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Simulation

Simulate DiffBot in Gazebo

As described in the Creating your own Gazebo ROS Package, it is common in ROS to create a package that contains all the world files and launch files used with Gazebo. These files are located in a ROS package named /MYROBOT_gazebo. For DiffBot the package is named diffbot_gazebo. Another example that follows best pratices is rrbot which can be found in the gazebo_ros_demos repository.

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fjp@ubuntu:~/git/diffbot/ros/src$ catkin create pkg diffbot_gazebo
Creating package "diffbot_gazebo" in "/home/fjp/git/diffbot/ros/src"...
Created file diffbot_gazebo/package.xml
Created file diffbot_gazebo/CMakeLists.txt
Successfully created package files in /home/fjp/git/diffbot/ros/src/diffbot_gazebo.

The diffbot_gazebo package contains a launch file to lauch a world in Gazebo and spawn the robot model, which is defined in the previously created diffbot_description package. For the launch files the convention is to have a folder named launch and for Gazebo world files a folder named world inside a package.

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fjp@ubuntu:~/git/diffbot/ros/src/diffbot_gazebo$ mkdir launch world

Inside the launch folder is the diffbot.launch.

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<launch>
    <!-- these are the arguments you can pass this launch file, for example paused:=true -->
    <arg name="paused" default="false"/>
    <arg name="use_sim_time" default="true"/>
    <arg name="gui" default="true"/>
    <arg name="headless" default="false"/>
    <arg name="debug" default="false"/>

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

In the world folder of the diffbot_gazebo package is the diffbot.world file:

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<?xml version="1.0" ?>
<sdf version="1.4">
  <world name="default">
    <include>
      <uri>model://ground_plane</uri>
    </include>
    <include>
      <uri>model://sun</uri>
    </include>
    <include>
      <uri>model://gas_station</uri>
      <name>gas_station</name>
      <pose>-2.0 7.0 0 0 0 0</pose>
    </include>
  </world>
</sdf>

With these files build the catkin workspace and source it to make the new diffbot_gazebo package visible to roslaunch:

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catkin build
source devel/setup.zsh

Then its possible to launch the diffbot.launch with:

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roslaunch diffbot_gazebo diffbot.launch

This will lead to the following output:

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roslaunch diffbot_gazebo diffbot.launch 
... logging to /home/fjp/.ros/log/6be90ef2-fdd8-11ea-9cb3-317fd602d5f2/roslaunch-tensorbook-393333.log
Checking log directory for disk usage. This may take a while.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

started roslaunch server http://tensorbook:32837/

SUMMARY
========

PARAMETERS
 * /gazebo/enable_ros_network: True
 * /rosdistro: noetic
 * /rosversion: 1.15.8
 * /use_sim_time: True

NODES
  /
    gazebo (gazebo_ros/gzserver)
    gazebo_gui (gazebo_ros/gzclient)

ROS_MASTER_URI=http://localhost:11311

process[gazebo-1]: started with pid [393352]
process[gazebo_gui-2]: started with pid [393357]
[ INFO] [1600950165.494721382]: Finished loading Gazebo ROS API Plugin.
[ INFO] [1600950165.495515766]: waitForService: Service [/gazebo/set_physics_properties] has not been advertised, waiting...
[ INFO] [1600950165.649461740]: Finished loading Gazebo ROS API Plugin.
[ INFO] [1600950165.650277038]: waitForService: Service [/gazebo_gui/set_physics_properties] has not been advertised, waiting...
[ INFO] [1600950166.640929113]: waitForService: Service [/gazebo/set_physics_properties] is now available.
[ INFO] [1600950166.659917502, 0.007000000]: Physics dynamic reconfigure ready.

Also, the Gazebo simulator will open a new window with the objects defined in diffbot.world except for the Gas station because it is a model that has to be downloaded first, which is happening in the background. TODO: gas station is not showing this way.

Empty world of DiffBot.

To get the Gas station or other available models it is possible to clone the gazebo models repository into your /home/your_username/.gazebo folder, e.g.:

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/home/fjp/.gazeb$ git clone osrf/gazebo_models
Then add this path inside Gazebo to insert these models into your world file.

Using ROS launch to Spawn URDF Robots

According to the Gazebo roslaunch tutorial the recommended way to spawn a robot into Gazebo is to use a launch file. Therefore, edit the diffbot.launch inside the diffbot_gazebo package by adding the following inside the <launch> </launch tag:

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   <!-- Load the URDF into the ROS Parameter Server -->
   <param name="robot_description"
       command="$(find xacro)/xacro --inorder '$(find diffbot_description)/urdf/diffbot.xacro'" />

   <!-- Run a python script to the send a service call to gazebo_ros to spawn a URDF robot -->
   <node name="urdf_spawner" pkg="gazebo_ros" type="spawn_model" respawn="false" output="screen"
       args="-urdf -model diffbot -param robot_description"/>

See also the complete diffbot.launch file.

This will open Gazebo simulator and show the DiffBot model:

Empty world including DiffBot.

