How to add Kinect sensor input to a URDF model?

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In your model for the kinect you need to create a fixed joint between the link that represents your model and the frame_id being published by the kinect node.

for example:

<link name="kinect_link">
  <visual>
    <geometry>
      <box size="0.064 0.121 0.0381" />
    </geometry>
    <material name="Blue" />
  </visual>
  <inertial>
      <mass value="0.0001" />
      <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="kinect_depth_joint" type="fixed">
  <origin xyz="0 0.028 0" rpy="0 0 0" />
  <parent link="kinect_link" />
  <child link="kinect_depth_frame" />
</joint>

<link name="kinect_depth_frame">
  <inertial>
      <mass value="0.0001" />
      <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="depth_optical_joint" type="fixed">
  <origin xyz="0 0 0" rpy="${-M_PI/2} 0 ${-M_PI/2}" />
  <parent link="kinect_depth_frame" />
  <child link="kinect_depth_optical_frame" />
</joint>

<link name="kinect_depth_optical_frame">
  <inertial>
      <mass value="0.0001" />
      <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>

Then based on the above urdf launch the kinect node with the following parameteres:

  <node pkg="openni_camera" type="openni_node" name="openni_camera" output="screen" respawn="true" >
    <param name="device_type" value="1" />
    <param name="registration_type" value="1" />
    <param name="point_cloud_resolution" value="1" />
    <param name="openni_depth_optical_frame" value="kinect_depth_optical_frame" />
    <param name="openni_rgb_optical_frame" value="kinect_rgb_optical_frame" />
    <param name="image_input_format" value="5" />
    <rosparam command="load" file="$(find openni_camera)/info/openni_params.yaml" />
  </node>