<?xml version="1.0" ?>
<sdf version="1.4">
  <world name="default">
    <scene>
      <ambient>0.5 0.5 0.5 1</ambient>
      <background>0.5 0.5 0.5 1</background>
      <shadows>false</shadows>
    </scene>
    <include>
      <uri>model://sun</uri>
    </include>
    <include>
      <uri>model://ground_plane</uri>
    </include>
    <physics type="ode">
      <gravity>0.0 0.0 -10.0</gravity>
      <ode>
        <solver>
          <type>quick</type>
          <iters>100</iters>
          <sor>1.3</sor>
        </solver>
        <constraints>
          <cfm>0.0</cfm>
          <erp>1.0</erp>
          <contact_max_correcting_vel>10000.0</contact_max_correcting_vel>
          <contact_surface_layer>0.0</contact_surface_layer>
        </constraints>
      </ode>
      <simbody>
        <accuracy>0.01</accuracy>
        <max_transient_velocity>0.01</max_transient_velocity>
        <contact>
          <stiffness>111.111</stiffness>
          <dissipation>0</dissipation>
        </contact>
      </simbody>
      <real_time_update_rate>0</real_time_update_rate>
      <max_step_size>0.001</max_step_size>
    </physics>
    <!-- a test of spring, see reference from ODE discussion
      http://www.ode.org/old_list_archives/2006-March/018014.html
      where stiffness and damping are related to joint limit cfm and erp
      public static void Calc(float springConstant, float dampingCofficient, float
      sprungMass, out float erp, out float cfm)
      {
                  float r = 2.0f * dampingCofficient * (float)Math.Sqrt(sprungMass
                            * springConstant);
                  float hk = Scene.OdeTimeStep * springConstant;

                  erp = hk / (hk + r);
                  cfm = 1.0f / (hk + r);
      }
      in the end,

        erp = dt * kp / ( dt * kp + kd )
        cfm = 1 / ( dt * kp + kd )

      or

        kp = erp / (cfm * h)
        kd = (1 - erp) / cfm
    -->
    <model name="model_0">
      <pose>0.0 0.0 1.1 0 0.0 0.0</pose>
      <link name="link_1">
        <velocity_decay>
          <linear>0.0</linear>
          <angular>0.0</angular>
        </velocity_decay>
        <pose>0.0 0.0 0.0 0.0 0.0 0.0</pose>
        <inertial>
          <pose>0.0 0.0 -0.5 0.0 0.0 0.0</pose>
          <inertia>
            <ixx>0.01</ixx>
            <ixy>0.0</ixy>
            <ixz>0.0</ixz>
            <iyy>0.01</iyy>
            <iyz>0.0</iyz>
            <izz>0.01</izz>
          </inertia>
          <mass>1.0</mass>
        </inertial>
        <visual name="visual_sphere">
          <pose>0.0 0.0 -0.5 0.0 0.0 0.0</pose>
          <geometry>
            <sphere>
              <radius>0.1</radius>
            </sphere>
          </geometry>
          <material>
            <script><uri>file://media/materials/scripts/gazebo.material</uri><name>Gazebo/Grey</name></script>
          </material>
        </visual>
        <collision name="collision_sphere">
          <pose>0.0 0.0 -0.5 0.0 0.0 0.0</pose>
          <max_contacts>250</max_contacts>
          <geometry>
            <sphere>
              <radius>0.1</radius>
            </sphere>
          </geometry>
        </collision>
        <gravity>true</gravity>
        <self_collide>true</self_collide>
        <kinematic>false</kinematic>
      </link>
      <joint name="joint_0" type="prismatic">
        <parent>world</parent>
        <child>link_1</child>
        <pose>0.0 0.0 0.0 0.0 0.0 0.0</pose>
        <axis>
          <limit>
            <lower>0</lower>
            <upper>0</upper>
            <stiffness>1.0e6</stiffness>
            <dissipation>0</dissipation>
          </limit>
          <xyz>0.0 0.0 1.0</xyz>
        </axis>
        <physics>
          <ode>
            <limit>
              <cfm>0.0</cfm>
              <erp>0.2</erp>
            </limit>
          </ode>
        </physics>
      </joint>
      <static>false</static>
    </model>
    <model name="model_1_prismatic">
      <pose>0.0 0.3 1.1 0 0.0 0.0</pose>
