model set to be immovable still move after collide with other model

edit

Goal:

I want to set base_link of robot arm fixed to the ground_plane of the world. When moving robot arm collides with surface of wall, because base_link is fixed to ground, arm has to stop moving(or rather, try to moving to penetrate but doesn't work) instead of the base_link moving away.

Background:

I have learnt how to fix box model onto the world from this question, by adding static tag of gazebo tag under robot tag in xacro, reference to using urdf in gazebo

<static>true</static>

However, it doesn't work for the robot arm, though I have done the same trick for robot arm's xacro.

<gazebo reference="base_link">
    <material>Gazebo/White</material>
    <!--test: set base_link fixed to world
     however, it doesn't work! -->
    <static>true</static>
  </gazebo>

Or try this one:

  <!--test: set seven_dof_arm immovable? However it doesn't work!!! -->
  <gazebo>
    <static>true</static>
  </gazebo>

When moving robot arm hits the surface of box, the base_link of robot arm is forced to move away.

Original pose of robot arm

Robot arm yields to the inter-force and base_link moves away