
          <surface>
            <friction>
              <ode>
                <mu>100000</mu>
                <mu2>100000</mu2>
                <slip1>0</slip1>
                <slip2>0</slip2>
              </ode>
            </friction>
            <bounce/>
            <contact>
              <ode/>
            </contact>
          </surface>
          <max_contacts>10</max_contacts>
        </collision>
        <visual name='visual'>
          <geometry>
            <cylinder>
              <radius>0.11</radius>
              <length>0.05</length>
            </cylinder>
          </geometry>
          <material>
            <script>
              <uri>file://media/materials/scripts/gazebo.material</uri>
              <name>Gazebo/FlatBlack</name>
            </script>
          </material>
        </visual>
        <velocity_decay>
          <linear>0</linear>
          <angular>0</angular>
        </velocity_decay>
        <self_collide>0</self_collide>
        <kinematic>0</kinematic>
        <gravity>1</gravity>
      </link>
      <joint name='left_wheel_hinge' type='revolute'>
        <pose>0 0 -0.03 0 -0 0</pose>
        <child>left_wheel</child>
        <parent>chassis</parent>
        <axis>
          <xyz>0 1 0</xyz>
          <limit>
            <lower>-1e+16</lower>
            <upper>1e+16</upper>
          </limit>
        </axis>
      </joint>
      <joint name='right_wheel_hinge' type='revolute'>
        <pose>0 0 0.03 0 -0 0</pose>
        <child>right_wheel</child>
        <parent>chassis</parent>
        <axis>
          <xyz>0 1 0</xyz>
          <limit>
            <lower>-1e+16</lower>
            <upper>1e+16</upper>
          </limit>
        </axis>
      </joint>
      <!-- Replaced Gazebo's differential drive plugin with the
           ROS-friendly variant -->
      <!--
      <plugin filename="libDiffDrivePlugin.so" name="diff_drive">
        <left_joint>left_wheel_hinge</left_joint>
        <right_joint>right_wheel_hinge</right_joint>
        <torque>5</torque>
      </plugin>
      -->
      <plugin name='differential_drive_controller'
              filename='libgazebo_ros_diff_drive.so'>
        <alwaysOn>true</alwaysOn>
        <updateRate>100</updateRate>
        <leftJoint>left_wheel_hinge</leftJoint>
        <rightJoint>right_wheel_hinge</rightJoint>
        <wheelSeparation>0.39</wheelSeparation>
        <wheelDiameter>0.15</wheelDiameter>
        <torque>5</torque>
        <commandTopic>cmd_vel</commandTopic>
        <odometryTopic>odom</odometryTopic>
        <odometryFrame>odom</odometryFrame>
        <robotBaseFrame>chassis</robotBaseFrame>
      </plugin>
      <pose>0 0 0 0 -0 0</pose>
      <static>0</static>
    </model>
    <state world_name='default'>
      <sim_time>85 304000000</sim_time>
      <real_time>85 849190220</real_time>
      <wall_time>1432260579 736436496</wall_time>
      <model name='ground_plane'>
        <pose>0 0 0 0 -0 0</pose>
        <link name='link'>
          <pose>0 0 0 0 -0 0</pose>
          <velocity>0 0 0 0 -0 0</velocity>
          <acceleration>0 0 0 0 -0 0</acceleration>
          <wrench>0 0 0 0 -0 0</wrench>
        </link>
      </model>
      <model name='pioneer2dx'>
        <pose>-0.103826 0.027961 2e-06 -2e-06 -8e-06 -0.094162</pose>
        <link name='chassis'>
          <pose>-0.103827 0.027961 0.160002 -2e-06 -8e-06 -0.094162</pose>
          <velocity>
            -0.000797 0.002229 0.003363 -0.024657 -0.007301 0.0023
          </velocity>
          <acceleration>0 0 0 0 -0 0</acceleration>
          <wrench>0 0 0 0 -0 0</wrench>
        </link>
        <link name='left_wheel'>
          <pose>0.011714 0.187806 0.110002 1.38258 1.57033 2.85912</pose>
          <velocity>
            -0.000816 0.001565 0.000275 -0.014307 -0.006987 0.002012
          </velocity>
          <acceleration>0 0 0 0 -0 0</acceleration>
          <wrench>0 0 0 0 -0 0</wrench>
        </link>
        <link name='right_wheel'>
          <pose>
            -0.020254 -0.150688 0.110002 -1.56886 1.51434 -0.092322
          </pose>
          <velocity>
            -0.000347 0.001559 0.008433 -0.014387 -0.003243 0.000126
          </velocity>
          <acceleration>0 0 0 0 -0 0</acceleration>
          <wrench>0 0 0 0 -0 0</wrench>
        </link>
      </model>
    </state>
    <gui fullscreen='0'>
      <camera name='user_camera'>
        <pose>5 -5 2 0 0.275643 2.35619</pose>
        <view_controller>orbit</view_controller>
      </camera>
    </gui>
  </world>
</sdf>
Open a terminal window and start the ROS middleware by entering:
roscore
Open another terminal window, cd to the folder containing pioneer2dx_ros.world and enter:
rosrun gazebo_ros gazebo -file pioneer2dx_ros.world
You should now have a Gazebo window opened with a Pioneer 2DX placed in the middle of an empty world.

Finally, open a third terminal window (it's the last one, I promise) and check if the differential drive topics have been published:
rostopic list
The following topics should be visible, among others:
/pioneer2dx/cmd_vel
/pioneer2dx/odom
You should now be able to get the robot moving by publishing messages to the /pioneer2dx/cmd_vel topic, e.g.
rostopic pub -1 /pioneer2dx/cmd_vel geometry_msgs/Twist \
'{linear: {x: 1.0, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 1.0}}'
Should get the robot running in a loop.

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