      <link name="link_1">
        <velocity_decay>
          <linear>0.0</linear>
          <angular>0.0</angular>
        </velocity_decay>
        <pose>0.0 0.0 0.0 0.0 0.0 0.0</pose>
        <inertial>
          <pose>0.0 0.0 -0.5 0.0 0.0 0.0</pose>
          <inertia>
            <ixx>0.01</ixx>
            <ixy>0.0</ixy>
            <ixz>0.0</ixz>
            <iyy>0.01</iyy>
            <iyz>0.0</iyz>
            <izz>0.01</izz>
          </inertia>
          <mass>1.0</mass>
        </inertial>
        <visual name="visual_cylinder">
          <pose>0.0 0.0 -0.5 0.0 0.0 0.0</pose>
          <geometry>
            <cylinder>
              <length>0.1</length>
              <radius>0.1</radius>
            </cylinder>
          </geometry>
          <material>
            <script><uri>file://media/materials/scripts/gazebo.material</uri><name>Gazebo/White</name></script>
          </material>
        </visual>
        <collision name="collision_cylinder">
          <pose>0.0 0.0 -0.5 0.0 0.0 0.0</pose>
          <max_contacts>250</max_contacts>
          <geometry>
            <cylinder>
              <length>0.1</length>
              <radius>0.1</radius>
            </cylinder>
          </geometry>
        </collision>
        <gravity>true</gravity>
        <self_collide>true</self_collide>
        <kinematic>false</kinematic>
      </link>
      <joint name="joint_0" type="prismatic">
        <parent>world</parent>
        <child>link_1</child>
        <pose>0.0 0.0 0.0 0.0 0.0 0.0</pose>
        <axis>
          <limit>
            <lower>0.0</lower>
            <upper>0.0</upper>
            <stiffness>333.3333</stiffness>
            <dissipation>0</dissipation>
          </limit>
          <xyz>0.0 0.0 1.0</xyz>
        </axis>
        <physics>
          <ode>
            <limit>
              <cfm>3.0</cfm>
              <erp>1.0</erp>
            </limit>
          </ode>
        </physics>
      </joint>
      <static>false</static>
    </model>
    <model name="model_1_revolute">
      <pose>-1.0 0.3 1.1 0 0.0 0.0</pose>
      <link name="link_1">
        <velocity_decay>
          <linear>0.0</linear>
          <angular>0.0</angular>
        </velocity_decay>
        <pose>0.0 0.0 0.0 0.0 0.0 0.0</pose>
        <inertial>
          <pose>1.0 0.0 0 0.0 0.0 0.0</pose>
          <inertia>
            <ixx>0.0001</ixx>
            <ixy>0.0</ixy>
            <ixz>0.0</ixz>
            <iyy>0.0001</iyy>
            <iyz>0.0</iyz>
            <izz>0.0001</izz>
          </inertia>
          <mass>1.0</mass>
        </inertial>
        <visual name="visual_sphere">
          <pose>1.0 0.0 0 0.0 0.0 0.0</pose>
          <geometry>
            <sphere>
              <radius>0.1</radius>
            </sphere>
          </geometry>
          <material>
            <script><uri>file://media/materials/scripts/gazebo.material</uri><name>Gazebo/White</name></script>
          </material>
        </visual>
        <collision name="collision_sphere">
          <pose>1.0 0.0 0 0.0 0.0 0.0</pose>
          <max_contacts>250</max_contacts>
          <geometry>
            <sphere>
              <radius>0.1</radius>
            </sphere>
          </geometry>
        </collision>
        <gravity>true</gravity>
        <self_collide>true</self_collide>
        <kinematic>false</kinematic>
      </link>
      <joint name="joint_0" type="revolute">
        <parent>world</parent>
        <child>link_1</child>
        <pose>0.0 0.0 0.0 0.0 0.0 0.0</pose>
        <axis>
          <limit>
            <lower>0.0</lower>
            <upper>0.0</upper>
            <stiffness>333.3333</stiffness>
            <dissipation>0</dissipation>
          </limit>
          <xyz>0.0 1.0 0.0</xyz>
        </axis>
        <physics>
          <ode>
            <limit>
              <cfm>3.0</cfm>
              <erp>1.0</erp>
            </limit>
          </ode>
        </physics>
      </joint>
      <static>false</static>
    </model>
