Gazebo kinect plugin needs <interial>

edit

I am trying to mount a kinect camera on the end-effector of a manipulator in the URDF. But the plug-in is loaded only when the reference frame has an <inertial> tag:

<!-- setup empty world -->
<link name="camera_depth_frame">
<inertial>
  <mass value="0.001" />
  <origin xyz="0 0 0" />
  <inertia ixx="0.0001" ixy="0.0" ixz="0.0"
           iyy="0.0001" iyz="0.0"
           izz="0.0001" />
</inertial>
</link>
<link name="camera_depth_optical_frame">
<inertial>
  <mass value="0.001" />
  <origin xyz="0 0 0" />
  <inertia ixx="0.0001" ixy="0.0" ixz="0.0"
           iyy="0.0001" iyz="0.0"
           izz="0.0001" />
</inertial>
</link>
<joint name="camera_depth_optical_joint" type="fixed">
  <origin xyz="0 0 0" rpy="${-M_PI/2} 0 ${-M_PI/2}" />
  <parent link="camera_depth_frame" />
  <child link="camera_depth_optical_frame" />
</joint>
<gazebo reference="camera_depth_frame">
  <sensor type="depth" name="camera">
    <always_on>true</always_on>
    <update_rate>20.0</update_rate>
    <camera>
      <horizontal_fov>${60.0*M_PI/180.0}</horizontal_fov>
      <image>
        <format>B8G8R8</format>
        <width>640</width>
        <height>480</height>
      </image>
      <clip>
        <near>0.05</near>
        <far>8.0</far>
      </clip>
    </camera>
    <plugin name="kinect_camera_controller" filename="libgazebo_ros_openni_kinect.so">
      <cameraName>camera</cameraName>
      <alwaysOn>true</alwaysOn>
      <updateRate>10</updateRate>
      <imageTopicName>color/image_raw</imageTopicName>
      <depthImageTopicName>depth/image_raw</depthImageTopicName>
      <pointCloudTopicName>depth/points</pointCloudTopicName>
      <cameraInfoTopicName>color/camera_info</cameraInfoTopicName>
      <depthImageCameraInfoTopicName>depth/camera_info</depthImageCameraInfoTopicName>
      <frameName>camera_depth_optical_frame</frameName>
      <baseline>0.1</baseline>
      <distortion_k1>0.0</distortion_k1>
      <distortion_k2>0.0</distortion_k2>
      <distortion_k3>0.0</distortion_k3>
      <distortion_t1>0.0</distortion_t1>
      <distortion_t2>0.0</distortion_t2>
      <pointCloudCutoff>0.4</pointCloudCutoff>
    </plugin>
  </sensor>
</gazebo>

If I remove the <inertial> tag, then the plugin is not loaded anymore. Why is that? None of the tutorials mention this.

Also, the plug-in is loaded only when the parent of the camera_depth_frame is fixed. As in, if it is mounted on an end-effector frame, then the plugin is not loaded. What is happening here?

I am using ROS-kinetic on Ubuntu 16.04.