My robot arm moves as soon as it's loaded in Gazebo

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Hi everyone,

I've loaded a robot arm in Gazebo. It's rappresented in urdf file and it appear correctly. I launch gazebo paused, but when I start the simulation the arm moves alone, without any applied force. It can be due by some contact (internal) force in the arm? Have you any suggests?

It seem that the arm goes to a new position, than it stays there.

Thank you!

Hi @nkoenig , and hi everyone.

I update my questions. I have an urdf directory, with the .xacro file. In this .xacro file I created link and joints eith this template:

<!-- ARM -->
<link name ="name_link">
<inertial>
  <origin xyz="-0.00543349 -0.04480023 0.01581062" rpy="0 0 0"/>
  <mass value="1.81745527"/>
  <inertia ixx="0.00571053"  ixy="0.00041083"  ixz="-0.00005130" iyy="0.01126908" iyz="-0.00003461" izz="0.01371197" />
</inertial>
<visual>
  <origin xyz="0 0 0" rpy="0 0 0"/>
  <geometry name="name_link_visual">
    <mesh filename="package://my_directory/meshes/mesh_name.STL" scale="0.001 0.001 0.001" />
  </geometry>
  <material name="material_name"/>
</visual>
<collision>
  <origin xyz="0 0 0" rpy="0 0 0"/>
  <geometry name="name_link_collision">
    <mesh filename="package://my_directory/meshes/simple/mesh_name.STL" scale="0.001 0.001 0.001" />
  </geometry>             
</collision>

</link>

And joints:

<joint name="joint_name" type="revolute">
<parent link="link_name1"/>
<child link="link_name2"/>
<origin xyz="0.036 0.0 -0.15" rpy="0 0 0" />
<axis xyz="0 1 0" />
<limit velocity="6.283" effort="60" lower="${-170.0*toRad}" upper="${34.0*toRad}" />
<dynamics friction="100" damping="10.0"/>       
</joint>

I used same meshes that are in meshes directory, and are .stl type. I tried to change joints friction and damping ( I increase them), and the behaviour change, but the motion of the arm remains. Do you think that this values can be the problem? I connect the arm with a fixed link, even if I connect them without contact, the arm begins to rotate. Can I modify something? Thank you.