    <model name="model_2_prismatic">
      <pose>0.0 0.6 1.1 0 0.0 0.0</pose>
      <link name="link_1">
        <pose>0.0 0.0 0.0 0.0 0.0 0.0</pose>
        <inertial>
          <pose>0.0 0.0 -0.5 0.0 0.0 0.0</pose>
          <inertia>
            <ixx>0.01</ixx>
            <ixy>0.0</ixy>
            <ixz>0.0</ixz>
            <iyy>0.01</iyy>
            <iyz>0.0</iyz>
            <izz>0.01</izz>
          </inertia>
          <mass>1.0</mass>
        </inertial>
        <visual name="visual_cylinder">
          <pose>0.0 0.0 -0.5 0.0 0.0 0.0</pose>
          <geometry>
            <cylinder>
              <length>0.1</length>
              <radius>0.1</radius>
            </cylinder>
          </geometry>
          <material>
            <script><uri>file://media/materials/scripts/gazebo.material</uri><name>Gazebo/FlatBlack</name></script>
          </material>
        </visual>
        <collision name="collision_cylinder">
          <pose>0.0 0.0 -0.5 0.0 0.0 0.0</pose>
          <max_contacts>250</max_contacts>
          <geometry>
            <cylinder>
              <length>0.1</length>
              <radius>0.1</radius>
            </cylinder>
          </geometry>
        </collision>
        <gravity>true</gravity>
        <self_collide>true</self_collide>
        <kinematic>false</kinematic>
      </link>
      <joint name="joint_0" type="prismatic">
        <parent>world</parent>
        <child>link_1</child>
        <pose>0.0 0.0 0.0 0.0 0.0 0.0</pose>
        <axis>
          <limit>
            <lower>0</lower>
            <upper>0</upper>
            <stiffness>166.66667</stiffness>
            <dissipation>0</dissipation>
          </limit>
          <xyz>0.0 0.0 1.0</xyz>
        </axis>
        <physics>
          <ode>
            <limit>
              <cfm>6.0</cfm>
              <erp>1.0</erp>
            </limit>
          </ode>
        </physics>
      </joint>
      <static>false</static>
    </model>
    <model name="model_2_revolute">
      <pose>-1.0 0.6 1.1 0 0.0 0.0</pose>
      <link name="link_1">
        <pose>0.0 0.0 0.0 0.0 0.0 0.0</pose>
        <inertial>
          <pose>1.0 0.0 0 0.0 0.0 0.0</pose>
          <inertia>
            <ixx>0.01</ixx>
            <ixy>0.0</ixy>
            <ixz>0.0</ixz>
            <iyy>0.01</iyy>
            <iyz>0.0</iyz>
            <izz>0.01</izz>
          </inertia>
          <mass>1.0</mass>
        </inertial>
        <visual name="visual_sphere">
          <pose>1.0 0.0 0 0.0 0.0 0.0</pose>
          <geometry>
            <sphere>
              <radius>0.1</radius>
            </sphere>
          </geometry>
          <material>
            <script><uri>file://media/materials/scripts/gazebo.material</uri><name>Gazebo/FlatBlack</name></script>
          </material>
        </visual>
        <collision name="collision_sphere">
          <pose>1.0 0.0 0 0.0 0.0 0.0</pose>
          <max_contacts>250</max_contacts>
          <geometry>
            <sphere>
              <radius>0.1</radius>
            </sphere>
          </geometry>
        </collision>
        <gravity>true</gravity>
        <self_collide>true</self_collide>
        <kinematic>false</kinematic>
      </link>
      <joint name="joint_0" type="revolute">
        <parent>world</parent>
        <child>link_1</child>
        <pose>0.0 0.0 0.0 0.0 0.0 0.0</pose>
        <axis>
          <limit>
            <lower>0</lower>
            <upper>0</upper>
            <stiffness>166.66667</stiffness>
            <dissipation>0</dissipation>
          </limit>
          <xyz>0.0 1.0 0.0</xyz>
        </axis>
        <physics>
          <ode>
            <limit>
              <cfm>6.0</cfm>
              <erp>1.0</erp>
            </limit>
          </ode>
        </physics>
      </joint>
      <static>false</static>
    </model>
    <model name="model_3_prismatic">
      <pose>0.0 0.9 1.1 0 0.0 0.0</pose>
      <link name="link_1">
        <pose>0.0 0.0 0.0 0.0 0.0 0.0</pose>
        <inertial>
