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Using gazebo_ros_control with SDF instead of URDF

asked 2015-12-28 16:50:48 -0600

l0g1x gravatar image

My robot has a closed loop linkage, and therefore I cant use a URDF file as URDF only supports tree strucutres. Since SDF supports graph structures, I need to use that in order to simulate my robot. With that being said, I have a 2 actuators as well as a servo that I need to control. Normally, if I was to use an URDF, I would setup the tags in the URDF and then just include the gazebo_ros_control plugin in the URDF. But since the SDF specifications ( do not support tags, im not sure how to get the gazebo ros control plugin to work with my SDF file.

All the ros_control tutorials are based upon URDF, although there are lines in the tutorials stating "for your URDF/SDF file".

Currently, im simply adding this tag to the bottom of my SDF file (i wont include the whole thing for space reasons, for now at least):

    <plugin name="gazebo_ros_control" filename="">


And when i launch the simulation, this is the error I receive in which i cant pinpoint where it comes from (searched through the ros_control plugin code):

[INFO] [WallTime: 1451341627.255617] [0.000000] Loading model xml from ros parameter
[INFO] [WallTime: 1451341627.264068] [0.000000] Waiting for service /gazebo/spawn_sdf_model
[INFO] [WallTime: 1451341628.176358] [0.000000] Calling service /gazebo/spawn_sdf_model
[INFO] [WallTime: 1451341629.094573] [0.001000] Spawn status: SpawnModel: Successfully spawned model
[spawn_model-3] process has finished cleanly
log file: /home/krystian/.ros/log/1fbfd810-adb2-11e5-9876-001c4239cab7/spawn_model-3*.log
[ INFO] [1451341632.830481140, 0.001000000]: Loading gazebo_ros_control plugin
[ INFO] [1451341632.830768827, 0.001000000]: Starting gazebo_ros_control plugin in namespace: /surus_sim
[ INFO] [1451341632.832180175, 0.001000000]: gazebo_ros_control plugin is waiting for model URDF in parameter [/robot_description] on the ROS param server.
[ERROR] [1451341633.054706078, 0.001000000]: Could not find the 'robot' element in the xml file
[ INFO] [1451341633.104100859, 0.001000000]: Loaded gazebo_ros_control.
[ INFO] [1451341633.150833544, 0.022000000]: waitForService: Service [/gazebo/set_physics_properties] is now available.
[ INFO] [1451341633.232730090, 0.053000000]: Physics dynamic reconfigure ready.

The error [ERROR] [1451341633.054706078, 0.001000000]: Could not find the 'robot' element in the xml file is what I cannot figure out as only a URDF file contains the <robot> tag and SDF file does not. surus_sim is the name of the model in the sdf as well.

Any help would be appreciated. Would anyone know if the gazebo_ros_control plugin is meant to work directly with a SDF file? Ive looked around and a few people asked the question a while back but received no responses.

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2 Answers

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answered 2016-01-26 23:49:28 -0600

l0g1x gravatar image

So i did manage to figure out a work around to this, but its super hacked..

So basically to at least be able to use ros_control to control 2 actuators (cant view it in rviz since its a closed linkage) you need 2 robot_description parameters. ros_control loads from robot_description so you need to specify the path to your URDF file (if you only have a SDF, you can reference my answer here about converting from SDF to URDF). You need to specify the URDF file so that you can insert the <transmission> tags and associate which joints you want to use. If you convert URDF -> SDF or vis versa, you will still have all the same joint and link names, for example:

In my URDF file:

      xyz="-0.5588 -0.010351 0"
      rpy="0 0 -3.9443E-31" />
      link="left_top_actuator_link" />
      link="left_bot_actuator_link" />
      xyz="-1 0 0" />
    <limit effort="100.0" lower="0" upper="1.4" velocity="0.2"/>

<transmission name="tran1">
  <joint name="left_mid_actuator_joint">
  <actuator name="motor1">

and then you can see in my SDF file that i will have the same joint name left_mid_actuator_joint

