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Converting URDF to SDF: <preserveFixedJoint> not working

asked 2018-11-19 12:28:37 -0500

FabianMene gravatar image

updated 2018-11-20 15:45:07 -0500

I am trying to convert a robot specified in URDF into SDF by using the URDF parser from Gazebo (gz sdf -p in.urdf > out.sdf).

By default, links connected by fixed joints in URDF get lumped together into single link, as SDF apparently didn't support fixed links in the past:[1]

By adding the tag <disableFixedJointLumping> this lumping is deactivated, and a fixed joint in SDF is approximated by a revolute joint with minPos=maxPos=0. This however doesn't work for me as the joint still allows minimal oscillations.

As I read here [2], a new tag <preserveFixedJoint> has supposedly been added that will make the SDF file have a true fixed joint. However, upon using the tag, the URDF parser throws the following warning:

Warning [] XML Element[preserveFixedJoint], child of element[joint] not defined in SDF. Ignoring[preserveFixedJoint]. You may have an incorrect SDF file, or an sdformat version that doesn't support this element.

Is this tag not part of the final build yet, or can i somehow update my SDFormat to get access to it? I am using Gazebo 7.0.0



Example URDF:

<robot name="robot">

<link name="base">
    <mass value="1"/>
    <inertia ixx="1" ixy="0" ixz="0" iyy="1" iyz="0" izz="1"/>
<joint name="base_to_link" type="fixed">
  <parent link="base"/>
  <child link="link"/>
<link name="link">
    <mass value="1"/>
    <inertia ixx="1" ixy="0" ixz="0" iyy="1" iyz="0" izz="1"/>
<gazebo reference="base_to_link">


Gets converted to:

 <joint name='base_to_link' type='revolute'>
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Can you post your URDF file? Also, did the resulting SDF output have fixed joints when using <preservefixedjoint>?

nkoenig gravatar imagenkoenig ( 2018-11-20 10:45:41 -0500 )edit

I edited the OP to include a simple example. In short, the tag is completely ignored. If I specify <disablefixedjointlumping> alongside, I get a revolute joint with 0 range, otherwise I don't get a joint at all, but the two links are lumped into a single unit.

FabianMene gravatar imageFabianMene ( 2018-11-20 15:48:07 -0500 )edit

1 Answer

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answered 2018-11-21 10:02:49 -0500

azeey gravatar image

The <preserveFixedJoint> tag was added in SDFormat 5.3.0 (2017-11-13) which came out after Gazebo 7.0.0 (2016-01-25). Your best bet would be to install the latest version of Gazebo. If you have to stay on Gazebo7, then use 7.13.1. Installation instructions can be found at

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Thank you, that clears it up. I tried building SDFormat 6.0.1 from source, which did not help with gazebo 7.0.0. Is there some kind of standalone tool that employs the SDFormat library to convert URDF to SDF besides using gazebo?

FabianMene gravatar imageFabianMene ( 2018-11-21 17:10:21 -0500 )edit

Unfortunately, the only tool I know to do this is `gz sdf`. However, you might be able to extract the snippet of code that does the conversion from

azeey gravatar imageazeey ( 2018-11-26 11:06:05 -0500 )edit

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Asked: 2018-11-19 12:28:37 -0500

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Last updated: Nov 21 '18