          <pose>0.0 0.0 -0.5 0.0 0.0 0.0</pose>
          <inertia>
            <ixx>0.01</ixx>
            <ixy>0.0</ixy>
            <ixz>0.0</ixz>
            <iyy>0.01</iyy>
            <iyz>0.0</iyz>
            <izz>0.01</izz>
          </inertia>
          <mass>1.0</mass>
        </inertial>
        <visual name="visual_cylinder">
          <pose>0.0 0.0 -0.5 0.0 0.0 0.0</pose>
          <geometry>
            <cylinder>
              <length>0.1</length>
              <radius>0.1</radius>
            </cylinder>
          </geometry>
          <material>
            <script><uri>file://media/materials/scripts/gazebo.material</uri><name>Gazebo/Red</name></script>
          </material>
        </visual>
        <collision name="collision_cylinder">
          <pose>0.0 0.0 -0.5 0.0 0.0 0.0</pose>
          <max_contacts>250</max_contacts>
          <geometry>
            <cylinder>
              <length>0.1</length>
              <radius>0.1</radius>
            </cylinder>
          </geometry>
        </collision>
        <gravity>true</gravity>
        <self_collide>true</self_collide>
        <kinematic>false</kinematic>
      </link>
      <joint name="joint_0" type="prismatic">
        <parent>world</parent>
        <child>link_1</child>
        <pose>0.0 0.0 0.0 0.0 0.0 0.0</pose>
        <axis>
          <limit>
            <lower>0</lower>
            <upper>0</upper>
            <stiffness>111.1111</stiffness>
            <dissipation>0</dissipation>
          </limit>
          <xyz>0.0 0.0 1.0</xyz>
        </axis>
        <physics>
          <ode>
            <limit>
              <cfm>9.0</cfm>
              <erp>1.0</erp>
            </limit>
          </ode>
        </physics>
      </joint>
      <static>false</static>
    </model>
    <model name="model_3_revolute">
      <pose>-1.0 0.9 1.1 0 0.0 0.0</pose>
      <link name="link_1">
        <pose>0.0 0.0 0.0 0.0 0.0 0.0</pose>
        <inertial>
          <pose>1.0 0.0 0 0.0 0.0 0.0</pose>
          <inertia>
            <ixx>0.01</ixx>
            <ixy>0.0</ixy>
            <ixz>0.0</ixz>
            <iyy>0.01</iyy>
            <iyz>0.0</iyz>
            <izz>0.01</izz>
          </inertia>
          <mass>1.0</mass>
        </inertial>
        <visual name="visual_sphere">
          <pose>1.0 0.0 0 0.0 0.0 0.0</pose>
          <geometry>
            <sphere>
              <radius>0.1</radius>
            </sphere>
          </geometry>
          <material>
            <script><uri>file://media/materials/scripts/gazebo.material</uri><name>Gazebo/Red</name></script>
          </material>
        </visual>
        <collision name="collision_sphere">
          <pose>1.0 0.0 0 0.0 0.0 0.0</pose>
          <max_contacts>250</max_contacts>
          <geometry>
            <sphere>
              <radius>0.1</radius>
            </sphere>
          </geometry>
        </collision>
        <gravity>true</gravity>
        <self_collide>true</self_collide>
        <kinematic>false</kinematic>
      </link>
      <joint name="joint_0" type="revolute">
        <parent>world</parent>
        <child>link_1</child>
        <pose>0.0 0.0 0.0 0.0 0.0 0.0</pose>
        <axis>
          <limit>
            <lower>0</lower>
            <upper>0</upper>
            <stiffness>111.1111</stiffness>
            <dissipation>0</dissipation>
          </limit>
          <xyz>0.0 1.0 0.0</xyz>
        </axis>
        <physics>
          <ode>
            <limit>
              <cfm>9.0</cfm>
              <erp>1.0</erp>
            </limit>
          </ode>
        </physics>
      </joint>
      <static>false</static>
    </model>
    <model name="model_4_prismatic_plugin">
      <pose>0.0 1.2 1.1 0 0.0 0.0</pose>
      <link name="link_1">
        <pose>0.0 0.0 0.0 0.0 0.0 0.0</pose>
        <inertial>
          <pose>0.0 0.0 -0.5 0.0 0.0 0.0</pose>
          <inertia>
            <ixx>0.01</ixx>
            <ixy>0.0</ixy>
            <ixz>0.0</ixz>
            <iyy>0.01</iyy>