<joint name='left_mid_actuator_joint' type='prismatic'>
    <xyz>-0.685505 -4.48206e-15 -0.728068</xyz>

You then want to load both the SDF to spawn the model in gazebo (this actually eventually gets converted into URDF anyway) and the URDF so that ros_control can find the <transmission> tags. Make sure the URDF is loaded into robot_description and the SDF is loaded into some other parameter which you then pass as an arguement to spawn_model

<param name="robot_description" textfile="$(find rmc_simulation)/surus_sim/robots/surus_sim.URDF" />
<param name="robot_description_sdf" textfile="$(find rmc_simulation)/surus_sim/robots/surus_sim.sdf" />

  args="-sdf -param robot_description_sdf -model surus_sim -x 0 -y 0.6 -z 0.3"

<rosparam file="$(find rmc_simulation)/gazebo_config/control/surus_control.yaml" command="load"/>

<node name="controller_spawner" 
    type="spawner" respawn="false" output="screen"
    args="joint1_position_controller joint2_position_controller joint_state_controller"/>

Where my yaml is:

  # Publish all joint states -----------------------------------
    type: joint_state_controller/JointStateController
    publish_rate: 50  

  # Position Controllers ---------------------------------------
    type: effort_controllers/JointPositionController
    joint: right_mid_actuator_joint
    pid: {p: 100.0, i: 0.01, d: 10.0}
    type: effort_controllers/JointPositionController
    joint: left_mid_actuator_joint
    pid: {p: 100.0, i: 0.01, d: 10.0}

Like i said, this is NOT the proper way of doing this. If someone could still post an answer as to how to directly include ros_control into a sdf, that would be splendid.

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Hi Krystian, Did you try to create a custom plugin?

Adnen gravatar image Adnen  ( 2016-04-04 08:30:07 -0600 )edit

@Adnen , I didnt create any custom plugin for this as I currently dont see any easy way of implementing a fix for this by creating a plugin. Did you have anything in mind?

l0g1x gravatar image l0g1x  ( 2016-04-13 21:27:12 -0600 )edit

@Krystian, I'm still a newbie in ROS and Gazebo, so when I tried your solution I couldn't spawn the model using two files. I followed the Gazebo tutorial and I created a model plugin. Now I can control the velocities of the joints via ROS Topics.

Adnen gravatar image Adnen  ( 2016-04-18 04:30:19 -0600 )edit

Have you played at all with getting this to be viewable inside gzweb? I was passed a ros based launch file to view a ros-controlled vehicle in gazebo. It works fine in gzclient, but I can't seem to figure out how to get the models to show up in gzweb...

srees304 gravatar image srees304  ( 2016-10-28 14:32:19 -0600 )edit

@Adnen Could you elaborate a bit more on how you created a model plugin to control velocities? Do you implement your own PID controller within the plugin or something like that? I am trying to control joints defined in an sdf using a pid controller receiveing commands from ros.

nathan_u gravatar image nathan_u  ( 2016-12-01 00:07:22 -0600 )edit

Hey ,@Krystian, may you introduce your method to convert sdf to urdf, I have trouble launching model in Gazebo.

rosroll gravatar image rosroll  ( 2017-08-07 01:46:57 -0600 )edit

you can refer:

it to solve the issue that URDF not support Closed loop chains.

Angel-J gravatar image Angel-J  ( 2019-11-01 04:47:57 -0600 )edit

Interesting workaround. I have 2 joints. For first I am trying to load controller of type velocity_controllers/JointVelocityController and second one is of type velocity_controller/JointPositionController. But only first one is loaded correctly and can be controlled. Second does not load correctly and gives error: Failed to initialize the controller. Did you encounter this error by any chance?

AidenPierce gravatar image AidenPierce  ( 2022-05-03 07:15:37 -0600 )edit

answered 2019-11-01 04:48:02 -0600

Angel-J gravatar image

updated 2019-11-01 04:49:35 -0600

you can refer:

it to solve the issue that URDF not support Closed loop chains.

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Asked: 2015-12-28 16:50:48 -0600

Seen: 7,665 times

Last updated: Nov 01 '19