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After understanding the implementation of screw joint in pr2's torso part. It is clear that URDF don't support screw joint, so to add the functionality of the same we have to use gazebo tags since SDF (For those who don't know:Under the hood gazebo converts urdf to sdf before spawning) totally supports it.

You also need to specify prismatic and continuous joints. Refer to pr2.urdf for full understanding. Following is the snippet from the mentioned above.

<gazebo>
<joint name="torso_lift_screw_torso_lift_joint" type="screw">
  <parent>torso_lift_link</parent>
  <child>torso_lift_motor_screw_link</child>
  <axis>
    <xyz>0 0 1</xyz>
  </axis>
  <thread_pitch>3141.6</thread_pitch>
  </joint>
 </gazebo>
 <transmission name="torso_lift_trans" type="pr2_mechanism_model/SimpleTransmission">
   <actuator name="torso_lift_motor"/>
<joint name="torso_lift_joint"/>
<mechanicalReduction>-47641.53</mechanicalReduction>
<simulated_actuated_joint name="torso_lift_motor_screw_joint" simulated_reduction="3141.6"/>
    </transmission>