            <iyz>0.0</iyz>
            <izz>0.01</izz>
          </inertia>
          <mass>1.0</mass>
        </inertial>
        <visual name="visual_cylinder">
          <pose>0.0 0.0 -0.5 0.0 0.0 0.0</pose>
          <geometry>
            <cylinder>
              <length>0.1</length>
              <radius>0.1</radius>
            </cylinder>
          </geometry>
          <material>
            <script><uri>file://media/materials/scripts/gazebo.material</uri><name>Gazebo/Green</name></script>
          </material>
        </visual>
        <collision name="collision_cylinder">
          <pose>0.0 0.0 -0.5 0.0 0.0 0.0</pose>
          <max_contacts>250</max_contacts>
          <geometry>
            <cylinder>
              <length>0.1</length>
              <radius>0.1</radius>
            </cylinder>
          </geometry>
        </collision>
        <gravity>true</gravity>
        <self_collide>true</self_collide>
        <kinematic>false</kinematic>
      </link>
      <joint name="joint_0" type="prismatic">
        <parent>world</parent>
        <child>link_1</child>
        <pose>0.0 0.0 0.0 0.0 0.0 0.0</pose>
        <axis>
          <limit>
            <lower>-1.0</lower>
            <upper>1.0</upper>
          </limit>
          <xyz>0.0 0.0 1.0</xyz>
        </axis>
      </joint>
      <link name="link_2">
        <pose>0.0 0.0 0.5 0.0 0.0 0.0</pose>
        <inertial>
          <pose>0.0 0.0 -0.5 0.0 0.0 0.0</pose>
          <inertia>
            <ixx>0.01</ixx>
            <ixy>0.0</ixy>
            <ixz>0.0</ixz>
            <iyy>0.01</iyy>
            <iyz>0.0</iyz>
            <izz>0.01</izz>
          </inertia>
          <mass>1.0</mass>
        </inertial>
        <visual name="visual_sphere">
          <pose>0.0 0.0 -0.5 0.0 0.0 0.0</pose>
          <geometry>
            <sphere>
              <radius>0.1</radius>
            </sphere>
          </geometry>
          <material>
            <script><uri>file://media/materials/scripts/gazebo.material</uri><name>Gazebo/Green</name></script>
          </material>
        </visual>
        <collision name="collision_sphere">
          <pose>0.0 0.0 -0.5 0.0 0.0 0.0</pose>
          <max_contacts>250</max_contacts>
          <geometry>
            <sphere>
              <radius>0.1</radius>
            </sphere>
          </geometry>
        </collision>
        <gravity>true</gravity>
        <self_collide>true</self_collide>
        <kinematic>false</kinematic>
      </link>
      <joint name="joint_1" type="prismatic">
        <parent>world</parent>
        <child>link_2</child>
        <pose>0.0 0.0 0.0 0.0 0.0 0.0</pose>
        <axis>
          <limit>
            <lower>-1.0</lower>
            <upper>1.0</upper>
          </limit>
          <xyz>0.0 0.0 1.0</xyz>
        </axis>
      </joint>
      <plugin name="spring_test" filename="libSpringTestPlugin.so">
        <joint_explicit>joint_0</joint_explicit>
        <kp_explicit>111.1111</kp_explicit>
        <kd_explicit>0</kd_explicit>
        <joint_implicit>joint_1</joint_implicit>
        <kp_implicit>111.1111</kp_implicit>
        <kd_implicit>0</kd_implicit>
      </plugin>
      <static>false</static>
    </model>
    <model name="model_5_soft_contact">
      <pose>0.0 1.2 0.0 0 0.0 0.0</pose>
      <link name="link_1">
        <pose>0.0 0.0 0.0 0.0 0.0 0.0</pose>
        <inertial>
          <pose>0.0 0.0 0.05 0.0 0.0 0.0</pose>
          <inertia>
            <ixx>0.01</ixx>
            <ixy>0.0</ixy>
            <ixz>0.0</ixz>
            <iyy>0.01</iyy>
            <iyz>0.0</iyz>
            <izz>0.01</izz>
          </inertia>
          <mass>1.0</mass>
        </inertial>
        <visual name="visual_sphere">
          <pose>0.0 0.0 0.1 0.0 0.0 0.0</pose>
          <geometry>
            <sphere>
              <radius>0.1</radius>
            </sphere>
          </geometry>
          <material>
            <script><uri>file://media/materials/scripts/gazebo.material</uri><name>Gazebo/Blue</name></script>