Moving the Robot

Note that the robot cannot be moved without having either a Gazebo plugin loaded or making use of ROS Control and its Gazebo plugin gazebo_ros_control, see also the Gazebo ROS Control Tutorial. Using the ROS Control and its Gazebo plugin is done in case of DiffBot. An alternative would be to use the existing differential_drive_controller Gazebo plugin without having to rely on ROS Control. The next section explains the diffbot_control package in more detail and how to setup the diff_drive_controller from the ros_controllers package.

Adding Sensors

To add sensors to a robot model make use of link and joint tags to define the desired location and shape, possibly using meshes. For the simulation of these sensor there exist common Gazebo plugins that can be used. See, the [Tutorial: Using Gazebo plugins with ROS] for existing plugins and more details how to use them. For a full list of plugins see also gazebo_ros_pkgs which is a package or interface for using ROS with the Gazebo simulator.

Camera

This section follows Gazebo tutorial Adding a Camera.

Laser (Lidar)

This section follows Gazebo tutorial Adding a Laser GPU.

Ultrasonic Ranger

See the source of the gazebo_ros_range plugin.

Inertial Measurement Unit (IMU)

This section follows Gazebo tutorial [Adding an IMU](http://gazebosim.org/tutorials?tut=ros_gzplugins#IMU(GazeboRosImu). Note use GazeboRosImuSensor?

Troubleshooting

A quick way to verify if the conversion from xacro to urdf to sdf is working is the following (source: Tutorial URDF in Gazebo): First convert the xacro model to a urdf model with the xacro command:

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xacro src/diffbot_description/urdf/diffbot.xacro -o diffbot.urdf

This will output the urdf into a file named diffbot.urdf in the current working directory.

Then use the gz command to create a sdf:

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# gazebo3 and above
gz sdf -p MODEL.urdf

