How to "glue" a camera to a pioneer 2DX robot

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I'm new in gazebo, and I have added a camera with a joint to a pioneer 2DX. After some minutes, the camera falls down to the floor.

When I open the world the model is:

After some minutes:

This is the SDF for that model:

<?xml version='1.0'?>
<sdf version='1.6'>
  <model name='pioneer2dx-camera'>
    <link name='chassis'>
      <pose frame=''>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>
        <pose frame=''>0 0 0 0 -0 0</pose>
      </inertial>
      <self_collide>0</self_collide>
      <kinematic>0</kinematic>
      <gravity>0</gravity>
      <visual name='visual'>
        <pose frame=''>0 0 0.04 0 -0 0</pose>
        <geometry>
          <mesh>
            <uri>model://pioneer2dx/meshes/chassis.dae</uri>
          </mesh>
        </geometry>
        <cast_shadows>1</cast_shadows>
        <transparency>0</transparency>
        <material>
          <emissive>0 0 0 1</emissive>
          <shader type='vertex'>
            <normal_map>__default__</normal_map>
          </shader>
        </material>
      </visual>
      <visual name='castor_visual'>
        <pose frame=''>-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>
          <ambient>0.1 0.1 0.1 1</ambient>
          <diffuse>0.1 0.1 0.1 1</diffuse>
          <specular>0.01 0.01 0.01 1</specular>
          <emissive>0 0 0 1</emissive>
          <shader type='vertex'>
            <normal_map>__default__</normal_map>
          </shader>
        </material>
        <transparency>0</transparency>
        <cast_shadows>1</cast_shadows>
      </visual>
      <collision name='castor_collision'>
        <laser_retro>0</laser_retro>
        <max_contacts>10</max_contacts>
        <pose frame=''>-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>
              <fdir1>0 0 0</fdir1>
              <slip1>1</slip1>
              <slip2>1</slip2>
            </ode>
            <torsional>
              <coefficient>1</coefficient>
              <patch_radius>0</patch_radius>
              <surface_radius>0</surface_radius>
              <use_patch_radius>1</use_patch_radius>
              <ode>
                <slip>0</slip>
              </ode>
            </torsional>
          </friction>
          <bounce>
            <restitution_coefficient>0</restitution_coefficient>
            <threshold>1e+06</threshold>
          </bounce>
          <contact>
            <collide_without_contact>0</collide_without_contact>
            <collide_without_contact_bitmask>1</collide_without_contact_bitmask>
            <collide_bitmask>1</collide_bitmask>
            <ode>
              <soft_cfm>0</soft_cfm>
              <soft_erp>0.2</soft_erp>
              <kp>1e+13</kp>
              <kd>1</kd>
              <max_vel>0.01</max_vel>
              <min_depth>0</min_depth>
            </ode>
            <bullet>
              <split_impulse>1</split_impulse>
              <split_impulse_penetration_threshold>-0.01</split_impulse_penetration_threshold>
              <soft_cfm>0</soft_cfm>
              <soft_erp>0.2</soft_erp>
              <kp>1e+13</kp>
              <kd>1</kd>
            </bullet>
          </contact>
        </surface>
      </collision>
      <collision name='collision'>
        <laser_retro>0</laser_retro>
        <max_contacts>10</max_contacts>
        <pose frame=''>0 0 0 0 -0 0</pose>
        <geometry>
          <box>
            <size>0.445 0.277 0.17</size>
          </box>
        </geometry>
        <surface>
          <friction>
            <ode>
              <mu>1</mu>
              <mu2>1</mu2>
              <fdir1>0 0 0</fdir1>
              <slip1>0</slip1>
              <slip2>0</slip2>
            </ode>
            <torsional>
              <coefficient>1</coefficient>
              <patch_radius>0</patch_radius>
              <surface_radius>0</surface_radius>
              <use_patch_radius>1</use_patch_radius>
              <ode>
                <slip>0</slip>
              </ode>
            </torsional>
          </friction>
          <bounce>
            <restitution_coefficient>0</restitution_coefficient>
            <threshold>1e+06</threshold>
          </bounce>
          <contact>
            <collide_without_contact>0</collide_without_contact>
            <collide_without_contact_bitmask>1</collide_without_contact_bitmask>
            <collide_bitmask>1</collide_bitmask>
            <ode>
              <soft_cfm>0</soft_cfm>
              <soft_erp>0.2</soft_erp>
              <kp>1e+13</kp>
              <kd>1</kd>
              <max_vel>0.01</max_vel>
              <min_depth>0</min_depth>
            </ode>
            <bullet>
              <split_impulse ...