          </material>
        </visual>
        <collision name="collision_sphere">
          <pose>0.0 0.0 0.1 0.0 0.0 0.0</pose>
          <max_contacts>250</max_contacts>
          <geometry>
            <sphere>
              <radius>0.1</radius>
            </sphere>
          </geometry>
          <surface>
            <friction>
              <ode>
                <mu>0.2</mu>
                <mu2>0.2</mu2>
                <fdir1>1.0 0 0</fdir1>
                <slip1>0</slip1>
                <slip2>0</slip2>
              </ode>
            </friction>
            <contact>
              <ode>
                <kp>111.1111</kp>
                <kd>0.0</kd>
                <max_vel>10000.0</max_vel>
                <min_depth>0.0</min_depth>
              </ode>
            </contact>
          </surface>
        </collision>
        <gravity>true</gravity>
        <self_collide>true</self_collide>
        <kinematic>false</kinematic>
      </link>
      <static>false</static>
    </model>
    <model name="model_6_prismatic_sdf">
      <pose>0.0 1.5 0.6  0 0 0</pose>
      <link name="link">
        <inertial>
          <inertia>
            <ixx>0.01</ixx>
            <ixy>0.0</ixy>
            <ixz>0.0</ixz>
            <iyy>0.01</iyy>
            <iyz>0.0</iyz>
            <izz>0.01</izz>
          </inertia>
          <mass>1.0</mass>
        </inertial>
        <visual name="visual">
          <geometry>
            <cylinder>
              <length>0.1</length>
              <radius>0.1</radius>
            </cylinder>
          </geometry>
          <material>
            <script><uri>file://media/materials/scripts/gazebo.material</uri><name>Gazebo/Green</name></script>
          </material>
        </visual>
        <collision name="collision">
          <geometry>
            <cylinder>
              <length>0.1</length>
              <radius>0.1</radius>
            </cylinder>
          </geometry>
        </collision>
      </link>
      <joint name="joint" type="prismatic">
        <parent>world</parent>
        <child>link</child>
        <pose>0.0 0.0 0.5  0 0 0</pose>
        <axis>
          <limit>
            <lower>-1.0</lower>
            <upper>1.0</upper>
          </limit>
          <xyz>0.0 0.0 1.0</xyz>
          <dynamics>
            <spring_stiffness>111.1111</spring_stiffness>
            <spring_reference>0.0</spring_reference>
            <damping>0.0</damping>
          </dynamics>
        </axis>
      </joint>
    </model>
    <model name="model_7_revolute_sdf">
      <pose>-1.0 1.5 0.6  0 0 0</pose>
      <link name="link">
        <inertial>
          <inertia>
            <ixx>0.01</ixx>
            <ixy>0.0</ixy>
            <ixz>0.0</ixz>
            <iyy>0.01</iyy>
            <iyz>0.0</iyz>
            <izz>0.01</izz>
          </inertia>
          <mass>1.0</mass>
        </inertial>
        <visual name="visual">
          <geometry>
            <sphere>
              <radius>0.1</radius>
            </sphere>
          </geometry>
          <material>
            <script><uri>file://media/materials/scripts/gazebo.material</uri><name>Gazebo/FlatBlack</name></script>
          </material>
        </visual>
        <collision name="collision">
          <geometry>
            <sphere>
              <radius>0.1</radius>
            </sphere>
          </geometry>
        </collision>
      </link>
      <joint name="joint" type="revolute">
        <parent>world</parent>
        <child>link</child>
        <pose>-1.0 0.0 0.0  0 0 0</pose>
        <axis>
          <limit>
            <lower>-10</lower>
            <upper>10</upper>
          </limit>
          <xyz>0.0 1.0 0.0</xyz>
          <dynamics>
            <spring_stiffness>111.1111</spring_stiffness>
            <spring_reference>0.0</spring_reference>
            <damping>0.0</damping>
          </dynamics>
        </axis>
      </joint>
    </model>
  </world>
</sdf>