DiffBot sdf. 
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<sdf version='1.7'>
  <model name='diffbot'>
    <link name='base_footprint'>
      <inertial>
        <pose>-0.012273 0 0.040818 0 -0 0</pose>
        <mass>5.5</mass>
        <inertia>
          <ixx>0.0387035</ixx>
          <ixy>0</ixy>
          <ixz>0.000552273</ixz>
          <iyy>0.0188626</iyy>
          <iyz>0</iyz>
          <izz>0.0561591</izz>
        </inertia>
      </inertial>
      <collision name='base_footprint_collision'>
        <pose>0 0 0 0 -0 0</pose>
        <geometry>
          <box>
            <size>0.001 0.001 0.001</size>
          </box>
        </geometry>
        <surface>
          <contact>
            <ode/>
          </contact>
          <friction>
            <ode/>
          </friction>
        </surface>
      </collision>
      <collision name='base_footprint_fixed_joint_lump__base_link_collision_1'>
        <pose>0 0 0.04 0 -0 0</pose>
        <geometry>
          <box>
            <size>0.3 0.15 0.02</size>
          </box>
        </geometry>
        <surface>
          <contact>
            <ode/>
          </contact>
          <friction>
            <ode/>
          </friction>
        </surface>
      </collision>
      <collision name='base_footprint_fixed_joint_lump__caster_link_collision_2'>
        <pose>-0.135 0 0.029 0 -0 0</pose>
        <geometry>
          <sphere>
            <radius>0.025</radius>
          </sphere>
        </geometry>
        <surface>
          <contact>
            <ode/>
          </contact>
          <friction>
            <ode/>
          </friction>
        </surface>
      </collision>
      <visual name='base_footprint_fixed_joint_lump__base_link_visual'>
        <pose>0 0 0.04 0 -0 0</pose>
        <geometry>
          <box>
            <size>0.3 0.15 0.02</size>
          </box>
        </geometry>
        <material>
          <script>
            <name>Gazebo/White</name>
            <uri>file://media/materials/scripts/gazebo.material</uri>
          </script>
        </material>
      </visual>
      <visual name='base_footprint_fixed_joint_lump__caster_link_visual_1'>
        <pose>-0.115 0 0.029 0 -0 0</pose>
        <geometry>
          <sphere>
            <radius>0.025</radius>
          </sphere>
        </geometry>
      </visual>
      <velocity_decay/>
      <velocity_decay/>
      <gravity>1</gravity>
      <velocity_decay/>
    </link>
    <joint name='front_left_wheel_joint' type='revolute'>
      <pose relative_to='base_footprint'>0.105 -0.085 0.04 0 -0 0</pose>
      <parent>base_footprint</parent>
      <child>front_left_wheel</child>
      <axis>
        <xyz>0 1 0</xyz>
        <limit>
          <lower>-1e+16</lower>
          <upper>1e+16</upper>
        </limit>
        <dynamics>
          <spring_reference>0</spring_reference>
          <spring_stiffness>0</spring_stiffness>
        </dynamics>
      </axis>
    </joint>
    <link name='front_left_wheel'>
      <pose relative_to='front_left_wheel_joint'>0 0 0 0 -0 0</pose>
      <inertial>
        <pose>0 0 0 0 -0 0</pose>
        <mass>2.5</mass>
        <inertia>
          <ixx>0.00108333</ixx>
          <ixy>0</ixy>
          <ixz>0</ixz>
          <iyy>0.00108333</iyy>
          <iyz>0</iyz>
          <izz>0.002</izz>
        </inertia>
      </inertial>
      <collision name='front_left_wheel_collision'>
        <pose>0 0 0 1.5708 -0 0</pose>
        <geometry>
          <cylinder>
            <length>0.02</length>
            <radius>0.04</radius>
          </cylinder>
        </geometry>
        <surface>
          <contact>
            <ode>
              <kp>1e+07</kp>
              <kd>1</kd>
            </ode>
          </contact>
          <friction>
            <ode>
              <mu>1</mu>
              <mu2>1</mu2>
              <fdir1>1 0 0</fdir1>
            </ode>
          </friction>
        </surface>
      </collision>
      <visual name='front_left_wheel_visual'>
        <pose>0 0 0 1.5708 -0 0</pose>
        <geometry>
          <cylinder>
            <length>0.02</length>
            <radius>0.04</radius>
          </cylinder>
        </geometry>
        <material>
          <script>
            <name>Gazebo/Grey</name>
            <uri>file://media/materials/scripts/gazebo.material</uri>
          </script>
        </material>
      </visual>
      <gravity>1</gravity>
      <velocity_decay/>
    </link>
    <joint name='front_right_wheel_joint' type='revolute'>
      <pose relative_to='base_footprint'>0.105 0.085 0.04 0 -0 0</pose>
      <parent>base_footprint</parent>
      <child>front_right_wheel</child>
      <axis>
        <xyz>0 1 0</xyz>
        <limit>
          <lower>-1e+16</lower>
          <upper>1e+16</upper>
        </limit>
        <dynamics>
          <spring_reference>0</spring_reference>
          <spring_stiffness>0</spring_stiffness>
        </dynamics>
      </axis>
    </joint>
    <link name='front_right_wheel'>
      <pose relative_to='front_right_wheel_joint'>0 0 0 0 -0 0</pose>
      <inertial>
        <pose>0 0 0 0 -0 0</pose>
        <mass>2.5</mass>
        <inertia>
          <ixx>0.00108333</ixx>
          <ixy>0</ixy>
          <ixz>0</ixz>
          <iyy>0.00108333</iyy>
          <iyz>0</iyz>
          <izz>0.002</izz>
        </inertia>
      </inertial>
      <collision name='front_right_wheel_collision'>
        <pose>0 0 0 1.5708 -0 0</pose>
        <geometry>
          <cylinder>
            <length>0.02</length>
            <radius>0.04</radius>
          </cylinder>
        </geometry>
        <surface>
          <contact>
            <ode>
              <kp>1e+07</kp>
              <kd>1</kd>
            </ode>
          </contact>
          <friction>
            <ode>
              <mu>1</mu>
              <mu2>1</mu2>
              <fdir1>1 0 0</fdir1>
            </ode>
          </friction>
        </surface>
      </collision>
      <visual name='front_right_wheel_visual'>
        <pose>0 0 0 1.5708 -0 0</pose>
        <geometry>
          <cylinder>
            <length>0.02</length>
            <radius>0.04</radius>
          </cylinder>
        </geometry>
        <material>
          <script>
            <name>Gazebo/Grey</name>
            <uri>file://media/materials/scripts/gazebo.material</uri>
          </script>
        </material>
      </visual>
      <gravity>1</gravity>
      <velocity_decay/>
    </link>
    <plugin name='gazebo_ros_control' filename='libgazebo_ros_control.so'>
      <robotNamespace>/diffbot</robotNamespace>
      <robotSimType>gazebo_ros_control/DefaultRobotHWSim</robotSimType>
    </plugin>
    <static>0</static>
    <plugin name='differential_drive_controller' filename='libgazebo_ros_diff_drive.so'>
      <legacyMode>1</legacyMode>
      <rosDebugLevel>Debug</rosDebugLevel>
      <publishWheelTF>0</publishWheelTF>
      <robotNamespace>/</robotNamespace>
      <publishTf>1</publishTf>
      <publishWheelJointState>0</publishWheelJointState>
      <alwaysOn>1</alwaysOn>
      <updateRate>100.0</updateRate>
      <leftJoint>front_left_wheel_joint</leftJoint>
      <rightJoint>front_right_wheel_joint</rightJoint>
      <wheelSeparation>0.3</wheelSeparation>
      <wheelDiameter>0.08</wheelDiameter>
      <broadcastTF>1</broadcastTF>
      <wheelTorque>30</wheelTorque>
      <wheelAcceleration>1.8</wheelAcceleration>
      <commandTopic>cmd_vel</commandTopic>
      <odometryFrame>odom</odometryFrame>
      <odometryTopic>odom</odometryTopic>
      <robotBaseFrame>base_footprint</robotBaseFrame>
    </plugin>
  </model>
</sdf>

In case the output looks like the following, there are most certainly missing <inertial> tags in the <link> tag. For the Gazebo simulator the <inertial> must be present, in order to simulate the dynamics of the robot. See also http://wiki.ros.org/urdf/XML/link and the Gazebo tutorials on URDF.

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<sdf version='1.7'>
  <model name='diffbot'/>
</sdf>