<sensor> elements in .urdf disappearing when converting to .sdf

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Hello everyone, I am working on the URDF description of the IIT COMAN robot, and I'm having some difficulties in inserting the gazebo force_torque sensor in my robot description. Inserting the element directly into the sdf file solves the problem, but at the moment we are working on the urdf description (which gets converted to sdf using gzsdf). Putting the element inside the .gazebo or inside the .urdf doesn't seem to make a difference. The description of the robot we are using is https://github.com/EnricoMingo/iit-co... Could someone give me instructions on how to correctly insert the sensor in the urdf or gazebo files? So far I tried the following syntax:

In the .gazebo file

<gazebo reference="LElbj">
    <cfmDamping>1</cfmDamping>
    <provideFeedback>true</provideFeedback>
    <sensor type="force_torque" name="LElbj_FT">
    <always_on>true</always_on>
    <visualize>true</visualize>
    <update_rate>1000</update_rate>
    </sensor>
</gazebo>

In the .gazebo, after including the right xmlns:sensor in the <robot> element

<gazebo reference="LElbj">
<cfmDamping>1</cfmDamping>
<provideFeedback>true</provideFeedback>
<sensor:force_torque name="LElbj_FT">
<always_on>true</always_on>
<visualize>true</visualize>
<update_rate>1000</update_rate>
</sensor:force_torque>

</gazebo>

In the .urdf

<joint name="RElbj" type="revolute">
<parent link="RShy"/>
<child link="RElb"/>
<origin xyz="0.0 0.0 -0.13519152999999998" rpy="0 0 0" />
<axis xyz="0 1 0" />
<limit velocity="4.0" effort="50" lower="-2.3562" upper="0.0" />
<sensor type="force_torque" name="Relbj_FT">
<always_on>true</always_on>
<visualize>true</visualize>
<update_rate>1000</update_rate>
</sensor>

</joint>

Obviously doing the same thing in the .sdf file works correctly (as shown in the force_torque_demo.world example). Any help would be greatly appreciated.