Fixed joint not working properly

edit

Hi everyone! I'm new in Gazebo. I created a laser rangefinder model and a robot model in which I want to use it. When I try to fix the imported model with fixed joint or revolute joint with 0 limit, and start the simulation, Gazebo shows the following:

If I don't use any joints, the sensor just falls far down. Without a sensor, the robot looks normal during the simulation, as it should:

Listing of laser model.sdf file:

<?xml version='1.0'?>
<sdf version='1.6'>
  <model name="VL6180">
    <link name='body'>    
      <visual name='visual'>
        <pose>-0.0024 -0.0014 0 0 0 0</pose>
        <geometry>
          <mesh>
            <uri> model://VL6180/meshes/STL/VL6180X.stl </uri>
            <scale> .001 .001 .001 </scale>
          </mesh>
        </geometry>
      </visual>
      <collision name='collision'>
        <pose> 0 0 .0005  0 0 0 </pose>
        <geometry>
          <box>
            <size>.0048 .0028 .001</size>
          </box>
        </geometry>
      </collision>
      <inertial>
        <pose>0 0 0 0 0 0</pose>
        <mass>0.00003</mass>
        <inertia>
          <ixx>0.00000000000000001</ixx>
          <ixy>0.00000000000000001</ixy>
          <ixz>0.00000000000000001</ixz>
          <iyy>0.00000000000000001</iyy>
          <iyz>0.00000000000000001</iyz>
          <izz>0.00000000000000001</izz>
        </inertia>
      </inertial>
      <sensor type="ray" name="sensor">
        <pose>0.0018 0 0 0 -1.5708 0</pose>
          <visualize>true</visualize>
          <update_rate> 30 </update_rate>
          <ray>
            <scan>
              <horizontal>
                <samples>50</samples>
                <resolution>1</resolution>
                <min_angle>-0.174533</min_angle>
                <max_angle>0.174533</max_angle>
              </horizontal>
              <vertical>
                <samples>50</samples>
                <resolution>1</resolution>
                <min_angle>-0.174533</min_angle>
                <max_angle>0.174533</max_angle>
              </vertical>
            </scan>
            <range>
              <min>0.000001</min>
              <max>0.6</max>
            </range>
          </ray>
      </sensor>
    </link>
  </model>
</sdf>

Listing of robot model.sdf file:

<?xml version='1.0'?>
<sdf version='1.6'>
  <model name="my_robot">
    <frame name="robot"></frame>

    <pose>0 0 0.25 0 0 0</pose>
   <!-- <include>
      <name>laser_1</name>
      <uri>model://VL6180</uri>
      <pose>0 0.0425 .003 3.14159 0 3.14159</pose>
    </include>
 -->

    <link name='sole'>
      <pose>0 0 0 1.5708 0 0</pose>
      <visual name='sole_visual'>
        <pose>0 0 0 0 0 0</pose>
        <geometry>
          <mesh>
            <uri> model://Robot_model/meshes/STL/Sole.stl </uri>
            <scale> .001 .001 .001 </scale>
          </mesh>
        </geometry>
      </visual>

      <collision name='sole_collision_box'>
        <pose> 0 .049 0   -1.5708 0 0 </pose>
        <geometry>
          <box>
            <size>.026 .016 .068</size>
          </box>
        </geometry>
      </collision>

      <collision name='sole_collision_cylinder'>
        <pose> 0 .0075 0   -1.5708 0 0 </pose>
        <geometry>
          <cylinder>
            <radius>.05</radius>
            <length>.015</length>
          </cylinder>
        </geometry>
      </collision>

      <inertial>
        <pose>0 .0128 0  0 0 0</pose>
        <mass>.370</mass>
        <inertia>
          <ixx>0.000270821</ixx>
          <ixy>0.00</ixy>
          <ixz>0.00</ixz>
          <iyy>0.000402393</iyy>
          <iyz>0.00000</iyz>
          <izz>0.000273382</izz>
        </inertia>
      </inertial>
    </link>

    <link name = 'cross'>
      <pose>0 0 0.0725 0 0 1.5708</pose>

      <visual name='cross_visual'>
        <pose>0 0 0 0 0 0</pose>
        <geometry>
          <mesh>
            <uri> model://Robot_model/meshes/STL/Cross.stl </uri>
            <scale> .01 .01 .01 </scale>
          </mesh>
        </geometry>
      </visual>

      <collision name='cross_collision'>
        <pose> 0 0 .0025 0 0 0 </pose>
        <geometry>
          <box>
            <size>.022 .014 .005</size>
          </box>
        </geometry>
      </collision>
      <inertial>
        <pose>0 ...