# Problem changing joint from fixed to revolute

Hello all. I am trying to simulate a UR arm with a robotiq 85 2F gripper on the end. I need to simulate a FT sensor between the wrist_3 link of the UR and the base_link of the gripper. I was going to use this tutorial for this. The FT sensor plugin is attached to a joint, as shown in the tutorial. Now the joint between the UR and the gripper was a fixed joint, which means that when my initial xacro is converted to sdf by gazebo, the fixed joint is ignored, which means the plugin cannot find it. To get around this problem, I changed the joint between the robot and the gripper to a revolute joint with the following limits as suggested here:

<limit velocity="6.5" effort="1000" lower="0" upper="0" />


The problem is, now the simulation does not run at all, with the sdf file missing all of the joints and links after the joint I converted from fixed to revolute (I checked with rosrun xacro xacro my_urdf.urdf.xacro > test.urdf followed by gz sdf -p test.urdf > test.sdf). Very frustrated. Any help would be greatly appreciated!

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I had the same problem I found the solution here: https://answers.ros.org/question/2584... As it shows: "Gazebo ignores links without inertial properties" Hence my issue was solved by calculating simple inertials (e.g. based on cylinder (https://en.wikipedia.org/wiki/List_of... )) and adding them to my robot urdf links:

  <inertial>
<origin xyz="0.0189 0.0375 0.0375" rpy="0 0 0"/> <!--COM-->
<mass value="0.3"/> <!--300g-->
<inertia ixx="7.0312e-04" ixy="0" ixz="0" iyy="4.3425e-04" iyz="0" izz="4.3425e-04"/> <!--realistic inertials-->
</inertial>

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