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Hi @myboyhood,

I had the same issue, and I ended in the same post you linked. I tried their solution and did not work so I decided to include in the gazebo tag a direct reference to the SDF format to avoid the conversion from URDF, because when I use it as a xacro, when converting form URDF to SDF the parser always warned me saying the light link did not exist, thus omiting the generation of the plugin. I will place here my approach but may not be the best.

<?xml version="1.0"?>

<robot name="mol" xmlns:xacro="http://www.ros.org/wiki/xacro">

    <!-- Base Link -->
    <link name="base_link">
        <collision>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <geometry>
                <box size="1 1 1"/>
            </geometry>
        </collision>

        <visual>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <geometry>
                <box size="1 1 1"/>
            </geometry>
        </visual>

        <inertial>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <mass value="1"/>
            <inertia
                ixx="1.0" ixy="0.0" ixz="0.0"
                iyy="1.0" iyz="0.0"
                izz="1.0"/>
        </inertial>
    </link>

    <gazebo>
      <model name='flashlight'>
        <pose>0 0 0 0 0 0</pose> 
        <link name='light_source'>
          <pose>0 0 0.0 0.01 0 0</pose>
          <inertial>
            <mass>0.0025</mass>
            <inertia>
              <ixx>1.0e-05</ixx>
              <ixy>0</ixy>
              <ixz>0</ixz>
              <iyy>1.0e-05</iyy>
              <iyz>0</iyz>
              <izz>1.0e-05</izz>
            </inertia>
          </inertial>
          <light name='lamp' type='spot'>
            <pose>0 0 0.0 0 0 0</pose>
            <attenuation>
              <range>15</range>
              <linear>0</linear>
              <constant>0.1</constant>
              <quadratic>0.01</quadratic>
            </attenuation>
            <diffuse>0.8 0.8 0.5 1</diffuse>
            <specular>0.8 0.8 0.5 1</specular>
            <spot>
              <inner_angle>1</inner_angle>
              <outer_angle>1.1</outer_angle>
              <falloff>1</falloff>
            </spot>
            <direction>0 0 -1</direction>
          </light>
          <visual name='lamp'>
            <geometry>
              <cylinder>
                <radius>0.019</radius>
                <length>0.005</length>
              </cylinder>
            </geometry>
            <transparency>0.0</transparency>
            <material>
              <ambient>1 1 1 1</ambient>
              <diffuse>1 1 1 1</diffuse>
              <specular>1 1 1 1</specular>
              <emissive>0.8 0.8 0.5 1</emissive>
            </material>
          </visual>
        </link>
        <plugin name='light_control' filename='libFlashLightPlugin.so'>
              <enable>true</enable>
              <light>
                <id>light_source/lamp</id>
                <duration>0.5</duration>
                <interval>0.5</interval>
              </light>
        </plugin>
      </model>
    </gazebo>
    <gazebo>
      <joint name='flashlight_joint' type='fixed'>
        <parent>base_link</parent>
        <child>flashlight::light_source</child>
      </joint>
    </gazebo>

</robot>

Also I found this reference from a DARPA challenge but they are using directly some ROS custom plugins instead of Gazebo ones. Furthermore, they were manage to implement a Xacro for Flashlights and Leds, you may want to take a look at that, maybe you find some inspiration to make up a good solution.